Eco-friendly mixed metal (Mg–Ni) ferrite nanosheets for efficient electrocatalytic water splitting

Eco-friendly and cost-effective catalysts with multiple active sites, large surface area, high stability and catalytic activity are highly desired for efficient water splitting as a sustainable green energy source. Within this line, a facile synthetic approach based on solventless thermolysis was employed for the simple and tunable synthesis of Ni1−xMgxFe2O4 (0 ≤ x ≤ 1) nanosheets. The characterization of nanosheets (via p-XRD, EDX, SEM, TEM, HRTEM, and SAED) revealed that the pristine ferrites (NiFe2O4 and MgFe2O4), and their solid solutions maintain the same cubic symmetry throughout the composition regulation. Elucidation of the electrochemical performance of the nanoferrite solid solutions showed that by tuning the local chemical environment of Ni in NiFe2O4 via Mg substitution, the intrinsic catalytic activity was enhanced. Evidently, the optimized Ni0.4Mg0.6Fe2O4 catalyst showed drastically enhanced HER activity with a much lower overpotential of 121 mV compared to the pristine NiFe2O4 catalyst. Moreover, Ni0.2Mg0.8Fe2O4 catalyst exhibited the best OER performance with a low overpotential of 284 mV at 10 mA/cm2 in 1 M KOH. This enhanced electrocatalytic activity could be due to improved electronic conductivity caused by the partial substitution of Ni2+ by Mg2+ in the NiFe2O4 matrix as well as the synergistic effect in the Mg-substituted NiFe2O4. Our results suggest a feasible route for developing earth-abundant metal oxide-based electrocatalysts for future water electrolysis applications.


Synthesis of Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) solid solutions
The Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) solid solutions of different stoichiometric compositions were prepared by solventless thermolysis of metal acetylacetonates.Briefly, for the synthesis of NiFe 2 O 4 nanoparticles, 0.10 g (0.39 mmol) of nickel acetylacetonate and 0.27 g (0.78 mmol) of iron acetylacetonate were mixed to form a solid mixture.The solid mixture was ground using a pestle and mortar for ≈ 15 min to homogenize the mixture.The precursor mixture was then placed into a ceramic boat, which was placed in a reactor tube.The reactor tube was then placed inside the carbolite tube furnace, followed by thermal treatment at 450 °C, at a heating rate of 20 °C per minute for 1 h.After 1 h of annealing, the heating was switched off, and the furnace was allowed to cool to room temperature.The reactor tube was taken out of the furnace upon cooling, and the product was collected for analysis without any post-treatment.Similarly, the synthesis of MgFe 2 O 4 nanoparticles was achieved by employing similar procedures except that magnesium acetylacetonate was used instead of nickel acetylacetonate and the amount of magnesium and iron complexes were maintained in the same mole ratio of 1:2.
For the synthesis of Ni 1−x Mg x Fe 2 O 4 (x = 0.2, 0.4, 0.6, 0.8) solid solutions, a known quantity of nickel acetylacetonate was partially substituted by appropriate amounts of magnesium acetylacetonate by adjusting the mole ratios of Mg and Ni in the intervals of 0.2, 0.4, 0.6, and 0.8, while keeping the amount of iron acetylacetonate unchanged in the reaction mixture.The reaction procedures for the entire series of solid solutions were kept similar to those employed to synthesize the ternary nickel and magnesium ferrites.

Characterization of the Ni 1−x Mg x Fe 2 O 4 nanocatalysts
Structural analysis of the Ni 1−x Mg x Fe 2 O 4 nanoparticles was ascertained by powder X-ray diffraction (p-XRD) analysis employing a Bruker AXS D8 Advance X-ray diffractometer.The instrument uses nickel-filtered Cu Kα radiation (λ = 1.5418Å) at 40 kV, 40 mA.SEM imaging was carried out on a ZEISS-Auriga Cobra SEM Field Emission Scanning Electron Microscope (FE SEM) while EDX elemental analysis was performed on a JEOL JSM-7500F Field Emission Scanning Electron Microscope (FE-SEM) equipped with Energy Dispersive X-ray spectroscopy (EDX).The SAED, TEM and HRTEM analyses were performed on a JEOL 2100 HRTEM at accelerating voltages of 200 kV.

Electrochemical characterization
The electrocatalytic property of the Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) was examined via a three-electrode system using a Versastat 4-500 electrochemical workstation (Princeton Applied Research, Oak Ridge, TN, USA).For the preparation of the working electrode, the electrode paste was synthesized using Ni 1−x Mg x Fe 2 O 4 material (80%), PVDF (10%), carbon black (10%) with N-methyl pyrrolidinone (NMP) solvent as active materials, binder, and conducting agent, respectively.The paste was dipped into the clean Ni foam and dried for 48 h.While Pt wire was used as a counter electrode, Hg/HgO was used as the reference electrode.To examine the performance of the electrocatalyst for HER and OER, linear sweep voltammetry (LSV) was carried out at a scan rate of 2 mV/s.Also, electrochemical impedance spectroscopy (EIS) was performed at the potential of 0.6 V (V, SCE) in the frequency range of 0.05 Hz-10 kHz at an applied AC amplitude of 10 mV.For the stability of electrocatalysts, chronoamperometry techniques were utilized at the potential of 0.57 V (V, SCE).All measurements for electrocatalysts were conducted using 1 M KOH electrolyte.

