Enhanced Electrochemical Nitrate-to-Ammonia Performance of Cobalt Oxide by Protic Ionic Liquid Modification.

Abstract

Electrochemical conversion of nitrate to ammonia is an appealing way for small-scale and decentralized ammonia synthesis and waste nitrate treatment. Currently, strategies to enhance the reaction performance through elaborate catalyst design have been well developed, but it is still of challenge to realize the promotion of reactivity and selectivity at the same time. Instead, a facile method of catalyst modification with ionic liquid to modulate the electrode surface microenvironment that mimic the role of the natural MoFe protein environment is found effective for the simultaneous improvement of NH3 yield rate and Faradaic efficiency (FE) at a low NaNO3 concentration of 500 ppm. Protic ionic liquid (PIL) N-butylimidazolium bis(trifluoromethylsulfonyl)imide ([Bim]NTf2) modified Co3O4-x is fabricated and affords the NH3 yield rate and FE of 30.23 ± 4.97 mg h-1 mgcat.-1 and 84.74 ± 3.43% at -1.71 and -1.41 V vs. Ag/AgCl, respectively, outperforming the pristine Co3O4-x. Mechanistic and theoretical studies reveal that the PIL modification facilitates the adsorption and activation of NO3- as well as the NO3--to-NH3 conversion and inhibits hydrogen evolution reaction competition via enhancing the Lewis acidity of the Co center, shuttling protons, and constructing a hydrogen bonded and hydrophobic electrode surface microenvironment.