English

• The electrochemical production of hydrogen fuel is currently considered an important method for addressing energy and climate issues^[1-3]. However, traditional water electrolysis is limited by the relatively slow kinetics of the oxygen evolution reaction at the anode, and its overall efficiency in energy utilization still needs to be improved. By changing the anode substrate to aliphatic compounds, it is possible to significantly reduce energy consumption and synthesize more valuable organic products^[4-8]. Among all organic substrates, the benzyl alcohol electro-oxidation reaction (BAOR) shows the advantages of low production cost and a wide range of raw material sources^[9-10]. Besides, the oxidation product of benzyl alcohol, benzoic acid, is widely used in numerous fields, including medicine, food, and the chemical industry^[11-13].

  Ni-based non-precious metal catalysts offer certain advantages, including low cost and strong tolerance to poisoning^[14-15]. Meanwhile, Ni-based catalysts need to adsorb OH^- at a higher potential to effectively catalyze the electrochemical oxidation of alcohols^[16-17]. Therefore, reducing the activation potential and enhancing the overall activity are important means for developing Ni-based BAOR catalysts. Adding other cations is a common method to enhance the activity of Ni-based catalysts^[9,17-20]. In particular, Ni-Co binary metal compounds are a common method for enhancing the performance of Ni-based electrocatalysts^[21-23]. The Co site has a strong adsorption effect on OH^-, which is conducive to the use of various anode electrocatalysts^[23-24]. Therefore, adding Co cations is an important means for optimizing Ni-based anode catalysts.

  By taking advantage of the anion effect, the charge distribution on the catalyst surface can be adjusted, thereby further enhancing the strengthening function of Co^[9,25-26]. Therefore, we introduced S anions into Ni-Co bimetallic compounds, enhanced the additional adsorption of Co, and resulted in the formation of a highly active Ni₂CoS₄ BAOR catalyst. The specific activity of the Ni₂CoS₄ BAOR catalyst was 271 mA·cm^-2 at 1.40 V (vs RHE), 3.3 times of Ni₂Co(OH)_x (83 mA·cm^-2), and 7.7 times of Ni(OH)₂ (35 mA·cm^-2). By S- anion effects, the charge distribution among different metals on the surface of the catalyst was re-adjusted, strengthening the positive charge distribution on the Co surface and enhancing its OH^- adsorption ability. Finally, the overall catalytic performance of Ni₂CoS₄ for BAOR was improved.

  1.   Experimental

  1.1   Materials and instruments

  Cobalt chloride hexahydrate (CoCl₂·6H₂O, 99%, Macklin), nickel chloride hexahydrate (NiCl₂·6H₂O, 99%, Macklin), potassium hydroxide (KOH, 99.999%, Aladdin), thiourea (99%, Aladdin), urea (99%, Aladdin), benzyl alcohol (99.9%, Aladdin) were obtained commercially and used without further purification.

  The X‐ray diffraction (XRD) patterns were obtained on a Rigaku D/Max 2500 VB2+/PC X-ray powder diffractometer equipped with Cu Kα radiation (λ=0.154 nm) operating at 40 kV and 40 mA with a 2θ range of 20° to 90°. The transmission electron microscopy (TEM) image was performed on a 2100F/F200X transmission electron microscope (200 kV). The valence states of the elements were recorded on a Thermo Fisher Escalab Xi+ X-ray photoelectron spectrometer (XPS) with a monochromatic Al Kα X-ray source. The binding energies derived from XPS measurements were calibrated to the C1s at 284.8 eV. The morphologies were obtained on a Thermo Fisher Scientific Apreo 2C scanning electron microscope. The elemental contents were tested by an Agilent 5100 inductively coupled plasma optical emission spectrometer (ICP-OES).

