Research of non platinum gas diffusion electrode preparation for anion exchange membrane fuel cells (tóm tắt)

ii ABSTRACT This research focused on the preparation of non-platinum electrodes applied for anion exchange membrane fuel cells AEMFCs.. Specifically, two attempts including the developm

ii

ABSTRACT

This research focused on the preparation of non-platinum electrodes applied for anion exchange membrane fuel cells (AEMFCs) Specifically, two attempts including the development of silver nanoparticles supported on functionalized carbon particles (Ag/C) used

as the cathode catalyst in AEMFCs and the study of the effects of PTFE content in gas diffusion substrate, microporous layer, cell temperature and inlet gas humidification on AEMFC performance were carried out The characterization results show that Ag/C catalyst was successfully synthesized by wet impregnation method For AEMFC performance evaluation, the experimental results showed that the peak power densities of the single AEMFC using Ag/C were only 3.5% lower than that using commercial Pt/C which was consistent with the cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements Therefore, the Ag/C can be used as the cathode catalyst to substitute the commercial Pt/C as the strategic cost reduction, so that a commercialized alkaline anion exchange membrane fuel cell can be realized

Moreover, the gas diffusion substrate (GDS) treated with polytetrafluoroethylene (PTFE) can offer not only an appropriate hydrophobic level but also robust supporting for the microporous layer and catalyst layer Thereby, well water management and catalyst usage

in the cell can be obtained during the cell operation The testing results showed that the best cell performance was achieved by employing the GDS with 30 wt.% PTFE content and MPL

at both anode and cathode sides of a single AEMFC Although PTFE treatment in the GDS

is beneficial for AEMFC performance, excessive PTFE embedment in the GDS will lead to

an adverse effect due to most of the pores on the GDS surface blocked by excessive PTFE particles, causing a severe hindrance of transport of reactant gas and water In addition, it is found that the AEMFC performance was strongly affected by the cell operating temperature and highly sensitive to humidification at both anode and cathode inlet gases Besides, back diffusion could partly support the water demand at the cathode once the water concentration gradient between the anode and cathode is formed These results suggest that the water management in AEMFCs plays a critical role in achieving a desirable cell performance

Key words: AEMFCs, Non-Pt catalyst, PTFE effect, water management

TABLE OF CONTENTS

ACKNOWLEDGEMENTS i

ABSTRACT ii

TABLE OF CONTENTS iii

LIST OF FIGURES vii

LIST OF TABLES xi

NOMENCLATURE xii

CHAPTER 1: INTRODUCTION 1

1.1 Fuel cell fundamentals 1

1.2 Anion exchange membrane fuel cells 7

1.2.1 Principle of anion exchange membrane fuel cells 7

1.2.2 Main components of AEMFCs 9

1.2.2.1 Bipolar Plate 10

1.2.2.2 Gasket 12

1.2.2.3 Gas Diffusion Layer 12

1.2.2.4 Catalyst Layer 12

1.2.2.5 Anion exchange Membrane 13

1.3 Motivation 13

1.4 Objectives and outline of dissertation 14

1.4.1 Dissertation objective 14

1.4.2 Dissertation outline 15

CHAPTER 2: LITERATURE REVIEW 16

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2.1 Fundamentals of oxygen reduction reaction 16

