In situ formation of Mn⁴⁺ over oxygen vacancy rich spongy manganese–cobalt oxide through sodium and NiP incorporation for the electrocatalytic oxygen evolution reaction†

Aneesh Kumar M. A.^a, A. Anantha Krishnan^a, Revathy B. Nair^b, Sreehari Harikumar^a, Najuma Noushad^a, Akshara Mohan^a, Sajith Kurian^b, S. R. Archana^d, M. Ameen Sha*^c and P. S. Arun*^a
aDepartment of Chemistry, St. John's College (Affiliated to University of Kerala), Anchal, Kollam, Kerala 691306, India. E-mail: arunps@stjohns.ac.in
^bDepartment of Chemistry, Mar Ivanios College (Affiliated to University of Kerala), Nalanchira, Thiruvanathapuram, Kerala 695015, India
^cDepartment of Chemistry, University of Kerala, Kariavattom Campus, Thiruvanathapuram, Kerala 695581, India. E-mail: shaalameen@gmail.com
^dDepartment of Chemistry, S.N. College, Varkala, Thiruvanathapuram, Kerala, India

First published on 8th July 2025

---

---

1. Introduction

Over the past decades, environmental concerns have prompted pivotal research on sustainable, clean energy resources such as hydrogen and other green fuels.¹ Among the various hydrogen production techniques, electrochemical methods are environmentally safe and dependable.² The water splitting reaction involves the hydrogen evolution reaction (HER) at the cathode and a sluggish oxygen evolution reaction (OER) at the anode. The complex four-electron transfer in the OER affects the efficiency of hydrogen production and large scale H₂ production cost because of the excess energy utilization due to the high overpotential and low exchange current density.³

Among the non-precious metals for the oxygen evolution reaction, manganese has garnered significant attention because of its multiple oxidation states and ability to mimic natural photosynthetic reactions.⁴ The ease of transition between different oxidation states via electron transfer characteristics and the formation of active redox couples makes the spinel MnCo₂O₄ system attractive for catalytic application.^5,6 Modifications in terms of surface morphology, particle size, and defect engineering to maximize electron transport are some of the strategies used to enhance the inherent catalytic activity of MnCo₂O₄.⁷ Heteroatom doping/incorporation, defect engineering and modification using various electrode supports are generally used to modulate the conductivity of the material.^8,9 Heteroatoms, such as Ce, Zn, and Ni, can be incorporated or doped into the MnCo₂O₄ system to improve its efficiency in electrocatalytic reactions.^10–12

An important strategy to enhance catalytic efficiency is to create oxygen vacancies through defect engineering.¹³ Oxygen vacancies in the MnCo₂O₄ system can cause variations in the electronic properties in such a way that facilitates the seamless transport of electrons to and from the catalyst surface.¹⁴ Defect engineering improves the multi-step adsorption and desorption processes of the OER.¹⁵ Cations with a lower oxidation state can be used as dopants in metal oxide systems to produce abundant oxygen vacancies.¹⁶ Oxygen vacancies reduce the energy gap between the metal 3d and O 2p orbitals in metal oxides and enhance the covalency of the metal–oxygen bond to favour electrocatalytic OER performance.^17,18 For instance, Co-doped-MnO₂@MnCo₂O_4.5 with oxygen vacancies and active sites has been reported to exhibit low OER overpotential, and ultrathin MnO₂ nanosheets with Mn³⁺ active sites impart half-metallicity, improving its electrocatalytic performance.^19,20 Sodium has been explored as a dopant and supporting agent to generate oxygen vacancies in metal–air batteries with enhanced performance.²¹ Sodium-modified Fe₂O₃–Al₂O₃ as an oxygen carrier has been used for hydrogen production, while Na-metal-doped Ni_(y)Fe_(1−y)O₂, Na-based metal–organic frameworks (MOFs) and Na–TiO₂ are used for water splitting reaction.^22–24 Furthermore, enhancement in electron mobility, a reduction in the bandgap and improvement in material stability have been realized using Na doping.^25,26 Hence, the incorporation of Na into an oxide system, especially into the spinels, can produce more oxygen vacancies and lower oxygen vacancy formation energy. These catalytic characteristics developed through Na doping highly influence electrode performance during electrocatalytic reactions.²⁷ However, the conductivity enhancement due to the oxygen vacancies generated via Na incorporation into oxide catalysts, especially into the spinel structures, is relatively underexplored for the electrocatalytic OER.

