Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

3.1 Variations of the electrode support material

The choice of the conductive substrate used as backbone for the MnO_x catalyst material greatly influences its WOC performance. In the case of MnO_x the application of solid MnO_x disks or rods prepared from powders is not feasible, because compared to other transition metal oxides commonly used for WOC, MnO_x generally show much higher electrical resistivities (Table 1). Therefore, it seems inevitable for MnO_x to avoid the transport of charge carriers through larger distances of the materials, which is most commonly achieved by applying just a thin layer of MnO_x catalyst on top of a much more conductive electrode support material. However, to find suitable support materials is far from straightforward, as they also have to fulfill a number of general requirements: (1) high electrical conductivity; (2) high chemical stability and (electrochemical) inertness up to at the high anodic potentials needed to drive the water oxidation reaction; (3) good stability even under the often drastic pH conditions of acidic or alkaline electrolysers; (4) a good thermal stability since MnO_x catalysts often require a sintering step at temperatures above 300 °C to establish good electrical contacts between the particles and the support; (5) a good availability and a low price; and (6) ideally a high surface area to increase the catalyst/electrolyte contact. In addition to this list, the experimentalist sometimes has to consider special requirements, such as transparency for UV/Vis light or X-rays in order to carry out spectroscopic studies. In the following we will present an overview of the most commonly used support materials for the electrochemical testing of MnO_x and mention possible advantages and disadvantages.

An electrochemical support material class that has very often been used in the context of WOC studies are transparent conductive oxides (TCOs). Typical examples are fluorine-doped/tin(IV)-alloyed indium(III) oxides (FTO/ITO), which are semiconducting oxide materials transparent to most visible light. Their n-type conductivity is either generated by oxygen vacancies, fluorine doping or by the substitution of the host metal by higher valency metal atoms (e.g. Sn in Sn:In₂O₃). For their use as conductive supports, thin layers of the TCOs are coated under high vacuum conditions onto glass substrates, resulting in glass slides with a thin conductive layer on top, which exhibit high degrees of transparencies for visible light. On their own, ITO and FTO can be considered to be WOC inactive both in NaOH and H₂SO₄ electrolytes (WOC onset potentials of +1.8 V and higher), making both materials suitable “innocent” substrates for the testing of water oxidation catalysts [67], [68], [69]. In comparison to ITO, FTO has a better chemical and thermal stability and lower price (making it the better choice for long-term experiments [70]), but both materials have often been used as substrates for WOC experiments [49], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83].

A further advantage of ITO or FTO substrates is their already mentioned transparency. As an example, one of our groups electrodeposited manganese oxides on ITO via different electrochemical coating protocols and monitored the changes of the Mn oxidation state of the deposited material by UV/Vis-spectroscopy [49], [75]. It was found that higher absorptions at 450 nm indicated a higher average oxidation state of the Mn ions and that during the pseudocapacitive charging of the electrode at potentials below the WOC onset, oxidation equivalents are accumulated due to Mn^II ⟶ Mn^III and Mn^III ⟶ Mn^IV transitions. In another, very influential work by the group of Nakamura [75], manganese oxide nanoparticles from the birnessite family were deposited on a FTO electrode to monitor the spectral changes during WOC at pH regimes ranging from 4 to 13. The recorded spectral changes at around 500 nm during the evolution of oxygen were explained by the accumulation of Mn³⁺ ions (from the oxidation of Mn²⁺-ions) at the electrode surface, which were identified as key elements of the electrochemical reaction. This work also represented a first in situ spectroscopic detection of intermediates during WOC by manganese oxides. Generally, an increase of the average oxidation state of the bulk of different MnO_x polymorphs during operation is likely occurring, as seen by post operando soft- and hard- X-ray absorption (XAS)-measurements [49], [84], [85]. A discussion about the relationship between the XAS absorption changes and parameters like the Mn oxide conductivity can be found in a recent publication by some of us [84].

In addition to the transparency advantage there are further benefits of using TCO materials: (1) a good thermal stability up to 800 °C in air [71], [72], [74], [75], [80], [83], [77], which allows sintering and the synthesis of crystalline MnO_x phases at high temperatures [61], [79]; (2) easy characterization of the catalyst layer by TEM [75], [79], [82]; (3) the possibility to determine the band gaps of the MnO_x materials using UV/Vis spectroscopy [78]. A major disadvantage of using TCO materials are the quite low electrical sheet conductivity of the conductive layers ranging from 0.10 to 0.25 S/□ [86]. This resistivity becomes a substantial factor at high current densities or large electrode areas, thus making the use of FTO or ITO as support material for industrial WOC applications unlikely [87]. Additionally, there are no convincing ways to “activate” FTO or ITO surfaces for the deposition of MnO_x layers and hence it is not uncommon that the contact between the catalyst and the conductive oxide is mechanically weak, leading to the partial or complete detachment of the catalyst film under WOC conditions. Overall, MnO_x/TCO-anodes have clear advantages for fundamental investigations like mechanistic studies but will most likely not be suitable for the construction of even medium sized devices.

Another conductive electrode support material class is elemental carbon, used in electrochemistry since the early 19^th century when Sir Humphrey Davy first used graphite electrodes for the production of alkaline metals. Since then, many different carbon materials have been developed for various electrochemical applications [88], including the role of support material for manganese oxide in WOC, for which they possess a number of profound advantages: (1) very good availability and (often) a low prize; (2) good chemical and electrochemical stability; (3) good thermal stability (up to 450 °C in air and up to 3000 °C in Ar); (4) high electrical conductivity (e.g. 2–3 10⁵ S m⁻¹ ∥ to the basal plane and 3 10² S m⁻¹ ⊥ to the basal plane of graphite) [89]; (5) high intrinsic surface areas for materials like carbon nanotubes (CNTs) or carbon fibers; (6) the possibility to modify the material surface for example by the introduction of oxygen functionalities in order to enhance the electric and mechanical contact between MnO_x and C; [90], [91], [92] (7) suitability for some spectroscopic techniques, e.g. vibrational or X-ray spectroscopies (XAS, XPS) [93], [94].

Some of the most commonly used carbon-based electrode materials are graphite rods or sheets, CNTs, carbon fibers (in the form of papers or felts), amorphous carbon (carbon blacks), graphene, highly oriented pyrolytic carbon (HOPG) and – most popular for electrochemical analysis – glassy carbon (GC). This material was discovered in 1962 and consists of randomly intertwined ribbons of graphitic planes prepared by heating different organic polymers under pressure in an inert gas atmosphere at 1000–3000 °C [88], [95], [96]. GC exhibits a high (electro)chemical stability, is commercially available and can be manufactured into many shapes such as rotating disk electrodes which have often been used as substrates for the characterization of MnO_x at WOC conditions [92], [97], [98]. On the other hand, GC has only a small intrinsic surface area (similar to the TCO supports described above). Some research groups increased the contact area between the catalyst and the electrolyte by first depositing MnO_x catalysts on CNTs and then coating GC disks with the obtained MnO_x/CNT hybrids [92], [98], [99]. Another possibility to increase the intrinsic surface areas is to use an electrode material that is based on carbon fibers, e.g. carbon fiber paper (CFP) or carbon felts (CF), which are well-known in electrocatalysis e.g. as gas diffusion layers (GDL) in polymer electrolyte (PEM) fuel cells or electrolysers [100], [101]. Furthermore, CFPs and CFs have also been used as catalyst supports in so called gas diffusion electrodes (GDE) [102], [103].

