Water Oxidation to Hydrogen Peroxide Over a Super-Aerophilic Graphite Catalyst.

Abstract

Two-electron water oxidation reaction (2e-WOR) to produce hydrogen peroxide (H₂O₂) is an attractive anode reaction with several merits. It can be paired with several large-scale cathode reactions that produce valuable chemical substances in an electrochemical cell. However, high-performing and reliable 2e-WOR anodic catalysts are yet to be fully developed. In this work, a rationally designed, inexpensive, robust, and selective graphite catalyst electrode is presented, made by following the key principle mechanisms of 2e-WOR. First, an aerophilic graphite-based electrode is created to leverage the challenges posed by the four-electron WOR, where the generated O₂ from this reaction is kept onto the electrode surface to shift the O intermediates binding on graphite in the direction of improved H₂O₂ generation. An initial improvement in H₂O₂ selectivity of seven fold is observed, albeit with no improved H₂O₂ generation rates. The stunted H₂O₂ generation is ascribed to poor activity from pristine graphite, courtesy of less active sites and low intrinsic O₂ binding in the electrolyte environment. Second, to improve and balance graphite's activity and selectivity, the structure of graphite is altered via different elemental doping (with N, S, B, and P atoms), a method that allows the retention of the O₂ on the graphite surface. The super-aerophilic B-doped graphite catalyst (optimum) reaches a maximum Faraday efficiency (FE) of 60.6 ± 2.6% with a production rate of 26.7 ± 0.6 µmol min^-1 cm^-2 (85.9 ± 2.2 mA cm^-2 partial current density) and excellent stability of over 120 h. In tandem, cathodic H₂ co-production is demonstrated with an FE of above 90%. This approach demonstrates a rational approach to designing inexpensive and robust 2e-WOR anode catalysts for H₂O₂ and the possibility of its use in chemical co-production at the cathode.