Synthesis and characterization of m doped tio2 (m=w, ir) materials as supports for platinum nanoparticles to improve catalytic activity and durability in fuel cells (luận án tiến sĩ kỹ thuật hóa học)

Nowadays, carbon-supported Platinum catalysts are widely utilized in fuel cell technologies, however, they exhibit some restrictions; namely, poor durability due to the corroded carbon l

VIETNAM NATIONAL UNIVERSITY - HO CHI MINH CITY

UNIVERSITY OF TECHNOLOGY

TAI THIEN HUYNH

(M=W, Ir) MATERIALS AS SUPPORTS FOR PLATINUM NANOPARTICLES TO IMPROVE CATALYTIC ACTIVITY

AND DURABILITY IN FUEL CELLS

DOCTORAL DISSERTATION

HO CHI MINH CITY, 2020

VIETNAM NATIONAL UNIVERSITY - HO CHI MINH CITY

UNIVERSITY OF TECHNOLOGY

TAI THIEN HUYNH

(M=W, Ir) MATERIALS AS SUPPORTS FOR PLATINUM NANOPARTICLES TO IMPROVE CATALYTIC ACTIVITY

AND DURABILITY IN FUEL CELLS

Major subject: Chemical Engineering

Major subject code: 62520301

Advisor: 1 ASSOC PROF VAN THI THANH HO

2 DR SON TRUONG NGUYEN

PLEDGE

I pledge that this dissertation is my own research under the direction of the Assoc Prof Van Thi Thanh Ho and Dr Son Truong Nguyen The research 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

Tai Thien Huynh

ABSTRACT

Low-temperature fuel cell systems have been drastically gaining attention because of their high energy production efficiency and near-zero emissions that can solve the serious reliance on fossil fuel In fuel cell technology, electrocatalysts play

an important role at anode electrode and cathode electrode which directly impact the fuel cell performance Nowadays, carbon-supported Platinum catalysts are widely utilized in fuel cell technologies, however, they exhibit some restrictions; namely, poor durability due to the corroded carbon leading to sintering/detachment and agglomeration of Pt nanocatalysts, sluggish kinetics of fuel anodic oxidation and oxygen reduction reaction (ORR), CO poisoning of active sites of platinum nanocatalyst at even low CO concentration (< 5 ppm)causing significant performance deterioration in the long-term operating condition of fuel cells

Up to now, developing robust electrocatalysts is still a major challenge for further commercialization of fuel cell technologies One of the most effective approaches to solve these problems is to use non-carbon materials, which have emerged as promising alternative catalyst supports due to the superior corrosion resistance in electrochemical media and strong interaction with Pt nanocatalysts and therefore, the electrocatalytic activity and stability of Pt-based catalysts can be significantly enhanced Among carbon-free supports, titanium dioxide (TiO2) material has gained considerable attention in fuel cell application owing to superior electrochemical stability, non-toxicity and affordability Furthermore, the strong metal-support interaction (so-called ―SMSI‖) between TiO2 support and Pt nanocatalyst is a synergistic effect resulting in the significant enhancement of both electrocatalytic activity and durability of this electrocatalyst The intrinsic low electrical conductivity of TiO2, however, is a major hindrance to be solved for its further application in fuel cell technologies Recently, doping strategy of titania with transition metals has come to be known as the best way to enhance both the electronic conductivity of TiO2 and electrochemical activity and durability of Pt-based catalysts for fuel cell application

To this end, I introduce the combination between Platinum nanocatalysts and M-doped TiO2 (M=W, Ir) supports, which were successfully synthesized by means of one-pot synthesis without surfactants/stabilizers or further heat treatment, to assemble robust 20 wt % Pt/Ti0.7M0.3O2 (M=W, Ir) catalysts for the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) Experimental results demonstrated that

20 wt % Pt/M-doped TiO2 (M=W, Ir) electrocatalysts are promising anodic and cathodic electrocatalysts for low-temperature fuel cells

In this work, a novel Pt catalyst supported on mesoporous Ti0.7W0.3O2, which exhibited high conductivity (2.2x10-2 S.cm-1) and large specific surface area (201.481

m2.g-1), was prepared successfully via rapid microwave-assisted polyol route It is found that uniform 3 nm spherical-like Pt of nano-form adhered homogeneously on the surface of Ti0.7W0.3O2 Intriguingly, the electrochemical surface area of the 20 wt % Pt/Ti0.7W0.3O2 was found to be ~90 m2.g-1Pt, which is profoundly higher than that of the commercial 20 wt % Pt/C (E-TEK) catalyst For MOR, the If/Ib ratio of the 20 wt

% Pt/Ti0.7W0.3O2 catalyst was found to be approximately 2.33, which is 2.5-fold higher than that of the commercial 20 wt % Pt/C (E-TEK) catalyst Similarly, the chronoamperometry data also revealed that the 20 wt % Pt/Ti0.7W0.3O2 catalyst possessed higher durability than the 20 wt % Pt/C (E-TEK) catalyst These aforementioned results indicated the much higher catalytic activity and better CO-poisoning tolerance toward MOR of the 20 wt % Pt/Ti0.7W0.3O2 electrocatalyst which could be due to the strong interaction (SMSI) between Pt and M-doped TiO2 support leading to the weak adsorption of carbonaceous species on the active sites of Pt and thus increasing the catalyst’s activity and stability for the MOR in the direct methanol fuel cell

For the first time, novel Ti0.7Ir0.3O2 support was prepared by means of a one-pot hydrothermal route as a catalyst support for Pt nanocatalysts to assemble robust electrocatalyst for both anodic and cathodic catalysts in low-temperature fuel cells For starter, the electrochemical surface area (ECSA) of the 20 wt % Pt/Ti0.7Ir0.3O2nanoparticles (NPs) catalyst was found to be ~96.98 m2.g-1Pt, which is higher the 20

wt % Pt/C (E-TEK) catalyst For MOR, the superior catalytic activity and CO tolerance of the 20 wt % Pt/Ti0.7Ir0.3O2 electrocatalyst compared to the 20 wt % Pt/C

(E-TEK) catalyst was demonstrated through the negative shift of 0.3 V, ~1.5-fold higher oxidation current density and ~1.87-fold higher If/Ib ratio of the 20 wt % Pt/C (E-TEK) catalyst For ORR, the 20 wt % Pt/Ti0.7Ir0.3O2 NPs electrocatalyst exhibited the good onset potential, which was positively shifted ~90 mV, and high electrocatalytic stability after 5000 cycling test compared to that of the 20 wt % Pt/C (E-TEK) catalyst Besides, ―electronic transfer mechanism‖, which does not appear in the conventional Pt/C catalyst, was founded in 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst that could interpret for these enhancements of the robust Pt/Ti0.7Ir0.3O2 NPs catalyst

