Platinum-trimer decorated cobalt-palladium core-shell nanocatalyst with promising performance for oxygen reduction reaction

Oxidized Ti Single Atoms and Co₃O₄ with Abundant Oxygen Vacancies Collaborating with Adjacent Pd Sites for an Efficient and Stable Oxygen Reduction Reaction

The current study addresses these key issues by providing the ensemble sites and creating oxygen vacancies in the oxidized Ti-single atoms. Here we report a novel heterogeneous catalyst comprising oxidized Ti-single atoms uniformly coated on the cobalt-oxide-supported Pd nanoparticles (denoted as CP@Ti-1), where the oxygen vacancies are introduced in oxidized Ti-single atoms as well as cobalt-oxide support. As-developed CP@Ti-1 catalyst demonstrates remarkable ORR activity with a mass activity (MA) of 9,725 mAmgTi -1 at 0.85 V vs RHE and 1,244 mAmgTi -1 at 0.90 V vs RHE in alkaline conditions. These values mark significant improvements over the commercial J.M.-Pt/C catalyst, outperforming it by 145- and 50-fold, respectively. Additionally, the CP@Ti-1 catalyst shows exceptional durability, maintaining 100% of its initial performance after 20, 000 cycles of accelerated degradation test (ADT). In-situ X-ray absorption spectroscopy (XAS) analysis reveals that the catalyst design promotes synergistic interactions between oxygen vacancies in Ti/Co atoms and adjacent Pd domains, facilitating key reactions in ORR such as oxygen splitting and hydroxide ion formation, respectively, enhancing overall catalytic efficiency. These insights promise significant advancements in both scientific research and industrial applications of ORR catalysis.

Selectivity in Chemiresistive Gas Sensors: Strategies and Challenges

The demand for highly functional chemical gas sensors has surged due to the increasing awareness of human health to monitor metabolic disorders or noncommunicable diseases, safety measures against harmful greenhouse and/or explosive gases, and determination of food freshness. Over the years of dedicated research, several types of chemiresistive gas sensors have been realized with appreciable sensitivities toward various gases. However, critical issues such as poor selectivity and sluggish response/recovery speeds continue to impede their widespread commercialization. Specifically, the mechanisms behind the selective response of some chemiresistive materials toward specific gas analytes remain unclear. In this review, we discuss state-of-the-art strategies employed to attain gas-selective chemiresistive materials, with particular emphasis on materials design, surface modification or functionalization with catalysts, defect engineering, material structure control, and integration with physical/chemical gas filtration media. The nature of material surface-gas interactions and the supporting mechanisms are elucidated, opening opportunities for optimizing the materials design, fine-tuning the gas sensing performance, and guiding the selection of the most appropriate materials for the accurate detection of specific gases. This review concludes with recommendations for future research directions and potential opportunities for further selectivity improvements.

Simultaneous Enhancement of the Activity and Durability of the Oxygen Reduction Reaction via Pd3Mo@Pt/C Catalysts

To overcome the limitations of conventional bimetallic catalysts in facilitating the oxygen reduction reaction (ORR), we employed density functional theory (DFT) screening to evaluate ternary Pd3X@Pt core@shell catalysts (X = transition metals), with the objective of increasing both the ORR activity and durability. Among the 25 candidates, Pd3Mo@Pt emerges as the most promising catalyst, showing a combination of a low limiting potential and a high dissolution potential. Experimental validation reveals that the carbon-supported Pd3Mo@Pt/C catalysts clearly exhibit exceptional mass activity (3.76 A mgPt-1) and specific activity (1.67 mA cm-2); these activities significantly surpass those of their Pt/C counterparts by factors of 10.2 and 3.18, respectively. Furthermore, these core@shell catalysts exhibit robust durability, while also exhibiting enhanced CO tolerance, as evidenced by CO stripping voltammetry. DFT calculations show that the superior activity and stability of Pd3Mo@Pt/C are attributed to the optimal modulation of the Pt surface electronic structures by the core elements, particularly Mo.

In/Outside Catalytic Sites of the Pore Walls in One-Dimensional Covalent Organic Frameworks for Oxygen Reduction Reaction

Pore channels play a decisive role in mass transport in catalytic systems. However, the influences of the location of catalytic sites inside or outside of the pore walls on the performance were still under-explored due, because it is difficult to construct sites anchored in or outside of pore walls. Herein, one-dimensional covalent organic frameworks with precisely anchored active sites were used to explore the effects of channels on a typical oxygen reduction reaction (ORR) catalysis. Electrocatalytic evaluations showed that single Pt sites located inside of the channels exhibited higher kinetic activity compared to those anchored outside. The in situ spectroscopic analysis revealed that local reconstruction of Pt-Cl breaking and potential-induced anion transport occurred more effectively inside the channels. The superior anion transportability and kinetic activity of the inside-channel active sites also facilitated *OH desorption during the ORR process outperforming their outside-channel counterparts. The results of this study provide strategies for designing active sites in porous catalysts for heterogeneous catalysis.

Fe─N4 and Fe7Co3 Nanoalloy Dual-Site Modulation by Skeleton Defect in N-Doped Graphene Aerogel for Enhanced Bifunctional Oxygen Electrocatalyst in Zinc-air Battery

The dual-site electrocatalysts formed by metal single atoms combines with metal nanoparticles represent a promising strategy to enhance both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance. Herein, defect engineering is applied to dual-site ORR and OER electrocatalysts. Its design, synthesis, structural properties, and catalytic performance experimentally and theoretically are insightfully studied for the single-atomic Fe─N4 and the adjacent Fe7Co3 nanoalloy (FeCoNA) as dual-site loading on nitrogen-doped graphene aerogel (Fe─N/FeCo@NGA). The high-density dual-sites, together with the good electronic conductivity of NGA, synergistically improve the electronic structure for superior electrocatalytic activity. The half-wave potential of Fe─N/FeCo@NGA in ORR is 0.92 V and the overpotential of it in OER is 1.58 V. Corresponding all-solid-state Zn-air battery demonstrates a peak power density of 147.6 mW cm-2 and charge/discharge durability for over 140 h. Theoretical calculations reveal that the single-atomic Fe-N4 and FeCoNA dual-site on the skeleton defect optimized NGA, further refine the local electronic structure, modulating the tensile force on the O─O bond in *OOH intermediate, leading to its spontaneous dissociation and facilitating a significantly reduced energy barrier. This work takes a promising shortcut in the application of defect engineering for the development of highly efficient dual-site bifunctional oxygen electrocatalysts with single atoms.

