Tungsten-doped cobalt sulfide/selendie as high-efficient electrocatalyst for outstanding overall water splitting

The development of earth-abundant and highly efficient bifunctional electrocatalysts is a pressing requirement for electrochemical water splitting. However, several critical challenges still need to be addressed. Element doping can effectively enhance the electrocatalytic activity by tuning the microstructure, morphology, and electronic structure. Therefore, this work rationally designs and prepares three-dimensional nanosphere-like structured W-doped CoS1.097/CoSe2 (W-CoS1.097/CoSe2) as efficient bifunctional electrocatalysts for overall water splitting. W-CoS1.097/CoSe2 exhibits super activities with an overpotential of 69.8 mV at −10 mA cm−2 for HER and 400.0 mV at 10 mA cm−2 for OER, respectively. This study provides a new approach for the design of dual-functional catalysts for alkaline water electrolysis of transition metals.


Introduction
With the excessive consumption of fossil fuels and their negative impact on environment, developing clean renewable energy as alternative has become an urgent task.Under this trend, hydrogen (H 2 ), an ideal renewable energy carrier, can be produced by electrochemical water splitting [1].In addition, electrocatalytic water splitting is regarded as one of the most promising and efficient methods for obtaining highpurity hydrogen [2].Nevertheless, the sluggish kinetics of water splitting limits the large-scale application of hydrogen production from water electrolysis.Thus, exploring highly active electrocatalysts for the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) is of great significance.So far, the noble metal-based catalysts such as Pt-based materials (for HER) and Ru/Irbased materials (for OER) have attracted significant attention due to their excellent electrocatalytic performance in improving reaction rates and reducing overpotential [3,4].However, the high cost and limited earth reserves limit their large-scale applications.Therefore, it is highly desirable to search for a facile method to develop high-performance transition metal-based materials for HER and OER to reduce the cost of hydrogen production from water electrolysis.
Cobalt-based catalysts have received widespread attention as homogeneous molecular catalysts that fulfill the low-overpotential requirements for both HER and OER [5][6][7][8].Moreover, cobalt-based compounds are deemed as the promising non-noble metal-based alternatives ascribed to the remarkable electrocatalytic activity in water splitting [9][10][11].Nowadays, an increasing number of efforts have been conducted toward the development of cobalt-based dichalcogenides with enhanced electrocatalytic activity.For example, Li et al. developed a cobalt-base phosphides within carbon nanofibers as bifunctional electrocatalysts to show ultrahigh activity for water splitting [12].Huang et al. designed F dopants triggered active sites in bifunctional cobalt sulfide@nickel foam, which possessed outstanding overall water splitting [13].Li et al. designed Ni/Co-based nanosheet arrays with a slight Tafel slope, which acquired favorable OER performance [14].Hu et al. synthesized a sort of Co-based interstitial compounds to boost alkaline and acidic hydrogen electrocatalysis [15].Hu et al. prepared situ cobalt-based heterojunctions to improve hydrogen evolution activity [16].However, the low conductivity-induced unsatisfactory activity of cobalt-based dichalcogenides has hindered the widespread application of these catalysts [17,18].Therefore, an accessibly manipulated and efficient strategy is urgently desired to enhance electron transfer [19][20][21].
The existing effective methods for improving catalytic performance can be broadly categorized into two routes.The first strategy is to design and synthesize catalysts with distinct morphologies and nanostructures such as nanoparticles [22,23], nanosheets [24,25], and mesopores [26,27].These novel designs serve to expand the surface area, increase the exposure of reactive surface sites, promote the diffusion of electrolytes, and accelerate the desorption of gas bubbles from the catalyst surface.Another approach entails tailoring the electronic configuration of the cobalt-based catalysts through the integration of vacancies, introduction of lattice strain, and doping with other elements, with the aim of augmenting the intrinsic catalytic activity of the system [28][29][30][31][32][33][34][35].
