Periodic gradient flow fields intensify mass transfer in vanadium redox flow batteries through enhanced under-rib convection

The surface flow field design is key to inducing pressure gradients that enhance convective mass transfer within the porous electrodes of vanadium redox flow batteries (VRFBs). However, conventional designs suffer from insufficient driving intensity and limited driving regions. These limitations lead to severe polarization losses, especially under low flow rates and high current densities. A novel periodic gradient flow field (PGFF) featuring convergent-divergent channels is proposed. By introducing periodic width variation, the PGFF establishes enhanced pressure differences between adjacent channels, which generate stronger local pressure gradients and thereby strengthen under-rib convection. Under low flow rate and high current density conditions, the PGFF maintains a reactant distribution uniformity of 79.94%, indicating superior mass transfer performance. Compared with the serpentine and gradient serpentine flow fields, it improves energy efficiency by 5.62% and 4.57%, respectively. Under the investigated operating conditions, the PGFF reached a minimum flow rate of 0.31 mL min −1 cm −2 and a maximum operable current density of 450 mA cm −2 . The PGFF achieved a 50.00% lower flow rate limit and a 20.00% higher current density limit than the benchmark flow fields, demonstrating enhanced operational tolerance under low flow rate and high current density conditions. In addition, a channel characteristic parameter ( γ FF ) is introduced as a geometric descriptor to elucidate the structure-performance relationship among flow field geometry, pressure difference between adjacent channels, and under-rib convection. This work provides both mechanistic insight and a design-oriented framework for high-performance VRFB flow field design.