Gobinath Pillai Rajarathnam

The influence of ionic liquid additives on zinc half-cell electrochemical performance in zinc/bromine flow batteries

Six ionic liquids were assessed for their suitability as alternative bromine-sequestering agents (BSAs) in zinc/bromine redox flow batteries (Zn/Br RFBs) via comparison against conventional BSA, 1-ethyl-1-methylpyrrolidinium bromide ([C2MPyrr]Br). These alternative BSAs included the bromide salts of the following cations: 1-ethyl-1-methylpiperidinium ([C2MPip]+), 1-ethyl-1-methylimidazolium ([C2MIm]+), 1-(2-hydroxyethyl)-3-methylimidazolium ([C2OHMIm]+), 1-ethylpyridinium ([C2Py]+) and 1-(2-hydroxyethyl)pyridinium ([C2OHPy]+). Cyclic and linear sweep voltammetry, as well as electrochemical impedance spectroscopy, were performed to understand the influence of electrolytes containing these ionic liquids on zinc half-cell electrochemical performance. Solutions with [C2Py]Br, [C2MIm]Br and [C2OHPy]Br improved zinc half-cell performance (highest-magnitude current, charge, maximum power and energy) when compared to those utilizing [C2MPyrr]Br. Electrolytes employing these BSAs also reduced the nucleation overpotential of zinc electrodeposition and stripping compared to those using [C2MPyrr]Br. Zinc electrodeposits obtained during charging from electrolytes containing the different BSAs were analyzed via scanning electron microscopy and X-ray diffraction. Scanning electron micrographs showed a strong relationship between the chemical structure of the BSA employed and the crystallinity of zinc electrodeposits, with solutions containing [C2OHMIm]Br, [C2Py]Br and [C2OHPy]Br producing more compact zinc deposits than those with other BSAs. These findings warrant further investigation of BSAs with delocalized cationic charge. While these compounds have been proposed for application in Zn/Br systems, they are also potentially adaptable to other types of RFBs, which employ the Br2/Br− redox couple and use electrolytes containing BSAs.

The influence of novel bromine sequestration agents on zinc/bromine flow battery performance

This study benchmarks cycle performance of electrolyte solutions containing novel bromine sequestration agents (BSA) in a zinc bromine flow battery. Five alternative BSA candidates – 1-ethyl-1-methylpiperidinium bromide ([C2MPip]Br), 1-ethylpyridinium bromide ([C2Py]Br), 1-(2-hydroxyethyl)-pyridinium bromide ([C2OHPy]Br), 1-ethyl-3-methylimidazolium bromide ([C2MIm]Br) and 1-(2-hydroxyethyl)-3-methylimidazolium bromide ([C2OHMIm]Br) were investigated under operational conditions typical for zinc bromine flow batteries. Results were compared to the conventional BSA, 1-ethyl-1-methylpyrrolidinium bromide ([C2MPyrr]Br). Electrolytes containing the various alternative BSAs were tested at bench scale in a full-cell battery setup under controlled electrolyte flow and temperature. The evaluated performance parameters were: voltaic efficiency, coulombic efficiency, energy efficiency and recoverable charge. A correlation between BSA–bromine bond strength and cycle performance was observed. Performance of the tested electrolytes varied widely, and gains in coulombic efficiency were generally offset by losses in voltaic efficiency. [C2Py]Br and [C2MIm]Br produced cycle performance improvements compared to the other BSAs studied.

Zinc Electrodeposition Morphology

The electrochemical performance of the zinc half-cell is strongly linked to the quality and morphology of zinc electrodeposits generated during the charging phase. The structure of the zinc plating also dictates performance characteristics such as efficiencies, charge densities and peak current values during the subsequent discharge phase. The previous chapter described and analyzed the considerations arising from chemical reactions occurring at the zinc-side electrode. Following from that point, this chapter describes the underlying reasons why different zinc plating morphologies are obtained under different conditions and how certain behavior such as dendritic growth can be detrimental to Zn/Br performance. Promising methods for solving such issues are then identified from a wide range of literature including studies directly related to redox flow batteries as well as from the highly established electroplating industry. The primary means of controlling zinc crystal structure involves the use of organic additives to achieve a specific growth template and rate. Additionally, the merits and drawbacks of alternative strategies such as controlling deposition rates are investigated in this chapter.

