Legitimate intermediates of oxygen evolution on iridium oxide revealed by in situ electrochemical evanescent wave spectroscopy

Iridium-Cooperated, Symmetry-Broken Manganese Oxide Nanocatalyst for Water Oxidation

The water oxidation reaction, the most important reaction for hydrogen production and other sustainable chemistry, is efficiently catalyzed by the Mn4CaO5 cluster in biological photosystem II. However, synthetic Mn-based heterogeneous electrocatalysts exhibit inferior catalytic activity at neutral pH under mild conditions. Symmetry-broken Mn atoms and their cooperative mechanism through efficient oxidative charge accumulation in biological clusters are important lessons but synthesis strategies for heterogeneous electrocatalysts have not been successfully developed. Here, we report a crystallographically distorted Mn-oxide nanocatalyst, in which Ir atoms break the space group symmetry from I41/amd to P1. Tetrahedral Mn(II) in spinel is partially replaced by Ir, surprisingly resulting in an unprecedented crystal structure. We analyzed the distorted crystal structure of manganese oxide using TEM and investigated how the charge accumulation of Mn atoms is facilitated by the presence of a small amount of Ir.

Electrocatalytic on-site oxygenation for transplanted cell-based-therapies

Implantable cell therapies and tissue transplants require sufficient oxygen supply to function and are limited by a delay or lack of vascularization from the transplant host. Previous exogenous oxygenation strategies have been bulky and had limited oxygen production or regulation. Here, we show an electrocatalytic approach that enables bioelectronic control of oxygen generation in complex cellular environments to sustain engineered cell viability and therapy under hypoxic stress and at high cell densities. We find that nanostructured sputtered iridium oxide serves as an ideal catalyst for oxygen evolution reaction at neutral pH. We demonstrate that this approach exhibits a lower oxygenation onset and selective oxygen production without evolution of toxic byproducts. We show that this electrocatalytic on site oxygenator can sustain high cell loadings (>60k cells/mm3) in hypoxic conditions in vitro and in vivo. Our results showcase that exogenous oxygen production devices can be readily integrated into bioelectronic platforms, enabling high cell loadings in smaller devices with broad applicability.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Electrochemical Preparation of Crystalline Hydrous Iridium Oxide and Its Use in Oxygen Evolution Catalysis

Even the most stable Ir-based oxides inevitably encounter a severe degradation problem during the oxygen evolution reaction (OER) in acid, resulting in quick formation of amorphous IrO_x layers on the catalyst surface. Unfortunately, there is still a lack of fundamental understanding of such hydrous IrO_x layers, including the atomic arrangement, key active structure, compositions, chemical stability, and so on. In this work, we demonstrate an electrochemical strategy to prepare two types of protonated iridium oxides with well-defined crystalline structures: one possesses a 2D layered structure (denoted as α-H_xIrO₃) and the other consists of 3D interconnected polymorphs (denoted as β-H_xIrO₃). Both protonated iridium oxides demonstrate superior electrochemical stabilities with 6 times suppressed Ir dissolution comparing to the initial Li₂IrO₃ and rutile IrO₂. It is hypothesized that the enriched protons and fast diffusions in these two protonated H_xIrO₃ crystal oxides may promote surface structural stability by suppressing the formation of high-valence Ir species at the solid-liquid interfaces during OER. Overall, the results of this work shed light on the role of proton dynamics toward the OER processes on the catalyst surface in acid media.

Time-Resolved Vibrational and Electronic Spectroscopy for Understanding How Charges Drive Metal Oxide Catalysts for Water Oxidation

Temporally resolved spectroscopy is a powerful approach for gaining detailed mechanistic understanding of water oxidation at robust Earth-abundant metal oxide catalysts for guiding efficiency improvement of solar fuel conversion systems. Beyond detecting and structurally identifying surface intermediates by vibrational and accompanying optical spectroscopy, knowledge of how charges, sequentially delivered to the metal oxide surface, drive the four-electron water oxidation cycle is critical for enhancing catalytic efficiency. Key issues addressed in this Perspective are the experimental requirements for establishing the kinetic relevancy of observed surface species and the discovery of the rate-boosting role of encounters of two or more one-electron surface hole charges, often in the form of randomly hopping metal oxo or oxyl moieties, for accessing very low-barrier O-O bond-forming pathways. Recent spectroscopic breakthroughs of metal oxide photo- and electrocatalysts inspire future research poised to take advantage of new highly sensitive spectroscopic tools and of methods for fast catalysis triggering.

