Introduction

Hydrogen is one of the most sustainable, and low-cost technologies for large-scale clean energy production to solve the global energy problem1. Fossil fuels consume a significant amount of natural resources and produce undesired products like CO2, which creates an alarming situation due to the greenhouse effect2. Using hydrogen and oxygen as fuel is considered an excellent candidate source of clean energy to meet the growing energy demands, climate change, etc3. The electro-catalytic water-splitting demonstrates a sustained energy source for generating hydrogen/oxygen at a large scale attributable to high purity, high efficiency and zero pollutant4,5. Several alloys and materials oxides are well-known candidates for developing hydrogen/oxygen electrodes. As a result, certain oxides with transition metals, such as spinel structures, have high electronic conductivity and exhibit significant electrochemical activity in the hydrogen/oxygen evolution reaction (HER/OER) because of their outstanding chemical and physical characteristics. A selection of parameters such as temperature, preparation method, substitution ions and pH of the precursor solution substantially improved the catalytic activity of spinels6,7,8. Accordingly, adjusting the spinel oxide samples' phase can optimise the hydrogen evolution functioning of the sample9,10,11,12.

MnCo2O4 has an inverse spinel structure in which Mn2+ ions and the Co2+ ions have occupied the octahedral (Oh, B) sites and are evenly distributed over the Oh and tetrahedral (Td, A) sites. The cation substitution into MnCo2O4 spinel can significantly alter its electrical and magnetic features because the electron transfer distance between B and B is short, which enhances the electron transfer and the electrical conductivity13,14. The effect of cation composition on the magnetic features of Co2−xBixMnO4 (0.0 ≤ x ≤ 0.3) mixed cubic spinel system was examined by Rajeevan et al15. The Ni2+ substitution on magnetic properties of MnCo2O4 was also studied by Wang et al16. The literature survey showed that the influence of cation substitution on the magnetic properties of MnCo2O4 is still low and needs more investigation.

Newly, numerous research has been dedicated to developing electrocatalysts for electrochemical Hydrogen Evolution Reaction (HER) utilizing nanomaterials. EWS (electrochemical water splitting) is a favorable methodology for hydrogen production17,18. To promote the development of hydrogen gas as fuel, a suitable catalyst with low-cost, natural abundance, excellent electrochemical activity, ease of preparation, and long-term stability for both HER19,20 and OER (oxygen evolution reaction) will be employed be employed be critical for electrochemical water splitting21.

Spinel oxides are emerging as a promising electrocatalyst class for EWS, including MnCo2O422, NiCo2O423, Co3O424, etc. The cobalt-based spinel oxides (MnCo2O4, NiCo2O4, ZnCo2O4, CuCo2O4) have demonstrated significant performance in electrocatalytic OER25. Moreover, MnCo2O4 has also been investigated as a potential applicant for energy storage applications, including batteries and supercapacitors, in terms of large capacitance, long cycling stability, and high operating voltage. These superior electrochemical properties are attributed to the multiple oxidation states (+2, +3, +4) of Mn and Co in magnetic spinel oxides26. Different types of spinel oxides (Co3O4, ZnCo2O4, MgV2O4, NiCo2O4, CuCo2O4, and NiMn2O4 have been explored as battery-type electrode materials in recent years due to multiple valence states and rich redox reactions27. Yan et al.28 synthesized the vanadium-doped nickel sulfide nanoflower via a hydrothermal approach for a high-performance supercapacitor. They observed that the increase in surface area and interfacial position of electrode materials promoted the rapid transport of electrolyte ions and electrons and improved the electrochemical performance after vanadium doping. The eco-friendly hydrogen generation by urea-assisted water electrolysis using spinel M2GeO4 (M = Fe, Co) as an active electrocatalyst was done by the Choi research group29. Alqarni et al.30 studied the synthesis and high current supercapacitor applications of vanadium intercalated spinel ferrite (Co0.5Ni0.5VxFe1.6− xO4) electrodes, and results showed a facile and low-cost production of vanadium doped ternary ferrite nanomaterials illustrated excellent high-rate electrochemical performance.

The present study demonstrated a novel V-doped MnCo microsphere spinel oxide (MC) synthesised via a hydrothermal route and investigated the physical properties and electrochemical features. The V doped MnCo microspheres spinel oxide was coated with glassy carbon electrodes to be used as cathodes to explore their performance for HER in an acidic medium. Vanadium has a variety of stable oxidation states (+2, +3, +4, and +5). Its high oxidation states (+4 and +5) can store charge in a positive potential range, thus supplying a favourable pseudo capacitance28,31. Boosting the specific capacitance of the electrode dramatically, ions with two (or more) ions of different valence states have higher charge storage capacity and more abundant redox reactions than most other transition metal ions29.