Nanomaterials for Zinc Batteries-Aerogels

Aqueous zinc batteries, mainly including Zn-ion batteries (ZIBs) and Zn-air batteries (ZABs), are promising energy storage systems, but challenges exist at their current stage. For instance, the zinc anode in aqueous electrolyte is impacted by anodic dendrites, hydrogen and oxygen precipitation, and some other harmful side reactions, which severely affect the battery's lifespan. As for traditional cathode materials in ZIBs, low electrical conductivity, slow Zn2+ ion migration, and easy collapse of the crystal structure during ion embedding and migration bring challenges. Also, the slower critical oxygen reduction reaction (ORR), for example, in ZABs shows unsatisfactory results. All these issues greatly hindered the development of zinc batteries. Aerogel materials, characterized by their high specific surface area, unique open-pore structure formed by nanoporous structures, and excellent physicochemical properties, have a positive role in cathode modification, electrode protection, and catalytic reactions in zinc batteries. This manuscript provides a systematic review of aerogel materials, highlighting advancements in their preparation and application for zinc batteries, aiming to promote the future progress and development of aerogel nanomaterials and zinc batteries.

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.

Dynamic Control of Asymmetric Charge Distribution for Electrocatalytic Urea Synthesis

Constructing dual catalytic sites with charge density differences is an efficient way to promote urea electrosynthesis from parallelNO 3 - ${\mathrm{NO}}_3^ - $ and CO2 reduction yet still challenging in static system. Herein, a dynamic system is constructed by precisely controlling the asymmetric charge density distribution in an Au-doped coplanar Cu7 clusters-based 3D framework catalyst (Au@cpCu7CF). In Au@cpCu7CF, the redistributed charge between Au and Cu atoms changed periodically with the application of pulse potentials switching between -0.2 and -0.6 V and greatly facilitated the electrosynthesis of urea. Compared with the static condition of pristine cpCu7CF (FEurea = 5.10%), the FEurea of Au@cpCu7CF under pulsed potentials is up to 55.53%. Theoretical calculations demonstrated that the high potential of -0.6 V improved the adsorption of *HNO2 and *NH2 on Au atoms and inhibited the reaction pathways of by-products. While at the low potential of -0.2 V, the charge distribution between Au and Cu atomic sites facilitated the thermodynamic C-N coupling step. This work demonstrated the important role of asymmetric charge distribution under dynamic regulation for urea electrosynthesis, providing a new inspiration for precise control of electrocatalysis.

Iridium Single Atoms to Nanoparticles: Nurturing the Local Synergy with Cobalt-Oxide Supported Palladium Nanoparticles for Oxygen Reduction Reaction

A ternary catalyst comprising Iridium (Ir) single-atoms (SA)s decorated on the Co-oxide supported palladium (Pd) nanoparticles (denoted as CPI-SA) is developed in this work. The CPI-SA with 1 wt.% of Ir exhibits unprecedented high mass activity (MA) of 7173 and 770 mA mgIr -1, respectively, at 0.85 and 0.90 V versus RHE in alkaline ORR (0.1 m KOH), outperforming the commercial Johnson Matthey Pt catalyst (J.M.-Pt/C; 20 wt.% Pt) by 107-folds. More importantly, the high structural reliability of the Ir single-atoms endows the CPI-SA with outstanding durability, where it shows progressively increasing MA of 13 342 and 1372 mA mgIr -1, respectively, at 0.85 and 0.90 V versus RHE up to 69 000 cycles (3 months) in the accelerated degradation test (ADT). Evidence from the in situ partial fluorescence yield X-ray absorption spectroscopy (PFY-XAS) and the electrochemical analysis indicate that the Ir single-atoms and adjacent Pd domains synergistically promote the O2 splitting and subsequent desorption of hydroxide ions (OH-), respectively. Whereas the Co-atoms underneath serve as electron injectors to boost the ORR activity of the Ir single-atoms. Besides, a progressive and sharp drop in the ORR performance is observed when Ir-clusters and Ir nanoparticles are decorated on the Co-oxide-supported Pd nanoparticles.

Tunable Pt-NiO interaction-induced efficient electrocatalytic water oxidation and methanol oxidation

Metal-support interaction engineering is considered an efficient strategy for optimizing the catalytic activity. Nevertheless, the fine regulation of metal-support interactions as well as understanding the corresponding catalytic mechanisms (particularly those of non-carbon support-based counterparts) remains challenging. Herein, a controllable adsorption-impregnation strategy was proposed for the preparation of a porous nonlayered 2D NiO nanoflake support anchored with different forms of Pt nanoarchitectures, i.e. single atoms, clusters and nanoparticles. Benefiting from the unique porous architecture of NiO nanosheets, abundant active defect sites facilitated the immobilization of Pt single atoms onto the NiO crystal, resulting in NiO lattice distortion and thus changing the valence state of Pt, chemical bonding, and the coordination environment of the metal center. The synergy of the porous NiO support and the unexpected Pt single atom-NiO interactions effectively accelerated mass transfer and reduced the reaction kinetic barriers, contributing to a significantly enhanced mass activity of 5.59 A mgPt -1 at an overpotential of 0.274 V toward the electrocatalytic oxygen evolution reaction (OER) while 0.42 A mgPt -1 at a potential of 0.7 V vs. RHE for the methanol oxidation reaction (MOR) in an alkaline system, respectively. This work may offer fundamental guidance for developing metal-loaded/dispersed support nanomaterials toward electrocatalysis through the fine regulation of metal-support interactions.

Highly effective degradation of ibuprofen by alkaline metal-doped copper oxides via peroxymonosulfate activation: Mechanisms, degradation pathway and toxicity assessments

Redox ratios of Cu2+/Cu+ and adsorbed oxygen species (Oads) have shown great activity toward radical generation by activating peroxymonosulfate (PMS). Herein, different alkaline metal oxides (CaO, MgO and BaO) and various amounts of CaO are incorporated into CuO, which could tune the main active sites of redox ratios of Cu2+/Cu+ and Oads. The results show that CaO-CuO-5% exhibits the outstanding performance of PMS activation toward ibuprofen (IBF) degradation with excellent kinetics (k = 0.812 min-1). The X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculation show that the CaO-CuO-5% has the higher electron density with superior electron transfer ability and lower PMS adsorption energy. Based on radical scavengers and electron paramagnetic resonance spectrometer (EPR), a nonradical process is proposed to play the dominant role. The degradation pathway and the corresponding toxicity of degraded intermediates with residue PMS after reaction is evaluated by LC-MS/MS and bioassay experiments, indicating the lower antagonistic influence on human hormone receptors after advanced oxidation process. Mitigation of the Cu leaching with cyclic stability can be achieved. This study provides a facile method to optimize high-performance catalysts to activate PMS and offer practical environmental applications in the remediation of emerging contaminants.

