VO2 as a Highly Efficient Electrocatalyst for the Oxygen Evolution Reaction

Herein, we report high electrocatalytic activity of monoclinic VO2 (M1 phase) for the oxygen evolution reaction (OER) for the first time. The single-phase VO2 (M1) nanoparticles are prepared in the form of uniformly covering the surface of individual carbon fibers constituting a carbon fiber paper (CFP). The VO2 nanoparticles reveal the metal-insulator phase transition at ca. 65 °C (heating) and 62 °C (cooling) with low thermal hysteresis, indicating a high concentration of structural defect which is considered a grain boundary among VO2 nanoparticles with some particle coalescence. Consequently, the VO2/CFP shows a high electrocatalytic OER activity with the lowest η10 (350 mV) and Tafel slope (46 mV/dec) values in a 1 M aqueous solution of KOH as compared to those of the vacuum annealed V2O5 and the hydrothermally grown VO2 (M1), α-V2O5, and γ′-V2O5. The catalytically active site is considered V4+ components and V4+/5+ redox couples in VO2. The oxidation state of V4+ is revealed to be more favorable to the OER catalysis compared to that of V5+ in vanadium oxide through comparative studies. Furthermore, the amount of V5+ component is found to be increased on the surface of VO2 catalyst during the OER, giving rise to the performance degradation. This work suggests V4+ and its redox couple as a novel active component for the OER in metal-oxide electrocatalysts.


Introduction
Renewable energy sources, such as wind and solar power, are providing an increasing share of the energy supply [1,2]. In particular, electrocatalytic water splitting is considered a promising strategy for the sustainable production of hydrogen from renewable energy sources [3][4][5][6][7]. However, overall water splitting suffers from the sluggish kinetics of anodic oxygen evolution reaction (OER) involving the four-electron oxidative half-reaction [8,9]. Therefore, an appropriate catalyst is necessary to accelerate the OER at low overpotentials (η) for enhanced energy conversion efficiencies [5]. The state-of-the-art OER catalysts are based on precious metals such as IrO 2 and RuO 2 with η close to 300 mV in a 1 M aqueous solution of KOH. However, the scarcity and high costs of those materials restrict their large-scale application [9]. In this regard, the earth-abundant transition-metal oxides based on Mn, Fe, Co, and Ni have been extensively investigated as alternative OER catalysts. In particular, Ni or Co-based layered double hydroxides (LDHs) exhibited high OER activities due to the oxidation or stabilization of Ni or Co ions forming the OER-active redox couples with the addition of Fe ion [8,[10][11][12]. Also, spinel-type oxides such as Co 3 O 4 and NiCo 2 O 4 and amorphous Co-based oxides were proposed as promising OER catalysts [8,9,13,14]. Meanwhile, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF) perovskite was identified as the most promising OER catalyst based on orbital principles with the e g occupation as a superior descriptor for oxygen evolution activity [15]. Other descriptor approaches of M-OH bond strength and (∆G O * − ∆G HO *) were suggested to find superior catalysts [16,17]. However, application of such descriptors is limited to experimental work and finding novel catalysts based on other transition metals such as Nb, Ta, and V [9,18]. Among transition metal Preparation of VO 2 (M1) nanoparticles by vacuum annealing: To prepare VO 2 nanoparticles coated on a carbon fiber paper (CFP), 0.1 g of V 2 O 5 particles (Sigma-Aldrich, St. Louis, MI, USA) was dispersed in 10 mL of ethanol, followed by ultrasonication for 1 h. A solution loading of 100 µL/cm 2 using a micropipette was cast onto a bare CFP (Toray Paper 120) substrate with dimensions of 2 cm × 1 cm size, which was cleaned by ultrasonication in acetone, deionized water, and ethanol. The solution-coated CFP was allowed to dry in air overnight and placed within an alumina boat, which was placed at the center of a 1-inch-diameter horizontal cold-wall quartz tube furnace equipped with a rotary pump system. For annealing, the tube furnace was heated to 950 • C at a ramp rate of 20 • C/min under vacuum condition (~10 mTorr). After holding at 950 • C for 10 min, the furnace was allowed to cool naturally to room temperature. The annealed coat on CFP was then removed from the center of the furnace for characterization and electrocatalytic evaluation. To prepare V 2 O 5 nanoparticles coated on CFP, the same V 2 O 5 -dispersion solution was cast onto a bare CFP substrate in the same way and allowed to dry in air overnight. Then, the solution-coated CFP was placed within the alumina boat placed at the furnace center, followed by annealing. The reactor was heated to a temperature of 950 • C at a ramp rate of 20 • C/min under an O 2 flow of 10 sccm and continuous vacuum pumping condition. After the temperature was held at 950 • C for 10 min, the furnace was then naturally cooled to room temperature.
Preparation of VO 2 (M1) nanoparticles f or comparison: VO 2 (M1) nanoparticles were synthesized hydrothermally by adding 300 mg of V 2 O 5 powder (Sigma-Aldrich) and 450 mg of oxalic acid (Fisher Scientific, Waltham, MA, USA) to 16 mL of deionized water (ρ = 18.2 MΩ/cm, purified using a Barnstead International NANOpure Diamond system) in a 23 mL polytetrafluoroethylene cup, as reported in previous work [38]. The reaction mixture was heated within an autoclave to 250 • C for 72 h. A matte-black powder was collected by vacuum filtration and washed with copious amounts of acetone and deionized water.
