Synthesis of Ultrathin PdCu Alloy Nanosheets Used as a Highly Efficient Electrocatalyst for Formic Acid Oxidation

A Biphasic Strategy to Synergistically Accelerate Activation and CO Spillover in Formic Acid Oxidation Catalysis

Developing highly efficient and carbon monoxide (CO)-tolerant platinum (Pt) catalysts for the formic acid oxidation reaction (FAOR) is vital for direct formic acid fuel cells (DFAFCs), yet it is challenging due to the high energy barrier of direct intermediates (HCOO* and COOH*) as well as the CO poisoning issues associated with Pt alloy catalysts. Here we present a versatile biphasic strategy by creating a hexagonal/cubic crystalline-phase-synergistic PtPb/C (h/c-PtPb/C) catalyst to tackle the aforementioned issues. Detailed investigations reveal that h/c-PtPb/C can simultaneously facilitate the adsorption of direct intermediates while inhibiting CO adsorption, thereby significantly improving the activation and CO spillover. As a result, h/c-PtPb/C showcases an outstanding FAOR activity of 8.1 A mgPt-1, which is 64.5 times higher than that of commercial Pt/C and significantly surpasses monophasic PtPb. Moreover, the h/c-PtPb/C-based membrane electrode assembly exhibits an exceptional peak power density of 258.7 mW cm-2 for practical DFAFC applications.

Boosting CO2 piezo-reduction via metal-support interactions in Au/ZnO based catalysts

Confronting the challenge of climate change necessitates innovative approaches for the reduction of CO2 emissions. Metal-support interaction has been widely demonstrated to enable greatly improved performances in thermal-catalytic, photocatalytic and electrocatalytic CO2 reduction. However, its applicability and specifically its role in the emerging piezo-electrocatalytic CO2 reduction are unknown, severely hampering the utilizations of piezo-electrocatalysis in CO2 conversion. Herein, by adopting Au particles supported on ZnO (Au/ZnO) as a paradigm, it is found that the metal-support interaction can remarkably improve the separation and transfer of piezo-carriers and enhance CO2 adsorption. As a result, Au/ZnO demonstrates a substantially boosted activity for piezo-electrocatalytic CO2 reduction and the optimal sample exhibits a 37.3% increase in CO yield compared to the pristine ZnO. The integration of metal-support interactions opens a new avenue to the design of advanced piezo-electrocatalysts for CO2 reduction.

Subnanometric Osmium Clusters Confined on Palladium Metallenes for Enhanced Hydrogen Evolution and Oxygen Reduction Catalysis

Highly efficient, cost-effective, and durable electrocatalysts, capable of accelerating sluggish reaction kinetics and attaining high performance, are essential for developing sustainable energy technologies but remain a great challenge. Here, we leverage a facile heterostructure design strategy to construct atomically thin Os@Pd metallenes, with atomic-scale Os nanoclusters of varying geometries confined on the surface layer of the Pd lattice, which exhibit excellent bifunctional properties for catalyzing both hydrogen evolution (HER) and oxygen reduction reactions (ORR). Importantly, Os5%@Pd metallenes manifest a low η10 overpotential of only 11 mV in 1.0 M KOH electrolyte (HER) as well as a highly positive E1/2 potential of 0.92 V in 0.1 M KOH (ORR), along with superior mass activities and electrochemical durability. Theoretical investigations reveal that the strong electron redistribution between Os and Pd elements renders a precise fine-tuning of respective d-band centers, thereby guiding adsorption of hydrogen and oxygen intermediates with an appropriate binding energy for the optimal HER and ORR.

