V6O13 Micro-Flower Arrays Grown In Situ on Ni Foam as Efficient Electrocatalysts for Hydrogen Evolution at Large Current Densities

Abstract

Developing a high-activity, robust and economic electrocatalyst for large-scale green hydrogen production is still of great significance. Herein, a novel V₆O₁₃ nanosheets self-assembled micro-flower array self-supporting electrode is synthesized using a facile one-pot hydrothermal route. Owing to the large electrochemically active surface area of a unique hierarchical micro-flower and the stable all-in-one structure, the as-prepared V₆O₁₃/NF electrode delivers impressive HER activity with extremely low overpotentials of 125 and 298 mV at large current densities of 100 and 1000 mA cm⁻², respectively, and a long-term durability for at least 90 h in an alkaline condition. This work extends the application of vanadium oxides to the realm of electrocatalytic hydrogen fuel production.

1. Introduction

Developing renewable green energy that is eco-friendly and has high efficiency has become an irresistible trend and a global challenge because of the overconsumption of the fossil fuels and excessive CO₂ emissions [1,2]. Nowadays, water electrolysis technology is widely recognized as a promising sustainable pathway to obtain clean, high-purity and high-efficiency hydrogen energy [3,4]. Until now, Pt and Pt-based materials were identified as state-of-the art electrocatalysts that only need quite low potentials to overcome the activation of the energy barrier for a hydrogen evolution reaction (HER) [5,6]. Due to their expensive cost and rare global reserves restricting their wide applications, it is urgent to develop non-precious metal electrocatalysts for the HER of green H₂ production.

Transition metal vanadium-based compounds, including carbides, sulfides and nitrides of vanadium, have been widely investigated as non-noble metal electrocatalysts due to the multivalent states of V metal, its low-cost position and metallic nature [7,8,9]. Among them, the vanadium oxide family includes V₂O₅, VO₂, V₂O₃, V₆O₁₃ and so on, and exhibits excellent electrochemical properties and rich active sites [10,11,12,13]. Zhang et al. constructed oxygen vacancy (O_V) to obtain a higher valence electron binding energy for the regulation of surface chemical states, leading to moderating the adsorption of H* at the V center, and thus optimizing V₂O₅ electrocatalytic activity and stability [14]. The oxygen evolution activity of VO₂ was studied by Yun-Hyuk Choi, proving that the performance of V⁴⁺ was more favorable to the OER catalysis than that of V⁵⁺ [15]. Ding et al. reported on the fabrication of 3D V₆O₁₃ nanotextiles as cathodes of lithium-ion batteries, exhibiting outstanding Li⁺ storage properties such as excellent rate capability, high capacity and stable cyclability [16]. Rao et al. proposed a V₂O₃-based hybrid electrocatalyst for a highly efficient bifunctional air electrode of a Zn–air battery with good cycling stability and mechanical flexibility [17]. Some research groups made great efforts on the OER activity and battery performance, while the performance of vanadium oxide in electrocatalytic hydrogen evolution is a big challenge that still requires more effort for boosting the electrocatalytic HER performance [18,19,20,21].

In this work, we report on the synthesis of a V₆O₁₃ nanosheets self-assembled micro-flower array grown in situ on Ni foam as a highly active and stable HER electrocatalyst at large current densities using a facile one-step hydrothermal method. Benefiting from the well-integrated nanoarray structure, the precise control of the vanadium source content and appropriate oxalic acid addition, the electron and charges transport rate were well motivated, the HER kinetics were accelerated and numerous exposed catalytic active sites were enlarged, thus the resulting V₆O₁₃/NF displayed a glorious HER performance at the large current densities of 100 mA cm⁻² and 1000 mA cm⁻² with an extremely low overpotential of 125 mV and 298 mV, as well as a superior durability for at least 90 h in an alkaline condition. More broadly, these findings inspire new insights into the design of remarkable vanadium oxide electrocatalysts for efficient hydrogen production in the future.