  1.2   Syntheses of Ni-based catalysts

  1.2.1   Synthesis of Ni₂CoS₄

  A mixture was prepared by combining 0.48 g NiCl₂·6H₂O, 0.24 g CoCl₂·6H₂O, 0.4 g of thiourea, and 28 mL of deionized water. Three pieces of Ni foams (1 cm×2.5 cm) were immersed in the mixture, which was then transferred into a 50 mL Teflon-lined stainless-steel autoclave. The sealed autoclave was heated at 120 ℃ for 8 h. After the reaction, the Ni foam substrates with deposited Ni₂CoS₄ were taken out, rinsed thoroughly, and dried, yielding the final Ni₂CoS₄ product.

  1.2.2   Syntheses of Ni₂Co(OH)_x and Ni(OH)₂

  A homogeneous solution was prepared by dissolving 0.48 g NiCl₂·6H₂O, 0.24 g CoCl₂·6H₂O, and 0.6 g urea in 28 mL of deionized water. Three pieces of Ni foams (1 cm×2.5 cm) were immersed in the solution, which was then transferred into a 50 mL Teflon-lined stainless‐steel autoclave. The sealed autoclave was heated at 120 ℃ for 8 h. After the reaction, the Ni foam substrates were collected, washed with ethanol and deionized water, and dried at 50 ℃ for 2 h to obtain Ni₂Co(OH)_x. Using the same procedure but without the addition of CoCl₂·6H₂O, pure Ni(OH)₂ was synthesized.

  1.3   Electrochemical measurements

  BAOR tests were performed in a standard three-electrode configuration using a Bio-Logic workstation. The electrolyte consisted of a 1.0 mol·L^-1 KOH solution containing 1.0 mol·L^-1 benzyl alcohol. A graphite rod and an Ag/AgCl electrode were employed as the counter and reference electrodes, respectively. The working electrode was prepared by depositing the catalyst onto Ni foam, with a fixed geometric area of 1 cm×1 cm. All measurements were carried out at a scan rate of 50 mV·s^-1. All the electrochemical reactions were carried out at 30 ℃ under a N₂ atmosphere. And through rotor agitation, the transfer of substances on the catalyst surface was accelerated.

  1.4   Computational method

  We have employed the Vienna Ab Initio Package (VASP) to perform all the density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew‐Burke‐ Ernzerhof (PBE) formulation. We have chosen the projected augmented wave (PAW) potentials to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 500 eV. Partial occupancies of the Kohn-Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10^-5 eV. A geometry optimization was considered convergent when the force change was smaller than 0.5 eV·nm^-1. Grimme′s DFT‐D3 methodology was used to describe the dispersion interactions. Adsorption energies (E_ads) were determined using the formula E_ads=E_ad/sub-E_ad-E_sub, where E_ad/sub represents the total energy of the optimized adsorbate/substrate system, E_ad is the energy of the isolated adsorbate, and E_sub is the energy of the pristine substrate.

  2.   Results and discussion

  2.1   Characterization

  Ni₂CoS₄ was synthesized through the hydrothermal method. Ni foam, NiCl₂·6H₂O, CoCl₂·6H₂O, and thiourea were added to a Teflon vessel and reacted for 8 h. Thiourea can provide the S element and regulate the pH. After that, the Ni-Co bimetallic sulfides precipitated on the Ni foam. To better study the structure and composition of this material, we conducted a series of characterization experiments.

  Firstly, XRD characterization was conducted, as shown in Fig. 1a. The obtained catalysts matched well with the peaks of Ni₂CoS₄ (PDF No.24-0334), which could be preliminarily concluded that Ni₂CoS₄ was synthesized. To further confirm the results, we also conducted ICP-OES characterization, and the results are shown in Table S1 (Supporting information). The molar ratio of Ni and Co was approximately 2∶1, consistent with the XRD results.

