Electrochemical behavior of N and Ar implanted highly oriented pyrolytic graphite substrates and activity toward oxygen reduction reaction

Abstract

In this paper nitrogen and argon implanted highly oriented pyrolytic graphite (HOPG) electrodes have been studied, with the aim of distinguishing whether the electrocatalytic activity of the modified electrodes is preferentially driven by chemical defects or by morphological defects. Modified HOPG electrodes have been prepared by ion implantation and fully characterized by photoemission spectroscopy and scanning tunneling microscopy. Several different N-based defects have been identified and characterized in terms of their thermal stability. The modified electrodes were electrochemically characterized considering the electron-transfer kinetics of two redox probes Ru(III)(NH3)6Cl3 and K4Fe(II)(CN)6: both types of electrodes show an increased performance in terms of standard rate constant k0 with respect to pristine HOPG for the oxidation of Fe(CN)64− and this effect has been exclusively related to a morphological effect. N-implanted HOPG electrodes show an increased reactivity toward oxygen reduction reaction (ORR), the onset potential being more positive with respect to both Ar-implanted HOPG and pristine HOPG (0.09 and 0.28V, respectively) and follow a four electrons reduction pathway to H2O. The results indicate that morphological as well as chemical defects are important factors for influencing the ORR kinetics. However, the enhanced ORR activity at N-HOPG suggests a pivotal role played by the N-based chemical defects.