Efficient Ammonia Electrosynthesis and Energy Conversion through a Zn-Nitrate Battery by Iron Doping Engineered Nickel Phosphide Catalyst

Abstract

The electrocatalytic nitrate reduction reaction (NO<inf>3</inf>-RR) to ammonia (NH<inf>3</inf>) offers a promising alternative approach for NH<inf>3</inf> production and nitrate-based voltaic cells which can deliver both electricity and NH<inf>3</inf> as products, are also highly attractive. However, nitrate-to-NH<inf>3</inf> conversion involves a proton-assisted multiple-electron transfer process with considerable kinetic barrier, underlying the need for efficient catalysts for the NO<inf>3</inf>^–RR. A Zn-nitrate battery is reported to enable a “killing three birds with one stone” strategy for energy supply, ammonia production and removal of pollutants with the iron doped nickel phosphide (Fe/Ni<inf>2</inf>P) as a NO<inf>3</inf>^–RR catalyst electrode. Iron doping induces a downshift of the d-band center of Ni atoms to the Fermi level, allowing the optimization of Gibbs free energies for reaction intermediates. The Fe/Ni<inf>2</inf>P catalyst exhibits 94.3% NH<inf>3</inf> Faradaic efficiency (FE) and nearly 100% nitrate conversion efficiency at –0.4 V vs. reversible hydrogen electrode (RHE). Further applying this highly efficient NO<inf>3</inf>^–RR electrocatalyst as the cathode material, a novel Zn-nitrate battery exhibits a power density of 3.25 mW cm^–2 and a FE of 85.0% for NH<inf>3</inf> production. This work enriches the application of Zn-based batteries in the field of electrocatalysis and highlights the promise of bimetal phosphide for the NO<inf>3</inf>^–RR.