English

• The huge fossil fuel consumption with economic development has led to an energy crisis and environmental problems. Therefore, the development of clean and sustainable energy technologies has been significant in recent years^[1]. Electrochemical water splitting is considered to be the most effective method for hydrogen and oxygen production, which is composed of two half-reactions, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)^[2]. However, the OER involves four electron-proton coupled reactions, which suffer from sluggish kinetic and high overpotential, thus hindering the overall energy-conversion efficiency and practical large-scale application^[3-5]. Therefore, it is necessary to seek a highly active and stable electrocatalyst for OER to accelerate the electrocatalytic process. Noble metal oxides such as IrO₂ and RuO₂ have been regarded as the most effective OER electrocatalysts for reducing the overpotential of water splitting. Nevertheless, their defects with high prices, limited reserves, and poor stability restrict their widespread application^[5]. Consequently, it is particularly important to design and develop a non-precious metal-based OER electrocatalyst with high activity, strong durability, and low cost.

  In recent years, non-precious metal-based (especially iron-, cobalt-, and nickel-based) OER electrocatalysts have shown great potential for boosting electrocatalytic performance^[6-8]. Oxides^[9], phosphides^[10], borides^[11], and sulfides^[12] have received considerable attention due to their low cost and excellent performance. Among the numerous OER electrocatalysts, metal-organic frameworks (MOFs), a class of organic-inorganic hybrid materials formed by the self-assembly of metal ions/clusters and polydentate organic bridging ligands, have become attractive catalysts in electrocatalysis because of their tunable composition and pore size, and abundant catalytic active sites, which can effectively reduce the kinetic barrier and facilitate the mass/charge transfer during electrocatalysis, thus showing excellent electrochemical performance^[13-16]. Tannic acid (TA) with abundant phenolic hydroxyl groups can easily chelate with metal ions as an excellent organic ligand and construct stable metal-phenolic networks on various substrates^[7,17]. For instance, Chen et al. have successfully synthesized Fe-tannic coordination network (Fe-Tan) nanoparticles covered on the oxidized iron foam (O-IF), which showed the remarkably boosted oxygen evolution electrocatalysis under irradiation of NIR laser^[18]. Wang et al. have controllably synthesized TA-metal ion coordination networks (e.g., TA-Fe, TA-Co, and TA-Ni) on diverse nanostructured substrates including Ni(OH)₂ nanosheets, Co(OH)F nanowire arrays, and nickel foam. The obtained hierarchical coordination complexes showed the bifunctional electrocatalytic activity toward the OER and HER^[19]. Shi et al. presented the fast and facile complex film coating on conductive carbon fiber paper (CFP) by coordinating TA with transition metal (Fe, Co, and Ni) ions as efficient OER electrocatalysts. Among different TA-metal complexes, TA-Ni₃Fe (TANF) showed the highest electrochemical OER activity^[20]. Electrochemical results showed that the appropriate Fe incorporation in the Co-based catalyst can regulate the electronic structure, create abundant oxygen vacancies, and expose more active sites, thereby promoting the catalytic activity^[7,12,21-22]. In addition, heteroatoms such as P, S, and B incorporated into MOF materials can improve the stability and the electrocatalytic OER performance of catalysts, ascribed to the altered charge or spin distribution, which can promote the oxygen adsorption or/and subsequent O—O bond cleavage^[21,23]. Taken together, the synergistic effect between the bimetallic species and heteroatom can generate abundant oxygen vacancies, expose more active sites, enhance electronic conductivity, and significantly improve the OER activity^[7,24].

  Herein, sulfur-doped iron-cobalt tannate (S-FeCoTA) nanorods were synthesized by a simple one-step hydrothermal method using TA as ligand precursors. Owing to the incorporation of sulfur, the electronic environment around iron and cobalt in S-FeCoTA was changed, resulting in oxygen vacancies and exposing more active sites. In addition, the interaction between S, Fe, and Co accelerates the electrochemical reaction kinetics of S-FeCoTA and facilitates the electron transfer at the electrode/electrolyte interface, thereby enhancing the electrocatalytic OER performance. As a result, S-FeCoTA showed the highest electrochemical OER activity with a low overpotential of 273 mV at a current density of 10 mA·cm^-2, which was superior to that of the commercial RuO₂. This work can provide some theoretical guidance and reference value for the synthesis of stable and efficient MOF-based electrocatalytic OER materials.

  1.   Experimental

  1.1   Materials

  Cobalt(Ⅱ) nitrate hexahydrate (Co(NO₃)₂·6H₂O), iron(Ⅲ) nitrate nonahydrate (Fe(NO₃)₃·9H₂O), TA, and NaOH were purchased from Chengdu Kelong Chemical Co., Ltd (China). Thioacetamide, polyethylene glycol 6000 (PEG 6000), and isopropanol were purchased from Chengdu Kelong Chemical Reagent Factory (China). KOH was purchased from Chengdu Jinshan Chemical Reagent Co., Ltd (China). All the reagents mentioned above were of analytical grade and used without further purification. Nafion solution (5%, mass fraction, Dupont D520), 20% platinum/carbon (Pt/C), and CFP were purchased from Shanghai Hesen Electric Co., Ltd (China). Ultrapure water (18.25 MΩ·cm) was prepared by an ultra-pure water system (UPT-Ⅱ-20T, Sichuan ULUPURE Ultrapure Technology Co., Ltd, China) and used throughout all the experiments.

  1.2   Preparation of S-FeCoTA

  S-FeCoTA was prepared by a facile one-step hydrothermal method. Typically, 17 g of TA was first dissolved in 6 mol·L^-1 NaOH solution to obtain 0.02 mol·L^-1 sodium tannate solution. Then 0.12 g of thioacetamide, 0.30 g of PEG 6000, and 25 mL ultrapure water were added into a mixed solution (10 mL) containing sodium tannate, Fe(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O with the molar ratio of 1∶6∶8 under continuously stirring for 30 min. The above mixture was then transferred to 100 mL of Teflon-lined stainless steel autoclave and reacted at 120 ℃ for 6 h. After cooling down to room temperature, the obtained suspension was centrifuged and washed with ultrapure water and anhydrous ethanol several times, respectively. The obtained sample was dried in an oven at 50 ℃. Similarly, the other comparative materials such as S-FeTA, S-CoTA, and FeCoTA were prepared by the same method of S-FeCoTA except for not adding the corresponding element, and S-FeCoTA₀ was synthesized without adding PEG 6000.

