Iridium-Based Multimetallic Nanoframe@Nanoframe Structure: An Efficient and Robust Electrocatalyst toward Oxygen Evolution Reaction

An integral composite electrode with low Ir loading for efficient acidic oxygen evolution

The development of low-Ir-loading oxygen evolution reaction catalysts with high performance and stability is a major challenge for advancing proton exchange membrane water electrolysis (PEMWE). In this study, we report an integral composite electrode by depositing TiO2 nanowires on titanium plate via an in situ growth method and subsequently loading Ir nanoparticles onto these nanowires (Ir-TiO2/TP). This approach not only reduces the usage of Ir but also significantly enhances the catalytic performance, with a low overpotential of 199 mV at 10 mA cm-2, while maintaining negligible activity decay over 440 hours at 10 mA cm-2 in 0.5 M H2SO4.

Recent Development of Ir- and Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers are one type of the most promising technologies for efficient, nonpolluting and sustainable production of high-purity hydrogen. The anode catalysts account for a very large fraction of cost in PEM water electrolyzer and also determine the lifetime of the electrolyzer. To date, Ir- and Ru-based materials are types of promising catalysts for the acidic oxygen evolution reaction (OER), but they still face challenges of high cost or low stability. Hence, exploring low Ir and stable Ru-based electrocatalysts for acidic OER attracts extensive research interest in recent years. Owing to these great research efforts, significant developments have been achieved in this field. In this review, the developments in the field of Ir- and Ru-based electrocatalysts for acidic OER are comprehensively described. The possible OER mechanisms are first presented, followed by the introduction of the criteria for evaluation of the OER electrocatalysts. The development of Ir- and Ru-based OER electrocatalysts are then elucidated according to the strategies utilized to tune the catalytic performances. Lastly, possible future research in this burgeoning field is discussed.

Nanoscale Engineering of 3D Intragaps and Compositions in Multimetallic Hollow Superstructures for Enhanced Catalysis

Designing and synthesizing multishelled metallic hollow nanostructures with intragaps and porous shells have received widespread attention for enhancing optical and catalytic properties. However, significant challenges remain in engineering these structures at the nanometer scale. Herein, we employed the galvanic replacement reaction (GRR) method to prepare multimetallic hollow superstructures with 3D cavities and distinct nanometer intragaps. By precise control of the intragap distances (1-10 nm) and composition distributions within a single entity, libraries of multimetallic hollow superstructures were constructed. Using the 4-nitrophenol reduction as a model reaction, triple-shell Au@Pt-Ag nanoparticles with approximately 1 nm intragaps exhibited a catalytic rate 211.6 times higher than that of commercial Pt/C catalysts. The nanoconfinement environment of multishelled structures not only increases active sites but also promotes electron delocalization of reactants, accelerating the hydrogenation process both thermodynamically and kinetically. Our work advances the rational synthesis of multishelled nanostructures, expanding their potential applications in catalysis, plasmonics, and biosensing.

Effect of Selective Metallic Defects on Catalytic Performance of Alloy Nanosheets

Defects in the crystal structure of nanomaterials are important for their diverse applications. As, defects in 2D framework allow surface confinement effects, efficient molecular accessibility, high surface-area to volume-ratio and lead to higher catalytic activity, but it is challenging to expose defects of specific metal on the surface of 2D alloy and find the correlation between defective structure and electrocatalytic properties with atomic precision. Herein, the work paves the way for the controlled synthesis of ultrathin porous Ir-Cu nanosheets (NSs) with selectively iridium (Ir) rich defects to boost their performance for acidic oxygen evolution reaction (OER). X-ray absorption spectroscopy reveals that the oxidized states of Ir in defects of porous NSs significantly impact the electronic structure and decline the energy barrier. As a result, porous Ir-Cu/C NSs deliver improved OER activity with an overpotential of 237 mV for reaching 10 mA cm-2 and exhibit significantly higher mass activity than benchmark Ir/C under acidic conditions. Therefore, the present work highlights the concept of constructing a selective noble metal defect-rich open structure for catalytic applications.

Sr-induced Fermi Engineering of β-FeOOH for Multifunctional Catalysis

Designing a multifunctional electrocatalyst to produce H2 from water, urea, urine, and wastewater, is highly desirable yet challenging because it demands precise Fermi-engineering to realize stronger π-donation from O 2p to electron(e-)-deficient metal (t2g) d-orbitals. Here a Sr-induced phase transformed β-FeOOH/α-Ni(OH)2 catalyst anchored on Ni-foam (designated as pt-NFS) is introduced, where Sr produces plenteous Fe4+ (Fe3+ → Fe4+) to modulate Fermi level and e--transfer from e--rich Ni3+(t2g)-orbitals to e--deficient Fe4+(t2g)-orbitals, via strong π-donation from the π-symmetry lone-pair of O bridge. pt-NFS utilizes Fe-sites near the Sr-atom to break the H─O─H bonds and weakens the adsorption of *O while strengthening that of *OOH, toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Invaluably, Fe-sites of pt-NFS activate H2-production from urea oxidation reaction (UOR) through a one-stage pathway which, unlike conventional two-stage pathways with two NH3-molecules, involves only one NH3-molecule. Owing to more suitable kinetic energetics, pt-NFS requires 133 mV (negative potential shift), 193 mV, ≈1.352 V, and ≈1.375 V versus RHE for HER, OER, UOR, and human urine oxidation, respectively, to reach the benchmark 10 mA cm-2 and also demonstrates remarkable durability of over 25 h. This work opens a new corridor to design multifunctional electrocatalysts with precise Fermi engineering through d-band modulation.