The higher oxidation states of Mn and Co in MnCo₂O₄ can provide favourable conditions for the OER because of the suppression of Jahn–Teller distortion of Mn and the capability of Co to participate in the adsorption–desorption kinetics of the reaction intermediates.²⁸ Silva et al. reported an MnCo₂O₄ nanoparticle catalyst on nickel foam with higher Co³⁺ and Mn⁴⁺ ion concentrations on its surface, with these ions serving as the primary promoters of the electrocatalytic process.²⁹ An integrated Ir–MnCo₂O_4.5 catalyst with Co³⁺ and Mn⁴⁺ ions demonstrated low overpotential during the acidic OER process.³⁰ The limitation to effective electron transfer in MnCo₂O₄ due to the Jahn–Teller distortion of Mn³⁺ was addressed by Jing Ge et al. by in situ growing Ni₂P nanosheets onto MnCo₂O₄.²⁸ Mn⁴⁺ abundant MnCo₂O₄ can regulate Jahn–Teller distortion and consequent enhancement in electron transport to provide a favourable surface for the OER.³¹

In the present work, we designed an MnCo₂O₄-based electrocatalytic OER system in such a way that MnCo₂O₄ exhibits a porous–spongy surface with a high active surface area, electrocatalytically favourable metal states and redox properties. A porous–spongy MnCo₂O₄ powder with abundant oxygen vacancies is developed by implementing a modified sol–gel method. The amount of oxygen vacancies is regulated by incorporating an optimum concentration of Na into the system. NiP is implemented as an effective electron transporter between the components and is deposited with Na–MnCo₂O₄ to produce an active Mn³⁺/Mn⁴⁺ redox couple with amorphous-crystalline phases in the OER electrode. The electronic synergy between the active metal species and suppressed Jahn–Teller distortion due to the NiP-generated Mn³⁺/Mn⁴⁺ redox couple can enhance the electrochemically active surface area to promote an efficient oxygen evolution reaction. The developed system has a lower Tafel slope of 70.2 mV dec⁻¹, overpotential of 214 mV at 10 mA cm⁻² and charge transfer resistance of 309 Ω at open circuit potential (OCP); additionally, this system is competent with other available systems (Table S1 of ESI†). Furthermore, it exhibited higher stability even after 1000 cycles of cyclic voltammetry.

2. Experimental materials and methods

2.1. Development of a spongy Na-incorporated MnCo₂O₄ and Na–MnCo₂O₄/NiP electrode

A modified sol–gel method using citric acid as a capping agent was employed to synthesize spongy MnCo₂O₄ and Na-incorporated MnCo₂O₄. Mn(NO₃)₂·4H₂O and Co(NO₃)₂·6H₂O were dissolved in a citric acid solution by maintaining an Mn:Co ratio of 1:2. The solution was stirred to make a sol; this sol was then heated at 70 °C ± 2 °C with continuous agitation at 500 rpm until a gel layer formed. The gel was dried in an air oven at 70 °C overnight. The dried powder was then calcined at 450 °C ± 5 °C at a heating rate of 3 °C per minute for 5 hours. After calcination, the sample was slowly cooled to room temperature. The resulting MnCo₂O₄ catalyst was finely powdered and stored in airtight bottles.

Na–MnCo₂O₄ was synthesized by varying the content of Na using sodium nitrate as the source. Different amounts of sodium nitrate were added to the dry MnCo₂O₄ powder to produce different Na–MnCo₂O₄ systems. The resultant mixtures were calcined in a furnace at 450 °C ± 5 °C for 3 hours at a heating rate of 3 °C per minute and, then, slowly cooled to room temperature. The different Na–MnCo₂O₄ systems were labelled as 0.5 Na–MnCo₂O₄, 1 Na–MnCo₂O₄, 3 Na–MnCo₂O₄, and 5 Na–MnCo₂O₄.

The Na–MnCo₂O₄ incorporated NiP electrodes were fabricated using an electroless deposition method reported by Revathy et al.³² The schematic description of the fabricated procedure is depicted in Fig. 1.

2.2. Material characterization

The powder catalysts and electrodes were comprehensively characterized through X-ray diffraction (XRD) analysis using a Bruker D8 advance twin–twin spectrometer, and the average crystallite sizes of the catalyst powders were calculated using the Scherrer formula³³ and X-ray photoelectron spectroscopy (XPS) analysis through a Thermo Scientific ECSALAB Xi+ photoelectron spectrometer with an Al K-alpha (1486.6 eV) X-ray source. The binding energy was calibrated using carbon 1s (284.6 eV) as a reference. FE-SEM and EDAX analyses were performed using a Carl Zeiss-Sigma 300 scanning electron microscope.

2.3. Electrochemical characterization and OER performance

The electrodes’ performance during the OER was monitored using a three-electrode system; in this system, the developed electrode and a Pt mesh (5 cm²) were used as working and counter electrodes, respectively, and Ag/AgCl/KCl was used as a reference electrode in an electrochemical workstation (CH16041E). Electrochemical impedance spectroscopy (EIS) analysis was performed in 1 M NaOH electrolyte at open circuit potential (OCP) over an AC frequency range of 1 Hz–100 kHz. The electrochemically active surface area (ECSA) of the developed electrode was analysed using CV results in the non-faradaic region at varying scan rates. OER performance was assessed using voltammetry at a scan rate of 10 mV s⁻¹ in 1 M NaOH electrolyte between a scanning window of 1.2–1.8 V vs. RHE. The electrochemical stability of the developed electrode during the OER was confirmed from the LSV results before and after conducting 1000 cyclic voltammetric cycles. Moreover, 1% Na–MnCo₂O₄ deposited on an NiP coating exhibited the best OER performance and is henceforth referred as Na–MnCo₂O₄/NiP.