Despite these many advantages, there are also some drawbacks of carbon materials like the possible vulnerability to corrosion. From a thermodynamic point of view, carbon is only stable within a small potential window in water, as for example the four-electron oxidation of C to CO₂ could occur already at potentials of E > 200 mV [104], and thus far below the thermodynamic potential of water oxidation. Fortunately, this reaction is kinetically severely hindered due to the complex sequence of reaction steps leading from graphite to CO₂, which is known to proceed via the formation of surface oxides at defect sites of the carbon material [105], [106], [107], [108]. One of our groups found that the number of such defect sites correlates directly with the degree of graphitization (quantified using Raman spectroscopy) and that graphitic carbon materials are therefore best chosen as MnO_x supports in order to obtain stable anodes for water oxidation [109]. Using in-operando membrane inlet-mass spectrometry (MIMS) to detect CO₂ as likely corrosion product, we observed that only graphitic carbon materials are sufficiently stable in water at the potentials needed for WOC by MnO_x (e.g. E ≥ 1.75 V at pH 7.0). On the other hand, low graphitized carbons suffered from corrosion, leading to a loss of contact between MnO_x and C (and thus a deactivation of such anodes) over time. On the other hand, it has been reported that highly graphitic substrates like GC or HOPG, the latter a highly pure and very ordered form of graphitic carbon, can be used with little corrosion problems even at potentials above 2 V [67]. However, a disadvantage of GC or HOPG are their low surface areas, so that additional modifications are usually needed to reach interesting WOC current densities (see above).

In summary, this short introduction on TCOs and carbon materials as possible conductive substrates for manganese oxides has illustrated that choosing the right material for this task can be quite challenging. For analytical studies where a flat, UV/Vis transparent support is important, TCO materials are for sure a good choice. On the other hand, nanostructured, graphitic carbon materials like CFP appear to be especially well suited as backbones for MnO_x catalysts if devices like electrolysers are the target of a study. Of course, many other types of conductive supports like gold, stainless steel, platinum or nickel foam have been used in this context as well [74], [94], [85], [110], [111], but none of them seems to have substantial advantages over TCOs or C to us, so that a detailed discussion of these substrate materials is omitted here.

Fig. 3:

Schematic representation of different preparation techniques for MnO_x-based electrodes for the OER.

3.2 Routes for electrode preparation

There are many ways to deposit manganese oxides on conductive electrode backbones with the aim to prepare anodes for electrochemical water oxidation (see Figure 3), such as e.g. electrodeposition [49], [61], [71], [72], [74], [78], [79], [81], [82], [93], [94], [97], [85], [110], [114], [115], [116], [117], dip-coating [118], drop-casting [92], [98], [99], [119], [120], spin-coating [121], spray-coating [76], [75], [80], [77], screen-printing [73], [77], [118], direct redox-deposition [98], [109], calcination of surfaces impregnated by manganese salts [37], [75], atomic layer deposition [122], [123], [124], [125], physical vapor deposition [126] or reactive sputtering (DC or AC sputtering) [127], [128].

As indicated by the large number of references cited above, the electrodeposition of manganese oxides has so far been the most widely used technique. Electrodeposition has the big advantage that the thickness and composition of the catalyst layer can be tuned by the deposition protocol. Furthermore, various post-deposition treatments even allow access to different MnO_x phases. As a result, there are nearly as many different electrodeposition protocols as publications. However, most of them have the following basic parameters in common: (1) the Mn oxides are deposited anodically, starting from an aqueous solution containing Mn²⁺ and acetate, nitrate, sulfate or chloride as counterions; (2) to increase the ionic conductivity of the electrolyte and in order to control the pH, additional ionic compounds like MgSO₄, Na₂SO₄, (Na/K)NO₃, NaCl, NaClO₄, acetate/borate buffer, etc. are often added. These additives are known to influence both the composition and the structure of the electrodeposited MnO_x, which in turn both have an effect on WOC activity and stability. However, systematic investigations on this topic are so far missing; (3) depositions are carried out at moderate pH values ranging from around pH 5.5 [61], [72], [78], [79] to pH 9.2 [94].

Even as these parameters already allow for substantial variations, the factor having the biggest influence on composition, structure and activity/stability of the MnO_x electrocatalyst is the electrochemical protocol. There are several ways to achieve the deposition of MnO_x, e.g. voltage cycling in the anodic potential range [49], [81], [82], [93], [97], constant anodic potentials high enough to oxidize Mn²⁺ or the application of a constant deposition current density (and thus deposition potentials which in some cases increase over time) [61], [72], [78], [79], [49], [71], [74], [82], [94], [114]. One of our groups studied the influence of the electrochemical protocol on the activity of the deposited manganese oxide in detail and found that a higher activity can be achieved if the catalyst is deposited via a voltage cycling protocol compared to one using a constant potential [49]. Several reasons for this behavior have been proposed: (1) the more active MnO_x obtained from the cycling protocol showed a lower average oxidation state of Mn ∼+3.8 when exposed to oxidizing potentials compared to Mn ∼+4.0 in the film deposited at constant potential; (2) in both cases, a layered, birnessite-type manganese oxide is formed, but its structure is significantly less ordered in the case of the cycling protocol. These results on oxidation states and structures were later confirmed in a study by Huynh et al., which additionally showed that a process where a further cathodic pulse is applied to the anodically deposited catalyst film leads to an even more active MnO_x catalyst [82], [129].

Despite the possible advantages of MnO_x electrodeposition at strongly varied electrochemical conditions, catalysts formed at constant potential or current density conditions are much more common and also reach high electrocatalytic activities. However, here the as-deposited manganese oxide films usually have to be activated further in a heat-treatment step to reach the best WOC results [61], [74], [78], [79], [94], [114], [130]. In work by another of our groups, an amorphous (most likely layered) manganese oxide film was first deposited at constant current density conditions and then annealed at 300 °C (in air), 500 °C (in air) or 600 °C (in argon), resulting in the formation of activated amorphous MnO_x, Mn₂O₃ or Mn₃O₄, respectively [78], [79]. In electrocatalysis experiments, the activated amorphous MnO_x- and the Mn₂O₃-materials showed the smallest onset potentials and highest catalytic currents.

Based on the results presented so far, one can conclude that electrodeposition protocols starting from aqueous Mn²⁺ solutions generally lead to the formation of amorphous, birnessite-type manganese oxides. These as-deposited materials can already be quite active in WOC (e.g. when prepared via a voltage cycling protocol) and can often be activated further by subsequent heat-treatment or cathodization steps.

