In situ observation of reactive oxygen species forming on oxygen-evolving iridium surfaces

Oxygen K-edge X-ray Absorption Spectra

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Kinetics of the Thermal Oxidation of Ir(100) toward IrO2 Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy

Using time-lapsed ambient-pressure X-ray photoelectron spectroscopy, we investigate the thermal oxidation of single-crystalline Ir(100) films toward rutile IrO₂(110) in situ. We initially observe the formation of a carbon-free surface covered with a complete monolayer of oxygen, based on the binding energies of the Ir 4f and O 1s core level peaks. During a rather long induction period with nearly constant oxygen coverage, the work function of the surface changes continuously as sensed by the gas phase O 1s signal. The sudden and rapid formation of the IrO₂ rutile phase with a thickness above 3 nm, manifested by distinct binding energy changes and substantiated by quantitative XPS analysis, provides direct evidence that the oxide film is formed via an autocatalytic growth mechanism that was previously proposed for PbO and RuO₂.

Liquefied Sunshine: Transforming Renewables into Fertilizers and Energy Carriers with Electromaterials

It has become apparent that renewable energy sources are plentiful in many, often remote, parts of the world, such that storing and transporting that energy has become the key challenge. For long-distance transportation by pipeline and bulk tanker, a liquid form of energy carrier is ideal, focusing attention on liquid hydrogen and ammonia. Development of high-activity and selectivity electrocatalyst materials to produce these energy carriers by reductive electrochemistry has therefore become an important area of research. Here, recent developments and challenges in the field of electrocatalytic materials for these processes are discussed, including the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the nitrogen reduction reaction (NRR). Some of the mis-steps currently plaguing the nitrogen reduction to ammonia field are highlighted. The rapidly growing roles that in situ/operando and quantum chemical studies can play in new electromaterials discovery are also surveyed.

New heteroleptic [Ni(ii) 1,1-dithiolate-phosphine] complexes: synthesis, characterization and electrocatalytic oxygen evolution studies

Four new heteroleptic Ni(ii) complexes with general formula [Ni(ii)(LL')] (L = 2-(methylene-1,1'-dithiolato)-5-phenylcyclohexane-1,3-dione (L1) and 2-(methylene-1,1'-dithiolato)-5,5'-dimethylcyclohexane-1,3-dione (L2); L' = 1,2-bis(diphenylphosphino)ethane (dppe) and bis(diphenylphosphino)monosulphide methane (dppms) have been synthesized and characterized by elemental analysis and spectroscopy (IR, UV-Vis, 1H, 13C{1H} and 31P{1H} NMR). All complexes 1-4 have also been characterized by PXRD and single crystal X-ray crystallography. The solid state molecular structures revealed distorted square planar geometry about the four-coordinate Ni(ii) metal centre together with rare NiH-C intra/intermolecular anagostic interactions in axial positions. In these complexes supramolecular structures have been sustained by non-covalent C-HO, C-OH-O, C-Hπ, C-Hπ (NiCS2, chelate), ππ and HH interactions. Their electrocatalytic properties have been investigated for oxygen evolution reaction (OER) in which complex 2 showed the highest activity with 10 mA cm-2 at the potential of 1.58 V vs. RHE. In addition, complex 2 also exhibits an OER onset potential at 1.52 V vs. RHE.

Influence of Surface Oxygen Vacancies and Ruthenium Valence State on the Catalysis of Pyrochlore Oxides

Proton exchange membrane (PEM) water electrolysis is a promising energy storage solution by electrochemically splitting water into hydrogen fuel and oxygen. However, the sluggish kinetics, high operating potential, and corrosive acidic environment during the oxygen evolution reaction (OER) require the use of scarce and costly Ir-based oxides, tremendously hampering its large-scale commercialization. Hence, developing active and stable anode catalysts with reduced precious-metal usage is desperately essential. For the first time, we report a group of Y_2-xBa_xRu₂O₇ pyrochlore oxides and employ them in acid OER and PEM electrolyzers. We reveal the mechanism for the promoted OER performance of Ba-doped Y₂Ru₂O₇ in which partially replacing Y³⁺ by Ba²⁺ in Y₂Ru₂O₇ greatly facilitates the hole-doping effect, which generates massive oxygen vacancy and multivalence of Ru⁵⁺/Ru⁴⁺, thus boosting the OER performance of Y_2-xBa_xRu₂O₇. This work provides an effective method and paradigm for improving the electrocatalytic property of pyrochlore oxides.

