Introduction   介绍

Over the past decades, research has shown that ignorance of the three-waste problem has resulted in tons of environmental problems.^1–3 Most organic reactions involve toxic and highly corrosive additives, as well as strong acids or bases, and are not compatible with the concept of green chemistry.^4–6 Due to these problems, electrocatalytic reactions are receiving more and more attention as a form of green and clean reaction method, with increasing applications in water pollution treatment, ammonia oxidation, and organic reactions powered by renewable energy sources.^7–12 Most electrochemical studies today focus on hydrogen evolution reactions (HER), but oxygen evolution reactions (OER) dominate in electrochemical systems.^13–18 The OER at the anode has high overpotential with a complex four-electron transfer reaction and slow kinetics.¹⁹ It is thermodynamically more favorable to replace the OER with an organic oxidation reaction over the anode, which can not only reduce the high overpotential caused by the OER when using traditional methods, but also produce high-value chemicals in a more environmentally friendly way.²⁰
在过去的几十年里，研究表明，对三废问题的无知导致了大量的环境问题。^1-3 大多数有机反应涉及有毒和强腐蚀性添加剂，以及强酸或强碱，与绿色化学的概念不相容。^4-6 由于这些问题，电催化反应作为一种绿色和清洁的反应方法越来越受到关注，越来越多地应用于水污染处理、氨氧化和由可再生能源驱动的有机反应。^{7 - 12} 如今，大多数电化学研究都集中在析氢反应 （HER） 上，但析氧反应 （OER） 在电化学系统中占主导地位。^13-18 岁阳极的 OER 具有高过电位，具有复杂的四电子转移反应和缓慢的动力学。¹⁹ 在热力学上，用阳极上的有机氧化反应代替 OER 更有利，这不仅可以减少使用传统方法时 OER 引起的高过电位，而且可以以更环保的方式生产高价值的化学品。²⁰

Due to the crucial role catalysts play in reactions, the design and preparation of electrocatalysts have been of great interest. Electrocatalysts based on noble metals are most commonly used in electrochemistry because they are efficient, robust, and able to speed up half-cell reactions.^21–23 However, they are relatively expensive and scarce, which limits their application in electrocatalysts.²⁴ To achieve high conversion and faradaic efficiency at a low potential, nickel foam with a three-dimensional porous structure has been chosen as the ideal electrode substrate to replace noble metal-based electrocatalysts.^25,26 3D porous nickel foam is characterized by its low cost, large active surface area, and high electrical conductivity, which enhance the convection of air bubbles on the electrode surface and prevent them from gathering, thus improving the recyclability of the material.^27–29 Meanwhile, the modification of electrocatalysts with nanostructures directly on electrode substrate materials is currently one of the most popular methods. This increases the contact between the electrocatalyst and the electrode substrate, which helps electrons transport more efficiently during the electrocatalytic process and thus improves the performance of the material.²⁶ The performance of 3D porous nickel foam is now mostly improved by metal oxides or metal salts. Huang et al. designed a novel electrode with an amorphous nanosheet structure (A–Ni–Co–H/NF) by using 0.304 g of CoCl₂·6H₂O and 0.178 g of urea as precursors.³⁰ A–Ni–Co–H/NF exhibited extremely high activity towards the oxidation of benzyl alcohol. Nickel oxyhydroxide, containing Co species in the electrode, was considered as an active substance that promotes the oxidation of benzyl alcohol. Ming et al. obtained porous transition metal nickel hydroxide/nickel foam electrodes by immersing 1 cm² of nickel foam in 0.1 M Ni(NO₃)₂.³¹ It is highly active towards benzyl alcohol oxidation, requiring a low potential of 1.33 V (vs. RHE) to reach a current density of 100 mA cm⁻². Cao et al. designed a Co₃O₄/NF electrode that also had activity towards benzyl alcohol, providing a current density of 86 mA cm⁻² at voltages above 1.5 V (vs. RHE).³² However, most electrocatalysts for organic reactions by electrochemical workstations require relatively large amounts of precursors to achieve superior electrocatalytic performance. More economical electrocatalysts urgently need to be investigated.
由于催化剂在反应中起着至关重要的作用，电催化剂的设计和制备引起了人们的极大兴趣。基于贵金属的电催化剂最常用于电化学，因为它们高效、稳定并且能够加速半电池反应。^21-23 岁然而，它们相对昂贵且稀缺，这限制了它们在电催化剂中的应用。²⁴ 为了在低电位下实现高转化率和法拉第效率，选择具有三维多孔结构的泡沫镍作为理想的电极衬底，以取代贵金属基电催化剂。^25,26 3D 多孔泡沫镍的特点是成本低、活性表面积大、导电性高，增强了气泡在电极表面的对流并防止它们聚集，从而提高了材料的可回收性。^27-29 岁同时，直接在电极衬底材料上对具有纳米结构的电催化剂进行改性是目前最流行的方法之一。这增加了电催化剂和电极衬底之间的接触，这有助于电子在电催化过程中更有效地传输，从而提高材料的性能。²⁶ 3D 多孔泡沫镍的性能现在主要通过金属氧化物或金属盐来改善。Huang 等人使用 0.304 g CoCl₂·6H₂O 和 0.178 g 尿素作为前驱体，设计了一种具有非晶纳米片结构 （A-Ni-Co-H/NF） 的新型电极。³⁰ A-Ni-Co-H/NF 对苯甲醇的氧化表现出极高的活性。 电极中含有 Co 种类的羟基氢氧化镍被认为是促进苯甲醇氧化的活性物质。Ming 等人通过将 1 cm² 的泡沫镍浸入 0.1 M Ni（NO₃）₂ 中获得了多孔过渡金属氢氧化镍/泡沫镍电极。³¹ 它对苯甲醇氧化具有高度活性，需要 1.33 V（ 相对于 RHE）的低电位才能达到 100 mA ^cm-2 的电流密度。Cao 等人设计了一种 Co₃O₄/NF 电极，该电极也对苯甲醇具有活性，在高于 1.5 V 的电压下提供 86 mA ^cm-2 的电流密度（ 相对于 RHE）。³² 然而，大多数电化学工作站用于有机反应的电催化剂需要相对大量的前驱体才能获得卓越的电催化性能。迫切需要研究更经济的电催化剂。

