Phase Engineering of a Ruthenium Nanostructure toward High-Performance Bifunctional Hydrogen Catalysis

The physicochemical properties and catalytic performance of transition metals are highly phase-dependent. Ru-based nanomaterials are superior catalysts toward hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR), but studies are mostly limited to conventional hexagonal-close-packed (hcp) Ru, mainly arising from the difficulty in synthesizing Ru with pure face-centered-cubic (fcc) phase. Herein, we report a crystal-phase-dependent catalytic study of MoO_x-modified Ru (MoO_x-Ru fcc and MoO_x-Ru hcp) for bifunctional HER and HOR. MoO_x-Ru fcc is proven to outperform MoO_x-Ru hcp in catalyzing both HER and HOR with much higher catalytic activity and more durable stability. The modification effect of MoO_x gives rise to optimal adsorption of H and OH especially on fcc Ru, which thus has resulted in the superior catalytic performance. This work highlights the significance of phase engineering in constructing superior electrocatalysts and may stimulate more efforts on phase engineering of other metal-based materials for diversified applications.

Protrusion-Rich Cu@NiRu Core@shell Nanotubes for Efficient Alkaline Hydrogen Evolution Electrocatalysis

The development of highly efficient and durable water electrolysis catalysts plays an important role in the large-scale applications of hydrogen energy. In this work, protrusion-rich Cu@NiRu core@shell nanotubes are prepared by a facile wet chemistry method and used for catalyzing hydrogen evolution reaction (HER) in an alkaline environment. The protrusion-like RuNi alloy shells with accessible channels and abundant defects possess a large surface area and can optimize the surface electronic structure through the electron transfer from Ni to Ru. Moreover, the unique 1D hollow structure can effectively stabilize RuNi alloy shell through preventing the aggregation of nanoparticles. The synthesized catalyst can achieve a current density of 10 mA cm^-2 in 1.0 m KOH with an overpotential of only 22 mV and show excellent stability after 5000 cycles, which is superior to most reported Ru-based catalysts. Density functional theory calculations illustrate that the weakened hydrogen adsorption on Ru sites induced by the alloying with Ni and active electron transfer between Ru and Ni/Cu are the keys to the much improved HER activity.

Recent Advances Regarding Precious Metal-Based Electrocatalysts for Acidic Water Splitting

Electrochemical water splitting has wide applicability in preparing high-density green energy. The Proton exchange membrane (PEM) water electrolysis system is a promising technique for the generation of hydrogen due to its high electrolytic efficiency, safety and reliability, compactness, and quick response to renewable energy sources. However, the instability of catalysts for electrochemical water splitting under operating conditions limits their practical applications. Until now, only precious metal-based materials have met the requirements for rigorous long-term stability and high catalytic activity under acid conditions. In this review, the recent progress made in this regard is presented and analyzed to clarify the role of precious metals in the promotion of the electrolytic decomposition of water. Reducing precious metal loading, enhancing catalytic activity, and improving catalytic lifetime are crucial directions for developing a new generation of PEM water electrolysis catalysts. A summary of the synthesis of high-performance catalysts based on precious metals and an analysis of the factors affecting catalytic performance were derived from a recent investigation. Finally, we present the remaining challenges and future perspectives as guidelines for practical use.

Highly Curved, Quasi-Single-Crystalline Mesoporous Metal Nanoplates Promote CC Bond Cleavage in Ethanol Oxidation Electrocatalysis

The ability to manipulate metal nanocrystals with well-defined morphologies and structures is greatly important in material chemistry, catalysis chemistry, nanoscience, and nanotechnology. Although 2D metals serve as interesting platforms, further manipulating them in solution with highly penetrated mesopores and ideal crystallinity remains a huge challenge. Here, an easy yet powerful synthesis strategy for manipulating the mesoporous structure and crystallinity of 2D metals in a controlled manner with cetyltrimethylammonium chloride as the mesopore-forming surfactant and extra iodine-ion as the structure/facet-selective agent is reported. This strategy allows for preparing an unprecedented type of 2D quasi-single-crystalline mesoporous nanoplates (SMPs) with highly curved morphology and controlled metal composition. The products, for example, PdCu SMPs, feature abundant undercoordinated sites, optimized electronic structures, excellent electron/mass transfers, and confined mesopore environments. Curved PdCu SMPs exhibit remarkable electrocatalytic activity of 6.09 A mg_Pd ^-1 and stability for ethanol oxidation reaction (EOR) compared with its counterpart catalysts and commercial Pd/C. More importantly, PdCu SMPs are highly selective for EOR electrocatalysis that dramatically promotes C-C bond cleavage with a superior C1 pathway selectivity as high as 72.1%.

