Freestanding Single-Atom-Layer Pd-Based Catalysts: Oriented Splitting of Energy Bands for Unique Stability and Activity

Ce Salt-Assisted Construction of Bilayer Cu-Ce-O Nanostructure Arrays on Cu Foil Enhances Methanol Oxidation Performance

The construction of highly ordered nanostructures on copper offers significant advantages in energy conversion electrocatalysis, particularly as a potential alternative to platinum-based precious metal catalysts in the methanol oxidation process to prevent catalytic poisoning. However, the self-assembly and ordered growth of nanostructures on copper has been a research challenge. Herein, we report a Ce salt-assisted electrooxidation strategy for the first time to prepare bilayer Cu-Ce-O nanostructures on copper, containing Cu2O-Ce2O3 nanorods on the top and Cu2O nanoparticles next to the substrate. The incorporation of Ce salts into the electrolyte enables the controllable oriented growth of nanostructures from cubic nanoparticles to nanorods. Moreover, the high activity surface area of nanorods demonstrates enhanced catalytic activity and long-term stability (6 h) in methanol oxidation, exhibiting a current density of 71.5 mA cm-2 and 72 mV lower onset overpotential compared to blank Cu foil. Density functional theory calculations also demonstrate that Cu2O enhances the adsorption of CH3OH. This strategy provides new insights into the rational construction of sophisticated copper-based nanostructures, showing promising potential for applications in methanol catalysis.

Nanoscale Confinement-Induced Atom-Milling Pd Nano-Sheets Into Ultra-Fine Pd3Co Alloys

Atomic regulation of nanocatalysts is of both scientific and technological importance in efficient hydrogen energy conversion technology. Here a feasible nanoscale confinement-induced atom-milling (NCAM) method is reported to synthesize and atomically mesh 1 nm thick Pd metallic films into ultra-fine Pd3Co nanoalloys. The nano-sized inner space of layered crystalline provides a nano-scaled limitation (1.3 nm) for atoms' collision, which forces the assembly of hydrogen or Co atoms into a Pd lattice to expand or contract crystal as atom-milling. The resulting ultra-fine Pd3Co nanoalloys exhibit nearly three times higher activity than commercial Pt/C for oxygen reduction reaction, and 2.3 times higher activity than Pd/C for formic acid electrooxidation reaction, for fuel cells, and wider electrocatalysis applications.

Atomic Layer Thickness Modulated the Catalytic Activity of Platinum for Oxygen Reduction and Hydrogen Oxidation Reaction

Reducing platinum (Pt) usage and enhancing its catalytic performance in the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) are vital for advancing fuel cell technology. This study presents the design and investigation of monolayer and few-layer Pt structures with high platinum utilization, developed through theoretical calculations. By minimizing the metal thickness from 1 to 3 atomic layers, an atomic utilization rate ranging from 66.66% to 100% is achieved, in contrast to conventional multilayer Pt structures. This reduction resulted in a unique surface coordination environment. These thinner structures exhibited nonlinear fluctuations in key electronic characteristics-such as the d-band center, surface charge, and work function-as the atomic layer thickness decreased. These variations significantly impacted species adsorption and the Pt-H2O interfacial structure, which in turn affected the catalytic activity. Notably, 1-layer Pt exhibited the best performance for HOR, while 3-layer Pt showed high activity for both HOR and ORR. The findings establish a clear relationship between atomic layer thickness, surface characteristics, adsorption behavior, electric double-layer structure, and catalytic performance in Pt systems. This research contributes to a deeper understanding of precision atomic-structured electrocatalyst design and paves the way for the development of highly effective, low-loading Pt-based catalytic materials.

Galvanic replacement mediated morphological adjustments boost nanoparticle performance in electrocatalytic alcohol oxidation

The PdPtCo core-shell nanostructure, fabricated via galvanic replacement by rapidly incorporating Pd into PtCo nanoalloys at room temperature, excels in alcohol oxidation (AOR). Synergistic effects lower Pt electron density, promoting C-C cleavage, while increased Co coverage reduces COads adsorption, enhancing CO tolerance. This work offers a novel strategy for high-performance AOR catalysts.

