Two-Dimensional Catalysts: From Model to Reality

Two-dimensional (2D) materials have been utilized broadly in kinds of catalytic reactions due to their fully exposed active sites and special electronic structure. Compared with real catalysts, which are usually bulk or particle, 2D materials have more well-defined structures. With easily identified structure-modulated engineering, 2D materials become ideal models to figure out the catalytic structure-function relations, which is helpful for the precise design of catalysts. In this review, the unique function of 2D materials was summarized from model study to reality catalysis and application. It includes several typical 2D materials, such as graphene, transition metal dichalcogenides, metal, and metal (hydr)oxide materials. We introduced the structural characteristics of 2D materials and their advantages in model researches. It emphatically summarized how 2D materials serve as models to explore the structure-activity relationship by combining theoretical calculations and surface research. The opportunities of 2D materials and the challenges for fundamentals and applications they facing are also addressed. This review provides a reference for the design of catalyst structure and composition, and could inspire the realization of two-dimensional materials from model study to reality application in industry.

Well-Defined PtCo@Pt Core-Shell Nanodendrite Electrocatalyst for Highly Durable Oxygen Reduction Reaction

The rational design of efficient electrocatalysts with controllable structure and composition is crucial for enhancing the lifetime and cost-effectiveness of oxygen reduction reaction (ORR). PtCo nanocrystals have gained attention due to their exceptional activity, yet suffer from stability issues in acidic media. Herein, an active and highly stable electrocatalyst is developed, namely 3D Pt7Co3@Pt core-shell nanodendrites (NDs), which are formed through the self-assembly of small Pt nanoparticles (≈6 nm). This unique structure significantly improves the ORR with an enhanced mass activity (MA) of 0.54 A mgPt -1, surpassing that of the commercial Pt/C (com-Pt/C) catalyst by three fold (0.17 A mgPt -1). The well-organized dendritic morphology, along with the Pt-rich shell, contributes significantly to the observed high catalytic activity and superior stability for acidic ORR, which exhibit a loss of 2.1% in MA and, impressively, an increase of 12.0% in specific activity (SA) after an accelerated durability test (ADT) of 40,000 potential-scanning cycles. This work offers insights for improving the design of highly stable Pt-based electrocatalysts for acidic ORR.

Freestanding Penta-Twinned Palladium Nanosheets

Control over the morphology of nanomaterials to have a 2D structure and manipulating the surface strain of nanostructures through defect control have proved to be promising for developing efficient catalysts for sustainable chemical and energy conversion. Here a one-pot aqueous synthesis route of freestanding Pd nanosheets with a penta-twinned structure (PdPT NSs) is presented. The generation of the penta-twinned nanosheet structure can be succeeded by directing the anisotropic growth of Pd under the controlled reduction kinetics of Pd precursors. Experimental and computational investigations showed that the surface atoms of the PdPT NSs are effectively under a compressive environment due to the strain imposed by their twin boundary defects. Due to the twin boundary-induced surface strain as well as the 2D structure of the PdPT NSs, they exhibited highly enhanced electrocatalytic activity for oxygen reduction reaction compared to Pd nanosheets without a twin boundary, 3D Pd nanocrystals, and commercial Pd/C and Pt/C catalysts.

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.

Atomically thin iridium nanosheets for oxygen evolution electrocatalysis

2D nanostructures of noble metals hold great potential for developing efficient electrocatalysts due to their high atom efficiency associated with their large specific surface area and abundant active sites. Here, we introduce a one-pot solvothermal synthesis method that can enable the fabrication of freestanding atomically thin Ir nanosheets. The thermal decomposition of a complex of Ir and a long-chain amine, which could readily be formed with the assistance of a strong base, under CO flow conditions successfully yielded Ir nanosheets consisting of 2-4 atomic layers. The prepared Ir nanosheets showed prominent activity and stability toward oxygen evolution electrocatalysis in acidic conditions, which can be attributed to their ultrathin 2D structure.

Insight into the ordering process and ethanol oxidation performance of Au-Pt-Cu ternary alloys

Direct ethanol fuel cells (DEFCs), which have been widely recognized as nontoxic and green energy conversion devices, show attractive application prospects for liquid hydrogen-carriers, due to the higher specific energy and lower toxicity of ethanol. Pt-based catalysts are widely used in DEFCs, while their poor poisoning resistance highlights the importance of composition and structure optimization. Herein, we synthesized a series of reduced graphene oxide supported ternary alloy AuxPt1-xCu3/rGO (x = 0-1) catalysts with excellent ethanol oxidation performance and a composition-dependent volcano plot trend of the ordering degree was observed and rationalized. The highest Pt-normalized mass activity of Au0.8Pt0.2Cu3/rGO is attributed to the optimized CO binding energy according to DFT calculations. This work not only provides an efficient EOR catalyst based on ordered alloys AuxPt1-xCu3 (x = 0-1), but also offers valuable insight into the role of a third metal in tuning the structure and function of alloys.

