Effect of local pH change on non-PGM catalysts – a potential-dependent mechanistic analysis of the oxygen reduction reaction

Abstract

Typically, the ORR shows higher activity in alkaline electrolytes than in acidic media on either platinum-group metal (PGM) or non-PGM catalysts, known from their outer sphere electron transfer (OSET) mechanism via adsorbed hydroxyl species on the platinum–carbon electrode. However, poor understanding of non-PGM catalysts' active sites and the electrical double-layer structure impede the deciphering of the ORR mechanism in alkaline and neutral electrolytes. The ORR on Fe/PPy-900 (synthesized from an Fe–polypyrrole composite) for a full pH range of electrolytes (prepared from HClO₄, KClO₄ and KOH) was studied using RRDE voltammetry. The ORR mechanism in intermediate pH electrolytes (5 < pH < 11) is elusive due to the concomitant rise in the surface (local) pH with applied potential. The quantitative estimation of potential-dependent local pH by leveraging the RRDE technique elucidates the electrical double layer structure followed by the ORR mechanism in intermediate pH electrolytes. The OSET mechanism operates in the intermediate pH electrolyte for the first 2-electron transfer, followed by the ISET mechanism for the subsequent reduction of adsorbed HO₂⁻ ions. The smaller ring current in intermediate pH electrolytes can be rationalised by the kinetic facility of the H₂O₂ reduction reaction. The increasing ring current in the electrolytes having a pH value above 11 is attributed to the inhibition of HO₂⁻ binding on the active sites due to the active sites being blocked by OH⁻ ions. The increasing ring current with pH (in the alkaline region) supports the above prediction of ISET via the chemisorbed HO₂⁻ intermediate.