Metal vanadium has a variety of oxidation states and different compositional forms.^33,34 Furthermore, vanadium has a large quantity of empty d-orbitals that can receive electrons from Ni elements, which promotes higher carrier concentrations for faster electron transportation.^35,36 Hence, vanadium can be a potentially superior catalytic material. Ammonium vanadate, the precursor of metal vanadium, consists of a vanadium oxide layer and NH₄⁺ ions.³⁷ The vanadium oxide layer was reported to have a high theoretical charge storage capacity.³⁷ Besides, NH₄⁺ ions are beneficial for increasing the intrinsic conductivity, expanding the layer spacing, and building N–H⋯O bonds, with properties that enhance the crystal structure.^38,39 This facilitates faster ion diffusion and transfer in solution and improves the stability of the electrode material.^38,39 Therefore, ammonium vanadate is a cheap, high performance precursor for electrode materials. However, there have been fewer experiments with vanadium for electrocatalytic organic reactions, mostly focusing on increasing the OER activity.^40,41
金属钒具有多种氧化态和不同的成分形式。^33,34 此外，钒具有大量空的 d 轨道，可以接收来自镍元素的电子，这促进了更高的载流子浓度，从而加快了电子传输速度。^35,36 因此，钒可能是一种潜在的优质催化材料。钒酸铵是金属钒的前体，由氧化钒层和 NH₄⁺ 离子组成。³⁷ 据报道，氧化钒层具有很高的理论电荷存储容量。³⁷ 此外，NH₄⁺ 离子有利于提高本征电导率、扩大层间距和构建 N-H⋯O 键，具有增强晶体结构的特性。^38,39 这有助于更快地在溶液中扩散和转移离子，并提高电极材料的稳定性。^38,39 因此，钒酸铵是一种廉价、高性能的电极材料前驱体。然而，使用钒进行电催化有机反应的实验较少，主要集中在提高 OER 活性上。^40,41 元

Inspired by the above, we prepared V–N supported electrocatalysts with a nickel foam substrate (VO-N/NF nanocomposite) by a one-step hydrothermal method and applied them to the electrocatalytic benzyl alcohol oxidation reaction. After confirming that the electrode was supported with N, Ni, V, and O elements, we tested the performance of the electrode through an electrochemical workstation. The result was that the VO-N/NF nanocomposite electrode with 0.2 mmol metal precursor showed high activity for benzyl alcohol oxidation. Meanwhile, we monitored and tested the electro-oxidation of benzyl alcohol, which generated excellent conversion and faradaic efficiency. Ultimately, the electrode was used for repeated electrocatalytic benzyl alcohol oxidation to confirm its cycling performance.
受上述启发，我们通过一步水热法制备了带有泡沫镍基材 （VO-N/NF nanocomposite） 的 V-N 负载电催化剂，并将其应用于电催化苯甲醇氧化反应。在确认电极由 N、Ni、V 和 O 元素支撑后，我们通过电化学工作站测试了电极的性能。结果是，具有 0.2 mmol 金属前驱体的 VO-N/NF 纳米复合电极显示出高苯甲醇氧化活性。同时，我们监测和测试了苯甲醇的电氧化，产生了出色的转化率和法拉第效率。最终，该电极用于重复电催化苯甲醇氧化，以确认其循环性能。