Another common route to immobilize MnO_x on conductive supports is to pre-synthesize the desired oxide phases as powders and then use drop-casting [92], [99], [119], spin-coating [121], spray-coating [76], [75], [80], [77] or screen-printing [73], [77], [118] methods to coat the chosen support. These procedures have in common that at first suspensions (“inks”) of the MnO_x have to be prepared prior to the deposition and these often contain additives like polyethylene oxide (PEO) [77], polytetrafluoroethylene (PTFE) [73], Nafion [98], [99], ethyl cellulose [73] or ethylene glycol [118] in order to obtain stable coatings. A major advantage of this overall strategy is that well characterized, pre-synthesized catalyst materials are used, for which it is often possible to control material properties like composition, oxide phase or morphology quite well. Thus, one can evaluate the influence of these parameters on the materials’ activities and stabilities under WOC conditions much better [119] and even carry out mechanistic studies, where a well characterized catalysts material is essential.

Screen-printing is a standard technique in electronics and it has also been used to prepare anodes for WOC [77], [73]. The procedure requires an ink containing polymeric binders which are evaporated or decomposed after printing in a sintering step. As a consequence of this, highly porous metal oxide coatings are often obtained, for which the thickness and the coated area can be controlled very well. In a study where a pre-synthesized Ca-birnessite was screen-printed onto FTO supports, one of our groups investigated the influence of parameters like oxide film thickness, binder type or sintering temperature on electrochemical water-oxidation activity and stability in a neutral phosphate electrolyte. It was found that a 30 min. annealing step at 450 °C is essential to remove the organic compounds of the ink and to obtain a mechanically stable catalyst coating. As might have been expected, the choice of organic binder also greatly affects the catalyst’s performance and here the best results were recorded for anodes prepared from PEO-containing inks. Subsequently, the relation between the layer thickness and the catalytic activity was investigated revealing a maximal current density for 10 μm thick oxide coatings. Catalyst layers thinner or a thicker than this value showed lower WOC current densities, a result that was interpreted as a hint that it is necessary to find a compromise between the lower electrical resistivity of thinner coatings and the increased number of active sites in thicker ones [93].

Spray-coating, known for the fabrication of large area thin films in the electronics industry, is another versatile and promising method for the preparation of manganese oxide electrodes. The group of Nakamura established a spray-coating process in which a dilute MnO_x colloid is first formed by the reduction of KMnO₄ by Na₂S₂O₃ in water. This solution was then repeatedly (up to 600 times) sprayed onto FTO serving as conductive substrate and afterwards calcinated at 500 °C to form a stable coating of δ-MnO₂. Alternatively it has been reported that stable and easily scalable MnO_x coatings can also be prepared by a layer by layer deposition of a manganese acetate solution on a conductive substrate (FTO) using an air-spray gun [83]. Subsequent annealing of this deposited precursor for 2 h at 500 °C in an O₂ atmosphere leads to the formation of a smooth and robust oxide film which was identified to be crystalline Mn₂O₃. The authors underline that the high reproducibility, the precise control of the film thickness, the high production speed and the simple scalability are all important advantages of this coating process. Electrodes of this type needed an overpotential η = 400 mV to reach a current density j = 10 mA cm⁻¹ in 1 M KOH.

Drop-casting has also been used as a rather simple route to coat conductive substrates by MnO_x [92], [98], [99], [120]. This process starts with dropping the catalyst containing solution onto the support followed by the spontaneous evaporation of the low boiling solvent. A high-temperature sintering step is omitted, so that conductive additives like Nafion are commonly added to the drop-casting solution in this process to yield mechanically stable and electronically well-connected catalyst coatings. As the method is carried out at mild conditions and it is especially easy to control the amount of catalyst loaded onto the conductive substrate, drop-casting has been regularly used to screen catalyst libraries or to study loading effects. Another advantage of the gentle coating procedure is its suitability to attach MnO_x materials to rotating-disc electrodes (RDEs), which one cannot heat to high temperatures [99], [98].

As an alternative to adding conductive polymers like Nafion, it is also quite common to use carbon powders like Vulcan XC-72 [120] or charcoal [119] to enhance the electric contact between MnO_x catalysts and support. An elegant way to extend this approach to nanostructured carbon materials of increased electrochemical active surface area was developed by Mette et al. [99] and further improved by Bergmann et al. [92]. The groups studied MnO_x which was deposited onto the surface of functionalized carbon nanotubes (fCNTs) either by (a) impregnation of the CNTs with Mn²⁺ followed by permanganate treatment or (b) impregnation with Mn²⁺ followed by a calcination step at 300 °C in air. Both techniques are cheap, well established, scalable and typically lead to the deposition of thin layers of the active materials. The MnO_x formed via route (a) was identified as an amorphous layered manganese oxide with an average oxidation state of +3.5, the one from route (b) as a mixture of β-MnO₂ and γ-MnO₂ with an oxidation state around +3.0. Both composites were attached via drop-casting onto GC-RDEs, where better WOC activities and stabilities at pH 7.0 were observed for the β-/γ-MnO₂/CNT system.

Fig. 4:

High surface area graphitized carbon fiber paper (CFP, Toray TGP-H-60) before (left) and after (right) the decoration with MnO_x via direct redox deposition from an acidic permanganate solution.

Motivated by these very promising first reports on MnO_x/C-anodes (see Figure 4) for water-oxidation, some of us developed an alternative way to prepare such hybrids, this time without the need to use binders or additives [109], [131]. Here, the carbon material does not only serve as conductive and mechanical backbone, but also as reducing agent for MnO₄⁻ to deposit MnO_x directly onto the surfaces of carbon materials (Figure 4). After a sintering step in air at 400 °C, the binder-free MnO_x coating was again identified as amorphous, potassium containing birnessite with an average oxidation state of +3.7. Loading optimized MnO_x/C-electrodes of this type showed promising electrocatalytic activities and stabilities when operated under neutral conditions. This method is cheap, easily scalable and applicable to many different carbon support materials, as e.g. shown by Antoni et al. who used fCNTs as carbon source [98].

As a final preparation route we would like to mention low-pressure methods like atomic layer deposition (ALD) [122], [123], [125], [132], physical or chemical vapour deposition (PVD/CVD) or sputtering [127], [124], [126]. All four techniques are technologically much more demanding than the previous methods, but allow the benefit of depositing catalytic MnO_x films in highly controlled ways. ALD can be used to prepare pin hole free monolayer to multilayer coatings with full control over the catalyst thickness on many substrates. The group of Pickrahn et al. deposited a 40 nm thin, planar MnO layer onto a glassy carbon substrate by using [Mn(EtCp)₂] as volatile manganese precursor and water as co-reactant [122], [132]. Subsequent annealing of the coating at 480 °C led to the formation of a very rough and porous Mn₂O₃ coating. Both MnO_x phases are active in electrochemical water-oxidation and exhibit nearly identical activities, most likely due to similar surface structures induced by the oxidative potentials applied at WOC conditions. Furthermore, the team was able to show that the application of their “ALD-MnO_x” on high surface area GC (HAS-GC), which has a much higher surface area, leads to an enhanced geometric activity indicating that the catalytic activity scales with the surface area. However, ALD is a very slow and expensive deposition technique and hence the MnO_x layers that are accessible in a reasonable time are only a few nanometers thick. Further possible disadvantages are corrosion and mechanical detachment, which are well known to occur for very thin MnO_x films [49]. However, both mentioned studies on ALD-MnO_x do not provide any long-term measurements.