Preparation of α-V 2 O 5 nanowires f or comparison: α-V 2 O 5 nanowires (with the average diameters of between 150-250 nm and lengths ranging from 1-100 µm) were synthesized according to a previously reported method [39]. In a typical reaction, 1.6 g of V 2 O 5 (Beantown Chemical, Hudson, NH, USA, 99.5%) were added to a 125 mL capacity Teflon-lined autoclave. Subsequently, 80 mL of deionized water (ρ = 18.2 MΩ/cm) and 8 mL of 2-propanol (ACS reagent grade) were added. The autoclave was heated at 210 • C for 48 h. Following the heating period, the autoclave was removed from the oven and allowed to cool autogenously. The resulting green to blue powder (V 3 O 7 ·H 2 O) was separated by filtration and washed with copious amounts of water and 2-propanol and allowed to dry in air overnight. The dried paper-like product was placed in a ceramic crucible and heated in open air in a muffle furnace at 350 • C for 72 h with stirring every 24 h. The resulting yellow-orange paper-like product was lightly ground and used in further experiments without any subsequent modification.
Preparation of γ -V 2 O 5 microrods f or comparison: The metastable γ -V 2 O 5 polymorph microrods (with average widths of 0.9-1.1 µm) were synthesized according to a previously reported synthesis [40]. In a typical reaction, about 1 g of α-V 2 O 5 was added to 40 mL of acetonitrile (MilliporeSigma, Burlington, MA, USA, <50 ppm H 2 O, 99.8%) under inert atmosphere. Subsequently, 1.25 molar equivalents (typically ca. 1 g) of LiI (Alfa Aesar, Haverhill, MA, USA, anhydrous, 98%) were added to the suspension. The reaction was allowed to proceed without stirring under inert atmosphere for 72 h. The dark green-blue γ-LiV 2 O 5 product was separated by vacuum filtration and washed with copious amounts of acetonitrile and 2-propanol and allowed to dry in air overnight. The γ-LiV 2 O 5 powder was then dispersed in 40 mL of acetonitrile under an inert atmosphere. Subsequently, 1.25 molar equivalents of NOBF 4 were added to the suspension, resulting in the complete topotactic deintercalation of lithium from the structure. The resulting orange/red powder was separated from the suspension by vacuum filtration, washed with copious amounts of acetonitrile and 2-propanol, and used without further modification.
Each 0.1 g of the prepared VO 2 (M1), α-V 2 O 5 , and γ -V 2 O 5 products was dispersed in 10 mL of ethanol, followed by ultrasonication for 1 h. Each solution loading of 100 µL/cm 2 using a micropipette was cast onto CFP and allowed to dry in air overnight.
Structural characterization: The morphology of the prepared materials was examined by field-emission scanning electron microscopy (FESEM) using a JEOL JSM-7500F instrument. Particles harvested from carbon fiber paper (CFP) substrate by ultrasonication in toluene and hydrothermally grown particles were examined by high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2010 instrument operated at an accelerating voltage of 200 keV. Phase assignment was performed with the help of X-ray diffraction (XRD) using a Bruker D8-Advance instrument equipped with a Cu Kα source (λ = 1.5418 Å) as well as by Raman microprobe analysis using a Jobin-Yvon HORIBA LabRAM HR800 instrument coupled to an Olympus BX41 microscope. Raman spectra were collected with excitation from the 514.5 nm line of an Ar-ion laser; the laser power was kept below 10 mW to minimize photooxidation. Differential scanning calorimetry (DSC) analysis was performed using a TA Instruments Q2000 instrument. The temperature was scanned from 0 to 100 • C and back again to 0 • C at ramp rates ranging from 1 to 15 • C/min. For DSC experiments, the VO 2 -deposited CFP was cut into small pieces and stacked in an aluminum T-Zero pan under a purge flow of Ar gas. A bare CFP was used as a reference. The chemical composition and oxidation states of the prepared materials were investigated by X-ray photoelectron spectroscopy (XPS, Omicron XPS) with Mg Kα radiation (1253.6 eV). Energy calibration was achieved by setting the C1s line from adventitious hydrocarbons to 284.8 eV. Vanadium K-edge X-ray absorption near-edge structure (XANES) spectra were collected at the Advanced Light Source (ALS) bending magnet beamline 10.3.2. V K-edge XANES spectra were recorded in fluorescence mode in the energy range 5450-5600 eV by continuously scanning a Si (111) monochromator (Quick XAS mode) from 20 eV below to 40 eV above the white line absorption. For XANES analysis, a suite of custom LabVIEW programs at the beamline was used to perform deadtime correction, energy calibration, glitch removal, pre-edge subtraction, and post-edge normalization. The Athena suite of programs in the IFEFFIT package was used to analyze the XANES spectra.