  1.3   Characterizations

  The morphologies of the materials were tested using a Regulus 8100 scanning electron microscope (SEM, Japan) with an accelerating voltage of 5.0 kV. The morphology and mapping images of S-FeCoTA were performed with a JEM-2100PLUS transmission electron microscopy (TEM, Japan) at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were recorded on a DX-2700 X-ray diffractometer (China) with Cu Kα radiation (λ=0.154 06 nm) under a working voltage of 40 kV, a working current of 30 mA, and a scanning range of 10°-70°. The surface elemental composition and element valence states of the materials were analyzed using an Escalab 250Xi X-ray photoelectron spectrometer (XPS, USA, Al Kα, 1 486.6 eV). Fourier transform infrared (FTIR) spectra measurements (KBr pellets) were performed on a WQF-510A FTIR spectrometer (China). The Brunauer-Emmett-Teller (BET) specific surface areas were measured by an ASAP 2460 surface area analyzer (USA), and the pore size distributions of the materials were calculated using the Barrette-Joyner-Halenda (BJH) method.

  1.4   Electrochemical measurements

  All the electrochemical tests were conducted on a CHI660E electrochemical workstation (Chenhua, Shanghai, China). The three-electrode system was performed in 1 mol·L^-1 KOH solution with a Pt foil as a counter electrode, a Hg/HgO electrode as the reference electrode, and a glassy carbon electrode (GCE, 3 mm in diameter) as the working electrode. 4.0 mg of S-FeCoTA was added into 125 μL of isopropanol, 375 μL of ultrapure water, and 10 μL of 5.0% Nafion solution, and the mixture was ultrasonically dispersed for about 30 min to obtain the homogenous catalyst ink. The S-FeCoTA electrode was prepared by the deposition of 4 μL catalyst ink on a smooth GCE with a loading of 0.45 mg·cm^-2 and dried naturally at room temperature.

  All the potentials relative to the Hg/HgO electrode (E′) were converted to the potentials relative to the reversible hydrogen electrode (E) according to the equation: E=E′+0.059 2pH+0.098. Linear sweep voltammetry (LSV) with a scan rate of 50 mV·s^-1 was conducted in the range of 0.92-1.72 V (vs RHE). Electrochemical impedance spectroscopy (EIS) was performed at 0.60 V (vs Hg/HgO) in the frequency range from 0.01 Hz to 100 kHz with an amplitude of 5 mV. To obtain the electrochemical double-layer capacitances (C_dl), cyclic voltammetry (CV) tests were carried out in the potential range of 0.3-0.4 V (vs Hg/HgO) at scan rates of 2, 5, 10, 20, 50, and 100 mV·s^-1, respectively. The stability was investigated via a chronoamperometry test at a current density of 10 mA·cm^-2. CV measurements for 1 000 cycles were performed at a scan rate of 50 mV·s^-1 in the range of 0-0.8 V (vs Hg/HgO), and then the LSV test of the collected post-cyclic material was conducted from 0.92 to 1.72 V (vs RHE). Energy consumption tests were conducted with a two-electrode system in 6 mol·L^-1 KOH solution at 70 ℃. The Faraday efficiency was obtained by calculating the ratio between the actual and theoretical amount of maximum oxygen production using volumetric analysis. Partical polarization curves were iR-corrected according to E_corr=E_mea-iR_s, where E_corr is the iR-corrected potential, E_mea is the measured potential, i is the current, and R_s is the equivalent series resistance extracted from the EIS measurement.

  2.   Results and discussion

  2.1   Morphology and structure

  The S-FeCoTA framework was prepared through a facile one-step hydrothermal strategy^[7,25]. The subtle morphologies of the as-prepared materials were observed by SEM. As shown in Fig. 1a, FeCoTA nanomaterials were stacked by many nanosheets. When an appropriate amount of S was introduced, S-FeCoTA nanomaterials were composed of a large number of interwoven nanorods with a loose surface and many voids (Fig. 1b), which can expose more active sites, provide electron transfer channels, and enhance the electrocatalytic OER properties^[26]. The TEM image further verified the interwoven nanorod shape of S-FeCoTA (Fig. 1d). The energy-dispersive X-ray spectroscopy (EDS) mapping images of S-FeCoTA (Fig. 1e) revealed the presence of Fe, Co, C, O, and S in the nanorods of S-FeCoTA and illustrated that all elements were uniformly distributed throughout the S-FeCoTA nanorods, suggesting the good coordination of Fe/Co ions with TA molecules. XRD patterns were analyzed to determine the crystal structures of S-FeCoTA and comparative materials such as S-FeTA, S-CoTA, FeCoTA, and S-FeCoTA₀. As shown in Fig. 2a, S-FeCoTA and comparative materials exhibited a weak and broad diffraction peak at around 20°, indicating their amorphous nature. The amorphous structure can offer more active sites, which facilitates electron transfer and enhances electrocatalytic activity^[27].

  Figure 1

  

  Figure 1.  SEM images of FeCoTA (a) and S-FeCoTA before (b) and after (c) 1 000 cycles; TEM image (d) and EDS mapping images (e) of S-FeCoTA

  下载: 全尺寸图片 幻灯片

  The structures and chemical bonds of S-FeCoTA and the comparative materials were analyzed by FTIR spectroscopy. As presented in Fig. 2b, in the FTIR spectrum of TA, the absorption peak at around 3 404 cm^-1 is attributed to the stretching vibration of O—H bonds^[28]. The peak near 1 716 cm^-1 belongs to the stretching vibration of the carboxylate (C=O) group^[29]. The peaks at 1 448-1 612 cm^-1 are assigned to aromatic stretches^[30]. The peaks at 1 031 and 757 cm^-1 are ascribed to C—O stretching vibration^[31] and out-of-plane bending vibration of C—H bonds in aromatic compounds^[32], respectively. As for FeCoTA, the peaks at 1 541, 1 400, 1 037, and 750 cm^-1 correspond to the C=C stretching, symmetric —COO stretching, phenolic C—O stretching, and C—H bending vibrations^[7,31,33-34], respectively. Compared with FeCoTA, except for the above absorption peaks, S-FeCoTA and S-FeCoTA₀ possessed characteristic peaks near 1 115 cm^-1, corresponding to C—S stretching vibration^[35-36]. The peak near 640 cm^-1 is attributed to Fe—O/Co—O bonds, further indicating that Fe/Co ions were successfully coordinated with the phenolic hydroxyl oxygen within TA^[7]. The FTIR spectra of S-FeTA and S-CoTA indicate that Fe/Co ions were coordinated with TA, and the sulfur was successfully incorporated into S-FeTA and S-CoTA, respectively.

