Self-healing catalysis in water

Copper-based electrocatalyst for hydrogen evolution in water

In aqueous pH 7 phosphate buffer, during controlled potential electrolysis (CPE) at -1.10 V vs. Ag|AgCl the literature square planar copper complex, [CuIILEt]BF4 (1), forms a heterogeneous deposit on the glassy carbon working electrode (GCWE) that is a stable and effective hydrogen evolution reaction (HER) electrocatalyst. Specifically, CPE for 20 hours using a small GCWE (A = 0.071 cm2) gave a turnover number (TON) of 364, with ongoing activity. During CPE the brownish-yellow colour of the working solution fades, and a deposit is observed on the small GCWE. Repeating this CPE experiment in a larger cell with a larger GCWE (A = 2.7 cm2), connected to a gas chromatograph, resulted in a TON of 2628 after 2.6 days, with FE = 93%, and with activity ongoing. After this CPE, the working solution had faded to nearly colourless, and visual inspection of the large GCWE showed a material had deposited on the surface. In a 'rinse and repeat test', this heterogeneous deposit was used for further CPE, in a freshly prepared working solution minus fresh catalyst, which resulted in similar ongoing HER activity to before, consistent with the surface deposited material being the active HER catalyst. EDS, PXRD and SEM analysis of this deposit shows that copper and oxygen are the main components present, most likely comprising copper and copper(I) oxide ((Cu2O)n) formed from 1. The use of 1 leads to a deposit that is more catalytically active than that formed when starting with a simple copper salt (control), likely due to it forming a more robustly attached deposit, which also enables the observed long-lived catalytic activity.

Atomistic mechanisms of water vapor-induced surface passivation

The microscopic mechanisms underpinning the spontaneous surface passivation of metals from ubiquitous water have remained largely elusive. Here, using in situ environmental electron microscopy to atomically monitor the reaction dynamics between aluminum surfaces and water vapor, we provide direct experimental evidence that the surface passivation results in a bilayer oxide film consisting of a crystalline-like Al(OH)3 top layer and an inner layer of amorphous Al2O3. The Al(OH)3 layer maintains a constant thickness of ~5.0 Å, while the inner Al2O3 layer grows at the Al2O3/Al interface to a limiting thickness. On the basis of experimental data and atomistic modeling, we show the tunability of the dissociation pathways of H2O molecules with the Al, Al2O3, and Al(OH)3 surface terminations. The fundamental insights may have practical significance for the design of materials and reactions for two seemingly disparate but fundamentally related disciplines of surface passivation and catalytic H2 production from water.

Optimal Coatings of Co3 O4 Anodes for Acidic Water Electrooxidation

Implementation of proton-exchange membrane water electrolyzers for large-scale sustainable hydrogen production requires the replacement of scarce noble-metal anode electrocatalysts with low-cost alternatives. However, such earth-abundant materials often exhibit inadequate stability and/or catalytic activity at low pH, especially at high rates of the anodic oxygen evolution reaction (OER). Here, the authors explore the influence of a dielectric nanoscale-thin oxide layer, namely Al2 O3 , SiO2 , TiO2 , SnO2 , and HfO2 , prepared by atomic layer deposition, on the stability and catalytic activity of low-cost and active but insufficiently stable Co3 O4 anodes. It is demonstrated that the ALD layers improve both the stability and activity of Co3 O4 following the order of HfO2 > SnO2 > TiO2 > Al2 O3 , SiO2 . An optimal HfO2 layer thickness of 12 nm enhances the Co3 O4 anode durability by more than threefold, achieving over 42 h of continuous electrolysis at 10 mA cm-2 in 1 m H2 SO4 electrolyte. Density functional theory is used to investigate the superior performance of HfO2 , revealing a major role of the HfO2 |Co3 O4 interlayer forces in the stabilization mechanism. These insights offer a potential strategy to engineer earth-abundant materials for low-pH OER catalysts with improved performance from earth-abundant materials for efficient hydrogen production.