Metallic nanostructures with low dimensionality for electrochemical water splitting

Metallic nanostructures with low dimensionality (one-dimension and two-dimension) possess unique structural characteristics and distinctive electronic and physicochemical properties including high aspect ratio, high specific surface area, high density of surface unsaturated atoms and high electron mobility. These distinctive features have rendered them remarkable advantages over their bulk counterparts for surface-related applications, for example, electrochemical water splitting. In this review article, we highlight the recent research progress in low-dimensional metallic nanostructures for electrochemical water splitting including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fundamental understanding of the electrochemistry of water splitting including HER and OER is firstly provided from the aspects of catalytic mechanisms, activity descriptors and property evaluation metrics. Generally, it is challenging to obtain low-dimensional metallic nanostructures with desirable characteristics for HER and OER. We hereby introduce several typical methods for synthesizing one-dimensional and two-dimensional metallic nanostructures including organic ligand-assisted synthesis, hydrothermal/solvothermal synthesis, carbon monoxide confined growth, topotactic reduction, and templated growth. We then put emphasis on the strategies adopted for the design and fabrication of high-performance low-dimensional metallic nanostructures for electrochemical water splitting such as alloying, structure design, surface engineering, interface engineering and strain engineering. The underlying structure-property correlation for each strategy is elucidated aiming to facilitate the design of more advanced electrocatalysts for water splitting. The challenges and perspectives for the development of electrochemical water splitting and low-dimensional metallic nanostructures are also proposed.

Operando XANES from first-principles and its application to iridium oxide

Density-functional theory calculations augmented with a continuum description of the electrochemical environment are implemented to simulated X-ray absorption spectra as a function of the applied potential.

Shining light on the solid–liquid interface: in situ/operando monitoring of surface catalysis

Many industrially important chemical transformations occur at the interface between a solid catalyst and liquid reactants. In situ and operando spectroscopies offer unique insight into the reactivity of such catalytically active solid–liquid interfaces.

Nitrogen-Doped Porous Carbon Derived from Biomass Used as Trifunctional Electrocatalyst toward Oxygen Reduction, Oxygen Evolution and Hydrogen Evolution Reactions

Tremendous developments in energy storage and conversion technologies urges researchers to develop inexpensive, greatly efficient, durable and metal-free electrocatalysts for tri-functional electrochemical reactions, namely oxygen reduction reactions (ORRs), oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs). In these regards, this present study focuses upon the synthesis of porous carbon (PC) or N-doped porous carbon (N-PC) acquired from golden shower pods biomass (GSB) via solvent-free synthesis. Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies confirmed the doping of nitrogen in N-PC. In addition, morphological analysis via field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) provide evidence of the sheet-like porous structure of N-PC. ORR results from N-PC show the four-electron pathway (average n = 3.6) for ORRs with a Tafel slope of 86 mV dec^-1 and a half-wave potential of 0.76 V. For OERs and HERs, N-PC@Ni shows better overpotential values of 314 and 179 mV at 10 mA cm^-2, and its corresponding Tafel slopes are 132 and 98 mV dec^-1, respectively. The chronopotentiometry curve of N-PC@Ni reveals better stability toward OER and HER at 50 mA cm^-2 for 8 h. These consequences provide new pathways to fabricate efficient electrocatalysts of metal-free heteroatom-doped porous carbon from bio-waste/biomass for energy application in water splitting and metal air batteries.