  Figure 2

  

  Figure 2.  XRD patterns (a) and FTIR spectra (b) of the samples

  下载: 全尺寸图片 幻灯片

  As exhibited in Fig.S1 (Supporting information), the N₂ adsorption-desorption isotherms of S-FeCoTA and FeCoTA obtained at 77 K show the typical type Ⅳ isotherm with an H3 hysteresis ring. The small slopes at relative pressure (p/p₀=0.5-1) accompanied by hysteresis of desorption indicate that S-FeCoTA and FeCoTA had a mesoporous structure and slit-shaped pores, which can provide plenty of interspace^[37] and is beneficial for mass transport during electrocatalysis OER^[38]. Especially at higher relative pressures (p/p₀ > 0.9), the adsorption curves of S-FeCoTA and FeCoTA increased sharply, implying the formation of larger slit-like pores and loosely packed particles in S-FeCoTA and FeCoTA^[39]. The average pore diameters of S-FeCoTA and FeCoTA were 12.6 and 9.4 nm, respectively. The Brunauer-Emmett-Teller (BET) specific surface area and pore volume of S-FeCoTA were measured to be 21 m²·g^-1 and 0.065 cm³·g^-1, respectively, while those of FeCoTA were 20 m²·g^-1 and 0.045 cm³·g^-1, respectively. The specific surface area of S-FeCoTA was almost the same as that of FeCoTA, suggesting that the incorporation of sulfur contributes less to the improvement of the specific surface area.

  The surface element composition and chemical valence states of FeCoTA and S-FeCoTA were analyzed by XPS. As illustrated in Fig.S2, Fe, Co, C, and O elements were presented in FeCoTA, while the XPS survey spectrum demonstrated the coexistence of Co, Fe, C, O, and S elements in S-FeCoTA. Fig. 3a shows the high-resolution Fe2p spectrum. For FeCoTA, two peaks located at 711.9 and 717.5 eV are assigned to the +3 oxidation state of the Fe species^[40]. The Fe2p region in S-FeCoTA showed two signals at 711.5 and 723.2 eV attributed to Fe³⁺2p_3/2 and Fe³⁺2p_1/2, respectively^[41-42]. In the Co2p XPS spectra (Fig. 3b), the Co2p spectrum of FeCoTA can be fitted with two main peaks at 780.8 and 796.7 eV assigned to Co²⁺2p_3/2 and Co²⁺2p_1/2, and two satellite peaks of Co²⁺ at 785.3 and 802.0 eV, respectively^[43-44]. In the Co2p region of S-FeCoTA, the two characteristic peaks of Co2p_1/2 and Co2p_3/2 appeared at 781.0 and 797.6 eV, together with two satellite peaks at 786.4 and 801.2 eV, respectively, implying the existence of a Co²⁺ oxidation state^[45-46]. Compared with the binding energies of Fe2p and Co2p in FeCoTA, the binding energy shifts of Fe2p and Co2p in S-FeCoTA were observed, which may originate from the electronegativity and polarization of sulfur^[47]. Sulfur has a lower electronegativity, and the anions are easily polarized and prone to bond with adjacent metal ions (Fe³⁺, Co²⁺) so that the electrons can be dispersed to balance the positive electric fields of Fe³⁺ and Co²⁺. Therefore, the S incorporation makes the binding energies of Fe2p and Co2p in S-FeCoTA shift and reduces the adsorption free energy of OH* and the free energy gap between O* and OH* intermediates (where ^* denotes an active site on the catalyst surface), thus improving the electrocatalytic performance^[47-49]. As presented in Fig. 3c, the C1s XPS spectra of FeCoTA and S-FeCoTA could be deconvoluted into three peaks. For S-FeCoTA, the peaks positioned at 284.5, 286.0, and 288.3 eV belong to C—C/C=C^[50], C—O/C—S^[51], and C=O/O—C=O^[52], respectively. The C1s binding energy of FeCoTA exhibited a slight shift from that of S-FeCoTA, further suggesting the modulated electronic structure on account of the polarization effect of doped sulfur^[53]. Two distinct characteristic peaks at 531.2 and 532.6 eV can be observed in the O1s XPS spectrum of S-FeCoTA (Fig. 3d) related to Fe—O/Co—O and C—O, respectively, implying successful coordination between the phenolic oxygen of TA and Fe³⁺/Co^{2+ [43,52,54]}. Due to the introduction of sulfur, the O1s binding energy of S-FeCoTA was slightly red-shifted compared with FeCoTA, which can induce lattice distortion and electronic redistribution, destabilize oxygen coordination, and facilitate the generation of oxygen vacancies, thereby improving the OER activity^[55-56]. The S2p XPS spectra of S-FeCoTA (Fig. 3e) can be fitted with three distinct characteristic peaks. The peaks at 163.0, 164.4, and 168.5 eV are attributed to S^2-, C—S, and oxidized sulfur^[47,57], respectively, demonstrating that sulfur is incorporated into the structure both as dopant ions and through covalent bonding with carbon. Furthermore, the presence of sulfur as S^2- suggests that it may replace oxygen in the metal-oxygen coordination environment, forming M—S (M: metal) bond that alter the local electronic structure of Fe and Co ions.

  Figure 3

  

  Figure 3.  XPS spectra of Fe2p (a), Co2p (b), C1s (c), O1s (d), and S2p (e)

  下载: 全尺寸图片 幻灯片

  2.2   Electrochemical performance

  As shown in Fig. 4a, the current density of S-FeCoTA was 162 mA·cm^-2 at 1.58 V (vs RHE), superior to those of other comparative materials. At a current density of 10 mA·cm^-2, the overpotential of S-FeCoTA was 273 mV, significantly better than those of S-CoTA (366 mV), S-FeCoTA₀ (323 mV), FeCoTA (326 mV), and RuO₂ (324 mV) (Fig. 4b), indicating that the prepared S-FeCoTA, with abundant active sites and optimized electronic structure due to sulfur doping and the regulation of PEG 6000, likely exhibitted superior OER activity, enabling better performance compared to RuO₂. While the overpotential of S-FeCoTA at a current density of 100 mA·cm^-2 was less than that of RuO₂ (Fig.S3), manifesting that the loosely packed interwoven nanorod morphology of S-FeCoTA may result in suboptimal mass transport pathways compared to the dense and highly conductive structure of RuO₂, leading to a performance decline at higher current densities. The Tafel slope is a critical parameter to examine the catalytic kinetics of electrocatalysts. The lower Tafel slope indicates a faster charge transfer of the OER process^[58]. According to the equation: η=a+blg j (η, a, b, and j represent the overpotential, Tafel constant, Tafel slope, and current density, respectively). The Tafel slopes were obtained by fitting the LSV curves (Fig. 4c). The Tafel slope of S-FeCoTA was 36 mV·dec^-1, lower than those of S-FeTA (135 mV·dec^-1), S-CoTA (91 mV·dec^-1), S-FeCoTA₀ (48 mV·dec^-1), FeCoTA (75 mV·dec^-1), and RuO₂ (88 mV·dec^-1), suggesting the fast OER kinetics and favorable catalytic activity of S-FeCoTA. Compared with the catalysts reported in the current literature (Table S1), the excellent OER performance of S-FeCoTA further demonstrates that the incorporation of S induces lattice distortion and creates defects, including oxygen vacancies, which enhances the exposure of active sites and improves the overall reactivity of the material. The electronic and structural modifications brought about by S doping reduce the adsorption-free energy of oxygen-containing intermediates (e.g., OH*), thereby lowering the reaction barriers and enhancing OER activity^[47-49].

