Heterostructure interface-engineered 3D/2D CoMn₂O₄/CoFe₂O₄/NF core/shell Bi-functional electrocatalytic nanomaterials for efficient overall water splitting application in alkaline media

Abstract

Developing a high-performance, affordable, reinforced, and stable bi-functional nanocatalyst for a sustainable energy conversion process is highly desirable for large-scale commercialization. Inspired by the benefits of heterostructure engineering, herein, we hierarchically constructed an interface-engineered 3D/2D (CoMn₂O₄/CoFe₂O₄) core/shell bi-functional nanohybrid material on a nickel foam (NF) substrate via a facile two-step solvothermal process. As evidenced by HR-TEM and XPS results, heterostructure interfaces were properly established, and strong electronic coupling was built at the heterostructure interfaces. Moreover, electrochemical studies revealed that the synergistic coupling effect expedites rapid charge transfer at the electrode/electrolyte interfaces, elevating accessible metal active sites and oxygen vacancies over the nanomaterial surface, and affords robust electrochemical stability to an electrode material. This combined effect greatly elevates electrochemical performance and facilitates remarkably low overpotential values of 104 mV and 233 mV and minimal Tafel slope values of 73.6 mV dec⁻¹ and 53.2 mV dec⁻¹ for the HER and OER, respectively. Moreover, an assembled two-electrode electrochemical cell setup exhibited a small cell voltage of 1.55 V with stupendous stability over 50 h at a current density of 10 mA cm⁻² in 1 M KOH alkaline media. This rational core/shell heterostructure interface-constructed bi-functional electrode will be a new avenue for scaling up the electrochemical reaction kinetics of new-generation energy-conversion technologies.

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1. Introduction

Extreme dwindling of fossil fuels, elevating energy scarcity and global warming alarm us to switch the pathway to sustainable energy conversion technologies.^1,2 Owing to its high calorific value and long-term sustainable energy supply characteristics, hydrogen (H₂) is considered an ideal candidate compared with other fuel resources.^3,4 Electrochemical water splitting (EWS) is a potent and attractive approach that has been widely employed over recent decades to harvest renewable, ultrapure, nontoxic, and eco-friendly H₂ from naturally abundant water resources.^5,6 Nonetheless, its sluggish and complex 4-stage oxygen evolution reaction (OER) at the anode and 2-stage hydrogen evolution reaction (HER) at the cathode increase the overpotential and hinder its overall commercial applications.^7,8 Presently, Pt-, Ir-, and Ru-doped nanomaterials furnish excellent EWS performances.^9,10 However, the high cost of these metals hampers the industrial-scale production of H₂. Therefore, over the past few decades, massive efforts have been devoted by researchers to construct non-precious, bi-functional electrocatalytic materials. Lately, first-row transition metal complexes such as transition metal oxides,¹¹ nitrides,¹² sulfides,¹³ phosphates,¹⁴ chalcogenides,¹⁵ carbides,¹⁶ and layered double hydroxides^17,18 have been found to exhibit outstanding electrochemical performances owing to their tunable electronic and morphological structures.¹⁹ Particularly, CoMn₂O₄ binary transition metal–oxide nanocatalysts have garnered distinct attention owing to their cost-effectiveness, ecological benignity, diverse structural morphologies, multiple oxidation states of Co⁺ and Mn⁺ ions and rapid redox reaction characteristics.^20–22 Likewise, the distinctive spinel structure of 2D CoFe₂O₄ underlies its notable electrochemical performance owing to its Co ion's larger oxidation potential, larger availability of surface electro-active species with tunable morphology and electronic states furnishing excellent electro catalytic OER property for EWS application.^23,24 Nevertheless, poor electrical conductivity, paucity of active sites and inferior structural stability have been severely limiting the wide-ranging applications of the above-mentioned metal oxides. However, to overcome these shortcomings, several strategies such as doping,²⁵ alloying,²⁶ templating,²⁷ composition modulating,²⁸ morphological modifications (combination of 1D, 2D, and 3D nanostructures), and heterostructure formation^29–32 have been applied. Amongst them, the heterostructural interface formation is an ingenious strategy that has gained interest in recent times as it triggers spontaneous electron transfer at the interface, increasing metal active sites and oxygen vacancies over the electrode surface, and improves structural and electrochemical stability to the level of electrocatalytic nanomaterials.^32–34 Generally, oxygen vacancies are primary active sites that facilitate optimizing adsorption–desorption binding energies with OER intermediates (OH*, O*, and OOH*) and effectively boost the overall reaction kinetics of electrodes.^35–37 Moreover, the core/shell (3D/2D) structure formation is an another ingenious strategy due to its high surface area, and a 3D core material serves as a scaffold with high surface area for steadily growing 2D ultra-thin nanosheets.³⁸ This 2D shell-like nanosheet wrapping over the 3D core structure further increases the electrochemical active surface area, which leads to rapid charge transfer for catalytic reactions by shortening the ion diffusion path.³⁹ Furthermore, the integration of highly efficient hydrogen evolution and oxygen evolution in electrocatalytic materials as a core–shell structure makes them bi-functional electrode materials and enhances the overall system efficiency.⁴⁰ Additionally, the shell-like encapsulation provides high stability in a harsh acidic and alkaline medium.⁴¹ In another context, polymer binders (Nafion, polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), etc.) greatly elevate the contact resistance at the catalyst/current collector interface, diminishing the metal active sites and declining ion transfer among the electrolyte/electrode surfaces.^42,43 By the way, the solvothermal method is a versatile technique to overcome these limitations and build up binder-free diverse nanoscale dimensions directly on electrically conductive substrates. Moreover, due to its facile, eco-friendliness, energy efficient with controlled nanostructures synthesis declares as an outstanding technique.^44,45

