The multifunctional tixw1 xo2 (x=0 5; 0 6; 0 7; 0 8) support for platinum to enhance the activity and co tolerance of direct alcohol fuel cells

VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY
HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

HAU QUOC PHAM

THE MULTIFUNCTIONAL TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8)
SUPPORT FOR PLATINUM TO ENHANCE THE ACTIVITY
AND CO-TOLERANCE OF DIRECT ALCOHOL FUEL CELLS

A dissertation submitted for the degree of
Doctor of Philosophy

HO CHI MINH CITY – 2022


VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY
HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

HAU QUOC PHAM

THE MULTIFUNCTIONAL TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8)
SUPPORT FOR PLATINUM TO ENHANCE THE ACTIVITY
AND CO-TOLERANCE OF DIRECT ALCOHOL FUEL CELLS
A dissertation submitted for the degree of
Doctor of Philosophy

Major subject: Chemical Engineering
Major subject code: 9520301

Independent Reviewer:
Independent Reviewer:

Reviewer: ASSOC. PROF. DR. TRAN VAN MAN
Reviewer: ASSOC. PROF. DR. NGUYEN DINH THANH
Reviewer: ASSOC. PROF. DR. NGUYEN NHI TRU

Advisor: 1. ASSOC. PROF. VAN THI THANH HO
2. ASSOC. PROF. SON TRUONG NGUYEN


PLEDGE
I pledge that this dissertation is my study under the direct guidance of Assoc. Prof.
Van Thi Thanh Ho and Assoc. Prof. Son Truong Nguyen. The study results and
conclusions in this dissertation are honest, and not copied from any one source and in any
form. The reference to the sources of documents (if any) has been cited and the reference
sources are recorded as prescribed.

Signature

Hau Quoc Pham

i


ABSTRACT
The worldwide environment has been getting worse day by day because of the
emission of various harmful pollutants into the environment from burning traditional
fossil fuels. Also, fossil fuels are limited resources and will be exhausted in the next
few decades, therefore, finding out sustainable and renewable energy sources has
sparked interest as future alternatives. Recently, direct alcohol fuel cells (DAFCs) have
been considered a promising green energy source in portable and transportation

applications due to their relatively simple infrastructure, portability, operation cost, easy
storage, and conveyance of alcohol fuels. Nonetheless, the sluggish oxidation kinetics
and “CO-like poisoning” effect of catalysts are limitations for commercializing DAFCs.
Alloying Pt with Ru is regarded as an efficient anodic DAFC catalyst owing to its high
electrochemical activity and great CO anti-poisoning ability. The Ru metal, however,
can be dissolved at the fuel cell operation potential, resulting in a decrease in the
electrocatalytic stability of this alloy catalyst. Furthermore, the high price and low
natural abundance of Ru are drawbacks of particular uses.
To address aforementioned problems, we fabricate TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8)
nanostructures as multifunctional support with co-catalytic functionality to prevent or
reduce the deterioration of anodic catalyst in DAFCs. Additionally, tuning morphology
and structure of metal catalyst are also combined to enhance the catalytic performance
of electrocatalyst, thereby promoting large-scale DAFC applications.
Various mesoporous TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) supports are fabricated by a
facile solvothermal route to comprehend the effect of doping tungsten concentration on
electrochemical properties of 20 wt% Pt/TixW1-xO2 catalysts for ethanol electrooxidation reaction (EOR). As a result, the surface area and electrical conductivity of asprepared supports are drastically increased with the doped tungsten contents of 20 at%
(Ti0.8W0.2O2) and 30 at% (Ti0.7W0.3O2). With rising the doped tungsten content up to 40
at% (Ti0.6W0.4O2), the electrical conductivity is almost unchanged, whereas the surface
area is remarkably decreased. This implies that the addition of proper doped tungsten
content into TiO2 lattices results in an enormous enhancement in both surface area and
electrical conductivity. Also, small-size Pt nanoparticles (NPs) are well-distributed on
ii


the support surface by a rapid microwave-assisted polyol route. In term of the EOR, 20
wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) catalyst shows the catalytic performance better
than the commercial 20 wt% Pt/C (E-TEK) catalyst. Among as-made catalysts, 20 wt%
Pt/Ti0.7W0.3O2 catalyst displays the highest mass activity (260. 23 mA mgPt-1) and largest
If/Ib ratio (1.34), which are 2.0- and 1.57-fold greater than those of 20 wt% Pt/C (ETEK) catalyst (130.62 mA mgPt-1 for mass activity and 0.85 for If/Ib value, respectively).
After 5000-cycling ADT, as-made catalysts show the mass activity loss about twice as

lower than the commercial catalyst that exhibits great EOR stability. Experimental
results demonstrate that TixW1-xO2 supports can be utilized as a suitable alternative for
the common carbon material in the function of catalyst support for fuel cells.
For the first time, a combination of using non-carbon nanosupport and tuning the
morphology and structure of metal catalyst is utilized to assemble a robust catalyst for
alcohol oxidation reaction (AOR). The one-dimensional (1D) Pt nanowires (NWs) are
successfully grown on the Ti0.7W0.3O2 surface by a simple chemical reduction route at
room temperature, only using formic acid as a reducing agent. These observational
results indicate that the 1D Pt NWs/Ti0.7W0.3O2 catalyst is a potential anodic catalyst for
the oxidation reaction of methanol (MOR) and ethanol (EOR), which can replace a
conventional Pt NPs/C catalyst. For instance, 1D Pt NWs/Ti0.7W0.3O2 catalyst exhibits
a low onset potential (~0.1 VNHE for MOR and ~0.2 VNHE for EOR), high mass activity
(355.29 mA mgPt-1 for MOR and 325.01 mA mgPt-1 for EOR), and impressive
electrochemical stability compared to the Pt NPs/C catalyst. The outstanding activity
and stability of 1D Pt NWs/Ti0.7W0.3O2 catalyst can be interpreted due to the unique
properties of 1D Pt nanostructures and advantages of Ti0.7W0.3O2, as well as synergetic
effects between 1D Pt NWs and Ti0.7W0.3O2 support.
More importantly, 1D-bimetallic Pt3Co NWs with a diameter of around 4 nm and
several tens of nanometers in the lengths are grown on the Ti07W0.3O2 by a templateand

surfactant-free

chemical

reduction

method.

