Conductivity-mediated in situ electrochemical reconstruction of CuO for nitrate reduction to ammonia

The electrocatalytic nitrate reduction reaction (NO₃RR) is an ideal NH₃ synthesis route with ease of operation, high energy efficiency, and low environmental detriment. Electrocatalytic cathodes play a dominant role in the NO₃RR. Herein, we constructed a carbon fiber paper-supported CuO nanoarray catalyst (CP/CuO) by an in situ electrochemical reconstruction method for NO₃^--to-NH₃ conversion. A series of characterization techniques, such as X-ray diffraction (XRD) and in situ Raman spectroscopy, unveil that CP/CuO is a polycrystalline-faceted composite copper nanocatalyst with a valence composition containing Cu⁰, Cu⁺ and Cu²⁺. CP/CuO shows more efficient NO₃^--to-NH₃ conversion than CP/Cu and CP/Cu₂O, which indicates that the coexistence of various Cu valence states could play a dominant role. CP/CuO with a suitable Cu²⁺ content obtained by adjusting the conductivity during the in situ electrochemical reconstruction process exhibited more than 90% faradaic efficiencies for the NO₃RR in a broad range of -0.3 to -1.0 V vs. RHE, 28.65 mg cm^-2 h^-1 peak ammonia yield, and stable NO₃RR efficiencies for ten cycles. These findings suggest that CP/CuO with suitable copper valence states obtained by fine-tuning the conductivity of the electrochemical reconstruction may provide a competitive cathode catalyst for achieving excellent activity and selectivity of NO₃^--to-NH₃ conversion.