Strong Metal-Support Interactions in Heterogeneous Oxygen Electrocatalysis

Molecular oxygen redox electrocatalysis involves oxygen reduction and evolution as core reactions in various energy conversion and environmental technology fields. Strong metal-support interactions (SMSIs) based nanomaterials are regarded as desirable and state-of-the-art heterogeneous electrocatalysts due to their exceptional physicochemical properties. Over the past decades, considerable advancements in theory and experiment have been achieved in related studies, especially in modulating the electronic structure and geometrical configuration of SMSIs to enable activity, selectivity, and stability. In this focuses on the concept of SMSI, explore their various manifestations and mechanisms of action, and summarizes recent advances in SMSIs for efficient energy conversion in oxygen redox electrocatalysis applications. Additionally, the correlation between the physicochemical properties of different metals and supports is systematically elucidated, and the potential mechanisms of the structure-activity relationships between SMSIs and catalytic performance are outlined through theoretical models. Finally, the obstacles confronting this burgeoning field are comprehensively concluded, targeted recommendations and coping strategies are proposed, and future research perspectives are outlined.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Toward Functionality and Deactivation of Metal-Single-Atom in Heterogeneous Photocatalysts

Single-atom heterogeneous catalysts (SAHCs) provide an enticing platform for understanding catalyst structure-property-performance relationships. The 100% atom utilization and adjustable local coordination configurations make it easy to probe reaction mechanisms at the atomic level. However, the progressive deactivation of metal-single-atom (MSA) with high surface energy leads to frequent limitations on their commercial viability. This review focuses on the atomistic-sensitive reactivity and atomistic-progressive deactivation of MSA to provide a unifying framework for specific functionality and potential deactivation drivers of MSA, thereby bridging function, purpose-modification structure-performance insights with the atomistic-progressive deactivation for sustainable structure-property-performance accessibility. The dominant functionalization of atomically precise MSA acting on properties and reactivity encompassing precise photocatalytic reactions is first systematically explored. Afterward, a detailed analysis of various deactivation modes of MSA and strategies to enhance their durability is presented, providing valuable insights into the design of SAHCs with deactivation-resistant stability. Finally, the remaining challenges and future perspectives of SAHCs toward industrialization, anticipating shedding some light on the next stage of atom-economic chemical/energy transformations are presented.

Construction of Pd-Te Intermetallic Compounds to Achieve Ultrastable Oxygen Reduction Activity

Palladium (Pd)-transition metal alloys have the potential to regulate the intermediate surface adsorption strength in oxygen reduction reactions (ORR), making them a promising substitute for platinum-based catalysts. Nonetheless, prolonged electrochemical cycling can lead to the depletion of transition metals, resulting in structural degradation and poor durability. Herein, the synthesis of alloy catalysts (Pd25%Te75%) containing Pd and the metalloid tellurium (Te) through a one-step reduction method is reported. Characterizations of powder X-ray photoelectron spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy demonstrated both uniform dispersion and strong binding force of elements within the PdTe alloy, along with providing crystallographic details of associated compounds. Based on density functional theory calculations, PdTe had a more negative d-band center than that of pure Pd, which reduces the adsorption capacity between active sites and intermediates in the ORR, and therefore enhances reaction kinetics. The Pd25%Te75% exhibited excellent ORR activity, and its onset and half-wave potentials were ∼0.98 and ∼0.90 V, respectively, at 1600 rpm within the O2-saturated 1.0 M KOH. Significantly, accelerated durability tests achieved exceptional stability, and half-wave potential just decayed by 4 mV after 30000 consecutive cycles. Moreover, this study aims to promote the preparation of Pd and metalloid alloys for other energy conversion applications.

Rearranging Spin Electrons by Axial-Ligand-Induced Orbital Splitting to Regulate Enzymatic Activity of Single-Atom Nanozyme with Destructive d-π Conjugation