The

1D-bimetallic

Pt3Co

NWs/Ti0.7W0.3O2 catalyst exhibits high mass activity (393.29 mA mgPt-1 for MOR and
341.76 mA mgPt-1 for EOR) and great electrochemical durability compared to
conventional Pt NPs/C catalyst. In addition, the CO-stripping result shows superior CO-

iii


tolerance of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst with the COads oxidation peak at 0.64
VNHE, which is much lower than that of the Pt NPs/C catalyst (0.78 VNHE). After 5000cycling ADT, the activity loss of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst is 10.68% of the
initial mass activity, which was 4.18-time lower than that of the Pt NPs/C catalyst
(44.66%), indicating superior stability retention of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst.
These enhancements are attributable to (i) advantages of 1D nanostructures with
abundant active catalytic sites facilitating the oxidation of adsorbed small organic
molecules; (ii) addition of Co promotes the removal of strongly bound intermediates on
Pt sites neighboring Co, resulting in boosting CO-tolerance of as-prepared catalyst; (iii)
synergic and electronic effects of compounds, Pt3Co NWs, and Ti0.7W0.3O2 support.
This work can open up an effective approach to enhance the performance of catalysts
with a decrease in Pt consumption for electrochemical energy conversion.

iv


TĨM T T LU N ÁN
Bi n đ i khí h u và ô nhi m môi tr

ng trên th gi i ngày càng tr nên t i t do s

phát th i c a các ch t ô nhi m t vi c đ t các nhiên li u hóa th ch truy n th ng. H n
n a, tr l

ng nghiên li u hóa th ch trên th gi i là gi i h n và s c n ki t trong vài

th p k t i, do đó, nhu c u tìm ki m các ngu n n ng l
tái t o đ

ng thay th s ch và có kh n ng

c u tiên. Pin nhiên li u s d ng tr c ti p alcohol (DAFCs) đang đ

c nghiên

c u và s d ng trong nhi u l nh v c nh v n chuy n và các thi t b c m tay do ít phát
th i khí nhà kính, hi u su t chuy n đ i n ng l

ng t

ng đ i cao, chi phí v n hành th p,

kh n ng l u tr và v n chuy n d dàng và an toàn c a nhiên li u alcohol. Tuy nhiên,
đ ng h c cho ph n ng oxi hóa ch m và s ng đ c CO c a xúc tác Pt là nh ng h n
ch chính nh h

ng tr c ti p t i hi u su t ho t đ ng c a DAFCs trong th i gian ho t

đ ng lâu dài. V t li u xúc tác h p kim Pt v i Ru đang đ

c s d ng nh xúc tác hi u

qu cho ph n ng oxi hóa nhiên li u alcohol do ho t tính xúc tác và kh n ng ch ng
ng đ c CO cao, nh ng s d hòa tan c a kim lo i Ru t i th ho t đ ng c a pin nhiên
li u d n t i s không n đ nh c a v t li u xúc tác này. Ngoài ra, giá thành cao và l
Ru t nhiên t

ng đ i th p c ng là m t nh

ng

c đi m c a xúc tác Pt-Ru.

gi i quy t v n đ này, tôi t ng h p và kh o sát đ c tính c a v t li u c u trúc nano
TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nh v t li u n n xúc tác đa ch c n ng v i vai trò đ ng
xúc tác đ c i thi n ho t tính và đ b n c a v t li u xúc tác trong pin nhiên li u s d ng
tr c ti p alcohol. Ngoài ra, vi c đi u khi n hình d ng và c u trúc c a kim lo i xúc tác
c ng đ

c k t h p trong lu n án này đ c i thi n ho t tính xúc tác c a v t li u xúc tác

đi n hóa cho ph n ng oxi hóa alcohol, thúc đ y s

ng d ng c a pin nhiên li u s d ng

tr c ti p alcohol.
V t li u n n TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) đ
nhi t đ kh o sát s

nh h

ng c a l

c t ng h p b ng ph

ng pháp dung

ng Vonfram (W) pha t p lên đ c tính xúc tác

đi n hóa c a v t li u xúc tác 20 wt% Pt/TixW1-xO2 cho ph n ng oxi hóa ethanol (EOR).
K t qu ch ra r ng di n tích b m t riêng và đ d n đi n c a v t li u n n t ng đáng k
khi l

ng W pha t p là 20 at% (Ti0.8W0.2O2) và 30 at% (Ti0.7W0.3O2), và khi t ng lên t i

40 at% thì đ d n đi n h u nh không thay đ i, trog khi đó di n tích b m t riêng gi m
rõ r t.

i u này ch ng t r ng khi thêm m t l
v

ng W pha t p tích h p d n t i s c i


thi n c di n tích b m t riêng và đ d n đi n c a v t li u n n TixW1-xO2. Bên c nh đó,
h t xúc tác Pt d ng c u v i kích th
n n thông qua ph

c nh c ng đ

c phân b t t trên b m t v t li u

ng pháp polyol có s h tr c a vi sóng.

