Atomic Iridium Incorporated in Cobalt Hydroxide for Efficient Oxygen Evolution Catalysis in Neutral Electrolyte

Dynamic chloride ion adsorption on single iridium atom boosts seawater oxidation catalysis

Seawater electrolysis offers a renewable, scalable, and economic means for green hydrogen production. However, anode corrosion by Cl- pose great challenges for its commercialization. Herein, different from conventional catalysts designed to repel Cl- adsorption, we develop an atomic Ir catalyst on cobalt iron layered double hydroxide (Ir/CoFe-LDH) to tailor Cl- adsorption and modulate the electronic structure of the Ir active center, thereby establishing a unique Ir-OH/Cl coordination for alkaline seawater electrolysis. Operando characterizations and theoretical calculations unveil the pivotal role of this coordination state to lower OER activation energy by a factor of 1.93. The Ir/CoFe-LDH exhibits a remarkable oxygen evolution reaction activity (202 mV overpotential and TOF = 7.46 O2 s-1) in 6 M NaOH+2.8 M NaCl, superior over Cl--free 6 M NaOH electrolyte (236 mV overpotential and TOF = 1.05 O2 s-1), with 100% catalytic selectivity and stability at high current densities (400-800 mA cm-2) for more than 1,000 h.

Catalyzing Sustainable Water Splitting with Single Atom Catalysts: Recent Advances

Electrochemical water splitting for sustainable hydrogen and oxygen production have shown enormous potentials. However, this method needs low-cost and highly active catalysts. Traditional nano catalysts, while effective, have limits since their active sites are mostly restricted to the surface and edges, leaving interior surfaces unexposed in redox reactions. Single atom catalysts (SACs), which take advantage of high atom utilization and quantum size effects, have recently become appealing electrocatalysts. Strong interaction between active sites and support in SACs have considerably improved the catalytic efficiency and long-term stability, outperforming their nano-counterparts. This review's first section examines the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER). In the next section, SACs are categorized as noble metal, non-noble metal, and bimetallic synergistic SACs. In addition, this review emphasizes developing methodologies for effective SAC design, such as mass loading optimization, electrical structure modulation, and the critical role of support materials. Finally, Carbon-based materials and metal oxides are being explored as possible supports for SACs. Importantly, for the first time, this review opens a discussion on waste-derived supports for single atom catalysts used in electrochemical reactions, providing a cost-effective dimension to this vibrant research field. The well-known design techniques discussed here may help in development of electrocatalysts for effective water splitting.

Design Principles of Single-Atom Catalysts for Oxygen Evolution Reaction: From Targeted Structures to Active Sites

Hydrogen production from electrolytic water electrolysis is considered a viable method for hydrogen production with significant social value due to its clean and pollution-free nature, high hydrogen production efficiency, and purity, but the anode oxygen evolution reaction (OER) process is complex and kinetically slow. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites often exhibit high catalytic activity and are expected to be extensively applied. The catalytic performance of OER can be further improved by precise regulation of the structure through electronic effects, coordination environment, heteroatomic doping, and so on. In this review, the mechanisms of OER under different conditions are introduced, the latest research progress of SACs in the field of OER is systematically summarized, and then the effects of various structural regulation strategies on catalytic performance are discussed, and principles and ideas for the design of SACs for OER are proposed. In the end, the outstanding issues and current challenges in this field are summarized.

Tungsten-Iron-Ruthenium Ternary Alloy Immobilized into the Inner Nickel Foam for High-Current-Density Water Oxidation

The pursuit of highly-active and stable catalysts in anodic oxygen evolution reaction (OER) is desirable for high-current-density water electrolysis toward industrial hydrogen production. Herein, a straightforward yet feasible method to prepare WFeRu ternary alloying catalyst on nickel foam is demonstrated, whereby the foreign W, Fe, and Ru metal atoms diffuse into the Ni foam resulting in the formation of inner immobilized ternary alloy. Thanks to the synergistic impact of foreign metal atoms and structural robustness of inner immobilized alloying catalyst, the well-designed WFeRu@NF self-standing anode exhibits superior OER activities. It only requires overpotentials of 245 and 346 mV to attain current densities of 20 and 500 mA cm-2 , respectively. Moreover, the as-prepared ternary alloying catalyst also exhibits a long-term stability at a high-current-density of 500 mA cm-2 for over 45 h, evidencing the inner-immobilization strategy is promising for the development of highly active and stable metal-based catalysts for high-density-current water oxidation process.