The Implications of Coupling an Electron Transfer Mediated Oxidation with a Proton Coupled Electron Transfer Reduction in Hybrid Water Electrolysis

Electrolysis of water is a sustainable route to produce clean hydrogen. Full water-splitting requires a high applied potential, in part because of the pH-dependency of the H₂ and O₂ evolution reactions (HER and OER), which are proton-coupled electron transfer (PCET) reactions. Therefore, the minimum required potential will not change at different pHs. TEMPO [(2,2,6,6-tetramethyl-1-piperidin-1-yl)oxyl], a stable free-radical that undergoes fast electro-oxidation by a single-electron transfer (ET) process, is pH-independent. Here, we show that the combination of PCET and ET processes enables hydrogen production from water at low cell potentials below the theoretical value for full water-splitting by simple pH adjustment. As a case study, we combined the HER with the oxidation of benzylamine by anodically oxidized TEMPO. The pH-independent electrocatalytic oxidation of TEMPO permits the operation of a hybrid water-splitting cell that shows promise to perform at a low cell potential (≈1 V) and neutral pH conditions.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Learning from Nature's Example: Repair Strategies in Light-Driven Catalysis

The continuous repair of subunits of the photosynthetic apparatus is a key factor determining the overall efficiency of biological photosynthesis. Recent concepts for repairing artificial photocatalysts and catalytically active materials within the realm of solar fuel formation show great potential in reshaping the research directions within this field. This perspective describes the latest advances, concepts, and mechanisms in the field of catalyst repair and catalyst self-healing and provides an outlook on which additional steps need to be taken to bring artificial photosynthetic systems closer to real-life applications.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

Dynamic Electrochemical Interfaces for Energy Conversion and Storage

Electrochemical energy conversion and storage are central to developing future renewable energy systems. For efficient energy utilization, both the performance and stability of electrochemical systems should be optimized in terms of the electrochemical interface. To achieve this goal, it is imperative to understand how a tailored electrode structure and electrolyte speciation can modify the electrochemical interface structure to improve its properties. However, most approaches describe the electrochemical interface in a static or frozen state. Although a simple static model has long been adopted to describe the electrochemical interface, atomic and molecular level pictures of the interface structure should be represented more dynamically to understand the key interactions. From this perspective, we highlight the importance of understanding the dynamics within an electrochemical interface in the process of designing highly functional and robust energy conversion and storage systems. For this purpose, we explore three unique classes of dynamic electrochemical interfaces: self-healing, active-site-hosted, and redox-mediated interfaces. These three cases of dynamic electrochemical interfaces focusing on active site regeneration collectively suggest that our understanding of electrochemical systems should not be limited to static models but instead expanded toward dynamic ones with close interactions between the electrode surface, dissolved active sites, soluble species, and reactants in the electrolyte. Only when we begin to comprehend the fundamentals of these dynamics through operando analyses can electrochemical conversion and storage systems be advanced to their full potential.

Water-Splitting Artificial Leaf Based on a Triple-Junction Silicon Solar Cell: One-Step Fabrication through Photoinduced Deposition of Catalysts and Electrochemical Operando Monitoring

Solar hydrogen generation via water splitting using a monolithic photoelectrochemical cell, also called artificial leaf, could be a powerful technology to accelerate the transition from fossil to sustainable energy sources. Identification of scalable methods for the fabrication of monolithic devices and gaining insights into their operating mode to identify solutions to improve performance and stability represent great challenges. Herein, we report on the one-step fabrication of a CoWO|ITO|3jn-a-Si|Steel|CoWS monolithic device via the simple photoinduced deposition of CoWO and CoWS as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalyst layers, respectively, onto an illuminated ITO|3jn-a-Si|Steel solar cell using a single-deposition bath containing the [Co(WS₄)₂]^2- complex. In a pH 7 phosphate buffer solution, the best device achieved a solar-to-hydrogen conversion yield of 1.9%. Evolution of the catalyst layers and that of the 3jn-a-Si light-harvesting core during the operation of the monolithic device are examined by conventional tools such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) together with a bipotentiostat measurement. We demonstrate that the device performance degrades due to the partial dissolution of the catalyst. Still, this degradation is healable by simply adding [Co(WS₄)₂]^2- to the operating solution. However, modifications on the protecting indium-doped tin oxide (ITO) layer are shown to initiate irreversible degradation of the 3jn-a-Si light-harvesting core, resulting in a 10-fold decrease of the performances of the monolithic device.

Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni²⁺/Ni³⁺ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s^–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Self-healing oxygen evolution catalysts

Electrochemical and photoelectrochemical water splitting offers a scalable approach to producing hydrogen from renewable sources for sustainable energy storage. Depending on the applications, oxygen evolution catalysts (OECs) may perform water splitting under a variety of conditions. However, low stability and/or activity present challenges to the design of OECs, prompting the design of self-healing OECs composed of earth-abundant first-row transition metal oxides. The concept of self-healing catalysis offers a new tool to be employed in the design of stable and functionally active OECs under operating conditions ranging from acidic to basic solutions and from a variety of water sources.

Large scale sustainable energy storage by water splitting benefits from performing the oxygen evolution reaction under a variety of conditions. Here, the authors discuss self-healing catalysis as a new tool in the design of stable and functionally active catalysts in acidic to basic solutions, and a variety of water sources

Proton-Coupled Electron Transfer: The Engine of Energy Conversion and Storage

Proton-coupled electron transfer (PCET) underpins energy conversion in chemistry and biology. Four energy systems are described whose discoveries are based on PCET: the water splitting chemistry of the Artificial Leaf, the carbon fixation chemistry of the Bionic Leaf-C, the nitrogen fixation chemistry of the Bionic Leaf-N and the Coordination Chemistry Flow Battery (CCFB). Whereas the Artificial Leaf, Bionic Leaf-C, and Bionic Leaf-N require strong coupling between electron and proton to reduce energetic barriers to enable high energy efficiencies, the CCFB requires complete decoupling of the electron and proton so as to avoid parasitic energy-wasting reactions. The proper design of PCET in these systems facilitates their implementation in the areas of (i) centralized large scale grid storage of electricity and (ii) decentralized energy storage/conversion using only sunlight, air and any water source to produce fuel and food within a sustainable cycle for the biogenic elements of C, N and P.

A self-healing catalyst for electrocatalytic and photoelectrochemical oxygen evolution in highly alkaline conditions

While self-healing is considered a promising strategy to achieve long-term stability for oxygen evolution reaction (OER) catalysts, this strategy remains a challenge for OER catalysts working in highly alkaline conditions. The self-healing of the OER-active nickel iron layered double hydroxides (NiFe-LDH) has not been successful due to irreversible leaching of Fe catalytic centers. Here, we investigate the introduction of cobalt (Co) into the NiFe-LDH as a promoter for in situ Fe redeposition. An active borate-intercalated NiCoFe-LDH catalyst is synthesized using electrodeposition and shows no degradation after OER tests at 10 mA cm⁻² at pH 14 for 1000 h, demonstrating its self-healing ability under harsh OER conditions. Importantly, the presence of both ferrous ions and borate ions in the electrolyte is found to be crucial to the catalyst’s self-healing. Furthermore, the implementation of this catalyst in photoelectrochemical devices is demonstrated with an integrated silicon photoanode. The self-healing mechanism leads to a self-limiting catalyst thickness, which is ideal for integration with photoelectrodes since redeposition is not accompanied by increased parasitic light absorption.

While self-healing catalysts may survive the harsh environments used for oxygen evolution, understanding how to develop such electrocatalysts remains a challenge. Here, authors find cobalt to promote the self-healing of leached iron centers in borate-intercalated nickel-iron-cobalt oxyhydroxides.

