Modern Aspects of Electrochemistry 40 (eBook)

Preface 7
Memories of Brian Evans Conway Editor 1955-2005 9
Table of Contents 14
1 PEM Fuel Cell Bipolar Plates 20
I. INTRODUCTION 20
1. NAFION MEA Based Bipolar Plate Problems 21
2. Polybenzimldazole/H3PO4 22
II. DEFINITION 22
1. Separator Plate 23
2. FlowField 24
3. Port and Port Bridges 25
4. Seals 26
5. Frame 27
III. BIPOLAR PLATE FEATURES 27
1. Tolerances 28
2. Thermal Management 28
3. Electrical Conduction 29
4. Water Management 30
5. Low Cost 32
6. Stable, Free from Corrosion Products 32
(i) Galvanic Corrosion 32
IV. MATERIALS AND PROCESSES 34
1. Comparison of Carbon and Metal 34
(i) Operational 35
(ii) Forming Cost 35
2. Carbon 37
(i) Molded Graphite 37
(ii) Paper 38
(iii) Stamped Exfoliated Graphite iGrafoil, Graflex) 38
3. Metal 38
(i) Forming Metal BipolarPlates 39
(ii) Intrinsically Corrosion Resistant Metals 40
(iii) Direct Coatings 40
(a) Plating 40
(b) Cladding 41
(c) Vapor deposition plasma, jet spray, CVD, laser ablation, LAFAD 41
(d) Flame spraying 41
(e) Thermal vapor deposition 41
(f) Thermally activated CVD 42
(g) Fast CVD 42
(h) Thermal laser assisted CVD (LCVD) 42
(i) Reactive ion beam-assisted electron beam-physical vapor deposition 42
(j) Reactive plasma spraying (RPS) 42
(k) Pulsed laser deposition (PLD) 43
(iv) Conductive Polymer Grafting 43
(a) Aluminum coupling 44
(b) Polymer matrix 48
(c) Accelerators 49
(d) Crosslinkers 50
(e) Graft initiators and regenerators 50
(f) Solvents 50
(g) Conductive fillers 52
REFERENCES 52
2 Basic Applications of the Analysis of Variance and Covariance in Electrochemical Science and Engineering 55
I. INTRODUCTION 55
II. BASIC PRINCIPLES AND NOTIONS 56
III. ANOVA: ONE-WAY CLASSIFICATION 58
1. Completely Randomized Experiment (CRE) 60
2. Randomized Block Experiment (RBE) 60
(i) Example 1: A Historical Perspective of Caustic Soda Production 61
(ii) Example 2: Metallic Corrosion 63
IV. ANOVA: TWO-WAY CLASSIFICATION 64
1. Null and Alternative Hypotheses 64
2. Illustration of Two-Way Classification: Specific Energy Requirement for an Electrolytic Process 65
V. ANOVA: THREE-WAY CLASSIFICATIONS 67
VI. ANOVA: LATIN SQUARES (LS) 69
VII. APPLICATIONS OF THE ANALYSIS OF COVARIANCE (ANCOVA) 71
1. ANCOVA with Velocity as Single Concomitant Variable 71
(i) Pattern A (CRE) 71
(ii) Pattern B(RBE) 74
2. ANCOVA with Velocity and Pressure Drop Acting as Two Concomitant Variables 76
3. Two Covariate-Based ANCOVA of Product Yields in a Batch and in a Flow Electrolyzer 76
4. Covariance Analysis for a Two-Factor, Single Cofactor CRE 78
VIII. MISCELLANEOUS TOPICS 80
1. Estimation of the Type II Error in ANOVA 80
2. Hierarchical Classification 82
3. ANOVA-Related Random Effects 84
4. Introductory Concepts of Contrasts Analysis 87
IX. FINAL REMARKS 89
ACKNOWLEDGMENTS 90
LIST OF PRINCIPAL SYMBOLS 90
Subscripts 90
Greek Symbols 91
REFERENCES 91
3 Nanomaterials in Li-Ion Battery Electrode Design 93
I. INTRODUCTION 93
II. TEMPLATES USED 96
1. Track-Etch Membranes 96
2. Alumina Membranes 98
3. Other Templates 99
II. NANOSTRUCTURED CATHODIC ELECTRODE MATERIALS 101
1. Electrode Fabrication 102
(i) Nanostructured Electrode 102
(ii) Control Electrodes 103
2. Structural Investigations 104
3. Electrochemical Characterization 105
(i) Cyclic Voltammetry 105
(ii) Rate Capabilities 107
III. NANOSTRUCTURED ANODIC ELECTRODES 109
1. Electrode Fabrication 110
(i) Nanostructured Electrodes 110
(ii) Control Electrodes 110
2. Structural Investigations 111
3. Electrochemical Investigations 113
V. NANOELECTRODE APPLICATIONS 115
1. Low-Temperature Performance 115
