Evolution Of Molecular Oxygen Research Articles

{Co4O4} Cubanes in a conducting polymer matrix as bio-inspired molecular oxygen evolution catalysts

Exploration of efficient molecular water oxidation catalysts for long-term application remains a key challenge for the conversion of renewable energy sources into fuels. Cuboidal {Co4O4} complexes keep attracting interest as molecular water oxidation catalysts as they combine features of both heterogeneous and homogeneous catalysis with bio-inspired motifs. However, the application of many cluster-based catalysts for the oxygen evolution reaction still requires new stabilization strategies. Drawing inspiration from the stabilizing effects of natural polymers, we introduce a conductive polymer-hybrid approach to covalently immobilize {Co4O4} cubane oxo clusters as oxygen evolution catalysts. Polypyrrole is applied as an efficient p-type conducting polymer that promotes hole transfer during the oxygen evolution reaction, resulting in higher turnover frequency compared to the pristine {Co4O4} oxo cluster and heterogeneous Co-oxide benchmarks. The asymmetric coordination of {Co4O4} not only mitigates catalyst decomposition pathways, but also increases the catalytic efficiency by exposing a directed cofacial dihydroxide motif during catalysis.

Tethering Cobalt Ions to BiVO4 Surface via Robust Organic Bifunctional Linker for Efficient Photoelectrochemical Water Splitting.

In the quest for efficient and stable oxygen evolution catalysts (OECs) for photoelectrochemical water splitting, the surface modification of BiVO4 is a crucial step. In this study, a novel and robust OEC, based on 3-(bis(pyridin-2-ylmethyl) amino) propanoic acid bifunctional linker known as dipicolyl alanine acid (DPAA) and cobalt ions, is prepared and fully characterized. The DPAA is anchored to the surface of BiVO4 and utilized to tether cobalt ions. The Co-DPAA/BiVO4 photoanode exhibits remarkable stability and efficiency toward photoelectrochemical water oxidation. Specifically, it showed anodic photocurrent increase of 7.1, 5.0, 3.0, and 1.3-fold at 1.23VRHE as compared to pristine BiVO4, DPAA/BiVO4, Co-BiVO4, and Co-Pi/BiVO4 photoanodes, respectively. The photoelectrochemical and IMPS studies revealed that the Co-DPAA/BiVO4 photoanode exhibits a longer transient decay time for surface-trapped holes, higher charge transfer kinetics, and charge separation efficiency compared to Co-Pi/BiVO4 and pristine BiVO4 photoelectrodes. This indicates that the Co-DPAA effectively reduces surface recombination and facilitates charge transfer. Moreover, at 1.23VRHE, the Co-DPAA/BiVO4 photoanode achieved a faradic efficiency of 92% for oxygen evolution reaction and could retain a turnover frequency of 3.65s-1. The- exhibited effeciency is higher than most of the efficient molecular oxygen evolution catalyst based on Ru.

The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII.

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.

High-capacity, nanocrystalline Li2RuO3-LiCoO2 cathodes for flexible solid-state thin film batteries

Safe, energy-dense batteries are needed to power the next generation of flexible, wearable microelectronic devices. Solid-state Li metal thin-film batteries (TFBs) offer compelling performance but their thin film cathodes require high temperature annealing for optimal electrochemical performance, making integration with low-cost, flexible, thermoplastic substrates difficult. In this study, thin film cathodes based on 0.8Li2RuO3-0.2LiCoO2-xLi2O (LRCO) are reported for the first time. LRCO thin-film cathodes prepared by radio-frequency (RF) sputtering provide excellent energy storage performance without requiring a thermal annealing step. X-ray diffraction shows that the LRCO is nanocrystalline, and X-ray photoelectron spectroscopy confirms the 4+ and 3+ oxidation states of Ru and Co, respectively. Energy storage performance as a function of the charge potential was investigated via galvanostatic cycling to identify the maximum safe potential to avoid molecular oxygen evolution from the cathode lattice. Nanocrystalline LRCO delivers a discharge capacity of >110 μAh cm−2 μm−1 at 0.3C which compares favorably to the state-of-art LiCoO2 thin-film cathode at 69 μAh cm−2 μm−1. The feasibility of fabricating and performance on flexible thermoplastic susbstrates is also demonstrated. On polyethylene terephthalate and polyimide susbtrates, the discharge capacity remains >100 μAh cm−2 μm−1 and the capacity retention is 97.5% over 120 cycles in a bent state.

