INFLUENCE OF THE MORPHOLOGICAL ABSORPTIVE PROPERTIES UPON THE PERFORMANCE OF PHOSPHORIC-ACID FUEL-CELL GAS-DIFFUSION ELECTRODES

The morphological and absorptive characteristics of the electrocatalyst layer of Teflon-bonded gas diffusion electrodes have been investigated. The catalyst layer microstructure of cathodes and anodes was varied by using different preparative conditions (320-360-degrees-C sintering temperature, T(s), and 20-70% Teflon content). From porosimetric analysis and phosphoric acid absorption, a set of parameters has been derived which gives an extensive description of the structure of the catalyst layer. The catalyst layer porosity and electrolyte volume fraction depend directly on the preparative conditions, while the agglomerate porosity and the macropore porosity are not influenced significantly. A ratio of unity between the gas and liquid fraction of the total gas porosity, i.e. a percentage acid occupation (PAO) of 50%, is a prerequisite for highest electrochemical performance, lowest Tafel slopes and oxygen gains for the O2 reduction and lowest over-potential for H-2 oxidation. A theoretical expression for the Pt utilization in the catalyst layer based upon morphological-absorptive properties has also been proposed and evaluated which agrees well with electrochemical measurements on the active Pt surface area using cyclic voltammetry. The PAO was also found to be a good property for correlating morphological-absorptive characteristics with Pt utilization.