Effect of RTILs on the Hydrogen Evolution Reaction in Alkaline Media
Authors: Amaral Luís , Sljukic Biljana , Santos Diogo M.F. , Sequeira Cesar A.C. 
Published: February 2020

The efficient production of high purity hydrogen gas (H2) has been considered of fundamental importance in solving global energy problems while avoiding environmental impact [1].  At the present time, most of the H2 production is achieved using the reforming of fossil- or biofuels. An attractive alternative is the production of H2 by water electrolysis. This is an advantageous method for H2 production because it is not dependent on fossil hydrocarbon sources and originates no carbon emissions. Also, the H2produced by this method is very pure, the process can be operated in small scale plants, and can rely exclusively in renewable primary energy sources.

Industrial water electrolysis cells typically employ nickel and other metal-based electrodes that operate in a potassium (or sodium) hydroxide solution at a concentration range of 6-9 M and temperature range of 60-80 ºC. The overall energy efficiency of electrolysis, which is partly related to the hydrogen evolution reaction (HER) and to the ohmic resistivity of the electrolytic bath, is relatively limited. Major technical problems with these cells are the low stability of the electrode materials and the low conductivity of the alkaline aqueous solutions [2], namely due to the formation of gas bubbles, which must be effectively eliminated from the solution by an optimized inter-electrode distance in an appropriate cell design.

In this context, the use of room temperature ionic liquids (RTILs) as electrolyte additives promises to be a viable alternative. RTILs may acceptably be defined as semi-organic salts composed entirely of organic cations and organic or inorganic anions at (or near) room temperature. Besides their wide range of fluidity, they possess high ionic conductivity, excellent thermal and chemical stability, high heat capacity and cohesive energy density [2,3]. Among them, BMI.BF4 and BMI.PF6 have been the subject of enormous amount of research. These imidazolium salt electrolytes have unique properties in terms of electrical conductance, chemical and electrochemical stability, and previous studies [4] showed that the production of H2 through water electrolysis using different electrodes, like stainless steel, nickel, platinum and low carbon steel, in presence of these RTILs led to higher H2production rates at lower cost operation. However, the electrodes showed corrosion and deactivation problems under some practical conditions. Therefore, after solving the remaining issues, it seems to be possible to use RTILs to increase the ionic conductivity of the alkaline electrolyte and to use cheaper and simple electrode materials with a high electrocatalytic performance in the HER.

Therefore, the main objectives of this study are to systematically evaluate the effect of RTILs addition on the KOH electrolyte conductivity through electrochemical impedance spectroscopy (EIS) studies, as well as the potential use of dissolved imidazolium species during HER on nickel and mild carbon steel. HER kinetics, in-situactivation of the metal electrodes, and detection of solution intermediates of the HER in KOH electrolyte with and without the addition of dissolved imidazolium liquids will be conducted by chronopotentiometry (CP), chronoamperometry (CA), cyclic voltammetry (CV), and rotating ring-disk electrode (RRDE) experiments.

[1] D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Maccio, A. Saccone, J.L. Figueiredo, Electrocatalytic approach for the efficiency increase of electrolytic hydrogen production: Proof-of-concept using platinum-dysprosium alloys, Energy 50 (2013) 486.

[2] C. Lagrost, D. Carrié, M. Vaultier, P. Hapiot, Reactivities of Some Electrogenerated Organic Cation Radicals in Room-Temperature Ionic Liquids:  Toward an Alternative to Volatile Organic Solvents?, J. Phys. Chem. A 107 (2003)745.

[3] M. Opallo, A. Lesniewski, A review on electrodes modified with ionic liquids, J. Electroanal. Chem. 656 (2011) 2.

[4] R.F. de Souza, J.C. Padilha, R.S. Gonçalves, M.O. de Souza, J. Rault-Berthelot, Electrochemical hydrogen production from water electrolysis using ionic liquid as electrolytes: Towards the best device, J. Power Sources 164 (2007) 792.