Amorphous RuPd bimetallene for hydrogen evolution reaction in acidic and alkaline conditions: a first-principles study

Metallene materials can provide a large number of active catalytic sites for the efficient use of noble metals as catalysts for hydrogen evolution reaction (HER), whereas the intrinsic activity on the surface is insufficient in crystal phase. The amorphous phase with an inherent long-range disorder can offer a rich coordinate environment and charge polarization on the surface is proposed for promoting the intrinsic catalytic activity on the surface of noble metals. Herein, we designed an amorphous RuPd (am-RuPd) structure by the first principles molecular dynamics method. The performance of the acidic HER on am-RuPd can have a huge enhancement due to the free energy change of hydrogen adsorption close to zero. In alkaline conditions, the H2O dissociation energy barrier on am-RuPd is just 0.49 eV, and it is predicted that the alkaline HER performance of am-RuPd will largely exceed that of Pt nanocrystalline sheets. This work provides a strategy for enhancing the intrinsic catalytic activity on the surface and a way to design an efficient HER catalyst based on metallene materials used in both acidic and alkaline conditions.

Atomic Ordering Engineering of Precious Metal Alloys in Liquid Phase Synthesis

Atomic ordering of noble metal alloys is an effective strategy for improving catalytic performance, yet the low-temperature synthesis of ordered alloys still faces significant challenges. The low-temperature liquid phase method has enormous potential for the synthesis of alloys; however, the atomic ordering mechanism of this process has not been thoroughly studied. Herein, we investigate the mechanism of the influence of metal precursors, reducing agents, solvents, and mixing modes of reactant regulating strategies on precious metal alloy ordering using this method. These regulating strategies are designed to change the coordination structure of metal complexes, affect the reduction potential of metals, and thus change the reduction order of metals and their arrangement in the alloy products. Notably, the reduction potential differences between metal complexes can be used to predict the ordering of the synthetic products (Pd-Cu, Pd-Cd, Pd-Sn, Pd-Pb, and Pt-Sn). This work provides an excellent platform for investigating atomic arrangement engineering.

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.

Seeded Synthesis of Hollow PdSn Intermetallic Nanomaterials for Highly Efficient Electrocatalytic Glycerol Oxidation

Intermetallic nanomaterials have shown promising potential as high-performance catalysts in various catalytic reactions due to their unconventional crystal phases with ordered atomic arrangements. However, controlled synthesis of intermetallic nanomaterials with tunable crystal phases and unique hollow morphologies remains a challenge. Here, a seeded method is developed to synthesize hollow PdSn intermetallic nanoparticles (NPs) with two different intermetallic phases, that is, orthorhombic Pd2 Sn and monoclinic Pd3 Sn2 . Benefiting from the rational regulation of the crystal phase and morphology, the obtained hollow orthorhombic Pd2 Sn NPs deliver excellent electrocatalytic performance toward glycerol oxidation reaction (GOR), outperforming solid orthorhombic Pd2 Sn NPs, hollow monoclinic Pd3 Sn2 NPs, and commercial Pd/C, which places it among the best reported Pd-based GOR electrocatalysts. The reaction mechanism of GOR using the hollow orthorhombic Pd2 Sn as the catalyst is investigated by operando infrared reflection absorption spectroscopy, which reveals that the hollow orthorhombic Pd2 Sn catalyst cleaves the CC bond more easily compared to the commercial Pd/C. This work can pave an appealing route to the controlled synthesis of diverse novel intermetallic nanomaterials with hollow morphology for various promising applications.

A Surfactant-Free and General Strategy for the Synthesis of Bimetallic Core-Shell Nanocrystals on Reduced Graphene Oxide through Targeted Photodeposition

Tunable physicochemical properties of bimetallic core-shell heterostructured nanocrystals (HNCs) have shown enormous potential in electrocatalytic reactions. In many cases, HNCs are required to load on supports to inhibit catalyst aggregation. However, the introduction of supports during the process of growing core-shell HNCs makes the synthesis much more complicated and difficult to control precisely. Herein, we reported a universal photochemical synthetic strategy for the controlled synthesis of well-defined surfactant-free core-shell metal HNCs on a reduced graphene oxide (rGO) support, which was assisted by the fine control of photogenerated electrons directly transferring to the targeted metal seeds via rGO and the precisely tuned adsorption capacity of the added second metal precursors. The surface photovoltage microscopy (SPVM) platform proved that photogenerated electrons flowed through rGO to Pd particles under illumination. We have successfully synthesized 24 different core-shell metal HNCs, i.,e., M_A@M_B (M_A = Pd, Au, and Pt; M_B = Au, Ag, Pt, Pd, Ir, Ru, Rh, Ni and Cu), on the rGO supports. The as-prepared Pd@Cu core-shell HNCs showed outstanding performance in the electrocatalytic reduction of CO₂ to CH₄. This work could shed light on the controlled synthesis of more functional bimetallic nanostructured materials on diverse supports for various applications.