A universal synthesis strategy of Pd-based trimetallic nanowires for efficient alcohol electrooxidation

Trimetallic nanowires (NWs) have drawn much attention in efficient alcohol oxidation reaction (AOR) due to their unique features, including high atomic utilization efficiency and fast electron transfer ability. However, a universal strategy to synthesize Pd-based trimetallic NWs with high catalytic performance is still lacking. Herein, we develop a universal method for facile synthesis of PdBiM (M = Pt, Ru, Ir, Co, Cu) NWs with excellent AOR activities. By taking PdBiPt NWs as an example, the formation mechanism was investigated, and it is found that introduction of bismuth (Bi) plays an important role in facilitating the formation of the NW structure. Moreover, the PdBiPt NWs deliver an outstanding performance toward both the ethanol oxidation reaction (EOR) and the methanol oxidation reaction (MOR). Density functional theory (DFT) calculations together with experimental results disclose that the moderate electronic structure of trimetallic PdBiPt NWs can optimize the adsorption of OHads and weaken the adsorption of COads, thereby leading to the substantially enhanced AOR performance. We believe that this work can inspire the design of multimetallic NWs as high-performance catalysts.

Amorphization Activated Multimetallic Pd Alloys for Boosting Oxygen Reduction Catalysis

Amorphous nanomaterials have drawn extensive attention owing to their unique features, while amorphization on noble metal nanomaterials still remains formidably challenging. Herein, we demonstrate a universal strategy to synthesize amorphous Pd-based nanomaterials from unary to quinary metals through the introduction of phosphorus (P). The amorphous Pd-based nanoparticles (NPs) exhibit generally promoted oxygen reduction reaction (ORR) activity and durability compared with their crystalline counterparts. Significantly, the quinary P-PdCuNiInSn NPs, benefiting from the amorphous structure and multimetallic component effect, exhibit mass activities as high as 1.04 A mgPd-1 and negligible activity decays of 1.8% among the stability tests, which are much better than values for original Pd NPs (0.134 A mgPd-1 and 28.4%). Experimental and theoretical analyses collectively reveal that the synergy of P-induced amorphization and the expansion of metallic components can considerably lower the free energy changes in the rate-determined step, thereby explaining the positive correlation with the catalytic activity.

Ultrathin PdPtP nanodendrites as high-activity electrocatalysts toward alcohol oxidation

PdPtP nanodendrites were prepared by a post-phosphating method. Due to their well-designed structure and composition, the EOR activity of the PtPdP NDs is significantly increased to 14.3 A mgPd+Pt-1, which is a significant improvement compared to commercial Pd/C catalysts. In addition, stability tests demonstrated their excellent catalytic activity and structural durability.

One-pot synthesis of 3D surface-wrinkled PdAu nanospheres for robust alcohols electrocatalysis

Three dimensional (3D) noble-metal nanomaterials with special surface structures have been regarded as high-performance catalysts for alcohol oxidation on account of their superior thermal stability, electrical conductivity and large specific surface area. Although extensive efforts have been devoted to the preparation of 3D Pd-based catalysts with superior activity and stability, designing a simple, effective and eco-friendly method remains a challenge. Herein, we developed a facile one-step coreduction strategy to synthesize a series of 3D surface-wrinkled PdAu nanospheres (NSs) with tunable Pd/Au atomic ratios and found a universal method to prepare surface-wrinkled PdM (M = Au, Pt, Cu and Pb) NSs. Benefiting from the function of the surfactant cetyltrimethylammonium chloride (CTAC), the synthesized PdAu NSs with different composition possess abundant surface wrinkles, which is beneficial for exposing more electroactive centers. Attributed to the unique geometric morphology and optimized atomic ratio, the PdAu-2 NSs exhibited an optimal mass activity (MA) of 8103 mA mg-1 and 5113 mA mg-1 for the ethylene glycol oxidation reaction (EGOR) and ethanol oxidation reaction (EOR), which was 6.1 and 4.1 times that of commercial Pd/C, respectively. Moreover, the PdAu-2 NSs exhibited superb stability after long-term current-time (i-t) and cyclic voltammetry (CV) tests of the EGOR and EOR. This work not only provides new avenues to engineer PdAu NSs with enhanced electrocatalytic performance but also offers guidance for extending to more 3D PdM (M = other metals) NSs with novel morphology applied to fuel cell fields.

Facile Synthesis of Multifunctional Ni(OH)2 -Supported Core-Shell Ni@Pd Nanocomposites for the Electro-Oxidation of Small Organic Molecules

In the domain of proton exchange membrane fuel cells (PEMFCs), the development of efficient and durable catalysts for the electro-oxidation of small organic molecules, especially of alcohols (methanol, ethanol, ethylene glycol, et al.) has always been a hot topic. A large number of related electrocatalysts with splendid performance have been designed and synthesized till now, while the preparation processes of most of them are demanding on experimental operations and conditions. Herein, we put forward a facile and handy method for the preparation of multifunctional Ni(OH)2 -supported core-shell Ni@Pd nanocomposites (Ni(OH)2 /Ni@Pd NCs) with the assistance of galvanic replacement reaction (GRR) at room temperature and ambient pressure. As expected, the Ni(OH)2 substrate can prevent the aggregation of core-shell (CS) Ni@Pd nanoparticles (NPs) and inhibit the formation of COads and further prevent Pd from being poisoned. The synergistic effect between CS Ni@Pd NPs and Ni(OH)2 substrate and the electronic effect between Pd shell and Ni core contribute to the outstanding electrocatalytic performance for methanol, ethanol, and ethylene glycol oxidation in alkaline condition. This study provides a succinct method for the design and preparation of efficient Pd-based electrocatalysts for alcohol electro-oxidation.

