pH Effect on Electrochemistry of Nitrogen-Doped Carbon Catalyst for Oxygen Reduction Reaction

In this work, the effect of pH on a nitrogen-doped ordered mesoporous carbon catalyst for the oxygen reduction reaction (ORR) is extensively investigated. Electrochemical methods, including cyclic voltammetry (CV), rotating ring-disk electrode (RRDE), and cathodic stripping voltammetry, are applied to investigate the electrochemical behavior in electrolyte solutions of different pHs (0-2, 7, 12-14). The CV result reveals that nitrogen-doped carbon has a variety of enriched reversible redox couples on the surface, and the pH has a significant effect. Whether these redox couples are electrochemically active or inactive to the ORR depends on the electrolyte used. In acid media, an oxygen molecule directly interacts with the redox couple, and its reduction proceeds by the surface-confined redox-mediation mechanism, yielding water as the product. Similarly, the first electron transfer in alkaline media is achieved by the surface-confined redox-mediation mechanism at the higher potentials. With decreasing potential, another parallel charge transfer process by the outer-sphere electron transfer mechanism gets pronounced, followed by parallel 2-e and 4-e reduction of oxygen. The proposed mechanisms are well supported by the following electrochemical results. At high potentials, the Tafel slope remains unchanged (60-70 mV dec(-1)) at all investigated pHs, and the reaction order of proton and hydroxyl ions is found to be 1 and -0.5, respectively, in acid and alkaline media. The electron transfer number is similar to 4 at high potentials in both acid and alkaline media; however, at higher pHs, it shows a considerable decrease as the potential decreases, indicating the change in the reaction pathway. Finally, the nitrogen-doped carbon catalyst shows performance in alkaline media superior to that in acid media. Such a gap in performance is rationalized by considering the chemical change in the surface at different pH values.