Multi-gradient electrodes for flow batteries: Synergizing transport and reaction for high-power performance

The flow battery is considered to be a promising technology for scalable, long-duration energy storage technology due to its inherently decoupled power and capacity. However, the performance of conventional graphite felt electrodes is limited by poor reaction kinetics and mass transport at the electrode interface, hindering battery efficiency and durability. Most seriously, the current uniform-modification electrodes are unable to match the pore structure and active interface with the non-uniform electrochemical reaction and mass transport characteristics along the thickness direction in the flow-by-type flow batteries. Herein, we propose a gravity-driven sedimentation strategy to constructure the multi-gradient electrode with ordered distributions of sub-micron pores and oxygen functional groups. This design matches the physicochemical properties of the electrode with the obviously-ununiform reaction and transport characteristics. Specifically, the gradually increasing sub-micron pores and oxygen functional groups from the flow field side toward the membrane side progressively increase the electrochemically active surface area, effective diffusion coefficient, and standard rate constant, enhancing both the reaction and mass transport processes towards the membrane side so as to decrease the concentration polarization in flow battery. Compared to the commercial electrode, it was found that the discharge power density, the energy efficiency, and the current density uniformity were markedly enhanced in flow battery with the developed electrode. In durability measurement, the flow battery assembled with multi-gradient electrode enables excellent long-term stability at the current density of 200 mA cm⁻² for over 1100 cycles, with an ultra-low decay rate of 0.0024 % per cycle. This work provides a promising method for preparing electrodes for a large-scale, long-term flow battery energy storage system.