Water Oxidation Half-reaction Research Articles

Photocatalytic Hydrogenation of Alkenes Using Water as Both the Reductant and the Proton Source.

Utilization of clean and low-cost water as the reductant to enable hydrogenation of alkenes is highly attractive in green chemistry. However, this research subject is considerably challenging due to the sluggish kinetics of the water oxidation half-reaction. It is also very difficult to avoid the undesired oxidation of alkenes because that this oxidation is far easier to occur than the desired oxidation of water from thermodynamic standpoint. Herein, this challenge is overcome by applying a cooperative catalysis where HCl is used as the cocatalyst to accelerate Pt/g-C3N4-catalyzed water oxidation and suppress the undesired oxidation of the alkene. This provides an example for using water as the reductant and the proton source to enable the photocatalytic hydrogenation of alkenes. The present method exhibits broad substrate applicability, and allows various arylethenes and aliphatic alkenes to undergo the hydrogenation smoothly.

Bias distribution and regulation in photoelectrochemical overall water-splitting cells.

The water oxidation half-reaction at anodes is always considered the rate-limiting step of overall water splitting (OWS), but the actual bias distribution between photoanodes and cathodes of photoelectrochemical (PEC) OWS cells has not been investigated systematically. In this work, we find that, for PEC cells consisting of photoanodes (nickel-modified n-Si [Ni/n-Si] and α-Fe2O3) with low photovoltage (Vph < 1V), a large portion of applied bias is exerted on the Pt cathode for satisfying the hydrogen evolution thermodynamics, showing a thermodynamics-controlled characteristic. In contrast, for photoanodes (TiO2 and BiVO4) with Vph > 1V, the bias required for cathode activation can be significantly reduced, exhibiting a kinetics-controlled characteristic. Further investigations show that the bias distribution can be regulated by tuning the electrolyte pH and using alternative half-reaction couplings. Accordingly, a volcano plot is presented for the rational design of the overall reactions and unbiased PEC cells. Motivated by this, an unbiased PEC cell consisting of a simple Ni/n-Si photoanode and Pt cathode is assembled, delivering a photocurrent density of 5.3±0.2mA cm-2.

Soluble Gd6Cu24 clusters: effective molecular electrocatalysts for water oxidation.

The water oxidation half reaction in water splitting for hydrogen production is extremely rate-limiting. This study reports the synthesis of two heterometallic clusters (Gd6Cu24-IM and Gd6Cu24-AC) for application as efficient water oxidation catalysts. Interestingly, the maximum turnover frequency of Gd6Cu24-IM in an NaAc solution of a weak acid (pH 6) was 319 s-1. The trimetallic catalytic site, H2O-GdIIICuII2-H2O, underwent two consecutive two-electron two-proton coupled transfer processes to form high-valent GdIII-O-O-CuIII2 intermediates. Furthermore, the O-O bond was formed via intramolecular interactions between the CuIII and GdIII centers. The results of this study revealed that synergistic catalytic water oxidation between polymetallic sites can be an effective strategy for regulating O-O bond formation.

Band structure engineering of Pd, Rh, Cu-Modified SrMoO4 for enhanced activity and selectivity in photocatalytic CO2 reduction to CH4

Photocatalytic CO2 reduction holds promise as a strategy to mitigate the energy crisis and greenhouse effects. However, devising cost-effective photocatalysts capable of efficiently and selectively converting CO2 to CH4 remains challenging. This study presents the synthesis of Pd, Rh, and Cu-modified SrMoO4 photocatalysts via a one-step solvothermal method. Our findings demonstrate increased CH4 activity and controlled selectivity through the modification with Pd, Rh, or Cu ions. It was determined that Pd, Rh, or Cu ions modified the SrMoO4 surface as -O-Pd-O-, -O-Rh-O-, or -O-Cu–O- species, increasing the adsorption sites and active regions on the catalyst surface. Moreover, these introduced -O-Pd-O-, -O-Rh-O-, or -O-Cu–O- species' energy level enhances visible light absorption and photogenerated charge carrier separation, and regulates the band structure for better alignment with water oxidation potential. This leads to greater water oxidation half-reaction efficiency and increased H+ generation in photocatalytic CO2 reduction. Consequently, more protons (H+) participate in CO2 photocatalytic reduction, thus promoting CH4 production and selectivity. These insights may guide the design of highly-selective photocatalysts for effective CO2 photoreduction through bandgap engineering strategies.