  Figure 4

  

  Figure 4.  LSV curves (a), overpotentials (b), and Tafel plots (c) of the samples

  Inset: Tafel plot of S-FeCoTA₀.

  下载: 全尺寸图片 幻灯片

  To further evaluate the OER performance of S-FeCoTA, the electrochemically active surface areas (ECSA) of the electrocatalysts were studied by electrochemical double-layer capacitance (C_dl) using CV (Fig.S4). The ECSA is proportional to the C_dl value, so a higher C_dl indicates a larger ECSA^[59]. As shown in Fig.S4f, the C_dl value of S-FeCoTA was 102.4 mF·cm^-2, which was higher than those of S-FeTA, FeCoTA, S-FeCoTA₀, and S-CoTA (0.8, 25.3, 53.5, and 82.2 mF·cm^-2, respectively), demonstrating that S-FeCoTA possessed a much higher ECSA and could provide more active sites, thereby enhancing the OER catalytic activity. EIS was performed to further estimate interfacial charge transfer kinetics. The semi-circle at the high-frequency region of the Nyquist plot corresponds to the charge transfer resistance (R_ct)^[60]. As presented in Fig. 5a and S5, S-FeCoTA had the smallest semi-circle diameter, which suggests that S doping effectively modulates the electronic structure of FeCoTA, thus accelerating the charge transfer and improving the electrocatalytic OER activity^[7,15]. The intrinsic electrocatalytic activity of S-FeCoTA can also be explored by the turnover frequency (TOF) at an overpotential of 300 mV. Assuming that both Fe and Co serve as OER active sites, as listed in Table S2, the TOF values of S-FeCoTA were higher than those of FeCoTA, further implying the superior electrocatalytic OER activity of S-FeCoTA. The durability of S-FeCoTA was evaluated using chronopotentiometry at a current density of 10 mA·cm^-2. As shown in Fig. 5b, the potential of S-FeCoTA was maintained at 1.48 V (vs RHE) for 15 h, suggesting a good stability of S-FeCoTA.

  Figure 5

  

  Figure 5.  (a) EIS and (b) chronopotentiometry curve of S-FeCoTA at a current density of 10 mA·cm^-2

  Inset in a: equivalent circuit, where CPE is the constant phase angle element and R_s is the solution resistance;
  Inset in b: LSV curves before and after 1 000 cycles

  下载: 全尺寸图片 幻灯片

  It is known that the morphology and structure of OER electrocatalysts may change after cycling. The morphology of S-FeCoTA after 1 000 cycles (Fig. 1c) was transformed from nanorods to nanosheets, and the nanosheets were packed more tightly. As shown in Fig. 2b, after 1 000 cycles, the absorption peaks near 1 545 and 1 400 cm^-1 were significantly weakened, and the peak at 617 cm^-1 became more prominent, which may be due to the partial collapse of the MOF structure during the cycling process and the formation of more Fe—O/Co—O bonds^[61-62]. The peak near 1 115 cm^-1 is inconspicuous, further suggesting that the C—S bonds may be broken and can not be observed because of the partial collapse of the MOF structure. Furthermore, the XPS survey spectrum of S-FeCoTA after 1 000 cycles (Fig. 3a) demonstrates that the high-resolution Fe2p region exhibited the binding energies at 712.1, 725.0, and 718.2 eV ascribed to Fe2p_3/2, Fe2p_1/2, and satellite peaks of Fe³⁺, respectively^[40,63]. The Co2p spectrum after 1 000 cycles (Fig. 3b) can be divided into the two main peaks at 780.6 and 796.7 eV, and the two satellite peaks located at 785.4 and 802.9 eV, respectively, which corresponds to Co²⁺2p_3/2 and Co²⁺2p_1/2^[7,43]. Compared to the O1s XPS spectrum of S-FeCoTA before 1 000 cycles, an additional peak of O1s of S-FeCoTA after 1 000 cycles (Fig. 3d) at 529.6 eV is attributed to O^2-[64], further indicating the partial collapse of the MOF structure and the formation of more Fe—O/Co—O bonds during the cycling process. The S2p spectrum after 1 000 cycles (Fig. 3e) showed three peaks at 163.1, 168.1, and 169.1 eV assigned to S^2- and sulfate^[65]. No fitting peak of the C—S bond was observed, which may be due to the breakage of the C—S bonds during cycling and is consistent with the FTIR results. Compared with S-FeCoTA (158 mA·cm^-2), the current density of S-FeCoTA after 1 000 cycles was 130 mA·cm^-2 with a decrease of 17.7% (the inset of Fig. 5b), which may be due to the C—S bond cleavage, structural collapse and morphological change after cycling, thus reducing the OER activity.

  In this study, we simulated the OER process using a two-electrode system at 70 ℃ in a 6 mol·L^-1 KOH solution. The experimental setup involved using 20% Pt/C as the cathode and S-FeCoTA as the anode, with a loading of 0.784 mg·cm^-2 on CFP. This configuration allowed us to evaluate the energy consumption effectively at current densities of 100 and 200 mA·cm^-2 over 10 000 s. The results, as shown in Fig. 6a and Table S3, demonstrate that the bias voltage and energy consumption of S-FeCoTA were lower than that of RuO₂, suggesting that S-FeCoTA is more suitable for commercial alkaline water electrolysis. To further evaluate the energy conversion efficiency of S-FeCoTA for OER, the Faraday efficiency was studied by chronopotentiometry at a stable current of 10 mA (Fig. 6b). The actual oxygen production of S-FeCoTA was obtained within 120 min, compared with the theoretical oxygen yield, the Faraday efficiency of S-FeCoTA was approximately 94%, indicating that S-FeCoTA had a high-efficiency current-to-oxygen conversion^[66].