Materials and methods

Synthesis of Mn spinel oxide microspheres (Vx-MnCo MCs)

The one-step hydrothermal process manufactured the Vx-MnCo (x ≤ 0.4) MCs. Stoichiometric amount of Co(NO3)2⋅6H2O, Mn(NO3)2⋅6H2O, VCl3, and urea (CH4N2O) were used as initial materials. The stoichiometric quantities of metal salts were thawed in 20 ml DI H2O by stirring at room temperature (RT), and then 1.2 g of urea (CH4N2O) were thawed in 30 ml DI water. Both mixtures were stirred and sonicated for 30 min. The solution was installed into an autoclave (stainless steel) and heated for 12 h at 180 °C. Finally, the solid product was washed with warm deionised water, filtered, and dried in an oven.

For the working electrode preparation, 80 µL of 5% Nafion solution and 4 mg of catalyst were dissolved in a mixture of de-ionized water and ethanol in 1 mL (4:1 in volume ratio). The ultrasonication was utilised for 30 min. to guarantee that the solution was fully dispersed. Then, 5 μL catalyst ink solution was deposited on the polished GC electrode and dried at 80 °C for 2 h to achieve the catalyst loading of 0.285 mg/cm2.

The crystal structure of the synthesized MCs was analyzed via a Rigaku Benchtop Miniflex XRD (X-ray diffractometer). The surface morphology and chemical composition were implemented with SEM (scanning electron microscopy) and FEI with the model Titan ST model HR-TEM, TEM (transmission and high-resolution electron microscopies) microscopes along with EDX (energy dispersive X-ray spectroscopy).

Electrochemical characterization

The Linear sweep voltammetry (LSV), Cyclic voltammetry (CV), Chronopotentiometry (CP), and Electrochemical impedance spectroscopy (EIS) were performed in a 0.5 M H2SO4 solution with a typical three-electrode system using an AutoLab PGSTAT302N electrochemical workstation. Glassy carbon (GC) electrodes coated with V-MnCo MCs were used as working electrodes, a saturated Ag/AgCl electrode was employed as the reference electrode, and the Pt wire was utilized as the counter electrode. Nernst equation was used to calibrate all the potentials to ERHE (Reversible hydrogen electrode potential) was used to calibrate all the potentials for HER measurements using Eq. (1)1.

$${\mathrm{ERHE }=\mathrm{ Eappl }+ 0.059\mathrm{ pH }+\mathrm{ E}}_{\mathrm{Ag}/\mathrm{AgCl}}^{\mathrm{o}}$$
(1)

where \({\mathrm{E}}_{\mathrm{Ag}/\mathrm{AgCl}}^{\mathrm{o}}\) is standard electrode potential equivalent to 0.198 V at 25 °C. The scan rate for LSV measurements was 5 mV s−1. The electrochemical chronopotentiometry was used to measure the stability of the V-MnCo MCs at a fixed current of −10 mA/cm2. EIS measurements were measured by the same potentiostat (PGSTAT302N Autolab) equipped with a frequency module analyzer (FRA2).

Results & discussion

Phase and morphology analysis

The phase purity analysis of Vx-MnCo MCs was investigated through powdered XRD analysis and refined by Match! software and full proof, as depicted in Fig. 1. The XRD pattern disclosed the prominent dominant peaks of cubic spinel oxide structure with relatively low-intensity peaks associated with the trace of MnO2. All diffraction peaks in Fig. 1 matched well with the standard international centre for diffraction data (file no. 23-1237) of MnCo2O4. Rietveld analysis of experimental XRD pattern data revealed the lattice parameters, cell volume, reliability factors and crystallite size, as seen in Table 1. The lattice parameters increased with increasing the 'x' due to the expansion in the lattice due to a mismatch in ionic radii. The crystallite sizes were calculated between 16.0 and 22.4 nm implementing Scherrer’s formula for the most intense peak (311)32.

Figure 1
figure 1

XRD powder patterns of Vx-MnCo (x ≤ 0.4) MCs.