Electrochemical CO2 Reduction to Methyl Formate/Formic Acid in a Dual Non-Aqueous/Aqueous Flow Cell Electrolyzer

Electrochemical conversion of CO2 has been very promising in neutralizing existing carbon in the atmosphere as well as creating a pathway to produce various fuels and chemicals. The use of a CO2 flow electrolyzer makes it possible to achieve high faradaic efficiencies at high current densities from pure CO2 or dilute flue gas feeds. CO2 solubility in methanol is higher than in aqueous solutions and with acidic methanol as a solvent, it is possible to generate a unique product, methyl formate. This work describes the use of mixed propylene carbonate/methanol solutions as a solvent for CO2 reduction using a gas diffusion electrode and an aqueous anolyte and optimization of a high mass flux CO2 electrolyzer to produce methyl formate using a Pb-catalyst at higher current densities. pH-dependent product distribution is also described. The partial current densities varied depending on the cell design; cells where charge passes through more solvent result in lower current density because methanol has a higher solution resistance than aqueous solutions. The methanolic catholyte is paired with an aqueous anolyte to promote a sustainable water oxidation half-reaction.

Coupling Benzylamine Oxidation with CO2 Photoconversion to Ethanol over a Black Phosphorus and Bismuth Tungstate S-Scheme Heterojunction.

Photoconversion of CO2 and H2O into ethanol is an ideal strategy to achieve carbon neutrality. However, the production of ethanol with high activity and selectivity is challenging owing to the less efficient reduction half-reaction involving multi-step proton coupled electron transfer (PCET), slow C-C coupling process, and sluggish water oxidation half-reaction. Herein, a two-dimensional/two-dimensional (2D/2D) S-scheme heterojunction consisting of black phosphorus and Bi2WO6 (BP/BWO) was constructed for photocatalytic CO2 reduction coupling with benzylamine (BA) oxidation. The as-prepared BP/BWO catalyst exhibits a superior photocatalytic performance toward CO2 reduction, with a yield of 61.3 μmol g-1 h-1 for ethanol (selectivity of 91%).In situ spectroscopic studies and theoretical calculations reveal that S-scheme heterojunction can effectively promote photogenerated carrier separation via the Bi-O-P bridge to accelerate the PCET process. Meanwhile, electron-rich BP acts as the active site and plays a vital role in the process of C-C coupling. In addition, the substitution of BA oxidation for H2O oxidation can further enhance the photocatalytic performance of CO2 reduction to C2H5OH. This work opens a new horizon for exploring novel heterogeneous photocatalysts in CO2 photoconversion to C2H5OH based on cooperative photoredox systems.

Photoreforming of Waste Polymers for Sustainable Hydrogen Fuel and Chemicals Feedstock: Waste to Energy.

Energy from renewable resources is central to environmental sustainability. Among the renewables, sunlight-driven fuel synthesis is a sustainable and economical approach to produce vectors such as hydrogen through water splitting. The photocatalytic water splitting is limited by the water oxidation half-reaction, which is kinetically and energetically demanding and entails designer photocatalysts. Such challenges can be addressed by employing alternative oxidation half-reactions. Photoreforming can drive the breakdown of waste plastics and biomass into valuable organic products for the production of H2. We provide an overview of photoreforming and its underlying mechanisms that convert waste polymers into H2 fuels and fine chemicals. This is of paramount importance from two complementary perspectives: (i) green energy harvesting and (ii) environmental sustainability by decomposing waste polymers into valuables. Competitive results for the generation of H2 fuel without environmental hazards through photoreforming are being generated. The photoreforming process, mechanisms, and critical assessment of the field are scarce. We address such points by focusing on (i) the concept of photoreforming and up-to-date knowledge with key milestones achieved, (ii) uncovering the concepts and challenges in photoreforming, and (iii) the design of photocatalysts with underlying mechanisms and pathways through the use of different polymer wastes as substrates.

