A Cobalt–Iron Double-Atom Catalyst for the Oxygen Evolution Reaction

Water dissociation on mixed Co-Fe oxide bilayer nanoislands on Au(111)

We investigate the hydroxylation behaviour of mixed Co-Fe oxide nanoislands synthesized on a Au(111) surface under exposure to water vapour at vacuum conditions. The pure Co and Fe bilayer oxides both become hydroxylated by water exposure in vacuum conditions, albeit to a very different extent. It is however an open question how mixed oxides, exposing sites with a mixed coordination to Fe and Co, behave. By forming surface O species with a mixed Fe/Co coordination, we can investigate the nature of such sites. By means of scanning tunnelling microscopy and x-ray photoelectron spectroscopy, we characterize a series of Co-Fe oxides samples with different Fe contents at the atomic scale and observe a scaling of the hydroxylation degree with the amount of Fe inside the Co-Fe oxides. Our results indicate that the Fe dopants within the Co-Fe oxides have opposing effects on edge and basal plane sites modifying the maximum hydroxylation degree of pure cobalt oxide, perturbing the original binding sites of H, releasing the absorbed H or blocking the diffusion pathway of H.

Dynamics and control of active sites in hierarchically nanostructured cobalt phosphide/chalcogenide-based electrocatalysts for water splitting

The rational design of efficient electrocatalysts for industrial water splitting is essential to generate sustainable hydrogen fuel. However, a comprehensive understanding of the complex catalytic mechanisms under harsh reaction conditions remains a major challenge. We apply a self-templated strategy to introduce hierarchically nanostructured "all-surface" Fe-doped cobalt phosphide nanoboxes (Co@CoFe-P NBs) as alternative electrocatalysts for industrial-scale applications. Operando Raman spectroscopy and X-ray absorption spectroscopy (XAS) experiments were carried out to track the dynamics of their structural reconstruction and the real catalytically active intermediates during water splitting. Our operando analyses reveal that partial Fe substitution in cobalt phosphides promotes a structural reconstruction into P-Co-O-Fe-P configurations with low-valence metal centers (M0/M+) during the hydrogen evolution reaction (HER). Results from density functional theory (DFT) demonstrate that these in situ reconstructed configurations significantly enhance the HER performance by lowering the energy barrier for water dissociation and by facilitating the adsorption/desorption of HER intermediates (H*). The competitive activity in the oxygen evolution reaction (OER) arises from the transformation of the reconstructed P-Co-O-Fe-P configurations into oxygen-bridged, high-valence CoIV-O-FeIV moieties as true active intermediates. In sharp contrast, the formation of such CoIII/IV-O-FeIII/IV moieties in Co-FeOOH is hindered under the same conditions, which outlines the key advantages of phosphide-based electrocatalysts. Ex situ studies of the as-synthesized reference cobalt sulfides (Co-S), Fe doped cobalt selenides (Co@CoFe-Se), and Fe doped cobalt tellurides (Co@CoFe-Te) further corroborate the observed structural transformations. These insights are vital to systematically exploit the intrinsic catalytic mechanisms of non-oxide, low-cost, and robust overall water splitting electrocatalysts for future energy conversion and storage.

Route to the Structure-Controlled Synthesis of Fe Nanobelts and Their Oxygen Evolution Reaction Application

Belt-shaped metal-organic frameworks (MOFs) have received extensive attention because of their unique structure. In this Communication, Fe-MOF nanobelts were synthesized by a solvothermal method with Fe²⁺ as the metal source and could not be obtained by using Fe³⁺ as the metal source. The final result shows that Fe²⁺ played a transitional role in the process of achieving belt-shaped and cubelike structural changes. Our work provides an idea for the synthesis of belt-shaped MOFs and promotes the development of electrocatalysts.