Unraveling Nanoscale Cobalt Oxide Catalysts for the Oxygen Evolution Reaction: Maximum Performance, Minimum Effort

The oxygen evolution reaction (OER) is a key bottleneck step of artificial photosynthesis and an essential topic in renewable energy research. Therefore, stable, efficient, and economical water oxidation catalysts (WOCs) are in high demand and cobalt-based nanomaterials are promising targets. Herein, we tackle two key open questions after decades of research into cobalt-assisted visible-light-driven water oxidation: What makes simple cobalt-based precipitates so highly active-and to what extent do we need Co-WOC design? Hence, we started from Co(NO₃)₂ to generate a precursor precipitate, which transforms into a highly active WOC during the photocatalytic process with a [Ru(bpy)₃]²⁺/S₂O₈^2-/borate buffer standard assay that outperforms state of the art cobalt catalysts. The structural transformations of these nanosized Co catalysts were monitored with a wide range of characterization techniques. The results reveal that the precipitated catalyst does not fully change into an amorphous CoO_x material but develops some crystalline features. The transition from the precipitate into a disordered Co₃O₄ material proceeds within ca. 1 min, followed by further transformation into highly active disordered CoOOH within the first 10 min. Furthermore, under noncatalytic conditions, the precursor directly transforms into CoOOH. Moreover, fast precipitation and isolation afford a highly active precatalyst with an exceptional O₂ yield of 91% for water oxidation with the visible-light-driven [Ru(bpy)₃]²⁺/S₂O₈^2- assay, which outperforms a wide range of carefully designed Co-containing WOCs. We thus demonstrate that high-performance cobalt-based OER catalysts indeed emerge effortlessly from a self-optimization process favoring the formation of Co(III) centers in all-octahedral environments. This paves the way to new low-maintenance flow chemistry OER processes.

Detection of high-valent iron species in alloyed oxidic cobaltates for catalysing the oxygen evolution reaction

Iron alloying of oxidic cobaltate catalysts results in catalytic activity for oxygen evolution on par with Ni-Fe oxides in base but at much higher alloying compositions. Zero-field ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) are able to clearly identify Fe⁴⁺ in mixed-metal Co-Fe oxides. The highest Fe⁴⁺ population is obtained in the 40–60% Fe alloying range, and XAS identifies the ion residing in an octahedral oxide ligand field. The oxygen evolution reaction (OER) activity, as reflected in Tafel analysis of CoFeO_x films in 1 M KOH, tracks the absolute concentration of Fe⁴⁺. The results reported herein suggest an important role for the formation of the Fe⁴⁺ redox state in activating cobaltate OER catalysts at high iron loadings.

The capturing of high valent iron in a catalytic reaction is important but difficult task. Here, the authors report identification of a high-valent Fe(IV)-species with different spectroscopic tools such as Mössbauer spectroscopy and X-ray absorption spectroscopy during the course of an oxygen evolving reaction.

Delivering the Full Potential of Oxygen Evolving Electrocatalyst by Conditioning Electrolytes at Near-Neutral pH

This study reports on the impact of identity and compositions of buffer ions on oxygen evolution reaction (OER) performance at a wide range of pH levels using a model IrOx electrocatalyst. Rigorous microkinetic analysis employing kinetic isotope effects, Tafel analysis, and temperature dependence measurement was conducted to establish rate expression isolated from the diffusion contribution of buffer ions and solution resistance. It was found that the OER kinetics was facile with OH- oxidation compared to H2 O, the results of which were highlighted by mitigating over 200 mV overpotential in the presence of buffer to reach 10 mA cm-2 . This improvement was ascribed to the involvement of the kinetics of the local OH- supply by the buffering action. Further digesting the kinetic data at various buffer pKa and the solution bulk pH disclosed a trade-off between the exchange current density and the Tafel slope, indicating that the optimal electrolyte condition can be chosen at a different range of current density. This study provides a quantitative guideline for electrolyte engineering to maximize the intrinsic OER performance that electrocatalyst possesses especially at near-neutral pH.