Electronic Modulation of Ru Nanosheet by d-d Orbital Coupling for Enhanced Hydrogen Oxidation Reaction in Alkaline Electrolytes

The alkaline polymer electrolyte fuel cells (APEFCs) hold great promise for using nonnoble metal-based electrocatalysts toward the cathodic oxygen reduction reaction (ORR), but are hindered by the sluggish anodic hydrogen oxidation reaction (HOR) in alkaline electrolytes. Here, a strategy is reported to promote the alkaline HOR performance of Ru by incorporating 3d-transition metals (V, Fe, Co, and Ni), where the conduction band minimum (CBM) level of Ru can be rationally tailored through strong d-d orbital coupling. As expected, the obtained RuFe nanosheet exhibits outstanding HOR performance with the mass activity of 233.46 A g_PGM ^-1 and 23-fold higher than the Ru catalyst, even threefold higher than the commercial Pt/C. APEFC employing this RuFe as anodic catalyst gives a peak power density of 1.2 W cm^-2 , outperforming the documented Pt-free anodic catalyst-based APEFCs. Experimental results and density functional theory calculations suggest the enhanced OH-binding energy and reduced formation energy of water derived from the downshifted CBM level of Ru contribute to the enhanced HOR activity.

Ru doping boosts electrocatalytic water splitting

Heteroatom doping plays a crucial role in improving the electrocatalytic performance of catalysts towards water splitting. Owing to the existence of Ru-O moieties, Ru is thus emerging as an ideal dopant for promoting the electrocatalytic performance for water splitting by modifying the electronic structure, introducing extra active sites, improving electronic conductivity, and inducing a strong synergistic effect. Benefitting from these advantages, Ru-doped nanomaterials have been widely investigated and employed as advanced electrocatalysts for water splitting, and many excellent Ru-doped electrocatalysts have been successfully developed. In an effort to obtain a better understanding of the influence of Ru doping on the electrocatalytic water splitting performance of nanocatalysts, we herein summarize the recent progress of Ru-doped electrocatalysts by focusing on the synthesis strategies and advantageous merits. Applications of these new materials in water electrolysis technology are also discussed with emphasis on future directions in this active field of research.

Highly dispersed ruthenium nanoparticles on nitrogen doped carbon toward efficient hydrogen evolution in both alkaline and acidic electrolytes

Efficient and inexpensive electrocatalysts toward the hydrogen evolution reaction (HER) play an important role in electrochemical water splitting. Herein, we report the synthesis of highly dispersed ruthenium nanoparticles (2.2 ± 0.4 nm) on nitrogen doped carbon (Ru/N-C) by chemical reduction of RuCl₃ on carbon in the presence of polyvinylpyrrolidone in combination with subsequent pyrolysis. Ru/N-C exhibits an excellent overpotential of 13.5 and 18.5 mV at 10 mA cm⁻² in 1.0 M KOH and 0.5 M H₂SO₄ aqueous solution, respectively, much better than and comparable to those of commercial Pt/C (38.0 and 10.0 mV). The exceptional HER activity arises from high surface area of ultrafine Ru nanoparticles and appropriate Ru electronic state tuned by nitrogen dopant. Furthermore, Ru/N-C demonstrates excellent durability in both alkaline and acidic condition relative to commercial Pt/C. We speculate that the nitrogen dopant might have coordinated with Ru and tightly anchored Ru nanoparticles, preventing them from agglomerating.

Ultrafine ruthenium nanoparticles on nitrogen doped carbon show exceptional activity toward the hydrogen evolution reaction in alkaline and acidic electrolytes.

Wet-chemical synthesis of two-dimensional metal nanomaterials for electrocatalysis

Two-dimensional (2D) metal nanomaterials have gained ever-growing research interest owing to their fascinating physicochemical properties and promising application, especially in the field of electrocatalysis. In this review, we briefly introduce the recent advances in wet-chemical synthesis of 2D metal nanomaterials. Subsequently, the catalytic performances of 2D metal nanomaterials in a variety of electrochemical reactions are illustrated. Finally, we summarize current challenges and highlight our perspectives on preparing high-performance 2D metal electrocatalysts.