Results and discussion
Structural analysis of Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) solid solutions A set of Ni 1−x Mg x Fe 2 O 4 nanocatalysts were synthesized by a direct solid-state thermolysis process.Figure 1a shows the typical powder X-ray diffraction (p-XRD) patterns of the as-prepared Ni 1−x Mg x Fe 2 O 4 .The diffraction peaks found in the pure ternary systems prepared at x = 0 and x = 1 are exclusively indexed with the cubic spinel crystal system having the space group Fd3̅ m.The pristine ferrites are consistent with the cubic phases of pure trevorite, NiFe 2 O 4 (ICDD # 01-086-2267) and magnesioferrite, MgFe 2 O 4 (ICDD # 01-089-3084) for x = 0 and x = 1, respectively.The p-XRD data for the nanoferrites with x = 0.2 to x = 0.8 indicate the formation of solid solution phases with variable stoichiometric composition of Ni 2+ and Mg 2+ in the spinel matrix.Notably, their diffraction peaks lie in between those of pure NiFe 2 O 4 and MgFe 2 O 4 , and these solid solutions maintained the same cubic symmetry throughout the composition regulation.
The lattice constants (a = b = c) of Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) nanospinels were ascertained from the p-XRD data by employing the formula shown by Eq. ( 1) and the results are shown in Table 1.
The lattice constants of NiFe 2 O 4 were found to be 8.313 Å, conforming to those reported in the standard data (8.337Å, ICDD #: 01-086-2267).After the incorporation of Mg 2+ , a slight increase in the values of the lattice parameters is observed, which is also ascribed to the slightly larger size of Mg 2+ (0.72 Å) relative to the Ni 2+ (0.69 Å) 45 .The lattice parameters computed for pure MgFe 2 O 4 (8.344Å) are also comparable with the values reported in standard data (8.369Å, ICDD #: 01-089-3084).The values of lattice parameters were then plotted as a function of Mg 2+ content (x) as shown in Fig. 1b.It is obvious that the lattice constant increases in a linear fashion with Mg 2+ inclusion from 8.313 Å for NiFe 2 O 4 to 8.344 Å for MgFe 2 O 4 .This linear relationship between the lattice parameters and Mg 2+ content is in agreement with Vegard's law 46 .The values of the lattice constants (1)     47 .Likewise, the data in Table 1 and Fig. 1b demonstrate that the cell volume increases monotonically with increasing magnesium content.All these findings confirm the successful inclusion of Mg 2+ into the crystal structure of NiFe 2 O 4 .The Debye-Scherrer formula (Eq.2), Ref. 48as employed to compute the average crystallite size of Ni 1−x Mg x Fe 2 O 4 samples.
In the formula, L = average crystallite size, λ = X-ray wavelength, β = full width at half maximum, and θ = Bragg's angle of the (311) plane.The average crystallite sizes of the as-prepared Ni 1−x Mg x Fe 2 O 4 nanoparticles vary between 10 and 20 nm (Table 1).The average crystallite size obtained for the pristine nickel ferrite was larger compared to those exhibited by magnesium-substituted samples.

Elemental compositional analysis
The composition and elemental distributions of Ni, Mg, Fe, and O were analyzed by energy-dispersive X-ray (EDX).The EDX results (Supplementary Information, Fig. S1) indicate the presence of Ni, Fe and O for x = 0, and Mg, Fe and O for x = 1 in the desired ratio.For the solid solution nanocrystals with compositions x = 0.2-0.8, the presence of Ni, Mg, Fe, and O was confirmed.A summary of the atomistic composition of each of the components in the alloyed nanoferrites is provided in Table S1.The stoichiometry of the elements obtained is consistent with the expected values within the substitution limits, suggesting that there is no side reaction or significant loss of the starting materials.In Fig. 2, the relationship between the amount of Ni 2+ and Mg 2+ detected from EDX with respect to the mole fraction of [Mg]/[Mg + Ni] in precursor feed indicates a decrease in nickel content with a linear increase in magnesium content.In addition, the EDX mapping of the as-prepared Ni 1−x Mg x Fe 2 O 4 solid solutions is given in Fig. 3, indicating that the distribution of the respective elements in the spinel structure is nearly uniform, ruling out the possibility of de-alloying or phase segregation.This also confirms the formation of the solid solution between NiFe 2 O 4 and MgFe 2 O 4 in the single-crystalline alloyed nanospinel.