A single step wet chemical approach to bifunctional ultrathin (ZnO)62(Fe2O3)38 dendritic nanosheets

At the ultrathin scale, nanomaterials exhibit interesting chemical and physical properties, like flexibility, and polymer-like rheology. However, to limit the dimensions of composite nanomaterials at the ultrathin level is still a challenging task. Herein, by adopting a new low temperature single step and single pot wet chemical approach, we have successfully fabricated two dimensional (2D) mixed oxide ZnO-Fe₂O₃ dendritic nanosheets (FZDNSs). Various control experimental outcomes demonstrate that precursor salts of both the metals are crucial for the formation of stable 2D FZDNSs. The obtained FZDNSs not only exhibit the best photoreduction performance but also much enhanced electrocatalytic performance. This work will provide a promising avenue for the synthesis of cost effective transition metal mixed oxide based 2D nanosheets having wide ranging applications.

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.

Ultrathin PdCu Nanosheet as Bifunctional Electrocatalysts for Formate Oxidation Reaction and Oxygen Reduction Reaction

The development of robust nonplatinum electrocatalysts to enhance the performance of formate oxidation reaction (FOR) and oxygen reduction reaction (ORR) is one of the key issues for the commercialization of direct formate fuel cells (DFFCs), but still challenging. Herein, a structurally controlled 3D flower-like PdCu nanosheet (NS) catalyst is synthesized by a method of oil bath reduction under mild conditions as a bifunctional electrocatalyst for DFFCs. Under the dual tuning on the composition and intermetallic phase, the PdCu nanosheet catalyst exhibits 8.67 times higher mass activity for anodic formate oxidation reaction than the commercial Pd/C. For the cathodic ORR, a positive shift half-wave potential is obtained for PdCu nanosheets exceeding Pt/C. Moreover, after a long-term stability test, the current density of the PdCu nanosheet catalyst for FOR and ORR can be well maintained with the least activity decay. When the PdCu NSs are used as optimized anode and cathode electrodes for DFFCs enable a peak power density as high as 53 mW cm^-2 at room temperature, which is about 1.3 times higher than that of the commercial Pd/C and Pt/C as anode and cathode electrodes. This work provides a potential strategy to improve the catalytic performance of non-Pt-based nanocatalysts.

Facet modulation of nickel-ruthenium nanocrystals for efficient electrocatalytic hydrogen evolution

Constructing highly active electrocatalysts towards hydrogen evolution reaction (HER) in both alkaline and acidic media is essential for achieving a sustainable energy economy. Here, a facile ethylene glycol reduction strategy was employed to design the nickel-ruthenium nanocrystals (Ni-Ru NC) with an exposed highly active Ru (101) facet as an efficient electrocatalyst for HER. Testings show Ni-Ru NC outperforms the benchmark catalyst Pt/C by delivering extraordinarily low overpotentials of 21.1 and 70.9 mV to drive 10 mA cm^-2 in acidic and alkaline solutions, respectively. The results of experimental and theoretical studies suggest that Ni can modulate the electronic structure of the Ru NC and optimize the hydrogen adsorption free energy on Ru's surface, which accelerates the charge transfer kinetics and enhances the HER performance. The study support the potential application of facet-modulated Ru-based HER eleccatalyst in an alkaline environment.