2. Results

2.1. Structural and Morphological Characterizations

The V₆O₁₃ nanosheets self-assembled micro-flower arrays grown in situ on nickel foam (denoted as V₆O₁₃/NF) were synthesized using a simple one-pot hydrothermal reaction, as illustrated in Figure 1. For comparative studies, a control sample was obtained under identical experimental conditions with the exception that no oxalic acid was added (denoted as VO_x/NF, Figure S1). As shown in Figure 2, the composition and crystal structure of the V₆O₁₃/NF materials scraped from NF were studied using X-ray diffraction (XRD). The obvious diffraction peaks were located at 25.4°, 26.8°, 30.1°, 33.5°, 45.6°, 45.8°, 49.5° and 59.8° and were attributed to (110), (003), (−401), (−311), (−601), (114), (020) and (−711) planes of the monoclinic V₆O₁₃ phase (PDF #89-0100), respectively. The results manifest that V₆O₁₃ was successfully synthesized on the surface of the NF substrate. For comparison, the VO_X/NF sample was obtained when H₂C₂O₄ was not added into the synthetic process. As shown in Figure S2A, it can be observed that some diffraction peaks located at 21.55°, 27.68° and 35.45°are indexed to (200), (202) and (311) crystal planes of the tetragonal V₄O₉ (PDF#23-0720) phase, and other diffraction peaks at 20.26°, 26.13° and 31.00° can be assigned to (001), (110) and (400) planes of orthorhombic V₂O₅ (PDF#41-1426). The results confirm that the existence of H₂C₂O₄ played an important role in the formation of the V₆O₁₃ phase.

Then, scanning electron microscopy (SEM) images were conducted to investigate the microstructure and morphology characteristics of the synthetic sample. Firstly, to investigate the effects of the content of the V source on the morphology of V₆O₁₃, three more samples were prepared by changing the addition of NH₄VO₃ (0.2, 0.5 and 1.0 g), denoted as sample-0.2 g, sample-1.0 g, and the remarkable as-prepared V₆O₁₃/NF, here denoted as sample-0.5 g. The SEM spectrum of sample-0.2 g indicated that a few nanosheets and agglomeration were formed on the surface of the NF and the self-assembly micro-flower arrays were not formed when there was not enough of NH₄VO₃ content (Figure 3A,B). As displayed in Figure 3C, the entire NF substrate of sample-0.5 g is equally and densely covered by the micro-flower arrays self-assembled with the V₆O₁₃ nanosheets. The magnificated SEM view in Figure 3D reveals that the structure of every micro-flower is formed by dense nanosheets stretching outward. It is speculated that with the increasing amount of the vanadium source, the much larger diameter of the nanosheets in sample-1.0 g evenly grows on the surface of the nickel foam. The sheets with micrometer scale will extend the charge transfer path and reduce the exposed electrochemical active area, which is not conducive to the hydrogen evolution reaction kinetics and efficient electrocatalytic activity (Figure 3E,F). Combined with the SEM images (Figure 3C,D), the V₆O₁₃/NF micro-flower array with the shape of a globular peony structure possesses smaller nanosheets, more accessible active sites and consequently facilitates the faster electrolyte and electron transportation, thus improving the catalytic HER performance [22,23].

Moreover, the transmission electron microscopy (TEM) image (Figure 4A) of V₆O₁₃/NF shows that the micro-flower array consisting of a nanosheet structure corresponds to its SEM results (Figure S2C). The HRTEM (Figure 4B) image of V₆O₁₃/NF displays distinct lattice fringes with a distance of 0.198 nm, which can be attributed to the (−601) crystal plane of V₆O₁₃. Furthermore, the selected area for the electron diffraction (SAED) pattern of V₆O₁₃/NF in Figure 4C shows a series of concentric rings, confirming its high crystallinity, which well coincides with the (110) and (020) plane of V₆O₁₃. The EDS mapping further confirms that V, O and Ni elements are homogeneously distributed over the whole V₆O₁₃/NF nanosheet (Figure 4D–G). According to Figure S2B–H, the micro-morphology and element composition of VO_X/NF are also observed. The SEM of VO_X/NF in Figure S2B and the insert image indicate that the size of the VO_X/NF nanosheets is much larger than that of V₆O₁₃/NF, and these nanosheets self-assemble into larger micron-flowers. Obviously, a too large size is not conducive to the efficient electrocatalytic reaction [24]. The TEM and HRTEM images show the clear lattice fringes with a crystal spacing of 0.199 nm corresponding to the (411) crystal plane of V₂O₅. The SAED pattern in Figure S2D inset exhibits the crystal plane composed of bright diffraction points, demonstrating the single crystal nature of VO_X/NF, which is attributed to the (200) and (110) planes of V₂O₅ and (200) plane of V₄O₉, and is consistent with the XRD result in Figure S2A. Combined with the corresponding EDS mapping (Figure S2E–H), it can be observed that the elements of V, O and Ni are uniformly dispersed in the VO_X/NF nanosheet.