Electroabsorption as a Probe of Electrons Injected into Nanocrystalline SnO2/TiO2 Core/Shell Thin Films

Core/shell nanoparticles comprised of a SnO2 core and a TiO2 shell, SnO2/TiO2, present in a mesoporous thin film, are of practical interest for applications in dye-sensitized water splitting as they outperform sensitized materials based on SnO2 or TiO2 (anatase or rutile polymorphs) for the water oxidation half-reaction. Here we report electroabsorption studies designed to quantify the surface electric field and the ability of the shell to screen an electric field from a surface anchored dye. The metal-to-ligand charge transfer (MLCT) absorbance of [Ru(bpy)2(4,4′-(PO3H2)2-2,2′-bipyridine)](PF6)2, abbreviated RuP, was indeed sensitive to electron injection into the oxide support. When a fixed charge of 1 mC was injected, the electric field magnitude (in MV/cm) increased in the order 0.15 TiO2 (rutile) < 0.35 TiO2 (anatase) ≪ 1.4 SnO2 (rutile). This order tracks the reported bulk dielectric constants of the oxide materials. Comparative studies with SnO2/TiO2 core/shell materials, fabricated by atomic layer deposition (ALD) of a TiO2 shell on a mesoporous SnO2 thin film, diminished the field strength by about a factor of 4 to values less than or about equal to that measured for anatase TiO2. Heat treating some SnO2/TiO2 core/shell materials at 450 °C resulted in a crystalline rutile TiO2 shell that screened the field most effectively, more so even than rutile TiO2 alone. Taken together, the data indicate that injected electrons reside within the SnO2 core region.

Electrochemical Reduction of Flue Gas CO2 in a Dual Methanol/Water Electrolysis System for the Synthesis of Methyl Formate

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of the nonaqueous solvent methanol allows for higher CO2 solubility and novel products through the incorporation of methanol as an intermediate. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with a faradaic efficiency for methyl formate as high as 75% measured in pH < 2. Furthermore, strategies to limit the concentration of formic acid and maintain the surface oxide of the catalyst have been shown to be effective at improving the steady-state durability of the CO2-to-methyl formate process. Tests with simulated flue gas as the feedstock have shown a high level of tolerance for the Pb-catalyzed electrolysis to SO2, NO, and dilute O2 contaminants. Finally, a comparative technoeconomic analysis will be discussed to highlight target performance metrics and the viability of alternate pathways for electrochemical conversion of CO2 to methyl formate.

Unveiling the Effect of Flue Gas Contaminants on Electrochemical Reduction of CO2 to Methyl Formate in Dual Methanol/Water Electrolysis System

Exploiting renewable energy sources to drive CO2 reduction electrochemically into value-added products has gained tremendous attention towards reducing the greenhouse effect. Upstream processes for capturing and purifying the CO2 raise the overall cost of the electrolysis system. Direct reduction of flue gas into value-added C2+ products has attained much consideration in recent years. However, the sensitivity of the cathode catalyst performance in the presence of flue gas contaminants, particularly O2, and strong competition from the hydrogen evolution reaction in aqueous electrolyte are major challenges for such an approach. Herein, the influence of flue gas contaminants on electrochemical reduction of CO2 in acidic nonaqueous methanol to a methyl formate product has been investigated on a Pb-catalyzed electrode in conjunction with aqueous anolyte for the promotion of a sustainable water oxidation half-reaction. The presence of 4% O2, 0.05% SO2, and 0.05% NO is shown to have minimal effect on the total faradaic efficiency of CO2 reduction products. CO2 concentration-dependent measurements reveal declining CO2 reduction faradaic efficiencies corresponding to the decrease in partial pressure, which is attributed to CO2 mass transfer limitations to the electrode surface. XPS analysis displays the relative stability of the Pb working electrode before and after the electrochemical operation during exposure to flue gas components, which further highlights the promising tolerance of this system for direct flue gas conversion. Figure 1