Amorphous High-entropy Non-precious metal oxides with surface reconstruction toward highly efficient and durable catalyst for oxygen evolution reaction

High-entropy materials (HEMs) have attracted extensive interests in exploring multicomponent systems for highly efficient and durable catalysts. Tuning composition and configuration of HEMs provides untapped opportunities for accessing better catalytic performance. Herein, we report three amorphous high-entropy transition metal oxides catalysts with uniform composition through a simple and controllable liquid phase non-equilibrium reduction method. The self-made catalyst FeCoNiMnBO_x exhibits excellent oxygen evolution performance, including a low overpotential (266 mV at 10 mA cm^-2), small Tafel slope (64.5 mV dec^-1) and extremely high stability (only 3.71% increase of potential after 100 h test and no current decay after cyclic voltammetry of 31,000 cycles). The outstanding performance can be attributed to the in-situ electrochemical activation induced surface reconstruction to form a stable oxyhydroxide surface layer, the cocktail effect (multi-metal synergy) brought by high entropy, and the advantages of amorphous structure itself. The outstanding catalytic properties of the new high-entropy amorphous metal oxide, as well as its advantages of low cost and simple preparation, suggest its great potential in water splitting.

Cobalt-Doped Carbon Nitride Frameworks Obtained from Calcined Aromatic Polyimines as Cathode Catalyst of Anion Exchange Membrane Fuel Cells

Cobalt-doped carbon nitride frameworks (CoNC) were prepared from the calcination of Co-chelated aromatic polyimines (APIM) synthesized from stepwise polymerization of p-phenylene diamine (PDA) and o-phthalaldehyde (OPAl) via Schiff base reactions in the presence of cobalt (II) chloride. The Co-chelated APIM (Co-APIM) precursor converted to CoNC after calcination in two-step heating with the second step performed at 100 °C lower than the first one. The CoNCs demonstrated that its Co, N-co-doped carbonaceous framework contained both graphene and carbon nanotube, as characterized by X-ray diffraction pattern, Raman spectra, and TEM micropictures. CoNCs also revealed a significant ORR peak in the current–voltage polarization cycle and a higher O₂ reduction current than that of commercial Pt/C in a linear scanning voltage test in O₂-saturated KOH_(aq). The calculated e-transferred number even reaches 3.94 in KOH_(aq) for the CoNC1000A900 cathode catalyst, which has the highest BET surface area of 393.94 m² g⁻¹. Single cells of anion exchange membrane fuel cells (AEMFCs) are fabricated using different CoNCs as the cathode catalysts, and CoNC1000A900 demonstrates a peak power density of 374.3 compared to the 334.7 mW cm⁻² obtained from the single cell using Pt/C as the cathode catalyst.

CO2 Reduction on Single-Atom Ir Catalysts with Chemical Functionalization

As promising catalytic systems, single-atom catalysts (SACs) demonstrate improved catalytic performance for electrochemical reactions. However, the pinning of metal atoms on surfaces usually depends on the adsorption on defects. In...

Electrochemical fabrication of an efficient cobalt–iron oxide/graphene heterostructure by a three-electrode system for electrocatalytic oxygen evolution reaction

Electrosynthesized cobalt–iron oxide/graphene heterostructure by a three-electrode electrolysis system for efficient oxygen evolution reaction.

Bi/Trinuclear Pt1&2Cu Clusters Assembly from Isolated Metal Atoms

We report a facile strategy in synthesizing uniform heterometallic bi/tri-atom clusters starting from mono-metallic atoms in liquid phase. Specifically, Pt1&2Cu bi/tri-atoms are prepared by reducing CuCl2 at preformed Pt1 atoms...

Multi-atom cluster catalysts for efficient electrocatalysis

This review presents recent developments in the synthesis, modulation and characterization of multi-atom cluster catalysts for electrochemical energy applications.

Enhanced oxygen evolution performance by partial phase transformation of cobalt/nickel carbonate hydroxide nanosheet arrays in Fe-containing alkaline electrolyte

Herein, we employ a partial phase conversion strategy to transform cobalt/nickel carbonate hydroxide (CoxNiyCH) nanosheet arrays in Fe-containing KOH electrolyte. The optimized sample exhibits a remarkable electrocatalytic activity (η50 =...

