English

• Recently, worsening global environment and exhausted non-renewable energy have forced human to seek clean and green renewable source as substitute for traditional fossil fuels [1]. As one of the most perspective members in new breed of energy carrier, H₂ can offer human being with adequate, sustainable and pollution-free energy supply, which has been considered as an important participant for the construction of energy infrastructure in the future [2, 3]. For H₂, to realize large scale application, production method is the most important segment [4]. Electrocatalytic water splitting is a clean and perspective way for H₂ production [5-7]. Compared with other H₂ production technology, it is a fascinating and promising method, which exhibits low cost, simple operation and zero discharge [8-10]. With promotion of electrocatalyst, HER occurs and generates H₂ efficiently [11-13]. Pt based materials are well-known HER electrocatalysts [14]. But their massive popularization in H₂ production is severely limited by expensive price, low reserve and poor stability [15-18]. Currently, design and exploration of stable, cut-price HER electrocatalyst with high efficiency becomes the first issue to achieve scalable H₂ production [19-21].

  As a prolific and high performance electrocatalyst, MoS₂ exhibits graphene shaped two-dimensional layer structure, which is regarded as a prospective choice for H₂ production [22-26]. Hexagonal symmetry MoS₂ (2H-MoS₂) and tetragonal phase MoS₂ (1T-MoS₂) are the most common phase in MoS₂ [27]. By and large, 2H-MoS₂ possesses more superior HER activity over 1T-MoS₂ [28]. But 2H-MoS₂ also has a fatal weakness in HER, improper H^* adsorption free energy (ΔG_H^*) [29]. For an electrocatalyst, HER activity is largely determined by its interaction with adsorbed H (H^*) [30]. In 2H-MoS₂, d orbital shows intensive adsorption towards H^*, which hinders it is desorption and postpones H₂ generation [31-35]. For 2H-MoS₂, an n-type semiconductor, to turn the situation around, building a p-n heterojunction with another p-type semiconductor, such as MoO₃, is a feasible approach [36]. As photocatalysts, after the construction of p-n heterojunction, an inner electric field forms with the direction from n-type MoS₂ to p-type MoO₃ [37]. This produces positive charged MoS₂ and negative charged MoO₃ in p-n heterojunction [38]. The electrostatic repulsion between positive charged MoS₂ and H^* facilitates its desorption and favors H₂ production.

  To synthesis MoS₂ electrocatalyst, polyoxometalates (POMs) are ideal precursors, which have achieved great successes due to unique structural features. Here, MoO₃/MoS₂, was synthesized through partial vulcanization of [(H₂DPA)(HDPA)(PMo₁₂O₄₀)]_n (DPA·PMo₁₂, DPA = dipyridylamine), a POM compound. Because of the existence of p-n heterojunction, MoO₃/MoS₂ exhibits superior HER activity over pure MoS₂ in both acidic and basic electrolyte. With MoO₃/MoS₂ as cathode material, an asymmetry Zn-H⁺ battery was built up, which achieves electric energy generation and H₂ production simultaneously. Its open circuit voltage achieves 1.11 V with short circuit current 151.4 mA/cm². The specific capacity and energy density of this battery reach 728 mAh/g and 759 Wh/kg at 10 mA/cm². Under this condition, its H₂ production rate achieves 364 µmol/h with Faradic efficiency 97.8%.

