Introduction

While hydrogen is considered as a highly promising alternative fuel for energy production and consumption systems due to its clean-burning properties, its relatively low volumetric energy density has hindered its sorption abilities under ambient conditions. Thus, the consideration of hydrogen storage is crucial for the widespread utilization of hydrogen due to its lower costs, less environmental damages, and the availability of the primary sources1,2,3. The combustion of hydrogen results in the production of water as a byproduct, thereby bestowing numerous environmental benefits upon it4,5. To address the demand for the advancement of sustainable energy, the focal point is directed towards the storage systems of clean energy, specifically hydrogen6,7. A significant amount of effort has been dedicated to the exploration of new nanomaterials that possess the ability to enhance the capacity for hydrogen storage across various operational conditions. The storage of hydrogen is achieved through three distinct methods, which comprise the storage of compressed gas within containers, the storage of hydrogen in the form of liquid or solid8. Due to the necessity for high pressure and the construction of sizable containers, the viability of compressed hydrogen storage is limited from an economic standpoint. Factors such as the substantial expense associated with the construction of a large hydrogen storage container, the requirement for ample space to accommodate the hydrogen storage containers, the considerable weight of the containers, and the elevated pressure required for hydrogen liquefaction contribute to this limitation9,10. The storage of hydrogen in liquid form, also known as cryogenic storage, necessitates a reduction in temperature to 20 °C. To ensure temperatures remain below freezing point, a hydrogen storage container must be adequately insulated11. Despite recent advancements in liquefaction techniques, this approach is not widely employed today due to its exorbitant cost. Conversely, solid-state hydrogen storage presents numerous advantages, encompassing enhanced safety measures, a heightened capacity for voluminous storage, as well as lower operating temperatures and pressures. Among the different methods and limitations mentioned, electrochemical hydrogen storage emerges as the most effective method amidst various approaches, due to its ability to produce and retain hydrogen in-situ, while functioning under normal pressure and temperature conditions12. Polyoxometalates (POMs) have recently attracted considerable interest in the field of hydrogen storage due to their notable chemical and structural variability, redox potential, surface charge, thermal stability, and adjustable acidity13,14. POMs are a class of inorganic compounds that consist of metal-oxo clusters, typically containing a combination of transition metals and oxygen atoms. Some common structures of POMs, e.g., Keggin [{XM12O40}3/4−] (X = P, Si, B and M = and M = Mo, W)15, sandwich [{M4(H2O)2(XW9O34)2}n− (X = P, Si, Ga, etc.)16, Anderson [{AM6O24}9/10−] (A = Mn, Fe, Cr, Al, I, etc. and M = Mo, W)17, Wells–Dawson [{X2M18O62}6−], (X = P, Si and M = Mo, W)18, Evans–Showell [{Co2Mo10H4O38}6]19, etc. Among their various configurations, Keggin-type POMs have been designed for the purpose of catalyzing the hydrogen storage systems. This is made possible by their possession of multiple redox-active metal centers and bridging oxygen atoms, which enable them to accept and denote electrons during the reaction20,21. Nevertheless, there are certain drawbacks associated with utilization of POMs in industrial applications, including a limited surface area (5 m2 g−1) and poor reusability due to its high solubility22,23. Hence, the immobilization of Keggin-type POMs on the surfaces of metal oxides and polymers emerges as a promising approach to enhance their catalytic properties and mechanical durability. To design an innovative heterogeneous catalyst that possesses high stability and impressive reusability, a range of materials based on metallic oxides were employed as careers for the support of POMs, e.g., MgCo2O4,NiZn2O4,CuCo2O4, NiCo2O4, etc24,25. The NiZn2O4 nanoparticles have become a preferred material for use in the heterogenization of POMs due to their non-toxic nature, low cost of fabrication, eco-friendly properties, and the specific capacities that arise from the synergistic impact of Ni and Zn ions26. In our research, we endeavor to investigate novel characteristics of Keggin-type POMs by substituting different metals. The Keggin structure consists of a spherical conglomerate of polymeric units of MO6 (M = Mo, W), wherein the corners and edges are shared, along with a central tetrahedron (PO4). Furthermore, POMs typically exhibit a weakly basic and nucleophilic nature, unless the surface charge is enhanced through metal substitution or by reducing the POMs. This research has validated that the incorporation of Zn metal into the tetrahedral unit of Keggin-type polyoxometalate leads to a substantial enhancement of the hydrogen storage. Herein, a new and efficient nanocomposite was synthesized based on Keggin-type [ZnW12O40]6− and NiZn2O4 with hydrogen storage capability. The physicochemical characteristics of the as-prepared nanocomposite was characterized through several analyses. Subsequently, the hydrogen storage capacity was examined via CV and CHP techniques, exploring various factors. This study presents a potential method for hydrogen storage using the ZnW12O40/NiZn2O4 nanocomposite.

