Manganese-based oxide electrocatalysts for the oxygen evolution reaction: a review

Abstract

The oxygen evolution reaction (OER), as an essential process in water decomposition and air batteries, has received increasing attention in the context of clean energy production and efficient energy storage. With their abundant composition and morphology, manganese-based oxides (MnO_x) offer great possibilities for the exploration and design of OER catalysts. In this paper, three classes of MnO_x materials, including MnO₂, Mn₂O_3, and Mn₃O₄, are systematically reviewed and their development and applications in OER systems are comprehensively presented. Subsequently, the effects of Jahn–Teller distortion and the question of the active site and stability of MnO_x in the OER are discussed, and the presence of Mn³⁺, which is considered essential for OER activity, and strategies for improving performance are proposed. This paper focuses on the impact of crystal structure, catalytic mechanisms, and design strategies on MnO_x.

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1. Introduction

The energy crisis caused by the exploitation and over-consumption of fossil fuels and the increasingly severe climate change have emerged as two prominent issues constraining human development.^1–3 Hydrogen can replace fossil fuels as the main energy source, and improving hydrogen production by electrolysis of water is one of the most promising sustainable methods. Nevertheless, the development of hydrogen evolution in water electrolysis is limited by the anodic oxygen evolution reaction (OER) as a consequence of its moderate multi-electron transfer process. The most commonly used electrocatalysts for the OER are noble metals and their oxides, such as Ir, Ru, IrO₂, and RuO₂, but their extreme cost and scarcity prevent their large-scale application. Therefore, the development of oxygen-evolving electrocatalysts with excellent catalytic properties, elevated stability, and low cost is essential.^4–7 Thus far, transition metal matrices (Fe, Co, etc.) and transition metal oxides, nitrides, carbides, hydroxides, and sulfides have been proven to have elevated catalytic activity in the field of water electrolysis.^8–11

Manganese-based oxides (MnO_x) are attractive materials in various fields due to their low price, natural abundance, and low toxicity. With its wide range of oxidation states, MnO_x can be broadly classified as MnO₂ (oxidation state of +4), Mn₃O₄ (mixed state), Mn₂O₃ (oxidation state of +3), and MnO (oxidation state of +2).¹² Take the most common MnO₂, for example, there are six crystal structures of MnO₂, which means that MnO₂ can be prepared by adjusting its preparation method to obtain catalysts with different surface areas and controllable morphology, and it is found that the conversion between different crystal structures of MnO₂ will improve its catalytic performance.¹³ However, there are still insufficient studies on MnO_x in the OER direction. The catalytic activity of MnO_x is extremely different from that of iron, cobalt, and nickel catalysts, whose limited active sites and low conductivity limit their development. Therefore, it is necessary to explore its catalytic properties in-depth and implement modification strategies such as defect engineering or heteroatom incorporation.¹⁴

Based on the current research progress, the conventional adsorption mechanism of the OER in an alkaline environment is first discussed in this paper. Second, the latest research progress of MnO_x (MnO₂, Mn₃O₄, and Mn₂O₃) as OER catalysts is reviewed. Of these MnO_x materials, MnO₂ is currently the most well studied, followed by Mn₃O₄ and Mn₂O₃. After that, the Jahn–Teller distortion, active sites, and stability issues in MnO_x catalysts and suggested ways to improve OER activity, including the introduction of vacancies and the formation of heterogeneous interfaces are discussed. We believe that this review can serve as a reference for the development of high-quality manganese oxide catalysts.

2. The OER mechanism

The reaction mechanism of the OER is thought to be through two possible pathways: the adsorption evolution mechanism (AEM) and lattice oxygen mechanism (LOM).¹⁵ The AEM is a more general mechanism for OER catalysts (Fig. 1a), and the LOM normally occurs in extremely covalent oxides.^4,16,17 In this section, we focus on the proportionality of AEM to different reaction intermediates. The AEM pathway is a four-coordinated proton-electron transfer reaction centered on metal ions, as shown in eqn (1)–(4).¹⁸ MOH, MO, and MOOH represent intermediates adsorbed on active sites. The hydroxy anion (OH⁻) is first adsorbed on the surface of the metal site (M) through the single electron oxidation process, forming adsorbed MOH at the M site. Then, MOH intermediates are oxidized to form MO substances. The subsequent O–O bond formation step is usually considered the rate-determining step for most catalysts in this step where MO reacts with another OH⁻ to form MOOH. Finally, MOOH is oxidized by a single electron transfer process, releasing O₂ and restoring the original M active site.¹⁹

OH⁻ + M → MOH + e⁻ (ΔG₁) (1)

MOH + OH⁻ → MO + H₂O + e⁻ (ΔG₂) (2)

MO + OH⁻ → MOOH + e⁻ (ΔG₃) (3)

MOOH + OH⁻ → O₂ + M + H₂O + e⁻ (ΔG₄) (4)

Fig. 1 (a) Schematic diagram of the OER mechanism indicated (using MnO₂ as an example). (b) Gibbs free energy and reaction coordinate diagram of active intermediates (horizontal line) of the OER. The blue line and red line represent the real (typical) catalyst and the ideal catalyst respectively. (c) Overpotential (−η^OER) Gibbs free binding energy difference with different reaction sites OOH* and O*. (a) Reproduced with permission from ref. 17. Copyright 2023, American Chemical Society. (b) Reproduced with permission from ref. 25. Copyright 2010, John Wiley and Sons. (c) Reproduced with permission from ref. 30. Copyright 2019, Royal Society of Chemistry.

The Gibbs free energy variation of the above four-electron transfer process can be calculated using the following eqn (5)–(8).^20,21

ΔG₁ = G_MOH − G_M − (G_OH⁻ − G_e⁻) (5)

ΔG₂ = G_MO + G_H2O(l) − G_MOH − (G_OH⁻ − G_e⁻) (6)

ΔG₃ = G_MOOH − G_MO − (G_OH⁻ − G_e⁻) (7)

ΔG₄ = G_M + G_O2(g) + G_H2O(l) − G_MOOH − (G_OH⁻ − G_e⁻) (8)

Each elementary step has the specific free energy of the corresponding intermediate. The step with the maximum free energy difference (G = Max [ΔG₁, ΔG₂, ΔG₃, ΔG₄]) is identified as the rate-determining step and the theoretical overpotential of the overall reaction (η = (G/e) −1.23 V).²² Specifically, catalytic activity is estimated from the magnitude of the potential determining step G^OER of the OER.²³ That is to say, all adsorption intermediates of MOH, MO, and MOOH should balance the fracture of the O–M bond and the formation of the O–O bond to ensure that the bonding strength of O–M is neither too strong nor too weak.²⁴ An ideal catalyst in thermodynamics requires that all four steps have the same order of reaction-free energy as shown in the figure (Fig. 1b).²⁵ Under this condition, all reaction-free energies are zero at the equilibrium potential (1.23 V), if ignoring the desorption step. Given the constant adsorption energy difference between MOH and MOOH (ΔG_MOOH = ΔG_MOH + 3.2 ± 0.2 eV), OER overpotential can be determined from MO adsorption energy, which means that the second or third process is the decisive step. According to Sabatier's principle, the ideal catalyst requires that the adsorption strength of key intermediates is neither too strong nor too weak.^26–28 As such, η^OER as (ΔG_MO − ΔG_MOH) leads to a universal volcanic shape relationship (Fig. 1c); an ideal OER electrocatalyst with optimum activity requires an intermediate binding strength and an ΔG_MO − ΔG_MOH value of 1.6 eV (corresponding to the peak of the volcano).^29–31

Although the electrocatalytic decomposition of water has been studied since the last century and the basic mechanism of the anodic OER has been revealed, there are still different debates on the mechanism of water decomposition.^32,33 Understanding the underlying mechanisms can only help us to find the source of active sites for different materials and to bring the catalyst performance close to the theoretical thermodynamic limit, which contributes to the understanding of the necessary reactions of MnO_x in the OER process. In the following subsections, we focus on MnO_x electrocatalysts with different crystal structures and their impact on electrochemical properties.

