Protic Ionic Liquids as Electrolytes for Metal-Hydride Batteries

Abstract

Nowadays, the most popular and effective energy storage systems are Li-ion batteries, despite some limitations such as high cost, flammability, temperature restrictions, and use of critical raw material (e.g. Co) [1]. This raises the challenge of the demand for alternative systems. One potential alternative is the Ni-MH battery, which are recognized for their good cyclability, competitive energy density, low memory effect, wide range of operating temperatures, low cost, and environmental friendliness [1]. Their practical use, however, is constrained due to their low specific energy density, high self-discharge rate, and electrode ageing [2,3].This work is focused on the study of the negative electrode of the Ni-MH battery and its interaction with the electrolyte. The objective aims to improve the performance of the negative electrode of Ni-MH batteries by substituting the conventional water-based KOH 8.7 M electrolyte by a Protic Ionic Liquid (PIL) to overcome aforementioned drawbacks [4]. PILs can exchange a labile proton, thereby supporting the main redox reaction, hydrogenation, at the negative electrode of Ni-MH battery.Thus, a series of twelve PILs or their combinations were synthesized and tested in a half-cell configuration. The charge/discharge cycle tests were carried out. Results showed that Pyrrolidinium Acetate (PyrrAc), Ethanolammonium Acetate (EOAAc) mixed with g-butyrolactone (GBL), and Diethanolammonium Acetate (DEOAAc) exhibited promising performance in half-cell configuration (Table 1).PyrrAc was shown to be an optimal electrolyte and has been further studied in terms of long-term performance and system degradation. The morphology of the aged negative electrodes was observed by Scanning Electron Microscopy (SEM) (Fig. 1). For instance, after 7 cycles of charge/discharge, formation of cracks was observed in the electrode (Fig. 1 (right)) due to hydrogen ab/desorption.The utilization of PyrrAc as electrolyte resulted in no initial capacity loss and, moreover, exhibited an increase in capacity during the first cycles, which can be recognized as an activation of the electrode (similar to the one observed in aqueous electrolytes). In order to obtain a complete understanding of the electrode stability, the electrolytes after cycling as well as calendar corrosion were studied using Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES).[1] M.A. Hannan, M.M. Hoque, A. Mohamed, and A. Ayob, Renewable and Sustainable Energy Reviews, 69 (2017) 771–789.[2] B. Puga et al., ChemElectroChem, 2 (2015) 1321–1330.[3] J. Matsuda, Y. Nakamura, E. Akiba, J Alloys Compd, 509 (2011) 7498–7503.[4] T. Meng, K. H. Young, D. F. Wong, and J. Nei, Batteries, 3 (2017) 4. Figure 1