i v i EOR, v t li u xúc

tác 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) th hi n hi u qu xúc tác t t h n so v i v t
li u xúc tác th

ng m i 20 wt% Pt/C (E-TEK). Trong s v t li u xúc tác t ng h p, v t

li u xúc tác 20 wt% Pt/Ti0.7W0.3O2 th hi n hoat tính cao nh t v i c

ng đ oxi hóa là

260. 23 mA mgPt-1 và t l If/Ib là 1.34, g p 2 và 1.57 l n so v i xúc tác th
wt% Pt/C (E-TEK) (c

ng đ oxi hóa là 130.62 mA mgPt-1 và t l If/Ib là 0.85. Sau

5000 vòng quét th tu n hòa, v t li u xúc tác t ng h p đ
th p h n kho ng 2 l n so v i xúc tác th
c a v t li u t ng h p đ

ng m i 20

c cho th y s suy ho t tính

ng m i, đi u này cho th y đ b n xúc tác t t

c. Nh ng k t qu trên cho th y r ng v t li u n n TixW1-xO2 có

th là s thay th thích h p cho v t li u carbon th

ng m i trong pin nhiên li u.

L n đ u tiên, v t li u xúc tác 1D Pt d ng s i (nanowires) đ
trên v t li u n n Ti0.7W0.3O2 b ng ph

c t ng h p thành công

ng pháp kh đ n gi n t i nhi t đ phòng s d ng

formic acid nh ch t kh . V t li u xúc tác 1D Pt NWs/Ti0.7W0.3O2 th hi n là v t li u
xúc tác ti m n ng cho q trình oxi hóa methanol (MOR) và ethanol (EOR) có th thay
th v t li u xúc tác truy n th ng Pt NPs/C. C th , xúc tác 1D Pt NWs/Ti0.7W0.3O2 th
hi n th kh i phát q trình oxi hóa nhiên li u th p (~0.1 VNHE cho MOR và ~0.2 VNHE
cho EOR), c

ng đ oxi hóa cao (355.29 mA mgPt-1 cho MOR và 325.01 mA mgPt-1

cho EOR), c ng nh đ b n xúc tác t t so v i v t li u xúc tác Pt NPs/C. S c i thi n
này đ

c gi i thích là do u đi m c a c u trúc nano 1D Pt và v t li u n n Ti0.7W0.3O2

c ng nh hi u ng liên h p gi a 1D Pt NWs và v t li u n n Ti0.7W0.3O2.
c bi t, xúc tác h p kim 1D Pt3Co d ng s i (NWs) v i đ
dài vài ch c nanomet c ng đ
ph

ng kính kho ng 4 nm và

c t ng h p thành công trên v t li u n n Ti0.7W0.3O2 b ng

ng pháp kh đ n gi n không s d ng khung m u hay ch t ho t đ ng b m t. V t

li u xúc tác 1D Pt3Co NWs/Ti0.7W0.3O2 th hi n ho t tính cao (393.29 mA mgPt-1 cho
MOR và 341.76 mA mgPt-1 cho EOR), và đ b n xúc tác t t so v i v t li u xúc tác
truy n th ng Pt NPs/C. Ngoài ra, ph
ng đ c CO v

ng pháp CO-stripping th hi n kh n ng ch ng

t tr i c a xúc tác 1D Pt3Co NWs/Ti0.7W0.3O2 v i peak oxi hóa COads t i

0.64 VNHE, th p h n đáng k so v i xúc tác Pt NPs/C (0.78 VNHE). Sau 5000 vòng quét

vi


th tu n hoàn, s suy gi m ho t tính c a v t li u xúc tác 1D Pt3Co NWs/Ti0.7W0.3O2 là
10.68%, th p h n 4.18 l n so v i xúc tác Pt NPs/C (44.66%), đi u này ch ra đ b n
xúc tác t t c a v t li u 1D Pt3Co NWs/Ti0.7W0.3O2. S c i thi n này có th là do (i) u
đi m c a c u trúc nano 1D v i nhi u v trí ho t hóa thúc đ y q trình oxi hóa các phân
methanol và ethanol; (ii) s xu t hi n c a Co thúc đ y vi c oxi hóa các s n ph m trung
gian trong su t q trình oxi hóa nhiên li u trên b m t Pt, d n t i t ng kh n ng ch ng
ng đ CO c a v t li u xúc tác t ng h p; (iii) hi u ng đi n t và liên h p gi a Pt3Co
NWs và v t li u n n Ti0.7W0.3O2. H

ng nghiên c u này có th m ra m t cách ti p c n

hi u qu đ nâng cao hi u qu xúc tác c a v t li u xúc tác đi n hóa v i s gi m l
Pt s d ng cho l nh v c chuy n hóa n ng l

ng đi n hóa.