Switchable wetting of oxygen-evolving oxide catalysts

The surface wettability of catalysts is typically controlled via surface treatments that promote catalytic performance. Here we report on potential-regulated hydrophobicity/hydrophilicity at cobalt-based oxide interfaces with an alkaline solution. The switchable wetting of single particles, directly related to their activity and stability towards the oxygen evolution reaction, was revealed by electrochemical liquid-phase transmission electron microscopy. Analysis of the movement of the liquid in real time revealed distinctive wettability behaviour associated with specific potential ranges. At low potentials, an overall reduction of the hydrophobicity of the oxides was probed. Upon reversible reconstruction towards the surface oxyhydroxide phase, electrowetting was found to cause a change in the interfacial capacitance. At high potentials, the evolution of molecular oxygen, confirmed by operando electron energy-loss spectroscopy, was accompanied by a globally thinner liquid layer. This work directly links the physical wetting with the chemical oxygen evolution reaction of single particles, providing fundamental insights into solid–liquid interfacial interactions of oxygen-evolving oxides.

Applying Active Learning to the Screening of Molecular Oxygen Evolution Catalysts.

The oxygen evolution reaction (OER) can enable green hydrogen production; however, the state-of-the-art catalysts for this reaction are composed of prohibitively expensive materials. In addition, cheap catalysts have associated overpotentials that render the reaction inefficient. This impels the search to discover novel catalysts for this reaction computationally. In this communication, we present machine learning algorithms to enhance the hypothetical screening of molecular OER catalysts. By predicting calculated binding energies using Gaussian process regression (GPR) models and applying active learning schemes, we provide evidence that our algorithm can improve computational efficiency by guiding simulations towards candidates with promising OER descriptor values. Furthermore, we derive an acquisition function that, when maximized, can identify catalysts that can exhibit theoretical overpotentials that circumvent the constraints imposed by linear scaling relations by attempting to enforce a specific mechanism. Finally, we provide a brief perspective on the appropriate sets of molecules to consider when screening complexes that could be stable and active for this reaction.

Mechanistic Understanding of Water Oxidation in the Presence of a Copper Complex by In Situ Electrochemical Liquid Transmission Electron Microscopy.

The design of molecular oxygen-evolution reaction (OER) catalysts requires fundamental mechanistic studies on their widely unknown mechanisms of action. To this end, copper complexes keep attracting interest as good catalysts for the OER, and metal complexes with TMC (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) stand out as active OER catalysts. A mononuclear copper complex, [Cu(TMC)(H2O)](NO3)2 (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), combined both key features and was previously reported to be one of the most active copper-complex-based catalysts for electrocatalytic OER in neutral aqueous solutions. However, the functionalities and mechanisms of the catalyst are still not fully understood and need to be clarified with advanced analytical studies to enable further informed molecular catalyst design on a larger scale. Herein, the role of nanosized Cu oxide particles, ions, or clusters in the electrochemical OER with a mononuclear copper(II) complex with TMC was investigated by operando methods, including in situ vis-spectroelectrochemistry, in situ electrochemical liquid transmission electron microscopy (EC-LTEM), and extended X-ray absorption fine structure (EXAFS) analysis. These combined experiments showed that Cu oxide-based nanoparticles, rather than a molecular structure, are formed at a significantly lower potential than required for OER and are candidates for being the true OER catalysts. Our results indicate that for the OER in the presence of a homogeneous metal complex-based (pre)catalyst, careful analyses and new in situ protocols for ruling out the participation of metal oxides or clusters are critical for catalyst development. This approach could be a roadmap for progress in the field of sustainable catalysis via informed molecular catalyst design. Our combined approach of in situ TEM monitoring and a wide range of complementary spectroscopic techniques will open up new perspectives to track the transformation pathways and true active species for a wide range of molecular catalysts.