X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the surface chemical composition and the electronic states of V₆O₁₃/NF and VO_X/NF. As indicated in Figure 5A, the survey XPS spectrum of V₆O₁₃/NF and VO_X/NF, the elemental peaks for Ni, O and V are observed. The high-resolution XPS spectra of O 1s in V₆O₁₃/NF can be resolved into three obvious peaks at 529.80 eV, 530.93 eV and 533.22 eV (Figure 5B), which are, respectively, belonging to the metal oxygen (M–O), hydroxyl oxide (OH⁻), and adsorbed oxygen (H₂O) [25,26]. Correspondingly, the M–O peak of O 1s region in V₆O₁₃/NF exhibits a slight positive shift of ~0.07 eV relative to that of VO_X/NF, and the relative intensity of the M–O peak in V₆O₁₃/NF is marginally higher than that of VO_X/NF, indicating that the V–O covalent bond in V₆O₁₃/NF is stronger, which is more conducive to increase the electron transfer rate, thus enhancing the capability of electron donation. The hydroxyl oxide in O 1s of V₆O₁₃/NF is slightly shifted by 0.03 eV to the positive binding energy as compared to VO_X/NF, and the relative intensity of adsorbed oxygen deriving from O 1s peak in V₆O₁₃/NF is greater than that of VO_X/NF, suggesting that the introduction of H₂C₂O₄ effectively accelerated the water decomposition process, providing more adequate contact between the electrode and electrolyte in the catalytic reaction.

As shown in Figure 5C, a close inspection discloses that the XPS spectra of Ni 2p and the Ni 2p spectra of V₆O₁₃/NF and VO_X/NF all exhibited two primary peaks: Ni 2p_1/2 (865–885 eV) and Ni 2p_3/2 (850–865 eV), originating from the spin–orbit splitting of Ni [27]. There are three peaks located at 852.20 eV, 854.42 eV and 859.18 eV, respectively, belonging to the Ni 2p_3/2 peaks of Ni²⁺, Ni³⁺ and the corresponding satellite peak in V₆O₁₃/NF. The peaks located at 869.58 eV, 872.60 eV and 880.15 eV ascribe to the Ni 2p_1/2 of Ni²⁺, Ni³⁺ and accompany satellite peaks in VO_X/NF [28]. The peaks located at 852.22 eV, 854.74 eV and 860.76 eV are assigned to Ni²⁺, Ni³⁺ and the shakeup satellite peak of Ni 2p_3/2 in VO_X/NF, respectively. Wherein three components are recognized at 869.34 eV, 872.52 eV and 880.10 eV belonged to Ni 2p_1/2 of VO_X/NF. It is obvious that the peak positions of Ni 2p in V₆O₁₃/NF slightly shifted (~0.39 eV) towards a lower binding energy direction compared with that of VO_X/NF, indicating that the electrons of Ni transferred to the other atoms may be due to the addition of oxalic acid that oxidizes part of the Ni atom.

