“Pseudo-Haber–Bosch” process for the production of green ammonia using a defect-rich and active Bi2MoO6/Cu8S5 electrocatalyst via an interrupted chemical reaction

Abstract

Developing catalysts that combine improved activity, selectivity, and stability for the electrochemical nitrogen reduction reaction (eNRR) remains a critical challenge for advancing NH₃ electrosynthesis. This study introduces a novel approach to green NH₃ synthesis via a “pseudo-Haber–Bosch process” that uses a high-performance, defect rich Bi₂MoO₆/Cu₈S₅ catalyst grown in situ on an NF (nickel foam) by a one-step hydrothermal method. The Bi₂MoO₆/Cu₈S₅ catalyst achieved a remarkable NH₃ yield rate of 6757 µg h⁻¹ cm⁻² or 2609 µg h⁻¹ mg_cat⁻¹ and a Faradaic efficiency (FE) of 57.9% at −0.6 V vs. RHE in 0.5 M Na₂SO₄, surpassing not only its Mo-free counterpart (Bi₂O₃/CuS₂), with an NH₃ formation rate of 3374 µg h⁻¹ cm⁻² and an FE of 28.9%, but also most existing catalysts reported in the literature. The addition of Mo, for an interrupted chemical reaction, introduced abundant ion vacancies, along with multiple redox-active sites, including V_O²⁺, V_Cu²⁻, and Bi_Bi²⁺, which acted as donors (Lewis bases) or acceptors (Lewis acids). These sites worked together to improve N₂–catalyst interactions, adsorption, and activation, promoting effective N₂ fixation under ambient conditions. An isotopic labeling time-dependent experiment using ¹⁵N₂ confirmed that the nitrogen atoms in the produced NH₃ came exclusively from the supplied ¹⁵N₂, ruling out contamination. The stack-cell reactor used in this study achieved a notable NH₃ formation rate of 2016 µg h⁻¹ cm⁻², with an FE of 38.8%, an energy efficiency (EE) of 48.5%, and a specific energy consumption (EC) of 23.2 kWh kg⁻¹ of NH₃ at an applied voltage of 1.9 V. The Bi₂MoO₆/Cu₈S₅ catalyst demonstrates outstanding selectivity and stability for sustainable electrochemical NH₃ synthesis, marking a significant step toward practical, industrially viable eNRR technology.