Interface Engineering of Co/CoMoN/NF Heterostructures for High-Performance Electrochemical Overall Water Splitting

The development of low-cost and high-efficiency catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte is still challenging. Herein, interfacial Co/CoMoN heterostructures supported on Ni foam (Co/CoMoN/NF) are constructed by thermal ammonolysis of CoMoO_x . In 1.0 m KOH solution, Co/CoMoN/NF heterostructures exhibit excellent HER activity with an overpotential of 173 mV at 100 mA cm^-2  and a Tafel slope of 68.9 mV dec^-1 . Density functional theory calculations indicate that the low valence state Co site acts as efficient water-dissociation promoter, while CoMoN substrate has favorable hydrogen adsorption energy, leading to an enhanced HER activity. The Co/CoMoN/NF heterostructures also achieve high OER activity with an overpotential of 303 mV at 100 mA cm^-2  and a Tafel slope of 56 mV dec^-1 . Using Co/CoMoN/NF heterostructures as the cathode and anode, the alkaline electrolyzer requires a low voltage of 1.56 V to reach the current density of 100 mA cm^-2  along with superior long-term durability. This study provides a new design strategy toward low-cost and excellent catalysts for water splitting.

Strong Oxide-Support Interaction over IrO2 /V2 O5 for Efficient pH-Universal Water Splitting

Constructing strong oxide-support interaction (SOSI) is compelling for modulating the atomic configurations and electronic structures of supported catalysts. Herein, ultrafine iridium oxide nanoclusters (≈1 nm) are anchored on vanadium oxide support (IrO2 /V2 O5 ) via SOSI. The as made catalyst, with a unique distorted IrO2 structure, is discovered to significantly boost the performance for pH-universal oxygen evolution reaction (OER). Based on experimental results and theoretical calculations, the distorted IrO2 active sites with flexible redox states in IrO2 /V2 O5 server as electrophilic centers balance the adsorption of oxo-intermediates and effectively facilitate the process of OO coupling, eventually propelling the fast turnover of water oxidation. As a result, IrO2 /V2 O5 demonstrates not only ultralow overpotentials at 10 mA cm-2 (266 mV, pH = 0; 329 mV, pH = 7; 283 mV, pH = 14) for OER, but also high-performance overall water electrolysis over a broad pH range, with a potential of mere 1.50 V (pH = 0), 1.65 V (pH = 7) or 1.49 V (pH = 14) at 10 mA cm-2 . In addition, SOSI can simultaneously secure the distorted active sites and thus remarkably improving the catalytic stability, making it a promising strategy to develop high-performance catalytic systems.

Superlattice in a Ru Superstructure for Enhancing Hydrogen Evolution

Superlattices are attracting extensive attention due to their unique properties. Nevertheless, the observations of superlattices are limited to those layered structures with weak interlayered interactions, and the effect of the superlattice in metal-based nanostructures on catalysis is unexplored yet. We here report a facile wet-chemical method for synthesizing two-dimensional Ru multilayered nanosheets (Ru MNSs) with a superlattice. Characterizations reveal that the superlattice is formed by stacking Ru layers with twisted angles from 2° to 30°. Owing to the strong synergy between the adjacent layers, Ru MNSs can serve as an efficient catalyst for the alkaline hydrogen evolution reaction (HER). Theoretical calculations reveal that the superlattice can induce the strain effect, which leads to lattice contraction and weak *H adsorption ability, as a result of improved HER performance. This work sheds new light on the utilization of the superlattice on enhancing catalysis in metal-based materials.

Electrocatalytic Water Splitting: From Harsh and Mild Conditions to Natural Seawater

Electrocatalytic water splitting is regarded as the most effective pathway to generate green energy-hydrogen-which is considered as one of the most promising clean energy solutions to the world's energy crisis and climate change mitigation. Although electrocatalytic water splitting has been proposed for decades, large-scale industrial hydrogen production is hindered by high electricity cost, capital investment, and electrolysis media. Harsh conditions (strong acid/alkaline) are widely used in electrocatalytic mechanism studies, and excellent catalytic activities and efficiencies have been achieved. However, the practical application of electrocatalytic water splitting in harsh conditions encounters several obstacles, such as corrosion issues, catalyst stability, and membrane technical difficulties. Thus, the research on water splitting in mild conditions (neutral/near neutral), even in natural seawater, has aroused increasing attention. However, the mechanism in mild conditions or natural seawater is not clear. Herein, different conditions in electrocatalytic water splitting are reviewed and the effects and proposed mechanisms in the three conditions are summarized. Then, a comparison of the reaction process and the effects of the ions in different electrolytes are presented. Finally, the challenges and opportunities associated with direct electrocatalytic natural seawater splitting and the perspective are presented to promote the progress of hydrogen production by water splitting.

Highly Dispersed Ru Nanoclusters Anchored on B, N Co-doped Carbon Nanotubes for Water Splitting

We developed an N, B co-doped carbon nanotubes as a substrate material to load 2-3 nm uniform Ru clusters. We rationally realized controllable N and B doping into carbon nanotubes....

