Self-Assembled Ruddlesden-Popper/Perovskite Hybrid with Lattice-Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

Tuning the high-entropy perovskite as efficient and reliable electrocatalysts for oxygen evolution reaction

Due to their unique electronic structure, atomic arrangement and synergistic effect, high-entropy materials are being actively pursued as electrocatalysts for oxygen evolution reaction (OER) in water splitting. However, a relevant strategy to improve high-entropy materials is still lacking. Herein, substitutional doping on the La-site in high-entropy perovskite La1-x Sr x (CrMnFeCoNi)0.2O3 is reported as an efficient OER catalyst. Sr doping is found to be crucial to enhancing the OER activity. The overpotential for the best catalyst La0.3Sr0.7(CrMnFeCoNi)0.2O3 is only 330 mV at 10 mA cm-2, achieving a reduction of 120 mV in overpotential compared to La(CrMnFeCoNi)0.2O3, which is attributed to the enhancement in intrinsic catalytic activity. Experimental evidences including in situ electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) indicate Sr doping induces the formation of high-valence Cr6+, Mn4+, Fe4+, Co4+ and Ni3+ species, which can accelerate the faster charge transfer at the interface, thereby increasing the intrinsic catalytic activity. The assembled two-electrode overall water splitting system operates stably at 10 mA cm-2 for 200 h without attenuation. This work offers an important method for developing a high-performance, high-entropy perovskite OER catalyst for hydrogen production by electrochemical water splitting.

Strategic Design and Insights into Lanthanum and Strontium Perovskite Oxides for Oxygen Reduction and Oxygen Evolution Reactions

Perovskite oxides exhibit bifunctional activity for both oxygen reduction (ORR) and oxygen evolution reactions (OER), making them prime candidates for energy conversion in applications like fuel cells and metal-air batteries. Their intrinsic catalytic prowess, combined with low-cost, abundance, and diversity, positions them as compelling alternatives to noble metal and metal oxides catalysts. This review encapsulates the nuances of perovskite oxide structures and synthesis techniques, providing insight into pivotal active sites that underscore their bifunctional behavior. The focus centers on the breakthroughs surrounding lanthanum (La) and strontium (Sr)-based perovskite oxides, specifically their roles in zinc-air batteries (ZABs). An introduction to the mechanisms of ORR and OER is provided. Moreover, the light is shed on strategies and determinants central to optimizing the bifunctional performance of La and Sr-based perovskite oxides.

Perovskite Oxides Toward Oxygen Evolution Reaction: Intellectual Design Strategies, Properties and Perspectives

Developing electrochemical energy storage and conversion devices (e.g., water splitting, regenerative fuel cells and rechargeable metal-air batteries) driven by intermittent renewable energy sources holds a great potential to facilitate global energy transition and alleviate the associated environmental issues. However, the involved kinetically sluggish oxygen evolution reaction (OER) severely limits the entire reaction efficiency, thus designing high-performance materials toward efficient OER is of prime significance to remove this obstacle. Among various materials, cost-effective perovskite oxides have drawn particular attention due to their desirable catalytic activity, excellent stability and large reserves. To date, substantial efforts have been dedicated with varying degrees of success to promoting OER on perovskite oxides, which have generated multiple reviews from various perspectives, e.g., electronic structure modulation and heteroatom doping and various applications. Nonetheless, the reviews that comprehensively and systematically focus on the latest intellectual design strategies of perovskite oxides toward efficient OER are quite limited. To bridge the gap, this review thus emphatically concentrates on this very topic with broader coverages, more comparative discussions and deeper insights into the synthetic modulation, doping, surface engineering, structure mutation and hybrids. More specifically, this review elucidates, in details, the underlying causality between the being-tuned physiochemical properties [e.g., electronic structure, metal-oxygen (M-O) bonding configuration, adsorption capacity of oxygenated species and electrical conductivity] of the intellectually designed perovskite oxides and the resulting OER performances, coupled with perspectives and potential challenges on future research. It is our sincere hope for this review to provide the scientific community with more insights for developing advanced perovskite oxides with high OER catalytic efficiency and further stimulate more exciting applications.

