Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential‐determining step. Subsequently, novel cobalt oxide‐based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water‐oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non‐ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox‐potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

The electronic structure of transition metal oxides governs the catalysis of many central reactions for energy storage applications such as oxygen electrocatalysis. Here we exploit the versatility of the perovskite structure to search for oxide catalysts that are both active and stable. We report double perovskites (Ln 0.5 Ba 0.5 )CoO 3 À d (Ln ¼ Pr, Sm, Gd and Ho) as a family of highly active catalysts for the oxygen evolution reaction upon water oxidation in alkaline solution. These double perovskites are stable unlike pseudocubic perovskites with comparable activities such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 À d which readily amorphize during the oxygen evolution reaction. The high activity and stability of these double perovskites can be explained by having the O p-band centre neither too close nor too far from the Fermi level, which is computed from ab initio studies.

REVIEWThis journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1-3 | 1 In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal-air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field. Broader ContextThe formation of chemical bonds is an energy dense mode of storing energy. In both nature and technology, the electrochemical generation and consumption of fuels is one of the most efficient routes for energy usage. Solar and electrical energy can be stored in chemical bonds by splitting water or metal oxides to produce hydrogen and metal. These compounds can then be oxidized to produce energy when coupled to the reduction of oxygen. However, these device efficiencies are severely limited by the catalysis of oxygen electrochemical processes -namely the oxygen reduction reaction and oxygen evolution reaction, which have slow kinetics. Non-precious transition metal oxides show promise as cost-effective substitutes for noble metals in commercially viable renewable energy storage and conversion devices. Furthermore, this class of materials has ben efitted from a wealth of spectroscopic and first-principles studies in the past few decades, providing the frameworks and theories needed to understand the electronic structure and design optimal catalysts. The incredibly diverse range of chemistries and physical properties that can be explored in oxide families afford numerous degrees of freedom for conducting systematic investigations relating intrinsic mat erial properties to catalytic performance. Here, we present background on the fundamental concepts in catalysis for the rational design of transition metal perovskite oxide catalysts for oxygen electrocatalysis and critically examine the current understanding a nd its impact on future directions of perovskite catalysts.

Electrochemical energy storage by making H2 an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state‐of‐the‐art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic‐ and background‐correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state‐of‐the‐art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

Perovskites such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF82) can be highly active for the oxygen evolution reaction (OER) upon water oxidation in alkaline solution. Here we report that BSCF82 can quickly undergo amorphization of its surface at OER potentials, which is accompanied by reduced surface concentrations of Ba 2+ and Sr 2+ ions as well as increased pseudocapacitive and OER currents. Such quick amorphization during OER was also observed for perovskite catalysts with similar OER activities such as Ba 0.5 Sr 0.5 Co 0.4 Fe 0.6 O 3−δ and SrCo 0.8 Fe 0.2 O 3−δ . In contrast, perovskite oxides with lower OER activities than BSCF82 did not undergo this transformation when subjected to identical electrochemical conditions. These findings demonstrate that the active chemistry and structure of oxide catalysts during OER can significantly differ from those of the as-synthesized material and that understanding how the oxide surface may change and impact the OER activity is critical to the design of highly active and stable OER catalysts.

In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn 4 Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-m-oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of Mn III sites and prevented formation of well-ordered and unreactive Mn IV O 2 . Structure-function relations in Mn-based wateroxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use. Broader contextThreatening global climate changes and unsecured supply of fossil fuels call for a global transition toward sustainable energyconversion systems. The storage of wind or solar energy by formation of energy-rich fuel molecules could play a central role in both transient storage of the intermittently provided energy and replacement of fossil fuels in the transportation sector. Whether hydrogen or a carbon-based fuel is the target, in any event the extraction of reducing equivalents and protons from water (that is, water oxidation) is pivotal. In search of water-oxidation catalysts that ultimately could play a role at a global scale, we and others are aiming at development of simple routes towards formation of Mn-based catalysts. Manganese excels by high availability and low toxicity; and in oxygenic photosynthesis, nature has demonstrated that a Mn-based catalyst can oxidize water efficiently. When aiming at an 'artificial leaf' with a Mn-based catalyst directly coupled to a solar-energy-converting material, the activity of the catalyst (per area) needs to cope with the incoming photon flux. Moreover the device design typically requires the use of benign synthesis and operation conditions, that is, temperatures close to room temperature and pH close to neutral. As an important first step, we report a simple protocol for e...

Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in-situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoints potentials of about 1.0 V (Co II 0.4Co III 0.6  all-Co III ) and 1.2 V (all-Co III  Co III 0.8Co IV 0.2) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveal that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in the amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo Co II Co III Co IV oxidationstate changes coupled to structural modification and deprotonation of Co-oxo bridges. By encounter of two (or more) Co IV ions, an active site is formed at which the O-O bondformation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity. EXPERIMENTAL XAS measurements -in-situ experimentCoCat-coated electrodes were prepared by electrodeposition in a separate electrochemical setup before start of the in-situ XAS measurements from a solution of 0.5 mM Co 2+ ions in 0.1 M KPi at pH 7 (deposition of about 50 nmol cm -1 of Co ions, see ESI for further details). The in-situ XAS measurements were performed at the SuperXAS beamline of the Swiss Light Source (SLS) in Villigen, Switzerland. The excitation energy was selected by a double-crystal monochromator (Si-111, detuning to 50 % intensity, scan range of 7650-8400 eV) and used to irradiate the backside of the ITO/PET electrode at an angle of 45°. The spot size of the X-ray beam on the sample was 5 mm × 1 mm. Due to employment of a large spot size (defocussed beam) and a rapid-scanning mode, the influence of radiation-induced modifications was negligible, as verified in control experiments. The cobalt K-edge fluorescence was monitored perpendicular to the incident beam by a scintillation detector (19.6 cm 2 active area, 51BMI/2E1-YAP-Neg, Scionix), which was shielded by a 25 μm iron foil agai...

While many perovskites remain crystalline during the oxygen evolution reaction (OER) in alkaline media, some highly active perovskites become amorphous. We studied the local structure changes of perovskites LaCoO3, Ba0.5Sr0.5Co0.8Fe0.2O3‑δ, and SrCo0.8Fe0.2O3‑δ before and after OER by X-ray absorption spectroscopy. No change in either local structure or OER activity was observed for LaCoO3, while considerably enhanced OER activities and the conversion of the local structure from corner-sharing octahedra to edge-sharing octahedra were noted for Ba0.5Sr0.5Co0.8Fe0.2O3‑δ and SrCo0.8Fe0.2O3‑δ as a result of the OER. Possible processes responsible for the structural change and enhanced OER activities are discussed.