Ir-Alloyed Ultrathin Ternary PdIrCu Nanosheet-Constructed Flower with Greatly Enhanced Catalytic Performance toward Formic Acid Electrooxidation

Recent advances of doping strategy for boosting the electrocatalytic performance of two-dimensional noble metal nanosheets

Benefiting from the ultrahigh specific surface areas, massive exposed surface atoms, and highly tunable microstructures, the two-dimensional (2D) noble metal nanosheets (NSs) have presented promising performance for various electrocatalytic reactions. Nevertheless, the heteroatom doping strategy, and in particular, the electronic structure tuning mechanisms of the 2D noble metal catalysts (NMCs) yet remain ambiguous. Herein, we first review several effective strategies for modulating the electrocatalytic performance of 2D NMCs. Then, the electronic tuning effect of hetero-dopants for boosting the electrocatalytic properties of 2D NMCs is systematically discussed. Finally, we put forward current challenges in the field of 2D NMCs, and propose possible solutions, particularly from the perspective of the evolution of electron microscopy. This review attempts to establish an intrinsic correlation between the electronic structures and the catalytic properties, so as to provide a guideline for designing high-performance electrocatalysts.

Sn-doped PdCu alloy nanosheet assemblies as an efficient electrocatalyst for formic acid oxidation

A ternary alloy catalyst has been confirmed to be an effective catalyst for anode catalysis in direct formic acid fuel cells, which can improve the electrocatalytic performance of the fuel cell by introducing commonly used metal elements to change the Pd electronic structure and can reduce the use of precious metals and the cost of catalyst production. In this study, PdCuSn Ns/C with a special 3D structure was synthesized by a simple two-step wet chemical method. The PdCuSn Ns/C catalyst prepared exhibits excellent catalytic activity and stability for the formic acid oxidation reaction (FAOR). The mass activity of 2420.1 mA mg-1Pd is 3.94 times that of the Pd/C catalyst. The improvement in the electrocatalytic performance stems from the introduction of Cu and Sn atoms and the unique 3D nanosheet structure, which changes the electronic structure of Pd to increase the reactive active site and accelerates the reaction mass transfer rate, and also reduces the content of precious metals, while improving the electrocatalytic performance. Therefore, the PdCuSn Ns/C catalyst has a promising future in the field of electrocatalysis.

Recent advances in nanoflowers: compositional and structural diversification for potential applications

In recent years, nanoscience and nanotechnology have emerged as promising fields in materials science. Spectroscopic techniques like scanning tunneling microscopy and atomic force microscopy have revolutionized the characterization, manipulation, and size control of nanomaterials, enabling the creation of diverse materials such as fullerenes, graphene, nanotubes, nanofibers, nanorods, nanowires, nanoparticles, nanocones, and nanosheets. Among these nanomaterials, there has been considerable interest in flower-shaped hierarchical 3D nanostructures, known as nanoflowers. These structures offer advantages like a higher surface-to-volume ratio compared to spherical nanoparticles, cost-effectiveness, and environmentally friendly preparation methods. Researchers have explored various applications of 3D nanostructures with unique morphologies derived from different nanoflowers. The nanoflowers are classified as organic, inorganic and hybrid, and the hybrids are a combination thereof, and most research studies of the nanoflowers have been focused on biomedical applications. Intriguingly, among them, inorganic nanoflowers have been studied extensively in various areas, such as electro, photo, and chemical catalysis, sensors, supercapacitors, and batteries, owing to their high catalytic efficiency and optical characteristics, which arise from their composition, crystal structure, and local surface plasmon resonance (LSPR). Despite the significant interest in inorganic nanoflowers, comprehensive reviews on this topic have been scarce until now. This is the first review focusing on inorganic nanoflowers for applications in electro, photo, and chemical catalysts, sensors, supercapacitors, and batteries. Since the early 2000s, more than 350 papers have been published on this topic with many ongoing research projects. This review categorizes the reported inorganic nanoflowers into four groups based on their composition and structure: metal, metal oxide, alloy, and other nanoflowers, including silica, metal-metal oxide, core-shell, doped, coated, nitride, sulfide, phosphide, selenide, and telluride nanoflowers. The review thoroughly discusses the preparation methods, conditions for morphology and size control, mechanisms, characteristics, and potential applications of these nanoflowers, aiming to facilitate future research and promote highly effective and synergistic applications in various fields.

