Investigations on physicochemical properties and electrochemical performance of graphite felt and carbon felt for iron-chromium redox flow battery

In this paper, polyacrylonitrile-based graphite felt (GF), carbon felt (CF) and the effect of thermal activation on them with or without the catalyst (BiCl₃) are comprehensively investigated for iron-chromium redox flow battery (ICRFB) application. The physical-chemical parameters of GF and CF after the thermal activation is affected significantly by their graphitization degree, oxygen functional groups, and surface area. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results manifest that GF and CF before and after the thermal activation have different electrocatalytic activities owing to oxygen functional groups number increase and the graphitization degree decrease. In terms of the capacity decay rate, as oxygen functional groups provide shorter electrocatalytic pathways than bismuth ions, the performance of GF and CF after the thermal activation is more ideal. As a result, GF before and after the thermal activation exhibits higher efficiency (EE: 86%) and better stability at a charge-discharge current density of 60 mA·cm⁻² than those of CF during charge-discharge cycling, as the dominant limitation in an ICRFB is ohmic and activation polarization. Therefore, GF after thermal activation together with the addition of BiCl₃ in the electrolyte is a more promising electrode material for ICRFBs application than CF.