Porous, Ultrathin PtAgBiTe Nanosheets for Direct Hydrazine Hydrate Fuel Cell Devices

Ultrathin 2D nanomaterials have attracted extensive attention due to their fascinating applications in sustainable and clean-energy-related devices, but obtaining ultrathin 2D multimetallic polycrystalline structures with large lateral dimensions remains a challenge. In this study, ultrathin 2D porous PtAgBiTe and PtBiTe polycrystalline nanosheets (PNSs) are obtained via a visible-light-photoinduced Bi2 Te3 -nanosheet-mediated route. The PtAgBiTe PNSs are assembled by sub-5 nm grains with widths beyond 700 nm. Strain and ligand effects originating from the porous, curly polycrystalline structure endow the PtAgBiTe PNSs with robust hydrazine hydrate oxidation reaction activity. Theoretical research demonstrates that the modified Pt activates the N-H bonds in N2 H4 during the reaction, and strong hybridization between Pt-5d and N-2p facilitates dehydrogenation while reducing energy consumption. The peak power densities of the PtAgBiTe PNSs in actual hydrazine-O2 /air fuel cell devices are boosted to 532.9/315.9 mW cm-2 , while those of the commercial Pt/C are 394.7/157.9 mW cm-2 . This work provides a strategy not only for preparing ultrathin multimetallic PNSs but also for finding promising electrocatalysts for actual hydrazine fuel cells.

Programmable Synthesis of High-Entropy Nanoalloys for Efficient Ethanol Oxidation Reaction

Controllable synthesis of nanoscale high-entropy alloys (HEAs) with specific morphologies and tunable compositions is crucial for exploring advanced catalysts. The present strategies either have great difficulties to tailor the morphology of nanoscale HEAs or suffer from narrow elemental distributions and insufficient generality. To overcome the limitations of these strategies, here we report a robust template-directed synthesis to programmatically fabricate nanoscale HEAs with controllable compositions and structures via independently controlling the morphology and composition of HEA. As a proof of concept, 12 kinds of nanoscale HEAs with controllable morphologies of zero-dimension (0D) nanoparticles, 1D nanowires, 2D ultrathin nanorings (UNRs), 3D nanodendrites, and vast elemental compositions combining five or more of Pd/Pt/Ag/Cu/Fe/Co/Ni/Pb/Bi/Sn/Sb/Ge are synthesized. Moreover, the as-prepared HEA-PdPtCuPbBiUNRs/C demonstrates the state-of-the-art electrocatalytic performance for the ethanol oxidation reaction, with 25.6- and 16.3-fold improvements in mass activity, relative to commercial Pd/C and Pt/C catalysts, respectively, as well as greatly enhanced durability. This work provides a myriad of nanoscale HEAs and a general synthetic strategy, which are expected to have broad impacts for the fields of catalysis, sensing, biomedicine, and even beyond.

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.

Directing Energy Flow in Core-Shell Nanostructures for Efficient Plasmon-Enhanced Electrocatalysis

Conjugating plasmonic metals with catalytically active materials with controlled configurations can harness their light energy harvesting ability in catalysis. Herein, we present a well-defined core-shell nanostructure composed of an octahedral Au nanocrystal core and a PdPt alloy shell as a bifunctional energy conversion platform for plasmon-enhanced electrocatalysis. The prepared Au@PdPt core-shell nanostructures exhibited significant enhancements in electrocatalytic activity for methanol oxidation and oxygen reduction reactions under visible-light irradiation. Our experimental and computational studies revealed that the electronic hybridization of Pd and Pt allows the alloy material to have a large imaginary dielectric function, which can efficiently induce the shell-biased distribution of plasmon energy upon illumination and, hence, its relaxation at the catalytically active region to promote electrocatalysis.

Promoting the Electrocatalytic Ethanol Oxidation Activity of Pt by Alloying with Cu

The development and commercialization of direct ethanol fuel cells requires active and durable electro-catalysts towards the ethanol oxidation reactions (EOR). Rational composition and morphology control of Pt-based alloy nanocrystals can not only enhance their EOR reactivity but also reduce the consumption of precious Pt. Herein, PtCu nanocubes (NCs)/CB enclosed by well-defined (100) facets were prepared by solution synthesis, exhibiting much higher mass activity (4.96 A mgPt−1) than PtCu nanoparticles (NPs)/CB with irregular shapes (3.26 A mgPt−1) and commercial Pt/C (1.67 A mgPt−1). CO stripping and in situ Fourier transform infrared spectroscopy (FTIR) experiments indicate that the alloying of Cu enhanced the adsorption of ethanol, accelerated the subsequent oxidation of intermediate species, and increased the resistance to CO poisoning of PtCu NCs/CB, as compared with commercial Pt/C. Therefore, alloying Pt with earth-abundant Cu under rational composition and surface control can optimize its surface and electronic structures and represents a promising strategy to promote the performance of electro-catalysts while reduce the use of precious metals.

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.

