The Influence of Trihaloacetic Acids on the Hydrogen Sorption Properties of Palladium Limited Volume Electrode in Aprotic Ionic Liquid: Towards Ionic Liquid-Based Electrolytes for Protonic Systems
Authors: Pająk Małgorzata , Monikowska Dorota , Jankowska Joanna , Hubkowska Katarzyna , Czerwiński Andrzej 
Published: December 2024

Ever-rising energy demand will result in increased need for efficient energy storage. Over the past few decades, nickel-metal hydride (Ni-MH) batteries are becoming increasingly popular due to a good cycle life and as they are more environmentally friendly and safer than other rechargeable battery systems. However, the electrode material corrosion caused by water-induced side reactions occurring in aqueous electrolytes is a common problem in such type of batteries. To overcome this disadvantage, many strategies have been exploited. Nowadays, efforts are being made to develop non-aqueous electrolytes suitable for Ni-MH batteries. In recent years, ionic liquids (ILs) have been suggested for electrolyte applications as an alternative option due to their unique properties important for battery technology [1]. However, the significant drawback of ILs is their relatively high viscosity, and thus low conductivity. The increase in conductivity can be obtained by addition of organic solvent to IL and recently, mixtures of organic acids and ILs are of interest to the battery technology. Therefore, the focus of this study was to check the applicability of ILs and their mixtures with haloacetic acids as non-aqueous electrolytes through the investigation of hydrogen sorption in palladium limited volume electrode (Pd-LVE). 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 hydrogen absorbing alloys [2].
The electrochemical experiments were conducted in 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MS]) under inert gas (Ar) using three-electrode system. Pd-LVE (ca. 0.5 μm thick) was a working electrode, Ag wire was a quasi-reference electrode, and Pt spiral was an auxiliary electrode. Pd-LVE were obtained electrochemically from the PdCl2 aqueous solution. To perform the electrochemical measurements cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP) techniques were used.
Physicochemical properties of IL-based electrolytes, i.e. dynamic viscosity, density, ionic conductivity, and water content were determined as they have crucial influence on electrochemical measurements results. The Pd-LVE was used as an indicative electrode to study the optimum concentration of acetic acid and its halogen derivatives allowing to obtain maximum hydrogen capacity. The capacity was expressed as hydrogen to palladium atomic ratio. The amount of hydrogen was determined based on hydrogen oxidation charge obtained by integration of oxidation peak in the CV curve, while Pd amount was calculated based on the Farraday’s law from the deposition charge. CV curves of Pd-LVE registered in selected binary mixtures of [EMIM][MS] with acetic acid and its halogen derivatives are presented in Fig. 1. As one can notice, an increase in hydrogen oxidation and reduction signals intensity is observed with increasing concentration of organic acid. The optimum acid concentration equals to 2 M in the case of mixtures of [EMIM][MS] with acetic acid, while for the mixtures of [EMIM][MS] with trifluoroacetic acid it is 0.5 M as trifluoroacetic acid is a stronger acid than acetic acid. Surprising were the results obtained for mixtures of [EMIM][MS] with trichloro- and tribromoacetic acid. In the case of trichloroacetic acid increasing acid concentration did not cause increased hydrogen capacity. In the case of tribromoacetic acid there is no visible peaks of hydrogen oxidation after gradual addition of acid to [EMIM][MS]. Moreover, dissolution of palladium were observed.
To explain unusual behavior of samples and electrochemical results, 1H NMR was used as a supporting technique. In the 1H NMR spectra additional peaks compared to spectrum of pure [EMIM][MS] are visible at 7.336 and 6.682 ppm for trichloro- and tribromoacetic acid, respectively. They are assigned to chloroform and bromoform formed as a result of haloacetic acid decarboxylation after mixing with [EMIM][MS]. To explain this phenomenon, computational studies were performed. The observed energy barriers correlate with the experimental observations of no decarboxylation in the [EMIM]+[FAC]−[MS-H] system, and the more efficient decarboxylation observed for Br than for Cl derivative.
Examined binary mixtures can be effectively used as electrolytes in the hydrogen sorption process. The maximum hydrogen capacity obtained for [EMIM][MS] mixtures with acetic and trifluoroacetic acid are similar (ca. 0.74) and comparable with those measured in aqueous solutions. The results of this research involving other haloacetic acids show that there is a need of synthesis of tailored ionic liquids suitable for the more efficient performance of the electrode material.

Funding: This work was funded by National Science Centre (NCN, Poland), grant number 2021/41/B/ST5/04047.
[1] M. Pająk et al. Energy Convers. Manag.: X 20 (2023) 100500
[2] K. Hubkowska et al. J. Solid State Electrochem. 28 (2024) 1159–1169

Figure 1