Metallene-related materials for electrocatalysis and energy conversion

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

Super-Branched PdCu Alloy for Efficiently Converting Carbon Dioxide to Carbon Monoxide

The alloying of noble metals with Cu is one of the most effective strategies for improving catalytic performance and reducing cost in electrocatalytic carbon dioxide reduction reactions (CO₂RR). Previous works usually focused on the influence of morphology and composition on the catalytic activity, but lacked the study of the valence state ratio of metals and the electron transfer behavior on alloys. In this work, PdCu-2 alloy (Pd/Cu molar ratio is 1:2) was obtained by a simple one-step solvothermal method, which can effectively convert CO₂ to CO with a maximum Faradaic efficiency (FE) of 85% at -0.9 V (vs. RHE). Then, the effect of the chemical state of Pd and Cu on the catalytic performance was investigated. The X-ray photoelectron spectroscopy (XPS) shows that the binding energy of Pd in PdCu alloy has a negative shift, which has affected the adsorption of key intermediates. When the proportion of oxidized state and zero-valent metal in the alloy is about 1:2, the PdCu alloy shows the best catalytic activity. In addition, the transient photovoltage (TPV) measurements further demonstrate that due to the introduction of Cu, the electron transfer rate of PdCu-2 becomes the slowest, which helps the accumulation of electrons on PdCu-2 and leads to the improvement of catalytic performance for electrocatalytic CO₂RR. This work can provide more insights into the alloy catalysts of electrocatalytic CO₂RR.

Promoting Effect of Cu on Pd Applied to the Hydrazine Electro-Oxidation and Direct Hydrazine Fuel Cells

Use of liquid fuels in fuel cells is advantageous due to the easier and safer handling, transportation, and storage. Among the different options, hydrazine is of interest since the formation of highly poisoning carbonaceous species is avoided, in addition to its high energy density. In the search for more active direct hydrazine fuel cells (DHFC), this study analyzes the influence of Cu as an auxiliary metal on Pd. Three different PdxCu/C (x = 3, 1, and 0.33) catalysts were prepared by chemical reduction with NaBH4. The materials were physiochemically characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Electrochemical analysis in a three-electrode glass cell and a single-cell DHFC was also carried out to study the impact on the electroactivity. Cu exerts a beneficial effect by reducing the adsorption energies of the adsorbed species and donating oxidized species for the completion of the hydrazine electro-oxidation, optimally balanced in the Pd1Cu/C (maximum power density of 180 mW cm−2). As a counterpoint, Cu slightly promotes the non-faradaic decomposition of hydrazine, seen by a larger H2 signal in mass spectroscopy in the anode exhaust at high current densities, which results in a slight loss in faradaic efficiency.

Precisely modulated 2D PdCu alloy nanodendrites as highly active peroxidase mimics for the elimination of biofilms

Pd-based nanomaterials are good candidates for antibacterial applications because of their high catalytic activity and good biocompatibility. Nonetheless, there is still much work to do to improve the catalytic activity of Pd nanomaterials as antibacterial agents, particularly for anti-biofilms. In this work, Cu was introduced into Pd to form a series of 2D PdCu alloy nanodendrites (PdCu NDs) as high-performance peroxidase mimics based on flexible control of compositions. Remarkably, catalytic kinetics show that the composition-dependent synergy in the PdCu NDs strongly enhances the peroxidase-like activity. The detailed theoretical study reveals that the tuning of the electrostatic adsorption and dissociative chemisorption of the H₂O₂ molecule on PdCu ND surfaces by the precise introduction of Cu plays a key role in obtaining superior peroxidase-like catalytic activity. Significantly, the distinct peroxidase-like properties of the fine-tuned PdCu NDs endow them with excellent biofilm elimination capability via the generation of hydroxyl radicals. This work offers a great opportunity to design noble metal nanozymes with enhanced performance, which might advance the development of nanozymes as a new class of highly efficient antibacterial agents.

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.