Bias-free solar hydrogen production at 19.8 mA cm−2 using perovskite photocathode and lignocellulosic biomass

Solar hydrogen production is one of the ultimate technologies needed to realize a carbon-neutral, sustainable society. However, an energy-intensive water oxidation half-reaction together with the poor performance of conventional inorganic photocatalysts have been big hurdles for practical solar hydrogen production. Here we present a photoelectrochemical cell with a record high photocurrent density of 19.8 mA cm−2 for hydrogen production by utilizing a high-performance organic–inorganic halide perovskite as a panchromatic absorber and lignocellulosic biomass as an alternative source of electrons working at lower potentials. In addition, value-added chemicals such as vanillin and acetovanillone are produced via the selective depolymerization of lignin in lignocellulosic biomass while cellulose remains close to intact for further utilization. This study paves the way to improve solar hydrogen productivity and simultaneously realize the effective use of lignocellulosic biomass.

The effect of flue gas contaminants on electrochemical reduction of CO2 to methyl formate in a dual methanol/water electrolysis system

CO 2 electroreduction to value-added products has promise as a scalable technique for mitigating climate change, but CO 2 purification requirements raise the overall process cost. Direct reduction of flue gas is thus an attractive approach, but the sensitivity of the catalyst activity to flue gas contaminants and increased hydrogen evolution with diluted CO 2 have been major challenges. Herein, flue gas electroreduction in a methyl formate synthesis route has been investigated on a Pb-catalyzed electrode in acidic methanol catholyte with an aqueous anolyte for promotion of a sustainable water oxidation half-reaction. Contaminant concentrations of 50 ppm SO 2 and NO each had a minimal effect on the product faradaic efficiencies, whereas 4% O 2 led to a notable improvement in partial current density for methyl formate attributed to the improved durability of the catalyst surface oxide. Decreased CO 2 concentration showed a corresponding decline in current density attributed to CO 2 mass transfer limitations. • Study of the effects of flue gas contaminants and CO 2 concentration • Pb-catalyzed methyl formate production from CO 2 and methanol • High-performance tolerance to SO 2 and NO impurities • Improved performance with dilute O 2 Increasing atmospheric CO 2 is contributing to anthropogenic climate change. Industrial utilization of CO 2 can add economic value to this waste product and provide a route to decreased greenhouse gas emissions. Direct electroreduction of post-combustion flue gas would bypass expensive and energy-intensive carbon capture technologies and improve the overall economics of the process. However, the contaminants in flue gas and the lower partial pressure of CO 2 can deleteriously affect the electrolyzer performance. Herein, the influence of flue gas components and combined simulated flue gas feedstock on the electrochemical reduction of CO 2 in a methyl formate synthesis route has been investigated on a Pb-catalyzed cathode in acidic methanol in conjunction with water at the anode for a sustainable reaction. The results show promise for a viable route to produce a chemical not reported in aqueous CO 2 electroreduction directly from the output of a fossil fuel utility or similar industrial process. The feasibility of electrochemical reduction of waste carbon dioxide from industrial flue gas was investigated in an uncommon methanol/water dual electrolyte approach targeting a methyl formate product. The work demonstrates a pathway to produce a chemical not formed in aqueous CO 2 reduction that is largely compatible with direct operation from flue gas.

