Insight into the valence state of sisal-like MoO2 nanosheet arrays for N2 electrolysis

Abstract Developing efficient electrocatalysts for direct N2 electrolysis that includes two key reactions of oxygen evolution reaction (OER) at the anode and nitrogen reduction reaction (NRR) at the cathode is an appealing yet challenging task. Herein, Ni foam (NF) supported sisal-like porous MoO2 nanosheet array (MoO2/NF) was synthesized through a facile hydrothermal and subsequent annealing process. It is shown that in the NRR process the Mo4+ in MoO2/NF is more favorable in an associative mechanism of NRR than Mo6+ in MoO3/NF, thus yielding an impressive ammonia electrosynthesis activity (NH3 yield rate: 9.56 μg h−1 cm−2; Faradaic efficiency: 5.14%) in 0.1 M KOH. Moreover, the sisal-like porous nanosheet array structure not only provides a high surface area for abundant accessible active sites, but also limits the coalescence of gas bubbles, enabling active sites better exposed to nitrogen by removing the small gas bubbles rapidly. The MoO2/NF also shows excellent performance for the OER in 0.1 M KOH, exhibiting a small overpotential of 340 mV to realize a current density of 10 mA cm−2. In two-electrode full N2 electrolysis, a remarkable ammonia production performance with an ammonia yield of 5.73 μg h−1 cm−2, an FE of 15.8%, and good long-term stability for at least 24 h is obtained.