Dual role of anions on the stability and activity of PtCo electrocatalyst for the oxygen reduction reaction

Abstract

This study investigates the influence of electrolyte anions on the oxygen reduction reaction (ORR) activity and stability of PtCo alloy electrocatalyst under varying electrochemical conditions. Using a combination of rotating disk electrode (RDE) and gas diffusion electrode (GDE) setups, we examined the interactions between weakly adsorbing (ClO- 4), moderately adsorbing (HSO-4, H₂PO-4), and strongly adsorbing (Cl⁻) anions and their effects on electrocatalytic performance. Experimental results revealed that anion adsorption impacts both the catalytic site occupation and the dissolution rates of Pt and Co during accelerated durability testing (ADT). Specifically, moderate anion adsorption, as observed with H₂PO-4, provided a protective effect by reducing metal dissolution while partially blocking catalytic sites. Conversely, strongly adsorbing anions like Cl⁻ significantly degraded the electrochemical surface area (ECSA), leading to a substantial decline in ORR activity. The stability tests showed that the addition of H₃PO₄ resulted in an improvement in ORR activity after 10,000 cycles, highlighting its stabilizing effect. In contrast, Cl⁻ caused pronounced Pt dissolution and particle agglomeration, as evidenced by X-ray fluorescence (XRF) and transmission electron microscopy (TEM) analyses. X-ray photoelectron spectroscopy (XPS) further confirmed changes in the electronic state of Pt, with phosphoric acid showing the least alteration, correlating with enhanced catalytic stability. These findings emphasize the dual role of anions in influencing both the intrinsic activity and durability of PtCo electrocatalysts. The insights gained can inform the design of tailored electrochemical microenvironments for optimizing ORR performance in practical fuel cell applications.