Palladium Nanoparticles-Modified AB5-Type Alloy in Ionic Liquid-Based Electrolytes for Novel Hydrogen Storage Systems

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Constantly growing demand for electricity storage devices has inclined scientists to develop new systems for accumulating energy and to improve those currently used. Portable devices and fully electric or hybrid vehicles require efficient and reliable rechargeable batteries. Among most common battery technologies Ni-MH batteries deserve special attention as they are considered well suited for these applications because of their high energy density, good cyclability, safety and wide operating temperature range [1]. Additionally, due to the absence of toxic metals and volatile solvents, they are one of the most environmentally friendly battery systems.As a negative electrode material various hydrogen-absorbing metals and alloys differing in hydrogen storage capacity, hydrogen sorption kinetics and hydride stability are applied. AB5-type alloys, where A is a mixture of rare earth elements while transition metals such as Ni, Al, Co and Mn constitute component B, can be designed or modified to provide optimum performance of the Ni-MH system [2]. The electrochemical properties of the alloy depend on the microstructure, the nature and amount of each element in the intermetallic compound, and the electrochemical processes taking place. Palladium, which is capable of hydrogen absorption on its own, is particularly interesting in terms of application to the modification of electrode material, especially in the form of nanoparticles (NPs). Decorating AB5-type alloy with Pd-NPs led to the improvement of discharge capacity and high rate dischargeability [3]. Moreover, Pd shows a catalytic effect on the kinetics of the charge transfer process at the electrode surface, increasing the activation rate of the metal alloy [4].Concentrated alkaline aqueous electrolytes used in Ni-MH batteries cause significant corrosion of the negative electrode material. This problem can be overcome by applying ionic liquid-based electrolytes, as ionic liquids (ILs) exhibit good conducting properties and a wide electrochemical stability window. In order to provide hydrogen source both protic ILs and mixtures of organic acids with ILs can be used.The focus of presented study was to evaluate the electrochemical performance of hydrogen absorbing Pd-NPs-modified AB5-type alloy in binary mixtures of 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MS]) with carboxylic acids: acetic and trifluoroacetic acid. These mixtures were chosen as their electrochemical and physicochemical properties were extensively studied in our previous works with the use of palladium limited volume indicative electrode (Pd-LVE) which revealed that they are promising electrolytes for hydrogen sorption measurements. The palladium content in the prepared electrode material is ca. 3.5 wt %, which is enough to improve the properties of the used alloy. In order to overcome the problems with rather high viscosity, low conductivity and high cost of ILs, we also preliminarily tested the prospect of using chosen ILs as additives to conventional aqueous KOH solutions.Developed systems were studied using electrochemical methods: cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP), which was used to determine the hydrogen absorption capacity of the electrode material. The measured capacity of AB5-Pd-NPs LVE in 2 M HAC/[EMIM][MS] binary mixture corresponds to 85.4% of the electrochemically determined capacity of pristine AB5 alloy in standard KOH aqueous solution, while in 1 M FAC/[EMIM][MS] significantly lower. The type of acid present in the acid/IL mixture does not influence the discharge potential main plateau, which is ca. 0.2 V, while a clear difference in charging potential can be observed (Figure 1 A). In addition to electrochemical analysis, SEM imaging was performed, which revealed noticeable changes in the electrode surface before and after electrochemical treatment in IL/acid mixtures, depending on the acid used (Figure 1 B-D). Acknowledgements This work was funded by National Science Centre (NCN, Poland), grant number 2021/41/B/ST5/04047.