Electrochemical characterization:
The oxygen evolution reaction (OER) performance of the prepared materials was evaluated using a three-electrode cell with the help of a Bio-Logic potentiostat (SP-200). All of the measurements were performed in a 1 M aqueous solution of KOH purged with N 2 gas. The vanadium oxides prepared on CFP were individually used as the working electrodes. A saturated calomel electrode (SCE) and a Pt foil were used as reference and counter electrodes, respectively. The potential versus SCE (E SCE ) was converted to the potential versus the reversible hydrogen electrode (RHE) (E RHE ) using the relation E RHE = E SCE + 1.0464 V [41]. Polarization curves for OER were measured using linear sweep voltammetry (LSV) in the range between 1.2 and 1.8 V versus RHE at a scan rate of 8 mV/s. The polarization curves were corrected for the ohmic potential drop (iR) losses, where R is the series resistance of the electrochemical cell as determined by electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were performed in the range between 200 kHz and 50 mHz using an AC amplitude of 25 mV. The double-layer capacitance (C dl ) of the samples was determined by cyclic voltammetry (CV) at scan rates between 20-100 mV/s. Gas chromatography (GC) analysis of gaseous product was performed on the headspace of sealed electrocatalytic cells with the three-electrode configuration in a 1 M KOH electrolyte solution. The cells were sealed under an Ar ambient. After application of a constant voltage of 1.6 V versus RHE for 30 min, the headspace was sampled using a syringe. An Agilent Trace 1300 GC equipped with a thermal conductivity detector and a custom-made 120 cm stainless steel column packed with Carbosieve-II from Sigma-Aldrich was used for analysis. The carrier gas was Ar. Identification of O 2 produced from electrolysis were accomplished by withdrawing 200 µL of the headspace using a 0.5 mL Valco Precision Sampling Syringe, Series A-2 equipped with a Valco Precision Sampling syringe needle with a five-point side port.

Results and Discussion
The VO 2 nanoparticles have been prepared through the vacuum annealing of the V 2 O 5 particles coated on a carbon fiber paper (CFP) at 950 • C, which is slightly higher than the melting point of V 2 O 5 (690 • C at standard temperature and pressure). The fieldemission scanning electron microscopy (FESEM) image in Figure 1a shows clearly that the produced VO 2 nanoparticles with the size of ca. 300 ± 76 nm in diameter uniformly cover the surface of individual carbon fibers (ca. 7 µm in diameter) constituting the CFP. The bare surface of CFP is comparatively shown with a smooth grain by FESEM images in Figure  S1 of the Supplementary material. More specifically, the VO 2 nanoparticles have some particle coalescence, which results from solidifying after slightly melting down (see the high-magnification image in Figure 1b). The phase of VO 2 nanoparticles obtained should be defined since the VO 2 possesses three polymorphs, M1 monoclinic, M2 monoclinic, and R rutile phases, across the phase transition at ca. 67 • C in the bulk [34][35][36][37]. Each of them has distinctive Raman spectral signatures arising from the pronounced differences in local symmetry, where the space groups for the R and M1 phases are P4 2 /mnm (D 4h 14 ) and P2 1 /c (C 2h 3 ), respectively [36,37]. In particular, the most stable M1 phase at room temperature is characterized by 18 Raman-allowed modes, 9 each of A g and B g symmetry [36,37,42,43]. In Figure 1c, the Raman modes acquired for our VO 2 nanoparticles at room temperature are assigned to the M1 monoclinic phase of VO 2 . The mode assignments denoted in Figure 1c are derived from group theory considerations and previously reported polarized Raman spectroscopy studies [42][43][44]. Furthermore, the XRD reflections acquired on CFP are indexed well to the M1 phase of VO 2 (Joint Committee on Powder Diffraction Standards (JCPDS) 43-1051), as shown in Figure S2. These results indicate that the VO 2 nanoparticles obtained here have a well-defined single M1 phase without any mixed phase. The structural characterization has been further performed using high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). Figure 1d,e show the lattice-resolved HRTEM image and the indexed SAED pattern of an individual VO2 (M1) nanoparticle harvested from the VO2/CFP sample by ultrasonication for 1 h in toluene (low-magnification TEM image of VO2 (M1) nanoparticles is shown in Figure S3a). The nanoparticle reveals the d-spacing of (011) lattice planes, which are the XRD main reflection of VO2 M1 monoclinic phase. The (200) lattice planes of VO2 M1 phase are also observed in another individual nanoparticle ( Figure S3b). These results corroborate the single crystalline nature of the individual nanoparticles.