vii

ng


ACKNOWLEDGEMENTS
First of all, I would like to express my deepest gratitude to my advisors, Assoc Prof.
Dr. Van Thi Thanh Ho, and Assoc Prof. Dr. Son Truong Nguyen for suggesting the
problem, supervising the work, and being a potential source of inspiration at each stage
of this dissertation research work. I would like to express my deep thankfulness to Prof.
Nam Thanh Son Phan supported me during this dissertation research work at the
University of Technology – Viet Nam University HCMC.
I would like to give deep thanks to Dr. Tai Thien Huynh for the collaboration during
4 years of working together. His enthusiasm and support are highly appreciated.
I would like to thank you for the support of the Faculty of Chemical Engineering –
University of Technology – Viet Nam University HCMC, the MANAR Laboratory –
Faculty of Chemical Engineering – University of Technology – Viet Nam University
HCMC, the Physical Chemistry Laboratory – Ho Chi Minh City University of Natural
Resources and Environment, the Applied Physical Chemistry Laboratory – University
of Science – Viet Nam University HCMC, and the Key Laboratory of Polymer and
Composite Materials – University of Technology – Viet Nam University HCMC.
My special thanks to my parents, and my girlfriend for understanding,
encouragement, and consistent support throughout my dissertation journey. Without

their enthusiastic support, I could not complete my research.
Finally, I acknowledge The Young Innovative Science and Technology Incubation
Program, managed by Youth Promotion Science and Technology Center, Hochiminh
Communist Youth Union, HCMC, Vietnam (Project No. 10/2018/H -KHCN-V ), and
Ph.D. Scholarship Programme of Vingroup Innovation Foundation (VINIF) (No.
VINIF.2019.TS.22, VINIF.2020.TS.108 and VINIF.2021.TS.016) and University
Scholarship of VNU – HCMC, 2019 for financial support.
I sincerely thank you all!

viii


TABLE OF CONTENTS
LIST OF TABLES ..................................................................................................... xiii
LIST OF FIGURES ................................................................................................... xiv
LIST OF SYMBOLS AND ABBREVIATIONS ..................................................... xix
MOTIVATION OF RESEARCH ............................................................................... 1
CHAPTER 1. INTRODUCTION AND LITERATURE REVIEW ......................... 2
1.1. Direct alcohol fuel cells (DAFCs) ........................................................................... 2
1.1.1. Overview of DAFC technologies ......................................................................... 2
1.1.2. Research history of direct alcohol fuel cells......................................................... 3
1.2. Alcohol electro-oxidation on the Pt nanocatalysts .................................................. 7
1.2.1. Methanol electro-oxidation reaction (MOR) on the Pt surface ............................ 7
1.2.2. Ethanol electro-oxidation reaction (EOR) on the Pt surface ................................ 9
1.3. Challenges of Pt-based catalyst in direct alcohol fuel cells .................................. 11
1.3.1. CO poisoning ...................................................................................................... 11
1.3.2. Carbon corrosion ................................................................................................ 12
1.3.3. Platinum dissolution and growth ........................................................................ 14
1.4. Non-carbon support for Pt-based electrocatalyst .................................................. 16
1.4.1. Advantages and challenges of non-carbon nanosupport for DAFCs ................. 16

1.4.2. State-of-the-art M-doped TiO2 support for alcohol electro-oxidation (AOR) ... 17
1.5. Tungsten-doped TiO2 nanosupport for direct alcohol fuel cells ........................... 19
1.6. One-dimensional (1D) Pt-based catalysts for the alcohol electro-oxidation ......... 20
1.6.1. Advantages and challenges of 1D Pt-based nanostructures ............................... 21
1.6.2. Preparation of 1D Pt-based electrocatalysts ....................................................... 22
1.6.3. State-of-the-art 1D Pt-based electrocatalysts ..................................................... 24
1.7. Strategy and research objectives ........................................................................... 26
ix


CHAPTER 2. MATERIAL AND EXPERIMENTS ............................................... 29
2.1. Chemicals .............................................................................................................. 29
2.2. Experimental Sections ........................................................................................... 29
2.2.1. Fabrication of non-carbon TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports ...... 29
2.2.2. Fabrication of 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) nanocatalysts............... 30
2.2.3. Fabrication of 1D Pt nanowires (NWs) on the Ti0.7W0.3O2 support ................... 31
2.2.4. Preparation of 1D-bimetallic Pt3Co NWs on the Ti0.7W0.3O2 support ............... 32
2.3. Physical Characterizations ..................................................................................... 33
2.3.1. X-ray diffraction (XRD) ..................................................................................... 33
2.3.2. X-ray photoelectron spectroscopy (XPS) ........................................................... 33
2.3.3. X-ray fluorescence (XRF) .................................................................................. 34
2.3.4. Transmission electron microscopy (TEM) ......................................................... 34
2.3.5. Scanning electron microscopy (SEM) with energy dispersive spectroscopy
(EDX) mapping measurement ...................................................................................... 34
2.3.6. Brunauer-Emmett-Teller (BET) method ............................................................ 35
2.3.7. Electrical conductivity measurement ................................................................. 35
2.4. Electrochemical Characterizations ........................................................................ 35
2.4.1. Preparation of electrocatalytic ink ...................................................................... 35
2.4.2. Electrochemical test............................................................................................ 36
2.4.3. Cyclic voltammetry (CV) test ............................................................................ 37

2.4.4. Alcohol electro-oxidation reaction (MOR, EOR) .............................................. 38
2.4.5. CO-stripping voltammetry test ........................................................................... 39
2.4.6. Accelerated durability test (ADT) ...................................................................... 39
2.4.7. Chronoamperometry measurement .................................................................... 40
CHAPTER 3. SYNTHESIS AND CHARACTERIZATION OF THE TixW1-xO2 (x
x


