Intrinsically robust cubic MnCoO_x solid solution: achieving high activity for sustainable acidic water oxidation

Abstract

This article reveals the promising potential of MnCoO_x solid solutions as an effective catalyst for the oxygen evolution reaction (OER). The solid oxide solution was synthesized by utilizing redox deposition of KMnO₄ and Co(NO₃)₂ on a carbon paper (CP) support with varying Co/Mn ratios. XRD and TEM analyses confirmed the homogeneous distribution of Co and Mn species in MnCoO_x. According to OER tests in acid electrolyte, the catalytic efficiency of Mn₁Co₅O_x surpassed that of Co₃O₄, MnO_x, and IrO₂, demonstrating overpotentials of 275 and 569 mV at a current density of 10 and 100 mA cm⁻², respectively. The investigation emphasizes the crucial roles of Co³⁺ and Mn⁴⁺ cations as well as oxygen vacancies (O_v) sites in facilitating the OER mechanism. These essential characteristics were duly observed in a Mn₁Co₅O_x solid solution catalyst. Furthermore, the introduction of a Mn dopant contributed to the durability of Mn₁Co₅O_x in acidic OER, thereby exhibiting outstanding catalytic stability for nearly 300 hours at 100 mA cm⁻² current density. In addition, DFT studies provided insight into how Mn₁Co₅O_x enhances the catalytic performance of the OER. In conclusion, this study offers valuable insight into designing efficient OER catalysts using MnCoO_x solid solutions as a promising candidate.

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Introduction

Using renewable energy sources for water electrolysis presents a reliable and sustainable approach to produce clean hydrogen (H₂) fuel.^1–4 In conventional electrolysis systems, the slow kinetics of their proton-coupled electron transfer electrochemical reaction and poor stability impede the progress of overall water splitting.^5–8 Thus, for optimizing the efficiency to meet such systems, the oxygen evolution reaction (OER) plays a decisive role.^9–11 The water electrolysis system featuring a proton exchange membrane (PEM) is attracting global interest due to its various benefits, for instance, elevated current densities, exceptionally pure gases, minimal ohmic losses, and superior compactness.^12,13 However, this technique requires electrocatalysts that resist corrosion and can function effectively in highly acidic environments. Due to their impressive durability and exceptional effectiveness, only Ir-based catalysts, especially IrO₂, are considered practical electrocatalysts for the OER in PEM devices.^14–16 Nevertheless, limited availability and high cost restrict their widespread applications. Moreover compared to Ir, ruthenium is approximately ten times less expensive, making it a desirable substitute for acidic OER.^17–19 RuO₂ has satisfactory OER activity in an acidic environment. However the OER long-term stability of Ru-based catalysts in acids or PEM reactors remains a significant issue.^20–24 As a result, it's an enormous task to develop OER catalysts that are both inexpensive and effective for applications in an acidic medium.

Several 3d transition metals, particularly cobalt-based catalysts, have been extensively studied as potential OER catalysts, such as Co₂TiO₄,²⁵ Co₃O₄,^26–28 CoFePbO_x,²⁹ and hetero-N-coordinated Co single atom catalysts.³⁰ Regardless, they are subjected to severe degradation under strong acidic and oxidative states. This results in a limited selection of OER catalysts with high abundance on earth that exhibit high activity and stability under acidic conditions.^31–34 An essential approach involves generating catalysts from transition metal oxides with enhanced resistance to acid-induced corrosion. For example, MnO_x has shown promise in acidic OER investigations by a self-regeneration mechanism under acidic conditions demonstrated by Huynh et al.³⁵ Meanwhile, Zhou et al.³⁶ discovered that the rutile alloy Mn–Sb–O systems can achieve 50 mA cm⁻² at an overpotential of 580 mV for acidic OER, but the stability of Mn–Sb oxides was not better than that of the artwork of IrO₂. Additionally, the correlation between cobalt oxides' active sites and the mechanism in acidic media is rarely studied. It is also reported that the intrinsic oxygen evolution reaction activity of Co₃O₄ can be enhanced by modifying its redox properties.³⁷ As a result, investigating the connection between redox properties and catalytic OER performance is intended to encourage mechanistic understanding.

