Mass transport and polarization limitations in high-temperature proton exchange membrane fuel cells (HT-PEMFCs) based on phosphoric acid (PA)-doped membranes arise due to several factors. These factors affect reactant delivery, catalyst kinetics, and overall cell performance. We have investigated the PA induced limiting effects caused due to PA migration and redistribution within the membrane electrode assembly. This leads to two major problems: PA poisoning of Pt catalyst, leading to kinetic losses, and excess PA can block catalyst layer pores, reducing the effective gas diffusion. Both fuel cell and proton pumping operation modes of HT-PEM devices are affected by these factors and ultimately determine overall performance. Proton transport in the catalyst layer is highly affected by the P.A re-distribution in electrode layer and the amount of P.A doping in the membrane. Furthermore, to sustain high current density (beyond 3 A/cm
2) at low voltage, some consideration in operating conditions is required like, selective humidification, back pressure etc.
In this study, we systematically analyzed the voltage contributions from each factor and applied electrochemical diagnostic methods to overcome the limitations of PA poisoning and proton transport. By using array of tools for improving electrochemical engineering and materials design such as reaction conditions and modifications in membranes, ionomers, catalysts and gas diffusion layers, we see that the performance for HT-PEM PA based devices can show drastic difference. This study will provide foundational knowledge to further the electrocatalyst and ionomer design to overcome the limiting factors.