Interestingly, even with low iridium doping concentration, the Ti0.9Ir0.1O2support possessed a high electronic conductivity of 1.6x10-2 S.cm-1, which was ~105times as high as pure TiO2 (1.37x10-7 S.cm-1), suggesting the efficient doping of iridium into TiO2 lattice The modified chemical reduction route utilized to fabricate the 20 wt % Pt/Ti0.9Ir0.1O2 electrocatalyst exhibited the good anchoring and uniform distribution of Pt nanoparticles (~3 nm) over Ti0.9Ir0.1O2 surface and thus eventually resulting in the high electrochemical surface area (~85 m2.g-1Pt) compared to that of the 20 wt % Pt/C (E-TEK) catalyst (~70 m2.g-1Pt) The cyclic voltammetry results in the methanol media revealed that the 20 wt % Pt/Ti0.9Ir0.1O2 exhibited superior electrocatalytic activity compared to the 20 wt % Pt/C (E-TEK) catalyst For instance, the 20 wt % Pt/Ti0.9Ir0.1O2 catalyst possessed a higher oxidation current density (~28.8 mA/cm2), a lower onset potential (~0.12 V) and a higher If/Ib ratio in comparison with the commercial 20 wt % Pt/C (E-TEK) catalyst It is worth noting that the chronoamperometry results also indicated that the 20 wt % Pt/Ti0.9Ir0.1O2 exhibited higher durability than the commercial 20 wt % Pt/C (E-TEK) catalyst This effective approach contributes to designing other advanced catalysts to revise conventional catalysts in low-temperature fuel cells

ACKNOWLEDGEMENTS

First of all, I would like to express my deepest gratitude to my advisors, Assoc Prof Dr Van Thi Thanh Ho and 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 deepest gratitude to Prof Nam Thanh Son Phan supported me during this dissertation research work at the HCMUT

I would like to give deep thanks to Mr Hau Quoc Pham for the collaboration during 3 years of working together The enthusiasm and generous support of him is highly appreciated

I would like to express my gratitude towards my students, Mr At Van Nguyen,

Ms Vi Thi Thuy Phan and Ms Anh Ngoc Tram Mai for their consistent support in this research Without them, the research process will not be as smooth and I also appreciate their valuable supports as well as help in achieving the results presented in this dissertation

I would like to thank the Faculty of Chemical Engineering - HCMUT, the MANAR Laboratory - Faculty of Chemical Engineering – HCMUT, the Physical Chemistry Laboratory – HCMUNRE, the Applied Physical Chemistry Laboratory – HCMUS and the Key Laboratory of Polymer and Composite Materials – HCMUT for their support during the research period

My special thanks to my parents, my wife and my children for their love, 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 17/2017/HĐ-KHCN-VƯ and Project No 10/2018/HĐ-KHCN-VƯ) for financial support

TABLE OF CONTENTS

PLEDGE ii

ABSTRACT iii

ACKNOWLEDGEMENTS vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS AND ABBREVIATIONS xix

THE MOTIVATION OF RESEARCH 1

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW 3

1.1 Fuel cell systems 3

1.1.1 Overview of fuel cell technologies 3

1.1.2 Proton Exchange Membrane Fuel Cell 8

1.1.3 Direct Methanol Fuel Cell 12

1.1.4 Challenges and current issues of fuel cell systems 15

1.2 Non-carbon support materials 17

1.2.1 Tungsten trioxide (WO3) material 18

1.2.2 Iridium dioxide (IrO2) material 20

1.2.3 Titanium dioxide (TiO2) material 20

1.2.4 Metal-doped TiO2 materials 23

1.3 W-doped TiO2 material 26

1.4 Ir-doped TiO2 material 27

1.5 Methods for synthesizing M-doped TiO2 materials 28

1.5.1 Sol-gel method 28

1.5.2 Hydrothermal method 28

1.5.3 Solvothermal method 29

1.5.4 Other methods 30

1.6 Methods for preparing Pt-based catalyst 30

1.6.1 Polyol method 30

1.6.2 Chemical reduction method 30

1.7 Objectives of thesis research 31

CHAPTER 2 MATERIALS AND EXPERIMENT 34

2.1 Materials 34

2.2 Experimental procedure 34

2.2.1 Synthesis of W-doped TiO2 35

2.2.2 Synthesis of 20 wt % Pt/Ti0.7W0.3O2 catalyst 36

2.2.3 Synthesis of Ir-doped TiO2 38

2.2.4 Synthesis of Pt/Ti0.7Ir0.3O2 catalyst 40

2.3 Characterization techniques 41

2.3.1 X-ray diffraction (XRD) 41

2.3.2 X-ray photoelectron spectroscopy (XPS) 41

2.3.3 Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) 42

2.3.4 Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HR-TEM) 43

2.3.5 Brunauer Emmett Teller (BET) surface area analysis 43

2.3.6 Electrical conductivity measurements 43

2.3.7 Electrode preparation and electrochemical measurements 44

2.3.8 Electrochemical characterization techniques 47

CHAPTER 3 HIGH CONDUCTIVITY AND SURFACE AREA OF Ti0.7W0.3O2 NANOSTRUCTURE SUPPORT FOR Pt NANOPARTICLES TOWARD ENHANCED METHANOL OXIDATION IN DMFC 53

3.1 Synthesis of Ti0.7W0.3O2 support 53

3.1.1 Effect of reaction temperature on W-doped TiO2 53

3.1.2 Effect of reaction time on W-doped TiO2 55

3.2 Characterization of the novel Ti0.7W0.3O2 support (optimum condition at 200oC for 10 hours) 60

3.2.1 The structure of Ti0.7W0.3O2 and un-doped TiO2 60

3.2.2 X-ray photoelectron spectroscopy (XPS) of Ti0.7W0.3O2 60

3.2.3 The morphology of Ti0.7W0.3O2 and un-doped TiO2 61

3.2.4 Elemental composition of Ti0.7W0.3O2 62

3.2.5 BET surface area of the Ti0.7W0.3O2 63

3.2.6 The electronic conductivity of the Ti0.7W0.3O2 65

3.3 Synthesis of the 20 wt % Pt/Ti0.7W0.3O2 catalyst 66

3.4 Electrochemical properties of the 20 wt % Pt/Ti0.7W0.3O2 catalyst 69

3.5 Conclusion 74

CHAPTER 4 NEW Ir DOPED TiO2 NANOSTRUCTURE SUPPORT FOR PLATINUM: ENHANCING CATALYTIC ACTIVITY AND DURABILITY FOR FUEL CELLS 75

4.1 Synthesis of the Ti0.7Ir0.3O2 support 75

4.1.1 Effect of reaction time on Ir-doped TiO2 75

4.1.2 Effect of reaction temperature on Ir-doped TiO2 78

4.1.3 Effect of pH value on Ir-doped TiO2 80

4.2 Novel Ti0.7Ir0.3O2 nanorod support prepared by a facile hydrothermal process: A

promising non-carbon support for Pt in PEMFC 84

4.2.1 Characterization of novel Ti0.7Ir0.3O2 nanorod support 84

4.2.2 Characterization of the 20 wt % Pt/Ti0.7Ir0.3O2 NRs catalyst 88

4.2.3 Electrochemical properties of the 20 wt % Pt/Ti0.7Ir0.3O2 NRs catalyst 90

4.2.4 Conclusions 95

4.3 Advanced nanoelectrocatalyst of Pt nanoparticles supported on robust Ti0.7Ir0.3O2 nanoparticles as a promising catalyst for fuel cells 96