Graphical Abstract

Locking the lattice oxygen in RuO2 to stabilize highly active Ru sites in acidic water oxidation

Ruthenium dioxide is presently the most active catalyst for the oxygen evolution reaction (OER) in acidic media but suffers from severe Ru dissolution resulting from the high covalency of Ru-O bonds triggering lattice oxygen oxidation. Here, we report an interstitial silicon-doping strategy to stabilize the highly active Ru sites of RuO2 while suppressing lattice oxygen oxidation. The representative Si-RuO2-0.1 catalyst exhibits high activity and stability in acid with a negligible degradation rate of ~52 μV h-1 in an 800 h test and an overpotential of 226 mV at 10 mA cm-2. Differential electrochemical mass spectrometry (DEMS) results demonstrate that the lattice oxygen oxidation pathway of the Si-RuO2-0.1 was suppressed by ∼95% compared to that of commercial RuO2, which is highly responsible for the extraordinary stability. This work supplied a unique mentality to guide future developments on Ru-based oxide catalysts' stability in an acidic environment.

Amorphous Ni-Fe-Mo Oxides Coupled with Crystalline Metallic Domains for Enhanced Electrocatalytic Oxygen Evolution by Promoted Lattice-Oxygen Participation

The crystalline/amorphous heterophase nanostructures are promising functional materials for biomedicals, catalysis, energy conversion, and storage. Despite great progress is achieved, facile synthesis of crystalline metal/amorphous multinary metal oxides nanohybrids remains challenging, and their electrocatalytic oxygen evolution reaction (OER) performance along with the catalytic mechanism are not systematically investigated. Herein, two kinds of ultrafine crystalline metal domains coupled with amorphous Ni-Fe-Mo oxides heterophase nanohybrids, including Ni/Ni0.5-a Fe0.5 Mo1.5 Ox and Ni-FeNi3 /Ni0.5-b Fe0.5-y Mo1.5 Ox , are fabricated through controllable reduction of amorphous Ni0.5 Fe0.5 Mo1.5 Ox precursors by simply tuning the amount of used reductant. Due to the suited component in metal domains, the special structure with dense crystalline/amorphous interfaces, and strong electronic coupling of their components, the resultant Ni-FeNi3 /Ni0.5-b Fe0.5-y Mo1.5 Ox nanohybrids show greatly enhanced OER activity with a low overpotential (278 mV) to reach 10 mA cm-2 current density and ultrahigh turnover frequency (38160 h-1 ), outperforming Ni/Ni0.5-a Fe0.5 Mo1.5 Ox , Ni0.5 Fe0.5 Mo1.5 Ox precursors, commercial IrO2 , and most of recently reported OER catalysts. Also, such Ni-FeNi3 /Ni0.5-b Fe0.5-y Mo1.5 Ox nanohybrids manifest good catalytic stability. As revealed by a series of spectroscopy and electrochemical analyses, their OER mechanism follows the lattice-oxygen-mediated (LOM) pathway. This work may shed light on the design of advanced heterophase nanohybrids, and promote their applications in water splitting, metal-air batteries, or other clean energy fields.

In-situ reconstruction of rock-like 3D hierarchical MIL-53(Fe) self-supporting electrode with oxygen vacancy induced ultra-long stable and efficient water oxidation

The exploitation of efficient, stable and cheap electrocatalyst for oxygen evolution reaction (OER) is very significant to the development of energy technology. In this study, Fe-based metal-organic frameworks (MIL-53(Fe)) self-supporting electrode with a 3D hierarchical open structure was developed through a semi-sacrificial strategy. The self-supporting electrode exhibits an excellent OER performance with an overpotential of 328 mV at 100 mA cm-2 in 1 M KOH, which is superior than that of IrO2 catalyst. Importantly, the optimized self-supporting electrode could operate at 100 mA cm-2 for 520 h without visible decrease in activity. It was also found that the structure of MIL-53(Fe) was in-situ self-reconstructed into oxyhydroxides during OER process. However, the 3D hierarchical open structure assembled with nano-microstructures kept well, which ensured the long-term stability of our self-supporting electrode for OER. Furthermore, density functional theory (DFT) calculations reveal that the FeOOH with rich oxygen vacancy transformed from MIL-53(Fe) plays a key role for the OER catalytic activity. And, the uninterrupted formation of oxygen vacancy during OER process ensures the continuous OER catalytic activity, which is the original source for the ultra-long stability of the self-supporting electrode toward OER. This work explores the way for the construction of efficient self-supporting oxygen electrodes based on MOFs.