  Figure 1

  

  Figure 1.  Characterization of Ni₂CoS₄: (a) XRD patterns; (b, c) SEM images; (d) elemental mappings of Ni, Co, and S on the surface of Ni foam; (e) HRTEM image; (f) elemental mappings of Ni, Co, and S in HRTEM image

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  We also conducted the SEM characterization, and the results are shown in Fig. 1b and 1c. It could be observed that there were nanorods with abundant defects on the surface of the Ni foam, and the diameter was approximately 100 nm. The rich defect structure can provide more active sites and a larger specific area. We also conducted EDS (energy dispersive X-ray spectroscopy) mappings (Fig. 1d). It can be found that Ni, Co, and S were uniformly distributed on the surface of Ni foam. Then, we placed the Ni foam loaded with Ni₂CoS₄ in ethanol for ultrasonic treatment. The resulting solution was loaded on an ultrathin copper net and subjected to high-resolution TEM (HRTEM) characterization (Fig. 1e). The lattice fringe spacing was 0.28 nm, corresponding to the Ni₂CoS₄ (311) plane. In HRTEM image, the uniform distribution of Ni, Co, and S could be observed. This conclusion was consistent with previous characterizations of XRD and ICP-OES. By integrating the relevant characterization methods, it can be confirmed that the successful synthesis of Ni₂CoS₄.

  Ni(OH)₂ and Ni₂Co(OH)_x were also synthesized as control samples. For Ni(OH)₂, the XRD pattern was shown in Fig. S1, which matched well with Ni(OH)₂ (PDF No.22-0444). We also conducted SEM images (Fig. S2), revealing the coverage of the nanosheet on the surface of the Ni foam. Ni and O were evenly distributed in the Ni foam as well. The XRD characterization of Ni₂Co(OH)_x (Fig. S3) showed that the peaks shifted compared to Ni(OH)₂. The result of ICP-OES indicates that the molar ratio of Ni to Co was approximately 2∶1. SEM images (Fig. S4) revealed nanorods with numerous surface defects. EDS mappings in Fig. S4c demonstrated the uniform distribution of Ni, Co, and O. Through ICP-OES, XRD, and SEM, the synthesis of Ni-Co hydroxides (Ni₂Co(OH)_x) can be preliminarily inferred. To further clarify the composition of the material, we also conducted HRTEM characterization (Fig. S5). The lattice fringe spacing was 0.22 nm between the Ni(OH)₂ (101) plane and the Co(OH)₂ (101) plane. Furthermore, by using EDS mappings at high magnification, Ni and Co were uniformly distributed. These results verified the successful synthesis of Ni₂Co(OH)_x.

  2.2   Electrochemical testing

  To study the performance of Ni-based catalysts with different components for BAOR, we conducted electrochemical tests in an electrolyte solution with 1.0 mol·L^-1 KOH and 0.1 mol·L^-1 benzyl alcohol. The loading amounts of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ were 10.9, 10.6, and 10.8 mg·cm^-2, respectively. In a 1.0 mol·L^-1 KOH solution without benzyl alcohol, it could be observed that the polarization curve exhibited a weak current at a low potential. When 0.1 mol·L^-1 benzyl alcohol was added, a significant anode current could be observed at 1.30 V (vs RHE) (Fig. S6). It can be concluded that the obtained Ni₂CoS₄ showed ideal catalytic activity for BAOR, and side reactions could be ignored.

  We tested and compared the polarization curves of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ in an electrolyte with 1.0 mol·L^-1 KOH and 0.1 mol·L^-1 benzyl alcohol (Fig. 2a). Compared with Ni₂Co(OH)_x and Ni(OH)₂, Ni₂CoS₄ showed a higher current density at the same potential, indicating better activity. The addition of Co can enhance the BAOR activity of the Ni-based catalyst, but the performance improvement was not significant. In contrast, Ni₂CoS₄ exhibited obviously higher current density and a lower onset potential than the other two catalysts. The specific activities of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ at 1.40 V (vs RHE) are summarized in Fig. 2b. The specific activity of Ni₂Co(OH)_x was 83 mA·cm^-2, which was slightly higher than that of Ni(OH)₂ (35 mA·cm^-2). Notably, Ni₂CoS₄ could reach 271 mA·cm^-2 at 1.40 V (vs RHE), which was 7.7 times that of Ni(OH)₂.