Most of the nanozymes have been obtained based on trial and error, for which the application is usually compromised by enzymatic activity regulation due to a vague catalytic mechanism. Herein, a hollow axial Mo-Pt single-atom nanozyme (H-MoN5@PtN4/C) is constructed by a two-tier template capture strategy. The axial ligand can induce Mo 4d orbital splitting, leading to a rearrangement of spin electrons (↑ ↑ → ↑↓) to regulate enzymatic activity. This creates catalase-like activity and enhances oxidase-like activity to catalyze cascade enzymatic reactions (H2O2 → O2 → O2•-), which can overcome tumor hypoxia and accumulate cytotoxic superoxide radicals (O2•-). Significantly, H-MoN5@PtN4/C displays destructive d-π conjugation between the metal and substrate to attenuate the restriction of orbitals and electrons. This markedly improves enzymatic performance (catalase-like and oxidase-like activity) of a Mo single atom and peroxidase-like properties of a Pt single atom. Furthermore, the H-MoN5@PtN4/C can deplete overexpressed glutathione (GSH) through a redox reaction, which can avoid consumption of ROS (O2•- and •OH). As a result, H-MoN5@PtN4/C can overcome limitations of a complex tumor microenvironment (TME) for tumor-specific therapy based on TME-activated catalytic activity.

Optimizing Intermediate Adsorption over PdM (M=Fe, Co, Ni, Cu) Bimetallene for Boosted Nitrate Electroreduction to Ammonia

Electrochemical reduction of nitrate to ammonia (NO3RR) is a promising and eco-friendly strategy for ammonia production. However, the sluggish kinetics of the eight-electron transfer process and poor mechanistic understanding strongly impedes its application. To unveil the internal laws, herein, a library of Pd-based bimetallene with various transition metal dopants (PdM (M=Fe, Co, Ni, Cu)) are screened to learn their structure-activity relationship towards NO3RR. The ultra-thin structure of metallene greatly facilitates the exposure of active sites, and the transition metals dopants break the electronic balance and upshift its d-band center, thus optimizing intermediates adsorption. The anisotropic electronic characteristics of these transition metals make the NO3RR activity in the order of PdCu>PdCo≈PdFe>PdNi>Pd, and a record-high NH3 yield rate of 295 mg h-1 mgcat -1 along with Faradaic efficiency of 90.9 % is achieved in neutral electrolyte on PdCu bimetallene. Detailed studies further reveal that the moderate N-species (*NO3 and *NO2) adsorption ability, enhanced *NO activation, and reduced HER activity facilitate the NH3 production. We believe our results will give a systematic guidance to the future design of NO3RR catalysts.

Single-atom Mo-tailored high-entropy-alloy ultrathin nanosheets with intrinsic tensile strain enhance electrocatalysis

The precise structural integration of single-atom and high-entropy-alloy features for energy electrocatalysis is highly appealing for energy conversion, yet remains a grand challenge. Herein, we report a class of single-atom Mo-tailored PdPtNiCuZn high-entropy-alloy nanosheets with dilute Pt-Pt ensembles and intrinsic tensile strain (Mo1-PdPtNiCuZn) as efficient electrocatalysts for enhancing the methanol oxidation reaction catalysis. The as-made Mo1-PdPtNiCuZn delivers an extraordinary mass activity of 24.55 A mgPt-1 and 11.62 A mgPd+Pt-1, along with impressive long-term durability. The planted oxophilic Mo single atoms as promoters modify the electronic structure of isolated Pt sites in the high-entropy-alloy host, suppressing the formation of CO adsorbates and steering the reaction towards the formate pathway. Meanwhile, Mo promoters and tensile strain synergistically optimize the adsorption behaviour of intermediates to achieve a more energetically favourable pathway and minimize the methanol oxidation reaction barrier. This work advances the design of atomically precise catalytic sites by creating a new paradigm of single atom-tailored high-entropy alloys, opening an encouraging pathway to the design of CO-tolerance electrocatalysts.

Importing Antibonding-Orbital Occupancy through Pd-O-Gd Bridge Promotes Electrocatalytic Oxygen Reduction

The active-site density, intrinsic activity, and durability of Pd-based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd2 O3 , Sm2 O3 , Eu2 O3 , and CeO2 ) by using RE metal-organic frameworks to tune the ORR performance of the Pd sites through the Pd-REx Oy interface interaction. Taking Pd-Gd2 O3 /C as a representative, it is identified that the strong coupling between Pd and Gd2 O3 induces the formation of the Pd-O-Gd bridge, which triggers charge redistribution of Pd and Gd2 O3 . The screened Pd-Gd2 O3 /C exhibits impressive ORR performance with high onset potential (0.986 VRHE ), half-wave potential (0.877 VRHE ), and excellent stability. Similar ORR results are also found for Pd-Sm2 O3 /C, Pd-Eu2 O3 /C, and Pd-CeO2 /C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd2 O3 promotes electron transfer through the Pd-O-Gd bridge, which induces the antibonding-orbital occupancy of Pd-*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd-Gd2 O3 is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd-Gd2 O3 /C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.