Compensating Electronic Effect Enables Fast Site-to-Site Electron Transfer over Ultrathin RuMn Nanosheet Branches toward Highly Electroactive and Stable Water Splitting

To improve the electroactivity and stability of electrocatalysts, various modulation strategies have been applied in nanocatalysts. Among different methods, heteroatom doping has been considered as an effective method, which modifies the local bonding environments and the electronic structures. Meanwhile, the design of novel two-dimensional (2D) nanostructures also offers new opportunities for achieving efficient electrocatalysts. In this work, Mn-doped ultrathin Ru nanosheet branches (RuMn NSBs), a newly reported 2D nanostructure, is synthesized. With the ultrathin and naturally abundant edges, the RuMn NSBs have exhibited bifunctionalities of hydrogen evolution reaction and oxygen evolution reaction with high electroactivity and durability in different electrolytes. Experimental characterizations have revealed that RuO bonds are shortened due to Mn doping, which is the key factor that leads to improved electrochemical performances. Density functional theory (DFT) calculations have confirmed that the introduction of Mn enables flexible modulations on the valence states of Ru sites. The inversed redox state evolutions of Ru and Mn sites not only improve the electroactivity for the water splitting but also the long-term stability due to the pinning effect of Ru sites. This work has provided important inspirations for the design of future advanced Ru-based electrocatalysts with high performances and durability.

Recent Progress in Advanced Electrocatalyst Design for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers hold great significance for renewable energy storage and conversion. The acidic oxygen evolution reaction (OER) is one of the main roadblocks that hinder the practical application of PEM water electrolyzers. Highly active, cost-effective, and durable electrocatalysts are indispensable for lowering the high kinetic barrier of OER to achieve boosted reaction kinetics. To date, a wide spectrum of advanced electrocatalysts has been designed and synthesized for enhanced acidic OER performance, though Ir and Ru based nanostructures still represent the state-of-the-art catalysts. In this Progress Report, recent research progress in advanced electrocatalysts for improved acidic OER performance is summarized. First, fundamental understanding about acidic OER including reaction mechanisms and atomic understanding to acidic OER for rational design of efficient electrocatalysts are discussed. Thereafter, an overview of the progress in the design and synthesis of advanced acidic OER electrocatalysts is provided in terms of catalyst category, i.e., metallic nanostructures (Ir and Ru based), precious metal oxides, nonprecious metal oxides, and carbon based nanomaterials. Finally, perspectives to the future development of acidic OER are provided from the aspects of reaction mechanism investigation and more efficient electrocatalyst design.

Manipulation on Two-Dimensional Amorphous Nanomaterials for Enhanced Electrochemical Energy Storage and Conversion

Low-carbon society is calling for advanced electrochemical energy storage and conversion systems and techniques, in which functional electrode materials are a core factor. As a new member of the material family, two-dimensional amorphous nanomaterials (2D ANMs) are booming gradually and show promising application prospects in electrochemical fields for extended specific surface area, abundant active sites, tunable electron states, and faster ion transport capacity. Specifically, their flexible structures provide significant adjustment room that allows readily and desirable modification. Recent advances have witnessed omnifarious manipulation means on 2D ANMs for enhanced electrochemical performance. Here, this review is devoted to collecting and summarizing the manipulation strategies of 2D ANMs in terms of component interaction and geometric configuration design, expecting to promote the controllable development of such a new class of nanomaterial. Our view covers the 2D ANMs applied in electrochemical fields, including battery, supercapacitor, and electrocatalysis, meanwhile we also clarify the relationship between manipulation manner and beneficial effect on electrochemical properties. Finally, we conclude the review with our personal insights and provide an outlook for more effective manipulation ways on functional and practical 2D ANMs.

Electrocatalytic Oxygen Evolution Reaction in Acidic Conditions: Recent Progress and Perspectives

The electrochemical oxygen evolution reaction (OER) is an important half-cell reaction in many renewable energy conversion and storage technologies, including electrolyzers, nitrogen fixation, CO₂ reduction, metal-air batteries, and regenerative fuel cells. Among them, proton exchange membrane (PEM)-based devices exhibit a series of advantages, such as excellent proton conductivity, high durability, and good mechanical strength, and have attracted global interest as a green energy device for transport and stationary sectors. Nevertheless, with a view to rapid commercialization, it is urgent to develop highly active and acid-stable OER catalysts for PEM-based devices. In this Review, based on the recent advances in theoretical calculation and in situ/operando characterization, the OER mechanism in acidic conditions is first discussed in detail. Subsequently, recent advances in the development of several types of acid-stable OER catalysts, including noble metals, non-noble metals, and even metal-free OER materials, are systematically summarized. Finally, the current key issues and future challenges for materials used as acidic OER catalysis are identified and potential future directions are proposed.