In contrast to ALD, physical vapour deposition offers much faster deposition rates and thus the option to prepare thicker and probably more durable catalyst films in a reasonable time. PVD can also be used to deposit films with different morphologies and compositions. Therefore, metal oxide layers with well-defined nano-architectures are accessible. Together with the Mohney group one of our teams deposited layers of elemental manganese with varying thicknesses on conductive ITO substrates by electron beam evaporation [126]. Subsequent oxidation of Mn in air for 16 h at 300 °C yielded very smooth, brown MnO_x-layers which were characterized as mixtures of MnO and Mn₃O₄ with thicknesses ranging from about 100–400 nm. The films only exhibited small catalytic currents due to their low surface areas, but as the long-term stabilities of these “PVD-MnO_x” anodes proved to be very good, they mark a good starting point for further improvements and optimizations (as e.g. already achieved by the introduction of Ca²⁺-ions into such films via a co-sputtering process) [127].

The deposition of thin manganese oxide films can also be achieved by reactive magnetron sputtering, a simple and efficient technique to prepare homogeneous, compact layers of materials on various supports (see Figure 5). In dependence of the substrate temperature, different Mn-oxides can be obtained: at RT, an amorphous MnO_x is formed, which changes its structure with increasing temperature to nano-crystalline γ-MnO₂ and at ∼350 °C transforms again into α-Mn₂O₃. Both the γ-MnO₂ and the α-Mn₂O₃ films showed similar catalytic OER activities with overvoltages of ∼380 mV for 10 mA cm⁻² under alkaline conditions (pH 13.8) [113], [112].

Fig. 5:

SEM (left) and TEM (right) images of a α-Mn₂O₃ layer deposited at 350 °C on TCO glass (F:SnO₂, FTO) by reactive magnetron sputtering. The layer homogeneously covers the FTO substrate. The Mn₂O₃ crystallites grown on top of the FTO substrate have a size of about 50 nm. The tooth-like morphology indicates a growth of facetted nanocrystals [112], [113].

In summary, there is a plethora of different possibilities to deposit manganese oxides on conductive materials. In combination with the large number of possible substrates this leads to nearly endless possibilities for the preparation of MnO_x-functionalized electrodes. Even as different MnO_x/substrate combination types clearly show varying catalytic activities and stabilities, it is currently not possible to identify a clear winner among the electrode preparation techniques, e.g. as it appears that every MnO_x/substrate pair has be optimized individually. However, as a general trend, MnO_x materials of low order deposited on conductive, nanostructured backbones of high surface areas are often very good candidates for well-performing manganese oxide-based water oxidation anodes.

3.3 Electrolyte variations and influences of the pH

In electrochemical water oxidation it is well known that the choice of the electrolyte has an enormous impact on the catalytic activity and stability of the metal oxide catalyst [133], [134]. To benefit from the very good conductivity of concentrated acids or bases, water oxidation electrocatalysis has commonly been studied for either very acidic or very alkaline conditions. Also some of the very early studies on WOC by MnO_x were conducted either in 1 M H₂SO₄ or 1 M KOH [37], [60] and significant differences between the performances at pH ∼0 and pH ∼14 were already found, but a reason for this phenomenon could not be given. Later, WOC by MnO_x was additionally studied in detail for less harsh, near neutral electrolytes, as it became clear that especially photoelectrochemical systems like artificial leaves demand conditions where the photoabsorber and/or junction materials are stable as well [103], [135]. For example, silicon is easily passivated (forming SiO₂) under alkaline conditions and corrodes in acid. As a result, many reports on the catalytic activity of manganese oxides operated close to pH 7 are available, where the influence of factors like the addition of buffering bases or the electrolyte concentration on the stability and activity of the MnO_x have been addressed.

As can be seen in the Pourbaix diagram for manganese, MnO_x show a good stability over a large pH range. They generally suffer less from corrosion under neutral to acidic conditions while this is a problem e.g. for NiO_x- or CoO_x-based electrodes [136], [137], [138]. As an example, Nocera and co-workers extensively investigated the electrocatalytic activity of electrodeposited MnO_x/FTO anodes in phosphate buffers of three different pH values (pH 2.2/7.0/12.2) [81]. The ionic strengths of the solutions were adjusted by adding KNO₃ in order to exclude pure conductivity effects. First, the influence of the buffer base concentration on the catalytic activity for the three pH values was tested resulting in a zero-order dependence of the activity on the phosphate ion concentration under all selected pH values. On the first view, this result indicates that the buffer base is not involved in the rate determining step of the mechanism for water oxidation at MnO_x, so that the activity of the catalyst cannot be increased by adding proton accepting ions. However, the reaction rate determination was carried out at relatively low current densities where the proton uptake by water molecules could be sufficient. Recently it was shown by some of us that the OER current for amorphous cobalt oxide catalysts at neutral pH strongly depend on the rate of the proton transfer to available bases [134]. This points to the fact, that especially at higher current densities (which are needed for a possible industrial applications) mass-transport limitations come into play and the availability of high concentrations of a proton accepting base would thus enhance OER activity. The aforementioned study also investigated the influence of the pH on the activity and the mechanism [81]. Kinetic analysis revealed two competing mechanisms under acidic and alkaline conditions and a competition between both at neutral pH. Higher activities were found for electrodes operated in alkaline pH, which was explained by the higher stability of Mn³⁺ ions at the oxide surface for pHs above 9.

In 2012, Nakamura and co-workers investigated the behavior of MnO_x electrodes under different pH conditions, but here with a special focus on changes of the MnO_x catalyst material during operation detected by in situ spectroelectrochemistry [75]. For pH >9, a non-Nernstian decrease of the onset potential was observed, leading to significantly lower overpotentials in alkaline media. These results were accompanied by a change in the optical absorption spectrum which the authors again assigned to an increase of the number of Mn³⁺ ions available as active sites for catalysis. At intermediate to acidic pH conditions, such Mn³⁺ sites are removed by the disproportionation into Mn²⁺ and Mn⁴⁺, so that Mn²⁺ has to be re-oxidised to Mn³⁺ prior to the catalysis. This study can be seen as a starting point for further work aiming to stabilize the active Mn³⁺ form under WOC conditions. As an example, the Nakamura group later modified spray-coated δ-MnO₂/FTO electrodes with an amine containing polymer (poly(allylamine) hydro-chloride, PAH) [76]. It is known that Mn–N bonds can stabilize Mn³⁺ against disproportionation and indeed a lower onset potential for the evolution of O₂ at pH 8 was detected.

The Nakamura group also investigated the influence of the addition of proton accepting bases on electrochemical WOC under neutral conditions [80]. It was found that the addition of 0.5 M pyridine to a 0.5 M Na₂SO₄-solution led to a large decrease of the onset potential by around 200 mV. As a control, pyridine was also added to a Na₂SO₄-electrolyte at pH 4 (where ∼95% of the pyridine molecules are protonated). Here, no enhancement of the catalytic activity was observed, indicating that the proton-accepting base is responsible for the activity gain and that a proton coupled electron transfer step might be involved in the rate-determining reaction step at neutral conditions. In the following, five pyridine derivatives with pK_a values ranging from ∼5 to 7.5 were evaluated as potential bases for WOC at pH 7.5. The best results were found for γ-collidine, which had the highest pK_a-value of the series (pK_a = 7.48), making it the best proton acceptor at these conditions and resulting in an impressive 15-fold higher O₂-evolution rate compared to WOC in the absence of bases. Although the studies of the Nakamura group clearly demonstrated that water-oxidation activity can be greatly enhanced by the addition of suitable bases, it is worth to mention that pyridines can be oxidized themselves at WOC conditions so that pyridine-containing electrolytes are not the best choice for this task [80].