4.3.1 Characterization of Ti0.7Ir0.3O2 nanoparticles 96

4.3.2 Characterization of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst 99

4.3.3 Electrochemical properties of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst 101

4.3.4.Conclusions 110

4.4 High conductivity of novel Ti0.9Ir0.1O2 support for Pt as a promising catalyst for low-temperature fuel cell applications 111

4.4.1 Characterization of the Ti0.9Ir0.1O2 support 111

4.4.2 Characterization of the 20 wt % Pt/Ti0.9Ir0.1O2 catalyst 115

4.4.3 Electrocatalytic properties of the 20 wt % Pt/Ti0.9Ir0.1O2 catalyst 116

4.4.4 Conclusions 120

CONTRIBUTIONS OF THIS DISSERTATION 121

LIST OF PUBLICATIONS 124

LIST OF CONFERENCES 124

LIST OF RESEARCH PROJECTS 125

REFERENCES 126

LIST OF TABLES

Table 1 1 Summary of main types of fuel cell systems 11 7

Table 1 2 The particle size and the electrochemical surface area of Pt/TiO2 and Pt/C electrocatalysts at potential 1.2 V for 0 hours and 80 hours 80 22

Table 2 1 Materials for this research 34

Table 2 2 The effect of reaction temperature on the synthesis of W-doped TiO2 36

Table 2 3 The effect of reaction time on synthesis of W-doped TiO2 36

Table 2 4 The effect of reaction time on the synthesis of Ir-doped TiO2 39

Table 2 5 The effect of reaction temperature on synthesis of Ir-doped TiO2 39

Table 2 6 The effect of pH value on synthesis of Ir-doped TiO2 39

Table 3 1 Electrochemical properties of the 20 wt % Pt/Ti0.7W0.3O2 and others electrocatalyst 72

Table 4 1 The methanol electro-oxidation characterization of our catalyst and other electrocatalysts in the previous studies 105

Table 4 2 The electrochemical properties of the 20 wt % Pt/Ti0.9Ir0.1O2 and other catalysts in the previous studies 118

LIST OF FIGURES

Figure 1 1 A series of experiments of William Grove 3

Figure 1 2 The basic structure of a fuel cell system 4

Figure 1 3 Applications of different fuel cells 5

Figure 1 4 Advantages of fuel cell systems compared to others generating power 6

Figure 1 5 Some commercialized cars using PEMFC 8

Figure 1 6 Proton Exchange Membrane Fuel Cell (PEMFC) 9

Figure 1 7 Polymer Electrolyte Membrane (PEM) fuel cell stacks 11

Figure 1 8 The direct methanol fuel cell (DMFC) system 12

Figure 1 9 The activity performance of electrocatalysts decrease 20 15

Figure 1 10 TEM of Pt/C catalyst before 0 cycle (a) and 3600 cycles (b) 23 16

Figure 1 11 The activity degradation of Pt/C catalyst 28 17

Figure 1 12 Mechanism transition of nanotube H-titanate to single-phase TiO2 with the different morphology and structure 84 21

Figure 1 13 The strong interaction between Pt and TiO2 (SMSI) 87 23

Figure 1 14 The ―electron transition‖ mechanism from Ti0.7Ru0.3O2 to Pt versus that Pt foil and the commercial Pt/C 95 25

Figure 1 15 The ―electron transition‖ mechanism of Pt/Ti0.7Mo0.3O2 catalyst 88 25

Figure 2 1 Process for preparing W-doped TiO2 35

Figure 2 2 Schematic drawing for synthesis Pt/Ti0.7W0.3O2 catalyst via microwave-assisted polyol route 37

Figure 2 3 Procedure for preparing Ir-doped TiO2 39

Figure 2 4 Schematic drawing for synthesizing Pt/Ti0.7Ir0.3O2 catalyst 40

Figure 2 5 The basic principle of XPS 42

Figure 2 6 Three-electrode electrochemical cell for measuring polarization curve 46

Figure 2 7 CV of Pt supported on carbon black in 0.5 M H2SO4 solution 48

Figure 2 8 ORR polarization curves Pt supported on carbon black in oxygen saturated 0.5 M H2SO4 electrolyte solution 49

Figure 2 9 The cyclic voltammogram of Pt/C catalyst for methanol oxidation 164 52

Figure 3 1 XRD profile of WTO_180_4; WTO_200_4; WTO_220_4 samples 53

Figure 3 2 TEM images of WTO_180_4 at different scale bar (a) 100 nm; (b) 50 nm 54 Figure 3 3 TEM images of WTO_200_4 at scale bar (a) 50 nm; (b) 20 nm 54

Figure 3 4 TEM images of WTO_220_4 at scale bar (a) 50 nm; (b) 20 nm 55

Figure 3 5 XRD profile of WTO_220_2, WTO_220_4, WTO_220_6 56

Figure 3 6 TEM images of (a) WTO_220_2, (b) WTO_220_4 and (c) WTO_220_6 57 Figure 3 7 XRD profile of WTO_200_4, WTO_200_6, WTO_200_8 and WTO_200_10 samples 58

Figure 3 8 TEM images of WTO_200_6 with scale bar (a) 50nm, (b) 20nm 58

Figure 3 9 TEM images of WTO_200_8 with scale bar (a) 50nm, (b) 20nm 59

Figure 3 10 TEM images of WTO_200_10 with scale bar (a) 50nm, (b) 20nm 59

Figure 3 11 XRD profile of WTO_200_10 and TO_200_10 in the 2range of (a) 20o -80o and (b) 20o-30o 60

Figure 3 12 X-ray photoelectron spectroscopy (XPS) of (a) Ti 2p and (b) W 4f 61

Figure 3 13 TEM images of (a) TiO2 and (b) Ti0.7W0.3O2 at 200oC for 10 hours 61

Figure 3 14 (a) SEM image, (b) EDX spectroscopy, (c) XRF spectroscopy and (d-f) elemental mapping of the Ti0.7W0.3O2 62

Figure 3 15 N2 adsorption/desorption isotherms of (a) un-doped TiO2; (b) Ti0.7W0.3O2; inset: the pore size distribution of catalyst supports 63