Experimental section

Chemicals and materials

Nickel foam was bought from Kunshan GuangJiaYuan New Materials Co., Ltd. Ammonium vanadate (NH₄VO₃, 99%), benzyl alcohol (99 wt%), benzoic acid (99 wt%), and benzaldehyde (99 wt%) were bought from Energy Chemical. Acetic acid (AR) and methanol (HPLC) were, respectively, bought from Sinopharm Chemical Reagent and Sigma-Aldrich. All chemical reagents were used without any further purification.

Synthesis of the VO-N/NF nanocomposite

Firstly, the nickel foam (1 cm × 4 cm × 1 mm) was pretreated with acetone, hydrochloric acid, and deionized water and then vacuum-dried. NH₄VO₃ (0.2 mmol) was taken in a clean beaker and deionized water (40 mL) was poured into the beaker to dissolve on a stirring table at room temperature. The solution was then poured into a PTFE liner into which the pretreated and dried nickel foam was added. Next, the liner was placed in a stainless steel autoclave and hydrothermally heated in an oven (140 °C, 6 h). After the hydrothermal process, when the stainless steel autoclave had cooled to room temperature, the nickel foam was cleaned with deionized water (DI water) and then dried in a vacuum oven at 50 °C for 12 h.

Characterization

X-ray diffraction (XRD) was carried out on a Bruker D8 Advance instrument from Germany to determine the elemental composition of the material. Transmission electron microscopy (TEM) images were obtained with an FEI Tecnai F20 microscope. The morphology and EDS images of the material were recorded with a Sigma 500 field emission scanning electron microscope (SEM). The surface chemical composition of the electrode material was acquired by means of an X-ray photoelectron spectrometer (XPS) with a Thermo ESCALAB 250Xi spectrometer. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was carried out with a PerkinElmer 8300 spectrometer.

Electrochemical investigation

All electrochemical tests were carried out on an Autolab PGSTAT128N electrochemical workstation with an anion-exchange membrane (N117 DuPont) on an H-type three-electrode system with 1.0 M KOH solution (40 mL). The VO-N/NF nanocomposite was prepared as the working electrode for the three-electrode system, and the saturated Ag/AgCl electrode and the platinum foil (1 cm × 1 cm) were used as the reference electrode and counter electrode, respectively. All electrochemical procedures related to the voltage determination were calibrated with respect to the reversible hydrogen electrode (RHE) according to the following equation: E_(vs. RHE) = E + E_{(vs. Ag/AgCl)} + 0.059 × pH. The linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV s⁻¹ and all the curves were manually IR-compensated to reduce the effect of solution resistance (R_s). R_s was obtained from the EIS test by selecting the first intercept of the curve in the horizontal axis coordinates. The corresponding corrected voltage values were obtained using E_corrected = E_measured − 85%iR_s.⁴² The related cyclic voltammetry (CV) and double-layer capacitance tests (C_dl) were performed at different scan rates over the voltage range of 0.9 V to 1.0 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10⁵ to 0.01 Hz at 1.425 V (vs. RHE). Chronoamperometry (IT) was carried out at 1.50 V (vs. RHE).

Product analysis

The electrocatalytic benzyl alcohol oxidation was carried out using a high performance liquid chromatography (HPLC) system (Agilent 1260) equipped with a C18 column and a UV-VIS detector. In order to separate the possible products existing in the mobile phase, the detection conditions were first screened and confirmed. Namely, during electrocatalytic alcohol oxidation, 20 μL of the reaction solution from the test procedure was dissolved in deionized water to make up 1 mL of solution at a flow rate of 0.6 mL min⁻¹, with methanol and 10% aqueous acetic acid (the volume ratio was 4 : 6) as the mobile phase. Apart from this, for the detection, the detection wavelength was set at 245 nm and the column temperature at 40 °C. Meanwhile, a certain volume of benzaldehyde was also observed to be produced during the electrocatalytic benzyl alcohol oxidation reaction. Secondly, based on the standard curve plots for the pure sample and the pure product (Fig. S1, ESI†), the peak times of benzyl alcohol, benzaldehyde, and benzoic acid were identified to be at around 10 min, 14 min, and 15 min, respectively.

