Direct Synthesis of Ammonia from Nitrate on Amorphous Graphene with Near 100% Efficiency

Abstract

Ammonia is an indispensable commodity in the agricultural and pharmaceutical industries. Direct nitrate-to-ammonia electroreduction is a decentralized route yet challenged by competing side reactions. Most catalysts are metal-based, and metal-free catalysts with high nitrate-to-ammonia conversion activity are rarely reported. Herein, it is shown that amorphous graphene synthesized by laser induction and comprising strained and disordered pentagons, hexagons, and heptagons can electrocatalyze the eight-electron reduction of NO3− to NH3 with a Faradaic efficiency of ≈100% and an ammonia production rate of 2859 µg cm−2 h−1 at −0.93 V versus reversible hydrogen electrode. X-ray pair-distribution function analysis and electron microscopy reveal the unique molecular features of amorphous graphene that facilitate NO3− reduction. In situ Fourier transform infrared spectroscopy and theoretical calculations establish the critical role of these features in stabilizing the reaction intermediates via structural relaxation. The enhanced catalytic activity enables the implementation of flow electrolysis for the on-demand synthesis and release of ammonia with >70% selectivity, resulting in significantly increased yields and survival rates when applied to plant cultivation. The results of this study show significant promise for remediating nitrate-polluted water and completing the NOx cycle.