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Table 1 Refined cell parameters of Vx-MnCo (x ≤ 0.4) MCs.
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The surface morphology and shape of Vx-MnCo MCs were investigated by SEM analysis with different magnifications, as shown in Fig. 2. The images exhibited the different sizes of spherical particles. The high magnification images revealed that the spherical particles consisted of highly agglomerated small cubic particles, giving rise to the spherical particles with a rough surface. However, an enormous aggregation of the particles was dominated for the Vx-MnCo MCs (x = 0.3) after the stability test, as depicted in Fig. 12b. To further confirm the morphology and structure of Vx-MnCo MCs (x = 0.3), TEM and HR-TEM were employed, as clear from Fig. 3 shows the assembling of spherical particles, and HR-TEM approved the spinel phase of the synthesized samples. The well-defined D-spacing of 0.28 and 0.18 nm are assigned to the (220) and (331) facets of Vx-MnCo MCs following the XRD results. The presence of Co, Mn, V and O elements in Vx-MnCo (x = 0.2 and 0.4) MCs was confirmed by the EDX analysis as illustrated in Fig. 4a,b. Moreover, the V content was increased, and Mn and Co concentration was suppressed in the EDX spectra from x = 0.2 and 0.4, which indicates that V was successfully doped into MnCo MCs and consistent with XRD and TEM analysis.

Figure 2
figure 2

SEM microstructural images of Vx-MnCo samples for (a) without V doping followed by x = (b) 0.1, (c) 0.2, (d) 0.3, and (e) 0.4 at different magnifications.

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Figure 3
figure 3

TEM (left) and HR-TEM (right) micrographs of Vx-MnCo (x = 0.3) MCs.

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Figure 4
figure 4

EDX spectrum of Vx-MnCo samples for x = (a) 0.2 MCs, and (b) 0.4 MCs.

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X-ray photoelectron spectroscopy analysis

The chemical composition and oxidation state of Vx-MnCo MCs (x = 0.3) magnetic nanoparticles were determined using XPS analysis. A comprehensive survey scan was conducted to identify the sample elements as presented in Fig. 5a. In addition, the deconvoluted characteristic peaks of each element were shown separately in Figs. 5b–f. The core level spectrum of the C 1s state was observed at 284.8 eV, further deconvoluted into three characteristic peaks, as shown in Fig. 5b. The peaks observed at binding energies of 284.8 eV, 286.3 eV, and 288.3 eV are assigned to C–C, C–O–C and O–C=O bonds, respectively. Figure 5c shows the core level peaks of Co at a binding energy of 779.7 eV and 794.7 eV, which correspond to Co 2p3/2 and Co 2p1/2, respectively33. The deconvolution of the core peak of Co 2p3/2 introduced two characteristic peaks at 779.5 eV and 780.9 eV corresponding to Co2+ and Co3+ states, respectively.

Figure 5
figure 5

XPS analysis of Vx-MnCo MCs (x = 0.3) sample represents (a) survey scan and core level spectrum of (b) C 1s, (c) Co 2p, (d) Mn 2p, (e) V 2p, (f) O 1s.

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Similarly, Co 2p1/2 spectrum was deconvoluted into two peaks at 794.5 eV and 795.9 eV indicating the presence of Co atoms at two different geometrical positions in the spinal structure. In addition, two satellite peaks of Co 2p3/2 and Co 2p1/2 spectra were also observed at 789.0 eV and 804.0 eV, respectively. Mn 2p3/2 and Mn 2p1/2 core spectra were observed at 641.6 eV and 653.2 eV, respectively34 as presented in Fig. 5d. As a result of deconvolution, the core spectra of Mn were split into characteristic peaks associated with Mn2+, and Mn3+ states, respectively35. The spin–orbit doublet of core level spectra of V 2p3/2 and V 2p1/2 were detected at 516.5 eV and 524.1 eV, respectively36, as shown in Fig. 5e. The further deconvolution of the V 2p core spectrum introduces two peaks in each case, indicating the presence of V3+ and V5+ ionic states. Figure 5f represents the deconvoluted spectrum of O 1 s species exhibiting characteristic peaks at 529.7 eV and 531.1 eV, respectively37. The high-intensity peak at 529.7 eV represents the metal oxide bonding, whereas the low-intensity broad peak centered around 531.2 eV is associated with the adsorbed hydroxyl groups and unreacted carbonates.