Universal Synthesis of Amorphous Metal Oxide Nanomeshes

Constructing the pore structures in amorphous metal oxide nanosheets can enhance their electrocatalytic performance by efficiently increasing specific surface areas and facilitating mass transport in electrocatalysis. However, the accurate synthesis for porous amorphous metal oxide nanosheets remains a challenge. Herein, a facile nitrate-assisted oxidation strategy is reported for synthesizing amorphous mesoporous iridium oxide nanomeshes (a-m IrOx NMs) with a pore size of ∼4 nm. X-ray absorption characterizations indicate that a-m IrOx NMs possess stretched Ir─O bonds and weaker Ir-O interaction compared with commercial IrO2. Combining thermogravimetric-fourier transform infrared spectroscopy with differential scanning calorimetry measurements, it is demonstrated that sodium nitrate, acting as an oxidizing agent, is conducive to the formation of amorphous nanosheets, while the NO2 produced by the in situ decomposition of nitrates facilitates the generation of pores within the nanomeshes. As an anode electrocatalyst in proton exchange membrane water electrolyzer, a-m IrOx NMs exhibit superior performance, maintaining a cell voltage of 1.67 V at 1 A cm-2 for 120 h without obvious decay with a low loading (0.4 mgcatalyst cm-2). Furthermore, the nitrate-assisted method is demonstrated to be a general approach to prepare various amorphous metal oxide nanomeshes, including amorphous RhOx, TiOx, ZrOx, AlOx, and HfOx nanomeshes.

Advances in Noble Metal Electrocatalysts for Acidic Oxygen Evolution Reaction: Construction of Under-Coordinated Active Sites

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.

Iridium single-atoms anchored on a TiO2 support as an efficient catalyst for the hydrogen evolution reaction

Single-atom catalysts (SACs) play a vital role in the hydrogen evolution reaction (HER) owing to the highly desirable atom efficiency and the minimal amount of precious metals. Herein, we use TiO2 nanosheets to anchor stable atomically dispersed iridium (Ir) to be used as a catalyst (Ir@TiO2) for the HER. The atomic dispersion of Ir on the TiO2 substrate is confirmed by aberration-corrected scanning transmission electron microscopy and it is anchored by numerous surface functional groups on abundantly exposed basal planes in TiO2. In acidic media, the Ir@TiO2 catalyst (1.35 wt% Ir) shows a low overpotential (41 mV at 10 mA cm-2), a small Tafel slope of 42 mV dec-1, and a decent durability for 1000 cycles of the HER with the polarization curve having only a 1 mV shift, which are comparable with those of a commercial Pt/C catalyst with 20 wt% Pt. This work paves a way to design Ir atomically anchored catalysts with low cost and high activity.

Recent Advances in Iridium-based Electrocatalysts for Acidic Electrolyte Oxidation

Ongoing research to develop advanced electrocatalysts for the oxygen evolution reaction (OER) is needed to address demand for efficient energy conversion and carbon-free energy sources. In the OER process, acidic electrolytes have higher proton concentration and faster response than alkaline ones, but their harsh strongly acidic environment requires catalysts with greater corrosion and oxidation resistance. At present, iridium oxide (IrO2) with its strong stability and excellent catalytic performance is the catalyst of choice for the anode side of commercial PEM electrolysis cells. However, the scarcity and high cost of iridium (Ir) and the unsatisfactory activity of IrO2 hinder industrial scale application and the sustainable development of acidic OER catalytic technology. This highlights the importance of further research on acidic Ir-based OER catalysts. In this review, recent advances in Ir-based acidic OER electrocatalysts are summarized, including fundamental understanding of the acidic OER mechanism, recent insights into the stability of acidic OER catalysts, highly efficient Ir-based electrocatalysts, and common strategies for optimizing Ir-based catalysts. The future challenges and prospects of developing highly effective Ir-based catalysts are also discussed.

Solar-Powered AEM Electrolyzer via PGM-Free (Oxy)hydroxide Anode with Solar to Hydrogen Conversion Efficiency of 12.44

Water electrolyzers powered by renewable energy are emerging as clean and sustainable technology for producing hydrogen without carbon emissions. Specifically, anion exchange membrane (AEM) electrolyzers utilizing non-platinum group metal (non-PGM) catalysts have garnered attention as a cost-effective method for hydrogen production, especially when integrated with solar cells. Nonetheless, the progress of such integrated systems is hindered by inadequate water electrolysis efficiency, primarily caused by poor oxygen evolution reaction (OER) electrodes. To address this issue, a NiFeCo─OOH has developed as an OER electrocatalyst and successfully demonstrated its efficacy in an AEM electrolyzer, which is powered by renewable electricity and integrated with a silicon solar cell.