Microstructure and morphological studies
.In a similar study, CoFe 2 O 4 exhibited 370 mV under similar electrolytic conditions.Hirai et al. reported that Mn 3 O 4 needed an overpotential of 600 mV to produce a current density of 10 mA/cm 2 in 1 M KOH solution.They further reported the synthesis of Mn 2.4 Co 0.6 O 4 which exhibited a high overpotential of 510 mV 56 .Also, Co 3 O 4 nanocubes fabricated by Chen et al. were reported to display an overpotential of 580 mV (at 10 mA/cm 2 ) in alkaline electrolytes 57 .Table S3    The diameter of the semicircle obtained at a lower frequency provides information on the ionic resistance of the electrolyte, indicating series resistance (R s ) with charge transfer resistance R ct .The R s for all the samples studied were the same, while R ct depends on the composition.The simplest equivalent circuit for these samples is where one resistance R s in series with C dl and R ct parallel to the C dl .The composition of Ni 1−x Mg x Fe 2 O 4 with X = 0 showed the highest charge-transfer resistance while with X = 0.8 displayed the lowest charge-transfer resistance.
Evaluation of the catalyst's electrochemical stability is important for practical water splitting applications.To explore the electrochemical stability of Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) electrocatalysts, chronoamperometry measurements were performed at 0.55 V. Remarkably, no significant changes in the current density were observed during 17 h tests, signifying excellent electrochemical stability of all Ni 1−x Mg x Fe 2 O 4 systems in the alkaline electrolyte (Fig. 5f).The stability of Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) electrocatalysts was further examined by continuous LSV scans in 1 M KOH.The results indicate the absence of significant change in the polarization curve after 1000 cycles, signifying superior stability of the nanocatalysts for both HER (Fig. 6a,b, and Fig. S3) and OER (Fig. 6c,d, and Fig. S4) in alkaline solution.

Conclusion
In conclusion, this study reports a composition-controlled fabrication of homogeneous Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) solid solutions by solventless pyrolysis method.Experimental investigation demonstrates that by regulating the molar composition of Mg and Ni in the preparation process, the physicochemical and electrochemical performance of the material were modified.The as-synthesized Ni 0.4 Mg 0.6 Fe 2 O 4 nanoparticles exhibited the best electrocatalytic activity for HER with an overpotential of only 121 mV which is much smaller compared to its analogues, at a current density of 10 mA/cm 2 and the electrode exhibits good stability during long-term electrolysis.Meanwhile, Ni 0.2 Mg 0.8 Fe 2 O 4 showed the best OER activity, requiring an overpotential of 284 mV to deliver the same current density within the window of potential examined.The outstanding electrocatalytic performance of these solid solutions is largely ascribed to the enhanced conductivity due to surfactant free surfaces, nanoparticulate nature and synergic effect of different metals (Mg, Ni and Fe) which either directly or indirectly

Figure 2 .Figure 3 .
Figure 2. Change in Mg and Ni content as a function of mole fraction of [Mg]/[Mg + Ni] in precursor feed.

Figure 4 .
Figure 4. TEM and SAED (insets) images of Ni 1−x Mg x Fe 2 O 4 solid solutions over the entire range, by solid-state decomposition of metal acetylacetonate complexes.
shows the comparison of the values of overpotentials Ni 0.2 Mg 0.8 Fe 2 O 4 with other non-precious metal catalysts.The values of the Tafel slope indicated in Fig. 5d were obtained in the range of 54-112 mV/dec.The low overpotential and small Tafel slope make Ni 0.2 Mg 0.8 Fe 2 O 4 a more promising OER catalyst.
13:22179 | https://doi.org/10.1038/s41598-023-49259-ywww.nature.com/scientificreports/The electrical conductivity of Ni 1−x Mg x Fe 2 O 4 (0 ≤ x ≤ 1) nanocatalyts was elucidated by electrochemical impedance spectroscopy.The Nyquist plot displayed in Fig. 5e shows that the pristine NiFe 2 O 4 nanoparticles possess a large semicircle, demonstrating poor electron transfer capability, compared to Ni 1−x Mg x Fe 2 O 4 (0.2 ≤ x ≤ 0.8) solid solutions.The lowest charge resistance values displayed by the solid solutions imply intimate contact between the current collector and Ni 1−x Mg x Fe 2 O 4 and is an indication of more swift charge transfer kinetics.The results confirm further that the incorporation of Mg in spinel NiFe 2 O 4 lattices contributed to the improvement of electrical conductivity via reduction of the charge transfer resistance, and consequently boosting the electrocatalytic properties of Ni 1−x Mg x Fe 2 O 4 electrodes.Of all electrode configurations investigated, Ni 0.2 Mg 0.8 Fe 2 O 4 shows the smallest semicircle, indicating superior conductivity, and hence high electrocatalytic activity towards water splitting.

Figure 6 .
Figure 6.Comparison of HER (a, b) and OER (c, d) polarization curve between LSV 1 curve and LSV 1 k curve for Ni 1−x Mg x Fe 2 O 4 (x = 0 and 0.6) electrodes.

Table 1 .
Lattice parameter (a), crystallite size (d), unit cell volume (V), and EDX composition of nanospinel Ni 1−x Mg x Fe 2 O 4 solid solutions at various magnesium contents (x).