Advances in heterogeneous single-cluster catalysis

Heterogeneous single-cluster catalysts (SCCs) comprising atomically precise and isolated metal clusters stabilized on appropriately chosen supports offer exciting prospects for enabling novel chemical reactions owing to their broad structural diversity with unparalled opportunities for engineering their properties. Although the pioneering work revealed intriguing performance trends of size-selected metal clusters deposited on supports, synthetic and analytical challenges hindered a thorough understanding of surface chemistry under realistic conditions. This Review underscores the importance of considering the cluster environment in SCCs, encompassing the development of robust metal-support interactions, precise control over the ligand sphere, the influence of reaction media and dynamic behaviour, to uncover new reactivities. Through examples, we illustrate the criticality of tailoring the entire catalytic ensemble in SCCs to achieve stable and selective performance with practically relevant metal coverages. This expansion in application scope transcends from model reactions to complex and technically relevant reactions. Furthermore, we provide a perspective on the opportunities and future directions for SCC design within this rapidly evolving field.

Advances in the synthesis and applications of 2D MXene-metal nanomaterials

MXenes, two-dimensional (2D) materials that consist of transition metal carbides, nitrides and/or carbonitrides, have recently attracted much attention in energy-related and biomedicine fields. These materials have substantial advantages over traditional carbon graphenes: they possess high conductivity, high strength, excellent chemical and mechanical stability, and superior hydrophilic properties. Furthermore, diverse functional groups such as -OH, -O, and -F located on the surface of MXenes aid the immobilization of numerous noble metal nanoparticles (NP). Therefore, 2D MXene composite materials have become an important and convenient option of being applied as support materials in many fields. In this review, the advances in the synthesis (including morphology studies, characterization, physicochemical properties) and applications of the currently known 2D MXene-metal (Pd, Ag, Au, and Cu) nanomaterials are summarized based on critical analysis of the literature in this field. Importantly, the current state of the art, challenges, and the potential for future research on broad applications of MXene-metal nanomaterials have been discussed.

Probing the atomically diffuse interfaces in Pd@Pt core-shell nanoparticles in three dimensions

Deciphering the three-dimensional atomic structure of solid-solid interfaces in core-shell nanomaterials is the key to understand their catalytical, optical and electronic properties. Here, we probe the three-dimensional atomic structures of palladium-platinum core-shell nanoparticles at the single-atom level using atomic resolution electron tomography. We quantify the rich structural variety of core-shell nanoparticles with heteroepitaxy in 3D at atomic resolution. Instead of forming an atomically-sharp boundary, the core-shell interface is found to be atomically diffuse with an average thickness of 4.2 Å, irrespective of the particle's morphology or crystallographic texture. The high concentration of Pd in the diffusive interface is highly related to the free Pd atoms dissolved from the Pd seeds, which is confirmed by atomic images of Pd and Pt single atoms and sub-nanometer clusters using cryogenic electron microscopy. These results advance our understanding of core-shell structures at the fundamental level, providing potential strategies into precise nanomaterial manipulation and chemical property regulation.

Subnanoscale Dual-Site Pd-Pt Layers Make PdPtCu Nanocrystals CO-Tolerant Bipolar Effective Electrocatalysts for Alcohol Fuel Cell Devices

Finding a high-performance low-Pt bipolar electrocatalyst in actual direct alcohol fuel cells (DAFCs) remains challenging and desirable. Here, we developed a crystalline PdPtCu@amorphous subnanometer Pd-Pt "dual site" layer core-shell structure for the oxygen reduction reaction (ORR) and alcohol (methanol, ethylene glycol, glycerol, and their mixtures) oxidation reaction (AOR) in an alkaline electrolyte (denoted D-PdPtCu). The prepared D-PdPtCu/C achieved a direct 4-electron ORR pathway, a full oxidation pathway for AOR, and high CO tolerance. The ORR mass activity (MA) of D-PdPtCu/C delivered a 52.8- or 59.3-fold increase over commercial Pt/C or Pd/C, respectively, and no activity loss after 20000 cycles. The D-PdPtCu/C also exhibited much higher AOR MA and stability than Pt/C or Pd/C. Density functional theory revealed the intrinsic nature of a subnanometer Pd-Pt "dual site" surface for ORR and AOR activity enhancement. The D-PdPtCu/C as an effective bipolar electrocatalyst yielded higher peak power densities than commercial Pt/C in actual DAFCs.

Recent Advances in Water-Splitting Electrocatalysts Based on Electrodeposition

Green hydrogen is being considered as a next-generation sustainable energy source. It is created electrochemically by water splitting with renewable electricity such as wind, geothermal, solar, and hydropower. The development of electrocatalysts is crucial for the practical production of green hydrogen in order to achieve highly efficient water-splitting systems. Due to its advantages of being environmentally friendly, economically advantageous, and scalable for practical application, electrodeposition is widely used to prepare electrocatalysts. There are still some restrictions on the ability to create highly effective electrocatalysts using electrodeposition owing to the extremely complicated variables required to deposit uniform and large numbers of catalytic active sites. In this review article, we focus on recent advancements in the field of electrodeposition for water splitting, as well as a number of strategies to address current issues. The highly catalytic electrodeposited catalyst systems, including nanostructured layered double hydroxides (LDHs), single-atom catalysts (SACs), high-entropy alloys (HEAs), and core-shell structures, are intensively discussed. Lastly, we offer solutions to current problems and the potential of electrodeposition in upcoming water-splitting electrocatalysts.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Atomic Scaled Depth Correlation to the Oxygen Reduction Reaction Performance of Single Atom Ni Alloy to the NiO2 Supported Pd Nanocrystal

This study demonstrates the intercalation of single-atom Ni (NiSA ) substantially reduces the reaction activity of Ni oxide supported Pd nanoparticle (NiO2 /Pd) in the oxygen reduction reaction (ORR). The results indicate the transition states kinetically consolidate the adsorption energy for the chemisorbed O and OH species on the ORR activity. Notably, the NiO2 /Ni1 /Pd performs the optimum ORR behavior with the lowest barrier of 0.49 eV and moderate second-step barrier of 0.30 eV consequently confirming its utmost ORR performance. Through the stepwise cross-level demonstrations, a structure-Eads -ΔE correspondence for the proposed NiO2 /Nin /Pd systems is established. Most importantly, such a correspondence reveals that the electronic structure of heterogeneous catalysts can be significantly differed by the segregation of atomic clusters in different dimensions and locations. Besides, the doping-depth effect exploration of the NiSA in the NiO2 /Pd structure intrinsically elucidates that the Ni atom doping in the subsurface induces the most fruitful NiSA /PdML synergy combining the electronic and strain effects to optimize the ORR, whereas this desired synergy diminishes at high Pd coverages. Overall, the results not only rationalize the variation in the redox properties but most importantly provides a precision evaluation of the process window for optimizing the configuration and composition of bimetallic catalysts in practical experiments.