In our previous investigation, we have reported the preparation of clusters based on cobalt through the bonding of ligands.^38,39 These clusters demonstrated exceptional electrochemical efficiency in the OER and oxygen reduction reaction (ORR). Thenceforth, we have been exploring the use of the potassium permanganate redox method to print cobalt–manganese oxide materials onto carbon paper (CP) surfaces. The Mn₁Co₅O_x prepared in our study shows notable distinctions from doped compounds. In this process, we employ the oxidizing properties of potassium permanganate to transform low-valent cobalt into a high-valent state, leading to the creation of a homogeneous solid solution of Mn₁Co₅O_x (which can be viewed as a Mn-rich doped Co₃O₄). A remarkable characteristic of the solid solution crystal structure is its ability to retain the crystal structure of the original solvent, thus maintaining identical composition, structure, and properties.⁴⁰ This solid solution belongs to a single phase and displays exceptional catalytic performance for the OER. The Mn₁Co₅O_x catalyst exhibited superior performance in an acid electrolyte, with overpotentials of 275 mV@10 mA cm⁻² and 569 mV@100 mA cm⁻². Moreover, the catalyst displayed remarkable catalytic stability for 300 hours at a current density of 100 mA cm⁻², which highlights the potential of MnCoO_x as a promising catalyst for electrocatalytic applications. Scheme 1 illustrates the utilization of MnCoO_x as a highly effective catalyst for OER mechanisms in acidic media. Furthermore, it highlights the novel aspects of this material in terms of material design and exceptional performance features in OER applications.

Scheme 1 Pictorial demonstration of the OER mechanism using MnCoO_x catalysts under acidic conditions and highlighting the paramount aspects of this study.

Experimental section

Electrocatalyst preparation

A 2 × 4 cm² CP substrate was cleaned using ultrasonication in ethanol and water, followed by treatment with HNO₃. MnCoO_x catalysts were synthesized using cobalt nitrate and potassium permanganate dissolved in water. The resulting solution was drop-cast onto CP and subsequently dried in an oven at 80 °C for 30 min. After rinsing, the CP was subjected to sonication for 10 seconds to remove the loosely attached particles. The drop-casting, drying, and cleaning steps were repeated to ensure complete and consistent coverage. The electrode precursor was then calcined at 300 °C for 2 hours, resulting in a catalyst loading of ∼2 mg on the CP substrate with a geometric area of 0.25 cm².

Characterization

The powder X-ray diffraction (XRD) patterns were obtained on an X-ray powder diffractometer (SmartLab, Rigaku Corporation) with Cu Kα radiation at 40 kV and 30 mA from 10° to 70° at a scan rate of 10° min⁻¹ to identify the phase structure of these electrodes. X-ray photoelectron spectroscopy (XPS) analyses were conducted on a Theta Probe system (base pressure: ∼10⁻⁸ Pa), equipped with a Phoibos 100 hemispherical analyzer and XR 50 X-ray source (SPECS GmbH) operated in the constant pass energy mode at 50 eV. Using CasaXPS, all spectra were corrected by using the C 1s signal (284.8 eV). Subsequently, peaks were fitted after a Shirley-type background subtraction utilizing a mixed Gauss–Lorentz method, with peak positions and full-width half maxima left unconstrained. The concentration of the surface Co³⁺ (Co³⁺% = A_Co³⁺/(A_Co³⁺ + A_Co²⁺)), Mn⁴⁺ (Mn⁴⁺% = A_Mn⁴⁺/(A_Mn⁴⁺ + A_Mn³⁺)), and surface oxygen vacancies (O_v% = A_Ov/(A_Ov + A_Ol + A_Oa)) was calculated based on the area under the peaks of Co 2p, Mn 2p, and O 1s, respectively. Note that A_i represents the peak area of the i species. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was carried out on an ICP-8100 (Shimadzu). The samples of MnCoO_x were dissolved in an aqua regia solution. The actual ratios of Co/Mn in the electrocatalyst were determined to be 1.53 (for Mn₁Co_1.5O_x), 2.46 (Mn₁Co_2.5O_x), 5.12 (Mn₁Co₅O_x), and 8.13 (Mn₁Co₈O_x) by ICP-OES. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) were carried out on a Thermo Fisher FEI Talos F200x (Thermo Fisher Scientific). X-ray absorption spectroscopy (XAS) measurements were performed on a laboratory device (easyXAFS300, easyXAFS LLC), which is based on Rowland circle geometries with spherically bent crystal analyzers (SBCA) and a silicon drift detector. Si (5,5,1) SBCA was used for Mn measurement at 35 kV, and Si (5,3,3) SBCA was used for Co measurement at 30 kV. The powder samples were thoroughly ground and mixed with polyethylene glycol using an agate mortar and pestle and pressed into Ø = 13 mm pellets. The pressed pellets were then sandwiched using Kapton tape.