Experimental section

Materials

All chemicals and solvents used in this investigation were procured from commercial sources and utilized in accordance with the specified instructions. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, \(\ge 98\%)\), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, \(97\mathrm{\%})\), sodium tungstate dihydrate (Na2WO4·2H2O, \(99\%\)), phosphate Buffer Saline (PBS) were purchased from Sigma-Aldrich. Moreover, potassium chloride (KCl, ≥ 99%), citric acid monohydrate (HOC(COOH)(CH2COOH)2·H2O), urea (CH4N2O), ethanol (EtOH, 96%), and potassium hydroxide (KOH, ≥ 98%) were provided from Merck chemical company.

Characterizations methods

The Fourier transform infrared (FT-IR) spectra were acquired using the Thermo-Nicolet-iS 10 spectrometer in the solid state from 400 to 4000 cm−1. The investigation of the optical characteristics was conducted through the utilization of a Shimadzu UV–Vis double beam spectrometer, specifically the UV-2450 model originating from Japan. This analysis was performed within the spectral range spanning from 200 to 700 nm. The measurement of X-ray diffraction (XRD) was executed by Bruker D8 Advance, which was outfitted with a crystal monochromator made of graphite. The XRD employed a voltage of 40 kV and a current of 30 mA, utilizing CuKα radiation with a wavelength of 0.15406 nm. The field-emission scanning electron microscope (FE-SEM) by LEO 1455 VP (voltage of 10.00 kV) was employed to examine the particle size and surface morphologies of the samples, along with an energy-dispersive X-ray spectroscopy (EDX) for elemental mappings. In addition, a simultaneous analysis of thermogravimetry (TGA) and derivative thermal gravimetry (DTG) was conducted on a NETZSCH STA 409 PC/PG Germany spectrometer. Additionally, the measurement of the specific surface area was carried out by BET device, specifically the Belsorp-Mini II, manufactured by Microtrace. The electrochemical analyses involving cyclic voltammetry (CV) and chronoamperometry (CHP) were performed using a Sama 500 potentiostat (Isfahan, Iran).

Preparation of the ZnW12O40@NiZn2O4 nanocomposite

Synthesis of Keggin-type [ZnW12O40]6– clusters

0.12 g (2.5 mmol) of Zn(NO3)2·6H2O and 1.38 g (25 mmol) of Na2WO4·2H2O were completely dissolved in 15 mL of DW, separately. Afterward, the prepared solution of Zn(NO3)2·6H2O was added gradually to Na2WO4·2H2O solution. Following this, the pH value was adjusted at 5.5 using buffer of acetate (1 M). The obtained solution was refluxed for 6 h and after cooling to room temperature, a saturated solution prepared from KCl (5.00 g in 10 mL of DW) was added and stirred for 60 min. Following a few days of aging, the resulting solid precipitate was washed using ethanol and water, and then was dried at 80 °C for 4 h.