Anoxic photogeochemical oxidation of manganese carbonate yields manganese oxide

The oxidation states of manganese minerals in the geological record have been interpreted as proxies for the evolution of molecular oxygen in the Archean eon. Here we report that an Archean manganese mineral, rhodochrosite (MnCO3), can be photochemically oxidized by light under anoxic, abiotic conditions. Rhodochrosite has a calculated bandgap of about 5.4 eV, corresponding to light energy centering around 230 nm. Light at that wavelength would have been present on Earth's surface in the Archean, prior to the formation of stratospheric ozone. We show experimentally that the photooxidation of rhodochrosite in suspension with light centered at 230 nm produced H2 gas and manganite (γ-MnOOH) with an apparent quantum yield of 1.37 × 10-3 moles hydrogen per moles incident photons. Our results suggest that manganese oxides could have formed abiotically on the surface in shallow waters and on continents during the Archean eon in the absence of molecular oxygen.

Discerning Activity and Inactivity in Earth‐Abundant Molecular Oxygen Evolution Catalysts

AbstractMost computational studies on the oxygen evolution reaction (OER) focus on systems which are known to catalyze this process experimentally. This work computationally examines a number of earth‐abundant metal (M=Mn, Fe, Co, Ni) complexes with tetradentate coordinating anionic ligands which, subject to the same experimental conditions, either evolve or do not evolve oxygen. We offer an account for the inactivity of catalysts and develop an effective thermodynamic descriptor to describe the activity of those catalysts that do evolve oxygen. Additionally, we describe how the functionalization of certain Fe OER catalysts can boost activity. Finally, to allow for the rapid evaluation of homogeneous OER catalysts within this family of complexes, high‐level heuristics to computationally screen these catalysts are detailed.

A Comparison of Electrode Surface Films Formed with Different Oxide Cathodes for Lithium-Ion Batteries

High-nickel, Li-rich, and high-voltage spinel electrodes are lithium-ion battery cathodes commonly considered as candidates for near-term commercialization. Each of these cathode materials can offer higher energy densities than conventional lithium cobalt oxide cathode materials, at least during initial cycles. High-Ni cathodes of the form LiNiaMnbCo1-a-bO2 have the same layered structure as current commercial cathodes with alternating transition metal and lithium layers between oxide slabs. With increasing Ni content, High-Ni cathode materials generally can provide higher discharge capacities, reaching upwards of 240 mAh g-1. Li-rich electrode materials (Li1+xMO) have a similar layered structure as High-Ni materials but include excess Li+ in the transition metal layer that can be accessed for much higher discharge capacities, even exceeding 300 mAh g-1. High-voltage spinel electrodes of the form LiMnaNi2-aO4 operate at high voltages providing high energy densities, though at much lower discharge capacities, ca. 140 mAh g-1. While all three of these electrode classes offer exciting possibilities for future lithium-ion battery development, they all suffer from the inherent instability of their surface reactivities. High-Ni cathodes suffer from high surface reactivity and a slow growth of the cubic rocksalt phase throughout cycling that impedes lithium intercalation. The activation of Li-rich electrodes involves a complex surface passivation involving the evolution of molecular oxygen and the formation of a spinel-like phase. High-voltage spinel electrodes operate outside the electrolyte stability region, so also suffer from rapid oxidative breakdown of electrolyte components and impedance growth. Aside from the cathode interphase, the solid electrolyte interphase formed at the anode can be affected by the surface chemistry of these various cathodes; for instance, transition-metal ions may dissolve from the cathode, migrate to the anode side where they are reduced, and act as a catalyst for further electrolyte reduction. This presentation describes our recent characterization of cathode and anode surface electrolyte interphases from cells with High-Ni, high-voltage spinel, and Li-rich electrodes. Half cells vs. lithium anodes, as well as full cells with up to 1,500 cycles are presented. For these studies, we employ time of flight – secondary ion mass spectrometry (TOF-SIMS), a powerful characterization technique pioneered by our group for characterization of battery materials. TOF-SIMS provides sub-nanometer depth resolution of specific chemical fragments over micron-sized image areas that can also be used in a depth profiling method, through layer-by-layer ablation of surface species to form a 3D image (Figure 1). TOF-SIMS is used in conjunction with standard characterization techniques, such as X-ray photoelectron spectroscopy (XPS), galvanostatic cycling data, and electrochemical impedance spectroscopy. Figure 1: A 3D TOF-SIMS image of cycled high-Ni cathode particles with transition metals indicated in green, as well as decomposed organics (C-, CH-, CH2 - ) indicated in red. Figure 1

How ultrafast X-ray pulses can reveal hidden secrets of photosynthesis

X-ray-based techniques are extremely versatile and can provide information regarding the atomic arrangement of atoms in a molecule and are often the method of choice for exploring the structures of proteins and their ligands. The photosynthetic splitting of water and evolution of molecular oxygen by plants and cyanobacteria is one of the key reactions in nature, which is catalysed by a metal site in a membrane-bound photosynthetic protein complex. In this article, we will describe how X-ray pulse lasers can simultaneously probe the overall atomic structure of the photosynthetic system and the electronic structure of a catalytic metal site under physiological conditions in real time.