Emerging beyond-graphene elemental 2D materials for energy and catalysis applications

Elemental two-dimensional (2D) materials have emerged as promising candidates for energy and catalysis applications due to their unique physical, chemical, and electronic properties. These materials are advantageous in offering massive surface-to-volume ratios, favorable transport properties, intriguing physicochemical properties, and confinement effects resulting from the 2D ultrathin structure. In this review, we focus on the recent advances in emerging energy and catalysis applications based on beyond-graphene elemental 2D materials. First, we briefly introduce the general classification, structure, and properties of elemental 2D materials and the new advances in material preparation. We then discuss various applications in energy harvesting and storage, including solar cells, piezoelectric and triboelectric nanogenerators, thermoelectric devices, batteries, and supercapacitors. We further discuss the explorations of beyond-graphene elemental 2D materials for electrocatalysis, photocatalysis, and heterogeneous catalysis. Finally, the challenges and perspectives for the future development of elemental 2D materials in energy and catalysis are discussed.

Hollow Hydrangea-Like CoRu/Co Architecture as an Excellent Electrocatalyst for Oxygen Evolution

Developing low-cost but efficient electrocatalysts to promote the sluggish kinetics of oxygen evolution from water splitting is essential for hydrogen production. In this study, a hierarchical hollow hydrangea-like CoRu/Co superstructure is constructed through a self-templating method by morphology-controlled pyrolysis of flower-like Ru-doped Co-based layered double hydroxides (LDH). The anchoring of Ru into Co-LDH is the key to the formation of well-defined hydrangea-like three-dimensional superstructure composed of CoRu/Co. The optimized CoRu/Co-M-350 with a low Ru loading of 3.0 wt% exhibits excellent catalytic performances in the oxygen evolution reaction (OER) with low overpotential (η₁₀ =192 mV) and excellent stability for 100 h at 100 mA cm^-2 in alkaline media, outperforming the benchmark RuO₂ and most reported electrocatalysts. The superior morphology and structural features of CoRu/Co-M-350 provide not only abundant accessible surface sites but also fast mass and electron transfer, thereby promoting OER catalysis. The present study provides a new synthetic route for preparing highly active OER electrocatalysts.

Facile Synthesis of PdCuRu Porous Nanoplates as Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction in Alkaline Medium

Ru is a key component of electrocatalysts for hydrogen evolution reaction (HER), especially in alkaline media. However, the catalytic activity and durability of Ru-based HER electrocatalysts are still far from satisfactory. Here we report a solvothermal approach for the synthesis of PdCuRu porous nanoplates with different Ru compositions by using Pd nanoplates as the seeds. The PdCuRu porous nanoplates were formed through underpotential deposition (UPD) of Cu on Pd, followed by alloying Cu with Pd through interdiffusion and galvanic replacement between Cu atoms and Ru precursor simultaneously. When evaluated as HER electrocatalysts, the PdCuRu porous nanoplates exhibited excellent catalytic activity and durability. Of them, the Pd24Cu29Ru47/C achieved the lowest overpotential (40.7 mV) and smallest Tafel slope (37.5 mV dec−1) in an alkaline solution (much better than commercial Pt/C). In addition, the Pd24Cu29Ru47/C only lost 17% of its current density during a stability test for 10 h, while commercial Pt/C had a 59.5% drop under the same conditions. We believe that the electron coupling between three metals, unique porous structure, and strong capability of Ru for water dissociation are responsible for such an enhancement in HER performance.

Nanoscaled and Atomic Ruthenium Electrocatalysts Confined Inside Super-Hydrophilic Carbon Nanofibers for Efficient Hydrogen Evolution Reaction

A series of Ru-based catalysts have been developed for the hydrogen evolution reaction (HER) by the facile impregnation of copious and eco-friendly bacterial cellulose (BC) with Ru(bpy)₃ Cl₂ (bpy = 2,2'-bipyridine) followed by pyrolysis. After the oxidation and molecular recomposition processes that occur within the BC precursors during pyrolysis, sub-2 nm Ru nanoparticles (NPs) and atomic Ru species confined within surface-oxidized N-doped carbon nanofibers (CNFs) can be observed in the derived catalysts. The surface oxidation of CNFs leads the derived catalysts with super hydrophilicity and water-absorbing capacity, and also provides dimensional confinement for the nanoscaled and atomic Ru species. With these added structural advantages and the component synergy, the derived catalysts show superior HER activities, for which the overpotentials are as low as 19.6 mV (1 m KOH) and 55.0 mV (0.5 m H₂ SO₄ ) for the most active case at the current density of 10 mA cm^-2 . Moreover, superior HER activity can be also achieved for the catalysts derived with a wide range of Ru loadings. Finally, the influence of Ru NP size on HER activity is investigated by density functional theory simulations. This method provides a reliable protocol for preparing highly active HER catalysts for scale-up applications.