Synthesis of NiZn2O4 nanoparticles

In order to synthesize NiZn2O4 ceramics27, the subsequent procedures were performed: Initially, Ni(NO3)2·6H2O (0.30 g) was dissolved in 20 mL of DW as a Ni precursor. Subsequently, this solution was added drop-wise to the Zn(NO3)2.6H2O solution (0.12 g in 20 mL of DW) with a molar ratio of 1:2 under magnetic stirring (referred to as solution A). Additionally, a mixture of citric acid solution (0.50 g in 15 mL of DW) and urea (0.32 g in 20 mL of DW) was prepared with a molar ratio of 1:2, employing sonication (referred to as solution B). The obtained solution was then heated to the temperature of 75 °C. Afterward, the combination of solutions A and B was carried out under constant agitation for 5 h at 80 °C, leading to the formation of a green gel. The resulting gel was subjected to calcination at a temperature of 500 °C for 4 h in a furnace, resulting in the generation of NiZn2O4 nanoparticles powder.

Synthesis of ZnW12O40/NiZn2O4 nanocomposite

In order to prepare the ZnW12O40/NiZn2O4 nanocomposite, the following method was performed via the sol–gel method. In a typical procedure, the gradual addition of Keggin-type ZnW12O40 solution (0.09 g in 20 mL of DW) to the prepared gel consisting of NiZn2O4 nanoparticles (prior to complete gelling) was conducted. Subsequently, the resulting solution was subjected to magnetic agitation at 80 °C for 60 min, leading to the formation of a gray gel. Following this, the gray gel that was acquired was subjected to calcination process in a furnace, wherein it was exposed to a temperature of 500 °C for 4 h, resulting in the formation of the ZnW12O40/NiZn2O4 powder.

Electrochemical hydrogen storage study

The CHP technique was employed to determine the hydrogen storage capacity of the ZnW12O40/NiZn2O4 nanocomposite electrodes in a three-electrode electrochemical cell comprising a reference electrode (Hg/HgO electrode), a counter electrode (Pt plate), and a working electrode (bare and modified copper foam). For the purpose of preparing the electrolyte solution, the aqueous electrolyte with a molarity of 6.0 M KOH was provided. To prepare 6 M KOH, 16.83 g of solid KOH was dissolved in distilled water and then added to the desired volume in a 50 mL of volumetric flask. A working electrode was created by coating a bare copper plate ((porous per inch) PPI: 95, 1 \(\times \) 2 cm2) with a thin layer of the ZnW12O40/NiZn2O4 nanocomposite powders at a temperature of 100 °C for 60 min, without the use of any binder. The working electrode was prepared in the following manner: the synthesized samples were dispersed in EtOH using an ultrasonic bath. Subsequently, the dispersion was applied onto the Cu foam through the drop cast method, and coated Cu foam was dried in an oven for 10 h. The electrochemical cell was assembled and operated at room temperature. By applying a constant current between the working and counter electrodes, the potential of the ZnW12O40@NiZn2O4 nanocomposite electrode was measured in relation to the reference electrode. In the following, the charge–discharge galvanostatic technique was executed on the SAMA-500 device at the designated current density of 2 mA. Moreover, The CV analysis for the achieved electrodes was carried out in analogous assembled cells at a scan rate of 0.10 Vs−1. Scheme 1 illustrates a diagrammatic representation of the procedure for preparing the working electrode and a view of the electrochemical cell.

Scheme 1. 
scheme 1

Schematic illustration of working electrode (sample/Cu substrate) and a view of the electrochemical cell.

