Comparative Study of Mass Transfer and Electrochemical Performance between Parallel-Flow and Flow-Field-Free Configurations in Commercial-Scale Vanadium Redox Flow Batteries

For a commercial-grade all-vanadium redox flow battery (VRFB) with an active area of 400 mm × 900 mm, a multiphysics model was established using COMSOL Multiphysics software to compare the flow, mass transfer, and electrochemical performance of the battery under two different electrolyte distribution structures: parallel flow channels and a flow-channel-free design. The results showed that under the parallel flow channel structure, the electrolyte flow velocity within the electrode was lower, mass transfer was dominated by diffusion, and the pressure drop was smaller. However, the electrolyte current density and VO₂⁺ concentration distribution were uneven, concentration polarization was more severe, overpotentials were higher, and both discharge voltage and voltage efficiency were lower. Under the flow-channel-free structure, the electrolyte velocity within the electrode was higher, mass transfer was dominated by convection, and the distribution of electrolyte current density and VO₂⁺ was more uniform. As the state of charge (SOC) increased, the overpotentials for both electrolyte distribution structures first decreased and then increased, reaching a minimum around 0.5. With increasing applied current density, both the discharge voltage and voltage efficiency gradually decreased.