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• INTRODUNTION

  With a rising global population, increasing energy demands and impending climate change, major concerns have been raised over the security of our energy future and have motivated a variety of researchers to develop alternative energy.^[1-3] Numerous innovative ideas have been proposed for achieving more efficient energy conversion or storage systems such as water electrolysis, fuel cells and metal-air batteries.^[4-6] They are two-electrode systems in which hydrogen evolution (HER) or oxygen reduction (ORR) occurs at the cathode and oxygen evolution (OER) or oxidation of certain chemical fuels at the anode. Due to sluggish kinetics and complex reaction pathways of water oxidation, including water oxidation activation, O–O bond formation, intermediate peroxide activation and O₂ liberation, ^{[7, 8]} it has been considered as a major bottleneck in the development of electrochemical energy conversion and storage system. Therefore, in the past few decades, tremendous efforts have been devoted to understanding the mechanism of this challenging reaction and improving the catalytic activity.^[9] Inspired by the biological photosystem II (PS II), Mn-based materials have been extensively studied as promising candidate catalysts for efficient water oxidation.^{[10, 11]}

  In this review, the central structure and catalytic cycle process of water oxidation in nature will first be introduced. Secondly, the structural characteristics of synthetic Mn-based heterogeneous catalyst are summarized, with emphasis on the understanding of reaction mechanisms and rate-determining steps. Finally, the strategy of understanding the catalytic mechanism of Mn-based water oxidation was prospected.

  THE OXYGEN-EVOLVING CENTER IN PHOTOSYSTEM II (PSII)

  In nature, water oxidation is completed by the oxygen-evolving center (OEC) in PS II.^[12-15] The OEC consists of an inorganic manganese-calcium-oxo cluster (Mn₄CaO₅), which is stabilized by a large number of the surrounding amino acid residues and water molecules (Figure 1a).^[16] The Mn₄CaO₅ central structure and surrounding environment cooperatively perform the water oxidation reaction. Compared with current artificial catalysts, the OEC exhibits more efficient and selective water oxidation catalytic activity, achieving a turnover frequency of 100 s^-1 for water oxidation with only 150–300 mV of overpotential.^[10] The possible reasons for its high catalytic activity are as follows. First, the Mn₄CaO₅ complex consists of a highly distorted Mn₃Ca-oxo cubane-like motif and one dangling Mn atom (Figure 1b).^[17] When the redox state changes between Mn(III) and Mn(V), this highly distorted structure might promote its atomic rearrangement in the water oxidation reaction. Second, researchers have found that the Ca(II) ion is essential for the high activity of OEC and may have a direct effect on the oxidation process of water.^[18] Third, the hydrogen bond network formed by the surrounding amino acid residues provides well-defined channels for the transfer of substrate water molecules and products such as H⁺ and O₂ (Figure 1c).^[17]

  Figure 1

  

  Figure 1.  Structure of the Mn₄CaO₅ cluster. (a) Crystal structure of the Mn₄CaO₅ cluster and its ligand environment. Reprinted with permission from ref. 16. Copyright 2017 American Chemical Society. (b) Bond length differences (in Å) between metal atoms and oxo bridges or water molecules. (c) Hydrogen bond network from the Mn₄CaO₅ cluster. Water molecules participating in the hydrogen bond network are depicted in orange, whereas those not participating are in grey. Reprinted with permission from ref. 19. Copyright 2011 Nature Publishing Group. (d) Kok cycle: the five intermediate states S_n (n = 0–4) during water oxidation. Reprinted with permission from ref. 19. Copyright 2021 American Chemical Society.

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  In water oxidation reaction, the OEC catalyzes water splitting into electrons, protons and dioxygen through a five-state cycle (Kok cycle, S_n, n = 0–4, where n is the number of accumulated oxidizing equivalents) (Figure 1d).^{[19, 20]} During the entire Kok cycle, each Mn atom in the tetrameric cluster is continuously oxidized from Mn(III) to Mn(IV) with simultaneous deprotonation of water. And O–O bond is generated in the S₃–S₄ step and then O₂ is released in the S₄–S₀ step. In recent years, the structural changes of Mn₄CaO₅ in each step of the Kok cycle have been determined by femtosecond X-ray free electron laser technology.^{[21, 22]} In addition, the atomic arrangement and oxidation state of S_n state were also revealed in the experiment. Only S₄ state has not been determined at present. Based on these latest results, it is strongly believed that one oxygen atom in O₂ comes from the O(5) atom of the Mn₄CaO₅ cluster, and the other oxygen atom comes from the water molecules entering through the surrounding channels.^[23]

  MANGANESE-BASED HETEROGENEOUS ELECTROCATALYSTS

  Finding a suitable descriptor is one important direction for the rational design of high-efficient catalysts. With the development of various water oxidation electrocatalysts, various descriptors based on experiments or calculations have been proposed.^[24] The adsorption energy of catalytic intermediates is the most representative descriptor in recent years, which indicates that the adsorption for the reaction intermediates should neither be too strong nor too weak. To simplify the water oxidation reaction step from the perspective of electron transfer, four surface intermediate species could be considered (*OH₂, *OH, *O and *OOH). The adsorption of these reaction intermediates can be linked together by a linear proportional relationship, because all reaction intermediates are bound to the surface of the heterogeneous catalyst through O atoms. According to the linear scale relationship, the catalytic activity can be expressed as a volcanic relationship with a single adsorption energy as a descriptor.^[25] However, studies have shown that this adsorption-energy based volcano approach is not well suitable for predicting the catalytic activity of Mn-based materials. According to the existing reports, there is a big difference between the calculated prediction and experimental results.^[26-29] Only considering the method based on adsorption energy is not enough to understand the activity origin of Mn-based catalysts. Therefore, it is necessary to understand the activity origin of Mn-based materials to characterize the exact structural properties and chemical states of these materials through experiments, and link them with catalytic activity. In the following section, we will introduce several Mn-based materials and discuss their structural properties and water oxidation mechanism.

