Microwave-assisted synthesized Co-doped Fe₃O₄ nanoparticles for superior electrocatalytic water splitting

Abstract

The inherent catalytic efficiency of Fe₃O₄ nanoparticles (Fe₃O₄ NPs) for water splitting is unsatisfactory owing to its limited electronic conductivity and inadequate active sites necessary for both oxygen and hydrogen evolution processes (OER and HER). These issues have prompted the investigation of diverse approaches for Fe₃O₄ NPs, including doping with transition metals. Herein, the Co-doped Fe₃O₄ NPs were loaded on Ni-foam using a microwave-assisted method, and characterized by various analytical techniques. The electrochemical investigations demonstrated that Co-doped Fe₃O₄ NPs exhibit exceptional OER and HER performance, with minimal overpotentials of 146 mV and 210 mV at a current density of 10 mA/cm², in contrast to Fe₃O₄ NPs, which showed overpotentials of 278 mV and 245 mV at the same current density. Theoretical investigations indicated that Co-doping substantially altered the electronic structure and optimised the active sites of Fe₃O₄ NPs, hence enhancing overall catalytic efficiency. This study presents an innovative approach for the synthesis of highly efficient, economical electrocatalysts for water splitting, with potential applications in clean energy generation and sustainable hydrogen production.

Introduction

The increasing global demand for clean and sustainable energy has driven significant interest in the development of efficient technologies for water splitting to generate hydrogen, a clean fuel with high energy content [1]. One of the most promising ways to make hydrogen is through electrocatalytic water splitting, which includes the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) [2]. However, the need for cheap and efficient electrocatalysts remains a key challenge in making this process economically viable. In this context, the design and optimization of electrocatalysts play a critical role in enhancing the overall efficiency of the water splitting reaction [3]. Iron oxide-based materials, particularly magnetite (Fe₃O₄), have garnered attention as promising candidates for water splitting due to their abundance, low cost, and eco-friendly nature [4–6].

Fe₃O₄ NPs possess electrochemical properties that make them promising electrocatalysts. Fe₃O₄ NPs have a mix of iron ions, Fe²⁺ and Fe³⁺, which lets them participate in redox reactions [7]. The mixed oxidation state makes electron transfer possible and improves the capability of the material to catalyse the water-splitting reaction [8]. However, the intrinsic catalytic ability of Fe₃O₄ NPs for water splitting is still not very favourable because of their low electronic conductivity [9], limited active sites, and relatively high overpotential for both the OER and HER. Because of these issues, several strategies to improve the performance of Fe₃O₄ NPs have been explored [10]. Doping Fe₃O₄ NPs with transition metals results in a notable enhancement of their catalytic function. Among the various additives, adding cobalt (Co) changes the electronic and structural features of the Fe₃O₄ NPs, which boosts their water splitting ability. Co-doping is also known to make other transition metal oxides more stable and reactive, which can lead to better OER and HER activity [11–15]. Moreover, adding Co to Fe₃O₄ NPs increases the number of active spots on the catalyst’s surface, which helps water molecules and reaction intermediates work together better, making HER and OER more effective [16, 17]. Although a cornucopia of reports on Co-doped Fe₃O₄ NPs for electrocatalytic water splitting exists, realising their full potential in terms of electrocatalytic water splitting performance has remained a challenge. Unlocking such untapped potential requires a fine optimization of preparation strategies, as they significantly influence the electronic structure, morphology, and surface properties of the Co-doped Fe₃O₄ NPs [8]. Changing the way Co-doped Fe₃O₄ NPs are made can boost their natural ability, leading to significant gains in how well they work and how stable they are. The future of this material lies in innovative synthesis path manipulations that will lead to the creation of highly efficient and robust catalysts for sustainable water-splitting applications [18, 19].

This study aims to create Co-doped Fe₃O₄ NPs using a microwave method and to assess how well they work for electrocatalytic OER and HER. The microwave-assisted approach allows for the rapid and controlled synthesis of Fe₃O₄ NPs, resulting in a uniform size distribution with high surface area, which can contribute to their superior catalytic activity. The electrocatalytic performance of the Co-doped Fe₃O₄ NPs was evaluated towards both OER and HER, demonstrating their remarkable performance in alkaline media. Further, using the theoretical calculations, the influence of Co doping on the structural, electronic, and electrochemical properties of Fe₃O₄ NPs was investigated, thus providing insight of potential of Co-doped Fe₃O₄ as an efficient electrocatalyst for both OER and HER. The results presented herein will contribute to the understanding of the underlying mechanisms driving the enhanced activity of Co-doped Fe₃O₄ and offer a promising approach for designing high-performance catalysts for water splitting.