Pd/Fe2O3 with Electronic Coupling Single-Site Pd-Fe Pair Sites for Low-Temperature Semihydrogenation of Alkynes

Dispersing single palladium atoms on a support is promising to minimize the usage of palladium and improve the selectivity for alkyne semihydrogenation, but its activity is often very low as a result of unfavorable H₂ activation. Here, we load palladium onto α-Fe₂O₃(012) to construct highly active and stable single-site Pd-Fe pairs with luxuriant d-electron domination near the Fermi level driven by strong electronic coupling and prove that Pd-Fe pairs cooperatively adsorb H₂ and dissociate an H─H bond, whereas solo Pd sites enable preferential desorption of C═C intermediate, thus achieving both high activity and high selectivity for alkyne hydrogenation. This catalyst exhibits state-of-the-art performance in purifying acetylene of ethylene stream, with 99.6% and 100% conversion and 96.7% and 94.7% selectivity at 353 and 393 K, respectively, and excellent stability with negligible activity decay after a 200 h test. This single-site pair inherits the advantage but overcomes the weakness of both Pd ensemble and single Pd atoms, enabling ultralow-Pd-loading catalysts for selective hydrogenation.

Carbonaceous Oxygen Evolution Reaction Catalysts: From Defect and Doping-Induced Activity over Hybrid Compounds to Ordered Framework Structures

Oxygen evolution reaction (OER) is expected to be of great importance for the future energy conversion and storage in form of hydrogen by water electrolysis. Besides the traditional noble-metal or transition metal oxide-based catalysts, carbonaceous electrocatalysts are of great interest due to their huge structural and compositional variety and unrestricted abundance. This review provides a summary of recent advances in the field of carbon-based OER catalysts ranging from "pure" or unintentionally doped carbon allotropes over heteroatom-doped carbonaceous materials and carbon/transition metal compounds to metal oxide composites where the role of carbon is mainly assigned to be a conductive support. Furthermore, the review discusses the recent developments in the field of ordered carbon framework structures (metal organic framework and covalent organic framework structures) that potentially allow a rational design of heteroatom-doped 3D porous structures with defined composition and spatial arrangement of doping atoms to deepen the understanding on the OER mechanism on carbonaceous structures in the future. Besides introducing the structural and compositional origin of electrochemical activity, the review discusses the mechanism of the catalytic activity of carbonaceous materials, their stability under OER conditions, and potential synergistic effects in combination with metal (or metal oxide) co-catalysts.

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

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

Toward Multicomponent Single-Atom Catalysis for Efficient Electrochemical Energy Conversion

Single-atom catalysts (SACs) have recently emerged as the ultimate solution for overcoming the limitations of traditional catalysts by bridging the gap between homogeneous and heterogeneous catalysts. Atomically dispersed identical active sites enable a maximal atom utilization efficiency, high activity, and selectivity toward the wide range of electrochemical reactions, superior structural robustness, and stability over nanoparticles due to strong atomic covalent bonding with supports. Mononuclear active sites of SACs can be further adjusted by engineering with multicomponent elements, such as introducing dual-metal active sites or additional neighbor atoms, and SACs can be regarded as multicomponent SACs if the surroundings of the active sites or the active sites themselves consist of multiple atomic elements. Multicomponent engineering offers an increased combinational diversity in SACs and unprecedented routes to exceed the theoretical catalytic performance limitations imposed by single-component scaling relationships for adsorption and transition state energies of reactions. The precisely designed structures of multicomponent SACs are expected to be responsible for the synergistic optimization of the overall electrocatalytic performance by beneficially modulating the electronic structure, the nature of orbital filling, the binding energy of reaction intermediates, the reaction pathways, and the local structural transformations. This Review demonstrates these synergistic effects of multicomponent SACs by highlighting representative breakthroughs on electrochemical conversion reactions, which might mitigate the global energy crisis of high dependency on fossil fuels. General synthesis methods and characterization techniques for SACs are also introduced. Then, the perspective on challenges and future directions in the research of SACs is briefly summarized. We believe that careful tailoring of multicomponent active sites is one of the most promising approaches to unleash the full potential of SACs and reach the superior catalytic activity, selectivity, and stability at the same time, which makes SACs promising candidates for electrocatalysts in various energy conversion reactions.