  The structure of DPA·PMo₁₂ was characterized by single X-ray analysis. In the fundamental unit, there exists one [PMo₁₂O₄₀]³⁻ cluster and two protonated DPA molecules (Fig. 1a). In [PMo₁₂O₄₀]³⁻ cluster, Mo(1) adopts distorted octahedron coordination mode with Mo(1)-O(1) = 2.303, Mo(1)-O(8) = 1.939, Mo(1)-O(13) = 1.913, Mo(1)-O(24) = 1.867, Mo(1)-O(25) = 2.048 and Mo(1)-O(31) = 1.996 Å. Mo(2) and Mo(8) exist near Mo(1), but with different bond lengths and angles. Such three MoO₆ octahedron links together with edge sharing mode and generates a Mo₃ unit (Fig. 1b). [PMo₁₂O₄₀]³⁻ cluster is composed by four Mo₃ units with vertex sharing mode. Adjacent [PMo₁₂O₄₀]³⁻ clusters are further connected by O(24) and generates one-dimensional chain-like structure with Mo(1)-O(24) = 1.867 Å and Mo(2)-O(24) = 1.863 Å (Fig. 1c). Protonated DPA molecules locate near this [PMo₁₂O₄₀]³⁻ chain. Intensive electrostatic attraction exists between DPA and [PMo₁₂O₄₀]³⁻. The structure of DPA·PMo₁₂ was studied with FTIR (Fig. S1 in Supporting information). The stretching at 804 and 864 cm⁻¹ originates from Mo-O-Mo. Mo=O appears at 945 cm⁻¹. The stretching located at 1051 cm⁻¹ is ascribed to P-O.

  Figure 1

  

  Figure 1.  (a) Fundamental unit of DPA·PMo₁₂. (b) Structure of [PMo₁₂O₄₀]³⁻ cluster. (c) Structure of one-dimensional chain-like structure of DPA·PMo₁₂.

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  The structure of vulcanized product was studied with powder X-ray diffraction (PXRD) at first. Three typical diffraction peaks belonging to 2H MoS₂ appear at 12.7°, 33.4° and 39.8°, which match with its (002), (100) and (103) planes (Fig. S2a in Supporting information) [39]. Compared with pure MoS₂, the (002) diffraction peak shifts towards low angle direction slightly and this implies the spacing between MoS₂ layers broadens, which can be ascribed to DPA insertion. Another peak located at 26.6° agrees with (040) plane of MoO₃ (35–0609). Further structural information is revealed by Raman spectrum. Two distinctive Raman peaks appear at 379.6 and 400.2 cm⁻¹ originates from E_2g¹ and A_g¹ of 2H MoS₂, which correspond with in-plane and out-of-plane vibration modes (Fig. S2b in Supporting information). Compared with MoS₂, E_2g¹ and A_g¹ both move towards low wave-number direction. The interval between E_2g¹ and A_g¹ peaks is 20.6 cm⁻¹, which is lower than pure MoS₂. This suggests MoS₂ exists with few layer and low dimension character [40]. Some additional peaks were also detected in Raman spectrum, which located at 658.3, 812.8 and 1002.2 cm⁻¹. Their appearance confirms the existence of MoO₃ in vulcanized products [41, 42].

  More structural information of MoO₃/MoS₂ was revealed by X-ray photoelectron spectroscopy (XPS). In survey spectrum, the distinctive peaks can be attributed to S 2p, Mo 3d, Mo 3p and O 1s (Fig. S3 in Supporting information). Based on the content of O and S, the ratio between MoO₃ and MoS₂ is 4.2:1. In high resolution Mo 3d spectrum, three kinds of peaks appear after simulation (Fig. S4a in Supporting information). A pair of intensive doublet peak located at 230.1 and 233.2 eV matches with Mo 3d_5/2 and Mo 3d_3/2 of 2H-MoS₂. The remained twin peaks emerged at 233.1 and 236.7 eV can be attributed to Mo 3d_5/2 and Mo 3d_3/2 of Mo⁶⁺ in MoO₃. This further confirms co-existence of MoO₃ and MoS₂. Besides above peaks, a low single peak emerged at 226.2 eV originates from S 1s in MoS₂. In high resolution S 2p spectrum, two peaks appeared at 162.8 and 164.4 eV correspond to S 2p_3/2 and S 2p_1/2 of MoS₂ in 2H phase (Fig. S4b in Supporting information) [43]. In MoO₃/MoS₂, Mo 3d_3/2 and Mo 3d_5/2 of MoS₂ move to negative side compared with pure MoS₂ (Fig. S5). This affirms intensive interaction exists between MoO₃ and MoS₂.