= 0.5; 0.6; 0.7; 0.8) NANOMATERIALS AS ROBUST NON-CARBON
SUPPORTS FOR PLATINUM NANOPARTICLES IN DIRECT ETHANOL
FUEL CELLS ............................................................................................................. 41
3.1. Characterization of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports ....... 41
3.1.1. Effect of differential Ti: W ratio on nanostructures ........................................... 41
3.1.2. Effect of differential Ti: W ratio on particle size and morphology .................... 42
3.1.3. Effect of differential Ti: W ratio on the surface area ......................................... 43
3.1.4. Effect of differential Ti: W ratio on the electrical conductivity ......................... 45
3.2. Characterization of 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7; 0.8) catalysts ................. 47
3.3. Electrochemical characterization of electrocatalysts............................................. 50
3.4. Conclusion ............................................................................................................. 58
CHAPTER 4. ONE-DIMENSIONAL Pt NANOWIRES ON Ti0.7W0.3O2
SUPPORT WITH EFFECTIVE ELECTRO-ACTIVITY FOR ALCOHOL
ELECTRO-OXIDATION .......................................................................................... 59
4.1. Formation of the 1D Pt nanowires on Ti0.7W0.3O2 nanoparticles .......................... 59
4.1.1. Effect of reduction times on the growth of 1D Pt nanowires ............................. 59
4.1.2. Effect of loading amount of Pt on the growth of 1D Pt nanowires .................... 61
4.1.3. A proposed mechanism for the formation of 1D Pt NWs/Ti0.7W0.3O2 ............... 62
4.2. Characterization of 1D Pt NWs/Ti0.7W0.3O2 catalyst ............................................ 63
4.3. Electrochemical characterization of 1D Pt NWs/Ti0.7W0.3O2 catalyst .................. 66
4.4. Conclusion ............................................................................................................. 79
CHAPTER

5.

SUPERIOR

CO-TOLERANCE

AND

STABILITY

FOR

ALCOHOL ELECTRO-OXIDATION REACTION OF 1D-BIMETALLIC
PLATINUM-COBALT NANOWIRES ON Ti0.7W0.3O2 NANOMATERIAL ...... 80
5.1. Characterization of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst ...................................... 80
5.2. Electrochemical properties of 1D Pt3Co NWs/Ti0.7W0.3O2 for MOR ................... 82
xi


5.3. Electrochemical properties of 1D Pt3Co NWs/Ti0.7W0.3O2 for EOR .................... 87
5.4. Conclusion ............................................................................................................. 91
CONCLUSIONS AND SCIENTIFIC CONTRIBUTION ...................................... 92
LIST OF PUBLICATIONS ....................................................................................... 94
LIST OF CONFERENCES ....................................................................................... 95
LIST OF RESEARCH PROJECTS ......................................................................... 96
REFERENCES ........................................................................................................... 97
Appendix A ................................................................................................................ 112
Appendix B ................................................................................................................ 120

xii


LIST OF TABLES
Table 1. 1. Thermodynamic data associated with the electrochemical oxidation of some
fuels (under standard conditions). .................................................................................. 2
Table 1. 2. Products of direct alcohol fuel cells. ............................................................ 6
Table 2. 1. Summary of reaction temperatures for preparing W-doped TiO2 supports.
...................................................................................................................................... 30
Table 2. 2. Summary of reaction times for preparing W-doped TiO2 supports. .......... 30
Table 2. 3. Summary of Ti: W ratios for preparing W-doped TiO2 supports. ............. 30
Table 2. 4. Summary of reaction times for fabricating 1D Pt NWs/Ti0.7W0.3O2. ......... 32
Table 2. 5. Summary of Pt loadings for fabricating 1D Pt NWs/Ti0.7W0.3O2. ............. 32
Table 3. 1. Summary of characterizations of non-carbon nanomaterials. .................... 46
Table 3. 2. Comparison of the EOR performance of Pt-based NPs catalysts. ............. 54
Table 3. 3. Comparison of EOR stability of catalysts after 5000 cycling test. ............ 56
Table 4. 1. Binding energies of Pt in electrocatalysts. ................................................. 66
Table 4. 2. Comparison of MOR performance of 1D Pt NWs/Ti0.7W0.3O2 catalyst. ... 70
Table 4. 3. Comparison of COads oxidation of 1D Pt NWs/Ti0.7W0.3O2 catalyst. ......... 72
Table 4. 4. Comparison of MOR stability of catalysts after 5000 cycling test. ........... 73
Table 4. 5. Comparison of EOR performance of 1D Pt NWs/Ti0.7W0.3O2 catalyst. .... 75
Table 4. 6. Comparison of EOR stability of catalysts after 5000 cycling test. ............ 77
Table 5. 1. Comparison of MOR performance of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst.
...................................................................................................................................... 85
Table 5. 2. Comparison of COads oxidation of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst. .. 87
Table 5. 3. Comparison of EOR performance of 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst.
...................................................................................................................................... 89

xiii

LIST OF FIGURES
Figure 1. 1. Scheme of direct alcohol fuel cells (DAFCs) ............................................. 3
Figure 1. 2. (a) Scheme of the first alkaline direct methanol fuel cells built by Kordesch
and Marko; and (b) part of a 60-W methanol-air battery with cylindrical air diffusion
electrodes built by Boveri et al. ...................................................................................... 4
Figure 1. 3. Proposed mechanism of parallel pathways for the methanol oxidation on
pure Pt’s surface in acidic media. ................................................................................... 7
Figure 1. 4. Proposed mechanism for the ethanol electro-oxidation on Pt surface in
acidic medium (all species with colored filling were detected either by IR reflectance
spectroscopy or by chromatographic analysis)............................................................... 9
Figure 1. 5. Electrosorption of methanol in an acidic medium. ................................... 11
Figure 1. 6. (a) Formation of radicals by the reaction of Pt, O2, and H2O; (b) carbon
corrosion in the presence of Pt, O2, and H2O. .............................................................. 13
Figure 1. 7. Carbon corrosion in the absence of Pt. ..................................................... 14
Figure 1. 8. Proposed mechanisms for Pt NP instability in fuel cells. ......................... 15
Figure 1. 9. The most stable substance under fuel cell cathode conditions at 80 oC. .. 17
Figure 1. 10. Effect of dopants in Pt/M-doped TiO2 (M = V, Cr, and Nb) catalysts on
the ORR performance. .................................................................................................. 18
Figure 1. 11. A summary of the kinds of quasi-one-dimensional nanostructures. ....... 21
Figure 1. 12. Our motivation and approach to enhance AOR performance................. 27
Figure 2. 1. The schematic illustration for fabricating W-doped TiO2 supports. ......... 29
Figure 2. 2. The schematic illustration for preparation of the 20 wt% Pt/TixW1-xO2. .. 31
Figure 2. 3. The schematic illustration of the synthesis of 1D Pt NWs/Ti0.7W0.3O2. ... 32
Figure 2. 4. Cyclic voltammogram curves of 20 wt% Pt/C (E-TEK) catalyst in 0.5 M
H2SO4 solution at a scan rate of 50 mV s-1. .................................................................. 38
xiv