PdAu Nanosheets for Visible-Light-Driven Suzuki Cross-Coupling Reactions

Combining a two-dimensional (2D) morphology and plasmonic photocatalysis represents an efficient design for light-driven organic transformations. We report a one-pot synthesis of surfactant templated PdAu nanosheets (NSs). Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses show the formation of 2D PdAu structures was initiated through nanoparticle seeds dispersed in the alkyl ammonium salt surfactant which acted as a template for the growth into NSs. The PdAu NSs were used for visible-light-enhanced Suzuki cross coupling. The PdAu bimetallic NSs outperformed monometallic Pd NSs and commercial Pd/C in room-temperature Suzuki cross-coupling reactions. The high catalytic activity is attributed to a combination of the 2D morphology giving rise to plasmon-enhanced catalysis and a high density of surface atoms, the electron-rich Pd surface due to alloying, and the presence of weakly bound amines. A comparative study of surfactant-assisted NSs and CO-assisted NSs was also carried out to assess the influence of surface ligands on the catalytic and photocatalytic enhancement of NSs with similar morphology. The surfactant-assisted NSs showed substantially superior performance compared to the CO-assisted for room-temperature Suzuki coupling reactions.

Quaternary PdCuNiP Porous Nanosheets with Enhanced Electrochemical Performance in the Ethanol Oxidation Reaction

The ability to manipulate metal electrocatalysts with satisfactory performance for the ethanol oxidation reaction (EOR) is promising but still unsatisfactory for practical application in direct ethanol fuel cells. Beyond traditional metal-metal alloys, we herein report a novel metal-nonmetal alloy electrocatalyst that takes advantage of quaternary PdCuNiP alloy composition and the ultrathin/porous nanosheet (NS) structure. The optimized PdCuNiP porous NSs feature more undercoordinated active sites and modified electron/function structures, enabling better antipoisoning ability. Under alkaline conditions, this electrocatalyst shows excellent electrochemical EOR performance with a high EOR activity of 4.05 A mg_Pd^-1 and a low activation energy of 21.2 kJ mol^-1, comparable to the state-of-the-art electrocatalysts reported in the literature. Meanwhile, PdCuNiP porous NSs are electrocatalytically active for electrochemical oxidation of other fuels (methanol, glycerol, and glucose), highlighting their great potential for various direct alcohol fuel cells. The findings reported here may put forward some insights into designing new functional electrocatalysts for various fuel cell electrocatalysis and beyond.

Noble-Metal-Based Hollow Mesoporous Nanoparticles: Synthesis Strategies and Applications

As second-generation mesoporous materials, mesoporous noble metals (NMs) are of significant interest for their wide applications in catalysis, sensing, bioimaging, and biotherapy owing to their structural and metallic features. The introduction of interior hollow cavity into NM-based mesoporous nanoparticles (MNs), which subtly integrate hierarchical hollow and mesoporous structure into one nanoparticle, produces a new type of hollow MNs (HMNs). Benefiting from their higher active surface, better electron/mass transfer, optimum electronic structure, and nanoconfinement space, NM-based HMNs exhibit their high efficiency in enhancing catalytic activity and stability and tuning catalytic selectivity. In this review, recent progress in the design, synthesis, and catalytic applications of NM-based HMNs is summarized, including the findings of the groups. Five main strategies for synthesizing NM-based HMNs, namely silica-assisted surfactant-templated nucleation, surfactant-templated sequential nucleation, soft "dual"-template, Kirkendall effect in synergistic template, and galvanic-replacement-assisted surfactant template, are described in detail. In addition, the applications in ethanol oxidation electrocatalysis and hydrogenation reactions are discussed to highlight the high activity, enhanced stability, and optimal selectivity of NM-based HMNs in (electro)catalysis. Finally, the further outlook that may lead the directions of synthesis and applications of NM-based HMNs is prospected.

PdPbAg alloy NPs immobilized on reduced graphene oxide/In2O3 composites as highly active electrocatalysts for direct ethylene glycol fuel cells

rGO-modified indium oxide (In2O3) anchored PdPbAg nanoalloy composites (PdPbAg@rGO/In2O3) were prepared by a facile hydrothermal, annealing and reduction method. Electrochemical tests showed that the as-prepared trimetallic catalyst exhibited excellent electrocatalytic activity and high resistance to CO poisoning compared with commercial Pd/C, mono-Pd and different bimetallic catalysts. Specifically, PdPbAg@rGO/In2O3 has the highest forward peak current density of 213.89 mA cm-2, which is 7.89 times that of Pd/C (27.07 mA cm-2). After 3600 s chronoamperometry (CA) test, the retained current density of PdPbAg@rGO/In2O3 reaches 78.15% of the initial value. Its excellent electrocatalytic oxidation performance is attributed to the support with large specific surface area and the strong synergistic effect of PdPbAg nanoalloys, which provide a large number of interfaces and achievable reactive sites. In addition, the introduction of rGO into the In2O3 matrix contributes to its excellent electron transfer and large specific surface area, which is beneficial to improving the catalytic ability of the catalyst. The study of this novel composite material provides a conceptual and applicable route for the development of advanced high electrochemical performance Pd-based electrocatalysts for direct ethylene glycol fuel cells.

