Reshaping hydrogen bond network in aqueous-aprotic hybrid electrolyte to achieve highly selective ambient ammonia synthesis

Abstract

Aqueous electrolytes are extensively applied in electrochemical nitrogen reduction reaction, while the overwhelming H2O always produce much protons, favoring the electron-stealing hydrogen evolution reaction (HER) and thus leaving the current ammonia synthesis much to be desired. Here, we propose an aqueous-aprotic hybrid electrolyte system by introducing trimethyl phosphate (TMP) as a cosolvent to achieve highly selective ammonia synthesis. TMP features higher Gutmann donors than that of H2O, preferring to serve as hydrogen bond (HB) acceptor and reshaping the HB network in the electrolyte. Molecular dynamics simulations suggest that, compared with H2O-H2O HB, the H2O-TMP HB exhibits longer lifetime and better stability. The intensified interaction between H2O and TMP weakens the interaction between H2O and H2O, which strengthens the O-H bond of H2O and makes it more difficult to be broken, thus greatly inhibiting the dissociation of H2O and leading to a suppressed HER activity. Correspondingly, a significantly improved NRR performance with a superior NH3 yield rate of 82.1 ± 2.7 μg h–1 mg–1 and an optimum Faradaic efficiency of 73.3 ± 2.7% is achieved in the H2O-TMP hybrid electrolyte, indicative of order of magnitude enhancement compared with that delivered in the conventional aqueous electrolyte.