DSpace at VNU: The development of mixture, alloy, and core-shell nanocatalysts with nanomaterial supports for energy conversion in low-temperature fuel cells

Nano Energy (2013) 2, 636–676

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

REVIEW

The development of mixture, alloy, and
core-shell nanocatalysts with nanomaterial
supports for energy conversion in
low-temperature fuel cells
Nguyen Viet Longa,b,c,d,e,h,n, Yong Yanga, Cao Minh Thif,
Nguyen Van Minhh, Yanqin Caoa, Masayuki Nogamia,d,g
a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Science,1295, Dingxi Road, Shanghai 200050, China
b
Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences,
Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan
c
Department of Education and Training, Posts and Telecommunications Institute of Technology, Nguyen Trai,
Ha Dong, Hanoi, Vietnam
d
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku,
Nagoya 466-8555, Japan
e
Laboratory for Nanotechnology, Ho Chi Minh Vietnam National University, Linh Trung, Thu Duc, Ho Chi Minh, Vietnam
f
Ho Chi Minh City University of Technology, 144/24 Dien Bien Phu, Ward 25, Binh Thach, Ho Chi Minh City, Vietnam
g

Nagoya Industrial Science Research Institute, Yotsuya, Chikusa-ku, Nagoya 464-0819, Japan
h
Hanoi National University of Education, Vietnam

Received 15 March 2013; received in revised form 16 May 2013; accepted 6 June 2013
Available online 25 June 2013

KEYWORDS

Abstract

Alloy and core-shell
nanoparticles;
Pt-Pd core-shell
nanostructure;
Supports;
Oxygen reduction
reaction (ORR);
Proton-exchange
membrane fuel cell

In this review, we present the development of Pt-based catalysts and the uses of Pt-based
bimetallic and multi-metallic nanoparticles with mixture, alloy and core-shell structures for
nanocatalysis, energy conversion, catalytic nanomaterials and fuel cells (FCs). The important
roles of the structure, size, shape, and morphology of Pt and Pd nanoparticles, which can be
engineered via chemistry and physics methods, are discussed. To reduce the high costs of FCs,
Pt-based mixture catalysts can be used with cheaper base metals. Importantly, Pt-based alloy
and core-shell catalysts with very thin Pt and Pt-Pd shells, Pt-noble-metal coatings or Pt-noblemetal skins can be used as Pt-based catalysts in FCs, typically low- and high-temperature

n

Corresponding author. Tel.: +86 21 52414321;
fax: 86 21 52414219; Mob.: +81(0)90 9930 9504; +84 (0)94 6293304
E-mail addresses: , nguyenviet
, (N.V. Long),
(M. Nogami).
2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
/>

The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
(PEMFC);
Direct methanol fuel
cell (DMFC)

637

proton-exchange membrane FCs (PEMFCs) and direct methanol FCs (DMFCs). On the basis of the
latest scientific reports and research results, new catalytic models of the possibilities and
relations of both Pt-based catalysts and supports, which are typically carbon, glasses, oxides,
ceramics, and composite nanosized nanomaterials, are proposed for the further investigation of
catalytic surface roles to achieve crucial improvements of Pt-based catalysts. The various
applications of Pt-based catalysts with specific supports in PEMFCs and DMFCs are also
discussed. The nanosystems of as-prepared Pt nanoparticles as well as Pt-based nanoparticles
with various mixture, alloy, and core-shell structures are of great importance to nextgeneration FCs. Low-cost Pt-based mixture, alloy, and core-shell nanoparticles have been
shown to have the advantages of excellently durability, reliability, and stability for realizing FCs
and their large-scale commercialization. The latest trend in the use of new non-Pt alloys or new
alloys without Pt but they have high catalytic activity as the same as to that of Pt catalyst has
been discussed. We propose a new method of atomic deformation, and surface deformation as
well as nanoparticle and structure deformation together with plastic and elastic deformation at
the micro- and nano-scale ranges by heat treatments at high temperature can be applied for

enhancement of catalytic activity, stability and durability of Pt catalyst and new non-Pt alloy
and oxide catalysts in future while the characteristics of size and shape can be retained.
Finally, there has been a great deal of demand to produce catalytic nanosystems of
homogeneous Pt-based nanoparticles because of their ultra-high stability, long-term durability,
and high reliability as well as the durable and stable nanostructures of Pt-based catalysts with
carbon, oxide and ceramic supports. Such materials can be utilized in FCs, and they pose new
challenges to scientists and researchers in the fields of energy materials and FCs. In addition,
the importance of Pt based nanoparticle heat treatment with, and without the nanoparticle
surface deformation or nuclei surface deformation is very crucial to discover a new robust Pt
based catalyst for alcohol FCs. The new urgently trend of producing various novel alloy catalysts
replacing Pt catalyst but similar catalytic activity is confirmed in the avoidance of the
dependence of Pt-noble-metal catalyst in both the cathode and the anode of FCs.
& 2013 Elsevier Ltd. All rights reserved.

Contents
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 637
Low-temperature fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638
Fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638
Proton-exchange membrane fuel cell (PEMFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638
Direct methanol fuel cells (DMFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639
Platinum catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 640
Characterization of Pt- and Pd-based nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643
Development of Pt-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 644
Development of Pt-Ru-based catalysts (PtxRuy and PtxRuy/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646
Development of Pt-Rh-based catalysts (PtxRhy and PtxRhy/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647
Development of Pt-Au-based catalysts (PtxAuy and PtxAuy/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647
Development of Pt-Cu-based catalysts (PtxCuy and PtxCuy/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647
Development of Pt-Ni-based catalysts (PtxNiy and PtxNiy/support). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648
Development of Pt-Co-based catalysts (PtxCoy and PtxCoy/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648
Development of Pt-Sn-based catalysts (PtxSny and PtxSny/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648

Development of Pt-Fe-based catalysts (PtxFey and PtxFey/support) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648
Development of Pt-and-Pd-based nanoparticles (PtxPdy and PtxPdy/support) . . . . . . . . . . . . . . . . . . . . . . . 649
Development of Pt- and Pd-based catalysts with carbon and oxide supports . . . . . . . . . . . . . . . . . . . . . . . . . . 661
Development of novel alloy-based catalysts (alloy and alloy/support) without Pt . . . . . . . . . . . . . . . . . . . . . . . 663
Stability and durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665

Introduction
Several known direct chemcal-electrical energy conversion
processes in various fuel cells (FCs) with high efficiency and
low pollutant emission have been studied [1,2]. Recently, the

U. S. Department of Energy Fuel Cell Technologies Program
(DOE Program) and the New Energy and Industrial Technology
Development Organization (NEDO Program) in Japan have
financially supported large research and development programs (R&D) concerning FCs and FC systems for stationary,


638
portable, and transportation applications, such as FCs for cars
and vehicles as well as for portable devices, such as laptop
computers and mobile devices [3–7]. In PEMFCs and DMFCs, Pt
catalysts are mainly used to provide catalytic activity, typically for reactions such as oxygen reduction reactions (ORRs)
or hydrogen evolution reactions (HERs)/hydrogen oxidation
reactions (HORs). In addition, very important ORRs have also
been examined in studies of Pt-based catalysts with transition
metals, typically Pt, Ir, Os, Pd, Rh, and Ru, for their applications for the enhancement of current density in FCs [8–10]. Ru,
Os, Rh, Ir, Pd, Pt, Ag, and Au are precious metals of great

importance in catalysis [11]. A Pt-based catalyst in the
catalyst layer is one of the main elements of powergeneration membrane-electrode-assembly (MEA) technology
in PEMFCs and DMFCs [7], which utilizes various protonconducting membranes, such as Nafion-type membranes
[7,18]. The roles of the dissolved gases (O2, H2, and N2) in
the solvent medium have been proven via their interactions
and reactions to certain shapes and morphologies of Pt and Pd
nanoparticles [12]. The specific catalytic properties of metal
nanoparticles on various supports with respect to the effects
of size, shape, morphology, porosity, surface, structure,
support, composition, and oxidation state have been discussed
previously [13]. Besides, it has been established that the
ability to control particle sizes is very crucial to create good
and robust Pd-based catalysts in place of Pt [14]. At present,
various proton-exchange membranes with high quality and
long-term stability are used for FC applications below or above
100 1C [15]. The best efforts toward improving the electrocatalytic activity of pure Pt catalysts have been conducted in
the process of testing Pt-based catalysts in DMFCs and
PEMFCs [16]. In addition, a large number of the various
support materials for PEMFC and DMFC electrocatalysts have
been reviewed [17,18]. Analogously, other noble-metal
electrocatalysts can be used in a promising structural
paradigm for DMFCs [19]. The catalyst layer is of great
importance to efforts to decrease the very high cost of FC
products, as it constitutes more than 50% of the cost. For
this reason, Pt and Pt-based alloys have been developed for
next-generation PEMFCs and DMFCs [6,20]. Pt-based nanowires can be used as potential electrocatalysts in PEMFCs
[21]. The improved manufacture of Pt nanoparticles with a
well-defined size, composition, and shape via chemistry can
lead to a very good catalyst with high selectivity and thermal
stability, especially in future FCs [22,23]. The challenges of

designing Pt-based electrocatalysts have been considered in
the context of automotive FC applications [24]. Moreover,
various novel Pt-based catalysts have been proposed for
PEMFCs [25]. Proposals and ideas for novel low-Pt-loading
catalysts in PEMFC or DMFC systems have been presented,
and the catalytic activity and stability of ORR catalysts that
use metal and bimetal nanoparticles in various FCs have
been compared [26,27]. The avenues for improving Pt- or
Pd-based catalysts involve shape- or size-dependent catalytic activity, instability and surface-area loss, dealloying
phenomena, and the synergistic effects of bimetallic catalysts. Incredible differences between the catalytic activity
of a homogeneous catalytic system of synthesized Pt nanoparticles and that of an inhomogeneous catalytic system of
synthesized Pt nanoparticles have been observed in the
relations between their preparation processes, structures
and properties. The rapid development of direct alcohol FCs

N.V. Long et al.
(DAFCs) has primarily involved the design and discoveries of
new materials and catalysts [28]. The HOR and MOR
mechanisms have been intensively studied with the goals
of improving the long-term durability, stability and cost of
PEMFCs and DMFCs. In particular, the price of FCs mainly
depends on the price of the Pt-based catalysts
or on the design of catalysts that use a low Pt weight or no
Pt at all.
In this review, we present the latest developments in asprepared Pt-based nanoparticles for use as the Pt-based
catalysts for alcohol FCs, particularly PEMFCs and DMFCs.
The issues of nanosized ranges of Pt-based nanoparticles are
discussed with respect to the related degrees of stability
and durability in alcohol FCs. The advantages of polyhedrallike and spherical-like shapes and morphologies are also
discussed for the purpose of identifying the best Pt-based

catalysts for various applications of growing concern. It is
certain that the issues of tuning, controlling, and shaping
Pt-based nanostructures within certain size and shape
ranges are usually much more difficult than controlling the
metal compositions of Pt-based nanostructures. To achieve
large-scale commercialization of FCs, various designed Pt
nanoparticles in the mixture, alloy and core-shell categories
have been tested to determine whether they meet the
demands of high catalytic activity. Importantly, in this
review, we propose new catalytic models for the use of
pure Pt-based nanoparticles on and inside of supports,
leading to improvement in the catalytic activity and sensitivity of Pt-based nanoparticles that are loaded on various
supports, such as carbon, oxide, and ceramic, to maximize
their durability and stability. The excellent recently
achieved advantages of Pt-based catalysts with mixture,
alloy and core-shell nanostructures have been confirmed in
testing and measurements.

Low-temperature fuel cells
Fuel cells
Proton-exchange membrane fuel cell (PEMFC)
In principle, a PEMFC with a polymer membrane electrolyte
and a pure Pt catalyst has a low operation temperature of
o90 1C. The electrochemical reactions that occur in a lowtemperature PEMFC are as follows [29,52].
Cathode: 1/2 O2+2H++2e--H2O (ORR);
Anode: H2-2H++2e- (HOR)

(1)

Overall reaction: 1/2 O2+H2-H2O (Fuel cell reaction) (2)

At present, Pt-metal catalysts are the most active toward the
hydrogen oxidation reaction (HOR) that occurs at the anode in
PEMFCs. To achieve a low-cost FC design, the very high Pt
catalyst loading must be decreased. Two strategies are under
investigation for reducing the Pt loading in PEMFCs: the
fabrication of binary and ternary Pt-based alloyed nanomaterials and the dispersion of Pt-based nanomaterials onto
high-surface-area substrates, such as carbon nanomaterials.
To reduce the cost associated with pure Pt catalysts, Pt-based
catalysts have been widely developed. At present, carbon
monoxide (CO) poisoning still occurs at the anode, and CO can
heavily adsorb on a Pt-based catalyst and block the hydrogen


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
oxidation. To improve the stability and activity of the HOR on
a pure Pt catalyst, different base metals can be added to
reduce CO poisoning. Because of such efforts, Pt- and Pdbased catalysts with various mixture, alloy, and core-shell
nanostructures have been developed. In addition, novel COtolerant catalysts will necessarily be developed in large
amounts but at low cost. Thus far, the Pt-based bimetallic
nanoparticles for Pt-based bimetallic catalysts that have been
studied are Pt-Ru, Pt-Fe, Pt-Cu, Pt-Mo, Pt-Ni, Pt-Sn, Pt-Re,
Pt-W, Pt-Ir, Pt-Os, Pt-Rh, Pt-Pd, Pt-Au, and Pt-Ag in mixture,
alloy and core-shell structures [55,376,377].
In addition to various alloy and core-shell nanostructures,
a wide variety of material mixtures can be used for the
development of PEMFCs. As a result, the catalytic activity and
stability of Pt-based alloy and core-shell catalysts are greatly
enhanced with respect to those of pure Pt catalysts.
In recent research, various binary, ternary and quaternary
Pt-based catalysts using metals such as Pt, Ru, Rh, Pd, Ir, Os,

Au, Ag, Cu, Ni, Fe, Co, Mn, Zn, Mo, Sn, and W have been
prepared for the purpose of obtaining catalysts with higher
catalytic activity and stability [2,56,57]. A shape transformation from Pt nanocubes to tetrahexahedra with a size of near
10 nm, leading to an influence on the catalytic activity of a Pt
nanoparticle catalyst, was observed in one study [58]. In
general, such tetrahexahedral Pt nanoparticles in this size
range had a high density of step atoms. They exhibited an
enhancement of electrocatalytic activity toward ethanol
oxidation [58]. However, the complexity of the preparation
processes was high. These catalysts can be used on various
carbon nanomaterials, such as CNT and Vulcan-XC-72R, to
provide a significant enhancement of catalytic activity.
Modified catalysts that do not contain Pt are being considered
to avoid the dependence on Pt metal. The use of various PtRu-alloy/C catalysts in the anodes of DMFCs has been
reviewed previously [56]. These catalysts can also be used
as CO-tolerant catalysts in the anodes of PEMFCs. In addition,
Pt-alloy catalysts have exhibited improved catalyst behavior
for novel cathodes of both PEFCs and DMFCs [56]. Because of
their high stability and durability, Pt-alloy catalysts can be
used for the large-scale commercialization of automotive FCs
[57]. The potential PEMFC applications of Pt- and Pt-Ru-based
catalysts of mixed metal nanoparticles have been discussed
previously [2,56], especially those of Pt/C and Pt-Ru/C
catalysts that use ordered mesoporous carbon [2,56,57].