Lately, several works were initiated to construct effective electrocatalysts for water splitting applications. For example, Junhee Lee et al., demonstrated the synthesis of various single and bimetallic composites of CoO, Mn₂O₃, Co₂MnO₄, and CoMn₂O₄ through morphological engineering via a solvothermal process. The Density Functional Theory (DFT) calculations clearly demonstrated that the CoMn₂O₄ bimetallic spinel structure possesses a lower energy barrier to the dissociation of water molecules to generate H* and deliver excellent HER performance of 132 mV at a current density of 10 mA cm⁻².⁴⁶ Similarly, via a core–shell structural strategy, Iryna Makarchuk and his co-workers constructed Fe₂O₃-magnetite as a core and CoFe₂O₄ spinel as a shell by a seed-mediated crystal growth technique. She varied the thickness ratios of the CoFe₂O₄ spinel structure and achieved outstanding OER performance at a shell thickness value of 0.65 nm.⁴⁷ Moreover, Jiahui Song et al. fabricated a NiO/CoFe₂O₄ heterostructure in situ. Here Ni was used as the low-oxophilic dopant for creating oxygen vacancies. These oxygen vacancies effectively provide optimal adsorption and desorption binding energies for OER intermediates and achieve a low overpotential of 270 mV to deliver a current density of 10 mA cm⁻² in 1 M KOH.⁴⁸

Inspired by the above-mentioned hypothesis on heterostructure-interfaced nanomaterials, we successfully fabricated binder-free, heterostructure-interfaced, 3D/2D core–shell CoMn₂O₄/CoFe₂O₄ bi-functional nanohybrids on conducting NF substrates. This core/shell heterostructured architecture induces a strong and synergistic electronic coupling effect among the CoMn₂O₄/CoFe₂O₄ counterpart nanomaterials, which greatly reduces the ion/electron transportation resistance and accelerates the electrochemical reaction. Furthermore, the surface modification process generates numerous active sites, which serve as active centers for the HER and OER intermediates as well as charge storage property. From these advantages of the effective core/shell heterostructure interface strategy, we achieved excellent HER and OER overpotential values of 104 mV and 233 mV and minimal Tafel slope values of 79.9 mV dec⁻¹ and 49.5 mV dec⁻¹. Besides, it delivered an outstanding cell voltage of 1.55 V at a current density of 10 mA cm⁻² in 1 M KOH alkaline media with 50 h robust electrochemical stability.