Continuous electrochemical water splitting from natural water sources via forward osmosis

Electrochemical water splitting stores energy as equivalents of hydrogen and oxygen and presents a potential route to the scalable storage of renewable energy. Widespread implementation of such energy storage, however, will be facilitated by abundant and accessible sources of water. We describe herein a means of utilizing impure water sources (e.g., saltwater) for electrochemical water splitting by leveraging forward osmosis. A concentration gradient induces the flow of water from an impure water source into a more concentrated designed electrolyte. This concentration gradient may subsequently be maintained by water splitting, where rates of water influx (i.e., forward osmosis) and effective outflux (i.e., water splitting) are balanced. This approach of coupling forward osmosis to water splitting allows for the use of impure and natural sources without pretreatment and with minimal losses in energy efficiency.

Alloying-realloying enabled high durability for Pt-Pd-3d-transition metal nanoparticle fuel cell catalysts

Alloying noble metals with non-noble metals enables high activity while reducing the cost of electrocatalysts in fuel cells. However, under fuel cell operating conditions, state-of-the-art oxygen reduction reaction alloy catalysts either feature high atomic percentages of noble metals (>70%) with limited durability or show poor durability when lower percentages of noble metals (<50%) are used. Here, we demonstrate a highly-durable alloy catalyst derived by alloying PtPd (<50%) with 3d-transition metals (Cu, Ni or Co) in ternary compositions. The origin of the high durability is probed by in-situ/operando high-energy synchrotron X-ray diffraction coupled with pair distribution function analysis of atomic phase structures and strains, revealing an important role of realloying in the compressively-strained single-phase alloy state despite the occurrence of dealloying. The implication of the finding, a striking departure from previous perceptions of phase-segregated noble metal skin or complete dealloying of non-noble metals, is the fulfilling of the promise of alloy catalysts for mass commercialization of fuel cells.

Early-stage formation of (hydr)oxo bridges in transition-metal catalysts for photosynthetic processes

Ab initio simulations have been used to assess reaction pathways for the formation of M–(hydr)oxo–M (M = Co, Mn, Ni) bridges from M(ii) aqueous solutions, as early-stage building blocks of transition-metal catalysts for oxygen evolution.

Recent advances in understanding oxygen evolution reaction mechanisms over iridium oxide

Recent spectroscopic and computational studies concerning the oxygen evolution reaction over iridium oxides are reviewed to provide the state-of-the-art understanding of its reaction mechanism.

Phosphate-assisted efficient oxygen evolution over finely dispersed cobalt particles supported on graphene

Finely dispersed cobalt particles supported on graphene activated by phosphate boosting highly efficient and stable oxygen evolution.

Elucidation of Factors That Govern the 2e-/2H+ vs 4e-/4H+ Selectivity of Water Oxidation by a Cobalt Corrole

Considering the importance of water splitting as the best solution for clean and renewable energy, the worldwide efforts for development of increasingly active molecular water oxidation catalysts must be accompanied by studies that focus on elucidating the mode of actions and catalytic pathways. One crucial challenge remains the elucidation of the factors that determine the selectivity of water oxidation by the desired 4e^-/4H⁺ pathway that leads to O₂ rather than by 2e^-/2H⁺ to H₂O₂. We now show that water oxidation with the cobalt-corrole CoBr₈ as electrocatalyst affords H₂O₂ as the main product in homogeneous solutions, while heterogeneous water oxidation by the same catalyst leads exclusively to oxygen. Experimental and computation-based investigations of the species formed during the process uncover the formation of a Co(III)-superoxide intermediate and its preceding high-valent Co-oxyl complex. The competition between the base-catalyzed hydrolysis of Co(III)-hydroperoxide [Co(III)-OOH]^- to release H₂O₂ and the electrochemical oxidation of the same to release O₂ via [Co(III)-O₂^•]^- is identified as the key step determining the selectivity of water oxidation.

