Vanadium redox flow batteries (VRFBs) are widely applied in energy storage systems (e.g., wind energy, solar energy), while the poor activity of commonly used carbon-based electrode limits their large-scale application. In this study, the graphene modified carbon felt (G/CF) with a large area of 20 cm × 20 cm has been successfully prepared by a chemical vapor deposition (CVD) strategy, achieving outstanding electrocatalytic redox reversibility of the VRFBs. The decorating graphene can provide abundant active sites for the vanadium redox reactions. Compared with the pristine carbon felt (CF) electrode, the G/CF composite electrode possesses more defective sites on surface, which enhances activity toward VO²⁺/VO ⁺₂ couple and electrochemical performances. For instance, such G/CF electrode delivered remarkable voltage efficiency (VE) of 88.4% and energy efficiency (EE) of 86.4% at 100 mA·cm⁻², much higher than CF electrode by 2.1% and 3.78%, respectively. The long-term cycling stability of G/CF electrode was further investigated and a high retention value of 47.6% can be achieved over 600 cycles. It is demonstrated that this work develops a promising and effective strategy to synthesize the large size of carbon electrode with high performances for the next-generation VRFBs.

Energy storage systems are crucial in the deployment of renewable energies. As one of the most promising solutions, redox flow batteries (RFBs) are still hindered for practical applications by low energy density, high cost, and environmental concerns. To breakthrough the fundamental solubility limit that restricts boosting energy density of the cell, we here demonstrate a new RFB system employing polysulfide and high concentrated ferricyanide (up to 1.6 M) species as reactants. The RFB cell exhibits high cell performances with capacity retention of 96.9% after 1,500 cycles and low reactant cost of $32.47/kWh. Moreover, neutral aqueous electrolytes are environmentally benign and cost-effective. A cell stack is assembled and exhibits low capacity fade rate of 0.021% per cycle over 642 charging-discharging steps (spans 60 days). This neutral polysulfide/ferricyanide RFB technology with high safety, long-duration, low cost, and feasibility of scale-up is an innovative design for storing massive energy.