Figure 3 16 Comparison of the surface area of Ti0.7W0.3O2 with other supports in the previous studies 64Figure 3 17 Comparison of the electronic conductivity of Ti0.7W0.3O2 with other supports in the previous studies 65Figure 3 18 XRD profile of the 20 wt % Pt/Ti0.7W0.3O2 catalyst 66Figure 3 19 (a) SEM image; (b) EDX spectroscopy and (c-e) the elemental mapping

of 20 wt % Pt/Ti0.7W0.3O2 catalysts 67Figure 3 20 (a, b) TEM, (c) HR-TEM of the20 wt % Pt/Ti0.7W0.3O2 catalyst 68Figure 3 21 X-ray photoelectron spectroscopy of (a) 20 wt % Pt/Ti0.7W0.3O2; (b) Pt 4f and (c) Ti 2p of the as-synthesized 20 wt % Pt/Ti0.7W0.3O2 catalyst 69Figure 3 22 Cyclic voltammogram of the electrocatalysts in N2-purged 0.5 M H2SO4

at a scan rate of 50 mV.s-1, inset: HR-TEM of Pt/Ti0.7W0.3O2 catalyst 70Figure 3 23 Cyclic voltammogram of methanol oxidation; inset: the onset oxidation potential of the Pt/Ti0.7W0.3O2 and Pt/C (E-TEK) catalysts in N2-purged 10 v/v %

CH3OH/0.5 M H2SO4 solution at a scan rate of 50 mV.s-1 71Figure 3 24 Cyclic voltammograms of (a) the 20 wt % Pt/Ti0.7W0.3O2 catalyst and (b) the 20 wt % Pt/C (E-TEK) catalyst for 2000 cycles in N2-purged 10 v/v % CH3OH/0.5

M H2SO4 solution with scan rate of 50 mV.s-1 73Figure 3 25 Chronoamperometry curves of the electrocatalysts in N2-purged 10 v/v %

CH3OH in 0.5 M H2SO4 solution at the oxidation potential 0.7 for 60 min 73Figure 4 1 The XRD profile of the as-synthesized Ti0.7Ir0.3O2 at 210oC, pH=0 for the different reaction times 75Figure 4 2 TEM images of the as-synthesized Ti0.7Ir0.3O2 at 210oC, pH=0 with reaction time (a) 12 hours; (b) 10 hours; (c) 8 hours and (d) 6 hours 76Figure 4 3 SEM image and EDX spectroscopy of Ti0.7Ir0.3O2 at 210oC for 8 hours 77Figure 4 4 The influence of reaction time to the electrical conductivity of materials 78

Figure 4 5 XRD profile of Ir-doped TiO2 at pH=0 for 10 hours with different reaction temperature 79Figure 4 6 TEM images of Ir-doped TiO2 for 10 hours at (a) 190oC and (b) 170oC 79Figure 4 7 SEM - EDX spectroscopy of Ti0.7Ir0.3O2 at 190oC for 10 hours 80Figure 4 8 XRD profile of the as-synthesized Ti0.7Ir0.3O2 at 210oC for 8 hours with the different pH value 81Figure 4 9 TEM images of Ti0.7Ir0.3O2 were prepared at 210oC for 8 hours with (a, b)

pH value of 0; (c, d) pH value of 1 and (e, f) pH value of 2 82Figure 4 10 (a) XRD profile and TEM image (inset) of Ti0.7Ir0.3O2 NR; (b) XRD profile of Ti0.7Ir0.3O2 NR in the range from 25o to 30o; (c-e) XPS of Ti0.7Ir0.3O2 NRs 85Figure 4 11 The TEM image of Ti0.7Ir0.3O2 NRs with different scale bar for overview (a,b) and local (high resolution) observation (c,d) 86Figure 4 12 SEM-EDX of the Ti0.7Ir0.3O2 support 87Figure 4 13 The comparison of electrical conductivity between Ti0.7Ir0.3O2 NRs and other non-carbon materials in the previous research 88Figure 4 14 X-ray diffraction pattern of 20 wt % Pt/Ti0.7Ir0.3O2 NRs 88Figure 4 15 TEM images of Pt nanoparticles deposited on Ti0.7Ir0.3O2 NRs with different scale bar for overview (a, b) and local observation (c, d) 89Figure 4 16 Cyclic voltammograms of Pt/Ti0.7Ir0.3O2 NRs catalysts and commercial Pt/C (E-TEK) and in 0.5 M H2SO4 at a scan rate of 50 mV.s-1 90Figure 4 17 The cyclic voltammograms after 2000 cycles of the 20 wt % Pt/Ti0.7Ir0.3O2 NRs catalyst and 20 wt % Pt/C (E-TEK) catalyst in 0.5 M H2SO4electrolyte solution at 25 oC at a scan rate of 50 mV.s-1 91Figure 4 18 The TEM images (a, b) of the 20 wt % Pt/Ti0.7Ir0.3O2 NRscatalyst and (c, d) the 20 wt % Pt/C (E-TEK) catalyst before and after the stability test 92

Figure 4 19 Polarization curves showed ORR current of the 20 wt % Pt/Ti0.7Ir0.3O2NRs catalyst and the 20 wt % Pt/C (E-TEK) catalyst with the scan rate of 1 mV.s-1; (inset) the onset potential evaluation 93Figure 4 20 ORR polarization curves after 2000 cyclic voltammetry cycles of (a) 20

wt % Pt/Ti0.7Ir0.3O2 NRs catalyst and (b) 20 wt % Pt/C (E-TEK) catalyst, (c) the mass activity before and after stabilizing test of the electrocatalysts 94Figure 4 21 (a) XRD profile and (b, c) TEM images of Ti0.7Ir0.3O2 NPs 96Figure 4 22 (a) XPS spectroscopy of Ti0.7Ir0.3O2 NPs; (b) XPS spectroscopy of Ti 2p and (c) XPS spectroscopy of Ir 4f in Ti0.7Ir0.3O2 nanoparticles 97Figure 4 23 (a) XRF spectroscopy; (b) EDX spectroscopy and (c-e) elemental mapping of Ti, Ir, O of Ti0.7Ir0.3O2 nanoparticles 98Figure 4 24 Comparison surface area the Ti0.7Ir0.3O2 and other non-carbon materials99Figure 4 25 (a, b) TEM images, (c) HR-TEM image, (d) SEM image, (e) EDX spectroscopy and (f-h) elemental mapping of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs 100Figure 4 26 The cyclic voltammograms of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs and 20 wt

% Pt/C (E-TEK) catalysts in N2-purged 0.5 M H2SO4 solution at a sweep rate of 25 mV.s-1; inset: the estimated ECSA value of electrochemical catalysts 101Figure 4 27 Cyclic voltammograms of (a) the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst and (b) the 20 wt % Pt/C (E-TEK) catalyst in N2-purged 0.5 M H2SO4 at a scan rate of

25 mV.s-1 after potential cycling over 2000 cycles 102Figure 4 28 TEM images of (a, b) the 20 wt % Pt/Ti0.7Ir0.3O2 NPs and (c, d) the 20

wt % Pt/C (E-TEK) catalyst before and after 2000 cyclic voltammetry (CV) cycles103Figure 4 29 Cyclic voltammograms of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst and 20

wt % Pt/C (E-TEK) catalyst in N2-purged 10 v/v % CH3OH/0.5 M H2SO4 solution at a scan rate of 25 mV.s-1; inset: the onset potential of electrocatalysts 104Figure 4 30 Cyclic voltammograms of (a) the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst and (b) the commercial 20 wt % Pt/C (E-TEK) catalyst in N2-purged 10 v/v%

