Novel Denitrification Fuel Cell for Energy Recovery of Nitrate-N and TN Removal Based on NH₄⁺ Generation on a CNW@CF Cathode

NO₃^- is an undesirable environmental pollutant that causes eutrophication in aquatic ecosystems, and its pollution is difficult to eliminate because it is easily converted into NH₄⁺ instead of N₂. Additionally, it is a high-energy substance. Herein, we propose a novel denitrification fuel cell to realize the chemical energy recovery of NO₃^- and simultaneous conversion of total nitrogen (TN) into N₂ based on the outstanding ability of NH₄⁺ generation on a three-dimensional copper nanowire (CNW)-modified copper foam (CF) cathode (CNW@CF). The basic steps are as follows: direct and highly selective reduction of NO₃^- to NH₄⁺ rather than to N₂ on the CNW@CF cathode, on which negative NO₃^- ions can be easily adsorbed due to their double-electron layer structure and active hydrogen ([H]) can be generated due to a large number of catalytic active sites exposed on CNWs. Then, NH₄⁺ is selectively oxidized to N₂ by the strong oxidation of chlorine free radicals (Cl^•), which originate from the reaction of chlorine ions (Cl^-) by photogenerated holes (h⁺) and hydroxyl radicals (OH^•) under irradiation. Then, the electrons from the oxidation on the photoanode is transferred to the cathode to form a closed loop for external power generation. Owing to the continuous redox loop, NO₃^- completely reduces to N₂, and the released chemical energy is converted into electrical energy. The results indicate that 99.9% of NO₃^- can be removed in 90 min, and the highest yield of electrical power density reaches 0.973 mW cm^-2, of which the nitrate reduction rates on the CNW@CF cathode is 79 and 71 times higher than those on the Pt and CF cathodes, respectively. This study presents a novel and robust energy recycling concept for treating nitrate-rich wastewater.