  Figure 6

  

  Figure 6.  Energy consumption test curves (a) and Faraday efficiencies (b) of S-FeCoTA

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In conclusion, S-FeCoTA nanorods were successfully synthesized with the S-doping strategy via a feasible one-step hydrothermal method. Due to the incorporation of S atoms, S-FeCoTA possessed the interwoven nanorods and moderated electronic structure, abundant oxygen vacancies, exposed active sites, and fast charge transfer rate, thus demonstrating excellent OER electroactivity with an overpotential of 273 mV at a current density of 10 mA·cm^-2, a small Tafel slope of 36 mV·dec^-1, and an outstanding Faraday efficiency of 94%. Considering the simple, low-cost, and easy-to-operate method in this work, introducing S atoms to promote a synergistic effect between elements can be a prospective approach for optimizing the electrocatalytic performances of MOFs for high-efficiency energy conversion applications.

  Supporting information is available at http://www.wjhxxb.cn

  
  
• 1. [1]

     FANG M, GAO X T, LIANG J, GUO B A, ZOU L, LU J, GAO Y, TSE E C M, LIU J X. Bioinspired NiFe-gallate metal-organic frameworks for highly efficient oxygen evolution electrocatalysis[J]. J. Mater. Chem. A, 2022, 10(13):  7013-7019. doi: 10.1039/D2TA01293F

  2. [2]

     RUI K, ZHAO G Q, CHEN Y P, LIN Y, ZHOU Q, CHEN J Y, ZHU J X, SUN W P, HUANG W, DOU S X. Hybrid 2D dual-metal-organic frameworks for enhanced water oxidation catalysis[J]. Adv. Funct. Mater., 2018, 28(26):  1801554. doi: 10.1002/adfm.201801554

  3. [3]

     HUANG J W, LUO W, YUAN X L, WANG J. Bimetallic MOF-derived oxides modified BiVO₄ for enhanced photoelectrochemical water oxidation performance[J]. J. Alloy. Compd., 2023, 930:  167397. doi: 10.1016/j.jallcom.2022.167397

  4. [4]

     FRIEBEL D, LOUIE M W, BAJDICH M, SANWALD K E, CAI Y, WISE A M, CHENG M J, SOKARAS D, WENG T C, ALONSO-MORI R, DAVIS R C, BARGAR J R, NORSKOV J K, NILSSON A, BELL A T. Identification of high active Fe sites in (Ni, Fe)OOH for electrocatalytic water splitting[J]. J. Am. Chem. Soc., 2015, 137(3):  1305-1313. doi: 10.1021/ja511559d

  5. [5]

     GAO T T, WU S W, LI X Q, LIN C H, YUE Q, TANG X M, YU S M, XIAO D. Phytic acid assisted ultra-fast in situ construction of Ni foam-supported amorphous Ni-Fe phytates to enhance catalytic performance for the oxygen evolution reaction[J]. Inorg. Chem. Front., 2022, 9(14):  3598-3608. doi: 10.1039/D2QI00924B

  6. [6]

     LIU Y Z, LI X T, SUN Q D, WANG Z L, HUANG W H, GUO X Y, FAN Z X, YE R Q, ZHU Y, CHUEH C C, CHEN C L, ZHU Z L. Freestanding 2D NiFe metal-organic framework nanosheets: Facilitating proton transfer via organic ligands for efficient oxygen evolution reaction[J]. Small, 2022, 18(26):  2201076. doi: 10.1002/smll.202201076

  7. [7]

     ZHOU G Y, MA Y R, GU C J, YANG J, PANG H, LI J, XU L, TANG Y W. Fe incorporation-induced electronic modification of Co-tannic acid complex nanoflowers for high-performance water oxidation[J]. Inorg. Chem. Front., 2022, 9(6):  1091-1099. doi: 10.1039/D1QI01630J

  8. [8]

     LV C H, REN Y Y, LI B B, LU Z M, LI L L, ZHANG X H, YANG X J, YU X F. 1, 2, 4-Triazole-assisted metal-organic framework-derived nitrogen-doped carbon nanotubes with encapsulated Co₄N particles as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries[J]. J. Colloid Interface Sci., 2023, 645:  618-626. doi: 10.1016/j.jcis.2023.04.106

  9. [9]

     ZHANG S F, WEI N, YAO Z J, ZHAO X Y, DU M, ZHOU Q S. Oxygen vacancy-based ultrathin Co₃O₄ nanosheets as a high-efficiency electrocatalyst for oxygen evolution reaction[J]. Int. J. Hydrog. Energy, 2021, 46(7):  5286-5295. doi: 10.1016/j.ijhydene.2020.11.072

  10. [10]

      LIU Y F, LI Y, WU Q, SU Z, WANG B, CHEN Y F, WANG S F. Hollow CoP/FeP₄ heterostructural nanorods interwoven by CNT as a highly efficient electrocatalyst for oxygen evolution reactions[J]. Nanomaterials, 2021, 11(6):  1450. doi: 10.3390/nano11061450

  11. [11]

      YUAN H F, WANG S M, GU X D, TANG B, LI J P, WANG X G. One-step solid-phase boronation to fabricate self-supported porous FeNiB/FeNi foam for efficient electrocatalytic oxygen evolution and overall water splitting[J]. J. Mater. Chem. A, 2019, 7(33):  19554-19564. doi: 10.1039/C9TA04076E

  12. [12]

      MAO X Q, LIU Y, CHEN Z Y, FAN Y P, SHEN P K. Fe and Co dual-doped Ni₃S₄ nanosheet with enriched high-valence Ni sites for efficient oxygen evolution reaction[J]. Chem. Eng. J., 2022, 427:  130742. doi: 10.1016/j.cej.2021.130742

  13. [13]

      LI C Q, LIU Y W, GUAN L H, LI K, WANG G, LIN Y Q. Understanding coordination modification strategy on metal organic framework-based system for efficient water oxidation[J]. Chem. Eng. J., 2020, 400:  125884. doi: 10.1016/j.cej.2020.125884

  14. [14]

      LIU M R, ZHENG X, TANG L, ZHANG M S, CHEN X, CAI Z X. The effect of replacing linker with carboxylic acid ligands on the oxygen evolution performance of metal-organic frameworks[J]. J. Alloy. Compd., 2023, 961:  170976. doi: 10.1016/j.jallcom.2023.170976

  15. [15]

      MI H T, LI L Y, ZENG C T, JIN Y H, ZHANG Q Q, ZHOU K L, LIU J B, WANG H. Cuboid-like phosphorus-doped metal-organic framework-derived CoSe₂ on carbon cloth as an advanced bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries[J]. J. Colloid Interface Sci., 2023, 633:  424-431. doi: 10.1016/j.jcis.2022.11.116

  16. [16]