Direct methanol fuel cells (DMFC)
Instead of hydrogen fuel, methanol is used in DMFCs. It is
known that DMFCs have a low operation temperature of 40–
100 1C when they are constructed using MEA technology
with a proton-exchange membrane, such as Nafion, as the

electrolyte, and there is a direct MOR at the anode of
DMFCs. Methanol offers advantages over hydrogen as a fuel,
including ease of transportation and storage and a high
theoretical energy density. Pt is the most promising candidate among the pure basic metals for application in DMFCs
because Pt exhibits the highest activity to the dissociative
adsorption of methanol. However, a pure Pt catalyst is
easily poisoned by CO, which is produced as a by-product of
the MOR at room temperature. Pt-based catalysts can be
used in the electrodes, both the anode and the cathode. In

639

principle, the operation of a DMFC primarily depends on the
chemical reactions at the electrodes, as follows.
Cathode: 3/2O2+6H++6e--3H2O (ORR);
Anode: CH3OH+H2O-CO2+6H++6e- (MOR)

(3)

Overall reaction: CH3OH+3/2O2-CO2 +2H2O (Fuel cell
reaction)
(4)
Currently, hydrogen/oxygen PEMFCs and DMFCs mostly employ
Pt-based catalysts for portable power generation, especially in
compact mobile devices. At present, alcohol technology has
been well developed for high-purity Pt catalysts. Therefore, the
surface kinetics of the Pt catalyst and its catalytic activity with
respect to hydrogen and alcohol fuels have been extensively
investigated and demonstrated [1–10].
To improve the performance of DMFCs and reduce their

costs, Pt-based catalysts must yet be considerably further
developed. Thus far, Pt-Ru-based catalysts have been very
successfully used for the cathode reaction in DMFCs, but
only at high cost [59–62]. The CO poisoning that is strongly
adsorbed on Pt atoms on the surfaces of Pt-Ru is addressed
by reduction by the Ru metal atoms, leading to the poisoned
Pt surface becoming very active to the MOR with the
following bi-functional mechanism [18,59,61].
Ru+H2O-Ru-OH+H++eÀ

(5)

Ru-OH+Pt-CO-Pt+Ru+CO2+H++e(General mechanism)

(6)

The above two equations are the most general mechanisms of CO-poisoning reduction in most Pt-based alloy
catalysts that use low concentrations of Ru or other various
base metals. Therefore, exploiting the bi-functional
mechanism (or various possible multi-functional mechanisms
for Pt-based multi-metal catalysts) for the reduction of CO
poisoning is a good way to improve FC systems, such as
PEMFCs and DMFCs, as a whole. Similarly, Pt-based bimetallic catalysts have been developed for DMFCs. These can be
many types of Pt-M-based catalysts, where M may be Co, Ni,
Fe, Cu, Cr, or other cheap and abundant metals. Thus, there
are many available Pt-based mixture catalysts, including
various binary, ternary and quaternary Pt-based catalysts.
Accordingly, a Pt-Ru-Rh-Ni-based catalyst has been prepared
for the sake of achieving a high MOR rate in DMFCs, although
it suffers from the high complexity of its composition.

Similar trends have led to a reduction in the amount of Pt
metal used [63] and the development of new Pt-based
catalysts with carbon nanomaterials, such as Pt/C, PtSn/C,
PtRu/C, and Pt/CeO2/C [381] as well as Pt/FeRu/C, Pt/
NiRu/C, and Pt/CoRu/C [382], with the goal of reducing the
total cost of DMFCs. We must develop Pt-based catalysts, or
good catalysts without Pt, that not only resist CO poisoning
but also prevent it and maintain CO poisoning in the MOR at
a suitable minimal level, or we must develop CO-tolerant Ptbased catalysts. In CO poisoning, intermediates that are
generated by oxygen reduction, such as hydroxyl and oxide
groups, can firmly adsorb on the surfaces of the nanocatalysts, which will decrease the overall performance of the
catalytic activity. The issue of CO poisoning can be understood by the mechanisms of Pt-COad and a second metal
atom, a third metal atom, and so forth reacting with their
OH groups, e.g., M-OH reacts to form CO2. This


640
characterization is a so-called bi-functional mechanism (Ptbased bimetallic nanoparticles with alloy and mixture structures) or a complex multi-functional mechanism (Pt-based
multi-metal nanoparticles). Accordingly, a simple solution is
to find a suitable metal that acts against CO poisoning and
can be used in Pt-based-alloy catalysts. The known Pt/C, Ptoxides/C, Pt-Ru/C, and Pt-Ru-oxides catalysts are promising
candidates for novel DMFC electrodes. The various carbon
nanomaterials used are carbon (C) black, C-nanotubes
(CNTs), C-nanofibers (CNFs), C-nanowires (CNWs), etc., and
the other various supports are oxides, glasses, ceramics,
composites, or mixtures, typically WO3, SnO2, SiO2, etc [64].
The primary issue facing Pt/C-based catalysts is the corrosion of the carbon by water over time [65]. Therefore, the
investigation of the catalytic behavior of metal-, bimetal-,
and multi-metal-based nanoparticles on carbon supports as
well as novel supports such as glass and ceramics is

important for future developments in nanocatalysis, energy
conversion, PEMFCs and DMFCs. At present, single metal
nanoparticles, such as Cu, Ag, Au, Pt, Pd, Ru, Rh, Ir, Os, Ni,
Fe, and Co, and the chemical and physical methods by which
they can be synthesized are of importance to various FC
sciences and technologies [66–69].
FC materials with non-homogeneous sizes and heteromorphology from several nm to μm can be synthesized with
the ultrasound method. However, the challenge is to obtain
a homogeneous size and morphology [70]. At present, pure
ultrafine Pt clusters of approximately 0.88 nm on commercial carbon can be used for the DMFC reactions [71].
However, a quick collapse in the nanostructure of Pt
nanoparticles and a corresponding decline in catalytic
activity have been discovered in the particle-size limit of
o1 nm [72]. This phenomenon affects the catalytic sensitivity and activity of Pt-based nanostructures. Most ultrafine
or very small metal nanoclusters exhibit very high catalytic
activity but no confirmed stability or durability; for example, very tiny Au and Pt clusters can be used to achieve
much higher ORR rates, but their stability and durability
cannot be reliably confirmed [73]. In addition, Pt nanoparticles of approximately 2–5 nm in size on carbon supports
are known to be the best catalysts for ORRs, and very small
Pt clusters exhibit very high catalytic activity for the fourelectron reduction of oxygen molecules [74]. At present,
metal and bimetal nanoparticles of different shapes have
high potential for applications in catalysis and energy
[75,76]. Among the noble nanoparticles, platinum (Pt) and
palladium (Pd) are of importance for their utilization in the
catalyst layers of PEMFCs and DMFCs in both the cathode
and the anode. In general, most noble-metal nanoparticles
can be shaped in size and morphology below or above
1000 nm; sizes of less than 10 or 20 nm are of particular
interest for their excellent potential for application in
catalysis, biology and medicine because of their large

quantum-size and surface effects. Small, strong adsorbates
or adsorbents (e.g., IÀ, CO, amines) are crucial for providing size and shape control during the synthesis of Pd and Pt
nanoparticles [77–80]. A Pt/C catalyst that uses functionalized ordered mesoporous carbon has been utilized for
DMFCs [81]. In addition, a graphene-nanoplate-Pt catalyst
has been demonstrated to serve as a high-performance
catalyst for DMFCs [82]. At the same time, various types
of new membranes have been developed for DMFCs with the

N.V. Long et al.
use of the above modified catalyst layers [83]. In many
cases, Pd nanoparticles can be replaced with Pt nanoparticles, despite alcohol the lower catalytic activity of the
latter. The effect of pseudo-halide thiocyanate ions on the
seed-mediated growth of Pd nanocrystals has been investigated [84,85]. In the synthesis of metal nanoparticles via
chemistry and physics, most of the metal nanoparticles are
shaped to size and morphology ranges of less than 10 nm,
approximately 100 nm, and 1000 nm or more [86]. In general, nanoparticles of a homogeneous size and morphology
offer excellent properties in practical applications. We
suggest that the ability to shape metal nanoparticles or
bimetallic nanoparticles of both noble and cheap metals to
size and morphology ranges of approximately 10 nm,
approximately 20 nm, and approximately 30 nm is extremely important to catalysis and FCs. The size and morphology of Pt nanoparticles or Pt-based nanoparticles can be
maintained by storing these nanosystems in various suitable
solvents. The issues of the dissolution of noble metals (Pt,
Au, Pd, Rh, and Ru) from the nanoparticles or the catalysts
in electro-chemical measurements and the experimental
conditions of DMFCs or PEMFCs must be further investigated. To improve the catalytic activity and durability of
pure Pt catalysts, the specific effects of synergistic, dealloying, Janus, and composition effects have been given
particular consideration in Pt- and Pd-based mixture, alloy
and core-shell nanoparticles. The most popular nanosystems
of Pt-based catalysts for alcohol FCs, PEMFCs and DMFCs are

those that utilize bimetallic nanoparticles. They include
PtxAuy, PtxRhy, PtxPdy, PtxCuy, PtxNiy, PtxFey, and PtxSny as
well as polymetal nanoparticles, such as PtRuRh. The
reduction of the very high cost of pure Pt catalysts the
focus of much current research. At the same time, it may
also be possible to enhance the catalytic activity for the
HOR, reduce CO poisoning (by using the various base metals
discussed above), and increase the ORR rate (for the sake of
a large current density), all of which contribute to the high
stability and durability of FCs. The composition of Pt-noblemetal- and cheap-metal-based catalysts can be adjusted to
obtain various desirable Pt-based catalysts of high catalytic
activity, high durability and high sensitivity by reducing the
level of CO poisoning.

Platinum catalyst
To date, pure Pt catalysts have been the most commonly
used catalysts for FCs. Novel FC catalysts (Pt-based catalysts and carbon or oxide supports) have been under
continuous development, but they have not completely
replaced Pt catalysts. In recent years, Pt nanoparticle
catalysts have played a key role in the sustainable hydrogen
economy because Pt is the best catalyst for the hydrogen
oxidation and oxygen reduction reactions at the anodes and
cathodes of FCs [23,29]. At present, many metal nanoparticles, such as Au, Ag, Pt, Pd, Cu, Rh, Rh, Ru, Ni, Co, Fe, and
Mo nanoparticles, are used in electrocatalysis. The controlled synthesis of noble- and cheap-metal nanoparticles in
particular size ranges of 10 nm (1–10 nm), 20 nm (1–20 nm),
30 nm (1–30 nm), etc. with well-controlled shapes and
morphologies, such as the polyhedral and polyhedral-like
categories (tetrahedra, octahedra, cubes, etc.) and the

The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
spherical and spherical-like categories, is critical for FC
electrocatalysts in DMFCs and PEMFCs, especially the synthesis of Pt nanoparticles in the range of 10 nm [1,2,29–31].
However, among the above-listed elements, Pt noble
metal is known to be the best metal electrocatalyst for
the FC reactions, which include the hydrogen evolution
reaction (HER)/hydrogen oxidation reaction (HOR), the
oxygen oxidation reaction (ORR), and the electrooxidation of carbon monoxide (CO). The issue of CO poisoning in the HOR on pure Pt catalysts is well known. The effect
of such CO poisoning of Pt-based catalysts at the anode
should be significantly reduced to achieve the best FC
performance. To minimize CO poisoning, Pt-based catalysts
can be heated at high temperatures in N2/H2 [134,135]. As
another method of dealing with this problem, Pt-based
bimetallic and multi-metal nanoparticles have been developed for use in the catalyst layer [389]. In this way, the CO
poisoning will be controlled by reduction by a second metal,
a third metal, or even more. This is an excellent method of
avoiding and preventing anode failure. In addition, highly
CO-poisoning-resistant catalysts must be developed for the
anode or fuel electrode. The surfaces of the Pt nanoparticles are very important to nanocatalysis. Polyhedral Pt
nanoparticles typically exhibit mainly low-index facets of
(100), (110), and (111), although they do include high-index
facets, (h k l). Nevertheless, in the typical TEM method, a
certain number of the (h k l) planes of the low- and highindex planes was determined in the selection rule for the
various types of fcc crystal structures because of the
limitations of the TEM method. In addition, certain peaks
have been characterized as (111), (200), (220), (311), and
(222) peaks by the XRD method [32]. The dependence of the
catalytic activity on the surfaces of the prepared Pt
nanoparticles has been determined for various categories
of the Pt nanostructures [33–35]. Furthermore, catalytically

active Pt atoms belonging to the low-index facets of (111),
(100) and (110) of the Pt nanoparticles have been shown to
have high stability and durability in nanocatalysis, as
indicated by electrochemical measurements, and good
reconstruction in the highest catalytic reactions in various
FCs [36–38].
Pt nanoparticles that have been engineered via facile and
successful preparation methods based on chemistry and
physics can be used for FC applications, such as PEMFCs
and DMFCs [66–70,155]. In typical electrochemical measurements of pure Pt catalysts, the electrode is usually swept
from E = - 0.2 to E= 1.0 V with respect to the saturated
hydrogen electrode (SHE). In such measurements, specific
regions are observed in the cyclic voltammogram that
exhibit catalytic activity and surface kinetics for the case
of pure Pt catalysts. Regarding hydrogen catalytic activity,
the HER on the Pt catalyst is described by the important
Volmer, Tafel, and Heyrovsky mechanisms. In addition, the
Volmer-Tafel and Volmer-Heyrovsky mechanisms also occur
in the complex combinations of the basic mechanisms. As a
rule, the surface kinetics and chemical activity that occur at
the surface of electrodes that contain pure Pt catalysts as
well as those that contain mixtures of Pt/supports are
characterized by the catalytic activity of Pt with respect
to hydrogen and oxygen atoms and to water [1–10,39–41]. In
the mechanisms of the catalytic activity, selectivity and
sensitivity of pure Pt nanoparticles with respect to

641

Figure 1 Trends in oxygen reduction activity as a function of

the oxygen binding energy. The highest ORR of Pt and Pd base
metals are theoretically calculated in the proof as important
factor to enhance the current density in fuel cells, PEMFC and
DMFC. Reprinted with permission from: J.K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J.R. Kitchin, T. Bligaard,
H. Jónsson, J. Phys. Chem. B 108 (2004) 17886-17892 [42].
Copyright © 2004 American Chemical Society.

