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 ∼20%, respectively. In the sulfate electrolyte, the primary reactive oxygen species (ROS) identified were OH radicals (Faradaic efficiency η = 2.4%) with trace levels of ozone and hydrogen peroxide (η < 0.01%) at Sb-SnO₂. In contrast, the primary ROS at Ni-Sb-SnO₂ was ozone (η = 9.3%) followed by OH radicals (η = 3.7%). In the chloride electrolyte, the production of hypochlorite (OCl ^-) was higher (η = 0.73%) than that of ozone (η = 0.13%) at Sb-SnO₂, whereas the level of ozone (η = 13.6%) was much higher than that of hypochlorite (η = 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.