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

Nano-Scale Engineering of Heterojunction for Alkaline Water Electrolysis

Alkaline water electrolysis is promising for low-cost and scalable hydrogen production. Renewable energy-driven alkaline water electrolysis requires highly effective electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the most active electrocatalysts show orders of magnitude lower performance in alkaline electrolytes than that in acidic ones. To improve such catalysts, heterojunction engineering has been exploited as the most efficient strategy to overcome the activity limitations of the single component in the catalyst. In this review, the basic knowledge of alkaline water electrolysis and the catalytic mechanisms of heterojunctions are introduced. In the HER mechanisms, the ensemble effect emphasizes the multi-sites of different components to accelerate the various intermedium reactions, while the electronic effect refers to the d-band center theory associated with the adsorption and desorption energies of the intermediate products and catalyst. For the OER with multi-electron transfer, a scaling relation was established: the free energy difference between HOO* and HO* is 3.2 eV, which can be overcome by electrocatalysts with heterojunctions. The development of electrocatalysts with heterojunctions are summarized. Typically, Ni(OH)2/Pt, Ni/NiN3 and MoP/MoS2 are HER electrocatalysts, while Ir/Co(OH)2, NiFe(OH)x/FeS and Co9S8/Ni3S2 are OER ones. Last but not the least, the trend of future research is discussed, from an industry perspective, in terms of decreasing the number of noble metals, achieving more stable heterojunctions for longer service, adopting new craft technologies such as 3D printing and exploring revolutionary alternate alkaline water electrolysis.

Unveiling the Critical Role of Surface Hydroxyl Groups for Electro-Assisted Uranium Extraction from Wastewater

The electro-driven extraction of uranium from fluorine-containing uranium wastewater is anticipated to address the challenge of separating fluoro-uranium complexes in conventional technologies. Herein, we developed hydroxy-rich cobalt-based oxides (CoOx) for electro-assisted uranium extraction from fluorine-containing wastewater. Relying on theoretical calculations and other spectral measurements, the hydroxy-rich CoOx nanosheets can enhance the affinity for uranium due to the existence of a substantial quantity of hydroxyl groups. Accordingly, the CoOx nanosheets exhibit outstanding U(VI) removal efficiency in the presence of fluorine ions. Through the utilization of X-ray absorption fine structure (XAFS), we confirm that hydroxy-rich CoOx nanosheets capture free uranyl ions to form a sturdy 2Oax-1U-3Oeq configuration, which can be achieved through electro-driven fluorine-uranium separation. Notably, for the first time, the whole reaction process of uranium species on the CoOx surface from the initial uranium single atom growth to uranium oxide nanosheets is monitored by aberration-corrected transmission electron microscopes (AC-TEM). This work provides a paradigm for the advancement of novel functional materials as electrocatalysts for uranium extraction, as well as a new approach for studying the evolution mechanism of uranium species.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Improved Electrocatalytic Activity and Stability by Single Iridium Atoms on Iron-based Layered Double Hydroxides for Oxygen Evolution

Full understanding to the origin of the catalytic performance of a supported nanocatalyst from the points of view of both the active component and support is significant for the achievement of high performance. Herein, based on a model electrocatalyst of single-iridium-atom-doped iron (Fe)-based layered double hydroxides (LDH) for oxygen evolution reaction (OER), we reveal the first completed origin of the catalytic performance of such supported nanocatalysts. Specially, besides the activity enhancement of Ir sites by LDH support, the stability of surface Fe sites is enhanced by doped Ir sites: DFT calculation shows that the Ir sites can reduce the activity and enhance the stability of the nearby Fe sites; while further finite element simulations indicate, the stability enhancement of distant Fe sites could be attributed to the much low concentration of OER reactant (hydroxyl ions, OH- ) around them induced by the much fast consumption of OH- on highly active Ir sites. These new findings about the interaction between the main active components and supports are applicable in principle to other heterogeneous nanocatalysts and provide a completed understanding to the catalytic performance of heterogeneous nanocatalysts.

Atomically Dispersed FeN2 P2 Motif with High Activity and Stability for Oxygen Reduction Reaction Over the Entire pH Range

The past decade has witnessed the great potential of Fe-based single-atom electrocatalysis in catalyzing oxygen reduction reaction (ORR). However, it remains a grand challenge to substantially improve their intrinsic activity and long-term stability in acidic electrolytes. Herein, we report a facile chemical vapor deposition strategy, by which high-density Fe atoms (3.97 wt%) are coordinated with square-planar para-positioned nitrogen and phosphorus atoms in a hierarchical carbon framework. The as-crafted atomically dispersed Fe catalyst (denoted Fe-SA/PNC) manifests an outstanding activity towards ORR over the entire pH range. Specifically, the half-wave potential of 0.92 V, 0.83 V, and 0.86 V vs. reversible hydrogen electrode (RHE) are attained in alkaline, neutral, and acidic electrolytes, respectively, representing the high performance among reported catalysts to date. Furthermore, after 30,000 durability cycles, the Fe-SA/PNC remains to be stable with no visible performance decay when tested in 0.1 M KOH and 0.5 M H2 SO4 , and only a minor negative shift of 40 mV detected in 0.1 M HClO4 , significantly outperforming commercial Pt/C counterpart. The coordination motif of Fe-SA/PNC is validated by density functional theory (DFT) calculations. This work provides atomic-level insight into improving the activity and stability of non-noble metal ORR catalysts, opening up an avenue to craft the desired single-atom electrocatalysts.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

Electrode/Electrolyte Synergy for Concerted Promotion of Electron and Proton Transfers toward Efficient Neutral Water Oxidation