Water Electrolysis in Saturated Phosphate Buffer at Neutral pH

Hydrogen production from renewable energy and ubiquitous water has a potential to achieve sustainability, although current water electrolyzers cannot compete economically with the fossil fuel-based technology. Here, we evaluate water electrolysis at pH 7 that is milder than acidic and alkaline pH counterparts and may overcome this issue. The physicochemical properties of concentrated buffer electrolytes were assessed at various temperatures and molalities for quantitative determination of losses associated with mass-transport during the water electrolysis. Subsequently, in saturated K-phosphate solutions at 80 °C and 100 °C that were found to be optimal to minimize the losses originating from mass-transport at the neutral pH, the water electrolysis performance over model electrodes of IrOx and Pt as an anode and a cathode, respectively, was reasonably comparable with those of the extreme pH. Remarkably, this concentrated buffer solution also achieved enhanced stability, adding another merit of this electrolyte for water electrolysis.

Atomic-Scale Dynamic Interaction of H_{2}O Molecules with Cu Surface

Atomic-scale interaction of water vapor with metal surfaces beyond surface adsorption under technologically relevant conditions remains mostly unexplored. Using aberration-corrected environmental transmission electron microscopy, we reveal the dynamic surface activation of Cu by H_{2}O at elevated temperature and pressure. We find a structural transition from flat to corrugated surface for the Cu(011) under low water-vapor pressure. Increasing the water-vapor pressure leads to the surface reaction of Cu with dissociated H_{2}O, resulting in the formation of a metastable "bilayer" Cu─O─H phase. Corroborated by density functional theory and ab initio molecular dynamics calculations, the cooperative O and OH interaction with Cu is responsible for the formation and subsurface propagation of this phase.

Synthetic Approaches to Metallo-Supramolecular CoII Polygons and Potential Use for H2O Oxidation

Metal-directed self-assembly has been applied to prepare supramolecular coordination polygons which adopt tetrahedral (1) or trigonal disklike topologies (2). In the solid state, 2 assembles into a stable halide-metal-organic material (Hal-MOM-2), which catalyzes H₂O oxidation under photo- and electrocatalytic conditions, operating with a maximum TON = 78 and TOF = 1.26 s^-1. DFT calculations attribute the activity to a Co^III-oxyl species. This study provides the first account of how Co^II imine based supramolecules can be employed as H₂O oxidation catalysts.

Template-stabilized oxidic nickel oxygen evolution catalysts

Earth-abundant oxygen evolution catalysts (OECs) with extended stability in acid can be constructed by embedding active sites within an acid-stable metal-oxide framework. Here, we report stable NiPbO_x films that are able to perform oxygen evolution reaction (OER) catalysis for extended periods of operation (>20 h) in acidic solutions of pH 2.5; conversely, native NiO_x catalyst films dissolve immediately. In situ X-ray absorption spectroscopy and ex situ X-ray photoelectron spectroscopy reveal that PbO₂ is unperturbed after addition of Ni and/or Fe into the lattice, which serves as an acid-stable, conductive framework for embedded OER active centers. The ability to perform OER in acid allows the mechanism of Fe doping on Ni catalysts to be further probed. Catalyst activity with Fe doping of oxidic Ni OEC under acid conditions, as compared to neutral or basic conditions, supports the contention that role of Fe³⁺ in enhancing catalytic activity in Ni oxide catalysts arises from its Lewis acid properties.

Operando Raman spectroscopy tracks oxidation-state changes in an amorphous Co oxide material for electrocatalysis of the oxygen evolution reaction