The Role of Surface Hydroxylation, Lattice Vacancies and Bond Covalency in the Electrochemical Oxidation of Water (OER) on Ni-Depleted Iridium Oxide Catalysts

The usage of iridium as an oxygen-evolution-reaction (OER) electrocatalyst requires very high atom efficiencies paired with high activity and stability. Our efforts during the past 6 years in the Priority Program 1613 funded by the Deutsche Forschungsgemeinschaft (DFG) were focused to mitigate the molecular origin of kinetic overpotentials of Ir-based OER catalysts and to design new materials to achieve that Ir-based catalysts are more atom and energy efficient, as well as stable. Approaches involved are: (1) use of bimetallic mixed metal oxide materials where Ir is combined with cheaper transition metals as starting materials, (2) use of dealloying concepts of nanometer sized core-shell particle with a thin noble metal oxide shell combined with a hollow or cheap transition metal-rich alloy core, and (3) use of corrosion-resistant high-surface-area oxide support materials. In this mini review, we have highlighted selected advances in our understanding of Ir–Ni bimetallic oxide electrocatalysts for the OER in acidic environments.

Rational Design of Rhodium-Iridium Alloy Nanoparticles as Highly Active Catalysts for Acidic Oxygen Evolution

The oxygen evolution reaction (OER) is pivotal for renewable energy conversion and storage devices, such as water electrolyzers and rechargeable metal-air batteries. However, the rational design of electrocatalysts with suitably high efficiencies and stabilities in strongly acidic electrolytes remains a significant challenge. Here, we show the demonstration of sub-10 nm, composition-tunable Rh-Ir alloy nanoparticles (NPs) prepared using a scalable microwave-assisted method as superior acidic OER catalysts. The OER activities showed a volcano-shaped dependence on Ir composition, with Ir-rich NPs (Ir ≥ 51%) achieving better OER performance than pure Ir NPs, as reflected by lower overpotentials and higher mass activities. Most significantly, Rh₂₂Ir₇₈ NPs achieved a maximum mass activity of 1.17 A mg^-1_Ir at a 300 mV overpotential in 0.5 M H₂SO₄, which corresponds to a 3-fold enhancement relative to pure Ir NPs, making it one of the most active reported OER catalysts under acidic conditions. Density functional theory calculations reveal that owing to the synergy of ensemble and electronic effects by alloying a small amount of Rh with Ir, the binding energy difference of the O and OOH intermediates is reduced, leading to faster kinetics and enhanced OER activity. Furthermore, Rh-Ir alloy NPs demonstrated excellent durability in strongly acidic electrolyte. This work not only provides fundamental understandings relating to composition-electrochemical performance relationships but also represents the rational design of highly efficient OER electrocatalysts for applications in acidic media.

Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy

Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1-2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area.

Ambient Fast Synthesis and Active Sites Deciphering of Hierarchical Foam-Like Trimetal-Organic Framework Nanostructures as a Platform for Highly Efficient Oxygen Evolution Electrocatalysis

Metal-organic frameworks (MOFs) have attracted tremendous interest due to their promising applications including electrocatalysis originating from their unique structural features. However, it remains a challenge to directly use MOFs for oxygen electrocatalysis because it is quite difficult to manipulate their dimension, composition, and morphology of the MOFs with abundant active sites. Here, a facile ambient temperature synthesis of unique NiCoFe-based trimetallic MOF nanostructures with foam-like architecture is reported, which exhibit extraordinary oxygen evolution reaction (OER) activity as directly used catalyst in alkaline condition. Specifically, the (Ni₂ Co₁ )_0.925 Fe_0.075 -MOF-NF delivers a minimum overpotential of 257 mV to reach the current density of 10 mA cm^-2 with a small Tafel slope of 41.3 mV dec^-1 and exhibits high durability after long-term testing. More importantly, the deciphering of the possible origination of the high activity is performed through the characterization of the intermediates during the OER process, where the electrochemically transformed metal hydroxides and oxyhydroxides are confirmed as the active species.

