DFT investigation of the polymer electrolyte membrane degradation caused by OH radicals in fuel cells
ABSTRACT Thermodynamic properties of the radicals originating from the reactions of the compounds modeling building blocks of the polymer membranes used in low temperature fuel cell applications with hydroxyl radicals and oxygen molecules are studied in water environment and room temperature. All characteristics of the compounds are calculated using the UB3LYP DFT method with spin unrestricted orbitals. The Polarizable Continuum Model (PCM) is employed to model the solvation of species by water.A degradation mechanism for the nonfluorinated aromatic polymers sPEEK and PSU, which leads to the loss of sulfonic acid groups, is proposed. The process starts with the addition of the OH radical to aromatic rings. At the next step, an oxygen molecule is attached to the cyclohexadienyl radical forming various OH–aromatic–OO type radicals. These species can act as direct precursors for the loss of sulfonic acid group. Alternatively, they can transform into bicyclic and/or epoxy-type radicals, which are labile towards the detachment of the sulfonic acid group. The presence of water favors the detachment reactions significantly. The processes lead to decreased proton conductivity and contribute to the reduction of the membrane performance in PEMFCs and DMFCs.
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ABSTRACT: Polymeric materials find applications in almost all types of fuel cells (FCs). For example, microstructural optimization of materials by poly(methyl methacrylate) microspheres in solid oxide fuel cells (SOFCs) or as the conductive binders for the preparation of carbon electrodes for the phosphoric acid fuel cells (PAFCs). However, these applications are only marginal and not intrinsically essential to the functioning of these types of fuel cells, and will not be discussed in this review. On the other hand, ion exchange polymers used for the preparation of the ion exchange membranes, separating anode and cathode compartment of proton exchange membrane fuel cells (PEMFCs), anion exchange membrane fuel cells (AEMFCs), and direct methanol fuel cells (DMFCs) are one of their major components, and investigations of them, conducted during the last two years, are discussed in the present review. The same is true for a few polymers used for complexation of enzymes and their attachment to electrodes of biofuel cells (BFCs). Copyright © 2007 John Wiley & Sons, Ltd.Polymers for Advanced Technologies 09/2007; 18(10):785 - 799. · 2.01 Impact Factor