Beyond Copper: Novel Ni Foam Catalysts for Sustainable Nitrate to Ammonia Electroreduction

Electrochemical nitrate reduction is considered a promising energy efficient approach to remove environmentally harmful nitrate from various types of wastewater while simultaneously producing a product with high added value: ammonia. One important factor to be accounted for is the choice of the catalyst, which is required not only to accelerate nitrate reduction but also to direct the product selectivity of the electrolysis toward ammonia production. To this end, herein, we demonstrate the fabrication of novel Ni foam catalysts produced by means of a dynamic hydrogen bubble template and additive assisted electrodeposition process. The resulting 3D foam morphology of the catalyst is demonstrated to crucially govern its overall catalytic performance. Post-electrolysis cross-sectional SEM-EDX analysis (K mapping) demonstrated complete wetting of the 3D foam structure by the electrolyte. Ni foams deposited within 20 s exhibited outstanding selectivity toward nitrate electroreduction: more than 95% of the Faradaic efficiency of ammonia production was achieved in the particularly low potential range from -0.1 to -0.3 V vs. RHE. Hydrogen was found to be the only minor by-product of the nitrate reduction. Intriguingly, no other nitrogen containing products (e.g., NO, N2O, and N2) formed during electrolysis, thus indicating a highly efficient nitrate-to-ammonia conversion process. This significant improvement over the use of a planar Ni foil reference (33% FE at -0.3 V vs. RHE) is attributable to (i) the effective suppression of the HER in this potential regime and (ii) a high surface density of active sites for the nitrate reduction formed during Ni foam electrodeposition under extreme experimental conditions, e.g., at an applied geometric current density of -3 A cm-2. Trapping intermediates inside the primary macroporosity contributed to the excellent catalytic performance of the Ni foams. Identical location (IL) SEM analyses demonstrated the excellent structural stability of the novel Ni foam during extended catalyst stressing. These superior characteristics have not previously been reported for Ni and Ni rich catalysts, thus making the novel foam type of catalyst a highly promising candidate for truly selective and energy efficient nitrate-to-ammonia electroreduction and a promising alternative to mature copper-based catalysts.