Design of a new nanocomposite based on Keggin-type [ZnW12O40]6− anionic cluster anchored on NiZn2O4 ceramics as a promising material towards the electrocatalytic hydrogen storage

Extensive research efforts have been dedicated to developing electrode materials with high capacity to address the increasing complexities arising from the energy crisis. Herein, a new nanocomposite was synthesized via the sol–gel method by immobilizing K6ZnW12O40 within the surface of NiZn2O4. ZnW12O40@NiZn2O4 was characterized by FT-IR, UV–Vis, XRD, SEM, EDX, BET, and TGA-DTG methods. The electrochemical characteristics of the materials were examined using cyclic voltammogram (CV) and charge–discharge chronopotentiometry (CHP) techniques. Multiple factors affecting the hydrogen storage capacity, including current density (j), surface area of the copper foam, and the consequences of repeated cycles of hydrogen adsorption–desorption were evaluated. The initial cycle led to an impressive hydrogen discharge capability of 340 mAh/g, which subsequently increased to 900 mAh/g after 20 cycles with a current density of 2 mA in 6.0 M KOH medium. The surface area and the electrocatalytic characteristics of the nanoparticles contribute to facilitate the formation of electrons and provide good diffusion channels for the movement of electrolyte ions throughout the charge–discharge procedure.


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).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 NiZn 2 O 4 nanoparticles
In order to synthesize NiZn 2 O 4 ceramics 27 , the subsequent procedures were performed: Initially, Ni(NO 3 ) 2 •6H 2 O (0.30 g) was dissolved in 20 mL of DW as a Ni precursor.Subsequently, this solution was added drop-wise to the Zn(NO 3 ) 2 .6H 2 O 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.

Electrochemical hydrogen storage study
The CHP technique was employed to determine the hydrogen storage capacity of the ZnW 12 O 40 /NiZn 2 O 4 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 × 2 cm 2 ) with a thin layer of the ZnW 12 O 40 /NiZn 2 O 4 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 ZnW 12 O 40 @NiZn 2 O 4 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.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, respectively 28 .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 phenomena 29 .These observations have thus provided valuable evidence concerning the influence of hydration on the structure of matter.The corresponding vibrational frequencies of the prepared [ZnW 12 O 40 ] 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-O b -W (corner-sharing bond), and W-Oc-W (edge-sharing bond), respectively (Fig. 1b) 30,31 .

Results and discussion
As illustrated in Fig. 1c, peaks associated with the presence of [ZnW 12 O 40 ] 6-were observed in the resulting asprepared nanocomposite, exhibiting a certain degree of displacement.Furthermore, it was observed that the characteristic [ZnW b/c → W 6+ ), respectively 32,33 .Furthermore, the shifts towards shorter hypochromic wavelengths (blue-shifts) can be seen compared to the pure components of as-prepared nanocomposite material (Fig. 2c).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.

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 (1) (αhυ)   110), ( 211), ( 220), (310), (311), (321), (320), (400), ( 420), ( 421), ( 511), ( 432), ( 442), ( 620), ( 541), ( 711), ( 633), ( 722), (653), and (842) (Fig. 4b) 38 39,40 .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 manner 41,42 : In this formula, β hkl 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 β hkl cosθ and 4sinθ for the ZnW 12 O 40 /NiZn 2 O 4 .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) NiZn 2 O 4 , (b) ZnW 12 O 4 , and (c) ZnW 12 O 40 /NiZn 2 O 4 nanocomposite were calculated as − 5.17 × 10 -3 , 2.42 × 10 -4 , and 1.6 × 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 ZnW 12 O 40 /NiZn 2 O 4 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 ZnW 12 O 40 /NiZn 2 O 4 was found to be about 33 nm, a measurement consistent with the dimensions obtained using the Scherer formula. (2)

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 ZnW 12 O 40 /NiZn 2 O 4 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

Specific surface area and pore size distribution
The absorption-desorption isotherm of N 2 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  1.

TGA-DTG analysis
The

Cyclic voltammetry
The electrochemical characteristics of the provided samples were examined by charge-discharge chronopotentiometry (CHP) and cyclic voltammetry (CV) methodologies.Figure 10   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 NiZn 2 O 4 nanoceramics exhibits the ability to undergo reversible and continuous multielectron redox reactions on anodic/cathodic sweeps.Furthermore, Keggin-type [ZnW 12 O 40 ] 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 W VI and W V (Fig. 10b).As seen in Fig. 10c, the redox process of the ZnW 12 O 40 / NiZn 2 O 4 electrode is under surface control.This phenomenon may be attributed to the presence of Keggin polyanions and the pore structure of the layer.