Ultra-low-loaded Ni-Fe Dimer Anchored to Nitrogen/Oxygen Sites for Boosting Electroreduction of Carbon Dioxide

Single-atom catalysts (SACs), as a novel emerging category in heterogeneous catalysis, have exhibited superb activity and selectivity within the scope of many catalytic reactions, originating from their nature of atomic dispersion. However, they are not appropriate for more complicated reactions that benefit from multi-metal promotion, such as the carbon dioxide reduction reaction (CO₂ RR). Atomic pair catalysts can provide a synergistic effect to break the intrinsic activity limit. Herein, inspired by theoretical prediction, a hetero-paired atomic-site catalyst (Ni/Fe-N/O-C) was developed for CO₂ RR. Typically, the trace-amount-loaded double-atom-site catalysts exhibited outstanding turnover frequencies (≈460 s^-1 ) surpassing reported ones by far. Interestingly, the loaded metal contents of the three M-N/O-C samples were extremely low, and Ni/Fe-N/O-C exhibited greatly improved durability compared with pure Ni-N/O-C or Fe-N/O-C and excellent CO selectivity above 80 % within a broad potential window of -1.4 to -1.7 V (vs. saturated calomel electrode, 99.8 % at -1.5 V). The superb performance of diatomic-site catalysts was attributed to the adjusted local environment and electron structure of the active center, which could decrease the reaction barrier of *COOH formation. This work presents new insights into manipulating electrocatalytic performance for the development of more sophisticated active sites.

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.

Synergistic Effect of Atomically Dispersed Ni-Zn Pair Sites for Enhanced CO2 Electroreduction

Dual-atom catalysts have the potential to outperform the well-established single-atom catalysts for the electrochemical conversion of CO₂ . However, the lack of understanding regarding the mechanism of this enhanced catalytic process prevents the rational design of high-performance catalysts. Herein, an obvious synergistic effect in atomically dispersed Ni-Zn bimetal sites is observed. In situ characterization combined with density functional theory (DFT) calculations reveals that heteronuclear coordination modifies the d-states of the metal atom, narrowing the gap between the d-band centre (ε_d ) of the Ni (3d) orbitals and the Fermi energy level (E_F ) to strengthen the electronic interaction at the reaction interface, resulting in a lower free energy barrier (ΔG) in the thermodynamic pathway and a reduced activation energy (E_a ) as well as fortified metal-C bonding in the kinetic pathway. Consequently, a CO faradaic efficiency of >90% is obtained across a broad potential window from -0.5 to -1.0 V (vs RHE), reaching a maximum of 99% at -0.8 V, superior to that of the Ni/Zn single-metal sites.

Hybrid Metal-Boron Diatomic Site Embedded in C2 N Monolayer Promotes C-C Coupling in CO2 Electroreduction

Double-atom catalyst (DAC) has gained much interest for its versatile tuning and synergistic effect of dual-atom active sites. Metal (M)-metal (M) diatomic sites, either homo- or heteronuclear, are typically researched. Hybrid metal-non-metal combined sites have rarely been studied and even the viability of such active sites are unknown. Herein, CO₂ electroreduction (CO₂ RR) is explored on M@X-C₂ N (M = Fe, Co, Ni, and Cu; X = S, P, and B) which renders naturally generated M-X diatomic site. Using spin-polarized density functional theory coupled with computational hydrogen electrode model, it is demonstrated that the functionality of hybrid M-B dual-atom center is superior over that of a single- or double-M center in driving CO₂ RR especially C-C coupling. Among metal-boron DACs studies, Fe@B-C₂ N (μ = 2μ_B ) exhibits the lowest free energy barrier of 0.17 eV in C-C coupling whereas Ni@B-C₂ N (μ = 0μ_B ) mainly produces CH₄ with the lowest barrier of 0.42 eV. Hence, the electronic spin state of M can be particularly important in modulating selectivity and C-C coupling barrier in CO₂ RR. Fe@B-C₂ N is predicted as the promising catalyst for CO₂ RR towards C₂₊ products owing partially to its enhanced spin state. The findings can enrich the design strategy of electrocatalysts normally running at ambient conditions.