2.2 Design of gas diffusion layer for low-temperature fuel cells 21

2.3 Methodology 30

2.3.1 Gas permeability of GDL 30

2.3.2 Porosity and pore size distribution 31

2.3.3 Through-plane electrical resistance of GDL 32

2.3.4 Hydrophobicity of GDL (wettability) 34

2.3.5 Polarization curve 34

2.3.6 Cyclic voltammetry 36

2.3.7 Rotating disk electrode voltammetry 39

2.3.8 Thermogravimetric analysis 41

2.3.9 Fourier-transform infrared spectroscopy 42

2.3.10 X-Ray Diffraction 42

2.3.11 Scanning electron microscopy 43

2.3.12 Energy dispersive X-ray spectroscopy 43

2.3.13 Transmission Electron Microscopy 44

CHAPTER 3: NON-PLATINUM CATHODE CATALYST FOR ANION EXCHANGE MEMBRANE FUEL CELLS 45

3.1 Introduction 45

3.2 Experimental 49

3.2.1 Ag/C catalyst synthesis 49

3.2.2 Synthesized catalyst characterization 50

3.2.3 Fuel cell test 51

3.3 Results and discussion 53

3.3.1 Synthesized catalyst characteristics 53

3.3.2 AEMFC performance results 62

3.4 Conclusions 64

CHAPTER 4: EFFECTS OF PTFE CONTENT IN THE GAS DIFFUSION SUBSTRATE AND MICROPOROUS LAYER 65

4.1 Introduction 65

4.2 Materials 68

4.3 Experiment 70

4.3.1 Physical characterization 70

4.3.2 Preparation of membrane electrode assembly 70

4.3.3 Single cell testing 71

4.4 Results and discussion 72

4.4.1 Effect of PTFE treatment in GDS and MPL in the GDL 72

4.4.2 Effect of PTFE treatment in GDS on the morphology of MPL 75

4.4.3 Effect of PTFE in GDS on the CL morphology 78

4.4.4 Single cell performance of AEMFC 81

4.5 Conclusions 87

CHAPTER 5: EFFECTS OF CELL TEMPERATURE AND REACTANT HUMIDIFICATION 89

5.1 Introduction 89

5.2 Experimental 93

5.3 Results and discussion 93

5.3.1 Effect of the cell operating temperature 93

5.3.2 Effect of inlet gas humidification 95

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5.4 Conclusions 99

CHAPTER 6: CONCLUSIONS AND FUTURE RESEARCH 101

6.1 Conclusions 101

6.2 Future research 103

REFERENCES 104

LIST OF FIGURES

Figure 1 1 Illustration of the working principle of a fuel cell 2Figure 1 2 Schematic diagram of AEMFCs 9Figure 1 3 Main components of a typical single AEMFC 11Figure 1 4 Schematic of different flow field patterns: (a) Serpentine, (b) parallel, (c)

parallel-serpentine, (d) interdigitated, (e) porous mesh and (f) spiral-serpentine [19] 11 Figure 2 1 The possible mechanism of all pathways for ORR on a metal catalyst surface 19Figure 2 2 Trends in oxygen reduction activity plotted as a function of both the O (a) and the OH (b) binding energy [33] 20Figure 2 3 Manufacturing process involved in conventional carbon paper-based GDL fabrication [36] 22Figure 2 4 SEM images of typical GDS made of (a) carbon fiber paper and (b) carbon cloth 22Figure 2 5 Flow chart of micromachining process for Ti GDL and SEM image Ti GDL with microholes [43] 25Figure 2 6 Schematic of gas diffusion medium made of copper foil used as GDL in

PEMFCs [44] 25Figure 2 7 (a) Fabrication steps of porous copper foil and (b) its typical SEM image [45] 26Figure 2 8 Schematic of stainless steel GDL design in comparison to carbon paper GDL [46] 26Figure 2 9 SEM images of gas diffusion substrate made of PA and Ti using 3D printing technique: (a) Plane-view and (b) cross-section [47] 28Figure 2 10 SEM images of: (a) sintered stainless steel fiber felt and (b) carbon paper TGP-H-060 [48] 28

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Figure 2 11 Plane-view and cross-sectional SEM images of titanium felts with three

different thickness, (A) top view of 350 um thickness titanium felt, (B) top view of 500 um thickness titanium felt, (C) top view of 1000 um thickness titanium felt, (D) cross section of

350 um thickness titanium felt, (E) cross section of 500 um thickness titanium felt, (F) cross section of 1000 um thickness titanium felt [49] 29Figure 2 12 Cross-sectional SEM images of the single-layered Ti foam GDL [50] 29Figure 2 13 Air permeability tester (Gurley tester) QD-B-003 31Figure 2 14 Schematic principle of the experimental apparatus for the through-plane

electrical conductivity measurement of GDL [56] 33Figure 2 15 Illustration of static contact angle formed by sessile liquid drops on a flat solid surface 34Figure 2 16 A typical polarization curve of a polymer membrane fuel cell with the three loss regions [59] 35Figure 2 17 Schematic principle of (a) CV excitation signal and (b) its cyclic

voltammogram 37Figure 2 18 Schematic diagram of a three-electrode system for CV measurements 38Figure 2 19 Schematic of (a) a typical RDE structure and (b) solution movement caused by rotation of an RDE [63] 41 Figure 3 1 FT-IR spectra of carbon particles; (a) Before surface treatment and (b) After surface treatment 54Figure 3 2 XRD patterns of different samples 55Figure 3 3 Surface morphology SEM images of (a) Bare carbon black (SEI mode) and (b) Ag/C (COMPO mode) 57Figure 3 4 TEM images of the Ag/C catalyst 58Figure 3 5 EDX pattern of (a) Bare carbon particles and (b) Ag/C catalyst 58

Figure 3 6 Thermogravimetric analysis graph of prepared Ag/C catalyst under N2 and O2

atmospheres 59Figure 3 7 Cyclic voltammograms of different catalysts for ORR in 1M KOH at scan rate

of 100 mVs-1 60Figure 3 8 Linear sweep voltammograms of different catalysts for ORR in O2-saturated 1