Boosting Bifunctional Catalysis by Integrating Active Faceted Intermetallic Nanocrystals and Strained Pt-Ir Functional Shells

PtIr-based nanostructures are fascinating materials for application in bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis. However, the fabrication of PtIr nanocatalysts with clear geometric features and structural configurations, which are crucial for enhancing the bifunctionality, remains challenging. Herein, PtCo@PtIr nanoparticles are precisely designed and fabricated with a quasi-octahedral PtCo nanocrystal as a highly atomically ordered core and an ultrathin PtIr atomic layer as a compressively strained shell. Owing to their geometric and core-shell features, the PtCo@PtIr nanoparticles deliver approximately six and eight times higher mass and specific activities, respectively, as an ORR catalyst than a commercial Pt/C catalyst. The half-wave potential of PtCo@PtIr exhibits a negligible decrease by 9 mV after 10 000 cycles, indicating extraordinary ORR durability because of the ordered arrangement of Pt and Co atoms. When evaluated using the ORR-OER dual reaction upon the introduction of Ir, PtCo@PtIr exhibits a small ORR-OER overpotential gap of 679 mV, demonstrating its great potential as a bifunctional electrocatalyst for fabricating fuel cells. The findings pave the way for designing precise intermetallic core-shell nanocrystals as highly functional catalysts.

Electrochemical Evolution of Ru-Based Polyoxometalates into Si,W-Codoped RuOx for Acidic Overall Water Splitting

Despite intensive studies over decades, the development of electrocatalysts for acidic water splitting still relies on platinum group metals, especially Pt and Ir, which are scarce, expensive, and poorly sustainable. Because such problems can be alleviated, Ru-based bifunctional catalysts such as rutile RuO2 have recently emerged. However, RuO2 has a relatively low activity for hydrogen evolution reactions (HER) and low stability for oxygen evolution reactions (OER) under acidic conditions. In this study, the synthesis of a RuOx -based bifunctional catalyst (RuSiW) for acidic water splitting via the electrochemical evolution from Ru-based polyoxometalates at cathodic potentials is reported. RuSiW consists of the nanocrystalline RuO2 core and Si,W-codoped RuOx shell. RuSiW exhibits outstanding HER and OER activity comparable to Pt/C and RuO2 , respectively, with high stability. Computational analysis suggests that the codoping of RuOx with W and Si synergistically improves the HER activity of otherwise poor RuO2 by shifting the d-band center and optimizing atomic configurations beneficial for proper hydrogen adsorption. This study provides insights into the design and synthesis of unprecedented bifunctional electrocatalysts using catalytically inactive and less explored elements, such as Si and W.

Metal Alloys-Structured Electrocatalysts: Metal-Metal Interactions, Coordination Microenvironments, and Structural Property-Reactivity Relationships

Metal alloys-structured electrocatalysts (MAECs) have made essential contributions to accelerating the practical applications of electrocatalytic devices in renewable energy systems. However, due to the complex atomic structures, varied electronic states, and abundant supports, precisely decoding the metal-metal interactions and structure-activity relationships of MAECs still confronts great challenges, which is critical to direct the future engineering and optimization of MAECs. Here, this timely review comprehensively summarizes the latest advances in creating the MAECs, including the metal-metal interactions, coordination microenvironments, and structure-activity relationships. First, the fundamental classification, design, characterization, and structural reconstruction of MAECs are outlined. Then, the electrocatalytic merits and modulation strategies of recent breakthroughs for noble and non-noble metal-structured MAECs are thoroughly discussed, such as solid solution alloys, intermetallic alloys, and single-atom alloys. Particularly, unique insights into the bond interactions, theoretical understanding, and operando techniques for mechanism disclosure are given. Thereafter, the current states of diverse MAECs with a unique focus on structural property-reactivity relationships, reaction pathways, and performance comparisons are discussed. Finally, the future challenges and perspectives for MAECs are systematically discussed. It is believed that this comprehensive review can offer a substantial impact on stimulating the widespread utilization of metal alloys-structured materials in electrocatalysis.