Mechanistic insight into the active centers of single/dual-atom Ni/Fe-based oxygen electrocatalysts

Single-atom catalysts with maximum metal utilization efficiency show great potential for sustainable catalytic applications and fundamental mechanistic studies. We here provide a convenient molecular tailoring strategy based on graphitic carbon nitride as support for the rational design of single-site and dual-site single-atom catalysts. Catalysts with single Fe sites exhibit impressive oxygen reduction reaction activity with a half-wave potential of 0.89 V vs. RHE. We find that the single Ni sites are favorable to promote the key structural reconstruction into bridging Ni-O-Fe bonds in dual-site NiFe SAC. Meanwhile, the newly formed Ni-O-Fe bonds create spin channels for electron transfer, resulting in a significant improvement of the oxygen evolution reaction activity with an overpotential of 270 mV at 10 mA cm⁻². We further reveal that the water oxidation reaction follows a dual-site pathway through the deprotonation of *OH at both Ni and Fe sites, leading to the formation of bridging O₂ atop the Ni-O-Fe sites.

The development of high performance dual-site single-atom catalysts is a promising research direction. Here, the authors report structural dynamics of dual-site nickel-iron single-atom oxygen electrocatalysts under reaction conditions, and proposes a dual-site pathway for the water oxidation reaction.

Single Iridium Atom Doped Ni2P Catalyst for Optimal Oxygen Evolution

Single-atom catalysts (SACs) with 100% active sites have excellent prospects for application in the oxygen evolution reaction (OER). However, further enhancement of the catalytic activity for OER is quite challenging, particularly for the development of stable SACs with overpotentials <180 mV. Here, we report an iridium single atom on Ni₂P catalyst (Ir_SA-Ni₂P) with a record low overpotential of 149 mV at a current density of 10 mA·cm^-2 in 1.0 M KOH. The Ir_SA-Ni₂P catalyst delivers a current density up to ∼28-fold higher than that of the widely used IrO₂ at 1.53 V vs RHE. Both the experimental results and computational simulations indicate that Ir single atoms preferentially occupy Ni sites on the top surface. The reconstructed Ir-O-P/Ni-O-P bonding environment plays a vital role for optimal adsorption and desorption of the OER intermediate species, which leads to marked enhancement of the OER activity. Additionally, the dynamic "top-down" evolution of the specific structure of the Ni@Ir particles is responsible for the robust single-atom structure and, thus, the stability property. This Ir_SA-Ni₂P catalyst offers novel prospects for simplifying decoration strategies and further enhancing OER performance.

Single-Atom Electrocatalysts for Multi-Electron Reduction of CO2

The multi-electron reduction of CO₂ to hydrocarbons or alcohols is highly attractive in a sustainable energy economy, and the rational design of electrocatalysts is vital to achieve these reactions efficiently. Single-atom electrocatalysts are promising candidates due to their well-defined coordination configurations and unique electronic structures, which are critical for delivering high activity and selectivity and may accelerate the explorations of the activity origin at atomic level as well. Although much effort has been devoted to multi-electron reduction of CO₂ on single-atom electrocatalysts, there are still no reviews focusing on this emerging field and constructive perspectives are also urgent to be addressed. Herein recent advances in how to design efficient single-atom electrocatalysts for multi-electron reduction of CO₂ , with emphasis on strategies in regulating the interactions between active sites and key reaction intermediates, are summarized. Such interactions are crucial in designing active sites for optimizing the multi-electron reduction steps and maximizing the catalytic performance. Different design strategies including regulation of metal centers, single-atom alloys, non-metal single-atom catalysts, and tandem catalysts, are discussed accordingly. Finally, current challenges and future opportunities for deep electroreduction of CO₂ are proposed.