Pt-Mediated Interface Engineering Boosts the Oxygen Reduction Reaction Performance of Ni Hydroxide-Supported Pd Nanoparticles

Fuel cells are considered potential energy conversion devices for utopia; nevertheless, finding a highly efficacious and economical electrocatalyst for the oxygen reduction reaction (ORR) is of great interest. By keeping this in view, we have proposed a novel design of a trimetallic nanocatalyst (NC) comprising atomic Pt clusters at the heterogeneous Ni(OH)₂-to-Pd interface (denoted NPP-70). The as-prepared material surpasses the commercial J.M.-Pt/C (20 wt %) catalyst by ∼ 166 and ∼19 times with exceptionally high specific and mass activities of 16.11 mA cm^-2 and 484.8 mA mg_Pt^-1 at 0.90 V versus reversible hydrogen electrode (RHE) in alkaline ORR (0.1 M KOH), respectively. On top of that, NPP-70 NC retains nearly 100% performance after 10k accelerated durability test (ADT) cycles. The results of physical characterization and electrochemical analysis confirm that atomic-scale Pt clusters induce strong lattice strain (compressive) at the Ni(OH)₂-to-Pd interface, which triggers the electron relocation from Ni to Pt atoms. Such charge localization is vital for O₂ splitting on surface Pt atoms, followed by the relocation of OH^- ions from the Pd surface. Besides, a sharp fall down in ORR performance (mass activity is 37 mA mg_Pt^-1 at 0.90 V versus RHE) is observed when the Pt clusters are decorated on the surface of NiO_x and Pd (denoted NPP-RT). In situ partial fluorescence yield mode X-ray absorption spectroscopy (PFY-XAS) was employed to reveal the ORR pathways on both configurations. The obtained results demonstrate that interface engineering can be a potential approach to boost the electrocatalytic activity of metal hydroxide/oxide-supported Pd nanoparticles and in turn allow Pd to be a promising alternative for commercial Pt catalysts.

Mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowire electrocatalyst for efficient oxygen reduction

The design of Pt-based nanoarchitectures with controllable compositions and morphologies is necessary to enhance their electrocatalytic activity. Herein, we report a rational design and synthesis of anisotropic mesoporous Pt@Pt-skin Pt₃Ni core-shell framework nanowires for high-efficient electrocatalysis. The catalyst has a uniform core-shell structure with an ultrathin atomic-jagged Pt nanowire core and a mesoporous Pt-skin Pt₃Ni framework shell, possessing high electrocatalytic activity, stability and Pt utilisation efficiency. For the oxygen reduction reaction, the anisotropic mesoporous Pt@Pt-skin Pt₃Ni core-shell framework nanowires demonstrated exceptional mass and specific activities of 6.69 A/mg_pt and 8.42 mA/cm² (at 0.9 V versus reversible hydrogen electrode), and the catalyst exhibited high stability with negligible activity decay after 50,000 cycles. The mesoporous Pt@Pt-skin Pt₃Ni core-shell framework nanowire configuration combines the advantages of three-dimensional open mesopore molecular accessibility and compressive Pt-skin surface strains, which results in more catalytically active sites and weakened chemisorption of oxygenated species, thus boosting its catalytic activity and stability towards electrocatalysis.

Atomic-Level Interface Engineering for Boosting Oxygen Electrocatalysis Performance of Single-Atom Catalysts: From Metal Active Center to the First Coordination Sphere

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the core reactions of a series of advanced modern energy and conversion technologies, such as fuel cells and metal-air cells. Among all kinds of oxygen electrocatalysts that have been reported, single-atom catalysts (SACs) offer great development potential because of their nearly 100% atomic utilization, unsaturated coordination environment, and tunable electronic structure. In recent years, numerous SACs with enriched active centers and asymmetric coordination have been successfully constructed by regulating their coordination environment and electronic structure, which has brought the development of atomic catalysts to a new level. This paper reviews the improvement of SACs brought by atom-level interface engineering. It starts with the introduction of advanced techniques for the characterizations of SACs. Subsequently, different design strategies that are applied to adjust the metal active center and first coordination sphere of SACs and then enhance their oxygen electrocatalysis performance are systematically illustrated. Finally, the future development of SACs toward ORR and OER is discussed and prospected.

Adsorption Energy in Oxygen Electrocatalysis

Adsorption energy (AE) of reactive intermediate is currently the most important descriptor for electrochemical reactions (e.g., water electrolysis, hydrogen fuel cell, electrochemical nitrogen fixation, electrochemical carbon dioxide reduction, etc.), which can bridge the gap between catalyst's structure and activity. Tracing the history and evolution of AE can help to understand electrocatalysis and design optimal electrocatalysts. Focusing on oxygen electrocatalysis, this review aims to provide a comprehensive introduction on how AE is selected as the activity descriptor, the intrinsic and empirical relationships related to AE, how AE links the structure and electrocatalytic performance, the approaches to obtain AE, the strategies to improve catalytic activity by modulating AE, the extrinsic influences on AE from the environment, and the methods in circumventing linear scaling relations of AE. An outlook is provided at the end with emphasis on possible future investigation related to the obstacles existing between adsorption energy and electrocatalytic performance.

Patterning the consecutive Pd3 to Pd1 on Pd2Ga surface via temperature-promoted reactive metal-support interaction

Atom-by-atom control of a catalyst surface is a central yet challenging topic in heterogeneous catalysis, which enables precisely confined adsorption and oriented approach of reactant molecules. Here, exposed surfaces with either consecutive Pd trimers (Pd₃) or isolated Pd atoms (Pd₁) are architected for Pd₂Ga intermetallic nanoparticles (NPs) using reactive metal-support interaction (RMSI). At elevated temperatures under hydrogen, in situ atomic-scale transmission electron microscopy directly visualizes the refacetting of Pd₂Ga NPs from energetically favorable (013)/(020) facets to (011)/(002). Infrared spectroscopy and acetylene hydrogenation reaction complementarily confirm the evolution from consecutive Pd₃ to Pd₁ sites of Pd₂Ga catalysts with the concurrent fingerprinting CO adsorption and featured reactivities. Through theoretical calculations and modeling, we reveal that the restructured Pd₂Ga surface results from the preferential arrangement of additionally reduced Ga atoms on the surface. Our work provides previously unidentified mechanistic insight into temperature-promoted RMSI and possible solutions to control and rearrange the surface atoms of supported intermetallic catalyst.