Experimental section

Materials

The materials, including iron (III) chloride hexahydrate (FeCl₃·6H₂O), cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), sodium hydroxide (NaOH), ethanol, and Ni-foam were purchased from Sigma Aldrich. All the chemicals were used without any further purification.

Preparation of Fe₃O₄ NPs

For the microwave-assisted synthesis of Fe₃O₄ NPs supported on Ni foam (2 × 2 cm²) [20], a precursor solution was made by dissolving 2.7 g of iron(III) chloride hexahydrate (FeCl₃·6H₂O) in 100 mL of deionized water. A 1 M sodium hydroxide (NaOH) solution was made and its 10 mL was slowly added to the FeCl₃ solution with constant stirring to induce precipitation. After 2 × 2 cm² Ni foam was soaked in the precursor solution for the uniform deposition of Fe₃O₄ nanoparticles, the resultant mixture was subjected to 600 W microwave irradiation for 10 min. This led to heat rapidly and Fe₃O₄ NPs deposition on the surface of Ni foam. Upon irradiation, the Fe₃O₄ nanoparticles-supported Ni foam was taken out from the microwave synthesizer and washed several times with deionized water and ethanol to eliminate excess reagents and dried in 60 °C to get the final Fe₃O₄ NPs@ Ni Foam.

Preparation of Co-doped Fe₃O₄ NPs

In microwave-assisted synthesis of Co-doped Fe₃O₄ NPs on nickel foam (2 × 2 cm² Ni foam) [21, 22], 2.7 g of iron(III) chloride hexahydrate (FeCl₃·6H₂O) and 1.0 g of cobalt(II) chloride hexahydrate (CoCl₂·6H₂O) were dissolved in 100 mL deionized water to create a mixed precursor solution. A 1 M sodium hydroxide (NaOH) solution was then prepared, and 10 mL of it was gradually added to the precursor solution under constant stirring to cause precipitation. The obtained solution was then used to impregnate the Ni foam of 2 × 2 cm² in dimensions to achieve equal deposition of the Co-doped Fe₃O₄ nanoparticles. The mixture was also irradiated for 10 min at 600 W in a microwave synthesizer where immediate heating occurred through microwave energy leading to the formation of Co-doped Fe₃O₄ NPs on the surface of Ni foam. The resulting nickel foam was thoroughly washed with deionized water and ethanol to remove free reagents and dried at a temperature of 60 °C to get the final Co-doped Fe₃O₄@ Ni foam. Figure 1 showed a schematic illustration of the synthesis of Co-doped Fe₃O₄ NPs for electrocatalytic water splitting.

Fig. 1.

A schematic illustration of the synthesis of Co-doped Fe₃O₄ NPs for electrocatalytic water splitting

Physical characterization

Characterization of the synthesized materials was carried out by X-ray diffraction (XRD) in the 2-theta range of 10–80 degrees using LabX XRD-6000 Shimadzu X-Ray diffractometer with Cu Kalpha source (lambda = 1.54 Å). Field emission scanning electron microscope (FESEM: SU 5000-Hitachi) was used for imaging morphology, elemental mapping, and energy dispersive X-ray analysis (EDAX, Oxford). Electrochemical measurements were performed on a CHI 663 A electrochemical workstation. A multipurpose electrochemical performance study in a three-electrode testing system was conducted in identical experimental conditions using 1 M KOH solution to examine the OER and HER activity of the prepared materials. This three-electrode system consisted of a saturated calomel electrode as reference electrode, a graphite rod as counter electrode, and the prepared materials as working electrode. The electrocatalytic OER and HER activity of the prepared materials was assessed using linear sweep voltammetry (LSV), Tafel slope analysis, and turnover frequency (TOF) calculations. We used cyclic voltammetry (CV) to further assess the cyclic stability of the prepared materials during OER and HER. Cyclic stability means how well the important factors like current density or overpotential are maintained after a certain number of cycles, showing how long the catalyst lasts under redox conditions. Therefore, we recorded and compared the LSV curves before and after 1000 CV cycles to evaluate the stability of the prepared materials.