Neutral water oxidation is a crucial half-reaction for various electrochemical applications requiring pH-benign conditions. However, its sluggish kinetics with limited proton and electron transfer rates greatly impacts the overall energy efficiency. In this work, we created an electrode/electrolyte synergy strategy for simultaneously enhancing the proton and electron transfers at the interface toward highly efficient neutral water oxidation. The charge transfer was accelerated between the iridium oxide and in situ formed nickel oxyhydroxide on the electrode end. The proton transfer was expedited by the compact borate environment that originated from hierarchical fluoride/borate anions on the electrolyte end. These concerted promotions facilitated the proton-coupled electron transfer (PCET) events. Due to the electrode/electrolyte synergy, Ir-O and Ir-OO- intermediates could be directly detected by in situ Raman spectroscopy, and the rate-limiting step of Ir-O oxidation was determined. This synergy strategy can extend the scope of optimizing electrocatalytic activities toward more electrode/electrolyte combinations.

Facet Engineering and Pore Design Boost Dynamic Fe Exchange in Oxygen Evolution Catalysis to Break the Activity-Stability Trade-Off

The oxygen evolution reaction (OER) plays a vital role in renewable energy technologies, including in fuel cells, metal-air batteries, and water splitting; however, the currently available catalysts still suffer from unsatisfactory performance due to the sluggish OER kinetics. Herein, we developed a new catalyst with high efficiency in which the dynamic exchange mechanism of active Fe sites in the OER was regulated by crystal plane engineering and pore structure design. High-density nanoholes were created on cobalt hydroxide as the catalyst host, and then Fe species were filled inside the nanoholes. During the OER, the dynamic Fe was selectively and strongly adsorbed by the (101̅0) sites on the nanohole walls rather than the (0001) basal plane, and at the same time the space-confining effect of the nanohole slowed down the Fe diffusion from catalyst to electrolyte. As a result, a local high-flux Fe dynamic equilibrium inside the nanoholes for OER was achieved, as demonstrated by the Fe57 isotope labeled mass spectrometry, thereby delivering a high OER activity. The catalyst showed a remarkably low overpotential of 228 mV at a current density of 10 mA cm-2, which is among the best cobalt-based catalysts reported so far. This special protection strategy for Fe also greatly improved the catalytic stability, reducing the Fe leaching amount by 2 orders of magnitude compared with the pure Fe hydroxide catalyst and thus delivering a long-term stability of 130 h. An assembled Zn-air battery was stably cycled for 170 h with a low discharge/charge voltage difference of 0.72 V.

Cyclodextrin-supported Co(OH)2 Clusters as Electrocatalysts for Efficient and Selective H2 O2 Synthesis

Co-based material catalysts have shown attractive application prospects in the 2 e- oxygen reduction reaction (ORR). However, for the industrial synthesis of H2 O2 , there is still lack of Co-based catalysts with high production yield rate. Here, novel cyclodextrin-supported Co(OH)2 cluster catalysts were prepared via a mild and facile method. The catalyst exhibited remarkable H2 O2 selectivity (94.2 % ~ 98.2 %), good stability (99 % activity retention after 35 h), and ultra-high H2 O2 production yield rate (5.58 mol gcatalyst -1 h-1 in the H-type electrolytic cell), demonstrating its promising industrial application potential. Density functional theory (DFT) reveals that the cyclodextrin-mediated Co(OH)2 electronic structure optimizes the adsorption of OOH* intermediates and significantly enhances the activation energy barrier for dissociation, leading to the high reactivity and selectivity for the 2 e- ORR. This work offers a valuable and practical strategy to design Co-based electrocatalysts for H2 O2 production.

Immobilizing Metal Nanoparticles on Hierarchically Porous Organic Cages with Size Control for Enhanced Catalysis

Incorporating metal nanoparticles (MNPs) into porous composites with controlled size and spatial distributions is beneficial for a broad range of applications, but it remains a synthetic challenge. Here, we present a method to immobilize a series of highly dispersed MNPs (Pd, Ir, Pt, Rh, and Ru) with controlled size (<2 nm) on hierarchically micro- and mesoporous organic cage supports. Specifically, the metal-ionic surfactant complexes serve as both metal precursors and mesopore-forming agents during self-assembly with a microporous imine cage CC3, resulting in a uniform distribution of metal precursors across the resultant supports. The functional heads on the ionic surfactants as binding sites, together with the nanoconfinement of pores, guide the nucleation and growth of MNPs and prevent their agglomeration after chemical reduction. Moreover, the as-synthesized Pd NPs exhibit remarkable activity and selectivity in the tandem reaction due to the advantages of ultrasmall particle size and improved mass diffusion facilitated by the hierarchical pores.