Magnetic properties

Figure 6a,b present the variations of magnetization against the applied magnetic field (M-H) for different Vx-MnCo (x ≤ 0.4) MCs registered under H = ± 70 kOe and at temperatures of 300 and 10 K, respectively. M-H results measured at room temperature showed almost linear curves for all samples, indicating their paramagnetic behaviors at 300 K. an apparent hysteresis loop-like feature could be observed in the M-H curves at low magnetic fields measured at 10 K as pointed out by the enlarged view in the inset of Fig. 6b. The opened hysteresis loops at 10 K for all prepared samples reflect the transformation to a ferrimagnetic state at lower temperatures. In spinel oxides, three main magnetic interactions exist among the tetrahedral (A) and octahedral (B) sites via the intermediate O2− ions through superexchange interactions. For instance, the weak superexchange interactions Co2+–O2– Co3+–O2– Co2+ maintained an antiferromagnetism effect in Co3O4 composition38. Substituting some Co3+ ions with Mn3+ ions to form a MnCo2O4 sample will significantly affect the magnetic exchange interactions. Indeed, the inclusion of Mn3+ ions having a larger size than Co3+ ions will distort the oxygen octahedron and hence frustrate the tetrahedral site, provoking an insufficient magnetic ordering in the MnCo2O4 (x = 0.0 in this case) MCs. Accordingly, the manifestation of ferrimagnetism in MnCo2O4 MCs at lower temperatures could be ascribed to the canting effect of antiferromagnetically ordered spins via structural distortions and breakdown of the equilibrium among the Co2+ sublattices’ antiparallel magnetization caused by the inclusion of Mn3+ ions39.

Figure 6
figure 6

Magnetic hysteresis loops of Vx-MnCo (x ≤ 0.4) MCs recorded at (a) T = 300 K and (b) T = 10 K.

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Catalysis and magnetism may be different manifestations of some more fundamental atomic properties. But the current chief applications of magnetism to catalysis are in structural studies of catalytic solids.

On the other hand, the variation in magnetic parameters is probably a result of the inclusion of vanadium ions that will cause inhomogeneity and intrinsic pinning of the domain walls. Due to the dissimilarity in ionic radii of vanadium and cobalt ions, a change in the exchange interactions could be caused by the local crystal fields. Such an effect may conduce to the generation of energy barriers that could considerably affect the magnetization reversal process at low temperatures.At low temperatures (10 K), the vanadium composition dependence of the magnetic parameters like magnetization Mmax achieved at 70 kOe, remanent magnetization (Mr), and coercive field (Hc) is presented in Fig. 7. The room temperature (300 K) magnetization Mmax achieved at 70 kOe was also presented in Fig. 7a. There is a continuous increase in the magnetization Mmax with increasing vanadium content in comparison to the parent sample at both 300 and 10 K. For different prepared compositions, Mmax (at 70 kOe) increased as the temperature decreased from 300 K down to 10 K. For example, Mmax (at 70 kOe) is equal to 2.0 and 3.2 emu/g at 300 K for samples with x = 0.0 and 0.4, respectively, and increased to about 11.5 and 20.0 emu/g at 10 K. As the temperature decreases, Mmax (at 70 kOe) increases since the thermal fluctuations contributing to demagnetization with be lowered at lower temperatures. Hence, Mmax (at 70 kOe) and Mr and Hc will rise. In other words, the thermal agitation will tend to decrease as the temperature decreases. Hence the easy magnetization rotation would be more dominant than the thermal agitations, leading to an increase in global magnetization magnitude. As the composition content (x) rises, it is found that Mmax (at 70 kOe) and Mr values increase simultaneously at low temperatures (T = 10 K). For example, Mmax (at ± 70 kOe) is about 2.3 emu/g (at 300 K) and 11.5 emu/g (at 10 K) for the non-doped sample (x = 0.0) and attained about 3.2 emu/g (at 300 K) and 20.0 emu/g (at 10 K) for the product with x = 0.4. Mr measured at 10 K is about 0.8 emu/g for the non-doped sample (x = 0.0) and attained about 1.58 emu/g for the product with x = 0.4. For the intermediate compositions, Mr values are about 13.1, 14.9, and 17.3 emu/g for samples with x = 0.1, 0.2, and 0.3, as shown in Fig. 7b, respectively.

Figure 7
figure 7

Evolutions in (a) Mmax (± 70 kOe) at 300 and 10 K, (b) Mr at 10 K, and (c) Hc at 10 K concerning “x”.

