Exceptionally active iridium evolved from a pseudo-cubic perovskite for oxygen evolution in acid

Fundamental understanding of the acidic oxygen evolution reaction: mechanism study and state-of-the-art catalysts

The oxygen evolution reaction (OER), as the anodic reaction of water electrolysis (WE), suffers greatly from low reaction kinetics and thereby hampers the large-scale application of WE. Seeking active, stable, and cost-effective OER catalysts in acidic media is therefore of great significance. In this perspective, studying the reaction mechanism and exploiting advanced anode catalysts are of equal importance, where the former provides guidance for material structural engineering towards a better catalytic activity. In this review, we first summarize the currently proposed OER catalytic mechanisms, i.e., the adsorbate evolution mechanism (AEM) and lattice oxygen evolution reaction (LOER). Subsequently, we critically review several acidic OER electrocatalysts reported recently, with focus on structure-performance correlation. Finally, a few suggestions on exploring future OER catalysts are proposed.

Constructing an Adaptive Heterojunction as a Highly Active Catalyst for the Oxygen Evolution Reaction

Electrochemical water splitting is of prime importance to green energy technology. Particularly, the reaction at the anode side, namely the oxygen evolution reaction (OER), requires a high overpotential associated with OO bond formation, which dominates the energy-efficiency of the whole process. Activating the anionic redox chemistry of oxygen in metal oxides, which involves the formation of superoxo/peroxo-like (O₂ )^{n -} , commonly occurs in most highly active catalysts during the OER process. In this study, a highly active catalyst is designed: electrochemically delithiated LiNiO₂ , which facilitates the formation of superoxo/peroxo-like (O₂ )^{n -} species, i.e., NiOO*, for enhancing OER activity. The OER-induced surface reconstruction builds an adaptive heterojunction, where NiOOH grows on delithiated LiNiO₂ (delithiated-LiNiO₂ /NiOOH). At this junction, the lithium vacancies within the delithiated LiNiO₂ optimize the electronic structure of the surface NiOOH to form stable NiOO* species, which enables better OER activity. This finding provides new insight for designing highly active catalysts with stable superoxo-like/peroxo-like (O₂ )^{n -} for water oxidation.

Assembly of a Highly Active Iridium-Based Oxide Oxygen Evolution Reaction Catalyst by Using Metal-Organic Framework Self-Dissolution

The proton exchange membrane (PEM) electrolyzer for hydrogen production has multiple advantages but is greatly restricted by expensive iridium and sluggish oxygen evolution reaction (OER) kinetics. The most promising way to reduce the precious metal loading is to design and develop highly active Ir-based catalysts. In this study, a versatile approach is reported to prepare a hybrid in the form of a catalyst-support structure (Fe-IrO_x@α-Fe₂O₃, abbreviated Ir@Fe-MF) by utilizing the self-dissolving properties of Fe-MIL-101 under aqueous conditions. The formation of this hybrid is mainly due to the Ir⁴⁺ and released Fe³⁺ ions coprecipitated to assemble into Fe-IrO_x nanoparticles, and the Fe³⁺ released from the inward collapse of the metal-organic framework (MOF) spontaneously forms α-Fe₂O₃. The prepared Ir@Fe-MF-2 hybrid exhibits enhanced catalytic activity toward OER with a lower onset potential and Tafel slop, and only 260 mV overpotential is required to drive the current density to reach 10 mA cm^-2. The performed characterizations clearly indicate that the IrO₆ coordination structure is changed significantly by Fe incorporated into the IrO₂ lattice. The performed X-ray adsorption spectra (XAS) provides evidence that Ir 5d orbital degeneracy is eliminated because of multiple orbitals being semi-occupied in the presence of Fe, which is mainly responsible for the enhancement of OER activity. These findings open an opportunity for the design and preparation of more efficient OER catalysts of transition metal oxides by utilization of the MOF materials. It should be highlighted that a long-term stability of this catalyst run at a high current density in acidic conditions still faces great challenges.

Toward Efficient Electrocatalytic Oxygen Evolution: Emerging Opportunities with Metallic Pyrochlore Oxides for Electrocatalysts and Conductive Supports

The design of active and stable electrocatalysts for oxygen evolution reaction is a key enabling step toward efficient utilization of renewable energy. Along with efforts to develop high-performance electrocatalysts for oxygen evolution reaction, pyrochlore oxides have emerged as highly active and stable materials that function as catalysts as well as conductive supports for hybrid catalysts. The compositional flexibility of pyrochlore oxide provides many opportunities to improve electrocatalytic performance by manipulating material structures and properties. In this Outlook, we first discuss the recent advances in developing metallic pyrochlore oxides as oxygen evolution catalysts, along with elucidation of their reaction mechanisms, and then introduce an emerging area of using pyrochlore oxides as conductive supports to design hybrid catalysts to further improve the OER activity. Finally, the remaining challenges and emerging opportunities for pyrochlore oxides as electrocatalysts and conductive supports are discussed.

