MODELING AND EXPERIMENTAL DIAGNOSTICS IN POLYMER ELECTROLYTE FUEL-CELLS

This paper presents a fit between model and experiment for well-humidi fied polymer electrolyte fuel cells operated to maximum current densit y with a range of cathode gas compositions. The model considers, in de tail, losses caused by: (i) interfacial kinetics at the Pt/ionomer int erface, (ii) gas-transport and ionic-conductivity Limitations in the c atalyst layer, and (iii) gas-transport limitations in the cathode back ing. Our experimental data were collected with cells that utilized thi n-film catalyst layers bonded directly to the membrane, and a separate catalyst-free hydrophobic backing layer. This structure allows a clea rer resolution of the processes taking place in each of these distingu ishable parts of the cathode. In our final comparison of model predict ions with the experimental data, we stress the simultaneous fit of a f amily of complete polarization curves obtained for gas compositions ra nging from 5 atm O-2 to a mixture of 5% O-2 in N-2, employing in each case the same model parameters for interfacial kinetics, catalyst-laye r transport, and backing-layer transport. This approach allowed us to evaluate losses in the cathode backing and in the cathode catalyst lay er, and thus identify the improvements required to enhance the perform ance of air cathodes in polymer electrolyte fuel cells. Finally, we sh ow that effects of graded depletion in oxygen along the gas flow chann el can be accurately modeled using a uniform effective oxygen concentr ation in the flow channel, equal to the average of inlet and exit conc entrations. This approach has enabled simplified and accurate consider ation of oxygen utilization effects.