Delamination-Assisted Ultrafast Wrinkle Formation in a Freestanding Film

Freestanding films provide a versatile platform for materials engineering thanks to additional structural motifs not found in films with a substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time on the nanoscopic scale. Here, we use time-resolved electron diffraction to directly observe light-activated wrinkling formation in freestanding La2/3Ca1/3MnO3 films. Via a "lock-in" analysis of oscillations in the diffraction peak position, intensity, and width, we quantitatively reconstructed how wrinkles develop on the time scale of lattice vibration. Contrary to the common assumption of fixed boundary conditions, we found that wrinkle development is associated with ultrafast delamination at the film boundaries. Our work provides a generic protocol to quantify wrinkling dynamics in freestanding films and highlights the importance of the film-substrate interaction in determining the properties of freestanding structures.

Single-Atom Heterogeneous Catalysts: Human- and AI-Driven Platform for Augmented Designs, Analytics and Reality-Enabled Manufacturing

Heterogeneous catalysts with targeted functionality can be designed with atomic precision, but it is challenging to retain the structure and performance upon the scaled-up manufacturing. Particularly challenging is to ensure the "atomic economy", where every catalytic site is most gainfully utilized. Given the emerging synergistic integration of human- and artificial intelligence (AI)-driven augmented designs (AD), augmented analytics (AA), and augmented reality manufacturing (AM) platforms, this minireview focuses on single-atom heterogeneous catalysts (SAHCs) and examines the current status, challenges, and future perspectives of translating atomic-level structural precision and data-driven discovery to next-generation industrial manufacturing. We critically examine the atomistic insights into structure-driven SAHCs functionality and discuss the opportunities and challenges on the way towards the synergistic human-AI collaborative data-driven platform capable of monitoring, analyzing, manufacturing, and retaining the atomic-scale structure and functions. Enhanced by the atomic-level AD, AA, and AM, evolving from the current high-throughput capabilities and digital materials manufacturing acceleration, this synergistic human-AI platform is promising to enable atom-efficient and atomically precise heterogeneous catalyst production.

Computational Insights and Design of Promising Ultrathin PdM Bimetallenes for Oxygen Reduction Electrocatalysis

Bimetallic Palladium-based catalysts as an alternative of Pt-free electrocatalysts play a vital role in electrocatalysis. The doping of transition metal (M) into the ultrathin Pd nanosheets is new promising strategy to regulate the reactivity and durability of surface Pd sites. In this work, an in-depth investigation of the origin oxygen reduction reaction (ORR) activity and stability over 2D ultrathin PdM bimetallenes is presented. The M doping can greatly modify the reactivity of the Pd site by changing the local Fermi softness (S_F(r) ). All PdM bimetallenes follow the dissociative 4e^- pathway, and a thorough screening identified several promising alternatives (PdTa, PdHf, PdZr, and PdNb) with much lower ORR overpotential than the pure Pd(111) metallene. The Pd-O bond length and the Fermi softness of surface Pd atoms are effective descriptors of the adsorption of O* key intermediates. The hetero-metal induced ligand effect plays the key role for the activity improvement, which modifies the electronic properties and surface reactivity of Pd by the Pd-M orbital hybridization and result in the decline of bonding filling between Pd 3d_z ² and O* 2p orbital. The computational insight provides useful guideline for future experimental realizations of bimetallic Pd-based nanoalloys in ORR and other electrocatalytic reactions.

Enhancement Mechanism of Pt/Pd-Based Catalysts for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is one of the key catalytic reactions for hydrogen fuel cells, biofuel cells and metal-air cells. However, due to the complex four-electron catalytic process, the kinetics of the oxygen reduction reaction are sluggish. Platinum group metal (PGM) catalysts represented by platinum and palladium are considered to be the most active ORR catalysts. However, the price and reserves of Pt/Pd are major concerns and issues for their commercial application. Improving the catalytic performance of PGM catalysts can effectively reduce their loading and material cost in a catalytic system, and they will be more economical and practical. In this review, we introduce the kinetics and mechanisms of Pt/Pd-based catalysts for the ORR, summarize the main factors affecting the catalytic performance of PGMs, and discuss the recent progress of Pt/Pd-based catalysts. In addition, the remaining challenges and future prospects in the design and improvement of Pt/Pd-based catalysts of the ORR are also discussed.