Meanwhile, it should be noticed that vanadium oxides have multiple vanadium oxidation states (V 2+ , V 3+ , V 4+ , V 5+ ) [24,25]. Among them, VO2 with the oxidation state of V 4+ exhibits the characteristic phase transition phenomenon at near 67 °C as mentioned earlier, which is distinct from compounds with the other oxidation states. Such property can be used for characterizing the phase and defect of the prepared VO2. In detail, the thermally and electrically induced metal-insulator phase transition in VO2 is accompanied by a considerable consumption (M1  R) or release (R  M1) of latent heat, given that it is a first-order transition [37,45]. The latent heat at the phase transition comprises an enthalpy component arising from the structural distortion of the lattice (which is compensated in part by a modulation of the phonon entropy) and conduction entropy of electrons [37,[45][46][47]. Figure 2a shows differential scanning calorimetry (DSC) profiles acquired at various scan rates for the prepared VO2 nanoparticles. The pronounced endothermic (M1  R) and exothermic (R  M1) DSC traces centered at around 65 °C and 62 °C are observed upon heating and cooling, respectively; the temperatures are recognized at Tmax which represents the temperature at the maximum height of the transition peak and is indicative of the maximum transformation rate [45]. Such DSC traces corroborate again that the prepared vanadium oxide is indeed VO2 with the metal-insulator phase transition. Figure 2b shows the evolution of hysteresis, which is defined as the difference between Tmax temperatures acquired upon heating and cooling across the scan rates. The Tmax upon heating is slightly altered between 65.2 °C and 65.7 °C across the measured scan rates, while the one The structural characterization has been further performed using high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). Figure 1d,e show the lattice-resolved HRTEM image and the indexed SAED pattern of an individual VO 2 (M1) nanoparticle harvested from the VO 2 /CFP sample by ultrasonication for 1 h in toluene (low-magnification TEM image of VO 2 (M1) nanoparticles is shown in Figure S3a). The nanoparticle reveals the d-spacing of (011) lattice planes, which are the XRD main reflection of VO 2 M1 monoclinic phase. The (200) lattice planes of VO 2 M1 phase are also observed in another individual nanoparticle ( Figure S3b). These results corroborate the single crystalline nature of the individual nanoparticles.
Meanwhile, it should be noticed that vanadium oxides have multiple vanadium oxidation states (V 2+ , V 3+ , V 4+ , V 5+ ) [24,25]. Among them, VO 2 with the oxidation state of V 4+ exhibits the characteristic phase transition phenomenon at near 67 • C as mentioned earlier, which is distinct from compounds with the other oxidation states. Such property can be used for characterizing the phase and defect of the prepared VO 2 . In detail, the thermally and electrically induced metal-insulator phase transition in VO 2 is accompanied by a considerable consumption (M1 → R) or release (R → M1) of latent heat, given that it is a first-order transition [37,45]. The latent heat at the phase transition comprises an enthalpy component arising from the structural distortion of the lattice (which is compensated in part by a modulation of the phonon entropy) and conduction entropy of electrons [37,[45][46][47]. Figure 2a shows differential scanning calorimetry (DSC) profiles acquired at various scan rates for the prepared VO 2 nanoparticles. The pronounced endothermic (M1 → R) and exothermic (R → M1) DSC traces centered at around 65 • C and 62 • C are observed upon heating and cooling, respectively; the temperatures are recognized at T max which represents the temperature at the maximum height of the transition peak and is indicative of the maximum transformation rate [45]. Such DSC traces corroborate again that the prepared vanadium oxide is indeed VO 2 with the metal-insulator phase transition. Figure 2b shows the evolution of hysteresis, which is defined as the difference between T max temperatures acquired upon heating and cooling across the scan rates. The T max upon heating is slightly altered between 65.2 • C and 65.7 • C across the measured scan rates, while the one upon cooling is almost constant with the values between 62.4 • C and 62.6 • C. As a result, it is found that the width of hysteresis in the prepared VO 2 nanoparticles here is relatively scan-rate-invariant compared to that of the hydrothermally prepared VO 2 nanoparticles previously [45]. The phase transitions in VO 2 upon heating and cooling are known to be mediated by defects such as oxygen vacancy, twin boundary, and grain boundary, which serve as phase nucleation sites [45,48]. The extent of supercooling of the high-temperature phase and superheating of the low-temperature phase (thermal hysteresis) can be decreased with increasing concentration of defects since the nucleation probability is increased with increasing defect density. Therefore, the low thermal hysteresis for the prepared VO 2 nanoparticles here indicates importantly that those nanoparticles possess a considerable number of structural defects. upon cooling is almost constant with the values between 62.4 °C and 62.6 °C. As a result, it is found that the width of hysteresis in the prepared VO2 nanoparticles here is relatively scan-rate-invariant compared to that of the hydrothermally prepared VO2 nanoparticles previously [45]. The phase transitions in VO2 upon heating and cooling are known to be mediated by defects such as oxygen vacancy, twin boundary, and grain boundary, which serve as phase nucleation sites [45,48]. The extent of supercooling of the high-temperature phase and superheating of the low-temperature phase (thermal hysteresis) can be decreased with increasing concentration of defects since the nucleation probability is increased with increasing defect density. Therefore, the low thermal hysteresis for the prepared VO2 nanoparticles here indicates importantly that those nanoparticles possess a considerable number of structural defects.  