Photocatalytic Overall Water Splitting Under Visible Light Enabled by a Particulate Conjugated Polymer Loaded with Palladium and Iridium

Polymer photocatalysts have received growing attention in recent years for photocatalytic hydrogen production from water. Most studies report hydrogen production with sacrificial electron donors, which is unsuitable for large-scale hydrogen energy production. Here we show that the palladium/iridium oxide-loaded homopolymer of dibenzo[b,d]thiophene sulfone (P10) facilitates overall water splitting to produce stoichiometric amounts of H2 and O2 for an extended period (>60 hours) after the system stabilized. These results demonstrate that conjugated polymers can act as single component photocatalytic systems for overall water splitting when loaded with suitable co-catalysts, albeit currently with low activities. Transient spectroscopy shows that the IrO2 co-catalyst plays an important role in the generation of the charge separated state required for water splitting, with evidence for fast hole transfer to the co-catalyst.

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir₂(pyalc)₂(H₂O)₄-(μ-O)]²⁺ (Ir_Molecular, pyalc = 2-(2′pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrO_x) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)–Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for Ir_Molecular contrasting with potential-dependent kinetics for IrO_x. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of Ir_Molecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

Progress of Nonprecious-Metal-Based Electrocatalysts for Oxygen Evolution in Acidic Media

Water oxidation, or the oxygen evolution reaction (OER), which combines two oxygen atoms from two water molecules and releases one oxygen molecule, plays the key role by providing protons and electrons needed for the hydrogen generation, electrochemical carbon dioxide reduction, and nitrogen fixation. The multielectron transfer OER process involves multiple reaction intermediates, and a high overpotential is needed to overcome the sluggish kinetics. Among the different water splitting devices, proton exchange membrane (PEM) water electrolyzer offers greater advantages. However, current anode OER electrocatalysts in PEM electrolyzers are limited to precious iridium and ruthenium oxides. Developing highly active, stable, and precious-metal-free electrocatalysts for water oxidation in acidic media is attractive for the large-scale application of PEM electrolyzers. In recent years, various types of precious-metal-free catalysts such as carbon-based materials, earth-abundant transition metal oxides, and multiple metal oxide mixtures have been investigated and some of them show promising activity and stability for acidic OER. In this review, the thermodynamics of water oxidation, Pourbaix diagram of metal elements in aqueous solution, and theoretical screening and prediction of precious-metal-free electrocatalysts for acidic OER are first elaborated. The catalytic performance, reaction kinetics, and mechanisms together with future research directions regarding acidic OER are summarized and discussed.

High crystallinity design of Ir-based catalysts drives catalytic reversibility for water electrolysis and fuel cells

The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation. Applying a reversible multifunctional electrocatalyst to the hydrogen electrode is a practical solution. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrO_x at potentials > 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrO_x layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Our analysis reveals that a metallic Ir subsurface under thin IrO_x layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design.

Reversible multifunctionality in electrocatalysts can allow voltage reversal during device operation. Here, authors design a crystalline Ir-based electrocatalyst with a thin reversible metallic-Ir/IrO_x layer that shows activity for O₂ evolution, H₂ evolution, and H₂ oxidation.

Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy

Electrochemical water splitting is a promising means to produce eco-friendly hydrogen fuels. Inspired by the Mn₄ CaO₅ cluster in nature, substantial works have been performed to develop efficient manganese (Mn)-based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn-oxo moieties. We identify water oxidation intermediates on the surface of Mn₃ O₄ nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential-dependent Raman peak was detected at 760 cm^-1 and assigned to the active Mn^IV =O species generated during water oxidation. Isotope-labeling experiments combined with scavenger experiments confirmed the generation of surface terminal Mn^IV =O intermediates in the Mn-oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.

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.