Theoretical studies

First principles calculations based on density functional theory (DFT) using the plane-wave pseudopotential route were carried out on the set models of Fe₃O₄ and Co-doped Fe₃O₄ with a 2 × 2 × 1 supercell using the Quantum ESPRESSO (QE) program. The PBE functional, which is the Perdew-Burke-Ernzerhof function, in the GGA approximation was used to describe the exchange-correlation energy. Ultrasoft pseudopotentials were used to describe the core-electron interactions. The optimization of all structures was performed until the force on one atom was less than 10^− 4 Ry/Bohr. The threshold for convergence of atomic forces was set to 0.03 eV/Å with an energy threshold being set to 10^− 5 Ry for all geometrical optimization. The Brillouin zone was sampled using an 8 × 10 × 6 k-point grid generated according to the Monkhorst-Pack scheme ensuring sufficient accuracy for total energy estimations. After convergence testing, a 250 Ry plane-wave cutoff and a 460 Ry charge density cutoff were chosen [23, 24].

In order to understand the states of spin polarization, density of states was performed. In addition, the projected DOS was calculated to analyze the contribution of specific atomic orbitals to the electronic structure. Charge density maps were computed to analyze the electronic distribution within the material, while emphasizing the impact of structural symmetry on the electronic properties. The charge density difference was calculated using the following formula (Eq. 1).

1

The catalytic activity of Fe₃O₄, and Co-doped Fe₃O₄ materials was assessed by computing the Gibbs free energy (ΔG) for the OER and HER based on adsorption energies.

Results and discussion

Characterization of the prepared Fe₃O₄, and Co-doped Fe₃O₄ NPs

The morphologies, surface properties, and elemental composition of the prepared Fe₃O₄ and Co-doped Fe₃O₄ NPs are investigated via FE-SEM and energy-dispersive X-ray (EDX) spectroscopy. The SEM picture of Fe₃O₄ NPs clearly indicated their uniform spherical morphology, having the particle size ranging from 10 to 20 nm with a very high percentage of monodispersed NPs [25]. Moreover, the prepared Fe₃O₄ NPs were agglomerated in lumps even though individually distinguishable, suggesting that the microwave-assisted synthesis route was effective in limiting growth while encouraging aggregation of particles under the process conditions (Fig. 2a–b.

Fig. 2.

a–b FESEM images, and c EDX spectrum of Fe₃O₄ NPs. d FESEM image, and e EDX spectrum of Co-doped Fe₃O₄ NPs. f XRD pattern for the Fe₃O₄, and Co-doped Fe₃O₄ NPs

The EDX spectrum (Fig. 2c) clearly indicated that the elemental composition was mostly Fe and O in line with the phase of Fe₃O₄, and no further peaks of major size, signifying the authenticity of the Fe₃O₄ NPs. Moreover, the atomic ratio of Fe to O confirmed by the EDX spectrum matched the expected stoichiometric ratio in Fe₃O₄, thereby giving yet another proof for its formation as Fe₃O₄ NPs. In a similar way, the SEM investigation of Co-doped Fe₃O₄ NPs indicated that its morphology was almost similar to Fe₃O₄ NPs with sizes of mostly between 12 and 25 nm (Fig. 2d). The increase in the size of Co-doped Fe₃O₄ NPs can be ascribed due to Co-doping, which has a higher ionic radius of Co²⁺ (0.745 Å) than that of Fe³⁺ (0.645 Å) [26]. The particles still have a well-defined and uniform shape with only minor aggregation, indicating that Co incorporation into Fe₃O₄ NPs did not influence the particle growth mechanism greatly but enhanced alongside stability for the Co-doped Fe₃O₄ NPs. The EDX spectrum of Co-doped Fe₃O₄ NPs confirmed the incorporation of Co ions successfully into the Fe₃O₄ lattice (Fig. 2e). The actual Co doping percentage in Co-doped Fe₃O₄ NPs was found to be 10–11%. Furthermore, the peaks of any possible phases of cobalt oxide, such as CoO or Co₃O₄ are absent in the EDX spectrum, which further confirmed that Co ions were incorporated into the Fe₃O₄ structure without separating Co-containing faces.