The oxidation state and stoichiometry of VO 2 have been characterized by X-ray photoelectron spectroscopy (XPS). Figure 2c,d show V 2p and O 1s spectra for the VO 2 nanoparticles, respectively. The V 2p 3/2 spectrum can be deconvoluted into two sub-peaks arising from the V 4+ (516.3 eV) and V 5+ (517.7 eV) components in Figure 2c, where the binding energy values for the spectra correspond to the reported values [49][50][51]. Noticeably, it indicates that the prepared VO 2 consists primarily of V 4+ component (87 at%) with the small amount of V 5+ component (13 at%). The main peak for O 1s located at 530.1 eV is assigned to the lattice oxygen comprising the VO 2 in Figure 2d [49,52]. The additional oxygen component located at 531.3 eV can be assigned to the surface-adsorbed oxygen or the C-O and C=O bonds coming from it [52]. Figure 2e plots V K-edge X-ray absorption spectra of the prepared VO 2 and contrasting α-V 2 O 5 , acquired by X-ray absorption near-edge structure (XANES) spectroscopy. For comparison, α-V 2 O 5 has been prepared in the form of nanowires by the hydrothermal method with the average diameters of between 150-250 nm and lengths ranging from 1-100 µm, according to a previously reported method [39]. The spectra consist of the pre-edge and white-line absorption features due to dipole-allowed transitions from V 1s to 3d states and from V 1s to 4p states, respectively. In Figure 2e, the VO 2 (composed of V 4+ ions) exhibits the characteristic V K-edge absorption spectrum with obviously different line shape, intensity, and peak position from α-V 2 O 5 (composed of V 5+ ions), indicating the discrepancy in the local symmetry and oxidation state of vanadium atoms between two types of V-O systems [53]. Specifically, a broader pre-edge peak for VO 2 is observed at 5468.04 eV, shifting toward lower energy as compared to that of α-V 2 O 5 (5469.13 eV) (see the inset of Figure 2e). On the other hand, a primary edge peak in the white-line absorption feature is more intensely observed at 5489.94 eV for VO 2 . A distinctive pre-edge feature observed in V K-edge spectrum of α-V 2 O 5 is essentially local in character and originates from the broken octahedral symmetry of the vanadium centers in V 2 O 5 [53]. The asymmetric broadening and shift of the area-weighted centroid to lower energy in the pre-edge feature across two V-O systems is a result of the reduction of the V 5+ sites to V 4+ sites. Upon such a reduction of vanadium sites, the electron remains localized within V 3d xy orbitals with stabilization of a small polaron and the excitation of core electrons requires less energy due to more screening charge at the excited atom, and thus the pre-edge feature is shifted to lower energy [21,22,53]. In addition to the red shift of the pre-edge peak position, the V 4+ character contributes to the broadening of the pre-edge peak due to the increase of octahedral symmetry precluding V 4p-3d hybridization, as shown in the inset of Figure 2e. As a result, it is found that the prepared VO 2 is considerably stoichiometric and somewhat free from the point defect such as oxygen vacancy. Hence, the principal defect associated with the low thermal hysteresis across the phase transition for VO 2 nanoparticles is considered grain boundaries among VO 2 nanoparticles with some particle coalescence, as observed in FESEM image of Figure 1a.
To investigate the effect of vanadium oxidation state (V 4+ /V 5+ ) on the electrocatalytic properties of vanadium oxide, we have further prepared V 2 O 5 nanoparticles covering the carbon fibers through the annealing of the V 2 O 5 particles coated on the CFP at 950 • C under an O 2 flow of 10 sccm and continuous vacuum pumping condition using a tube furnace. FESEM images in Figure S4a,b reveal the morphology of the V 2 O 5 nanoparticles covering uniformly the surface of individual carbon fibers constituting the CFP, where the morphology and particle size are similar to those of the VO 2 nanoparticles. Only slightly faceted shape in the V 2 O 5 nanoparticles is observed as compared with the VO 2 nanoparticles ( Figure S4b). The Raman bands acquired for the prepared V 2 O 5 nanoparticles in Figure S4c are well matched with the Raman-active modes of V 2 O 5 reported in the literature [54,55]. XPS V 2p and O 1s spectra for the prepared V 2 O 5 nanoparticles are shown in Figure S4a,b, respectively. The XPS result demonstrates that the prepared V 2 O 5 nanoparticles are composed primarily of V 5+ component (80 at%) with the small amount of V 4+ component (20 at%). Besides, VO 2 (M1), α-V 2 O 5 , and γ -V 2 O 5 have been separately prepared by the hydrothermal methods for comparison, followed by drop-casting onto CFP. The agglomeration of the hydrothermally grown VO 2 (M1) nanoparticles in a few nanometer size is shown by TEM in Figure S5a,b. The hydrothermally grown VO 2 (M1) nanoparticles are characterized to possess the same ratio of V 4+ to V 5+ by XPS ( Figure S5c,d), which is a higher amount of V 5+ component compared to the VO 2 nanoparticles prepared by the vacuum annealing. The α-V 2 O 5 nanowires with the average diameters of between 150-250 nm and lengths ranging from 1-100 µm and the metastable γ -V 2 O 5 polymorph microrods with average widths of 0.9-1.1 µm were synthesized separately (their XRD patterns and FESEM images are shown in Figure S6).