Ir-Sn pair-site triggers key oxygen radical intermediate for efficient acidic water oxidation

The anode corrosion induced by the harsh acidic and oxidative environment greatly restricts the lifespan of catalysts. Here, we propose an antioxidation strategy to mitigate Ir dissolution by triggering strong electronic interaction via elaborately constructing a heterostructured Ir-Sn pair-site catalyst. The formation of Ir-Sn dual-site at the heterointerface and the resulting strong electronic interactions considerably reduce d-band holes of Ir species during both the synthesis and the oxygen evolution reaction processes and suppress their overoxidation, enabling the catalyst with substantially boosted corrosion resistance. Consequently, the optimized catalyst exhibits a high mass activity of 4.4 A mgIr-1 at an overpotential of 320 mV and outstanding long-term stability. A proton-exchange-membrane water electrolyzer using this catalyst delivers a current density of 2 A cm-2 at 1.711 V and low degradation in an accelerated aging test. Theoretical calculations unravel that the oxygen radicals induced by the π* interaction between Ir 5d-O 2p might be responsible for the boosted activity and durability.

Strain-Induced Electronic Structure Modulation on MnO2 Nanosheet by Ir Incorporation for Efficient Water Oxidation in Acid

Oxygen electrochemistry plays a key role in renewable energy technologies, such as fuel cells and electrolyzers, but its slow kinetics limits the performance and the commercialization of such devices. Here, a strained MnO2 nanosheet induced by Ir incorporation is developed with optimized electronic structure by a simple hydrothermal method. With the incorporation of Ir, the strain induces elongated Mn─O bond length, and thereby tuning the electronic structure to favor the oxygen evolution reaction (OER) performance. The obtained catalyst exhibits an excellent mass activity of 5681 A g-1 at an overpotential of 300 mV in 0.5 m H2 SO4 , and reaches 50 and 100 mA cm-2 at overpotentials of only 240 and 277 mV, respectively. The catalyst is also stable even at 300 mA cm-2 in 0.5 m H2 SO4 . Using the nanosheet as the OER catalyst and the Pt/C as the hydrogen evolution reaction catalyst, a two-electrode electrolyzer achieves 10 mA cm-2 with only a cell voltage of 1.453 V for overall water splitting in 0.5 m H2 SO4 . This strategy enables the material with high feasibility for practical applications on hydrogen production.

Efficient and sustainable water electrolysis achieved by excess electron reservoir enabling charge replenishment to catalysts

Suppressing the oxidation of active-Ir(III) in IrOx catalysts is highly desirable to realize an efficient and durable oxygen evolution reaction in water electrolysis. Although charge replenishment from supports can be effective in preventing the oxidation of IrOx catalysts, most supports have inherently limited charge transfer capability. Here, we demonstrate that an excess electron reservoir, which is a charged oxygen species, incorporated in antimony-doped tin oxide supports can effectively control the Ir oxidation states by boosting the charge donations to IrOx catalysts. Both computational and experimental analyses reveal that the promoted charge transfer driven by excess electron reservoir is the key parameter for stabilizing the active-Ir(III) in IrOx catalysts. When used in a polymer electrolyte membrane water electrolyzer, Ir catalyst on excess electron reservoir incorporated support exhibited 75 times higher mass activity than commercial nanoparticle-based catalysts and outstanding long-term stability for 250 h with a marginal degradation under a water-splitting current of 1 A cm-2. Moreover, Ir-specific power (74.8 kW g-1) indicates its remarkable potential for realizing gigawatt-scale H2 production for the first time.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

Glutathione-Mediated Synthesis of Yolk-Shell AuAg Nanostructures Containing a Spherical Core and Cuboctahedral Skeletons and Their Applications in Plasmonic Catalysis

Frame/skeleton-like nanostructures are of great value in plasmonic catalysis as a result of the synergetic structural advantages arising from both maximized surface atomic exposure and efficient incident light absorptions. Herein, we report the size-tunable fabrication of yolk-shell AuAg nanoparticles containing a spherical core and cuboctahedral skeletons (AuAg YSCNSs), together with the exploration of their applications for assisting the reduction of 4-nitrophenol (4-NP) under ultraviolet-visible (UV-vis) light irradiation. The use of glutathione (GSH) at an appropriate amount to mediate the galvanic replacement reaction between Au@Ag core-shell nanocubes and HAuCl4 is found to be crucial in regulating the shape evolution. Their sizes could be readily tuned by altering the edge lengths of Au@Ag core-shell nanocubes. When working as the photocatalyst assisting the reduction of 4-NP, the AuAg YSCNSs exhibit a higher apparent rate constant under UV-vis light irradiation. The current work demonstrates the feasibility to create skeleton-like noble metal nanocrystals with the shape largely deviated from that of the original template via the "top-down" carving strategy by introducing non-metallic surface doping, which could be potentially extended to other noble metals or alloys.

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

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

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Understanding the Structural Evolution of IrFeCoNiCu High-Entropy Alloy Nanoparticles under the Acidic Oxygen Evolution Reaction

High-entropy alloy (HEA) nanoparticles are promising catalyst candidates for the acidic oxygen evolution reaction (OER). Herein, we report the synthesis of IrFeCoNiCu-HEA nanoparticles on a carbon paper substrate via a microwave-assisted shock synthesis method. Under OER conditions in 0.1 M HClO₄, the HEA nanoparticles exhibit excellent activity with an overpotential of ∼302 mV measured at 10 mA cm^-2 and improved stability over 12 h of operation compared to the monometallic Ir counterpart. Importantly, an active Ir-rich shell layer with nanodomain features was observed to form on the surface of IrFeCoNiCu-HEA nanoparticles immediately after undergoing electrochemical activation, mainly due to the dissolution of the constituent 3d metals. The core of the particles was able to preserve the characteristic homogeneous single-phase HEA structure without significant phase separation or elemental segregation. This work illustrates that under acidic operating conditions, the near-surface structure of HEA nanoparticles is susceptible to a certain degree of structural dynamics.