IrW nanobranches as an advanced electrocatalyst for pH-universal overall water splitting

Developing highly efficient and durable electrocatalysts for overall water splitting over a wide pH range is of great interest for practical applications, but still remains a challenge. Specifically, to the best of our knowledge, a 3-in-1 electrocatalyst that can efficiently catalyze overall water splitting in acidic, alkaline, and neutral electrolytes has not been reported so far. Herein, we report the colloidal synthesis of well-dispersed IrW nanobranches with branch architectures and describe how the morphology varies with the amount of W doping. As expected, they exhibit outstanding catalytic performance and durability for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at all pH values, which are much higher than those of Ir nanoparticles (NPs), and most reported state-of-the-art electrocatalysts. More importantly, when further used as both an anode and cathode for overall water splitting in 0.1 M HClO4, 0.1 M KOH, and 1.0 M PBS (phosphate buffer solution), cell voltages of 1.58, 1.60, and 1.73 V, respectively, were achieved at a current density of 10 mA cm-2. The present work opens up a new avenue for designing electrocatalysts for pH-universal overall water splitting, especially for application in highly corrosive acidic media and neutral media with limited ionic concentrations.

Iridium-Based Catalysts for Solid Polymer Electrolyte Electrocatalytic Water Splitting

Chemical energy conversion/storage through water splitting for hydrogen production has been recognized as the ideal solution to the transient nature of renewable energy sources. Solid polymer electrolyte (SPE) water electrolysis is one of the most practical ways to produce pure H₂ . Electrocatalysts are key materials in the SPE water electrolysis. At the anode side, electrode materials catalyzing the oxygen evolution reaction (OER) require specific properties. Among the reported materials, only iridium presents high activity and is more stable. In this Minireview, an application overview of single iridium metal and its oxide catalysts-binary, ternary, and multicomponent catalysts of iridium oxides and supported composite catalysts-for the OER in SPE water electrolysis is presented. Two main strategies to improve the activity of an electrocatalyst system, namely, increasing the number of active sites and the intrinsic activity of each active site, are reviewed with detailed examples. The challenges and perspectives in this field are also discussed.

The Oxidation of Platinum under Wet Conditions Observed by Electrochemical X-ray Photoelectron Spectroscopy

During the electrochemical reduction of oxygen, platinum catalysts are often (partially) oxidized. While these platinum oxides are thought to play a crucial role in fuel cell degradation, their nature remains unclear. Here, we studied the electrochemical oxidation of Pt nanoparticles using in situ XPS. When the particles were sandwiched between a graphene sheet and a proton exchange membrane that is wetted from the back, a confined electrolyte layer was formed, allowing us to probe the electrocatalyst under wet conditions. We show that the surface oxide formed at the onset of Pt oxidation has a mixed Pt^δ+/Pt²⁺/Pt⁴⁺ composition. The formation of this surface oxide is suppressed when a Br-containing membrane is chosen due to adsorption of Br on Pt. Time-resolved measurements show that oxidation is fast for nanoparticles: even bulk PtO₂·nH₂O growth occurs on the subminute time scale. The fast formation of Pt⁴⁺ species in both surface and bulk oxide form suggests that Pt⁴⁺-oxides are likely formed (or reduced) even in the transient processes that dominate Pt electrode degradation.

Improved oxygen evolution activity of IrO2 by in situ engineering of an ultra-small Ir sphere shell utilizing a pulsed laser

Noble metal-based catalysts are vital electrocatalysts for the oxygen evolution reaction (OER), which is a half reaction among multiple renewable energy-related reactions. To fully exploit their potential as efficient OER catalysts, we developed a fast one-step strategy to engineer a unique nanostructure for the benchmark catalyst IrO2 utilizing an ultra-fast pulse laser, through which a shell of ultra-small Ir spheres with a diameter of ca. 2 nm is in situ engineered around the IrO2 core. The creation of the Ir sphere shell not only increases the electrochemical surface area, but also improves the electrical conductivity of the electrocatalyst. The as-engineered IrO2@Ir architecture exhibits extremely high electrocatalytic activity towards the OER, revealing an overpotential of 255 mV at 10 mA cm-2 and Tafel slope of 45 mV dec-1. These values are much lower than those observed for the unmodified structure. Furthermore, the catalytic performance is the best among all the noble metal-based OER catalysts. This work may open a new avenue to efficiently improve the catalytic activity of noble metal-based catalysts and significantly advance the development in the energy industry.