Transition metal oxides are of high interest in both energy storage (batteries) and production of non-fossil fuels by (photo)electrocatalysis. Their functionally crucial charge (oxidation state) changes and electrocatalytic properties are best investigated under electrochemical operation conditions. We established operando Raman spectroscopy for investigation of the atomic structure and oxidation state of a non-crystalline, hydrated, and phosphate-containing Co oxide material (CoCat), which is an electrocatalyst for the oxygen evolution reaction (OER) at neutral pH and is structurally similar to LiCoO₂ of batteries. Raman spectra were collected at various sub-catalytic and catalytic electric potentials. ²H labeling suggests Co oxidation coupled to Co-OH deprotonation at catalytic potentials. ¹⁸O labeling supports O-O bond formation starting from terminally coordinated oxygen species. Two broad bands around 877 cm^-1 and 1077 cm^-1 are assigned to CoCat-internal H₂PO₄ ^-. Raman peaks corresponding to terminal oxide (Co=O) or reactive oxygen species were not detectable; 1000-1200 cm^-1 bands were instead assigned to two-phonon Raman scattering. At an increasingly positive potential, the intensity of the Raman bands decreased, which is unexpected and explained by self-absorption relating to CoCat electrochromism. A red-shift of the Co-O Raman bands with increasing potentials was described by four Gaussian bands of potential-dependent amplitudes. By linear combination of Raman band amplitudes, we can follow individually the Co(2+/3+) and Co(3+/4+) redox transitions, whereas previously published x-ray absorption spectroscopy analysis could determine only the averaged Co oxidation state. Our results show how electrochemical operando Raman spectroscopy can be employed as a potent analytical tool in mechanistic investigations on OER catalysis.

Cation insertion to break the activity/stability relationship for highly active oxygen evolution reaction catalyst

The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li₂IrO₃ which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/stability tradeoff normally encountered for OER catalysts.

Renewable hydrogen production from water will require understanding and improving the oxygen evolution reaction (OER) on catalyst surfaces. Here, authors report α-Li₂IrO₃ to transform into a hydrated birnessite phase under OER conditions that exhibits enhanced OER performances and durabilities.

New Versatile Synthetic Route for the Preparation of Metal Phosphate Decorated Hydrogen Evolution Photocatalysts

Photocatalytic hydrogen generation will benefit from the realization of more active but less expensive cocatalysts compared with noble metal counterparts. Herein we developed a universal vapor deposition method that selectively uses the thermal decomposition products of sodium hypophosphite as a phosphorus source for the fabrication of inexpensive and highly efficient metal phosphate (MPi) modified CdS nanorods. We find that the modification with a bimetal phosphate (i.e., 5 wt % NiCoPi) leads to an activity enhancement by a factor of approximately 52 in boosting visible-light-driven hydrogen evolution relative to the pristine CdS nanorods. The photocatalyst exhibits a high hydrogen generation rate of 13.44 mmol·g^-1·h^-1, which is much higher than that of its single metal counterparts (NiPi, 8.70 mmol·g^-1·h^-1; CoPi, 5.79 mmol·g^-1·h^-1) and 1 wt % Pt modified CdS (1.33 mmol·g^-1·h^-1). Its apparent quantum efficiency reaches 23.5% at 420 nm. Furthermore, it also shows remarkable photostability for eight consecutive cycles of photocatalytic activity tests with total reaction time of 24 h. The excellent photocatalytic performance of the photocatalyst is believed to be associated with the in situ formed Ni^ICoP and NiCo^IIIPi cocatalysts, which not only play an important role in photogenerated charge separation but also provide highly active catalytic reaction sites for the corresponding hydrogen evolution reaction and the sacrificial agent oxidation reaction.

Role of electrolyte composition on the acid stability of mixed-metal oxygen evolution catalysts

Acid stability in catalysts that promote the oxygen evolution reaction (OER) involves an interplay between electrolyte and catalyst composition, both of which must be judiciously selected in order to promote activity and durability.

Artificial Photosynthesis at Efficiencies Greatly Exceeding That of Natural Photosynthesis