  Mn Oxide-based Heterogeneous Electrocatalysts. Since manganese oxide was found to have high catalytic activity under alkaline conditions, the study of electrocatalytic water oxidation behavior of Mn-based materials has been widely concerned by researchers.^[30-33] Some early reports indicated that Mn-based heterogeneous catalysts have high potential in water oxidation reaction, however the origin of activity was not discussed in detail. Through the synthesis of various Mn oxide materials as a comparative study of model systems, it can simply and clearly reveal the active structural motif. For example, Suib's group prepared manganese oxides of different structures (α-, β-, δ-MnO₂ and amorphous manganese oxide) (Figure 2a), and systematically studied the samples to determine their catalytic activities at high current density under alkaline conditions.^[34] Surprisingly, the experimental results show that the catalytic activity strongly depends on the crystallographic structures and exhibits the following order: α-MnO₂ > amorphous manganese oxide > β-MnO₂ > δ-MnO₂. Notably, α-MnO₂ has the highest activity (the overpotential is 490 mV under 10 mA·cm^-2, 0.1 M KOH electrolyte, amorphous manganese oxide (590 mV), β-MnO₂ (600 mV), δ-MnO₂ (740 mV)). They suggested that the high activity is due to the following aspect: α-MnO₂ has a mixed oxidation state, unique tunnel structure and abundant protonation sites. These results indicate that the pore structure of materials may be an important factor affecting the catalytic activity, because ion transport becomes more important at higher current density. Our group also systematically studied the unusual hexagonal Mn(OH)F rings^[35] and the unusual network of α-MnO₂ nanowires^[36] with structure-induced hydrophilicity and conductivity. It is systematically studied that the high porosity of the 3D superstructure can improve the substrate diffusion efficiency and the unique network structure brings high hydrophilicity and conductivity to enhance the catalytic performance.

  Figure 2

  

  Figure 2.  (a) Structures of Mn oxides: α-MnO₂ (2 × 2 tunnel), β-MnO₂ (1 × 1 tunnel), δ-MnO₂ (layered), and amorphous manganese oxides (AMO). Reprinted with permission from ref. 34. Copyright 2014 American Chemical Society. (b) Demonstration of the crystal structure and d-orbital electronic configuration of Mn₂O₃ and γ-MnOOH. (c) Description of corner-shared Mn(III) octahedra in δ-MnO₂ (top) and the biological Mn₄CaO_x cluster (bottom). Reprinted with permission from ref. 37. Copyright 2016 American Chemical Society.

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  Dismukes's team studied the geometric effect of Mn(III)O₆ octahedrons on electrocatalytic water oxidation.^[37] Through comparative study of γ-MnOOH, Mn₂O₃, β-MnO₂, δ-MnO₂ and Mn₃O₄ crystalline manganese oxide, it is found that the D_3d symmetrical triangular anti-prism Mn(III)O₆ of Mn₂O₃ exhibits higher water oxidation reaction activity. The reason for the higher water oxidation activity of Mn₂O₃ may also be due to its higher Mn(III) content.^[38] And the tetragonal twisted Mn(III)O₆ with D_4h symmetry of γ-MnOOH shows moderate activity. And β-MnO₂ shows poor catalytic activity. Most notably, the catalytic activity of hexagonal polymorph δ-MnO₂ (HexBir) is much higher than the triclinic polymorph of δ-MnO₂ (TriBir). By comparing the structural characteristics, it can be found that the hexagonal polymorph δ-MnO₂ (HexBir) has some corner-sharing Mn(III)O₆ structure, which is the reason for its excellent catalytic performance (Figure 2c). Through correlation with the high catalytic activity of γ-MnOOH, they further perceived the importance of the corner-sharing Mn(III) content in δ-MnO₂, rather than the total content of Mn(III). In addition, the author further explored the similarity between the corner-sharing Mn(III) species structure in the synthetic catalyst and the dangling Mn(III) of the biological OEC (Figure 2c, bottom). They believe that the active site structure and electronic structure of synthetic artificial catalysts similar to those of biological OEC are the reason of their high catalytic activity.

  The geometry of Mn(III) octahedra was studied in greater detail by comparing γ-MnOOH and Mn₂O₃ which contain only Mn(III) species. As shown in Figure 2b, γ-MnOOH only contains the Mn(III)O₆ type structure of Mn(III) in its unit cell. This configuration is affected by the Jahn-Teller distortion to form a tetragonal elongation (D_4h type) octahedron. This distortion causes each D_4h type octahedron to form four short and two long Mn–O bonds, and individual D_4h units are connected to three other Mn(III)O₆ octahedra by corner-sharing bridges. In contrast, Mn₂O₃ contains five symmetric non-equivalent Mn(III)O₆ sites in its unit cell. In three of the five sites, due to the distortion of the D_4h symmetry, all Mn–O bonds have different lengths, with two long and four short. In the other two sites, all bond lengths are very similar, indicating D_3d symmetry of the triangular anti-prism. The sites are connected to another two or three Mn(III)O₆ by corner sharing bridges, and thus these sites can be compared with the typical γ-MnOOH. The surface of Mn(III)O₆ is located above the threefold axis and maintains the D_3d symmetry formed by three oxo bridges. Therefore, there is an e_g¹ electronic configuration. Since the triangular ligand field of the fixed oxo bridge suppresses the Jahn-Teller distortion of the quadrilateral, the Mn–O bond length is expected to be almost equal. The three surface-bound water-oxidizing oxygen sites are considered to be relatively flexible, which could be beneficial to catalytic activity if the oxygen release from this site is rate-limiting.

