Analysis and outlook of mass transport in ultralow Pt loading proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) for automotive propulsions have been regarded as an ideal and the most promising alternative to replace fossil-fuel based internal combustion engines due to the high performance, high energy efficiency and zero-emission. However, the high cost has severely impeded the commercialization of PEMFCs, especially the high cost of Pt-based precious metal catalyst used for the cathode oxygen reduction reaction (ORR). It is critical to reducing the amount of Pt and thus decreasing the cost of fuel cell stacks, which would lead to numbers of new challenges. On the one hand, the reduction of Pt load could increase the oxygen transport resistance in cathode catalyst layers (CCLs). In fact, the oxygen transport resistance could be divided into several constituent parts, i.e., oxygen transport resistances in CCLs, gas diffusion layers (GDLs) and the flow channels, where the transport resistance in CCLs is much higher than other resistances. Further, CCLs are constructed as heterogeneous composites of ORR electrocatalysts, conductive polymer such as perfluorinated sulfonic acid (PFSA) ionomer as well as the pore space determined. Hence, the oxygen transport resistance in CCLs includes the bulk transport resistance caused by oxygen diffusion in the nanopore and the local transport resistance caused by oxygen permeation through the ultrathin ionomer film covering on the Pt surface. Local oxygen transport behavior could be divided into three processes: (1) Oxygen adsorption from gas phase into ionomer, (2) oxygen diffusion inside the ionomer, and (3) oxygen adsorption from the ionomer to Pt surfaces. As the Pt loading decreases, local oxygen transport resistance would increase significantly, thus leading to a dramatical performance degradation at high current density of fuel cells. In specific, the oxygen adsorption at gas/ionomer interface used to be considered as Henry's adsorption. However, it is proved recently that this type of adsorption should be quasi-logarithmic, due to the finiteness of the adsorption site on ionomer surface. In addition, the bulk oxygen transport resistance depends on the porosity and pore size distribution inside the electrodes. We summarized plenty of experimental results of effective oxygen diffusivities reported by different research groups, and found that all the experimental values are obviously smaller than the calculated results based on the classical Bruggeman approximation. It is believed that the effective porosity plays a more important role relative to the geometrical porosity in the estimation of bulk oxygen transport resistance in PEMFCs. On the other hand, confining effect of the ultrathin ionomer film could weaken the separation of hydrophilic and hydrophobic phases and then cut down the pathway of proton migration. There are two proton conduction mechanisms in CCLs: Grotthuss hopping and vehicular mechanism. The proton conduction in CCLs could be affected by numerous factors, such as temperature, ionomer structure, thickness of ionomer film, as well as cation contaminants in the ionomer film. Both high oxygen transport resistance and low proton conduction in ultra-low Pt membrane electrode assemblies would damage the performance and the lifetime of PEMFCs. In this review, the bulk and local transport behavior, proton transport in CCLs, as well as the corresponding influence on fuel cell performance are systematically analyzed, and a series of coping strategies are presented.