Synergistic effects in oxygen evolution activity of mixed iridium-ruthenium pyrochlores

Electrocatalysis: From Planar Surfaces to Nanostructured Interfaces

The reactions critical for the energy transition center on the chemistry of hydrogen, oxygen, carbon, and the heterogeneous catalyst surfaces that make up electrochemical energy conversion systems. Together, the surface-adsorbate interactions constitute the electrochemical interphase and define reaction kinetics of many clean energy technologies. Practical devices introduce high levels of complexity where surface roughness, structure, composition, and morphology combine with electrolyte, pH, diffusion, and system level limitations to challenge our ability to deconvolute underlying phenomena. To make significant strides in materials design, a structured approach based on well-defined surfaces is necessary to selectively control distinct parameters, while complexity is added sequentially through careful application of nanostructured surfaces. In this review, we cover advances made through this approach for key elements in the field, beginning with the simplest hydrogen oxidation and evolution reactions and concluding with more complex organic molecules. In each case, we offer a unique perspective on the contribution of well-defined systems to our understanding of electrochemical energy conversion technologies and how wider deployment can aid intelligent materials design.

Recent Progress in Balancing the Activity, Durability, and Low Ir Content for Ir-Based Oxygen Evolution Reaction Electrocatalysts in Acidic Media

Proton exchange membrane (PEM) electrolysis faces challenges associated with high overpotential and acidic environments, which pose significant hurdles in developing highly active and durable electrocatalysts for the oxygen evolution reaction (OER). Ir-based nanomaterials are considered promising OER catalysts for PEM due to their favorable intrinsic activity and stability under acidic conditions. However, their high cost and limited availability pose significant limitations. Consequently, numerous studies have emerged aimed at reducing iridium content while maintaining high activity and durability. Furthermore, the research on the OER mechanism of Ir-based catalysts has garnered widespread attention due to differing views among researchers. The recent progress in balancing activity, durability, and low iridium content in Ir-based catalysts is summarized in this review, with a particular focus on the effects of catalyst morphology, heteroatom doping, substrate introduction, and novel structure development on catalyst performance from four perspectives. Additionally, the recent mechanistic studies on Ir-based OER catalysts is discussed, and both theoretical and experimental approaches is summarized to elucidate the Ir-based OER mechanism. Finally, the perspectives on the challenges and future developments of Ir-based OER catalysts is presented.

Recent advances in iridium-based catalysts with different dimensions for the acidic oxygen evolution reaction

Proton exchange membrane (PEM) water electrolysis is considered a promising technology for green hydrogen production, and iridium (Ir)-based catalysts are the best materials for anodic oxygen evolution reactions (OER) owing to their high stability and anti-corrosion ability in a strong acid electrolyte. The properties of Ir-based nanocatalysts can be tuned by rational dimension engineering, which has received intensive attention recently for catalysis ability boosting. To achieve a comprehensive understanding of the structural and catalysis performance, herein, an overview of the recent progress was provided for Ir-based catalysts with different dimensions for the acidic OER. The promotional effect was first presented in terms of the nano-size effect, synergistic effect, and electronic effect based on the dimensional effect, then the latest progress of Ir-based catalysts classified into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) catalysts was introduced in detail; and the practical application of some typical examples in the real PEM water electrolyzers (PEMWE) was also presented. Finally, the problems and challenges faced by current dimensionally engineered Ir-based catalysts in acidic electrolytes were discussed. It is concluded that the increased surface area and catalytic active sites can be realized by dimensional engineering strategies, while the controllable synthesis of different dimensional structured catalysts is still a great challenge, and the correlation between structure and performance, especially for the structural evolution during the electrochemical operation process, should be probed in depth. Hopefully, this effort could help understand the progress of dimensional engineering of Ir-based catalysts in OER catalysis and contribute to the design and preparation of novel efficient Ir-based catalysts.

Active and durable R2MnRuO7 pyrochlores with low Ru content for acidic oxygen evolution

The production of green hydrogen in water electrolyzers is limited by the oxygen evolution reaction (OER). State-of-the-art electrocatalysts are based on Ir. Ru electrocatalysts are a suitable alternative provided their performance is improved. Here we show that low-Ru-content pyrochlores (R₂MnRuO₇, R = Y, Tb and Dy) display high activity and durability for the OER in acidic media. Y₂MnRuO₇ is the most stable catalyst, displaying 1.5 V at 10 mA cm^-2 for 40 h, or 5000 cycles up to 1.7 V. Computational and experimental results show that the high performance is owed to Ru sites embedded in RuMnO_x surface layers. A water electrolyser with Y₂MnRuO₇ (with only 0.2 mg_Ru cm^-2) reaches 1 A cm^-2 at 1.75 V, remaining stable at 200 mA cm^-2 for more than 24 h. These results encourage further investigation on Ru catalysts in which a partial replacement of Ru by inexpensive cations can enhance the OER performance.