Similarly, the V 2p spectrum also had two modes including V 2p_3/2 and V 2p_1/2. As shown in Figure 5D, V₆O₁₃/NF displays four well-fitted peaks of V 2p_3/2 focusing on 513.99 eV, 515.10 eV, 516.25 eV and 516.95 eV, respectively, and are assigned to V²⁺, V³⁺, V⁴⁺ and V⁵⁺, and the characteristic peaks at 521.65 eV, 522.75 eV and 523.94 eV binding energy belong to V²⁺, V³⁺ and V⁴⁺ [29]. Interestingly, vanadium in VO_X/NF only has three valence states, the corresponding peak positions are located at 513.58 eV, 514.79 eV and 515.85 eV, respectively, and are assigned to V²⁺, V³⁺ and V⁴⁺. An extra V⁵⁺ appearing in V₆O₁₃/NF compared to VO_X/NF may be due to the air oxidation [30]. V₂O₅ and V₄O₉ have higher valence states of vanadium as +5 and +4.5 than +4.33 of V₆O₁₃. It might be reasonable to suppose that oxalic acid is a key raw material for the synthesis of V₆O₁₃ because of the fact that V₂O₅ and V₄O₉ have been reduced to V₆O₁₃ by oxalic acid with expecting reactions, V₂O₅ + x HCO₂₋ > V₂O_5−x/2 + x/2 H₂O + x CO₂. The peak positions of V 2p in V₆O₁₃/NF were slightly positively shifted relative to VO_X/NF, combined with the peak position of O 1s negative shift, which means the electrons were transferred from V to O when V₆O₁₃ was synthesized. The above results indicated that the construction of V₆O₁₃ nanoarchitecture changes the valence state of the V element, regulates the electron structure and promotes the electron transfer between V, O and Ni, eventually accelerating the reaction kinetics of HER with an enhanced catalytic activity.

2.2. Electrocatalytic HER Performance of the as-Synthesized Catalysts

The HER performance of V₆O₁₃/NF was conducted using a standard three-electrode system in 1 M KOH solution to evaluate the electrocatalytic activity. Here, the binder-free self-supporting electrodes were used directly as the working electrodes with the same exposed electrocatalytic area (the details of the experiment concerning the HER measurements are described in the following Section 3.5). Firstly, to investigate the effect of different contents of the vanadium source on the HER performance of V₆O₁₃/NF, sample-0.2 g, sample-0.5 g and sample-1.0 g were prepared by changing the amount of NH₄VO₃ (0.2, 0.5 and 1.0 g), respectively, and the contrast LSV curve is shown in Figure S3. The result reveals that sample-0.5 g (V₆O₁₃/NF) shows the best HER performance, indicating that the amount of NH₄VO₃ has a significant effect on the catalytic HER activity performance. Combined with the SEM results of sample-0.2 g, sample-0.5 g and sample-1.0 g in Figure 3, the experimental results concluded that regulating the amount of the vanadium source can adjust the morphology and seriously affect the electrocatalytic hydrogen evolution performance. Furthermore, the XPS spectra of sample-0.2 g, sample-0.5 g and sample-1.0 g are displayed in Figure S4, and it can be clearly seen that the V peaks move positively towards the direction of high binding energy, while the Ni and O peaks shift negatively, indicating that more and more electrons are transferred from V to Ni and O when the content of NH₄VO₃ is increased, which is further demonstrated by the amount of the V source that can regulate the electronic structure.

In order to further explore the hydrogen evolution performance of the as-prepared materials, the blank nickel foam (NF), V₆O₁₃/NF, VO_X/NF and commercial 25 wt% Pt/C/NF were compared in identical conditions. Figure 6A records the linear sweep voltammetry (LSV) with iR-correction of all samples. The V₆O₁₃/NF displays optimal HER activity, achieving large current densities of 100, 500 and 1000 mA cm⁻² at a quite low overpotential of 125 mV, 228 mV and 298 mV, respectively, which are superior to the counterparts, such as VO_X/NF (155 mV, 268 mV and 362 mV) and NF (169 mV, 297 mV and 419 mV), ranking in first place among the reported vanadium oxide catalysts for the HER performance (Table S1) [31,32,33], and benefiting from the synergistic effect between exotic oxalic acid and V, O and Ni at optimum conditions. In particular, according to the LSV polarization curves, the HER activity of V₆O₁₃/NF manifests even better than that of 25 wt% Pt/C/NF commercial electrode in electrocatalytic activity at the large current of 1000 mA cm⁻² (46 mV for 100 mA cm⁻², 190 mV for 500 mA cm⁻², 331 mV for 1000 mA cm⁻²). In addition, the Tafel plots were fitted from the LSV curves to reveal the fast HER kinetics toward the various electrodes. According to Figure 6B, the Tafel slope of V₆O₁₃/NF, VO_X/NF, NF and commercial 25 wt% Pt/C/NF are displayed, the value of the Tafel slope in V₆O₁₃/NF (93 mV/dec) is much smaller than that of VO_X/NF (122 mV/dec) and the blank NF (159 mV/dec). In alkaline environments, when the Tafel slope is less than 120 mV/dec, the Volmer step mainly determines the rate of hydrogen evolution, indicating that V₆O₁₃/NF electrode firstly involves H⁺ adsorption behavior in the HER process, resulting in V₆O₁₃/NF possessing the most active HER process among them [34,35].