2. Materials and methods

2.1 Chemicals and reagents

Cobalt nitrate hexahydrate [Co(NO₃)₂·6H₂O] (99.99%), manganese nitrate tetrahydrate (Mn (NO₃)₂·4H₂O) (99.99%), iron nitrate nanohydrate [Fe(NO₃)₃·9H₂O] (99.99%), ammonium fluoride (NH₄F) (99.99%), urea [CO(NH₂)₂], ethylene glycol [(CH₂OH)₂], DI water, and nickel foam (NF) were used in this experiment.

2.2 Synthesizing procedure

2.2.1 Synthesis of CoMn₂O₄ bud-like nanostructures. First, a 3 cm × 2 cm NF substrate was treated with a 3 M concentrated HCl solution via a continuous ultrasonification process for 15 min in order to eliminate the surface oxidation layer. Furthermore, it was processed for about 10 min with ethanol and DI water successively. Then, it was dried at 65 °C over 6 h.

The preparation procedure of the CoMn₂O₄/NF sample primarily involved that successive addition of 1 mmol of Co(NO₃)₂·6H₂O, 2 mmol of Mn(NO₃)₂·4H₂O, 12 mmol of NH₄F, and 24 mmol of CO(NH₂)₂, followed by dissolving in 85 mL of distilled water and stirring for 30 min to obtain a homogeneous solution (Scheme 1). The resulting solution was transferred into a 100 mL Teflon autoclave. Then, the cleaned NF substrate was kept in a slanting position in the precursor solution and sealed with a stainless steel covering. The reaction temperature was maintained at 120 °C for 24 h in the furnace. Afterward, it was cooled down to room temperature, and the synthesized CoMo/NF-precursor was cleaned with ethanol and DI water consecutively to remove impurities. Subsequently, the CoMo/NF-precursor was dried at 60 °C for 12 h. Then, the cleaned CoMo/NF-precursor was calcined at 300 °C for 2 h to obtain a CoMn₂O₄/NF electrode material.

Scheme 1 Schematic of the fabrication procedure of a CoMn₂O4/CoFe₂O₄ core/shell nanostructure on an Ni-foam substrate.

2.2.2 Synthesis of CoMn₂O₄/CoFe₂O₄/NF flower bud-like nanostructures. To grow 2D CoFe₂O₄ nanosheets over the CoMn₂O₄/NF nanobud-like morphology, 40 mL solution of 2 mmol of Co(NO₃)₂·6H₂O, 4 mmol of Fe(NO₃)₃·9H₂O, and 24 mmol of CO(NH₂)₂ were added to a mixture of ethanol and DI water taken in 1/1 ratio and vigorously stirred for 30 min. Then, the resulting solution was transferred to a 50 mL Teflon autoclave with a slantingly fixed CoMn₂O₄/NF substrate. Then, it was maintained in a hot-air oven at 120 °C for 7 h. Later, it was washed with ethanol and DI water sequentially and dried for 6 h. The cleaned substrate was calcined at 400 °C for 2 h to construct a CoMn₂O₄/CoFe₂O₄/NF flower bud-like morphological heterostructure. Furthermore, for comparison, CoFe₂O₄/NF nanosheets were grown over the NF substrate following the above-mentioned procedure, except for the use of a cleaned bar NF substrate instead of CoMn₂O₄/CoFe₂O₄/NF in the autoclave chamber.

2.3 Physical characterization

The crystalline phases of the prepared electrodes were investigated using a Rigaku Smart Lab X-ray diffractometer (XRD) with Cu-k-alpha radiation (λ = 0.1542 nm). Furthermore, a Thermal Field emission scanning electron microscope (TFE-SEM, Model: JEOL JSM-7800F, 0.01–30 kV, 0.1–1.2 nm resolution) and a high-resolution tunneling electron microscope (HR-TEM, Model: JEOL JEM-2010, 80–200 kV) were used for analyzing the morphological changes of the nanostructures, EDS elemental mapping, crystalline lattice plane, and the SAED pattern of all the fabricated electrode materials. Moreover, the surface ionic states and composition of the synthesized samples were identified using an X-ray photoelectron spectrometer (XPS model: ULVAC-PHI, PHI 5000 Versa Probe, 10 keV, C-60 ion beam).

