Enhancing Vanadium redox flow batteries performance through local compression ratio adjustment using stiffness gradient carbon felt electrodes

Abstract

Vanadium redox flow batteries (VRFBs) are assembled by compressing the electrodes to reduce the contact resistance between electrodes and bipolar plates and prevent liquid electrolyte leakage. However, an excessive compression ratio can impede the mass transfer and cause high-pressure drops in the electrolyte, ultimately reducing energy and system efficiencies. Accordingly, conventional stacks require a compromised compression ratio of 20 to 30%, resulting in substantial contact resistance and lower cell performance. To address this issue, we proposed a new approach to adjust the local compression ratio of carbon felt electrodes by creating a gradient in compressive stiffness along the through-thickness direction. We achieved this by selectively treating the carbon felt electrode to increase the interaction of carbon fibers in a treated region. This approach deforms the electrode non-uniformly, increasing the electrical contact point between the electrode and bipolar plate in regions with higher deformation and improving the convection and diffusion of ions at the interface between the electrode and membrane in regions with lower deformation, where porosity is relatively high. In a VRFB single-cell test, the electrode with the stiffness gradient demonstrated the highest energy efficiency (86.5%) under a current density of 100 mA∙cm−2, 10% higher than that of neat CF.