Strain-modulated Ru-O Covalency in Ru-Sn Oxide Enabling Efficient and Stable Water Oxidation in Acidic Solution

RuO2 is one of the benchmark electrocatalysts used as the anode material in proton exchange membrane water electrolyser. However, its long-term stability is compromised due to the participation of lattice oxygen and metal dissolution during oxygen evolution reaction (OER). In this work, weakened covalency of Ru-O bond was tailored by introducing tensile strain to RuO6 octahedrons in a binary Ru-Sn oxide matrix, prohibiting the participation of lattice oxygen and the dissolution of Ru, thereby significantly improving the long-term stability. Moreover, the tensile strain also optimized the adsorption energy of intermediates and boosted the OER activity. Remarkably, the RuSnOx electrocatalyst exhibited excellent OER activity in 0.1 M HClO4 and required merely 184 mV overpotential at a current density of 10 mA cm-2 . Moreover, it delivered a current density of 10 mA cm-2 for at least 150 h with negligible potential increase. This work exemplifies an effective strategy for engineering Ru-based catalysts with extraordinary performance toward water splitting.

Synergistic dual-phase air electrode enables high and durable performance of reversible proton ceramic electrochemical cells

Reversible proton ceramic electrochemical cells are promising solid-state ion devices for efficient power generation and energy storage, but necessitate effective air electrodes to accelerate the commercial application. Here, we construct a triple-conducting hybrid electrode through a stoichiometry tuning strategy, composed of a cubic phase Ba0.5Sr0.5Co0.8Fe0.2O3-δ and a hexagonal phase Ba4Sr4(Co0.8Fe0.2)4O16-δ. Unlike the common method of creating self-assembled hybrids by breaking through material tolerance limits, the strategy of adjusting the stoichiometric ratio of the A-site/B-site not only achieves strong interactions between hybrid phases, but also can efficiently modifies the phase contents. When operate as an air electrode for reversible proton ceramic electrochemical cell, the hybrid electrode with unique dual-phase synergy shows excellent electrochemical performance with a current density of 3.73 A cm-2 @ 1.3 V in electrolysis mode and a peak power density of 1.99 W cm-2 in fuel cell mode at 650 °C.

Interface engineering of Ruddlesden-Popper perovskite/CeO2/carbon heterojunction for rechargeable zinc-air batteries

The development of noble metal-based bifunctional electrocatalysts is the key to driving the sluggish oxygen reduction/evolution reaction (ORR/OER) for rechargeable zinc-air battery applications. There is an urgent need to design and construct robust and cost-efficient bifunctional electrocatalysts. Herein, an interface engineering strategy of Ruddlesden-Popper (RP) perovskite/CeO2/carbon heterojunction with core-shell nanostructures is described. Ce-based metal-organic framework derived CeO2-C nanosheets are decorated on the surface of RP type perovskite Pr3Sr(Ni0.5Co0.5)3O10-δ (PSNC) nanofibers. Benefiting from the favorable conductivity, abundant oxygen vacancies and strong interfacial coupling, the hierarchical CeO2-C/PSNC electrode delivers a half-wave potential of 0.78 V (ORR), and an OER overpotential of 370 mV at 10 mA cm-2, respectively. A liquid rechargeable zinc-air battery (ZAB) assembled with CeO2-C/PSNC electrocatalysts as the air cathode exhibits a peak power density of 161 mW cm-2 and a long-term cycling life over 219 h. In addition, the CeO2-C/PSNC-based all-solid-state cable-type ZAB provides a high open-circuit voltage (∼1.44 V), good flexibility and durability. Our study opens a new insight into the design of efficient electrocatalysts for rechargeable ZABs by constructing hierarchical heterojunctions.

Improvement of oxygen reduction activity and stability on a perovskite oxide surface by electrochemical potential

The instability of the surface chemistry in transition metal oxide perovskites is the main factor hindering the long-term durability of oxygen electrodes in solid oxide electrochemical cells. The instability of surface chemistry is mainly due to the segregation of A-site dopants from the lattice to the surface. Here we report that cathodic potential can remarkably improve the stability in oxygen reduction reaction and electrochemical activity, by decomposing the near-surface region of the perovskite phase in a porous electrode made of La1-xSrxCo1-xFexO3 mixed with Sm0.2Ce0.8O1.9. Our approach combines X-ray photoelectron spectroscopy and secondary ion mass spectrometry for surface and sub-surface analysis. Formation of Ruddlesden-Popper phase is accompanied by suppression of the A-site dopant segregation, and exsolution of catalytically active Co particles onto the surface. These findings reveal the chemical and structural elements that maintain an active surface for oxygen reduction, and the cathodic potential is one way to generate these desirable chemistries.