Figure 2. 5. Cyclic voltammogram curves of 20 wt% Pt/C (E-TEK) catalyst in 10 v/v%

CH3OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1. ......................................... 39
Figure 3. 1. XRD patterns of TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports in the
2range from 20o to 80o at a step size of 0.02o. ........................................................... 42
Figure 3. 2. XRD patterns of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) nanosupports in
the 2range from 22o to 32o at a step size of 0.02o. ..................................................... 42
Figure 3. 3. TEM images of (a) undoped TiO2, (b) Ti0.8W0.2O2; (c) Ti0.7W0.3O2; (d)
Ti0.6W0.4O2; and (e) Ti0.5W0.5O2 supports. .................................................................... 43
Figure 3. 4. Comparison of surface area of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8)
nanosupports with reported non-carbon nanomaterials. ............................................... 44
Figure 3. 5. Electrical conductivity of various TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) supports.
...................................................................................................................................... 45
Figure 3. 6. X-ray diffraction (XRD) patterns of the 20 wt% Pt/TixW1-xO2 (x = 0.6; 0.7;
0.8) electrocatalyst in the 2range from 20o to 80o at a step size of 0.02o. ................. 48
Figure 3. 7. TEM images of (a) Pt/Ti0.8W0.2O2, (b) Pt/Ti0.7W0.3O2, and (c) Pt/Ti0.6W0.4O2.
...................................................................................................................................... 49
Figure 3. 8. High-resolution Pt 4f spectrum of (a) Pt/Ti0.8W0.2O2, (b) Pt/Ti0.7W0.3O2, (c)
Pt/Ti0.6W0.4O2 and (d) Pt/C (E-TEK) catalysts. ............................................................ 50
Figure 3. 9. Cyclic voltammograms of different electrocatalysts in N2-saturated 0.5 M
H2SO4 aqueous solution at a scan rate of 50 mV s-1..................................................... 51
Figure 3. 10. Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%
C2H5OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1. .......................... 52
Figure 3. 11. Onset potential of different catalysts in N2-saturated 10 v/v% C2H5OH/0.5
M H2SO4 solution at a scan rate of 50 mV s-1. ............................................................. 53
Figure 3. 12. The mass activity of catalysts in N2-saturated 10v/v% C2H5OH/0.5 M
H2SO4 solution at a scan rate of 50 mV s-1. .................................................................. 53
Figure 3. 13. Cyclic voltammograms of catalysts before and after 5000 cycling test in
xv


N2-saturated 10v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1....... 55

Figure 3. 14. Chronoamperogams of catalysts in N2-saturated 10 v/v% C2H5OH/0.5 M
H2SO4 aqueous solution at the fixed potential of 0.7 V for 7200s. .............................. 57
Figure 3. 15. Effect of the tungsten doping content on electrocatalytic performance of
Pt/TixW1-xO2 catalysts for ethanol electro-oxidation reaction. ..................................... 58
Figure 4. 1. XRD patterns of 1D Pt NWs/Ti0.7W0.3O2 catalyst with the reaction time (60
hours; 72 hours, and 84 hours) in the 2range from 20o to 80o at a step size of 0.02o.
...................................................................................................................................... 59
Figure 4. 2. TEM images of the 1D Pt NWs/Ti0.7W0.3O2 catalysts at different reduction
times of (a) 60 hours, (b) 72 hours, and (c) 84 hours. .................................................. 60
Figure 4. 3. XRD patterns of 1D Pt NWs/Ti0.7W0.3O2 catalyst with different Pt amounts
(40 wt%, 50 wt%, and 60 wt%) in the 2range in 20o-80o at a step size of 0.02o. ..... 61
Figure 4. 4. TEM images of the 1D Pt NWs/Ti0.7W0.3O2 catalysts at the different Pt
loading of (a) 40 wt%, (b) 50 wt%, and (c) 60 wt%. ................................................... 62
Figure 4. 5. XRD patterns of the 1D Pt NWs/Ti0.7W0.3O2 catalyst in the 2range from
20o to 80o at a step size of 0.02o. .................................................................................. 63
Figure 4. 6. (a, b) TEM images, (c) HR-TEM image, and (d) Energy-dispersive X-ray
(EDX) spectroscopy of the as-obtained 1D Pt NWs/Ti0.7W0.3O2 electrocatalysts. ...... 64
Figure 4. 7. High-resolution of Pt 4f spectrum of (a) 1D Pt NWs/Ti0.7W0.3O2, (b) Pt
NWs/C, (c) Pt NPs/C, (d) Comparison of binding energies of Pt 4f spectrum in catalysts.
...................................................................................................................................... 65
Figure 4. 8. Cyclic voltammograms of different catalysts in N2-saturated 0.5 M H2SO4
aqueous solution at a scan rate of 50 mV s-1. ............................................................... 66
Figure 4. 9. Cyclic voltammograms before and after 5000-cycling ADT of different
electrocatalysts in N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s-1. .... 67
Figure 4. 10. Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%
CH3OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1. ......................................... 68
xvi