3. Results and discussion

3.1. Physicochemical characteristics of the Na–MnCo₂O₄/NiP electrodes

The variations in the XRD results of the bare spongy MnCo₂O₄ and different Na–MnCo₂O₄ powder systems are shown in Fig. 2(a). The diffraction peaks obtained for spongy MnCo₂O₄ matched with the peaks of the MnCo₂O₄ spinel structure. The peaks at 2 theta values of 31.8°, 37°, 38.5°, 44.6°, 55.0°, 59.1° and 64.9° are attributed to the (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) lattice, respectively, and effectively match with the JCPDS #23-1237, indicating that the cations in the system possess similar lattice parameters as those in the spinel.³⁴ No additional response was obtained after Na incorporation because of the high dispersion of Na at low concentrations on the spongy MnCo₂O₄ surface. The average crystallite sizes of MnCo₂O₄ and MnCo₂O₄ with Na contents of 0.5%, 1%, 3%, and 5% were 9.26 nm, 11.34 nm, 11.16 nm, 11.28 nm, and 13.01 nm, respectively, as shown in Fig. 2(c). Only a slight increase in the average crystal size after Na incorporation was observed because of the proper incorporation of Na into spongy MnCo₂O₄.³⁴ A moderate Na concentration (1% and 3%) resulted in a low crystallite size, while the lower and higher concentrations resulted in large crystallite formation. The moderate Na content in the system shows smaller crystallites; this is potentially caused by the introduction of lattice strain via Na doping and consequent hindering of crystal growth.³⁵ The catalyst with 1% Na possessed the lowest average crystallite size among the studied catalysts. A significant peak shift was observed with an increment in the concentration of Na incorporation, suggesting the effective incorporation of Na into the spongy MnCo₂O₄ structure. The crystallite size increment with Na content is attributed to the aggregation of particles.³⁶ The crystallinity of the MnCo₂O₄ spinel structure was maintained after Na incorporation as evidenced from the nature of the XRD pattern of the different samples.

The absence of Na/Na₂O diffraction peaks and the broadening of MnCo₂O₄ diffraction peaks with increasing Na concentration confirms the successful Na incorporation onto the spongy MnCo₂O₄. The XRD patterns of NiP, spongy MnCo₂O₄/NiP, and Na–MnCo₂O₄/NiP are presented in Fig. 2(b). The prominent diffraction peaks at 2θ values of about 44° and 50.5° are attributed to the diffraction from the (111) and (200) crystal planes, respectively. The crystalline nature of MnCo₂O₄ and Na–MnCo₂O₄ combined with the amorphous nature of NiP, as evidenced by the sharpening of the corresponding responses in MnCo₂O₄/NiP and Na–MnCo₂O₄/NiP. However, the loading of Na–MnCo₂O₄ onto NiP resulted in the formation of the amorphous and crystalline phases on the surface.

The spongy MnCo₂O₄ catalyst's composition and elemental distribution were examined using EDAX analysis and mapping, and results are provided in Fig. S1 and S2 of ESI.† The catalyst composed of Mn, Co, and O exhibited even distribution of the elements without any impurity. The elements surrounding the pores of the spongy MnCo₂O₄ are mainly oxygen with Co and Mn at an atomic percentage ratio of 2:1. The EDAX spectra (Fig. 2(d)) of Na–MnCo₂O₄/NiP shows distinct peaks corresponding to sodium, nickel and phosphorus, in addition to Mn, Co, and O; moreover, evidence for the uniform distribution of sodium is observed throughout the electrode (Fig. 3).

The XPS results of the spongy MnCo₂O₄, Na–MnCo₂O₄ and Na–MnCo₂O₄/NiP electrodes are presented in Fig. 4. The XPS spectrum of Mn 2p of spongy MnCo₂O₄ exhibited the Mn 2p_3/2 peak between a lower energy band of 643.4 eV and a higher energy band of 654.9 eV; this peak was attributed to Mn³⁺ with oxygen vacancies.³⁷ When incorporating Na in spongy MnCo₂O₄, the Mn 2p_3/2 and 2p_1/2 values slightly decreased to 642.1 eV and 653.7 eV, respectively, as observed in Fig. 4(a). This shift indicates the enhanced electron density of Mn³⁺, resulting in decreased Mn–O covalency.³⁷ After the electroless NiP coating to develop Na–MnCo₂O₄/NiP electrodes, a new peak emerged at 645.8 eV, corresponding to Mn⁴⁺, in addition to the Mn³⁺ peak at 641.7 eV, as observed in Fig. 4(a).^38,39 The development of Mn⁴⁺ establishes the Mn³⁺/Mn⁴⁺ redox couple in the Na–MnCo₂O₄/NiP electrode and accelerates electron transport by suppressing Jahn–Teller distortion.^28,31

As shown in Fig. 4(b), the Co 2p spectrum of spongy MnCo₂O₄ demonstrate the formation of the Co²⁺/Co³⁺ redox couple, as confirmed by the peaks at 782.8 eV and 781.5 corresponding to the Co²⁺ and Co³⁺ oxidation states.^11,40