  The morphology of MoO₃/MoS₂ was surveyed with scanning electron microscopy (SEM), which shows flower like appearance (Fig. 2a). These flowers are constructed by two dimensional nanosheet as leaf. The size of flower leaf ranges from 200 nm to 300 nm with thickness 20–30 nm (Fig. 2b). Transmission electron microscopy (TEM) reveals more morphology information of MoO₃/MoS₂. TEM image also confirms the flower appearance of MoO₃/MoS₂, which is in accordance with SEM result (Fig. 2c). High resolution TEM (HRTEM) reveals few layer character of MoS₂ with thickness less than 10 nm. The lattice spacing about 0.62 nm is found in MoS₂, which matches with its (002) plane perfectly (Fig. 2d) [44]. In addition, new lattice spacing about 0.35 nm is detected in HRTEM image, which can be ascribed to (040) plane of MoO₃ [45]. High angle annular dark field (HAADF) image and elemental mapping suggest S and O distribute uniformly (Fig. 2e). This further confirms co-existence of MoO₃ and MoS₂ on the sample.

  Figure 2

  

  Figure 2.  (a, b) SEM images of MoO₃/MoS₂. (c) TEM image of MoO₃/MoS₂. (d) High resolution TEM image of MoO₃/MoS₂. (e) Annular dark-field TEM image and S, O elemental mapping of MoO₃/MoS₂.

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  The band structure of MoO₃/MoS₂ was studied with UPS and UV–vis spectra. At first, their valence band energies (E_VB) were calculated based on UPS spectra (Figs. 3a and b). For MoS₂ and MoO₃, their secondary electron cutoff energies (abbreviated as E_cutoff) are 17.01 and 16.14 eV. Based on the equation Φ = hν - (E_cutoff - E°_F), corresponding work functions (Φ) are 4.21 and 5.08 eV, respectively. In this equation, hν represents the energy of excited photo (21.22 eV) and E°_F means Fermi level of the spectrometer after calibration [46]. UPS spectra can also reflect the difference between E_VB and Fermi level (E_F). For MoS₂ and MoO₃, the difference values are 1.05 and 2.74 eV. So, E_VB values are −5.26 and −7.82 eV for MoS₂ and MoO₃, respectively. Their band gaps (E_g) can be calculated based on Tauc plots. For MoS₂ and MoO₃, E_g values are 2.2 and 3.6 eV (Figs. 3c and d). So, the conduction bond energy (C_B) was calculated, which are −3.06 and −4.22 eV for MoS₂ and MoO₃ [47]. The position of valence and conduction bands confirms p-n heterojunction has generated. In p-n heterojunction, an inner electric field forms with the direction from n-type MoS₂ to p-type MoO₃. This produces positive charged MoS₂ and negative charged MoO₃ in p-n heterojunction (Fig. 3e). The electrostatic repulsion between positive charged MoS₂ and H^* facilitates its desorption and favors H₂ production.

  Figure 3

  

  Figure 3.  Binding energy cutoff region of UPS spectra for (a) MoS₂, (b) MoO₃. Tauc plots of (c) MoS₂, (d) MoO₃. (e) Schematic diagram of p-n heterojunction.

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  HER activity of MoO₃/MoS₂ was studied by three-electrode equipment in 1 mol/L KOH. During experiment, linear sweep voltammetry (LSV) curve was measured at 5 mV/s. For MoO₃/MoS₂, the η₁₀, η₁₀₀ and η₂₀₀ (overpotentials to achieve 10, 100 and 200 mA/cm²) values are 68, 173 and 198 mV (Fig. 4a). As for pure MoS₂, η₁₀ is 151 mV. Moreover, it demands 325 and 393 mV to obtain 100 and 200 mA/cm², which are higher obviously than MoO₃/MoS₂. Tafel plot can also reveal the HER activity. For MoO₃/MoS₂, Tafel slope is 66 mV/dec (Fig. 4b). The value is lower than pure MoS₂ (94 mV/dec). This implies to drive similar current MoO₃/MoS₂ needs much lower voltage than pure MoS₂. Under acidic condition, the excellent HER activity of MoO₃/MoS₂ is still retained. In 0.5 mol/L H₂SO₄, η₁₀ is 193 mV (Fig. S6 in Supporting information). This is lower than pure MoS₂ (244 mV). For MoO₃/MoS₂, η₁₀₀ and η₂₀₀ are 287 and 334 mV, which are also lower pure MoS₂. The merit of MoO₃/MoS₂ is also indicated by low Tafel slope of 89 mV/dec (Fig. S7 in Supporting information). In contrast, Tafel slope of MoS₂ is 105 mV/dec. The performance of MoO₃/MoS₂ is comparable with Pt/C and other HER electrocatalysts (Fig. S8 and Table S1 in Supporting information). In neutral electrolyte (1 mol/L Na₂SO₄), HER activity of MoO₃/MoS₂ becomes poor.

