As global energy demands are rapidly increasing, the development of a sustainable energy system is of fundamental importance in the near term. A strong possibility has been pointed as the “hydrogen economy”, consisting in using hydrogen (H₂) as an energy vector. Hydrogen attractiveness arises from the fact that it may be burned to produce heat or reacted with air in a fuel cell to produce electricity. This could be a solution to global energy problems, while minimizing the environmental impact (1).

An attractive technology for the production of H₂ is water electrolysis, which is not dependent on fossil hydrocarbon sources and originates no carbon emissions. Moreover, it allows the production of very pure H₂, being able to rely exclusively on renewable primary energy sources. However, the relatively limited overall energy efficiency of industrial water electrolysis cells still hinders its wide application. Industrial water electrolyzers typically employ nickel (Ni) or other metal-based electrodes, operating in 6 to 9 M potassium (or sodium) hydroxide electrolyte in the 60 - 80 ºC temperature range. The hydrogen evolution reaction (HER) and the ohmic drop in the bath are closely related to the low energy efficiency. Additional technical issues are the low stability and corrosion of the electrode materials and other cell resistances associated with the electrolyzer under operating conditions (2). The efficient removal of H₂ bubbles from the solution is also a challenge.

Room temperature ionic liquids (RTILs) are salts composed of organic cations and organic or inorganic anions at (or near) room temperature. They are characterized by a wide range of fluidity, high ionic conductivity, excellent thermal and chemical stability, as well as high heat capacity and cohesive energy density (2,3).

They may be used in replacement of aqueous electrolytes or in mixture with conventional ones and, therefore, are interesting candidates for application in water electrolyzers. RTILs with several imidazolium-based cations have been studied for water electrolysis, originating electrolytes with unique properties in terms of electrical conductance, chemical and electrochemical stability, and leading to beneficial effects on the HER using different electrode materials (4). Increased currents were also reported, as well as a significant decrease of the overall impedance in the RTILs-added solutions (5). The enhancement of the HER was related to surface pre-adsorption of the RTIL additives, stabilizing the intermediate hydrogen atoms and modifying adsorption and charge transfer processes at the metal-electrolyte interface (5).

In the present work, bromide-based RTILs are studied as electrolyte additives for water electrolysis in alkaline media. Several ionic liquids are investigated, such as 1-butyl-3-ethylimidazolium bromide (beim Br), 1,3-dibutylimidazolium bromide (bbim Br), 1-butyl-3-hexylimidazolium bromide (bhim Br), 1-butyl-3-octylimidazolium bromide (boim Br) and 1,3-diethylimidazolium bromide (eeim Br), which are composed by the same anion (Br^-) and different imidazolium-based cations. The effect of the bromide anion and its combined effect with the different cations when used as electrolyte additives for water electrolysis, particularly on the HER kinetics, were previously unknown.

The electrolysis performance after addition of small amounts of these ionic liquids to the KOH electrolyte is herein investigated and compared with that of IL-free KOH electrolyte. Electrochemical techniques such as linear scan voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were used. Measurements were performed in the 25 - 80 ºC temperature range, using a Pt foil electrode in 8 M KOH electrolyte, with and without the addition of the bromide-based RTILs.

The additives beneficial or detrimental effects on the HER are reported and discussed considering the RTILs composition, namely the effect of the bromide anion and the effect of each of the studied cations on the electrode processes. The reported results may contribute to the enhancement of the performance of industrial alkaline water electrolyzers.

1. D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone and J.L. Figueiredo, Energy, 50, 486 (2013).
2. C. Lagrost, D. Carrié, M. Vaultier and P. Hapiot, J. Phys. Chem. A, 107, 745 (2003).
3. M. Opallo and A. Lesniewski, J. Electroanal. Chem. 656, 2 (2011).
4. R.F. de Souza, J.C. Padilha, R.S. Gonçalves, M. O. de Souza and J. Rault-Berthelot, J. Power Sources, 164, 792 (2007).
5. L. Amaral, D.S.P. Cardoso, B. Šljukić, D.M.F. Santos and C.A.C. Sequeira, J. Electrochem. Soc. 164 (4) F427 (2017).