Atomically Structural Regulations of Carbon-Based Single-Atom Catalysts for Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂ RR) converting CO₂ into value-added chemicals and fuels to realize carbon recycling is a solution to the problem of renewable energy shortage and environmental pollution. Among all the catalysts, the carbon-based single-atom catalysts (SACs) with isolated metal atoms immobilized on conductive carbon substrates have shown significant potential toward CO₂ RR, which intrigues researchers to explore high-performance SACs for fuel and chemical production by CO₂ RR. Especially, regulating the coordination structures of the metal centers and the microenvironments of the substrates in carbon-based SACs has emerged as an effective strategy for the tailoring of their CO₂ RR catalytic performance. In this review, the current in situ/operando study techniques and the fundamental parameters for CO₂ RR performance are first briefly presented. Furthermore, the recent advances in synthetic strategies which regulate the atomic structures of the carbon-based SACs, including heteroatom coordination, coordination numbers, diatomic metal centers, and the microenvironments of substrates are summarized. In particular, the structure-performance relationship of the SACs toward CO₂ RR is highlighted. Finally, the inevitable challenges for SACs are outlined and further research directions toward CO₂ RR are presented from the perspectives.

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.

Atomically Dispersed Reactive Centers for Electrocatalytic CO2 Reduction and Water Splitting

Developing electrocatalytic energy conversion technologies for replacing the traditional energy source is highly expected to resolve the fossil fuel exhaustion and related environmental problems. Exploring stable and high-efficiency electrocatalysts is of vital importance for the promotion of these technologies. Single-atom catalysts (SACs), with atomically distributed active sites on supports, perform as emerging materials in catalysis and present promising prospects for a wide range of applications. The rationally designed near-range coordination environment, long-range electronic interaction and microenvironment of the coordination sphere cast huge influence on the reaction mechanism and related catalytic performance of SACs. In the current Review, some recent developments of atomically dispersed reactive centers for electrocatalytic CO2 reduction and water splitting are well summarized. The catalytic mechanism and the underlying structure-activity relationship are elaborated based on the recent progresses of various operando investigations. Finally, by highlighting the challenges and prospects for the development of single-atom catalysis, we hope to shed some light on the future research of SACs for the electrocatalytic energy conversion.

Which is Better for Nanomedicines: Nanocatalysts or Single-Atom Catalysts?

With the rapid advancements in nanotechnology and materials science, numerous nanomaterials have been used as catalysts for nanomedical applications. Their design and modification according to the microenvironment of diseases have been shown to achieve effective treatment. Chemists are in pursuit of nanocatalysts that are more efficient, controllable, and less toxic by developing innovative synthetic technologies and improving existing ones. Recently, single-atom catalysts (SACs) with excellent catalytic activity and high selectivity have attracted increasing attention because of their accurate design as nanomaterials at the atomic level, thereby highlighting their potential for nanomedical applications. In this review, the recent advances in nanocatalysts and SACs are briefly summarized according to their synthesis, characterizations, catalytic mechanisms, and nanomedical applications. The opportunities and future scope for their development and the issues and challenges for their application as nanomedicine are also discussed. As far as it is known, the review is the systematic comparison of nanocatalysts and SACs, especially in the field of nanomedicine, which has promoted the development of nanocatalytic medicine.

Co and Pt Dual-Single-Atoms with Oxygen-Coordinated Co-O-Pt Dimer Sites for Ultrahigh Photocatalytic Hydrogen Evolution Efficiency

The platinum single-atom-catalyst is verified as a very successful route to approach the size limit of Pt catalysts, while how to further improve the catalytic efficiency of Pt is a fundamental scientific question and is challenging because the size issue of Pt is approached at the ultimate ceiling as single atoms. Here, a new route for further improving Pt catalytic efficiency by cobalt (Co) and Pt dual-single-atoms on titanium dioxide (TiO₂ ) surfaces, which contains a fraction of nonbonding oxygen-coordinated Co-O-Pt dimers, is reported. These Co-Pt dimer sites originate from loading high-density Pt single-atoms and Co single-atoms, with them anchoring randomly on the TiO₂ substrate. This dual-single-atom catalyst yields 13.4% dimer sites and exhibits an ultrahigh and stable photocatalytic activity with a rate of 43.467 mmol g^-1 h^-1 and external quantum efficiency of ≈83.4% at 365 nm. This activity far exceeds those of equal amounts of Pt single-atom and typical Pt clustered catalysts by 1.92 and 1.64 times, respectively. The enhancement mechanism relies on the oxygen-coordinated Co-O-Pt dimer coupling, which can mutually optimize the electronic states of both Pt and Co sites to weaken H* binding. Namely, the "mute" Co single-atom is activated by Pt single-atom and the activity of the Pt atom is further enhanced through the dimer interaction. This strategy of nonbonding interactive dimer sites and the oxygen-mediated catalytic mechanisms provide emerging rich opportunities for greatly improving the catalytic efficiency and developing novel catalysts with creating new electronic states.

