Multi-Component and Nanoporous Design toward RuO2-Based Electrocatalyst with Enhanced Performance for Acidic Water Splitting

Developing electrocatalysts with excellent activity and stability for water splitting in acidic media remains a formidable challenge due to the sluggish kinetics and severe dissolution. As a solution, a multi-component doped RuO2 prepared through a process of dealloying-annealing is presented. The resulting multi-doped RuO2 possesses a nanoporous structure, ensuring a high utilization efficiency of Ru. Furthermore, the dopants can regulate the electronic structure, causing electron aggregation around unsaturated Ru sites, which mitigates Ru dissolution and significantly enhances the catalytic stability/activity. The representative catalyst (FeCoNiCrTi-RuO2) shows an overpotential of 167 mV at 10 mA cm-2 for oxygen evolution reaction (OER) in 0.5 m H2SO4 solution with a Tafel slope of 53.1 mV dec-1, which is among the highest performance reported. Moreover, it remains stable for over 200 h at a current density of 10 mA cm-2. This work presents a promising approach for improving RuO2-based electrocatalysts, offering a crucial advancement for electrochemical water splitting.

In Situ Modulation of Oxygen Vacancies on 2D Metal Hydroxide Organic Frameworks for High-Efficiency Oxygen Evolution Reaction

The discovery of non-precious catalysts for replacing the precious metal of ruthenium in the oxygen evolution reaction (OER) represents a key step in reducing the cost of green hydrogen production. The 2D d-MHOFs, a new 2D materials with controllable oxygen vacancies formed by controlling the degree of coordination bridging between metal hydroxyl oxide and BDC ligands are synthesized at room temperature, exhibit excellent OER properties with low overpotentials of 207  mV at 10 mA cm-2. High-resolution transmission electron microscopy images and density functional theory calculations demonstrate that the introduction of oxygen vacancy sites leads to a lattice distortion and charge redistribution in the catalysts, enhancing the OER activity of 2D d-MHOFs comprehensively. Synchrotron radiation and in situ Raman/Fourier transform infrared spectroscopy indicate that part of oxygen defect sites on the surface of 2D d-MHOFs are prone to transition to highly active metal hydroxyl oxides during the OER process. This work provides a mild strategy for scalable preparation of 2D d-MHOFs nanosheets with controllable oxygen defects, reveals the relationship between oxygen vacancies and OER performance, and offers a profound insight into the basic process of structural transformation in the OER process.

3D flower-like bimetallic Ni-Co metal-organic framework as an electrocatalyst for the oxygen evolution reaction

The rational design and facile preparation of a catalyst with high activity, strong durability and low consumption for the oxygen evolution reaction (OER) is an ongoing challenge in water splitting to generate clean and renewable H2 fuel. Herein, bimetallic metal-organic frameworks (MOFs) with a uniform morphology, controlled metal ratio and low crystallinity were constructed using a simple and reliable one-step solvothermal method. The three-dimensional (3D) flower-like MOF (F-Ni1Co4-BTC) with a Ni to Co molar ratio of 1 : 4 coordinated with 1,3,5-benzenetricarboxylic acid exhibited excellent OER catalytic activity compared with its corresponding counterparts, which can be attributed to the establishment of the exquisite morphology, the proportion of the dual-metal center, and the formation of active intermediates. Furthermore, when F-Ni1Co4-BTC was directly grown on carbon cloth (F-Ni1Co4-BTC/CC), it achieved an obvious improvement in electrochemical performance, affording a low overpotential of 292 mV at a current density of 10 mA cm-2, a small Tafel slope (48 mV dec-1), and excellent mechanical durability in an alkaline electrolyte, which is due to the integrated electrode attained richer active sites and faster electron transfer rate with the introduction of highly conductive carbon cloth. Our work offers a promising strategy to tailor the properties of bimetallic MOFs and the possibility of highly efficient earth-abundant catalysts for practical applications.

In Situ Raman Study of Surface Reconstruction of FeOOH/Ni3S2 Oxygen Evolution Reaction Electrocatalysts

Construction of heterojunctions is an effective strategy to enhanced electrocatalytic oxygen evolution reaction (OER), but the structural evolution of the active phases and synergistic mechanism still lack in-depth understanding. Here, an FeOOH/Ni3S2 heterostructure supported on nickel foam (NF) through a two-step hydrothermal-chemical etching method is reported. In situ Raman spectroscopy study of the surface reconstruction behaviors of FeOOH/Ni3S2/NF indicates that Ni3S2 can be rapidly converted to NiOOH, accompanied by the phase transition from α-FeOOH to β-FeOOH during the OER process. Importantly, a deep analysis of Ni─O bond reveals that the phase transition of FeOOH can regulate the lattice disorder of NiOOH for improved catalytic activity. Density functional theory (DFT) calculations further confirm that NiOOH/FeOOH heterostructure possess strengthened adsorption for O-containing intermediates, as well as lower energy barrier toward the OER. As a result, FeOOH/Ni3S2/NF exhibits promising OER activity and stability in alkaline conditions, requiring an overpotential of 268 mV @ 100 mA cm-2 and long-term stability over 200 h at a current density of 200 mA cm-2. This work provides a new perspective for understanding the synergistic mechanism of heterogeneous electrocatalysts during the OER process.