The electrocatalytic OER characteristics of the vanadium oxides prepared on CFP have been investigated in a 1 M aqueous solution of KOH, using a conventional three-electrode setup. Figure 3a displays linear polarization curves, which have been corrected for ohmic potential drop (iR) losses. Bare CFP is contrasted as a control and is essentially catalytically inert toward OER. Remarkably high OER performance for the VO 2 (M1) nanoparticles prepared by the vacuum annealing on CFP is found with an overpotential of 350 mV, reaching a current density of 10 mA/cm 2 (η 10 ) and a Tafel slope of 46 mV/dec (Figure 3b). The hydrothermally grown VO 2 (M1) nanoparticles prepared by drop-casting on CFP exhibit the second-highest OER activity with a η 10 value of 460 mV and a Tafel slope of 114 mV/dec. As compared to the VO 2 samples, V 2 O 5 samples represent relatively low OER activities with higher η 10 values more than 490 mV and higher Tafel slope values more than 130 mV/dec. Among the V 2 O 5 samples, γ -V 2 O 5 shows the lowest η 10 value of 490 mV, but its Tafel slope value (134 mV/dec) is roughly equal to that of α-V 2 O 5 (131 mV/dec), indicating their same OER kinetics. Nanomaterials 2022, 12, x FOR PEER REVIEW 8 of (20 at%). Besides, VO2 (M1), α-V2O5, and γ′-V2O5 have been separately prepared by th hydrothermal methods for comparison, followed by drop-casting onto CFP. The agglom eration of the hydrothermally grown VO2 (M1) nanoparticles in a few nanometer size shown by TEM in Figure S5a,b. The hydrothermally grown VO2 (M1) nanoparticles a characterized to possess the same ratio of V 4+ to V 5+ by XPS ( Figure S5c,d), which is higher amount of V 5+ component compared to the VO2 nanoparticles prepared by the va uum annealing. The α-V2O5 nanowires with the average diameters of between 150-25 nm and lengths ranging from 1-100 μm and the metastable γ′-V2O5 polymorph microrod with average widths of 0.9-1.1 μm were synthesized separately (their XRD patterns an FESEM images are shown in Figure S6). The electrocatalytic OER characteristics of the vanadium oxides prepared on CF have been investigated in a 1 M aqueous solution of KOH, using a conventional thre electrode setup. Figure 3a displays linear polarization curves, which have been correcte for ohmic potential drop (iR) losses. Bare CFP is contrasted as a control and is essential catalytically inert toward OER. Remarkably high OER performance for the VO2 (M1) n noparticles prepared by the vacuum annealing on CFP is found with an overpotential 350 mV, reaching a current density of 10 mA/cm 2 (η10) and a Tafel slope of 46 mV/de (Figure 3b). The hydrothermally grown VO2 (M1) nanoparticles prepared by drop-castin on CFP exhibit the second-highest OER activity with a η10 value of 460 mV and a Taf slope of 114 mV/dec. As compared to the VO2 samples, V2O5 samples represent relative low OER activities with higher η10 values more than 490 mV and higher Tafel slope valu more than 130 mV/dec. Among the V2O5 samples, γ′-V2O5 shows the lowest η10 value 490 mV, but its Tafel slope value (134 mV/dec) is roughly equal to that of α-V2O5 (13 mV/dec), indicating their same OER kinetics.  We further measured the double layer capacitance (C dl ) for the samples using a cyclic voltammetry (CV) method to estimate the electrochemically active surface area (ECSA), which is directly proportional to C dl [18,33]. The voltammograms for the VO 2 (M1) nanoparticles prepared by the vacuum annealing on CFP have been collected at various scan rates (20-100 mV/s) in the potential range of 0.75-1.35 V versus reversible hydrogen electrode (RHE) (a potential range with no Faradaic current), where the current is preponderantly due to the charging of the double layer (and not due to water oxidation), as shown in Figure 3c. The differences (∆j) of anodic and cathodic current densities at 1.0 V vs. RHE for the CV plot are shown as a function of the scan rate in Figure 3d. The slope of ∆j vs. scan rate plot is equal to a value of 2C dl . The CV and ∆j vs. scan rate plots acquired for the α-V 2 O 5 , γ -V 2 O 5 , and commercial V 2 O 5 samples prepared on CFP are shown in Figure S7 for comparison. As a result, the VO 2 (M1) nanoparticles prepared by the vacuum annealing, which have the lowest η 10 (350 mV) and Tafel slope (46 mV/dec) values, are found to exhibit the highest C dl value of 4298.43 µF/cm 2 . By contrast, α-V 2 O 5 , γ -V 2 O 5 , and commercial V 2 O 5 show relatively low C dl values of 742.54, 748.98, and 491.42 µF/cm 2 , respectively ( Figure S7). These experimental results definitely indicate that VO 2 shows higher OER activity than V 2 O 5 . In other words, it is concluded that V 4+ in vanadium oxide composes a catalytically active site for the OER with high intrinsic activity. A much higher OER activity of the VO 2 (M1) nanoparticles prepared directly on CFP by the vacuum annealing compared to the hydrothermally grown and drop-casted ones can be explained by their improved coverage and adhesion on the carbon fibers (i.e., improved morphological factor by process innovation, facilitating charge transfer) as well as their higher V 4+ content (87% for the former and 50% for the latter, according to the XPS result). For V 2 O 5 catalysts, a slightly higher OER activity (lower η 10 ) of γ -V 2 O 5 than α-V 2 O 5 , despite their identical OER mechanism with the roughly same Tafel slope value, is regarded to result from a smaller particle size and concurrently higher ECSA (higher C dl