hydrogen, there are surface chemical and physical differences that arise as the pure Pt-metal catalyst changes into
PtO oxide, but these differences may exist only on the
surfaces. The important effects of the thickness of the PtO
oxide, which are relevant to the inverse change of the PtO
oxide into pure Pt-metal catalyst during the general processes at the electrodes, have not yet been thoroughly
examined. Therefore, further study of the formation of Pt-H
and Pt-O through the catalytic activity of Pt with H and O is
very crucial and may lead to the enhancement of the
catalytic activity of Pt-based catalysts, which, in turn, will
improve low-temperature FCs, PEMFCs and DMFCs. In acidic
electrolytes, the ORR is observed to follow two main
pathways, one with four electrons transferred directly and
one with two electrons transferred consecutively [39–41]. To
evaluate the catalytic activity of the prepared nanocatalysts, the electrochemical active surface area (ECA) of the
various pure Pt-based catalysts has been calculated to be
QH0.21 Â LPt [39]. Here, the specific charge transfer (QH)
due to hydrogen adsorption and desorption is calculated as
QH = (QTÀQDL)/2, where QT denotes the total amount of
charge during hydrogen adsorption and desorption on Pt
sites, and QDL is related to the charge due to the doublelayer capacitance. The area within the curve in the relevant
region can provide QT and QDL and can be obtained by taking
the area under the same region, but with upper and lower
boundaries of horizontal lines passing through a data point

outset of the hydrogen desorption/adsorption waves. A
conversion factor of approximately 0.21 (in mC cmÀ2) can
be used for a monolayer of hydrogen. In this context, the
value of LPt corresponds to the loading of the Pt catalyst on
the glassy carbon surface (in mg cmÀ2) [39–41].
As discussed previously, the catalytic characterization of
Pt nanoparticles has involved investigation into the size,
shape, morphology, and structure of Pt nanostructures as
well as various effects of composition modifications. To
obtain a performance enhancement with respect to pure Pt


642
catalysts, Pt-based catalysts of various categories, such as
mixture, alloy, and core-shell structures, have been developed. In the operation of FCs, the most important concerns
are that the ECA and the QH should be high and that the
loading LPt should be low. At present, the phenomena of
the ORR kinetics and mechanisms that occur on Pt catalysts
have been intensively investigated, but a very high overpotential loss has been observed, indicating that hydrogen
peroxide (H2O2) is formed before the formation of water
molecules. Therefore, very high loadings of Pt must be
used for the operation of FCs with large currents. It is
known that the Pt catalyst has exhibited the highest
activity with respect to the ORR mechanism. In recent
years, much research has been conducted with the goal of
understanding the ORR in catalytic systems with Pt catalysts that are designed to use the minimal level of ultralow Pt loading. However, the challenges of low Pt-catalyst
loading, high performance, durability, and cost-effective
design in FC systems remain very crucial for the large-scale
commercialization of such systems. The cyclic voltammetry (CV) results of various Pt nanoparticles (spherical,
cubic, hexagonal and tetrahedral-octahedral morphologies) in HClO4 or H2SO4 have illustrated the strong structural sensitivity of as-prepared Pt nanoparticles. The most

basic and stable (111), (100) and (110) planes with high
densities of highly active Pt atoms have been confirmed in
the active sites of specific catalytic activity, such as in the
edges, corners, and terraces [36,111,112]. Further investigations of the HER, HOR, and ORR mechanisms in asprepared catalyst layers are crucial to obtaining high
currents. Nanostructured catalysts must have high hydrogen solubility and reactivity. In addition, fast, sensitive,
and stable hydrogen desorption/adsorption could be very
crucial for FCs. Currently, catalysts that are designed for
FCs must have a high and stable ORR rate. To obtain large
current densities in PEMFCs and DMFCs, we must study the
ORR mechanism in detail and develop novel Pt-based
catalysts. Interesting studies have performed densityfunctional-theory (DFT) calculations of the energies of
the surface intermediates for a number of metals, both
expensive, rare, noble metals such as Pt, Pd, Au, and Ag
and abundant, cheap metals such as Cu, Ni, Fe, and Co, as
shown in Figure 1 [42,43]. As a result, a clear volcanoshaped relationship was established between the rate of
the cathode reaction and the oxygen-adsorption (Oad)
energy. From this useful model, which involves the dband center or d-state of various base metals, Pt and Pd
were determined to be the two elements that are the best
choices for cathode materials (Figure 1). It is likely that a
Pd-based catalyst can replace a Pt-based catalyst in the
cathode for the ORR in PEMFCs and DMFCs, reducing the
dependence on Pt, which is an expensive and rare precious
metal [42–44]. To enhance the catalytic activity, stability
and durability of the catalysts, various Pt/support catalysts have been studied as part of the continuous development of low-temperature FCs, such as PEMFCs and
DMFCs. Pt, Pd, and Pt- and Pd-based bimetallic nanoparticles with sizes of 10 nm and 20 nm on carbonnanomaterial supports have been evaluated for potential
applications involving the direct methanol oxidation reaction (MOR). It has been found that pure Pt nanoparticles
must be highly dispersed on the supports to obtain the best

N.V. Long et al.
catalytic activity for the operation of FCs. In catalyst

engineering, the microwave-assisted polyol method has
been used for the preparation of Pt/C, Ru/C and PtRu/C
catalysts for the MOR [45]. PtRu/C electrocatalysts and
PtRu-graphitic mesoporous carbons (GMCs) have been
synthesized for evaluation for MOR applications. The
results indicate that the role of the various pore sizes of
the GMCs is especially important in determining the
performance of DMFCs [46]. In one study, it was found
that the electrodeposition of Au, Pt, and Pd metal nanoparticles on carbon nanotubes (CNTs), such as singlewalled CNTs (SW-CNTs), could be performed via a twoelectrode arrangement. An issue of concern for Pt/C
catalysts is the corrosion of the carbon by water, which
can cause a significant decrease in the catalytic activity in
both PEMFCs and DMFCs. In the future, we expect that
novel supports (metals, alloys, oxides, and ceramics) with
the same catalytic activity as carbon will be developed
that can replace the carbon supports. However, carbon
supports are of importance to low-temperature FC catalysts [47]. Accordingly, a microwave-heated polyol synthesis of a Pt/CNTs catalyst for methanol electro-oxidation
has been presented [48]. Pt is expensive, and Pd can be
used to replace Pt in many cases [49–51]. Some authors
have proven that by adding a very small amount of Pt (5 at
%) to a Pd-based catalyst, the HOR activity of the Pd-based
catalyst can increase to nearly the same as that of a pure
Pt catalyst. These results can serve as a foundation for the
suitable utilization of noble Pt metal in FCs [52,378], for
example, Pd-based electrocatalysts with a thin layer of Pt
(5 wt%) [378]. The catalytic properties of as-prepared Pt
nanoparticles are strongly affected by the nature of their
surface structure and internal structure, including factors
such as roughness, sharpness, flatness, smoothness, porosity, the atomic density of the particle surface, chemical
bonding, and chemical and structural changes [53]. In the
recent research, Pt-based metallic and bimetallic nanoparticles with alloy, core-shell, and mixture nanostructures

have been synthesized and developed for the purposes of
catalysis, energy conversion, environmental friendliness,
and FCs. In Figure 2, pure as-prepared Pt nanoparticles
that were prepared with shape-controlled synthesis are
shown to be in the size range of 10 nm with a highly
homogeneous distribution in size and morphology, which is
required for a good characterization of a catalytic system.
In most cases, the chemical reactivity can be increased
through nanostructuring because of the resultant increase
in the ratio of reactive surface atoms to non-participating
bulk atoms. In a particle with a diameter of 20 nm, only
approximately 10% of the atoms are on the surface, while
in a particle with a diameter of 1 nm, the proportion of
reactive surface atoms is approximately 99% [54]. In
nature, a given number of larger Pt particles has a much
higher durability and stability than the (larger) number of
small Pt nanoparticles with the same total weight of Pt
metal, but their catalytic activity is much smaller than
that of the small Pt particles. In other words, a nanoparticle with a very small particle size, in the range of 10 nm,
has a larger relative number of surface atoms and therefore a higher catalytic activity compared to a larger
particle, but it is clear that very small particles have less
structural durability and stability. Because of the high


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion

643

Figure 2 (a)-(f) TEM images of the uniform Pt nanoparticles by a modified polyol method. The homogeneous polyhedral Pt
nanoparticles of main tetrahedral, cubic, and octahedral morphologies and truncated shapes and morphologies were clearly observed in

the size range of 10 nm. This proves that highest quantum-size effect in catalytic activity, sensitivity, and selectivity are achieved in the
size range of 10 nm. The surface attachments among the Pt nanoparticles were observed. Scale bars: (a)-(c) 100 nm; (d) 50 nm; (e) and
(f) 20 nm. Reprinted with permission from: N.V. Long, C.M. Thi, M. Nogami, M. Ohtaki, Novel issues of morphology, size, and structure of
Pt nanoparticles in chemical engineering: surface attachment, aggregation or agglomeration, assembly, and structural changes, New J.
Chem. 36 (2012) 1320-1334 [53]. Copyright © 2012 Royal Society of Chemistry (RSC), Thomas Graham House.

quantum-size effect, the optimization of these factors is
extremely important to improve the catalytic activity and
the cost of the Pt-metal loading of catalysts.

Characterization of Pt- and Pd-based
nanoparticles
At present, Pt- and Pd-based nanosystems that can be
prepared with simple chemical methods with homogeneous
morphology, shape, size, and structure in the nanosized range
of 10 nm are extremely important to catalysis and nextgeneration FCs [87–89]. The shape-controlled synthesis of
single metal nanoparticles, with an emphasis on various Pt
nanostructures, is discussed, along with its crucial role in the
electrocatalysis of anodic reactions in PEMFCs. A clear shape
and size –dependence of catalytic activity has been demonstrated [33]. Various methanol-tolerant Pd nanocubes have
been compared in terms of their catalytic activity for ORR in
H2SO4 electrolyte, showing that the catalytic activity of all
types of Pd nanocubes is better than that of normal Pd
nanoparticles [88]. The MOR can be used as a probe reaction
on Pt dendrites and cubes to determine their effect on the
MOR as a function of particle shape and morphology [90]. Pt/C
catalysts have been very successfully used for PEMFCs [91]. A
more active Pt/C catalyst for DMFCs has been developed [92].
However, for the 10-nm size range, hetero-characterizations

of shape, morphology, size, structure, and surface are also
very crucial to understanding the natures of nanoclusters,
nanocrystals and homogeneous and non-homogeneous nanosystems for FCs. The ability to control the size and shape of Pt
or Pd nanoparticles is very crucial to catalysis and FCs,
especially DMFCs and PEMFCs [93–96]. A nanoporous Pd rod
catalyst was developed for MOR [97]. In general, ethylene
glycol (EG) (and other various alcohols) has been shown to be
one of the most important organic compounds used as a
chemical intermediate in a large number of industrial processes. EG can be used as the reducing agent or the solvent for
the successful syntheses of various metal, bimetal, or multimetal nanoparticles [98]. The kinetics of the ORR on a Pd
catalyst in acid media is very crucial to enhance the performance of PEMFCs [99]. The effect of the nature of the
precursor has been investigated with regard to the performance of Pd-Co catalysts for DMFCs [100]. A strategy for
controlling bimetallic nanostructures, e.g., Pd-Au, by seedmediated co-reduction has been proposed [101]. Structuresensitive catalysis in Pt catalysts has been confirmed in the
(111), (100), and (110) low-index facets [102,103]. The shapedependent catalytic properties of Pt nanoparticles depend on
their specific crystal nanostructures [104]. However, the highindex facets of metal nanoparticles, such as the (557) and
(730) surfaces of Pt nanoparticles, also play a crucial role in
morphology-dependent catalysts [105]. Tetrahexahedral Pt
nanoparticles of high catalytic activity with average sizes of


644
53, 100, 126, and 144 nm exhibit 24 high-index facets, such as
the {730}, {210}, and/or {520} surfaces, with a large density of
atomic steps and dangling bonds [106]. Because of the higher
concentration of surface steps, kinks, islands, terraces, and
corners in their surfaces and morphologies, superior catalytic
performance has been obtained for un-sharp Pt nanoparticles
[107,108]. The instability and surface-area loss of Pt/C
electrocatalysts at high voltages in low-temperature FCs have
been shown [109]. Pt and Pt-based catalysts have both been

considered for use in low-temperature FCs. Meso/nanoporous
metal structures possess much higher surface areas and higher
catalytic activities than non-porous catalysts, but their stability and durability have yet to be proven [110]. Highly active
selective Pt-based catalysts have been proposed, with an
emphasis on the various roles of the kinks and steps [35] and
the active sites on Pt nanoparticles [108,109].
It is remarkable that the complexity of the surface,
surface structure and morphology of Pt nanoparticles has
been clearly confirmed. In addition, various possible visible
(h k l) indices have been assigned to fcc structures of
specific (h k l) low and high indices, including (111), (200),
(220), (311), (222), (400), (311), (420), (422), (333), (511),
(440), (531), (442), (600), (620) and (533) [32]. The synthesis methods for nanoparticles of Pd and its alloys have been
discussed in the context of FC applications [52,113]. The
general methods for the shape-controlled synthesis of Pd
nanocrystals in aqueous solutions with size and morphology
control have been presented previously [114]. Clearly, Pt
and Pd nanoparticles are of importance to catalysis. Their
size, shape and morphological transformations are crucial
for the long-term stability of Pt- and Pd-based FC technologies. The complex morphologies of Pt and Pd nanoparticle
catalysts over the entire nanoscale have been studied from
both theoretical and experimental perspectives. The relative stability of nanoparticles of various shapes and sizes has
been found for various Pt and Pd nanostructures [115].
Hollow-structured nanoparticles with an appropriate voidto-total-volume ratio can be stable at high temperatures
with respect to an increasing stable-void size with increasing temperature [116,117]. Pd-based catalysts are used for
alcohol oxidation in half-cells and in direct alcohol FCs
[118]. In the DFT method, computational results of the
catalytic reactions at the surfaces are used for comparison
with experiments. The catalytic activity may be tuned by
engineering the electronic structure of the active surface by

changing its composition and structure [43,44]. Bimetallic
Pd-Pt nanoparticles have exhibited a significantly enhanced
electrocatalytic activity with respect to pure Pt or Pd
nanoparticles [119]. At present, DMFCs are a key enabling
technology for the direct conversion of chemical energy into
electrical energy. Using DFT calculations, the ORR on model
electrodes has been studied. The reactivity has been found
for a set of monometallic and bimetallic transition-metal
surfaces, both flat and stepped, that included Pt-based
alloys with Ru, Sn, and Cu as well as non-precious alloys,
overlayer structures, and modified edges. Pt-Cu surfaces are
promising anode catalysts for DMFCs [120]. At present,
various metals (Pt, Ru, Rh, Pd, Os, Ir, Au, Ag, Fe, Co, Ni,
Cu, and Mn) are recognized as promising electrode materials
for FC anodes because of the predictions of quantum
mechanical calculations and DFT, especially in SOFCs
[121]. Because of the modifications of the surface catalytic

N.V. Long et al.
properties of noble-metal surfaces induced by the dealloying phenomenon, the electrocatalytic Pt mass activity of
dealloyed Pt-Cu core-shell particles for the ORR is higher
than that of a Pt electrocatalyst by more than a factor of 4,
and it therefore meets the performance targets for FC
cathodes [122,123]. The catalytic activity and selectivity for
hydrogen of various Pt(h k l) facets of Pt catalysts have been
confirmed, especially the hydrogen adsorption on Pt(100),
Pt(111), and Pt(110), because of the specific characterization of the corresponding Pt-metal nanoparticles [36,124–
127]. Meso-structured Pt films have been prepared that
exhibit high catalytic activity and stability for the ORR
[128]. Nanostructured tungsten-carbide/carbon composites

have been synthesized with a microwave-heating method to
serve as supports for platinum catalysts for methanol
oxidation [129]. Highly dispersed Pt nanoparticles supported
on poly(ionic liquid)-derived hollow carbon spheres have
been studied for the enhancement of the MOR [130]. Hollow
graphite carbon spheres have also been used as Pt-catalyst
supports in DMFCs [131]. The significant influence of the
properties of CNF supports on the ORR behavior has been
observed in proton-conducting-electrolyte-based DMFCs
[132]. At present, the design of Pt/C-based-electrocatalyst-support materials with “dense-erythrocyte-like (DEL)”
and “hollow-porous-microsphere (HPM)” morphologies
synthesized by spray drying is under development for highperformance PEMFC applications [133].