Figure 4. 11. Onset potential of different catalysts in N2-saturated 10 v/v% CH3OH/0.5

M H2SO4 solution at a scan rate of 50 mV s-1. ............................................................. 69
Figure 4. 12. The mass activity of different catalysts in N2-saturated 10 v/v%
CH3OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1. ........................... 70
Figure 4. 13. CO-stripping cyclic voltammograms of differential catalysts in 0.5 M
H2SO4 aqueous solution at a scan rate of 50 mV s-1..................................................... 71
Figure 4. 14. Cyclic voltammograms of catalysts before and after 5000 cycling test in
N2-saturated 10v/v% CH3OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1. ....... 72
Figure 4. 15. Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%
C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1. ........................................ 74
Figure 4. 16. A comparison of the mass activity of different catalysts in N2-saturated 10
v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1................................ 74
Figure 4. 17. Cyclic voltammograms of catalysts before and after 5000 cycling test in
N2-saturated 10v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1....... 76
Figure 4. 18. Chronoamperogams of catalysts in N2-saturated 10 v/v% C2H5OH/0.5 M
H2SO4 aqueous solution at the fixed potential of 0.7 V for 7200s. .............................. 78
Figure 4. 19. Schematic illustration for improvement of 1D Pt NWs/Ti0.7W0.3O2
catalysts toward methanol and ethanol electrochemical oxidation. ............................. 79
Figure 5. 1. XRD patterns of the 1D Pt3Co NWs/Ti0.7W0.3O2 catalyst in the 2range
from 20o to 80o at a step size of 0.02o. .......................................................................... 81
Figure 5. 2. (a, b) TEM and (c) HR-TEM images of the Pt3Co NWs/Ti0.7W0.3O2 catalyst.
...................................................................................................................................... 81
Figure 5. 3. X-ray fluorescence (XRF) spectroscopy of 1D Pt3Co NWs/Ti0.7W0.3O2.. 82
Figure 5. 4. Cyclic voltammograms of different catalysts in N2-saturated 0.5 M H2SO4
aqueous solution at a scan rate of 50 mV s-1. ............................................................... 83
Figure 5. 5. Cyclic voltammograms of different catalysts in N2-saturated 10 v/v%
CH3OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1. ........................... 84
xvii


Figure 5. 6. The mass activity of different catalysts in N2-saturated 10 v/v% CH3OH/0.5

M H2SO4 aqueous solution at a scan rate of 50 mV s-1. ............................................... 85
Figure 5. 7. CO-stripping cyclic voltammograms of differential catalysts in 0.5 M
H2SO4 aqueous solution at a scan rate of 50 mV s-1..................................................... 86
Figure 5. 8. Cyclic voltammograms of catalysts in N2-saturated 10 v/v% C2H5OH/0.5
M H2SO4 solution at a scan rate of 50 mV s-1. ............................................................. 88
Figure 5. 9. The Mass activity of different catalysts in N2-saturated 10 v/v%
C2H5OH/0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s-1. .......................... 88
Figure 5. 10. Cyclic voltammograms of catalysts before and after 5000 cycling test in
N2-saturated 10v/v% C2H5OH/0.5 M H2SO4 solution at a scan rate of 50 mV s-1....... 90
Figure 5. 11. Chronoamperogams of the as-obtained catalysts in N2-purged 10 v/v %
C2H5OH/0.5 M H2SO4 solution at 0.70 V for 7200 s. .................................................. 91