According to previous research, most of these manipulations could modify electronic structures and optimize the electron band gaps and surface energy to boost their electrocatalytic performance.The tactic of introducing metal elements, possessing varying electronegativities, into materials has been validated as a successful approach to modulate the electronic structure at the atomic level, which can not only promote water dissociation but also optimize the adsorption capacity for diverse reaction intermediates, thus leading to improve electrocatalytic performance [36][37][38].For example, in our previous woks, nickel-doped Co 4 S 3 hollow nanocube/ nitrogen-doped V 2 CT x MXene nanosheet was presented for finishing highly efficiency overall water splitting at high current density [39].Cu-doped FeOOH nanoclusters to modify double-shelled porous g-C 3 N 4 was proposed, which showed effective removal of organic dyes [40].Liquid nitrogen quenching combined with doping engineering was proposed to achieve optimizing hydrogen evolution reaction [41].
To further boost overall water splitting performance of cobalt-based dichalcogenide catalysts, the efficient interface engineering through integration with other metal catalysts is deemed as a meaningful strategy.The focal point of interface engineering is to manipulate the surface characteristics of materials in such a way that they possess the optimal adsorption energy for reaction intermediates, which strikes a harmonious balance between excessive and minimal adsorption interactions [42].Meanwhile, interface engineering could effectively regulate the electronic structure through interfacial bonding and lattice strain, which would appropriately optimize the binding energy of targeted intermediates like hydrogen [43].For example, in our previous reports, the heterostructure between CoSe 2 and CoO was synthesized, which exhibited promising electrocatalyst performance with an ultra-low overpotential of 71.50 mV at current density of −10 mA cm −2 for HER and 257.60 mV 10 mA cm −2 for OER [44].Yu et al. successfully prepared NiO/RuO 2 heterojunction nano-sheets in situ grown on nickel foam [45].Wu et al. created heterostructure between MoS 2 and Co 9 S 8 , which exhibited remarkable catalytic efficacy in the context of hydrogen evolution reactions.Specifically, the system demonstrated an ultra-low overpotential of 73 mV at a current density of −10 mA cm −2 along with a Tafel slope of 78 mV dec −1 [46].
In light of the aforementioned considerations, a controllable and reliable method has been presented, which combines doping strategy, unique morphology, and interface engineering.The prepared W-CoS 1.097 /CoSe 2 is a promising electrocatalyst to resolve limited active sites and low conductivity.Metal atom doping entails the enhancement of the hydrogen evolution reaction (HER) activity of transition metal sulfides, along with the impact of the neighboring S atoms' local bond order on the hydrogen binding energy to the active sites.Meanwhile, interface engineering can effectively regulate the electronic structure of the active sites, thereby leading to favorable adsorption-free energies of the reaction intermediates during the HER/OER processes.Surprisingly, the resulting W-CoS 1.097 /CoSe 2 showcases remarkable activity for HER and OER with low overpotential.In addition, the electrolytic tank utilizing the bifunctional catalyst exhibits outstanding overall performance.The study provides an effective way to rationally design and synthesize renewable catalysts.

Experimental section
Preparation of cobalt glycerolate solid microspheres A total of 1091 mg cobalt nitrate was added into 12 mL glycerol and 40 mL isopropanol mixture solution.Afterwards, the mixed solution was transferred into a Teflon autoclave and heated at a constant temperature of 130 °C for 6 h.After the reactor was cooled to room temperature naturally, the precipitated product was washed with anhydrous ethanol by centrifugation, then the precipitated product was dried at 60 °C for 12 h.The resulting precipitation was cobalt glycerolate solid microspheres (recorded as CoG).
Preparation of W-CoG microspheres Firstly, 150 mg cobalt glycerol was added into 80 mL deionized water and 20 absolute ethyl alcohol; whereafter, 150 mg Na 2 WO 4 •2H 2 O was dispersed into cobalt glycerol solution to obtain solution A. Then, solution A was maintained at room temperature for 60 min.Finally, the W-CoG is achieved.