Au-Stabilized Nanoporous PdCuAu Alloys Exhibiting Outstanding Catalytic Activity and Durability for the Formic Acid Oxidation Reaction

Metallic Pd is widely recognized as an efficient electrocatalyst for the formic acid oxidation reaction (FAOR), which unfortunately suffers from poor durability owing to Pd dissolution and CO poisoning. The present work describes an effective method to enhance Pd durability by alloying with Cu and Au. Cu could provide surface OH at low potentials to remove poisonous CO for improved CO resistance. Au, the most inert metal, was added to reduce Pd and Cu dissolution. Moreover, alloying with Cu and Au could also modulate the electronic structure of Pd which is just profitable for the FAOR. The constructed PdCuAu with a nanoporous structure exhibits a specific activity of 14.9 mA cm^-2 and a Pd mass activity of 6012 A g^-1, which is ∼15 times and ∼14 times higher than those of commercial Pd/C. While these two electrocatalysts were used as fuel cell anodes, the maximum power density of the PdCuAu anode (Pd loading 10 μg cm^-2) is 93.2 mW cm^-2 and the value of the Pd/C anode (Pd loading 1.2 mg cm^-2) is 60.3 mW cm^-2. The power efficiency of Pd has been notably increased by 185 times in this home-made nanoporous PdCuAu ternary alloy electrocatalyst.

Assembly of bimetallic (Au-Ag)FON composite films at liquid/solid interfaces and their tunable optical properties

The regular structure provided by two-dimensional (2D) structural colloidal crystals is widely accepted to provide an ideal template that ensures that plasmonic bimetallic composite nanostructures are uniform. Herein, we report an effective method for fabricating bimetallic Au-Ag composite films loaded on the surfaces of 2D polystyrene@polyacrylic acid (PS@PAA) colloidal crystals. PS@PAA particles coated with uniform Ag particle layers (AgFON) were produced by a simple and effective sputtering-deposition technique, after which the galvanic replacement (GR) reaction was used to produce a bimetallic (Au-Ag)FON composite film at the liquid/solid interface in aqueous HAuCl₄. The morphology and relative contents of the bimetallic (Au-Ag)FON composite film can be regulated by changing the kinetic factors that control the GR reaction, including the concentration and pH of the HAuCl₄ solution, and the reaction time. We demonstrated that the fabricated bimetallic (Au-Ag)FON composite has localized surface plasmon resonance (LSPR) properties that can be regulated by varying the composite structure and Ag/Au composition. On the one hand, the regular 2D colloidal crystal structure provides an ideal template for preparing Au-Ag composite films, which ensures that the optical signals of plasmonic Au-Ag composite films are reproducible. On the other hand, the synergy between Ag and Au in the bimetallic alloy composite film ensures stable and tunable LSPR performance. Furthermore, the prepared 2D ordered (Au-Ag)FON Au-Ag bimetallic material is expected to be used in sensing and catalysis applications.

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.

Well-designed internal electric field from nano-ferroelectrics promotes formic acid oxidation on Pd

Pd-Based catalysts are considered the most efficient catalysts in direct formic acid fuel cells. However, the poisoning and dissolution of Pd in acidic systems limit its commercialization. Here, we propose an all-in-one solution for the anti-dissolution and anti-poisoning properties of palladium. A novel structured catalyst, Pd nanoparticles embedded in a carbon layer internally decorated with tourmaline nanoparticles (TNPs), is proposed for formic acid oxidation (FAO). The internal electric field strength of the catalysts is readily regulated by controlling the amount of TNPs. Remarkably, the prepared catalyst exhibits as high as 3.9 times mass activity (905 A g^-1) compared with the commercial Pd/C catalyst. The significant improvement in the electrocatalytic performance of the catalyst is mainly due to the polarized electric field of TNPs causing charge transfer from Pd to tourmaline, which weakens the O-H bond of HCOOH and the bond between Pd and CO_ad. Another advantage brought by the internal polarized electric field is that it facilitates water dissociation to produce OH_ad, thereby improving the anti-poisoning ability of the catalyst in acidic media. Moreover, the firmly anchored Pd nanoparticles can avoid dissolution and agglomeration during long-term use. 80.2% mass activity remained after the accelerated durability test.