Electrochemical measurements

All the electrochemical measurements were carried out on an electrochemical workstation CHI 760E using a standard three-electrode system with a platinum electrode (Tianjin Aida Hengsheng Technology Development Co., Ltd.) as the counter electrode and Hg/Hg₂Cl₂ electrode (saturated KCl solution) as the reference electrode. The potential of the Hg/Hg₂Cl₂ reference electrode was corrected using a reversible hydrogen electrode (RHE) after the experiments. The working electrode was cycled for 20 seconds at 100 mV s⁻¹ in 0.5 M H₂SO₄ (pH 0) to activate the electrode and then linear sweep voltammetry (LSV) was recorded at a scan rate of 5 mV s⁻¹. The current density was calculated based on the geometric area (0.25 cm²) of the electrode exposed to the electrolyte solutions. All the potentials were referred to the reversible hydrogen electrode by calibrating the reference electrodes using Pt sheets (purchased from Aldrich) as both the working and counter electrode in corresponding solutions. The iR compensation was performed manually after the measurement for 80% of the resistance.

DFT

In this work, all the calculations were conducted using the Viena ab initio simulation package (VASP),^41,42 and the structure optimization was based on density functional theory (DFT). The modeling of exchange–correlation energy was performed with the implemented generalized gradient approximation (GGA), proposed by Perdew–Burke–Ernzerhof (PBE).^43,44 The valence electronic states were expanded with the plane waves; and the core–valence interactions were described using the projector augmented wave (PAW) approach,⁴⁵ with the corresponding cutoff kinetic energies set to 400 eV. The DFT+U method with U = 2.0 eV and U = 3.2 eV was used to describe the 3d orbital of Co and Mn, respectively.^46,47 The Brillouin zone was set with a Gamma-centered k-point grid of 1 × 1 × 1 (ref. ⁴⁸), and to further improve the convergence of the simulated systems, a Gaussian functionalized Fermi surface, with a broadening magnitude of 0.1 eV, was used. The structures were first relaxed with the implemented conjugate-gradient algorithm, until the convergence tolerance of Hellmann–Feynman forces and energy on each atom was less than 0.05 eV Å⁻¹ and 10⁻⁴ eV per atom, respectively. Moreover, due to the periodic boundary conditions (PBC), there exist non-negligible interactions among periodical images; to reasonably overcome this, a separate vacuum layer with a thickness of 15 Å along the z-direction was applied. Also, the van der Waals (vdW) interactions were described by using the Grimme's DFT-D3 scheme with the application of dispersion correction.^49,50 In addition, for all the calculations, spin polarization was considered.

The Gibbs free energy was calculated via the following equation:

ΔG = E_DFT + ZPE-TS (1)

where ΔG, E_DFT, ZPE and TS represent the calculated Gibbs free energy, total energy obtained by DFT calculations, zero point energy correction and entropy correction, respectively.

The OER pathways are depicted as follows:

* + H₂O → OH* + H⁺ + e⁻ (2)

OH* + OH⁻ → O* + H₂O + e⁻ (3)

O* + H₂O → OOH* + H⁺ + e⁻ (4)

OOH* + H₂O → * + H⁺ + e (5)

The adsorption Gibbs free energy was calculated as follows:

ΔG_(O*) = E_(O*) − E_(*) − E_(H2O) + E_(H2) (6)

ΔG_(OH*) = E_(OH*) − E_(*) − E_(H2O) + 1/2E_(H2) (7)

ΔG_(OOH*) = E_(OOH*) − E_(*) − 2E_(H2O) + 3/2E_(H2) (8)

where the E_(*), E_(O*), E_(OH*) and E_(OOH*) are the total energies for pristine systems, and adsorbed O*, OH*, and OOH*, respectively.⁵¹

Results and discussion

Synthesis and structural and morphological characterization of MnCoO_x solid solution

