The role of nanoporosity in oxygen reduction reaction under elevated mass transport: Porous vs core-shell

Abstract

Reliable assessment of electrocatalytic performance of novel materials to determine the oxygen reduction (ORR) activity plays a pivotal role in systematic-driven design of tailored composites. Unfortunately rotating disc electrode technique, typically employed for this purpose, is incapable to accurately predict the behaviour of promising candidates in membrane electrode assemblies (MEAs) which are finally used in fuel cells. Instead, miniature electrochemical setups based on floating electrode, which mimics MEA’s three-phase boundary active sites, has recently been recognized as an adequate diagnostics substitute. Compared to conventional RDE the working electrode operating under floating regime makes the acquisition of catalysts’ behaviour at low potentials easily achieved without being limited by the solubility and/or mass transport of O₂ in aqueous electrolyte. Accordingly, the present study employs a modified version of the floating electrode methodology (MFE) to accurately investigate the effect of electrocatalyst nanostructure on high-current density ORR performance. Two morphologically distinct platinum-based de-alloyed nanoparticle samples—porous and non-porous core–shell analogues—are compared. The analysis reveals that at the high current density region (< 0.8 V vs RHE) porous nanoparticles demonstrate significantly worse ORR specific activities in comparison to core–shell analogues. On the other hand, the performance is reversed at low current densities (> 0.8 V vs RHE) supporting the results from the RDE analysis. The observed trend is attributed to a reduction in the utilization of active surface area in nanoporous catalysts with increasing overpotential.