Controlled Synthesis of Carbon-Supported Pt-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells are playing an increasing role in postpandemic economic recovery and climate action plans. However, their performance, cost, and durability are significantly related to Pt-based electrocatalysts, hampering their large-scale commercial application. Hence, considerable efforts have been devoted to improving the activity and durability of Pt-based electrocatalysts by controlled synthesis in recent years as an effective method for decreasing Pt use, and consequently, the cost. Therefore, this review article focuses on the synthesis processes of carbon-supported Pt-based electrocatalysts, which significantly affect the nanoparticle size, shape, and dispersion on supports and thus the activity and durability of the prepared electrocatalysts. The reviewed processes include (i) the functionalization of a commercial carbon support for enhanced catalyst–support interaction and additional catalytic effects, (ii) the methods for loading Pt-based electrocatalysts onto a carbon support that impact the manufacturing costs of electrocatalysts, (iii) the preparation of spherical and nonspherical Pt-based electrocatalysts (polyhedrons, nanocages, nanoframes, one- and two-dimensional nanostructures), and (iv) the postsynthesis treatments of supported electrocatalysts. The influences of the supports, key experimental parameters, and postsynthesis treatments on Pt-based electrocatalysts are scrutinized in detail. Future research directions are outlined, including (i) the full exploitation of the potential functionalization of commercial carbon supports, (ii) scaled-up one-pot synthesis of carbon-supported Pt-based electrocatalysts, and (iii) simplification of postsynthesis treatments. One-pot synthesis in aqueous instead of organic reaction systems and the minimal use of organic ligands are preferred to simplify the synthesis and postsynthesis treatment processes and to promote the mass production of commercial carbon-supported Pt-based electrocatalysts.

Graphical Abstract

This review focuses on the synthesis process of Pt-based electrocatalysts/C to develop aqueous one-pot synthesis at large-scale production for PEMFC stack application.

Corrosion Chemistry of Electrocatalysts

Electrocatalysts are the core components of many sustainable energy conversion technologies that are considered the most potential solution to the worldwide energy and environmental crises. The reliability of structure and composition pledges that electrocatalysts can achieve predictable and stable performance. However, during the electrochemical reaction, electrocatalysts are influenced directly by the applied potential, the electrolyte, and the adsorption/desorption of reactive species, triggering structural and compositional corrosion, which directly affects the catalytic behaviors of electrocatalysts (performance degradation or enhancement) and invalidates the established structure-activity relationship. Therefore, it is necessary to elucidate the corrosion behavior and mechanism of electrocatalysts to formulate targeted corrosion-resistant strategies or use corrosion reconstruction synthesis techniques to guide the preparation of efficient and stable electrocatalysts. Herein, the most recent developments in electrocatalyst corrosion chemistry are outlined, including corrosion mechanisms, mitigation strategies, and corrosion syntheses/reconstructions based on typical materials and important electrocatalytic reactions. Finally, potential opportunities and challenges are also proposed to foresee the possible development in this field. It is believed that this contribution will raise more awareness regarding nanomaterial corrosion chemistry in energy technologies and beyond.

Optimization of SnPd Shell Configuration to Boost ORR Performance of Pt-Clusters Decorated CoOx@SnPd Core-Shell Nanocatalyst

Fuel cells are expected to bring change to the whole human race when commercialized, however, the sluggish kinetics of oxygen reduction reaction (ORR) severely hampers their commercial viability. Thus far, platinum (Pt) based catalysts are nearly inevitable due to the harsh redox environment of fuel cells. Thus, minimizing Pt metal loading and increasing Pt utilization is a paramount factor for realizing fuel cell technologies. In this context, herein, we developed a multi-metallic nanocatalyst (NC) comprising Pt-clusters (1 wt.%) decorated SnPd composite shell over cobalt-oxide core crystal underneath (denoted as CSPP). For optimizing the ORR performance of the as-prepared NC, we further modulated the configuration of the SnPd shell. In the optimum case, when the Sn/Pd ratio is 0.5 (denoted as CSPP 1005), the ORR mass activity (MA) is 3034.7 mA mgPt−1 at 0.85 V vs. RHE in 0.1 M KOH electrolyte, which is 45-times higher than the commercial Johnson Matthey-Pt/C (J.M.-Pt/C; 20 wt.% Pt) catalyst (67 mA mgPt−1). The results of physical inspections along with electrochemical analysis suggest that such high performance of CSPP 1005 NC can be attributed to the synergistic collaboration between Pt-clusters, PtPd nanoalloys, and adjacent SnPd domains, where Pt-clusters and PtPd nanoalloys promote the O2 adsorption and subsequent splitting, while the SnPd shell favours the OH− relocation step. We believe that the obtained results will open a new avenue for further exploring the high-performance Pt-based catalysts with low Pt-loading and high utilization.

Co-Existence of Atomic Pt and CoPt Nanoclusters on Co/SnOx Mix-Oxide Demonstrates an Ultra-High-Performance Oxygen Reduction Reaction Activity

An effective approach for increasing the Noble metal-utilization by decorating the atomic Pt clusters (1 wt.%) on the CoO₂@SnPd₂ nanoparticle (denoted as CSPP) for oxygen reduction reaction (ORR) is demonstrated in this study. For the optimum case when the impregnation temperature for Co-crystal growth is 50 °C (denoted as CSPP-50), the CoPt nanoalloys and Pt-clusters decoration with multiple metal-to-metal oxide interfaces are formed. Such a nanocatalyst (NC) outperforms the commercial Johnson Matthey-Pt/C (J.M.-Pt/C; 20 wt.% Pt) catalyst by 78-folds with an outstanding mass activity (MA) of 4330 mA mg_Pt^-1 at 0.85 V vs. RHE in an alkaline medium (0.1 M KOH). The results of physical structure inspections along with electrochemical analysis suggest that such a remarkable ORR performance is dominated by the potential synergism between the surface anchored Pt-clusters, CoPt-nanoalloys, and adjacent SnPd₂ domain, where Pt-clusters offer ideal adsorption energy for O₂ splitting and CoPt-nanoalloys along with SnPd₂ domain boost the subsequent desorption of hydroxide ions (OH^-).

Enhanced the synergistic degradation effect between active hydroxyl and reactive oxygen species for indoor formaldehyde based on platinum atoms modified MnOOH/MnO2 catalyst

Maintaining high activity during prolonged catalysis is always the pursuit in catalytic degradation of organic pollutants. For indoor formaldehyde (HCHO) degradation, the accumulation of intermediates is the major factor limiting the conversion of HCHO to final product CO₂ (HCHO-to-CO₂ conversion) and long-lasting catalysis. Herein, a three-dimensional radialized nanostructure catalyst self-assembled by MnOOH/MnO₂ nanosheets anchored with Pt single atoms (Pt_SA-MnOOH/MnO₂ with a trace platinum loading amount of 0.09%) is developed by thermally assisted two-step electrochemical method, which achieves enhanced CO₂ production in catalytic HCHO degradation at the room temperature by the collaborative action of active hydroxyl (OH*) and active oxygen species (O₂*). By boosting intermediates' decomposing, the catalyst implements real-time HCHO-to-CO₂ conversion (∼85.7%) and long-term continuous HCHO removal (∼98%) during 100 h in a 15 ppm HCHO atmosphere at 25 °C under a weight hourly space velocity of 30000 mL/g_cat∙h. Density functional theory calculation shows that the formation energy of O₂* from O₂ over Pt_SA-MnOOH/MnO₂ is nearly half lower than that over Pt-MnO₂ catalyst. And decomposing accumulated intermediates gives the credit to OH* species sustainably generated by the combined action of MnOOH and O₂*. The synergistic action between Pt_SA and MnOOH contributes to the continuous production of O₂* and OH* for enhancing CO₂ production in indoor catalytic formaldehyde degradation.