Further, to investigate the crystalline structure, phase purity, and structure transformations of Fe₃O₄ and Co-doped Fe₃O₄ NPs, X-ray diffraction (XRD) studies were conducted. The XRD pattern of Fe₃O₄ NPs (Fig. 2e) displayed the characteristic peaks at 2θ values of 30.2°, 35.5°, 43.2°, 53.6°, 57.1°, 62.8°, 72.2°, and 75.1° which can be ascribed due to the diffraction planes (220), (311), (400), (422), (511), (440), (533), and (731) respectively. These diffraction peaks matched with the standard XRD pattern of Fe₃O₄ indexed to face-centered cubic (FCC) crystal structure consistent with JCPDS card No. 19–0629. The richness of strong, sharp peaks indicated that the microwave-assisted synthesis conditions highly promoted the formation of a pure phase with minimum structural defects [27, 28]. Furthermore, none of the other peaks were present in the XRD pattern of Fe₃O₄ NPs, indicating their phase purity without any observable traces of other iron oxides or impurities. On the other hand, the changes in XRD peaks were significant when Co was doped to Fe₃O₄; that is, the characteristic peaks of the Fe₃O₄ NPs structure remained in the XRD pattern of Co-doped Fe₃O₄ (Fig. 2e) but showed some slight attributed shifting in few peaks (mentioned by * in the XRD pattern). The diffraction peaks are recorded in exactly the same 2θ values as pure Fe₃O₄ despite the slight shifting, and lowering in the intensity of the peaks. Such changes in position and intensity have been caused due to replacement of Fe³⁺ ions with Co²⁺ ions in the octahedral sites of the magnetite structure. Such substitutions are known to alter the lattice parameter and thus change the inter-planar spacing, resulting in subtle shifts of the diffraction peaks. The shifts were observed to have occurred most noticeably round (311) and (400) planes; this was attributed to the difference in ionic radii between Fe³⁺ (0.645 Å) and Co²⁺ (0.745 Å), indicating successful doping of Co into the structure of Fe₃O₄ [29]. The XRD pattern observed showed the expected match for Co-doped nanoparticles with those of Fe₃O₄, so according to the JCPDS card No. 19–0629 for Co-doped particles, both hold the same spinel structure. The Co-doping did not end up with the formation of any secondary phase, which means that Co was well incorporated in the Fe₃O₄ lattice. As a support for this assertion, no other peaks were shown in connection with cobalt oxide (CoO, JCPDS No. 47-1741) or with cobalt ferrite (CoFe₂O₄, JCPDS No. 22-1086) in the XRD patterns of the prepared NPs.

Electrocatalytic OER performance

The Fe₃O₄, being a mixed-valent iron oxide with spinel crystal structure, possesses catalytic value towards OER. Nevertheless, OER activity of Fe₃O₄ is equivalent to restricted overpotential, slow reaction kinetics, and inferior stability in comparison to more traditional catalysts such as RuO₂ and IrO₂. Co-doping of Fe₃O₄ can be promising approach to overcome these limitations. The doping with Co inserts Co ions into the Fe₃O₄ lattice, causing electronic structure and local coordination environment modification of iron centers, which results in significant promotion of electrochemical properties. In Co-doped Fe₃O₄, cobalt ions are generally in octahedral positions within the spinel lattice, promoting electronic conductivity and enhancing active site density for OER [30]. This structural anomaly also bestows cobalt with a good electronic structure for OER catalysis, hence improving electron transfer and lowering the energy barrier during OER. Overpotential is one of the parameters used to measure OER electrocatalyst efficiency. The electrochemical results indicated Co-doped Fe₃O₄ to have much lower value of overpotential of 146 mV compared to pure Fe₃O₄ at 278 mV, i.e., affirming the superior catalytic activity of Co-doped Fe₃O₄ NPs (Fig. 3a). The overpotential of 146 mV for Co-doped Fe₃O₄ is significantly below state-of-the-art benchmarks for non-precious metal-based OER catalysts, thus demonstrating a potential route towards an inexpensive yet efficient alternative for OER process.