Tripodal Pd metallenes mediated by Nb2C MXenes for boosting alkynes semihydrogenation

2D metallene nanomaterials have spurred considerable attention in heterogeneous catalysis by virtue of sufficient unsaturated metal atoms, high specific surface area and surface strain. Nevertheless, the strong metallic bonding in nanoparticles aggravates the difficulty in the controllable regulation of the geometry of metallenes. Here we propose an efficient galvanic replacement strategy to construct Pd metallenes loaded on Nb₂C MXenes at room temperature, which is triggered by strong metal-support interaction based on MD simulations. The Pd metallenes feature a chair structure of six-membered ring with the coordination number of Pd as low as 3. Coverage-dependent kinetic analysis based on first-principles calculations reveals that the tripodal Pd metallenes promote the diffusion of alkene and inhibit its overhydrogenation. As a consequence, Pd/Nb₂C delivers an outstanding turnover frequency of 10372 h^-1 and a high selectivity of 96% at 25 ^oC in the semihydrogenation of alkynes without compromising the stability. This strategy is general and scalable considering the plentiful members of the MXene family, which can set a foundation for the design of novel supported-metallene catalysts for demanding transformations.

Metallene-related materials for electrocatalysis and energy conversion

As a member of graphene analogs, metallenes are a class of two-dimensional materials with atomic thickness and well-controlled surface atomic arrangement made of metals or alloys. When utilized as catalysts, metallenes exhibit distinctive physicochemical properties endowed from the under-coordinated metal atoms on the surface, making them highly competitive candidates for energy-related electrocatalysis and energy conversion systems. Significantly, their catalytic activity can be precisely tuned through the chemical modification of their surface and subsurface atoms for efficient catalyst engineering. This minireview summarizes the recent progress in the synthesis and characterization of metallenes, together with their use as electrocatalysts toward reactions for energy conversion. In the Synthesis section, we pay particular attention to the strategies designed to tune their exposed facets, composition, and surface strain, as well as the porosity/cavity, defects, and crystallinity on the surface. We then discuss the electrocatalytic properties of metallenes in terms of oxygen reduction, hydrogen evolution, alcohol and acid oxidation, carbon dioxide reduction, and nitrogen reduction reaction, with a small extension regarding photocatalysis. At the end, we offer perspectives on the challenges and opportunities with respect to the synthesis, characterization, modeling, and application of metallenes.

Recent advances in the design of single-atom electrocatalysts by defect engineering

Single-atom catalysts (SACs) with isolated metal atoms dispersed on supports have attracted increasing attention due to their maximum atomic utilization and excellent catalytic performance in various electrochemical reactions. However, SACs with a high surface-to-volume ratio are fundamentally less stable and easily agglomerate, which weakens their activity. In addition, another issue that restricts the application of SACs is the low metal loading. Defect engineering is the most effective strategy for the precise synthesis of nanomaterials to catch and immobilize single atoms through the modulation of the electronic structure and coordination environment. Herein, in this mini-review, the latest advances in designing SACs by defect engineering have been first highlighted. Then, the heteroatom doping or intrinsic defects of carbon-based support and anion vacancies or cation vacancies of metal-based supports are systematically evaluated. Subsequently, the structure–activity relationships between a single-atom coupled defect structure and electrocatalytic performance are illustrated by combining experimental results and theoretical calculations. Finally, a perspective to reveal the current challenges and opportunities for controllable preparation, in situ characterization, and commercial applications is further proposed.