value) of γ -V 2 O 5 than α-V 2 O 5 ( Figure S6). Indeed, much bulkier commercial V 2 O 5 particles with the lowest ECSA (lowest C dl ) show the lowest OER activity. However, it should be noted that these V 5+ -constituted V 2 O 5 catalysts reveal much lower OER activities compared to the V 4+ -constituted VO 2 catalyst. According to the theoretically proposed mechanism, the OER proceeds in the four steps in a basic environment as per [8]: where * denotes a surface adsorption site. The adsorption energies for intermediates of OH *, O *, and OOH *, formed in the reaction steps, determine the efficiency of the catalyst; i.e., the lowest overpotential is achieved when the energies of the OH * → O * (Equation (2)) and O * → OOH * (Equation (3)) steps are equalized. Thus, the lower energy disparity between two steps on the adsorption sites of the catalyst is, the higher efficiency of the catalyst is. Meanwhile, the entire water-splitting cycle can be divided into a metal oxidation step and metal reduction step with O 2 evolution, where the oxidation state and redox kinetics of the transition metal play important roles in determining the OER efficiency by affecting the adsorption energy [8]. When the kinetics of the metal reduction with O 2 evolution step is slow or rapid, the metal oxidation maintains a high-valent or low-valent state at the OER potential, respectively. Here, when the kinetics of the metal-reduction step is controlled to maintain the metal oxidation state with a high intrinsic OER activity at the OER potential, the OER activity of the catalyst is enhanced. Therefore, in this work, the high OER activity of VO 2 is attributed to the preservation of highly OER active V 4+ components and V 4+/5+ redox couples in VO 2 . Likewise, V 3+ , Co 3+ , Fe 3+ , Ni 3+ , and their redox couples are known to work as the OER-active species in certain crystal structures [8,18]. More recently, the amorphous VO x and CoVO x with V 4+ were also reported to have high OER activities [9]. A high OER activity of the VO 2 (M1) with η 10 of 350 mV and Tafel slope of 46 mV/dec is found to be comparable to those of various electrocatalysts reported in the literature (see Table S1): e.g., η 10 of 390 mV for CoO x , η 10 of 465 mV for amorphous VO x , η 10 of 410 mV for V 2 O 5 , and 534 mV for Co 3 O 4 . In particular, the VO 2 (M1) shows a much lower level of Tafel slope value compared to the nickel-, cobalt-, and manganesebased catalysts as well as the other vanadium-based catalysts such as VOOH, amorphous VO x , and V 6 O 13 . The η 10 of VO 2 (M1) is even comparable to those of state-of-the-art OER catalysts, RuO 2 and IrO 2 with η 10 of around 300 mV. The oxygen evolution from the VO 2 (M1) nanoparticles prepared by the vacuum annealing on CFP has been corroborated by gas chromatography (GC) analysis. The resulting GC trance in Figure S8 shows the only peak eluting from the column at 2.29 min, corresponding to O 2 . The long-term stability test of the VO 2 (M1) catalyst has been performed by 1000 repeated CV sweeps in a 1 M aqueous solution of KOH in the range between 0.75 and 1.60 V versus RHE at a scan rate of 100 mV/s. As sown in Figure 4a, the VO 2 (M1) catalyst exhibits stable performance with almost exactly superimposable polarization curves after 1000 sweeps. The FESEM image of the catalyst acquired after the long-term stability test is shown in Figure 4b. Compared with the morphology before 1000 sweeps in Figure 1b, there seemed to be no remarkable difference. To further investigate the surface state of the VO 2 (M1) catalyst after OER, XPS spectra were acquired for the catalyst after OER under a constant voltage of 1.6 V versus RHE for 30 min in a 1 M KOH electrolyte solution (Figure 4c,d). Noticeably, the V 5+ component on the surface of the catalyst is found to be largely increased from 13% to 76% after OER, which is close to V 2 O 5 . Such an oxidation of vanadium during OER again corroborates a relatively poor OER activity of V 5+ site in VO 2 (M1), although the overpotential and morphology for the catalyst did not change obviously until 1000 CV sweeps. Similarly, the metal oxidation behavior involving a performance degradation is found in the several literature reports. For example, the surface of VOOH catalyst is observed to oxidize from V 3+ to V 5+ after OER [18]. Also, Kim et al. reported the surface oxidation from Co 2+ to Co 3+ after OER in amorphous cobalt phyllosilicate catalyst [14].
the high OER activity of VO2 is attributed to the preservation of highly OER active V 4+ components and V 4+/5+ redox couples in VO2. Likewise, V 3+ , Co 3+ , Fe 3+ , Ni 3+ , and their redox couples are known to work as the OER-active species in certain crystal structures [8,18]. More recently, the amorphous VOx and CoVOx with V 4+ were also reported to have high OER activities [9]. A high OER activity of the VO2 (M1) with η10 of 350 mV and Tafel slope of 46 mV/dec is found to be comparable to those of various electrocatalysts reported in the literature (see Table S1): e.g., η10 of 390 mV for CoOx, η10 of 465 mV for amorphous VOx, η10 of 410 mV for V2O5, and 534 mV for Co3O4. In particular, the VO2 (M1) shows a much lower level of Tafel slope value compared to the nickel-, cobalt-, and manganese-based catalysts as well as the other vanadium-based catalysts such as VOOH, amorphous VOx, and V6O13. The η10 of VO2 (M1) is even comparable to those of state-of-the-art OER catalysts, RuO2 and IrO2 with η10 of around 300 mV.