Highly Active Pyrochlore-Type Praseodymium Ruthenate Electrocatalyst for Efficient Acid-Water Oxidation

To produce directly combustible hydrogen from water, highly active, acid-resistant, and economical catalysts for oxygen evolution reaction (OER) are needed. An electrocatalyst based on praseodymium ruthenate (Pr2Ru2O7) is presented here that greatly outperforms RuO2 for acid-water oxidation. Specifically, at 10 mA cm-2, this electrocatalyst presents a low overpotential (η) of 213 mV and markedly superior stability. Moreover, Pr2Ru2O7 presents a significant rise in turnover frequency (TOF) and a highly intrinsic mass activity of 1618.8 A gRu-1 (η = 300 mV), exceeding the most commonly reported acid OER catalysts. Density functional theory calculations and electronic structure study demonstrate that the Ru 4d-band center related to the longer Ru-O bond with a large radius of Pr ion in this pyrochlore is lower than that in RuO2, which would optimize the binding between the adsorbed oxygen species and catalytic metal sites and enhance the catalytic intrinsic activity.

Toward Superb Perovskite Oxide Electrocatalysts: Engineering of Coupled Nanocomposites

A significant issue that bedeviled the commercialization of renewable energy technologies, ranging from low-temperature water electrolyzers to high-temperature solid oxide cells, is the lack of high-performance catalysts. Among various candidates that could tackle such a challenge, perovskite oxides are rising-star materials because of their unique structural and compositional flexibility. However, single-phase perovskite oxides are challenging to satisfy all the requirements of electrocatalysts concurrently for practical applications, such as high catalytic activity, excellent stability, good ionic and electronic conductivities, and superior chemical/thermo-mechanical robustness. Impressively, perovskite oxides with coupled nanocomposites are emerging as a novel form offering multifunctionality due to their intrinsic features, including infinite interfaces with solid interaction, tunable compositions, flexible configurations, and maximum synergistic effects between assorted components. Considering this new configuration has attracted great attention owing to its promising performances in various energy-related applications, this review timely summarizes the leading-edge development of perovskite oxide-based coupled nanocomposites. Their state-of-art synthetic strategies are surveyed and highlighted, their unique structural advantages are highlighted and illustrated through the typical oxygen reduction reaction and oxygen evolution reactions in both high and low-temperature applications. Opinions on the current critical scientific issues and opportunities in this burgeoning research field are all provided.

Understanding of Oxygen Redox in the Oxygen Evolution Reaction

The electron-transfer process during the oxygen evolution reaction (OER) often either proceeds solely via a metal redox chemistry (adsorbate evolution mechanism (AEM), with metal bands around the Fermi level) or an oxygen redox chemistry (lattice oxygen oxidation mechanism (LOM), with oxygen bands around the Fermi level). Unlike the AEM, the LOM involves oxygen redox chemistry instead of metal redox, which leads to the formation of a direct oxygen-oxygen (OO) bond. As a result, such a process is able to bypass the rate-determining step, that is, OO bonding, in AEM, which highlights the critical advantage of LOM as compared to the conventional AEM. Thus, it has been well reported that LOM-based catalysts are able to demonstrate higher OER activities as compared to AEM-based catalysts. Here, a comprehensive understanding of the oxygen redox in LOM and all documented and possible characterization techniques that can be used to identify the oxygen redox are reviewed. This review will interpret the origins of oxygen redox in the reported LOM-based electrocatalysts and the underlying science of LOM-induced surface reconstruction in transition metal oxides. Finally, perspectives on the future development of LOM electrocatalysts are also provided.

Perovskite-based electrocatalysts for oxygen evolution reaction in alkaline media: A mini review

Water electrolysis is one of the attractive technologies for producing clean and sustainable hydrogen fuels with high purity. Among the various kinds of water electrolysis systems, anion exchange membrane water electrolysis has received much attention by combining the advantages of alkaline water electrolysis and proton exchange membrane water electrolysis. However, the sluggish kinetics of the oxygen evolution reaction, which is based on multiple and complex reaction mechanisms, is regarded as a major obstacle for the development of high-efficiency water electrolysis. Therefore, the development of high-performance oxygen evolution reaction electrocatalysts is a prerequisite for the commercialization and wide application of water electrolysis systems. This mini review highlights the current progress of representative oxygen evolution reaction electrocatalysts that are based on a perovskite structure in alkaline media. We first summarize the research status of various kinds of perovskite-based oxygen evolution reaction electrocatalysts, reaction mechanisms and activity descriptors. Finally, the challenges facing the development of perovskite-based oxygen evolution reaction electrocatalysts and a perspective on their future are discussed.

