Screening of effective electrolyte additives for zinc-based redox flow battery systems

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.