Rational Design of an Iridium-Tungsten Composite with an Iridium-Rich Surface for Acidic Water Oxidation

Developing highly active and stable water oxidation catalysts with reduced cost in acidic media plays a critical role in clean energy technologies such as fuel cells and electrolyzers. Precious iridium-based oxides are still the only oxygen evolution reaction (OER) catalysts with reasonable activity and stability in acid. Herein, we design iridium-tungsten composites with a metallic tungsten-rich core and an iridium-rich surface by the sol-gel method followed by hydrogen reduction. The thus obtained iridium-tungsten catalyst shows much higher intrinsic water oxidation activity (100 mA/mg_Ir at an overpotential of 290 mV) and stability (100 h at 10 mA/cm²_geom) together with reduced iridium content (33 wt % only) as compared with pure iridium oxide. An operando method using H₂O₂ as a probe molecule is developed to determine the relative adsorption strength of the reaction intermediates (*OH and *OOH) in the OER process. Detailed characterization shows that the tungsten-incorporated surface not only modulates the adsorption energy of oxygen intermediates on iridium but also enhances the stability of iridium species in acid, while the metallic tungsten core exhibits high electrical conductivity, all of which collectively give rise to the much enhanced catalytic performance of iridium-tungsten composite in acidic water oxidation. A single-membrane electrode assembly is further prepared to demonstrate the advantages and potential application of iridium-tungsten composite in practical proton exchange membrane electrolyzers.

An Ir/Ni(OH)2 Heterostructured Electrocatalyst for the Oxygen Evolution Reaction: Breaking the Scaling Relation, Stabilizing Iridium(V), and Beyond

Developing efficient electrocatalysts for the oxygen evolution reaction (OER) is highly challenging for hydrogen production from water splitting, due to the high energy barrier for OO bond formation and the restriction of the scaling relation between the multiple reaction intermediates. In order to simultaneously address these concerns, an Ir/Ni(OH)₂ heterostructure with abundant heterointerfaces is deliberately designed as an efficient electrocatalyst system, with Ir nanoparticles (NPs) homogeneously confined on the Ni(OH)₂ nanosheets. The strong electronic interaction and chemical bonding across the interface between the Ir and Ni(OH)₂ can effectively stabilize the metastable electrophilic Ir(V) species, which is vital to boost the formation of OO bonds. Meanwhile, the adsorption of the multiple intermediates is synergistically optimized at the heterointerface, which breaks the restrictive scaling relation and substantially accelerates the OER kinetics. In addition, the severe agglomeration of Ir species is greatly mitigated by the confinement effect, ensuring the structural integrity of the catalyst and the constant exposure of active sites. Owing to its well-defined multifunctional interfaces, the Ir/Ni(OH)₂ heterostructure exhibits exceptional OER activity and durability in alkaline media. The present results highlight the significance of heterostructure interface engineering toward the rational design and development of advanced electrocatalysts for the OER and beyond.

Water oxidation intermediates on iridium oxide electrodes probed by in situ electrochemical SHINERS

Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is applied to the study of a state-of-the-art water oxidation electrocatalyst, IrO_x, during oxygen evolution.

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.

Evidence for tetranuclear bis-μ-oxo cubane species in molecular iridium-based water oxidation catalysts from XAS analysis

Latest EXAFS results suggest a ‘dimer-of-dimers’ as the dominant resting state of Ir-pyalk WOCs in aqueous solution.

Element strategy of oxygen evolution electrocatalysis based on in situ spectroelectrochemistry

Oxygen evolution electrocatalysis has received extensive attention due to its significance in biology, chemistry, and technology. However, it is still unclear how the abundant 3d-elements can be used to drive the four-electron oxidation of water as efficiently as in Nature. In this Feature Article, we will propose a design strategy concerning the optimization of the charge accumulation process based on our ongoing spectroelectrochemical study on Mn, Fe, and Ir oxygen evolution catalysts. Spectroscopic identification of the reaction intermediates showed that the activity of MnO₂ and Fe₂O₃ was dictated by the generation of Mn³⁺ and Fe⁴⁺, whereas in the case of IrO_x, the activity did not correlate with the valence change of Ir. The efficiency of charge accumulation through valence change is closely linked with the spin configuration of the metal center, because charge disproportionation, which was found to inhibit charge accumulation in the high-spin 3d metals, requires an electron in the e_g orbital. In addition to directly increasing the overpotential through the generation of an unstable intermediate, charge disproportionation inhibits charge accumulation by dissipating the total oxidative energy of the system. A favorable charge accumulation process may also be beneficial for electrode kinetics due to the enhanced coupling between reaction rates and electrochemical driving force. The model proposed in this study may help explain why low-spin 4d/5d rare metals are often more active than the abundant high-spin 3d materials for multi-electron transfer reactions in general, and provides new insight into how active 3d-metal catalysts can be synthesized by optimizing the energetics of both bond formation and charge accumulation.