(i) Electrode Fabrication 115
(ii) Strategy 116
(iii) Electrochemical Results 117
(iv) Electronic Conductivity 119
(v) Cycle Life 120
2. Variations on a Synthetic Theme 120
(i) Nanocomposite of LiFePO4/Carbon 120
(a) Electrode fabrication 121
(b) Methods 122
(c) Imaging 122
(d) Carbon analysis 124
(e) Electrochemistry 125
(ii) Improving Volumetric Capacity 127
(a) Strategy 128
(b) Electrode morphology 129
(c) Rate capabilities 132
VI. CARBON HONEYCOMB 135
1. Preparation of Honeycomb Carbon 136
2. Electrochemical Characterization 139
VII. CONCLUSIONS 141
ACKNOWLEDGEMENTS 141
REFERENCES 142
4 Direct Methanol Fuel Cells: Fundamentals, Problems and Perspectives 145
I. INTRODUCTION 145
II. OPERATING PRINCIPLE OF THE SPE-DMFC 146
III. ELECTRODE REACTION MECHANISMS IN SPE-DMFCS 150
1. Anodic Oxidation of Methanol 150
2. Cathodic Reduction of Oxygen 157
IV. MATERIALS FOR SPE-DMFCS 158
1. Catalyst Materials 158
(i) Anode Catalysts 158
(ii) Oxygen Reduction Catalysts 167
(iii) Membrane Materials 174
V. DIRECT METHANOL FUEL CELL PERFORMANCE 181
1. DMFC Stack Performance 193
2. Alternative Catalysts and Membranes in the DMFC 196
3. Alkaline Conducting Membrane and Alternative Oxidants 201
VI. CONVENTIONAL VS. MIXED-REACTANT SPE-DMFCS 203
VII. MATHEMATICAL MODELLING OFTHEDMFC 210
1. Methanol Oxidation 213
2. Empirical Models for Cell Voltage Behaviour 216
3. Membrane Transport 220
4. Effect of Methanol Crossover on Fuel Cell Performance 222
5. Mass Transport and Gas Evolution 223
6. DMFC Electrode Modelling 227
7. Cell Models 228
8. Single Phase Flow 230
9. Two- and Three-Dimensional Modelling 231
10. Dynamics and Modelling 233
11. Stack Hydraulic and Thermal Models 233
VIII. CONCLUSIONS 234
LIST OF SYMBOLS 235
Superscripts and Subscripts 235
Greek Symbols 236
REFERENCES 236
5 Review of Direct Methanol Fuel Cells 246
I. INTRODUCTION 246
II. ANODE KINETICS 249
1. Reaction Mechanism 249
2. Methanol Oxidation Catalysts 250
(i) Platinum and Platinum Catalyst Structure 250
(ii) Platinum and Platinum Alloy Catalyst Performance 257
III. OXYGEN REDUCTION REACTION CATALYSTS 264
IV. HIGH TEMPERATURE MEMBRANES 265
V. METHANOL CROSSOVER 270
1. Magnitude of Crossover 270
2. Effect of CO2 Crossover 275
3. Mixed-Potential Effects 277
4. Novel Membranes to Reduce Methanol Crossover 278
VI. DMFC MODELING REVIEW 281
1. One-Dimensional Models 282
2. Two-Dimensional and Three-Dimensional Models 290
VII. SUMMARY 295
REFERENCES 297
6 Direct Numerical Simulation of Polymer Electrolyte Fuel Cell Catalyst Layers 302
I. INTRODUCTION 302
II. DIRECT NUMERICAL SIMULATION (DNS) APPROACH 305
1. Advantages and Objectives of the DNS Approach 306
2. DNS Model- Idealized 2-D Microstructure 307
3. Three-Dimensional Regular Microstructure 310
4. Results and Discussion 316
(i) 2-D Model: Kinetics- vs. Transport-Limited Regimes 316
(ii) Comparison ofthe Polarization Curves between 2-D and 3-D Simulations 321
III. THREE-DIMENSIONAL RANDOM MICROSTRUCTURE 322
1. Random Structure 323
2. Structural Analysis and Identification 324
3. Governing Equations 328
4. Boundary Conditions 331
5. Results and Discussion 333
IV. DNS MODEL - WATER TRANSPORT 337
1. Water Transport Mechanism 338
2. Mathematical Description 340
3. Results and Discussion 344
(i) Inlet-Air Humidity Effect 344
(ii) Water Crossover Effect 347
(iii) Optimization of Catalyst Layer Compositions 348
V. 3-D CORRELATED MICROSTRUCTURE 350
1. Stochastic Generation Method 350
2. Governing Equations, Boundary Conditions and Numerical Procedure 351
3. Results and Discussion 354
VI. CONCLUSIONS 357
ACKNOWLEDGEMENTS 357
REFERENCES 358
Index 359

