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media. In view of the initial attempts to use carbon-based bipolar electrodes in Zn-Ce stacks, the positive electrode reaction has mainly been studied at carbon composite substrates [124- 127]. The Zn(II)/Zn redox couple has fast kinetics in methanesulfonate acid and low overpotentials are observed for the deposition and redissolution reactions [127]. This is shown in a cyclic voltammogram for Zn(II) methanesulfonate at glassy carbon in Figure 9. An overpotential of ca. 100 mV is required to drive Zn nucleation; the actual deposition starts at a potential of –1.19 V vs. SCE. In the reverse direction, the voltammogram shows the oxidation current peak due to an unhindered Zn(II) dissolution at –1.07 V vs. SCE [128]. A laboratory RFB using a carbon-polyvinyl ester (PVE) composite as negative electrode along with a negative electrolyte composition of 1.5 mol dm–3 Zn(CH3SO3)2 + 1 mol dm–3 CH3SO3H has been characterized [127]. Cell potential profiles over a current density range between 10 and 50 mA cm–2 were obtained for different Zn(II) and H+ concentration, operational temperature, the presence of electrolytes additives and increasing mean linear electrolyte flow rate. The battery showed better performance at a current density of 50 mA cm–2 and 323 K using In(III) as inhibitor of H2 evolution. Dendrite-free Zn deposits were obtained at current efficiencies over 90% and voltage efficiencies of ca. 70%. The electrodeposition of Zn at polyvinylidene fluoride (PVDF) and high density polyethylene (HDPE) carbon-polymer composite was investigated by another group [126]. Zn nucleation was related to the standard rate constant for the Zn(II)/Zn reaction and the diffusion coefficient of Zn(II) ions. Nuclei density was estimated and confirmed to increase as a function of overpotential. Charge/discharge cycling at rotating disc electrodes using 1.5 mol dm–3 Zn(CH3SO3)2 in 5.7 mol dm–3 CH3SO3H at 333 K showed that the deposition and dissolution reactions required only small overpotentials. In the same experiments, the PVDF material 21PDF Image | hybrid redox flow batteries with zinc negative electrodes
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