Nanoarchitectonics of Metallene Materials for Electrocatalysis

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

Modulate the metallic Sb state on ultrathin PdSb-based nanosheets for efficient formic acid electrooxidation

Incorporation of oxophilic metals into Pd-based nanostructures has shown great potential in small molecule electrooxidation owing to their superior anti-poisoning capability. However, engineering the electronic structure of oxophilic dopants in Pd-based catalysts remains challenging and their impact on electrooxidation reactions is rarely demonstrated. Herein, we have developed a method for synthesizing PdSb-based nanosheets, enabling the incorporation of the Sb element in a predominantly metallic state despite its high oxophilic nature. Moreover, the Pd₉₀Sb₇W₃ nanosheet serves as an efficient electrocatalyst for the formic acid oxidation reaction (FAOR), and the underlying promotion mechanism is investigated. Among the as-prepared PdSb-based nanosheets, the Pd₉₀Sb₇W₃ nanosheet exhibits a remarkable 69.03% metallic state of Sb, surpassing the values observed for the Pd₈₆Sb₁₂W₂ (33.01%) and Pd₈₃Sb₁₄W₃ (25.41%) nanosheets. X-ray photoelectron spectroscopy (XPS) and CO stripping experiments confirm that the Sb metallic state contributes the synergistic effect of their electronic and oxophilic effect, thus leading to an effective electrooxidation removal of CO and significantly enhanced FAOR electrocatalytic activity (1.47 A mg^-1; 2.32 mA cm^-1) compared with the oxidated state of Sb. This work highlights the importance of modulating the chemical valence state of oxophilic metals to enhance electrocatalytic performance, offering valuable insights for the design of high-performance electrocatalysts for electrooxidation of small molecules.

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Recent progress on the synthesis of metal alloy nanowires as electrocatalysts

Benefiting from both one-dimensional (1D) morphology and alloy composition, metal alloy nanowires have been exploited as advanced electrocatalysts in various electrochemical processes. In this review, the synthesis approaches for metal alloy nanowires are classified into two categories: direct syntheses and syntheses based on preformed 1D nanostructures. Ligand systems that are of critical importance to the formation of alloy nanowires are summarized and reviewed, together with the strategies imposed to achieve the co-reduction of different metals. Meanwhile, different scenarios that form alloy nanowires from pre-synthesized 1D nanostructures are compared and contrasted. In addition, the characterization and electrocatalytic applications of metal alloy nanowires are briefly discussed.

Curved Porous PdCu Metallene as a High-Efficiency Bifunctional Electrocatalyst for Oxygen Reduction and Formic Acid Oxidation

Designing high-efficiency and newly developed Pd-based bifunctional catalytic materials still faces tremendous challenges for oxygen reduction reaction (ORR) and formic acid oxidation reaction (FAO). Metallene materials with unique structural features are considered strong candidates for enhancing the catalytic performance. In this work, we synthesized copper-doped two-dimensional curved porous Pd metallene nanomaterials via a simplistic one-pot solvothermal method. The updated catalysts served as sturdy bifunctional electrocatalysts for cathodal ORR and anodic FAO. In particular, the developed PdCu metallene exhibits excellent half-wave potential (0.943 V vs RHE) and mass activity (MA) (1.227 A mg_Pt^-1) in alkaline solutions, which are 1.09 and 6.26 times higher than those of commercial Pt/C, respectively, indicating that the nanomaterials have abundant active sites, displaying surpassing catalytic performance for oxygen reduction. Furthermore, in an acidic formic acid electrolyte, PdCu metallene exhibits prominent MA with a value of 0.905 A mg_Pd^-1, which is 2.76 times that of commercial Pd/C. The remarkable bifunctional catalytic performance of metallene materials can be attributed to the special structure and electronic effects. This work shows that metallene materials with curved and porous properties provide a scientific idea for the development and design of efficient and steady electrocatalysts.

Hollow Carbon Nanorod Confined Single Atom Rh for Direct Formic Acid Electrooxidation

Nearly theoretical 100% atomic utilization (supposing each atom could serve as independent sites to play a role in catalyz) of single-atom catalysts (SACs) makes it highly promising for various applications. However, for most SACs, single-atom sites are trapped in a solid carbon matrix, which makes the inner parts hardly available for reaction. Herein, a hollow N-doped carbon confined single-atom Rh (Rh-SACs/HNCR) is developed via a coordination-template method. Both aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy mapping confirm the uniform distribution of Rh single atoms. Owning to the unique hollow structure and effective carbon confinement, excessive conversion from pyridinic/pyrrolic N to graphic N is hindered. As a proof of concept, Rh-SACs/HNCR exhibits superior activity, stability, selectivity, and anti-poisoning capability in formic acid oxidation reaction compared with the counterpart Rh/C, Pd/C, and Pt/C catalysts. This work provides a powerful strategy for synthesizing hollow carbon confined single-atom catalysts apply in various energy-related systems.