The conversion of benzyl alcohol was calculated as follows:

The equation for the oxidative selectivity of benzyl alcohol was as follows:

The yield of benzoic acid was calculated as follows:

The formula of faradaic efficiency is , where n is the actual number of moles of benzoic acid produced as the main oxidation product of benzyl alcohol; z is the number of electrons transferred for the reaction in this organic oxidation reaction, which was 4 here; F is Faraday's constant (96 485 C mol⁻¹); and Q is the actual amount of electricity passing through the circuit.

Results and discussion

Characterization of the VO-N/NF nanocomposite

The VO-N/NF nanocomposite obtained through a simple one-step hydrothermal method shows a clear change in the color of the nickel foam surface before and after the reaction (Fig. S2, ESI†), which changes from the original silver color to yellow, indicating the formation of a new substance on the surface of the electrode. To determine the morphological characteristics and specific composition of the electrode, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used. Fig. S3 (ESI†) reveals the SEM image of the nickel foam electrode, from which the morphology of the nickel foam can be observed. The surface of the pretreated nickel foam, without a hydrothermal reaction, can be observed to be smooth. In contrast, a clear layer of nanosheets supported at the surface of the nickel foam skeleton can be observed over the vanadium–nitrogen supported nickel foam obtained after hydrothermal treatment (Fig. 1). At the same time, these nanosheets tend to stack irregularly and form porous layers (Fig. 1d), which facilitates active site exposure of the electrode. The TEM image (Fig. 2a) appears to confirm the disorderly stacked nanosheet structure on the electrode surface. This means that new substances are formed on the surface of the electrode. The crystallinity of the VO-N/NF nanocomposite electrode was measured using powder X-ray diffraction (XRD). From the XRD pattern (Fig. S4, ESI†), it is evident that there are distinct diffraction peaks at 44.54°, 51.89° and 76.42°, which represent the (111), (200), and (220) crystal planes of Ni (JCPDS Card No. 87-0712).⁴³ Apart from this, no other obvious diffraction peaks are found in Fig. S3 (ESI†), which suggests that vanadium is probably distributed in the framework of nickel foam and tend to form amorphous nanosheets.^30,44 Concurrently, three different lattice fringes can be traced in the HRTEM image (Fig. 2b and c), which, respectively, correspond to the (111), (220) and (200) lattice planes of Ni with a spacing of 0.204 nm, 0.124 nm and 0.176 nm. This means that V and N elements are probably distributed on the electrode and form an amorphous nanocomposite on the surface of the nickel foam framework.

To confirm the specific composition of the electrodes, energy dispersive spectroscopy (EDS) mapping provides a distribution of the elements present in the electrode material. It is distinct that the nitrogen, oxygen, nickel, and vanadium elements are evenly distributed over the nickel foam in a 4 : 22 : 21 : 3 ratio (Fig. 3 and Table S1, ESI†). In addition, the chemical composition of the VO-N/NF nanocomposite electrode surface was also analyzed by X-ray photoelectron spectroscopy (XPS). The corresponding total XPS spectrum of the electrode (Fig. S5, ESI†) confirmed the existence of the Ni, V, O, and N elements. Fig. 4a shows the corresponding XPS spectrum of Ni 2p with five obvious characteristic peaks. The Ni²⁺ 2p_3/2 and 2p_1/2 peaks are located at 855.56 eV and 872.80 eV, while the corresponding satellite peaks are at 861.02 eV and 878.40 eV.^25,43 Besides, there is a small peak at 852.50 eV corresponding to Ni⁰ at the 2p_3/2 orbital from the nickel foam substrate.^30,45 Hence, Ni²⁺ can be viewed as the partial oxidation of nickel that occurs when ammonium vanadate hydrolyzes to form H⁺ in the hydrothermal process.²⁵Fig. 4b presents the XPS spectrum of V 2p. Two apparent peaks can be observed at around 525 eV and 517 eV, attributed to the 2p_1/2 and 2p_3/2 orbitals, which can be split into three subpeaks, i.e., 515.72 eV, 516.73 eV, and 517.66 eV, corresponding to V³⁺, V⁴⁺, and V⁵⁺, indicating the existence of V–N, V–N–O, and V–O.^46–48 In the O 1s spectrum shown in Fig. 4c, the subpeak at 532.92 eV corresponds to the H₂O peak due to water absorption at the electrode surface, while those at 530.52 eV correspond to the metal–O bonds.^49–51 The characteristic peaks at 398.18 eV and 400.07 eV in the N 1s spectrum shown in Fig. 4d, respectively, correspond to metal–N species and N–H bonds.^52,53 The results of the above tests all prove that V and N elements are successfully supported on the nickel foam electrodes and tend to form the amorphous nanocomposite on it by a hydrothermal method. Therefore, the electrode with V and N elements supported on it is successfully prepared.