Highly Curved, Quasi-Single-Crystalline Mesoporous Metal Nanoplates Promote CC Bond Cleavage in Ethanol Oxidation Electrocatalysis

The ability to manipulate metal nanocrystals with well-defined morphologies and structures is greatly important in material chemistry, catalysis chemistry, nanoscience, and nanotechnology. Although 2D metals serve as interesting platforms, further manipulating them in solution with highly penetrated mesopores and ideal crystallinity remains a huge challenge. Here, an easy yet powerful synthesis strategy for manipulating the mesoporous structure and crystallinity of 2D metals in a controlled manner with cetyltrimethylammonium chloride as the mesopore-forming surfactant and extra iodine-ion as the structure/facet-selective agent is reported. This strategy allows for preparing an unprecedented type of 2D quasi-single-crystalline mesoporous nanoplates (SMPs) with highly curved morphology and controlled metal composition. The products, for example, PdCu SMPs, feature abundant undercoordinated sites, optimized electronic structures, excellent electron/mass transfers, and confined mesopore environments. Curved PdCu SMPs exhibit remarkable electrocatalytic activity of 6.09 A mg_Pd ^-1 and stability for ethanol oxidation reaction (EOR) compared with its counterpart catalysts and commercial Pd/C. More importantly, PdCu SMPs are highly selective for EOR electrocatalysis that dramatically promotes C-C bond cleavage with a superior C1 pathway selectivity as high as 72.1%.

Insight into the effects of the crystal phase of Ru over ultrathin Ru@Pt core-shell nanosheets for methanol electrooxidation

Coating a second metal on the surface of ultrathin 2D nanosheets (NSs) could induce lattice strain and modify the electronic structure, thereby changing the surface reactivity. Herein, we report the effects of different crystal phases of Ru on the electrocatalytic performance of ultrathin Ru@Pt core-shell NSs for the methanol oxidation reaction (MOR). Importantly, Ru with a novel face-centered-cubic phase was found to have more effect on the electronic structure of Pt than Ru with a conventional hexagonal close-packed phase, thereby leading to improved electrocatalytic activity toward the MOR under acidic and basic conditions. It is believed that the strategy presented here would offer a new approach to the construction of bimetallic core-shell nanostructures with various promising applications.

Synergistic effect of oxidation etching and phase transformation triggered by controllable ion-bath microenvironments toward constructing ultra-thin porous nanosheets for accelerated industrial water splitting at high current density

Precisely tailoring the structure of inorganic materials at the micron and nanometer scales, especially in collaboration with component customization to design efficient, stable and low-cost transition-metal-based catalysts for industrial electrocatalytic water splitting (EWS) is a key renewable energy technology, but still facing a daunting challenge. Here, the controllable escape of Ni atom is adopted to disturb the hydrothermal ion-bath environment, thereby resulting in the coexistence of high valence Ni and Fe ions. Combined with a one-step hydrothermal coordination strategy, the timeline-adjusted ion-bath microenvironment can effectively trigger the phase transformation of carbonate hydroxide hydrate nanosheets (NFCH) to nickel ferrite intercalated NFCH ultra-thin porous nanosheets (NF-CH-O). Thanks to the high-energy phase boundary synergistic effect and the rapid mass transfer advantages of ultra-thin porous nanostructures, the as-prepared NF-CH-O nanosheets exhibit remarkable oxygen and hydrogen evolution reaction (OER/HER) catalytic activity and stability, with low overpotentials of 207/191 mV at 50 mA cm^-2, respectively, as well as the activity retention for 100 h. The alkaline water electrolyzer set up with NF-CH-O as both anodic and cathodic electrodes only requires a cell potential of 1.688 V to reach 50 mA cm^-2 in a continuous operation of 100 h. More impressively, NF-CH-O only requires overpotentials of 266, 292 mV and 1.877 V to drive high current densities up to 500 mA cm^-2 for OER, HER and EWS, respectively, and exhibits excellent stability with a reduction in the activity of less than 10% over cycles of more than 65 h. This work highlights the room-temperature controllable ion-bath oxidative etching strategy to design efficient bifunctional catalysts with ultra-thin porous structure and high-current-density activity. Meanwhile, combined with the advantages of direct growth on the substrate for mass production, such meticulous consideration of nanostructured design will be more competitive in the H₂-production industry.

A highly efficient three-solvent methodology for separating colloidal nanoparticles

This study has established a three-solvent methodology for separating nanomaterials, such as monometallic nanoparticles, miscible and immiscible nanoalloys. After systematically investigating the separation methods in two-solvent and three-solvent systems, a three-solvent theoretical model has been proposed to thoroughly reveal the centrifugation mechanism of colloidal particles. PVP plays an important role in the formation of emulsion droplets as the key factor for separation. Based on the three-solvent model, a novel solvent system has been discovered, with low-toxicity solvents and high separation efficiency. This study can open a new path for the separation of colloidal particles in both research and industrial fields and further promote the development of functional nanomaterials in greener pathways.

One-Pot Aqueous Synthesis of Porous Hollow PdCu Alloy Nanoparticles for Enhanced Ethanol Electrooxidation

Bimetallic PdCu porous hollow nanoparticles (PHNs) with hierarchical nanostructures and well-alloyed compositions were precisely synthesized through a one-pot aqueous synthetic route. Bimetallic PdCu PHNs exhibited multiple enhancement synergies and thus performed well in ethanol oxidation electrocatalysis with remarkable activity and stability. This work expedites rational design and synthesis of the high-hierarchy alloy electrocatalysts for fuel cell electrocatalysis.