Unraveling the Lithium Hydroxide Formation Reaction Mechanism in Lithium-O2 Batteries: A Theoretical Study

It is important to identify the electronic factors that might enhance or hinder the formation of discharge/charge products or parasitic species within the complex electrolytic media of a lithium air battery. Although some reaction mechanisms have been proposed, the underlying reaction process by which LiOH formation occurs in the presence of LiI and water is not well understood. Similarly, the chemical decomposition reaction of LiOH, which leads toward molecular oxygen evolution still remains unknown. Herein is presented a theoretical study of the chemical reactions that may take place in the electrolytic media of a Li/O2 battery with dimethoxyethane (DME) solvent in the presence of water and the additive LiI. The results show that water is the most energetically accessible source of protons to obtain LiOH. The catalytic effect of iodide in the O-O peroxide bond scission could be ascribed to a halogen-bond interaction between the iodine atom and an oxygen nucleophilic site. The reaction pathway involving hydrogen peroxide was found to be a viable route accounting for the lithium hydroxide decomposition. Finally, the results for Li2O2 reactive species and a (Li2O2)4 cluster model were both compared.

Investigation of Ni­Al hydroxide with silver addition as an active substance of alkaline batteries

Layered double hydroxides with different ratios of nickel and aluminum in the presence of Ag + ions and without silver have been synthesized: Ni: Al – 80 %: 20 %, Ni: Al: Ag – 80 %: 15 %: 5 % and 75 %: 15 %: 5 %. The obtained nickel hydroxide powders have a structure similar to α-Ni(OH) 2 with a large number of crystal lattice defects. As a result of galvanostatic charge-discharge cycling, it was revealed that the addition of silver in the chemical synthesis stage increases the hydroxide utilization coefficient at fast discharges but decrease it at slow discharges. A possible mechanism that explains the influence of added silver during synthesis on discharge characteristics of hydroxide powders was proposed. The mechanism is that silver oxide, which is a semiconductor, is mixed with hydroxide and increases the specific conductivity of the powder. Increased electrical conductivity has a positive effect on charge effectiveness, because the initial phase has lower electrical conductivity than the oxidized form – NiOOH. Because the charge involves two processes – the main process of active material charging and evolution of molecular oxygen, the electrical conductivity would play a key role in the electrode charging. At low electrical conductivity and fast charge, the current would primarily be consumed by the side process of oxygen evolution. In case of slow charges, additional electrical conductivity due to the presence of silver oxide would not have a great effect on charge effectiveness, because under such conditions the own conductivity of hydroxide is sufficient. Additionally, the presence of silver oxide would decrease the hydroxide content, which in turn would decrease the utilization coefficient that is calculated from the total mass of the powder.

Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is ...

Exploring the LiOH Formation Reaction Mechanism in Lithium–Air Batteries

Identifying the electronic factors that enhance or hinder the formation of primary discharge/charge products or secondary parasitic species is crucial for defining the fundamental chemical reactions that may take place within the complex electrolytic media of a lithium–air battery. For example, several reaction mechanisms have been proposed to explain the formation of the LiOH discharge product in the presence of LiI and water; however, none of these have been demonstrated or fully understood. A similar situation exists for the decomposition reaction, which leads toward molecular oxygen evolution. Herein we present a mechanistic theoretical study of the reactions taking place in the electrolytic media of a Li–O2 battery with dimethoxyethane (DME) in the presence of both water and the LiI additive. The results reveal that water is the most energetically favorable source of protons yielding LiOH. The effect of iodide in lithium peroxide bond scission can be ascribed to a halogen-bond interaction. The reacti...

Sunlight-driven water splitting using hematite nanorod photoelectrodes.