Bromine-Side Electrode Functionality

The previous two chapters dealt with establishing a sound understanding of zinc-related physical and electrochemical processes, with a special focus on controlling electrodeposition in order to achieve optimal performance in the zinc half-cell. A similar approach can be taken for in-depth study of the bromine half-cell, with the aim of developing novel strategies and/or adapting existing methods of optimizing the Br–/Br2 redox to suit the Zn/Br electrochemical environment, thereby significantly improving Zn/Br system performance. This chapter reviews literature pertaining to relevant studies of reactions at the bromine-side electrode (both for within the Zn/Br system as well as different but related environments) to establish strong understanding of the fundamental physical and electrochemical processes that occur during charge and discharge phases of the battery. Materials challenges for conventional Zn/Br systems are highlighted and we review the viability of opportunities to improve electrode functionality through methods such as strategic catalyst doping leading to enhanced electrochemical performance.

Revisiting Zinc-Side Electrochemistry

On the basis of a reasonable understanding of Zn/Br redox flow battery systems obtained from the previous chapter, it is possible to formulate a sound strategy to carry out in-depth studies of each Zn/Br half-cell (i.e. the zinc and bromine sides). The knowledge obtained from such investigations would in turn enable researchers to test and identify methods of individually optimizing each half-cell to achieve significantly better overall performance. This chapter presents a deeper understanding of zinc-side electrochemical processes occurring in the Zn/Br during charge/discharge cycling, collating and reviewing relevant literature pertaining to this area from the field of flow batteries and others, such as studies on industrial electroplating. The problems faced by earlier generations of Zn/Br systems due to the utilization of metallic electrodes are highlighted, followed by a description of the attractiveness and viability of employing carbon-based electrode stacks instead. Finally, a detailed look is taken at zinc-side redox mechanisms and the kinetics of related reactions, leading into methods of catalytically enhancing electrode performance.

Description of the Zn/Br RFB System

In order to make beneficial changes to the Zn/Br flow battery system, it is necessary first to understand its present structure and functional status, including the level of performance for typical systems, the operating mechanisms as well as the conventional materials and methods of construction. The previous chapter introduced and discussed the need for reliable large-scale electrical energy storage and the role of redox flow batteries for such purposes. This chapter describes the physical architecture of the Zn/Br system (i.e. electrode stack, membrane separator, electrolyte flow schematic), as well as the conventional electrolyte solution employed and the dominant chemical redox reactions occurring during charge and discharge processes at each electrode. Design considerations are detailed, such as the safe storage and treatment of bromine evolved, together with important operating practices such as tracking state-of-charge. Finally, electrochemical and overall operational performance characteristics are discussed with regard to maximizing the specific energy of the Zn/Br flow battery and scaling-up next-generation systems from benchtop testing to commercial use.

Strategies for Studying and Improving the Zn/Br RFB

From the problems outlined for each technological challenge described in previous chapters, some promising strategies have been formulated for increasing knowledge about and improving the electrochemical and physical processes of Zn/Br systems, particularly at the electrode–electrolyte interface. This chapter presents a condensed collation of these focused strategies, aimed at improving Zn/Br flow battery technology. New-found understanding from fundamental studies would allow clear identification of promising investigative pathways and reduce the time and effort involved in developing tailor-made solutions to reduce or circumvent internal sources of losses (e.g. due to undesirable side reactions), consequently reducing costs while improving operating efficiencies and practical specific energy. For maximum gain, proposals are made for short-term research on two fronts, namely computer modeling and electrochemical studies. That combination would allow rapid discovery and implementation of solutions, both for developing novel materials and for characterizing the Zn/Br system’s behavior under various combinations of physicochemical conditions. Simulations using sophisticated modeling techniques with adjustments based on accurate empirical parameters and correlations would significantly minimize the time and cost of the experimental investigations required to develop suitable materials for use in Zn/Br batteries. These simulations include periodic density functional calculations and multi-physics models of the system. On the experimental front, impedance spectroscopy is a sensitive and highly informative technique that can be used to both study and track even minor changes to Zn/Br system behavior contingent upon variations of chemical composition, physical arrangements and operating conditions.