The synthetic route of MnCoO_x solid solutions with varying Co/Mn ratios is illustrated in Fig. 1a; they were synthesized directly on CP by a redox reaction of Co²⁺ and MnO₄⁻, followed by annealing in air. XRD analysis was performed to determine the crystalline structure of MnCoO_x (Fig. 1b). Six strong diffraction lines were observed at 2θ ∼19.0°, 31.3°, 38.5°, 44.8°, 59.3°, and 65.2°, which correspond to the planes (111), (220), (311), (400), (511) and (440) of cubic Co₃O₄ in the Fd-3m (227) space group (JCPDS #42-1467).^52,53 Furthermore, due to the significant dispersion of manganese ions throughout the cobalt framework, no reflections corresponding to MnO_x species (e.g. MnO, Mn₂O₃, MnO₂, etc.) were found in the XRD patterns of these MnCoO_x samples even at a relatively high concentration of MnO_x (∼40 mol%). By examining the XRD patterns of MnCoO_x catalysts with varying doping levels, a slight shift towards lower angles of around 36.8° for (311) was observed with the increasing amount of Mn dopants (Fig. S1†), indicating that the Mn cations were well doped into the lattice of Co₃O₄ to form MnCoO_x solid solutions, and we also obtained the XRD pattern of Mn₁Co₅O_x annealed at different temperatures. The results showed that the crystalline phase of the Co₃O₄ structure was formed at 200 °C. And the crystallinity was improved when the sample was calcined at high temperatures. Of note, no peaks for Mn oxides were observed even after the annealing at 500 °C, indicating the thermal stability of Mn₁Co₅O_x composites (Fig. S2†).

Fig. 1 Structural and morphological characterization of MnCoO_x electrocatalysts: (a) schematic illustration of the synthesis process, (b) XRD patterns, (c) TEM image of the Mn₁Co₅O_x catalyst, (d) linear-scanning profiles of the cross-section of the Mn₁Co₅O_x catalyst, and (e) HAADF-STEM image with EDS elemental mapping (cobalt, green; manganese, purple; oxygen, pink). XANES spectra of (f) Co K-edge and (g) Mn K-edge. Corresponding Fourier transforms of (h) Co K-edge and (i) Mn K-edge EXAFS spectra for Mn₁Co₅O_x and the standard references.

Subsequently, the MnCoO_x solid solutions were analyzed by TEM to determine their elemental composition and morphology. TEM images revealed that the majority of oxide particles ranged in size from 6 to 8 nm (as depicted in Fig. S3†). The lattice fringes observed in Fig. 1c exhibit an interplanar distance of 0.288 nm, 0.245 nm, and 0.467 nm, which correspond to the (220), (311), and (111) planes of cubic Co₃O₄, respectively.^54–56 Intriguingly, no lattice fringes for MnO_x species were observed, indicating that Mn was successfully doped within the lattice of Co₃O₄. Furthermore, the linear-scanning profiles depicted in Fig. 1d also confirmed that Mn atoms were successfully distributed within the lattice of Co₃O₄. Uniform lattice fringes can be seen in the line sweep area, and the distribution of elements obtained in the line sweep area also show that Co and Mn exist simultaneously with the content of Co more than that of Mn. The elemental mapping, illustrated in Fig. 1e, provides more evidence that Co and Mn species are homogeneously distributed within the MnCoO_x solid solutions.

X-ray absorption near edge structure (XANES) spectra, obtained at the Co and Mn K-edge, were utilized to examine the electronic structure and local coordination of Mn₁Co₅O_x.³⁹ As displayed in Fig. 1f, the peak intensity of Mn₁Co₅O_x is similar to that of Co₃O₄ and different from that of Co-foil. Meanwhile, it can also be observed that the pre-line of Mn₁Co₅O_x completely coincides with that of Co₃O₄. This confirms that Co²⁺ and Co³⁺ co-exist as the major valence states of Co species in Mn₁Co₅O_x. In contrast, the XANES spectra at the Mn K-edge revealed that the peak intensity of the white-line for Mn₁Co₅O_x is significantly higher than that of the reference Mn₃O₄ and Mn₂O₃ (Fig. 1g). Moreover, the pre-line for Mn₁Co₅O_x shifted toward higher binding energies compared to that of Mn₂O₃, indicating that the average valence of Mn species in Mn₁Co₅O_x should be higher than +3. This also suggests a state of electron deficiency near the Mn site, which is related to the use of potassium permanganate oxidation. To further investigate the coordination environment of the Mn₁Co₅O_x catalyst, R-space extended X-ray absorption fine structure (EXAFS) fitting analysis was performed at Co and Mn K-edges, as shown in Fig. 1h and i. Mn₁Co₅O_x exhibited an expansion of the Co–O bond distance from 1.81 Å to 1.83 Å compared to the control group of CoOOH. In contrast Mn₁Co₅O_x showed a reduction of the Co–O bond distance from 1.91 Å to 1.83 Å when compared to that of Co₃O₄.⁵⁷ Furthermore, Mn₁Co₅O_x exhibited a reduced Mn–O bonding distance from 1.80 Å to 1.67 Å relative to that of Mn₂O₃, plausibly due to Mn's valence state being higher than +3. Specifically, the Co–O bonds in Mn₁Co₅O_x had an electron density similar to that of Co₃O₄, which is slightly larger than CoOOH and relatively less than Co₃O₄ in length. The Mn–O bonds in Mn₁Co₅O_x with low electron density is smaller than that of Mn₂O₃, which facilitated the formation of a solid-solution structure with an approximately uniform M–O bond length and accelerated electron transfer between Mn and Co atoms, promoting the catalytic activity in the OER (vide infra).

