Effect of High Local Diffusive Mass Transfer on Acidic Oxygen Reduction of Pt Catalysis

In this study, we utilized a platinum ultramicroelectrode (UME) as a model platform for platinum electrocatalysts in acidic electrolytes to study the effects of local mass transfer on the oxygen reduction reaction (ORR), which plays a significant role in fuel cell with reduced pt loading. Finite element simulations showed that the UME exhibits size-dependent ultrathin diffusion layers during the electrochemical process. Submicron-scale UMEs can achieve ultrahigh localized mass transfer, which is unattainable through other experimental techniques. By conducting catalytic experiments under various mass transfer conditions, we found that the mass transfer limiting current is significantly lower than the value predicted by the four-electron process equation. Additionally, the apparent electron transfer number (napp) decreases as the mass transfer coefficient (m0) increases. Furthermore, as m0 increases, the half-wave potential shifts toward more negative values, allowing for the evaluation of the intrinsic activity of the catalysts over a broader potential range. Due to the UME technique's capacity to conveniently control local mass transfer, we foresee its potential application in understanding the effects of chemical microenvironments on complex electrochemical reactions, including ORR and other processes.