Nanotubular TiO x N y -Supported Ir Single Atoms and Clusters as Thin-Film Electrocatalysts for Oxygen Evolution in Acid Media

A versatile approach to the production of cluster- and single atom-based thin-film electrode composites is presented. The developed TiO _x N _y -Ir catalyst was prepared from sputtered Ti-Ir alloy constituted of 0.8 ± 0.2 at % Ir in α-Ti solid solution. The Ti-Ir solid solution on the Ti metal foil substrate was anodically oxidized to form amorphous TiO₂-Ir and later subjected to heat treatment in air and in ammonia to prepare the final catalyst. Detailed morphological, structural, compositional, and electrochemical characterization revealed a nanoporous film with Ir single atoms and clusters that are present throughout the entire film thickness and concentrated at the Ti/TiO _x N _y -Ir interface as a result of the anodic oxidation mechanism. The developed TiO _x N _y -Ir catalyst exhibits very high oxygen evolution reaction activity in 0.1 M HClO₄, reaching 1460 A g^-1 _Ir at 1.6 V vs reference hydrogen electrode. The new preparation concept of single atom- and cluster-based thin-film catalysts has wide potential applications in electrocatalysis and beyond. In the present paper, a detailed description of the new and unique method and a high-performance thin film catalyst are provided along with directions for the future development of high-performance cluster and single-atom catalysts prepared from solid solutions.

Revealing the Formation Mechanism and Optimizing the Synthesis Conditions of Layered Double Hydroxides for the Oxygen Evolution Reaction

Layered double hydroxides (LDHs), whose formation is strongly related to OH^- concentration, have attracted significant interest in various fields. However, the effect of the real-time change of OH^- concentration on LDHs' formation has not been fully explored due to the unsuitability of the existing synthesis methods for in situ characterization. Here, the deliberately designed combination of NH₃ gas diffusion and in situ pH measurement provides a solution to the above problem. The obtained results revealed the formation mechanism and also guided us to synthesize a library of LDHs with the desired attributes in water at room temperature without using any additives. After evaluating their oxygen evolution reaction performance, we found that FeNi-LDH with a Fe/Ni ratio of 25/75 exhibits one of the best performances so far reported.

Through the Self-Optimization process to achieve high OER activity of SAC catalysts within the framework of TMO3@G and TMO4@G: A High-Throughput theoretical study

High-throughput DFT calculations are performed to explore the oxygen evolution reaction (OER) catalytic activity of a series of 2D graphene-based systems with TMO₃ or TMO₄ functional units. By screening the 3d/4d/5d transition metal (TM) atoms, a total of twelve TMO₃@G or TMO₄@G systems had extremely low overpotential of 0.33 ∼ 0.59 V, in which the V/Nb/Ta atom in VB group and Ru/Co/Rh/Ir atom in VIII group served as the active sites. The mechanism analysis reveals that the filling of outer electrons of TM atom can play an important role in determining the overpotential value by affecting the ΔG_O* value as an effective descriptor. Especially, in addition to the general situation of OER on the clean surface of the systems containing the Rh/Ir metal centers, the self-optimization process of TM-sites was carried out, and it made most of these single-atom catalysts (SAC) systems to have high OER catalytic activity. All these fascinating findings can contribute to an in-depth understanding of the OER catalytic activity and mechanism of the excellent graphene-based SAC systems. This work will facilitate the design and implementation of non-precious and highly efficient OER catalysts in the near future.

Alkaline Media Regulated NiFe-LDH-Based Nickel–Iron Phosphides toward Robust Overall Water Splitting

The search for low-cost, high-performance, and robust stability bifunctional electrocatalysts to substitute noble metals-based counterparts for overall water splitting to generate clean and sustainable hydrogen energy is of great significance and challenges. Herein, a high-efficient bi-functional nickel–iron phosphide on NiFe alloy foam (denoted as e-NFP/NFF) with 3D coral-like nanostructure was controllably constructed by means of alkali etching and the introduction of non-metallic atoms P. The unique superhydrophilic coral-like structure can not only effectively facilitate the exposure of catalytic active sites and increase the electroactive surface area, but also accelerate charge transport and bubble release. Furthermore, owing to the synergistic effect between the bicomponent of nickel–iron phosphides as well as the strong electronic interactions of the multiple metal sites, the as-fabricated catalyst behaves with excellent bifunctional performance for the hydrogen evolution reaction (overpotentials of 132 and 286 mV at 10 and 300 mA·cm−2, respectively) and oxygen evolution reaction (overpotentials of 181 and 303 mV at 10 and 300 mA·cm−2, respectively) in alkaline electrolytes. Impressively, cells with integrated e-NFP/NFF electrodes as a cathode and anode require only a low cell voltage (1.58 V) to drive a current density of 10 mA·cm−2 for overall water splitting, along with remarkable stability in long-term electrochemical durability tests. This study provides a tunable synthetic strategy for the development of efficient and durable non-noble metal bifunctional catalysts based on the construction of an elaborate structure framework and rational design of the electronic structure.