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The magnetization values continuously increase with vanadium substitution. However, the variation in Hc value showed an anomaly at x > 0.2. The Hc increases first with increasing vanadium content (x) up to 0.2 but then drops with additional rising vanadium content (x > 0.2), as shown in Fig. 7c. It is well-known that coercivity Hc dramatically depends on the crystallite size40,41. According to XRD results, it is found that the crystallite size increases with increasing x content up to 0.2 but falls with further increasing x content. At 10 K, Hc is about 1212.1 Oe for the non-doped sample (x = 0.0), which progressively increases up to 1371.6 and 1390.3 Oe for the products with x = 0.1 and 0.2, respectively, and starts to slightly decrease with the further increase in vanadium content (Hc = 1230.2 Oe for x = 0.3 and Hc = 1214.5 Oe for x = 0.4). The registered Hc values are non-negligible, indicating the complex magnetic features of the present samples at low temperatures. The variations in Mmax (at 70 kOe) could be well-explained by the distribution of cations in different sites and hence, the variation in the net magnetic moment of the whole system.

For non-substituted MnCo2O4 MCs, the cations are distributed among Td and Oh sites as follows: (Co3+)A[Co2+Mn3+]BO4. The Td sites are occupied by Co3+ ions (spin S = 2 and g = 2) with a magnetic moment of about\(;close\mu \left(A\right)=4.9{\mu }_{B}\). On the other side, the Oh sites are occupied by Co2+ and Mn3+ ions with high spin state S = 3/2 and 2, respectively. Hence, the magnetic moment of B site is equal to the \({\mu \left(B\right)=\sqrt{{\left({\mu }_{{Co}^{2+}}\right)}^{2}+{\left({\mu }_{{Mn}^{2+}}\right)}^{2}}=\sqrt{{\left(3.87\right)}^{2}+{\left(4.9\right)}^{2}}=6.24\mu }_{B}\). Accordingly, it could be noticed that the orbital angular momentum of the magnetic ions situated at B sites is completely quenched. Upon vanadium doping, the cations are distributed as \({\left({Co}^{3+}\right)}_{A}{\left[{Co}_{2-x}^{2+}{V}_{x}^{2+}{Mn}^{3+}\right]}_{B}{O}_{4}\) where vanadium ions occupy Oh sites once they substitute some ions of Co2+. In this configuration, the three ions residing in the B site display magnetic moments of \(\mu \left({Co}^{2+}\right)=3.87{\mu }_{B}\), \(\mu \left({V}^{2+}\right)=1.73{\mu }_{B}\), and \(\mu \left({Mn}^{2+}\right)=4.9{\mu }_{B}\). The substitution of some magnetic Co2+ ions residing in B sites by some V2+ ions with lower magnetic moments will reduce the magnetic moment of B sites. Theoretically, the net magnetic moment of the whole system will decrease with increasing x content since the magnetic moment of A sites is constant. However, this is different in the present study, where an increment in magnetization with vanadium substitution has been observed. This indicates that other factors govern the magnetic behavior of present samples. Accordingly, further investigations will be done by our research group as further study.

Figure 8 displays the temperature (10 K ≤ T ≤ 325 K) dependence of magnetization (M–T) for different Vx-MnCo (x ≤ 0.4) MCs carried out under FC and ZFC conditions. A dc-magnetic field of about Happ = 100 Oe has been applied. Hereafter, M–T curves measured under ZFC, and FC conditions were noted as MZFC(T) and MFC(T) curves, respectively. The general shape of MZFC(T) curves is practically comparable for different products. The marked transformation in the slope of both MZFC(T) and MFC(T) plots indicates the well-known ferrimagnetic Curie temperature Tc. These magnetic transitions at Tc emphasize the ferrimagnetic behavior of the prepared products. For Vx-MnCo (x ≤ 0.4) MCs, Tc values are lesser than those registered for bulk MnCo2O4 MCs (Tc ~ 185 K), which is mainly attributed to the effects of finite size42. The Vx-MnCo (x ≤ 0.4) MCs display a distinctive separation between the MZFC(T) and MFC(T) curves at a certain temperature noted as Tsp < Tc (Fig. 8). MFC(T) plot declines progressively with the rising temperature and merges more than MZFC(T) plot. The magnetization’ thermal irreversibility is usually detected under Curie temperature. This is because of the existence of high magneto-crystalline anisotropy in prepared samples. The observed irreversibility is comparable to the previous findings for MnCo2O443 and certain ferrimagnetic and ferromagnetic oxides.