Synergistic Effect of Grain Boundaries and Oxygen Vacancies on Enhanced Selectivity for Electrocatalytic CO2 Reduction

Dual-engineering involved of grain boundaries (GBs) and oxygen vacancies (VO) efficiently engineers the material's catalytic performance by simultaneously introducing favorable electronic and chemical properties. Herein, a novel SnO2 nanoplate is reported with simultaneous oxygen vacancies and abundant grain boundaries (V,G-SnOx/C) for promoting the highly selective conversion of CO2 to value-added formic acid. Attributing to the synergistic effect of employed dual-engineering, the V,G-SnOx/C displays highly catalytic selectivity with a maximum Faradaic efficiency (FE) of 87% for HCOOH production at -1.2 V versus RHE and FEs > 95% for all C1 products (CO and HCOOH) within all applied potential range, outperforming current state-of-the-art electrodes and the amorphous SnOx/C. Theoretical calculations combined with advanced characterizations revealed that GB induces the formation of electron-enriched Sn site, which strengthens the adsorption of *HCOO intermediate. While GBs and VO synergistically lower the reaction energy barrier, thus dramatically enhancing the intrinsic activity and selectivity toward HCOOH.

Guided Design of Efficient Oxygen Evolution Catalysts Using Patent Analysis

The facile and rapid design of efficient oxygen evolution reaction (OER) catalysts holds paramount significance for energy conversion devices, such as water electrolyzers and fuel cells. Despite substantial progress in catalyst synthesis and performance exploration, the design and selection processes remain inefficient. In this context, we integrate patent analysis with catalyst design, leveraging the scholarly research functionalities within patent analyses to aid in the design and synthesis of a NiFeRu-carbon catalyst as a high-performance OER catalyst. The results demonstrate that the NiFeRu-Carbon catalyst with low Ru loading (0.3 wt %) exhibits an overpotential of only 219 mV at 10 mA cm-2 under alkaline conditions, and after continuous operation for 200 h, the overpotential only attenuates by 15 mV. The incorporation of high-valence Ru dopants elevated the intrinsic activity of individual catalytic sites within NiFe-layered double hydroxides (LDHs). During the catalytic process, the partial dissolution of Ru might lead to the generation of numerous oxygen vacancies within NiFe- LDH, thereby enhancing the catalyst's activity and stability.

Recent advanced strategies for bimetallenes toward electrocatalytic energy conversion reactions

Designing low-dimensional nanomaterials is vital to address the energy and environmental crisis by means of electrocatalytic conversion reactions. Bimetallenes, as an emerging class of 2D materials, present promise for electrocatalytic conversion reactions. By leveraging atomically thin layers, bimetallenes present unsaturated surface coordination, high specific surface area and high conductivity, which are all indispensable features for heterogeneous electrochemical reactions. However, the intrinsic activity and stability of bimetallenes needs to be improved further for bimetallene electrocatalysts, due to the higher demands of practical applications. Recently, many strategies have been developed to optimize the chemical or electronic structure to accommodate transfer of reactants, adsorption or desorption of intermediates, and dissociation of products. Considering that most such work focuses on adjusting the structure, this review offers in-depth insight into recent representative strategies for optimizing bimetallene electrocatalysts, mainly including alloying, strain effects, ligand effects, defects and heteroatom doping. Moreover, by summarizing the performance of bimetallenes optimized using various strategies, we provide a means to understand structure-property relationships. In addition, future prospects and challenges are discussed for further development of bimetallene electrocatalysts.

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.

Phase Engineering of Nanostructural Metallic Materials: Classification, Structures, and Applications

Metallic materials are usually composed of single phase or multiple phases, which refers to homogeneous regions with distinct types of the atom arrangement. The recent studies on nanostructured metallic materials provide a variety of promising approaches to engineer the phases at the nanoscale. Tailoring phase size, phase distribution, and introducing new structures via phase transformation contribute to the precise modification in deformation behaviors and electronic structures of nanostructural metallic materials. Therefore, phase engineering of nanostructured metallic materials is expected to pave an innovative way to develop materials with advanced mechanical and functional properties. In this review, we present a comprehensive overview of the engineering of heterogeneous nanophases and the fundamental understanding of nanophase formation for nanostructured metallic materials, including supra-nano-dual-phase materials, nanoprecipitation- and nanotwin-strengthened materials. We first review the thermodynamics and kinetics principles for the formation of the supra-nano-dual-phase structure, followed by a discussion on the deformation mechanism for structural metallic materials as well as the optimization in the electronic structure for electrocatalysis. Then, we demonstrate the origin, classification, and mechanical and functional properties of the metallic materials with the structural characteristics of dense nanoprecipitations or nanotwins. Finally, we summarize some potential research challenges in this field and provide a short perspective on the scientific implications of phase engineering for the design of next-generation advanced metallic materials.