Electrocatalytic performance

All electrochemical tests were carried out in a three-electrode system with an H-shaped electrolytic cell. Firstly, to exclude the quantity effect of the VO-N/NF nanocomposite, LSV was performed to determine its electrocatalytic performance with different concentrations. The LSV curves for nickel foam electrodes with different V-supported concentrations are shown in Fig. S6a and b (ESI†). The 0.2-VO-N/NF nanocomposite has the largest slope, which indicates faster current density growth, smaller changes in overpotential, and better electrocatalytic performance. Fig. S6c (ESI†) shows the electrochemical impedance spectroscopy results of the VO-N/NF nanocomposite with different concentrations of precursors. It can be observed that the R_ct of the 0.2-VO-N/NF nanocomposite is 0.863 Ω, which is lower than the electrode impedance under other loading conditions (Table S2, ESI†). This makes the 0.2-VO-N/NF nanocomposite the optimum support choice. The specific content of V in the electrode was determined using ICP-OES (Table S3, ESI†). Then, the data for the VO-N/NF nanocomposite electrodes were then analyzed with cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy.

In Fig. 5a, the LSV curve of the VO-N/NF nanocomposite is measured with and without 0.1 M benzyl alcohol. It takes over 1.70 V (vs. RHE) to achieve a current density of 100 mA cm⁻² without benzyl alcohol. At the same time, an obvious oxidation peak can also be seen at 1.41 V (vs. RHE), which is due to the oxidation of Ni²⁺.⁵⁴ A current density of 100 mA cm⁻² at 1.395 V (vs. RHE) was measured when 0.1 M benzyl alcohol was added to the electrolyte, indicating that replacing the anodic OER with an alcohol oxidation reaction can reduce the overpotential. On the other hand, if the electrocatalytic benzyl alcohol oxidation reaction is carried out using NF alone, the current density at 1.472 V (vs. RHE) is only 37 mA cm⁻² as shown by the LSV curves of NF under alcohol oxidation and OER conditions (Fig. S7a, ESI†). This explains why nickel foam does not react well for electrocatalytic alcohol oxidation. The VO-N/NF nanocomposite has a Tafel slope of 29.76 mV dec⁻¹ for electrocatalytic benzyl alcohol oxidation, which is smaller than 155.22 mV dec⁻¹ for the OER (Fig. 5b). This indicates that the VO-N/NF nanocomposite electrode has a faster increase in current density, performs the alcohol oxidation reaction faster, and has a better catalytic activity than the OER. Moreover, this phenomenon is also because alcohol oxidation has more thermodynamic advantages. It can replace the OER with alcohol oxidation to speed up the kinetics and reduce the potential required to reach a certain current density. The Tafel slope for nickel foam is 58.70 mV dec⁻¹ for alcohol oxidation and 72.40 mV dec⁻¹ for the oxygen evolution reaction (Fig. S7b, ESI†). This reveals that, when nickel foam is used as the electrode, substituting an alcohol oxidation reaction for the OER reduces the Tafel slope and overpotential. According to the comparison above, the Tafel slope of NF for alcohol oxidation is larger than that of the VO-N/NF nanocomposite, indicating that supporting nickel foam with vanadium–nitrogen lowers the overpotential and improves the Tafel slope. In addition, to determine the charge transfer rate of the VO-N/NF nanocomposite electrode for alcohol oxidation, the electrochemical impedance spectroscopy (EIS) of the NF and VO-N/NF nanocomposite was performed. It can be noticed from Fig. 5c that the R_ct of the VO-N/NF nanocomposite is approximately 0.863 Ω, which is much smaller than that of nickel foam (88.63 Ω) for the electrocatalytic benzyl alcohol oxidation reaction (Table S4, ESI†). This demonstrates the smaller charge transfer resistance of the VO-N/NF nanocomposite electrode for the alcohol oxidation reaction, which effectively speeds up charge transfer. In contrast, the small semicircle of the VO-N/NF nanocomposite for alcohol oxidation in Fig. S7c (ESI†) could indicate a faster charge transfer rate for benzyl alcohol oxidation than for the OER.

