Electrolytic nitrate reduction using Co3O4 rod-like and sheet-like cathodes with the control of (220) facet exposure and Co2+/Co3+ ratio

Co3O4 rod and sheet-like films with different crystal facet exposure and Co2+/Co3+ ratio are synthesized on Ti substrate through a facile hydrothermal method with the adjusting of hydrothermal pressure and duration. Low pressure and short hydrothermal duration is likely to obtain rod-like film with almost pure (220) facet exposure, while the sheet-like film with the mixture of (220) and (222) facets is formed with relatively high pressure. Moreover, the Co2+/Co3+ ratio is 2.067 for Co3O4 rods and 1.717 for Co3O4 sheets. The FT-IR and the XPS results indicate that the improvement of Co2+/Co3+ ratio possibly results from the doping of CO32−/OH−, and the introduction of oxygen vacancies. The films are employed as cathodes for electrocatalytic NO3−-N reduction. Because of the larger electrochemical active area and the lower internal resistance, better conversion of NO3−-N to NH4+-N is obtained by Co3O4 sheets film at constant voltage condition. However, the results are opposite when the constant current is applied. The larger exposure of (220) facet and the more amount of Co2+ and oxygen vacancies result in the better NO3−-N removal and NH4+-N conversion kinetics of Co3O4 rods film than that of Co3O4 sheets.