3. Application of manganese-based oxides in the OER

The oxygen-evolving complex (OEC) of photosystem II uses catalytic Mn₄CaO_n clusters to generate molecular oxygen from water. It is inspired by this MnO_x catalyst that is receiving more and more attention.^34,35 Preliminary studies have shown that manganese oxides enhance water oxidation due to their elevated thermodynamic stability, suitability of specific surface areas, and oxidation enthalpy.^36,37 Moreover, with the introduction of additional characterization techniques and the development of electrochemistry, more stable and efficient materials have been designed and synthesized to improve the catalytic performance of MnO_x. Recent advances in MnO_x as an OER electrode material are discussed in detail in this section.

3.1 MnO₂

MnO₂ and its composites have been widely used in supercapacitor and battery applications due to their low cost, environmentally friendly nature, high operating voltage, and theoretical capacity. Notably, MnO₂ exists in a variety of crystalline polycrystalline forms, such as α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂ (Fig. 2), and the crystal structure is diversified and controllable.^38–40 The MnO₂ crystal consists of [MnO₆] octahedral units with oxygen atoms at the top of the six corners of the octahedron and manganese atoms in the center. These octahedral units are connected by common corners or edges, resulting in the formation of various tunneling and layered structures. MnO₂ crystals (α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂) can be classified into three categories: a one-dimensional tunneling structure, a two-dimensional layered structure (δ-MnO₂), and a three-dimensional network structure formed by the common sides of [MnO₆] octahedra λ-MnO₂.^41,42 As a consequence of the multivalent state of Mn and a large number of crystalline phases, MnO₂ invariably contains impurities from multiple polycrystalline phases, a complex structure–property relation.⁴³ Different crystalline phases significantly affect the catalytic properties of MnO₂ and, in particular, the crystal structure of MnO₂ plays a key role in determining the catalytic properties. The OER activity is reported to follow the order α-MnO₂ > β-MnO₂ > δ-MnO₂.^44,45 What follows is an elaboration of the physical and chemical properties of MnO₂ electrocatalysts with different crystalline phases in terms of catalytic activity, electron/ion conductivity, and electrochemical properties.

Fig. 2 Polyhedral representation of several manganese oxide crystals (α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂). Reproduced with permission from ref. 40. Copyright 2019, American Chemical Society.

3.1.1 α-MnO₂. α-MnO₂ has a one-dimensional 2 × 2 (0.46 × 0.46 nm²) tunnel structure, which acts as a channel for ions and molecules. The presence of water and cations in the α-MnO₂ crystal structure acts as a support tunnel, otherwise, the structure would collapse and transform into another crystal, β-MnO₂, which is the most intuitive difference between the two crystal structures.⁴⁶ Manganese centers are mixed valence (+3 and +4), where charge neutrality can be maintained by cations within the tunnel structure.⁴⁷ In addition, the tunnel space in α-MnO₂ can also alleviate the strain induced by the ion insertion/desorption process, so that α-MnO₂ has excellent structural stability.⁴⁸ Extensive characterization shows that the OER catalytic performance of MnO₂ is greatly dependent on the crystal structure and follows the order of α-MnO₂ > β-MnO₂ > δ-MnO₂, as shown in Fig. 3a. This is due to the existence of α-MnO₂ active sites in the edge-shared [MnO₆] unit, the suitable tunnel size, large surface area providing high accessibility to water molecules, existence of a large number of di-μ-oxo bridges as protonation sites in α-MnO₂ and the mixed valence of Mn (AOS = 3.7); at the same time, in situ electrochemical impedance spectroscopy (EIS) showed the smallest charge transfer resistance compared with alternative crystal forms of MnO₂.⁴⁵

Fig. 3 (a) Comparison of the overpotential of MnO₂ at 10 mA cm⁻². (b) XRD patterns of MnO₂ with different IL additions and different reaction times. (c) Raman spectra of MnO₂–NF at different hydrothermal times. (d) Resistivity measurements of several types of MnO₂. (a) Reproduced with permission from ref. 45. Copyright 2014, American Chemical Society. (b) Reproduced with permission from ref. 49. Copyright 2019, Elsevier. (c) Reproduced with permission from ref. 50. Copyright 2019, Elsevier. (d) Reproduced with permission from ref. 51. Copyright 2019, Elsevier.

Gu used ionic liquids (ILs) as a structural inducer to transform the crystalline structure and morphology of the reaction product from β-MnO₂ nanorods to α-MnO₂ nanowires with a high specific surface area (Fig. 3b), and X-ray absorption near edge structure (EXANES) and X-ray photoelectron spectroscopy (XPS) analyses show that the transformed α-MnO₂ has a high Mn III content and an abundance of oxygen vacancies, which is beneficial for the OER. At 10 mA cm⁻², the overpotential is 394 mV and the Tafel slope is low at 49 mV ·dec⁻¹. Also, Gu's research has shown that the crystal structure has a much more decisive and critical influence on the catalytic OER than the surface area.⁴⁹ Subsequently, Zhou's group similarly used the structural transformation of MnO₂ to induce its phase transition from δ to α by merely adjusting the hydrothermal time (Fig. 3c). The transformed α-MnO₂ has nanowire and nanosheet morphologies that expand the specific surface area and thus expose more catalytically active sites; the higher content of Mn III ions and oxygen vacancies detected by XPS and electron paramagnetic resonance (EPR) will contribute to better catalytic performance; and density functional theory (DFT) calculations show that α-MnO₂ has a more suitable energy band gap (which leads to better electron conductivity and transfer).⁵⁰ Ni et al. reported that single-atom Ag-doped MnO₂ nanowires can catalyze both the OER and oxygen reduction reaction (ORR), due to the strong Ag–O bond weakening the surrounding Mn–O bond, leading to significant crystal distortion, which likely favorably modulates the binding energy of the reaction intermediates, resulting in higher activity. Moreover, the synergistic effect of Ag and MnO₂ should enhance the electrocatalytic activity, and the metallic and transition metal oxides modify the electronic structure of the metal and induce a shift of the d-band center. XPS studies confirm that Ag doping produces a large number of surface oxygen vacancies, which open the octahedral walls of MnO₂ and facilitate ion diffusion. And the resistivity of α-MnO₂ was also measured with a four-point probe, which decreased by more than 20 times after Ag doping (Fig. 3d).⁵¹

3.1.2 β-MnO₂. β-MnO₂ uses a 1 × 1 tunnel structure consisting of corner connected [MnO₆] octahedral chains. Among the various polymorphisms of MnO₂, β-MnO₂ has long been considered an inert electrocatalyst for oxygen conversion due to its elevated thermodynamic stability and perfect lattice structure.⁵² Although both β-MnO₂ and α-MnO₂ have tunnel structures, it has much lower OER activity, and this difference in activity is related to their structures. β-MnO₂ has the smallest BET surface area and porosity due to its small tunnel size, which cannot accommodate any cations, and its crystal structure has the highest Mn⁴⁺ content. Comparatively, α-MnO₂ has a larger tunnel structure to accommodate cations (K⁺) and water, and therefore has a much higher BET surface area and porosity; more importantly, α-MnO₂ has mixed valence (Mn³⁺/Mn⁴⁺) in it, which makes it a better catalyst for OER performance, and it may contribute to the OER process by following a similar redox cycle between the Mn³⁺ and Mn⁴⁺ OER process.^45,53,54

In order to improve the catalytic activity of β-MnO₂, Gu introduced Ru³⁺ precursor salt in the hydrothermal reaction of β-MnO₂. The crystal splitting phenomenon caused by doping drastically affected the OER and ORR activity of β-MnO₂. After doping, the obtained nanorods exhibit a fragmented structure as shown in Fig. 4a, and the Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) analysis indicates that the Ru-doped sites are mainly interstitial sites within the tunneling structure of β-MnO₂, with some exposure on the surface during crystal splitting, which is the main reason for the elevated OER activity. To explore the active sites for its reaction, DFT confirmed that crystal splitting exposes the atomically dispersed Ru–O species on the MnO₂ surface as OER active sites, while the induced lattice strain and Mn³⁺ state, as well as the abundant oxygen vacancies, can further enhance the ORR activity of the catalyst, which eventually leads to an OER/ORR potential difference of only 0.63 V.⁵⁵ Yao et al. controlled the concentration of KCl and synthesized β-MnO₂ crystals with a bipyramidal structure with tunable morphology using a simple hydrothermal method (Fig. 4b). In situ studies revealed that the K⁺ cation influences the morphology of β-MnO₂ crystals by affecting the formation of the α-K_0.5Mn₄O₈ intermediate phase and the subsequent phase transition process. The elevated concentration of K⁺ cations reduced the prism length of β-MnO₂ crystals by affecting the formation of α-K_0.5Mn₄O₈ nanowires and the tunneling transition process. They speculated that if a suitable surfactant is available to limit the directional attachment of K_0.5Mn₄O₈ nanowires while maintaining the K⁺ cations, it is possible to significantly reduce the size of the β-MnO₂ structure and improve its electrochemical properties. Their work undoubtedly brings current ideas for the design of such materials for applications in supercapacitors, fuel cells, and catalysts.⁵⁶