Assembly of Alloyed PdCu Nanosheets and Their Electrocatalytic Oxidation of Ethanol

Two-dimensional (2D) nanostructured catalysts have attracted great attention in many important fields, including energy applications and chemical industry. In this study, PdCu nanosheet assemblies (NSAs) have been synthesized and investigated as electrocatalysts for direct ethanol fuel cells in an alkaline medium. A great number of active sites on the nanosheets of PdCu NSAs for ethanol electro-oxidation are exposed, where the electron structures are optimized combined with the second element copper. Electrochemical measurements show that PdCu NSA1 exhibits excellent catalytic activity (2536 mA mg^-1) and cyclic stability compared to PdCu NSA2 (1700 mA mg^-1) and PdCu NSA3 (1436 mA mg^-1), much higher than commercial Pd/C. Kinetics studies on the electrolysis of ethanol suggest that PdCu NSAs should be more favorable at higher catalytic temperatures, higher concentrations of ethanol, and low pH value environments. The unique composition and structures PdCu NSA1 would result in the lowest energy barrier in the rate-controlling step of the ethanol oxidation reaction (EOR), confirmed by density functional theory (DFT). The formation mechanism of PdCu NSAs and their excellent electrocatalytic activity toward EOR have been discussed and analyzed.

Controllable Synthesis of Web-Footed PdCu Nanosheets and Their Electrocatalytic Applications

Morphological control of noble-metal-based nanocrystals has attracted enormous attention because their catalytic behaviors can be optimized well by adjusting the size and shape. Herein, the controllable synthesis of web-footed PdCu nanosheets via a facile surfactant-free method is reported. It is discovered that the Cu(II) precursor in this synthetic system displays a critical role in growing branches along the lateral of nanosheets. This work demonstrates a Pd-based alloy nanoarchitecture for efficient and stable electrocatalysis of both ethanal and formic acid oxidation reactions.

Amorphous/Crystalline Heterophase Ruthenium Nanosheets for pH-Universal Hydrogen Evolution

To design and synthesize heterophase noble-metal materials is of crucial importance owing to their unique structure and apparent properties. Ruthenium (Ru) is one of the most active candidates for hydrogen evolution reaction because of its low price compared with other precious metals, which is favorable for industrial hydrogen cycle operation. In this study, free-standing amorphous/crystalline Ru nanosheets are facilely synthesized through a controlled annealing method. Charge redistribution occurs at the phase interface because of the work function difference between amorphous and crystalline domains. The resulting structure and property are conductive to the adsorption and dissociation of water molecules, associated with optimized hydrogen interaction and enhanced binding between Ru atoms. Accordingly, electrochemical measurements demonstrate that the amorphous/crystalline heterophase Ru exhibits improved hydrogen evolution efficiency as compared with pure amorphous Ru and pure crystalline Ru, at pH-universal conditions. Specifically, only 16.7 mV overpotential is required to reach 10 mA cm^-2 in 1.0 m KOH. Meanwhile, the heterophase structure displays a higher stability during operation than pure amorphous and crystalline structures. This study demonstrates the importance of phase engineering, broadens the Ru-based material family, and provides more insights for developing efficient metal materials.

Two-dimensional copper based colloidal nanocrystals: synthesis and applications

Two-dimensional (2D) semiconductor nanocrystals display unconventional physical and opto-electronic properties due to their ultrathin and unique electronic structures. Since the success of Cd-based photoemissive nanocrystals, the development of sustainable and low-cost nanocrystals with enhanced electronic and physical properties has become a central research theme. In this context, copper-based semiconductor 2D nanocrystals, the cost-effective and eco-friendly alternative, exhibit unique plasmonic resonance, transport properties, and high ionic conductivity beneficial for sensing, energy storage, conversion, and catalytic applications. This review summarizes recent progress in the colloidal synthesis, growth mechanisms, properties, and applications of 2D copper-based nanostructures with tunable compositions, dimensions, and crystal phases. We highlight the growth mechanisms concerning their shape evolution in two dimensions. We analyse the effectiveness of cation exchange as a tool to synthesize multinary nanocrystals. Based on the preparation of Cu-based chalcogenide and non-chalcogenide compositions, we discuss synthesis control achieved via colloidal approaches to allow dimension tunability, phase engineering, and plasmonic and thermoelectric property optimization. Furthermore, their potential in various applications of catalysis, energy storage, and sensing is reviewed. Finally, we address the current challenges associated with 2D Cu-based nanocrystal development and provide an outlook pertaining to unexplored research areas.