Figure 6C shows the multi-step chronoamperometric curve for V₆O₁₃/NF to evaluate the HER stability from 50 mV to 500 mV of the overpotential, and each gradient lasts for 500 s. The result verifies that in the whole test range, the current density remains high in durability, which can be further validated by the LSV polarization curve in Figure 6D. Obviously, after continuously working for 90 h in an alkaline condition, the polarization curve of V₆O₁₃/NF displays an ignorable loss for the cathode current and there is only an unremarkable change. Moreover, V₆O₁₃/NF exhibits long-time electrocatalytic stability at 50 mA cm⁻² current density without marked fluctuation over 90 h, wherein the electrode exhibited outstanding durability in Figure 6E.

Furthermore, the electrochemically active surface area (ECSA) is another evaluation index to assess the electrocatalytic activity, and the cyclic voltammogram (CV) curves (Figure S5) of V₆O₁₃/NF and VO_x/NF were recorded to evaluate the ECSA, which was correlated with the electrochemical double-layer capacitance (C_dl). The C_dl value could be linearly fitted from the corresponding CV, which was recorded at different scan rates from 20 to 120 mV/s. According to Figure 7A, the C_dl values of NF, VO_X/NF and V₆O₁₃/NF are quite different, in which the C_dl values of V₆O₁₃/NF are calculated to be 7.14 mF cm⁻², which is larger than that of VO_X/NF (2.65 mF cm⁻²) and pure NF (0.53 mF cm⁻²), respectively, indicating that the V₆O₁₃/NF exposes more electrocatalytic active sites. Then, the electrochemical impedance spectroscopy (EIS) results in Figure 7B verify that the V₆O₁₃/NF possesses the smaller semicircle dimension relative to VO_X/NF and NF electrodes, suggesting that it has relatively low charge transfer resistance (R_ct). Based on the fitted equivalent circuit inset in Figure 7B, the charge transfer resistance (R_ct) is fitted from the Nyquist plot in Table S2, and the values for V₆O₁₃/NF are calculated to be lower (1.819 Ω) than those for VO_X/NF (2.067 Ω) and NF (6.101 Ω), suggesting that V₆O₁₃/NF is beneficial to speed up the electron transport and HER kinetics. The results above suggested that V₆O₁₃/NF electrocatalysts were more efficient in the aggrandizement of catalytic reactive sites, favoring the Volmer-Heyrovsky step, thus enhancing the HER electrocatalytic performance. Therefore, the experimental results displayed the appropriate vanadium source content and oxalic acid addition could boost the electron transport rate, accelerate the HER reaction kinetics and expose numerous catalytic active sites, thus enhancing the HER electrocatalytic performance of V₆O₁₃/NF [36].

3. Materials and Methods

3.1. Chemicals and Reagents

All chemicals and reagents used in this work, including ammonium metavanadate (NH₄VO₃), polyaniline (PAN), oxalic acid (H₂C₂O₄) and potassium hydroxide (KOH), were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). without further purification treatment. Nickel foam (NF, 1.5 mm of thickness, 0.23 g cm⁻³ of density) was available from Suzhou Jiashide metal foam Co., Ltd. (Suzhou, China). The deionized water (18.25 M Ω of resistivity) was purified through the PALL PURELAB system (Beijing, China). NF (Nickel foam) was cut into a rectangle of 1 × 5 cm² and immersed in 3 mol L⁻¹ hydrochloric acid and acetone for ultrasonic cleaning for 10 min. Finally, the foam was washed alternately with ethanol and deionized water several times, then dried under vacuum for 6 h at 40 °C for pretreatment to ensure that the organics and impurities were removed.