Fig. 6:

(a) Chronoamperometry traces for Ca-birnessite/FTO electrodes immersed in 0.1 M phosphate buffer (pH 7, blue trace), 0.1 M borate buffer (pH 9.2, black trace), 0.1 M methyl phosphonate (pH 7, red trace), 0.1 M imidazolium buffer (pH 7.4, green trace), 0.1 M acetate buffer (pH 4.8, orange trace) and 0.1 M potassium sulfate (pH 7.0, pink trace). In all cases a constant potential of +1.71 V vs. RHE (η = 480 mV) was applied. (b) Chronopotentiometry experiments conducted at a constant current density of j = 1 mA cm⁻², the concentration of the phosphate buffer electrolyte has been varied as follows: black – 0.033 M, red – 0.1 M, green – 0.2 M, blue – 0.3 M, cyan – 0.4 M, purple – 0.5 M, yellow – 0.6 M, brown – 0.7 M [139].

Inspired by these reports, some of us tested the influence of five different 0.1 M electrolyte systems in the pH-range between ∼5 and 9 on the electrocatalytic performance of screen-printed Ca-birnessite/FTO electrodes [139]. We found that an operation in phosphate buffer at pH 7 resulted in the best activity and stability within the series (see Figure 6) and found several possible reasons to explain this trend: (1) the phosphate buffer exhibits a higher proton affinity than e.g. acetate or (even more so) sulfate [134]; (2) the 0.1 M phosphate buffer solution exhibited the highest ionic conductivity of all tested systems; (3) methyl phosphonate showed an oxidation event at around +1.8 V meaning that it is not completely redox stable. After identifying phosphate as the preferred buffer system, the dependence of the WOC current density on the phosphate buffer concentration were investigated using chronopotentiometry for j = 1 mA cm⁻² and pH 7. It was found that it was possible to further lower the necessary overpotential by around 100 mV when the phosphate buffer concentration was increased from 0.1 to 0.5 M (see Figure 6). This result is contradicting the observations of the Nocera group for low current densities [81], but is in line with the findings of the teams of Nakamura and Dau [80], [134] and confirms the importance of a suitable base especially at higher current densities where mass-transport becomes the rate limiting factor.

It can be concluded, that phosphate buffers are especially well-suited electrolytes for water oxidation by manganese oxides under neutral conditions which explains why they have been the first choice by many different groups for WOC at pH ∼7 [49], [71], [74], [78], [81], [82], [93], [77], [92], [94], [109], [114], [116], [121], [126], [128], [140]. The main advantages of this buffer system are: (1) good availability and affordable price; (2) good redox stability over a wide potential range; (3) due to the three acid constants of phosphoric acid (pK_a1 = 2.1; pK_a2 = 7.2; pK_a3 = 12.4), it can be used over nearly the entire pH range; (4) the high solubility in water allows for making phosphate buffers with high concentrations and high ionic conductivities.

In addition to the development of MnO_x catalysts for water electrolysis under neutral pH, highly alkaline conditions (for example 1 M KOH or NaOH) have been extensively studied as well [61], [72], [73], [83], [97], [98], [99], [110], [119], [120], [122], [124], [128], [132]. These electrolytes also show numerous advantages like (1) high solubility leading to the possibility of reaching very high ionic conductivities; (2) no redox stability issue as OH⁻ is derived from the water; and (3) very high reactivity with protons. As a result, WOC by MnO_x generally proceeds at higher rates in highly alkaline electrolytes compared to pH ∼7 and the best MnO_x electrodes reach the benchmark current density of j = 10 mA cm⁻² already at overpotentials η = 350 mV when operated in 1 M KOH (α-Mn₂O₃/FTO [78] and K_0.1MnO_x/CFP, see also Table 2). Additionally, higher stabilities have been reported for electrodes operated under these conditions [81], [131], [77], [75], [142]. In a study by two of our groups, the long-term stabilities of MnO_x anodes was tested at pH 2.5/7.0/12.0. There, significantly higher stabilities were observed for electrodes operated at pH 12.0, explained by a suppressed formation of permanganate which was found to be higher for electrodes operated at pH 7.0 and 2.5 [131]. However, the remarkable activity gains found for NiFeO_x when operated in alkaline electrolysers are not matched by MnO_x, so that very alkaline electrolytes do not seem to be the particular strength of MnO_x used for WOC.

For the acidic regime, Lewis and co-workers tested the WOC activity of a NiMnSbO_x prepared by sputter deposition in 1 M H₂SO₄ [138]. Although the best catalyst needed a rather high overpotential of η = 730 mV to reach a current density of 10 mA cm⁻², a remarkable long-term stability (>1 week) was found. Li et al. confirmed this observation for a carbon fiber paper substrate coated by MnO_x, which could be operated in a PEM-electrolyser setup (H₂SO₄, pH ∼2) at a current density of 10 mA cm⁻² (E = 1.73 V) for more than 8000 h (!) without a clear decrease of the activity [141]. Furthermore, the group determined the potential window where MnO_x can be operated at pH 2 to lie between +1.4 and 1.75 V. At lower or higher potentials, UV/Vis spectroelectrochemical measurements indicated the formation of Mn²⁺ or MnO₄⁻, respectively.

In summary, the choice of the right proton accepting, redox innocent electrolyte system for manganese oxide based water-oxidation is both important and challenging. Under near neutral conditions, an optimization of the electrolyte is especially rewarding and phosphate buffers have proven to be a very good choice. Towards both alkaline and acidic media, MnO_x show remarkable stabilities and reasonable activities over the whole pH range, making these WOC catalysts true “all-rounders” suitable for many different applications and devices.

3.4 Structure/activity relationships forelectrochemical WOC

The oxygen evolution reaction is a very complex four-electron/four-proton redox process involving multiple bond cleavages and the formation of the O–O double bond. A detailed understanding of WOC mechanisms exists only for two systems: molecular ruthenium catalysts like the “blue dimer” and the OEC, biology’s CaMn₄O_x-cluster catalyzing water-oxidation in photosynthesis [143], [144], [145], [146], [147]. For heterogeneous catalysts like manganese oxides, on the other hand, our mechanistic knowledge is still in its infancy, so that even the nature of the WOC active sites in MnO_x catalysts is largely unknown. As a first indication, researchers try to correlate data about the bulk structures of metal oxides with their electrochemical WOC activities [148], [149], [150], [151], [152]. For the case that this leads to convincing results, it is then hoped to start a knowledge-guided search for more efficient catalysts. Generally, one has to admit that such a “rational catalyst design” is so far not possible for WOC by metal oxides. In the case of manganese oxides, however, there is a certain advantage: as biology “chose” a manganese-oxido cluster as WOC active site, an enormous wealth of information about this particular catalytic mechanism taking place at a MnO_x active site has been accumulated over decades of research [15], [20], [153], [154], [155] and can now be used for a better understanding of synthetic MnO_x catalysts as well.