Furthermore, a double-layer capacitance test (C_dl) was carried out for nickel foam and the VO-N/NF nanocomposite according to cyclic voltammetry. The pattern of the C_dl plot (Fig. S8, ESI†) shows that the V-supported nickel foam electrode has a C_dl value of 2.00 mF cm⁻², which is larger than the C_dl value of nickel foam. This declares more active sites on the VO-N/NF nanocomposite than the NF.⁵⁵ Meanwhile, to eliminate the effect of voltage on electrocatalytic benzyl alcohol oxidation, the conversion of the material and other parameters were measured at different voltages without any other oxygen evolution reaction. Electrocatalytic benzyl alcohol oxidation efficiency decreases at voltages of 1.46 V and 1.54 V. This is because the oxygen evolution reaction becomes more intense at these voltages (Fig. 5d). In the range of 1.48–1.52 V, the conversion, selectivity, yield, and FE do not change significantly, so the conclusion can be drawn that the oxidation performance of benzyl alcohol is almost not affected by voltage within this voltage range. The chronoamperometry tests were performed at 1.50 V as an intermediate value to eliminate the impact of adjacent voltages. From the above electrochemical workstation tests, it is evident that the performance of the VO-N/NF nanocomposite is superior to that of nickel foam. And it also verifies the claim that vanadium and nitrogen elements supported at the surface help nickel foam perform better.

After measuring the electrocatalytic performance of the VO-N/NF nanocomposite electrode, the electrocatalytic benzyl alcohol oxidation reaction was further investigated. Chronoamperometry tests were carried out at a potential of 1.50 V (vs. RHE) as it is the maximum potential at which no significant anodic oxygen evolution reaction occurs. The electrocatalytic benzyl alcohol oxidation reaction test was followed by high performance liquid chromatography (HPLC) to determine the specific content of the benzyl alcohol oxidation product. The reaction solution was then tested at intervals and observed to produce benzaldehyde during the reaction, along with benzyl alcohol and all products. As shown in Fig. 5e, benzyl alcohol decreased gradually during the electrochemical alcohol oxidation reaction. As the reaction reached 5000 s, benzyl alcohol oxidation was almost complete, but the benzaldehyde yield was still at 3.15%. By extending the electrochemical reaction time to 6000 s, the benzoic acid yield reached 99.1% and the benzyl alcohol conversion reached 99.7%. The faradaic efficiency also reached 99.3%. Besides, the benzyl alcohol oxidation was the main reaction at 1.50 V (vs. RHE) over nickel foam without the anodic oxygen evolution reaction (Fig. S7a, ESI†). Therefore, the electrocatalytic benzyl alcohol oxidation of nickel foam was performed at 1.50 V (vs. RHE). With nickel foam as the electrode, Fig. S9 (ESI†) provides the variation in the content of each element over time during electrocatalytic benzyl alcohol oxidation. It takes more than 15 000 seconds to oxidize most of the benzyl alcohol because nickel foam has a low current during the benzyl alcohol oxidation reaction. The final conversion of benzyl alcohol oxidized by nickel foam is 98.6%, the yield is 97.9%, and the faradaic efficiency is 95.7%. This comparison highlights that the efficiency of electrocatalytic benzyl alcohol oxidation with the VO-N/NF nanocomposite is better than that with bare nickel foam. To verify the stability of the electrodes, chronoamperometry tests and linear sweep voltammetry were carried out five times. Fig. 5f illustrates the trend of conversion, yield, selectivity, and faradaic efficiency during multiple electrocatalysis reactions. The electrocatalytic effectiveness of the VO-N/NF nanocomposite does not decrease significantly during the multiple reactions, and the faradaic efficiency remained around 99%, proving that the VO-N/NF nanocomposite has good cyclability. The corresponding chronoamperometry tests and LSV curves did not change significantly after several electrocatalytic benzyl alcohol oxidation reactions (Fig. S10 and S11, ESI†), which represents the good stability of the VO-N/NF nanocomposite.

To demonstrate the excellent performance of our prepared electrodes, we summarize the catalysts developed in recent years for electrocatalytic benzyl alcohol oxidation and chemically assisted hydrogen evolution in Tables S5 and S6 (ESI†). As compared to VO-N/NF nanocomposite electrodes, most catalysts require a large amount of specific precursors in order to achieve a certain current density or a high conversion rate under a low voltage. However, the preparation of the VO-N/NF electrode with three-dimensional and amorphous nanocomposite structures was achieved by using a simple one-step hydrothermal method with only 0.2 mmol precursor. Meanwhile, the conversion of benzyl alcohol and faradaic efficiency are 99.7% and 99.3%, respectively. This indicates the good performance of the VO-N/NF nanocomposite electrode for the benzyl alcohol oxidation reaction.

