Selective and sustainable nitrate-to-ammonia conversion with near-unity faradaic efficiency: Sb-induced phase evolution in Ti-supported Cu-Co spinel and HER suppression

The electrochemical nitrate reduction to ammonia (NRA) offers an environmentally friendly solution to nitrate pollution while producing the vital chemical ammonia. In this work, we design and synthesize a novel electrocatalyst with a multiphase crystal structure supported on a dimensionally stable Ti substrate (CuCo y O x -Sb z /Ti) via a facile sol-gel process for NRA. The study showed that while all Cu ratios showed measurable activity, CuCo 2 O x exhibited prominent spinel reflections consistent with the Cu 0.3 Co 2.7 O 4 reference pattern and provided the optimal base for antimony addition. XRD peak shifts and changes in the surface Co 2+ /Co 3+ ratio derived from XPS are consistent with possible copper incorporation into a Co 3 O 4 -derived spinel. The electrochemical properties obtained from LSV and EIS show that the CuCo 2 O x -Sb 0.2 electrocatalyst has the highest apparent NRA activity among the investigated Cu-Co-Sb compositions (β c = 120 mV.dec -1 ) and the lowest charge transfer resistance (R ct = 1.57 Ω cm 2 ). CuCo 2 O x -Sb 0.2 exhibits excellent NRA performance, achieving an impressive NH 3 yield rate of 20,790 μg.h -1 .cm -2 and a FE of 99.4% at -0.8 V vs. RHE in alkaline electrolyte. The coexistence of the Cu-containing spinel and CuSb 2 O 6 phases may contribute to the enhanced NRA performance. During the separate 24-h electrolysis test, the electrocatalyst-maintained nitrate-to-ammonia conversion and reached an NH 3 selectivity of 98.4%, despite measurable metal dissolution. During the 24-h full-cell test, at a current density of 100 mA cm -2 , the cell potential remained around 3.75 V. This behavior supports stable operation of the system during simultaneous NRA and OER.