Co–Ni bimetallic oxide with tuned surface oxygen vacancies efficiently electrocatalytic reduction of nitrate to ammonia

The electrocatalytic reduction reaction of nitrate (NO₃^-) to ammonia (NH₃) provides an efficient and clean NH₃ production method, which has the potential to replace the traditional industrial preparation methods. However, the limited activity and Faraday efficiency (FE) of existing catalysts impede the practical application of this technology. Herein, in this work, a high-performance catalyst with high NH₃ yield and FE was fabricated. Co-Ni bimetallic oxide (NiCo₂O₄) catalysts with tuned surface oxygen vacancies (OVs) contents were prepared by changing the heating rate during calcination, NiCo₂O₄ calcined at a heating rate of 5 °C/min (NiCo₂O₄-5) possessed the highest surface OVs content. Experimental studies showed that NiCo₂O₄ with higher surface OVs had better NO₃RR activity, inhibited the production of nitrite (NO₂^-), and exhibited higher selectivity to NH₃. Among prepared catalysts, NiCo₂O₄-5 demonstrated superior performance in electrocatalytic reduction of NO₃^- to NH₃, achieving a high NH₃ FE (94.4 %) and yield (193.2 mmol/h g^-1) at a suitable applied voltage. Besides, in situ Fourier transform infrared spectroscopy analysis suggested that NiCo₂O₄-5 preferentially followed the NO₃RR pathway as follows: *NO₃ → *HNO₃ → *NO₂ → *HNO₂ → *NO → *HNO → *N → *NH → *NH₂ → *NH₃.