Metal organic framework derived perovskite/spinel heterojunction as efficient bifunctional oxygen electrocatalyst for rechargeable and flexible Zn-air batteries

Interface engineering strategy has been developed to design efficient catalysts for boosting electrocatalytic performance in past few decades. Herein, heterojunctions of PrCoO₃/Co₃O₄ nanocages (PCO/Co₃O₄ NCs) with atomic-level engineered interfaces and rich oxygen vacancies are proposed for Zn-air batteries. The synthesized product shows exceptional bifunctional activity and robust stability towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The enhanced catalytic capacity is primary attributed to the synergistic effect of PCO/Co₃O₄, evidenced by the experimental results and theoretical calculations. More importantly, the PCO/Co₃O₄ NCs assembled liquid Zn-air battery exhibits a power density of 182 mW cm^-2 and a long-term operation of 185 h. When assembled into solid-state cable type battery, this newly designed catalyst also reaches a stable open circuit voltage (1.359 V) and a peak power density of 85 mW cm^-3. Our findings provide essential guidelines of engineering heterostructured electrocatalysts for future wearable electronic devices.

Puffing ultrathin oxides with nonlayered structures

Two-dimensional (2D) oxides have unique electrical, optical, magnetic, and catalytic properties, which are promising for a wide range of applications in different fields. However, it is difficult to fabricate most oxides as 2D materials unless they have a layered structure. Here, we present a facile strategy for the synthesis of ultrathin oxide nanosheets using a self-formed sacrificial template of carbon layers by taking advantage of the Maillard reaction and violent redox reaction between glucose and ammonium nitrate. To date, 36 large-area ultrathin oxides (with thickness ranging from ~1.5 to ~4 nm) have been fabricated using this method, including rare-earth oxides, transition metal oxides, III-main group oxides, II-main group oxides, complex perovskite oxides, and high-entropy oxides. In particular, the as-obtained perovskite oxides exhibit great electrocatalytic activity for oxygen evolution reaction in an alkaline solution. This facile, universal, and scalable strategy provides opportunities to study the properties and applications of atomically thin oxide nanomaterials.

Enhanced charge separation in La2NiO4 nanoplates by coupled piezocatalysis and photocatalysis for efficient H2 evolution

Photocatalytic hydrogen evolution is one promising method for solar energy conversion, but the rapid charge recombination limits its efficiency. To this end, in this work, grain size, and hence the charge carrier migration path, is reduced by lowering the synthesis temperature of two-dimensional visible light-responsive La₂NiO₄ perovskite. Interestingly, the hydrogen yield for the piezoelectric response of La₂NiO₄ under only 40 kHz ultrasonic vibration is as high as 680 μmol h^-1 g^-1, which is 80 times that under only 600 mW cm^-2 visible light irradiation. More surprisingly, the hydrogen production rate under both light illumination and ultrasonic vibration is 129 times higher than under visible light irradiation alone. Clearly, a synergistic effect exists between piezocatalysis and photocatalysis. The hydrogen production activity of the samples with water splitting can reach 1097 μmol h^-1 g^-1 without any sacrificial reagent or co-catalyst, when the light intensity reaches about 1000 mW cm^-2, which is a much higher hydrogen evolution rate by piezo-photocatalysis than is achieved by either piezocatalysis or photocatalysis individually. Further analysis indicates that the internal electric field generated by deformation of the La₂NiO₄ edge under piezoelectric action facilitates the directional separation and migration of photogenerated charges, which in turn significantly enhances the efficiency of use of photogenerated charges for hydrogen production. The investigation here provides a novel approach to design a new reaction system for hydrogen production by coupling multiple external physical fields.

