New PEMFC active layer architectures for optimized Pt utilization rate,
Résumé
Before proton exchange membrane fuel cell (PEMFC) technology can gain a significant share of the electrical power market, important issues have to be addressed. In a PEMFC, energy conversion is achieved with costly Pt-based catalysts, and the low oxygen reduction reaction (ORR) kinetics implies high Pt loadings at the cathode. Electrode reactions can occur only at confined spatial sites, the triple-phase boundaries, where the reactant, the ionic conducting polymer, and the electronic conducting substrate are present on the same platinum particle. Efforts to increase the amount of such active sites are therefore of paramount importance in enhancing the cell performance by increasing the platinum utilization efficiency in 3D Pt/C porous electrodes. In current systems, triple-phase boundaries are achieved by adding Nafion ionomer to the catalytic ink used for the preparation of the active layer. Most of the investigations concerning the effect of the amount of ionomer in fuel cell electrodes concluded that the optimal Nafion content depends on the fuel cell working point; high Nafion content increases the electrode active area and the cell performance at low current densities, whereas it induces mass transport limitations at high current densities because the pores fill up. To balance both of these antagonist effects, a compromise of ca. 30 wt % Nafion is often used in electrodes. Moreover, the Pt utilization efficiency in current commercially offered prototype fuel cell electrodes remains very low (20-30%), and reaching higher utilization efficiency is still a crucial and therefore very active research topic. The combination of the limiting ORR kinetics at the cathode with the low Pt utilization efficiency becomes detrimental to the cell performance and cost. To overcome these limitations, the current electrode active-layer architecture paradigm has to be abandoned. The transposition of the triple-phase boundaries on the molecular scale by grafting ionic conducting molecules directly either onto platinum nanoparticles or carbon support leads to nanocomposite materials with great potency as cathode catalysts. This nanomaterial design is expected to allow creating a proton-conducting pathway between platinum active sites and the conducting membrane. The possibility of modifying the catalytic powder without altering its catalytic activity and selectivity is very attractive for potentially lowering the electrode material processing and PEMFC system costs by increasing the metal utilization efficiency, by avoiding the addition of an ionomer to the active layer, and by making possible the use of membrane alternatives to Nafion as a solid electrolyte.