Considering the incorporation of Na in spongy MnCo₂O₄, the binding energy of Co 2p corresponding to Co²⁺ decreased to 782.2 eV and that corresponding to Co³⁺ decreased to 780.3 eV. The electronic shift of Co 2p is similar to that of Mn 2p because of the similar impact of Na incorporation. However, in Na–MnCo₂O₄/NiP, the binding energy value of Co²⁺ increased to 782.6 eV and that of Co³⁺ increased to 781.2 eV, as shown in Fig. 4(b). No noticeable difference was observed in the binding energy values of Mn³⁺, but Mn⁴⁺ was present on the electrode after NiP deposition. In the Ni 2p spectrum, a low-energy band at 856.6 eV (Ni 2p_3/2) is attributed to the Ni³⁺ oxidation state in Na–MnCo₂O₄/NiP, as shown in (ESI† Fig. S3(a)).⁴¹ The Ni 2p peak of NiP was at 856.8 eV (Ni 2p_3/2) corresponds to the Ni³⁺ ions present in it; this value presents a higher binding energy value than that of Na–MnCo₂O₄/NiP and indicates that the electron density around Ni decreased via Na–MnCo₂O₄ incorporation. The Na 1s peak was observed at 1071.3 eV for Na–MnCo₂O₄, while the value was slightly higher (1071.9 eV) for Na–MnCo₂O₄/NiP (ESI† Fig. S3(c)); these results indicate electron transfer from Na to NiP.⁴² The P 2p peak of NiP shifted from 133.6 to 133.1 eV when Na–MnCo₂O₄ was incorporated into NiP, as shown in ESI† Fig. S3(b).⁴³ These electronic shifts suggest a change in the electronic environment of the NiP system due to Na–MnCo₂O₄ incorporation. In Na–MnCo₂O₄/NiP, the electrons are transferred from Co 2p and Na 1s to the NiP system and showed electron shuttling among Na, Mn, Co, and NiP, resulting in favourable electronic environments and surface electron transport to enhance the OER. In addition, the incorporation of Na resulted in high oxygen storage capacity and altered the oxygen coordination environment with the Mn and Co centres; thereby modifying their electronic properties, redox behaviour and OER kinetics.²¹

Oxygen vacancy formation and its variations with different systems were confirmed by the O 1s spectrum, as shown in Fig. 4(c). In spongy MnCo₂O₄, two peaks at 530.3 and 531.9 eV correspond to lattice oxygen and oxygen vacancy formation, respectively. The peak corresponding to oxygen vacancy exhibited a shift from 531.9 to 531.8 eV due to Na incorporation. The corresponding lattice oxygen peak shifted to 530.0 eV. In Na–MnCo₂O₄, the peak area corresponds to oxygen vacancies and is lower compared with that in spongy MnCo₂O₄; this is due to the diffusion of Na into the oxygen vacancies, which will enhances the conductivity of spongy MnCo₂O₄.⁴⁴ In Na–MnCo₂O₄/NiP, the lattice oxygen peak shifted to 531.0 eV, the oxygen vacancy peak shifted to 532.3 eV, and the peak area of oxygen vacancies significantly reduced.

The surface morphology and other surface characteristics of the catalyst and electrode were investigated using FE-SEM at different magnifications, and some representative images are presented in Fig. 5. The MnCo₂O₄ sample has a spongy appearance and contains a large number of pores throughout its surface. The uniformly distributed porous MnCo₂O₄ can provide a large surface area, facilitating enhanced electrochemical reactivity.⁴⁵ The images further revealed the presence of polydisperse grains, indicating different crystal morphologies (Fig. 5(e)). The heterogeneity in grain morphology and large surface area with abundant metal sites contribute to conductivity and thereby result in enhanced electrocatalytic activity.⁴⁶ The morphology and surface features are maintained even after Na incorporation and NiP deposition. The Na incorporation strategy and the electroless coating procedure do not disturb the basic surface characteristics and morphological features although the development strategy uses high temperature calcination and chemical reduction steps.

3.2. Investigation of the electrochemical performance of the Na–MnCo₂O₄/NiP electrode

Performance of the various catalysts and catalytic electrodes was studied in detail, and the electrodes with different Na concentrations (0.5%, 1%, 3%, and 5%) were studied to determine the optimal Na incorporated MnCo₂O₄ electrode for the OER. The EIS results for all the electrodes are shown in Fig. S4 (ESI†). Among the studied electrode systems, the 1% Na–MnCo₂O₄/NiP electrode exhibited the lowest overpotential values during LSV analyses. The electrochemical results of the electrode were then examined in detail and compared with the other studied electrodes, such as NiP and MnCo₂O₄/NiP.

The EIS results provide critical information on the interfacial charge transfer resistance and double layer capacitance of the systems. The Nyquist plots and equivalent circuit are illustrated in Fig. 6(a). Notably, similar equivalent circuits have been reported in comparable systems, including those involving sodium, with dissolution/corrosion and photoelectrochemical behaviours.^47,48 A very low interfacial charge transfer resistance (R_ct) value of 309 Ω for the Na–MnCo₂O₄/NiP electrode at OCP was observed because of the greater charge transport characteristics of the electrode components obtained by the implemented material design strategy. MnCo₂O₄/NiP and NiP exhibited an R_ct value of 738 Ω and 1002 Ω, respectively. The double layer capacitance (C_dl) determined using the same equivalent circuit for Na–MnCo₂O₄/NiP was as high as 16.89 × 10⁻⁶ F; this value was higher than that of the MnCo₂O₄/NiP (14.4 × 10⁻⁶ F) and NiP (4.194 × 10⁻⁶ F) electrodes. The Na–MnCo₂O₄/NiP electrode exhibited higher double layer capacitance due to the deposition of porous spongy MnCo₂O₄ on the NiP coating.

