Highly active iron phosphide catalysts for selective electrochemical nitrate reduction to ammonia

The electrochemical reduction reaction of the nitrate ion (NO3-), a widespread water pollutant, to valuable ammonia (NH3) is a promising approach for environmental remediation and green energy conservation. The development of high-performance electrocatalysts to selectively reduce NO3- wastes into value-added NH3 will open up a different route of NO3- treatment, and impose both environmental and economic impacts on sustainable NH3 synthesis. Transition metal phosphides represent one of the most promising earth-abundant catalysts with impressive electrocatalytic activities. Herein, we report for the first time the electrocatalytic reduction of NO3- using different phases of iron phosphide. Particularly, FeP and Fe2P phases were successfully demonstrated as efficient catalysts for NH3 generation. Detection of the in-situ formed product was achieved using electrooxidation of NH3 to nitrogen (N2) on a Pt electrode. The Fe2P catalyst exhibits the highest Faradaic efficiency (96 %) for NH3 generation with a yield (0.25 mmol h-1 cm - 2 or 2.10 mg h-1 cm-2) at - 0.55 V vs. reversible hydrogen electrode (RHE). The recycling tests confirmed that Fe2P and FeP catalysts exhibit excellent stability during the NO3- reduction at - 0.37 V vs. RHE. To get relevant information about the reaction mechanisms and the fundamental origins behind the better performance of Fe2P, density functional theory (DFT) calculations were performed. These results indicate that the Fe2P phase exhibits excellent performance to be deployed as an efficient noble metal-free catalyst for NH3 generation.