Electrocatalytic OER in acidic solution

To assess the electrochemical activity of MnCoO_x samples towards electrocatalytic water splitting, OER tests of the as-synthesized nanocomposites deposited on CP were performed in a solution of 0.5 M H₂SO₄ (pH 0) employing a conventional three-electrode setup. Before the measurements, the reference electrode was calibrated relative to the reversible hydrogen electrode (RHE) (Fig. S4†).⁵⁸ Electrocatalytic measurements suggested that Mn₁Co₅O_x exhibited superior OER response and outperformed its MnCo_yO_x (y: 1.5, 2.5, and 8) counterparts (Fig. 2a). Apparently, the Mn₁Co₅O_x catalyst demonstrated the highest current density and lowest Tafel slope (Fig. S5†). We further investigated the effect of annealing temperatures in the range of 200 to 500 °C and discovered that Mn₁Co₅O_x displays optimal OER activity after annealing at 300 °C for 2 h (Fig. S6†). Moreover, the OER polarization curves demonstrated that the Mn₁Co₅O_x composite exhibited superior catalytic efficiency compared to Co₃O₄, MnO_x,, a CP support and IrO₂, indicating that the OER occurred over the Mn₁Co₅O_x surface (Fig. 2b).

Fig. 2 Catalytic OER performance. (a) OER polarization curves with different Mn/Co ratios of MnCoO_x. (b) OER polarization curves of Mn₁Co₅O_x, Co₃O₄, MnO_x, IrO₂, and CP in a 0.5 M H₂SO₄ solution. (c) Overpotentials of Mn₁Co₅O_x (pink), IrO₂ (orange) and Co₃O₄ (blue) at a current density of 10 and 100 mA cm⁻². (d) Tafel plots derived from (a). (e) OER polarization curves of Mn₁Co₅O_x before and after iR correction. (f) Nyquist plots of Mn₁Co₅O_x at E_OCV, E_j=10, and E_j=20. (g) O 1s XPS spectra of MnCoO_x catalysts. (h) The behavior of E (V vs. RHE) at 200 mA cm⁻² in different O_v populations.

Specifically, the Mn₁Co₅O_x showed overpotentials of 275 and 569 mV at a current density of 10 and 100 mA cm⁻², respectively (Fig. 2c). These values are in good agreement with those of IrO₂ (296 mV@10 mA cm⁻² and 629 mV@100 mA cm⁻²) and are much lower than those of Co₃O₄ (371 mV@10 mA cm⁻² and 653 mV@100 mA cm⁻²). Such improved catalytic OER activity of Mn₁Co₅O_x can be ascribed to the cooperation between the Mn and Co components. Additionally, the surface of Mn₁Co₅O_x effectively interacts with the intermediates through an oxygen atom, and the bonding interactions (–Mn–O–Co–O–) in the intermediates appear to be essential in determining the OER activity as a whole.^59–63 The superior OER activity over the Mn₁Co₅O_x surface stems from its smallest Tafel slopes of 52 mV dec⁻¹, which is considerably lower than those of Co₃O₄ (65 mV dec⁻¹), IrO₂ (94 mV dec⁻¹) and MnO_x (187 mV dec⁻¹), hence suggesting the favorable OER kinetics of Mn₁Co₅O_x (Fig. 2d). Furthermore, after iR compensation, the maximum current density of the Mn₁Co₅O_x catalyst can reach up to 1 A cm⁻² at 2.0 V (vs. RHE), as shown in Fig. 2e. It is worth noting that similar results were also obtained from the test on the PtTi substrate, indicating that the current originated from the activity of Mn₁Co₅O_x (Fig. S7†).