  Figure 2

  

  Figure 2.  Electrochemical performance of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ in 1.0 mol·L^-1 KOH with 1.0 mol·L^-1 benzyl alcohol: (a) polarization curves; (b) current densities at 1.40 V (vs RHE); (c) Nyquist plots; (d) C_dl; (e) chronopotentiometry curves at 50 mA·cm^-2 after IR correction

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  We also calculated the turnover frequency (TOF), as shown in Fig. S7a. It can be observed that at the same potential, Ni₂Co(OH)_x showed a higher TOF compared to Ni(OH)₂, while the TOF of Ni₂CoS₄ showed a more significant improvement. To conduct a quantitative analysis, we also calculated the TOF of the three catalysts at 1.40 V (vs RHE) (Fig. S7b), and the TOF values of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ were 0.31, 0.11, and 0.05 s^-1, respectively. This result indicated that the active center of Ni₂CoS₄ had the highest intrinsic activity. By combining the linear sweep voltammetry (LSV) curve and the TOF calculation results, it can be observed that, on the one hand, the Co-cation effects can enhance the intrinsic activity of the Ni sites; on the other hand, the S-anion effects can further strengthen the cationic effect of Co, enhancing the optimization effect of Co on the charge distribution and activity of the catalyst, achieving further improvement in overall performance.

  By comparing the electrochemical impedance spectroscopy (EIS) of the three catalysts (Fig. 2c), the kinetic processes of the three catalysts can be studied. The EIS test was set to a frequency of 10⁵ Hz to 50 mHz at a potential of 1.40 V (vs RHE). The faster kinetic process corresponds to a smaller charge transfer resistance (R_ct). The R_ct value of Ni₂CoS₄ was only 1.34 Ω, smaller than those of Ni₂Co(OH)_x and Ni(OH)₂, indicating a faster kinetic process. By analyzing the cyclic voltammogram (CV) curves obtained at different scan rates within the range of 1.20 to 1.40 V (vs RHE) (Fig. S8), the C_dl of the catalyst can be studied. A larger C_dl corresponds to a higher electrochemically active area. Based on the fitting results, the C_dl values for Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ were 23.1, 14.4, and 8.8 mF·cm^-2, respectively (Fig. 2d). The multi-defect structure of Ni₂CoS₄ significantly increased the contact area between the catalyst and the reactants, which is beneficial for the catalytic process of BAOR.

  Stability is another crucial indicator for catalyst performance. At the current density of 50 mA·cm^-2, we conducted a chronopotentiometry test to study the long-term stability of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂. As shown in Fig. 2e, during the 10‐hour test, both Ni₂Co(OH)_x and Ni(OH)₂ exhibited significant performance degradation. However, the working potential of Ni₂CoS₄ remained consistently at around 1.34 V (vs RHE), with a reduction of only 10 mV, demonstrating significantly higher stability.

  2.3   Catalytic mechanism

  Based on the relevant electrochemical test results, it can be concluded that the Co cation can optimize the activity of Ni-based BAOR catalyst to a certain extent, while the S anion can further enhance the performance.

  The alcohol electro-oxidation reaction catalyzed by Ni-based catalysts needs to be carried out under alkaline conditions^[27-28]. The OH^- absorbed on Ni-based active sites to form higher-valent Ni(Ⅲ) centers, which further catalyze the electrochemical oxidation of benzyl alcohol to benzoic acid^[27]. The stronger OH-adsorption ability can help to improve the overall performance of Ni-based BAOR catalysts. By analyzing the Bode diagrams obtained from EIS at different potentials, the changes of oxidation species on the surface of Ni-based catalyst can be studied^[18,29-30]. The phase angle change at lower frequencies often corresponds to the change of oxidation species on the surface. A lower phase angle variation potential is often associated with a lower adsorption potential and a lower catalyst activation potential^[29,31]. The Bode diagrams of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ catalysts are shown in Fig. 3, and the phase angle change potentials can be obtained. The Co cations reduced the phase angle change potential of the Ni-based catalysts. The introduction of S anions could further decrease the external electrical energy required for this process. This demonstrates that the synergistic anion-cation effects of S and Co can effectively reduce the energy required for the adsorption process, thereby enhancing the overall activity of the catalyst.