An Efficient Electrocatalyst (PtCo/C) for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is paid much more attention because of the high overpotential required for driving the four-electron process in the field of storage and sustainable energy conversion, including fuel cell applications. In this paper, PtCo nanoparticles encapsulated on carbon supports were prepared by a simple modified polyol method with ethylene glycol. Structural as well as electrochemical characterizations illustrated that the PtCo/C electrocatalysts had a minimum particle size of 4.8 nm, which is close to the commercial 40 wt% Pt/JM. Moreover, the electrochemical measurements indicated that ORR activity was competitive with the commercial 40 wt% Pt/JM catalyst. The synthesis method is a critical way to produce PtCo/C catalysts for use in polymer electrolyte membranes in fuel cells (PEMFCs).

Redispersion strategy for high-loading carbon-supported metal catalysts with controlled nuclearity

Supported low-nuclearity metal catalysts integrating single atoms or small clusters have emerged as promising materials for diverse applications. While sophisticated synthetic methods provide a high level of nuclearity control in the subnanometre regime, these routes do not fulfil the requirements for translation into industrial practice of (i) effectiveness for high metal contents and (ii) facile scalability. Herein, we present a gas-phase redispersion strategy consisting of sequential C2H2 and HCl treatments to gradually disperse Ru, Rh and Ir nanoparticles supported on commercial activated carbon with metal content up to 10 wt% and initial average sizes of ≈ 1 nm into small clusters and eventually single atoms. Avoidance of nanoparticle surface overchlorination, which hinders C2H2 adsorption, is identified as key for the redispersion process, as demonstrated by the inefficacy of both C2H2-HCl cofeeding and inverse sequence (i.e., HCl first) treatments. Precise size control (±0.1 nm) is enabled by regulating the number of C2H2-HCl cycles. Detailed characterisation by X-ray absorption spectroscopy, electron paramagnetic resonance and time-resolved mass spectrometry reveals that the redispersion occurs via a layer-by-layer mechanism. Specifically, the migration of surface chlorinated metal species to the carbon support is induced by the C2H2 treatment, depleting accessible surface Cl atoms, while the subsequent HCl treatment rechlorinates the cluster surface. The strategy paves the way for the generation of high-density metal sites with tuneable nuclearity for tailored applications.

Exploring the Effect of Pd on the Oxygen Reduction Performance of Pt by In Situ Raman Spectroscopy

Directly monitoring the oxygen reduction reaction (ORR) process in situ is very important to deeply understand the reaction mechanism and is a critical guideline for the design of high-efficiency catalysts, but there is still lack of definite in situ evidence to clarify the effect between adsorbed intermediates and the strain/electronic effect for enhanced ORR performance. Herein, in situ surface-enhanced Raman spectroscopy (SERS) was employed to detect the intermediates during the ORR process on the Au@Pd@Pt core/shell heterogeneous nanoparticles (NPs). Direct spectroscopic evidence of the *OOH intermediate was obtained, and an obvious red shift of the *OOH frequency was identified with the controllable shell thickness of Pd. Detailed experimental characterizations and density functional theory (DFT) calculations demonstrated that such improved ORR activity after inducing Pd into Au@Pt NPs can be attributed to the optimized adsorbate-substrate interaction due to the strain and electronic effect, leading to a higher Pt-O binding energy and a lower O-O binding energy, which was conducive to O-O dissociation and promoted the subsequent reaction. Notably, this work illustrates a relationship between the performance and strain/electronic effect via the intermediate detected by SERS and paves the way for the construction of ORR electrocatalysts with high performance.

Lens-Shaped Carbon Particles with Perpendicularly-Oriented Channels for High-Performance Proton Exchange Membrane Fuel Cells

Two-dimensional sheet-like mesoporous carbon particles are promising for maximizing the number of active sites and the mass transport efficiency of proton exchange membrane fuel cells (PEMFCs). Herein, we develop a series of lens-shaped mesoporous carbon (LMC) particles with perpendicularly oriented channels (diameter = 60 nm) and aspect ratios (ARs) varying from 2.1 to 6.2 and apply them for the fabrication of highly efficient PEMFCs. The membrane emulsification affords uniform-sized, lens-shaped block copolymer particles, which are successfully converted into the LMC particles with well-ordered vertical channels through hyper-cross-linking and carbonization steps. Then, an ultralow amount (1 wt %) of platinum (Pt) is loaded into the particles. The LMC particles with higher ARs are packed with a higher density in the cathode and are better aligned on the cathode surface compared to the LMC particles with lower ARs. Thus, the well-ordered channels in the particles facilitate the mass transport of the reactants and products, significantly increasing the PEMFC performance. For example, the LMC particles with the AR of 6.2 show the highest initial single cell performance of 1135 mW cm^-2, and the cell exhibits high durability with 1039 mW cm^-2 even after 30 000 cycles. This cell performance surpasses that of commercial Pt/C catalysts, even at 1/20 of the Pt loading.

Electrocatalysts for the Oxygen Reduction Reaction: From Bimetallic Platinum Alloys to Complex Solid Solutions

The oxygen reduction reaction has been the object of intensive research in an attempt to improve the sluggish kinetics that limit the performance of renewable energy storage and utilization systems. Platinum or platinum bimetallic alloys are common choices as the electrode material, but prohibitive costs hamper their use. Complex alloy materials, such as high-entropy alloys (HEAs), or more generally, multiple principal component alloys (MPCAs), have emerged as a material capable of overcoming the limitations of platinum and platinum-based materials. Theoretically, due to the large variety of active sites, this new kind of material offers the opportunity to identify experimentally the optimal binding site on the catalyst surface. This review discusses recent advances in the application of such alloys for the oxygen reduction reaction and existing experimental challenges in the benchmarking of the electrocatalytic properties of these materials.

