Non-precious Co3O4-TiO2/Ti cathode based electrocatalytic nitrate reduction: Preparation, performance and mechanism

The presence of high nitrate (NO3−) concentration in natural water constitutes a serious issue to the environment and human health. Therefore, the development of low-cost, stable non-precious metal catalysts is imminent for efficient NO3- reduction. In this study, we prepared a Co3O4-TiO2/Ti cathode via combining sol-gel and calcination methods and evaluated its performance for electrocatalytic NO3- reduction. The dispersion of the Co3O4 catalyst particles was improved by the addition of PVP to the coating liquid. The presence of anatase could effectively stabilize Co3O4 and prevent the releasing of toxic Co ions into the solution. The Co3O4-TiO2/Ti cathode with the optimized performance for NO3- reduction could be prepared by four times coating at calcination temperature of 500 °C. The electrocatalytic reduction of NO3- was negligibly impacted by solution pH in the range of 3.0–9.0, while it could be facilitated by elevating the current density from 2.5 to 25 mA cm2. Ammonium ions were the main final NO3- reduction product, and the presence of Cl- was capable to oxidize ammonium ions to N2 due to the electrochemical production of reactive chlorine species. The electrochemical analyses, scavenging experiments and density functional theory calculations collectively confirm that NO3- reduction was mainly induced by the Co2+–Co3+–Co2+ redox process instead of being directly resulted from the electrons generated at the cathode. Unlike noble metal (e.g., Pd and Ag) based catalytic reaction systems, in the present Co3O4 mediated electrocatalytic reaction process, atomic H* would more favorably turn to H2 by Heyrovsky and Tafel routes and therefore contributed marginally to the NO3- reduction. Generally, this study provided a new paradigm for designing the stable and cost-effective cathode for NO3- reduction.