Recent advances in fuel cell reaction electrocatalysis based on porous noble metal nanocatalysts

As the center of fuel cells, electrocatalysts play a crucial role in determining the conversion efficiency from chemical energy to electrical energy. Therefore, the development of advanced electrocatalysts with both high activity and stability is significant but challenging. Active site, mass transport, and charge transfer are three central factors influencing the catalytic performance of electrocatalysts. Endowed with rich available surface active sites, facilitated electron transfer and mass diffusion channels, and highly active components, porous noble metal nanomaterials are widely considered as promising electrocatalysts toward fuel cell-related reactions. The past decade has witnessed great achievements in the design and fabrication of advanced porous noble metal nanocatalysts in the field of electrocatalytic fuel oxidation reaction (FOR) and oxygen reduction reaction (ORR). Herein, the recent research advances regarding porous noble metal nanocatalysts for fuel cell-related reactions are reviewed. In the discussions, the inherent structural features of porous noble metal nanostructures for electrocatalytic reactions, advanced synthetic strategies for the fabrication of porous noble metal nanostructures, and the structure-performance relationships are also provided.

Nonmetal-doping of noble metal-based catalysts for electrocatalysis

In response to the shortage of fossil fuels, efficient electrochemical energy conversion devices are attracting increasing attention, while the limited electrochemical performance and high cost of noble metal-based electrode materials remain a daunting challenge. The electrocatalytic performance of electrode materials is closely bound with their intrinsic electronic/ionic states and crystal structures. Apart from the nanoscale design and conductive composite strategies, heteroatom doping, particularly for nonmetal doping (e.g., hydrogen, boron, sulfur, selenium, phosphorus, and tellurium), is also another effective strategy to greatly promote the intrinsic activity of the electrode materials by tuning their atomic structures. From the perspective of electrocatalytic reactions, the effective atomic structure regulation could induce additional active sites, create rich defects, and optimize the adsorption capability, thereby contributing to the promotion of the electrocatalytic performance of noble metal-based electrocatalysts. Encouraged by the great progress achieved in this field, we have reviewed recent advancements in nonmetal doping for electrocatalytic energy conversion. Specifically, the doping effect on the atomic structure and intrinsic electronic/ionic state is also systematically illustrated and the relationship with the electrocatalytic performance is also investigated. It is believed that this review will provide guidance for the development of more efficient electrocatalysts.

Electrocatalysis of gold-based nanoparticles and nanoclusters

Gold (Au)-based nanomaterials, including nanoparticles (NPs) and nanoclusters (NCs), have shown great potential in many electrocatalytic reactions due to their excellent catalytic ability and selectivity. In recent years, Au-based nanostructured materials have been considered as one of the most promising non-platinum (Pt) electrocatalysts. The controlled synthesis of Au-based NPs and NCs and the delicate microstructure adjustment play a vital role in regulating their catalytic activity toward various reactions. This review focuses on the latest progress in the synthesis of efficient Au-based NP and NC electrocatalysts, highlighting the relationship between Au nanostructures and their catalytic activity. This review first discusses the parameters of Au-based nanomaterials that determine their electrocatalytic performance, including composition, particle size and architecture. Subsequently, the latest electrocatalytic applications of Au-based NPs and NCs in various reactions are provided. Finally, some challenges and opportunities are highlighted, which will guide the rational design of Au-based NPs and NCs as promising electrocatalysts.

Ultrathin PdCu alloy nanosheet-assembled 3D nanoflowers with high peroxidase-like activity toward colorimetric glucose detection

Enzyme-mimetic properties of nanomaterials can be efficiently tuned by controlling their size, composition, and structure. Here, ultrathin PdCu alloy nanosheet-assembled three-dimensional (3D) nanoflowers (Pd₁Cu_x NAFs) with tunable surface composition are obtained via a generalized strategy. In presence of H₂O₂, the as-synthesized Pd₁Cu_x NAFs can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to the oxidized form of TMB (oxTMB) with a characteristic absorption peak at 652 nm. Interestingly, Pd₁Cu_x NAFs show obviously composition-dependent peroxidase-like catalytic activities because of the synergistic interaction of nanoalloy. Additionally, different from 2D Pd nanosheets, the distinctive 3D superstructures are featured with rich approachable sites and proper layer spacing, which are in favor of fast mass transport and electron transfers during the catalytic process. Among the studied Pd₁Cu_x NAFs, the Pd₁Cu_1.7 NAFs show the highest enzyme-like activities and can be successfully applied for the colorimetric detection of glucose with a low detection limit of 2.93 ± 0.53 μM. This work provides an efficient avenue to fabricate PdCu NAF nanozymes in biosensing toward glucose detection. Two-dimensional (2D) PdCu ultrathin nanosheet-assembled 3D nanoflowers (Pd₁Cu_x NAFs) with tunable surface composition exhibit substantially enhanced intrinsic peroxidase-like catalytic activities. The Pd₁Cu_1.7 NAFs are successfully used as peroxidase mimic catalyst for the colorimetric detection of glucose with low detection limit of 2.93 μM.

