Cation influence of new imidazolium-based ionic liquids on hydrogen production from water electrolysis

Aqueous solutions of three new imidazolium-based ionic liquids, imidazolium hydrogenosulfate, methylimidazolium hydrogenosulfate, and butylimidazolium hydrogenosulfate, have been tested as electrolytes in the hydrogen evolution reaction (HER) by water electrolysis. Tafel analyses have been conducted on each system and reveal that the mechanism of HER in Pt cathode is Volmer–Tafel, where the determining step is the hydrogen desorption at the catalytic surface. The electrolytic solutions have a lower pH value and higher ionic conductivity when the size of the substituent in the imidazole ring increases. Likewise, the activation energy of HER decreases and the exchange current increases with increasing size of the substituent in the imidazole ring of the ionic liquid used in the electrolytic solution. Therefore, the experimental results indicated that butylimidazolium hydrogenosulfate ionic liquid is the most effective, among those tested, as an aqueous electrolyte for HER. The determination of the potential of zero charge of the three electrolytes indicates that the specific adsorption is favored by the increase of the molar mass of the cation, however without the blocking the active sites of the Pt cathode. The adsorbed cations provide an alternative mechanism to the Tafel step with a lower activation energy for the HER, thus characterizing the catalytic process of this reaction.