The efficiency of nanostructures for photoelectrochemical water-splitting is fundamentally governed by the capability of the surface to sustain the reaction without electron trapping or recombination by photogenerated holes. This brief review will summarize the latest progress on hematite, designed with columnar morphology via chemical synthesis, for photoelectrochemical cell application. The columnar morphology efficiently minimizes the number of defects, grain boundaries, and surface traps normally present on the planar morphology. The major drawback related to hole diffusion through the solid/liquid interface was addressed by using high annealing temperature combined with dopant addition. A critical view and depth of understanding of these two parameters were discussed focusing on the molecular oxygen evolution mechanism from the sunlight-driven water oxidation reaction.

(Invited) Insight into Polyoxometalate-Induced Catalysis at Semiconducting Oxides

One of the major limitations in photoelectrochemical water splitting with use of semiconducting oxide is their sluggish surface water oxidation kinetics. Identification of a molecular oxygen evolution catalysts active especially in highly corrosive media is not a trivial case. N-type mesoporous tungsten trioxide, WO3, is one of few stable semiconductor materials able to photo-electrochemically split water under solar light irradiation but it’s operational acidic environment restricts use of any catalyst to the precious metals. In an attempt to overcome this limitation, Keggin‐type polyoxometalate, [PW12O40]3− and [PMo12O40]3−, anions have been employed to act as highly effective oxygen evolution reaction molecular catalysts in photoelectrochemical water splitting devices. This the first example of oxidatively stable in acid media non noble metal water splitting catalyst. Combined with semitransparent Na‐doped nanoporous WO3 film photoanode, they allow achieving under simulated AM 1.5G sunlight exceptionally high water oxidation photocurrent of 4.5 mA cm−2 at 1.1 V versus reversible hydrogen electrode. The surface and structure modifications of a WO3 photoanode which led to this performance, in parallel with the mechanism of the enhancement will be presented and discussed in details. M. Sarnowska, K. Bienkowski, R. Solarska, P. Barczuk, J.Augustynski; Highly Efficient and Stable Solar Water Splitting at (Na)WO3 Photoanodes in Acidic Electrolyte Assisted by Non-noble Metal Oxygen Evolution Catalyst; Advanced Energy Materials 10.1002/aenm201600526

(Invited) Beyond Li-Ion Batteries: Why? , to Where?

The field of advanced batteries faces today two main challenges: development of power sources for electro-mobility (i.e., electrochemical propulsion) and development of storage technologies for load-leveling applications (i.e., large scale batteries). Our starting point in this presentation is state-of-the-art Li-ion batteries. Using carbonaceous anodes and lithiated transition metal cathodes with selected combinations of transition metals may offer very suitable batteries for full EVs. Ni rich Li[NiCoMn]O2, Mn and Li rich Li1+x[NiMnCo]1-xO2 cathode materials operating in the right potential windows can provide Li-ion batteries high enough energy density as needed for full electrochemical propulsion. Also, by combining Li-Ti-O anodes with LiMPO4 olivine cathodes it is possible to demonstrate very suitable battery systems for stationary large energy storage applications. We will review briefly the updated frontier of Li-ion battery technology. Despite the high capability of this battery technology, there are so many applications for power sources in modern life, what promote very strongly a search for more new battery systems. The use of sulfur cathodes in Li batteries can increase considerably their gravimetric energy density, however, the volumetric energy density of Li-sulfur battery systems is not advantageous compared to advanced Li ion batteries. The good news are that in recent years, we see a nice progress in developing durable composite sulfur cathodes. It seems that there are 3 main approaches: encapsulating sulfur in activated carbon matrices, the use of substrates to which the LiSn species formed by sulfur reduction adsorb very strongly and the use of composite cathodes with high loading of sulfur and semi-permeable membranes that avoid transport of LiSn species to the negative electrodes. There is intensive work on Li-oxygen systems. We see progress in development of electro-catalysis for these systems. We will demonstrate electro-catalytic electrodes in Li-oxygen cells, which anodic reactions (Li-peroxide oxidation and molecular oxygen evolution) occur at very low over-potential, that does not endanger the stability of the electrolyte solutions. We demonstrate also red-ox mediators in solutions that facilitate both ORR and OER. Despite the frustration from the slow progress made towards practical directions, it is important to continue investing efforts in Li-oxygen systems. The community should understand that a long-term research is still required in order to see practical horizons to Li-air battery systems. A main problem is the reactivity of all kinds of polar-aprotic solvents towards superoxide and peroxide species formed by oxygen reduction, in the presence of the super-electrophilic Li ions. The intensive work on both Li-S and Li-oxygen systems is connected to a renaissance in efforts to develop practical Li metal anodes for rechargeable batteries. However, intensive studies that were carried out during more than 3 decades, concluded more than 15 years ago that there is no way to cycle Li metal anodes in practical rechargeable Li battery systems. Even if dendrites formation is avoided, continuous detrimental reactions between the active metal and any type of relevant electrolyte solution is inevitable. We will examine briefly what are the chances to overcome the intrinsic problems in using Li metal anodes in practical rechargeable batteries. We may be able to suggest practical alternative anodes for non-aqueous batteries based on sulfur or oxygen cathodes. Discussing other options for “beyond Li-ion batteries” usually includes Na-ion and magnesium batteries. In fact, the plethora of relevant Na-ion insertion cathodes available today is a good surprise, especially since many of them demonstrate excellent kinetics. However, there is no way that Na-ion systems can rival Li-ion battery systems in terms of energy density. Hence, Na-ion systems may be relevant for large energy storage applications, that can benefit from the high abundance of Na in earth crust. However for such uses, demonstrating high stability during prolonged cycling is mandatory. Consequently, a major challenge in R&D of Na-ion batteries is to exhibit the durability required for load-leveling applications. Finally, we will mention the status of R&D of rechargeable Mg battery systems: excellent work was done so far on the anode-solutions part. Several families of electrolyte solutions with wide electrochemical windows, in which Mg anodes are fully reversible were demonstrated. We have not seen yet cathodes materials that function well in the solutions in which Mg anodes are fully reversible, except the low voltage/low capacity Chevrel phase cathode materials. Hence, these systems still require long term research work, especially towards high voltage/high capacity cathode materials. A main incentive to develop rechargeable Mg batteries may relate to advantages in safety and high volumetric energy density.

