Nanoparticle-Agminated Copper Nanospheres Featuring Tailored Grain Boundary Densities for Enhanced Ammonia Electrosynthesis

Abstract The electrochemical nitrate reduction reaction (NITRR) represents a high-potential avenue for the sustainable synthesis of ammonia (NH3) and for helping to maintain the balance of the nitrogen cycle. Nevertheless, the quest for highly efficient electrocatalysts continues to pose a persistent challenge. Herein, we report a combined chelated co-precipitation and electrochemical reduction strategy for in situ surface reconstruction on carbon nanotubes, converting Cu4(OH)6SO4 precursors (x-Cu4(OH)6SO4/CNTs, x = 0.9, 1.0, 1.1) into nanoparticle-aggregated copper nanospheres (x-Cu NS/CNTs). Notably, adjusting alkali concentration during co-precipitation enables precise control over precursor crystallinity, leading to post-electroreduction x-Cu NS catalysts with tailored grain boundary densities on CNTs. Thanks to the abundant grain boundaries on its surface, which offer highly effective catalytic active sites for the electroreduction of nitrate, the resulting 1.0-Cu NS/CNT catalyst exhibits an outstanding Faradaic efficiency for NH3 synthesis (FENH3), reaching 91.3% at –0.9 V vs RHE. Moreover, a homemade NITRR‖ZnOR battery system, utilizing a 1.0-Cu NS/CNTs cathode paired with a Zn foil anode, achieved a peak power density of 4.2 mW cm–2. This research offers an effective approach for the controllable fabrication of copper-based electrocatalysts with abundant surface grain boundaries, aimed at enhancing the electroreduction of nitrate to NH3.