Development of Pt-based catalysts
In all research on Pt-based catalysts in FCs, the catalytic
activity, reliability, durability and stability of the prepared
Pt-based catalysts should be evaluated alongside their
accompanying reduction of the high cost of such catalysts.
In one work, the authors have successfully synthesized Pt
nanoparticles with various modified polyol methods with
size-and morphology-control processes [53]. However, the
size of the Pt nanoparticles must be controlled within the
size range of 10 nm or 20 nm, while the uniform shapes and
morphologies must be controlled to manifest in sphericallike and polyhedral-like shapes and morphologies for
better catalytic behavior. In one study, pure Pt catalysts
with as-prepared Pt nanoparticles of both polyhedral-like
and spherical-like morphologies were investigated for their
catalytic properties in methanol. The results showed that
the electrocatalytic performance of spherical-like and
rough Pt nanoparticles is better than that of polyhedral

and sharp Pt nanoparticles. Pt nanoparticles have specific
fringe lattices, 0.910 nm for the distance of the (100)
planes, and 0.235 nm for the distance of the (111) planes.
Special surface-dependence catalytic properties have been
confirmed for various polyhedral and polyhedral-like
morphologies as well as spherical and spherical-like
morphologies in the size range of 8-16 nm (20 nm) for both
of the two cases above, with ECA values of $10.53 m2/g for
the sharp and polyhedral-like Pt nanoparticles and 14.370
m2/g for the un-sharp and spherical-like Pt nanoparticles.
The catalytic activity of the as-prepared Pt nanoparticles
was measured in a 0.1 M HClO4+1 M CH3OH solution. A
stable voltammogram was attained after 10 cycles of
sweeping a potential range of -0.2 to 1.0 V for both


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
samples. The two typical oxidation peaks can be clearly
observed; one is between 0.6 and 0.7 V in the forward
scan, and the other is at approximately 0.50 V in the
reverse scan. The two peaks are directly related to the
oxidation of methanol and its associated intermediate
species. In the reverse scan, the oxidation peak at 0.50 V
can be related to the removal of the residual carbon
species formed in the forward scan. The peak current
density in the forward scan represents the activity of a
catalyst during CH3OH dehydrogenation. For the prepared
catalyst samples, the peak current density for the Pt
nanoparticles with non-sharp and spherical shapes
(14.90x10-4 A/cm2) was confirmed to be 1.51 times higher

than that for the Pt nanoparticles with sharp and polyhedral shapes (9.90x10-4 A/cm2). The heat treatment of
as-prepared Pt nanoparticles has been confirmed to promote advantageous electrochemical features [134]. Heat
treatment plays an important role in improving catalytic
activity. First, the as-prepared PVP-Pt nanoparticles must
be washed and cleaned to remove the PVP. Then, the pure
Pt nanoparticles must be heated at 300 1C or higher. In this
way, the sizes, shapes, and morphologies of the polyhedral
Pt nanoparticles can be maintained during such high heat
treatment. However, the removal of PVP only by heat
treatment at 300 1C without washing causes a significant
variation in the Pt nanoparticles, creating a sharply polyhedral shape and morphology. Therefore, methods that are
capable of removing the PVP without changing the characterization of the as-prepared Pt nanoparticles are stringently required to obtain good catalytic performance. The
self-aggregation and assembly of the as-prepared polyhedral Pt nanoparticles have been studied in the formation of
large Pt particles accompanied by the decrease of catalytic
activity [135,136]. Therefore, the physics and chemistry
methods for controlling the size and morphology of Pt
nanoparticles for electrocatalysis in FCs are very important
to the overall performance of the catalytic system [137–
139], for example, to ensure that the prepared Pt nanoparticles are within the desirable size range of 10 nm (110 nm), 20 nm (1-20 nm), or 30 nm. Therefore, the synthesis and characterization of metallic and bimetallic nanoparticles with alloy, core-shell, and mixture nanostructures
are of great importance to electrocatalysis, energy conversion, and FCs. Thus far, we have successfully achieved
size and shape control during the synthesis of various Pt,
Rh, Pd NPs or Pt-Pd and Pt-Au bimetallic nanoparticles for
catalysis and direct energy conversion by utilizing modified
polyol methods. Certainly, both precious and cheaper
metal nanoparticles, typically Au, Ag, Fe, Co, etc., can
be synthesized for catalysis, energy and FCs. Therefore,
metal-, bi-metal- and multi-metal-based NPs show promise
for practical applications. Facile methods of synthesis
based on chemistry and physics can be used to prepare
NPs with controlled sizes, shapes, morphologies, and

structures. In particular, Pt- and Pd-based nanoparticles
as well as the combination of Pt and Pd NPs of certain
sizes, shapes, structures, and compositions have great
prospects for use in PEMFCs, DMFCs, and other energy
applications. Catalysts that use Pt- or Pd-based NPs can
improve future FCs. Therefore, continuous efforts are
ongoing to engineer Pt- and Pd-based alloy and core-shell
NPs in a variety of compositions using multi-metals (Co, Ni,

645

Fe, Cu …) as well as oxides, ceramics, and glasses.
Scientists must study the catalytic activity, selectivity,
durability, and stability of these materials for application
to next-generation FCs. Pt clusters, nanoclusters, and
nanoparticles can be synthesized with simple chemistry
and physics methods, such as the modified polyol method,
with or without the assistance of chemical compounds
(typically sodium iodide or silver nitrate) for controlling
the synthesized nanostructures. Pt clusters, nanoclusters
and Pt nanoparticles can be easily synthesized in the
nanosize range of approximately 10 nm or 20 nm. Certainly, the issues of size and morphology must be intensively studied in further catalytic investigations of Pt
nanoparticles of 10 nm and 20 nm, or even 30 nm or larger,
to confirm the catalytic durability, stability, and activity of
Pt-NP-based catalysts. In this respect, the research results
on the use of a low-weight loading of Pt metal in novel
robust and efficiently designed catalysts provide a good
foundation for the large-scale commercialization of FCs.
Therefore, the controlled synthesis of Pt- and Pd-based
nanoparticles is crucial to reducing the Pt and Pd loading

catalysts while preserving large quantum-size or shape
effects. Such nanoparticles can be used as catalytic Pt
and Pd shells for important bimetallic nanosystems to
reduce the high costs of FC systems that use noble-metal
catalysts. In addition, the controlled synthesis of metal,
bimetal, multi-metal, and multi-component NPs has been
studied in the fields of catalysis, biology, and medicine.
Here, we mainly focus on the controlled synthesis of novel
Pt- or Pd-based alloy or core-shell nanoparticles, or
“noble-metal-based core-shell nanosystems” with noblemetal or oxide shells, and their potential applications. It is
certain that bimetallic or multi-metallic nanoparticles
with novel homogeneous alloys and core-shell nanostructures (e.g., Pt-Fe-, Pt-Cu-, and Pt-Ni-based catalysts) can
be easily synthesized. However, at present, homogeneous
core-shell nanosystems pose considerable challenges to
researchers and scientists. Accordingly, other authors have
reported a study of the lattice-strain control of the
catalytic activity in the dealloying of core-shell Pt-Cu
catalysts that was conducted with the goal of reducing
the Pt loading significantly or developing a new method of
lattice-strain control [140]. A synergistic effect of Pt-Pd
core-shell bimetallic nanoparticles has been discovered to
enhance catalytic activity and sensitivity. Thus, the synergistic core-shell effects of Pt- and Pd-based bimetallic
catalysts and the dealloying effects of Pt-based core-shell
catalysts are very important to the creation of efficient Ptand Pd-based catalysts for developing sustainable and
renewable energy sources with various FC technologies.
Therefore, it is still necessary to develop novel metal and/
or oxide nanoparticles, nanosized structures, and various
FC materials. Similarly, multi-metal NPs are promising
catalysts for next-generation FCs that can be synthesized
with simple chemical and physical methods. Such NPs can

be used in catalysis, energy conversion, and FCs, and they
also have prospects for low-cost applications in the fields
of thermoelectric materials, biology and medicine. We
suggest that the use of noble-metal thin shells (Pt, Pd,
Rh, and Ru, possibly in combination with Ag and Au) and
cheap-metal thin shells is of great importance to core-shell
nanoparticles that are engineered for catalysis, catalysts,


646
and FCs because such thin shells protect the cores of the
nanosystems and provide an avenue for reducing the cost
of expensive catalysts. The recent development of non-Pt
catalysts for the ORR has become important to the largescale commercialization of various FCs [141]. Synthesis
methods that are capable of producing Pt- or Pd-based
bimetallic nanoparticles with various sizes and shapes as
well as various alloy and core-shell nanostructures have
been used to produce such nanoparticles for catalysis. The
core-shell structures of bimetallic nanoparticles are very
important in Pt- and Pd-based catalysts for FCs [142–149].
At present, the alloy surfaces of Pt- and Pd-based bimetallic alloys, for example, the important formula of Pt3M
catalysts (where M stands for Fe, Ni, Co, V, or Ti), are the
subject of intensive study [150–152]. The catalytic activity
of Pt/GC electrodes for CO monolayer oxidation has been
tested. The CO stripping voltammetry provided a fingerprint of the particle-size distribution and the extent of
particle agglomeration in carbon-supported Pt catalysts
[153]. The issue of CO poisoning can addressed with the use
of Pt/CeO2/ZrO2 in H2 FCs. Core-shell nanoparticles are
classified into various core-shell structures, including inorganic-inorganic, inorganic-organic, organic-inorganic,
organic-organic, core/multishell, and movable-core/hollow-shell. Among these nanoparticles, bimetallic nanoparticles are some of the most important nanoparticles that

can serve as catalysts for various FC reactions [154]. The
synergistic effects of bimetallic catalysts have been investigated. Pt-based core-shell bimetallic nanoparticles have
been shown to demonstrate much better catalytic activity
than Pt nanoparticles. Alloyed and core-shell bimetallic
nanoparticles have higher catalytic activities than metallic
nanoparticles of single metals because of synergistic
effects or the bi-functional mechanism that acts between
two different metals or within bimetallic nanoparticles and
catalysts [155–157]. The nanoshell provides sites for catalytic activity. At the same time, the core element exerts an
electronic effect (a ligand effect) on the nanoshell element because the surface atoms of the nanoshell are
coordinated to the nanocore in their catalytic reactions.
Therefore, the nanoshell is an important factor in controlling the catalytic properties. Core-shell bimetallic structures induce a greater suppression of adsorbed poisonous
species (CO species). They modify the electronic band
structure to create better surface adsorption. Therefore,
the electrocatalysis at the surfaces of a pure Pt catalyst
must be further studied with respect to the combinations
and mixtures of various elements that can be used in
nanostructured catalysts for FCs [155]. Clear evidence of
catalytic enhancement can be observed in the electrochemical data of Pt-Pd core-shell bimetallic nanoparticles
with respect to pure Pt-Pd alloy and core-shell catalysts.
Bimetallic Pt-M/CNTs catalysts (where M stands for Fe, Co,
or Ni) for DMFCs have been developed to enhance the
catalytic activity with respect to the MOR. Among them,
the Pt-Co/CNT catalyst exhibits the best catalytic activity
and stability, especially in terms of its anti-poisoning and
long-term cycle abilities [159]. In addition, Pt-Co/CNT
catalysts have been prepared by microwave-assisted synthesis [158], and the synergistic activity of Au-Pt alloy
catalysts has been studied [160]. The important synergistic
effects of Pt-based core-shell bimetallic nanoparticles,

N.V. Long et al.
which are capable of providing significant enhancements
to catalytic activity, must be intensively investigated in
both theory and practice to evaluate potential catalysts
for future FCs.

Development of Pt-Ru-based catalysts (PtxRuy and
PtxRuy/support)
The most successful but high-cost present Pt-Ru-based
catalysts with various supports for DMFCs have been discussed with respect to improving their activity and optimizing the exploration of new catalysts with a low Pt-metal
content [59,161]. The segregation of core-shell Ru-Pt nanoparticles has been discovered in NMR studies [162]. PAMAMstabilized Pt-Ru nanoparticles have been used to catalyze
the MOR [163]. Pt/Ru nanoparticles (2.5 nm) can be easily
synthesized in high-temperature and high-pressure fluids
[164]. The MOR of Pt-Ru catalysts deposited on a HOPG
substrate by sequential and simultaneous electrodeposition
has been studied in aqueous sulfuric acid [165]. PtRu/C
catalysts have been prepared for DMFC applications [166].
CO-tolerant Pt-Ru–MoOx/carbon nanofibers have been used
for DMFCs [167]. The catalytic activity of a Pt-Ru catalyst
for DMFCs has been enhanced by repetitive redox treatments [168]. The relative advantages and disadvantages of
metal shells versus alloy shells primarily arise from the
issues involved in their synthesis. Pt/C and Pt-Ru/C catalysts show high catalytic activity but carry high costs [169].
Pt-Ru electrocatalysts supported on ordered mesoporous
carbon have been evaluated for use in DMFCs [170]. Novel
Pt-based catalysts with Pt/mesoporous carbon nanocomposites imbued with Ni or Co nanoparticles have been used for
FCs [171]. From the perspective of their electrocatalytic
activity, Pt- and Ru-based bimetallic catalysts with 30 at%
Ru has been found to be the most active electrode for
methanol, ethanol and ethylene-glycol oxidation [172].
Methanol crossover has been discussed in the context of

DMFCs. In the past, Pt-Ru catalysts have exhibited good
performance for use in DMFCs [173]. Pt-Ru nanowire catalyst material in the anode of a DMFC has been used to
enhance the performance of the DMFC [174]. Electrochemical impedance studies on PtRuNi/C and PtRu/C anode
catalysts in an acid medium have demonstrated that these
catalysts can be used for DMFCs. The catalytic activity of
Pt-Ru-Ni/C with respect to the MOR is higher than that
of Pt-Ru/C, and its tolerance to CO is also better than that
of Pt-Ru/C [175]. In one example of the effects of composition on Pt-based catalysts, the quaternary Pt-Ru-Sn-W/C
catalyst for DMFCs can deliver significant currents under
half-cell operation, but it can also cause ohmic losses
because of the presence of semi-insulating metal oxides,
which limit the single-cell performance [176].
In general, Pt/SWCNTs and PtRu/SWCNTs have exhibited
high ORR rates [177]. The performance of PtRu/C catalysts
with respect to the MOR has been improved with sensitization and activation treatments [178]. Pt–Ru/C catalysts that
use carbon fibers a with stacked-cup-type structure have
also exhibited good MOR catalytic behavior [179]. Various
PtRu/C catalysts were fabricated using surfactants for
studies of the MOR [180]. A novel type of
polyoxometalate-stabilized Pt-Ru/MWCNTs catalyst has


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
been prepared for use in DMFCs [181]. Pt-Ru catalysts have
been stabilized against the dissolution of Ru with the
incorporation of Au [182]. A Pt-Ru-TiO2 photoelectrocatalyst
has also been used for the MOR [183]. The microwaveassisted polyol method has been investigated for the preparation of Pt/C, Ru/C and PtRu/C nanoparticles and its
application in the electro-oxidation of methanol. Pt-Ru/C
catalysts for the MOR have also been synthesized with a
two-stage polyol reduction process [45,184]. A novel COtolerant PtRu core-shell-structured electrocatalyst with a

Ru-rich core and a Pt-rich shell has been studied with
respect to the hydrogen oxidation reaction and its implications for PEMFCs [185]. Additionally, Pt and Pt-Ru electrocatalysts supported on carbon xerogels have been evaluated
for use in DMFCs [186]. For DMFCs, a kinetic analysis of
carbon-monoxide and methanol oxidation has been performed on high performance carbon-supported Pt-Ru electrocatalysts [187]. The studies discussed above demonstrate
that alloy and core-shell structures are of interest for the
improvement of the overall performance of DMFCs. Mesoporous Pt and Pt/Ru alloy electrocatalysts for the MOR have
been prepared [188]. In addition, the performance of Ptbased core-shell catalysts has been studied for methanol
and ethylene-glycol oxidation [189]. Recently, nanosized PtRu/C catalysts have also been shown to exhibit superior
catalytic activities for alcohol reactions [190], such as
methanol and ethanol oxidation.

