Effects of porosity variation in different linear and combination modes in the electrode on the performance of all‑vanadium flow battery

To improve the performance of all‑vanadium flow battery, the electrode porosity is arranged in different linear variations and combination forms, in which the electrolyte flow in the electrode, polarization characteristics, system efficiency (SE), energy efficiency (EE) and dissipative power ratio (DPR) are investigated among the sixteen combined electrodes with the porosity variation in range of ε = 0.9–0.5 and inlet velocity of Q = 10 ml/min in simulations. A novel electrode with the variable porosity in the diagonal direction along flow channel is developed to favor the permeation of electrolyte in the porous electrode. The change amplitude and offset of activation overpotential at 50% state of charge (SOC) are related to the porosity variation in electrode, and the lower concentration overpotential occurs in the area with larger porosity due to more electrolyte supply for the electrochemical reactions under lower permeation resistance. The Kirchhoff's Current law (KCL) in equivalent circuit model is employed to analyze the capacity variation with different porosity combinations in the electrode accounting for the effective capacity ratio of electrode in the consideration of porosity gap at the junction. The 66.9% system efficiency and 16.2% dissipative power ratio occur in the mode with combined electrode of horizontal linear increasing porosity and vertical linear increasing porosity, in which the higher system efficiency and lower dissipative power ratio can be obtained than those of other mentioned modes. • Linear porosity variation in the horizontal, vertical and diagonal directions • Analysis on the overpotential and efficiency of single and combined electrode • Porosity mutation has great impact on the performance of the battery. • The Kirchhoff's Current law is coupled to quantify the theoretical capacity.