Porous composite electrodes catalyst layer

A number of different methods exist for the production of catalyst layers [97-102]. They use variations in composition (contents of carbon, Pt, PFSI, PTFE), particle sizes and pds of highly porous carbon, material properties (e.g., the equivalent weight of the PFSI) as well as production techniques (sintering, hot pressing, application of the catalyst layer to the membrane or to the gas-diffusion layer, GDL) in order to improve the performance. The major goal of electrode development is the reduction of Pt and PFSI contents, which account for substantial contributions to the overall costs of a PEFC system. Remarkable progress in this direction has been achieved during the last decade [99, 100], At least on a laboratory scale, the reduction of the Pt content from 4.0 to 0.1 mg cm-2 has been successfully demonstrated. 

An important supplementary tool for performance analysis of PEFC electrodes is the study of the complex impedance, as it provides a tool to monitor changes of electrode function upon variation of its composition. It can help to detect in real time the structural changes due to spontaneous or current-induced repartitioning of the elements of the porous dual percolation network, that could lead to phase segregation and catalyst layer degradation. 

The potential benefit of impedance studies of porous GDEs for fuel applications has been stressed in Refs. 141, 142. A detailed combined experimental and theoretical investigation of the impedance response of PEFC was reported in Ref. 143. Going beyond these earlier approaches, which were based entirely on numerical solutions, analytical solutions in relevant ranges of parameters have been presented in Ref. 144 which are convenient for the treatment of experimental data. It was shown, in particular, how impedance spectroscopy could be used to determine electrode parameters as functions of the structure and composition. The percolation-type approximations used in Ref. 144, were, however, incomplete, having the same caveats as those used in Ref. 17. Incorporation of the refined percolation-type dependencies, discussed in the previous section, reveals effects due to varying electrode composition and, thus, provides diagnostic tools for optimization of the catalyst layer structure. 

The functions of porous electrodes in fuel cells are 1) to provide a surface site for gas ionization or de-ionization reactions, 2) to provide a pathway for gases and ions to reach the catalyst surface, 3) to conduct water away from the interface once these are formed, and 4) to allow current flow. A membrane electrode assembly (MEA) forms the core of a fuel cell and the key electrochemical reactions take place in the MEA. MEA performance is severely affected by electrode composition, structure, and geometry, and especially by cathode structure and composition, due to poor oxygen reduction kinetics and transport liniitations of the reactants in the cathode catalyst layer. 

There is more to catalyst layer operation than electrocatalysis, a lot more The design of fuel cell electrodes with high performance, long lifetime, and low cost is about embedding the catalyst, usually the most expensive and least stable material in the cell, into a porous composite host medium. It turns out that material selection and structural design of the host medium is as important as that of the catalyst material itself. 

At a critical value of the fraction of objects of one type, these objects would form an extended cluster that connects the opposite external faces of the sample. At this so-called percolation threshold, the corresponding physical property represented by the connected objects would start to increase above zero. Thereby percolation theory establishes constitutive relations between composition and structure of heterogeneous media and their physical properties of interest. For porous electrodes or catalyst layers in PEFC, these properties are electrical conductivities of electrons and protons, diffiisivities of gaseous reactants and water vapor, and liquid water permeability. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

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