Development of Pt-Rh-based catalysts (PtxRhy and
PtxRhy/support)
At present, Rh-Pt bimetallic catalysts with various core-shell,
alloy, and monometallic nanoparticle structures can be
successfully synthesized by polyol reduction [191] to produce
robust Rh-Pt-based catalysts for PEMFCs and DMFCs, but only
at high cost. In this type of Rh-Pt-based catalyst, the size
effect of the Rh-Pt bimetallic nanoparticles is sensitive to the
catalytic activity of CO oxidation. It has been discovered that
surface segregation plays an important role [192]. Hence, Rh
metal can play a beneficial role in reducing CO poisoning or
enhancing catalytic activity and stability via the synergistic
effect.

Development of Pt-Au-based catalysts (PtxAuy and
PtxAuy/support)
We suggest that Pt-Au catalysts (Pt-Au mixture, alloy, and
core-shell catalysts or catalysts that use Au clusters) may be
very promising candidates for FCs because of the enormous

availability by weight of Au metal on our planet [193–195].
An unalloyed bimetallic Au-Pt/C catalyst has been found to
be effective for the ORR [196]. The MOR mechanism on Pt
that has been spontaneously deposited on unsupported and
carbon-supported Ru nanoparticles has been presented for
Pt-Ru/C catalysts [197,198]. Pt-Au nanoparticles can be
used as catalysts for the ORR in the presence of methanol
[199]. The electrochemical stability of mesoporous Pt-Au
alloys toward the MOR has been found to be highly improved
relative to that of nonporous Pt and mesoporous Pt films.
Therefore, Pt-Au alloy films may be good catalysts to use for
DMFCs [200-202]. In one different work, Pt-Au nanoparticles

647

with the random self-assembly of Pt-Au nanoparticles have
successfully been synthesized and discovered [203]. Pt-Au/
CNT catalysts for the ORR have been investigated in a
comparison between Pt/Au and PtAu nanoparticles for the
determination of methanol-tolerance issues [204]. The MOR
has been studied with Au-Pt core-shell nanoparticles/C
synthesized with the epitaxial growth method [205], and
Au-Pt/C catalysts have also been studied for the ORR [206].
Various catalysts with Pt monolayers (or a thin coating or a
thin skin of Pt monolayers) have been developed and proven
to exhibit high stability and durability [207,208]. Submonolayer Pt-shell/Pd-core nanoparticles have exhibited
high HOR activity [209]. At present, Pt-covered-MWCNTs
are under development for the investigation of the ORR in
FC applications [210]. PtRu electrocatalysts with ultra-low
Pt and Ru content are under development for the MOR

[211]. An efficient Pd-Co-Pt core-shell catalyst/C with a thin
Pt shell has been developed for the MOR [212], and Pd-Pt/C
electrocatalysts have been developed that show good
modifications of the Pt or Pt-Pd monolayer but also demonstrate the CO-stripping voltammetry phenomenon [285].
The preferential CO oxidation in hydrogen has been studied
with respect to the reactivity of Pt-based core-shell nanoparticles [213], and excellent results on the use of Pt-based
alloy and core-shell nanostructures for PEMFCs and DMFCs
have been obtained [213–217]. Pt/M catalysts with coreshell structures (where M stands for Ru, Rh, Ir, Pd, or Au)
have been prepared and investigated for the determination
of both the CO coverage and the reduction of CO poisoning.
The shells were very thin Pt layers. In a comparison of
catalytic activity, the CO poisoning on the surfaces of Pt-M
catalysts in hydrogen-rich environments showed the following relation, which can be attributed to the composition
effect: Ru/PtoRh/PtoIr/PtoPd/PtoPtoAu/Pt. The Pt/
Ru-based catalyst exhibited the highest catalytic activity
because it had the weakest CO binding on its surface [213].
In the effort to improve the electrocatalytic properties of
FC cathodes, a volcano-shaped relationship was confirmed
between the rate of the cathode reaction and the oxygen
adsorption energy for the best catalytic activity of the Pt-Pd
bimetal [218,219]. The ORR of Pd-Co-Pt/C catalysts has
been studied; these catalysts exhibit a high ORR rate
because of their enhanced activity and stability with
respect to monolayer Pt [220]. It has been proven that the
evidence for a synergistic effect that has been discovered in
bimetallic nanoparticles indicates an enhancement of catalytic activity and selectivity [156,155,294,295].

Development of Pt-Cu-based catalysts (PtxCuy and
PtxCuy/support)
Thus far, Pt-Cu-based catalysts have been developed for

low-temperature FCs, PEMFCs, and DMFCs. They have
become promising electrocatalyst candidates because of
their high durability and stability during operation and
testing [221–224]. The catalytic activity can be controlled
via the lattice-strain effects in dealloyed core-shell FC
catalysts [140,221]. Pt-Cu catalysts have exhibited catalytic
activity four times higher than that of a Pt catalyst with the
same Pt mass activity for the ORR because of dealloying into
the core-shell structure. Cu-Pt core-shell catalysts with


648
nano-carbon supports can be used for PEMFCs [222,223]. In
most cases, supported Cu-M (where M stands for Pt, Pd, Ru,
or Rh) bimetal nanocatalysts have been studied simultaneously in an effort to identify the best catalytic performances [224].

Development of Pt-Ni-based catalysts (PtxNiy and
PtxNiy/support)
In recent years, PtxNiy- and PtxNiy/support-based catalysts
have been considered for their potential use in PEMFCs and
DMFCs because of their high catalytic stability and durability
[224–229]. Ni-Pt core-shell nanoparticles with thin Pt shells
prepared with the polyol method are 10 nm in total size, and
they are promising cathode catalysts for use in PEMFCs that
have a low Pt content but high catalytic activity [225].
Nanoporous Pt/Ni surface alloys of approximately 3 nm in
size have been prepared, which exhibited superior ORR
activity and long-term durability compared to a commercial
Pt/C catalyst [226]. The effect of the stabilizer on the
properties of a synthetic Ni-Pt core-shell catalyst for PEMFCs

has been investigated [227]. The effects of acid treatment of
Pt-Ni alloy nanoparticles-graphene on the kinetics of the ORR
in acidic and alkaline solutions have been studied [228]. Pt-Ni
alloy/C catalysts have shown high catalytic activity toward
the ORR [229]. The scientists show that Pt-Ni alloy nanoporous
nanoparticles have been prepared with a facile chemical
dealloying process using nanocrystalline alloys as precursors
[230]. As a result, Pt-Ni alloy/C has shown superior catalytic
properties compared to alloyed nanoparticles because of its
large surface area and small pores, which provide a significant
catalytic enhancement, which is very crucial to the operation
of alcohol FCs, PEMFCs and DMFCs.

Development of Pt-Co-based catalysts (PtxCoy and
PtxCoy/support)
It has been shown that Pt-Co-based catalysts can successfully substitute for pure Pt catalysts. Most research regarding PtxCoy and PtxCoy/support for PEMFCs and DMFCs has led
to the discovery of the suitable composition of Pt and Co for
the highest catalytic activity, considering the type of
supports used [321–351]. In these catalysts, Co can play
the role of reducing the CO poisoning or act as a synergistic
agent for the enhancement of the catalytic activity. Co-Pt
core-shell nanoparticles can be used as cathode catalysts
(Co-Pt/C catalysts) for PEMFCs or DMFCs [231–251]. The
utilization of Pt metal in conjunction with cheap Co metal is
important to reducing the cost of FCs [231–233,390]. Pt-Co/
C catalysts with low Pt content have been prepared, and
they can be used for PEMFCs and DMFCs [224]. Pt-Co
catalysts with polyphosphazene-coated CNT supports have
been studied for use in DMFCs [235]. The effect of thermal
treatment on the structure and surface composition of PtCo electrocatalysts can be exploited for application in

PEMFCs operating under automotive conditions [236]. Moreover, Pt, Pt-Ni and Pt-Co supported catalysts can be used to
achieve high ORR rates in PEMFCs [237]. Pt-Ni-Co-based
catalysts have been prepared for the ORR in which the
electrocatalytic activity has been enhanced through the
manipulation of structure parameters, such as the lattice

N.V. Long et al.
strain, the surface oxidation state, and the distribution
[238]. Pt-Ni and Pt-Co alloy catalysts have been studied for
use in PEMFCs and DMFCs [239–242]. Ni-Pt core-shell nanocatalysts have exhibited an enhancement of catalytic
activity for the ORR [243]. Pt-Ni bimetallic bundles prepared with a seed-based diffusion method exhibited methanol oxidation and catalytic activity that was 3.6-fold higher
than that observed for conventional Pt nanoparticles [244],
and the same method has been used to produce
Pt-Co/C catalysts for PEMFCs [245]. The main role of Co in
the MOR mechanism of the Pt/C catalyst has been presented
[246]. The effects of thermal annealing on the properties of
a Corich core-Ptrich shell/C catalyst with respect to the
implications for the ORR have been presented [247]. The
electrodeposition of PVA-protected PtCo electrocatalysts
has been developed to achieve high ORR rates in H2SO4
[248] and the synthesis of PtCo nanowires for the MOR
[249]. Among the Pt-based catalysts, PtxCoy/C is well suited
to high-performance use in the cathode of hightemperature PEMFCs [250]. The catalytic activity of Pt-Coalloy-nanoparticle-decorated functionalized MWCNTs can be
used to achieve high ORR rates in PEMFCs [251]. In a
different study [395], the authors proved that Pt-Co/Cbased catalysts that use graphitic carbon supports can be
ranked in order of durability as follows: Pt3Co/GrC4Pt/
GrC4Pt3Co/Non-GrC4Pt/Non-GrC. Research results regarding catalytic activity and sensitivity should always be considered alongside the corresponding results regarding
durability, stability, and reliability when evaluating the
suitability of Pt-based catalysts for use in low-temperature
FCs, PEMFCs and DMFCs. For example, based on the values of

ECSA and ORR activity of as-synthesized PtxCoy alloys after
electrochemical treatment in 0.1 M HClO4, the Pt-massbased activities (jmass) increase in the order of Pt(HT) (heat
treatment)oPtCooPt3CooPtCo3 at comparable particle
sizes [397].

Development of Pt-Sn-based catalysts
(PtxSny and PtxSny/support)
Pt-Sn nanoparticles with different particle sizes (1-9 nm)
and metal compositions (Sn content of 10-80 mol%) and with
various organic capping agents have been synthesized [252].
In the case of Pt-Sn catalysts with supports, the Pt-Sn/C
catalysts were directly applied to DMFCs [253]. The MOR at
a lead (Pb) electrode modified with Pt, Pt-Ru and Pt-Sn
microparticles dispersed in poly(o-phenylenediamine)
(PoPD) film has been investigated. The catalytic activity of
the Pt particles was found to be enhanced when Ru was codeposited in the polymer film, and the enhancement effect
was even greater in the case of Sn [254]. So far, the
development of PtxSny, and PtxSny/support has been
focused on the discovery of their relative catalytic activity
as well as on the optimal catalytic activity for PEMFCs
and DMFCs.

Development of Pt-Fe-based catalysts (PtxFey and
PtxFey/support)
At present, PtxFey and PtxFey/support are of great importance because of their ultra-high durability, ultra-high


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
stability, and high reliability. Fe nanoparticles can be used
as durable cores for Pt that will survive on very long time

scales, but there are few reports regarding such uses of
PtxFey and PtxFey/support. They have potential for application in both low- and high-temperature FCs. A significant
improvement of the Pt-Fe catalyst has been achieved by
adjusting the Fe composition to obtain the optimal concentrations for pure Pt-Fe catalysts with mixture, alloy and
core-shell structures [255–259]. In addition, Pt-Fe-based
catalysts are of interest for use in PEMFCs and DMFCs
because of their great durability, stability, and reliability
in the FC reactions. Among the combinations and mixtures
of Pt, Pt/C, and Pt-Fe/C, a category of Pt-Fe/C with a fixed
Pt:Fe ratio of 1.2:1 exhibits the highest FC performance at
90 1C and an oxygen back pressure in the cell of 0.2 MPa
[255–259]. Pt-Fe/C catalysts of high catalytic activity for
the ORR have been prepared [260]. In addition, Pt-Fe
catalysts have been used for the ORR in low-temperature
DMFCs [261]. The stability of Pt–Fe/C catalysts for the ORR
has been discussed in relation to the catalyst composition
[262], and a similar study has been performed with the
addition of Co [394]. Ternary Pt-Fe-Co alloy multi-metal
catalysts prepared by electrodeposition exhibited the best
catalytic mass activity for a structure of Pt85Fe10Co5 when
Pt-Fe-Co catalysts such as Pt, Pt97Fe3, Pt94Fe5Co1,
Pt94Fe6Co2, Pt88Fe8Co4, Pt86Fe10Co5, Pt83Fe12Co5, and
Pt78Fe15Co7 were considered [394]. A similar investigation
of Pt1-xMx-based catalysts (where M stands for Fe or Ni and
0oxo1) prepared with the sputtering method for use in
PEMFCs has been conducted [396]. Importantly, the relative
catalytic activities are very crucial to determine the types
of Pt-based catalysts that can be used for low-temperature
FCs, PEMFCs, and DMFCs.

