The effects of temperature and membrane thickness on the performance of aqueous alkaline redox flow batteries using napthoquinone and ferrocyanide as redox couple

The mixture of naphthoquinone-4-sulfonic acid sodium salt and 2-hydroxy-naphthoquinone (NQSO) and ferrocyanide dissolved in potassium hydroxide (KOH) electrolyte was used as catholyte and anolyte, respectively. We evaluated the effects of temperature and membrane thickness on the performance of aqueous organic redox flow batteries (AORFB) using the NQSO and ferrocyanide dissolved in alkaline electrolyte. Regarding temperature effect, when the electrochemical properties of NQSO and ferrocyanide are evaluated with 25 and 40 °C, their redox reactivity is enhanced with increased temperature due to the proportional relation of reaction rate and temperature. In addition, their electron transfer rate is also improved with increased temperature due to the proportional relation of electron transfer rate and temperature. These are proven by Nyquist plots showing the reciprocal relationship of resistance and temperature. In AORFB full cell tests performed at 25 and 40 °C, although capacity decay rate observed at 40 °C (0.067 Ah·L⁻¹ per cycle) is larger than that observed at 25°C (0.034 Ah·L⁻¹ per cycle), energy efficiency (EE) was improved from 86% at 25 °C to 89% at 40 °C. Regarding membrane thickness effect, the performance of AORFB using thin Nafion 212 membrane is better than that of AORFBs using thick Nafion 117 and Nafion 1110 membranes in voltage efficiency (VE) and EE, while its capacity retention is vice versa. This is because thinner membrane induces lower resistance.