Water activation and splitting by single anionic iridium atoms

Mass spectrometric analysis of anionic products that result from interacting Ir^- with H₂O shows the efficient generation of [Ir(H₂O)]^- complexes and IrO^- molecular anions. Anion photoelectron spectra of [Ir(H₂O)]^-, formed under various source conditions, exhibit spectral features that are due to three different forms of the complex: the solvated anion-molecule complex, Ir^-(H₂O), as well as the intermediates, [H-Ir-OH]^- and [H₂-Ir-O]^-, where one and two O-H bonds have been broken, respectively. The measured and calculated vertical detachment energy values are in good agreement and, thus, support identification of all three types of isomers. The calculated reaction pathway shows that the overall reaction Ir^- + H₂O → IrO^- + H₂ is exothermic. Two minimum energy crossing points were found, which shuttle intermediates and products between singlet and triplet potential surfaces. This study presents the first example of water activation and splitting by single Ir^- anions.

Iridium single atoms incorporated in Co3O4 efficiently catalyze the oxygen evolution in acidic conditions

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co3O4), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir>4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co3O4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.

Isolating Contiguous Ir Atoms and Forming Ir-W Intermetallics with Negatively Charged Ir for Efficient NO Reduction by CO

The lack of efficient catalysts with a wide working temperature window and vital O₂ and SO₂ resistance for selective catalytic reduction of NO by CO (CO-SCR) largely hinders its implementation. Here, a novel Ir-based catalyst with only 1 wt% Ir loading is reported for efficient CO-SCR. In this catalyst, contiguous Ir atoms are isolated into single atoms, and Ir-W intermetallic nanoparticles are formed, which are supported on ordered mesoporous SiO₂ (KIT-6). Notably, this catalyst enables complete NO conversion to N₂ at 250 °C in the presence of 1% O₂ and has a wide temperature window (250-400 °C), outperforming the comparison samples with Ir isolated-single-atomic-sites and Ir nanoparticles, respectively. Also, it possesses a high SO₂ tolerance. Both experimental results and theoretical calculations reveal that single Ir atoms are negatively charged, dramatically enhancing the NO dissociation, while the Ir-W intermetallic nanoparticles accelerate the reduction of the N₂ O and NO₂ intermediates by CO.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

Tailoring Bond Microenvironments and Reaction Pathways of Single-Atom Catalysts for Efficient Water Electrolysis

Single-Atom Sites (SASs) are commonly stabilized and influenced by neighboring atoms in the host; disclosing the structure-reactivity relationships of SASs in water electrolysis are the grand challenges originating from the enormous support materials with complex structures. Through a multidisciplinary view of the design principles, synthesis strategies, characterization techniques, and theoretical analysis of structure-performance correlations, this timely review is dedicated to summarizing the most recent progress in tailoring bond microenvironments on different supports and discussing the reaction pathways and performance advantages of different SAS structures for water electrolysis . The essences and mechanisms of how SAS structures influence their electrocatalysis and the critical needs for their future developments are discussed. Finally, the challenges and perspectives are also provided to stimulate their practically widespread utilization in water-splitting electrolyzers.

Photothermal-Driven High-Performance Selective Hydrogenation System Enabled by Delicately Designed IrCo Nanocages

The incorporation of a transition metal into a noble metal for the formation of nanoalloys paves a potential way to modulate the electronic structures and spatial arrangement modes, thereby manipulating the target catalysis under the desired reaction pathways. Herein, a top-down synthetic route to fabricate IrCo nanoalloys with delicately designed compositions and morphologies at an extremely low calcination temperature of 200 °C is reported, which efficiently breaks through the thermodynamic limitations caused by the large atomic radii and electronegativity discrepancies between Co and Ir. A high-performance selective hydrogenation system enabled by the synthesized IrCo nanoalloys and the light irradiation is further established. Significantly, the unique properties of IrCo alloy, involving the special capability of generating local heating rather than hot electrons under light irradiation (the hot-electron effect was considered detrimental to hydrogenation reactions), as well as the highly polarized surface which aids in the hydrogen transfer from borane-ammonia complex (AB) to 4-nitrostyrene (4-NS) are discovered.

Confining High-Valence Iridium Single Sites onto Nickel Oxyhydroxide for Robust Oxygen Evolution

Enhancing activity and stability of iridium- (Ir-) based oxygen evolution reaction (OER) catalysts is of great significance in practice. Here, we report a vacancy-rich nickel hydroxide stabilized Ir single-atom catalyst (Ir₁-Ni(OH)₂), which achieves long-term OER stability over 260 h and much higher mass activity than commercial IrO₂ in alkaline media. In situ X-ray absorption spectroscopy analysis certifies the obvious structure reconstruction of catalyst in OER. As a result, an active structure in which high-valence and peripheral oxygen ligands-rich Ir sites are confined onto the nickel oxyhydroxide surface is formed. In addition, the precise introduction of atomized Ir not only surmounts the large-range dissolution and agglomeration of Ir but also suppresses the dissolution of substrate in OER. Theoretical calculations further account for the activation of Ir single atoms and the promotion of oxygen generation by high-valence Ir, and they reveal that the deprotonation process of adsorbed OH is rate-determining.