Ultrathin Twisty PdNi Alloy Nanowires as Highly Active ORR Electrocatalysts Exhibiting Morphology-Induced Durability over 200 K Cycles

Even though the anion exchange membrane fuel cells have many advantages, the stability of their electrocatalysts for oxygen reduction reaction (ORR) has remained remarkably poor. We report here on the ultrathin twisty PdNi-alloy nanowires (NWs) exhibiting a very low reaction overpotential with an E_1/2 ∼ 0.95 V versus RHE in alkaline media maintained over 200 K cycles, the highest ever recorded for an electrocatalyst. The mass activity of the used NWs is >10 times higher than fresh commercial Pt/C. Therein, Ni improves the Pd d-band center for a more efficient ORR, and its leaching continuously regenerates the surface active sites. The twisty nanowire morphology imparts multiple anchor points on the electrode surface to arrest their detachment or coalescence and extra stability from self-entanglement. The significance of the NW morphology was further confirmed from the high-temperature durability studies. The study demonstrates that tailoring the number of contact points to the electrode-surface may help realize commercial-grade stability in the highly active electrocatalysts.

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.

A ternary PdNiMo alloy as a bifunctional nanocatalyst for the oxygen reduction reaction and hydrogen evolution reaction

Mo0.2Pd3Ni/NC shows superior electrochemical performances for both ORR and HER due to the charge redistribution among Ni, Pd, and Mo, tuning the electronic structure of Pd.

Promoting Oxygen Reduction via Crafting Bridge-bonded Oxygen Ligands on Iron Single-Atom Catalyst

Single-atom Fe-N-C catalysts with Fe-N4 coordination structures hailed as the most promising candidates are prohibited by the severe aggregation and migration of metal atoms. Bonding confine strategies can effectively regulate...

One-pot synthesis of Au-based nanocrystals via a platinum group metal anion controlled growth strategy in citrate medium

Finding a facile manufacturing method of Au-based low PGM content nanocrystals by exploring the reaction process of a series of PGM anions with the in situ Au NW templates.

In Situ Precise Tuning of Bimetallic Electronic Effect for Boosting Oxygen Reduction Catalysis

Tuning intermediate adsorption energy by shifting the d-band center offers a powerful strategy to tailor the reactivity of metal catalysts. Here we report a potential sweep method to grow Pd layer-by-layer on Au with the capability to in situ measure the surface structure through an ethanol oxidation reaction. Spectroscopic characterizations reveal charge-transfer induced valence band restructuring in the Pd overlayer, which shifts the d-band center away from the Fermi level compared to bulk Pd. Precise overlayer control gives the optimal bimetallic surface of two monolayers (ML) Pd on Au, which exhibits more than 370-fold mass activity enhancement in oxygen reduction reaction (at 0.9 V vs. reversible hydrogen electrode) and 40 mV increase in half-wave potential compared to the Pt/C. Tested in a homemade Zn-air battery, the 2-ML-Pd/Au/C exhibits a maximum power density of 296 mW/cm² and specific activity of 804 mAh/g_Zn, much higher than Pt/C with the same catalyst loading amount.

Platinum single-atom catalyst coupled with transition metal/metal oxide heterostructure for accelerating alkaline hydrogen evolution reaction

Single-atom catalysts provide an effective approach to reduce the amount of precious metals meanwhile maintain their catalytic activity. However, the sluggish activity of the catalysts for alkaline water dissociation has hampered advances in highly efficient hydrogen production. Herein, we develop a single-atom platinum immobilized NiO/Ni heterostructure (PtSA-NiO/Ni) as an alkaline hydrogen evolution catalyst. It is found that Pt single atom coupled with NiO/Ni heterostructure enables the tunable binding abilities of hydroxyl ions (OH*) and hydrogen (H*), which efficiently tailors the water dissociation energy and promotes the H* conversion for accelerating alkaline hydrogen evolution reaction. A further enhancement is achieved by constructing PtSA-NiO/Ni nanosheets on Ag nanowires to form a hierarchical three-dimensional morphology. Consequently, the fabricated PtSA-NiO/Ni catalyst displays high alkaline hydrogen evolution performances with a quite high mass activity of 20.6 A mg-1 for Pt at the overpotential of 100 mV, significantly outperforming the reported catalysts.

Hierarchically porous Fe,N-doped carbon nanorods derived from 1D Fe-doped MOFs as highly efficient oxygen reduction electrocatalysts in both alkaline and acidic media

Rationally designing low-cost yet highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in both alkaline and acidic media remains highly challenging. Herein, we report the facile synthesis of Fe,N-doped carbon nanorods (denoted as Fe-N/C-NR) with abundant hierarchical pores and highly active sites by the pyrolysis of a one-dimensional (1D) Fe-doped zeolitic imidazolate framework (Fe-ZIF-8) as a self-sacrificing template. The unique 1D nanoarchitecture of the resultant Fe-N/C-NR can provide fast electron and electrolyte transport towards exposed active sites, and their hierarchically porous structures with large surface areas can efficiently facilitate mass diffusion and increase the density of exposed active sites. Furthermore, it is demonstrated that the coexistence of highly dispersed Fe-Nx sites and Fe3C/Fe nanoparticles (NPs) in these electrocatalysts can provide a large number of desired catalytic centers with highly intrinsic activity and structural stability. As a result, the optimized 5Fe-N/C-NR exhibits excellent catalytic activity for the ORR, with a high half-wave potential (E1/2) of 0.90 V vs. RHE in alkaline medium, superior to that of commercial Pt/C (0.86 V vs. RHE), and also a high E1/2 of 0.81 V vs. RHE in acidic medium, comparable to that of commercial Pt/C (0.81 V vs. RHE). Moreover, its robust ORR durability can far surpass that of commercial Pt/C in both acidic and alkaline media, further highlighting the merit of this MOF-templated strategy. Our findings might shed light on the rational design of cost-effective and highly efficient ORR electrocatalysts for practical applications.

Directly Probing the Local Coordination, Charge State, and Stability of Single Atom Catalysts by Advanced Electron Microscopy: A Review

The drive for atom efficient catalysts with carefully controlled properties has motivated the development of single atom catalysts (SACs), aided by a variety of synthetic methods, characterization techniques, and computational modeling. The distinct capabilities of SACs for oxidation, hydrogenation, and electrocatalytic reactions have led to the optimization of activity and selectivity through composition variation. However, characterization methods such as infrared and X-ray spectroscopy are incapable of direct observations at atomic scale. Advances in transmission electron microscopy (TEM) including aberration correction, monochromators, environmental TEM, and micro-electro-mechanical system based in situ holders have improved catalysis study, allowing researchers to peer into regimes previously unavailable, observing critical structural and chemical information at atomic scale. This review presents recent development and applications of TEM techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs. Aberration corrected TEM imaging routinely achieves sub-Ångstrom resolution to reveal the atomic structure of materials. TEM spectroscopy provides complementary information about local composition, chemical bonding, electronic properties, and atomic/molecular vibration with superior spatial resolution. In situ/operando TEM directly observe the evolution of SACs under reaction conditions. This review concludes with remarks on the challenges and opportunities for further development of TEM to study SACs.

