Shift of the Reactive Species in the Sb SnO2-Electrocatalyzed Inactivation of E-coli and Degradation of Phenol: Effects of Nickel Doping and Electrolytes

The electrocatalytic behavior and anodic performance of Sb-SnO2 and nickel-doped Sb-SnO2 (Ni-Sb-SnO2) in sodium sulfate and sodium chloride electrolytes were compared. Nickel-doping increased the service lifetime by a factor of 9 and decreased the charge transfer resistance of the Sb-SnO2 electrodes by 65%. More importantly, Ni doping improved the electrocatalytic performance of Sb-SnO2 for the remediation of aqueous phenol and the inactivation of E. coli by a factor of more than 600% and similar to 20%, respectively. In the sulfate electrolyte, the primary reactive oxygen species (ROS) identified were OH radicals (Faradaic efficiency eta = 2.4%) with trace levels of ozone and hydrogen peroxide (eta < 0.01%) at Sb-SnO2. In contrast, the primary ROS at Ni-Sb-SnO2 was ozone (eta = 9.3%) followed by OH radicals (eta = 3.7%). In the chloride electrolyte, the production of hypochlorite (OCl-) was higher (7 eta = 0.73%) than that of ozone (eta = 0.13%) at Sb-SnO2, whereas the level of ozone (eta = 13.6%) was much higher than that of hypochlorite (eta = 0.24%) at Ni-Sb-SnO2. Based on the shift of the reactive species, the primary effect of Ni doping is to catalyze the six-electron oxidation of water to ozone and inhibit the competing one or two-electron oxidation of water (generation of OH radicals, hydrogen peroxides, and hypochlorites). A range of electrochemical and surface analyses were performed, and a detailed mechanism was proposed.