Development of Pt-and-Pd-based nanoparticles
(PtxPdy and PtxPdy/support)
At present, Pt-and-Pd-based nanoparticles can be very
successfully prepared with various chemistry and physics
methods. The synthesized nanoparticles can be binary,
ternary, multi-metal, and multi-component and can have
various mixture, alloy, and core-shell nanostructures with
various sizes, shapes and morphologies. Scientists have
commonly investigated such nanoparticles in the most
suitable experimental and theoretical ranges of characterization, preparation and synthesis, structure, and properties
of desirable Pt-based catalysts. Among the types of Ptbased catalysts that are suitable for use in PEMFCs and
DMFCs, the Pt-and-Pd-based catalyst is one of the most
important catalysts that can be used in both the cathodes
and the anodes. Pt-and-Pd-based catalysts have been continuously developed for testing in FCs, PEMFCs and DMFCs
for large-scale commercialization [263–301,391]. In recent
years, a Pt/Co-Ru/C catalyst has been shown to exhibit a
higher catalytic activity for the MOR than Pt on Fe-Ru/C or
Ni-Ru/C, and its performance is closer to that of a commercial Pt-Ru catalyst with a slightly higher metal loading
and a high cost [263]. Pt-Pdx-Cuy/C core-shell catalysts have
exhibited high catalytic activity for the ORR in PEMFCs
[264]. In addition, a low-Pt-content Pd45Pt5Sn50 cathode
catalyst has been developed for use in PEMFCs [265]. The

649

high performance and stability of Pd-Pt-Ni nanoalloy electrocatalysts have been tested in PEMFCs [266]. Ru-free,
carbon-supported, Co- and W-containing binary and ternary
Pt catalysts have been developed for the anodes of DMFCs
[267]. Carbon-Nb0.07Ti0.93O2-composite-supported Pt-Pd
catalysts of good catalytic activity and stability have been

prepared for the ORR in PEMFCs [268], as have efficient
pseudo-core-shell PdCu-Pt/C catalysts for DMFCs [269] and
Pd-Co-Mo catalysts for the ORR in PEMFCs [270]. Pt-based
ternary catalysts for low-temperature FCs have been
reported, and the very good characterization of Pt-Pdbased catalysts has been confirmed, according to their
electrochemical properties. The complexity of multi-metal
and multi-component catalysts should be significantly simplified by synthesis and preparation methods. A significant
challenge is to find novel cheap catalysts that can replace
the high-cost Pt catalysts. Among the known Pt-based
catalysts, Pt-Pd-based catalysts have proven to be successful for use in commercial PEMFCs and DMFCs. In particular,
core-shell Pt modified Pd/C is an active and durable
electrocatalyst for the oxygen reduction reaction in PEMFCs
[273]. The enhancement of the ORR activity of Pt-Pd/C
bimetallic catalysts can be achieved the preparation of Ptenriched surfaces in acid media and the use of a core-shell
structure [274,275]. In this context, Pd and Pt-Pd catalysts
can be used in DMFCs [276]. Some evidence of an epitaxial
overgrowth of Pt-on-Pd nanocrystals has been observed
[277]. Pd-Pt core-shell nanowire catalysts with ultra-thin
Pt shells on Vulcan XC-72 carbon supports have exhibited a
significant enhancement of the electrocatalytic performance with respect to the ORR [278]. The preparation of
Pt-Pd alloys with the one-pot solvothermal method with
selective shapes and enhanced electrocatalytic activities
has been presented [279], and Pt-Pd core-shell nanoparticles have been easily synthesized via the supermolecular
route [278]. Pd-Pt bimetallic nanodendrites with high
activity for the ORR have been reported [281]. The controlled synthesis of Pd-Pt alloy hollow structures with
enhancement of the catalytic activities for the ORR has
been confirmed [282,283]. It has been observed that PAMAM
dendrimer-encapsulated Pd-Pt nanoparticles exhibiting a
slight core-shell morphology can be synthesized with the
successive method using an aqueous NaBH4 solution [284].

The ORR was significantly enhanced by the Pt monolayers on
a Pd-Au alloy [285]. In an extensive study, the reactiondriven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles was investigated with respect to the changes in the
structure, size, shape, and morphology of the nanoparticles
[286]. The important discovery of the high hydrogen storage
of Pd and Pt nanoparticles and their core-shell structures
has been reported [287–289]. With the addition of Pt
coating, Pd nanotubes can be used as excellent electrocatalysts for the ORR [290]. In many investigations, various
Pt-and-Pd-based catalysts with specific alloy nanostructures
and bimetallic core-shell nanoparticles have successfully
been synthesized [291–295].
In typical works, Pt and Pt-Pd bimetallic nanoparticles
with polyhedral core-shell morphologies are precisely
synthesized by the reduction of Pt and Pd precursors at a
certain temperature in ethylene glycol with silver nitrate as
the structure-controlling agent, as shown in Figure 3 [291–
295]. Such Pt nanoparticles exhibit well-shaped polyhedral


650

N.V. Long et al.

Figure 3 (A)-(C) Pt nanoparticles in the range of 20 nm, and Pt-Pd core-shell nanoparticles in the range of 30 nm. The best
synergistic effect is found in the Pt-Pd bimetallic core-shell nanoparticles. Reprinted with permission from: V.L. Nguyen, M. Ohtaki,
T. Matsubara, M.T. Cao, M. Nogami, New experimental evidences of Pt-Pd bimetallic nanoparticles with core-shell configuration and
highly fine-ordered nanostructures by high-resolution electron transmission microscopy, J. Phy. Chem. C 116 (2012) 12265-12274.
© (2011) American Chemical Society. (D) Pt-Pd core-shell nanoparticles in the range of 30 nm. Reprinted with permission from: N.
V. Long, T. Asaka, T. Matsubara, M. Nogami, Shape-controlled synthesis of Pt-Pd core-shell nanoparticles exhibiting polyhedral
morphologies by modified polyol method, Acta Materialia, 59 (7), 2011, 2901-2907 [291]. Copyright © (2011) Elsvier Publishers.

The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
morphology with fine and specific nanostructures in the size
range of 20 nm. Important evidence of core-shell configurations of Pt-Pd core-shell nanoparticles can be clearly
obtained with HRTEM measurements. The results of HRTEM
imaging have shown that core-shell Pt-Pd nanoparticles in
the size range of 25 nm with polyhedral morphology form
thin Pd shells of 3 nm in thickness as atomic Pd layers grow
on the Pt cores during synthesis. High-resolution TEM images
of Pt-Pd bimetallic nanoparticles have shown that the
Frank-van der Merwe and Stranski-Krastanov growth modes
coexist in the nucleation and growth of the Pd shells on the
as-prepared Pt cores, indicating a good lattice match.
Experimental evidence of the deformations of lattice
fringes and lattice-fringe patterns has been found in Pt
and Pt-Pd core-shell nanoparticles. The interesting renucleation and recrystallization phenomena at the attachments, connections, and bondings between the
nanoparticles have been revealed to form a very good
lattice match.
Several specialists indicated that the porous structures of
hollow spherical sandwich PtPd/C catalysts for the MOR can
be fabricated by electrostatic self-assembly in polyol solution
[296]. The role of a composition with a Pd:Pt ratio of 3:1 has
been studied in Pd3Pt1/C for highly methanol-tolerant ORR
catalysis [297]. The effect of the Pt precursor on the
morphology and catalytic performance of Pt-impregnated
Pd/C for the ORR in PEMFCs has been investigated [298].
Scientists have demonstrated the temperature dependence of
methanol oxidation and product formation on Pt- and Pdmodified Pt electrodes in an alkaline medium [299]. In
general, Pd-Pt/C catalysts have shown high durability and
stability for the MOR in DMFCs [300]. Recently, the nanocage

effect has been found in hollow Pt-Pd nanoparticles during
electrocatalysis testing [301]. Further computations regarding
catalysis have been developed to identify the interactions and
catalytic activities of Pt-O and Pt-H as well as Pt and various
other species. Through the DFT approach, it has been found
that the level of Pt content in catalysts that use Pd-Pt alloys
should be appropriately modified to achieve an efficient ORR
[283]. The maximum of the ORR volcano curve is predicted to
occur for Pd3Fe-Pd3Pt and Pd3Mn-Pd3Pt core-shell catalysts by
DFT calculations. Compared with commercial Pt catalysts,
Pd3M/Pd3Pt catalysts have a lower Pt content but show
favorable ORR activity and selectivity [302]. A computational
and experimental study has been carried out to understand
the volcano behavior of the ORR of PdM-PdPt/C (where M
stands for Pt, Ni, Co, Fe, or Cr) core-shell catalysts. The result
is that the core-shell catalysts exhibit a high methanol
tolerance, which is important for use in PEMFCs and DMFCs,

651

in the volcano approach [303]. To study the kinetics and
mechanisms of the ORR, DFT calculations and molecular
dynamics simulations have been widely used in designing Ptand Pd-based catalysts for catalysis and FCs. Based on the
Gupta empirical potential and DFT calculations, the adsorption of CO, O, OH, and O2 on Pd-Pt clusters with 55 atoms has
been studied using molecular simulations. In this study,
Pd43Pt12 with a three-shell onion-like structure exhibited
the highest relative stability with respect to both DFT and
Gupta levels. In addition, these clusters showed the weakest
CO, O, OH, and O2 adsorption strength compared with the
Pt55, Pd55, and Pd13Pt42 clusters, indicating good catalytic

activities toward the ORR for the better Pt-based electrocatalysts [304]. The issues surrounding O2 on icosahedral NiPt12 core-shell nanoparticles have been investigated using ab
initio DFT calculations. A high catalytic activity of the Ni-Pt
core-shell nanoparticles was predicted for the highly active
ORR [305]. Molecular dynamics simulations can be used to
investigate the thermal stabilities of Pt-Pd core-shell nanoparticles with different core sizes and shell thicknesses. Twostage melting occurs during the continuous heating of bimetallic nanoparticles. The melting behaviors at the atomic
level of bimetallic and multi-metallic nanoparticles have been
identified [306,307]. In a study of the size effect, Pd-Pt
nanoparticles of 1.5-5.5 nm with controllable core-shell
structures were successfully prepared with a hydrogensacrificial protective strategy [308]. A study of methanol
decomposition over Pt-M bimetallic catalysts (where M stands
for Au, Pd, Ru, or Fe) has been presented. The oxidation state
and activity of the Pt were found to be influenced by the
addition of the secondary metal. In this study, PtO was found
to be highly stable [13,309]. The structural conversion and
formation of Pt, Pt-O, and PtO2 needs to be further studied
with respect to the catalytic activity in the oxidation states;
this is of particular interest for the MOR mechanism. Pt and
Pt-Ru can be supported on porous nanostructured materials,
such as mesoporous carbon and metal oxides, for use in DMFCs
and PEMFCs [310]. Porous Pt nanocubes of 20 to 80 nm
originating from small nanoparticles of 10 to 20 nm have also
exhibited high catalytic activity for the methanol oxidation
reaction (MOR) [311]. It is certain that bimetals such as Au-Pt,
Pt-Au, and Fe3O4-Au-Pt nanoparticles are catalytically active
for the ORR and the MOR, and their behavior must be studied
in further catalytic investigations to obtain better catalytic
characterizations [312–314]. The metal monolayers on Pt-WC
catalysts (Pt-WC and Pt-W2C) used for hydrogen production
from water electrolysis can be supported on low-cost transition-metal carbides [315]. The kinetics of the hydrogen
oxidation reaction have been determined for a Pt/WC

Figure 4 (A) HRTEM images of Pt-Pd core-shell. The thin Pd shells protect polyhedral Pt cores. The nucleation and growth of Pd
shells are controlled by a chemical synthesis. Scale bars: (a)-(c) 20 nm. (d) 5 nm. (e) 5 nm. (f) 2 nm. (B) Surface kinetics and
mechanism of cyclic voltammogram of Pt nanocatalysts, and Pt-Pd core-shell nanocatalysts on glassy carbon electrode in N2-bubbled
0.5 M H2SO4 electrolyte (scan rate: 50 mV sÀ1). (C) Cyclic voltammograms of as-prepared Pt-Pd core-shell nanocatalysts in 0.5 M
H2SO4 in the ranges of E=À0.2 V to E =1.0 V and E =À0.2 V to E=0.2 V. Cyclic voltammograms of as-prepared Pt nanocatalysts in
0.5 M H2SO4 in the ranges of E=À0.2 V to E=1.0 V and E=À0.2 V to E=0.2 V. Cyclic voltammograms of as-prepared Pt-Pd core-shell
nanocatalysts in 0.5 M H2SO4 in the ranges of E= À0.2 V to E =1.0 V and E=À0.2 V to E=0.2 V. (D) Cyclic voltammograms towards
methanol electro-oxidation of Pt nanocatalyst and Pt-Pd nanocatalyst. (E) Chronoamperometry data of Pt and Pt-Pd nanoparticles.
Electrolyte solution of 0.5 M H2SO4+1.0 M CH3OH and polarization potential about E =0.5 V. Reprinted with permission from: Long, N.
V., Ohtaki, M., Hien, T.D., Randy, J., Nogami, M., A comparative study of Pt and Pt-Pd core-shell nanocatalysts, 2011, Electrochimica
Acta 56 (25), pp. 9133-9143 [294]. Copyright © (2011) Elsvier Publishers.


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N.V. Long et al.