2.4 Electrochemical measurements

The electrochemical performances of all the synthesized electrodes were measured using a CHI instrument. The typical HER and OER assessment of an electrode was carried out in a N₂-saturated 1 M KOH electrolyte using a three-electrode configuration set-up, in which the synthesized electrodes served as the anode material, a platinum wire as the cathode and a Hg/HgO calomel electrode as the reference electrode. All expressed potentials were relative to the Reversible hydrogen electrode (RHE) following the equation of E(RHE) = E(Hg/HgO) + 0.098 + 0.0591 pH. A scan rate of 5 mV s⁻¹ was applied to the analysis of LSV characteristics of all electrodes, and all polarization curves were iR compensated. Furthermore, the general equation is η = b log I + a, where η is the over potential value, I is the current density, and b represents the Tafel slope value, which is used for measuring the Tafel slope reaction kinetics. Different scan rates in the range of 20–120 mV s⁻¹ were used for the cyclic voltammetry (CV) experiment conducted in the non-faradaic region, and the chemical double layer capacitance (C_dl) value was evaluated as Δj = |j_a − j_c|/2, where, j_a and j_c are the anodic and cathodic current densities. Moreover, the electrochemical active surface area (ESCA) of the electrodes was assessed using the formula ESCA = C_dl/C_S, where C_S is the specific capacitance, and the general C_dl value was referred from previously reported articles.^49,50 The frequency range of 0.1–100 kHz with an amplitude of 10 mV was provided to investigate the electrochemical impedance spectroscopy (EIS) property. The electrochemical stability of a final composite electrode was assessed by chronoamperometric (i–t) technique at a constant working potential of 1.55 V with a current density of 10 mA cm⁻².

3. Results and discussion

3.1 Morphological and electronic structural characterization

The evolution of the nanostructural morphology of the core/shell CoMn₂O₄/CoFe₂O₄/NF electrocatalyst was investigated by Thermal Field Emission-Scanning Electron Microscope (TFE-SEM) analysis. As depicted in Fig. 1(a–c), the CoMn₂O₄ nanocomposite emerged as a 3D bud-like structural morphology, and the alignment of the nanostructure was quite homogeneous with firm anchoring over the NF substrate. Afterwards, the CoFe₂O₄ nanocomposite material comprised 2D petals like nanosheets structural morphology, which was corrugated nature with interlinked each other over the NF surface presented at Fig. 1(d–f). More interestingly, after the hierarchical construction of 2D petals such as CoFe₂O₄ over the 3D nanobuds such as CoMn₂O₄, both were well conjoined with each other and exhibited flower bud-like core/shell CoMn₂O₄/CoFe₂O₄/NF nano morphological structures, as illustrated in Fig. 1(g–i). These heterostructural interface construction enlarges the electrode/electrolyte contact surface area and improves the electrochemical reaction.

Fig. 1 Low- and high-magnification FE-SEM images of (a–c) CoMn₂O₄/NF nanobuds, (d–f) CoFe₂O₄/NF nanosheets and (g–i) CoMn₂O₄/CoFe₂O₄/NF flower bud-like nanomaterials.

Furthermore, the TEM image in Fig. 2a illustrates that the CoMn₂O₄/CoFe₂O₄ composite material holds a flower bud-like nanomorphological structure, whereas the HR-TEM images displayed in Fig. 2b further display the development of heterostructural interfaces among the CoMn₂O₄ and CoFe₂O₄ counterpart materials. The marked portions (I, II, III, and IV) were enlarged to indisputably identify the heterostructural interface formation, in which the interplanar spacing values of (0.497 nm), (0.251 nm), (0.303 nm), (0.259 nm), and (0.481 nm) were associated with the crystalline lattice planes of CoMn₂O₄ and CoFe₂O₄ nanocomposite materials.^51–54 Moreover, the numerous bright spots with diminished rings appeared in the SAED pattern of the CoMn₂O₄/CoFe₂O₄ composite, as shown in Fig. 2c, which confirmed the slightly polycrystalline nature of the final composite material and the CoMn₂O₄ (101), (112), (211) and (224) lattice planes and the CoFe₂O₄ (440), (311) and (111) lattice planes, well consistent with the HR-TEM crystalline planes. The Energy-Dispersive X-ray (EDX) spectra with their corresponding elemental mappings are demonstrated in Fig. 2(d and e), which confirmed that all elements were present in the final CoMn₂O₄/CoFe₂O₄ nanocomposite material.