To order to enhance our understanding of the electrode kinetics during the OER, electrochemical impedance spectroscopy (EIS) was conducted at open circuit voltage (OCV), E_j=10, and E_j=20 with frequencies ranging from 10⁶ Hz to 10 mHz (Fig. 2f). The equivalent circuit used to model the data is shown in the inset of Fig. 2f. The electrical circuit consists of the solution resistance (R_u), film resistance (R_f), charge transfer resistance (R_ct), and diffusion resistance (W_o). The capacitive elements used, CPE1 and CPE2, represent the constant phase element of the film and the electrical double layer, respectively. The R_ct of Mn₁Co₅O_x was measured to be 18.11 Ω, 7.56 Ω, and 2.72 Ω at OCV, E_j=10, and E_j=20, respectively (here, “j” stands for current density). The smaller R_ct means better electron-transfer kinetics during the OER. Moreover, we examined the intrinsic activity of the catalyst, as characterized by its electrochemical active surface area (ECSA), which is linearly proportional to the double-layer capacitance (C_dl) derived from cyclic voltammetry (CV) curves with various sweep rates in the non-faradic region (Fig. S8 and S9†). As shown in Fig. S10,† the C_dl value of Mn₁Co₅O_x was significantly larger than that of Co₃O₄, indicating an increase in active sites available for the OER.

Oxygen vacancies (O_v) exist widely in transition metal oxides due to their low formation energy. The formation energy of O_v increases with the energy difference between the upper edge of the O 2p band and the Fermi level. Thus, only a certain amount of O_v can be created in oxides and they have a direct influence on the M–O bonds near the O_v. They can promote the adsorption of water molecules on the catalyst surface, thereby improving the reactivity of the active site and OER performance. Therefore, we obtained XPS spectra to analyze the O_v in MnCoO_x; the binding energy peaks at 530.2 eV, 531.2 eV and 532.7 eV belong to the lattice oxygen (O_l), oxygen vacancy (O_v) and adsorbed oxygen (O_a) respectively (Fig. 2g). The Mn₁Co₅O_x sample had the highest O_v proportion of 32.6%, followed by Mn₁Co_2.5O_x at 27.5%, Mn₁Co_1.5O_x at 24.0%, and Mn₁Co₈O_x at 20.9%, as shown in Table S1.† Correspondingly, the reaction potentials at 200 mA cm⁻² decreased with the increase in oxygen vacancy content, with E (Mn₁Co₅O_x) < E (Mn₁Co_2.5O_x) < E (Mn₁Co_1.5O_x) < E (Mn₁Co₈O_x). Next, we correlated the O_v concentration with the corresponding OER activity. As shown in Fig. 2h, the OER activity over these MnCoO_x composites increases proportionally to the surface O_v proportion, i.e., Mn₁Co₅O_x with the highest O_v proportion showed the best activity (lowest E_j=20 value). These results distinctly indicate that the presence of surface O_v in these MnCoO_x composites plays a crucial role in promoting OER activity, which is further substantiated by the DFT studies, vide infra.

The ability of the MnCoO_x catalyst to withstand acidic electrolytes is a crucial aspect for practical applications. The chronopotentiometry (CP) measurements were carried out to evaluate its long-term stability at varied constant current densities of 100, 200, 500, and 1000 mA cm⁻² in 0.5 M H₂SO₄ (pH 0), as shown in Fig. S11.† As evidenced by the potential remaining at around 1.97 V@100 mA cm⁻² (for ∼300 hours) and 2.01 V@200 mA cm⁻² (∼110 hours), as shown in Fig. 3b, our results demonstrate a remarkable durability of the Mn₁Co₅O_x catalyst, compared to various other reported catalysts (Fig. 3a and Table S2†). These experimental outcomes substantiate that Mn₁Co₅O_x electrocatalysts exhibited promising electrochemical activity and superior durability for the OER.

Fig. 3 Stability of MnCoO_x. (a) Comparison of the durability of Mn₁Co₅O_x catalysts in acidic solution with that of other reported catalysts. Of note, samples 1–5 were measured at 100 mA cm⁻², and the others, at 10 mA cm⁻². (b) Potential required to reach 100 and 200 mA cm⁻² in 0.5 M H₂SO₄ for Mn₁Co₅O_x measured using chronopotentiometry method. (c) Chronopotentiometric measurement of the Mn₁Co₅O_x catalyst at a constant OER current density of 200 mA cm⁻² recorded in an H-type water electrolysis cell in 0.5 M H₂SO₄ with the anode and cathode separated by a Nafion 115 membrane at room temperature.

Furthermore, the Mn₁Co₅O_x catalyst demonstrated a ∼15-fold increase, which is a significantly longer lifetime compared to that of Co₃O₄ at 100 mA cm⁻² (Fig. S12†), thus indicating that the Mn dopant substantially enhances the stability of MnCoO_x during the acidic OER. For further investigations, chronopotentiometry measurements of the Mn₁Co₅O_x catalyst were performed while maintaining a constant OER current density of 200 mA cm⁻² using a water electrolysis cell designed in an H-type configuration, featuring a Nafion 115 membrane to separate the anode and cathode (Fig. 3c). The results showed a monotonic increase in the E_j=200 in the case of the Mn₁Co₅O_x catalyst from 666 mV to 825 mV over 100 hours of continuous operation.

