Investigation of the impact of the flow mode in all-vanadium-redox-flow battery using macroporous and mesoporous carbon electrodes

Abstract

Energy storage in vanadium redox flow batteries (VRFBs) is significantly impacted by both the cell design and the kinetics of electron transfer at the electrode/electrolyte interfaces. In this work, a novel VRFB flow field was designed and evaluated, allowing a portion of the flow to go through the porous electrodes in the direction of current flow. It was found that, when using standard carbon paper electrodes with moderate hydrophilicity, as compared to flow-by or only flow-through, concurrent 50 % flow-through and 50 % flow-by mode decreased the voltage loss by 46 % and increased the voltage efficiency from 74 % to 80 % compared to flow-by only (flow rate of 60 ml s−1). The voltage efficiency and energy efficiency were also enhanced to 82 % and 68 %, respectively, in the flow-through mode. This was likely due to enhanced mass transfer and increased utilization of the internal electrode surface area, in turn lowering the overpotential during both charging and discharging. To further determine the benefits of flow-through conditions, the effect of a binder-free, hydrophilic, and mesoporous carbon scaffold electrode (NCS85) was also investigated under flow-through mode and compared with the standard carbon paper electrodes with moderate hydrophilicity. NCS 85 has monodisperse, 3-D interconnected 85 nm pore diameters and thus a much higher surface area, resulting in a further increase in voltage and energy efficiency to 86 and 65 %, respectively. Heat treatment of the NCS85 to graphitize it and thus lower its resistance showed further improvements when using the flow-through mode, giving a voltage efficiency of 94 % and energy efficiency of 72 %. These improvements show the benefits that can be realized by optimizing both flow mode and carbon electrode porosity and permeability on VRFB performance.