Chloride-Mediated Transformation of Copper Nanoparticles to Nanosponges for Nitric Acid Reduction at Low Concentrations.

Acidic nitrate electroreduction reaction (NO₃ ^-RR) offers a promising route for a sustainable nitrogen cycle. However, achieving high selectivity and efficiency under low-concentration acidic conditions remains challenging. Herein, it is demonstrated that Cu nanosponge can adsorb low-concentration nitric acid (HNO₃) and efficiently convert it to ammonia (NH₃). The Cu nanosponge is prepared by Cl^--induced reconstruction of porous Cu nanoparticles obtained through dealloying. In a Cl^--containing HNO₃ solution, porous Cu nanoparticles undergo chemical oxidation to form CuCl, which reconstructs into a nanosponge through migration and electrochemical reduction, consisting of nanoparticle-supported nanosheets. The nanosponge features abundant porous structures and numerous nanoparticle-nanosheet interfaces, creating a large active surface area and providing adsorption and reaction sites for NO₃ ^-. The optimized Cu nanosponge exhibits a 92% FE for NH₃ at -0.4 V versus RHE and 90% yield of NH₃ in 0.03 M HNO₃, significantly outperforming Cu nanoparticle (only 66 and 47%). In situ Raman spectroscopy confirms that the nanosponge structure not only enhances NO₃ ^- adsorption but also stabilizes the key NO₂ ^- intermediate. Furthermore, industrial wastewater is simulated to convert low concentrations of nitrate into ammonium nitrate products, which are applied to plant cultivation, effectively promoting plant growth.