Unveiling the Reaction Mechanism of Electrochemical Nitrate Reduction to Ammonia on a Cu/Cu ₂ O Heterojunction Catalyst

Sustainable ammonia synthesis is critical for advancing the hydrogen economy, as ammonia functions as a carbon-neutral hydrogen carrier with high volumetric energy density and benefits from well-established global infrastructure for storage and transport. Electrochemical nitrate reduction to ammonia (e-NO3RRA) emerges as an environmentally benign approach by utilizing renewable electricity to convert nitrate contaminants-common in agricultural runoff and industrial effluents-into valuable ammonia, thereby addressing both energy and environmental challenges. In this study, we engineered a multiphase Cu/Cu2O catalyst supported on polyvinylidene fluoride (PVDF), exploiting PVDF’s chemical inertness and porous morphology to provide a stable and high-surface-area platform to evaluate the catalytic performance of the multiple Cu electrocatalyst. Comprehensive characterization using advanced techniques confirmed the successful fabrication and structural integrity of the Cu/Cu2O/PVDF electrodes. Mechanistic insights were obtained through in situ surface-enhanced Raman spectroscopy (SERS) and Fourier transform infrared (FTIR) spectroscopy, complemented by density functional theory (DFT) calculations. These analyses revealed that the Cu2O phase preferentially facilitates the initial nitrate deoxygenation step to nitrite by stabilizing key reaction intermediates via in situ formation of Cu(OH)2. Concurrently, DFT calculations revealed that the metallic Cu phase promotes efficient hydrogen adsorption and subsequent hydrogen spillover, which are essential for the hydrogenation of nitrite intermediates to ammonia. This synergistic interaction between Cu, Cu2O, and Cu(OH)2 phases optimizes the adsorption–desorption energetics, thereby enhancing reaction kinetics and selectivity. Electrochemical evaluation demonstrated that the Cu/Cu2O/PVDF catalyst achieved a Faradaic efficiency (FE) of 85.62% for NH3 at −0.233 V, markedly surpassing Ag, Pd, Zn, Fe, and Mn-based PVDF catalysts. The superior performance is attributed to the minimized competing hydrogen evolution reaction (HER) and enhanced phase stability under operational conditions. Isotope-labeling experiments using 15N-nitrate and proton nuclear magnetic resonance (1H NMR) spectroscopy unequivocally confirmed that the ammonia produced originated exclusively from nitrate reduction, with no detectable hydrazine or other nitrogenous byproducts, indicating high reaction selectivity.