In order to find out the changes in the V level in the VO-N/NF nanocomposite electrode after the electrocatalytic alcohol oxidation reaction, the electrodes were tested by ICP-OES. A decline in the V content can be noticed in several reactions, while the V content has dropped after five electrocatalytic tests (Table S7, ESI†). Moreover, from Fig. S12 (ESI†), the surface of the VO-N/NF nanocomposite electrode exhibits more obvious flaky cracks after electrocatalytic oxidation of benzyl alcohol and becomes rougher. This is due to the loss of the V element from the VO-N/NF nanocomposite, leading to surface reconstruction. Apart from this, Fig. S13 (ESI†) refers to the distribution of various elements on the VO-N/NF nanocomposite electrode after electro-oxidation. It can be clearly observed that the content of V and N elements has significantly decreased, and the distribution is not as dense as before, which also confirms the loss of the V element. In addition, XPS was used to determine the chemical composition of the electrode surface after five electrocatalytic benzyl alcohol oxidation reactions. Fig. S14a (ESI†) shows the Ni 2p spectrum, in which the peaks found at 855.59 eV and 872.89 eV, respectively, correspond to the 2p_3/2 and 2p_1/2 orbitals of Ni²⁺, and the peaks at 857.04 eV and 874.35 eV correspond to Ni³⁺. This demonstrates that NiOOH species exist and contribute to the electrocatalytic oxidation of all alcohols such as benzyl alcohol.^56,57 Aside from the satellite peak, the small peak of Ni⁰ at 852.50 eV disappeared. According to Fig. S14b (ESI†), there are no new peaks from multiple electrochemical reactions for the V species. The subpeaks at 515.6 eV, 515.63 eV, and 517.60 eV belong to V³⁺, V⁴⁺, and V⁵⁺ on the 2p_3/2 orbital, and the peaks observed at 523.10 eV, 524.13 eV, and 525.10 eV attribute to V³⁺, V⁴⁺, and V⁵⁺ on the 2p_1/2 orbital. However, two peaks of V 2p_3/2 and 2p_1/2 orbitals can be found with much lower intensity and resolution due mainly to V being dissolved in water during the oxidation process.^47,58,59 Fig. S14c (ESI†) shows the XPS spectrum of O 1s. In addition to the peaks located at 532.60 eV and 530.81 eV, which are still assigned to H₂O and the metal–O bond, an additional subpeak appears at 531.89 eV, which belongs to –OH. This further demonstrates the successful formation of NiOOH.⁶⁰ The XPS spectrum of N 1s is shown in Fig. S14d (ESI†). When compared to the electrode without electrocatalytic alcohol oxidation, there is no more significant difference, indicating that no new N bonds were formed during the multiple reactions.

In order to prove that N is conducive to electrocatalytic benzyl alcohol oxidation, the VO/NF electrode without N element was prepared according to the literature.^61,62 The ICP-OES results of the as-prepared VO/NF electrode are displayed in Table S8 (ESI†), which show that no apparent difference between the V contents of 0.2-VO/NF and the 0.2-VO-N/NF nanocomposite can be observed, so the following tests on its performance should not be impacted. As shown in Fig. S15 (ESI†), the EDS mapping image of VO/NF shows only V, O, and Ni elements. Furthermore, the XPS survey spectrum (Fig. S16, ESI†) shows that Ni, V, and O coexist. The comparison between the fine spectrum of VO/NF and Fig. 3 implies that the difference only lies in the V 2p and N 1s spectra. LSV was performed on the VO/NF electrode to ensure the significance of the N element. As shown in Fig. S17a (ESI†), a current density of 100 mA cm⁻² can only be reached at voltages up to 1.624 V (vs. RHE). Compared with Fig. S7a (ESI†), it is not surprising to find that the VO/NF electrode lacking N element needs a potential lower than NF, but higher than the VO-N/NF nanocomposite electrode to attain a certain current density. This states that VO/NF performs superior to NF and inferior to the VO-N/NF nanocomposite. Based on the LSV curve, we selected 1.50 V (vs. RHE) as the voltage for the chronoamperometry test to monitor the oxidation process (Fig. S17b, ESI†). When the reaction reached 11 000 s, the benzyl alcohol conversion, the benzoic acid yield, and the faradaic efficiency were low, which were 89.1%, 72.6%, and 71.9%. In addition, to accurately verify the role of support elements, a N/NF electrode was also prepared. According to the EDS mapping image (Fig. S18, ESI†), it can be seen that only N, Ni, and O elements exist in the electrode. The LSV curve obtained by using the N/NF electrode shows that it needs 1.585 V to achieve a current density of 100 mA cm⁻² (Fig. S19a, ESI†). A chronoamperometry test was conducted at a voltage of 1.50 V (vs. RHE) to ensure the accuracy of the experiment. From Fig. S19b (ESI†), the conversion, yield, and faradaic efficiency of benzyl alcohol oxidation, respectively, reached 97.5%, 95.8%, and 81.3% in 12 000 s. To summarize, the addition of N and V elements helps decrease the reaction time and improve the activity of the electrocatalytic alcohol oxidation reaction.