The OER activity of the catalyst is generally governed by factors such as the electrochemically active surface area (ECSA) and the number of active electrons available for the reaction. To gain insights into the intrinsic activity of the catalytic electrodes, the ECSA and C_dl values were evaluated by analysing the CV results in the non-faradaic region at different scan rates. The ECSA is directly related to the number of active sites/the number of electrons available on the surface.

Different catalytic characteristics and the factors governing the OER activity of the different catalysts are compared using the ECSA and C_dl values. The C_dl values of Na–MnCo₂O₄/NiP, MnCo₂O₄/NiP and NiP are 0.79, 0.47 and 0.29 mF cm⁻², respectively, and the ECSA values of Na–MnCo₂O₄/NiP, MnCo₂O₄/NiP and NiP are 19.75, 11.73 and 7.26 cm², respectively. Na–MnCo₂O₄/NiP exhibited the highest surface area compared to the other samples.

The electron transfer kinetics of the OER and the electrocatalytic performance of the fabricated electrodes were analysed using Tafel curves. The Tafel slope values for Na–MnCo₂O₄/NiP, MnCo₂O₄/NiP and NiP were 70.2 mV dec⁻¹, 77.2 mV dec⁻¹, and 108 mV dec⁻¹, respectively. Na–MnCo₂O₄/NiP exhibited a lower Tafel slope compared with the other electrodes, as confirmed by the lower interfacial charge transfer resistance of Na–MnCo₂O₄/NiP.⁴⁹ In addition to this result, Na–MnCo₂O₄/NiP possessed faster electron transfer kinetics during the OER because of the presence of higher ECSA, sodium diffusion into oxygen vacancies, amorphous crystalline phases and the Mn³⁺/Mn⁴⁺ redox couple, as evidenced from the XPS results.

Electrocatalytic OER efficiency was studied by comparing the OER overpotential values. The overpotential values of NiP, MnCo₂O₄/NiP, and 1-Na–MnCo₂O₄/NiP were 320 mV, 240 mV and 214 mV, respectively (Fig. 7). The 1-MnCo₂O₄/NiP electrode exhibited the lowest overpotential compared with its NiP counterparts and all the other studied electrode systems. These overpotential responses and enhanced OER activity are mainly due to the amorphous–crystalline interactions, Na diffusion to the oxygen vacancies, and the highly active Mn⁴⁺ state; these factors facilitate the Mn³⁺/Mn⁴⁺ redox couple for efficient OER activity. In LSV, an anodic peak was observed between 1.3 V and 1.5 V vs. RHE; this peak corresponded to the oxidation of Ni. In the Na–MnCo₂O₄/NiP composite, Ni was in the +3-oxidation state and acted as one of the active sites for the OER. These Ni³⁺ ions underwent oxidation to form an active NiOOH intermediate, initiating the OER process. Among the various concentrations investigated, the 1-Na–MnCo₂O₄/NiP composite exhibited the formation of the active NiOOH intermediate at the lowest overpotential compared with the other electrodes. These results reveal that the optimum incorporation of sodium facilitates rapid formation of NiOOH at low overpotential, thereby enhancing the OER rate.

The long-term electrochemical stability of the Na–MnCo₂O₄/NiP electrode was thoroughly evaluated using LSV analysis before and after 1000 consecutive cycles of cyclic voltammetry (CV), as shown in the ESI† results (Fig. S5 and S6). Only a slight shift in LSV was observed after 1000 cycles of CV. In addition, the XRD and FE-SEM results (Fig. S7 and S8 of ESI†) revealed no significant change in the crystallographic or morphological characteristics of the electrode even after 1000 cycles of CV. These results confirmed that Na–MnCo₂O₄ exhibits high stability even after 1000 cycles of CV (Fig. S5 and S6 of ESI†).

