The Role of Oxygen Vacancies in Copper Amended Titanium Dioxide-Carbon Nanofiber Electrodes in Electrochemical Nitrate Reduction in Water

Novel catalysts and supports are being explored to enhance electrocatalytic activity and Faradaic efficiency (FE) for nitrate reduction to ammonia. Copper has emerged as a promising catalyst because of its ability to activate nitrate reduction to nitrite, and TiO2 has emerged as a promising electrode support because it can be treated to produce oxygen vacancies (OVs) that enhance nitrate adsorption at defect sites. However, the interplay between OVs in TiO2, copper species, and nitrate remains insufficiently understood, limiting improvements in electrochemical-based nitrate removal from drinking water. To address this challenge, a conductive electrospun carbon nanofiber (CNF) containing TiO2 (TiO2/CNFs) was deposited with Cu and subsequently reduced under H2 at different temperatures to create electrodes with various OV densities. Results show an increasing trend in activity, FE, and NH4+ selectivity with increasing reduction temperatures and mass loadings of TiO2; the nitrate reduction activity (5.9 L gCu–1 min–1) of the best-performing electrode (i.e., Cu/50%TiO2/CNF@700C, containing 50 wt % TiO2 and reduced at 700 °C) is 10 times that of an electrode prepared using the same Cu loading and deposition method but on activated carbon cloth support. Results from X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy indicate that rutile content increases with reduction temperature and that Cu substitutes into the TiO2/rutile lattice; the results also suggest that Cu substitution creates OVs that enhance nitrate adsorption at electrode reactive sites. This work is pivotal because it identifies important design principles for developing highly active and efficient TiO2-based electrodes for water treatment.