Nanostructured Metal Phosphide Based Catalysts for Electrochemical Water Splitting: A Review

Amongst various futuristic renewable energy sources, hydrogen fuel is deemed to be clean and sustainable. Electrochemical water splitting (EWS) is an advanced technology to produce pure hydrogen in a cost-efficient manner. The electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the vital steps of EWS and have been at the forefront of research over the past decades. The low-cost nanostructured metal phosphide (MP)-based electrocatalysts exhibit unconventional physicochemical properties and offer very high turnover frequency (TOF), low over potential, high mass activity with improved efficiency, and long-term stability. Therefore, they are deemed to be potential electrocatalysts to meet practical challenges for supporting the future hydrogen economy. This review discusses the recent research progress in nanostructured MP-based catalysts with an emphasis given on in-depth understanding of catalytic activity and innovative synthetic strategies for MP-based catalysts through combined experimental (in situ/operando techniques) and theoretical investigations. Finally, the challenges, critical issues, and future outlook in the field of MP-based catalysts for water electrolysis are addressed.

Facile synthesis MnCo2O4.5@C nanospheres modifying PbO2 energy-saving electrode for zinc electrowinning

The oxygen evolution reaction kinetics in industrial zinc electrowinning is sluggish, resulting in low electrocatalytic activity and substantial energy expenditure (about one-third of energy was wasted due to the strong polarization effect). Herein, the paper described a core-shell structured MnCo₂O_4.5@C modified PbO₂ electrode through the pyrolysis and co-electrodeposition as a promising candidate for zinc electrowinning. As a result, the obtained Pb-0.2%Ag/α-PbO₂/β-PbO₂-MnCo₂O_4.5@C composite electrode showed a sandwich-like structure, where Pb-0.2%Ag as a core, α-PbO₂ as a mid-layer, and β-PbO₂-MnCo₂O_4.5@C served as an electrocatalytic layer. It also possessed improved OER catalytic activity, only required 680 mV to achieve a current density of 50 mA cm^-2 and a Tafel slope of 216.04 mV dec^-1 in an acidic solution containing 50 g L^-1 Zn²⁺ and 150 g L^-1 H₂SO₄. The current efficiency increased by 0.7% and the cell voltage reduced by 360 mV as compared to a conventional Pb-0.76%Ag alloy electrode, leading to a remarkable energy-consumption reduction of 283.5 kW h for producing per ton metallic zinc. Furthermore, Pb-0.2%Ag/α-PbO₂/β-PbO₂-MnCo₂O_4.5@C exhibited a prolonged service life, which worked about 44 h under an ultra-high current density of 2 A cm^-2. Hence, this paper provides the strategy to design and construct non-precious, high-performance catalyst for electrolysis and other applications.

In-Situ Synthesis of Carbon-Encapsulated Atomic Cobalt as Highly Efficient Polysulfide Electrocatalysts for Highly Stable Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been considered as one of the most promising electrochemical energy storage systems because of their high energy density. However, a series of issues severely limit the practical performances of Li-S batteries such as low conductivity, significant volume change, and shuttle effect. The hollow carbon spheres with huge voids and high electrical conductivity are promising as sulfur hosts. Unfortunately, the nonpolar nature of carbon materials cannot prevent the shuttle effect effectively. In this case, the atomic cobalt is introduced to a nitrogen-doped hollow carbon sphere (ACo@HCS) through polymerization and controlled pyrolysis. The atomic cobalt dopants not only act as active sites to restrict the shuttle effect, but also can promote the kinetics of the sulfur redox reactions. ACo@HCS acting as sulfur host exhibits a high discharge capacity (1003 mAh g^-1 ) at a 1.0 C rate after 500 cycles, and the corresponding decay rate is as low as 0.002% per cycle. This exciting work paves a new way to design high-performance Li-S batteries.

Oxygen Vacancy and Core-Shell Heterojunction Engineering of Anemone-Like CoP@CoOOH Bifunctional Electrocatalyst for Efficient Overall Water Splitting

Constructing cost-efficient and robust bifunctional electrocatalysts for both neutral and alkaline water splitting is highly desired, but still affords a great challenge, due to sluggish hydrogen/oxygen evolution reaction (HER/OER) kinetics. Herein, an in situ integration engineering strategy of oxygen-vacancy and core-shell heterojunction to fabricate an anemone-like CoP@CoOOH core-shell heterojunction with rich oxygen-vacancies supported on carbon paper (CoP@CoOOH/CP), is described. Benefiting from the synergy of CoP core and oxygen-vacancy-rich CoOOH shell, the as-obtained CoP@CoOOH/CP catalyst displays low overpotentials at 10 mA cm^-2 for HER (89.6 mV/81.7 mV) and OER (318 mV/200 mV) in neutral and alkaline media, respectively. Notably, a two-electrode electrolyzer, using CoP@CoOOH/CP as bifunctional catalyst to achieve 10 mA cm^-2 , only needs low-cell voltages in neutral (1.65 V) and alkaline (1.52 V) electrolyte. Besides, systematically experimental and theoretical results reveal that the core-shell heterojunction efficiently accelerates the catalytic kinetics and strengthens the structural stability, while rich oxygen-vacancies efficiently decrease the kinetic barrier and activation energy, and reduce the energy barrier of the rate-determining-step for OER intermediates, thus intrinsically boosting OER performance. This work clearly demonstrates that oxygen-vacancy and core-shell heterojunction engineering provide an effective strategy to design highly-efficient non-precious, bi-functional electrocatalysts for pH-universal water splitting.