Honeycomb-Like Nitrogen-Doped Carbon 3D Nanoweb@Li2 S Cathode Material for Use in Lithium Sulfur Batteries

Current lithium-ion batteries have a low theoretical capacity that is insufficient for use in emerging electric vehicles and energy-storage systems. The development of lithium-sulfur batteries utilizing Li₂ S cathodes would be a promising option to overcome the capacity limitation. In this work, new three-dimensional (3D) honeycomb-like N-doped carbon nanowebs (HCNs) have been synthesized through a facile aqueous solution route for use as a cathode material in lithium-sulfur batteries. The Li₂ S@HCNs cathode delivers a high discharge capacity of approximately 815 mAh g^-1 after 65 cycles at 0.1 C, along with a superior rate capacity of approximately 568 mAh g^-1 even at 2 C. The outstanding electrochemical rate performance is ascribed to their unique 3D honeycomb-like nanoweb structure, consisting of nanowires derived from polypyrrole. These properties greatly enhance the electrochemical reaction kinetics by providing efficient electron pathways and hollow channels for electrolyte transport. Nitrogen doping in the carbon nanowebs also considerably improves the chemisorption properties by tuning affinity between sulfur and oxygen functional groups on the carbon framework. The simple synthesis strategy and the resulting unique electrode structure could present a new avenue in nanostructure research for high-performance lithium-sulfur batteries.

Silicon based photoelectrodes for photoelectrochemical water splitting

Solar water splitting using Si photoelectrodes in photoelectrochemical (PEC) cells offers a promising approach to convert sunlight into sustainable hydrogen energy, which has recently received intense research. This review summarizes the recent advances in the development of efficient and stable Si photoelectrodes for solar water splitting. The definition and representation of efficiency and stability for Si photoelectrodes are firstly introduced. We then present several basic strategies for designing highly efficient and stable Si photoelectrodes, including surface textures, protective layer, catalyst loading and the integration of the system. Finally, we highlight the progress that has been made in Si photocathodes and Si photoanodes, respectively, with emphasis on how to integrate Si with protective layer and catalyst.

Evolution of Oxygen-Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X-Ray Spectroscopy

Manganese oxide (MnO_x ) electrocatalysts are examined herein by in situ soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L-edge XAS data with previous mechanistic studies indicates Mn^IV is the highest oxidation state involved in the catalytic mechanism. MnO_x is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn-O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K-edge provide complementary evidence for such a transition. This step is crucial for the formation of O₂ from water.

Uncovering The Role of Oxygen in Ni-Fe(OxHy) Electrocatalysts using In situ Soft X-ray Absorption Spectroscopy during the Oxygen Evolution Reaction

In-situ X-ray absorption spectroscopy (XAS) at the oxygen K-edge was used to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(O_xH_y) electrocatalyst in alkaline pH. We show the rise of a pre-peak feature at 529 eV in the O K-edge spectra, correlated to the appearance of a shoulder at the Ni L₃-edge and formation of oxidized Ni^3+/4+-O. Then, for the first time, we track the spectral changes in a dynamic fashion in both the soft and hard X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. The pre-peak feature at the O K-edge likely signifies formation of an electron deficient oxygen site. The electrophilic oxygen species appears and disappears reversibly in correlation with the Ni²⁺ ↔ Ni^3+/4+ process, and persists during OER catalysis as long the metal is oxidized. Our study provides new insight into OER electrocatalysis: Before onset of the O-O bond formation step, the catalytic oxyhydroxide has accumulated electron deficiencies by both, oxidation of transition metal ions and formation of partially oxidized oxygen sites.

Breaking Long-Range Order in Iridium Oxide by Alkali Ion for Efficient Water Oxidation