CH3OH/0.5 M H2SO4 solution at a scan rate of 25 mV.s-1 after 2000 cyclic voltammetry (CV) cycles 106Figure 4 31 Chronoamperometry curves of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst and the 20 wt % Pt/C (E-TEK) catalysts in N2-purged 10 v/v% CH3OH/0.5 M H2SO4solution at the oxidation potential of 0.7 V for 60 min 107Figure 4 32 (a) Polarization curves of the 20 wt % Pt/Ti0.7Ir0.3O2 NPs and 20 wt % Pt/C (E-TEK) catalyst; ORR curves after 5000 cycling test of (b) the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst and (c) the 20 wt % Pt/C (E-TEK) catalyst 108Figure 4 33 XPS spectroscopy of (a) the 20 wt % Pt/Ti0.7Ir0.3O2 NPs catalyst; (b)

109

Figure 4 34 (a) X-ray diffraction pattern of Ti0.9Ir0.1O2 support and the 20 wt % Pt/Ti0.9Ir0.1O2 catalyst; (b) XRD pattern of Ti0.9Ir0.1O2 in the range from 24o to 30o 111Figure 4 35 The XPS spectroscopy of O 1s core level of Ti0.9Ir0.1O2 112Figure 4 36 TEM images of (a) Ti0.9Ir0.1O2 NPs, (b) un-doped TiO2 NPs, (c) Pt/Ti0.9Ir0.1O2 and (d) HR-TEM image of Pt/Ti0.9Ir0.1O2 catalyst 113Figure 4 37 (a, b) SEM/ EDX and (c – e) elemental mapping of Ti0.9Ir0.1O2 114Figure 4 38 (a, b) SEM/ EDX and (c – e) elemental mapping of Pt/Ti0.9Ir0.1O2 116Figure 4 39 The CV curves of the 20 wt % Pt/Ti0.9Ir0.1O2 and 20 wt % Pt/C (E-TEK)

in N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV.s-1 117Figure 4 40 The CV curves; inset: the onset potential of the 20 wt % Pt/Ti0.9Ir0.1O2 and 20 wt % Pt/C (E-TEK) in N2-saturated 10 v/v% CH3OH/0.5 M H2SO4 solution at

a scan rate of 50 mV.s-1 117Figure 4 41 The cyclic voltammetry curves after 2000 cycles of (a) the 20 wt % Pt/Ti0.9Ir0.1O2 and (b) the 20 wt % Pt/C (E-TEK) in N2-saturated 10 v/v% CH3OH/0.5

M H2SO4 solution at a scan rate of 50 mV.s-1 119

Figure 4 42 The CA curves of the 20 wt % Pt/Ti0.9Ir0.1O2 and 20 wt % Pt/C (E-TEK)

in N2-saturated 10 v/v % CH3OH/0.5 M H2SO4 solution at the oxidation potential 0.7

V for 60 min 120

LIST OF SYMBOLS AND ABBREVIATIONS

THE MOTIVATION OF RESEARCH

Nowadays, countries all over the world are developing toward industrialization, leading to demand for energy to get higher and higher Besides that, climate change and pollution issues resulting from utilizing fossil fuel become more and more serious These problems require us to discover renewable resources that are environmentally friendly, possibly replacing fossil fuels Among them, fuel cells are sources of electricity that are consistent with current energy trends and promising in the energy industry because they possess several dominant merits such as high efficiency, low noise along with near-zero pollution and high adaptability to different energy levels for various devices [1, 2]

The structure of fuel cell consists of three basic compartments: an anode electrode, a cathode electrode, and an electrolyte Anode and cathode electrodes are made of carbon and covered with a catalyst layer to enhance fuel cell performance The electrolyte between the cathode electrode and the anode electrode is used to transfer ions between two electrodes The conventional catalysts are Pt nanoparticles

or Pt-M alloys However, there exist several obstacles preventing fuel cells from commercialization, namely the poor stability of the carbon electrode and high cost of the Pt catalyst [3-5] More importantly, severe corrosion of carbon electrode leads to loss and agglomeration of Pt NPs on the support, thus declining fuel cells of performance Moreover, the weak interaction between the carbon support and the Pt nanocatalyst also contributes to the dissolution of Pt nanoparticles bringing about the significant decrease of electrochemical surface area (ECSA) of the catalyst, hence the declined performance of fuel cells under long-term operation [6] One effective approach to these aforementioned problems is to use carbon-free support due to the high structural and electrochemical durability in an acidic and oxidative environment and the strong interaction between them and Pt nanocatalyst [3]

For these reasons, we propose to conduct the research on ―Synthesis and characterization of M-doped TiO 2 (M=W, Ir) materials as supports for platinum nanoparticles to improve catalytic activity and durability in fuel cells‖ New stable

and effective supports were fabricated for Pt nanocatalyst to enhance its catalytic

activity and stability, resulting in improving the performance of fuel cells This research orientation contributes to not only economically due to a decrease in the composition of noble Pt metal in catalysts but also environmentally owing to the green energy source potential of fuel cells

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW

1.1 Fuel cell systems

1.1.1 Overview of fuel cell technologies

The concepts related to fuel cells were demonstrated in the early nineteenth century

by Humphry Davy This was followed by pioneering work on what were to become fuel cell systems by the scientist Christian Friedrich Schönbein In 1839, the basic operating principle of fuel cells was discovered by William Grove through a series of

experiments (Figure 1.1) which he termed a gas voltaic battery, ultimately proving

that electricity could be created from an electrochemical reaction between hydrogen and oxygen over a Platinum catalyst This principle, which he discovered remains unchanged today

Figure 1 1 A series of experiments of William Grove

so as to produce electricity continuously as long as resources are supplied Unlike

engines or conventional batteries, a fuel cell does not need recharging and produce only power and drinking water Thus, fuel cells have been regarded as clean and potential sources of electrical power for the future [8, 9].

Figure 1 2 The basic structure of a fuel cell system

(https://www.doitpoms.ac.uk/tlplib/fuel-cells/high_temp_sofc.php)

A fuel cell system consists of an electrolyte in contact with an anode electrode

(negative electrode) and a cathode electrode (positive electrode) (Figure 1.2) The

electrolytes used in fuel cells include acid, base and molten salt Nafion membrane is widely used in fuel cells with the aim of allowing penetration of appropriate ions but not electrons In addition, the catalyst layer can be placed between the electrolyte and the electrode or directly utilized as an electrode or deposited on the surface of electrodes Pure Pt or alloy of Pt and metals such as Ni, Ru, Co, etc or carbon supported catalysts in the forms of Pt/C or Pt-M/C are commonly used as catalysts The fuel cell system can generate different energy levels when joining fuel cells with each other Thus, fuel cell systems can supply energy source in the range of 1

Watt to 10 Megawatt, meeting several applications in their lifetime (Figure 1.3) At

low energy levels below 50 W, fuel cells can be used for mobile phones, laptops or any other type of personal electronic equipment In the 1 kW – 100 kW range a fuel

cell can be used to power vehicles both domestic and military Public transportation is also a target for fuel cells, along with any APU application In the 1 MW – 10 MW range fuel cells can be used to convert energy for distributed power uses (grid-quality AC) [8-10]

