The commercialization of PEFCs depends significantly on reduction of the costly precious metal content in fuel-cell electrodes. To achieve this goal, a high-performance low-loaded electrode must be utilized; and there is a need to understand the key transport resistances and processes within such a structure. The catalyst layer represents the most complex system in a PEFC due to the existence of multiple phases and a heterogeneous structure. Within catalyst layers, gas transport dominates the performance at high current densities, resulting in mass-transport limitations. To quantify these resistances, limiting-current measurements can be used. In this talk, we report on such mass-transport resistances utilizing a hydrogen-pump limiting current under various operating conditions including gas pressure and gas chemical identity for various low-loaded catalyst structures. In this fashion, one can separate the various resistances into molecular versus Knudsen diffusion and bulk versus non-bulk resistances.

The in-situ method based on H₂limiting current measurements was used for the characterization of catalyst-layer resistances in porous Pt/C based electrodes. The parameters studied were the ionomer to carbon ratio, the platinum loading, and the influence of different carbon supports. The effect of platinum loading on hydrogen transport resistance shows that the catalyst-layer resistance is inversely proportional to the roughness factor of the electrode as shown in the figure. The dominant transport mechanism is estimated to be Knudsen diffusion for vulcan-based catalyst layers. An increase of molecular diffusion can be observed by diluting Vulcan black by Ketjen black. Overall, the technique allows for quantification of the local transport resistances within the catalyst layer using a simplified experiment, thereby suggesting that the ionomer film is key in controlling the resistance. Furthermore, the observed trends can be used to guide optimization of novel low-loaded fuel-cell electrodes.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and by CRADA agreement LB08003874 between Lawrence Berkeley National Laboratory and Toyota Motor Company.

Figure 1. Total cell resistance as a function of inverse roughness factor at 40°C, 0.5psi. The catalyst layer resistance is inversely proportional to the roughness factor of the electrode, as represented by the linear regression function.