Fig. 7:

(a) Current densities J and (b) oxygen mass signals (at low J) as a function of the applied potentials E for sputtered RuO₂ (green), amorphous MnO_x (dotted), amorphous MnO_x annealed at 573 K (red), Mn₂O₃ (black) and Mn₃O₄ (blue) films deposited on FTO (black open circles) in 1 M phosphate buffer (pH 7.0). Reprinted from Ref. [79] with permission by the American Chemical Society.

Already some early studies dealing with the electrochemical performance of manganese oxides as water oxidation catalysts revealed a possible relationship between the MnO_x-phase and its WOC activity [37], [60]. As mentioned before, Tamura and co-workers prepared mixed MnO₂/Mn₂O₃-electrodes and investigated their catalytic performances in comparison with pure MnO₂ or Mn₂O₃. Interestingly, they found that the oxide mixture was more active than both pure phases on their own and concluded that both Mn³⁺ and Mn⁴⁺ centers might be involved in WOC. However, no evidence for this claim (e.g. from UV/Vis or X-Ray absorption spectroscopy) was available at this time.

Over the last decades, many research groups investigated possible correlations between manganese oxide phases and their catalytic activities and generally found large influences of the catalysts’ bulk structures on WOC rates [79], [92], [120], [122], [128]. For example, one of our teams systematically tested the electrocatalytic activities of electrochemically deposited amorphous manganese oxide (MnO_x/FTO, annealed at 573 K to increase its activity), Mn₂O₃/FTO and Mn₃O₄/FTO [79]. The latter two were obtained by further calcination steps from the amorphous MnO_x and showed high crystallinities, low surface areas (determined by capacitance measurements), but still significant film porosities. Even as all three manganese oxides acted as water-oxidation catalysts above E = +1.8 V in phosphate buffer at pH 7, clearly lower onset potentials and higher current densities were detected for MnO_x/FTO and Mn₂O₃/FTO (∼570 mV at 10 mA cm⁻², see Figure 7). This was explained by structurally more flexible Mn-sites well suited for WOC (in the case of the amorphous MnO_x) and the especially low specific resistance of Mn₂O₃ (Table 1). These results are in line with numerous other studies which generally indicate that MnO [121], [122], [132], Mn₃O₄ [82], [97], [126], [128] or MnO₂-polymorphs [73], [92], [99], [120], [156], are generally less active in electrochemical WOC than Mn₂O₃ [61], [71], [79], [83], [119], [122], [128], [132] or amorphous (mostly layered) MnO_x [49], [72], [74], [75], [81], [82], [83], [77], [109], [120], [141], [157].

However, it is hardly possible to derive the nature of the catalytically active sites in such materials from analytical methods like XRD or SEM/TEM which characterize the bulk material’s crystallographic phase or morphology. More advanced techniques like X-ray absorption spectroscopy (XAS) or X-ray photoelectron spectroscopy (XPS) were therefore used in a number of studies, especially for nanocrystalline or highly disordered systems, where diffraction methods yield little information.

From the analysis of surface-sensitive analytical data (like XPS), several groups concluded that an amorphous MnO_x layer is often formed on top of crystalline oxide materials like Mn₂O₃ at WOC conditions [48], [79]. For example, in our study mentioned above (Figure 7) we detected an increase of the average Mn oxidation state from +3 to +4 at the surface of the Mn₂O₃ catalyst by XPS. Similarly, Gorlin et al. and also Lange and co-workers reported the formation of a less-ordered birnessite-like structure on the surface of MnO_x particles after an application of oxidizing potentials [116], [128]. These results were recently confirmed by Tesch et al. who showed by in situ Mn L-edge XAS studies that a birnessite was the catalytically dominant phase during water oxidation catalysis by a mixture of Mn₃O₄/Mn₂O₃/birnessite at potentials E > 1.45 V. Furthermore, resonant inelastic X-ray scattering (RIXS) indicated that Mn oxidation never seems to exceed the +4 state, but rather that a partial O ⟶ Mn^IV charge transfer seems to occur at high oxidation potentials [85].

A detailed study concerning the WOC active sites of MnO_x materials for the subsequent development of rationally designed catalysts was carried out by one of our groups in 2012 [49]. The structural parameters of a WOC-inactive and an active MnO_x film, obtained from UV/Vis and Mn K-edge XAS measurements, were compared. Both materials were characterized as amorphous MnO_x exhibiting slightly different morphologies but roughly identical surface areas. The average oxidation states of the two samples differed and were estimated to be +3.8 for the active and +4.0 for the inactive material. Furthermore, the shape of the X-ray absorption near-edge structure (XANES) spectrum revealed that the inactive film was composed of a regularly assembled, layered birnessite-type MnO_x, whereas the active oxide exhibited a higher disorder and a more heterogeneous ligand environment. These conclusions were supported by the information gained from extended X-ray absorption fine structure (EXAFS) spectra, in which both materials are found to be built up from Mn^III/IVO₆-octahedra. However, while these building blocks are mainly connected via di-μ-oxido bridges to form a well-ordered layer in the inactive oxide, the active catalyst contains many mono-μ-oxido bridged Mn ions, resulting in a lack of long-range order (Figure 8). From this observation it was concluded that layered Mn^III/IVO_x of low order with many terminal coordination sites for reactive water species show high water oxidation activities, while in well-ordered MnO_x such readily (de)protonatable μ₂-O(H) groups are not available as WOC active sites. The special importance of bridging oxido ligands for WOC has also been noted for other metal oxides like CoO_x or NiO_x and is also a central feature of WOC by the OEC [146], [148], [149], [151], [152], [158], [159], [160], [161]. Finally, DFT calculations in combination with XAS data also confirmed that more active MnO_x films are formed by the interconnection of small, planar Mn-oxido sheets cross-linked by out-of-plane Mn atoms introduced by disorder, which can be arranged in closed cubane-like units very similar to the one found in the OEC [162].

Fig. 8:

X-ray absorption spectra of an WOC active (orange) and inactive (green) MnO_x. The inset shows the edge region of the spectrum (XANES), from which the average oxidation state of the Mn ions was estimated by a comparison with Mn reference compounds. Each peak in the Fourier transformed EXAFS spectra relates to a specific structural motif that is schematically depicted (O in red, Mn in purple). The spectra obtained by EXAFS simulations are shown as black lines. Reprinted from Ref. [49] with permission by the Royal Society of Chemistry.

Fig. 9:

XANES and EXAFS spectra of the OEC within the enzyme complex Photosystem II and of an electrodeposited active MnO_x. (a) XANES and (b) EXAFS of the Mn₄Ca complex for the four semi-stable intermediates of the S-state cycle. (c) XANES and (d) EXAFS of electrodeposited MnO_x where oxidation state changes were induced by the indicated potentials (given vs. NHE). For each FT peak the corresponding structural motif is schematically shown (Mn: magenta, O: red). Reprinted from Ref. [93] with permission by the Royal Society of Chemistry.