3.3. Mechanism of the OER on Na–MnCo₂O₄/NiP

An electron transport mechanism during the OER at the Na–MnCo₂O₄/NiP electrode follows a metal site lattice oxygen mechanism (LOM) (Fig. 8). The presence of the higher-valent metal ion Mn⁴⁺ and its redox couples are primarily responsible for the reaction paths.^9,50 The high-valent Mn⁴⁺ species, coupled with abundant oxygen vacancies and a low overpotential of 214 mV, indicate that the OER follows the lattice oxygen mechanism (LOM).^9,51 The Mn⁴⁺ ions in the catalytic system were formed through NiP deposition, resulting in Mn³⁺/Mn⁴⁺ redox couple formation. Na incorporation into MnCo₂O₄ caused the diffusion of Na into the oxygen vacancies. Furthermore, the oxygen vacancies effectively reduced the energy barrier for H₂O adsorption on the Co³⁺ site in MnCo₂O₄; this was a crucial and indispensable step in the OER mechanism.³⁷ The delocalization of electrons at the neighbouring metal sites of oxygen vacancies in Na–MnCo₂O₄/NiP, which previously occupied the O 2p orbital, significantly enhanced surface reactivity.¹⁶ As a result, the introduction of oxygen vacancies in Na–MnCo₂O₄/NiP generated a higher density of active sites and ultimately led to enhanced electrocatalytic performance for water oxidation.⁵² The high-valent Mn⁴⁺ active site initially adsorbed OH⁻ and underwent deprotonation, producing O*. This reactive oxygen species then reacted with lattice oxygen, resulting in the elimination of O–O* in the second step, and an oxygen vacancy was created. Since Na incorporation into the catalyst significantly reduced oxygen vacancy formation energy, the elimination of O–O* and the OER became more effective.^53,54 In addition, the Mn⁴⁺ in the Mn³⁺/Mn⁴⁺ redox couple could reduce Jahn–Teller distortion and provided empty e_g orbital for efficient desorption of O₂.^55,56 Moreover, XPS data revealed the existence of Ni³⁺, and the higher valence state of Ni was favourable for OER activity. During the initial stage of the OER, Ni³⁺ ions formed an active NiOOH intermediate, which enhanced the rate of the OER.^57,58 The reaction involved three-electron transfer occurring in the rate determine step, as evidenced from the Tafel slope value of 70.2 mV dec⁻¹.^49,59

4. Conclusions

Porous spongy MnCo₂O₄ with Mn³⁺/Mn⁴⁺ redox couples and a suitable electronic environment with an optimum concentration of oxygen vacancies was fabricated through the incorporation of Na into the MnCo₂O₄ system and its deposition on NiP coating. Modified sol–gel derived spongy MnCo₂O₄ was calcined with a Na source to generate electron rich Mn and Co species at its lower oxidation states. The catalytic system and its electronic environment were modulated by treating the system in a reducing NiP bath and through electroless deposition. The Mn³⁺/Mn⁴⁺ and Co²⁺/Co³⁺ redox couples, diffusion of Na into oxygen vacancies and suppressed Jahn–Teller distortion provided an optimal electronic environment for the electrocatalytic OER by accelerating the oxygen desorption rate.

The enhanced charge transport characteristics in the Na–MnCo₂O₄/NiP electrode were shown by low charge transfer resistance at open circuit potential (R_ct = 309 Ω) and the high electrocatalytically active surface area (19.75 cm²). The electrode demonstrated significant OER performance in alkaline media, an OER overpotential of 214 mV at 10 mA cm⁻² and a Tafel slope of 70.2 mV dec⁻¹. These results were comparable to those of other related electrode materials and surpassed those of its NiP counterparts. During the OER on Na–MnCo₂O₄/NiP, NiOOH intermediate formation occurred at lower overpotential than that of the other studied electrodes; this formation initiated the OER and resulted in an enhanced OER. Na–MnCo₂O₄/NiP possessed high stability, even after 1000 cycles of cyclic voltammetry, because of the electrochemical tolerance of NiP systems and the surface coverage of the electrode with Na–MnCo₂O₄. This novel material design strategy included Na incorporation into an MnCo₂O₄ catalyst and the catalyst's deposition on an NiP coating to generate an OER electrode with enhanced OH adsorption, improved O₂ desorption kinetics and high OER performance.

Author contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. All the authors contributed equally.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

The authors gratefully acknowledge DST, New Delhi, for the facility support under the DST-FIST programme at St. John's College, Anchal, Kerala, India. A. A. K., R. B. N. and S. H. thank University of Kerala for research fellowships. The authors express our gratitude towards CLIF-Kerala University, STIC Kochi and FE-SEM Lab-Bannari Amman Institute of Technology for providing material characterization data.