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Results and discussion

Characterization of ZnW12O40@NiZn2O4 nanocomposite

FT-IR spectra

The successful preparation of the ZnW12O40@NiZn2O4 nanocomposite was accomplished through the FT-IR spectroscopic analysis. The sequence FT-IR spectra of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite can be found in Fig. 1 with a spectral range of 400–4000 cm−1. As pointed out in Fig. 1a, the characteristic peaks at 525 and 1137 cm−1 are attributed to the stretching vibrations of Zn–O and Ni–O–Zn groups, respectively28. Additionally, the characteristic peak at 869 cm−1 is assigned to the stretching vibration band of Ni–O–H functional group. Furthermore, the absorption bands positioned at 3430 and 1634 cm−1 are associated with the bending and stretching vibrations of H–O–H molecules, respectively. It is worth noting that the absorption of air moisture during the preparation of sample pellets is the cause of these phenomena29. These observations have thus provided valuable evidence concerning the influence of hydration on the structure of matter. The corresponding vibrational frequencies of the prepared [ZnW12O40]6– anionic clusters are observed at 1106, 938, 869, and 785 cm−1, which are related to the stretching vibrations of Zn-Oa (central bond), W = Od (terminal bond), W–Ob–W (corner-sharing bond), and W–Oc–W (edge-sharing bond), respectively (Fig. 1b)30,31. As illustrated in Fig. 1c, peaks associated with the presence of [ZnW12O40]6– were observed in the resulting as-prepared nanocomposite, exhibiting a certain degree of displacement. Furthermore, it was observed that the characteristic [ZnW12O40]6– peaks displayed a partial coverage by the ZnW12O40/NiZn2O4 bands at 1348, 1454, and 482 cm−1. The findings provided an initial validation for the nanocomposite of [ZnW12O40]6– supported on NiZn2O4 ceramics solid. As a result, the successful immobilization of Keggin-type POM on NiZn2O4 matrix can be effectively demonstrated via the FT-IR surveys.

Figure 1
figure 1

FT-IR spectra of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite.

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UV–Vis analysis

In order to investigate the charge transfer mode of the materials, the UV–Vis spectra of the (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite were displayed in Fig. 2. As illustrated in Fig. 2a, the absorption peaks in the region of 300–400 nm are possibly caused due to the ligand to metal charge transfer (LMCT) of oxygen (2p) to metal species (O2− → Zn2+ and/or O2− → Ni2+) and π → π* transitions of metals. According to the spectrum of Keggin-type [ZnW12O40]6– (Fig. 2b), absorption peaks at 257 and 315 nm can be attributed to the LMCT of tetrahedral oxygen (2p) to tungsten (O2−(2p) → W6+) and bridge oxygens (2p) to tungsten (O2−b/c → W6+), respectively32,33. Furthermore, the shifts towards shorter hypochromic wavelengths (blue-shifts) can be seen compared to the pure components of as-prepared nanocomposite material (Fig. 2c).

Figure 2
figure 2

UV–Vis spectra of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4.

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Bandgap determination

The estimation of the bandgap energies of the materials has been conducted using the classical Tauc relation, in adherence to the subsequent methodology34:

$$ \mathop {{(}\alpha {\text{h}}\upsilon )}\nolimits^{\frac{1}{n}} = A(h\upsilon - Eg) $$
(1)

where A represents a constant known as the band tailing parameter, \(\alpha \) denotes the absorption coefficient, Eg refers to the energy of the optical band gap, h represents the Planck constant, v signifies the frequency of the photon, and n represents the power factor of the transition mode35. The curves of (αhv)2 versus hv for (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite are pointed out in Fig. 3. The relationship between (αhv)2 and hυ (energy) exhibits a linear pattern within a specific area. Extending this linear pattern will result in the intersection of the (hυ)-axis, providing the determination of the optical energy gap (Eg). Based on the calculations conducted, the bandgap energies of NiZn2O4, Keggin-type [ZnW12O40]6, and ZnW12O40/NiZn2O4 nanocomposite were determined to be 5.92, 5.54, and 5.50, correspondingly. Lower band gap of as-prepared nanocomposite typically exhibits higher carrier densities compared to materials with larger band gaps. This is because a smaller band gap allows more electrons to be excited from the valence to the conduction band, resulting in a higher concentration of free charge carriers.

Figure 3
figure 3

Band gap energies (Eg) of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 using experimental data of UV–Vis.