Sodium-Decorated Amorphous/Crystalline RuO2 with Rich Oxygen Vacancies: A Robust pH-Universal Oxygen Evolution Electrocatalyst

The oxygen evolution reaction (OER) is a key reaction for many electrochemical devices. To date, many OER electrocatalysts function well in alkaline media, but exhibit poor performances in neutral and acidic media, especially the acidic stability. Herein, sodium-decorated amorphous/crystalline RuO₂ with rich oxygen vacancies (a/c-RuO₂ ) was developed as a pH-universal OER electrocatalyst. The a/c-RuO₂ shows remarkable resistance to acid corrosion and oxidation during OER, which leads to an extremely high catalytic stability, as confirmed by a negligible overpotential increase after continuously catalyzing OER for 60 h at pH=1. Besides, a/c-RuO₂ also exhibits superior OER activities to commercial RuO₂ and most reported OER catalysts under all pH conditions. Theoretical calculations indicated that the introduction of Na dopant and oxygen vacancy in RuO₂ weakens the adsorption strength of the OER intermediates by engineering the d-band center, thereby lowering the energy barrier for OER.

Recent progress in water-splitting electrocatalysis mediated by 2D noble metal materials

Two-dimensional (2D) nanostructures have enabled noble-metal-based nanomaterials to be promising electrocatalysts toward overall water splitting due to their inherent structural advantages, including a high specific surface active area, numerous low-coordinated atoms, and a high density of defects and edges. Moreover, it is also disclosed that the electronic effect and strain effect within 2D nanostructures also benefit the further promotion of the electrocatalytic performance. In this review, we have focused on the recent progress in the fabrication of advanced electrocatalysts based on 2D noble-metal-based nanomaterials toward water splitting electrocatalysis. First, fundamental descriptions about water-splitting mechanisms, some promising engineering strategies, and major challenges in electrochemical water splitting are given. Then, the structural merits of 2D nanostructures for water splitting electrocatalysis are also highlighted, including abundant surface active sites, lattice distortion, abundant surface defects, electronic effects, and strain effects. Additionally, some representative water-splitting electrocatalysts have been discussed in detail to highlight the superiorities of 2D noble-metal-based nanomaterials for electrochemical water splitting. Finally, the underlying challenges and future opportunities for the fabrication of more advanced electrocatalysts for water splitting are also highlighted. We hope that this review article provides guidance for the fabrication of more efficient electrocatalysts for boosting industrial hydrogen production via water splitting.

Engineering Ruthenium-Based Electrocatalysts for Effective Hydrogen Evolution Reaction

The investigation of highly effective, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is a prerequisite for the upcoming hydrogen energy society. To establish a new hydrogen energy system and gradually replace the traditional fossil-based energy, electrochemical water-splitting is considered the most promising, environmentally friendly, and efficient way to produce pure hydrogen. Compared with the commonly used platinum (Pt)-based catalysts, ruthenium (Ru) is expected to be a good alternative because of its similar hydrogen bonding energy, lower water decomposition barrier, and considerably lower price. Analyzing and revealing the HER mechanisms, as well as identifying a rational design of Ru-based HER catalysts with desirable activity and stability is indispensable. In this review, the research progress on HER electrocatalysts and the relevant describing parameters for HER performance are briefly introduced. Moreover, four major strategies to improve the performance of Ru-based electrocatalysts, including electronic effect modulation, support engineering, structure design, and maximum utilization (single atom) are discussed. Finally, the challenges, solutions and prospects are highlighted to prompt the practical applications of Ru-based electrocatalysts for HER.

Supramolecular Anchoring Strategy for Facile Production of Ruthenium Nanoparticles Embedded in N-Doped Mesoporous Carbon Nanospheres for Efficient Hydrogen Generation

Because of the favorable mass transport and increased available active sites, the rational design and preparation of porous carbon structures are essential but still challenging. Herein, a novel and facile supramolecular anchoring strategy was developed to achieve the embedding of ruthenium (Ru) nanoparticles in N-doped mesoporous carbon nanospheres through pyrolyzing the precursor formed by coordination assembly between metal ions and zinc gluconate (G(Zn)). Featuring rich hydroxyl groups, the G(Zn) can effectively chelate Ru³⁺ via metal-oxygen bonds to form 3D supramolecular nanospheres, and meanwhile, mesopores in carbon nanospheres were expanded after subsequent pyrolysis thanks to the volatilization of zincic species at high temperature. As a demonstration, the best-performing catalyst displayed extraordinary activity for the hydrogen evolution reaction (HER) with a small overpotential of 43 mV versus reversible hydrogen electrode (vs RHE) at 10 mA/cm² and a Tafel slope of 39 mV/dec, which was superior to that of commercial Pt/C in alkaline medium. Theoretical calculations revealed that the catalytic activity was significantly promoted by the strong electronic coupling between Ru nanoparticles and N-doped porous carbon, which increased the electron transfer capability and facilitated the adsorption and dissociation of H₂O to realize an efficient HER.

