Nano-Polycrystalline Cu Layer Interlaced with Ti³⁺-Self-Doped TiO₂ Nanotube Arrays as an Electrocatalyst for Reduction of Nitrate to Ammonia

The electrochemical nitrate reduction reaction (NO 3 RR) is considered as a promising strategy to degrade nitrate-containing wastewater and synthesize recyclable ammonia at atmospheric pressure and room temperature. In this work, the copper oxides-derived nano-polycrystalline Cu (NPC Cu) was integrated with Ti 3+ -self-doped TiO 2 nanotube arrays (NTA) to fabricate the NPC Cu/H–TiO 2 NTA. Ti 3+ -self-doped TiO 2 NTAs and the NPC Cu facilitate electron transfer and mass transportation and create abundant active sites. The unique nanostructure in which Cu nano-polycrystals interlace with the TiO 2 nanotube accelerates the electron transfer from the substrate to surface NPC Cu. The density functional theory calculations confirm that the built-in electric field between Cu and TiO 2 improves the adsorption characteristic of the NPC Cu/H–TiO 2 NTA, thereby converting the endothermic NO 3 – adsorption step into an exothermic process. Therefore, the high NO 3 – conversion of 98.97%, the Faradic efficiency of 95.59%, and the ammonia production yield of 0.81 mg cm –2 h –1 are achieved at −0.45 V vs reversible hydrogen electrode in 10 mM NaNO 3 (140 mg L –1 )–0.1 M Na 2 SO 4 . This well-designed NPC Cu/H–TiO 2 NTA as an effective electrocatalyst for the 8e – NO 3 RR possesses promising potential in the applications of ammonia production.