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XRD data

The investigation of the XRD pattern of materials was conducted in order to examine the phase and dimensions of both grains and nanoparticles based on Rayleigh scattering (Fig. 4). According to the data presented in Fig. 4a, the prominent diffraction peaks observed at 2θ = 31.5, 34.3, 36.1, 43.1, 47.7, 56.8, 62.9, 68.1, and 69.0° can be attributed to the crystallographic planes denoted as (100), (002), (400), (102), (200), (110), (103), (112), and (201) within the hexagonal phase of NiZn2O436,37. Furthermore, there are prominent diffraction peaks observed at angles of 2θ = 8.3°, 11.8°, 18.8°, 20.5°, 23.8°, 2.26°, 28.5°, 30.3°, 33.8°, 34.8°, 36.9°, 40.6°, 41.2°, 45.9°, 48.4°, 50.3°, 57.4°, 58.7°, 60.0°, 66.4°, and 73.7°, which correspond to the crystal planes of (100), (110), (211), (220), (310), (311), (321), (320), (400), (420), (421), (511), (432), (442), (620), (541), (711), (633), (722), (653), and (842) (Fig. 4b)38. These peaks are indicative of the presence of potassium zinc tungsten oxide and suggest the existence of Keggin-type [ZnW12O40]6− species, as confirmed by the JCDD card No. 00-043-0001. The diffraction pattern of the ZnW12O40/NiZn2O4 nanocomposite reveals the presence of the majority of characteristic peaks associated with ZnW12O40-heteropolyoxo and NiZn2O4 within the structure of the nanocomposite, albeit with a slight shift (Fig. 4c). Upon formation of the composite, there exists the potential for alterations in individual crystal planes and potential variations in grain population orientations that result in crystal planes exhibiting specific Miller index orientations. Additionally, it is plausible that the peaks of the pure amorphous background material may coincide with similar peaks. The displacement and disappearance of certain [ZnW12O40]6– peaks can primarily be attributed to the rearrangement of Keggin clusters anions during the self-assembly process of [ZnW12O40]6– species and ceramic ions. Furthermore, the variation in atomic radius between the NiZn2O4 nanoparticles and the [ZnW12O40]6– serves as another factor leading to the reduction in intensity and shift of multiple peaks. Ultimately, the utilization of the Debye-Scherer formula enables the determination of the average crystallite size of the ZnW12O40/NiZn2O4 nanocomposite, which is calculated to be approximately 32.9 nm39,40. Besides, the X-ray peak broadening analysis was implemented to investigate the dimensions of the crystals and the lattice strain through the Williamson–Hall (W–H) in the subsequent manner41,42:

$$ \beta_{hkl} {\text{cos}}\theta { = }\frac{k\lambda }{D} \, + 4\varepsilon {\text{sin}}\theta $$
(2)
Figure 4
figure 4

XRD patterns of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite.

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In this formula, βhkl corresponds to the broadening of the instrumental peak, D represents the size of the crystallites (nm), K is the factor of shape which is equivalent to 0.89, λ corresponds to the wavelength of X-rays (0.15406 nm), θ denotes the reflection angle, and ε stands for the strain in the lattice. The plot in Fig. 5 illustrates the relationship between βhklcosθ and 4sinθ for the ZnW12O40/NiZn2O4. By employing linear regression, the measurements for the size of the crystalline particles (D) and the crystal lattice (ε) were ascertained through the inclination of the line. The lattice strain for (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite were calculated as − 5.17 \(\times \) 10–3, 2.42 \(\times \) 10–4, and 1.6 \(\times \) 10–3, respectively. The presence of a horizontal line exhibiting both positive and negative slopes can be ascribed to the phenomenon of lattice expansion and compression, respectively. The W–H curve of the ZnW12O40/NiZn2O4 nanocomposite exhibits a positive slope, indicating lattice expansion resulting from slight variations in the ionic radius of the constituent elements. Additionally, the size of the crystal in the ZnW12O40/NiZn2O4 was found to be about 33 nm, a measurement consistent with the dimensions obtained using the Scherer formula.

Figure 5
figure 5

W–H plots of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite.