Ultrahigh Stable Methanol Oxidation Enabled by a High Hydroxyl Concentration on Pt Clusters/MXene Interfaces

Anchoring platinum catalysts on appropriate supports, e.g., MXenes, is a feasible pathway to achieve a desirable anode for direct methanol fuel cells. The authentic performance of Pt is often hindered by the occupancy and poisoning of active sites, weak interaction between Pt and supports, and the dissolution of Pt. Herein, we construct three-dimensional (3D) crumpled Ti₃C₂T_x MXene balls with abundant Ti vacancies for Pt confinement via a spray-drying process. The as-prepared Pt clusters/Ti₃C₂T_x (Ptc/Ti₃C₂T_x) show enhanced electrocatalytic methanol oxidation reaction (MOR) activity, including a relatively low overpotential, high tolerance to CO poisoning, and ultrahigh stability. Specifically, it achieves a high mass activity of up to 7.32 A mg_Pt^-1, which is the highest value reported to date in Pt-based electrocatalysts, and 42% of the current density is retained on Ptc/Ti₃C₂T_x even after the 3000 min operative time. In situ spectroscopy and theoretical calculations reveal that an electric field-induced repulsion on the Ptc/Ti₃C₂T_x interface accelerates the combination of OH^- and CO adsorption intermediates (CO_ads) in kinetics and thermodynamics. Besides, this Ptc/Ti₃C₂T_x also efficiently electrocatalyze ethanol, ethylene glycol, and glycerol oxidation reactions with comparable activity and stability to commercial Pt/C.

Spin engineering of single-site metal catalysts

Single-site metal atoms (SMAs) on supports are attracting extensive interest as new catalytic systems because of maximized atom utilization and superior performance. However, rational design of configuration-optimized SMAs with high activity from the perspectives of fundamental electron spin is highly challenging. Herein, N-coordinated Fe single atoms are successfully distributed over axial carbon micropores to form dangling-FeN₄ centers (d-FeN₄). This unique d-FeN₄ demonstrates much higher intrinsic activity toward oxygen reduction reaction (ORR) in HClO₄ than FeN₄ without micropore underneath and commercial Pt/C. Both theoretical calculation and electronic structure characterization imply that d-FeN₄ endows central Fe with medium spin (t_2g⁴ e_g¹), which provides a spin channel for electron transition compared with FeN₄ with low spin. This leads to the facile formation of the singlet state of oxygen-containing species from triplet oxygen during the ORR, thus showing faster kinetics than FeN₄. This work provides an in-depth understanding of spin tuning on SMAs for advanced energy catalysis.

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Single-site FeN₄ species are designed to dangle over axial carbon micropores (d-FeN₄)

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d-FeN₄ shows much superior oxygen reduction reactivity to traditional FeN₄

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d-FeN₄ facilitates the formation of singlet-state oxygen-containing species with optimized spin states by micropore

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This work provides in-depth understanding of spin tuning for advanced catalyst design

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

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

Metal-metal interactions in correlated single-atom catalysts

Single-atom catalysts (SACs) include a promising family of electrocatalysts with unique geometric structures. Beyond conventional ones with fully isolated metal sites, an emerging class of catalysts with the adjacent metal single atoms exhibiting intersite metal-metal interactions appear in recent years and can be denoted as correlated SACs (C-SACs). This type of catalysts provides more opportunities to achieve substantial structural modification and performance enhancement toward a wider range of electrocatalytic applications. On the basis of a clear identification of metal-metal interactions, this review critically examines the recent research progress in C-SACs. It shows that the control of metal-metal interactions enables regulation of atomic structure, local coordination, and electronic properties of metal single atoms, which facilitate the modulation of electrocatalytic behavior of C-SACs. Last, we outline directions for future work in the design and development of C-SACs, which is indispensable for creating high-performing new SAC architectures.

Multisize CoS2 Particles Intercalated/Coated-Montmorillonite as Efficient Sulfur Host for High-Performance Lithium-Sulfur Batteries

The chemisorption and catalysis of lithium polysulfides (LiPSs) are effective strategies to suppress the shuttle effect in lithium-sulfur (Li-S) batteries. Herein, multisize CoS₂ particles intercalated/coated-montmorillonite (MMT) as an efficient sulfur host is synthesized. As expected, the obtained S/CoS₂ @MMT cathode achieves an absorption-catalysis synergistic effect through the polar MMT aluminosilicate sheets and the well-dispersed nano-micron CoS₂ particles. Furthermore, efficient interlamellar ion pathways and interconnected conductive network are constructed within the composite host due to the intercalation/coating of CoS₂ in/on MMT. Therefore, the S/CoS₂ @MMT cathode achieves an outstanding rate performance up to 5C (∼548 mAh g^-1 ) and a high cycling stability with low capacity decay of 0.063 and 0.067 % per cycle for 500 cycles at 1C and 2C, respectively. With a higher sulfur loading of 4.0 mg cm^-2 , the cathode still delivers satisfactory rate and cycling performance. It shows that the CoS₂ @MMT host has great application prospects in Li-S batteries.