xviii


LIST OF SYMBOLS AND ABBREVIATIONS
AOR

Alcohol electro-oxidation reaction

ADT

Accelerated durability test

BET

Brunauer-Emmett-Teller

CA

Chronoamperometry

CV

Cyclic voltammetry

DAFCs

Direct alcohol fuel cells

DEFCs

Direct ethanol fuel cells

DMFCs

Direct methanol fuel cells

ECSA

Electrochemical surface area

EDX

Energy dispersive spectroscopy

EOR

Ethanol electro-oxidation reaction

GCE

Glassy carbon electrode

HR-TEM

High-resolution transmission electron microscopy

JCPDS

Joint Committee on Powder Diffraction Standards

MOR

Methanol electro-oxidation reaction

NHE

Normal hydrogen electrode

NPs

Nanoparticles

NWs

Nanowires

SMSI

Strong metal-support interaction

RHE

Reversible hydrogen electrode

TEM

Transmission electron microscopy

XPS

X-ray photoelectron spectroscopy

XRD

X-ray diffraction

XPS

X-ray fluorescence

0D

Zero-dimensional

1D

One-dimensional

xix


MOTIVATION OF RESEARCH
Nowadays, the worldwide environment has been getting worse day by day because
of the emission of various harmful pollutants into the environment from burning
traditional fossil fuels. In addition, fossil fuel resource is non-renewable and will be
exhausted in the next few decades, therefore, finding out green and renewable power
sources have been sparked interest as future alternatives. Since William R. Grove
discovered that electricity could be generated directly from gaseous hydrogen and
oxygen, a fuel cell has been extensively developed into many different types such as
alkaline fuel cell, phosphoric acid, molten carbonate, solid oxide, and proton-exchange
membrane fuel cells. Of the multitude of fuel cells available, direct alcohol fuel cells
(DAFCs) use liquid and low-cost renewable fuels that have increasingly become
important for portable and transportation applications owing to their relatively simple
infrastructure, portability, operation cost, and facile storage, and conveyance.
In DAFC systems, nanocatalysts play a significantly important role in the half-cell
oxidation and reduction processes at anode and cathode electrodes, respectively. Up to
now, zero-dimensional (0D) Pt nanoparticles (NPs) on carbon support are currently
utilized as state-of-the-art DAFC catalysts, but the slow anodic oxidation kinetics and
CO-poisoning effect of Pt are large limitations for commercializing DAFCs.
Additionally, carbon corrosion can also cause loss and agglomeration of Pt NPs,
resulting in declining fuel cell performance. Recently, using carbon-free support and
tuning the metal catalyst structure have emerged as efficient approaches to address the
problems of catalysts.
For these mentioned reasons, we propose the study entitled: “The Multifunctional
TixW1-xO2 (x = 0.5; 0.6; 0.7; 0.8) Support for Platinum to enhance the activity and
CO-tolerance of Direct Alcohol Fuel Cells”. This efficient approach can improve
electrocatalytic activity and stability of anodic catalysts, promoting large-scale DAFC
applications and reducing the dependence on fossil fuels.

1


CHAPTER 1. INTRODUCTION AND LITERATURE REVIEW
1.1. Direct alcohol fuel cells (DAFCs)
1.1.1. Overview of DAFC technologies
A fuel cell is an electrochemical device that continuously and directly converts the
chemical energy of externally supplied fuel into electrical energy. Hydrogen is the most
common fuel due to its fast oxidation kinetic and high efficiency of hydrogen/oxygen
fuel cells, which was first explored by William Grove in 1839 [1-3]. However, hydrogen
is not a primary fuel, meaning it has to be produced from other sources; namely, natural
gas reforming, oil or coal gasification, and water electrolysis [1]. In addition, the
production and distribution difficulty of clean hydrogen are challenges for the use of
hydrogen-feel fuel cells.
Table 1. 1. Thermodynamic data associated with the electrochemical oxidation of some
fuels (under standard conditions). [4]

Fuel

Standard

- Go
(kJ mol )
-1

theoretical
potential, Eo

Reversible

Energy density

- Ho

(Wh L )

(kJ mol )

-1

-1

energy
efficiency

Hydrogen

0

0.000

180 (@1000 psi, 25 oC) 285.8

0.830

Methanol

9.3

0.016

4820 (100 wt%)

726

0.967

Ethanol

97.3

0.084

6280 (100 wt%)

1367

0.969

2-Propanol

186.3

0.107

7080 (100 wt%)

2005.6

0.971

8.78

0.009

5800 (100 wt%)

1189.5

0.990

Ethylene
glycol

In the current era, alcohols (e.g., methanol and ethanol) emerge as a promising
alternative because they are liquid under ambient temperature and pressure, making
their facile storage and distribution facile and safe [1, 3, 5]. Furthermore, alcohols have
a high energy density (4000 – 7000 Wh L-1) which is more similar to those of
hydrocarbons and gasoline (9800 Wh L-1) [4], as listed in Table 1.1. In the last decades,
there has been extensive research on direct methanol fuel cells (DMFCs) for portable
2


power applications at low to moderate temperatures because methanol has the simplest
alcohol structure, consisting of only one carbon atom, and thus it is easier to oxidize at
anode electrode and shows higher selectivity towards CO2 formation during oxidation
process compared to other alcohol molecules (Figure 1. 1). However, the main source
of methanol world production is 90% from natural gas [6].

Figure 1. 1. Scheme of direct alcohol fuel cells (DAFCs)
Among the other alcohols, ethanol is an attractive alternative to methanol as a fuel
for fuel cells because ethanol is a renewable fuel and can be produced in large quantities
from farm products and biomass. Ethanol is non-toxic and its energy density (6280 Wh
L-1) is higher than that of methanol (4820 Wh L-1). The problem with ethanol as a fuel
is that the complete oxidation of ethanol molecules requires cleavage of the C-C bond
in it, which is difficult on the state-of-the-art catalyst at temperatures lower than 100
C. This typically results in incomplete oxidation of ethanol, which decreases the fuel

o

cell efficiency and can produce toxic by-products or electrode deactivation.
1.1.2. Research history of direct alcohol fuel cells

3


The detailed research of the electrochemical oxidation process of methanol and other
organic compounds at platinum anodes in aqueous alkaline electrolytes was firstly
reported by Muller in 1922 [7]. By laboratory investigations, alkaline direct methanol
fuel cells (ADMFCs) were firstly built by Kordesch’s group in 1951. However, its main
trouble was cross-leakage of the methanol to cathode electrode harming noble metal
catalyst. Based on ADMFCs, Boveri et al. [8] developed a 6 V, 10 A battery for flashing
sea buoy that consisted of ten cylindrical cells with each cell having 18 pairs of
electrodes connected in parallel, as shown in Figure 1. 2.

Figure 1. 2. (a) Scheme of the first alkaline direct methanol fuel cells built by Kordesch
and Marko [9]; and (b) part of a 60-W methanol-air battery with cylindrical air diffusion
electrodes built by Boveri et al. [8]
During the 1960s, Exxon-Alsthom in France designed alkaline and buffer electrolyte

DMFCs; however, carbonation by complete methanol oxidation into CO2 was a
problem, which forced pull out the research in the late 1970s [10]. Therefore, since the
late 1960s and 1970s, pioneering studies on the MOR were performed in acidic media
and explored that MOR kinetic in acidic media was slower than in alkaline media [11].
In the late 1950s and early 1980s, DMFCs with concentrated sulfuric acid were
developed by Shell Research Center (England) and Hitachi Research Laboratories
4