Deep Eutectic Solvent Synthesis of Perovskite Electrocatalysts for Water Oxidation

Oxide perovskites have attracted great interest as materials for energy conversion due to their stability and structural tunability. La-based perovskites of 3d-transition metals have demonstrated excellent activities as electrocatalysts in water oxidation. Herein, we report the synthesis route to La-based perovskites using an environmentally friendly deep eutectic solvent (DES) consisting of choline chloride and malonic acid. The DES route affords phase-pure crystalline materials on a gram scale and results in perovskites with high electrocatalytic activity for oxygen evolution reaction. A convenient, fast, and scalable synthesis proceeds via assisted metathesis at a lower temperature as compared to traditional solid-state methods. Among LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 perovskites prepared via the DES route, LaCoO3 was established to be the best-performing electrocatalyst for water oxidation in alkaline medium at 0.25 mg cm-2 mass loading. LaCoO3 exhibits current densities of 10, 50, and 100 mA cm-2 at respective overpotentials of approximately 390, 430, and 470 mV, respectively, and features a Tafel slope of 55.8 mV dec-1. The high activity of LaCoO3 as compared to the other prepared perovskites is attributed to the high concentration of oxygen vacancies in the LaCoO3 lattice, as observed by high-resolution transmission electron microscopy. An intrinsically high concentration of O vacancies in the LaCoO3 synthesized via the DES route is ascribed to the reducing atmosphere attained upon thermal decomposition of the DES components. These findings will contribute to the preparation of highly active perovskites for various energy applications.

Quenching as a Route to Defect-Rich Ru-Pyrochlore Electrocatalysts toward the Oxygen Evolution Reaction

Defects have a significant impact on the electrocatalysts performance. Introducing defect structures in metal oxides such as pyrochlores and perovskites has proved to be an effective strategy to enhance electrocatalytic activity. However, it is hard to build numerous defect sites in such high-temperature oxides due to the strong metal-oxygen bonds and the so-called self-purification effect, which becomes increasingly important as the particle size reduced to the nanoscale. Here, a facile strategy is demonstrated to fabricate defect-rich yttrium ruthenate oxides Y₂ Ru₂ O_{7- δ} with the pyrochlore structure (denoted D_rich -YRO) by the liquid nitrogen (<-196 °C) quenching. Owing to the almost instantaneous cooling in oxygen-deficient condition, a large number of defects-including oxygen vacancies, grain boundaries, pores and surficial disorder-are preserved in the room temperature material and act as electrocatalytic active sites for oxygen evolution. As a result, D_rich -YRO shows excellent catalytic activity and high electrochemical stability, along with a high performance in the operation of proton exchange membrane electrolyzer. The quenching strategy employed in this work provides a facile approach for constructing defect-rich structures in high-temperature oxides and should lead to new applications in energy conversion and storage devices for such materials.

High-Level Oxygen Reduction Catalysts Derived from the Compounds of High-Specific-Surface-Area Pine Peel Activated Carbon and Phthalocyanine Cobalt

Non-platinum carbon-based catalysts have attracted much more attention in recent years because of their low cost and outstanding performance, and are regarded as one of the most promising alternatives to precious metal catalysts. Activated carbon (AC), which has a large specific surface area (SSA), can be used as a carrier or carbon source at the same time. In this work, stable pine peel bio-based materials were used to prepare large-surface-area activated carbon and then compound with cobalt phthalocyanine (CoPc) to obtain a high-performance cobalt/nitrogen/carbon (Co-N-C) catalyst. High catalytic activity is related to increasing the number of Co particles on the large-specific-area activated carbon, which are related with the immersing effect of CoPc into the AC and the rational decomposed temperature of the CoPc ring. The synergy with N promoting the exposure of CoNx active sites is also important. The Eonset of the catalyst treated with a composite proportion of AC and CoPc of 1 to 2 at 800 °C (AC@CoPc-800-1-2) is 1.006 V, higher than the Pt/C (20 wt%) catalyst. Apart from this, compared with other AC/CoPc series catalysts and Pt/C (20 wt%) catalyst, the stability of AC/CoPc-800-1-2 is 87.8% in 0.1 M KOH after 20,000 s testing. Considering the performance and price of the catalyst in a practical application, these composite catalysts combining biomass carbon materials with phthalocyanine series could be widely used in the area of catalysts and energy storage.

Design principles of noble metal-free electrocatalysts for hydrogen production in alkaline media: combining theory and experiment

Water electrolysis is a promising solution to convert renewable energy sources to hydrogen as a high-energy-density energy carrier. Although alkaline conditions extend the scope of electrocatalysts beyond precious metal-based materials to earth-abundant materials, the sluggish kinetics of cathodic and anodic reactions (hydrogen and oxygen evolution reactions, respectively) impede the development of practical electrocatalysts that do not use precious metals. This review discusses the rational design of efficient electrocatalysts by exploiting the understanding of alkaline hydrogen evolution reaction and oxygen evolution reaction mechanisms and of the electron structure-activity relationship, as achieved by combining experimental and computational approaches. The enhancement of water splitting not only deals with intrinsic catalytic activity but also includes the aspect of electrical conductivity and stability. Future perspectives to increase the synergy between theory and experiment are also proposed.