Fig. 3.

a OER polarization curves, b Tafel slop, c TOF, and d OER polarization curves before and after 1000 cycles, for the prepared Fe₃O₄, and Co-doped Fe₃O₄ NPs

The Tafel slope is a very important parameter for the evaluation of the kinetics of reaction. The smaller the Tafel slope, the quicker the OER kinetics, and therefore an effective catalyst. The prepared Co-doped Fe₃O₄ NPs showed a Tafel slope of 39 mV/dec, which is significantly less than the 62 mV/dec for Fe₃O₄ NPs (Fig. 3b). The Large change in Tafel slope in Co-doped Fe₃O₄ NPs is the proof towards improved catalytic activity as well as rapid electronic transfer in the OER reaction. Improved kinetics of Co-doped Fe₃O₄ NPs can be attributed to augmented electronic conductivity and favourably influenced reaction path due to Co-doping [31]. Another important parameter in establishing the efficiency of these specific electrocatalysts is the turnover frequency, which is the number of oxygen molecules evolved per active site per unit time. The TOF measure increased for Co-doped Fe₃O₄ NPs compared to that for Fe₃O₄ NPs, which could signify the confirmation of the proposition that such Co-doping of Fe₃O₄ boosted the catalytic activity in OER (Fig. 3c). The improved TOF of Co-doped Fe₃O₄ can be attributed to the synergistic effect between Fe and Co. This leads to having more active sites and better electronic transfer through the OER process. Moreover, the stability is one of the key factors that most catalytic materials suffer from, when in the application of supplying the severe conditions under which they split water, with most catalysts ultimately lead to long term deactivation as a result of dissolution of one catalyst, restructuring of the surface, or the formation of some unwanted phases. The Co-doped Fe₃O₄ NPs showed remarkable stability, further demonstrated by their inverse OER performance after 1000 cycles. The comparison made of the results of cycles 1 and 1000 show that Co-doped Fe₃O₄ were almost similar, thereby confirming excellent cyclic stability (Fig. 3d). Sustainability of this type of stability is paramount in real-world applications, in that a catalyst must perform at a very high level over a longer period [15].

Electrocatalytic HER performance

The HER electrochemical results showed that Fe₃O₄ NPs have an overpotential of 245 mV, while Co-doped Fe₃O₄ NPs have a much lower value of 210 mV, which shows that adding cobalt makes the catalyst work better. This overpotential reduction of 35 mV unequivocally showed that Co-doped Fe₃O₄ was more active than Fe₃O₄ NPs towards HER (Fig. 4a). The incorporation of cobalt into the Fe₃O₄ structure changes its electronic nature, which may lower the energy barrier to hydrogen adsorption and desorption, two significant steps in the HER process. Tafel slopes, which are an important way to measure reaction kinetics, showed more proof that Co-doped Fe₃O₄ was more active. A lower Tafel slope reflected more favourable reaction kinetics or a hydrogen-evolving catalyst that operated effectively. The Tafel slope of Fe₃O₄, to a large extent, was anticipated to be in the region of 110 mV/dec. Whereas Co-doped Fe₃O₄ NPs showed lower Tafel slope value of 79 mV/dec (Fig. 4b). The reduced Tafel slope strongly suggested that Co-doping elevated the charge transfer character of Fe₃O₄, ensuring improved proton reduction and hydrogen evolution [32]. Therefore, improved kinetics resulted from Co-induced changes in the electronic structure and conductivity favourable to electron-proton coupling, a key step in effective HER. Besides its remarkable overpotential and Tafel slope, the turnover frequency (TOF) of Co-doped Fe₃O₄ is what renders it significantly better than Fe₃O₄ as a demonstration of catalysis. The higher TOF of Co-doped Fe₃O₄ supported that doping with cobalt enhances the active sites available or enhances the intrinsic activity of an active site to evolve hydrogen (Fig. 4c). Such an increase in TOF indicates that Co-doped Fe₃O₄ would better utilize its active sites than pure Fe₃O₄ with an obvious improvement in HER performance [33]. Furthermore, with an almost identical HER performance between 1000 cycles and the first cycle (Fig. 4d), exemplary cyclic stability was displayed by the Co-doped Fe₃O₄ NPs-the utmost stability of the catalyst is one of the significant reasons that suggested Co-doped Fe₃O₄ can, for long periods of use, be an eminent candidate for maintaining high catalytic performance without any major degradation.

