Performance Characteristics of the Zn-Ce Hybrid Redox Flow Battery

Redox flow batteries (RFB) offer a potential solution to the implementation of medium to large scale energy storage (10 kWh - 10 MWh) since they have the advantage of cost, flexibility, rapid response and safety over other alternatives such as the lithium ion and sodium-sulfur batteries. The zinc-cerium hybrid RFB is one of the most promising RFB systems due to its high open circuit cell voltage (E_cell = 2.4 V) and moderately high energy density (20 – 35 kWh dm^-3). The investigations here examined the impact of operating parameters such as charging/discharge current density, electrolyte composition, flow velocity, temperature and the use of different carbon composite electrodes on the performance of the Zn-Ce RFB.  The chemical stability of the positive electrode material in the strongly oxidising Ce(III)/Ce(IV) is an issue here and although certain treated carbon surfaces have shown some promise, long term electrode stability has only been achieved so far with Pt or Pt/Ir coatings on titanium. On the negative electrode side, the impact of charging current density (0.5 mA cm^-2 to 100 mA cm^-2) and temperature (25°C to 60°C) on the nature and morphology of the zinc deposit from concentrated (~2.5 M) Zn²⁺ solutions in methanesulfonic acid (MSA) were initially investigated using an electrochemical Hull cell. The measurements revealed that on a number of carbon composite electrodes (BMA5, PPG86 and BPP4) from the SGL Group, the best compact and adherent deposits were produced over a narrow current density range. Flow cell investigations were then carried out to evaluate the influence of these parameters on the cell performance.  The RFB used here comprised the carbon composite electrode (100 cm²) as the negative terminal in contact with the Zn²⁺/MSA solution. A Pt/Ti mesh served as the positive electrode in contact with an electrolyte containing 0.8 M total cerium content in 3.5 M MSA. A Nafion® 117 cation exchange membrane separated the two electrolytes in the flow cell. The reservoirs containing the two electrolytes were immersed in a water bath whose temperature was maintained constant through a Gallenkamp water circulator.  Consistently, high (>90%) coulombic efficiencies (η_C) were obtained from the charge-discharge cycles. High zinc concentrations yielded energy efficiencies of ~64% at currents of 1 A over the temperature range 45°C - 60°C. Under these conditions, the duration of the charging cycles, from 10 min to 2 hours, had little impact on the energy efficiency. Nevertheless, variations in η_C and in particularly, the voltage efficiency (η_V) were observed on charging at different current densities although flow velocity had little impact on these efficiencies.  Investigations are also currently under way to examine the kinetics of the cerium half-cell so that the cell capacity can be improved by using additives as well as mixed anion electrolytes.