Electrochemical nitrogen reduction reaction over gallium – a computational and experimental study

Abstract

Ga was identified earlier as one of the “overlooked” metals for catalyzing the electrochemical nitrogen reduction reaction (ENRR). We investigate here the electrocatalytic activity of Ga towards the nitrogen reduction reaction. We used a combination of molecular modelling and simulations using periodic density functional theory calculations (DFT), and experimental ENRR measurements. The ENRR was found to proceed via an associative mechanism where the first PCET to dinitrogen forming the surface adsorbed N₂H* species is the overpotential limiting step. The bare Ga cathode has a high overpotential (>2 V (SHE)) for the ENRR. We also investigated the effect of a water-in-salt electrolyte (WISE) on the rate of ammonia formation. The addition of an Li salt lowers the overpotential to 1.88 V (SHE). DFT calculations revealed that the H-adatom was more favorably bound than the N-adatom, and the hydrogen evolution reaction (HER) is expected to dominate at high cathodic potentials. Experimental ENRR tests corroborate our results wherein no significant NH₃ formation was detected. The low electrochemical activity of Ga is attributed to poor binding and activation of N₂ which originates from an electropositive surface charge distribution.