Figure 1 3 Applications of different fuel cells

(https://slideplayer.com/slide/10896547/)

Compare to other power devices, fuel cells possess several advantages (Figure 1 4):

 A high power conversion efficiency: the efficiency increasing with lower load is considered an important characteristic for transportation applications where load operation is the key and an internal combustion engine (ICE) run

at reduced efficiency in low load conditions

 Very low gas emission: The actual emission of fuel cell systems depends on the fuel fed For instance, the fuel as pure hydrogen results in true zero-emission performance since the only reaction product is water Even if natural gas or petrol is employed as a fuel through a reforming route, CO2emission will be much lower than an internal combustion engine (ICE) In addition, no toxic nitrogen oxides (NOx), sulfur oxides (SOx) are created

 Low noise level: The operation of fuel cell systems based on the electrochemical reaction without any moving parts Fuel cell technologies, with all the necessary systems for cooling, power conversion, and fueling as well as air supply will emit some noise, mainly because of the air compressor

 System scalability: Because of their construction, fuel cell systems are modular power generators resulting in efficient systems to be built for power levels from several Watts to several Mega Watts

Figure 1 4 Advantages of fuel cell systems compared to others generating power

(http://archive.siliconchip.com.au/cms/A_30527/article.html)

Based on the electrolyte, fuel cell systems are divided into many types, namely Proton Exchange Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC), Solid Oxide Fuel Cell (SOFC), Alkaline Fuel Cell (AFC), Phosphoric Acid Fuel Cell (PAFC), etc Among them, the most studied and applied fuel cells are currently Proton Exchange Membrane Fuel Cell (PEMFC) and Direct Methanol Fuel Cell (DMFC) with the advantages of low operation temperature (~80oC), high efficiency in term of using energy (50-80%) and quick start-up time [1, 2]

Table 1 1 Summary of main types of fuel cell systems [11]

Electrolyte Ion Exchange

Membranes

Mobilized or Immobilized Potassium Hydroxide

Immobilized Liquid Phosphoric Acid

Immobilized Liquid Molten Carbonate

Components Carbon-based Carbon-based Carbon-based Stainless-based Ceramic Ceramic

Process Gas + Electrolyte Calculation

Process Gas + Independent Cooling Medium

Internal Reforming + Process Gas

Internal Reforming + Process Gas

Internal Reforming + Process Gas

1.1.2 Proton Exchange Membrane Fuel Cell

Proton Exchange Membrane Fuel Cell (PEMFC) has been studied and employed in many fields like mobile phones, laptops, vehicles, transportation, military equipment [12] Major carmakers applied PEMFC to manufacture clean and environmentally-benign cars with several advanced features, namely low operation temperature (50-100C), high energy conversion efficiency (50–60%) and near-zero emission in

comparison with conventional cars [13] (Figure 1 5)

Figure 1 5 Some commercialized cars using PEMFC

A proton-exchange membrane fuel cell system consists of an anode electrode and a cathode electrode in which occurred hydrogen oxidation reaction and oxygen

reduction reaction, respectively (Figure 1 6) The electrodes are separated by an

electron non-conducting and gas-tight electrolyte membrane which only allows H+ions to pass The surface, exposed to the electrolyte membrane is covered with a Pt nanocatalyst for the oxidation process in the anode electrode and reduction process in the cathode electrode Adjacent to the catalyst layer is the conductor and the porous diffusion layer, allowing gas flows (oxygen and hydrogen) to move to the catalyst layer to perform reactions and afterward water product to be discharged into the outdoors

Compared to other fuel cells, the PEMFC generate more power for a given volume

or weight of the fuel cell This high-power density characteristic makes it compact and lightweight and its operating temperature less than 100 ºC, which allows rapid start-up These traits plus the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for automotive power applications Other advantages result from the electrolyte being a solid material, compared to a liquid The sealing of the anode and cathode gases is simpler with a solid electrolyte, and therefore, less expensive to manufacture The solid electrolyte is also better immune to difficulties with orientation and has fewer problems with corrosion, compared to many of the other electrolytes, thus leading to a longer cell and stack life [14]

Figure 1 6 Proton Exchange Membrane Fuel Cell (PEMFC)

(https://www.tech-etch.com/photoetch/fuelcell.html)

In PEMFC, the membrane is placed in the center of the fuel cell with a thickness of under a few hundred micrometers Polymer solid electrolyte acts as a thin insulator layer and a barrier between two electrodes, facilitating the proton transfer process and obtaining high current density

The essence of the PEMFC is the energy transformation from the reaction between hydrogen and oxygen to generate electricity (1.1) and the byproduct is water

2H2 + O2 2H2O + power (1.1) This above overall reaction happens in two spatially separated half reactions, occurring at the anode and the cathode electrode hand side and given by:

 At the anode electrode: The hydrogen molecules diffused into the surface

of the catalyst layer splitting into hydrogen ions (protons) and electrons (1.2) Next, hydrogen ions (proton) permeate the electrolyte across to the cathode while the electrons flow through an external circuit to the cathode to generate electricity

is therefore referred to a catalyst layer [8, 14]

Adjacent to the catalyst layer on both sides of the membrane is a porous, electrically conducting gas diffusion layer (GDL) It allows reaction gases (i.e hydrogen and oxygen) to flow to the reaction sites on the catalyst layer and product water to flow back out The gas diffusion layers are held in place by gas impermeable, electrically

conductive plates Gas channels are machined into these so-called bipolar plates in order to distribute hydrogen and oxygen evenly over the membrane surface

From the anode inlet, hydrogen is distributed over the whole cell surface from whence it diffuses across the gas diffusion layer to the catalyst layer Upon contact with the catalyst in the anode compartment, the hydrogen atoms are split into a proton (H+) and an electron (e-) each The electron cannot cross the membrane and leaves the fuel cell as an electric current through the gas diffusion layer and the bipolar plate The proton, on the other hand, is transported across the membrane to the cathode side On the cathode side, oxygen enters the fuel cell, is distributed over the cell surface by the gas channels, and diffuses to the catalyst layer and also in the presence of the catalyst; the oxygen atom combines with the proton and the electron to form water The product water then diffuses back through the gas diffusion layer and leaves the fuel cell through the gas channels [8, 9, 14]

Figure 1 7 Polymer Electrolyte Membrane (PEM) fuel cell stacks

(http://www.graphitestore.com/fuel-cells)

The heat generated during the electrochemical reaction is transferred to the bipolar plates by conduction From there, it is either transmitted to the air through specifically designed cooling fins or is evacuated from the system through a cooling liquid (CL) circulating within the bipolar plates Depending on the current density (i.e the current

per unit of cell surface area), electrical voltages between 1 V and 0.5 V are typically achieved The fuel cell’s characteristics and operating conditions determine the maximum current density and power that can be achieved per cell In order to increase the power and voltage of a fuel cell, the number (nFC) of single cells is stacked to form

the so-called fuel cell stack (Figure 1.7) Such a configuration corresponds to an

electrical series connection of independent elements The reaction gases and cooling liquid supply to the cells through a parallel network of supply channels