M KOH Scan rate: 10 mV s-1 Rotation rate: 2400 rpm; (a) Steady state polarization

curves; (b) Tafel plots 61Figure 3 9 Polarization and power density curves of a single AEMFC for different cathode catalysts 63 Figure 4 1 The polarization and power density curves of single cell testing using GDL-10/GDL-10 with different catalyst loadings 71Figure 4 2 Pore-size distribution curves of (a) GDS-0, GDS-30, and GDL-30 and (b) GDL-

0, GDL-10, and GDL-30 measured by using mecury intrusion porosimetry 75Figure 4 3 Plane-view (a)-(e) and cross-sectional SEM images (f)-(j) of GDS-0, GDS-30, GDL-0, GDL-10, and GDL-30, respectively 77Figure 4 4 SEM images of GDL-30 [(a) back side, (b) plane-view, (c) cross-section] and GDL-40 [(d) back side, (e) plane-view, (f) cross-section] 78

Figure 4 5 Plane-view SEM images of (a) GDS-30, (b) GDL-0, and (c) GDL-30 and their

respective images after coating CL of (d)-(f) 79Figure 4 6 Cross-sectional SEM images of (a) GDS-30, (b) GDL-0, and (c) GDL-30 as well as their respective images after coating CL of (d)-(f) 80Figure 4 7 The polarization and power density curves of single cell testing using GDL-40/GDL-40, GDL-30/GDL-30, GDL-10/GDL-10, and GDL-0/GDL-0 83Figure 4 8 The polarization and power density curves of single cell testing using GDS-30/GDS-30, GDS-30/GDL-30, and GDL-30/GDS-30, and GDL-30/GDL-30 84

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Figure 4 9 The polarization and power density curves of single cell testing using (a) 0/GDL-0 and GDS-0/GDS-0, (b) GDL-30/GDL-0, GDL-30/GDS-0, GDL-0/GDL-30, and GDS-0/GDL-30, (c) GDL-30/GDL-30, GDL-30/GDL-10, GDL-30/GDL-0, GDL-0/GDL-

GDL-30, and GDL-10/GDL-30 86 Figure 5 1 Schematic diagram of water transport in AEMFCs 90Figure 5 2 (a) Polarization and power density curves of AEMFCs operated at the cell temperature of 60 C, 65C, and 70C with the optimized dew points of anode/cathode inlet gases at 60C/65C (60/60/65), 62C/67C (65/62/67), and 65 C/70C (70/65/70), respectively; (b) Temperature gap between optimized dew points and cell temperature 94Figure 5 3 Polarization and power density curves of AEMFCs operated at the cell

temperature of (a) 60 C, (b) 65 C, and (c) 70 C with different dew points of

anode/cathode inlet gases (The symbol of 60/55/65 represents the cell temperature and dew points of anode and cathode are 60 C, 55C and 65 C, respectively) 97

LIST OF TABLES

Table 1 1 Deployment status for applications in 2020 according to the Deployment

Strategy Europe [4] 3

Table 1 2 Classification of fuel cells based on employed electrolytes 6

Table 3 1 Physical properties of GDL-280 52

Table 3 2 Electrochemical parameters derived from CV and LSV measurements 62

Table 3 3 The peak power density of AEMFC using different cathode catalysts 63

Table 4 1 Experimental parameters of five prepared samples 69

Table 4 2 Physical properties of different GDLs 74

Table 5 1 IEC and water uptake of some selected AEMs 99

GDLs Gas diffusion layers

GWe Gigawatt electrical

GDE Gas diffusion electrode

GCE Glassy carbon electrode

IEC Ion exchange capacity

LSV Linear sweep voltammetry

MPL Microporous layer

MEA Membrane electrode assembly

MCFCs Molten carbonate fuel cells

MIP Mercury intrusion porosimetry

ORR Oxygen reduction reaction

OCV Open circuit voltage

Pt/C Platinum supported on carbon

PTFE Polytetrafluoroethylene

PAFC Phosphoric acid fuel cell

PEMFCs Proton exchange membrane fuel cells

PAN Polyacrylonitrile

PSD Pore size distribution

RHE Reversible hydrogen electrode

RDE Rotating disc electrode

RE Reference electrode

rpm Revolutions per minute

SOFCs Solid oxide fuel cells

SHE Standard hydrogen electrode

SEM Scanning electron microscope

TGA Thermogravimetric analysis

TEM Transmission electron microscopy

WE Working electrode

wt.% Weight percentage

XRD X-ray diffraction

𝐼𝑐 ORR current density

𝑖𝑂02 Exchange current density of ORR

𝑛𝑂 number of electrons transferred in the rate determining step in ORR

𝑶 Transfer coefficient in ORR

𝒄 Overpotential of ORR

F Faraday constant

T Temperature

D Diameter of the intruded pore

 Surface tension of mercury

θM Contact angle between mercury and the pore surface

P Pressure

Rcop Bulk resistances of the copper plate

RGDL Bulk resistances of GDL

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