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
catalyst with a low content of Pt nanoparticles [316]. Noblemetal (Pt, Pd, Ru, Au, Ag) nanoparticles and engineered
nanoparticles that use combinations of noble metals with
cheaper metals (Ni, Fe, Cu...) can be used as mixture, alloy,
and core-shell catalysts for FCs. For high-temperature catalytic reactions up to 750 1C, Pt-metal cores coated with
mesoporous silica shells of Pt-SiO2 can be used as highly active
catalysts that are equally as effective as bare Pt metal for
ethylene hydrogenation and CO oxidation [317]. Potential
applications of metal and bimetal nanoparticles to energy
conversion have been discussed with a particular focus on
electrocatalysts for PEMFCs and for thermoelectric energy

conversion. For this purpose, Pd-Pt core-shell nanoalloys
protected by a perfluorinated sulfonic acid ionomer can be
used [318–320]. The evolution of the structure and chemistry
of bimetallic nanoparticle catalysts under catalytic reaction
conditions must be intensively investigated [321] because it
exerts an important influence on the durability and stability
of the FC as a whole. Core-shell PdPt-Pt/C catalysts have
been observed to exhibit high catalytic activity because of
the structure of their Pd-Pt alloy cores and its synergistic
effects with the thin Pt coatings [322,323]. In addition, Pd-Pt
core-shell nanowire catalysts with high catalytic activity for
the MOR have been used for PEMFCs and DMFCs [324]. The
catalytic activities of Pt and Pt-Pd nanoparticles in polypyrrole films for the ORR have been compared against the very
beneficial advantages of Pt-Pd nanoparticles [325]. The role
of Pd loading in Pd-Pt catalysts used to dope mesoporous
hollow core-shell carbon has been discussed in the context of
the performance of PEMFCs [326]. This study illustrated the
great advantage of a high porosity accompanied by relatively
high stability and durability [326]. A novel hydrogenabsorption site in the hetero-interface of the Pd core and
the Pt shell of Pd-Pt core-shell bimetallic nanoparticles has
been identified [227], and hydrogen insertion in Pd-core/Ptshell cubo-octahedral nanoparticles has been observed [328].
In a study of the composition effect, Co5Pt95 and Pd16Pt84
were synthesized and found to be excellent electrocatalysts
for the MOR with mass activities of 1417 and 1790 mA/mg Pt,
respectively, which are much higher than that of a high-cost
Ru-Pt catalyst [329]. Another study investigated Pd-Pt alloys
with controlled compositions ranging from Pd88Pt12 to
Pd34Pt66. These alloy NPs were found to be much more active
and stable for the MOR, and their activities were Pd/Ptcomposition-dependent, with alloys containing 40-60% Pt
demonstrating the optimal activity and stability [330]. In

most cases, Pt-Pd/C has been shown to exhibit better
durability than Pt/C [383], and Pt-Pd/C demonstrates superior catalytic activity for methanol and ethanol oxidation
[384]. In the better surface C modification of the catalysts,
Pd/C or Pt-Pd/C catalyst can be prepared with the modification of the Pt monolayers or the Pt shell, as well as Pt/C or
Pd-Pt/C catalyst with the modification of Pd monolayers or
the Pd shell [385]. Pd-Pt alloy catalysts have been used for
methanol-tolerant catalysis of the ORR. The highest mass and
specific activities for the ORR using Pd-Pt/C catalysts have
been found for a Pd:Pt atomic ratio of 1:2. Pd-Pt alloy
catalysts of this atomic ratio have exhibited enhanced
methanol tolerance during the ORR with respect to Pt/C
catalysts [331]. Pd-Co/C catalysts have been developed for
use for the ORR in place of Pt-Pd/C catalysts [332]. A study of
potential oscillations in PEMFCs using a Pd-Pt/C anode has

653

provided an improved understanding of the catalytic activity
of Pd-Pt/C [333].
To date, the considerable advantages of core-shell structures have been established in various reports. Therefore,
scientists and researchers are using the advantages of coreshell structures to improve the catalytic activity and the
efficient utilization of Pt in PEMFCs and DMFCs, which is
important to the low-cost, large-scale commercialization of
such FCs. Through ultrasound-assisted polyol synthesis,
Pd4Co core-shell catalysts have been produced that show
high catalytic activity towards the ORR, comparable to that
of Pt catalysts [334]. In one work, the charge redistribution
in Pt-based core-shell nanoparticles was found to promote
the ORR [335]. The ORR activity of well-defined core-shell
nanocatalysts has also been investigated in relation to

particle-size, facet, and Pt-shell-thickness effects [336].
Recently, a simple route has been used to synthesize novel
Fe3O4-Pt core-shell nanostructures with high electrocatalytic activity, which has promising implications for core-shell
catalysts with Pt layers on oxide- and ceramics-based
nanoparticles; however, the ability to ensure the homogeneity of core-shell catalysts and the size range of Pt-based
catalysts during synthesis still poses a challenge to
researchers, even though Pt nanoparticles anchored on
graphene-encapsulated Fe3O4 magnetic nanospheres
(38075 nm) have been prepared to serve as robust catalysts for the MOR [337,338]. The performance of low-Ptcontent electrodes of PEMFCs can be appropriately controlled with the use of a Fe2O3-Pt/C core-shell catalyst
prepared with an in situ anchoring strategy [339]. In
particular, an Fe-based cathode catalyst has been found
to perform very well, with an enhanced power density of
0.75 W cmÀ2 at 0.6 V, in PEMFCs [340].
In this work, the synthesis of Pt (4-8 nm) and Pt-Pd coreshell nanoparticles (15-25 nm) is presented. Pt-Pd core-shell
catalysts possess catalytic properties far superior to those of
Pt catalysts. Pt-Pd core-shell catalysts exhibit fast and
highly stable catalytic activity for hydrogen. Methanol
oxidation is significantly enhanced by Pt-Pd core-shell
catalysts, with a current density much higher than that of
Pt catalysts. Fascinatingly, the size effect is not as important as the nanostructure effect. The improvement of fast,
stable, sensitive hydrogen adsorption is very crucial for FCs
[294,295]. According to the scholars' predictions and
assumptions, particular Pt-Pd core-shell catalysts with thin
Pt shells, thin Pd shells, or thin Pt-Pd alloy shells over thick
Pt cores or thick Pd cores have the special property that the
Pt or Pd cores cause an inherently preferential synergistic
effect with the thin Pt, Pd, or Pt-Pd shells, leading to high
catalytic activity, sensitivity, and selectivity with respect to
to the fast, efficient, and robust HOR, ORR, and MOR in
PEMFCs, and DMFCs. Such behavior is very crucial for the

design of desirable Pt-based catalysts. In one interesting
research, Pt-Pd core-shell nanostructures with high catalytic activity for methanol oxidation have been successfully
prepared. We have used pure Pt catalysts produced from asprepared Pt nanoparticles of 10 nm (4-8 nm) and pure Pt-Pd
core-shell catalysts produced from as-prepared Pt-Pd coreshell nanoparticles of 30 nm (15-25 nm) to perform a
comparison of catalytic activity (Figure 4). We have thereby
proven that the Pt-Pd core-shell catalysts possess catalytic
properties that are far superior to those of the Pt catalysts


654
because of the advantages provided by core-shell structures, shapes, and morphologies. We also found that the PtPd core-shell catalysts exhibited fast and highly stable
catalytic activity for hydrogen, leading to the possible
improvement of Pt- based catalysts via a novel phenomenon
of fast hydrogen adsorption through their specific core-shell
structures. The MOR activity was significantly enhanced by
the Pt-Pd core-shell catalysts, with a current density much
higher than that of the Pt catalysts. It was also discovered
that the size effect in the size limit of 10 nm is not as
important as the core-shell nanostructuring effect in the
size range of 30 nm (15-25 nm). Therefore, the fast, stable,
and sensitive hydrogen adsorption of Pt-based core-shell
structures is very crucial for PEMFCs and DMFCs. In addition,
the problem of CO poisoning was not observed in cyclic
voltammetry (CV) measurements because of the effective
reduction of weak and strong CO intermediates by the Pt
and core-shell Pt-Pd catalysts. The observed ORR activity of
the Pt catalyst was more sluggish than that of the Pt-Pd
core-shell catalyst. In this case, the fast enhancement of
the ORR on the electrode with Pt-Pd core-shell catalyst
because of its fast hydro-adsorption/desorption was clearly

observed. The fast hydro-absorption/desorption rates are
the most significant advantages of the core-shell bimetallic
nanoparticles. Therefore, Pt-Pd core-shell catalysts can
significantly increase the rates of the HER, HOR, and ORR
as well as providing some defense against CO poisoning. The
mechanism of the reduction of the CO poisoning coverage
occurred at the active sites of the catalytic activity of the
Pt and Pt-Pd core-shell catalysts because of proper catalyst
preparation. The mechanism of the reduction of the CO
poisoning coverage on Pt-Pd core-shell nanoparticles is
known to arise from the synergistic effects between the
core (Pt or Pd) and the shell (Pt or Pd) of Pt-based
bimetallic nanoparticles; these effects are the reason that
Pt-based core-shell bimetallic nanoparticles are the best Ptbased nanostructures for the electrocatalysis of hydrogen
and methanol in PEMCs and DMFCs. In the catalyst preparation, the as-prepared Pt, Pd, and Pt-Pd nanoparticles were
heated at approximately 300 1C in H2/N2, which resulted in
a good characterization of the size, surface, structure, and
morphology of the catalysts. The suitable temperature
range for the heat treatment should be chosen depending
on the intended application in various FCs, for example, the
ORR catalytic activity. This choice of the temperature range
depends on the operating temperature of the FCs, which are
characterized by the chemical activity occurring at the
electrode surface. In the forward sweep, the first region
assigned to the hydrogen desorption is crucial to confirm the
catalytic activity of Pt catalysts. The slow kinetics of the
hydrogen desorption in the case of the Pt catalyst was
confirmed in the cell before the stabilization of the CV was
achieved, from the first cycle to the twentieth cycle, and
the fast kinetics of the hydrogen desorption was determined

in the case of the Pt-Pd core-shell catalyst. Indeed, the
results demonstrated the good desorption and adsorption of
hydrogen for both the Pt catalyst and the Pt-Pd core-shell
catalyst, providing evidence of good catalytic activity in
both catalysts following the preparation process and heat
treatment. To evaluate the catalytic activity of the prepared catalysts, the ECSA of the Pt catalyst was calculated
to be 10.5 m2 g-1, and that of the Pt-Pd core-shell catalyst

N.V. Long et al.
was found to be 27.7 m2 g-1 in the catalytic investigations.
The Pt-Pd core-shell catalyst, which had a stable and high
catalytic activity, showed a very high initial current of
j = 1.29 Â 10À3 A cmÀ2, and 30.01% of the current remained
after 2 h of polarization. This was much higher than the Pt
catalyst, which had an initial current of j= 4.33 Â 10À4 A
cmÀ2, and 3.67% of the current remained after 2 h of
polarization. Therefore, the more stable Pt-Pd core-shell
catalyst (15 min) showed the higher initial current, and it
still had a current of approximately 1.5 x 10À3 A/cm2 after
2 h of polarization. In this research, the as-prepared Pt, Pd,
Pt-Pd catalysts were heat treated at approximately 300 1C
to obtain high catalytic activity and stability because the
nanostructures of the treated nanoparticles possess very
high hardness. However, the good characterization of the
as-prepared nanoparticles in terms of their surface, structure, size, shape, and morphology should be maintained
during heat treatment at high temperature. After heat
treatment (or sintering) at 300 1C, the Pt-Pd alloy and
core-shell catalysts exhibited good activity and stability in
the desirable nanostructures. The heat treatment is necessary to ensure the highly robust catalytic activity, long-term
stability and durability of the as-prepared nanoparticles.

The pure Pt-Pd core-shell catalysts demonstrated a significant enhancement of activity and selectivity for the ORR in
FCs compared to those of the pure Pt catalysts.
In all the interesting research, the catalytic activity and
stability of Pt-based catalysts has strongly depended on the
effects of heat treatment at 300 1C and the removal of the
poly(vinylpyrrolidone) (PVP) polymer from the surfaces of
the Pt-based nanoparticles. A significant enhancement of
the electrocatalytic activity toward the MOR of polyhedral
Pt nanoparticles has been achieved by removing the capping
agents. A pure Pt catalyst was obtained by heat treatment
at 300 1C while maintaining a good characterization in terms
of size, structure, shape and morphology. According to the
observed electrocatalytic property and activity of the Pt
nanoparticles, the ECA was approximately 6.75 m2/g for the
washed-only Pt nanoparticles, 8.56 m2/g for the directlyheated-only Pt nanoparticles, and 10.53 m2/g for the
washed and heated Pt nanoparticles. The cyclic voltammograms of the methanol electro-oxidation of polyhedralshaped Pt nanoparticles were investigated for different
methods of PVP removal by heat treatment. The electrolyte
solution was 0.1 M HClO4+1.0 M CH3OH, and the scan rate
was 50 mV/s. We discovered that the peak current density
in the forward scan (i(f)) serves as a benchmark for the
catalytic activity of the Pt nanoparticles during methanol
dehydrogenation. For the prepared catalyst samples, the i
(f) values were 7.62 Â 10À4, 8.75 Â 10À4, and 9.90 Â 10À4 A/
cm2 for the washed-only, heated-only, and washed and
heated samples, respectively [135]. When a new catalyst
is prepared and tested for catalytic activity, the asprepared polyhedral Pt nanoparticles in the size range of
10 nm can be used as the standard catalyst for any
confirmation of catalytic activity because of their high
catalytic activity [136]. In these works, Pt-Pd alloy and
core-shell bimetallic nanoparticles were synthesized, and

the epitaxial growth mode of the Pd-monolayer shells on the
Pt nanocores was observed. Pt-Pd and Pd-Pd core-shell
nanoparticles with thin Pt or Pd nanoshells in the form of
monolayers exhibit excellent electrocatalytic behavior for


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
DMFCs. Interestingly, the size effects on the catalytic
activity, selectivity, and sensitivity are not as important as
the effects of morphology and nanostructure.
In previous research, it has also been observed that Pt-Pd
alloy and core-shell bimetallic nanoparticles can be simply
synthesized (Figure 8). The epitaxial growth mode of the
Pd-monolayer shells on the Pt nanocores can be controlled.
Pt-Pd and Pd-Pd core-shell nanoparticles with controllable
thin Pt or Pd nanoshells in various monolayer forms exhibit
excellent electrocatalytic behavior for DMFCs. Interestingly,
the size effects on the catalytic activity are not as
important as the effects of morphology and nanostructure.
The Pt-and-Pd-based core-shell catalysts in the range of
30 nm demonstrated suitable properties, such as a high
current density, and resisted CO poisoning much more
effectively than various other Pt, Pd, Pt-Pd, and Pd-Pd
bimetallic catalysts in the various forms of single metal (Pt
or Pd) nanoparticles and bimetallic nanoparticles with alloy,
core-shell, and mixture structures [293,294,295]. In addition, the phenomenon of the CO stripping effect can not be
observed in any of electrochemical experiments because of
the careful preparation processes and heat treatments of
the pure Pt and Pt-Pd catalysts that were used. The past
experiments include the following: Category (1) - alloy

nanoparticles with polyhedral, spherical, near-polyhedral,
and near-spherical shapes and morphologies of 10 nm (510 nm) in size; Category (2) – mono-nanoparticles (Pt, Pd)
and bimetallic nanoparticles (Pt-Pd) with polyhedral, spherical, near-polyhedral, and near-spherical shapes and
morphologies of 10 nm (5-10 nm) in size for small nanoparticles and 30 nm (20-30 nm) in size for large nanoparticles;
Category (3) – Pt-Pd core-shell nanoparticles with good
polyhedral shapes and morphologies of 30 nm (15-25 nm)
in size with thin shells of approximately 3 nm in thickness;
Category (4) – Pd-Pt core-shell nanoparticles with good
spherical or near-spherical shapes and morphologies of
20 nm (6-16 nm) in size; Category (5) – Pt-Pd alloy and
core-shell nanoparticles with large and irregular shapes and
morphologies of various size ranges of 10 nm, 20 nm, 30 nm,
and up to 40 nm; and Category (6) – Pd-Pt alloy and coreshell nanoparticles with large and irregular shapes and
morphologies of 30 nm (15-30 nm) in size. Importantly, the
Pt-based bimetallic nanoparticles with core-shell structures
appear to be the most advantageous in terms of their
significantly enhanced catalytic activity and sensitivity for
both the HER and/or HOR mechanisms and the ORR and/or
MOR mechanisms (Figure 5). In comparison to the Pt-Pd
nanostructures in alloy, cluster and mixture forms, the
electrocatalytic properties were significantly enhanced for