Fig. 4.

a HER polarization curves, b Tafel slop, c TOF, and d HER polarization curves before and after 1000 cycles, for the prepared Fe₃O₄, and Co-doped Fe₃O₄ NPs

The main causes of enhanced HER activity in Co-doped Fe₃O₄ are known to be several factors: First, Co alters the electronic structure of Fe₃O₄, raising the density of states over the Fermi level, thereby promoting hydrogen adsorption and desorption and lowering the overpotential for the reaction. Apart from this, Co-doping increases the active sites relevant to proton reduction, leading to increased overall catalytic activity [34]. These active sites promote proton-electron coupling more effectively, thus accelerating HER process. In addition, Co may modify the electronic environment of Fe₃O₄, which could stabilize reaction intermediates and increase the overall efficiency of the catalyst. In general, co-doping improves the electronic conductivity of Fe₃O₄ and increases the effective charge transfer during the reaction, leading to the reduced Tafel slope observed for Co-doped Fe₃O₄.

The theoretical studies of Fe₃O₄ and Co-doped Fe₃O₄ notably elucidated the fundamental mechanism of OER and HER processes. Co incorporation into the Fe₃O₄ structure has evidently transformed the catalyst’s electronic property and instantly enhanced the catalytic activity. Figure 5a–b showed the side and top-views of the optimized structure of Fe₃O₄ and Co-doped Fe₃O₄. The differences in charge density between over the surface of Co-doped Fe₃O₄ (Fig. 6a) and the electron valence distribution profile (Fig. 5c–d) more than just complimentary indicated the pivotal role that Co played in the electronic structure modulation of Fe₃O₄ for eventual amelioration of catalytic activity. One striking feature observed from the charge density difference is that Co doping very much alters the electronic distribution in the Fe₃O₄ lattice as compared to the pristine one [35]. The charge density difference also indicated the redistribution of electron density in the vicinity of the Co-dopants, thereby indicating that the Co atoms donate or accept electrons affecting the local electronic environment. Such redistribution accumulates charge density close to Co-sites, indicating that Co really played a role in enhancing the local electron density that boosted the charge transfer processes during the electrocatalytic reactions. This modified electronic distribution is one of the important factors for improving the catalysing features of Co-doped Fe₃O₄ toward OER and HER process.

Fig. 5.

Top-view and side-view of the optimized a Fe₃O₄, and b Co-doped Fe₃O₄ structures. Valance electron profile of the a Fe₃O₄, and b Co-doped Fe₃O₄ structures

Fig. 6.

a Top-view and side-view of the charge density difference over the optimized Co-doped Fe₃O₄ structure along with its fully spaced structure. Density of state of b Fe₃O₄, and c Co-doped Fe₃O₄ structure. Projected density of state of d Fe₃O₄, and e Co-doped Fe₃O₄ structure

The electron valence distribution thus also supported the electronic enhancement of Co-doped Fe₃O₄. From this graph, it can be seen that the Co-doped Fe₃O₄ possesses an increased electron distribution versus that of pristine Fe₃O₄, while the electron density at the Co sites is even higher. Such observation corroborates that Co doping could generate more localized electronic states that might facilitate reactant adsorption and thus OER and HER kinetics [36]. A higher density of electrons in Co-doped Fe₃O₄ could be a signature of a more propitious electronic structure that promotes proton-coupled electron transfer; this is vital for both OER and HER. Another critical observation is the enormous difference in the distribution of the densities of states electronic (DOS) over the Fermi level for the Fe₃O₄ and its Co-doped Fe₃O₄. DOS provides the number of electronic states available at every energy level. In Fe₃O₄, the electronic structure is mainly dominated by the 3d states of Fe ions. So, the incorporation of Co into the Fe₃O₄ lattice seems to drastically alter this electronic structure, manifested in an increase in the DOS near and above the Fermi level in the Co-doped Fe₃O₄. Figure 6b–c clearly showed that the Co-doped Fe₃O₄ displayed a higher density of electronic states at the Fermi level than the undoped one. The increase in DOS near the Fermi level suggested that replacing Fe ions with Co ions introduced new electronic states near the Fermi level. This DOS enhancement for Co-doped Fe₃O₄ could lead to increased electronic conductivity, lowering the energy barrier for the rapid electron movement during the electrochemical reactions. Further, the Co(d) orbital of Co-doped Fe₃O₄ highly contributed to its DOS as compared to Fe(d) orbital. Besides this, the Co(d) orbital was more localized and therefore contributed to charge transfer processes more than the Fe(d) orbital (Fig. 6d–e). This, in turn, reflected the enhanced catalytic activity of Co-doped Fe₃O₄. Consequently, this demonstration shed light on the significance of Co(d) orbital in boosting the catalyst’s electronic conductivity and improving the efficiency of electrocatalytic performance.