1.1.3 Direct Methanol Fuel Cell

The direct methanol fuel cell (DMFC) has been considered as one of the most promising systems due to it using liquid fuel, which can be distributed at nearby gas stations Furthermore, DMFC exhibits high efficiency and low emission compared to the internal combustion engine (ICE)

Figure 1 8 The direct methanol fuel cell (DMFC) system

(https://www.mae.ust.hk/~mezhao/Areas.html)

DMFC has been widely utilized in portable applications such as mobile phones, laptops, vehicles owing to the low operating temperature (60-90oC) and using direct methanol as fuel In DMFC, the pure methanol is mixed with steam and fed directly into the cell at the anode and thus the methanol is converted to carbon dioxide and

hydrogen ions Here too the electrons are then pushed around an external circuit to produce electricity (before returning to the cathode) whilst the hydrogen protons pass

across the electrolyte to the cathode, as so occurs in a standard PEM fuel cell (Figure

1 8) To promote the electrochemical reactions, electrocatalysts, usually Pt

nanocatalysts, are also required

Thermodynamics of direct methanol fuel cell:

At the anode electrode: Under activity of catalysts, usually Pt or PtRu, methanol is

oxidized to generate electrons and hydrogen ions (protons) (1.4) CO2 is eliminated and hydrogen ions permeate the electrolyte across to the cathode while the electrons flow through an external circuit to the cathode to generate electricity

In actuality, the oxidation reaction of methanol in DMFC is a series of complex – reaction The reaction rate of the methanol oxidation reaction (MOR) significantly depends on electrocatalysts at the electrodes For instance, the MOR with Pt-M (M =

Ru, Mo, Ir) electrocatalysts work as follows:

Methanol is adsorbed on the surface of Platinum (1.5):

Then splitting into hydrogen ions (protons) and electrons (1.6 – 1.9):

Pt-(CH3OH)ads → Pt-(CH3O)ads + H+ + e- (1.6) Pt-(CH3O)ads → Pt-(CH2O)ads + H+ + e- (1.7) Pt-(CH2O)ads → Pt-(CHO)ads + H+ + e- (1.8) Pt-(CHO)ads → Pt-(CO)ads + H+ + e- (1.9) Water is adsorbed (1.10):

At the cathode electrode: Oxygen, usually in the form of air, is supplied and

combines with the hydrogen ions (protons) and the electrons to generate water (1.13)

Various reaction intermediates may be formed during the methanol oxidation [15] Some of these (CO-like) species are irreversibly adsorbed on the surface of the electrocatalyst and severely poison Pt, which has the effect of significantly reducing the efficiency of fuel consumption and the power density of the fuel cells Thus, it is vital to develop new electrocatalysts to inhibit the poisoning and increase the electro-oxidation rate by at least two to three orders of magnitude Other species may be released with the consequent decrease of fuel efficiency, whereas, an efficient catalyst must allow complete oxidation to CO2 Principal by-products in the methanol oxidation are formaldehyde and formic acid Methyl formate and other substances have been found in traces DMFC is characterized by two slow reactions, i.e methanol electro-oxidation and oxygen reduction with the further drawback of the presence of a mixed potential at the cathode determined by the methanol crossover Another concern

is that, even though significant progress has been made in enhancing the electrocatalysis of the four-electron transfer oxygen reduction reaction at low temperatures, the overpotential of this reaction at desired current densities (e.g 500 mA.cm-2) is still about 400 mV in H2/air fuel cells and increases by about 50-100 mV

in DMFC because of the effect caused by methanol, which crosses over from the anode electrode to the cathode electrode [16, 17]

1.1.4 Challenges and current issues of fuel cell systems

Nowadays, fuel cell technologies are widely employed in a variety of applications due to higher power conversion efficiency, eco-friendly and the low operation temperature (50 – 100oC) and fast start-up [1, 2, 12, 18] Besides these aforementioned advantages, these systems still face major obstacles for further commercialization, such as high price and poor durability Under long-term operating conditions, the performance of fuel cells is significantly degraded, resulting from corrosion of support material, bringing about the dissolution, agglomeration and/or detachment of metal nanocatalysts [19] Recently, carbon has been widely employed as support materials owing to the large surface area, high electronic conductivity and pore structure Nevertheless, this substrate is significantly corroded under harsh operation conditions with high potential (~ 1.23 V vs SHE), especially under startup/stop condition of fuel

cells (Figure 1 9) [20] A great deal of effort has been devoted to improve the

durability of support material for electrocatalyst as well as enhance the activity and the poisoning resistance of catalysts [21]

Figure 1 9 The activity performance of electrocatalysts decrease [20]

The corrosion of the carbon support under electrochemical media of fuel cells (1.15) [22] results in the decreased electrochemical surface area (ECSA) of Pt nanocatalyst and thus the performance of fuel cells is significantly degraded

C + H2O CO2 + 4H+ + 4e– Eo =0.207 V vs RHE (1.15)

Meier’s group [23] proved the detachment and agglomeration of Pt nanocatalyst

from carbon support which is the direct consequence of oxidation carbon (Figure 1

10) This is a major mechanism causing the performance degradation of Pt/C catalyst

[24, 25]

Figure 1 10 TEM of Pt/C catalyst before 0 cycle (a) and 3600 cycles (b) [23]

Beside the corroded carbon, the dissolution of the Pt-based catalyst [26, 27] is considered a major cause for decreasing ECSA, increasing particle size and dissolution

of Pt nanocatalyst into the electrolyte (membrane) The dissolution of Pt nanocatalyst could take place directly (1.16) or indirectly (1.17 – 1.18)

Pt Pt 2+ + 2e– Eo = 1,188V (1.16)

Pt + H2O PtO+ 2H+ + 2e– , Eo = 0,980V (1.17) PtO + 2H+ Pt2+ + H2O + 2e– , Eo = 0,980V (1.18)

An Ostwald ripening and agglomeration are also considered to be the main causes

of the activity degradation of Pt/C catalyst The smaller particle of Pt catalyst dissolves into the electrolyte and agglomerates by different surface energy and concentration on the surface support The agglomeration is a different explanation for

the increasing particle size of Pt catalyst in fuel cell systems (Figure 1 11)

Figure 1 11 The activity degradation of Pt/C catalyst [28]

To overcome these aforementioned problems of the carbon support, a lot of efforts have been devoted to designing robust support such as carbon nanotube (CNT, 1D) [29, 30], graphene (2D) [31, 32] or 3D carbon structure [33] These materials exhibited the improved durability versus that of conventional carbon; however, they are still corroded at high potential and during long-term operation because they are still based

on the carbon material [34] Therefore, finding out novel materials with high durability and strong interaction with metal nanocatalysts, high electrical conductivity, and large surface area as well as a stable structure for further commercialization of fuel cells will

be of great benefit

To solve these problems of carbon supports, development of non-carbon material such as Magnéli-Phase oxides (TinO2n-1) [35-37], titanium oxides [38-41], cerium oxide [42-44] and niobium oxide [45-48] as well as tungsten oxides (WOx) [49-52] have emerged as promising supports, which could replace carbon substrate due to their superior properties; namely, high corrosion- resistance, high stability and strong interaction with Pt nanocatalysts