Fig. 4 (a) Schematic diagram of Ru doping-induced crystal splitting of β-MnO₂ and its OER and ORR sites. (b) The schematic diagram illustrates the directional attachment and phase transformation process. During the phase transition, K⁺ stable cations are expelled from the crystal structure. (c) Schematic diagram of the synthesis of 0.5Fe–Mn–O nanosheets. (d) Linear sweep voltammetry (LSV) in 1 M KOH aqueous solution. (e) Overpotential and Tafel slope of MnO₂ compounded OER electrocatalysts. (a) Reproduced with permission from ref. 55. Copyright 2021, American Chemical Society. (b) Reproduced with permission from ref. 56. Copyright 2018, Elsevier. (c and d) Reproduced with permission from ref. 59. Copyright 2022, Elsevier.

3.1.3 δ-MnO₂. Birnessite is also expressed as δ-MnO₂ and has a two-dimensional (2D) layered structure. Structurally, the octahedral co-edge element [MnO6] in δ-MnO₂ consists of stable Mn⁴⁺ and unstable Mn³⁺. In contrast to the stable Mn⁴⁺, the unstable Mn³⁺ is responsible for hydroxide ion adsorption and oxygen evolution; therefore, Mn³⁺ is considered to be the active site in Mn-based oxide catalysis.⁵⁷ However, due to the inherently low conductivity and Mn³⁺ concentration, the OER performance of δ-MnO₂ is still lower than that of catalysts based on Fe–Co–Ni benchmarks.

The OER activity of δ-MnO₂ primarily depends on the chemical composition, crystal structure, and morphology.⁵⁸ Cheng et al. prepared ultra-thin amorphous δ-MnO₂/ε-Fe₂O₃ heterojunction nanosheets with oxygen vacancies, denoted as 0.5Fe–Mn–O (Fig. 4c–d), and the catalyst shows high output OER activity, which requires only 299 and 322 mV overpotential to drive current densities of 100 and 300 mA cm⁻² and a Tafel slope of only 35.1 mV dec⁻¹. Determination of its indicated chemical state by XPS revealed a higher Mn³⁺ concentration and proportion of defective oxygen in 0.5Fe–Mn–O compared to δ-MnO₂, which was attributed to additional distorted [MnO₆] structural units in δ-MnO₂ caused by evolved intercalated ε-Fe₂O₃. The observations further confirm that 0.5Fe–Mn–O consists of δ-MnO₂/ε-Fe₂O₃ heterojunctions with abundant Jahn–Teller active Mn³⁺, oxygen vacancies, and Fe³⁺ doping, which facilitates the initiation of interlayer cooperative catalysis and improves OER performance. Further DFT calculations revealed that the oxygen vacancies (Fe–Vo–Mn) immediately adjacent to the Fe and Mn atoms are the optimal OER active sites for the 0.5Fe–Mn–O catalyst, indicating that the oxygen vacancies play an important role in the modified framework for the OER.⁵⁹ The two-dimensional LDH(+)–Birnessite(−) hybrid catalyst synthesized by Chen et al. exhibited advanced OER catalytic activity and stability with an overpotential of only 407 mV at a current density of 400 mA cm⁻² and a Tafel of only 43 mV dec⁻¹, indicating that this hybrid catalyst has good potential for electrolysis of water for hydrogen production. The interlayer NiFe LDH (+) also changes the electronic structure of δ-MnO₂ and generates an electric field between NiFe LDH (+) and δ-MnO₂(−), which leads to a significant reduction of the overpotential of the OER. DFT calculations confirm the modulation of NiFe LDH during the OER, and the upward shift of Fe-3d orbitals in LDH promotes the electron transfer from δ-MnO₂ to LDH, which greatly enhances the performance of the OER.⁶⁰

Interlayer cations play an essential role in tuning the electronic structure of δ-MnO₂ materials. An effective method for increasing the catalytic activity of layered manganite oxides was developed by McKendry et al. through cooperative doping of a two-dimensional lattice and interlayer. The doping of the MnO₂ lattice with cobalt allows for more efficient hole transport to the active sites of the interlayer iron oxides; meanwhile, the iron hydrazine intercalation procedure reduces the particle size and increases the concentration of Mn³⁺ defects. With the addition of intra-layer cobalt and inter-layer iron, a highly efficient OER catalyst was obtained with an overpotential potential drop of 425 mV compared to birnessite.⁶¹ Pu synthesized efficient catalysts containing nickel in two-dimensional layered metal oxides (δ-MnO₂) by a simple ion-exchange reaction. The intercalation mechanism and oxidation states of nickel and manganese have been explored using the synchrotron X-ray absorption spectroscopy (XAS) technique. It was shown that the intercalation of nickel occurs at the Mn³⁺ site by reducing Mn³⁺ to Mn²⁺ and that the intercalated nickel is the active site for the OER. The synthesis scheme of this work can be broadly applied to provide ideas for the structural development of other layered metal oxides.⁶²

In Fig. 4e, we list the overpotentials and Tafel plots of MnO₂ compounded with other materials in recent years, most of which are limited to around 300 mV, and their range of application and catalytic activity is not superior. Although MnO₂ has great potential as a non-precious metal oxide catalyst in the field of water electrolysis, it still suffers from problems such as the small number of active sites. Meanwhile, some researchers have designed different MnO₂ nanomorphologies to increase the number of active sites on the surface, with nanowires exhibiting the highest OER mass activity due to the highest specific surface area and nanotubes exhibiting the highest specific activity due to the high Mn³⁺ surface concentration/surface defects.^63–65 On the other hand, in addition to the use of monoatomic metals and MnO₂ composites to enhance catalytic activity, Xu et al. used MnO₂-NWRS/CNTs, a composite of carbon nanotubes and MnO₂, to show good catalytic activity as well as conductivity in the ORR and OER.⁶⁶ We believe that the development of MnO₂ in the field of OER electrocatalysts will be accelerated with the preparation of more advanced materials and the design of chemical compositions based on the existing foundation.

3.2 Mn₂O₃

In the family of MnO_x, Mn₂O₃ has attracted extensive attention in the field of supercapacitors and batteries due to its excellent energy intensity and simple synthesis process. However, there are few types of studies on electrocatalysis.^67,68 The crystal structure of Mn₂O₃ is shown in Fig. 5a, where the Mn³⁺ cation is coordinated to the O anion in an octahedral, orthogonal crystal system.⁶⁹ The OER activity of Mn₂O₃ is attributed to the presence of a Jahn–Teller distorted shared edge octahedron occupied by Mn³⁺ (d⁴) centers with significantly elongated Mn–O bonds. Due to the asymmetric occupation of e_g orbitals, the bond energy is reduced and the Mn³⁺–O octahedron is activated to participate in the OER.⁷⁰ It has been reported that the overpotential value of Mn₂O₃ is considerably higher than that of other transition metal oxides, and due to the special d-electron structure of Mn₂O₃, it is very difficult to synthesize pure phases of Mn₂O₃.⁷¹ Zhao monitored the OER process on the Mn₂O₃ catalyst surface using in situ liquid TEM and observed the development of oxygen nanobubbles around the catalyst in space. In addition, in situ TEM observed that the thickness of the amorphous layer on the Mn₂O₃ surface is in a variable state, and they conjectured that the high-valence Mn⁴⁺ should be reduced to Mn³⁺ in the generation of molecular O₂. This suggests that the amorphous layer on the Mn₂O₃ surface is the main active region as well as that the valence of Mn is constantly changing, possibly to be more adapted to the different steps of the OER process.⁷² The poor electronic conductivity and relatively low OER dynamics of Mn₂O₃ should be improved by combining with an additional state with higher OER activity. Bigiani used the first row of transition metal (Fe, Co, Ni) oxide nanoparticles (NPs) as OER electrocatalysts in alkaline media. Through plasma-assisted fabrication, Fe₂O₃, Co₃O₄, and NiO NPs can be efficiently dispersed into Mn₂O₃ and form tight oxidation interfacial contacts, which can enhance charge carrier transport and facilitate the diffusion of reactants and products.⁷³ The unique hollow RuO₂/Mn₂O₃ composite fiber prepared by Yoon shows excellent OER electrocatalytic performance in alkaline media.⁷⁴ Another research group, Yang et al. synthesized Ni₂P/Mn₂O₃ ultra-high electrochemically active area catalysts by the electrostatic spinning technique, which required only a low overpotential of 280 mV to provide 10 mA cm⁻² current density, superior to many previously reported Mn₂O₃ catalysts. The large aspect ratio of the one-dimensional nanofibers facilitates electrolyte penetration and oxygen bubble release, and the introduction of Mn modulates the electronic structure of Ni₂P and greatly improves the OER performance.⁷⁵