In a further set of experiments, one of our teams compared the oxidation state and structural dynamics of an active, an inactive and a Ca²⁺-doped MnO_x film with the biological catalyst by means of electrochemical quasi in situ, freeze-quench XAS complemented by time-resolved detections of XAS, UV/Vis and impedance spectra (Figure 9). The combined data revealed changes of the average Mn oxidation state upon the application of a WOC potential from around +3.4 to +3.9, which is comparable to the changes known for the OEC during operation [93]. Such changes of the average Mn oxidation state during and/or after operation have also been found in other studies [73], [77], [128]. Interestingly, the inactive MnO_x film reached an all-Mn⁴⁺-state while for the active film some residual Mn³⁺-ions were found even after extended operation at +1.45 V. We propose that these trapped Mn³⁺-states are essential for the formation of structurally highly flexible local clusters that could resemble the active sites of water oxidation catalysis. Additionally, we would like to stress that this study again indicates that the stabilization of a fraction of Mn³⁺-centers at water oxidation potentials seems to be decisive for catalytic activity.

In summary, amorphous or nanocrystalline, highly disordered and often layered manganese(III, IV) oxides seem to be especially well suited for electrochemical water oxidation. These materials show an ability to accumulate oxidizing equivalents by bulk oxidation state changes and might contain functional units of molecular dimensions (like metal oxido-cubanes) as also found in the OEC. Additionally, they also often feature beneficial materials properties like high surface areas and/or porous structures.

6.1 What does the biological OER catalyst – the OEC – look like?

In biology, light-driven water oxidation is catalyzed by means of high-valent manganese ions bound to the proteins of photosystem II (PSII) [14], [19], [161], [198], [199], [200]. Most likely, all photosynthetic organisms capable of water oxidation, that is, all plants, algae and cyanobacteria, are using a highly similar machinery and specifically the same metal cluster for WOC. The core of the biological catalyst is a compact Mn4III/IVCa(μ3−O)4(μ2−O) moiety (where μ₂-O and μ₃-O indicate an oxygen atom in bridging position between 2 and 3 metal ions). Its structure may be conceived as a small fragment carved out of an extended manganese calcium oxide, e.g. from the mineral marokite (CaMn₂O₄) [57]. In line with this “oxide character” of the biological catalyst, all further donor atoms to Mn and Ca are also oxygens (with the exception of the nitrogen atom N-His332, see Figure 13) [201], [203], as a further four water or hydroxide molecules are terminally coordinated to Mn1 (W1, W2) and the calcium ion (W3, W4) [201], [204]. Looking beyond these first-shell ligands, we find that the Mn₄Ca-oxido cluster is partially surrounded by protein ligands and partially by a protein-internal water cluster comprising, inter alia, the terminal water ligands of the Mn1 and the Ca ion. Grossly simplified, the catalytic site of biological water oxidation consists of a tiny manganese-calcium oxide of well-defined structure anchored by protein ligands and partially exposed to water (which is both solvent and substrate of the water oxidation reaction).

6.2 What determines the specific structure of the OEC’s oxide core and how is it formed?

The metal-oxido core is formed in a process that may be denoted as oxidative self-assembly, but traditionally is called “photoactivation” in photosynthesis research [205]. Driven by light, a redox-active tyrosine residue of PSII called Tyr_Z is oxidized and in turn oxidizes Mn²⁺ ions that need to be present, at low concentrations, in the solvent environment of PSII. The oxidation of four Mn²⁺ ions eventually yields four Mn^III/IV ions, which are bound to the protein and interconnected by di-μ-oxido bridges. However, unlike in synthetic oxides, nuclearity and structure of this metal-oxido core are determined by ligands from the protein’s side-chains. Six carboxylates serve as bidentate ligands bridging between two metal ions each, complemented by one monodentate imidazole ligand (His332, Figure 13). In the absence of the Mn₄Ca-cluster, these ligating, evolutionary conserved residues are arranged in almost the same way as in its presence, suggesting that the structure of the apoprotein shapes the metal-oxido core [202]. On the other hand, it was recently reported that a synthetic complex closely resembling the OEC in structure, oxidation and spin states can also be prepared without the use of a complex ligand system [25]. A notable difference between this synthetic complex and the OEC is the absence of an open coordination site at Mn1, which is mostly believed to play a crucial role in biological water oxidation. Indeed, the synthetic compound is not active in WOC. Changes of UV/Vis absorption spectra suggest that spurious amounts of water cause a structural modification possibly associated with WOC activity, but clear evidence on the character of the structural change and its relation WOC activity is lacking [25].

In conclusion, the OEC is formed from dissolved Mn²⁺ ions by means of an oxidative self-assembly process in which details of its structure are determined by the specific protein environment. Similar structures may also form in the absence of a complex ligand system, either via conventional preparation routes for molecular coordination compounds or during the (electro)synthesis of amorphous manganese oxides, as already discussed above. We note that all electrodeposition protocols and many other synthesis protocols for obtaining manganese oxides may be viewed as oxidative self-assembly processes, so that there is no basic conceptual difference compared to the OEC’s assembly during photoactivation.

6.3 Accumulation of oxidizing equivalents by Mn – a mechanistic principle in biological WOC

In many mechanistic models of electrochemical water oxidation, the direct stepwise oxidation of water molecules (i.e. [O²⁻] → [O⁻] → [O]) has been assumed [146]. This involves, inter alia, that two “[O⁻]”couple to a peroxide species before two further oxidation steps transform the peroxide into a dioxygen molecule. Moreover, typically this is assumed to happen at the surface of well-ordered transition metal oxides, which are largely inert regarding changes in atomic structure, protonation states of bridging oxides and metal oxidation states. Whether this type of model correctly describes electrochemical water oxidation by electrocatalytic materials is debatable – but this is not the subject of the present review. In PSII, a clearly different mechanistic paradigm is now generally accepted: three oxidation equivalents are sequentially accumulated by oxidation of manganese ions; once a fourth oxidation equivalent is available at the nearby tyrosine, the final water-oxidation/O₂-formation step of the catalytic cycle starts [14], [161], [199], [200]. Also in PSII research, the formation of oxyl radicals or peroxide species at earlier steps of the catalytic cycle has been discussed for decades [206], [207], [208], [209], but it is incompatible with the now largely undisputed spectroscopic results on the oxidation-state changes of manganese ions in PSII [210], [211], [212], [213], [214], [215], [216].

