Ni-loaded Co-NC catalysts for promoting electrocatalytic nitrate reduction to ammonia†

Abstract

The Haber–Bosch process, the traditional method of ammonia synthesis, uses hydrogen derived from steam reforming of hydrocarbons; and involves harsh operating conditions of high temperatures (300–600 °C) and pressures (200–400 atm), expending a vast amount of energy each year. Recently, there has been a lot of interest in the electrochemical nitrogen reduction process (NRR) to NH₃, which is inspired by natural microbial nitrogen fixation. However, the stable NN violent hydrogen evolution reaction hindered the development of the NRR. In comparison, the electrocatalytic nitrate reduction reaction (NO₃RR) has significant advantages. The much lower dissociation energy of NO (204 kJ mol⁻¹) is required; nitrate is widespread in surface water. Herein, an electrocatalyst loaded with Ni onto CoZn@ZIF by a simple impregnation method is reported, which possesses a nitrogen-doped graphitic carbon structure after pyrolytic carbonization. Detailed experiments showed that the NiCo-NC catalyst significantly accelerated the NO₃RR compared to Co-NC. NiCo-NC exhibited remarkable NO₃RR activity. At −0.6 V and −1.1 V, ammonia yields of 5.01 mg cm⁻² h⁻¹ and 10.12 mg cm⁻² h⁻¹ were obtained, with FEs reaching 92.75% and 96.65%, respectively. The catalyst showed excellent electrochemical stability in 24-hour electrolysis experiments and five-cycle stability tests. Meanwhile, ¹⁵N isotope labeling experiments further verified the source of N in NH₄⁺ from NO₃⁻.