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

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 ii 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 Fuel cell fundamentals 1.2 Anion exchange membrane fuel cells 1.2.1 Principle of anion exchange membrane fuel cells 1.2.2 Main components of AEMFCs 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 iii 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 iv 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 v 5.4 Conclusions 99 CHAPTER 6: CONCLUSIONS AND FUTURE RESEARCH 101 6.1 Conclusions 101 6.2 Future research 103 REFERENCES 104 vi LIST OF FIGURES Figure 1 Illustration of the working principle of a fuel cell Figure Schematic diagram of AEMFCs Figure Main components of a typical single AEMFC 11 Figure 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 The possible mechanism of all pathways for ORR on a metal catalyst surface 19 Figure 2 Trends in oxygen reduction activity plotted as a function of both the O (a) and the OH (b) binding energy [33] 20 Figure Manufacturing process involved in conventional carbon paper-based GDL fabrication [36] 22 Figure SEM images of typical GDS made of (a) carbon fiber paper and (b) carbon cloth 22 Figure Flow chart of micromachining process for Ti GDL and SEM image Ti GDL with microholes [43] 25 Figure Schematic of gas diffusion medium made of copper foil used as GDL in PEMFCs [44] 25 Figure (a) Fabrication steps of porous copper foil and (b) its typical SEM image [45] 26 Figure Schematic of stainless steel GDL design in comparison to carbon paper GDL [46] 26 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