Recent Progress of Ultrathin 2D Pd-Based Nanomaterials for Fuel Cell Electrocatalysis

Pd- and Pd-based catalysts have emerged as potential alternatives to Pt- and Pt-based catalysts for numerous electrocatalytic reactions, particularly fuel cell-related reactions, including the anodic fuel oxidation reaction (FOR) and cathodic oxygen reduction reaction (ORR). The creation of Pd- and Pd-based architectures with large surface areas, numerous low-coordinated atoms, and high density of defects and edges is the most promising strategy for improving the electrocatalytic performance of fuel cells. Recently, 2D Pd-based nanomaterials with single or few atom thickness have attracted increasing interest as potential candidates for both the ORR and FOR, owing to their remarkable advantages, including high intrinsic activity, high electron mobility, and straightforward surface functionalization. In this review, the recent advances in 2D Pd-based nanomaterials for the FOR and ORR are summarized. A fundamental understanding of the FOR and ORR is elaborated. Subsequently, the advantages and latest advances in 2D Pd-based nanomaterials for the FOR and ORR are scientifically and systematically summarized. A systematic discussion of the synthesis methods is also included which should guide researchers toward more efficient 2D Pd-based electrocatalysts. Lastly, the future outlook and trends in the development of 2D Pd-based nanomaterials toward fuel cell development are also presented.

PdAgPt Corner-Satellite Nanocrystals in Well-Controlled Morphologies and the Structure-Related Electrocatalytic Properties

The functions of heterogeneous metallic nanocrystals (HMNCs) can be undoubtedly tuned by controlling their morphologies and compositions. As a less-studied kind of HMNCs, corner-satellite multi-metallic nanocrystals (CSMNCs) have great research value in structure-related electrocatalytic performance. In this work, PdAgPt corner-satellite nanocrystals with well-controlled morphologies and compositions have been developed by temperature regulation of a seed-mediated growth process. Through the seed-mediated growth, the morphology of PdAgPt products evolves from Pd@Ag cubes to PdAgPt corner-satellite cubes, and eventually to truncated hollow octahedra, as a result of the expansion of {111} facets in AgPt satellites. The growth of AgPt satellites exclusively on the corners of central cubes is realized with the joint help of Ag shell and moderate bromide, and hollow structures form only at higher reaction temperatures on account of galvanic displacement promoted by the Pd core. In view of the different performances of Pd and Pt toward formic acid oxidation (FAO), this structure-sensitive reaction is chosen to measure electrocatalytic properties of PdAgPt HMNCs. It is proven that PdAgPt CSMNCs display greatly improved activity toward FAO in direct oxidation pathway. In addition, with the help of AgPt heterogeneous shells, all PdAgPt HMNCs exhibit better durability than Pd cubes and commercial Pt.

Ultrasmall PdPtCo trimetallic nanorings with enriched low-coordinated edge sites and optimized compositions for effective oxygen reduction electrocatalysis

Ultrasmall two-dimensional Pd₅₅Pt₁₈Co₂₇ trimetallic alloy nanorings with enriched low-coordinated edge sites exhibit greatly enhanced ORR electrocatalytic activity.

General synthesis of Pd-pm (pm = Ga, In, Sn, Pb, Bi) alloy nanosheet assemblies for advanced electrocatalysis

Owing to the synergistic compositional and structural advantages, ultrathin bimetallic nanosheet assembly nanostructures are widely recognized as advanced catalysts for alcohol electrooxidation reaction. Although numerous efforts have been made, the fabrication of well-defined ultrathin bimetallic nanosheet assemblies (NSAs) at large scale is still a tough challenge. Herein, a universal synthetic approach has been proposed to produce a series of well-defined Pd-pm (pm = Ga, In, Sn, Pb, Bi) alloy NSAs. Due to multiple merits of their unique 3D flower-like nanostructure and alloyed crystalline features, the self-supported Pd-pm NSAs show excellent electrocatalytic performance for the methanol oxidation reaction (MOR) and glycerol oxidation reaction (GOR). Given the eco-friendly synthetic concept, facile universality, and outstanding electrocatalytic properties of the generated bimetallic Pd-pm NSAs, we believe that this method could be employed for building more advanced nanocatalysts toward efficient electrocatalytic applications.