  Nocera's research team conducted a detailed study on a disordered δ-MnO₂ catalyst system.^[39] They found that the enhancement of catalytic activity began with a structural phase transformation through the charge comproportionation of the original δ-MnO₂ film with generated Mn(OH)₂ to produce a Mn₃O₄-like intermediate structure (δ-Mn(IV)O₂ + 2Mn(II)(OH)₂ → Mn(II, III)₃O₄ + 2H₂O). The continuous anode potential further oxidizes the Mn₃O₄-like intermediate structure into disordered δ-MnO₂. In addition, in-situ XAS measurement results show that Mn(III) groups still exist in the catalyst during the water oxidation reaction but with a lower coordination number. The coordination number of Mn–O is reduced to four, and the bond length is relatively stable, indicating that Mn(III) may cooperate with a tetrahedral coordination field. The tetrahedral ligand field of Mn(III) inhibits further oxidation and the valence state is still trapped at Mn(III) even at a high applied anode potential. The researchers proposed a unique structural motif of the tetrahedral Mn(III) intermediate species in the disordered δ-MnO₂ located next to the Mn(IV)O₆ octahedron. The calculation of the electronic structure of this unique local coordination structure motif shows that the reconstruction of Mn 3d and O 2p states contributes to the formation of oxygen holes. In other words, the trapped Mn(III) species in the tetrahedral ligand field cause local strain to distort the Mn–O coordination bond nearby, resulting in a narrower gap between the highest occupied and lowest unoccupied orbitals of the oxygen radical. Therefore, they believe that this unique local structure in the disordered MnO₂ facilitates the generation of highly active Mn-oxyl radical species, thereby enhancing the electrochemical catalytic activity.

  Mn Phosphate-based Heterogeneous Electrocatalysts. According to previous studies, it is found that Mn(III) is an important active intermediate in Mn-based catalysts.^{[29, 39-42]}. In a neutral solution, the Mn(III) intermediate will undergo a disproportionation reaction to form Mn(II) and Mn(IV) species, making Mn(III) unable to exist.^[43-45] In order to solve this problem, a large number of asymmetric structures are introduced into the Mn geometry. The asymmetry in the crystal frameworks can tolerate J-T distortion and thereby stabilize the Mn(III) intermediate.^[46] Although transition metal phosphate compounds have been widely investigated as cathode materials for lithium-ion batteries, there have been few attempts to study the OER catalytic performance of phosphate-containing manganese-based crystals.^[47] In this chapter, we will introduce several Mn-based phosphate materials and discuss their asymmetric structures.

  The hydrated Mn(II) phosphate (Mn₃(PO₄)₂·3H₂O) as a water oxidation catalyst was investigated by Nam et al.^[48] They synthesized a new crystal structure as water oxidation catalyst and identified the OER catalytic activity under neutral conditions (the overpotential is 680 mV under 0.1 mA·cm^-2, 0.5 M phosphate buffer at pH = 7.0). They verified that the bulky phosphate polyhedron induces a less-ordered Mn geometry in Mn₃(PO₄)₂·3H₂O, and its structure is similar to that of catalytic active Mn(III)-containing materials. Computational analysis revealed that distinctive structural features of Mn₃(PO₄)₂·3H₂O could successfully stabilize Mn(III) during water oxidation, leading to superior catalytic activity under neutral conditions. In detail, they first focused on the crystal structure of Mn₃(PO₄)₂·3H₂O. As shown in Figure 3a, Mn(2) atoms are connected to each other by edge O–O sharing. The Mn(2) and Mn(3) atoms are bridged by edge O–O_w (oxygen atom in a water molecule) sharing, and the Mn(2) atoms are linked to Mn(4) atoms by O_w vertex sharing. The Mn(5) atoms share two oxygen atoms with each other, forming Mn₂O₈ dimers. The Mn(6) atom does not share any atoms with other manganese atoms. Spherical cluster around the P(1) site illustrates the asymmetrical arrangement of manganese atoms in the crystal. Because crystal structure is considered to be one of the major sources of catalytic capacity, especially for Mn containing catalysts, the stabilization of Mn(III) in the OER process and the initial content of Mn(III) in the crystal structure are very important, as evident in OEC and Mn₂O₃.^[41] They observe the effect of the oxidation of Mn(II) ions inside the catalyst to Mn(III) on the crystal structure by calculation and simulation. Calculations show that there is a certain degree of J-T distortion in Mn₃(PO₄)₂·3H₂O to stabilize Mn(III). At the same time, the average pressure when all Mn atoms are in the Mn(III) configuration is measured to measure the stability of Mn(III). When Mn(III) atoms are abundant, greater pressure means the tendency of lattice distortion and instability. The magnitude of pressure follows the order MnO₂ > MnO > Mn₃(PO₄)₂·3H₂O. Therefore, they conclude that the structural elasticity of the stable Mn(III) state could explain the high catalytic activity of Mn₃(PO₄)₂·3H₂O. On the surface of the terminal body, the exposed Mn(2), Mn(4) and Mn(5) atoms are 6-, 6- and 5-coordination, respectively (Figure 3b). In order to compare the activity of Mn ions on each surface, they were taking electrons one by one from the system and relaxing the structure. The results show that 5-fold Mn atoms have stronger catalytic activity than the 6-fold Mn ones. In addition, due to the presence of PO₄ dangling bonds near the Mn atom, some Mn(II)–H₂O bonds are found on the surface and underground to transfer hydrogen to the dangling bonds, resulting in Mn(III)-OH^- (Figure 3c). In fact, the phosphate group can participate in PCET as a proton acceptor in the OER catalysis process.^[49] Although whether the catalyst follows the PCET reaction in the OER catalysis process requires more experimental evidence, they thought that phosphate ions can promote the PCET reaction to oxidize the Mn-OH₂ center according to the calculation results. Therefore, in addition to the structural flexibility facilitating the J-T distortion and the 5-fold Mn(II) atoms that are more conducive to oxidation than the 6-fold Mn(II), the surface catalytic effect increases owing to the PO₄ dangling bonds near the Mn atoms.

  Figure 3

  

  Figure 3.  (a) Structure of Mn₃(PO₄)₂·3H₂O. (b) (100) Surface structure of Mn₃(PO₄)₂·3H₂O. (c) Bonding geometry of the J-T distorted surface Mn(2) atom of oxidized manganese phosphate, in which some Mn(II) atoms were intentionally oxidized to Mn(III). Reprinted with permission from ref. 48. Copyright 2014 American Chemical Society.