The oxygen evolution from the VO2 (M1) nanoparticles prepared by the vacuum annealing on CFP has been corroborated by gas chromatography (GC) analysis. The resulting GC trance in Figure S8 shows the only peak eluting from the column at 2.29 min, corresponding to O2. The long-term stability test of the VO2 (M1) catalyst has been performed by 1000 repeated CV sweeps in a 1 M aqueous solution of KOH in the range between 0.75 and 1.60 V versus RHE at a scan rate of 100 mV/s. As sown in Figure 4a, the VO2 (M1) catalyst exhibits stable performance with almost exactly superimposable polarization curves after 1000 sweeps. The FESEM image of the catalyst acquired after the long-term stability test is shown in Figure 4b. Compared with the morphology before 1000 sweeps in Figure 1b, there seemed to be no remarkable difference. To further investigate the surface state of the VO2 (M1) catalyst after OER, XPS spectra were acquired for the catalyst after OER under a constant voltage of 1.6 V versus RHE for 30 min in a 1 M KOH electrolyte solution (Figure 4c,d). Noticeably, the V 5+ component on the surface of the catalyst is found to be largely increased from 13% to 76% after OER, which is close to V2O5. Such an oxidation of vanadium during OER again corroborates a relatively poor OER activity of V 5+ site in VO2 (M1), although the overpotential and morphology for the catalyst did not change obviously until 1000 CV sweeps. Similarly, the metal oxidation behavior involving a performance degradation is found in the several literature reports. For example, the surface of VOOH catalyst is observed to oxidize from V 3+ to V 5+ after OER [18]. Also, Kim et al. reported the surface oxidation from Co 2+ to Co 3+ after OER in amorphous cobalt phyllosilicate catalyst [14].  The phase transition of VO 2 was investigated in this work only to identify an accurate preparation of the VO 2 (M1) nanoparticles. Since the phase transition temperature was found to be ca. 65 • C (heating) and 62 • C (cooling), the effect of phase transition on the OER in electrocatalytic systems operating at room temperature was ignored. Although the relationship between phase transition and electrocatalytic (including the OER) activity in VO 2 goes beyond the subject of this study, it can be another huge topic requiring further investigation.

Conclusions
In summary, in this work, the VO 2 (M1) is found to possess high electrocatalytic OER activity and stability for the first time. For this, the single-phase VO 2 (M1) nanoparticles, uniformly covering the surface of individual carbon fibers constituting the CFP, have been prepared through the vacuum annealing technique. The thermal analysis corroborates that the prepared VO 2 nanoparticles reveal the metal-insulator phase transition at ca. 65 • C (heating) and 62 • C (cooling) with the low thermal hysteresis, indicating a high concentration of structural defect which is considered grain boundaries among VO 2 nanoparticles with some particle coalescence. Consequently, the prepared VO 2 (M1) nanoparticles on CFP show a high electrocatalytic OER activity with the lowest η 10 (350 mV) and Tafel slope (46 mV/dec) values in a 1 M aqueous solution of KOH as compared to those of the vacuum annealed V 2 O 5 and the hydrothermally grown VO 2 (M1), α-V 2 O 5 , and γ -V 2 O 5 . The catalytically active site is considered V 4+ components and V 4+/5+ redox couples in VO 2 . The oxidation state of V 4+ is revealed to be more favorable to the OER catalysis compared to that of V 5+ in vanadium oxide through comparative studies. Furthermore, the amount of V 5+ component is found to be increased on the surface of VO 2 catalyst during OER, giving rise to the performance degradation. This work suggests V 4+ and its redox couple as a novel active component for OER in metal-oxide electrocatalysts.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano12060939/s1, References [9,10,14,[18][19][20][56][57][58][59] are cited in the Supplementary Materials. Figure S1. FESEM images of a bare carbon fiber paper (CFP). (b) illustrates the high-magnification image of a single carbon fiber. Figure Figure S7. Cyclic voltammograms acquired at various scan rates for (a) α-V 2 O 5 , (c) γ'-V 2 O 5 , and (e) commercial V 2 O 5 prepared on CFP. The differences in current density (∆j = j a − j c ) at 0.9 or 1.0 V versus RHE are plotted as a function of the scan rate for (b) α-V 2 O 5 , (d) γ'-V 2 O 5 , and (f) commercial V 2 O 5 prepared on CFP. The C dl values are extrapolated from a linear fit to the plot. Figure S8. Gas chromatogram (GC) of the generated O 2 gas for the VO 2 (M1) nanoparticles prepared by the vacuum annealing on CFP. For GC analysis, the O 2 gas was captured after application of a constant voltage of 1.6 V versus RHE for 30 min in a 1 M KOH electrolyte solution on the headspace of the electrocatalytic cell sealed under an Ar ambient with a three-electrode system. Table S1. Comparison of the electrocatalytic OER activity for metal oxide catalysts.