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 ( ∼ 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.
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 electrode 46,47 .The resultant reactions can be attributed to the quasi-reversible electrochemical reaction as follows: 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: 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   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 NiZn 2 O 4 nanoparticles experienced a significant increase from approximately 110 to 420 mAh/g after 20 cycles (Fig. 12a).Furthermore, the Keggin-type [ZnW 12 O 40 ] 6 cluster exhibited a notable discharge capacity ( ∼ 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 ZnW 12 O 40 @NiZn 2 O 4 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 ZnW 12 O 40 @NiZn 2 O 4 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 H 2 molecules.Further, it can be inferred that the remarkable efficacy of electrochemical hydrogen storage can be ascribed to the synergistic influence of ZnW 12 O 40 and NiZn 2 O 4 nanostructures.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 ZnW 12 O 40 /NiZn 2 O 4 nanocomposite under experimental condition.

Conclusions
In the current study, a new nanocomposite was synthesized successfully through the incorporation of [ZnW 12 O 40 ] 6-anionic clusters onto NiZn 2 O 4 particles and applied in the electrochemical hydrogen storage process.The as-prepared nanocomposite indicates a spherical morphology with an average diameter of 12 O 40 ] 6-peaks displayed a partial coverage by the ZnW 12 O 40 /NiZn 2 O 4 bands at 1348, 1454, and 482 cm −1 .The findings provided an initial validation for the nanocomposite of [ZnW 12 O 40 ] 6-supported on NiZn 2 O 4 ceramics solid.As a result, the successful immobilization of Keggin-type POM on NiZn 2 O 4 matrix can be effectively demonstrated via the FT-IR surveys.Vol:.(1234567890)Scientific Reports | (2024) 14:11038 | https://doi.org/10.1038/s41598-024-61871-0www.nature.com/scientificreports/UV-Vis analysis In order to investigate the charge transfer mode of the materials, the UV-Vis spectra of the (a) NiZn 2 O 4 , (b) ZnW 12 O 4 , and (c) ZnW 12 O 40 /NiZn 2 O 4 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 (O 2− → Zn 2+ and/or O 2− → Ni 2+ ) and π → π* transitions of metals.According to the spectrum of Keggin-type [ZnW 12 O 40 ] 6-(Fig.2b), absorption peaks at 257 and 315 nm can be attributed to the LMCT of tetrahedral oxygen (2p) to tungsten (O 2− (2p) → W 6+ ) and bridge oxygens (2p) to tungsten (O 2− Scheme 1. .Schematic illustration of working electrode (sample/Cu substrate) and a view of the electrochemical cell.
. These peaks are indicative of the presence of potassium zinc tungsten oxide and suggest the existence of Keggin-type [ZnW 12 O 40 ] 6− species, as confirmed by the JCDD card No. 00-043-0001.The diffraction pattern of the ZnW 12 O 40 /NiZn 2 O 4 nanocomposite reveals the presence of the majority of characteristic peaks associated with ZnW 12 O 40 -heteropolyoxo and NiZn 2 O 4 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 [ZnW 12 O 40 ] 6-peaks can primarily be attributed to the rearrangement of Keggin clusters anions during the self-assembly process of [ZnW 12 O 40 ] 6-species and ceramic ions.Furthermore, the variation in atomic radius between the NiZn 2 O 4 nanoparticles and the [ZnW 12 O 40 ] 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 ZnW 12 O 40 /NiZn 2 O 4 nanocomposite, which is calculated to be approximately 32.9 nm
illustrates the CV curves of the (a) NiZn 2 O 4 , (b) ZnW 12 O 4 , and (c) ZnW 12 O 40 @NiZn 2 O 4 in relation to the potential/current of the redox reaction
Vol.:(0123456789) Scientific Reports | (2024) 14:11038 | https://doi.org/10.1038/s41598-024-61871-0www.nature.com/scientificreports/Preparation The resulting gel was subjected to calcination at a temperature of 500 °C for 4 h in a furnace, resulting in the generation of NiZn 2 O 4 nanoparticles powder.Synthesis of ZnW 12 O 40 /NiZn 2 O 4 nanocompositeIn order to prepare the ZnW 12 O 40 /NiZn 2 O 4 nanocomposite, the following method was performed via the sol-gel method.In a typical procedure, the gradual addition of Keggin-type ZnW 12 O 40 solution (0.09 g in 20 mL of DW) to the prepared gel consisting of NiZn 2 O 4 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 ZnW 12 O 40 /NiZn 2 O 4 powder.

Characterization of ZnW 12 O 40 @NiZn 2 O 4 nanocomposite
, the elemental Mapping of constituents in the ZnW 12 O 40 /NiZn 2 O 4 aligns with the percentages documented by the EDX analysis.Also, elemental Mapping also displayed that metal oxide nanoparticles were decorated with [ZnW 12 O 40 ] 6-anionic clusters.

Table 2 .
Comparison of electrochemical hydrogen storage between ZnW 12 O 40 /NiZn 2 O 4 and other nanomaterials.a Porous carbon.b Condition of experiment: [KOH] (aq) = 6.0 M, a scan rate of 100 mV s −1 , current density of 2 mA, 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.