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  The direct relationship between the asymmetric structures in manganese-based catalysts and their electrocatalytic activity for water oxidation reaction was also supported by our group. We report a manganese(II) phosphate nanosheet assembly with asymmetric out-of-plane Mn centers from the transformation of amine-intercalated nanoplates for efficient electrocatalytic water oxidation in neutral aqueous solutions (the overpotential is 563 mV under 1 mA·cm^-2, 0.05 M phosphate buffer at pH = 7.0) (Figure 4a).^[50] The crystal structure of the MnPi is illustrated in Figure 4b. This structure contains six different Mn sites (Mn(1), Mn(2), Mn(3), Mn(4), Mn(5) and Mn(6)). First, 6-coordinated Mn(1), Mn(2), Mn(3) and Mn(4) octahedrons are connected on the a-b axis to form a dense layer parallel to the (001) plane. Then, the 5-coordinate Mn(5) triangular bipyramid and 6-coordinate Mn(6) octahedron are located outside the (001) plane to form another layer. It can be noted that Mn(5) and Mn(6) have one and two terminal coordinated water molecules, respectively. The terminal water on Mn(5, 6) can participate in water oxidation to form O₂. After O₂ is released, these coordination sites will be used by new water molecules. In MnPi, the Mn(5) trigonal bipyramid is highly distorted with one terminal water-coordination. This structure is essentially promising for water oxidation. And Mn(5) is 5-coordinated, which has the highest tolerance to Mn(III) distortion. The first-principles density functional theory (DFT) calculations were performed to determine the active Mn center in MnPi. The charge difference on the Mn atoms is simulated and calculated. In Figure 4c, the top three Mn sites with the largest reduction valence charge are marked with yellow circles. Two of them are the 6-coordinated Mn(6) sites with two terminal water molecules, and the remaining one is the 5-coordinated Mn(5) site with one terminal water molecule. The coordinated terminal water molecules further promote the oxidation of the corresponding Mn center by providing a disposable proton. In summary, the advantages of Mn(5) and Mn(6) centers are mainly to coordinate with the terminal water molecules to provide vacancies for OER substances. They are also considered as the out-of-plane Mn centers, which is similar to that of the dangling Mn4 (Figure 1b) in the Mn cluster in OEC. And they can be oxidized at a low potential to form intermediate state Mn(III).

  Figure 4

  

  Figure 4.  (a) Ultrathin MnPi nanosheet assembly with native asymmetric out-of-plane Mn centers (red) derived from the transformation of layered phosphates for electrocatalytic water oxidation. (b) The crystal structure of MnPi with six different Mn sites, including Mn(1-4) layers and out-of-plane Mn(5, 6) centers. (c) The unit cell of MnPi used to calculate the preference of the oxidation of different Mn sites. The three Mn sites that can be most easily oxidized are marked with yellow circles. Reprinted with permission from ref. 50. Copyright 2018 Royal Society of Chemistry.

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  On the basis of the previous work, the relationship between the amount of Mn(III) in Mn-based phosphate catalysts and their electrocatalytic activity for the water oxidation was further supported by our group.^[51] We reported two autologous phosphates (Mn₂P₂O₇ and Mn₃(PO₄)₂, the overpotentials are 555 and 630 mV under 1 mA·cm^-2, 0.05 M phosphate buffer at pH = 7.0) obtained from the same parent material for electrocatalytic water oxidation. The two autologous materials have similar morphology, crystallinity and surface areas, which make them a good platform for studying water oxidation in response to the Mn coordination environment. Both materials have monoclinic structures. In Mn₂P₂O₇, there is only one type of 6-coordinated Mn(II) center. And there are three types of Mn(II) centers in the Mn₃(PO₄)₂ crystal. One is the 6-coordinated (Mn(1)) octahedron and the other is the 5-coordinated (Mn(2), Mn(3)) trigonal bipyramid. The electrocatalytic water oxidation of the synthesized manganese phosphates was studied in 0.05 M phosphate buffer (pH = 7.0) by cyclic voltammetry (CV), and the catalytic performance of Mn₂P₂O₇ is better than that of Mn₃(PO₄)₂ with the same Mn valence. The presence of Mn(III) intermediate in Mn₂P₂O₇ and Mn₃(PO₄)₂ catalysts during electrolysis is identified by the determination of the Mn(III)-pyrophosphate (pp) complex, which has a characteristic ligand-to-metal charge transfer (LMCT) absorption at 258 nm.^{[52, 53]} In contrast, the relative absorption of Mn₂P₂O₇ catalyst solution is very obvious, indicating that the structure can provide more amount Mn(III). According to the electrochemical and spectroscopic results, it has been straightforwardly observed that highly asymmetric geometry of Mn₂P₂O₇ can stabilize the active Mn(III) with J-T distortion and form abundant active centers. In addition to stabilize the central manganese structure, phosphate can also form a rich, extensive and continuous hydrogen bond network to accelerate the proton transfer rate, which further promotes the electrocatalytic water oxidation.^[54]

  Mn-based Heterogeneous Electrocatalysts with Heteroatom Incorporation. Introducing heteroatoms into Mn-based materials is also one of the hotspots of research in recent years, although it is difficult to fully understand the exact role of each element.^[55-58] Nam's group selected the pyrophosphate-based Mn compound Li₂MnP₂O₇ as a model system and investigated the role of the Mn valence in the catalytic activity (LiMnP₂O₇, 5.0 A/g at an overpotential value of 680 mV in 0.5 M phosphate buffer at pH = 7.0).^[43] By controlling the content of lithium, the valence of Mn in the material is changed (Figure 5a). The XANES spectrum of Mn K-edge shows that the average oxidation state of Mn increases as the Li content decreases (Figure 5c). The catalytic current density and exchange current density increased proportionally with the amount of Mn(III) species, while the Tafel slopes are similar, indicating that the number of active sites of catalyst is directly related to the Mn(III) species (Figure 5b). In addition, the Baur distortion index is also calculated, which can reflect the degree of asymmetry of the local Mn octahedron. Interestingly, the catalytic activity and Baur distortion index showed similar linear dependence on the amount of Mn(III) species, which means the amount of Mn(III) species and asymmetric local coordination of Mn are directly related to the activity of Mn-based heterogeneous catalysts in electrochemical water oxidation.