"3 Nanomaterials in Li-Ion Battery Electrode Design (p. 75-76)

Charles R. Sides and Charles R. Martin*

I. INTRODUCTION

Li-ion batteries have generated great interest as lightweight, portable, rechargeable power sources over the last decade. Their introduction in 1990 by T. Nagaura and K. Tozawa of SonyTec Inc. fueled the explosion of personal electronic devices. Li-ion batteries are now the power source of choice for laptops, cell phones, and digital cameras. The public has quickly embraced this technology, which accounts for an approximately $3 billion annual market. 2 Despite (or perhaps as a result of) the commercial success of these batteries, a global research initiative exists to improve the existing design.

The goal of which is to apply this technology to more demanding and exotic uses, such as the electric component of hybrid vehicles, low-temperature applications, and power supplies for MEMs. However, the current design cannot adequately satisfy the power requirements of such systems, due to the inability to deliver a sufficient quantity of charge at high discharge currents. 3 This chapter will detail the efforts of laboratories, ours in particular, to incorporate the field of nanomaterials to improve upon Li-ion batteries.

Li-ion batteries operate by reversibly intercalating charge in each of two electrodes. Intercalation is the process by which a guest species (Li+) is able to reversibly enter/exit a host structure, causing little or no difference to the lattice of the host. These electrodes are separated by an ion-conductive electrolyte. Upon discharge, the Li-ions deintercalate from the low-potential electrode, migrate through the electrolyte, and insert into the highpotential electrode. The ions then rely on solid-state diffusion to fill the non-surface intercalation sites.

Obeying the governing laws of charge neutrality, electrons compensate for the movement of the ions. If current flow is reversed (from cathode to anode), Li-ions insert into the low-potential electrode and the system is charged. The low-potential electrode is the anode and the high-potential electrode is the cathode. This convention (adopted from the discharge process) is obeyed regardless of the direction of current flow. In the analysis of a battery system, both the ionic conductivity and electronic conductivity must be considered. Nanomaterials are advantageous in both regards.

The Martin research group has pioneered the nanofabrication strategy of template synthesis." This general method has been used to synthesize nanostructures of a variety of materials such as gold 5-8 carbon 9-11 semiconductors 12,13 polymers 14,15 and Li-ion battery electrodes,II,13,16-28 our focus here. In general, this method involves deposition of a precursor material into a micro- or nanoporous template. This template is typically commercially available track-etch polymer filters or anodized alumina, though others have been demonstrated. Depending on both the porediameter and the specific chemical interactions between the pore wall and the precursor, the resulting structures may be tubes (hollow) or wires (solid). These structures are referred to as "nano", if one or more of their dimensions are on the nanoscale « 100 nm). The aspect ratio (length / width), though, is often on the order of 10.

In this embodiment of Li-ion electrodes, a precursorimpregnated polycarbonate template membrane is attached to a section of metal foil. The foil has dual-functionality as it serves as a substrate during synthesis and as a common current collector during electrochemical characterization. The precursor is processed (typically, by aging or heating) into the desired product. Often in the case of battery materials, the template is then removed by plasma etching or dissolution. The result is an electrode that consists of structures that mirror the geometry (length, diameter, and number density) of the pores of the template."