Single-Atom Catalysts: A Sustainable Pathway for the Advanced Catalytic Applications

A heterogeneous catalyst is a backbone of modern sustainable green industries; and understanding the relationship between its structure and properties is the key for its advancement. Recently, many upscaling synthesis strategies for the development of a variety of respectable control atomically precise heterogeneous catalysts are reported and explored for various important applications in catalysis for energy and environmental remediation. Precise atomic-scale control of catalysts has allowed to significantly increase activity, selectivity, and in some cases stability. This approach has proved to be relevant in various energy and environmental related technologies such as fuel cell, chemical reactors for organic synthesis, and environmental remediation. Therefore, this review aims to critically analyze the recent progress on single-atom catalysts (SACs) application in oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and chemical and/or electrochemical organic transformations. Finally, opportunities that may open up in the future are summarized, along with suggesting new applications for possible exploitation of SACs.

Cooperativity in supported metal single atom catalysis

The discussion concerning cooperativity in supported single-atom (SA) catalysis is often limited to the metal-support interaction, which is certainly important, but which is not the only lever for modifying the catalytic performance. Indeed, if the interaction between the SA and the support, which can be seen as a solid ligand presenting its own specificities that fix the first coordination sphere of the metal, plays a central role as in homogeneous catalysis, other factors can strongly contribute to modification of the activity, selectivity and stability of SAs. Therefore, in this mini-review, we briefly summarize the importance of the support (oxide, carbon or a second metal) in SA photo- electro- and thermal-catalysis (support-assisted operation), and concentrate on other types of cooperativities that in some cases enable previously impossible reaction pathways on supported metal SAs. This includes topics that are not specific to SA catalysis, such as metal-ligand or heterobimetallic cooperativity, and cooperativity which is SA-specific such as nanoparticle-SA or mixed-valence SA cooperativity.

Fe doped amorphous single layered vanadyl phosphate nanosheets as highly efficient electrocatalyst for water oxidation

The search for earth-abundant water oxidation electrocatalysts with low-cost and high-performance is essential to the energy conversion field. Well defined, rational designed two-dimensional materials have attracted enormous interest in light of much more specific surface areas and unique electronic properties. Herein, we report a facile two-phase solvothermal approach for the synthesis of Fe doped amorphous single-layered (~0.85 nm) vanadyl phosphate nanosheets (Fe-Am VOPO₄). The obtained electrocatalyst exhibits excellent OER electrocatalytic performance, only require overpotential of 215 mV and 270 mV to reach current densities of 10 and 100 mA cm^-2 in 1.0 M KOH electrolyte, and long-term electrochemical stability of 40 h. This work strikes out a path of synthesis of graphene-like materials with amorphous phase, and explores a new type of phosphate for efficient OER electrocatalysts.

Spatial Confinement of a Carbon Nanocone for an Efficient Oxygen Evolution Reaction

A major bottleneck of large-scale water splitting for hydrogen production is the lack of catalysts for the oxygen evolution reaction (OER) with low cost and high efficiency. In this work, we proposed an electrocatalyst of a curved carbon nanocone embedded with two TMN₄ active sites (TM = transition metal) and used first-principles calculations to investigate their OER mechanisms and catalytic activities. In the particular spatial confinement of a curved nanocone, we found that the distance between intermediates adsorbed on two active sites is shorter than the distance between these two active sites. This finding can be used to enhance OER activity by distance-dependent interaction between intermediates through two different mechanisms. The first mechanism in which an O₂ molecule is generated from two neighboring *O intermediates exhibits a linear activity trend, and the lowest overpotential is 0.27 V for the FeN₄ system. In the second mechanism, selective stabilization of the *OOH intermediate is realized, leading to a new scaling relationship (ΔG_*OOH = ΔG_*OH + 3.04 eV) associated with a modified OER activity volcano (theoretical volcano apex at 0.29 V). The studied mechanisms of the spatial confinement of a carbon nanocone provide a new perspective for designing efficient OER catalysts.