Chelating adsorption-engaged synthesis of ultrafine iridium nanoparticles anchored on N-doped carbon nanofibers toward highly efficient hydrogen evolution in both alkaline and acidic media

Developing highly efficient and stable precious metal electrocatalysts toward hydrogen evolution reaction (HER) is crucial for energy application, while it is still challenging to achieve highly dispersed ultrafine metal nanoparticles on some promising supports to synergistically promote their electrocatalytic performance. Herein, we propose a feasible chelating adsorption-engaged strategy by introducing de-doped polyaniline with abundant amino groups to immobilize ultrafine iridium (Ir) nanoparticles on their derived N-doped carbon nanofibers (Ir-NCNFs). Experimental results demonstrate that the synthesized Ir-NCNFs can effectively promote the charge transfer and expose more electrochemical active sites, which eventually accelerate the reaction kinetics. Thus, the synthesized Ir-NCNFs catalyst exhibits admirable HER activities in both alkaline and acidic conditions with overpotentials of only 23 and 8 mV, which are even superior or close to the benchmark Pt/C catalyst. Furthermore, the synthesized Ir-NCNFs catalyst also exhibits a long-term durability. This study affords a reliable means to construct high-performance supported ultrafine metal nanocatalysts for electrocatalytic applications to alleviate the growing demand for energy conversion.

Nanoarchitectonics of Metallene Materials for Electrocatalysis

Controlling the synthesis of metal nanostructures is one approach for catalyst engineering and performance optimization in electrocatalysis. As an emerging class of unconventional electrocatalysts, two-dimensional (2D) metallene electrocatalysts with ultrathin sheet-like morphology have gained ever-growing attention and exhibited superior performance in electrocatalysis owing to their distinctive properties originating from structural anisotropy, rich surface chemistry, and efficient mass diffusion capability. Many significant advances in synthetic methods and electrocatalytic applications for 2D metallenes have been obtained in recent years. Therefore, an in-depth review summarizing the progress in developing 2D metallenes for electrochemical applications is highly needed. Unlike most reported reviews on the 2D metallenes, this review starts by introducing the preparation of 2D metallenes based on the classification of the metals (e.g., noble metals, and non-noble metals) instead of synthetic methods. Some typical strategies for preparing each kind of metal are enumerated in detail. Then, the utilization of 2D metallenes in electrocatalytic applications, especially in the electrocatalytic conversion reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, fuel oxidation reaction, CO₂ reduction reaction, and N₂ reduction reaction, are comprehensively discussed. Finally, current challenges and opportunities for future research on metallenes in electrochemical energy conversion are proposed.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Structurally-supported PtCuCo nanoframes as efficient bifunctional catalysts for oxygen reduction and methanol oxidation reactions

Developing highly durable and active catalysts with the morphology of structurally robust nanoframes toward oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in acidic environment is crucial but still a great challenge to completely achieve in a single material. Herein, PtCuCo nanoframes (PtCuCo NFs) with internal support structures as enhanced bifunctional electrocatalysts were prepared by a facile one-pot approach. PtCuCo NFs exhibited remarkable activity and durability for ORR and MOR owing to the ternary compositions and the structure-fortifying frame structures. Impressively, the specific/mass activity of PtCuCo NFs were 12.8/7.5 times as large as that of commercial Pt/C for ORR in perchloric acid solution. For MOR in sulfuric acid solution, the mass/specific activity of PtCuCo NFs was 1.66 A mg_Pt^-1/4.24 mA cm^-2, which was 5.4/9.4 times as large as that of Pt/C. This work may provide a promising nanoframe material to develop dual catalysts for fuel cells.

Coaxial Nanofiber IrOx@SbSnOx as an Efficient Electrocatalyst for Proton Exchange Membrane Dehumidifier

Development of efficient catalysts for oxygen evolution reaction (OER) remains challenging in PEM dehumidifier or vapor electrolyzer. This study developed novel coaxial IrO_x@SbSnO_x nanofiber (NF) catalysts by electrospinning using a dual-channel needle. This method ensures the fibrous structure and the uniform loading of Ir oxide on the support of antimony tin oxide (ATO). IrO₂@SbSnO_x nanoparticles were synthesized for comparison. Characterizations showed that the active area and charge transfer resistance of NF was 1.47 times and 17.72% of that of commercial ones, respectively. The overpotential of NF at 10 mA·cm^-2 was 359 mV, much smaller than that of commercial IrO₂ (418 mV). In addition, the reaction overpotential of NF increased by only 38 mV after 1000 cyclic voltammetry cycles, indicating good electrochemical stability. To explore the enhancement mechanism, first-principles calculations were conducted for theoretically simulating the hetero-structures. Based on d-band theory, the structure formed between ATO and IrO₂ can effectively weaken the adsorption of oxygen intermediates on the catalyst surface, which reduces the OER energy barrier from 1.705 to 1.632 eV, causing an over 15% decrease of overpotential after loading on ATO. As a practical attempt, we applied the new catalysts in real PEM assembly for air dehumidification and found that the performance was improved by about 2 times compared with that using commercial catalysts. This study provides a research direction for the design of one-dimensional NF catalysts and their using in PEM applications.