Fig. 2 (a) Low-magnification TEM image. (b) High-resolution TEM lattice planes (1–4 magnified portions). (c) SAED pattern. (d and e) FE-SEM energy-dispersive spectroscopy (EDX) spectra with elemental mappings for the final CoMn₂O₄/CoFe₂O₄ nanocomposite.

As disclosed in Fig. 3a, the three dominant peaks at 44.58°, 51.99° and 76.46° in all the synthesized samples apparently affirmed the presence of a metal NF substrate (JCPDS card no. 04–0850). The reflections arising at 18.32°, 29.6°, 33.35°, 36.67°, 58.95°, 61.02°, and 64.93° are well analogous with the (101), (112), (103), (211), (321), (224), and (400) characteristic lattices of the spinal CoMn₂O₄ nanostructure (JCPDS card no. 01-077-0471).⁵⁵ Similarly, five characteristic peaks at 28.3°, 31.4°, 36.6°, 58.4°, and 63.8° were ascribed to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 0 0) hexagonal phases of CoFe₂O₄ (JCPDS card no. 022-1086).⁵⁶ Moreover, all identified peaks with no obvious peaks from other crystalline phases of the CoMn₂O₄/CoFe₂O₄/NF sample clearly demonstrated that the final core/shell material was constructed with phase purity and well organized. Moreover, it is well united with HR-TEM crystalline lattice planes, which further ensures systemized heterostructural interface formation.

Fig. 3 (a) XRD-patterns. (b) XPS survey and corresponding (c) Co 2p, (d) Mn 2p, (e) Fe 2p, and (f) O 1s spectra of CoMn₂o₄, CoFe₂O₄, and CoMn₂O₄/CoFe₂O₄ nanocomposite materials.

The intrinsic electrocatalytic reactivity of an electrode material was highly associated with the electronic configuration of metal ions, which was analyzed by the XPS technique. The wide-scan survey spectrum in Fig. 3b depicts the presence of elements Co, Mn, Fe, and O in the final cmo/cfo/nf composite material. The electronic configuration of the Co 2p spectra of the CoMn₂O₄ nanomaterial shown in Fig. 3c was de-convoluted into two primary oxidation states: Co³⁺ at 2p_3/2 (780.46 eV) and 2p_1/2 (795.95 eV) and Co²⁺ at 2p_3/2 (782.28 eV) and 2p_1/2 (797.06 eV) binding energies with two shakeup satellite peaks (786.30 eV and 802.97 eV),^57,58 whereas the binding energies (638.41 eV)/(650.27 eV) and (640.30 eV)/(651.82 eV) at Fig. 3d should be attributed to the de-convoluted ionic states of (Mn³⁺ and Mn²⁺) the Mn 2p spectra.⁵⁹ As displayed in Fig. 3e, the fitted 4 main peaks at (710.77 eV)/(724.19 eV) and (713.63 eV)/(726.86 eV) binding energies were ascribed to the Fe²⁺ and Fe³⁺ species of the 2p_3/2 and 2p_1/2 spin orbitals along with two satellite shake-up peaks at 715.22 eV and 734.75 eV.^60,61 Furthermore, the O 1s spectrum embraced 3 peaks at 529.83 eV, 531.09 eV, and 533.2 eV in Fig. 3f, which were related to the formation of the Metal–Oxygen bond (O–M*), Oxygen vacancies (O_vac), and surface-adsorbed water molecules (O_c).⁶² The changes in binding energy at heterogeneous interfaces greatly influence the charge transport property as well as metal active centers during the electrocatalytic reaction.^63–65 Analogously, in our case, after hierarchical nanomorphological reconstruction of CoFe₂O₄ nanosheets grown over a pristine CoMn₂O₄ nanobud structure, numerous heterostructure interfaces were formed as mentioned by HRTEM and XPS analysis, which effectively modulated the electronic configuration of the final CoMn₂O₄/CoFe₂O₄/NF composite material. Essentially, binding energy levels of Co and Mn ions shifted positively towards higher levels by 0.36 eV and 0.41 eV.^66,67 Simultaneously, for Fe ions, it shifted negatively to lower levels by 0.63 eV. These positive and negative binding energy shifts clearly confirmed that strong electronic coupling developed amongst CoMn₂O₄/CoFe₂O₄ counterparts. Furthermore, the heterostructure formation created an electric field at the heterostructure interfaces and induced rapid charge transfer from the electron-abundant (CoMn₂O₄) composite to the e⁻ inadequate (CoFe₂O₄) material,⁶⁸ which may promote the expeditious charge transfer characteristics during the electrochemical reaction. Additionally, electron paramagnetic resonance (EPR) spectroscopy was performed to confirm the presence of oxygen vacancies in each individual material. The CoMn₂O₄/CoFe₂O₄ core/shell composite exhibited a significantly higher concentration of oxygen vacancies than the individual components, as depicted in Fig. S1.† This enhancement arises because the formation of the CoFe₂O₄ shell onto the CoMn₂O₄ core induces interfacial strain and strong electronic coupling. These effects weaken the metal–oxygen bonds near the interface, facilitating oxygen loss and increasing surface oxygen vacancies.^69–71 These increasing oxygen vacancies were employed as an active center for OER intermediates (OH*, O*, and OOH*).