Study of the utilized MnCoO_x catalyst

This study investigated the activity loss of MnCoO_x/CP electrocatalysts during the electrochemical process by comparing the pristine and utilized catalysts. The XRD analysis was used for comparison, which shows that the peak intensities at 2θ = 31.3° (for Co₃O₄ (220) facet), 44.8° (Co₃O₄ (400)), and 59.3° (Co₃O₄ (551)) were less intense in the utilized Mn₁Co₅O_x/CP catalyst (Fig. 4a), hence indicating the deformation of the MnCoO_x oxide structure, significant to the deactivation of the OER activity. In contrast, the CP support can be identified at 2θ = 26.5°. Furthermore, the TEM image of the utilized Mn₁Co₅O_x catalyst showed an amorphous layer of about 1 nm thickness, as shown in Fig. 4b and S13,† indicating the formation of Co(OH)_x hydroxide species. Furthermore, XPS was applied to investigate and compare the surface composition and oxidation state of the pristine and utilized MnCoO_x catalysts. In the high-resolution spectra of Co 2p, the deconvoluted binding energies (B.E.) at 781.72 eV and 780.40 eV can be assigned to Co²⁺ 2p_3/2 and Co³⁺ 2p_3/2 species, respectively (Fig. 4c).^52,64,65 Also, for the Mn 2p spectra shown in Fig. 4d, the B.E. peaks at ∼642.4 eV and 653.7 eV can be associated with Mn³⁺ species.^64,66 In comparison, other peaks at 644.0 and 655.3 eV are affiliated with Mn⁴⁺.⁶⁶ As listed in Table S1,† the Co³⁺ and Mn⁴⁺ species were reduced to Co²⁺ and Mn³⁺ during the OER process. The O 1s B.E. peaks of fresh Mn₁Co₅O_x can be identified by two components centered at 530.1 eV and 531.8 eV (Fig. S15†), depicting the O_l and O_v species.^67,68 Interestingly, the hydroxyl species (O_H) was found at ∼533.1 eV in the utilized Mn₁Co₅O_x catalyst, signifying the surface amorphous structure of the utilized oxide electrocatalyst, which is well consistent with the observations in the XRD and STEM analyses. These results imply that the Co³⁺ and Mn⁴⁺ species and O_v sites in the MnCoO_x solid solution catalysts play an important role in the OER, which is consistent with the previous literature studies.^67,69

Fig. 4 (a) XRD pattern of the pristine and utilized Mn₁Co₅O_x/CP electrocatalysts. (b) STEM image of utilized Mn₁Co₅O_x. XPS spectra of the pristine and utilized Mn₁Co₅O_x catalysts: (c) Co 2p and (d) Mn 2p. Electrolysis conditions: a 0.5 M H₂SO₄ solution for 10 h at 100 mA cm⁻² at 25 °C.

The stability of Mn₁Co₅O_x primarily arises from the incorporation of Mn into the crystal structure of Co₃O₄. The results of XAS and XPS suggested that the addition of a small amount of Mn enhances the conductivity of Co₃O₄ by increasing the number of 3d orbital electrons. Simultaneously, the presence of Mn reduces the number of outer orbital electrons in the local vicinity, leading to a decreased local lattice constant and improved stability of the local structure. This, in turn, enhances the overall stability of the crystal structure. TEM images of the spent MnCoO_x sample after a 10 hour reaction test reveals the presence of a small amorphous layer on the catalyst surface, while the remaining regions retain the crystal structure characteristics. XRD analysis of the employed catalyst confirms that MnCoO_x still maintains an overall crystal structure, as depicted in Fig. 4a. The results from TEM and XRD indicate that the sustained catalytic durability originates from the doped crystal structure.⁷⁰

To investigate the ion release levels of catalysts during stability testing, Co₃O₄ and Mn₁Co₅O_x catalysts were subjected to testing at a current density of 100 mA cm⁻². The final electrolyte was collected and prepared for inductively coupled plasma (ICP) analysis to determine the concentrations of Co and Mn ions in the solution. Upon comparative analysis of the results presented in Table S3,† it is worth noting that the Co₃O₄ catalyst exhibited a concentration of 89 ppm Co cations in the solution following a 14 hour stability test. In contrast, the Mn₁Co₅O_x catalyst showed low concentrations of 38 ppm Co and 6 ppm Mn cations in the solution after a 25 hour stability testing period. These outcomes revealed that the addition of Mn enhances the structural durability of the catalyst, retarding the release of Co cations in the solution and thus extending the lifespan of the catalyst.