Effects of Annealing Temperature on the Oxygen Evolution Reaction Activity of Copper-Cobalt Oxide Nanosheets

Developing high performance, highly stable, and low-cost electrodes for the oxygen evolution reaction (OER) is challenging in water electrolysis technology. However, Ir- and Ru-based OER catalysts with high OER efficiency are difficult to commercialize as precious metal-based catalysts. Therefore, the study of OER catalysts, which are replaced by non-precious metals and have high activity and stability, are necessary. In this study, a copper–cobalt oxide nanosheet (CCO) electrode was synthesized by the electrodeposition of copper–cobalt hydroxide (CCOH) on Ni foam followed by annealing. The CCOH was annealed at various temperatures, and the structure changed to that of CCO at temperatures above 250 °C. In addition, it was observed that the nanosheets agglomerated when annealed at 300 °C. The CCO electrode annealed at 250 °C had a high surface area and efficient electron conduction pathways as a result of the direct growth on the Ni foam. Thus, the prepared CCO electrode exhibited enhanced OER activity (1.6 V at 261 mA/cm²) compared to those of CCOH (1.6 V at 144 mA/cm²), Co₃O₄ (1.6 V at 39 mA/cm²), and commercial IrO₂ (1.6 V at 14 mA/cm²) electrodes. The optimized catalyst also showed high activity and stability under high pH conditions, demonstrating its potential as a low cost, highly efficient OER electrode material.

CoMn phosphide encapsulated in nitrogen-doped graphene for electrocatalytic hydrogen evolution over a broad pH range

CoMn phosphide encapsulated in a nitrogen-doped graphene core-shell structure was successfully prepared with Prussian Blue complexes as the precursor and P atom doping at a high temperature. The core-shell heterostructure demonstrates an impressive catalytic performance of hydrogen evolution reaction over a broad pH range, i.e., the overpotentials reach 159, 190 and 279 mV at a current density of 20 mA cm^-2 in the conditions of pH = 0, 14 and 7, and maintains excellent stability.

Structural tuning of heterogeneous molecular catalysts for electrochemical energy conversion

Heterogeneous molecular catalysts based on transition metal complexes have received increasing attention for their potential application in electrochemical energy conversion. The structural tuning of first and second coordination spheres of complexes provides versatile strategies for optimizing the activities of heterogeneous molecular catalysts and appropriate model systems for investigating the mechanism of structural variations on the activity. In this review, we first discuss the variation of first spheres by tuning ligated atoms; afterward, the structural tuning of second spheres by appending adjacent metal centers, pendant groups, electron withdrawing/donating, and conjugating moieties on the ligands is elaborated. Overall, these structural tuning resulted in different impacts on the geometric and electronic configurations of complexes, and the improved activity is achieved through tuning the stability of chemisorbed reactants and the redox behaviors of immobilized complexes.

Tuning the Selective Adsorption Site of Biomass on Co3 O4 by Ir Single Atoms for Electrosynthesis

The electrosynthesis from 5-hydroxymethylfurfural (HMF) is considered a green strategy to achieve biomass-derived high-value chemicals. As the molecular structure of HMF is relatively complicated, understanding the HMF adsorption/catalysis behavior on electrocatalysts is vital for biomass-based electrosynthesis. The electrocatalysis behavior can be modulated by tuning the adsorption energy of the reactive molecules. In this work, the HMF adsorption behavior on spinel oxide, Co₃ O₄ is discovered. Correspondingly, the adsorption energy of HMF on Co₃ O₄ is successfully tuned by decorating with single-atom Ir. It is observed that compared with bare Co₃ O₄ , single-atom-Ir-loaded Co₃ O₄ (Ir-Co₃ O₄ ) can enhance adsorption with the CC groups of HMF. The synergetic adsorption can enhance the overall conversion of HMF on electrocatalysts. With the modulated HMF adsorption, the as-designed Ir-Co₃ O₄ exhibits a record performance (with an onset potential of 1.15 V_RHE ) for the electrosynthesis from HMF.