  Figure 3

  

  Figure 3.  Bode plots of (a) Ni₂CoS₄, (b) Ni₂Co(OH)_x, and (c) Ni(OH)₂ in 1.0 mol·L^-1 KOH with 1.0 mol·L^-1 benzyl alcohol

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  The density of states (DOS) of the three catalysts was calculated to study the effects of S and Co on the charge distribution (Fig. 4a-4c, and calculation models are shown in Fig. S9). Compared with Ni(OH)₂ without Co, the total DOS of Ni-Co diohydroxide provided a higher electron cloud density near the Fermi level, which is beneficial for their adsorption ability. As for Ni₂CoS₄, due to the lattice expansion of S, the d-band center of Ni₂CoS₄ (-1.115 eV) was also closer to the Fermi level than that of Ni₂Co(OH)_x (-1.310 eV). This electron distribution facilitates the adsorption of OH* on the catalyst, thereby reducing the reaction energy barrier^[9,32].

  Figure 4

  

  Figure 4.  DOS of (a) Ni(OH)₂, (b) Ni₂Co(OH)_x, and (c) Ni₂CoS₄; (d) Calculated model for OH^- adsorption of Ni₂CoS₄; (e) Calculated adsorption energy of OH^- of Ni(OH)₂, Ni₂Co(OH)_x, and Ni₂CoS₄

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  In addition, we also calculated the adsorption energy of OH^- on Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂, and the calculation models are shown in Fig. 4d and S10. The adsorption energy of OH^- in Ni₂Co(OH)_x was only -0.839 eV, lower than -0.398 eV of Ni(OH)₂. Through calculation, it can be found that the Co sites showed a strong adsorption capacity for OH^-, which can provide additional active centers and increase the concentration of OH* on the surface. Meanwhile, in Ni₂CoS₄, the adsorption energy of OH^- on the Co sites was even lower (-1.724 eV). The redistribution of charges caused by the S enhanced the additional adsorption capacity of Co and accelerated the activation process. The results of the adsorption energy also confirm the calculations of DOS.

  We investigated the oxidation states and chemical environments through XPS. Fig. 5a shows the Ni2p spectra of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂. It can be observed that a clear Ni^δ+2p_3/2 peak at 852.7 eV in Ni₂CoS₄^[33-35], but could not be detected in Ni₂Co(OH)_x and Ni(OH)₂. The peaks at 852.7 eV are mainly observed in Ni-based sulfides^[17,35-37]. It might be due to the charge redistribution caused by the S-anion effect.

  Figure 5

  

  Figure 5.  High-resolution XPS spectra of (a) Ni2p, (b) Co2p, and (c) S2p in Ni-based catalysts; High-resolution XPS spectra of (d) Ni2p, (e) Co2p, and (f) S2p in Ni₂CoS₄ before and after stability test

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  Furthermore, we also analyzed the valence states of Co in Ni₂CoS₄ and Ni₂Co(OH)_x. As shown in Fig. 5b, for Ni₂CoS₄, the peak at 780.8 eV can be attributed to Co³⁺2p_3/2, and the characteristic peak of Co²⁺2p_3/2 was located at 782.5 eV^[38-40]. In Ni₂CoS₄, the content (atomic fraction) of Co³⁺ was 57%, and that of Co²⁺ was 43%. The content of Co³⁺ in Ni₂CoS₄ was higher than that in Ni₂Co(OH)_x (44%). This indicates that the oxidation state of the Co sites in Ni₂CoS₄ was more positive and conducive to the adsorption of OH^-. Besides, the peaks in Fig. 5c near 162 eV correspond to S—M, where M=Co and Ni^[30,33].