3.2. Synthesis of V₆O₁₃/NF

In this work, V₆O₁₃/NF was synthesized using a facile one-step hydrothermal method with 0.5 g of NH₄VO₃, 0.5 g of polyaniline (PAN) and 0.1 g of oxalic acid (H₂C₂O₄) dissolved in 30 mL ultrapure water, after magnetic stirring for 30 min; then, the cleaned nickel foam was added. Next, the above solution was loaded into the 50 mL high-pressure reaction kettle and heated at 180 °C for 14 h. Once cooled down to room temperature, the composite covering on the NF surface was produced, carefully cleaned alternately with ultrapure water and ethanol several times, and then dried in a vacuum for 10 h. Then, V₆O₁₃/NF self-supporting electrodes were obtained (as shown in Figure 1).

3.3. Synthesis of VOx/NF

The synthetic strategy of VO_X/NF was similar to that of V₆O₁₃/NF (0.5 g of NH₄VO₃, 0.5 g of polyaniline (PAN), except for removing H₂C₂O₄ in the synthetic process.

3.4. Synthesis of Sample-0.2 g, Sample-0.5 g and Sample-1.0 g

To investigate the effects of the content of the V source on V₆O₁₃, three more samples were prepared by changing the addition of NH₄VO₃ (0.2, 0.5 and 1.0 g), denoted as sample-0.2 g, sample-1.0 g and the remarkable as-prepared V₆O₁₃/NF, here denoted as sample-0.5 g, respectively.

3.5. General Characterizations

X-ray diffraction (XRD, D/max2200 V) data were recorded to analyze the phase and crystal texture of the samples with Cu Κα radiation (λ = 0.15406 nm). During the XRD test, the test angle was set to 10~80° and the scanning speed was set to 5°/min. In order to ensure the accuracy of the results, the self-supporting film material (VO_X/NF) was pressed flat before the test, and the powder material (V₆O₁₃/NF) was ground for refinement. The test results were analyzed and processed using MDI Jade 6.0 and Diamond software. Field emission scanning electron microscopy (FESEM, S4800, Hitachi, Tokyo, Japan) was performed to analyze the surface morphologies and nanostructure characteristics. Prior to testing, the self-supporting electrode samples were pasted to the conductive glue for observation. Transmission electron microscopy (TEM, Tecnai G2 F20S-TWIN, Tokyo, Japan) was employed to characterize the microstructure and morphologies. Prior to taking the TEM measurements, the samples were dissolved and dispersed in ethanol solution through ultrasound, then the liquid was dropped on the micro-grid and dried at room temperature. The elemental chemical compositions and bonding configuration were analyzed using an X-ray photoelectron spectrometer (XPS) (Thermo Fisher Scientific ESCALAB 250Xi, Waltham, MA, USA) with Al Κα radiation.

3.6. Electrochemical Tests

In this work, all electrochemical tests were measured using a three-electrode system on a CHI electrochemical workstation (ChenHua Instrument, Inc., Shanghai, China, CHI 660E). The as-prepared self-supporting electrode with a test surface area of 0.20 cm², carbon rod and Hg/HgCl₂ saturated calomel electrode were, respectively, employed as the working electrode, counter electrode and reference electrode, and were immersed in 1.0 M KOH during measurement. Following measurement, the load of V₆O₁₃/NF, as the working electrode, was 7 mg/cm² with a test surface area of 0.20 cm². Therefore, as the contrast working electrode, 1.41 mg of commercial 25 wt% Pt/C should have been dissolved in the mixed solution of isopropanol and Nafion, the mixed solution was ultrasonically and evenly coated on the NF work electrode surface, then the commercial 25 wt% Pt/C/NF was prepared. All of the potentials were converted to the reversible hydrogen electrode (RHE) as a reference based on the classical Nernst equation (E (RHE) = E (Hg/HgCl₂) + 0.0591 × pH + 0.2415). The polarization curves were recorded based on linear sweep voltammetry at the scanning rate of 5 mV s⁻¹ with 85% iR-correction. The electrochemical impedance spectroscopy (EIS) was performed with an overpotential of 100 mV when the frequency scope ranged from 0.01 Hz to 1 × 10⁵ Hz. Electrochemical active surface areas (ECSAs) were converted from collecting cyclic voltammetry (CV) at different scanning rates (20 mV s⁻¹, 40 mV s⁻¹, 60 mV s⁻¹, 80 mV s⁻¹, 100 mV s⁻¹ and 120 mV s⁻¹) in the non-Faraday zone. The double-layer capacitance (C_dl) was evaluated at the potential −0.95 V. The electrochemical stability of the synthetic material was estimated at a galvanostatic measurement at a current density of 100 mA cm⁻².