Enhanced Photoelectrochemical Water Splitting of Black Silicon Photoanode with pH‐Dependent Copper‐Bipyridine Catalysts

Since the water oxidation half-reaction requires the transfer of multi-electrons and the formation of O-O bond, it's crucial to investigate the catalytic behaviours of semiconductor photoanodes. In this work, a bio-inspired copper-bipyridine catalyst of Cu(dcbpy) is decorated on the nanoporous Si photoanode (black Si, b-Si). Under AM1.5G illumination, the b-Si/Cu(dcbpy) photoanode exhibits a high photocurrent density of 6.31 mA cm-2 at 1.5 VRHE at pH 11.0, which is dramatically improved from the b-Si photoanode (1.03 mA cm-2 ) and f-Si photoanode (0.0087 mA cm-2 ). Mechanism studies demonstrate that b-Si/Cu(dcbpy) has improved light-harvesting, interfacial charge-transfer, and surface area for water splitting. More interestingly, b-Si/Cu(dcbpy) exhibits a pH-dependent water oxidation behaviour with a minimum Tafel slope of 241 mV/dec and the lowest overpotential of 0.19 V at pH 11.0, which is due to the monomer/dimer equilibrium of copper catalyst. At pH ∼11, the formation of dimeric hydroxyl-complex could form O-O bond through a redox isomerization (RI) mechanism, which decreases the required potential for water oxidation. This in-depth understanding of pH-dependent water oxidation catalyst brings insights into the design of dimer water oxidation catalysts and efficient photoanodes for solar energy conversion.

Size-Dependent Electrocatalytic Water Oxidation Activity for a Series of Atomically Precise Nickel-Thiolate Clusters.

The development of renewable energy technologies is critical for reducing global carbon emissions. Water splitting offers a promising renewable energy mechanism by converting water into H2 and O2 gas, which can directly power fuel cells or be utilized as chemical feedstocks. To increase the efficiency of water splitting, catalysts must be developed for the water reduction and water oxidation half-reactions. To promote rational catalyst design, atomically precise metal clusters (APMCs) with earth-abundant metals provide a framework for developing both structure-activity relationships and cost-effective catalysts. Previous reports on the water oxidation activity of nickel-thiolate clusters [Nin(SR)2n] have not developed a systematic description of a possible size-activity relationship. Utilizing recent advancements in preparative chromatography for isolating APMCs, we have synthesized a series of Nin(SR)2n (n = 4, 5, or 6) clusters as electrocatalysts for the oxygen evolution reaction. We discovered a clear size-activity and size-stability trend, with intrinsic activity and stability increasing with cluster size. Using density functional theory, we found that intrinsic activity is inversely correlated to intermediate binding energy, and by extension the oxidation potential of each cluster. Our work demonstrates the ability of APMCs to uncover previously unknown structure-activity relationships that can guide future catalyst design.

Electrochemical Reduction of Flue Gas CO2 in a Dual Methanol/Water Electrolysis System for the Synthesis of Methyl Formate

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of the nonaqueous solvent methanol allows for higher CO2 solubility and novel products through the incorporation of methanol as an intermediate. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with a faradaic efficiency for methyl formate as high as 75% measured in pH < 2. Tests with simulated flue gas as the feedstock have shown a high level of tolerance for the Pb-catalyzed electrolysis to SO2, NO, and dilute O2 contaminants.

Tunable Carrier Transfer of Polymeric Carbon Nitride with Charge-Conducting CoV2O6∙2H2O for Photocatalytic O2 Evolution.

Photocatalytic water splitting is one of the promising approaches to solving environmental problems and energy crises. However, the sluggish 4e− transfer kinetics in water oxidation half-reaction restricts the 2e− reduction efficiency in photocatalytic water splitting. Herein, cobalt vanadate-decorated polymeric carbon nitride (named CoVO/PCN) was constructed to mediate the carrier kinetic process in a photocatalytic water oxidation reaction (WOR). The photocatalysts were well-characterized by various physicochemical techniques such as XRD, FT-IR, TEM, and XPS. Under UV and visible light irradiation, the O2 evolution rate of optimized 3 wt% CoVO/PCN reached 467 and 200 μmol h−1 g−1, which were about 6.5 and 5.9 times higher than that of PCN, respectively. Electrochemical tests and PL results reveal that the recombination of photogenerated carriers on PCN is effectively suppressed and the kinetics of WOR is significantly enhanced after CoVO introduction. This work highlights key features of the tuning carrier kinetics of PCN using charge-conducting materials, which should be the basis for the further development of photocatalytic O2 reactions.