Fig. 5 (a) Mn₂O₃ crystal structure. (b) Crystal structure of Mn₃O₄, octahedra of [MnO₆], and d-orbital splitting conformation. (c) Schematic representation of the liquid phase exfoliation method with Mn₃O₄ formation. (d) LSV curves and Tafel slope plots. (c) The OER process on the MnFe₂O₄/NF catalyst. (a) Reproduced with permission from ref. 88. Copyright 2021, John Wiley and Sons. (b) Reproduced with permission from ref. 89. Copyright 2020, John Wiley and Sons. (c) Reproduced with permission from ref. 78. Copyright 2022, Elsevier. (d) Reproduced with permission from ref. 82. Copyright 2020, Elsevier. (e) Reproduced with permission from ref. 87. Copyright 2021, Elsevier.

3.3 Mn₃O₄

Mn₃O₄ belongs to the spinel group, where divalent and trivalent manganese ions are distributed in two distinct lattice sites. The oxygen ions are closely arranged in a cubic form, with divalent manganese ions occupying tetrahedral voids and trivalent manganese ions occupying octahedral voids (Fig. 5b).⁷⁶ The general formula of the spinel type is AB₂O₄, which consists of a tetrahedral cation A with a divalent charge and an octahedral cation B with a trivalent charge. The spinel structure is nicely conductive due to the excellent electron transfer between the different oxidation states A²⁺ and B³⁺.⁷⁷ Gowda et al. carried out an in-depth study of the origin of the different active sites of the OER and ORR in Mn₃O₄. They synthesized two-dimensional Mn₃O₄ by liquid phase exfoliation (LPE), as shown in Fig. 5c, and DFT calculations confirmed the presence of two sites, Mn(III) and Mn(II), on the (112) oriented surface of Mn₃O₄, which are highly active for the OER and ORR, respectively.⁷⁸ In the early studies on Mn₃O₄, electrochemical and spectroscopic techniques were used to characterize their reactivity in the OER.⁷⁹ For example, MnO_x films obtained by electrodeposition have a different composition and polycrystalline morphology; after cyclic activation, the Mn(OH)₂ generated by the cathode causes a phase transition of (δ-MnO₂) to an intermediate product of (α-Mn₃O₄) type, and the catalytic activity of MnO_x after activation is much higher than that of the original MnO_x. It has been shown that different manganite systems in the literature have different OER activity curves, as different manganite compositions and crystals may be introduced by the preparation methods.⁸⁰

Recently, the synergetic chemical coupling effect and interface structure engineering have played a crucial role in the development of Mn₃O₄ materials. Guo enabled the chemical coupling of active CoO nanoclusters with Mn₃O₄ octahedra; the strong interaction between the two components promotes the formation of a high-energy interface Mn–O–Co and a highly oxidized state of CoO, and this high-energy structure has a large number of active sites that facilitate rapid charge transfer. The starting potential and catalytic kinetic analysis of the catalysts indicate that the OER activity of CoO/Mn₃O₄ is mainly derived from CoO, while the coupling of CoO to Mn₃O₄ further enhances the catalytic performance and provides a higher catalytic activity than the Ru/C OER activity.⁸¹ Mn₃O₄ is the most stable form of MnO_x at high temperatures and can be used as a suitable substrate for the construction of composite structures with other active materials. Wang constructed one-dimensional Mn₃O₄/NiCo₂S₄ rod-like nanomaterials based on the two-phase composite of metal oxide and sulfide, and its catalytic activity is shown in Fig. 5d. According to the Kirkendall effect, the outward diffusion of Mn⁴⁺ and O²⁻ and the sulfidation process of NiCo₂O₄ proceed almost simultaneously, which leads to the formation of a considerable number of oxygen defects at the interface of Mn₃O₄ and NiCo₂S₄ because the diffusion rate of Mn⁴⁺ is much faster than the diffusion rate of O²⁻. The chemical reaction at the interface leads to the formation of vacancies and defects, which can serve as catalytically active sites.⁸² Huang's work prepared hybrid catalysts of nanoparticles Co₃O₄/Mn₃O₄ anchored on (reduced graphene oxide) rGO substrates. Although Co₃O₄ has excellent ORR/OER activity, it is very unstable at high current densities, and the introduction of Mn₃O₄ exactly addresses this issue. Meanwhile, the large specific surface area provided by the layered rGO facilitates the contact between the catalyst and the electrolyte, thus improving the utilization of active sites and the reaction kinetics. In addition, DFT calculations showed that after coupling Mn₃O₄ (211) with Co₃O₄(111), the ΔG_MOH→OH value on the Mn₃O₄ (211) surface of Co₃O₄/Mn₃O₄ was close to −1.23 eV, which is the conceptual value of ΔG_MOH→OH for the catalyst at 0.4 V equilibrium potential at pH = 14, yielding that the Co₃O₄/Mn₃O₄ Mn₃O₄ (211) surface is the active surface for the ORR/OER.⁸³ Cho and his team demonstrated a breakthrough synthesis of 4 nm Mn₃O₄ nanoparticle (NPs) catalysts with an overpotential of only 395 mV at 10 mA cm⁻², a performance superior to any other Mn-, Fe-, Co-, and Ni-based electrocatalysts reported so far under neutral conditions. They then further optimized their catalytic performance by Ni doping, and XRD analysis showed Ni doping-induced lattice distortion of the Mn₃O₄ NPs, which also led to the production of distorted octahedral Mn(III) species, enhancing the number of active sites in the OER. According to electrokinetic studies, the OER catalytic mechanism of pristine and 5% Ni-doped Mn₃O₄ NPs is similar, namely involving reversible one-electron and one-PCET processes prior to the RDS. This implies that the doping of Ni in Mn₃O₄ NPs does not affect the OER mechanism under neutral conditions. The enhanced activity of Ni doping may be due to the promotion of this step of O–O bond formation.^84–86

Some research groups use low-cost transition metal spinel materials as OER catalysts. Kim synthesized Mn–Fe-based materials with a spinel structure and proposed the OER mechanism on the MnFe₂O₄/NF electrode during water electrolysis in an alkaline solution (Fig. 5e). The OH⁻ ions decomposed in water combined with the lattice oxygen of Fe²⁺ and the FeO in MnFe₂O₄ is oxidized to Fe–OOH (Fe³⁺). In the next step, Fe–OOH continues to be converted to Fe–OO by OH⁻, and then O₂ is oxidized and desorbed. After O₂ is generated, oxygen vacancies are formed in the iron lattice of the anode catalyst. At this point, Mn³⁺ with strong reducing power moves the oxygen in its lattice through the spinel lattice into the vacancies generated around Fe²⁺, which is then reduced to Mn²⁺. After that, the oxygen vacancies formed in the Mn²⁺ lattice are rapidly filled with OH⁻ ions in water and regenerated into Mn³⁺ ions. Through this process, the MnFe₂O₄ spinel lattice is restored to its initial state. In conclusion, the synergistic oxygen transfer process between Fe²⁺ (or Fe³⁺) and Mn³⁺ (or Mn²⁺) catalytic species in the MnFe₂O₄/NF of the OER anode contributes to the long-term stability of the catalytic electrode for the OER.⁸⁷

4. MnO_x issues and strategies

MnO_x is a potential OER catalyst whose oxides exist in more than 30 different stable polycrystalline forms, and the crystal structure and variable valence states are key to improving its electrostatic properties.^44,90,91 However, their electrocatalytic activity is still not sufficient to replace noble metal oxides. To explore efficient MnO_x electrocatalysts, it is essential to clarify the dominant factors controlling the electrocatalytic activity of these oxides.⁹² Above we have comprehensively reviewed the progress of MnO₂, Mn₃O₄, and Mn₂O₃ materials, many of which have encountered several common problems such as Jahn–Teller distortion, active sites, stability, etc. These critical issues have hindered the understanding of catalytic reaction mechanisms and the development of efficient and stable catalysts.

