Vanadium flow batteries (VFBs) have received high attention for large-scale energy storage due to their advantages of flexibility design, long cycle life, high efficiency, and high safety. However, commercial progress of VFBs has so far been limited by its high cost induced by its low power density. Ultrathin carbon paper is believed to be a very promising electrode for VFB because it illustrates super-low ohmic polarization, however, is limited by its low electrocatalytic activity. In this paper, a kind of carbon paper (CP) with super-high electrocatalytic activity was fabricated via a universal and simple CO activation method. The porosity and oxygen functional groups can be easily tuned via this method. The charge transfer resistance (denoting the electrochemical polarization) of a VFB with CP electrode after CO activation decreased dramatically from 970 to 120 mΩcm. Accordingly, the energy efficiency of a VFB with activated carbon paper as the electrode increased by 13% as compared to one without activation and reaches nearly 80% when the current density is 140 mAcm. This paper provides an effective way to prepare high-performance porous carbon electrodes for VFBs and even for other battery systems.

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A high power density vanadium flow battery (VFB) is essential for reducing the cost of a VFB system. The power density of a VFB is currently limited by the negative half-cell since the negative V2+/V3+ reaction has sluggish reaction kinetics and high activation overpotential. In this work, a silver (Ag) nanoparticle-decorated graphite felt (GF) was designed and prepared by the electrodeposit method for VFB applications. By regulating the deposition current density and loading amount of metallic Ag, the Ag nanoparticles with a size of 30–80 nm are evenly deposited on the carbon fibers. Combining high electrical conductivity and high electrocatalytic activity of Ag nanoparticles, the activation energy for the V2+/V3+ redox reaction is reduced dramatically. Thus, the reaction kinetics of the V2+/V3+ redox couple is accelerated, and a very low charge-transfer resistance of only 1.743 Ω is obtained for GF-Ag-3, which is much lower than that of pristine GF (525 Ω). Accordingly, the voltage efficiency and energy efficiency with GF-Ag-3 can reach 85 and 82% in a VFB single cell, respectively, at a current density of 200 mA cm–2. This paper shows an effective and simple way to prepare a highly effective electrocatalyst toward the V2+/V3+ redox reaction.