Energy-Efficient and Self-Powered Green Ammonia Synthesis by Electrochemical Nitrate Reduction Combined with Hydrazine Oxidation

Abstract

To achieve the global goal of carbon neutrality, recently, emphasis has been placed on developing green ammonia production method to replace the Haber–Bosch process. Nitrate reduction reaction (NO3RR) has received considerable attention, especially for electrochemically producing ammonia from nitrate and simultaneously purifying wastewater. This study first demonstrates that the combination of NO3RR with hydrazine oxidation reaction (HzOR) is an energy efficient green ammonia production method, which overcomes the sluggish water oxidation limitation. Tungsten phosphide (WP) nanowires (NWs) are prepared as cathode NO3RR electrocatalysts, which exhibit a high Faradaic efficiency in both neutral (≈93%) and alkaline (≈85%) media. Furthermore, they show a high bifunctional activity in anodic reactions and exhibit a low potential 0.024 V for generating a current density of 10 mA cm−2 in HzOR. The overall NO3RR-HzOR required an impressively low potential of 0.24 V for generating a current density of 10 mA cm−2; this potential is much lower than those required for NO3RR-OER (1.53 V) and NO3RR-UOR (1.31 V). A self-powered ammonia production system, prepared by assembling an NO3RR-HzOR with a perovskite solar cell, displays a high ammonia production rate of 1.44 mg cm−2 h−1. A single PV cell provides enough driving voltage in the PV-EC due to low required potential. This system facilitates unassisted green ammonia synthesis with a low energy consumption and also allows upcycling of wastewater to produce useful fuel.