      FAN C B, ZHANG X Y, GUO F, XING Z Y, WANG J L, LIN W Y, TAN J, HUANG G M, ZONG Z A. Design of five two-dimensional Co-metal-organic frameworks for oxygen evolution reaction and dye degradation properties[J]. Front. Chem., 2022, 10:  1044313. doi: 10.3389/fchem.2022.1044313

  17. [17]

      DENG Y J, WANG G H, SUN K L, CHI B, SHI X D, DONG Y Y, ZHENG L, ZENG J H, LI X H, LIAO S J. Highly effective and stable doped carbon catalyst with three-dimensional porous structure and well-covered Fe₃C nanoparticles prepared with C₃N₄ and tannic acid as template/precursors[J]. J. Power Sources, 2019, 417:  117-124. doi: 10.1016/j.jpowsour.2019.02.022

  18. [18]

      CHEN M, ZHANG Z C, ZENG C M, JIANG J, GAO H J, AI L H. Synergistically boosting oxygen evolution performance of iron-tannic electrocatalyst via localized photothermal effect[J]. Colloid Surf. A‒Physicochem. Eng. Asp., 2022, 638:  128248. doi: 10.1016/j.colsurfa.2022.128248

  19. [19]

      WANG Y Q, CHEN S, ZHAO S Y, CHEN Q W, ZHANG J T. Interfacial coordination assembly of tannic acid with metal ions on three-dimensional nickel hydroxide nanowalls for efficient water splitting[J]. J. Mater. Chem. A, 2020, 8(31):  15845. doi: 10.1039/D0TA02229B

  20. [20]

      SHI Y M, YU Y, LIANG Y, DU Y H, ZHANG B. In situ electrochemical conversion of an ultrathin tannin nickel iron complex film as an efficient oxygen evolution reaction electrocatalyst[J]. Angew. Chem.‒Int. Edit., 2019, 58(12):  3769-3773. doi: 10.1002/anie.201811241

  21. [21]

      CHEN H X, LI Y W, LIU H J, JI Q H, ZOU L J, GAO J K. Metal-organic framework-derived sulfur and nitrogen dual-doped bimetallic carbon nanotubes as electrocatalysts for oxygen evolution reaction[J]. J. Solid State Chem., 2020, 288:  121421. doi: 10.1016/j.jssc.2020.121421

  22. [22]

      HAN Z, LIN S Y, FENG J J, ZHANG L, ZHANG Q L, WANG A J. Transitional metal alloyed nanoparticles entrapped into the highly porous N-doped 3D honeycombed carbon: A high-efficiency bifunctional oxygen electrocatalyst for boosting rechargeable Zn-air batteries[J]. Int. J. Hydrog. Energy, 2021, 46(37):  19385-19396. doi: 10.1016/j.ijhydene.2021.03.086

  23. [23]

      TANG C, ZHANG Q. Nanocarbon for oxygen reduction electrocatalysis: Dopants, edges, and defects[J]. Adv. Mater., 2017, 29(13):  1604103. doi: 10.1002/adma.201604103

  24. [24]

      SAMANTA A, RAJ C R. Catalyst support in oxygen electrocatalysis: A case study with CoFe alloy electrocatalyst[J]. J. Phys. Chem. C, 2018, 122(28):  15843-15852. doi: 10.1021/acs.jpcc.8b02830

  25. [25]

      CHENG L, HOU P M, LIU C B. Tannic acid-copper metal-organic frameworks decorated graphene oxide for reinforcement of the corrosion protection of waterborne epoxy coatings[J]. Mater. Corros., 2022, 73(10):  1666-1675. doi: 10.1002/maco.202213189

  26. [26]

      ZHOU G Y, WU X M, ZHAO M M, PANG H, XU L, YANG J, TANG Y W. Interfacial engineering-triggered bifunctionality of CoS₂/MoS₂ nanocubes/nanosheet arrays for high-efficiency overall water splitting[J]. ChemSusChem, 2021, 14(2):  699-708. doi: 10.1002/cssc.202002338

  27. [27]

      XUAN C J, WANG J, XIA W W, ZHU J, PENG Z K, XIA K D, XIAO W P, XIN H L L, WANG D L. Heteroatoms (P, B, S) incorporated NiFe-based nanocubes as efficient electrocatalysts for oxygen evolution reaction[J]. J. Mater. Chem. A, 2018, 6(16):  7062-7069. doi: 10.1039/C8TA00410B

  28. [28]

      SONG X Z, ZHAO Y H, ZHANG F, NI J C, ZHANG Z, TAN Z Q, WANG X F, LI Y Q. Coupling plant polyphenol coordination assembly with Co(OH)₂ to enhance electrocatalytic performance towards oxygen evolution reaction[J]. Nanomaterials, 2022, 12(22):  3972. doi: 10.3390/nano12223972

  29. [29]

      ZHANG Z Y, SUN Y, ZHENG Y D, HE W, YANG Y Y, XIE Y J, FENG Z X, QIAO K. A biocompatible bacterial cellulose/tannic acid composite with antibacterial and anti-biofilm activities for biomedical applications[J]. Mater. Sci. Eng. C, 2020, 106:  110249. doi: 10.1016/j.msec.2019.110249

  30. [30]

      GE W J, CAO S, SHEN F, WANG Y Y, REN J L, WANG X H. Rapid self-healing, stretchable, moldable, antioxidant and antibacterial tannic acid-cellulose nanofibril composite hydrogels[J]. Carbohyd. Polym., 2019, 224:  115147. doi: 10.1016/j.carbpol.2019.115147

  31. [31]

      JIANG L, LIU Z M, YUAN Y, WANG Y J, LEI J X, ZHOU C L. Fabrication and characterization of fatty acid/wood-flour composites as novel form-stable phase change materials for thermal energy storage[J]. Energy Build., 2018, 171:  88-99. doi: 10.1016/j.enbuild.2018.04.044

  32. [32]

      SONG P, JIANG S C, REN Y J, ZHANG X, QIAO T K, SONG X F, LIU Q M, CHEN X S. Synthesis and characterization of tannin grafted polycaprolactone[J]. J. Colloid Interface Sci., 2016, 479:  160-164. doi: 10.1016/j.jcis.2016.06.056

  33. [33]

      MUNJAL S, KHARE N, NEHATE C, KOUL V. Water dispersible CoFe₂O₄ nanoparticles with improved colloidal stability for biomedical applications[J]. J. Magn. Magn. Mater., 2016, 404:  166-169. doi: 10.1016/j.jmmm.2015.12.017

  34. [34]