Electronic Interaction between In Situ Formed RuO2 Clusters and a Nanoporous Zn3V3O8 Support and Its Use in the Oxygen Evolution Reaction

The catalytic activity and durability of RuO₂ clusters toward the oxygen evolution reaction (OER) are strongly associated with their support; however, how the electronic interaction would enhance the catalytic performance is still not quite clear. Herein, hierarchical nanoporous and single-crystal Zn₃V₃O₈ nanosheets are adopted to anchor in situ formed RuO₂ clusters. X-ray photoelectron analysis reveals significant binding energy changes of both Ru and V due to the creation of strong Ru-O-V bonding interaction, which would lead to the reconstruction of the electronic structure of the Zn₃V₃O₈ matrix and RuO₂ clusters. The ultrastrong electronic interaction also results in superior OER activity, indicated by a small overpotential at 10 mA cm^-2 (228 mV) and a shallow Tafel slope of 46 mV dec^-1. First-principles simulation further reveals the synergistic effect derived from the unique RuO₂@Zn₃V₃O₈ couple, which effectively regulates the electronic structure for the OER process. In addition, the created interfacial chemical bond and the confined microporous structure of the Zn₃V₃O₈ substrate could prevent the RuO₂ clusters from detachment and aggregation, making the nanocomposite a promising long-term stable OER electrocatalyst.

High-Performance Perovskite Composite Electrocatalysts Enabled by Controllable Interface Engineering

Single-phase perovskite oxides that contain nonprecious metals have long been pursued as candidates for catalyzing the oxygen evolution reaction, but their catalytic activity cannot meet the requirements for practical electrochemical energy conversion technologies. Here a cation deficiency-promoted phase separation strategy to design perovskite-based composites with significantly enhanced water oxidation kinetics compared to single-phase counterparts is reported. These composites, self-assembled from perovskite precursors, comprise strongly interacting perovskite and related phases, whose structure, composition, and concentration can be accurately controlled by tailoring the stoichiometry of the precursors. The composite catalyst with optimized phase composition and concentration outperforms known perovskite oxide systems and state-of-the-art catalysts by 1-3 orders of magnitude. It is further demonstrated that the strong interfacial interaction of the composite catalysts plays a key role in promoting oxygen ionic transport to boost the lattice-oxygen participated water oxidation. These results suggest a simple and viable approach to developing high-performance, perovskite-based composite catalysts for electrochemical energy conversion.

Correlation between Oxygen Vacancies and Oxygen Evolution Reaction Activity for a Model Electrode: PrBaCo2 O5+δ

The role of the perovskite lattice oxygen in the oxygen evolution reaction (OER) is systematically studied in the PrBaCo₂ O_5+δ family. The reduced number of physical/chemical variables combined with in-depth characterizations such as neutron dif-fraction, O K-edge X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), magnetization and scanning transmission electron microscopy (STEM) studies, helps investigating the complex correlation between OER activity and a single perovskite property, such as the oxygen content. Larger amount of oxygen vacancies appears to facilitate the OER, possibly contributing to the mechanism involving the oxidation of lattice oxygen, i.e., the lattice oxygen evolution reaction (LOER). Furthermore, not only the number of vacancies but also their local arrangement in the perovskite lattice influences the OER activity, with a clear drop for the more stable, ordered stoichiometry.

Double Perovskite Cobaltites Integrated in a Monolithic and Noble Metal-Free Photoelectrochemical Device for Efficient Water Splitting

Water photoelectrolysis has the potential to produce renewable hydrogen fuel, therefore addressing the intermittent nature of sunlight. Herein, a monolithic, photovoltaic (PV)-assisted water electrolysis device of minimal engineering and of low (in the μg range) noble-metal-free catalysts loading is presented for unassisted water splitting in alkaline media. An efficient double perovskite cobaltite catalyst, originally developed for high-temperature proton-conducting ceramic electrolyzers, possesses high activity for the oxygen evolution reaction in alkaline media at room temperatures too. Ba_1-xGd_1-yLa_x+yCo₂O_6-δ (BGLC) is combined with a NiMo cathode, and a solar-to-hydrogen efficiency of 6.6% in 1.0 M NaOH, under 1 sun simulated illumination for 71 h, is demonstrated. This work highlights how readily available earth-abundant materials and established PV methods can achieve high performance and stable and monolithic photoelectrolysis devices with potential for full-scale applications.

