WANG Limin, WANG Wenqi1b, WANG Kangli, CHENG Tao, YAO Yaqi, YANG Yaxuan, ZHOU Enze, LIU Yuxuan, LI Jianfeng, LU Xubin
Electrodes are key components of vanadium redox flow batteries (VRFB)，and electrode properties largely govern the conversion efficiency between electrical and chemical energy.However，commercial graphite felt (GF) electrodes are dominated by inert sp2 carbon structures，leading to poor wettability，insufficient active sites，and slow charge-transfer kinetics，which severely constrain the VO2＋/VO2＋half-cell reaction.Although plasma treatments can introduce oxygen- and nitrogen-containing functional groups within short durations，substantial variations in plasma atmosphere，treatment time，and substrate pretreatment have limited comparability among reported results.Therefore，this study adopted a unified high-temperature activation baseline at 1 800 ℃ and systematically constructed a two-dimensional parameter window of plasma atmosphere，namely Ar，O2，and N2，together with treatment time，to develop a rapid and reproducible surface-engineering strategy for GF electrodes.Commercial carbon felt (CF) was first activated at 1 800 ℃ for 1 h under an inert atmosphere to obtain GF，while pristine CF was retained as a reference.Subsequently，GF was treated by Ar plasma for 1，3，5 min，by O2 plasma for 20，30，40 s，and by N2 plasma for 8，10，12 min，respectively.Fiber morphology and near-surface elemental composition were examined by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX).Wettability was evaluated by water contact angle measurements.Surface defects and graphitization were analyzed by Raman spectroscopy，and surface oxygen-containing functional groups and nitrogen configurations were identified by X-ray photoelectron spectroscopy (XPS).Electrochemical behaviors in the VO2＋/VO2＋half-cell were investigated in 3 mol/L H2SO4＋0.1 mol/L VOSO4 using cyclic voltammetry (CV)，electrochemical impedance spectroscopy (EIS)，and distribution of relaxation times(DRT) analysis to resolve charge-transfer and mass-transport contributions.Cycling stability was evaluated by 200 consecutive CV cycles at 25 mV/s.High-temperature activation increased the graphitization degree of GF.However，both CF and GF remained hydrophobic.After Ar，O2，and N2 plasma treatments，water droplets spread instantaneously and completely on the felt surface，indicating a transition from hydrophobic to fully wetting behavior.SEM-EDX and XPS results indicated that O2 plasma treatment for 30 s enriches polar oxygen-containing functional groups，including hydroxyl，carbonyl，and carboxyl species，on the fiber surface，while exerting a limited influence on the structural order of the carbon framework.In the VO2＋/VO2＋half-cell，the corresponding electrode exhibited the highest anodic/cathodic peak current densities and the lowest charge-transfer resistance (Rct).In DRT spectra，the high-frequency relaxation peak associated with charge transfer was markedly weakened，consistent with accelerated interfacial reaction kinetics.For N2 plasma treatment for 10 min，XPS test results confirmed the coexistence of pyridinic-N，pyrrolic-N，graphitic-N species，and high-binding-energy nitrogen species.A moderate increase in defect density was achieved while maintaining a relatively continuous sp2 framework，and the improvement was reflected by increased peak current densities and decreased Rct.In contrast，Ar plasma treatment for 3 min mainly introduced moderate surface defects via physical bombardment.As a result，peak current densities increased and Rct decreased relative to pristine GF，whereas the enhancement remained weaker than that obtained from the O2 and N2 systems.Regarding cycling stability，the optimal plasma-treated electrodes Ar＿GF (TAr ＝3 min)，O2＿GF (TO2 ＝30 s)，and N2＿GF (TN2 ＝10 min) exhibited cathodic peak current decay rates of 16.7%，13.9%，and 13.4%，respectively，after 200 cycles，whereas the corresponding anodic peak current density decay rates were smaller，at 5.9%，0.2%，and 1.0%，respectively.Meanwhile，the peak-topeak separation (ΔEp) increased by 0.10～0.13 V，corresponding to a relative increase of approximately 14.7%～18.8%.After 200 cycles，the CV profiles still display relatively symmetric anodic and cathodic peaks，indicating that interfacial kinetics showed measurable attenuation within the investigated cycling range，whereas the overall electrochemical response remained stable，without evidence of pronounced deactivation or severe interfacial passivation.Under a unified 1 800 ℃ high-temperature activation baseline，Ar，O2，and N2 plasma treatments effectively improved VO2＋/VO2＋half-cell interfacial kinetics on GF electrodes by regulating surface defects，polar oxygen-containing functional groups，and nitrogen-doping configurations.Among the investigated conditions，O2 plasma treatment for 30 s yielded the fastest interfacial reaction kinetics，whereas N2 plasma treatment for 10 min provided a more balanced optimization between electrochemical activity and structural integrity.The two-dimensional window of atomosphere and treatment time provided targeted guidance for plasma-based surface engineering of GF electrodes and offer a practical basis for positive electrode optimization in VRFBs.
