Defect-driven cobalt hydroxide with capacitive properties for enhanced nitrogen reduction catalysis

Ammonia (NH₃) with high hydrogen capacity (17.6 wt%) has drawn worldwide attention as an alternative energy carrier. Among sustainable NH₃ synthesis technologies, the electrochemical N₂ reduction reaction (NRR) driven by renewable energy generation is considered as an attractive pathway for artificial N₂ fixation. Inexpensive and efficient catalysts for NRR involving six electron-proton transfers are desired in the developing ammonia-based energy systems. In this work, a non-noble metal electrocatalyst of defect-rich Co(OH)₂ nanosheets grown on Co foam is obtained by an electrochemical oxidation process, showing a high NH₃ yield of 20.25 μg h⁻¹ cm⁻². A noticeable capacitive property of 924 mF cm⁻² demonstrates the successful formation of Co(OH)₂ with a high electrochemical surface area, and the high availability for Na⁺ adsorption on the surface of as-formed Co(OH)₂. Theoretical calculations show that oxygen vacancies benefit to reduce the energy barrier of rate-determining step for NRR on the surface of Co(OH)₂. The Na ⁺ adsorption on the surface of Co(OH)₂ with oxygen vacancies will further facilitate the suppression of hydrogen evolution reaction (HER) and promote the selectivity of NRR. The attention to the adsorption of alkali metal ions on the surface of capacitive materials will broaden the horizon for rational designs of NRR catalysts.