Numerical study of deep eutectic solvent electrolyte-based vanadium-iron redox flow battery with three-dimensional multi-layer porous electrode

Porous electrode is recognized as one of the crucial components in redox flow battery (RFB). During the operation of RFBs, the porosity distribution in porous electrode has a great effect on electrochemical performance. However, the conventional uniform porosity will lead to a low electrolyte velocity and insufficient active sites in the near membrane region in deep eutectic solvents (DES) electrolyte-based RFBs, thus causing a contradiction between electrolyte flow resistance and electrochemical reaction resistance. To address this issue, this work develops a modified three-dimensional multi-layer porous electrode structure to achieve a balance between electrolyte flow resistance and electrochemical reaction resistance, so as to improve the performance of RFB. Using finite element method to simulate the operation of a single RFB with proposed electrodes under different fluxes and discharging current densities, the numerical results reveal that the multi-layer porous electrodes can reduce the overpotential in the near membrane region and achieve the lowest total power loss under the flow flux of 1 mL center dot$$ \cdotp $$min(-1). Moreover, polarization power loss, pumping loss and total power loss are numerically compared to attain the optimal electrode structure under different operating conditions. The multi-layer electrode structure can offer a novel pathway in electrode structure design, thereby helping the RFB stack operation reduce unnecessary power loss by adjusting different operating conditions. Novelty statement The conventional porous electrode with uniform porosity will cause a contradiction between electrolyte flow resistance and electrochemical reaction resistance. A three-dimensional multi-layer porous electrode structure is proposed to balance the inconsistency, and total power loss including polarization power loss and pumping loss is numerically compared to attain the optimal electrode structure. It is found that the proposed electrode structure can improve the electrolyte velocity in near membrane side region and can reduce the power loss at some operation conditions.