4.1 Jahn−Teller distortion

Jahn–Teller distortion is very common in Mn-based materials, and it is believed that the presence of Mn³⁺ can induce this phenomenon, thereby explaining the structural degradation of Mn-based electrode materials.⁹³ The high spin electron configuration of the Mn(III) (t_2g)³(e_g)¹ octahedron in the octahedral unit usually experiences Jahn–Teller distortion, resulting in long and flexible Mn–O bonds, for the OER, (e_g)¹ is considered to be the best electronic configuration for transition metal-based catalysts.⁹⁴ Compared with manganese(IV), the d⁴ electron configuration of manganese(III) is more inclined to show the Jahn–Teller effect. In addition, based on the strong Jahn–Teller effect, the transition from Mn⁴⁺ to Mn³⁺ inevitably prolongs the Mn–O bond and causes electron redistribution on the surface.^95,96

Kang et al. stabilized the Jahn–Teller active Mn³⁺ species by replacing the Mn⁴⁺ ions with the more electronegative Ru⁴⁺ ions (Fig. 6a), which led to the stabilization of the Jahn–Teller active Mn³⁺ species and also provided the electronegative Ru sites. Due to bond competition, the adjacent (Mn–O) bond is weakened by (Ru–O), leading to distorted [MnO₆] octahedra with elongated (Mn–O) bond distances, which stabilizes the Mn³⁺ ion. Although Ru substitution also provides electrocatalytically active Ru sites for α-MnO₂, this contribution is smaller than the stabilization of Mn³⁺ species caused by adjusting the (Mn–O) bond covalency.⁹⁷ Yang proposed a strategy to thermally induce structural deformation of calcium manganese ore (CaMnO₃, denoted as CMO) to improve the oxygen catalytic activity of CMO. When O atoms were partially extracted from CMO (denoted as D-CMO), the structure of [MnO₆] octahedra was deformed due to the Jahn–Teller effect (Fig. 6b). They used simulations to model the reaction pathway of the OER (Fig. 6c). The Mn cation on the surface can coordinate with OH⁻ to form an Mn–OH bond, O–Mn–O(OH) is the starting point of the OER process, while the third step with the formation of OOH* (O* + OH⁻ → OOH* + e) is the rate-determining step (RDS), and the RDS shows that the theoretical overpotential of the OER for D-CMO (0.74 V) is lower than that of CMO (0.82 V). When O₂ is adsorbed on D-CMO, the average number of electron transfers from Mn to O atoms is more than that of CMO. This means that the Mn–O and strong interactions are shorter, which contributes to the activation of O₂ and reduces the Gibbs free energy of the activation step (* + O₂ → O₂*).⁹⁸ The above examples demonstrate that the strategy of inducing [MnO₆] octahedral deformation using the Jahn–Teller effect can provide a new and effective way to improve the electrocatalytic properties of MnO_x.

Fig. 6 (a) Schematic diagram of the evolution of the adjacent (Mn–O) bond when the (Ru⁴⁺–O) bond is incorporated into the α-MnO₂ lattice. (b) Schematic diagram of the crystal structure process of O in CaMnO₃ being extracted. (c) Reaction pathways and corresponding reaction free energy diagrams for the OER of CMO(121) and D-CMO(121) at 1.23 V potential. (a) Reproduced with permission from ref. 97. Copyright 2018, Elsevier. (b and c) Reproduced with permission from ref. 98. Copyright 2021, Elsevier.

4.2 Active sites

The second is the question of active sites in MnO_x. A remarkable correlation between the amount of Mn³⁺ and the catalytic activity of the OER has been reported. Therefore, strategies to increase the activity of MnO_x must take into account the enhancement and stabilization of the Mn³⁺ species amount. Numerous previous studies have also improved the catalytic performance by introducing Mn(III) in different crystalline phases and found that a large amount of Mn(II) and Mn(III) was oxidized to Mn(IV) in electrocatalytic tests, leading to a decrease in activity.^99,100

Back in 2012, Takashima's group found that the low catalytic activity of MnO₂ under neutral conditions can be attributed to the instability of Mn³⁺, whose accumulation on the catalytic surface requires Mn²⁺ oxidation at a potential above 1.4 V, as shown in Fig. 7a. They effectively stabilized the Mn³⁺ species by the formation of N–Mn bonds through the coordination of amine groups with the Mn sites on the MnO₂ electrode surface, and in situ spectroelectrochemical techniques demonstrated that stabilization of surface-associated intermediate Mn³⁺ species is an effective strategy to reduce the electrolytic water overpotential of MnO₂.¹⁰¹ Later, Park, et al. in 2014 chose a pyrophosphate-based Mn compound (Li₂MnP₂O₇) as a model system and observed the effect of the Mn(III) state itself on water oxidation catalysis by adjusting the manganese valence in Li₂MnP₂O₇. Remarkably, the OER catalytic performance increases continuously as the average oxidation state of manganese in Li_2−xMnP₂O₇ increases from 2 to 3, as shown in Fig. 7b.¹⁰²

Fig. 7 (a) Schematic illustration of a current density (j) vs. potential (U) curve for a pristine δ-MnO₂ electrode under neutral pH conditions. (b) Increase in the average oxidation state of Mn from 2 to 3 in Li_2−xMnP₂O₇ and schematic representation of the catalytic performance. (c) Schematic diagram of the band structure of the activated catalyst system derived from the “α-Mn₃O₄-3e” model. (d) Schematic diagram of the oxidation state of Mn ions in the δ-MnO₂ reaction at different pH values. (e) Proposed mechanism for the OER on MnO_x under (left) acidic and (right) alkaline pH conditions. (f) Schematic free energy profiles of overpotential results versus the difference between the adsorption energies of O* and OH* intermediates for MnO₂, etc. in acid media. (g) A schematic diagram of the charge redistribution process in Mn_0.73Ru_0.27O_2−δ. (a) Reproduced with permission from ref. 101. Copyright 2012, American Chemical Society. (b) Reproduced with permission from ref. 102. Copyright 2014, American Chemical Society. (c) Reproduced with permission from ref. 105. Copyright 2018, Proceedings of the National Academy of Sciences of the United States of America. (d) Reproduced with permission from ref. 107. Copyright 2012, American Chemical Society. (e) Reproduced with permission from ref. 108. Copyright 2014, American Chemical Society. (f and g) Reproduced with permission from ref. 109. Copyright 2022, RSC Publishing.