Figure 8
figure 8

M–T graph (temperature dependence of magnetization) of Vx-MnCo (x ≤ 0.4) MCs measured under FC and ZFC conditions and a dc-magnetic field of 100 Oe.

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Furthermore, all products showed maximum magnetizations peak at a temperature noted as Tp (lower than the Curie temperature) in the MZFC(T) curves, and afterwards, it falls to shallow values. Typically, the appearance of a peak around TP could be ascribed to either the spin-glass state or the blocking phenomenon caused by the finite sizes and surficial effects in—Vx-MnCo (x ≤ 0.4) MCs. Moreover, the presence of such a peak could be explained based on the Hopkinson effect, a competing effect between the applied magnetic field and the magneto-crystalline anisotropy that varies with the temperature. Like what was reported in the above M-H part, the impact of domain wall pinning could also influence the shape of MZFC(T) curves. Indeed, by heating the specimen with a low magnetic field application, the domain walls’ mobility could be raised. This means that the domain wall pinning effect will be diminished with the increase in temperature. Hence the walls could be moved easily toward the direction of the applied magnetic field, which will provoke a small increment in the magnetization.

Above Tc, the product will be demagnetized; hence, the magnetization drops to almost zero at Tc. On the other hand, the thermal agitation will tend to increase as the temperature increases, leading to a reduction in magnetization. Nevertheless, at T < Tc, the rotation of easy magnetization could be more dominant than the thermal agitations, leading to a global rise in magnetization.

Consequently, a maximum peak is detected in the MZFC(T) curves below Tc44. Furthermore, it was evident that Tc shifts progressively towards higher temperatures with increasing “x”. This could be correlated with the slight increment of crystallites/grains size as the vanadium content increases45. The variations in the critical temperatures Tsp and Tp as a function of vanadium content are shown in Fig. 9. It is noticed that Tp and Tsp shift gradually towards higher temperatures with increasing “x”. For instance, Tp is about 27.9 K for non-doped MCs, which has enhanced steadily from 28.7, 29.3, 29.7, to 30.0 K for x = 0.1, 0.2, 0.3, and 0.4 compositions, respectively.

Figure 9
figure 9

Variations in Tp and Tsp temperatures versus vanadium content.

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Hydrogen evolution reaction studies

The electrochemical performance of bare and various concentrations of Vx-MnCo (x ≤ 0.4) MCs towards HER were assessed in 0.5 M H2SO4 electrolyte with a three-electrode configuration. The LSV measurements were carried out to investigate the electrocatalytic activity of representative electrodes. Figure 10a depicts the LSV acquired by scanning the samples within 0 to −0.5 V versus RHE at a 10 mV/s scan rate. It is worth mentioning that Vx-MnCo (x = 0.3) MCs electrocatalyst has the lowest onset potential (−78.9 mV), making it the most efficient catalyst among Vx-MnCo MCs. The designated Vx-MnCo (x = 0.3) MCs exhibits remarkable HER activity in the overpotential of merely 85.9 mV to offer 10 mA cm−2 current density among other different composition of Vx-MnCo MCs. These results revealed that Vx-MnCo (x = 0.3) MCs performed very well as an HER electrocatalyst which could be attributed to the deprotonation of water molecules liberating a significant number of H species the Vx-MnCo MCs readily absorb. Afterwards, it establishes chemical connections with nearby electrons or adsorbed hydrogen (Hads), and both of these processes coincide, resulting in the formation of H2 molecules. The remaining samples exhibited relatively low performance, possibly due to the fewer available active sites for the adsorption of hydrogen species and reduced electrochemical surface area. V doping is crucial to achieving enhanced HER because adding V increases the electrochemical surface area and results in many defective sites, which are advantageous for the rapid transfer of charges. Figure 10b illustrates a comparison plot of normalized overpotential values at 10 mA/cm2 for several samples. The lowest overpotential value of 85.9 mV exhibited by the Vx-MnCo (x = 0.3) MCs electrocatalyst demonstrates the effectiveness of the composite in catalyzing HER (Table 2).

Figure 10
figure 10

(a) LSV curves of Vx-MnCo (x ≤ 0.4) MCs; (b) comparison plot between different catalysts of Vx-MnCo MCs and their respective normalized overpotentials at the current density of 10 mA/cm2; (c) CV curves recorded in non-Ohmic region of −0.25 to −0.35 V versus RHE at different scan rates for Vx-MnCo MCs (x = 0.3); (d) current density versus scan rate plot to estimate the value of Cdl for all the prepared samples.