Lattice Matching Growth of Conductive Hierarchical Porous MOF/LDH Heteronanotube Arrays for Highly Efficient Water Oxidation

The conjugation of metal-organic frameworks (MOFs) into different multicomponent materials to precisely construct aligned heterostructures is fascinating but elusive owing to the disparate interfacial energy and nucleation kinetics. Herein, a promising lattice-matching growth strategy is demonstrated for conductive MOF/layered double hydroxide (cMOF/LDH) heteronanotube arrays with highly ordered hierarchical porous structures enabling an ultraefficient oxygen evolution reaction (OER). CoNiFe-LDH nanowires are used as interior template to engineer an interface by inlaying cMOF and matching two crystal lattice systems, thus conducting a graft growth of cMOF/LDH heterostructures along the LDH nanowire. A class of hierarchical porous cMOF/LDH heteronanotube arrays is produced through continuously regulating the transformation degree. The synergistic effects of the cMOF and LDH components significantly promote the chemical and electronic structures of the heteronanotube arrays and their electroactive surface area. Optimized heteronanotube arrays exhibit extraordinary OER activity with ultralow overpotentials of 216 and 227 mV to deliver current densities of 50 and 100 mA cm^-2 with a small Tafel slope of 34.1 mV dec^-1 , ranking it among the best MOF and non-noble-metal-based catalysts for OER. The robust performance under high current density and vigorous gas bubble conditions enable such hierarchical MOF/LDH heteronanotube arrays as promising materials for practical water electrolysis.

Layered double hydroxide-based electrocatalysts for the oxygen evolution reaction: identification and tailoring of active sites, and superaerophobic nanoarray electrode assembly

The electrocatalytic oxygen evolution reaction (OER) is a critical half-cell reaction for hydrogen production via water electrolysis. However, the practical OER suffers from sluggish kinetics and thus requires efficient electrocatalysts. Transition metal-based layered double hydroxides (LDHs) represent one of the most active classes of OER catalysts. An in-depth understanding of the activity of LDH based electrocatalysts can promote further rational design and active site regulation of high-performance electrocatalysts. In this review, the fundamental understanding of the structural characteristics of LDHs is demonstrated first, then comparisons and in-depth discussions of recent advances in LDHs as highly active OER catalysts in alkaline media are offered, which include both experimental and computational methods. On top of the active site identification and structural characterization of LDHs on an atomic scale, strategies to promote the OER activity are summarised, including doping, intercalation and defect-making. Furthermore, the concept of superaerophobicity, which has a profound impact on the performance of gas evolution electrodes, is explored to enhance LDHs and their derivatives for a large scale OER. In addition, certain operating standards for OER measurements are proposed to avoid inconsistency in evaluating the OER activity of LDHs. Finally, several key challenges in using LDHs as anode materials for large scale water splitting, such as the issue of stability and the adoption of membrane-electrode-assembly based electrolysers, are emphasized to shed light on future research directions.

Efficient Water Splitting Actualized through an Electrochemistry-Induced Hetero-Structured Antiperovskite/(Oxy)Hydroxide Hybrid

Exploring active, stable, and low-cost bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is crucial for water splitting technology associated with renewable energy storage in the form of hydrogen fuel. Here, a newly designed antiperovskite-based hybrid composed of a conductive InNNi₃ core and amorphous InNi(oxy)hydroxide shell is first reported as promising OER/HER bifunctional electrocatalyst. Prepared by a facile electrochemical oxidation strategy, such unique hybrid (denoted as EO-InNNi₃ ) exhibits excellent OER and HER activities in alkaline media, benefiting from the inherent high-efficiency HER catalytic nature of InNNi₃ antiperovskite and the promoting role of OER-active InNi(oxy)hydroxide thin film, which is confirmed by theoretical simulations and in situ Raman studies. Moreover, an alkaline electrolyzer integrated EO-InNNi₃ as both anode and cathode delivers a low voltage of 1.64 V at 10 mA cm^-2 , while maintaining excellent durability. This work demonstrates the application of antiperovskite-based materials in the field of overall water splitting and inspires insights into the development of advanced catalysts for various energy applications.