Atomic-Level Tuning of Graphite Skeletal Structure via N-Doping for Selective Electrocatalytic Oxygen Reduction in Acidic Solutions

Carbon and its allotropes are popular as metal-free catalysts in various practical applications. Hydrogen peroxide (H₂O₂) is an important chemical from home disinfectants to rocket fuels. The selective electrochemical two-electron reduction of oxygen (2e^–-ORR) has been considered a green and clean alternative to the conventional anthraquinone process. This study involves the development of low-cost, robust, and efficient N-doped graphite (NGr) electrocatalysts for the on-site generation of H₂O₂. NGr was synthesized from commercial graphite and urea as nitrogen sources by thermal annealing under various conditions. Various state-of-the-art techniques, including X-ray photoelectron spectroscopy, were employed to reveal the underlying structure of NGr prepared with substantial amounts of pyridinic-N (pN) and pyrrolic-N (prN) sites. The catalytic performance of these materials was tested with hydrodynamic voltammetry of ORR in H₂SO₄ solutions at a rotating ring disk electrode. The onset potential, electrochemically active surface area, number of electrons involved, Tafel slope, reaction rate constant, and other parameters related to the ORR were evaluated and compared. The selectivity of 2e^–-ORR varied from 20% to 60%, and sparsely distributed pN sites were found to support the 2e^–-ORR. The origin of ORR selectivity of the NGr was elucidated by density functional theory calculation of electron density distribution using the Gaussian16 software package.