Atomic Crystal Facet Engineering of Core-Shell Nanotetrahedrons Restricted under Sub-10 Nanometer Region

Simultaneously engineering the size and surface crystal facets of bimetallic core-shell nanocrystals offers an effective route to not only reduce the extravagance of innermost core metal and maximize the utilization efficiency of shell atoms but also strengthen the core-to-shell interaction via ligand and/or strain effects. Herein, we systematically study the architecture transition and crystal facet engineering at the atomic level on the surface of sub-5 nm Pd(111) tetrahedrons (Ths), aimed at embodying how the variations in the local facet and shape of a sub-10 nm core-shell structure affect its surface geometrical properties and electronic structures. Specifically, surface atomic replication is predominant when the shell metal deposits less than five atomic layers, thus forming a series of Pd@M (M = Pt, Ru, and Rh) core-shell Ths enclosed by (111) facets (∼6.8 nm), while over five atomic layers, spontaneous facets tropism of each metal is predominant, where Pt atoms still follow fcc-(111) packing, Ru atoms select hcp-phase stacking, and Rh atoms choose fcc-(100) crystallization, respectively. In particular, Pt atoms take a seamless geometrical transformation from Pd@Pt Ths into Pd@Pt truncated octahedrons (TOhs, ∼7.6 nm). As a proof-of-concept application, such sub-10 nm core-shell architectures with Pt skin show a component-dependent relationship toward oxygen reduction reaction (ORR), where the catalytic activity follows the order of Pd@Pt(111) TOhs (E_1/2 = 0.916 V, 1.632 A mg_Pt^-1) > Pd@Pt(111) Ths > Pt black. Meanwhile the Ru skin show a facet-dependent relationship toward acidic hydrogen evolution reaction (HER) where the catalytic activity follows the order of Pd@Ru(111) Ths > Pd@Ru(hcp) Ths > Pd Ths.

Carbon Nitride Anchored on a Nitrogen-Doped Carbon Nanotube Surface for Enhanced Oxygen Reduction Reaction

Replacement of noble-metal-based electrocatalysts with inexpensive alternatives that have similar high performance will benefit the development of fuel cell technology. A metal-free composite of carbon nitride (CN) with nitrogen-doped carbon nanotubes (NCNTs) and its application as an electrocatalyst for the oxygen reduction reaction are reported for the first time in this study. The fabricated material (CN/NCNTs) formed a close interface between the two components and showed a comparable electrocatalytic activity in an alkaline medium to that of a conventional platinum-loaded carbon, superior tolerance to crossover effects, and excellent stability. The electrocatalytic efficiency of the CN/NCNT composite was nearly 100% of the four-electron (4 e^-) pathway. The high performance of the current electrocatalyst was derived from its features, such as a relatively large surface area, high content of pyridinic nitrogen, and superior conductivity.

Synthetic strategies of supported atomic clusters for heterogeneous catalysis

Supported atomic clusters with uniform metal sites and definite low-nuclearity are intermediate states between single-atom catalysts (SACs) and nanoparticles in size. Benefiting from the presence of metal–metal bonds, supported atomic clusters can trigger synergistic effects among every metal atom, which contributes to achieving unique catalytic properties different from SACs and nanoparticles. However, the scalable and precise synthesis and atomic-level insights into the structure–properties relationship of supported atomic clusters is a great challenge. This perspective presents the latest progress of the synthesis of supported atomic clusters, highlights how the structure affects catalytic properties, and discusses the limitations as well as prospects.

Supported atomic clusters with precise nuclearity are intermediate states between single-atom catalysts and nanoparticles in size. Here the authors summarize and discuss synthetic strategies of supported atomic clusters with unique catalytic properties for heterogeneous reactions.

Role of Substrate in Au Nanoparticle Decoration by Electroless Deposition

Decoration of nanostructures is a promising way of improving performances of nanomaterials. In particular, decoration with Au nanoparticles is considerably efficient in sensing and catalysis applications. Here, the mechanism of decoration with Au nanoparticles by means of low-cost electroless deposition (ELD) is investigated on different substrates, demonstrating largely different outcomes. ELD solution with Au potassium cyanide and sodium hypophosphite, at constant temperature (80 °C) and pH (7.5), is used to decorate by immersion metal (Ni) or semiconductor (Si, NiO) substrates, as well as NiO nanowalls. All substrates were pre-treated with a hydrazine hydrate bath. Scanning electron microscopy and Rutherford backscattering spectrometry were used to quantitatively analyze the amount, shape and size of deposited Au. Au nanoparticle decoration by ELD is greatly affected by the substrates, leading to a fast film deposition onto metallic substrate, or to a slow cluster (50–200 nm sized) formation on semiconducting substrate. Size and density of resulting Au clusters strongly depend on substrate material and morphology. Au ELD is shown to proceed through a galvanic displacement on Ni substrate, and it can be modeled with a local cell mechanism widely affected by the substrate conductivity at surface. These data are presented and discussed, allowing for cheap and reproducible Au nanoparticle decoration on several substrates.

Recent Progress in Engineering the Atomic and Electronic Structure of Electrocatalysts via Cation Exchange Reactions

In the past few decades, tremendous advances have been made in electrocatalysis due to the rational design of electrocatalysts at the nanoscale level. Further development requires engineering electrocatalysts at the atomic level, which is a grand challenge. Here, the recent advances in cation exchange strategy, which is a powerful tool for fine-tuning atomic structure of electrocatalysts via surface faceting, heteroatom doping, defects formation, and strain modulation, are the main focus. Proper atomic structure engineering effectively adjusts the electronic structure, and thus enhances the electronic conductivity and facilitates the adsorption/desorption of reaction intermediates. By virtue, the cation exchange strategy greatly boosts the intrinsic and apparent activities of electrocatalysts and shows a great potential toward design of new energy conversion devices, such as water splitting devices and metal-air batteries. It is believed that cation exchange offers new insights and opportunities for the rational design of a new generation of electrocatalysts.

Recent Advancements and Future Prospects of Noble Metal-Based Heterogeneous Nanocatalysts for Oxygen Reduction and Hydrogen Evolution Reactions

The oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) both are key electrochemical reactions for enabling next generation alternative-power supply technologies. Despite great merits, both of these reactions require robust electrocatalysts for lowering the overpotential and promoting their practical applications in energy conversion and storage devices. Although, noble metal-based catalysts (especially Pt-based catalysts) are at the forefront in boosting the ORR and HER kinetics, high cost, limited availability, and poor stability in harsh redox conditions make them unfit for scalable use. To this end, various strategies including downsizing the catalyst size, reducing the noble metal, and increasing metal utilization have been adopted to appropriately balance the performance and economic issues. This mini-review presents an overview of the current state of the technological advancements in noble metal-based heterogeneous nanocatalysts (NCs) for both ORR and HER applications. More specifically, we focused on establishing the structure–performance correlation.