Reducing the Pt loading, especially on the cathode side, is very important for reducing the cost of proton exchange membrane fuel cells. However, lowering the Pt loading causes high voltage losses, but the underlying mechanism has not been fully understood. In this study, a new analytical catalyst layer model is established to study the oxygen reactive transport process, in which effect of carbon particle overlap is considered. Results from the analytical model show that carbon particle overlap leads to increase of ionomer thickness, reduction of ionomer specific surface area, and finally increase of oxygen transport resistance. By considering carbon particle overlap, an increased ~100 nm effective ionomer thickness is observed, and creates ~1500 s m-1 ionomer resistance. The order of magnitude of such ionomer resistance is equivalent to that of the Pt surface resistance measured in previous experimental studies, but there is no clarified source of this Pt surface resistance. As a result, it is reasonable to consider that carbon particle overlap may cause one of the sources of the oxygen transport resistance, which provides new insights for further alleviating the oxygen transport resistance under low Pt loading.
Keywords catalyst layer, low Pt loading, carbon particle overlap, ionomer film, oxygen transport resistance