Recently, Ti-based oxides, especially Titanium dioxide (TiO2) have been gaining attention due to beneficial cost, high electrochemical and chemical stability as well as the strong interplay between it and Pt catalyst, suitable for a support under long-term operating fuel cells [20] Taking Ti4O7 support as an example, Ioroi et al [53] reported Pt/Ti4O7 electrocatalyst also possessed the negligible change of ECSA and the high electrochemical activity for both hydrogen oxidation reaction and oxygen reduction reaction versus that the conventional Pt/C catalyst at 80oC

Furthermore, Son Truong Nguyen’s group [54] synthesized Magnéli-Phase oxides (TinO2n-1) as support material for an alkaline ethanol fuel cell system via heat treatment with commercial anatase-TiO2 under H2 at 1050oC for 6 hours Experimental results indicated that TinO2n-1 displayed the superior durability in comparison with carbon black under alkaline media

Ioroi et al [35] also prepared TiOx support and Pt/TiOx catalyst which applied in PEMFC for the ORR Experimental results indicated that the Pt/TiOx possessed significantly high stability compared to that of the conventional catalyst under high potential (> 1.0 V) The activity for ORR was 2-fold higher than that of the traditional Pt/XC-72 Besides, the ECSA change of Pt/TiOx was negligible after 10.000 cycles, meanwhile, the Pt/XC-72 catalysts exhibited a loss of ECSA of approximately 30 % –

50 % after 10.000 cycles Other studies of Krishnam’s group [36] and Geng’s group

[37] also proved the high durability of TinO2n-1 material versus the carbon support under electrochemical media Nevertheless, Magnéli-Phase oxides (TinO2n-1) possessed the low surface area, which drags it out of fuel cell applications

1.2.1 Tungsten trioxide (WO 3 ) material

Tungsten trioxide (WO3), a commercially available material, is usually utilized as conducting oxide WO3 is a n-type semiconductor with a band gap of around 2.6 – 2.8

eV [55] Chhina et al [56] reported that a Pt/WO3 catalyst possessed the high stability towards the ORR versus that of Pt/XC-72 catalysts Furthermore, B Rajesh et al [57] also proved the Pt/WO3 exhibiting the high activity for ORR in comparison with pure Pt; however, the activity for the MOR is still lower than that of the commercial Pt/C catalyst, resulting from the agglomeration of Pt/WO3 catalysts

Cui et al [58] proved that Pt/mesoporous WO3, which possessed high surface area, displayed the superior activity for MOR in comparison with the commercial 20 wt % Pt/C and 20 wt % PtRu/C catalyst at potential 0.5 – 0.7 V vs NHE The electrochemical activity towards MOR was significantly improved due to the strong interaction between Pt nanocatalyst and WO3 support resulting in an enhanced CO-tolerance possibility of Pt/WO3 [59, 60] Furthermore, Micoud et al [61] showed that the Pt/WO3 catalyst exhibited the high CO-tolerance possibility versus those of Pt/C and PtRu/C electrocatalysts by observing the CO oxidation current at low potential (0.1 V vs RHE)

The amount of Pt loading on the support also plays an important role in improving the electrochemical activity of catalysts Reducing Pt NPs anchored on the substrate without reducing performance of electrocatalysts is the current pursuit of the scientists for further commercialization of fuel cell technologies For the Pt/WO3 catalyst, reducing Pt loading by more than 50 % resulting in the electrochemical activity of MOR as high as the pure Pt with spherical morphology of 50 nm to 150 nm in diameter [62] In addition, the morphology and structure of WO3 support also directly impact the electrochemical activity of Pt nanocatalysts toward MOR Ganesan and Lee [63] prepared spheral WO3 support with micrometer size for Pt catalyst and indicated that the electrochemical activity and durability for MOR was higher than the 20 wt % PtRu/Vulcan XC-72 and 20 wt % PtRu/C with micrometer size

Furthermore, Barczuk et al [64] fabricated the catalysts with the same PtRu loading

on different WO3 supports and indicated that the Pt/mesoporous WO3, which had a high surface area and pore structure, exhibited the highest electrochemical activity toward MOR Tungsten oxide, however, is still dissolved under acidic media, which is the major obstacle dragging it out of fuel cell technologies To improve the durability

of WO3 materials, Raghuveer and Viswanathan [65] prepared Ti-doped WO3 material, which exhibited the improved durability but the electronic conductivity of this material was decreased

1.2.2 Iridium dioxide (IrO 2 ) material

Iridium dioxide (IrO2), a promising non-carbon support, has been obtaining attention due to its high corrosion resistance, high electrical conductivity, and high stability under acidic and electrochemical media [66-68] In the fuel cell technology, Pt/IrO2 catalyst was studied as electrocatalyst for the ORR [69-71] Nevertheless, the poor distribution of Pt nanocatalysts on the surface of IrO2 support and the low electrocatalytic activity, resulting from the dissolution of Pt/IrO2 under the electrochemical media, was a major obstacle of these electrocatalysts [67] The low electrocatalytic activity towards ORR of Pt/IrO2 catalysts could be due to the agglomeration of IrO2, resulting in the high Ohmic resistance and the electron transport between the Pt nanocatalysts [67] To overcome this problem, the developed strategy to improve the electrical conductivity is to add Ir nanoparticles into IrO2materials [72, 73]

Pt/Irx(IrO2)10-x electrocatalysts were prepared by Kong et al [72], which demonstrated that the Pt/Ir3(IrO2)7 electrocatalysts exhibited the largest ECSA of 24.74

m2.g-1 and the highest activity toward ORR of around 21.71 mA.mg-1 at 0.85 V The improved performance of Pt/Ir3(IrO2)7 catalyst was attributable to the strong interaction between Ir nanoparticles and IrO2, bringing about the enhanced electronic conductivity of this material The activity towards ORR of Pt/Ir-IrO2 catalyst is higher than Pt/IrO2 catalyst resulting from not only the improved electronic conductivity but also the strong interplay between Pt and Ir nanoparticles [73] The high price and short-time stability of iridium metal, however, are major hindrances dragging it out of fuel cell applications

1.2.3 Titanium dioxide (TiO 2 ) material

Titanium dioxide (TiO2) has been widely utilized in a variety of applications such as sensor [74, 75], solar cell [76, 77], photocatalysis [78] due to the high electrochemical and chemical stability as well as easily controlled size and structure, commercially available materials, affordability, and non-toxicity [79] For fuel cell technology, titanium dioxide (TiO2) was studied due to the high durability under acidic and aqueous media [80-82] Titanium dioxide (TiO2) has three crystalline phases: anatase