Preparation of the W-CoS 1.097 /CoSe 2 Firstly, 100 mg W-CoG was placed in a porcelain boat, and 1000 mg mixed powder (Se:S=2:1) was placed in the upstream position.The furnace temperature was raised to 450 °C for 1 h under the protection of nitrogen at the rate of ~2 °C min −1 .After cooling to room temperature, the W-CoS 1.097 /CoSe 2 precursor was obtained.

Electrochemical measurements
The measurements on the electrochemical performances were performed using a CHI 660E electrochemical workstation in a standard threeelectrode mode.All electrochemical performance tests were performed in 1.0 M KOH (pH=14) solution.The Ni foam was firstly immersed in 1.0 M diluted acetone solution in an ultrasound bath for 50 min, then washed thoroughly with ethyl alcohol, water successively for twice, and finally dried.Then, the working electrode was prepared by passing the slurry containing 10 mg of W-CoS 1.097 /CoSe 2 , 1.8 mg of acetyleneblack, and a drop of PTFE in 1 mL absolute ethanol.After that, the electrode was made by heating at 60 °C for 40 min.The as-synthesized W-CoS 1.097 /CoSe 2 slurry was coated onto the preprocessed Ni foam by applying suitable compressive force.The working electrode, reference electrode, and counter electrode are self-supporting electrode carrying electrocatalysts (1 ×1 cm), saturated calomel electrode, and graphite rods, respectively.The HER potentials used in this work are corrected with a reversible hydrogen electrode according the following equation: The following well-known equation was used to calculate the Tafel slope of the catalysts.
where "b" is the Tafel slope, "j" is the current density, and "a" is a constant.Before the linear scanning voltammetry (LSV) test, the electrocatalyst needs to be activated.Cyclic voltammetry (CV) technology is used and the scan rate is 100 mV/s until a stable CV curve appears.

Results and discussion
The synthesis process of the W-CoS 1.097 /CoSe 2 microsphere is demonstrated in Scheme 1, and the specific manufacturing and preparation process is given in the Experimental section.Firstly, the well-defined spherical-like cobalt glycerolate with diameter of approximately 2 μm is prepared as sacrificial template.Subsequently, Na 2 WO 4 •2H 2 O is engaged as the chemical etching agent and tungsten source.The hydrolysis of the mixed solution provides both H + and OH − , and H + reacts with the cobalt glycerolate to produce Co 2+ , and the OH − promotes the precipitation of W and Co 2+ to form precipitate [47,48]; the sample is completely etched to form porous W-CoG microsphere after the reaction [49].Finally, the W-CoS 1.097 /CoSe 2 microsphere is obtained via sulfidation and selenization simultaneously.
The structure of the W-CoS 1.097 /CoSe 2 is analyzed by scanning electron microscopy Figure 1a and b clarifies the structure of cobalt glycerolate, which possesses a smooth microsphere structure with 2 μm.CoG microspheres with rough surface are obtained by Na 2 WO 4 •2H 2 O etching/ doping process after the W doping (Fig. 1c, d).Finally, the W-CoS 1.097 /CoS 2 microsphere is obtained via the technical route of sulfurating and selenizing W-cobalt glycerolate synchronously.Meanwhile, the corresponding morphology is similar to W-cobalt glycerolate in Fig. 1e and f, while the surface becomes much rougher, which could increase surface area and thereby offer rich active sites for electrochemical overall water splitting [50,51].Furthermore, the EDS mapping was performed to reveal distribution area from Co, W, S, and Se (Fig. 1g-k).The EDS shows that these elements are distributed on the entire area without apparent element aggregations or separations.