Cr-Doped Pd Metallene Endows a Practical Formaldehyde Sensor New Limit and High Selectivity

Electrochemical sensors for detecting micromolecule organics are desirable for improving the perception of environmental quality and human health. However, currently, the electrochemical sensors for formaldehyde are substantially limited on the market due to the long-term unsolved problems of the low electrooxidation efficiency and CO poisoning issue of commercial Pd catalysts. Here, a 2D Cr-doped Pd metallene (Cr-Pdene) with few atomic layers is shown as an advanced catalyst for ultrasensitive and selective sensing of formaldehyde via a highly efficient formaldehyde electrooxidation. It is found that the doping of Cr into Pd metallene can efficiently optimize the electronic structure of Pd and weaken the interaction between Pd and CO, providing an anti-poisoning means to favor CO₂ production and suppress CO adsorption. The Cr-Pdene-based electrochemical sensor exhibits one order of magnitude higher detection range and, especially, much higher anti-interference for formaldehyde than that of the conventional sensors. Most importantly, it is demonstrated that the Cr-Pdene can be integrated into commercializable wireless sensor networks or handheld instruments for promising applications relating to the environment, health, and food.

Regulating Pd nanosheets by W-doping for electrochemical nitrate reduction to ammonia

PdW nanosheets exhibit a high NO3RR activity, attributed to the W-alloying-induced upshift of the d-band center of Pd to enhance the NO3− activation and reduce the energy barriers of the NO3RR process.

Electronic and lattice strain dual tailoring for boosting Pd electrocatalysis in oxygen reduction reaction

Deliberately optimizing the d-band position of an active component via electronic and lattice strain tuning is an effective way to boost its catalytic performance. We herein demonstrate this concept by constructing core-shell Au@NiPd nanoparticles with NiPd alloy shells of only three atomic layers through combining an Au catalysis with the galvanic replacement reaction. The Au core with larger electronegativity modulates the Pd electronic configuration, while the Ni atoms alloyed in the ultrathin shells neutralize the lattice stretching in Pd shells exerted by Au cores, equipping the active Pd metal with a favorable d-band position for electrochemical oxygen reduction reaction in an alkaline medium, for which core-shell Au@NiPd nanoparticles with a Ni/Pd atomic ratio of 3/7 exhibit a half-wave potential of 0.92 V, specific activity of 3.7 mA cm^-2, and mass activity of 0.65 A mg^-1 at 0.9 V, much better than most of the recently reported Pd-even Pt-based electrocatalysts.

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

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

High-Index Faceted PdPtCu Ultrathin Nanorings Enable Highly Active and Stable Oxygen Reduction Electrocatalysis

Ultrathin nanosheet catalysts deliver great potential in catalyzing the oxygen reduction reaction (ORR), but encounter the ceiling of the surface atomic utilizations, thus presenting a challenge associated with further boosting catalytic activity. Herein, a kind of PdPtCu ultrathin nanorings with increased numbers of electrocatalytically active sites is reported, with the purpose of breaking the activity ceiling of conventional catalysts. The as-made PdPtCu nanorings possess abundant high-index facets at the edge of both the exterior and interior surfaces. An ultrahigh electrochemical active surface area of 92.2 m² g^-1 _PGM is achieved on this novel catalyst, much higher than that of the commercial Pt/C catalyst. The optimized Pd₃₉ Pt₃₃ Cu₂₈ /C shows a great enhanced ORR activity with a specific activity of 2.39 mA cm^-2 and a mass activity of 1.97 A mg^-1 _PGM at 0.9 V (versus RHE), as well as superior durability within 30 000 cycles. Density function theory calculations reveal that the high-index facets and alloying Cu atoms can optimize the oxygen adsorption energy, explaining the enhanced ORR activity. Overcoming a key technical barrier in sub-nanometer electrocatalysts, this work successfully introduces the hollow structures into the ultrathin nanosheets, heralding the exciting prospects of high-performance ORR catalysts in fuel cells.

