Regulating flow field design on carbon felt electrode towards high power density operation of vanadium flow batteries

• Carving flow field design on felts enhance mass transfer and reduce pressure drop. • Parallel flow based felt outperforms pristine interdigitated flow based felts. • Simulation and experiment confirm the superiority parallel flow based felt design. • A 70% system efficiency is predicted for 32 kW stack with parallel flow based felt. Flow batteries promise a great practice to integrate with renewable energy sources in electric grid applications. However, high power density operation of flow batteries remains a challenge due to mass transport limitation and flow resistance in porous carbon felt electrode, which urges the need of advanced flow design to synergistically lower concentration polarization and reduce pressure drop. Herein, we realize a remarkably enhanced power density operation for vanadium flow batteries by regulating flow field design on carbon felt electrodes. Finite element analyses firstly reveal significantly reduced pressure drop, well-distributed reactant and promoted flow velocity on carbon felts with parallel and interdigitated flow designs. On the basis of measured local mass transfer coefficients, both interdigitated and parallel based felts exhibit a notable reduction in simulated concentration polarization at 200 mA cm -2 with parallel flow outperforming interdigitated design. Experimental validations further confirm a superior voltage efficiency of 78% and significantly enhanced discharge capacity at 200 mA cm -2 for the flow cell adopting parallel based felts. Finally, dynamic modelling and simulation of an industrial-scale 32 kW stack highlight a desirable system efficiency of ca. 70% for the parallel flow felt design at 200 mA cm -2 , signifying a great potential of regulating flow field on carbon felts for design and scale-up of practical flow battery stacks.