In recent years, more scholarly studies have pointed out that Mn³⁺ is the active center of the OER, and its electronic configuration is (t_2g)³(e_g)¹, which plays an important role in the water oxidation reaction because the e_g orbital is involved in the σ-bonding to the anion adsorbate, the occupancy of this orbital directly determines the binding energy of the reactants and leads to a volcano-like activity profile.⁹⁴ Higher Mn³⁺ content in the manganese group is beneficial for enhancing the adsorption of –OH at the catalyst surface, and higher surface oxidation valence states are also beneficial for improving the OER activity of manganese-based materials.¹⁰³ In studying the OER activity of β-MnO₂ on the (101) and (110) faces, Nakamura's group found that the interlayer charge ratios of Mn²⁺ and Mn⁴⁺ produced two Mn³⁺ species on the (101) face, resulting in 11 times more Mn³⁺ coverage on the (101) face than on the (110) face, and thus a higher OER activity on the (101) face.¹⁰⁴ The study by Chan et al. explained that the presence of Mn³⁺ enhances the catalytic effect of the OER. They observed through Raman spectroscopy that the Mn³⁺ tetrahedron (T_d) produces local strain on the oxide lattice. This strain causes the O 2p valence band to be higher than the Mn³⁺ tetrahedron (T_d) and Mn⁴⁺ octahedron (O_h) valence bands and leads to the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to decrease, while the reduced HOMO–LUMO gap promotes the oxygen evolution reaction. As shown in Fig. 7c, the Mn³⁺ ions at the Td site are unable to form Mn⁴⁺, which causes a rearrangement of the Mn-d and O 2p states and promotes the formation of oxygen vacancies, factors that facilitate the OER to produce oxygen.¹⁰⁵ Moreover, in the study of spinel structures, the nature of their OER activity is also closely related to the valence states of Mn. Wei found that Mn in the octahedral sites was identified as the active site of MnCo₂O₄ and that the ORR/OER activity of MnCo₂O₄ exhibited a volcanic shape as a function of the Mn valence state in the octahedra, while the apex of the ORR/OER volcanic trend was located at the position of the Mn valence state ≈ +3.¹⁰⁶

4.3 Stability of MnO_x

Experimental and theoretical work on MnO_x catalysts has emphasized the role of Mn(III) as an essential intermediate state for achieving catalytic reactions. Regardless of the initial Mn oxidation state in the catalyst, Mn(III) is generated and participates in the water oxidation process. Therefore, the question of the stability and reconstruction of Mn³⁺ in MnO_x needs to be discussed next.¹⁰²

MnO_x can act as an effective electrocatalyst under strongly alkaline conditions but is inefficient under neutral and acidic conditions. In a 2012 review, Takashima et al. investigated the OER process in MnO₂ and found fundamentally different mechanisms for the OER in acidic, neutral, and basic conditions. At pH < 9, the larger overpotential is due to the rapid consumption of Mn³⁺ by the disproportionation reaction, as shown by the following reaction:

2Mn³⁺(aq) + 2H₂O → MnO₂(s) + Mn²⁺(aq) + 4H⁺

while at pH ≥ 9, the disproportionation of Mn³⁺ is effectively inhibited, which leads to a very easy accumulation of Mn³⁺ in the MnO₂ layer, resulting in a large decrease in the overpotential; moreover, in this pH region, the comproportionation reaction between Mn²⁺ and Mn⁴⁺ contributes to the regeneration process of Mn³⁺ (Fig. 7d). Therefore, the pH of the electrolyte has a significant effect on the productivity and accumulation of Mn³⁺, which further affects the electrocatalytic performance.¹⁰⁷ Subsequently, Huynh et al. performed a kinetic study of the OER mechanism of MnO_x over a range of pH values under acidic, neutral, and alkaline conditions. Tafel and reaction order analyses reflect two competing mechanisms: a single-electron single-proton PCET pathway, which dominates under alkaline conditions, and an Mn³⁺ disproportionation process, which dominates under acidic conditions (Fig. 7e). Another significant point of this study is that it shows that MnO_x has a high intrinsic and functional kinetic stability in acidic electrolytes and its ability to remain self-healing for electrolytes with a pH greater than zero, and MnO_x can undergo the OER in extremely acidic environments.¹⁰⁸ As for the development of MnO_x in acidic media, Wang's group has made new progress. They developed Mn–Ru solid-solution oxides with oxygen vacancies (named Mn_1−xRu_xO_2−δ), which is currently the better-performing MnO_x catalyst in acidic electrolytes. Among the synthesized catalysts, Mn_0.73Ru_0.27O_2−δ with the optimized electronic structure only requires an overpotential of 208 mV to produce a current density of 10 mA cm⁻² in an acidic electrolyte and a Tafel slope of 65.3 mV dec⁻¹, which exceeds RuO₂ and most noble metal-based OER electrocatalysts. The volcano diagram and the difference between the overpotential results and the adsorption energies of the O* and OH* intermediates are shown in Fig. 7f. The Mn_0.73Ru_0.27O_2−δ model is at the peak of the volcano diagram, indicating the lowest overpotential value for the OER of all models studied. DFT calculations show that the introduction of Mn can alter the RDS transition from O* → OOH* to OOH* → OO*. In addition, EXANES shows that the valence of the Mn species increases significantly after the OER process, while the valence of the Ru species remains almost unchanged, indicating that the Mn atom is converted into an electron donor and that the Ru species is the true OER active site (Fig. 7g).¹⁰⁹

Normally speaking, the (t_2g)³(e_g)¹ orbital and exceptionally stable Mn³⁺ ions in MnO_x are extremely vital to improving their catalytic performance; on the other side, more and more studies have been carried out on the octahedral unit of [MnO₆] because the structural changes caused by octahedral distortion can strengthen the orbital interaction between the Mn center and oxygen intermediate, thus considerably promoting the oxygen catalytic kinetics. Therefore, to design high-catalytic activity manganese-based catalysts, it is necessary to consider factors such as the active sites of the electronic orbitals. In the following part of this chapter, we propose some improvements for reference, such as defect engineering (introducing vacancies), interface engineering (heterogeneous interfaces), etc.

4.4 Improvement strategies

4.4.1 Defects caused by vacancies. As a means to further improve the electronic properties of two-dimensional materials, researchers are increasingly concerned about the relationship between defect engineering (vacancy) and structure performance. Previous studies have shown that appropriate vacancies can alter the physical properties (Fermi energy levels, electrical conductivity, etc.) and adsorption properties of materials. Therefore, the introduction of vacancy defects is an effective strategy to improve the catalytic performance of MnO_x.^110–113 Lu adjusted the electronic structure of MnO₂ by inserting transition metal cations and increased the content of Mn³⁺ and oxygen vacancies (Fig. 8a). With the insertion of Fe, Co, and Ni cations into the MnO₂ layer, the Mn ions of MnO₂ transform from the stable d³ (Mn⁴⁺) state to the unstable deeply rotated d⁴ state (Mn³⁺) generating extra oxygen vacancies in the MnO₂ structure according to the equation.

–Mn⁴⁺–O₂–Mn⁴⁺– → –Mn³⁺–*–Mn³⁺–+1/2O₂

where * represents the formation of oxygen vacancies. It was further shown by XPS that the cation-embedded MnO₂/NF material has a higher concentration of oxygen vacancies than before, thus improving the ability of the material to adsorb free OH⁻ when performing catalytic reactions. The group also concluded that the synergistic effect of oxygen vacancies and Mn³⁺ ions after the introduction of cations was the main reason for the superior catalyst performance, with the modulation effect being more pronounced than that of Co²⁺ and Ni²⁺ due to the higher oxidation valence of Fe³⁺, which generates more Mn³⁺ sites. ¹⁰³ Zhao grew two monolayer-thick δ-MnO₂ nanosheets on nickel foam using the in situ growth method, and the OER properties can be attributed to the ultrathin δ-MnO₂ nanosheets containing a large number of oxygen vacancies, which promote the formation of Mn³⁺ active sites, resulting in semiconducting properties and strong H₂O adsorption capacity.¹¹⁴ Liu et al. introduced anionic oxygen and cationic manganese vacancies (VO and VMn) into MnO₂ nanosheets. To elucidate the synergistic effect of VMn and VO on improving HER/OER kinetics, the calculation of electrochemically active surface area (ECSA) shows that oxygen vacancies enhance the active site, while the impedance indicates that VMn production is beneficial for improving conductivity. The possible HER and OER mechanisms are depicted in Fig. 8b. Under alkaline conditions, water molecules are adsorbed on the bridge site of two adjacent Mn atoms located in the DV (double vacancy), which then splits with the help of nearby O to produce H₂ and free OH⁻. Similar to the OER, the dissociated OH⁻ is adsorbed on the same bridge sites to form the Mn–O(H)–Mn intermediate. Subsequently, Mn–O(H)–Mn combines with OH⁻ to release water molecules, which then become Mn–O–Mn. The metastable Mn–O–Mn intermediate will combine with OH⁻ to produce Mn–OOH and eventually release O₂. DV-MnO₂ exhibited excellent catalytic performance with the assistance of synergistic double vacancies, with HER and OER overpotentials of 59 mV and 260 mV, respectively, and Tafel slopes of 63 mV dec⁻¹ and 40 mV dec⁻¹.¹¹⁵ These studies suggest that oxygen vacancies can modulate the catalyst energy band structure and act as active sites for the OER, while the introduction of cations mainly modulates the active center and increases the content of Mn³⁺ and oxygen vacancies. In addition to these strategies for cations and oxygen vacancies, the introduction of additional anions (e.g. S) can control the electronic spin state of transition metals and thus increase their OER/ORR catalytic activity,¹¹⁶ although such studies are not common in MnO_x, which we consider to be an equally effective strategy.