Single-Atom-Layer Catalysis in a MoS2 Monolayer Activated by Long-Range Ferromagnetism for the Hydrogen Evolution Reaction: Beyond Single-Atom Catalysis

Single-atom-layer catalysts with fully activated basal-atoms will provide a solution to the low loading-density bottleneck of single-atom catalysts. Herein, we activate the majority of the basal sites of monolayer MoS₂ , by doping Co ions to induce long-range ferromagnetic order. This strategy, as revealed by in situ synchrotron radiation microscopic infrared spectroscopy and electrochemical measurements, could activate more than 50 % of the originally inert basal-plane S atoms in the ferromagnetic monolayer for the hydrogen evolution reaction (HER). Consequently, on a single monolayer of ferromagnetic MoS₂ measured by on-chip micro-cell, a current density of 10 mA cm^-2 could be achieved at the overpotential of 137 mV, corresponding to a mass activity of 28, 571 Ag^-1 , which is two orders of magnitude higher than the multilayer counterpart. Its exchange current density of 75 μA cm^-2 also surpasses most other MoS₂ -based catalysts. Experimental results and theoretical calculations show the activation of basal plane S atoms arises from an increase of electronic density around the Fermi level, promoting the H adsorption ability of basal-plane S atoms.

Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity

As low-cost electrocatalysts for oxygen reduction reaction applied to fuel cells and metal-air batteries, atomic-dispersed transition metal-nitrogen-carbon materials are emerging, but the genuine mechanism thereof is still arguable. Herein, by rational design and synthesis of dual-metal atomically dispersed Fe,Mn/N-C catalyst as model object, we unravel that the O₂ reduction preferentially takes place on Fe^III in the FeN₄ /C system with intermediate spin state which possesses one e_g electron (t_2g4e_g1) readily penetrating the antibonding π-orbital of oxygen. Both magnetic measurements and theoretical calculation reveal that the adjacent atomically dispersed Mn-N moieties can effectively activate the Fe^III sites by both spin-state transition and electronic modulation, rendering the excellent ORR performances of Fe,Mn/N-C in both alkaline and acidic media (halfwave positionals are 0.928 V in 0.1 M KOH, and 0.804 V in 0.1 M HClO₄), and good durability, which outperforms and has almost the same activity of commercial Pt/C, respectively. In addition, it presents a superior power density of 160.8 mW cm⁻² and long-term durability in reversible zinc–air batteries. The work brings new insight into the oxygen reduction reaction process on the metal-nitrogen-carbon active sites, undoubtedly leading the exploration towards high effective low-cost non-precious catalysts.

The working mechanism of several low-cost electrocatalyst materials is still arguable. Here the authors show a model Fe,Mn/N-C catalyst where the oxygen reduction preferentially takes place on Fe(III) sites with the intermediate spin state (t2g4 eg1) caused by the adjacent Mn-N moieties.

Pd nanoparticles loaded onto a TiO2–C heterostructure via a photochemical strategy for efficient oxygen reduction reaction

In this paper, we successfully reduced Pd to TiO2–C heterostructure (Pd/TiO2–C catalyst) by photochemical reduction method. The directional transfer of electrons from TiO2–C to Pd catalyst strengthens MA and improves the durability of ORR.

Atomic Dispersion and Surface Enrichment of Palladium in Nitrogen-Doped Porous Carbon Cages Lead to High-Performance Electrocatalytic Reduction of Oxygen

Metal-nitrogen-carbon (MNC) nanocomposites have been hailed as promising and efficient electrocatalysts toward oxygen reduction reaction (ORR), due to the formation of MN_x coordination moieties. However, MNC hybrids are mostly prepared by pyrolysis of organic precursors along with select metal salts, where part of the MN_x sites are inevitably buried in the carbon matrix. This limited accessibility compromises the electrocatalytic performance. Herein, we describe a wet-impregnation procedure by facile thermal refluxing, whereby palladium is atomically dispersed and enriched onto the surface of hollow, nitrogen-doped carbon cages (HNC) forming Pd-N coordination bonds. The obtained Pd-HNC nanocomposites exhibit an ORR activity in alkaline media markedly higher than that of metallic Pd nanoparticles, and the best sample even outperforms commercial Pt/C and relevant Pd-based catalysts reported in the literature. The results suggest that atomic dispersion and surface enrichment of palladium in a carbon matrix may serve as an effective strategy in the fabrication of high-performance ORR electrocatalysts.