3.2 Electrochemical investigation

Charge transfer characteristics of the fabricated electrodes were investigated by the Electrochemical Impedance spectroscopy (EIS) technique, as displayed in Fig. 4a. Minimal charge transfer resistance (R_ct) specifies rapid charge transfer at the electrode/electrolyte interface. The Nyquist plots were interpreted in conjunction with the corresponding equivalent electrical circuit model. The lowest R_ct value of 118 Ω was obtained for the final core/shell CoMn₂O₄/CoFe₂O₄/NF heterostructure material compared to the bar NF substrate (283 Ω) and other two nanostructures, CoMn₂O₄/NF (139 Ω), CoFe₂O₄/NF (147 Ω), respectively. The formation of heterostructural interfaces built an electric field among the counterpart nanocomposites and shortened the charge transfer pathway for rapid charge transfer characteristics to the electrocatalytic reaction.

Fig. 4 (a) EIS-Nyquist plots. (b) C_dl (Δj vs. scan rate) graph. (c) ESCA. (d) LSV-HER. (e) Tafel-HER. (f) LSV-OER. (g) Tafel-OER for all the synthesized electrode materials. (h) LSV-two electrode cell and (i) chronoamperometry (i–t) graph for the final CoMn₂O₄/CoFe₂O₄/NF electrocatalytic material.

The electrochemical active surface area (ESCA) is a crucial factor that demonstrates the number of active sites involved in the electrochemical reaction and is also directly proportional to the chemical double-layer capacitance (C_dl) of the electrode material. Cyclic voltammetry (CV) is a comfiest and immense technique performed in the non-faradaic region at different scan rates of 20–120 mV s⁻¹, as shown in Fig. S2† to estimate the C_dl values of the synthesized electrode materials. The obtained C_dl values are depicted in Fig. 4b: for the bar NF substrate, it was 1.08 mF cm⁻², and for CoMn₂O₄/NF, CoFe₂O₄/NF, and CoMn₂O₄/CoFe₂O₄/NF nanostructures, they were 9.9 mF cm⁻², 3.6 mF cm⁻², and 17.4 mF cm⁻², respectively. As illustrated in the histogram in Fig. 4c, the heterostructured CoMn₂O₄/CoFe₂O₄/NF nanocomposite entails more active sites of 435 cm⁻² than the other CoMn₂O₄/NF (247 cm⁻²) and CoFe₂O₄/NF (90 cm⁻²) nanostructured samples, and the bar NF substrate (27 cm⁻²). The altered morphology and redistributed charge at the heterostructure interfaces lead to increased oxygen vacancies as well as diverse metal active sites across the electrode surface, which act as a major active site for HER as well as OER process.

