A study of different carbon composite materials for the negative half-cell reaction of the zinc cerium hybrid redox flow cell

An investigation into the suitability of several carbon composites materials as substrates for the negative electrode in the zinc-cerium redox flow cell has been carried out. The zinc deposition process was carried out in a methanesulfonic acid electrolyte over the temperature range between 25 °C and 60 °C. Elevated temperatures increased the kinetics of the zinc deposition and dissolution reactions and also shifted the onset (nucleation) potential at more positive values. Galvanostatic charge/discharge cycles were performed in order to test the performance of these composite materials under a variety of operating conditions. For all the materials, the highest charge/discharge coulombic efficiencies (∼95%) were found at elevated temperatures. Yet, the voltage efficiencies were found to be higher at lower temperatures (viz. 25 °C). Increased methanesulfonic acid concentrations promoted the hydrogen evolution reaction which in turn led to lower coulombic efficiencies. Three of the composite carbon materials tested (viz. PVE, PVDF, HDPE-1) were found to be robust with no surface deterioration or loss in efficiency observed over 250 cycles. Scanning electron microscopy revealed the formation of zinc clusters/grains on all the substrates tested whilst dendritic and granular growth was also present on some of the carbon samples.