High Valence M-Incorporated PdCu Nanoparticles (M = Ir, Rh, Ru) for Water Electrolysis in Alkaline Solution

The high-valence metal catalysts show extraordinary talent in various electrochemical reactions. However, there is no facile method to synthesize high-valence noble metal-based materials. Herein, we synthesized the different high valence noble metal M-incorporated PdCu nanoparticles (M = Ir, Ru, Rh) by the assistant of Fe³⁺ and exhibit excellent performance for water electrolysis. In 0.1 M KOH, the OER and HER mass activities of Ir₁₆-PdCu/C were 50.5 and 16.5 times as much as PdCu/C, and achieved a current density of 10 mA cm^-2 at 1.63 V when worked for overall water splitting. DFT calculation revealed that the incorporating of high valence Ir could optimize the binding energy of the intermediate products, and promote the evolution of oxygen and hydrogen. Ex situ XPS shows that the huge amount of oxidized Ir (V) formed in OER could promote the formation of O-O bonds.

Decoration of Ru/RuO2 hybrid nanoparticles on MoO2 plane as bifunctional electrocatalyst for overall water splitting

Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the two branches of artificial overall water splitting (OWS), in which the reaction efficiency usually depends on different specific catalysts. Although effective bifunctional electrocatalyst for OWS (HER and OER) are highly desired, designing and constructing such suitable materials is full of challenges to overcome several difficulties, involving slow kinetics, differences in catalytic mechanisms, large overpotential values, and low round-trip efficiencies. In this work, we reported a new bifunctional electrocatalyst Ru/RuO₂-MoO₂ catalyst (RRMC) via a redox solid phase reaction (RSPR) strategy to achieve the high electrocatalytic activity of OWS. Briefly, due to the restricted transport behavior of atoms in solid state precursor, the designed redox reaction occurred between the adjacent part of RuO₂ and MoS₂, forming Ru/RuO₂ hybrid NPs and MoO₂ plane. Therefore, the newly formed Ru/RuO₂ hybrid NPs and MoO₂ plane were tightly combined and used as an electrocatalyst for OWS. Benefiting from the exposed active sites and optimized electronic structure, the RRMC sample annealed at 500 °C (RRMC-500) exhibited low overpotential for HER (18 mV) and for OER (260 mV) at 10 mA cm^-2 under alkaline conditions. Especially, a low cell voltage of 1.54 V was required at 10 mA cm^-2 under alkaline condition.

Electronic Modulation of Non-van der Waals 2D Electrocatalysts for Efficient Energy Conversion

The exploration of efficient electrocatalysts for energy conversion is important for green energy development. Owing to their high surface areas and unusual electronic structure, 2D electrocatalysts have attracted increasing interest. Among them, non-van der Waals (non-vdW) 2D materials with numerous chemical bonds in all three dimensions and novel chemical and electronic properties beyond those of vdW 2D materials have been studied increasingly over the past decades. Herein, the progress of non-vdW 2D electrocatalysts is critically reviewed, with a special emphasis on electronic structure modulation. Strategies for heteroatom doping, vacancy engineering, pore creation, alloying, and heterostructure engineering are analyzed for tuning electronic structures and achieving intrinsically enhanced electrocatalytic performances. Lastly, a roadmap for the future development of non-vdW 2D electrocatalysts is provided from material, mechanism, and performance viewpoints.

A strategy for preparing high-efficiency and economical catalytic electrodes toward overall water splitting

Electrolyzing water technology to prepare high-purity hydrogen is currently an important field in energy development. However, the preparation of efficient, stable, and inexpensive hydrogen production technology from electrolyzed water is a major problem in hydrogen energy production. The key technology for hydrogen production from water electrolysis is to prepare highly efficient catalytic, stable and durable electrodes, which are used to reduce the overpotential of the hydrogen evolution reaction and the oxygen evolution reaction of electrolyzed water. The main strategies for preparing catalytic electrodes include: (i) choosing cheap, large specific surface area and stable base materials, (ii) modulating the intrinsic activity of the catalytic material through elemental doping and lattice changes, and (iii) adjusting the morphology and structure to increase the catalytic activity. Based on these findings, herein, we review the recent work in the field of hydrogen production by water electrolysis, introduce the preparation of catalytic electrodes based on nickel foam, carbon cloth and new flexible materials, and summarize the catalytic performance of metal oxides, phosphides, sulfides and nitrides in the hydrogen evolution and oxygen evolution reactions. Secondly, parameters such as the overpotential, Tafel slope, active site, turnover frequency, and stability are used as indicators to measure the performance of catalytic electrode materials. Finally, taking the material cost of the catalytic electrode as a reference, the successful preparations are comprehensively compared. The overall aim is to shed some light on the exploration of high-efficiency and economical electrodes in energy chemistry and also demonstrate that there is still room for discovering new combinations of electrodes including base materials, composition lattice changes and morphologies.

