Cr3C2/Cr2O3 dual-phase surface engineering of graphite felt for enhanced conductivity and catalytic activity in vanadium redox flow batteries

Abstract

Vanadium redox flow batteries (VRFBs) are promising for large-scale energy storage, yet their performance is often constrained by sluggish kinetics and ohmic losses of conventional graphite felt electrodes. Herein, we report a sequential chromium carbide-based surface engineering strategy involving thermal treatment, chromium precursor impregnation and high-temperature carburization to form conductive Cr₃C₂ anchored to the graphite felt fibers, and a subsequent post-oxidation-assisted hydrophilization (HPZ) step that generates a thin Cr₂O₃ surface layer while restoring surface polarity. XRD, SEM, and TEM analyses indicate the presence of Cr₃C₂ on the graphite fibers with an oxidized surface layer, resulting in a Cr₃C₂/Cr₂O₃ dual-phase surface that provides a favorable balance between electronic conductivity and electrolyte wettability compared with a Cr₂O₃-only modification. XPS reveals pronounced surface deoxygenation and defect enrichment after carburization, whereas HPZ reintroduces oxygen-containing functional groups. Electrochemical impedance spectroscopy demonstrates concurrent mitigation of kinetic and ohmic losses: the TT electrode exhibits an R_ct of 11.86 Ω and an R_s of 0.225 Ω, while the CB + HPZ electrode delivers much lower values of 2.8 Ω and 0.025 Ω, respectively. CV analysis further supports a more balanced and reversible vanadium redox response for CB + HPZ than OX. Consequently, the engineered electrode achieves ∼88% energy efficiency at 50 mA cm⁻² and sustains operation at 250 mA cm⁻² with ∼64% energy efficiency. Long-term cycling at 100 mA cm⁻² maintains energy efficiency above 80.8% with 97.6% retention over 200 cycles, while coulombic efficiency remains consistently >98%, indicating stable performance under prolonged operation. Post-cycling SEM/EDS confirms that Cr-containing particles remain on the graphite felt fibers after 200 cycles, supporting electrode durability.