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A dual oxidative approach using O2 plasma followed by treatment with H2 O2 to impart oxygen functional groups onto the surface of a graphite felt electrode. When used as electrodes for an all-vanadium redox flow battery (VRB) system, the energy efficiency of the cell is enhanced by 8.2 % at a current density of 150 mA cm(-2) compared with one oxidized by thermal treatment in air. More importantly, by varying the oxidative techniques, the amount and type of oxygen groups was tailored and their effects were elucidated. It was found that O-C=O groups improve the cells performance whereas the C-O and C=O groups degrade it. The reason for the increased performance was found to be a reduction in the cell overpotential after functionalization of the graphite felt electrode. This work reveals a route for functionalizing carbon electrodes to improve the performance of VRB cells. This approach can lower the cost of VRB cells and pave the way for more commercially viable stationary energy storage systems that can be used for intermittent renewable energy storage.

We have demonstrated a new ferri/ferrocyanide -polysulfide (Fe/S) flow battery, which employs less corrosive, relatively environmentally benign neutral alkali metal ferri/ferrocyanide and alkali metal polysulfides as the active redox couples. A cobalt nanoparticle -decorated graphite felt was used at the polysulfide side as the catalyst. Excellent electrochemical performance was successfully acquired in the Fe/S flow cells with high cell efficiencies (99% coulombic efficiency and ∼74% energy efficiency) and good cycling stability over extended charge/discharge operations. The positive half-cell appears to be the performance -limiting side in the Fe/S flow battery determined by using a carbon cloth probe. The inexpensive redox materials and possibly cell part materials can lead to reduced capital cost, making the Fe/S flow battery a promising cost-effective energy storage technology candidate.