3.2.1 Electrocatalytic HER performance. Electrocatalytic performance of the synthesized samples was characterized by a linear sweep voltammetry technique in a N₂-saturated 1 M KOH electrolyte solution at a scan rate of 5 mV s⁻¹. A series of LSV polarization curves for different nanostructures are illustrated in Fig. 4d. Initially, the HER for the bar NF substrate was unsatisfactory, resulting in a larger overpotential value of 235 mV. On the contrary, for CoMn₂O₄/NF and CoFe₂O₄/NF nanostructured materials, a discernible augmentation of HER activity can be noticed, at 127 mV and 155 mV, respectively, due to the specific morphological structure with moderate electrical conductivity of the counterpart materials. As anticipated, the final core–shell CoMn₂O₄/CoFe₂O₄/NF Heterostructured nanohybrid material affords an outstanding performance of 104 mV due to its high ionic conductivity with various metal active sites. To further gain insights into the electrocatalytic mechanism of the electrodes, Tafel slope analysis was performed from LSV polarization cures. Generally, two principal steps are involved in the HER in alkaline media: Volmer-water dissociation step (1) and Heyrovsky (2) or Tafel (3) H₂ evolution process.⁷²

Volmer: H₂O + e⁻ → H_ads + OH⁻ (1)

Heyrovsky: H₂O + H_ads + e⁻ → H₂ + OH⁻ (2)

Tafel: H_ads + H_ads → H₂ (3)

In particular, the Volmer water dissociation is a more sluggish and energy-consuming reaction step under alkaline conditions owing to its deficiency of active sites for the H–OH bond severing and H* adsorption as well as the lack of electron transfer characteristics of an electrode material. In our case, the bar NF substrate exhibits a larger slope value of 141.4 mV dec⁻¹. Besides, CoMn₂O₄/NF and CoFe₂O₄/NF nanostructures over the NF substrate delivered moderate slope values of 115.3 mV dec⁻¹ and 104.8 mV dec⁻¹, respectively, which clearly elucidated that the Volmer step is a rate-determining step.⁷³ As shown in Fig. 4e, the final CoMn₂O₄/CoFe₂O₄/NF electrocatalytic material delivered a very minimal slope value of 73.6 mV dec⁻¹, apparently attesting that the construction of core/shell heterostructure interfaces enormously accelerated the HER kinetics of the electrode material. Furthermore, (Mn²⁺) ions on the CoMn₂O₄/CoFe₂O₄/NF electrode surface, additionally serve as a H* adsorption active sites.⁷⁴ Moreover, synergistic coupling effects among the pristine CoMn₂O₄ and CoFe₂O₄ materials facilitate rapid (H*, OH⁻*) ion transfer among the electrode/electrolyte interfaces, which further accelerated the HER kinetics.

3.2.2 Electrocatalytic OER performance. Simultaneously, the electrochemical OER performance of the as-prepared electrodes was examined under similar conditions of 1 M KOH electrolyte via the LSV polarization technique. Fig. 4f depicts the typical anodic polarization curve of all the fabricated samples, in which the core–shell heterostructured CoMn₂O₄/CoFe₂O₄/NF nanohybrid yields a low overpotential value of 231 mV compared to other counterparts, namely bar NF substrate (419 mV), CoMn₂O₄/NF (273 mV) and CoFe₂O₄/NF (338 mV). In order to investigate the OER kinetics of the electrode materials, Tafel analysis was performed. The fabricated electrocatalytic materials and their corresponding slope values were NF (155.1 mV dec⁻¹), CoMn₂O₄/NF (124.8 mV dec⁻¹), CoFe₂O₄/NF (62.7 mV dec⁻¹), and CoMn₂O₄/CoFe₂O₄/NF (53.2 mV dec⁻¹), respectively. The minimal slope value of the final composite material elucidates that the construction of the core/shell CoMn₂O₄/CoFe₂O₄/NF heterostructure interfaces immensely amplifies the OER kinetics, as displayed in Fig. 4g. As mentioned in the XPS data, the surface reconstruction process generates more oxygen defects over the electrode surface and serves as active sites for oxygen reaction intermediates (OH*, O*, and OOH*) during the OER activity.^75,76