  Figure 4

  

  Figure 4.  (a) LSV curves; (b) Tafel slopes of MoO₃/MoS₂ and MoS₂ in 1 mol/L KOH. (c) Capacitive current as function of scan rates for MoO₃/MoS₂ and MoS₂ in 1 mol/L KOH. (d) EIS of MoO₃/MoS₂ and MoS₂. (e) Chronoampero-metric test of MoO₃/MoS₂ in 1 mol/L KOH. (f) Calculated ΔG_H^* for MoO₃/MoS₂, MoS₂ and Pt.

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  For MoO₃/MoS₂, to explore the root of outstanding HER activity over pure MoS₂, electrochemical active surface area (ECSA) is a landmark parameter. ECSA value is in directly proportional to electrochemical double-layer capacitance (C_dl) and can be simulated based on cyclic voltammetry (CV) curves under different scanning rates. CV tests of MoO₃/MoS₂ and pure MoS₂ were measured from 5 mV/s to 40 mV/s (Fig. S9 in Supporting information). The C_dl value of MoO₃/MoS₂ receives up to 71.4 mF/cm² (Fig. 4c). As for pure MoS₂, C_dl is only 44.9 mF/cm². This reveals MoO₃/MoS₂ can provide more active sites in HER. Electrochemical impedance spectroscopy (EIS) illustrates charge transfer resistance of MoO₃/MoS₂ is lower than pure MoS₂ in HER process (Fig. 4d). This ensures timely electron transportation property. Stability was investigated by chronoamperometry test at 120 mV in 1.0 mol/L KOH. After 96 h experiment, barely current decay is detected in MoO₃/MoS₂ (Fig. 4e). Stability of MoO₃/MoS₂ is revealed by successive HER test. After 8000 cycles scanning from 0 to −0.2 V, η₁₀₀ only shifts from 173 mV to 178 mV (Fig. S10 in Supporting information). The unique HER stability of MoO₃/MoS₂ is also kept under acidic condition. LSV curves almost coincide before and after 8000 cycles HER experiment (Fig. S11 in Supporting information). Outstanding HER stability is benefit for its real application in H₂ production.

  In MoO₃/MoS₂, to discover the internal relationship between MoS₂ and DPA as well as get to the deep insight into structure and HER activity, ΔG_H^* was calculated. It is well known that ΔG_H^* has been considered as a desired parameter to descript HER activity of electrocatalysts. In general, an ideal HER electrocatalyst has a ΔG_H^* value near 0 eV [48]. Too negative ΔG_H^* means excessive adsorption between electrocatalyst and H^*. On the contrary, if ΔG_H^* is higher than 0 eV, the interaction is too weak for HER. The ΔG_H^* value of pure MoS₂ is −0.32 eV (Fig. 4f). This means the adsorption of MoS₂ towards H^* is too intensive, which impedes its leaving and abates HER activity [49]. So, it is significant to weaken the adsorption of MoS₂ towards H^*. In MoO₃/MoS₂, the ΔG_H^* value becomes 0.12 eV and can almost be comparable with Pt (−0.09 eV). This implies the generation of p-n heterojunction with MoO₃ can relax the adsorption towards H^* and enhances HER activity.

