Metal-organic framework-derived nitrogen-doped highly disordered carbon for electrochemical ammonia synthesis using N2 and H2O in alkaline electrolytes

Abstract

Ammonia (NH3) is considered an important chemical for both agriculture fertilizer and renewable energy. The conventional Haber-Bosh process to produce NH3 is energy intensive and leads to significant CO2 emission. Alternatively, electrochemical synthesis of ammonia (ESA) through the nitrogen reduction reaction (NRR) by using renewable electricity has recently attracted significant attention. Herein, we report a metal-organic framework-derived nitrogen-doped nanoporous carbon as an electrocatalyst for the NRR. It exhibits a remarkable production rate of NH3 up to 3.4 × 10−6 mol cm−2 h−1 with a Faradaic efficiency (FE) of 10.2% at −0.3 V vs. RHE under room temperature and ambient pressure using aqueous 0.1 M KOH electrolyte. Increasing the temperature to 60 °C further improves production rates to 7.3 × 10−6 mol cm−2 h−1. The stability of the nitrogen-doped carbon electrocatalyst was demonstrated during an 18-h continuous test with constant production rates. First principles calculations were used to elucidate the possible active sites and reaction pathway. The moiety, which consists of three pyridinic N atoms (N3) adjacent with one carbon vacancy embedded in a carbon layer, is able to strongly adsorb N2 and further realize N≡N triple bond dissociation for the subsequent protonation process. The rate-determining step of the NRR is predicted to be the adsorption and bond activation of N2 molecule. Increasing overpotentials is favorable for the protonation process during NH3 generation. Further doping Fe into the nitrogen-doped carbon likely blocks the N3 active sites and facilitates the hydrogen evolution reaction, a strong competitor to the NRR, thus yielding negative effect on ammonia production. This work provides a new insight into the rational design and synthesis of nitrogen-doped and defect-rich carbon as efficient NRR catalysts for NH3 synthesis at ambient conditions.