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Table 2 HER parameters for different catalysts of Vx-MnCo (x ≤ 0.4) MCs electrodes.
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Electron spectroscopy for chemical analysis (ESCA) is a significant activity parameter relating to electrocatalytic performance, such as double layer capacitance (Cdl) and the specific capacitance (Cs)46. To confirm the assumption mentioned above for higher activity of Vx-MnCo (x = 0.3) MCs, the ECSA was measured for bare and Vx-MnCo MCs electrodes. The following Eq. (2) was utilized to calculate the ECSA.

$$\mathrm{ECSA }=\mathrm{ Cdl }/\mathrm{ Cs}$$
(2)

Cdl values were derived using the non-Faradaic potential region in CV curves, and Cs were assumed to be 35 µF cm−2 47. Figure 10c depict the CV curves obtained for the Vx-MnCo (x = 0.3) MCs electrocatalyst at scan speeds ranging from 10 to 120 mV/s in the potential range of −0.25 to −0.35 V versus RHE. Figure 10d illustrates the linear fit achieved by fitting the current density at a potential of −0.30 V versus RHE against several scan rates. The slope of these straight lines (Fig. 10d) was used to determine the value of Cdl. The ESCA and Cdl values for all the prepared samples of Vx-MnCo MCs have been displayed in Table 2. The electrocatalyst Vx-MnCo (x = 0.3) MCs exhibited outstanding performance with an ECSA value of 11.4 cm2. To further examine the catalysts' number of active sites (N), the following Eq. (3) was utilized to calculate,

$$\mathrm{N }=\mathrm{ Qs }/\mathrm{ F}$$
(3)

where Qs is the surface charge density was derived by integrating the charge of each CV curve over the whole potential range, the half value of the charge was obtained as the Qs values, and F is the Faraday constant (96,485 C mol−1)48. According to the Qs values, the calculated number of active sites for all the prepared samples of Vx-MnCo (x ≤ 0.4) MCs were 1.31 × 10–7, 1.35 × 10–7, 1.77 × 10–7, 2.41 × 10–7, and 1.82 × 10–7 mol cm−2, respectively. This result confirms that the electrocatalyst Vx-MnCo (x = 0.3) MCs has more active sites for adsorbing the hydrogen molecules responsible for HER.

The kinetics of the electrocatalytic process was investigated using Tafel analysis, which relates the connection between potential and current density. The corresponding Tafel equation, as shown in Eq. (4)

$$\upeta =\mathrm{ a }+\mathrm{ b}\times \mathrm{log}(\mathrm{j})$$
(4)

where Tafel constant, a; the Tafel slope, b; the current density, j; overpotential, η47. Figure 11a illustrates Tafel plots for Vx-MnCo MCs, with corresponding slopes of 131, 115, 103, 84, and 96 mV/dec, respectively. Vx-MnCo MCs (x = 0.3) electrocatalyst exhibited Tafel slope values as low as 84 mV/dec, further supporting the assumption that this material adheres to the Volmer–Heyrovsky rate-determining step in HER. The values of the Tafel slopes represent the reaction route encountered during the catalytic performance. An initial stage also called the Volmer reaction, involves the hydrogen atom absorbing a discharged proton off the surface of the electrode. In the Heyrovsky process, Hads will combine with a proton and an electron to produce new hydrogen molecules. The Tafel route suggests recombining two Hads species yielding a hydrogen molecule. The Volmer–Heyrovsky pathway consists of the Volmer and Heyrovsky processes and is the most prevalent mechanism for the HER to proceed48,49. The calculated Tafel slope of 84 mV/dec for the most effective electrocatalyst Vx-MnCo MCs (x = 0.3) signals that the reaction proceeds via the rate-determining Volmer–Heyrovsky step.

Figure 11
figure 11

(a) Tafel plot for Vx-MnCo (x ≤ 0.4) MCs; (b) Nyquist plot for all the prepared samples; (c) LSV curves of Vx-MnCo (x = 0.3) MCs swept before and after 1000 CV cycles; (d) CP curve of Vx-MnCo MCs (x = 0.3) MCs observed for a prolonged period of 36 h.