DFT calculations

Density functional theory (DFT) calculations were performed to elucidate further the fundamental mechanism behind the enhanced OER activity of Mn₁Co₅O_x. We considered the plausibility of Mn doping sites to locate their low energy configurations and calculated the configurations of 10 random bulk Mn₁Co₅O_x molecules, as shown in Fig. S16,† with the isomers arranged from low to high relative energies (high to low stability). This proves that the chosen lowest energy configuration remains relatively thermodynamically stable. The band structure of bulk Co₃O₄ was calculated, and the results indicate a band gap of 1.32 eV (Fig. S17†), demonstrating semiconductor behavior. Furthermore, we also calculated the OER activity of bare Co₃O₄(311), MnCoO_x(311) and O_v_MnCoO_x(311), respectively, and the configurations of the calculated OER are shown in Fig. S18 and S19.† The free energy diagrams of the OER at 0 V and 1.23 V are shown in Fig. 5a for Co₃O₄(311), MnCoO_x(311) and O_v_MnCoO_x(311). In contrast, the formation of the intermediate O* has the highest activation energy, which is thermodynamically unfavorable. However, after Mn doping,⁷¹ the overpotential of the OER is sharply reduced from 2.79 eV (Co₃O₄(311)) to 0.82 eV (MnCoO_x(311)).

Fig. 5 (a) Reaction-free energies of the OER pathways at 0 V and 1.23 V. (b) The d-band center of Co active sites.

Furthermore, the overpotential of the OER decreases to 0.29 eV upon the introduction of O_v. The d-band center is typically used as a descriptor to explain the direct interaction of the catalyst with the adsorbate, and the OER activity is regulated by modulation of the d-band center energy level (E_d). Therefore, we also calculated the projected density of states (PDOS) of the active site Co for Co₃O₄(311) and MnCoO_x(311), respectively. The results are shown in Fig. 5b; the incorporation of Mn can significantly elevate the d-band center position of Co₃O₄ (E_d = −2.05 eV) to near the Fermi level (E_d = −1.78 eV). Interestingly, after the introduction of O_v, the position of the d-band center at the active site is equilibrated (E_d = −1.88 eV). During the OER process, a moderate adsorption strength of intermediate species can lower the energy barrier of the OER. Therefore, the incorporation of Mn and the introduction of O_v can stabilize reaction intermediates, thus reducing the energy barrier of the OER and enhancing catalytic activity, as shown in Fig. S20 and S21.† (ref. ⁷²) The results of the theoretical calculations also agree with the experimental phenomena.

Conclusions

The study highlights the promising potential of MnCoO_x solid solutions as an efficient catalyst for the OER. The solid solutions were synthesized by redox deposition of KMnO₄ and Co(NO₃)₂ on a CP support under varying Co/Mn ratios. The crystalline nature of MnCoO_x was evaluated by XRD analysis, and TEM analysis verifies the homogeneous distribution of Co and Mn species. OER tests of MnCoO_x solid solutions deposited on CP revealed that Mn₁Co₅O_x was the most efficient catalyst, outperforming Co₃O₄, MnO_x, the CP support, and IrO₂, suggesting its practical application in electrocatalysis. Moreover, Mn₁Co₅O_x exhibited excellent durability and increased active sites, making it a promising candidate for OER applications. Following the XPS analysis, this study showed that Co³⁺, Mn⁴⁺, and O_v sites in the MnCoO_x solid solution catalyst play an essential role in the OER, with the Mn dopant significantly enhancing the catalyst's stability during acidic OER. Corroborative DFT computations validated the empirical observations and provided insight into how Mn₁Co₅O_x improves OER catalytic performance. This study offers valuable guidance for designing efficient OER catalysts, highlighting MnCoO_x solid solutions as a promising candidate.

Author contributions

G. L. and S. G. designed the concept of this work. J. J. Z. and Y. Zhao performed major experiments. J. J. Z. and analyzed the experimental data. J. J. Z. and S. G. wrote the first manuscript. A. R., Z. U. D. B., and G. L. wrote the final manuscript. L. X. performed the computational study. Y. C. performed TEM. All the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (22172167).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta05233h

‡ J. J. Z., A. R. and Y. Z. contributed equally to this work.

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