Compensating Electronic Effect Enables Fast Site-to-Site Electron Transfer over Ultrathin RuMn Nanosheet Branches toward Highly Electroactive and Stable Water Splitting

To improve the electroactivity and stability of electrocatalysts, various modulation strategies have been applied in nanocatalysts. Among different methods, heteroatom doping has been considered as an effective method, which modifies the local bonding environments and the electronic structures. Meanwhile, the design of novel two-dimensional (2D) nanostructures also offers new opportunities for achieving efficient electrocatalysts. In this work, Mn-doped ultrathin Ru nanosheet branches (RuMn NSBs), a newly reported 2D nanostructure, is synthesized. With the ultrathin and naturally abundant edges, the RuMn NSBs have exhibited bifunctionalities of hydrogen evolution reaction and oxygen evolution reaction with high electroactivity and durability in different electrolytes. Experimental characterizations have revealed that RuO bonds are shortened due to Mn doping, which is the key factor that leads to improved electrochemical performances. Density functional theory (DFT) calculations have confirmed that the introduction of Mn enables flexible modulations on the valence states of Ru sites. The inversed redox state evolutions of Ru and Mn sites not only improve the electroactivity for the water splitting but also the long-term stability due to the pinning effect of Ru sites. This work has provided important inspirations for the design of future advanced Ru-based electrocatalysts with high performances and durability.

A highly efficient atomically thin curved PdIr bimetallene electrocatalyst

The multi-metallene with an ultrahigh surface area has great potential in precise tuning of surface heterogeneous d-electronic correlation by surface strain effect for the distinctive surface electronic structure, which is a brand new class of promising 2D electrocatalyst for sustainable energy device application. However, achieving such an atomically thin multi-metallene still presents a great challenge. Herein, we present a new synthetic method for an atomic-level palladium-iridium (PdIr) bimetallene with an average thickness of only ∼1.0 nm for achieving superior catalysis in the hydrogen evolution reaction (HER) and the formic acid oxidation reaction (FAOR). The curved PdIr bimetallene presents a top-ranked high electrochemical active area of 127.5 ± 10.8 m² g_Pd+Ir⁻¹ in the reported noble alloy materials, and exhibits a very low overpotential, ultrahigh activity and improved stability for HER and FAOR. DFT calculation reveals that the PdIr bimetallene herein has a unique lattice tangential strain, which can induce surface distortion while concurrently creating a variety of concave-convex featured micro-active regions formed by variously coordinated Pd sites agglomeration. Such a strong strain effect correlates the abnormal on-site active 4d¹⁰-t_2g-orbital Coulomb correlation potential and directly elevates orbital-electronegativity exposure within these active regions, resulting in a preeminent barrier-free energetic path for significant enhancement of FAOR and HER catalytic performance.

Emerging beyond-graphene elemental 2D materials for energy and catalysis applications

Elemental two-dimensional (2D) materials have emerged as promising candidates for energy and catalysis applications due to their unique physical, chemical, and electronic properties. These materials are advantageous in offering massive surface-to-volume ratios, favorable transport properties, intriguing physicochemical properties, and confinement effects resulting from the 2D ultrathin structure. In this review, we focus on the recent advances in emerging energy and catalysis applications based on beyond-graphene elemental 2D materials. First, we briefly introduce the general classification, structure, and properties of elemental 2D materials and the new advances in material preparation. We then discuss various applications in energy harvesting and storage, including solar cells, piezoelectric and triboelectric nanogenerators, thermoelectric devices, batteries, and supercapacitors. We further discuss the explorations of beyond-graphene elemental 2D materials for electrocatalysis, photocatalysis, and heterogeneous catalysis. Finally, the challenges and perspectives for the future development of elemental 2D materials in energy and catalysis are discussed.

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

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

Ultrathin PdAuBiTe Nanosheets as High-Performance Oxygen Reduction Catalysts for a Direct Methanol Fuel Cell Device

Ultrathin 2D metal nanostructures have sparked a lot of research interest because of their improved electrocatalytic properties for fuel cells. So far, no effective technique for preparing ultrathin 2D Pd-based metal nanostructures with more than three compositions has been published. Herein, a new visible-light-induced template technique for producing PdAuBiTe alloyed 2D ultrathin nanosheets is developed. The mass activity of the PdAuBiTe nanosheets against the oxygen reduction reaction (ORR) is 2.48 A mg_Pd ^-1 , which is 27.5/17.7 times that of industrial Pd/C/Pt/C, respectively. After 10 000 potential cyclings, there is no decrease in ORR activity. The PdAuBiTe nanosheets exhibit high methanol tolerance and in situ anti-CO poisoning properties. The PdAuBiTe nanosheets, as cathode electrocatalysts in direct methanol fuel cells, can thus give significant improvement in terms of power density and durability. In O₂ /air, the power density can be increased to 235.7/173.5 mW cm^-2 , higher than that reported in previous work, and which is 2.32/3.59 times higher than Pt/C.

