Ionic Liquids As Tunable Electrolytes for Protonic Systems
Authors: Pająk Małgorzata , Hubkowska Katarzyna , Czerwiński Andrzej 
Published: July 2021

INTRODUCTION

The unique properties of ionic liquids (ILs) i.e. high thermal and chemical stability, non-flammability, low vapor pressure, and wider electrochemical window compared to traditional solvents, as well as the prospect that they can be tailored or custom designed for specific requirements make them all the more interesting. In recent years, ILs with organic cations have been suggested for electrolyte applications. Ionic liquids seems to be promising electrolytes in protonic systems as they exhibit good proton conducting properties.
The motivation for the presented research was dictated by the growing interest in the field of the use of ionic liquids as electrolytes in metal-hydride batteries (Ni-MH) and other proton batteries (eg. based on the Si anode). The electrolytes based on ionic liquids can reduce the problem of the anode material (eg. AB5 alloy) corrosion in aqueous basic solutions, which may result in the improvement of the operation parameters of the proton batteries.
The focus of this study was to check the applicability of ILs as non-aqueous electrolytes through the investigation of hydrogen sorption in Pd limited volume electrode (Pd-LVE) and AB5-LVE. Since protic ILs and their mixtures with other solvents have been recently proposed as electrolytes in metal-hydride batteries, Pd-LVE is a very good model system to carry out the preliminary study of hydrogen electrosorption in ionic liquids in view of the further research with the use of multicomponent alloys.

EXPERIMENTAL

The electrochemical experiments were conducted in selected, commercially available protic and aprotic ionic liquids (i.a. diethylmethylammonium triflate (DEMA-TFO), ethylammonium nitrate (EAN), 1-ethyl-3-methylimidazolium methanesulfonate (EMIm-MS) under inert gas (Ar) using three-electrode system. Pd-LVE (ca. 0.5 μm thick) or AB5-LVE (ca. 50 μm thick) was a working electrode, Ag wire was a quasi-reference electrode, and Pt or Au spiral was an auxiliary electrode. Pd-LVE were obtained electrochemically from the PdCl2 aqueous solution. AB5 alloy (LaMMNi4.1Al0.3Mn0.4Co0.45) was obtained from VARTA Microbatteries GmbH. To aprotic ionic liquid (EMIm-MS) organic acid of the same anion as in the IL (methanesulfonic acid, HMS) was added as a source of hydrogen and to increase the ionic conductivity of the obtained electrolyte. Cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP) techniques were used to perform the electrochemical measurements. The electrodes were subjected to a hydrogen pretreatment procedure (HPP) and standard hydrogen cyclic voltammetry electrosorption experiment to obtain hydrogen absorption isotherms.

RESULTS AND DISCUSSION

It was observed, that the reversibility of the hydrogen sorption in Pd is significantly lower in ILs than in aqueous solutions, which may be caused by hindered proton transport due to few reasons. The first one is the high viscosity of IL, the second one is the difference of proton source, and finally the possible interactions of IL ions with the Pd electrode. Multiple hydrogen sorption in ILs (HPP) increases the reversibility of the investigated process. Preliminary HPP performed in aqueous solution of sulfuric acid facilitates further hydrogen sorption only in some cases (i.a. in EAN). Thus, it was found that the rate of hydrogen absorption is controlled rather by surface than the bulk processes. The maximum hydrogen sorption capacity (H/Mmax) obtained for Pd in protic IL (i.e. DEMA-TFO) and aprotic IL with addition of organic acid (EMIm-MS + 0.5 M HMS) is comparable with those obtained in aqueous solutions of acids and bases, reaching the value of ca 0.7. Our preliminary studies of hydrogen sorption in the AB5 alloy in a mixture of an aprotic IL with the organic acid (EMIm-MS + 1 M HMS) revealed that, as in case of Pd-LVE, the electrochemical reversibility of this process is much lower in comparison with the electrosorption measurements performed in concentrated base (Fig.1), which is due to the significant difference in ion conductivity and dynamic viscosity of the tested electrolytes.

Fig. 1. Cyclic voltammetry behavior of AB5 electrode in EMIm-MS + 1 M HMS, inset: cyclic voltammetry of AB5 electrode in 6 M KOH; first cycles; 2 mVs-1.

CONCLUSIONS

Examined protic and aprotic ionic liquids can be effectively used as electrolytes in the hydrogen sorption process as we demonstrated by the exemplary Pd-LVE electrode. The max. H/Pd ratios obtained for DEMA-TFO and nitrate protic ILs are similar (0.6-0.7) and comparable with those measured in aqueous solutions of acids and bases. Hydrogen electrosorption from aprotic ionic liquids (eg. in EMIM-MS) with different concentration of acid gives ability to tune the maximum hydrogen capacity. The max. H/Pd ratios obtained for EMIm-MS (acid concentration ca. 0.5 M) equals to ca. 0.74 (this is the same value that was obtained for this type of electrode from acidic aqueous media).

Figure 1