NiMoFe/Cu nanowire core-shell catalysts for high-performance overall water splitting in neutral electrolytes

A bifunctional NiMoFe/Cu NW core-shell catalyst assembled into a practical solar-driven overall water splitting system leads to an unprecedented solar-to-hydrogen (STH) efficiency of 10.99% in neutral electrolytes, attributed to the synergic combination of a unique 3D self-supported core-shell architecture and rapid electron/mass transfer properties.

Facile Synthesis of Amorphous MoCo Lamellar Hydroxide for Alkaline Water Oxidation

To find an oxygen evolution reaction (OER) catalyst with satisfactory catalytic performance and affordable cost is of great importance to the development of many new energy devices. In this work, a simple and effective strategy was developed to synthesize a series of amorphous MoCo lamellar hydroxide through one-step chemical co-precipitation. Systematic investigations showed that different functional agents (2-methylimidazole, NaOH, NH₄ OH) in the fabrication process resulted in different micromorphology of the catalyst, thus influencing its electrocatalytic performance. Also, adding various amounts of Mo could influence the intrinsic catalytic properties. Samples synthesized with appropriate functional agent addition and optimized Mo addition exhibited amorphous nature and bent nanosheet morphology, as well as highest intrinsic catalytic activity, showing a low overpotential of 290 mV at 10 mA cm^-2 and a small Tafel slope of 55 mV dec^-1 in 1 m KOH solution. Additionally, the catalytic performance of the sample showed just small decay after 50 h chronopotentiometry test and 3000 cyclic voltammetry cycles, exhibiting the ultra-stable catalytic activity of the catalyst. This work provides a possible large-scale commercial production strategy of OER catalysts with promising performance and low fabrication cost.

Unraveling the Function of Metal-Amorphous Support Interactions in Single-Atom Electrocatalytic Hydrogen Evolution

Amorphization of support in single-atoms catalysts is a less researched concept for promoting catalytic kinetics through modulating the metal-support interaction (MSI). We modeled single-atom ruthenium (Ru SAs ) supported on amorphous cobalt/nickel (oxy)hydroxide (Ru-a-CoNi) to explore the favorable MSI between Ru SAs and amorphous skeleton for alkaline hydrogen evolution reaction (HER). Differing from the usual crystal counterpart (Ru-c-CoNi), the electrons on Ru SAs are facilitated to exchange among local configurations (Ru-O-Co/Ni) of Ru-a-CoNi since flexibly amorphous configuration induces the possible d-d electrons transfer and medium-to-long range p-π orbitals coupling, further intensifying the MSI. It enables Ru-a-CoNi with enhanced water dissociation, alleviated oxophilicity, and rapid hydrogen migration, which results in superior durability and HER activity of Ru-a-CoNi, wherein only 15 mV can deliver 10 mA cm - 2 , significantly lower than 58 mV of Ru-c-CoNi.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Elucidating the Role of Single-Atom Pd for Electrocatalytic Hydrodechlorination

In this study, we loaded Pd catalysts onto a reduced graphene oxide (rGO) support in an atomically dispersed fashion [i.e., Pd single-atom catalysts (SACs) on rGO or Pd₁/rGO] via a facile and scalable synthesis based on anchor-site and photoreduction techniques. The as-synthesized Pd₁/rGO significantly outperformed the Pd nanoparticle (Pd_nano) counterparts in the electrocatalytic hydrodechlorination of chlorinated phenols. Downsizing Pd_nano to Pd₁ leads to a substantially higher Pd atomic efficiency (14 times that of Pd_nano), remarkably reducing the cost for practical applications. The unique single-atom architecture of Pd₁ additionally affects the desorption energy of the intermediate, suppressing the catalyst poisoning by Cl^-, which is a prevalent challenge with Pd_nano. Characterization and experimental results demonstrate that the superior performance of Pd₁/rGO originates from (1) enhanced interfacial electron transfer through Pd-O bonds due to the electronic metal-support interaction and (2) increased atomic H (H*) utilization efficiency by inhibiting H₂ evolution on Pd₁. This work presents an important example of how the unique geometric and electronic structure of SACs can tune their catalytic performance toward beneficial use in environmental remediation applications.

