Impacts of Perchloric Acid, Nafion, and Alkali Metal Ions on Oxygen Reduction Reaction Kinetics in Acidic and Alkaline Solutions

Abstract

Fundamental understandings on the impacts induced by anions and cations on oxygen reduction reaction (ORR) are of great interest in designing more efficient catalysts and identifying reasons for discrepancies in activities measured in different protocols. In this study, the specific adsorption of ClO₄^–, Nafion ionomer, and cations on Pt/C, Pd/C, and transition metal, N codoped carbon-based (Me–N–C) catalysts, and their effects on the ORR kinetics were systematically investigated. It was found that ClO₄^– had a negligible impact on the ORR activity of Pt/C possibly due to its weak adsorption. Nafion ionomers, on the other hand, showed a significant poisoning effect on the bulk Pt electrode. Its impact on Pt/C, however, is negligible even with a very high I/C ratio (1.33) in acidic solutions. The three catalysts showed different behaviors in alkaline solutions. The noncovalent interaction between hydrated cations and surface OH groups was found on Pt/C and had an obvious impact on the ORR kinetics. This noncovalent interaction, however, was not observed on Pd/C, which showed the same ORR activity in all three electrolytes (LiOH, NaOH, and KOH). The ORR activity of Me–N–C increased following the order of KOH < NaOH < LiOH. This trend is totally opposite to that of Pt/C. The mechanisms for the material-dependent activity trend in different cation solutions were discussed