  Figure 5

  

  Figure 5.  (a) Simulated crystal structures of the Li₂MnP₂O₇, Li_1.75MnP₂O₇, Li_1.5MnP₂O₇ and LiMnP₂O₇ unit cells. The inset shows the local Mn environment around the Mn₂O₉ subunit. Mn²⁺ and Mn³⁺ atoms in the unit cell are shown in blue and yellow, respectively. (b) Polarization-corrected cyclic voltammetry curves of LiMnP₂O₇ (green), Li_1.5MnP₂O₇ (blue), Li_1.7MnP₂O₇ (red) and Li₂MnP₂O₇ (black) in 0.5 M sodium phosphate buffer (pH 7.0). (c) XANES Mn K-edge spectra of Li_2-xMnP₂O₇ (x = 0, 0.3, 0.5, 1) powders. The inset shows the average oxidation state of Mn atoms gradually changes. Reprinted with permission from ref. 43. Copyright 2014 American Chemical Society.

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  Furthermore, they reported the spectroscopic characterization of new intermediates during the water oxidation reaction of Mn-based heterogeneous catalysts and assigned them as low-spin Mn(IV)-oxo species (Figure 6).^[59] It shows that the engineering of the local distortions via Ni atom substitution in Mn₃O₄ NPs manipulates the spin state of the reaction intermediates during water oxidation reaction (Ni-Mn₃O₄ NPs/NiO, the overpotential is 420 mV under 1 mA·cm^-2, 0.5 M phosphate buffer at pH = 7.0). Through a combination of experiment and computation, they explored the effect of Ni atom substitution on the intermediate species of nanoscale manganese oxide NPs, and found that the Ni substitution enables the compression of surface Mn octahedron, thereby producing low-spin Mn(IV)-oxo intermediate species in the water oxidation reaction.

  Figure 6

  

  Figure 6.  DFT calculations to evaluate the impact of Ni substitution in Mn oxide. (a) The (001) and (231) surface structures of Mn₃O₄. (b) Schematic illustration of the surface Mn active site. (c) The impact of Ni substitution on the axial Mn–O distance and rhombicity of equatorial Mn–O distances of the Mn₃O₄ (001) and (231) surface active sites. (d) The UV-Vis spectra of the high-spin and low-spin Mn(IV) site cluster models. Reprinted with permission from ref. 59. Copyright 2020 Nature Publishing Group.

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  So far, two widely accepted OER mechanisms include adsorbate evolution mechanism and lattice oxygen oxidation mechanism.^{[60, 61]} No matter which OER mechanism the catalyst surface follows, the formation of O–O bond can follow two different ways, acid-base nucleophilic attack and direct radical coupling.^{[61, 62]} For acid-base nucleophilic attack pathway, there is an inherent linear scaling relation (LSR) between the adsorption energy of *OOH and *OH intermediates. The inherent LSR for most catalytic materials imposes a theoretical overpotential ceiling, limiting the development of efficient electrocatalysts.^{[60, 63]} Therefore, current research efforts are mainly directed to overcome the limitation from such LSR for developing practical electrocatalysts. Wang's group used Na_xMn₃O₇ with adjustable amount of Na⁺ as model to unlock the specific coordination configuration that can regulate the barrier symmetry between O–H bond cleavage and *OOH formation on the basis of overcoming the LSR between *OOH and *OH (Figure 7).^[64] Through a combination of in-situ spectroscopy-based characterization and first-principles calculations, they revealed that the number of Na⁺ is critical to the overall activity improvement. In terms of electronic effect, the O–O bond formation is promoted as the number of Na⁺ reduces, because of the increased number of oxygen holes in |O_2p upon activating lattice oxygen, and the relative barrier between O–H bond cleavage and O–O bond formation is regulated. In terms of geometric effect, the overpotential ceiling increases as the number of Na⁺ reduces due to the weakening of Na⁺-specific stabilizing effect on the pendant oxygen in *OOH. As a result of the above two opposite effects, an intermediate level of Na⁺ mediation (NaMn₃O₇) exhibits the optimum OER activity (NaMn₃O₇, the overpotential is 280 mV under 0.25 mA·cm^-2 in O₂-saturated 1 M KOH electrolytes at pH = 13.8). In addition, the oxygen precipitation mechanism and active site were verified. Resolving the near-surface structures under electrochemical condition of the catalyst is a prerequisite for understanding the OER mechanism and related active site. The in-situ X-ray photoelectron spectroscopy (XPS) measurements were performed on NaMn₃O₇. The pH-dependent experiment and Raman spectra further demonstrate it works in a decoupled proton/electron route with the presence of negatively charged oxidized oxygen species. By combining all results, they demonstrate that the activation of lattice oxygen leads to the enhanced OER activity, as the Fermi level enters the |O_2p states for Na_xMn₃O₇ (x < 2) due to the charge compensation and redistribution, creating the reactive oxygen radicals on the surface which behave as electrophilic centers prone to nucleophilic attack from the oxygen lone pairs of OH^-. This work provides a guideline for the development of better catalysts towards water oxidation or other oxidative reactions through tuning both lattice oxygen reactivity and scaling relation.

  Figure 7

  

  Figure 7.  (a) Schematic formation of oxygen holes in |O_2p lone-pair states for Na_xMn₃O₇. (b) Projected density of states of Na_xMn₃O₇ slabs (x = 2, 1.5, 1 and 0.5). (c) Partial charge density projected on O atoms by the shaded region shown in Figure 7b. (d) Scheme of rational design of better Na_xMn₃O₇ electrocatalysts. Reprinted with permission from ref. 64. Copyright 2021 Nature Publishing Group.