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Morphological observations

The FE-SEM analysis was employed to analyze the surface morphology, estimate the size of the particles, determine the orientation of the particles on the substrate, and identify potential particle accumulation. Figure 6 illustrates the surface morphology obtained from the FE-SEM analysis of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40/NiZn2O4 nanocomposite. The arrangement of particles in NiZn2O4 is observed to be in the form of clumps, resulting in a spherical and relatively uniform shape (Fig. 6a). The surface image of ZnW12O40–heteropolyoxo revealed irregular accumulations of spherical particles in the nanometer scale (Fig. 6b)43. In addition, the FE-SEM image of ZnW12O40/NiZn2O4 nanocomposite exhibits that the polyoxometalate is heterogeneously immobilized on NiZn2O4 with a spherical morphology (Fig. 6c,d). These findings indicate that [ZnW12O40]6– species was anchored on the surface of NiZn2O4 nanoparticles. Additionally, the particle size distribution can be analyzed through the histogram depicted in Fig. 6e. Based on the findings, it is evident that the particles are in the range of 30–35 nm. It should be noted that the average size of ZnW12O40/NiZn2O4 nanoparticles is about 33 nm.

Figure 6
figure 6

FE-SEM images of (a) NiZn2O4, (b) ZnW12O4, (c,d) ZnW12O40/NiZn2O4, and (d) the particle size distribution of the ZnW12O40/NiZn2O4 nanoparticles.

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Mapping and EDX micrographs

The dispersion of elements was evaluated through Mapping analysis, which was further confirmed by EDX micrographs. The examination conducted by the EDX analysis successfully demonstrates the integration of O, K, W, Ni, and Zn within the structure of the ZnW12O40/NiZn2O4 with corresponding weight percentages of 29.91, 16.52, 16.83, 27.74, and 9.00 wt%, respectively (Fig. 7i). As illustrated in Fig. 7a–h, the elemental Mapping of constituents in the ZnW12O40/NiZn2O4 aligns with the percentages documented by the EDX analysis. Also, elemental Mapping also displayed that metal oxide nanoparticles were decorated with [ZnW12O40]6– anionic clusters.

Figure 7
figure 7

Elemental mapping images (a–h) and (i) EDX micrographs of the ZnW12O40/NiZn2O4.

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Specific surface area and pore size distribution

The absorption–desorption isotherm of N2 gas and surface characteristics, inclusive of the surface area and pore diameter of the materials were evaluated by the BET and BJH techniques. Figure 8 exhibits the absorption–desorption isotherm of type (II) according to the IUPAC classification, which is characterized by the presence of a microporous or non-porous structure. The specific surface area and pore volume of as-prepared nanocomposite were determined to be 16.572 m2 g−1 and 0.023584 cm3 g−1, respectively. Additionally, the findings derived from the BET analysis indicated an average pore diameter of 5.69 nm for the ZnW12O40/NiZn2O4 as a potential material for the electrocatalytic hydrogen storage. The specific surface area and porous volume of the ZnW12O40/NiZn2O4 are summarized in Table 1.

Figure 8
figure 8

(a) Adsorption–desorption isotherm of N2 and (b) BJH pore size distribution of ZnW12O40/NiZn2O4 nanocomposite.

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Table 1 Porosity features obtained from BET analysis of ZnW12O40/NiZn2O4 nanocomposite.
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TGA-DTG analysis

The TGA-DTG analysis of [ZnW12O40]6− anionic clusters during pyrolysis in the N2 atmosphere is illustrated in Fig. 9. The TGA curve of [ZnW12O40]6− exhibits several distinct peaks, signifying that Keggin-type ZnW12O40 experiences several notable weight reduction processes whilst undergoing pyrolysis (Fig. 9a). A significant weight loss (3.90%) was observed at room temperature up to 300 °C, which could be mainly related to the decomposition of adsorbed water molecules. Moreover, a notable weight loss (3.79%) was attributed to the desorption of crystallization water of H6[ZnW12O40]·6H2O clusters incorporated into the ZnW12O40 nanostructure during synthesis. The enhanced mass reduction (92.31%) observed at 559 °C can be ascribed to the withdrawal of oxygen from ZnW12O40, resulting in its decomposition44. Moreover, the TGA-DTG curve for ZnW12O40/NiZn2O4 nanocomposite are illustrated in Fig. 9b. The curve shows a two-step decomposition process. The first weight loss (2.79%) at ~ 100 °C is corresponds to the elimination of adsorbed water during the synthesis of as-prepared ZnW12O40/NiZn2O4 nanocomposite. The second major weight loss (56.3%) between 250 and 450 °C corresponds to the elimination of capping agent, such as citric acid and urea according to synthesis45.

