Amorphous nickel–iron hydroxide nanosheets for effective electroreduction of nitrate to ammonia

Abstract

Nitrates leached from agricultural runoff and industrial wastewater seriously pollute surface and underground water. Converting nitrate (NO₃⁻) into ammonia (NH₄⁺) by electrochemical methods not only addresses these water pollution issues but also offers a route to produce ammonia. However, an insufficient availability of highly efficient electrocatalysts for selective synthesis of ammonia has hindered the development of this technique. Currently, although many studies exist where single/double metallic nanosheets have been developed to enhance the activity of nitrate reduction to ammonia, rarely has any work focused on exploring the influence of an amorphous single/double metallic hydroxide. Herein, we report on a facile method to prepare a series of nickel–iron hydroxide crossed-nanosheets with an amorphous structure. Electrocatalytic nitrate activity tests showed that the sample with an optimal Ni/Fe ratio (1 : 1) exhibited high selectivity (90.6%) and faradaic efficiency (74.8%) for ammonia synthesis. ¹⁵N isotope labelling experiments confirmed the origin of ammonia from nitrate. Both ¹H nuclear magnetic resonance (NMR) spectroscopy and UV-Vis spectrophotometry showed almost the same amount of generated ammonia. The online differential electrochemical mass spectrometry (DEMS) results revealed the intermediates during the electroreduction of nitrate to ammonia. XPS results demonstrated the significant role of oxygen vacancies in improving the electrocatalytic performance. Moreover, the structure and activity of materials could be maintained well after long-term testing.