Synergistic effect of p-type and n-type dopants in semiconductors for efficient electrocatalytic water splitting

The main challenge for acidic water electrolysis is the lack of active and stable oxygen evolution catalysts based on abundant materials, which are globally scalable. Iridium oxide is the only material which is active and stable. However, Ir is extremely rare. While both active materials and stable materials exist, those that are active are usually not stable and vice versa. In this work, we present a new design strategy for activating stable materials originally deemed unsuitable due to a semiconducting nature and wide band gap energy. These stable semiconductors cannot change oxidation state under the relevant reaction conditions. Based on DFT calculations, we find that adding an n-type dopant facilitates oxygen binding on semiconductor surfaces. The binding is, however, strong and prevents further binding or desorption of oxygen. By combining both n-type and p-type dopants, the reactivity can be tuned so that oxygen can be adsorbed and desorbed under reaction conditions. The tuning results from the electrostatic interactions between the dopants as well as between the dopants and the binding site. This concept is experimentally verified on TiO2 by co-substituting with different pairs of n- and p-type dopants. Our findings suggest that the co-substitution approach can be used to activate stable materials, with no intrinsic oxygen evolution activity, to design new catalysts for acid water electrolysis.

Spin-related symmetry breaking induced by half-disordered hybridization in BixEr2-xRu2O7 pyrochlores for acidic oxygen evolution

While acidic oxygen evolution reaction plays a critical role in electrochemical energy conversion devices, the sluggish reaction kinetics and poor stability in acidic electrolyte challenges materials development. Unlike traditional nano-structuring approaches, this work focuses on the structural symmetry breaking to rearrange spin electron occupation and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Herein, we propose an atomic half-disordering strategy in multistage-hybridized Bi_xEr_2-xRu₂O₇ pyrochlores to reconfigure orbital degeneracy and spin-related electron occupation. This strategy involves controlling the bonding interaction of Bi-6s lone pair electrons, in which partial atom rearrangement makes the active sites transform into asymmetric high-spin states from symmetric low-spin states. As a result, the half-disordered Bi_xEr_2-xRu₂O₇ pyrochlores demonstrate an overpotential of ~0.18 V at 10 mA cm⁻² accompanied with excellent stability of 100 h in acidic electrolyte. Our findings not only provide a strategy for designing atom-disorder-related catalysts, but also provides a deeper understanding of the spin-related acidic oxygen evolution reaction kinetics.

While water electrolysis offers a potential path for renewable hydrogen fuel, water oxidation electrocatalysts typically suffer from poor stabilities in acid. Here, authors prepare ruthenium-based pyrochlores and demonstrate promising activities and durabilities for acidic water electro-oxidation.

A yolk-shell structure construction for metal-organic frameworks toward an enhanced electrochemical water splitting catalysis

NiFe-based transition metal catalysts are widely used in electrocatalysis, especially in the field of water splitting, due to their excellent electrochemical performance. Herein, a simple method was designed to synthesize a Ni MOF based on nickel foam and it was modified with Fe. After the introduction of Fe, the resulting material exhibits an obvious yolk-shell structure, which greatly increases the specific surface area and facilitates the construction of active sites. At the same time, the synergy between Ni and Fe is conducive to optimizing the electronic structure and effectively improving the poor stability of the MOF. As a result, the synthesized Ni MOF-Fe-2 only needs an overpotential of 229 mV to achieve the OER at a current density of 10 mA cm^-2, which is better than most reported transition metal-based electrocatalysts. To our surprise, it showed extraordinary stability under the voltage used for water splitting.

Material design and surface chemistry for advanced rechargeable zinc-air batteries

Zinc–air batteries (ZABs) have been considered as a next-generation battery system with high energy density and abundant resources. However, the sluggish multi-step reaction of the oxygen is the main obstacle for the practical application of ZABs. Therefore, bifunctional electrocatalysts with high stability and activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are greatly required to promote the catalytic reaction. In this review, we first explain the reaction mechanism of the ZABs, mainly focusing on multiple oxygen intermediates. Then, the latest studies on bifunctional electrocatalysts for the air cathodes and their progress of the ZABs are discussed with following aspects: platinum group metal, metal-free, transition metal, and metal compound-derived electrocatalysts. Finally, we highlight the advanced ZAB systems with the design of the full-temperature range operation, the all-solid-state, and the newly reported non-alkaline electrolyte, summarizing the remaining challenges and requirements of the future research directions.

This work reviews latest research on the bifunctional electrocatalysts for air cathodes, introducing the advanced zinc–air batteries with the full-temperature range operation, all-solid-states, and newly reported non-alkaline electrolytes.