Understanding the electrolytic generation of sulfate and chlorine oxidative species with different boron-doped diamond anodes

Abstract

The electrochemical generation of several oxidative species was studied at the surfaces of five commercial boron-doped diamond anodes with different doping levels (100–8000 ppm). These insights can open the possibility of tailoring anodes for a more efficient application in environmental remediation processes. All materials evaluated were characterized by linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, contact angle, and scanning electron microscopy, as well as by bulk electrolysis. As a result, it was confirmed that the boron doping level influences the physical and electrochemical properties of the electrodes, indicating distinct behavior of the electrodes on the production of chlorine and sulfate oxidative species. The higher the boron doping, the lower is the crystallite size, and the higher is the conductivity, the hydrophilic behavior, and the electron-transfer activity. Voltammetric characterization demonstrates that low boron doping favors the formation of hydroxyl radicals, while high doping levels favor the direct electrochemical oxidation of sulfate or chloride. Moreover, when operating at high overpotentials in bulk electrolysis (typical conditions in environmental applications), the formation of chlorine and sulfate oxidative species is favored at low boron doping levels. This behavior is attributed to the very efficient mediated formation of these oxidants from the hydroxyl radicals, whose production is promoted with these electrodes at those conditions. It means that only operating at much softer conditions, the unique direct generation of hydroxyl oxidant occurs, opening a way for the potential prevention of perchlorate formation during disinfection by using highly boron-doped diamond anodes.