Figure 9
figure 9

TGA–DTG curve of (a) Keggin-type [ZnW12O40]6– and (b) ZnW12O40/NiZn2O4 nanocomposite from room temperature to 600 °C.

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Electrochemical study results

Cyclic voltammetry

The electrochemical characteristics of the provided samples were examined by charge–discharge chronopotentiometry (CHP) and cyclic voltammetry (CV) methodologies. Figure 10 illustrates the CV curves of the (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40@NiZn2O4 in relation to the potential/current of the redox reaction within the range of − 8.0 to − 1.0 V at a constant scan rate of 100.0 mV/s for 20 cycles. As depicted in Fig. 10a, the CV measurements of NiZn2O4 nanoceramics exhibits the ability to undergo reversible and continuous multi-electron redox reactions on anodic/cathodic sweeps. Furthermore, Keggin-type [ZnW12O40]6– anionic clusters possess the capability to perform reversible and continuous multi-electron redox, and there are also tungsten ions with different valence states of WVI and WV (Fig. 10b). As seen in Fig. 10c, the redox process of the ZnW12O40/NiZn2O4 electrode is under surface control. This phenomenon may be attributed to the presence of Keggin polyanions and the pore structure of the layer.

Figure 10
figure 10

Cyclic voltammetry curves of (a) NiZn2O4, (b) ZnW12O40, and (c) ZnW12O40/NiZn2O4 as working electrodes.

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Chronopotentiometry measurements

A set of CHP measurements were conducted in order to examine the electrochemical properties of materials. The measurements were carried out with the following specifications: [KOH](aq) = 6.0 M, a scan rate of 100 mV s−1, and a three-electrode system comprised of a working counter (Pt) and a reference electrode (Hg/HgO), along with a working electrode formed from the prepared products as active materials. For detailed examination of hydrogen storage capacity, an investigation was conducted on the electrochemical hydrogen storage of bare copper foam, which served as a substrate. It was imperative to present a desorption diagram of the first cycle of the copper foam to demonstrate that it does not affect the discharge capacity of the as-prepared nanocomposite. As indicated in Fig. 11, the discharge capacity of the bare Cu foam revealed a negligible capacity for hydrogen storage (\(\sim \) 0.9 mAh/g), which can be disregarded in all stages of the test. As a result, the copper foam can be used as a substrate throughout all electrochemical hydrogen storage procedures.

Figure 11
figure 11

Electrochemical hydrogen storage capacity of copper substrate before coating.

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During the adsorption process, splitting of the water within the electrolyte occurs in immediate proximity to the working electrode. The electrolyte is comprised of H+ ions that have the potential to be assimilated by the samples that have been prepared on the working electrode or lead to the reformation and dispersion of hydrogen molecules on the surface of the electrode46,47. The resultant reactions can be attributed to the quasi-reversible electrochemical reaction as follows:

$$ {\text{Sample }} + {\text{ mH}}_{{2}} {\text{O}} + {\text{me}}^{ - } \leftrightarrow {\text{sample }} + {\text{mH}}^{ + } + {\text{mOH}}^{ - } $$
(3)
$$ {\text{4OH}}^{ - } \leftrightarrow {\text{O}}_{{2}} + {\text{2H}}_{{2}} {\text{O }} + {\text{4e}}^{ - } $$
(4)