From the results above, we suspect that the V⁵⁺ of the ammonium vanadate reacts with nickel foam during the hydrothermal process, which results in V⁵⁺ being reduced to V³⁺ and V⁴⁺, while Ni is oxidized to Ni²⁺. Based on experiments and the literature, we propose the following possible reasons for the high activity of the VO-N/NF electrode during the benzyl alcohol electrocatalytic oxidation reaction. Firstly, during the electrocatalytic process, vanadium element with multiple oxidation states causes a change in the valence of other metal elements on the electrode surface, which promotes the rapid formation of Ni³⁺.⁵⁵ A high-valent nickel-based component (NiOOH) is proved as the key to reduce the overpotential of the benzyl alcohol oxidation reaction and improve the electrocatalytic activity.^30,58,63,64 Second, nitrogen can adjust the electronic structure of the adjacent atoms, which contributes to speeding up the electrochemical reaction.^65,66 In addition, the introduction of V and N elements improves the number of active sites on the electrode surface (Fig. S8, ESI†), which enables the electrode to have outstanding electrochemical activity. Finally, during the stability test of the VO-N/NF electrode, the vanadium element gradually dissolved in the electrolyte.⁴⁷ Fig. S20 (ESI†) shows the XPS spectra of the V element before and after the electrocatalytic benzyl alcohol oxidation reaction, and it can be observed that the intensity of V 2p is significantly reduced. Apart from this, the significant reduction of the V element in the EDS mapping image of the VO-N/NF electrode after the reaction also confirms the dissolution of the V element. At the same time, due to the element dissolution, the VO-N/NF electrode surface is reconstructed, resulting in the appearance of cracks and rough morphology (Fig. S12, ESI†). Also because of this, defects may form on the surface of the electrode, increasing the number of active sites on the VO-N/NF nanocomposite, thereby enhancing the electrocatalytic activity of the electrode.^55,67–69 In summary, it is the synergy of all the above factors that makes the VO-N/NF nanocomposite electrode perform better than bare nickel foam.

Conclusions

In this work, we developed a simple one-step hydrothermal method for preparing an electrode based on V–N elements supported at the surface of nickel foam (VO-N/NF nanocomposite) and then applied it to the electrocatalytic benzyl alcohol oxidation reaction. The results obtained from the tests reveal that the VO-N/NF nanocomposite electrode with a low metal precursor content has high electrocatalytic activity for benzyl alcohol oxidation. This is mainly due to the exposure of the active sites of the catalyst by element dissolution during the reaction and the excellent catalytic activity of the high-valent nickel-based compounds, as well as the addition of V and N elements that can maintain a good performance during the benzyl alcohol electrocatalytic oxidation reaction. This reduces the overpotential by replacing the original oxygen evolution reaction with alcohol oxidation, allowing benzoic acid to be obtained cleanly. Aside from this, superior yields and faradaic efficiency of benzoic acid can be obtained at higher current density, which is cleaner than the conversion of benzyl alcohol to benzoic acid using a classical method. The electrochemical system allows us to obtain high value-added benzoic acid at a low potential and produce hydrogen as the only by-product, opening up more possibilities for electrocatalytic organic reactions.

Author contributions

Handan Chen, Kejie Chai and Weiming Xu contributed to the design of the study. Handan Chen and Zhifei Zhu conducted the experiments and data analysis. Kejie Chai and Weiming Xu provided technical advice and result interpretation. Handan Chen and Yizhen Zhu conducted the catalyst characterization and corresponding data processing. Handan Chen, Zhifei Zhu and Mei Kuai conducted the catalysis performance test and corresponding data processing. Handan Chen, Kejie Chai and Weiming Xu wrote the manuscript and prepared the ESI† material, and all authors commented on and amended both documents. All authors discussed and contributed to the work.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by the Sci-Tech Project of Zhejiang (LGG21B020001), the Natural Science Foundation of China (22178078) and the Sci-Tech Project of Hangzhou (20201203B137).

References

Footnote