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The intense electrocatalytic activity presented by the small semicircle generally shows fast electron transfer at the catalyst electrolyte interface. As illustrated in Fig. 11b, the EIS analysis was conducted to understand the catalyst's efficiency in transferring charges across the interface in 0.5 M H2SO4 electrolyte solution. The Nyquist plot of Vx-MnCo MCs with an η of 85.9 mV and a frequency range of 0.1 Hz to 105 Hz is depicted in Fig. 11b. Vx-MnCo MCs (x = 0.3) electrocatalysts exhibited a lower charge transfer resistance than other catalysts, indicating that it significantly enhanced the HER performance, demonstrating a better ability to transfer electrons at the electrocatalysts/electrolyte interface.

Figure 11c depicts the LSV used to evaluate the stability of the Vx-MnCo (x = 0.3) MCs electrocatalyst before and after CV analysis. As seen in Fig. 11c, after 1000 cycles of CV study, the LSV curve of Vx-MnCo (x = 0.3) MCs electrocatalyst demonstrates almost a change of 4 mV in the overpotential, which may be due to the shredding of material from the electrode surface caused by the bubble formation. CP analysis was utilized to further confirm the long-term stability of the electrocatalyst by delivering a constant current of -10 mA/cm2 for 36 h in an acidic solution, illustrated in Fig. 11d. The little disruption observed in the CP curve of Vx-MnCo (x = 0.3) MCs is due to the formation and concentration of bubbles during H2 release at the electrode surface. However, the electrocatalyst exhibited excellent potential value retention even after 36 h of electrocatalysis at a current density of −10 mA/cm2. Following the stability tests, the structure, morphology, and composition of the Vx-MnCo (x = 0.3) MCs were re-evaluated (Fig. 12). However, the morphology was exhibited chunky of highly agglomerated cubic particles.

Figure 12
figure 12

(a) XRD spectrum; (b) EDX spectrum and SEM image of Vx-MnCo (x = 0.3) MCs after stability test.

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Furthermore, no significant changes in the structure and composition were observed. This provides conclusive evidence that Vx-MnCo (x = 0.3) MCs electrocatalysts are highly stable. These findings demonstrate that Vx-MnCo (x = 0.3) MCs is a high-performance HER electrocatalyst suitable for future hydrogen fuel cell technology. The overpotential comparison of electrocatalytic HER performance of Vx-MnCo (x = 0.3) MCs with other HER catalysts reported in the literature is listed in Table 3.

Table 3 The overpotential comparison of electrocatalytic HER performance of MnVxCo2−xO4 with other HER catalysts in an acidic (0.5 M H2SO4) medium was reported in literatures.
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Conclusion

In summary, Vx-MnCo (x ≤ 0.4) MCs were synthesized effectively using a simple hydrothermal process. These sample' structure, morphology, magnetic properties, and their possible usage as efficient HER electrocatalysts were investigated. Both structural and morphological analyses done by XRD, HR-TEM and TEM confirmed the formation Vx-MnCo (x ≤ 0.4) MCs with minor impurity of MnO2. The different products display a paramagnetic behaviour at higher temperatures and a ferrimagnetic behaviour at lower temperatures. It is detected that the Curie temperature (Tc) and peak temperature in MZFC curves increase with the rise in vanadium concentration. From both M-H and M-T results, an increase in the global magnitude of magnetization is detected with decreasing the temperature, which is mainly correlated with the reduction in thermal agitation. M-H results show that Mmax (± 70 kOe) values at both 300 and 10 K and Mr value increase as the vanadium content increases. The Hc value increases with increasing "x" up to 0.2, then drops with a further rise in “x” above 0.2. The Vx-MnCo (x = 0.3) MCs electrode has outstood HER activity much better than its other counterparts. The optimized catalyst Vx-MnCo (x = 0.3) MCs electrode requires a low onset potential of 78.9 mV, low overpotential of 85.9 mV at 10 mA/cm2, and a slight Tafel slope of 84 mV/dec in 0.5 M H2SO4 electrolyte. The excellent performance of Vx-MnCo (x = 0.3) MCs is due to the increased ECSA and electron transport capacity at the electrode/electrolyte interface.

These results revealed that Vx-MnCo (x = 0.3) MCs are a promising candidate electrode for the electrochemical HER in large-scale commercial hydrogen fuel cell applications. Further, long-term testing has shown that this electrode keeps high quality even after 1,000 CV cycles and 36 h of CP testing. Also, with the magnetic property advantage, the magnetic field-induced electrochemical catalytic property of these catalysts may provide a new perspective for the field of magnetoelectrochemistry in future.