Notes and references

1. Y. Zhang, J. Liu, Y. Xu, C. Xie, S. Wang and X. Yao, Chem. Soc. Rev., 2024, 53, 10620–10659 RSC .
2. Y. Zhu, Z. Tang, L. Yuan, B. Li, Z. Shao and W. Guo, Chem. Soc. Rev., 2024, 54, 1027–1092 RSC .
3. C. Hu, L. Zhang and J. Gong, Energy Environ. Sci., 2019, 12, 2620–2645 RSC .
4. D. G. Nocera, Acc. Chem. Res., 2012, 45, 767–776 CrossRef CAS PubMed .
5. P. Wang, S. Zhang, Z. Wang, Y. Mo, X. Luo, F. Yang, M. Lv, Z. Li and X. Liu, J. Mater. Chem. A, 2023, 11, 5476–5494 RSC .
6. H. Shi, K. Yang, F. Wang, Y. Ni and M. Zhai, CrystEngComm, 2021, 23, 7141–7150 RSC .
7. A. T. A. Ahmed, S. Sekar, S. S. Khadtare, N. T. Rochman, S. Lee, H. Kim, D. Y. Kim, H. Im and A. S. Ansari, J. Electroanal. Chem., 2023, 946, 117716 CrossRef CAS .
8. R. S. Redekar, A. T. Avatare, J. L. Chouhan, K. V. Patil, O. Y. Pawar, S. L. Patil, A. A. Bhoite, V. L. Patil, P. S. Patil and N. L. Tarwal, Chem. Eng. J., 2022, 450, 137425 CrossRef CAS .
9. A. A. Krishnan, S. Harikumar, M. A. Aneesh Kumar, R. B. Nair, S. Kurian, M. Ameen Sha and P. S. Arun, Catal. Sci. Technol., 2024, 14, 6155–6175 RSC .
10. A. Rebekah, S. Anantharaj, C. Viswanthan and N. Ponpandian, Int. J. Hydrogen Energy, 2020, 45, 14713–14727 CrossRef CAS .
11. A. Rebekah, E. A. Kumar, C. Viswanathan and N. Ponpandian, Int. J. Hydrogen Energy, 2020, 45, 6391–6403 CrossRef CAS .
12. X. Huang, H. Zheng, G. Lu, P. Wang, L. Xing, J. Wang and G. Wang, ACS Sustainable Chem. Eng., 2019, 7, 1169–1177 CrossRef CAS .
13. K. Xiao, Y. Wang, P. Wu, L. Hou and Z. Q. Liu, Angew. Chem., Int. Ed., 2023, 62, e202301408 CrossRef CAS PubMed .
14. W. Shi, Y. Zhang, L. Bo, X. Guan, Y. Wang and J. Tong, Inorg. Chem., 2021, 60, 19136–19144 CrossRef CAS PubMed .
15. W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich and Y. Shao-Horn, Energy Environ. Sci., 2015, 8, 1404–1427 RSC .
16. K. Zhu, F. Shi, X. Zhu and W. Yang, Nano Energy, 2020, 73, 104761 CrossRef CAS .
17. A. Sarkar and G. G. Khan, Nanoscale, 2019, 11, 3414–3444 RSC .
18. Y. Liu, W. Wang, X. Xu, J. P. Marcel Veder and Z. Shao, J. Mater. Chem. A, 2019, 7, 7280–7300 RSC .
19. J. Jia, L. Li, X. Lian, M. Wu, F. Zheng, L. Song, G. Hu and H. Niu, Nanoscale, 2021, 13, 11120–11127 RSC .
20. Y. Zhao, C. Chang, F. Teng, Y. Zhao, G. Chen, R. Shi, G. I. N. Waterhouse, W. Huang and T. Zhang, Adv. Energy Mater., 2017, 7, 1700005 CrossRef .
21. Q. Ma, Z. Chen, S. Zhong, J. Meng, F. Lai, Z. Li, C. Cheng, L. Zhang and T. Liu, Nano Energy, 2021, 81, 105622 CrossRef CAS .
22. B. Weng, F. Xu, C. Wang, W. Meng, C. R. Grice and Y. Yan, Energy Environ. Sci., 2017, 10, 121–128 RSC .
23. W. Chen, J. M. Zhang, Q. L. Xia, Y. Z. Nie and G. H. Guo, Phys. Chem. Chem. Phys., 2020, 22, 16007–16012 RSC .
24. V. S. J. Abish, D. H. Raja and D. J. Davidson, Mater. Lett., 2024, 362, 136210 CrossRef .
25. P. Huang, X. Feng, C. Zhou, X. Xu, G. Wang, H. Hu and F. Hu, Chem. Eng. J., 2024, 500, 156846 CrossRef CAS .
26. F. Qian, B. Zhao, M. Guo, Z. Wu, W. Zhou and Z. Liu, Sep. Purif. Technol., 2021, 256, 117583 CrossRef CAS .
27. A. Anantha Krishnan, A. K. Aneesh, R. B. Nair, R. Sivaraj, A. Lamiya, P. K. Jishnu, S. Kurian, T. Mathew, M. Ameen Sha and P. S. Arun, New J. Chem., 2022, 46, 22256–22267 RSC .
28. J. Ge, W. Zhang, J. Tu, T. Xia, S. Chen and G. Xie, Small, 2020, 16, 1–9 Search PubMed .
29. T. R. Silva, R. A. Raimundo, V. D. Silva, J. R. D. Santos, L. S. Ferreira, A. J. M. Araújo, F. J. A. Loureiro, F. F. da Silva, D. P. Fagg and D. A. Macedo, Colloids Surf., A, 2023, 672, 131626 CrossRef CAS .
30. K. Hua, X. Li, Z. Rui, X. Duan, Y. Wu, D. Yang, J. Li and J. Liu, ACS Catal., 2024, 14, 3712–3724 CrossRef CAS .
