Investigation of the zinc-cerium hybrid flow battery

Abstract

Electrochemical energy storage devices such as batteries, including redox flow batteries (RFB’s) are among the leading electricity energy storage technologies for the future because of their lower operating cost, scale-ability and versatility. This study was concerned with the evaluation of the zinc-cerium hybrid redox flow battery system and obtaining its optimum operating parameters. The clear advantage that the zinc-cerium flow cell has over other current flow battery systems such as the all-vanadium RFB is that the cell voltage is ~2.5 V (c.f. 1.3 V for the VRFB) and therefore greater power can be delivered from a single unit.During this project, the half-cell reactions were studied on a variety of electrode substrates, using different electrolyte compositions and temperature. The efficiency of the zinc-half cell reaction was examined on several carbon-based substrates and on three of these (the PVE, HDPE and PVDF composites), coulombic efficiencies of c.a. 95% were obtained. Nucleation studies indicated that a high density of nuclei (~106) was formed on these carbon substrates from solutions containing 1.5 mol dm3 zinc ions in 2.7 mol dm3 methane sulfonic acid. This led to the formation of more uniform deposits and minimised dendritic growth. The surfaces of the substrates following numerous zinc deposition-dissolution cycles were characterised using SEM and optical microscopy. Although some deterioration of the electrode surface was found in most of the substrates examined, the PVE, HDPE and PVDF were not severely affected and good cycling performance could be maintained over 200 cycles. During these cycles, hydrogen evolution was not a serious issue because of the inhibition of this reaction on the electrodeposited zinc substrate.The study on the positive half-cell reaction, that of the Ce(IV)/Ce(III) was carried out using a range of Pt and Pt-Ir based coatings on a titanium base. The high positive potential (~1.4 V) of this reaction eliminated the use of carbon-based substrates due to oxidation issues. Higher temperatures (60°C) were found to favour this reaction (viz. higher io and D values) but the solubility of the cerium species in the methanesulfonic acid was found to be dependent on both temperature and on the acid concentration/water content. The charge discharge current densities that could be applied on these substrates were not greater than ±25 mA cm2 due to the OER. The V values recorded were ca. 85%.The temperature range between 45C and 55C was found to be best for operating the redox flow cell while it was found that the coulombic efficiency increased with zinc concentration (C ~95% at 2 mol dm3 Zn2+). The V did not exceed 70% in the flow cell but in the case of the undivided system (i.e with no membrane separator), the values found were ca. 77%. The values obtained for the flow cell were ca. 55% for the PVE and PVDF carbon composite electrodes at current densities of ±10 mA cm2.