Custom-made bromide-based ionic liquids as electrolyte additives for enhancing hydrogen evolution in alkaline water electrolysis

Abstract

Two bromide-based ionic liquids (ILs), 1-butyl-3-ethylimidazolium bromide [Beim][Br] and 1,3-diethylimidazolium bromide [Eeim][Br], were prepared and their effect as alkaline electrolyte additives investigated for the hydrogen evolution reaction (HER). Electrochemical characterization was performed using platinum electrodes in IL-free 8 M KOH electrolyte and after addition of 1 or 2 vol.% of each ionic liquid. The addition of 2 vol.% of [Beim][Br] led to higher current densities. The home-made bromide-based additives changed the HER kinetics, with slightly higher Tafel slopes and small changes on the charge transfer coefficient and exchange current density being among the observed effects. Impedance measurements showed a strong decrease of the overall impedance of the system in the ionic liquid-added alkaline electrolytes, with the Nyquist plots being dominated by the intermediate frequency relaxation related with the adsorbed H. The bromide-based ionic liquids are shown to improve the electrolyte properties and to enhance the alkaline electrolysis efficiency.