Oxygen electrochemistry plays a critical role in clean energy technologies such as fuel cells and electrolyzers, but the oxygen evolution reaction (OER) severely restricts the efficiency of these devices due to its slow kinetics. Here, we show that via incorporation of lithium ion into iridium oxide, the thus obtained amorphous iridium oxide (Li-IrO _x) demonstrates outstanding water oxidation activity with an OER current density of 10 mA/cm² at 270 mV overpotential for 10 h of continuous operation in acidic electrolyte. DFT calculations show that lithium incorporation into iridium oxide is able to lower the activation barrier for OER. X-ray absorption characterizations indicate that both amorphous Li-IrO _x and rutile IrO₂ own similar [IrO₆] octahedron units but have different [IrO₆] octahedron connection modes. Oxidation of iridium to higher oxidation states along with shrinkage in the Ir-O bond was observed by in situ X-ray absorption spectroscopy on amorphous Li-IrO _x, but not on rutile IrO₂ under OER operando conditions. The much more "flexible" disordered [IrO₆] octahedrons with higher oxidation states in amorphous Li-IrO _x as compared to the periodically interconnected "rigid" [IrO₆] octahedrons in crystalline IrO₂ are able to act as more electrophilic centers and thus effectively promote the fast turnover of water oxidation.

Vibrational sum-frequency generation spectroscopy of electrode surfaces: studying the mechanisms of sustainable fuel generation and utilisation

The electrocatalytic oxidation of water coupled to the reduction of carbon dioxide, to make carbon based products, or the reduction of protons to provide hydrogen, offers a sustainable route to generating useful fuels.

Efficient oxygen evolution on mesoporous IrOx nanosheets

Amorphous iridium oxide (IrO_x) is a promising catalyst that has high activity in the oxygen evolution reaction (OER) over a broad range of pH values.

The fate of O2 in photocatalytic CO2 reduction on TiO2 under conditions of highest purity

Modification of P25-TiO₂ with IrO_x allowed the detection of gas-phase O₂ during photocatalytic CO₂ reduction with H₂O. The effect on the overall CO₂ conversion on P25 is discussed.

Ir-Ni Bimetallic OER Catalysts Prepared by Controlled Ni Electrodeposition on Irpoly and Ir(111)

The alteration of electrocatalytic surfaces with adatoms lead to structural and electronic modifications promoting adsorption, desorption, and reactive processes. This study explores the potentiostatic electrodeposition process of Ni onto polycrystalline Ir (Irpoly) and assesses the electrocatalytic properties of the resulting bimetallic surfaces. The electrodeposition resulted in bimetallic Ni overlayer (OL) structures and in combination with controlled thermal post-deposition annealing in bimetallic near-surface alloys (NSA). The catalytic oxygen evolution reaction (OER) activity of these two different Ni-modified catalysts is assessed and compared to a pristine, unmodified Irpoly. An overlayer of Ni on Irpoly showed superior performance in both acidic and alkaline milieu. The reductive annealing of the OL produced a NSA of Ni, which demonstrated enhanced stability in an acidic environment. The remarkable activity and stability improvement of Ir by Ni modification makes both systems efficient electrocatalysts for water oxidation. The roughness factor of Irpoly is also reported. With the amount of deposited Ni determined by inductively coupled plasma mass spectrometry (ICP-MS) and a degree of coverage (monolayer) in the dependence of deposition potential is established. The density functional theory (DFT) assisted evaluation of H adsorption on Irpoly enables determination of the preferred Ni deposition sites on the three low-index surfaces (111), (110), and (100).

Metallic Iridium Thin-Films as Model Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)—Morphology and Activity

Iridium (Ir) oxide is known to be one of the best electrocatalysts for the oxygen evolution reaction (OER) in acidic media. Ir oxide-based materials are thus of great scientific interest in current research on electrochemical energy conversion. In the present study, we applied Ir metal films as model systems for electrochemical water splitting, obtained by inductive heating in a custom-made setup using two different synthesis approaches. X-ray photoelectron spectroscopy (XPS) and selected area electron diffraction (SAED) confirmed that all films were consistently metallic. The effects of reductive heating time of calcined and uncalcined Ir acetate films on OER activity were investigated using a rotating disk electrode (RDE) setup. The morphology of all films was determined by scanning electron microscopy (SEM). The films directly reduced from the acetate precursor exhibited a strong variability of their morphology and electrochemical properties depending on heating time. The additional oxidation step prior to reductive heating accelerates the final structure formation.