      WU Q J, SI D H, SUN P P, DONG Y L, ZHENG S, CHEN Q, YE S H, SUN D, CAO R, HUANG Y B. Atomically precise copper nanoclusters for highly efficient electroreduction of CO₂ towards hydrocarbons via breaking the coordination symmetry of Cu site[J]. Angew. Chem.‒Int. Edit., 2023, 62(36):  e202306822. doi: 10.1002/anie.202306822

  35. [35]

      CRISTIANI F, DEVⅡLANOVA F A, VERANI G. Zinc(Ⅱ), cadmium(Ⅱ) and mercury(Ⅱ) halide pyrrolidine-2-thione complexes[J]. Transit. Met. Chem., 1977, 2:  50-52. doi: 10.1007/BF01402680

  36. [36]

      HALLAM H E, JONES C M. Molecular configuration and interactions of the amide group-Ⅲ infrared spectra of thio- and seleno-lactams between 1 400 and 550 cm^-l and the nature of the νCS and νCSe vibrations[J]. Spectroc. Acta Pt. A‒Molec. Spectr., 1969, 25(11):  1791-1798. doi: 10.1016/0584-8539(69)80207-2

  37. [37]

      GUO D K, HAN S C, WANG J C, ZHU Y F. MIL-100-Fe derived N-doped Fe/Fe₃C@C electrocatalysts for efficient oxygen reduction reaction[J]. Appl. Surf. Sci., 2018, 434:  1266-1273. doi: 10.1016/j.apsusc.2017.11.230

  38. [38]

      HONG W, KITTA M, XU Q. Bimetallic MOF-derived FeCo-P/C nanocomposites as efficient catalysts for oxygen evolution reaction[J]. Small Methods, 2018, 2(12):  1800214. doi: 10.1002/smtd.201800214

  39. [39]

      翟好英, 邹自力, 李明宇, 张理元, 周文俊. 硼、磷共掺杂铁钴双金属材料的制备及其电催化析氧性能[J]. 无机化学学报, 2023,39,(4): 627-636. ZHAI H Y, ZOU Z L, LI M Y, ZHANG L Y, ZHOU W J. Synthesis of boron and phosphorus co-doped Fe-Co bimetallic materials for electrocatalytic oxygen evolution[J]. Chinese J. Inorg. Chem., 2023, 39(4):  627-636.

  40. [40]

      LI D R, PAN Z Q, TAO H, LI J C, GU W S, LI B T, ZHONG C L, JIANG Q Y, YE C Q, ZHOU Q W. Self-derivation-behaviour of substrate realizing enhanced oxygen evolution reaction[J]. Chem. Commun., 2020, 56(82):  12399-12402. doi: 10.1039/D0CC05253A

  41. [41]

      ZHANG Z Y, XU Y Q, WU M, LUO B F, HAO J H, SHI W D. Homogeneously dispersed cobalt/iron electrocatalysts with oxygen vacancies and favorable hydrophilicity for efficient oxygen evolution reaction[J]. Int. J. Hydrog. Energy, 2021, 46(21):  11652-11663. doi: 10.1016/j.ijhydene.2021.01.063

  42. [42]

      SAHOO S, SAHOO P K, MANNA S, SATPATI A K. A novel low cost nonenzymatic hydrogen peroxide sensor based on CoFe₂O₄/CNTs nanocomposite modified electrode[J]. J. Electroanal. Chem., 2020, 876:  114504. doi: 10.1016/j.jelechem.2020.114504

  43. [43]

      XIE M W, MA Y, LIN D M, XU C G, XIE F Y, ZENG W. Bimetal-organic framework MIL-53(Co-Fe): An efficient and robust electrocatalyst for the oxygen evolution reaction[J]. Nanoscale, 2020, 12(1):  67-71. doi: 10.1039/C9NR06883J

  44. [44]

      DONG S F, TAN Z K, CHEN Q S, HUANG G C, WU L, BI J H. Cobalt quantum dots as electron collectors in ultra-narrow bandgap dioxin linked covalent organic frameworks for boosting photocatalytic solar-to-fuel conversion[J]. J. Colloid Interface Sci., 2022, 628:  573-582. doi: 10.1016/j.jcis.2022.08.047

  45. [45]

      SU K Y, YU Z H, LI M M, YANG S, LIANG Y J, TANG Y W, QIU X Y. Three-dimensional nickel cobalt phosphide nanocrosses with well-defined axial arms for efficient oxygen evolution reaction[J]. Chem.‒Eur. J., 2023, 29(32):  e202300398. doi: 10.1002/chem.202300398

  46. [46]

      LI K K, CUI J R, YANG Q, WANG S L, LUO R M, RODAS-GONZALEZ A, WEI P Y, LIU L. A new sensor for the rapid electrochemical detection of ractopamine in meats with high sensitivity[J]. Food Chem., 2023, 405:  134791. doi: 10.1016/j.foodchem.2022.134791

  47. [47]

      KIM C, KIM S H, LEE S, KWON I, KIM S H, KIM S, SEOK C, PARK Y S, KIM Y. Boosting overall water splitting by incorporating sulfur into NiFe (oxy) hydroxide[J]. J. Energy Chem., 2022, 64:  364-371. doi: 10.1016/j.jechem.2021.04.067

  48. [48]

      BOPPELLA R, TAN J W, YUN J W, MANORAMA S V, MOON J. Anion-mediated transition metal electrocatalysts for efficient water electrolysis: Recent advances and future perspectives[J]. Coord. Chem. Rev., 2021, 427:  213552. doi: 10.1016/j.ccr.2020.213552

  49. [49]

      WANG T Y, NAM G, JIN Y, WANG X Y, REN P J, KIM M G, LIANG J S, WEN X D, JANG H, HAN J T, HUANG Y H, LI Q, CHO J. NiFe (oxy) hydroxides derived from NiFe disulfides as an efficient oxygen evolution catalyst for rechargeable Zn-air batteries: The effect of surface S residues[J]. Adv. Mater., 2018, 30(27):  1800757. doi: 10.1002/adma.201800757

  50. [50]

      CHEN Z Z, HOU J G, ZHOU J, HUANG P, WANG H Q, XU C X. Carbon shell coated hollow NiCoSe_x composite as high-performance anode for lithium storage[J]. Rare Metals, 2021, 40(11):  3185-3194. doi: 10.1007/s12598-021-01748-7

  51. [51]

      YANG Y L, MA G Z, HUANG J X, NAN J M, ZHEN S Y, WANG Y, LI A J. Hollow MnO₂ spheres/porous reduced graphene oxide as a cathode host for high-performance lithium-sulfur batteries[J]. J. Solid State Chem., 2020, 286:  121297. doi: 10.1016/j.jssc.2020.121297

  52. [52]