Sunlight is an abundant energy source for a sustainable society. Indeed, photosynthetic organisms harness solar radiation to build the world around us by synthesizing energy-rich compounds from water and CO₂. However, numerous energy conversion bottlenecks in the natural system limits the overall efficiency of photosynthesis; the most efficient plants do not exceed solar storage efficiencies of 1%. Artificial photosynthetic solar-to-fuels cycles may occur at higher intrinsic efficiencies, but they typically terminate at hydrogen, with no process installed to complete the cycle for carbon fixation. This limitation may be overcome by interfacing solar-driven water splitting to H₂-oxidizing microorganisms. To this end, hybrid biological-inorganic constructs have been created to use sunlight, air, and water as the only starting materials to accomplish carbon fixation in the form of biomass and liquid fuels. This artificial photosynthetic cycle begins with the Artificial Leaf, which accomplishes the solar process of natural photosynthesis-the splitting of water to hydrogen and oxygen using sunlight-under ambient conditions. To create the Artificial Leaf, an oxygen evolving complex of Photosystem II was mimicked, the most important property of which was the self-healing nature of the catalyst. Self-healing catalysts permit water splitting to be accomplished using any water source, which is the critical development for (1) the Artificial Leaf, as it allows for the facile interfacing of water splitting catalysis to materials such as silicon, and (2) the hybrid biological-inorganic construct, called the Bionic Leaf, as it allows for the facile interfacing of water splitting catalysis to bioorganisms. Hydrogenases in the bioorganism allow the hydrogen to be coupled to NADPH and ATP production, thus allowing the solar energy from water splitting to be converted into cellular energy to drive cellular biosynthesis. In the design of the hybrid system, water splitting catalysts must be designed that support hydrogen generation at low applied potential to ensure a high energy efficiency while avoiding reactive oxygen species. Using the tools of synthetic biology, a bioengineered bacterium, Ralstonia eutropha, converts carbon dioxide from air, along with the hydrogen produced from such catalysts of the Artificial Leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. The Bionic Leaf operates at solar-to-biomass and solar-to-liquid fuels efficiencies that greatly exceed the highest solar-to-biomass efficiencies of natural photosynthesis.

Morphological and Structural Evolution of Co3O4 Nanoparticles Revealed by in Situ Electrochemical Transmission Electron Microscopy during Electrocatalytic Water Oxidation

Unveiling the mechanism of electrocatalytic processes is fundamental for the search of more efficient and stable electrode materials for clean energy conversion devices. Although several in situ techniques are now available to track structural changes during electrocatalysis, especially of water oxidation, a direct observation, in real space, of morphological changes of nanostructured electrocatalysts is missing. Herein, we implement an in situ electrochemical Transmission Electron Microscopy (in situ EC-TEM) methodology for studying electrocatalysts of the oxygen evolution reaction (OER) during operation, by using model cobalt oxide Co₃O₄ nanoparticles. The observation conditions were optimized to mimic standard electrochemistry experiments in a regular electrochemical cell, allowing cyclic voltammetry and chronopotentiometry to be performed in similar conditions in situ and ex situ. This in situ EC-TEM method enables us to observe the chemical, morphological, and structural evolutions occurring in the initial nanoparticle-based electrode exposed to different aqueous electrolytes and under OER conditions. The results show that surface amorphization occurs, yielding a nanometric cobalt (oxyhydr)oxide-like phase during OER. This process is irreversible and occurs to an extent that has not been described before. Furthermore, we show that the pH and counterions of the electrolytes impact this restructuration, shedding light on the materials properties in neutral phosphate electrolytes. In addition to the structural changes followed in situ during the electrochemical measurements, this study demonstrates that it is possible to rely on in situ electrochemical TEM to reveal processes in electrocatalysts while preserving a good correlation with ex situ regular electrochemistry.

Selective Production of Oxygen from Seawater by Oxidic Metallate Catalysts

Although the emphasis of water splitting is typically on hydrogen generation, there is a value in the oxygen byproduct especially for life support in field operations. For such applications, the production of a pure, unadulterated oxygen stream is highly desired under environmental conditions. Here, we demonstrate that self-healing oxygen evolution catalysts composed of cobalt or nickel are capable of selectively producing oxygen from both 0.5 M NaCl solutions and seawater. Differential electrochemical mass spectrometry demonstrates the absence of halogen in the product stream, and chemical analysis shows the production of only minute amounts of hypohalous acid.

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while ¹⁶O-to-¹⁸O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co^II/III and Co^III/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co^IV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

Structural and functional role of anions in electrochemical water oxidation probed by arsenate incorporation into cobalt-oxide materials

Arsenate ions are incorporated in amorphous cobalt oxide catalysts at the periphery of the lattice or substituting cobalt ions.