Construction of steady-active self-supported porous Ir-based electrocatalysts for the oxygen evolution reaction

Developing highly active and stable oxygen evolution reaction (OER) catalysts for water electrolysis remains a great challenge. A self-supported Ir nanocatalyst was prepared via a self-assembly method. Its porous structure and residual metal incorporation contributed to its high activity and stability for the OER in acid.

Hollow Heterostructured Nanocatalysts for Boosting Electrocatalytic Water Splitting

The implementation of electrochemical water splitting demands the development and application of electrocatalysts to overcome sluggish reaction kinetics of hydrogen/oxygen evolution reaction (HER/OER). Hollow nanostructures, particularly for hollow heterostructured nanomaterials can provide multiple solutions to accelerate the HER/OER kinetics owing to their advantageous merit. Herein, the recent advances of hollow heterostructured nanocatalysts and their excellent performance for water splitting are systematically summarized. Starting by illustrating the intrinsically advantageous features of hollow heterostructures, achievements in engineering hollow heterostructured electrocatalysts are also highlighted with the focus on structural design, interfacial engineering, composition regulation, and catalytic evaluation. Finally, some perspective insights and future challenges of hollow heterostructured nanocatalysts for electrocatalytic water splitting are also discussed.

Machine-Learning-Assisted Discovery of High-Efficient Oxygen Evolution Electrocatalysts

Iridium oxide (IrO₂) is the predominant electrocatalyst for the oxygen evolution reaction (OER), but its low efficiency and high cost limit its applications. In this work, we have developed a strategy by combination of high-throughput density functional theory (DFT) and machine learning (ML) techniques for material discovery on IrO₂-based electrocatalysts with enhanced OER activity. A total of 36 kinds of metal dopants are considered to substitute for Ir to form binary and ternary metal oxides, and the most stable surface structures are selected from a total of 4648 structures for OER activity evaluation. Utilizing the neural network language model (NNLM), we associate the atomic environment with the formation energies of crystals and free energies of OER intermediates, and finally a series of potential candidates have been screened as the superior OER catalysts. Our strategy could efficiently explore promising electrocatalysts, especially for evaluating complex multi-metallic compounds.

Oxygen Evolution Electrocatalysts for the Proton Exchange Membrane Electrolyzer: Challenges on Stability

Hydrogen generated by proton exchange membrane (PEM) electrolyzer holds a promising potential to complement the traditional energy structure and achieve the global target of carbon neutrality for its efficient, clean, and sustainable nature. The acidic oxygen evolution reaction (OER), owing to its sluggish kinetic process, remains a bottleneck that dominates the efficiency of overall water splitting. Over the past few decades, tremendous efforts have been devoted to exploring OER activity, whereas most show unsatisfying stability to meet the demand for industrial application of PEM electrolyzer. In this review, systematic considerations of the origin and strategies based on OER stability challenges are focused on. Intrinsic deactivation of the material and the extrinsic balance of plant-induced destabilization are summarized. Accordingly, rational strategies for catalyst design including doping and leaching, support effect, coordination effect, strain engineering, phase and facet engineering are discussed for their contribution to the promoted OER stability. Moreover, advanced in situ/operando characterization techniques are put forward to shed light on the OER pathways as well as the structural evolution of the OER catalyst, giving insight into the deactivation mechanisms. Finally, outlooks toward future efforts on the development of long-term and practical electrocatalysts for the PEM electrolyzer are provided.

Recent advances in hollow nanomaterials with multiple dimensions for electrocatalytic water splitting

Electrocatalytic water splitting has great research prospects in the production of green hydrogen energy, and electrocatalysts are the prerequisite. As widely employed efficient electrocatalysts, hollow nanostructures have attracted a lot of research attention due to their excellent catalytic activity and structural stability. Moreover, the abundant catalytically active sites and tunable morphology also make hollow nanomaterials promising electrocatalysts for water splitting. Despite these advantages, the industrial applications of these hollow nanocatalysts are impeded by limitations like the lack of effective synthesis methods and unclear formation mechanisms. Therefore, extensive efforts have been devoted to the development of efficient synthesis strategies to boost the development of more efficient hollow electrocatalysts, and great progress has been achieved in recent years. To gain a better understanding of the rapid development of hollow nanocatalysts for water splitting, we herein organize a review to summarize the recent synthetic methods and advantages of hollow materials with different dimensions. The specific advantages of hollow nanomaterials in electrocatalytic water splitting, such as abundant active sites, a stable structure, high mass transfer efficiency, and reduced aggregation of catalytic particles, are also summarized. Finally, the challenges and prospects of hollow nanostructures with multiple dimensions in electrocatalytic water splitting are further explored.