Water Oxidation in Natural Photosynthesis

The photosynthetic water oxidation reaction is energetically demanding and mechanistically complex because of the difficulties in managing the four electron, four proton redox chemistry required for the evolution of molecular oxygen starting from two water molecules. The reaction takes place in Photosystem II (PS II), a multi-subunit membrane protein present in plants, algae, and cyanobacteria. This sunlight-driven reaction is catalyzed by an oxygen-evolving complex (OEC), that consists of an oxo-bridged four Mn and one Ca cluster. O2 is formed and released only after four oxidation equivalents are accumulated at the OEC. The structure of the Mn4CaO5 cluster has been studied by various spectroscopic and diffraction methods. The recent XRD study by Umena et al.[1] has shown the oxo-bridged Mn4Ca cluster structure at 1.9 Å resolution. Based on this high-resolution XRD structure, there have been efforts to obtain chemically optimized structures and structural changes of the Mn4CaO5 cluster during the catalytic cycle using spectroscopic parameters and theoretical approaches. EXAFS spectra of the PS II S states show that the structure of the Mn4CaO5 cluster changes during the catalytic cycle.[2] In particular, the short Mn-Mn distances change in the range of 2.7 to 2.9 Å. Such changes in oxygen-bridged Mn-Mn distances can reflect several chemical parameters; Mn oxidation state changes, protonation state changes of bridging oxygens, ligation modes (e.g. bidentate/monodentate), as well as fundamental changes in geometry. We have also used femtosecond X-ray spectroscopy and crystallography to study the catalytic process of the OEC.[3] The femtosecond X-ray pulses of the free-electron laser allows us to out-run X-ray damage at room temperature, and the time-evolution of the photo-induced reaction can be probed using a visible laser-pump followed by the X-ray-probe pulse. We will discuss a possible water oxidation mechanism based on these results.

Dissociation rate constants for oxygen at temperatures up to 11000 K

Measurements of the absorption and vibrational temperature evolution of molecular oxygen behind the shock wave front in regimes with temperatures near the front varying over the range 4000–11 000 K made it possible to determine the dissociation rate constant for O2 molecules under both thermal equilibrium and nonequilibrium conditions. The dependence of the dissociation rate constant on the ratio Tv/T, that is, on the deviation from the thermal equilibrium conditions, is demonstrated experimentally. The corresponding expression for the two-temperature dissociation rate constant is proposed.