One-Pot Synthesis of Ternary Alloy Hollow Nanostructures with Controlled Morphologies for Electrocatalysis

The rational design and synthesis of multimetallic hollow nanostructures (HNSs) have been attracting great attention due to their structural and compositional advantages for application in electrocatalysis. Herein, the one-pot synthesis of Pd-Pt-Ag ternary alloy HNSs with controllable morphologies through a self-templating approach without any pre-synthesized templates is reported. Simultaneous reduction of multiple metal precursors by ascorbic acid in the presence of cetyltrimethylammonium chloride (CTAC) yielded initially metastable Pd-Ag nanocrystals, which can act as a self-template, and subsequent galvanic replacement and reduction led to the formation of final Pd-Pt-Ag HNSs. The size and hollowness (the ratio of inner cavity diameter to outer diameter) of the HNSs could be tuned through control over the concentration of CTAC. This can be attributed to the manipulated reduction kinetics of multiple metal precursors with the change in the CTAC concentration. The prepared Pd-Pt-Ag HNSs exhibited improved catalytic performance for ethanol electro-oxidation due to their large active surface areas and ternary alloy composition.

Facile Synthesis of PdCuRu Porous Nanoplates as Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction in Alkaline Medium

Ru is a key component of electrocatalysts for hydrogen evolution reaction (HER), especially in alkaline media. However, the catalytic activity and durability of Ru-based HER electrocatalysts are still far from satisfactory. Here we report a solvothermal approach for the synthesis of PdCuRu porous nanoplates with different Ru compositions by using Pd nanoplates as the seeds. The PdCuRu porous nanoplates were formed through underpotential deposition (UPD) of Cu on Pd, followed by alloying Cu with Pd through interdiffusion and galvanic replacement between Cu atoms and Ru precursor simultaneously. When evaluated as HER electrocatalysts, the PdCuRu porous nanoplates exhibited excellent catalytic activity and durability. Of them, the Pd24Cu29Ru47/C achieved the lowest overpotential (40.7 mV) and smallest Tafel slope (37.5 mV dec−1) in an alkaline solution (much better than commercial Pt/C). In addition, the Pd24Cu29Ru47/C only lost 17% of its current density during a stability test for 10 h, while commercial Pt/C had a 59.5% drop under the same conditions. We believe that the electron coupling between three metals, unique porous structure, and strong capability of Ru for water dissociation are responsible for such an enhancement in HER performance.

Rapid Aqueous Synthesis of Large-Size and Edge/Defect-Rich Porous Pd and Pd-Alloyed Nanomesh for Electrocatalytic Ethanol Oxidation

In this work, a facile aqueous synthesis strategy was used (complete in 5 min at room temperature) to produce large-size Pd, PdCu, and PdPtCu nanomeshes without additional organic ligands or solvent and the volume restriction of reaction solution. The obtained metallic nanomeshes possess graphene-like morphology and a large size of dozens of microns. Abundant edges (coordinatively unsaturated sites, steps, and corners), defects (twins), and mesopores are seen in the metallic ultrathin structures. The formation mechanism for porous Pd nanomeshes disclosed that they undergo oriented attachment growth along the ⟨111⟩ direction. Owing to structural and compositional advantages, PdCu porous nanomeshes with certain elemental ratios (e. g., Pd₈₇ Cu₁₃ ) presented enhanced electrocatalytic performance (larger mass activity, better CO tolerance and stability) toward ethanol oxidation.

Solvent-Mediated Shell Dimension Reconstruction of Core@Shell PdAu@Pd Nanocrystals for Robust C1 and C2 Alcohol Electrocatalysis

The core@shell structure dimension of the Pd-based nanocrystals deeply impacts their catalytic properties for C1 and C2 alcohol oxidation reactions. However, the precise simultaneous control on the synthesis of core@shell nanocrystals with different shell dimensions is difficult, and most synthesis on Pd-based core@shell nanocatalysts involves the surfactants participation by multiple steps, thus leads to limited catalytic properties. Herein, for the first time, a facile one-step surfactant-free strategy is developed for shell dimension reconstruction of PdAu@Pd core@shell nanocrystals by altering volume ratios of mixed solvents. The Pd-based sunflower-like (SL) and coral grass-like (CGL) nanocrystals are obtained with different 2D hexagonal nanosheet assembles and 3D network shells, respectively. Benefitting from the clean surface shell of 2D ultrathin nanosheets structure, high atom utilization efficiency, and robust electronic effect. The PdAu@Pd SL achieves the ascendant methanol/ethanol/ethylene glycol oxidation reaction (MOR/EOR/EGOR) activities, much higher than Pd/C catalysts, as well as the improved antipoisoning ability. Notably, this one-step construction shell dimension of PdAu@Pd core@shell catalysts not only provide a significant reference for the improvement of surfactant-free synthetic routes, but also shed light on the advanced engineering on shell dimensions in core@shell nanostructures for electrocatalysis and so forth.

