Abstract
The purpose of this work is to assess the suitability of potential electrolyte additives for zinc morphology control and improved electrochemical performance of the zinc electrode for application in zinc based redox flow battery (RFB) systems. Based on existing literature in the field, sixteen candidates are selected, including four metallic additives, two non-ionic surfactants and ten quaternary ammonium compounds. The electrochemical performance of the zinc electrode is assessed using cyclic voltammetry, linear sweep voltammetry and zinc half-cell cycling tests using chronopotentiometry. Zinc electrodepositions are carried out using chronopotentiometry in order to assess the effect of additives on zinc morphology with scanning electron microscopy. Based on zinc reduction and oxidation reaction potentials, the cycling efficiencies, and the effect on zinc morphology, the most promising additives of those tested are tetraethylammonium hydroxide and tetraethylammonium bromide. Both provide smooth and compact zinc deposits and zinc electrode coulombic efficiencies of 95–97% without leading to significant changes in the zinc reduction/oxidation overpotentials, yielding anodic and cathodic current densities of 77–78 mA cm−2 and 31–32 mA cm−2 at overpotentials of ± 50 mV, respectively. In a zinc-nickel flow cell, these additives provide energy efficiencies of 78–79%, compared with 69% without an additive.
Highlights
The research and development of zinc based redox flow batteries (Zn-redox flow battery (RFB)) commenced in the mid-1970s with the zinc-chlorine and zinc-bromine systems
Electrolyte additives are a promising solution to these challenges, with suitable additives enabling a compact and uniform zinc electrodeposition, improving the cycle life of the battery by allowing more complete dissolution of the zinc deposition on each cycle, depressing the build-up of materials on the electrode and maintaining the Zn(II) concentration at a stable level
The low charge efficiency is caused by incomplete oxidisation of the deposited zinc and the competing hydrogen evolution reaction during zinc deposition
Summary
The research and development of zinc based redox flow batteries (Zn-RFBs) commenced in the mid-1970s with the zinc-chlorine and zinc-bromine systems. Electrolyte additives are a promising solution to these challenges, with suitable additives enabling a compact and uniform zinc electrodeposition, improving the cycle life of the battery by allowing more complete dissolution of the zinc deposition on each cycle, depressing the build-up of materials on the electrode and maintaining the Zn(II) concentration at a stable level. For zinc systems in acidic media, previous work demonstrates compact and dendrite free zinc electrodepositions in methanesulfonic acid with or without the addition of electrolyte additives [11]. Producing such zinc morphologies is more challenging from alkaline electrolytes
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