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Molten Salt-Derived RuO2 Nanocrystals and Nanowires: Unveiling Correlations of Morphology, Microstructure, and Electrocatalytic Performance

Ruthenium oxide (RuO2), due to its comparable binding energy with *H and cost-effectiveness against Pt, has emerged as a pivotal electrocatalyst for oxygen evolution reaction (OER). In the present study, RuO2 nanocrystals (NCs) and nanowires (NWs) were obtained by a molten salt process and the morphology, crystal structure, and local bonding features were examined by using electron microscopy and X-ray absorption spectroscopy. From the electrochemical measurement, both RuO2 NCs and NWs exhibit favorable stability and activity toward oxygen evolution reaction in an alkali medium, althought NCs exhibit higher activity, which is likely attributed to the larger surface area and the high local structural disorder. The theoretical calculation reveals that RuO2 NWs with a primary (110) orientation show a higher overpotential due to its d-band center's proximity to the Fermi level versus (101). The present work suggests that the molten salt process could be an efficient method for producing metal oxide catalysts with tailorable geometry and performances.

Facile deposition of FeNi/Ni hybrid nanoflower electrocatalysts for effective and sustained water oxidation

Bimetallic iron-nickel (FeNi) compounds are widely studied materials for the oxygen evolution reaction (OER) owing to their high electrocatalytic performance and low cost. In this work, we produced thin films of the FeNi alloy on nickel foam (NF) by using an aerosol-assisted chemical deposition (AACVD) method and examined their OER catalytic activity. The hybrid FeNi/Ni catalysts obtained after 1 and 2 h of AACVD deposition show improved charge transfer and kinetics for the OER due to the strong interface between the FeNi alloy and Ni support. The FeNi/Ni-2h catalyst has higher catalytic activity than the FeNi/Ni-1h catalyst because of its nanoflower morphology that provides a large surface area and numerous active sites for the OER. Therefore, the FeNi/Ni-2h catalyst exhibits low overpotentials of 300 and 340 mV at 50 and 500 mA cm-2 respectively, and excellent stability over 100 h, and ∼0% loss after 5000 cycles in 1 M KOH electrolyte. Furthermore, this catalyst has a small Tafel slope, low charge transfer resistance and high current exchange density and thus surpasses the benchmark IrO2 catalyst. The easy, simple, and scalable AACVD method is an effective way to develop thin film electrocatalysts with high activity and stability.

Structurally Disordered RuO2 Nanosheets with Rich Oxygen Vacancies for Enhanced Nitrate Electroreduction to Ammonia

Electrochemical reduction of nitrate pollutants to ammonia has emerged as an attractive alternative for ammonia synthesis. Currently, many strategies have been developed for enhancing nitrate reduction to ammonia (NRA) efficiency, but the influence of the degree of structural disorder is still unexplored. Here, carbon-supported RuO₂ nanosheets with adjustable crystallinity are synthesized by a facile molten salt method. The as-synthesized amorphous RuO₂ displays high ammonia Faradaic efficiency (97.46 %) and selectivity (96.42 %), greatly outperforming the crystalline counterparts. The disordered structure with abundant oxygen vacancies is revealed to modulate the d-band center and hydrogen affinity, thus lowering the energy of the potential-determining step (NH₂ *→NH₃ *).

Cobalt Single Atom Incorporated in Ruthenium Oxide Sphere: A Robust Bifunctional Electrocatalyst for HER and OER

The development of highly active and stable bifunctional noble-metal-based electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is a crucial goal for clean and renewable energy, which still remains challenging. Herein, we report an efficient and stable catalyst comprising a Co single atom incorporated in an RuO₂ sphere for HER and OER, in which the Co single atom in the RuO₂ sphere was confirmed by XAS, AC-STEM, and DFT. This tailoring strategy uses a Co single atom to modify the electronic structures of the surrounding Ru atoms and thereby remarkably elevates the electrocatalytic activities. The catalyst requires ultralow overpotentials, 45 mV for HER and 200 mV for OER, to deliver a current density of 10 mA cm^-2 . The theoretical calculations reveal that the energy barriers for HER and OER are lowered after incorporation of a cobalt single atom.