Unveiling the origins of the activity gap between rotating disk electrodes and membrane electrode assemblies: Pt seed-mediated iridium-doped octahedral platinum nickel catalysts for proton exchange membrane fuel cells†

Abstract

Proton exchange membrane fuel cells (PEMFCs) offer energy solutions of high efficiency and low environmental impact. However, the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode limit their commercialization. Pt-based electrocatalysts, particularly octahedral (oh)PtNi bimetallic catalysts doped with additional transition metals, stand out as promising candidates for enhancing ORR rates and overall cell performance. A key challenge in the development and validation of active oh PtNi electrocatalysts is the unsuccessful translation of laboratory-scale catalyst test results, typically assessed using the rotating disk electrode (RDE) method, to practical applications in membrane electrode assembly (MEA) for PEMFCs. Here, we consider a new family of Ir-doped octahedral ORR fuel cell catalysts with very high RDE-based Pt mass activities. First, we designed the catalysts and tuned the catalyst layer properties to achieve the new state-of-the-art performance for oh-PtNi catalysts in PEMFCs. Still, a significant decrease in relative performance with respect to Pt/C when transitioning from RDE into an MEA-based cathode environment was observed. Thus, to better understand this performance loss, we investigated the effects of ionomer–catalyst interactions by adjusting the I/C ratio, the effect of temperature by applying RDE under high temperature, and the effects of acidity and high current density by applying and introducing the floating electrode technique (FET) to shaped nanoalloys. A severe detrimental effect was observed for high I/C ratios, with a behaviour contrasting reference commercial catalysts, while the negative effect of high temperatures was enhanced at low I/C. Based on this analysis, our study not only demonstrates a catalyst with enhanced ORR activity and specifically higher electrochemical surface area (ECSA) among oh-PtNi catalysts, but also provides valuable insights into overcoming MEA implementation challenges for these advanced fuel cell catalysts.