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  WATER OXIDATION MECHANISM OF Mn-BASED HETEROGENEOUS ELECTROCATALYSTS

  In addition to the knowledge of constructing stable intermediate states with different structures discussed, exploring the entire mechanism cycle and determining the slowest step are very important to further understand the OER mechanism of Mn-based catalysts. We first describe the basic steps of the electrochemical water oxidation reaction and explain the concept of rate-determining steps (RDS). Next, we summarized the reaction mechanisms of several representative Mn-based catalysts.

  The Basic Steps and Rate-determining Steps of Electrochemical Water Oxidation. Electrochemical water oxidation to O₂ is a multi-step reaction involving transfer of four electrons and four protons, including electrochemical steps and chemical steps. Electrochemical steps involve PCET, which includes both electron transfer (ET) and proton transfer (PT); the chemical steps mainly include the formation of O–O bonds, the release of O₂ and the charge rearrangement of catalyst materials.^{[65, 66]} The oxidizing power accumulates on the catalyst material to form high-valent active intermediate species in the electrochemical oxidation steps before the O–O bond formation step.^[67] First, the PCET process could be carried out in two ways: stepwise sequential electron and proton transfer (PT-ET or ET-PT) or concerted proton and electron transfer. During the electrochemical oxidation process, different intermediate species could be formed and some are unstable with charge rearrangement occurring rapidly, thereby reducing the concentration of active intermediate species. For example, the charge rearrangement of Mn(III) is closely related to the reaction conditions such as pH and the surface structure of the catalyst material.^[68] The charge disproportionation of Mn(III) in δ-MnO₂ is a representative example of a charge rearrangement process (Figure 8). And Mn(III) species is 11-fold higher on (101) surfaces compared with on the (110) surfaces in β-MnO₂ because Mn(III) species are favorably generated by the charge ratio between the layers of Mn(II) and Mn(IV) on the (101) plane. Therefore, it is important to explore the stability of the intermediate. For biological OEC, the O–O bond formation step in the OER process is still unknown. The formation of O–O bond can follow two different ways, acid-base nucleophilic attack and direct radical coupling. In the former case, a substrate water molecule or hydroxide ion nucleophilically attacks the high-valent metal-oxo/oxyl species and is deprotonated to generate a hydroperoxo intermediate, while in the latter case, two high-valent metal-oxo/oxyl species directly couple with each other to form a bridging peroxo intermediate. For example, Spiccia's research team studied the O–O bond formation pathway on a Ca decorated δ-MnO₂ catalyst, including both acid-base nucleophilic attack and direct radical coupling.^[69]

  Figure 8

  

  Figure 8.  Schematic image of surface valency changes on (101) and (110) surfaces of β-MnO₂ after the reduction of surface Mn atoms. Reprinted with permission from ref. 68. Copyright 2018 John Wiley and Sons.

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  The total reaction rate is usually determined by the elementary step with the slowest reaction rate. This slowest step is known as the RDS. Due to the different types of catalysts and reaction conditions, both electrochemical and chemical steps may become RDS. The Tafel slope observed in the electrochemical kinetic experiments provides information on the catalytic activity and RDS. Besides, the reaction-order analysis, such as pH or concentration dependence, can help to identify RDS by prompting whether there are protons or electrolytes involved in RDS. Combining RDS obtained by electrochemical kinetic analysis and in-situ spectroscopy to determine the information of active intermediates can provide a more comprehensive understanding of the reaction mechanism.

  Mechanism Cycle of Mn-Based Heterogeneous Electrocatalyst for Water Oxidation. According to the number of Mn atoms constituting the active site, the proposed mechanisms can be divided into two categories. The first is the monomeric active site. The manganese undergoes sequential oxidation steps to form the high-valent Mn-oxo species before O–O bond formation via the acid-base nucleophilic attack pathway. The second is the multimeric active site. It is particularly important to understand the charge rearrangement processes between the manganese atoms. Highly active Mn-oxyl species are formed through oxygen-centered oxidation and O–O bond formation proceeds via the direct radical coupling pathway.^[67]

  Based on both electrochemical kinetic analysis and various spectroscopic results, an overall water oxidation mechanistic scheme was constructed for Mn species on the nanoparticle surface. Nam's group proposed that monomeric Mn(IV)=O species were responsible for the slowest O–O bond formation step through acid-base nucleophilic attack pathway after generation through the PCET of Mn(II)-OH₂ and Mn(III)-OH species (Figure 9a).^[68] The RDS change from a charge accumulation step to the O–O bond formation step also corresponds well with the experimental results reported by Dau's group, ^[70] which verified the stable charge accumulation of oxidizing power through the fast Mn-centered redox-state change (Mn(III)-Mn(IV)) in catalytically active manganese oxides. Nocera's group performed electrochemical kinetic analysis of layered manganese oxide (δ-MnO₂) over the entire pH range (Figure 9b).^[71] Tafel and reaction order analyses were performed by measuring the current-voltage, phosphate concentration-dependent current density and pH-dependent current density characteristics of the catalyst. Under alkaline conditions, they suggested that the surface of the catalyst consisted of a mixture of Mn(III) and Mn(IV) valence states. They suggested that Mn(III)-OH is converted to Mn(IV)=O via a PCET pre-equilibrium step before the O–O bond formation through the direct coupling as the RDS. Under acidic conditions, based on previously observed unstable Mn(III) species on the surface of δ-MnO₂ below pH 9.0, they assigned that the cross-site proton coupled charge disproportionation of Mn(III) is RDS. Four Mn(III)-OH species disproportionated to give two Mn(II)-OH₂ and two Mn(IV)=O; and the adjacent Mn(IV)=O then coupled to form the O–O bond. The low catalytic activity of δ-MnO₂ under acidic conditions originated from the small proportion of four adjacent Mn(III)-OH species on the catalyst surface. At near-neutral pH, they proposed that the reaction mechanism of δ-MnO₂ was a mixture of the two competing pathways presented above rather than one specific mechanism.