3D Anisotropic Au@Pt-Pd Hemispherical Nanostructures as Efficient Electrocatalysts for Methanol, Ethanol, and Formic Acid Oxidation Reaction

Anisotropic 3D nanostructures exhibit excellent electrocatalytic activity and stability due to their heterogeneous elemental distribution and unsymmetrical configuration. However, it is still a huge challenge to combine anisotropically distributed elements and anisotropic morphologies within one 3D nanostructure. Herein, 3D Au@Pt-Pd hemispherical nanostructures (Au@Pt-Pd H-Ss) are fabricated as highly efficient electrocatalysts for oxidation reaction, which present heterogenous element distribution and anisotropic morphology. It is demonstrated that the non-uniform adsorption of BO₂ ^- on Au-CTA⁺ surface, as well as the simulated lower formation energy of Pt-Pd atoms for Au-CTA⁺ -BO₂ ^- , basically contribute to the eventual formation of Au@Pt-Pd H-Ss. Impressively, the unique anisotropic Au@Pt-Pd H-Ss exhibit superior electrocatalytic activity and durability for methanol, ethanol, and formic acid oxidation reaction compared with commercial Pt/C and previously reported noble-metal based electrocatalysts. Especially, the mass activity of Au@Pt-Pd H-Ss for MOR is 4.38 A mg_Pt+Pd ^-1 , which is about 2.0 and 4.7 times that of Au@Pt-Pd spherical nanostructures (Au@Pt-Pd Ss) and commercial Pt/C catalyst, respectively. This work provides an important reference for the design and preparation of 3D anisotropic and high-efficiency electrocatalysts.

Phase-Controlled Synthesis of Pd-Se Nanocrystals for Phase-Dependent Oxygen Reduction Catalysis

Searching for highly efficient oxygen reduction reaction (ORR) electrocatalysts for fuel cell technology, in which the crystal structure plays a powerful role in regulating the electrocatalysis, is urgent yet challenging. Herein, we have explored the active and stable Pd-Se alloy electrocatalysts with controlled phase toward alkaline ORR. The phase-controlled Pd-Se nanoparticles (NPs) show interesting phase-dependent electrocatalytic performance, in which the Pd₁₇Se₁₅ NPs/C exhibits much better ORR performance than its counterpart, Pd₇Se₄ NPs/C, and the commercial Pd/C and Pt/C. Based on the detailed analysis, Pd in Pd₁₇Se₁₅ possesses more Se atom coordination and a higher valence state, thus providing a stronger capacity for the absorption of oxygenated species. DFT further reveals more charge transfer from the Pd₁₇Se₁₅ surface to the *OOH intermediate, which is the reason for the activity enhancement.

Surface Plasmon Resonance Boost Electrocatalytic Alcohol Oxidation over Three-Dimensional PdM (M = Au, Ag, Cu) Nanosheet Assemblies

Photoelectrocatalytic nanomaterials are promising for direct alcohol fuel cells, but the construction of high-efficiency catalysts remains difficult. We herein successfully synthesized three-dimensional (3D) PdM nanosheet assemblies (PdM NSAs, M = Au, Ag, and Cu) through a seed-mediated growth method, which displayed a typical 3D nanoflower morphology assembled from many two-dimensional ultrathin nanosheets. Due to the open 3D structure and the synergistic and electronic effects between Pd and Ag, the optimized PdAg NSAs showed the highest mass activity (9378 mA mg^-1) for the ethylene glycol oxidation reaction. More interestingly, when irradiated with visible light, the mass activity increased to 14 590 mA mg^-1, 12.1 times higher than that of the commercial Pd/C (1205 mA mg^-1). In addition, the as-obtained catalysts also showed better long-term durability than that of the commercial Pd/C under the condition of with or without visible-light illumination. This work highlights the utilization of light energy in designing excellent photoelectrocatalysts to promote the photoelectrocatalytic performance of anode catalysts for fuel cells.

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.

Highly Porous Au-Pt Bimetallic Urchin-Like Nanocrystals for Efficient Electrochemical Methanol Oxidation

Highly porous Au–Pt urchin-like bimetallic nanocrystals have been prepared by a one-pot wet-chemical synthesis method. The porosity of urchin-like bimetallic nanocrystals was controlled by amounts of hydrazine used as reductant. The prepared highly porous Au-Pt urchin-like nanocrystals were superior catalysts of electrochemical methanol oxidation due to high porosity and surface active sites by their unique morphology. This approach will pave the way for the design of bimetallic porous materials with unprecedented functions.

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.

Metallenes as functional materials in electrocatalysis

2D metals, metallenes, feature exciting opportunities at the forefront of electrocatalysis. We bring to attention metallene preparation techniques and modification strategies for the derivation of highly functional metallenes in key electrocatalytic applications.

Two-dimensional multimetallic alloy nanocrystals: recent progress and challenges

In this highlight article, the recent progress on the preparation and application of multimetallic alloy nanocrystals with 2D nanostructures is systematically reviewed, as well as perspectives on future challenges and opportunities.

Hierarchical defective palladium-silver alloy nanosheets for ethanol electrooxidation

Tuning the chemical composition and surface structure of electrodes is demonstrated as a feasible and effective strategy to tailor advanced catalysts for energy electrocatalysis. In this work, hierarchical palladium-silver alloy nanosheets (PdAg NS) with the thickness ~7 atoms and rich atomic defects are successfully prepared, using the carbon monoxide (CO) confinement approach. The optimized Pd₇Ag₃ NS/C exhibits 8.8 times higher catalytic peak current density and much better stability toward ethanol electrooxidation than Pd NS/C catalyst. The catalytic enhancement mechanism could be attributed to the synergetic effects among optimized electronic structure of Pd, novel architecture, and rich atomic defects.