In the process of desorption, the release of H + ions occur from the samples and they subsequently react with OH− ions, resulting in the formation of H–O–H molecules. The elucidation of potential reactions taking place during the discharge process can be presented as follows:

$$ {\text{Sample }} + {\text{ mH}}^{ + } + {\text{mOH}}^{ - } \leftrightarrow {\text{sample }} + {\text{ mH}}_{{2}} {\text{O }} + {\text{me}}^{ - } $$
(5)
$$ {\text{O}}_{{2}} + {\text{2H}}_{{2}} {\text{O }} + {\text{4e}}^{ - } \leftrightarrow {\text{4OH}}^{ - } $$
(6)

The hydrogen storage capacity (HSC) of the materials can be calculated using the following equation (Eq. (7)): HSC = It/m, where I represent the applied current (mA), t is the charge–discharge time (h), and m is the amount of the coated nanocomposite (g)48. Figure 12a–c illustrates the electrochemical hydrogen storage of (a) NiZn2O4, (b) ZnW12O4, and (c) ZnW12O40@NiZn2O4 nanocomposite as active materials on working electrodes. After carrying out multiple absorption–desorption cycles at varying currents, a current of 2 mA was identified as the optimal value. The discharge capacity of each cycle demonstrates an increase compared to the preceding cycle when a current of 2 mA is administered. This positive trend persists until the capacity attains its highest achievable value. This enhancement can be attributed to the emergence of new sites that facilitate the absorption–desorption of hydrogen. In the applied of current of 2 mA, the discharge capacity of the NiZn2O4 nanoparticles experienced a significant increase from approximately 110 to 420 mAh/g after 20 cycles (Fig. 12a). Furthermore, the Keggin-type [ZnW12O40]6 cluster exhibited a notable discharge capacity (\(\sim \) 360 mAh/g) during the initial cycle, and the storage capability progressively amplified to about 790 mAh/g after 20 cycles (Fig. 12b). As seen in Fig. 12c, the discharge capacity of the ZnW12O40@NiZn2O4 nanocomposite has demonstrated a significant enhancement from 340 to 900 mAh/g under identical experimental conditions after 20 runs. The data presented in this research demonstrates a positive correlation between the surface area and the electrochemical hydrogen storage capacity. As a result, the ZnW12O40@NiZn2O4 nanocomposite proves to be a preferred active material for hydrogen adsorption and desorption due to its larger surface area, allowing for a stronger interaction with H2 molecules. Further, it can be inferred that the remarkable efficacy of electrochemical hydrogen storage can be ascribed to the synergistic influence of ZnW12O40 and NiZn2O4 nanostructures.

Figure 12
figure 12

Electrochemical discharge curves of (a) NiZn2O4, (b) ZnW12O40, and (c) ZnW12O40/NiZn2O4 nanocomposite at a current density of 2 mA.

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The discharge capacity of several materials was documented in Table 2 alongside their corresponding formula structures. Among the diverse nanoparticles and composites, the optimum efficiency was associated with ZnW12O40/NiZn2O4 nanocomposite under experimental condition.

Table 2 Comparison of electrochemical hydrogen storage between ZnW12O40/NiZn2O4 and other nanomaterials.
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Conclusions

In the current study, a new nanocomposite was synthesized successfully through the incorporation of [ZnW12O40]6– anionic clusters onto NiZn2O4 particles and applied in the electrochemical hydrogen storage process. The as-prepared nanocomposite indicates a spherical morphology with an average diameter of approximately 32.9 nm, which was calculated by the Scherer equation. Different parameters, such as discharge capacity of the bare copper foam, and the influence of repeated hydrogen adsorption–desorption cycles were discussed. The electrochemical findings revealed that the ZnW12O40/NiZn2O4 nanocomposite exhibits a remarkable capacity and outstanding reversibility in comparison to individual ZnW12O40 and NiZn2O4 nanoparticles for the purpose of hydrogen storage. The highest hydrogen discharge capability of the ZnW12O40/NiZn2O4 was achieved \(\sim \) 340 mAh/g during the 1st cycle, and the storage capacity enhanced to approximately 900 mAh/g at the end of 20 cycles using a current density of 2 mA. Hence, the ZnW12O40/NiZn2O4 nanocomposite can effectively contribute to the field of energy storage as a promising and innovative candidate material.