Porous Noble Metal Electrocatalysts: Synthesis, Performance, and Development

Active sites (intrinsic activity, quantity, and distribution), electron transfer, and mass diffusion are three important factors affecting the performance of electrocatalysts. Composed of highly active components which are built into various network structures, porous noble metal is an inherently promising electrocatalysts. In recent years, great efforts have been made to explore new efficient synthesis methods and establish structural-performance relationships in the field of porous noble metal electrocatalysis. In this review, the very recent progress in strategies for preparing porous noble metal, including innovation and deeper understanding of traditional methods is summarized. A discussion of relationship between porous noble metal structure and electrocatalytic performance, such as accessibility of active sites, connectivity of skeleton structures, channels dimensions, and hierarchical structures, is provided.

Advances in hydrogen production from electrocatalytic seawater splitting

As one of the most abundant resources on the Earth, seawater is not only a promising electrolyte for industrial hydrogen production through electrolysis, but also of great significance for the refining of edible salt. Despite the great potential for large-scale hydrogen production, the implementation of water electrolysis requires efficient and stable electrocatalysts that can maintain high activity for water splitting without chloride corrosion. Recent years have witnessed great achievements in the development of highly efficient electrocatalysts toward seawater splitting. Starting from the historical background to the most recent achievements, this review will provide insights into the current state, challenges, and future perspectives of hydrogen production through seawater electrolysis. In particular, the mechanisms of overall water splitting, key features of seawater electrolysis, noble-metal-free electrocatalysts for seawater electrolysis and the underlying mechanisms are also highlighted to provide guidance for fabricating more efficient electrocatalysts toward seawater splitting.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

Exclusive Strain Effect Boosts Overall Water Splitting in PdCu/Ir Core/Shell Nanocrystals

Core/shell nanocatalysts are a class of promising materials, which achieve the enhanced catalytic activities through the synergy between ligand effect and strain effect. However, it has been challenging to disentangle the contributions from the two effects, which hinders the rational design of superior core/shell nanocatalysts. Herein, we report precise synthesis of PdCu/Ir core/shell nanocrystals, which can significantly boost oxygen evolution reaction (OER) via the exclusive strain effect. The heteroepitaxial coating of four Ir atomic layers onto PdCu nanoparticle gives a relatively thick Ir shell eliminating the ligand effect, but creates a compressive strain of ca. 3.60%. The strained PdCu/Ir catalysts can deliver a low OER overpotential and a high mass activity. Density functional theory (DFT) calculations reveal that the compressive strain in Ir shell downshifts the d-band center and weakens the binding of the intermediates, causing the enhanced OER activity. The compressive strain also boosts hydrogen evolution reaction (HER) activity and the strained nanocrystals can be served as excellent catalysts for both anode and cathode in overall water-splitting electrocatalysis.

Direct Self-Assembly of Hierarchically Grown Rhodium Thin Films for Electrocatalytic Hydrogen Evolution Reaction

Thin films of metallic rhodium (Rh) are developed on two different supports, nickel foam (NF) and titanium foil (Ti), and evaluated for electrochemical hydrogen evolution reaction (HER). The electrodes are prepared by aerosol-assisted chemical vapor deposition technique using a Rh diethyldithiocarabamte precursor for three distinct time periods of 40, 80, and 120 min at 500 °C. The film consists of phase pure metallic Rh with hierarchical flower-like morphology. The structural features of such nanostructures can be modulated by adjusting the growth time. The HER catalytic performance data for the optimized films (i.e., with the deposition time of 80 min) suggest that the Rh deposited on Ti foil (Rh/Ti) catalyze the reaction substantially faster than Rh deposited on Ni foam (Rh/NF). To produce current density of 100 mA cm−2, the Rh/NF needed over potential of 263 mV, while the Rh/Ti electrode required only 175 mV. In spite of lower electrical conductivity, caused by the bare Ti foil, the Rh/Ti electrode exhibits superior HER performance. The Tafel slopes of Rh/NF and Rh/Ti electrodes are determined to be 52 and 42 mV dec−1, while the turnover frequencies are estimated to be 1.1 and 37.3 s−1 at over potential of 260 mV.