The pH and Potential Dependence of Pb-Catalyzed Electrochemical CO2 Reduction to Methyl Formate in a Dual Methanol/Water Electrolyte.

The conversion of waste CO2 to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of non-aqueous solvents like methanol allows for higher CO2 availability and novel products. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with higher pH conditions leading to formate and low pH favoring methyl formate. The potential dependence of the product distribution in acidic catholyte was also investigated, with a faradaic efficiency for methyl formate as high as 75 % measured at -2.0 V vs. Ag/AgCl.

Pyrochlore-structural Nd2Ta2O5N2 photocatalyst with an absorption edge of over 600 nm for Z-scheme overall water splitting.

A pyrochlore-structural oxynitride Nd2Ta2O5N2 with a visible light absorption edge of ca. 620 nm was explored for photocatalytic water splitting. Dual functions of Nd2Ta2O5N2 were confirmed by proton reduction and water oxidation half-reactions through separately loading Pt or CoOx as a cocatalyst in the presence of the corresponding sacrificial reagent. Finally, the platinum modified sample (Pt/Nd2Ta2O5N2) was prepared and employed as the H2-evolving photocatalyst to fabricate an effective photocatalytic Z-scheme overall water splitting together with PtOx/WO3 as the O2-evolving photocatalyst and IO3-/I- as shuttle ions under visible light irradiation.

SiCP4 Monolayer with a Direct Band Gap and High Carrier Mobility for Photocatalytic Water Splitting.

Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP2 and SiCP4 monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP4 monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼104 cm-1) and high carrier mobility (∼105 cm2 V-1 s-1) of SiCP2 and SiCP4 facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP2 and SiCP4 monolayers promising candidates for applications in photocatalytic water splitting.

Tuning the Catalytic Water Oxidation Activity through Structural Modifications of High-Nuclearity Mn-oxo Clusters [Mn18M] (M = Sr2+, Mn2+)

The water oxidation half-reaction is considered the bottleneck in the development of technological advances to replace fossil fuels with sustainable and economically affordable energy sources. In natural photosynthesis, water oxidation occurs in the oxygen evolving complex (OEC), a manganese-oxo cluster {Mn4CaO5} with a cubane-like topology that is embedded within a redox-active protein environment located in photosystem II (PS II). Therefore, the preparation of biomimetic manganese-based compounds is appealing for the development of efficient and inexpensive water oxidation catalysts. Here, we present the water oxidation catalytic activity of a high-nuclearity mixed-metal manganese-strontium cluster, [MnIII12MnII6Sr(μ4-O8)(μ3-Cl)8(HLMe)12(MeCN)6]Cl2∙15MeOH (Mn18Sr) (HLMe = 2,6-bis(hydroxymethyl)-p-cresol), in neutral media. This biomimetic mixed-valence cluster features different cubane-like motifs and it is stabilized by redox-active, quinone-like organic ligands. The complex displays a low onset overpotential of 192 mV and overpotentials of 284 and 550 mV at current densities of 1 mA cm−2 and 10 mA cm−2, respectively. Direct O2 evolution measurements under visible light-driven water oxidation conditions demonstrate the catalytic capabilities of this cluster, which exhibits a turnover frequency of 0.48 s−1 and a turnover number of 21.6. This result allows for a direct comparison to be made with the structurally analogous Mn-oxo cluster [MnIII12MnII7(µ4-O)8(µ3-OCH3)2(µ3-Br)6(HLMe)12(MeOH)5(MeCN)]Br2·9MeCN·MeOH (Mn19), the water oxidation catalytic activity of which was recently reported by us. This work highlights the potential of this series of compounds towards the water oxidation reaction and their amenability to induce structural changes that modify their reactivity.