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Unformatted text preview: Transport in Proton Conductors for Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology Klaus-Dieter Kreuer,* ,‡ Stephen J. Paddison, § Eckhard Spohr, # and Michael Schuster ‡ Max-Planck-Institut fu ¨r Festko ¨rperforschung, Heisenbergstr.1, D-70569 Stuttgart, Germany, Department of Chemistry, University of Alabama in Huntsville, Huntsville, Alabama 35899, and Forschungszentrum Ju ¨lich, D-52425 Ju ¨lich, Germany Received March 4, 2004 Contents 1. Introduction 4637 2. Theoretical Methodologies and Simulation Tools 4640 2.1. Ab Initio Quantum Chemistry 4641 2.2. Molecular Dynamics 4642 2.2.1. Classical Molecular Dynamics and Monte Carlo Simulations 4643 2.2.2. Empirical Valence Bond Models 4644 2.2.3. Ab Initio Molecular Dynamics (AIMD) 4645 2.3. Poisson- Boltzmann Theory 4645 2.4. Nonequilibrium Statistical Mechanical Ion Transport Modeling 4646 2.5. Dielectric Saturation 4647 3. Transport Mechanisms 4648 3.1. Proton Conduction Mechanisms 4648 3.1.1. Homogeneous Media 4648 3.1.2. Heterogeneous Systems (Confinement Effects) 4655 3.2. Mechanisms of Parasitic Transport 4661 3.2.1. Solvated Acidic Polymers 4661 3.2.2. Oxides 4665 4. Phenomenology of Transport in Proton-Conducting Materials for Fuel-Cell Applications 4666 4.1. Hydrated Acidic Polymers 4666 4.2. PBI- H 3 PO 4 Adducts 4668 4.3. Heterocycle-Based Systems 4669 4.4. Oxides 4669 5. Recent Approaches toward New Proton-Conducting Materials for Fuel-Cell Applications 4669 6. Acknowledgement 4674 7. References 4675 1. Introduction The electrolyte is the heart of any fuel cell. Ideally, this component effectively separates the anode and cathode gases and/or liquids and mediates the elec- trochemical reaction occurring at the electrodes through conducting a specific ion at very high rates during the operation of the fuel cell. In other words, transport through such electrolytes must be fast and highly selective; these two properties are frequently at odds with each other. Proton-conducting materials are used as the electrolyte for low- and intermediate- temperature fuel cells, which are currently attracting significant interest (i.e., polymer electrolyte mem- brane or proton exchange membrane fuel cells (PEMFCs)), direct methanol fuel cells (DMFCs), phosphoric acid fuel cells (PAFCs), and alkaline fuel cells (AFCs)). However, these are not true “single ion conductors”, in that the transport phenomena are more complex, involving significant parasitic trans- port of other species. This review is concerned with the transport properties of presently available and emerging materials that have the potential to be used as the electrolyte for fuel cells soon. Strategies for the development of novel materials with transport properties approaching those of an “ideal separator”, especially in the intermediate temperature range, are also discussed....
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