Metal-Organic Framework-Derived Carbon Electrocatalysts for Nitrogen Reduction

Abstract

Alternatively, electrochemical synthesis of ammonia (ESA) through the nitrogen reduction reaction (NRR) by using renewable electricity has recently attracted significant attention. Herein, we represent our new progress in the development of a new type of carbon electrocatalysts for the NRR, which is derived from a metal-organic framework precursors. The resulting nitrogen and/or metal co-doped carbon are highly disordered, which demonstrated encouraging catalytic activity and stability in both alkaline and acidic electrolytes. In particular, the zinc containing Zeolitic imidazolate frameworks-8 (ZIF-8) derived carbon catalysts exhibit 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 transition metals such as 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.