Fig. 8 (a) Schematic illustration of the formation of Fe–MnO₂/NF. (b) Schematic illustration of the HER and OER processes of DV–MnO₂ under alkaline conditions. (c) HRTEM images of Ni_xS_y@MnO_xH_y nanorods. (d) Comparison of LSV curves for Ni_xS_y@MnO_xH_y before and after the 5000-cycle stability test. (e) HRTEM images (f) schematic catalytic mechanism of Mn₂Co₂C/MnO. (g) Schematic illustrating the preparation of the 1D MnO@CNT@Co–N/C hierarchical nanostructure. (h) Probable OER mechanistic pathway (a) Reproduced with permission from ref. 103. Copyright 2021, Elsevier. (b) Reproduced with permission from ref. 115. Copyright 2021, John Wiley and Sons. (c and d) Reproduced with permission from ref. 120. Copyright 2022, Springer Nature. (e and f) Reproduced with permission from ref. 122. Copyright 2020, American Chemical Society. (g) Reproduced with permission from ref. 123. Copyright 2021, Royal Society of Chemistry. (h) Reproduced with permission from ref. 124. Copyright 2022, American Chemical Society.

4.4.2 Interfacial heterostructure. Engineering of interfaces is considered one of the most promising strategies for the development of efficient electrocatalysts. The construction of atomically heterogeneous interfaces can effectively tune the electronic and band structure of the catalysts to provide appropriate adsorption/desorption behaviors that improve the electrocatalytic performance.^117–119

Ni_xS_y@MnO_xH_y/NF constructed by Wang exhibits excellent activity and stability in alkaline solutions. Since the Ni_xS_y electrocatalyst is easily oxidized to the corresponding metal oxides/(oxygen) hydroxyl oxides during the OER, the MnO_x shell protection strategy can effectively improve its catalyst stability. The integration of MnO_xH_y and Ni_xS_y forms a heterostructure with abundant Mn–S bonds(Fig. 8c), leading to stronger electronic interactions, and its activity may be the conversion of NiS into amorphous Ni species (NiO_xH_y), while the synergistic interaction between MnO_xH_y and Ni_xS_y significantly accelerates the kinetics of HER and OER processes and enhances the charge transfer, and the OER performance remains intact after 5000 CV cycles (Fig. 8d).¹²⁰ Niu et al. constructed nanowire porous structures using low nitrogen acetic acid (NTA) as a chelating agent to stabilize Mn²⁺ and Co²⁺ metal ions. MnO_x alone is not effective as a bifunctional catalyst due to its insufficient OER activity and electron transfer rate limitation, so the synergistic effect between the metal Co and MnO_X was exploited to design bifunctional electrophiles for OER and ORR catalysts. In particular, a strong charge exchange between Co (200) and MnO_X (200) occurs at the heterogeneous interface between metal Co and MnO_X. This means that the O atoms can easily gain electrons from the Co atoms, which leads to a change in the electronic structure of pristine manganites, and charge redistribution facilitates the formation of abundant catalytically active sites on the OER and ORR surfaces.¹²¹ Meng prepared Mn₂Co₂C/MnO catalysts with cubic porous structures by pyrolysis, and TEM revealed the presence of abundant heterogeneous interfaces between Mn₂Co₂C and MnO (Fig. 8e), while XPS confirmed that MnO has an elevated oxidation state [Mn^(2+δ)+], which is favorable for OH⁻ adsorption and subsequent desorption processes. In addition, extended EXAFS measurements further revealed the presence of disordered Mn/O atoms and dangling bonds at the heterogeneous interfaces, which act as additional active sites for adsorption/desorption (Fig. 8f).¹²² Interfacial engineering is not a combination of any two catalysts. As mentioned above, there are interfaces that increase the conductivity of a single component, others that increase the active site, and a combination that achieves small reaction activation energies by modulating the energy band structure. Shell–core interfaces have also been designed to take advantage of the durability of MnO_x. However, the active sites of MnO_x heterojunctions are mostly catalysts of another set of components, and how to design catalysts with MnO_x as the active center is a future trend.

4.4.3 Combination with MOFs. Metal–organic frameworks (MOFs) consist of coordination bonds between metal ions and various organic linkages, displaying tunable pore size and composition, extremely ordered porous crystal structures, and high surface areas.^125,126 These characteristics make MOFs promising for applications in catalysis, energy storage, and conversion.^127–129

MnO_x can be used not only as a framework for metal–organic structures but also as a catalyst for the ORR/OER, which is commonly combined with conductive materials (conductive polymers or carbon materials) to enhance their electrical conductivity and catalytic properties. Li grew polypyrrole (PPy) nanotubes (NTs) and zeolite imidazole framework-67 (ZIF-67) sequentially on MnO₂ NTs as the substrate and oxidant, and subsequently formed a one-dimensional (1D) layered ternary nanocomposite by sintering, as shown in the flow chart in Fig. 8g. The MnO_x particles separated in carbon nanotubes (CNTs) have a mixed valence of Mn^2+/4+, which can greatly facilitate the diffusion of electrolytes and electron transfer in redox reactions. In addition, the Co–N/C formed on CNTs provides multiple catalytically active sites (e.g., Co–N_x, Co–O, and C–N molecules), and the MnO@CNT@Co–N/C catalysts exhibit better activity and durability compared with commercial catalysts of noble metals.^123,130 Wang introduced Mn species into the zeolite-imidazole frameworks (ZIFs) and then further pyrolyzed the Mn-containing bimetallic ZIFs to synthesize Co@Co₄N and MnO embedded in porous N-doped carbon nanocubes. The introduction of Mn species not only increased the surface content of pyridine/graphitized N and Co₄N in Co@Co₄N/MnO-NC but also converted during calcination to the MnO phase. These high pyridine/graphitic N and moderate MnO species provide efficient catalytic performance for the ORR, while the uniformly distributed Co@Co₄N nanoparticles ensure an abundance of active sites for the OER.^131,132 Selvasundarasekar et al. constructed a bimetallic ZIF catalyst containing Co and Mn metal ions and formed the fibers by the electrospinning technique. Molecular orbital studies evaluated the active centers of the bimetals and found that the Co²⁺ concentration inhibits the Jahn–Teller distortion produced by Mn³⁺, while OER activity is mainly attributed to Mn³⁺ ions and a moderate amount of Co²⁺ leads to a stronger overlap between the anti-bound Mn³⁺ e_g orbitals and oxygen adsorbed O 2p orbitals, enhancing the shorter binding nature of the OER intermediates on the catalytic surface. And a detailed OER mechanistic pathway was hypothesized (Fig. 8h). First, Co²⁺ is initially stabilized by the high oxidation of Mn(IV), then it is reduced to Mn³⁺, after which OH⁻ ions are adsorbed on the Mn³⁺ catalytic surface and subsequently it is converted to oxides and oxyhydroxides. Finally, O₂ is desorbed from the catalytic surface. The work of these individuals provides ideas for the search for active centers in bimetallic site catalysts.¹²⁴

4.4.4 MnO_x composites. Finally, we show some other MnO_x composites in Table 1, such as MnO_x/carbon composites, MnO_x/metal oxide composites, and MnO_x/phosphide composites. The Fe₃O₄/MnO_x, CoP/MnO, and MnO_x/NiFe-LDH composites in Table 1 show good electrocatalytic activity. In this case, the molar ratio of the manganese to oxygen atoms deviates significantly from the stoichiometric ratio of the composition, where the manganese ions have mixed valence. In addition, Mn⁴⁺ and Mn³⁺ species enriched in MnO_x also act as active catalytic centers, which can greatly accelerate electrocatalytic activity.^133–135 On the other hand, this is attributed to chemical coupling and synergistic effects between metal oxides. It is also possible that the smaller volume of the manganite may also provide additional active sites and accelerate the generation of cavity charge carriers.^136–138 These unique properties confer exceptional electrocatalytic properties to the MnO_x composites.