Vacancy-engineered catalysts for water electrolysis

Vacancy-engineered electrocatalysts showing various effects on improving performances toward water electrolysis.

A review on fundamentals for designing oxygen evolution electrocatalysts

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

Charge transfer and spillover phenomena in ceria-supported iridium catalysts: A model study

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO₂(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO₂(111) system are accompanied by charge transfer and partial reduction of CeO₂(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO₂(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO₂(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO₂(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO₂(111) represents the dominant reaction channel upon annealing in H₂ above 450 K. The interaction of Ir/CeO₂(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H₂. The oxidation of Ir nanoparticles under NAP conditions yields stable IrO_x nanoparticles. The stability of Ir and IrO_x nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

In Situ Observation of Nanoparticle Exsolution from Perovskite Oxides: From Atomic Scale Mechanistic Insight to Nanostructure Tailoring

Understanding and controlling the formation of nanoparticles at the surface of functional oxide supports is critical for tuning activity and stability for catalytic and energy conversion applications. Here, we use a latest generation environmental transmission electron microscope to follow the exsolution of individual nanoparticles at the surface of perovskite oxides, with ultrahigh spatial and temporal resolution. Qualitative and quantitative analysis of the data reveals the atomic scale processes that underpin the formation of the socketed, strain-inducing interface that confers exsolved particles their exceptional stability and reactivity. This insight also enabled us to discover that the shape of exsolved particles can be controlled by changing the atmosphere in which exsolution is carried out, and additionally, this could also produce intriguing heterostructures consisting of metal-metal oxide coupled nanoparticles. Our results not only provide insight into the in situ formation of nanoparticles but also demonstrate the tailoring of nanostructures and nanointerfaces.

Enhanced Iridium Mass Activity of 6H-Phase, Ir-Based Perovskite with Nonprecious Incorporation for Acidic Oxygen Evolution Electrocatalysis

One of the key objectives in PEM electrolysis technology is to reduce iridium loading and to improve iridium mass activity at the side of oxygen evolution electrocatalysis. 6H-phase, Ir-based perovskite (6H-SrIrO₃) is known to be a promising alternative to the IrO₂ catalyst, and developing effective strategies to further enhance its catalytic performance is needed. Here we present that a significant enhancement in electrocatalytic activity for the oxygen evolution reaction of 6H-SrIrO₃ can be achieved by cobalt incorporation. A suitable amount of cobalt dopants results in a decreased formation temperature of 6H-SrIrO₃ from 700 to 500 °C and thereby a decreased thickness of platelike particles for the material. Besides the morphological effect, the cobalt incorporation also increases the coverage of surface hydroxyl groups, regulates the Ir-O bond covalency, and modulates the oxygen p-band center of the material. This synergistic optimization of the morphological, surface, and electronic structures makes the cobalt-doped 6H-SrIrO₃ catalyst give a 3-fold increase in iridium mass activity for oxygen evolution reaction in comparison with the undoped 6H-SrIrO₃ under acidic conditions.

Intercalated Iridium Diselenide Electrocatalysts for Efficient pH-Universal Water Splitting

Developing bifunctional catalysts for both hydrogen and oxygen evolution reactions is a promising approach to the practical implementation of electrocatalytic water splitting. However, most of the reported bifunctional catalysts are only applicable to alkaline electrolyzer, although a few are effective in acidic or neutral media that appeals more to industrial applications. Here, a lithium-intercalated iridium diselenide (Li-IrSe₂ ) is developed that outperformed other reported catalysts toward overall water splitting in both acidic and neutral environments. Li intercalation activated the inert pristine IrSe₂ via bringing high porosities and abundant Se vacancies for efficient hydrogen and oxygen evolution reactions. When Li-IrSe₂ was assembled into two-electrode electrolyzers for overall water splitting, the cell voltages at 10 mA cm^-2 were 1.44 and 1.50 V under pH 0 and 7, respectively, being record-low values in both conditions.

Perovskite Oxide Based Electrodes for High-Performance Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an attractive strategy for the large-scale production of renewable hydrogen from water. Developing cost-effective, active and stable semiconducting photoelectrodes is extremely important for achieving PEC water splitting with high solar-to-hydrogen efficiency. Perovskite oxides as a large family of semiconducting metal oxides are extensively investigated as electrodes in PEC water splitting owing to their abundance, high (photo)electrochemical stability, compositional and structural flexibility allowing the achievement of high electrocatalytic activity, superior sunlight absorption capability and precise control and tuning of band gaps and band edges. In this review, the research progress in the design, development, and application of perovskite oxides in PEC water splitting is summarized, with a special emphasis placed on understanding the relationship between the composition/structure and (photo)electrochemical activity.