Interest in redox flow batteries (RFBs) has been growing as they present a unique way of storing large amounts of energy compared to conventional non-flow batteries. Non-flow batteries, which utilize non-aqueous organic solvents that are volatile and carry some form of toxicity to humans. The use of aqueous electrolytes is important in these effects. However, one drawback of aqueous electrolyte is the smaller electrochemical stability window (ESW) due to water electrolysis. Relatively recently, the variations in the ESW of aqueous electrolytes were investigated using a Pt electrode, and it was found that neutral and concentrated electrolyte solutions have the largest ESW. As a more practical alternative to a Pt electrode, we have investigated the variations using glassy carbon electrodes. The ESW of aqueous electrolytes under acidic, basic, and neutral conditions were tested using cyclic voltammetry with a glassy carbon electrode with respect to the saturated calomel electrode (SCE). The Nernst equation predicts hydrogen and oxygen evolution under neutral conditions to begin between the onset potentials of these reactions under acidic and basic conditions. Our results did not follow this prediction and instead showed that the onset potential of hydrogen evolution was close to those under basic conditions and oxygen evolution was close to those under acidic conditions for the uses of glassy carbon electrodes in a neutral Na2SO4 solution. Consequently, an electrochemical stability window of 3.26 V under neutral conditions was achieved, spanning from –1.64 to 1.62 V vs. the SCE, which is larger than the ESW under both acidic and basic conditions. This result was similar to reported phenomena on platinum electrodes using NaNO3 as an electrolyte, where an ESW of 2.04 V was achieved. The larger ESW of neutral electrolyte solutions recorded with glassy carbon electrodes makes an aqueous electrolyte solution more competitive against non-aqueous electrolyte solutions and a strong contender for applications in redox flow batteries.

Electrochemical stability window of glassy carbon electrodes for applications in redox flow batteries

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