Controllable Construction of IrCo Nanoclusters and the Performance for Water Electrolysis

Finding a suitable catalyst is an important research direction in hydrogen (H2) production from water electrolysis. We report a synthetic method to obtain IrxCo/C clusters by polyol reduction. The catalyst is small in size and can be evenly distributed. The Ir3Co/C cluster catalyst had very good activity under acidic conditions. The overpotential of the best-performing Ir3Co/C cluster for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is only 290 mV and 91 mV when 10 mA cm−2 and 100 mA cm−2. The catalyst performance may be improved because of the synergistic effect and the small size of the prepared catalyst, which accelerates proton transfer. This approach offers a strategy to reduce costs while improving catalytic activity.

Nesting of multiple polyhedral plasmonic nanoframes into a single entity

The development of plasmonic nanostructures with intricate nanoframe morphologies has attracted considerable interest for improving catalytic and optical properties. However, arranging multiple nanoframes in one nanostructure especially, in a solution phase remains a great challenge. Herein, we show complex nanoparticles by embedding various shapes of three-dimensional polyhedral nanoframes within a single entity through rationally designed synthetic pathways. This synthetic strategy is based on the selective deposition of platinum atoms on high surface energy facets and subsequent growth into solid platonic nanoparticles, followed by the etching of inner Au domains, leaving complex nanoframes. Our synthetic routes are rationally designed and executable on-demand with a high structural controllability. Diverse Au solid nanostructures (octahedra, truncated octahedra, cuboctahedra, and cubes) evolved into complex multi-layered nanoframes with different numbers/shapes/sizes of internal nanoframes. After coating the surface of the nanoframes with plasmonically active metal (like Ag), the materials exhibited highly enhanced electromagnetic near-field focusing embedded within the internal complicated rim architecture.

The spatial configuration of nanostructure building blocks determines the physical and optical properties of their superstructures. Here, the authors report on complex nanoparticles in which different geometric forms of nanoframes are nested into a single entity by multistep chemical reactions.

Heterogeneous Component Au (Outer)-Pt (Middle)-Au (Inner) Nanorings: Synthesis and Vibrational Characterization on Middle Pt Nanorings with Surface-Enhanced Raman Scattering

We report a synthetic approach for heterometallic (Au-Pt-Au) nanorings with intertwined triple rings (NITs), wherein three differently sized metal circular nanorings concentrically overlap in a single entity. The synthetic method allows one to control the component of core nanorings (Au or Pt) with a tunable gap distance. The narrow circular nanogaps between inner and outer Au rings strongly enhance the electromagnetic near-field via intraparticle coupling of localized surface plasmon resonance, which realizes surface-enhanced Raman scattering (SERS) at the single-particle level. Importantly, when the component of the middle ring is Pt, in situ SERS measurement for electrochemical reactions on Pt domains could be monitored with electrochemical potential variations due to the near-field focusing that is assisted by plasmonically active inner and outer Au nanorings, which is not feasible with pure Pt nanoparticle systems. The resulting NIT systems are robust and may benefit the synthesis of complicated nanostructures, giving myriad applications.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Core-Shell Nanostructured Ru@Ir-O Electrocatalysts for Superb Oxygen Evolution in Acid

The design of highly active and durable catalysts for the sluggish anodic oxygen evolution reaction (OER) in acid remains an urgent yet challenging goal in water electrolysis. Herein, a core-shell nanostructured Ru@Ir-O catalyst with tensile strains and incorporated oxygens is introduced in the Ir shell that holds an extremely low OER overpotential of 238 mV at 10 mA cm^-2 in acid. The material also shows a remarkable 78-fold higher mass activity than the conventional IrO₂ at 1.55 V in 0.5 M H₂ SO₄ . Structural characterization and theoretical calculations reveal that the core-shell interaction and tensile strain cause band position shift and charge redistribution. These electronic factors furthermore optimize the bonding strength of O* and HOO* intermediates on the surface, yielding significantly boosted OER activity relative to the conventional IrO₂ .