Heterostructuring Mesoporous 2D Iridium Nanosheets with Amorphous Nickel Boron Oxide Layers to Improve Electrolytic Water Splitting

2D heterostructures exhibit a considerable potential in electrolytic water splitting due to their high specific surface areas, tunable electronic properties, and diverse hybrid compositions. However, the fabrication of well-defined 2D mesoporous amorphous-crystalline heterostructures with highly active heterointerfaces remains challenging. Herein, an efficient 2D heterostructure consisting of amorphous nickel boron oxide (Ni-B_i ) and crystalline mesoporous iridium (meso-Ir) is designed for water splitting, referred to as Ni-B_i /meso-Ir. Benefiting from well-defined 2D heterostructures and strong interfacial coupling, the resulting mesoporous dual-phase Ni-B_i /meso-Ir possesses abundant catalytically active heterointerfaces and boosts the exposure of active sites, compared to their crystalline and amorphous mono-counterparts. The electronic state of the iridium sites is tuned favorably by hybridizing with Ni-B_i layers. Consequently, the Ni-B_i /meso-Ir heterostructures show superior and stable electrochemical performance toward both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline electrolyte.

Anchoring Sites Engineering in Single-Atom Catalysts for Highly Efficient Electrochemical Energy Conversion Reactions

Single-atom catalysts (SACs) have been at the frontier of research field in catalysis owing to the maximized atomic utilization, unique structures and properties. The atomically dispersed and catalytically active metal atoms are necessarily anchored by surrounding atoms. As such, the structure and composition of anchoring sites significantly influence the catalytic performance of SACs even with the same metal element. Significant progress has been made to understand structure-activity relationships at an atomic level, but in-depth understanding in precisely designing highly efficient SACs for the targeted reactions is still required. In this review, various anchoring sites in SACs are summarized and classified into five different types (doped heteroatoms, defect sites, surface atoms, metal sites, and cavity sites). Then, their impacts on catalytic performance are elucidated for electrochemical reactions based on their distance from the metal center (first coordination shell and beyond). Further, SACs anchored on two typical types of hosts, carbon- and metal-based materials, are highlighted, and the effects of anchoring points on achieving the desirable atomic structure, catalytic performance, and reaction pathways are elaborated. At last, insights and outlook to the SAC field based on current achievements and challenges are presented.

Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrO_x) due to its superior performance and excellent stability. However, large scale applications using IrO_x are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHf_xO_y) electrocatalyst for OER. The IrHf_xO_y electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrO_x electrocatalyst. The highest intrinsic mass activity of the IrHf_xO_y catalyst in acid conditions was calculated as 6950 A g_IrOx^-1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrO_x and IrHf_xO_y catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir-O, and lowers free energies for OER intermediates that accelerate the OER process.

Heuristic Iron-Cobalt-Mediated Robust pH-Universal Oxygen Bifunctional Lusters for Reversible Aqueous and Flexible Solid-State Zn-Air Cells

Rechargeable aqueous zinc-air cells (ZACs) promise an extremely safe and high energy technology. However, they are still significantly limited by sluggish electrochemical kinetics and irreversibility originating from the parasitic reactions of the bifunctional catalysts and electrolytes. Here, we report the preferential in situ building of interfacial structures featuring the edge sites constituted by FeCo single/dual atoms with the integration of Co sites in the nitrogenized graphitic carbon frameworks (FeCo SAs@Co/N-GC) by electronic structure modulation approach. Compared to commercial Pt/C and RuO₂, FeCo SAs@Co/N-GC reveals exceptional electrochemical performance, reversible redox kinetics, and durability toward oxygen reduction and evolution reactions under universal pH environments, i.e., alkaline, neutral, and acidic, due to synergistic effect at interfaces and preferred charge/mass transfer. The aqueous (alkaline, nonalkaline, and acidic electrolytes) ZACs constructed with a FeCo SAs@Co/N-GC cathode tolerate stable operations, have significant reversibility, and have the highest energy densities, outperforming those of noble metal counterparts and state-of-the-art ZACs in the ambient atmosphere. Additionally, flexible solid-state ZACs demonstrate excellent mechanical and electrochemical performances with a highest power density of 186 mW cm^-2, specific capacity of 817 mAh g_Zn^-1, energy density of 1017 Wh kg_Zn^-1, and cycle life >680 cycles with extremely harsh operating conditions, which illustrates the great potential of triphasic catalyst for green energy storage technologies.

Rational Design of Single-Atom Site Electrocatalysts: From Theoretical Understandings to Practical Applications

Atomically dispersed metal-based electrocatalysts have attracted increasing attention due to their nearly 100% atomic utilization and excellent catalytic performance. However, current fundamental comprehension and summaries to reveal the underlying relationship between single-atom site electrocatalysts (SACs) and corresponding catalytic application are rarely reported. Herein, the fundamental understandings and intrinsic mechanisms underlying SACs and corresponding electrocatalytic applications are systemically summarized. Different preparation strategies are presented to reveal the synthetic strategies with engineering the well-defined SACs on the basis of theoretical principle (size effect, metal-support interactions, electronic structure effect, and coordination environment effect). Then, an overview of the electrocatalytic applications is presented, including oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, oxidation of small organic molecules, carbon dioxide reduction reaction, and nitrogen reduction reaction. The underlying structure-performance relationship between SACs and electrocatalytic reactions is also discussed in depth to expound the enhancement mechanisms. Finally, a summary is provided and a perspective supplied to demonstrate the current challenges and opportunities for rational designing, synthesizing, and modulating the advanced SACs toward electrocatalytic reactions.