Characterization of Structural Corrosion of Graphite in Oxygen Evolution Reaction by Differential Electrochemical Mass Spectrometry

Electrocatalytic oxygen evolution reaction (OER) is not only a half-reaction of water-splitting to produce hydrogen, but also an important half-reaction of electrocatalytic carbon dioxide reduction, nitrogen reduction, nitrate reduction, organic small molecule reduction, and metal-air batteries. The OER process is driven by a four-electron mechanism. Due to the slow and complex kinetics of the electrocatalytic OER and its strong oxidation characteristics, studying the oxidative reconstruction rules and catalytic mechanisms of electrode materials in the OER process is of great significance to improve the efficiency of the OER. As a kind of metal-free electrode materials, carbon materials are widely used in electrocatalysis, which has become a potential electrocatalyst for OER due to their low price, abundant reserves, and high activity stability. At present, oxygen-containing functional groups on the surface of carbon materials have been proven to be active sites for OER. However, the deactivation mechanism of carbon materials at higher oxidation potentials during the OER process remains unclear due to a lack of understanding of the active site evolution mechanism. Correctly identifying the active sites of carbon materials under oxygen evolution conditions has become a research hotspot in the field. However, ex-situ characterization techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) are difficult to reflect the catalytic behavior of carbon materials under working conditions. In this study, differential electrochemical mass spectrometry (DEMS) was used to monitor the structural changes of carbon material during the OER process with graphite as the model material. The effects of applied voltage and pH value of electrolyte on graphite oxidation were investigated respectively. By changing the applied potential and the pH value of the electrolyte, we found that graphite firstly oxidizes itself to form oxygen-containing functional groups under acidic (pH 0), neutral (pH 7), and alkaline (pH 14) conditions. As the potential increases, the graphite begins to produce CO₂ and CO at 1.6 V vs. RHE over the full pH value range, and the content of CO₂ gradually augments as the potential increases. The generation potential of O₂ is higher than that of CO₂ under acidic condition, and the opposite is true under alkaline condition. Therefore, graphite can be used as an OER catalyst under alkaline condition in a certain potential range. This work not only reveals the structural transformation rules and corresponding evolution products of carbon materials in the full pH value range under electrochemical oxidation conditions, but also proposes a feasible potential operating range when carbon materials are used as OER catalysts.