31. Z. Xiao, F. Xia, L. Xu, X. Wang, J. Meng, H. Wang, X. Zhang, L. Geng, J. Wu and L. Mai, Adv. Funct. Mater., 2022, 32, 2108244 CrossRef CAS .
32. R. B. Nair, A. A. Krishnan, A. M. A. Kumar, S. Rajendran, S. Harikumar, C. Vidhya, M. Ameen Sha, T. Mathew, S. Kurian and P. S. Arun, Energy Adv., 2024, 3, 2790–2800 RSC .
33. A. O. Bokuniaeva and A. S. Vorokh, J. Phys. Conf. Ser., 2019, 1410, 012057 CrossRef CAS .
34. T. Y. Ma, Y. Zheng, S. Dai, M. Jaroniec and S. Z. Qiao, J. Mater. Chem. A, 2014, 2, 8676–8682 RSC .
35. A. Nandy, A. Roychowdhury, T. Kar, D. Das and S. K. Pradhan, RSC Adv., 2016, 6, 20609–20620 RSC .
36. S. Rajendran, S. Saju, S. S. Mani, A. K. Asoka, A. Saha, P. S. Arun, B. Ghosh, T. Mathew and C. S. Gopinath, J. Mater. Chem. A, 2024, 13, 2105–2120 RSC .
37. K. Zeng, W. Li, Y. Zhou, Z. Sun, C. Lu, J. Yan, J. H. Choi and R. Yang, Chem. Eng. J., 2021, 421, 127831 CrossRef CAS .
38. X. Ge, Y. Liu, F. W. T. Goh, T. S. A. Hor, Y. Zong, P. Xiao, Z. Zhang, S. H. Lim, B. Li, X. Wang and Z. Liu, ACS Appl. Mater. Interfaces, 2014, 6, 12684–12691 CrossRef CAS PubMed .
39. W. Wang, L. Kuai, W. Cao, M. Huttula, S. Ollikkala, T. Ahopelto, A. P. Honkanen, S. Huotari, M. Yu and B. Geng, Angew. Chem., Int. Ed., 2017, 56, 14977–14981 CrossRef CAS PubMed .
40. D. S. Baji, H. S. Jadhav, S. V. Nair and A. K. Rai, J. Solid State Chem., 2018, 262, 191–198 CrossRef CAS .
41. M. Li, J. Wang, X. Guo, J. Li, Y. Huang, S. Geng, Y. Yu, Y. Liu and W. Yang, Appl. Surf. Sci., 2021, 536, 147909 CrossRef CAS .
42. M. A. Muñoz-Márquez, M. Zarrabeitia, E. Castillo-Martínez, A. Eguía-Barrio, T. Rojo and M. Casas-Cabanas, ACS Appl. Mater. Interfaces, 2015, 7, 7801–7808 CrossRef PubMed .
43. L. J. Gao, X. L. Song, J. T. Ren and Z. Y. Yuan, Inorg. Chem. Front., 2022, 9, 1964–1972 RSC .
44. M. Jiang, D. Xu, B. Yang, C. Zhang and M. Cao, Adv. Mater. Interfaces, 2021, 8, 2100188 CrossRef CAS .
45. X. Kong, T. Zhu, F. Cheng, M. Zhu, X. Cao, S. Liang, G. Cao and A. Pan, ACS Appl. Mater. Interfaces, 2018, 10, 8730–8738 CrossRef CAS PubMed .
46. H. Ding, H. Liu, W. Chu, C. Wu and Y. Xie, Chem. Rev., 2021, 121, 13174–13212 CrossRef CAS PubMed .
47. A. Guerrero, J. Bisquert and G. Garcia-Belmonte, Chem. Rev., 2021, 121, 14430–14484 CrossRef CAS PubMed .
48. K. Vance, M. Aguayo, A. Dakhane, D. Ravikumar, J. Jain and N. Neithalath, Int. J. Concr. Struct. Mater., 2014, 8, 289–299 CrossRef CAS .
49. S. Anantharaj, K. Karthick and S. Kundu, Mater. Today Energy, 2017, 6, 1–26 CrossRef .
50. R. Chen, Z. Wang, S. Chen, W. Wu, Y. Zhu, J. Zhong and N. Cheng, ACS Energy Lett., 2023, 8, 3504–3511 CrossRef CAS .
51. X. Wang, H. Zhong, S. Xi, W. S. V. Lee and J. Xue, Adv. Mater., 2022, 34, 2107956 CrossRef CAS PubMed .
52. S. Ghosh and R. N. Basu, Nanoscale, 2018, 10, 11241–11280 RSC .
53. C. Huang, J. Zhou, D. Duan, Q. Zhou, J. Wang, B. Peng, L. Yu and Y. Yu, Chin. J. Catal., 2022, 43, 2091–2110 CrossRef CAS .
54. J. S. Hong, H. Seo, Y. H. Lee, K. H. Cho, C. Ko, S. Park and K. T. Nam, Small Methods, 2020, 4, 1900733 CrossRef CAS .
55. C. Wei, Z. Feng, G. G. Scherer, J. Barber, Y. Shao-Horn and Z. J. Xu, Adv. Mater., 2017, 29, 1606800 CrossRef PubMed .
56. M. Wang, K. Chen, J. Liu, Q. He, G. Li and F. Li, Catalysts, 2018, 8, 138 CrossRef .
57. Y. Sun, T. Zhang, C. Li, K. Xu and Y. Li, J. Mater. Chem. A, 2020, 8, 13415–13436 RSC .
58. Z. Yang, Y. Zhang, C. Feng, H. Wu, Y. Ding and H. Mei, Int. J. Hydrogen Energy, 2021, 46, 25321–25331 CrossRef CAS .
59. H. Zhang, A. W. Maijenburg, X. Li, S. L. Schweizer and R. B. Wehrspohn, Adv. Funct. Mater., 2020, 30, 2003261 CrossRef CAS .

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj02163d

This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2025