Intermetallic Pd3Pb ultrathin nanoplate-constructed flowers with low-coordinated edge sites boost oxygen reduction performance

Although tremendous efforts have been devoted to exploring non-Pt based electrocatalysts toward the oxygen reduction reaction (ORR), achievements in both catalytic activity and durability are still far from satisfactory. Here, we report a facile approach for the synthesis of intermetallic Pd3Pb ultrathin nanoplate-constructed flowers. Such highly opened hierarchical nanostructures with an ordered phase and low-coordinated edge sites exhibited a substantially enhanced activity toward the ORR. Especially, the intermetallic Pd3Pb nanoflowers achieved a record-breaking mass activity (1.14 mA μgPd-1) in an alkaline solution at 0.9 V vs. a reversible hydrogen electrode among the reported Pd-based ORR electrocatalysts to date, which was 1.8, 3.9 and 11.4 times higher than those of intermetallic Pd3Pb nanocubes, Pd3Pb dendrites and commercial Pt/C, respectively. More importantly, the intermetallic Pd3Pb nanoflowers also showed a higher durability with only 23.7% loss in mass activity after 10 000 cycles compared to the commercial Pt/C (35% loss in mass activity) due to their chemically stable intermetallic structures.

Superior Ethanol Oxidation Electrocatalysis Enabled by Ternary Pd-Rh-Te Nanotubes

Designing and elaborating cost-efficient Pd-based electrocatalysts for direct ethanol fuel cells is thought to be a significant approach to obliterating the challenge of large-scale practical application of fuel cells. Herein, our group creates a novel class of one-dimensional (1D) PdRhTe nanotubes (NTs) by using H₂PdCl₄ and RhCl₃ as metal precursors and Te nanowires (NWs) as the reductant and sacrificial template. Strikingly, the as-obtained PdRhTe ternary nanomaterials with a unique 1D nanotube structure display a high specific activity of 6.53 mA cm^-2 and a mass activity of 2039.2 mA mg^-1 for the ethanol oxidation reaction (EOR) in alkaline media, which are 1.25 (1.6) and 1.77 (8.0) times those of PdTe/C and (Pd/C), respectively. More significantly, further electrochemical measurements such as CA and successive CV confirm that the optimized PdRhTe NTs display desirable durability and negligible activity decay. Taking advantage of physicochemical characterizations and electrochemical measurements, we reasonably reveal that the outstanding electrocatalytic performances are derived from the unique geometric structure and synergistic effect. The introduction of Rh facilitates the cleavage of C-C bonds, increasing the self-stability of PdRhTe NTs. In general terms, this work should provide new orientations to synthesize cost-efficient electrocatalysts by a sacrificial template method.

Electrochemical Dealloying-Assisted Surface-Engineered Pd-Based Bifunctional Electrocatalyst for Formic Acid Oxidation and Oxygen Reduction

Synthesis of non-Pt bifunctional electrocatalyst for the anodic oxidation of liquid fuel and cathodic reduction of oxygen is of great interest in the development of energy conversion devices. We demonstrate a facile room-temperature synthesis of surface-engineered trimetallic alloy nanoelectrocatalyst based on Co, Cu, and Pd by thermodynamically favorable transmetallation reaction and electrochemical dealloying. The quasi-spherical Co _xCu _yPd _z trimetallic catalysts were synthesized by the thermodynamically favorable reaction of K₂PdCl₄ with sheetlike Co _mCu _n bimetallic alloy nanostructure. The surface engineering of Co _xCu _yPd _z was achieved by electrochemical dealloying. The surface-engineered alloy electrocatalyst exhibits excellent bifunctional activity toward formic acid oxidation reaction (FAOR) and oxygen reduction reaction (ORR) at same pH. The elemental composition and lattice strain control the electrocatalytic performance. The elemental composition-dependent compressive strain weakens the adsorption of oxygen-containing species and favors the facile electron transfer for FAOR and ORR. The engineered alloy electrocatalyst of Co_0.02Cu_13.8Pd_86.18 composition is highly durable and delivers high mass-specific activity for ORR and FAOR. It delivers mass-specific activities of 1.50 and 0.202 A/mg_Pd for FAOR and ORR, respectively, in acidic pH. The overall performance is superior to that of as-synthesized Pd and dealloyed bimetallic Co_2.7Pd_97.3 and Cu_5.61Pd_94.39 nanoelectrocatalysts.