Further studies were aimed at analysing the catalytic activity of Co-doped Fe₃O₄ and Fe₃O₄ towards OER and HER through the optimization of reaction intermediates on Co sites and Fe sites. Following the calculations, it was clear that each step in the OER and HER mechanisms has a lower energy barrier on Co sites compared to Fe sites. It’s very important to note that the binding energy of key intermediates (O* and H*) on a Co site is lower, which means that intermediates are more likely to stick to the Co site. This fact was also supported by the charge density difference calculations for the Co-OOH and Fe-OOH adducts over the Co-doped Fe₃O₄, indicating higher electron transfer in the Co-OOH adduct (Fig. 7a). Lower energy barriers at the Co sites presented a prime advantage for catalysis over the Co-doped Fe₃O₄ because promoting the overall reaction via lower activation energy required reduction of activation energy for each step of the pathway. The difference in energy barriers between Co and Fe sites indicated that Co was indeed the primary active site in Co-doped Fe₃O₄ for OER and HER (Fig. 7b–c). Thus, the lowered energy barriers were beneficial to the catalyst during proton-electron transfer steps of the catalytic processes [37]. Therefore, Co-doped Fe₃O₄ outperformed pure Fe₃O₄ in terms of low overpotential and fast kinetics for both OER and HER.

Fig. 7.

a Charge density difference over the Co-OOH, and Fe-OOH adducts over the Co-doped Fe₃O₄ catalyst. b OER free energy diagram, and c HER free energy diagram, calculated at each active site of the Fe₃O₄ and Co-doped Fe₃O₄ catalysts (* represents the active site)

Overall, the theoretical insights on Fe₃O₄ and Co-doped Fe₃O₄ catalysts indicated that Co could greatly improve the electronic structure and catalysis of Fe₃O₄. When doped into Fe₃O₄, Co redistributed the charge density, enhancing the electron valence distribution over the catalyst and widening the density of states around the Fermi level for enhanced electronic conductivity. The electronic conductivity and catalytic performance of Co-doped Fe₃O₄ were mainly enhanced by Co(d) sites. Further optimization of intermediates for OER and HER over Co and Fe sites suggested that Co sites decreased the energy barriers drastically for every step in the reaction, making Co-doped Fe₃O₄ a better catalyst for both OER and HER compared to native Fe₃O₄. It has a strong correlation with the findings on catalytic performance and thereby hints toward further applications in energy storage and conversion, which require efficient electrocatalysts.

Conclusion

The theoretical and experimental work showed that the Co-doped Fe₃O₄ catalyst worked better than pure Fe₃O₄ for both OER and HER. This suggests that Co-doped Fe₃O₄ could be a very good choice for highly efficient electro-catalysis. Experimental evidence showed that Co-doped Fe₃O₄ had a lower overpotential value than its parent compound, Fe₃O₄, which demonstrated a remarkable difference in overpotential between these two materials, with the Co-doped Fe₃O₄ showing an overpotential of 146 mV and 210 mV for OER and HER as compared to a higher value of 278 mV and 245 mV by the parent compound, respectively. The Co-doped Fe₃O₄ had a very low Tafel slope of 39 mV/dec compared to 62 mV/dec for the Fe₃O₄, which means that the reaction kinetics were speed up. It’s also important to note that the Co-doped Fe₃O₄ has a higher turnover frequency (TOF) and very good cyclic stability, meaning that its performance doesn’t drop much up to 1,000 cycles. This again shows that it lasts longer. Theoretical results showed that addition of Co to Fe₃O₄ made the catalytic activity much better. This is because Co is now part of the Fe₃O₄ matrix. The optimized OER and HER intermediates implied reduced energy barriers to each step over the Co sites of Co-doped Fe₃O₄, indicating Co was the main active site for both the OER and HER in the Co-doped Fe₃O₄ catalyst. This study indicated that doping transition metals into transition metal oxides can improve their performance in electrocatalytic reactions.

Author contributions

A.S. write the pre-draft of the manuscript. K.A. provided the comments, and supervised the project throughout.

Funding

Not applicable.

Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.