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Materials for Low-Temperature Fuel Cells

  • Erscheinungsdatum: 19.11.2014
  • Verlag: Wiley-VCH
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Materials for Low-Temperature Fuel Cells

There are a large number of books available on fuel cells; however, the majority are on specific types of fuel cells such as solid oxide fuel cells, proton exchange membrane fuel cells, or on specific technical aspects of fuel cells, e.g., the system or stack engineering. Thus, there is a need for a book focused on materials requirements in fuel cells. Key Materials in Low-Temperature Fuel Cells is a concise source of the most important and key materials and catalysts in low-temperature fuel cells. A related book will cover key materials in high-temperature fuel cells. The two books form part of the 'Materials for Sustainable Energy & Development' series. Key Materials in Low-Temperature Fuel Cells brings together world leaders and experts in this field and provides a lucid description of the materials assessment of fuel cell technologies. With an emphasis on the technical development and applications of key materials in low-temperature fuel cells, this text covers fundamental principles, advancement, challenges, and important current research themes. Topics covered include: proton exchange membrane fuel cells, direct methanol and ethanol fuel cells, microfluidic fuel cells, biofuel cells, alkaline membrane fuel cells, functionalized carbon nanotubes as catalyst supports, nanostructured Pt catalysts, non-PGM catalysts, membranes, and materials modeling. This book is an essential reference source for researchers, engineers and technicians in academia, research institutes and industry working in the fields of fuel cells, energy materials, electrochemistry and materials science and engineering. Associate Professor Bradley Ladewig is an academic in the Department of Chemical Engineering at Monash University, Australia, where he leads a research group developing membrane materials and technologies for clean energy applications. He has a wide range of experience as a chemical engineering researcher, including in membrane development for direct methanol fuel cells, testing and modeling of combined heat and power PEM fuel cell systems, and desalination membrane development. Recently he has worked on several collaborative projects in the field of direct carbon fuel cells, metal organic framework materials as gas sorbents and membrane components, and low-cost microfluidic sensors based on paper and thread substrates. He is a Fellow of the Institution of Chemical Engineers. Professor San Ping Jiang is a professor at the Curtin Centre for Advanced Energy Science and Engineering, Curtin University, Australia and Adjunct Professor of the Huazhong University of Science and Technology, China. He also holds Visiting/Guest Professorships at Wuhan University of Technology, University of Science and Technology of China (USTC), Sichung University, and Shandong University. Dr. Jiang has broad experience in both academia and industry, having held positions at Nanyang Technological University, the CSIRO Manufacturing Science and Technology Division in Australia, and Ceramic Fuel Cells Ltd (CFCL). His research interests encompass solid oxide fuel cells, proton exchange and direct methanol fuel cells, and direct alcohol fuel cells. Professor Yushan Yan is Distinguished Engineering Professor in the Department of Chemical and Biomolecular Engineering at the University of Delaware. Prior to that he was a Professor at The University of California, Riverside, and before that worked for AlliedSignal Inc. as a Senior Staff Engineer and Project Manager. His research focuses on zeolite thin films for semiconductors and aerospace applications and new materials for cheaper and durable fuel cells. He is co-Founder and Director of the start-up companies Full Cycle Energy and Zeolite Materials Solutions (ZSM).

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Materials for Low-Temperature Fuel Cells

2
Alkaline Anion Exchange Membrane Fuel Cells

Rhodri Jervis and Daniel J.L. Brett
2.1 Fuel Cells

Fuel cells represent a potentially integral technology in a greener electricity-based energy economy. Converting chemical energy directly into electricity with no moving parts and no particulate or greenhouse gas emissions at point of operation, they can offer higher efficiencies than combustion and greater energy storage and reduced "charge" times compared with batteries. While they retain few of the disadvantages of existing electricity generation technologies, a major barrier to commercialization and widespread use at present is cost. The key working part of a fuel cell, the membrane electrode assembly (MEA), comprises a catalyst, usually containing platinum, and an ionic polymer membrane, both of which contribute significantly to the overall cost of a fuel cell. This chapter will concentrate on the potential for alkaline anion exchange membrane (AAEM) fuel cells to provide a route to reduced costs and help realize commercial ubiquity of fuel cells in various energy sectors. We will first discuss the basic principles of the more common acidic PEM fuel cells and the thermodynamics and kinetics of the electrochemical reactions governing their operation, before explaining the key differences in AAEM fuel cells and how they might provide an advantage over the more established technology.

This basic idea of the fuel cell goes back to as far as 1839 when Swansea-born physicist Sir William Gove realized that reverse electrolysis of water was possible. However, development from this concept was slow and it was not until the 1960s and the Apollo Space Programme that fuel cells became practicable, in the form of aqueous alkaline electrolyte fuel cells. Aqueous electrolyte-based fuel cells have many disadvantages for portability causing recent focus to shift toward solid electrolytes, in particular toward polymer electrolyte membrane (PEM) fuel cells. These employ an ionomer, which is a polymer containing an ionic functional group in the monomer, as the electrolyte in order to allow hydrogen ion transport through a nonaqueous medium. Recent improvements in membrane technology, and in particular the performance of the industry standard Nafion membranes, have made PEM fuel cells a major focus of research. The alkaline analog of the more common PEM fuel cell uses a hydroxide-conducting membrane in an attempt to exploit the superior cathode kinetics of alkaline systems and ultimately reduce the catalyst's contribution to the cost of fuel cells.
2.2 PEM Fuel Cell Principles

The main elemental principle of a fuel cell is the direct electrochemical redox reaction that produces the electrical current. In the hydrogen/oxygen fuel cell, the redox reaction is composed of two electrochemical half equations - the hydrogen oxidation reaction (HOR) at the anode:
(2.1)
and the oxygen reduction reaction (ORR) at the cathode:
(2.2)
These combine to give the overall redox reaction:
(2.3)
Hydrogen is fed into the anode and air/oxygen into the cathode through flowfields and diffuses through a gas diffusion layer (GDL) to the catalyst layer where the gas, catalyst, and electrolyte meet in what is called a triple-phase boundary. It is here where the HOR and ORR occur on the anode and the cathode, respectively, separated by the polymer electrolyte membrane. The protons generated by the HOR diffuse through the electrolyte to react at the cathode and the electrons generated, impeded by the insulating polymer, travel through the external circuit creating a current ( Figure 2.1 ).

Figure 2.1 Schematic of an acidic PEM fuel cell.
2.2.1 Equilibrium Kinetics

The HOR and ORR reactions occurring at the electrodes of fuel cell

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