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.