Super-Hydrophilic Leaflike Sn4P3 on the Porous Seamless Graphene-Carbon Nanotube Heterostructure as an Efficient Electrocatalyst for Solar-Driven Overall Water Splitting

Water splitting using renewable energy resources is an economic and green approach that is immensely enviable for the production of high-purity hydrogen fuel to resolve the currently alarming energy and environmental crisis. One of the effective routes to produce green fuel with the help of an integrated solar system is to develop a cost-effective, robust, and bifunctional electrocatalyst by complete water splitting. Herein, we report a superhydrophilic layered leaflike Sn₄P₃ on a graphene-carbon nanotube matrix which shows outstanding electrochemical performance in terms of low overpotential (hydrogen evolution reaction (HER), 62 mV@10 mA/cm², and oxygen evolution reaction (OER), 169 mV@20 mA/cm²). The outstanding stability of HER at least for 15 days at a high applied current density of 400 mA/cm² with a minimum loss of potential (1%) in acid medium infers its potential compatibility toward the industrial sector. Theoretical calculations indicate that the decoration of Sn₄P₃ on carbon nanotubes modulates the electronic structure by creating a higher density of state near Fermi energy. The catalyst also reveals an admirable overall water splitting performance by generating a low cell voltage of 1.482 V@10 mA/cm² with a stability of at least 65 h without obvious degradation of potential in 1 M KOH. It exhibited unassisted solar energy-driven water splitting when coupled with a silicon solar cell by extracting a high stable photocurrent density of 8.89 mA/cm² at least for 90 h with 100% retention that demonstrates a high solar-to-hydrogen conversion efficiency of ∼10.82%. The catalyst unveils a footprint for pure renewable fuel production toward carbon-free future green energy innovation.

Design and Synthesis of Hollow Nanostructures for Electrochemical Water Splitting

Electrocatalytic water splitting using renewable energy is widely considered as a clean and sustainable way to produce hydrogen as an ideal energy fuel for the future. Electrocatalysts are indispensable elements for large-scale water electrolysis, which can efficiently accelerate electrochemical reactions occurring at both ends. Benefitting from high specific surface area, well-defined void space, and tunable chemical compositions, hollow nanostructures can be applied as promising candidates of direct electrocatalysts or supports for loading internal or external electrocatalysts. Herein, some recent progress in the structural design of micro-/nanostructured hollow materials as advanced electrocatalysts for water splitting is summarized. First, the design principles and corresponding strategies toward highly effective hollow electrocatalysts for oxygen/hydrogen evolution reactions are highlighted. Afterward, an overview of current reports about hollow electrocatalysts with diverse architectural designs and functionalities is given, including direct hollow electrocatalysts with single-shelled, multi-shelled, or open features and heterostructured electrocatalysts based on hollow hosts. Finally, some future research directions of hollow electrocatalysts for water splitting are discussed based on personal perspectives.

Discovering and Demonstrating a Novel High-Performing 2D-Patterned Electrode for Proton-Exchange Membrane Water Electrolysis Devices

Proton-exchange membrane water electrolysis (PEMWE) produces hydrogen with high efficiency and purity but uses high-loading platinum-group metal (PGM) catalysts. Such concerns call for the development of novel electrode architectures to improve catalyst utilization and mass activity, thus promoting PEMWE cost competitiveness for large-scale implementation. In this study, we demonstrated, for the first time, a novel two-dimensional (2D)-patterned electrode with edge effects to address these challenges. The edge effect was induced by membrane properties, potential distribution, and counter electrode coverage and could be optimized by tuning the catalyst layer dimensions. To achieve identical PEMWE performance, the optimal pattern saved the 21% anode PGM catalyst compared with the conventional catalyst fully covered electrode. The PGM catalyst could be further reduced by 61% to boost mass activity with no significant performance loss. The results also indicated that the electrode uniformity in PEMWE cells might not be as critical as that in PEM fuel cells. The novel 2D-patterned electrode could effectively reduce PGM catalyst loading, accelerating affordable and large-scale production of hydrogen and other value-added chemicals via electrolysis.

Electronic and architecture engineering of hammer-shaped Ir–NiMoO4-ZIF for effective oxygen evolution

Electronic and architecture engineering is realized via doping an ultralow amount of Ir into NiMoO4-ZIF hammers to achieve outstanding electrocatalytic OER performance.

Electrospun one-dimensional electrocatalysts for boosting electrocatalysis

Electrocatalytic reaction plays a crucial role in determining the energy conversion efficiency in advanced technology. However, it is limited by the sluggish reaction kinetics and high energy barrier. These shortcomings...

Noble Metal-Based Multimetallic Nanoparticles for Electrocatalytic Applications

Noble metal-based multimetallic nanoparticles (NMMNs) have attracted great attention for their multifunctional and synergistic effects, which offer numerous catalytic applications. Combined experimental and theoretical studies have enabled formulation of various design principles for tuning the electrocatalytic performance through controlling size, composition, morphology, and crystal structure of the nanoparticles. Despite significant advancements in the field, the chemical synthesis of NMMNs with ideal characteristics for catalysis, including high activity, stability, product-selectivity, and scalability is still challenging. This review provides an overview on structure-based classification and the general synthesis of NMMN electrocatalysts. Furthermore, postsynthetic treatments, such as the removal of surfactants to optimize the activity, and utilization of NMMNs onto suitable support for practical electrocatalytic applications are highlighted. In the end, future direction and challenges associated with the electrocatalysis of NMMNs are covered.