Driving Surface Redox Reactions in Heterogeneous Photocatalysis: The Active State of Illuminated Semiconductor-Supported Nanoparticles during Overall Water-Splitting

Materials used for photocatalytic overall water splitting (POWS) are typically composed of light-absorbing semiconductor crystals, functionalized with so-called cocatalytic nanoparticles to improve the kinetics of the hydrogen and/or oxygen evolution reactions. While function, quantity, and protection of such metal(oxide) nanoparticles have been addressed in the literature of photocatalysis, the stability and transients in the active oxidation-state upon illumination have received relatively little attention. In this Perspective, the latest insights in the active state of frequently applied cocatalysts systems, including Pt, Rh/Cr₂O₃, or Ni/NiO_x, will be presented. While the initial morphology and oxidation state of such nanoparticles is a strong function of the applied preparation procedure, significant changes in these properties can occur during water splitting. We discuss these changes in relation to the nature of the cocatalyst/semiconductor interface. We also show how know-how of other disciplines such as heterogeneous catalysis or electro-catalysis and recent advances in analytical methodology can help to determine the active state of cocatalytic nanoparticles in photocatalytic applications.

Iridium-Tungsten Alloy Nanodendrites as pH-Universal Water-Splitting Electrocatalysts

The development of highly efficient and durable electrocatalysts for high-performance overall water-splitting devices is crucial for clean energy conversion. However, the existing electrocatalysts still suffer from low catalytic efficiency, and need a large overpotential to drive the overall water-splitting reactions. Herein, we report an iridium-tungsten alloy with nanodendritic structure (IrW ND) as a new class of high-performance and pH-universal bifunctional electrocatalysts for hydrogen and oxygen evolution catalysis. The IrW ND catalyst presents a hydrogen generation rate ∼2 times higher than that of the commercial Pt/C catalyst in both acid and alkaline media, which is among the most active hydrogen evolution reaction (HER) catalysts yet reported. The density functional theory (DFT) calculations reveal that the high HER intrinsic catalytic activity results from the suitable hydrogen and hydroxyl binding energies, which can accelerate the rate-determining step of the HER in acid and alkaline media. Moreover, the IrW NDs show superb oxygen evolution reaction (OER) activity and much improved stability over Ir. The theoretical calculation demonstrates that alloying Ir metal with W can stabilize the formed active iridium oxide during the OER process and lower the binding energy of reaction intermediates, thus improving the Ir corrosion resistance and OER kinetics. Furthermore, the overall water-splitting devices driven by IrW NDs can work in a wide pH range and achieve a current density of 10 mA cm^-2 in acid electrolyte at a low potential of 1.48 V.

Single-Shot near Edge X-ray Fine Structure (NEXAFS) Spectroscopy Using a Laboratory Laser-Plasma Light Source

We present a proof of principle experiment on single-shot near edge soft X-ray fine structure (NEXAFS) spectroscopy with the use of a laboratory laser-plasma light source. The source is based on a plasma created as a result of the interaction of a nanosecond laser pulse with a double stream gas puff target. The laser-plasma source was optimized for efficient soft X-ray (SXR) emission from the krypton/helium target in the wavelength range from 2 nm to 5 nm. This emission was used to acquire simultaneously emission and absorption spectra of soft X-ray light from the source and from the investigated sample using a grazing incidence grating spectrometer. NEXAFS measurements in a transmission mode revealed the spectral features near the carbon K-α absorption edge of thin polyethylene terephthalate (PET) film and L-ascorbic acid in a single-shot. From these features, the composition of the PET sample was successfully obtained. The NEXAFS spectrum of the L-ascorbic acid obtained in a single-shot exposure was also compared to the spectrum obtained a multi-shot exposure and to numerical simulations showing good agreement. In the paper, the detailed information about the source, the spectroscopy system, the absorption spectra measurements and the results of the studies are presented and discussed.