Facile Synthesis of Pd and PdPtNi Trimetallic Nanosheets as Enhanced Oxygen Reduction Electrocatalysts

While bimetallic 2D metallic nanomaterials are widely synthesized and used as electrocatalysts with enhanced performance, trimetallic 2D structures are less commonly reported. In this work, a facile wet chemical method for synthesizing Pd nanosheets and PdPtNi trimetallic alloy nanosheets is developed. Without the introduction of gaseous CO and pressurized equipment, Pd nanosheets with a thickness of ≈2.85 nm and sizes in the range of 1-2 µm can be obtained. The facile synthesis conditions allow for a comprehensive study of the nanosheet growth mechanism. It is found that 2D growth is closely related to the product of solvent decomposition and the additive ligand diethylenetriamine. Further, by depositing Pt and Ni onto the Pd nanosheets, trimetallic nanosheets with tunable compositions can be obtained and applied as oxygen reduction reaction electrocatalysts. Typically, the Pd₉ Pt₁ Ni₁ nanosheets have the highest half-wave potential of 0.928 V (vs reversible hydrogen electrode), which is 34 mV higher than that of commercial Pt/C and 28 mV higher than that of Pd/C, and also have high durability.

Enhanced Electrocatalytic Activity of Alloyed Palladium-Lead Nanoparticles toward Electrooxidation of Ethanol

Although many researchers have made great efforts to pursue promising high-efficiency electrocatalysts, a formidable challenge remains for designing excellent palladium-based electrocatalysts for commercializing direct liquid fuel cells. This study reports the synthesis of bimetallic PdPb nanoparticles (NPs) via a mixed solution containing cetyl trimethyl ammonium bromide as the capping agent. Alloyed PdPb NPs are formed, where the size of the NPs increases as Pb atoms are introduced gradually. However, Pd₃Pb NPs are obtained with the same molar ratio of Pd and Pb in the raw systems. Among all of the as-made NPs, Pd₉Pb₁ NPs exhibit superior catalytic activity (2620 mA mg^-1) toward ethanol electrooxidation, 4.3 times higher than commercial Pd/C catalysts (613 mA mg^-1). The overall rate of the EOR for PdPb NPs is determined, demonstrating that the electrocatalytic activity of the PdPb NPs increases at high catalytic temperatures, in high pH environments, and/or at high ethanol concentrations.

Recent advances in phosphorus containing noble metal electrocatalysts for direct liquid fuel cells

Direct liquid fuel cells (DLFCs) are considered as satisfactory alternatives to traditional fossil fuels owing to their unique advantages, e.g. environmental friendliness and easy storage. Noble metal catalysts are widely used to improve the efficiency of DLFCs. However, the high cost, low utilization and poor stability of noble metals restricted their practical applications. Therefore, it is of great significance to explore cost-effective electrocatalysts and further improve their electrocatalytic performance. Reducing the content of noble metals by adding low-priced phosphorus (P) has been considered as an effective strategy, which is able to enhance their electrocatalytic activity and anti-poisoning ability through effectively changing the electronic density of active sites. In the past few years, tremendous P containing catalysts have been synthesized and utilized in DLFCs. In this review, we summarize the fundamentals of electrochemical reactions and present recent progress in P containing noble metal catalysts for DLFCs, including the discussion of their shape, composition and the relationship between P and active sites. Finally, the challenges and some potential directions in this field are pointed out.

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.