  Figure 9

  

  Figure 9.  (a) Suggested mechanism of manganese oxide nanoparticles for water oxidation. Reprinted with permission from ref. 68. Copyright 2017 American Chemical Society. (b) Proposed mechanism for the OER on MnO_x in (left) acidic and (right) alkaline conditions. Reprinted with permission from ref. 71. Copyright 2014 American Chemical Society.

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  Recent reports proposed interesting hypothetical water oxidation pathways that involve high-valent Mn species. Naruta and Zahran suggested that Mn(V)=O is a highly active species of the catalyst. When the electrolyte was changed from aqueous phosphate buffer to a non-aqueous solution, the RDS changed from ET to the O–O bond formation chemical step.^[72] Combined with the results of electrochemical kinetic analysis and kinetic isotope effect experiment, they suggested that Mn(V)=O is generated via the sequential oxidation steps of Mn(II)-OH₂ → Mn(III)-OH → Mn(IV)=O → Mn(V)=O and is the active species for the RDS of O–O bond formation by the acid-base nucleophilic attack pathway in non-aqueous electrolyte solution. Sun's group prepared c-disordered δ-MnO₂ by annealing electrodeposited manganese oxides, then got their electrochemical kinetic parameters.^[42] However, the RDS remained unclear because of the difficulty of interpreting the derived species originating from the strong interactions between various redox sites. They performed ex-situ infrared spectroscopy and detected the intermediate species of Mn(VII)=O, believing that at least four Mn sites are involved in the oxidation reaction of water, and the Mn(VII)=O intermediate is generated through charge rearrangement.

  Mechanism Cycle of Mn-based Homogeneous Electrocatalyst for Water Oxidation. Compared with material catalysts, the structures of molecular catalysts are clearer. The structure-func tion relationships of molecular catalysts can be systematically studied.^[73-76] For O–O bond formation step, three different possibilities have been discussed (Figure 10a): (1) an acid-base (AB) mechanism involving the nucleophilic attack of water or hydroxide on a formal Mn(V)-oxo species, (2) a radical coupling (RC) of two bridging metal-oxo species, or (3) a RC of the dangling Mn(IV)-O^• radical with a bridging oxygen in the manganese cluster.^{[77, 78]} Despite experimental and theoretical model studies, the actual mechanism for the formation of molecular oxygen at OEC has stayed ambiguous to date. Recently, an excellent system is established, in which Mn-oxo has considerable activity for water oxidation and meanwhile has satisfactory stability for characterizations and reactivity studies.^[79] By studying Mn tris(pentafluorophenyl)corrole 1 (Figure 10b), we provided evidence for a Mn-based WNA mechanism. The Mn(IV)-peroxo intermediate was identified by UV-vis spectra, EPR, infrared, high-resolution mass spectrometry and isotope-labeling experiments, confirming the nucleophilic attack of the O–O bond formation mechanism. We identified the formation of Mn(V)(O) at the Mn(V/IV) potential prior to catalyzing water oxidation and Mn(IV)-peroxo at the potential to initiate water oxidation. On the basis of these results, we can propose the mechanism of water oxidation with 1 (Figure 10c). Mn(V)(O) can be generated electrochemically (black line) and chemically (gray line). The reaction of Mn(V)(O) with hydroxide is fast, leading to the formation of the monovalent anion of Mn(III)-OOH, which is then oxidized by unreacted Mn(V)(O) and deprotonated with excess hydroxide to give a Mn(IV)-peroxo species (blue line). However, nucleophilic attack of water on Mn(V)(O) is so extremely slow that further oxidation of Mn(V)(O) is required to become more active to react with water under electrocatalytic conditions, and the formal Mn(VI)(O) describes this 1e^--oxidized Mn(V)(O) species. According to calculations, WNA on Mn(VI)(O) is dynamically feasible to generate Mn(IV)-peroxide (red line in Figure 10c), which can then be oxidized or disproportionated to produce O₂ and Mn(III). We also studied WNA on metal-oxo units with similar coordination environments, which suggested a similar WNA mechanism involved in water oxidation.^{[80, 81]}

  Figure 10

  

  Figure 10.  (a) Proposed O–O bond formation mechanisms in chemistry and biology. Reprinted with permission from ref. 78. Copyright 2021 Royal Society of Chemistry. (b) Molecular structure of 1 and (c) Proposed mechanisms for water oxidation with 1. Reprinted with permission from ref. 79. Copyright 2021 American Chemical Society.

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  CONCLUDING REMARKS

  The reason why Mn-based heterogeneous catalysts have attracted much attention in water oxidation is mainly that the biological photosystems only use four manganese atoms to achieve the highest turnover frequency. Inspired by natural photosystems, designing highly efficient, stable, cost-effective and environmentally-friendly electrocatalysts is the goal for efficient water oxidation. However, despite pioneering efforts of many groups in the development of Mn-based heterogeneous catalysts, there are still huge gaps between synthetic catalysts and biological photosystems in terms of performance and understanding of the mechanism at the atomic level. To address this, we thought that it is necessary to summarize the internal structural characteristics of the synthetic catalyst to clarify the specific structural characteristics and whether they are related to the OER process, so as to further understand the OER mechanism. In the water oxidation reaction, the OEC catalyzes water splitting into electrons, protons, and dioxygen through a five-state cycle. The atomic arrangements and oxidation state in every S_n state except the S₄ state have been experimentally revealed. However, we still do not have any direct experimental evidence for the O–O bond formation step in the biological system, even though it is the most important and RDS. In this regard, the investigation of heterogeneous catalysts can provide useful information to understand the mechanism by capturing intermediates and decoupling kinetic parameters.^[82]

  In order to better understand the oxidation process of water, the reaction rate constant of each step needs to be measured. However, the complexity of the catalyst surface makes the situation difficult. Furthermore, the fact that electrons are transferred between the catalyst surface and the electrode should always be considered.^[83] In this review, considerable knowledge has been obtained from the analysis of thermodynamically stable crystals of manganese materials, but there is still a lot of room to make metastable phases, especially on the surface of the catalyst. Additionally, we would like to bring attention to the effect of entropy during water oxidation, as proposed recently.^[84] In addition to enthalpy-based methods to control the combination of intermediates on the catalyst surface, the important contribution of entropy mainly involving water molecules needs to be considered. All in all, we hope that the review can provide useful guidance for the future research directions of Mn-based water oxidation catalysts. More exciting science will be revealed in the future, and the input of scientists and engineers in related research fields will be highly needed.