      ZHAO T, ZHONG D Z, HAO G Y, ZHAO Q. Vacancy engineering and hydrophilic construction of CoFe-MOF for boosting water splitting by in situ phytic acid treatment[J]. Appl. Surf. Sci., 2023, 607:  155079. doi: 10.1016/j.apsusc.2022.155079

  53. [53]

      ZHAO C X, LI B Q, ZHAO M, LIU J N, ZHAO L D, CHEN X, ZHANG Q. Precise anionic regulation of NiFe hydroxysulfide assisted by electrochemical reactions for efficient electrocatalysis[J]. Energ. Environ. Sci., 2020, 13(6):  1711-1716. doi: 10.1039/C9EE03573G

  54. [54]

      SAIKIA D, DEKA J R, LIN C W, LAI Y H, ZENG Y H, CHEN P H, KAO H M, YANG Y C. Insight into the superior lithium storage properties of ultrafine CoO nanoparticles confined in a 3 D bimodal ordered mesoporous carbon CMK-9 anode[J]. ChemSusChem, 2020, 13(11):  2952-2965. doi: 10.1002/cssc.202000009

  55. [55]

      YAO Y C, MA Z T, DOU Y B, LIM S Y, ZOU J Z, STAMATE E, JENSEN J O, ZHANG W J. Random occupation of multi-metal sites in transition metal-organic frameworks for boosting water oxidation[J]. Chem.‒Eur. J., 2022, 28(14):  e202104288. doi: 10.1002/chem.202104288

  56. [56]

      ZHANG J J, WANG H H, ZHAO T J, ZHANG K X, WEI X, JIANG Z D, HIRANO S I, LI X H, CHEN J S. Oxygen vacancy engineering of Co₃O₄ nanocrystals through coupling with metal support for water oxidation[J]. ChemSusChem, 2017, 10(14):  2875-2879. doi: 10.1002/cssc.201700779

  57. [57]

      LUO C, HU E Y, GASKELL K J, FAN X L, GAO T, CUI C Y, GHOSE S, YANG X Q, WANG C S. A chemically stabilized sulfur cathode for lean electrolyte lithium sulfur batteries[J]. Proc. Natl. Acad. Sci. U. S. A., 2020, 117(26):  14712-14720. doi: 10.1073/pnas.2006301117

  58. [58]

      WANG J S, LIU J, ZHANG B, CHENG F, RUAN Y J, JI X, XU K, CHEN C, MIAO L, JIANG J J. Stabilizing the oxygen vacancies and promoting water-oxidation kinetics in cobalt oxides by lower valence-state doping[J]. Nano Energy, 2018, 53:  144-151. doi: 10.1016/j.nanoen.2018.08.022

  59. [59]

      ZHU Z W, ZHANG Y D, WANG H, XIA B Y, YOU B. Improved corrosion-resistance and regulated electro-state of elastic polyaniline coated nickel phosphide for efficient water oxidation[J]. ChemCatChem, 2022, 14(23):  e202201100. doi: 10.1002/cctc.202201100

  60. [60]

      SUNADA S, MAJIMA K, AKASOFU Y, KANEKO Y. Corrosion assessment of Nd-Fe-B alloy with Co addition through impedance measurements[J]. J. Alloy. Compd., 2006, 408-412:  1373-1376. doi: 10.1016/j.jallcom.2005.04.039

  61. [61]

      LI C Q, WANG G, LI K, LIU Y W, YUAN B B, LIN Y Q. FeNi-based coordination crystal directly serving as efficient oxygen evolution reaction catalyst and its density functional theory insight on the active site change mechanism[J]. ACS Appl. Mater. Interfaces, 2019, 11(23):  20778-20787. doi: 10.1021/acsami.9b02994

  62. [62]

      RUAN H D, FROST R L, KLOPROGGE J T. The behavior of hydroxyl units of synthetic goethite and its dehydroxylated product hematite[J]. Spectroc. Acta Pt. A‒Molec. Biomolec. Spectr., 2001, 57(13):  2575-2586. doi: 10.1016/S1386-1425(01)00445-0

  63. [63]

      GUO C Y, LIU X J, GAO L F, KUANG X, REN X, MA X J, ZHAO M Z, YANG H, SUN X, WEI Q. Fe-doped Ni₂P nanosheets with porous structure for electroreduction of nitrogen to ammonia under ambient conditions[J]. Appl. Catal. B‒Environ., 2020, 263:  118296. doi: 10.1016/j.apcatb.2019.118296

  64. [64]

      ZOU X, FANG G J, QIN P L, WANG H J, SONG Z C, WANG H N, LONG H, WAN Q. Improved interface properties and reliability for Hf-In-Zn-O semiconductor capacitors with an electric-double-layer gate dielectric by inserting a HfO₂ interlayer[J]. Thin Solid Films, 2013, 540:  261-265. doi: 10.1016/j.tsf.2013.06.007

  65. [65]

      HOU J H, LEI Y G, WANG F, MA X H, MIN S X, JIN Z L, XU J. In-situ photochemical fabrication of transition metal-promoted amorphous molybdenum sulfide catalysts for enhanced photosensitized hydrogen evolution[J]. Int. J. Hydrog. Energy, 2017, 42(16):  11118-11129. doi: 10.1016/j.ijhydene.2017.01.235

  66. [66]

      LIU Y H, JIN Z Y, LI P P, TIAN X Q, CHEN X J, XIAO D. Boron- and iron-incorporated α-Co(OH)₂ ultrathin nanosheets as an efficient oxygen evolution catalyst[J]. Electrochem. Commun., 2018, 5(4):  593-597.

• 

  Figure 1  SEM images of FeCoTA (a) and S-FeCoTA before (b) and after (c) 1 000 cycles; TEM image (d) and EDS mapping images (e) of S-FeCoTA

  下载: 全尺寸图片 幻灯片
  

  Figure 2  XRD patterns (a) and FTIR spectra (b) of the samples

  下载: 全尺寸图片 幻灯片
  

  Figure 3  XPS spectra of Fe2p (a), Co2p (b), C1s (c), O1s (d), and S2p (e)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  LSV curves (a), overpotentials (b), and Tafel plots (c) of the samples

  Inset: Tafel plot of S-FeCoTA₀.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) EIS and (b) chronopotentiometry curve of S-FeCoTA at a current density of 10 mA·cm^-2

  Inset in a: equivalent circuit, where CPE is the constant phase angle element and R_s is the solution resistance;
  Inset in b: LSV curves before and after 1 000 cycles

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Energy consumption test curves (a) and Faraday efficiencies (b) of S-FeCoTA

  下载: 全尺寸图片 幻灯片
•