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

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

Heterostructured Pd/Ti/Pd Thin Films as Highly Efficient Catalysts for Methanol and Formic Acid Oxidation

Finding a highly efficient catalyst for proton exchange membrane fuel cells is still the subject of extensive research. This article describes heterostructured Pd/Ti/Pd bimetallic thin films prepared using a strain-release technology as electrocatalysts for fuel cells. With their particular structure, these materials exhibit intriguing electrocatalytic activity toward the oxidation of both methanol and formic acid, yielding current densities of 0.17 and 0.56 A mg^-1_Pd, much superior to that of the commercial Pd black catalyst. Moreover, the Pd/Ti/Pd thin films display a low onset oxidation potential and extremely high current retention in both acidic and alkaline media. The carbon monoxide poisoning resistance is also significantly enhanced, thus contributing to ultrahigh stability in the long-term electrocatalytic processes. Their encouraging performance implies that such composites could be potential materials for energy conversion in the fuel cell field.

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

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

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.

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

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

Single-parameter-tuned synthesis for shape-controlled gold nanocrystals stimulated by iron carbonyl

Shape-controlled synthesis is essential for functional nanomaterials, allowing deeper insights intothe relationship between the structures and the catalytic properties. Synthesis of nanocrystals with particular morphologies are usually studied independently among various synthetic methods, those underline that different surface capping ligands or shape-directing agents bring about disparate shapes. However, a single quantitative parameter method is still lacking to realize precise control of well-defined morphology nanocrystals, especially anisotropic structures, which is essential to understanding the growth process of nanocrystals. Herein, we proposed a single-parameter-tuned synthesis strategy for preparation of shape-controlled gold nanocrystals by regulating the amount of iron carbonyl, by which we produced highly monodisperse Au nanocrystals with various shapes in organic phase including nanoplates (diameter of 16.02 ± 1.13 nm and thickness of 5.35 ± 0.58 nm), nanorods (length of 37.53 ± 3.73 nm and width of 5.26 ± 0.37 nm) and nanospheres (diameter of 8.26 ± 0.38 nm). The single-parameter-tuned method reveals the dual roles of iron carbonyl for controlling the shapes of gold nanocrystals including reductant and oxidative etchant and empowers versatility in synthetic methodology for other noble metals. Moreover, catalytic activity shifting in shapes of nanocrystals was revealed based on the reduction of 4-nitrophenol, showing that the as-synthesized Au nanoplates displayed the enhanced catalytic performance with the lowest activation energy. Our work provides a brand-new pathway for shape-controlled synthesis of noble-metal nanocrystals and has a strong practical value in application fields.

Bidirectional controlling synthesis of branched PdCu nanoalloys for efficient and robust formic acid oxidation electrocatalysis

Through a two-way control of hexadecyl trimethyl ammonium bromide (CTAB) and hydrochloric acid (HCl), the PdCu nanoalloys with branched structures are synthesized in one step by hydrothermal reduction and used as electrocatalysts for formic acid oxidation reaction (FAOR). In this two-way control strategy, the CTAB is used as a structure-oriented surfactant, while a certain amount of HCl is used to control the reaction kinetics for achieving gradual growth of multi-dendritic structures. The characterizations including scanning transmission electron microscope (STEM), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) suggest that PdCu nanoalloys with unique multi-dendritic branches have favorable electronic structure and lattice strain for electrocatalyzing the oxidation of formic acid. In specific, among the electrocatalysts with different Pd/Cu ratios, the Pd₁Cu₁ branched nanoalloys have the largest electrochemically active surface area (ECSA) and the best performance for the FAOR. The catalytic activity of the Pd₁Cu₁ branched nanoalloys is 2.4 times that of commercial Pd black. After the chronoamperometry test, the Pd₁Cu₁ branched nanoalloys still maintain their original morphologies and higher current density than that of the commercial Pd black. In addition, in the CO-stripping tests, the initial oxidation potential and the oxidation peak potential of the PdCu branched nanoalloys for CO adsorption are lower than those of commercial Pd balck, evincing their better anti-poisoning performance.

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.