Table 1 Summary of preparation methods and OER properties of MnO_x composites^a

MnOx composite catalysts	Overpotential (mV)	Tafel slope (mV dec^−1)	Catalyst morphology/support template/catalyst loading	Synthetic method	Ref.
a CFP: carbon fiber paper; FTO: fuorine-doped tin oxide; NF: nickel foam; GCE: glassy carbon electrode; RDE: rotating disk electrode.
CoO/MnO	280	67.9	Porous nanowires and nanosheets/NF/—	Hydrothermal and vacuum annealing	139
CoP/MnO	230	95	One-dimensional hollow nanofibers/carbon paper/2 mg cm^−2	Electrospinning technology, thermal annealing, and phosphating process	140
CoFe@CNT/MnO	193	82	Nanoparticles/CFP/—	Hydrothermal and CVD pyrolysis	141
MnOx/NiFe-LDH	174	48	Nanosheets/FTO/—	Atomic layer deposition (ALD) and hydrothermal	142
MnOx/NiFe-LDH/NF	265	73	Nanosheets/NF/—	Hydrothermal and electrodeposition	143
Fe3O4@MnOx	188	77.6	Hierarchical flower-like core–shell microsphere/NF/—	Hydrothermal	144
MnOx/NiFeP/NF	247	59.8	Nanosheets/NF/—	Hydrothermal phosphating and electrodeposition	145
MnO/Co-CNTs	420	59	1-D hollow tube and particles/glassy carbon/—	High-temperature sintering and growth on carbon nanotubes	146
MnO/Co/PGC	300	77	Nanoparticles/rotating disk electrode RDE/—	Hydrothermal and high-temperature calcination	147
Co/MnO/NC	379	101	Nanometric grain size/GCE/30 μg cm^−2	Solvothermal and calcination annealing	148
Mn2Co2C/MnO	320	80	Nanocubes and nanoparticles/glassy carbon electrode/—	Coprecipitation and pyrolysis	122
Co/MnO@NC	235	53	Nanowire and nanoparticles/—	Solvothermal and pyrolysis	121
Mn3O4@CS/CP	290	95	Spheres/glassy carbon/204 μg cm^−2	Hydrothermal and high-temperature calcination	149
Mn3O4/CoP	306	51.8	Nanorod and nanoparticles/NF/1 mg cm^−2	Hydrothermal	150
MnFe2O4/NF	310	65	Particles/NF/40 mg cm^−2	Hydrothermal	151
Mn3O4/NiCo2S4	320	79	1D rod-like structure and sheet-like/glassy carbon/0.38 mg cm^−2	Hydrothermal and annealing	152
Co3O4/Mn3O4	360	78.2	Nanoparticles/carbon paper/0.382 mg cm^−2	Hydrothermal	153
RuO2/(Co, Mn)3O4	270	77	Nanoparticles/commercial carbon cloth/—	Hydrothermal	154
N–C/Mn2O3	347	75.5	Star-like structure with rough surface/NF/—	Hydrothermal and annealing	155
MnO/CoMn alloy@N	300	—	Nanoparticles/glassy carbon/0.28 mg cm^−2	Annealing	156
Co/MnO@N–C	350	84.5	The loosened foam-like/glassy carbon/0.6 mg cm^−2	Hydrothermal and pyrolysis	157
MnO/Co@NGC	326	90	Cubical/glassy carbon/—	Pyrolysis	158

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5. Summary and perspectives

This feature article first explains the conventional adsorption mechanism of the OER in alkaline environments, then presents the progress of different MnO_x studies on the OER, and presents the catalytic mechanism of MnO_x with several strategies for improvement. As far as the current study is concerned, MnO_x as an anode catalyst can reduce the OER excess potential and is an electrocatalyst that could be developed for the electrolysis of water. For MnO₂, the catalytic activity depends primarily on the crystal structure, morphology, size, pore structure, and surface oxidation state of the manganese atoms. It has been found that the catalytic activity of α-MnO₂ in the OER is higher than that of β-MnO₂ and δ-MnO₂ in early research. More importantly, the large interlayer spacing of MnO₂ allows for the insertion of additional ions, which affects its catalytic performance. Next, for Mn₂O₃, the presence of distorted octahedra occupied by the Mn³⁺(d⁴) center elongates the Mn–O bond due to the Jahn–Teller effect in Mn₂O₃, and the Mn³⁺–O octahedra are activated to participate in the OER. Due to its special d-electron structure, Mn₂O₃ has a much higher overpotential than other transition metal oxides. In this respect, the poor electronic conductivity and relatively low OER dynamics of Mn₂O₃ are mostly improved by combining them with other catalysts with higher OER activity. Mn₃O₄ with a spinel structure has tetrahedral and octahedral centers occupied by Mn³⁺and Mn²⁺ ions, respectively, and it has been demonstrated that Mn³⁺ and Mn²⁺ in Mn₃O₄ affect its OER and ORR performance. In addition, the spinel oxide MnFe₂O₄, also with significant OER catalytic activity, is mentioned in this paper.

Current research applications of MnO_x in electrochemistry are mainly in the direction of lithium batteries and capacitors, while in the direction of electrocatalysis, their activity is still distant from that of noble metals and transition metals such as Fe, and Ni. In this paper, while describing the current research progress, we propose the following points to further improve the electrochemical properties of MnO_x. (1) Continue to deepen the research on the catalytic mechanism of MnO_x. The mixing of multiple valence states of Mn leads to a catalytic center that is still not well defined for MnO_x, and the difference in OER activity may be due to differences in the surface electronic states of Mn³⁺. Recent studies have proposed that the e_g orbital of Mn³⁺ is involved in the σ-bonding with the anionic adsorbent; the higher the Mn³⁺ content in the manganese group, the more favorable it is to enhance the adsorption of –OH on the catalyst surface. As a consequence, increasing the content of Mn³⁺ ions is an effective strategy to improve its OER performance. (2) On the other hand, most of the research on MnO_x has been done in alkaline environments as the Mn³⁺ ion can be unstable due to disproportionation reactions in acidic environments. However, the development of MnO_x for the OER in acidic media has made great progress and the obtained MnO_x has good catalytic properties. Therefore, we believe that the development of MnO_x in acidic media is a positive direction.

In terms of enhancing the catalytic activity of MnO_x, this can be done by introducing vacancies and forming heterogeneous interfaces. First, for the problem of insignificant catalytic activity, vacancies can activate adjacent metal sites as fresh active sites for electrocatalysis and promote the OER by synergistic interaction with Mn ions; meanwhile, the presence of a large number of oxygen vacancies in MnO_x promotes the formation of Mn³⁺ active sites, which leads to their semiconducting nature and optimizes the local electronic configuration. Second, for low conductivity, the preparation of atomic heterojunctions can effectively modulate the electronic and band structure of the catalysts to provide moderate adsorption/desorption behavior, and MnO_x can form heterojunctions that effectively prevent the recombination of electrons and holes and thus improve their conductivity. MOFs can be used as catalysts due to their tunable pore structure, high specific surface area, and multiple coordination sites, while MnO_x can be used as a metal backbone for organic structures and also as a catalyst for the OER. Finally, we propose that recombining MnO_x catalysts with other electrically active materials or conducting substrates is also an effective strategy for fabricating high-performance electrocatalysts.

In summary, the development of MnO_x materials with high stability and good catalytic properties is our objective. We believe that with the continued maturation of material preparation techniques, the gradual evolution of characterization methods, the exploration of reasonable structures, and the breakthroughs in related electrocatalytic reactions, commercial applications of MnO_x can eventually be realized.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51971058 and 52071072).

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