6.4 Two crucial properties of the OEC

In this review, it is not possible to cover all properties of the OEC relevant for its function. Instead, we will focus on (a) the energetics (redox potentials) and (b) the role of the electronic coupling between metal ions facilitated by di-μ-oxido bridges.

a) In PSII, the oxidized (radical) form of a redox-active tyrosine (Tyr_Z^ox) serves as the oxidant driving water-oxidation catalysis. Its midpoint potential (E₀^TyrZ) has been estimated to be +1.1–1.2 V at pH ∼5.5, which consequently constitutes a strict upper limit for the metal-ion oxidation potential (E₀^M/M+) in the process of accumulating oxidation equivalents [14], [217]. However, too low potentials for E₀^M/M+ could also be detrimental, because a large potential difference E₀^TyrZ − E₀^M/M+ would correspond to large energetic losses for the respective oxidation steps. Therefore, typical values of E₀^M/M+ within a rather narrow window below 1.1 V (E₀^TyrZ) and above 0.9 V (thermodynamic water-oxidation potential at pH ∼5.5) are required for light-driven WOC in PSII.

b) For an efficient O–O bond formation step, the accumulation of oxidizing equivalents by spatially separated and electronically decoupled metal ions would be problematic, because oxidizing equivalents accumulated by electronically insulated metal ions could hardly support the concerted four-electron chemistry of the water-oxidation/O₂-formation step. Although systematic experimental and theoretical studies on this point are largely lacking, we hypothesize that di-μ-oxido bridging facilitates the needed electronic coupling between the individual metal sites. However, there also is a downside to electronically strongly coupled metal sites: a one-electron oxidation step of the metal cluster would raise E₀^M/M+ for a second oxidation step to a prohibitively high level (by several 100 mV). This can be prevented by appropriate coupling to charge-compensating deprotonation or other modifications of ligand groups, as discussed in more detail elsewhere [199], [218]. Prime candidates for such potential-lowering steps are the deprotonation of terminal water ligands as well as the transformation of μ-OH to μ-O ligands, as likely occurring in the S₀—S₁ transition of the biological metal cluster.

6.5 Comparison of biological WOC with WOC by transition metal oxides

So far, we have outlined crucial structural and functional properties of the biological catalyst for water oxidation in PSII. On these grounds we would now like to highlight some striking similarities of the OEC to (some) synthetic manganese oxides that are active in WOC:

(i) highly disordered MnO_x of the birnessite or todorokite type are very active catalysts and also share crucial structural motifs with the OEC [49], [55], [59], [93], [94].

(ii) active MnO_x can be synthesized (inter alia) by electrodeposition, a process that may be described as oxidative self-assembly. Similar to the self-assembly of the OEC, it also involves the oxidation of dissolved Mn²⁺ to Mn^3+/4+ ions and often results in a material rich in internal water molecules and stabilized by extensive di-μ-oxido bridges between Mn^3+/4+ ions [49].

(iii) Manganese oxides which are active in WOC can accumulate oxidizing equivalents by oxidation of Mn³⁺ to Mn⁴⁺ ions, a process which is most likely coupled to charge-compensating deprotonation steps also involving a rearrangement of the μ-oxido bridges between Mn^3+/4+ [93].

(iv) in spite of a beneficial influence of Mn³⁺ ions on WOC reaction rates, it is likely that the O–O bond formation step is taking place between O-atoms bound to Mn⁴⁺ ions.

Despite this remarkable set of similarities, the overpotential requirements of synthetic MnO_x for reasonable water-oxidation rates (η ∼400 mV) are still significantly higher than that of the biological catalyst (at least in the near-neutral pH regime), which points towards unnecessarily high redox potentials E₀^M/M+ in the synthetic oxides. Moreover, the formation of the specific structural motifs that facilitate fast water oxidation may be a rare (statistical) event, so that the actual number of WOC active sites in MnO_x catalysts might be quite low [93]. Though details are so far unknown, it is likely that these active sites contain Mn³⁺ ions also at catalytic potentials [93]. A key property of the specific, evolutionary optimized environment of the OEC in the protein may also be the optimal tuning the Mn³⁺/Mn⁴⁺ redox potential and this might be one aspect explaining the OEC’s exceptional catalytic performance.

In the last decade, there have been numerous fruitful investigations of WOC by first-row transition metal oxides (or oxyhydroxides) using a combination of electrochemical methods and X-ray spectroscopy at the respective metal K-edges. Most of these studies focused on catalyst materials based on CoO_x, (Fe)NiO_x and MnO_x (plus binary oxides). Above, we have already summarized the structural and functional similarities between biological water-oxidation by the OEC and WOC by MnO_x. Surprisingly, all the listed similarities, (i)–(iv), have also been found for amorphous cobalt and (iron) nickel oxides, which in addition also exhibit local structures and oxidation state changes of the metal ions similar to WOC-active MnO_x [10], [146], [149], [158], [219], [220], [221], [222], [223], [224], [225]. In clear contrast, amorphous iron-only oxides (FeO_x) do not exhibit these similarities and are WOC-inactive.

The common features of amorphous oxides (or oxyhydroxides) of manganese, cobalt or nickel can be explained by scrutinizing the midpoint potentials of the relevant redox transitions. In these catalyst materials, the potentials of the M²⁺ → M³⁺ and M³⁺ → M⁴⁺ transitions are typically only slightly above the equilibrium potential of water oxidation. There is still a significant uncertainty in the precise values of these potentials, but all three MO_x catalysts seem to be able to accumulate oxidizing equivalents via the formation of a sizeable fraction of M⁴⁺ ions at potential within about 200–500 mV above the equilibrium water-oxidation potential. In clear contrast, the Fe²⁺ → Fe³⁺ transitions in (WOC-inactive) FeO_x materials proceed far below the water-oxidation equilibrium potential and the Fe³⁺ → Fe⁴⁺ seemingly is not reached within a potential range that is of relevance for efficient water oxidation. Interestingly, the central importance of such “critical redox couples” for electrochemical WOC by transition metal oxides has already been proposed by Rasiyah und Tseung about 25 years ago [226].

6.6 Nature’s metal-ion choice for WOC during the evolution of photosynthetic water oxidation

Not only amorphous manganese oxides, but also oxide materials based on cobalt and nickel can act as water-oxidation catalysts and all seem to share the same set of crucial properties with the OEC, including the option of facile oxidative self-assembly. Thus, we suggest that in the evolution of oxygenic photosynthesis, Nature could also have selected cobalt or nickel for the catalysis of water-oxidation, as they are not lacking any unique required chemical properties. On the other hand, the use of a metal like iron was not an option because the typical oxidation potentials of the Fe²⁺ → Fe³⁺ and Fe³⁺ → Fe⁴⁺ transitions are too low for an accumulation of oxidizing equivalents during WOC (and also not well suited for controlled oxidative self-assembly).

There is, however, one clear difference between manganese and the other two “hot candidates” cobalt and nickel: its good bio-availability, today and even more so at prebiotic times. For example, the water of lakes or rivers typically contains about 20 times more dissolved Mn (∼5 ppb) than Ni (∼0.3 ppb) or Co (∼0.2 ppb) [227]. At prebiotic times, the Mn ion concentration in seawater was most likely even more than four orders of magnitude higher than that of Co or Ni [228]. Nature’s choice of manganese could therefore simply be explained by the comparably high ability of Mn²⁺ ions in the early earth’s seawater compared to the only two currently conceivable “bioinorganic alternatives” cobalt and nickel (the extreme scarcity of iridium and ruthenium excludes these otherwise very well suited elements for WOC in biology). Additionally, we would like to point once again at the summary of WOC performances shown in Figure 12, which illustrates that among the earth-abundant materials, MnO_x-catalysts might be best choice for an operation at near-neutral pH-values – an important boundary condition for cellular environments. Thus, for plants and algae a manganese-based water-oxidation catalysts might represent the best compromise between bio-availability, self-assembly properties and WOC performance at pH ∼7, so that element no. 25 became the “Mighty Manganese” of oxygenic photosynthesis.