655

the observed Pt-Pd bimetallic core-shell nanoparticles
because of the very strong synergistic effect that appears
in well-shaped core-shell morphologies and nanostructures
[294,295] and various other advantages of the core-shell
structure [215,341,342,370].
In our electrochemical data and measurements of Pt-Pd

catalyst with various nanostructures, we have found that
Pt-Pd alloy, Pt-Pd cluster and mixture, Pt-Pd core-shell
(15 min), Pd-Pt core-shell, Pt-Pd core-shell, and Pd-Pt coreshell catalysts have ECA values (m2/g) of 12.7, 11.5, 27.7,
17.7, 13.6, and 14.2 m2/g, respectively. They exhibited E
(V) values of 0.7, 0.67, 0.63, 0.65, 0.77, and 0.73 V,
respectively, according to the current response. The peak
current in the forward scan, if or i(forward) (A/cm2), was
found to be 1.4 Â 10À3, 9.8 Â 10À4, 1.5 Â 10À3, 1.4 Â
10À3, 9.5 Â 10À4, or 1.0 Â 10À3 A/cm2, respectively. The
current remaining after 2 h of polarization at 0.5 V was
found to be 29.40%, 38.70%, 30.00%, 23.80%, 23.80%, or
3.68%, respectively. Among the various Pt-Pd alloy, cluster,
mixture, and core-shell nanostructures that were investigated, the Pt-Pd core-shell catalyst with the most stable
configuration exhibited the highest ECA value (27.7 m2/g)
and the highest initial current (1.29 x 10À3 A/cm2 or 1.5 x
10À3 A/cm2) in two separate experiments, and approximately 30% of the current remained after 2 h of polarization
[295]. The as-prepared Pt-Pd catalysts with uniform coreshell structures exhibited far superior catalytic, selective,
sensitive, and quick activity to the as-prepared catalysts
with single, alloy and mixture structures.
Observations of most of the core-shell bimetallic nanoparticles indicated that the Frank-van der Merwe (FM) and
Stranski-Krastanov (SK) growth modes coexist in the nucleation, growth, and formation of the shells on the cores. It is
predicted that one of these two growth modes will become
distinctly more favorable than the other in the formation of
the thin shells of core-shell nanoparticles and nanostructures. In our research, we have described a strategy for
improving the catalytic activity of Pt-based catalysts
through the use of Pt-and-Pd-based alloy and core-shell
nanoparticles [294,295]. In addition, Pd-Pt nanoparticles
have shown high hydrogen solubility because of the coexistence of the Pd(2H), Pt(2H), Pt/Pd(2H) and Pt-Pd(2H)
hydride phases in the very tiny Pt-Pd nanoparticles. This is
an inherent property of Pd and Pt metals [287,288]. In this

context, Pt and Pd metals exhibit a very high strong
inherent interaction with hydrogen. For example, at room
temperature, Pd metal has the unusual property of absorbing up to 900 times its own volume of hydrogen [53,392].
Therefore, a crucial question for scientists is how to explain

Figure 5 The pure Pt-Pd mixture, alloy and core-shell catalysts for direct methanol fuel cells. The best catalytic activity and
sensitivity is found in the Pt-Pd bimetallic core-shell catalyst. (A1) (a)-(c) TEM images of alloy Pt-Pd nanoparticles. (A1) (d)-(i) TEM
images of mixture Pt-Pd nanoparticles. (A2)-(A3) (a)-(c). TEM and HRTEM images of Pt-Pd core-shell nanoparticles (Category 3). (A3)
(d)-(i) TEM and HRTEM images of Pd-Pt core-shell nanoparticles (Category 4). (B) Cyclic voltammograms of Pt-Pd nanoparticles with
their different configurations. Electrolyte solution was 0.5 M H2SO4 (Scan rate: 50 mV/s). (C) Cyclic voltammograms towards
methanol electro-oxidation of Pt-Pd nanoparticles with their different configurations. Electrolyte solution was 0.5 M H2SO4+1.0M
CH3OH (Scan rate: 50 mV/s). (D) Chronoamperometry data of Pt-Pd nanoparticles with their different configurations. Electrolyte
solution is 0.5 M H2SO4+1.0 M CH3OH. The polarization potential was 0.5 V. Reprinted with permission from: N.V. Long, T.D. Hien, T.
Asaka, M. Ohtaki, M., Nogami, M., Synthesis and characterization of Pt-Pd alloy and core-shell bimetallic nanoparticles for direct
methanol fuel cells (DMFCs): Enhanced electrocatalytic properties of well-shaped core-shell morphologies and nanostructures, Int.
J. Hydrogen Energy 36(14) (2011) 8478-8491 [295]. Copyright © (2011) Elsvier Publishers.


656

N.V. Long et al.


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion
the mechanisms of hydrogen adsorption and desorption that
occur both on the surface and in the bulk volume of very
tiny Pt nanoparticles. We can use some concentration of
suitable metal atoms on the surfaces of as-prepared Pt
nanoparticles for the quick reduction of CO poisoning. The
question is whether the hydrogen activity in FCs occurs only

on the surfaces of the nanoparticles or in the active
catalytic sites of the bulk volume of the nanoparticles
through their specific hydride phases [287,288,343].
In the DFT calculations of model electrodes, Pt-Ru and PtSn alloys can potentially show good activity for the MOR. In
comparison with Pt-Ru and Pt-Sn alloys, it is predicted that
Pt-Cu alloys can be promising anode catalysts on a common
volcano plot [120,344]. Thus, all the recent research
indicates that the catalytic activity and stability of Pt-Pd
mixture, alloy and core-shell catalysts can be ranked as
follows: Pt-Pd bimetallic core-shell catalysts4Pt-Pd bimetallic alloy catalysts4Pt-Pd bimetallic mixture catalysts
44 pure Pt catalysts in terms of the electrochemical
surface area (ECA) and the current. Of course, Pt-Pd
catalysts perform much better than pure Pt catalysts,
despite the fact that the as-prepared Pt nanoparticles have
the largest quantum-size effect in the range of 10 nm
(1-10 nm) as well as good characterization in terms of
structure, size, shape, morphology, and surface that undergoes no changes as the electrocatalytic reactions proceed.
This is an important conclusion for one typical category of
the various PtxPdy-based catalysts. To obtain good characterizations of the desirable ranges of composition and
structure as well as the desirable elements of the preparation processes for catalysts such as Pt3M catalysts (where M
stands for Fe, Ni, Co, V, or Ti), we can generally rank most

657

of the Pt-based catalysts in the order of the best choices for
PEMFCs and DMFCs; these are typically bimetallic catalysts,
such as PtxAuy [193–195], PtxAgx [393], alloyed PtmAg
nanostructures (atomic Pt/Ag ratio m = 0.03-1.0), PtxRhy
[191–192], PtxRuy [161–190], PtxCuy [121–124], PtxNiy [224–
230], PtxCoy [321–351], and PtxSny [352–354], as well as Ptbased alloy and core-shell catalysts [150–152]. In addition,

the as-prepared nanoparticles must be hardened with highheat treatments without causing any changes of size,
structure, or surface characterizations. Recently, asprepared Pd-Pt bimetallic nanodendrites have exhibited
high activity for the ORR [281].
Figure 6 shows nanoparticle and nanostructure models and
as-prepared Pt-based nanoparticles for potential applications
in PEMFCs and DMFCs. The roles of Pt atoms, Pt nanoclusters,
and very small Pt nanoparticles in the ranges of 1-10 nm, 120 nm, and 1-30 nm for the attainment of high catalytic
sensitivity can be understood from this figure. Thus far, most
Pt-based engineered catalysts have belonged to our proposed
models, depicted in Figure 7. However, there have been no
intensive and comprehensive research programs with the
intent of performing a definitive comparison among the
above models with respect to the stability, durability and
reliability of the engineered catalysts as well as the degree of
interaction between the Pt-based catalysts and their supports according to the definition of surface electrocatalysis.
The main pieces of evidence for catalytic activity have been
determined on the surfaces of Pt-based metal, bimetal, and
multi-metal catalysts with respect to stable and durable
facets, i.e., low- and high-index planes (h k l) of the pure Pt
catalyst, such as (111), (110), and (100) [36]. In the
nanosized range of several nanometers, especially near

Figure 6 Nanoparticles for catalysis & energy conversion. The ideas of the new nanostructures of the designed catalysts. The new
methods of designing new catalyst, especially with the use of Pt nanoparticles or Pt nanoclusters on cheap metal nanoparticles. The
The core-shell structure is an economic solution for reducing high cost of Pt. (A1), (A2-1), and (A2-2) HRTEM images of polyhedral Pt
nanoparticles. (A1), (A7) Reprinted with permission from: N.V. Long, M. Ohtaki, T.D. Hien, R. Jalem, M. Nogami, Synthesis and
characterization of polyhedral and quasi-sphere non-polyhedral Pt nanoparticles: effects of their various surface morphologies and
sizes on electrocatalytic activity for fuel cell applications, J. Nanopart. Res. 13 (2011), 5177–5191 [134]. © Springer, Part of Springer
Science +Business Media. (A2-1) N.V. Long, M. Ohtaki, M. Nogami, Control of Morphology of Pt Nanoparticles and Pt-Pd Core-Shell
Nanoparticles, J. Novel Carbon Resource Sci., Kuyshu university, 3 (2011) 40–44 [373]. (A2-2) (A6-2) Reprinted with permission from:

Nguyen, V.L., Ohtaki, M., Matsubara, T., Cao, M.T., Nogami, M., 2012, New experimental evidences of Pt-Pd bimetallic nanoparticles
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self-aggregation and assembly, J. Colloid Interface Sci. 359 (2011) 339–350 [136]. Copyright © Elsvier Publishers. (A5) S. Khanal, G.
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by Cs-Corrected STEM, J. Phys. Chem. C 116 (2012), 23596–23602 [374]. Copyright © American Chemical Society. (A6-1) Reprinted
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658
1 nm, we suggest that it is not possible to exactly determine
the active sites of highly active Pt atoms of the (111), (110),
and (100) low-index planes for the electrocatalysis of hydrogen, methanol and ethanol. A nice approach to determining
the total number of Pt atoms on the surface of one Pt
nanoparticle will be the ideal manner in which to explain the
electrocatalysis of all as-prepared Pt nanoparticles. Recently,
sharp tetrahexahedral Pt-based catalysts with 24 high-index

facets, typically the (730), (210), and/or (520) surfaces, have
exhibited extremely high catalytic activity for formic acid
and ethanol because of the large density of atomic steps and
dangling bonds on their surfaces [106].
This result indicates that the property of catalytic
activity mainly occurs at the surfaces of Pt-based catalysts.

N.V. Long et al.
The important roles of the combinations and mixtures of
pure Pt catalysts and their various supports as well as the
possible degrees of interaction between the pure Pt catalyst
and its supports are ignored. A good catalyst with the
optimal composition and morphology can be effectively
synthesized and tested for use in FCs with the research
models presented above. We suggest that Pt-metal-based
nanoparticles (in which the other metal can be Fe, Co, Ni,
Sn, Ru, Rh, Pd, Os, Au, or Ag) with mixture, alloy and coreshell structures in various size ranges from the nanoscale to
the microscale can be successfully prepared, especially
noble-metal-based nanoparticles (Pt, Pd, Pt-Au, Pt-Ag) in
the size range of 1-10 nm, which can be fully or partially
supported on high-surface-area carbon materials to form


The Development of Mixture, Alloy, and Core-Shell Nanocatalysts with Nanomaterial Supports for Energy Conversion

659

Figure 7 The new ideas and models of Pt loaded supports for significant improvement of the catalyst layer and find the best Pt based
catalysts. The shapes and morphology of Pt nanoparticles: Cube, octahedra, tetrahedra, sphere, hexagonal, rectangle, rod, flower,
polyhedra, and various other shapes and morphologies. The models of the pure Pt catalyst with the supports are proposed. The very

important roles of surface catalysis of the firm attachments and bondings of the prepared Pt nanoparticles and the supports: The
definitions of fully and partial surface catalysis of the prepared Pt nanoparticles are proposed. The supports of low and high porosity for
the integration of the Pt based catalysts can be prepared. The surfaces of Pt nanoparticles can be on or inside the surfaces of the
supports or the co-existence both on and inside the surfaces of the support. The good durable and stable characterization of structure,
size, shape, and morphology of Pt nanoparticles should be kept in the supports. We suppose that various mechanisms of totally inelastic
collisions between the Pt based nanoparticles with the various supports are the best ways of the good integration of the pure Pt catalyst
supported on the various supports for high catalytic activity as well as heat treatments for high durability and stability.

robust catalysts for use in FCs. Pt-based core-shell catalysts
with thin catalytic shells, coatings or skins, such as Pt, Pd,
Pt-Pd, Pt-Au, Pt-Ag, Pt-Ru, and Pt-Rh, can be very economically used in DMFCs and PEMFCs and incur very low costs in
comparison to pure Pt catalysts.
In fact, according to the proposed catalytic models of
catalytic activity and stability in the catalytic layers of FCs
(typically PEMFCs and DMFCs), one of the most important

and efficient methods for designing a catalyst is to utilize
the various collision effects among the prepared Pt-based
nanoparticles and the related supports to achieve the
highest possible distribution of the as-prepared Pt-based
nanoparticles on the supports, which can be materials such
as carbon nanomaterials, oxides, and ceramics, as shown in
our proposals in Figure 7. The Pt nanoparticles that are
used to form pure Pt catalysts, which are typically in the


660

N.V. Long et al.

Figure 8 Highlights and achievements of Pt based loaded carbon supports for the enhancements of the designed catalysts. TEM
images of the platinum nanoparticles with the uniformly dispersion on the carbon support with only a few agglomerations and the
corresponding histograms, demonstrating a very narrow size distribution. The Pt average diameter of the particles is approximately
2nm for all samples, indicating very small particles and a particle size independent of the support. ECSAs of Pt/CNTs, Pt/OCNTs,
Pt/NCNTs, and Pt/NCNTs-CVD catalysts were estimated by CO-stripping and found to be 102.37 m2 gPtÀ1, 116.41 m2 gPtÀ1, 111.62
m2 gPtÀ1 and, 122.25 m2 gPtÀ1, respectively. Reprinted with permission from: D.Z. Mezalira, M. Bron, High Stability of Low Pt
Loading High Surface Area Electrocatalysts Supported on Functionalized Carbon Nanotubes, J. Power Sources 231 (2012) 113-121
[354]. Copyright © (2011) Elsvier Publishers.

size ranges of 10 nm, 20 nm, and 30 nm and have polyhedral or polyhedral-like shapes and morphologies or spherical or spherical-like shapes and morphologies, can be
used as the catalytic standard for PEMFCs and DMFCs in
various comparisons of catalytic activity. However, we
believe that pure Pt-based catalysts should been entirely
incorporated on the supports (carbon, oxides, ceramics, or
mixed supports), according to catalytic models in Figure 7,

for the optimal electrocatalysis at the Pt surfaces. In the
models describing well-designed Pt catalysts, we need
various supports of very high porosity as well as both small
and large pores inside and on the surfaces of the supports. In
this way, the pure Pt nanoparticles can be well integrated
into the supports, partially on the surfaces of the supports
and partially inside the supports. To achieve the best
performance of the catalytic activity, the mixture of the