4. Conclusions

In summary, a novel V₆O₁₃/NF nanosheets self-assembled micro-flower array grown in situ on nickel foam was successfully synthesized using a facile one-step hydrothermal method. V₆O₁₃/NF showed excellent performance of electrocatalytic HER activity at the large current density of 100 mA cm⁻² with a considerably low overpotential of 125 mV and a small Tafel slope (93 mV dec⁻¹) as well as a superior HER catalytic stability for at least 90 h. Even more, at the larger current density of 1000 mA cm⁻², the overpotential was only 298 mV, which was superior to the commercial Pt/C/NF electrodes and most recently reported vanadium oxide materials. The highly efficient hydrogen evolution performance benefited from the unique hierarchical micro-flower, the stable well-integrated nanoarray structure and the precise control of vanadium source content, as well as from the appropriate oxalic acid addition that motivated the electron and charge transport rate, accelerated the HER reaction kinetics and enlarged the numerous exposed active catalytic sites, thus enhancing the HER electrocatalytic performance of V₆O₁₃/NF. The work we reported here on modulating the morphological and electronic structure extends the application of vanadium oxides as highly efficient electrocatalysts in the energy conversion and storage fields.

Author Contributions

Conceptualization, J.H., L.C. and L.F.; conceived the experiments, Y.X.; performed the major experiments and wrote the manuscript, Y.X.; performed a part of the measurements, Y.X., Y.W. and Y.Z.; review & editing, Y.X., K.K. and Y.L. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

Figure 1. Synthetic strategy and schematic illustration of the construction of V6O13/NF.

Figure 2. XRD pattern of V6O13/NF.

Figure 3. SEM images of (A,B) sample-0.2 g; (C,D) sample-0.5 g; and (E,F) sample-1.0 g.

Figure 4. (A) TEM image of V6O13/NF; (B) HRTEM image of V6O13/NF; (C) SAED pattern; and (D–G) aberration-corrected TEM image and the corresponding elemental mapping images of V6O13/NF.

Figure 5. High-resolution XPS spectra: (A) survey; (B) O 1s; (C) Ni 2p; and (D) V 2p of V6O13/NF and VOX/NF.S.

Figure 6. (A) HER polarization curves and (B) Tafel slopes of NF, V6O13/NF and VOX/NF; (C) multi-step chronoamperometric curves of HER over V6O13/NF in 1.0 M KOH solution; (D) polarization curves recorded for V6O13/NF before and after the 90 h I-t test; and (E) I-t curve obtained for HER of V6O13/NF in 1.0 M KOH at a constant current density of 50 mA cm−2.

Figure 7. (A) Double-layer capacitance (Cdl) for the evaluation of ECSA and (B) Nyquist plots recorded at an overpotential of 50 mV of NF, V6O13/NF and VOX/NF.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13050914/s1, Figure S1: Synthetic strategy and schematic illustration of the construction of VO_X/NF; Figure S2: (A) XRD pattern of the as-synthesized VO_X/NF; (B,C) SEM and TEM images of the VO_X/NF nanosheets grown on NF substrate with different magnifications; (D) HRTEM image of VO_X/NF, with SAED pattern shown in the inset; and (E–H) TEM image and the corresponding elemental mapping images of VO_X/NF. Figure S3: HER LSV curves of sample-0.2, sample-0.5 and sample-1.0. Figure S4: XPS spectra of (A) sample-0.2 g, (B) sample-0.5 g and (C) sample-1.0 g for HER. Figure S5: CV curves of V6O13/NF (A) and VOX/NF (B). Table S1: Comparison of the electrocatalytic activity of V₆O₁₃/NF with previously reported vanadium oxide electrocatalysts. Table S2: Fitted data from Nyquist plots of the as-synthesized samples in the electrocatalytic HER test.

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