A composite electrode with gradient pores for high-performance aqueous redox flow batteries

Large surface areas while maintaining a low mass transport resistance is a critical criterion for the optimal design of electrode structures for aqueous redox flow batteries. However, for conventional micro-scale electrode structures, increasing surface areas will lead to an increase in the mass transfer resistance. In this work, a composite electrode with a gradient porosity distribution is fabricated through combining two different carbon felt layers of different porosities. The smaller-porosity layer, offering a larger surface area, is placed adjacent to the membrane, while the larger-porosity layer, providing a smaller mass transfer resistance is placed on the flow field side. The thickness ratio of the two layers is adjusted in terms of the battery performance while applied in the vanadium redox flow battery. It is demonstrated that the battery with the structure-optimized composite electrode achieves a high energy efficiency of 82.7 % at 200 mA·cm⁻² at an electrolyte flow rate of 30 mL·min⁻¹, and delivers a discharge capacity of 240 mAh at 400 mA·cm⁻², which is 2.18 times that of the conventional graphite felt electrode. This work offers an idea for the structural design of high-performance aqueous redox flow batteries.