Operando Evidence for a Universal Oxygen Evolution Mechanism on Thermal and Electrochemical Iridium Oxides

Progress in the development of proton exchange membrane (PEM) water electrolysis technology requires decreasing the anode overpotential, where the sluggish multistep oxygen evolution reaction (OER) occurs. This calls for an understanding of the nature of the active OER sites and reaction intermediates, which are still being debated. In this work, we apply synchrotron radiation-based near-ambient pressure X-ray photoelectron and absorption spectroscopies under operando conditions in order to unveil the nature of the reaction intermediates and shed light on the OER mechanism on electrocatalysts most widely used in PEM electrolyzers-electrochemical and thermal iridium oxides. Analysis of the O K-edge and Ir 4f spectra backed by density functional calculations reveals a universal oxygen anion red-ox mechanism regardless of the nature (electrochemical or thermal) of the iridium oxide. The formation of molecular oxygen is considered to occur through a chemical step from the electrophilic O^I- species, which itself is formed in an electrochemical step.

Highly active nano-sized iridium catalysts: synthesis and operando spectroscopy in a proton exchange membrane electrolyzer

A stable and cost effective oxygen evolution reaction (OER) catalyst is crucial for the large-scale market penetration of proton exchange membrane (PEM) water electrolyzers. We show that the synthesis of iridium nanoparticles in either low purity ethanol or water, or in the absence of a surfactant, is detrimental to the electrocatalytic properties of the materials. Adding NaBH4 in excess improves the purity of the catalyst enhancing the OER activity up to 100 A gIr-1 at 1.51 V vs. RHE, the highest value reported so far for high purity Ir nanoparticles. The measured OER activity correlates with the capacitive current rather than with the charge corresponding to the IrIII/IrIV oxidation peak. Operando near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on membrane electrode assemblies (MEAs) with the synthesized catalysts reveals a metallic core surrounded by a thin layer of IrIII/IV oxides/hydroxides. Oxidation of IrIII leaves behind a porous ultrathin layer of IrIV oxides/hydroxides, which dominate the surface during the OER, while IrV was not detected.

Faradaic oxygen evolution from SrTiO3 under nano- and femto-second pulsed light excitation

During photocatalytic water oxidation, n-SrTiO₃(100) demonstrated near 100% Faradaic efficiency for O₂ evolution with nano- (30 ns) and femto- (150 fs) second pulsed laser excitation of the band gap, despite surface rearrangements attributed to the high peak power (300 MW cm^-2). Therefore, these results establish a methodology for tracking intermediates of the water oxidation cycle at the n-SrTiO₃(100) surface from the picosecond time scales of charge transfer through to the millisecond time scales of O₂ evolution.

Intercalation of nanostructured CeO2 in MgAl2O4 spinel illustrates the critical interaction between metal oxides and oxides

Heterogeneous catalytic oxidation arises from the prerequisite oxygen activation and transfer ability of metal oxide catalysts. Thus, engineering intercalated nanounits and heterophase metal oxide structures, and forming interstitial catalyst supports at the nanoscale level can drastically alter the catalytic performances of metal oxides. This is particularly important for ceria-based nanomaterial catalysts, where the interactions of reducible ceria (CeO₂) and nonreducible oxides are fundamental for the preparation of enhanced catalysts for oxygen-involved reactions. Herein, we intercalated nanostructured CeO₂ in the bulk phase of magnesium aluminate spinel (MgAl₂O₄, referred to as MgAl), produced the interstitial effect between CeO₂ nanoparticles and MgAl crystallites, thus boosting their oxygen transfer and activation capability. This nanoscaled intercalation engineering significantly enhanced the number and quality of tight contact points between the nanostructured CeO₂ and MgAl units. Therefore, the oxygen storage/release capability (OSC) is exceptionally improved as revealed by various characterizations and catalytic carbon oxidation reaction. A mechanism similar to the Mars-van Krevelen process at the nanoscale level was invoked to explain the catalytic oxidation mechanisms. The reactive oxygen species of gaseous O₂ originate formed the bulk of the as-obtained nanomaterial, where strong interactions between the CeO₂ and MgAl components occured, which were subsequently released and diffused to the catalyst-interface at elevated temperatures. Silver supported on Ce-MgAl produced an approximately 4-fold higher concentration of active oxygen species than Ag/MgAl, and gives the optimum low-temperature oxidation at 229 °C. This study verifies the importance of the redox performance of ceria-spinel with enhanced OSC, which validates that the arrangement of contacts at the nanoscale can substantially boost the catalytic reactivity without varying the microscale structure and properties of spinel.