  Compared to the XPS characterization results before and after the stability test, we investigated the changes in the valence states of elements on the catalyst surface after a long period of testing. The oxidation state of Ni slightly increased after the reaction, with a positive shift of 0.3 eV. Some Ni sites were partially oxidized, and Ni^δ+ gradually disappeared (Fig. 5d). Meanwhile, the chemical valence state of Co slightly decreased, and the content (atomic fraction) of Co³⁺ changed to 54% (Fig. 5e). The S underwent a more significant oxidation, with the content of S—M decreasing and the content of S—O increasing (Fig. 5c, 5f). The changes in the oxidation states of S and Co were likely to be the cause of catalyst deactivation.

  Furthermore, SEM images shown in Fig. S11 after the stability test, it can be observed that after a long period of testing, Ni₂CoS₄ still maintained a well‐ dispersed nanorod structure, and the microscopic morphology changes were not significant. The XRD patterns of Ni₂CoS₄ after BAOR (Fig. S12) indicated that the overall composition of Ni₂CoS₄ was basically consistent with Ni₂CoS₄ (PDF No.24-0334), and the material composition changes were not obvious. The influences of morphology and phase transformation on the stability of the catalyst were not significant.

  By combining with the adsorption energy calculation, it can be found that the S-anionic effects provide more positive charge centers for Co, thereby effectively enhancing the additional adsorption capacity of Co for OH^-. This achieves the enrichment of OH* species at a lower potential, which is beneficial to the overall improvement of activity.

  3.   Conclusions

  Combining electrochemical tests, theoretical calculations, and material characterization, we can observe that in the Ni-based BAOR catalyst, the Co cations can utilize their strong adsorption ability for OH^- to provide abundant surface species and a lower onset potential for the activation of the Ni active centers. The S-anion effects can achieve the redistribution of the charge, further enhance the additional adsorption capacity of Co sites, and significantly improve the activity of the Ni-based catalyst. By combining the dual regulation of Co and S, an efficient Ni₂CoS₄ catalyst for BAOR was ultimately obtained.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  Characterization of Ni₂CoS₄: (a) XRD patterns; (b, c) SEM images; (d) elemental mappings of Ni, Co, and S on the surface of Ni foam; (e) HRTEM image; (f) elemental mappings of Ni, Co, and S in HRTEM image

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  Figure 2  Electrochemical performance of Ni₂CoS₄, Ni₂Co(OH)_x, and Ni(OH)₂ in 1.0 mol·L^-1 KOH with 1.0 mol·L^-1 benzyl alcohol: (a) polarization curves; (b) current densities at 1.40 V (vs RHE); (c) Nyquist plots; (d) C_dl; (e) chronopotentiometry curves at 50 mA·cm^-2 after IR correction

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  Figure 3  Bode plots of (a) Ni₂CoS₄, (b) Ni₂Co(OH)_x, and (c) Ni(OH)₂ in 1.0 mol·L^-1 KOH with 1.0 mol·L^-1 benzyl alcohol

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  Figure 4  DOS of (a) Ni(OH)₂, (b) Ni₂Co(OH)_x, and (c) Ni₂CoS₄; (d) Calculated model for OH^- adsorption of Ni₂CoS₄; (e) Calculated adsorption energy of OH^- of Ni(OH)₂, Ni₂Co(OH)_x, and Ni₂CoS₄

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  Figure 5  High-resolution XPS spectra of (a) Ni2p, (b) Co2p, and (c) S2p in Ni-based catalysts; High-resolution XPS spectra of (d) Ni2p, (e) Co2p, and (f) S2p in Ni₂CoS₄ before and after stability test

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