  
  ACKNOWLEDGEMENTS: The project was supported by the Starting Research Funds of Shaanxi Normal University and the National Natural Science Foundation of China (21872092). COMPETING INTERESTS
  The authors declare no competing interests.
  ADDITIONAL INFORMATION
  For submission: https://mc03.manuscriptcentral.com/cjsc
  Full paper can be accessed via
  http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0024
  
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  Figure 1  Structure of the Mn₄CaO₅ cluster. (a) Crystal structure of the Mn₄CaO₅ cluster and its ligand environment. Reprinted with permission from ref. 16. Copyright 2017 American Chemical Society. (b) Bond length differences (in Å) between metal atoms and oxo bridges or water molecules. (c) Hydrogen bond network from the Mn₄CaO₅ cluster. Water molecules participating in the hydrogen bond network are depicted in orange, whereas those not participating are in grey. Reprinted with permission from ref. 19. Copyright 2011 Nature Publishing Group. (d) Kok cycle: the five intermediate states S_n (n = 0–4) during water oxidation. Reprinted with permission from ref. 19. Copyright 2021 American Chemical Society.

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  Figure 2  (a) Structures of Mn oxides: α-MnO₂ (2 × 2 tunnel), β-MnO₂ (1 × 1 tunnel), δ-MnO₂ (layered), and amorphous manganese oxides (AMO). Reprinted with permission from ref. 34. Copyright 2014 American Chemical Society. (b) Demonstration of the crystal structure and d-orbital electronic configuration of Mn₂O₃ and γ-MnOOH. (c) Description of corner-shared Mn(III) octahedra in δ-MnO₂ (top) and the biological Mn₄CaO_x cluster (bottom). Reprinted with permission from ref. 37. Copyright 2016 American Chemical Society.

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  Figure 3  (a) Structure of Mn₃(PO₄)₂·3H₂O. (b) (100) Surface structure of Mn₃(PO₄)₂·3H₂O. (c) Bonding geometry of the J-T distorted surface Mn(2) atom of oxidized manganese phosphate, in which some Mn(II) atoms were intentionally oxidized to Mn(III). Reprinted with permission from ref. 48. Copyright 2014 American Chemical Society.

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  Figure 4  (a) Ultrathin MnPi nanosheet assembly with native asymmetric out-of-plane Mn centers (red) derived from the transformation of layered phosphates for electrocatalytic water oxidation. (b) The crystal structure of MnPi with six different Mn sites, including Mn(1-4) layers and out-of-plane Mn(5, 6) centers. (c) The unit cell of MnPi used to calculate the preference of the oxidation of different Mn sites. The three Mn sites that can be most easily oxidized are marked with yellow circles. Reprinted with permission from ref. 50. Copyright 2018 Royal Society of Chemistry.

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  Figure 5  (a) Simulated crystal structures of the Li₂MnP₂O₇, Li_1.75MnP₂O₇, Li_1.5MnP₂O₇ and LiMnP₂O₇ unit cells. The inset shows the local Mn environment around the Mn₂O₉ subunit. Mn²⁺ and Mn³⁺ atoms in the unit cell are shown in blue and yellow, respectively. (b) Polarization-corrected cyclic voltammetry curves of LiMnP₂O₇ (green), Li_1.5MnP₂O₇ (blue), Li_1.7MnP₂O₇ (red) and Li₂MnP₂O₇ (black) in 0.5 M sodium phosphate buffer (pH 7.0). (c) XANES Mn K-edge spectra of Li_2-xMnP₂O₇ (x = 0, 0.3, 0.5, 1) powders. The inset shows the average oxidation state of Mn atoms gradually changes. Reprinted with permission from ref. 43. Copyright 2014 American Chemical Society.

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  Figure 6  DFT calculations to evaluate the impact of Ni substitution in Mn oxide. (a) The (001) and (231) surface structures of Mn₃O₄. (b) Schematic illustration of the surface Mn active site. (c) The impact of Ni substitution on the axial Mn–O distance and rhombicity of equatorial Mn–O distances of the Mn₃O₄ (001) and (231) surface active sites. (d) The UV-Vis spectra of the high-spin and low-spin Mn(IV) site cluster models. Reprinted with permission from ref. 59. Copyright 2020 Nature Publishing Group.

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  Figure 7  (a) Schematic formation of oxygen holes in |O_2p lone-pair states for Na_xMn₃O₇. (b) Projected density of states of Na_xMn₃O₇ slabs (x = 2, 1.5, 1 and 0.5). (c) Partial charge density projected on O atoms by the shaded region shown in Figure 7b. (d) Scheme of rational design of better Na_xMn₃O₇ electrocatalysts. Reprinted with permission from ref. 64. Copyright 2021 Nature Publishing Group.

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  Figure 8  Schematic image of surface valency changes on (101) and (110) surfaces of β-MnO₂ after the reduction of surface Mn atoms. Reprinted with permission from ref. 68. Copyright 2018 John Wiley and Sons.

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  Figure 9  (a) Suggested mechanism of manganese oxide nanoparticles for water oxidation. Reprinted with permission from ref. 68. Copyright 2017 American Chemical Society. (b) Proposed mechanism for the OER on MnO_x in (left) acidic and (right) alkaline conditions. Reprinted with permission from ref. 71. Copyright 2014 American Chemical Society.

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  Figure 10  (a) Proposed O–O bond formation mechanisms in chemistry and biology. Reprinted with permission from ref. 78. Copyright 2021 Royal Society of Chemistry. (b) Molecular structure of 1 and (c) Proposed mechanisms for water oxidation with 1. Reprinted with permission from ref. 79. Copyright 2021 American Chemical Society.

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