  The excellent HER activity and stability of MoO₃/MoS₂ makes it become an ideal cathode material for asymmetry Zn-H⁺ battery. This asymmetry Zn-H⁺ battery is built by anode chamber and cathode chamber. In anode chamber, 4.0 mol/L KOH serves as electrolyte and Zn slice is used as anode. In cathode chamber, 1.0 mol/L H₂SO₄ acts as electrolyte and MoO₃/MoS₂ is employed as cathode. Two chambers are separated by bipolar membrane, which supplies anion/cation and keep persistent current during discharge process (Fig. S12 in Supporting information). I-V curve of Zn-H⁺ battery illustrates with the increasing of current, voltage decays gradually (Fig. S13a in Supporting information). Open circuit voltage and short circuit current of Zn-H⁺ battery reach 1.11 V and 151.4 mA/cm². When discharge at 20 mA/cm², the voltage of Zn-H⁺ battery achieves 0.91 V. If discharge current increases to 60 mA/cm², its voltage still retains at 0.69 V. Peak power density of this Zn-H⁺ battery reaches 47.6 mW/cm² (Fig. S13b in Supporting information). This is comparable with Zn-air batteries (Table S2 in Supporting information). Discharge performance of Zn-H⁺ battery was studied at 10 mA/cm². Based on the mass of Zn consumed during discharge, specific capacity of Zn-H⁺ battery reaches 728 mAh/g (Fig. S13c in Supporting information). In this process, the energy density is 759 Wh/kg. H₂ generation ability was also assessed at 10 mA/cm² for 2 h (Fig. S13d in Supporting information). During this process, about 728 µmol H₂ is produced, which is close to the value in theory with Faradic efficiency 97.8%. More importantly, with this current density, Zn-H⁺ battery can also provide voltage of 0.97 V. This further confirms Zn-H⁺ battery can achieve electric energy generation and H₂ production simultaneously. But as a primary battery, Zn-H⁺ battery cannot be recharged.

  In summary, p-n heterojunction was constructed successfully in MoO₃/MoS₂ with POM compound as precursor. The inner electric field in p-n heterojunction facilitates H^* desorption and improves HER activity. MoO₃/MoS₂ exhibits outstanding HER activity in both acidic and basic electrolyte with low overpotential and Tafel slope. A Zn-H⁺ battery is assembled with MoO₃/MoS₂ as cathode material, which achieves electricity generation and H₂ production simultaneously. When discharge at 10 mA/cm², its specific capacity, energy density and H₂ production rate reach 728 mAh/g, 759 Wh/kg and 364 µmol/h Faradic efficiency achieves 97.8% in this process. We anticipate p-n heterojunction based material will play an important role in future H₂ production and electric energy generation.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by National Natural Science Foundation of China (No. 22171039); Fundamental Research Funds for the Central University (No. N2025035).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.071.

  
  
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• 

  Figure 1  (a) Fundamental unit of DPA·PMo₁₂. (b) Structure of [PMo₁₂O₄₀]³⁻ cluster. (c) Structure of one-dimensional chain-like structure of DPA·PMo₁₂.

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  Figure 2  (a, b) SEM images of MoO₃/MoS₂. (c) TEM image of MoO₃/MoS₂. (d) High resolution TEM image of MoO₃/MoS₂. (e) Annular dark-field TEM image and S, O elemental mapping of MoO₃/MoS₂.

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  Figure 3  Binding energy cutoff region of UPS spectra for (a) MoS₂, (b) MoO₃. Tauc plots of (c) MoS₂, (d) MoO₃. (e) Schematic diagram of p-n heterojunction.

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  Figure 4  (a) LSV curves; (b) Tafel slopes of MoO₃/MoS₂ and MoS₂ in 1 mol/L KOH. (c) Capacitive current as function of scan rates for MoO₃/MoS₂ and MoS₂ in 1 mol/L KOH. (d) EIS of MoO₃/MoS₂ and MoS₂. (e) Chronoampero-metric test of MoO₃/MoS₂ in 1 mol/L KOH. (f) Calculated ΔG_H^* for MoO₃/MoS₂, MoS₂ and Pt.

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