To gain an in-depth insight into the chemical constituent and electronic configuration, X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS) tests of the W-CoS 1.097 /CoSe 2 catalyst are carried out.Figure 3a presents the XRD patterns of the obtained samples.Significantly, the diffraction peaks of W-CoS 1.097 /CoSe 2 emerge at 30.9, 34.6, and 57.1, which coincide well with the (110), (111), and (211) planes, respectively, signifying the successful preparation of CoSe 2 .Moreover, the peaks of CoS 1.097 are located at 52.5, 72.9, and 80.3, which coincide well with the (500), (613), and (623) planes, respectively.The diffraction peaks about W species in W-CoS, W-CoSe, and W-CoS 1.097 / CoSe 2 could not be found, suggesting the dopant of W and further confirming the previous results.In addition, The XPS further analyze the elemental states and chemical compositions.Figure 3b shows the existence of W, Co, S, and Se elements in W-CoS 1.097 /CoSe 2 .Intriguingly, the W element incorporated into W-CoS 1.097 /CoSe 2 mainly exists at 35.5 and 37.7 eV in Fig. 3c.The result proves that W is doped into W-CoS 1.097 /CoSe 2 microsphere successfully.The Co 2p pattern of W-CoS 1.097 /CoSe 2 is well confirmed by two doublets and two satellites; e.g., one doublet at 778.9 and 794.0 eV represents 2p 3/2 and 2p 1/2 of Co 3+ and the other doublet at 781.2 and 797.0 eV are associated with 2p 3/2 and 2p 1/2 of Co 2+ , indicating the coexistence of Co 3+ and Co 2+ as shown in Fig. 3d [55].The spectrum of S 2p (Fig. 3e) reveals three main peaks; the fitted peaks at 161.5 and 165.6 eV are assigned to S 2p 1/2 and S 2p 3/2 , while the peak at 174.0 eV is the typical characteristic of sulfur ions with metal ions [56].The peaks of Se 3d 5/2 and Se 2d 3/2 are located at 55.0 and 55.9 eV (Fig. 3f), respectively; the oxygen signal is owing to surface oxide contamination [57,58].
The surface area and porous structure of the resultant samples are further characterized by N 2 adsorption/desorption measurements.Figure 4a  The results indicate that the reasonable design and construction with abundant active electrons can significantly strengthen the OER activity of the catalysts.The faradic efficiency for the gases evolved in the OER is 86.68% (Fig. S3). Figure 5g presents that the current density reveals no significant degradation after 24 h.The remarkable electrocatalytic activity and durability indicate that W-CoS 1.097 /CoSe 2 is at the top of nonprecious metal-based electrocatalysts.To further analyze the practical applicability of W-CoS 1.097 /CoSe 2 , the electrocatalyst is used as two electrodes for a full-cell water-splitting process in 1.0 M KOH.The LSV curve at 5 mV/s reveals that the W-CoS 1.097 /CoSe 2 ||W-CoS 1.097 / CoSe 2 catalyst exhibits excellent catalytic activity and requires a low voltage of 1.58 V at 10 mA cm −2 (Fig. 5h).Furthermore, the long cycling stability of the catalyst is shown in Fig. 5i.The catalyst maintains satisfactory stability after 24 h of the durability test.Meanwhile, the SEM of W-CoS 1.097 /CoSe 2 after HER demonstrates the excellent stability (Fig. S4).These results indicate that W-CoS 1.097 /CoSe 2 ||W-CoS 1.097 /CoSe 2 can be used as an advanced catalyst for high-efficiency bifunctional overall water splitting.The potential limitations or drawbacks associated with using CoS 1.097 /CoSe 2 electrocatalyst in practical applications are the relatively unsatisfactory OER performance.However, CoS 1.097 /CoSe 2 acquires desirable HER and overall water splitting behavior.

Conclusion
In summary, the 3D nanosphere W-CoS 1.097 /CoSe 2 with rough surface area is fabricated via a hydrothermal and calcine strategy.The conductivity has been markedly improved due to the doping, thereby leading to the enhanced ability of charge transfer compared to the bare sample.The W-CoS 1.097 /CoSe 2 with an optimal dopant is used as an electrochemistry catalyst in 1.0 M KOH, and ultra-low overpotential and Tafel slope are achieved.According to the experiment, the critical role of W doping in the electrocatalytic overall water-splitting behavior of CoS 1.097 /CoSe 2 is illuminated in this work.This work will pave a new way for developing high-performance and low-cost metal-based bifunctional electrocatalysts.