3.2.3 Overall water splitting performance. Encouraged by the fascinating electrocatalytic HER and OER activity of the synthesized electrocatalytic materials, it was assembled as a cathode and anode simultaneously in the electrocatalytic cell (1 M KOH electrolyte), as demonstrated in Fig. 4h, to assess the overall water splitting (OWS) performance of each electrode material, in which the performance of CoMn₂O₄ gradually declined over time, whereas CoFe₂O₄ demonstrated excellent stability without any noticeable degradation over a period of 50 hours, as shown in Fig. S3.† Interestingly, after the formation of a core/shell heterostructure between these two promising materials, the encapsulation of CoMn₂O₄ with sheet-like CoFe₂O₄ shells effectively suppressed the structural degradation of CoMn₂O₄. This is attributed to the strong electronic coupling between the core and shell components, which enhances the overall structural integrity of the core/shell CoMn₂O₄/CoFe₂O₄/NF heterostructured nano-hybrid composite. Moreover, the electrolyte cell only requires 1.55 V at a current density of 10 mA cm⁻², and also delivers continuous and stupendous electrochemical stability over 50 h, maintained at the constant current density of 10 mA cm⁻², as demonstrated in Fig. 4i.

After the chronoamperometry stability test, the structural stability of the CoMn₂O₄/CoFe₂O₄/NF electrocatalysts was inspected using the FE-SEM images shown in Fig. S4(a and b)† and the TEM image shown in Fig. S5.† From these results, it can be understood that the formation of strong heterostructural interfaces among the core/shell nanostructure prohibited morphological deformation. Furthermore, the LSV characteristics shown in Fig. 4h represent that the cell voltage increases by only 1.60 V after 50 h of continuous electrochemical stability. More impressively, the electrochemical performance of the core/shell CoMn₂O₄/CoFe₂O₄/NF heterostructured nanomaterial surpasses that of many recently reported bifunctional metal oxide composites (Table ST1†).

Eventually, the superior electrochemical energy conversion characteristics of our present work arise from several morphological and electronic structural modifications. (1) Morphological modification of the construction of a shell (2D CoFe₂O₄ nano sheets) over the core (3D CoMn₂O₄/NF nanobud)-like structure generously increases the specific surface area of the overall electrode material, which leads to the amplification of huge ion/electron transfer at the electrode–electrolyte interface. This numerous ion/electron transfer enhances H₂ and O₂ evolution. (2) As explained in the XPS technique, strong electronic coupling between two functional materials (CoMn₂O₄/CoFe₂O₄) builds an electric field at the heterostructural interfaces, which serves as a pathway for rapid electron transfer from the inner core (CoMn₂O₄/NF) to the shell (CoFe₂O₄) structure and supplies more electrons to the electrochemical HER and OER kinetics. (3) Moreover, these structural and electronic modifications primarily alter the binding energy levels and increase the Co³⁺ ion concentrations and oxygen vacancies over the surface of an electrode material. These species, along with the remaining Fe and Mn metal ions on the electrode surface, serve as active sites for the HER and OER intermediates. (4) The strong electronic coupling among the core and shell materials, together with substrate support, provides firm anchoring and ensures excellent structural and chemical stability in the final CoMn₂O₄/CoFe₂O₄/NF core–shell architecture.

4. Conclusion

In summary, a novel, free-standing, homogeneous CoMn₂O₄/CoFe₂O₄ heterostructure interface-assisted core–shell bi-functional electrocatalytic nanomaterial has been developed on a conducting bar NF substrate via a facile two-step solvothermal synthesis route. Through this rational core/shell heterostructure interface construction strategy, the CoMn₂O₄/CoFe₂O₄/NF nanohybrid demonstrated excellent HER and OER overpotential values of 104 mV and 233 mV and small Tafel slope values of 73.6 mV dec⁻¹ and 53.2 mV dec⁻¹. Furthermore, it delivered a remarkable cell voltage of 1.55 V with 50 h of stupendous stability at a constant current density of 10 mA cm⁻² in 1 M KOH alkaline media. This high-performance CoMn₂O₄/CoFe₂O₄/NF heterostructure interface-engineered bi-functional electrocatalyst will be a beneficial contestant for new-generation energy conversion gadgets.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be available upon reasonable request.

Acknowledgements

The authors of Chaoyang University of Technology thank the National Science and Technology Council of Taiwan for supporting this work (Grant No.: MOST 111-2221-E-324-004-MY3). We thank the Instrument Center of National Chung Hsing University for help with FESEM, HRTEM, ESCA, and EPR measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nr01307k

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