Fig. 1 Fig. 2 Fig. 3
Fig. 1 SEM images of cobalt glycerolate (a, b) and W-cobalt glycerolate microsphere (c, d).W-CoS 1.097 /CoS 2 microsphere (e, f).The EDS mapping of W-CoS 1.097 /CoSe 2 microsphere (g-k) confirms the existence of numerous mesopores in W-CoS 1.097 /CoSe 2[59,60].Particularly, W-CoS 1.097 /CoSe 2 acquires the high specific surface area of 27.0 m 2 /g.The corresponding pore size distribution curve indicates that the W-CoS 1.097 /CoSe 2 possesses large pore volume and is dominated by mesopores.The existence of mesopores greatly enhance the specific surface area of W-CoS 1.097 /CoSe 2 and provide plenty of active sites, while the mesopores are conducive to create more structural defects and transport the reaction intermediates.The high specific surface area and large pore volume of W-CoS 1.097 /CoSe 2 are expected to facilitate the mass transport and expose more active surface sites for electrochemical reactions.The HER performance of W-CoS 1.097 /CoSe 2 is studied in 1.0 M KOH solution.The LSV curves of W-CoS 1.097 / CoSe 2 at 5 mV/s clearly exhibit an electrocatalytic activity superior compared to other samples.In particular, the W-CoS 1.097 /CoSe 2 delivers low overpotentials of 69.8 mV at −10 mA cm −2 (Fig.5a), which outperforms other samples.From the Tafel slopes shown in Fig.5b, W-CoS 1.097 /CoSe 2 exhibits the Tafel slope of 135.9 mV dec −1 , which is lower than those of CoS 1.097 /CoSe 2 (176.7 mV dec −1 ), CoS 1.097 (220.6 mV dec −1 ), CoSe 2 (240.0 mV dec −1 ), and Ni-foam (251.5 mV dec −1 ), indicating the fast HER kinetics of the heterostructure W-CoS 1.097 /CoSe 2 .Additionally, W-CoS 1.097 /CoSe 2 shows higher catalytic performance than un-doped CoS 1.097 /CoSe 2 .This result shows that introducing W atoms into CoS 1.097 /CoSe 2 could enhance the catalytic activity of CoS 1.097 /CoSe 2 [59].The electrochemical impedance spectroscopic (EIS) measurements are performed in a frequency range from 0.05 to 10 5 Hz with an AC voltage amplitude of 5.0 mV.The Nyquist plots are shown in Fig. 5c; the charge transfer resistance of W-CoS 1.097 /CoSe 2 is the smallest among the examined catalysts, suggesting the fastest electron transfer during HER reaction.More importantly, W-CoS 1.097 /CoSe 2 owns excellent durability for HER reaction in the 1.0 M KOH condition, as indicated by the i-t test in Fig. 5d and the current density reveals no significant degradation after 24 h.The remarkable electrocatalytic activity and durability indicate that W-CoS 1.097 / CoSe 2 is in the top tier of other nonprecious metal-based electrocatalysts.For OER performance, the LSV curves at 5 mV/s indicate the significant enhancement in the OER activity of W-CoS 1.097 /CoSe 2 after W-modification (Fig. 5e).Among the prepared CoS 1.097 /CoSe 2 , W-CoS 1.097 /CoSe 2 , CoS 1.097 , CoSe 2 , and Ni-foam samples, W-CoS 1.097 /CoSe 2 exhibits the best OER electrocatalytic performance of 400.0 mV at 10 mA cm −2 .From the Tafel slopes shown in Fig. 5f, W-CoS 1.097 /CoSe 2 exhibits the lowest Tafel slope of 21.3 mV dec −1 , lower than those of CoS 1.097 /CoSe 2 (27.4 mV dec −1 ), CoS 1.097 (89.8 mV dec −1 ), CoSe 2 (92.7 mV dec −1 ), and Ni-foam (114.1 mV dec −1 ), indicating the fastest OER kinetics within W-CoS 1.097 /CoSe 2 .