Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high activity of 3.3 A/mg(Pt) for the oxygen reduction reaction

High electrocatalytic performance inspired by crystalline/amorphous interface in PtPb nanoplate

Nanoscale PtPb catalysts with core-shell structure have been actively explored in recent years owing to their outstanding catalytic activity. We report on a new class of PtPb nanoplate (NP) catalyst with a novel structure realized by ion irradiation modification, which contains an interface formed by a crystalline phase and an amorphous phase simultaneously in an annular state. Significantly, the PtPb NP with the new structure shows superior catalytic activity towards the methanol oxidation reaction (MOR). The specific activity of PtPb NPs with the new structure reaches 4.32 mA cm-2 towards the MOR and the mass activity reaches 1.31 A mg-1, which is 1.9-fold and 1.4-fold greater than those for the original crystalline PtPb NPs, respectively. The outstanding catalytic activity could be attributed to the presence of the interface between a crystalline phase and an amorphous phase with a special electronic structure created by ion irradiation. Density functional theory calculations reveal that the novel interface activates the C-H and O-H bonds, leading to high electrocatalytic activity, and optimizes the adsorption of hydroxyl and intermediates on the surface to facilitate the oxidation reaction. The novel structure with an interface formed by a crystalline phase and an amorphous phase opens up a new approach to improve electrocatalytic activity.

Dendrite-Embedded Platinum-Nickel Multiframes as Highly Active and Durable Electrocatalyst toward the Oxygen Reduction Reaction

Pt-based nanoframe catalysts have been explored extensively due to their superior activity toward the oxygen reduction reaction (ORR). Herein, we report the synthesis of Pt-Ni multiframes, which exhibit the unique structure of tightly fused multiple nanoframes and reinforced by an embedded dendrite. Rapid reduction and deposition of Ni atoms on Pt-Ni nanodendrites induce the alloying/dealloying of Pt and Ni in the overall nanostructures. After chemical etching of Ni, the newly formed dendrite-embedded Pt-Ni multiframes show an electrochemically active surface area (ECSA) of 73.4 m² g_Pt^-1 and a mass ORR activity of 1.51 A mg_Pt^-1 at 0.93 V, which is 30-fold higher than that of the state-of-the-art Pt/C catalyst. We suggest that high ECSA and ORR performances of dendrite-embedded Pt-Ni multiframes/C can be attributed to the porous nanostructure and numerous active sites exposed on surface grain boundaries and high-indexed facets.

Pt-Pd Bimetal Popcorn Nanocrystals: Enhancing the Catalytic Performance by Combination Effect of Stable Multipetals Nanostructure and Highly Accessible Active Sites

Exploration of highly efficient electrocatalysts is significantly urgent for the extensive adoption of the fuel cells. Because of their high activity and super stability, Pt-Pd bimetal nanocrystals have been widely recognized as one class of promising electrocatalysts for oxygen reduction. This article presents the synthesis of popcorn-shaped Pt-Pd bimetal nanoparticles with a wide composition range through a facile hydrothermal strategy. The hollow-centered nanoparticles are surrounded by several petals and concave surfaces. By exploring the oxygen reduction reaction on the carbon supported Pt-Pd popcorns in perchloric acid solution, it is found that compared with the commercial Pt/C catalyst the present catalysts display superior catalytic performances in aspects of catalytic activity and stability. More importantly, the Pt-Pd popcorns display minor performance degradations through prolonged potential cycling. The enhanced performances can be mainly attributed to the unique popcorn structure of the Pt-Pd components, which allows the appearance and long existence of the high active sites with more accessibility. The present work highlights the key roles of accessible high active sites in the oxygen reduction reaction, which will ultimately guide the design of highly durable Pt-Pd catalysts.

NiOOH Exfoliation-Free Nickel Octahedra as Highly Active and Durable Electrocatalysts Toward the Oxygen Evolution Reaction in an Alkaline Electrolyte

A layered β-NiOOH crystal with undercoordinated facets is an active and economically viable nonnoble catalyst for the oxygen evolution reaction (OER) in alkaline electrolytes. However, it is extremely difficult to enclose the β-NiOOH crystal with undercoordinated facets because of its inevitable crystal transformation to γ-NiOOH, resulting in the exfoliation of the catalytic surfaces. Herein, we demonstrate {111}-faceted Ni octahedra as the parent substrates whose surfaces are easily transformed to catalytically active β-NiOOH during the alkaline OER. Electron microscopic measurements demonstrate that the horizontally stacked β-NiOOH on the surfaces of Ni octahedra has resistance to further oxidation to γ-NiOOH. By contrast, significant crystal transformation and thus the exfoliation of the γ-NiOOH sheets can be observed on the surfaces of Ni cubes and rhombic dodecahedra (RDs). Electrocatalytic measurements show that the β-NiOOH formed on Ni octahedra exhibits highly enhanced OER durability compared to the Ni cubes, Ni RDs, and the state-of-the-art Ir/C catalysts.

Rational Design and Synthesis of Low-Temperature Fuel Cell Electrocatalysts

Abstract

Recent progresses in proton exchange membrane fuel cell electrocatalysts are reviewed in this article in terms of cathodic and anodic reactions with a focus on rational design. These designs are based around gaining active sites using model surface studies and include high-index faceted Pt and Pt-alloy nanocrystals for anodic electrooxidation reactions as well as Pt-based alloy/core–shell structures and carbon-based non-precious metal catalysts for cathodic oxygen reduction reactions (ORR). High-index nanocrystals, alloy nanoparticles, and support effects are highlighted for anodic catalysts, and current developments in ORR electrocatalysts with novel structures and different compositions are emphasized for cathodic catalysts. Active site structures, catalytic performances, and stability in fuel cells are also reviewed for carbon-based non-precious metal catalysts. In addition, further developmental perspectives and the current status of advanced fuel cell electrocatalysts are provided.

Graphical Abstract

Electrochemical processes on solid shaped nanoparticles with defined facets

This 2007 Chemistry Nobel prize update covers scientific advances of the past decade in our understanding of electrocatalytic processes on surfaces of nanoscale shape-controlled polyhedral solids. It is argued that the field of chemical reaction processes on solid surfaces has recently been paying increasing attention to the fundamental understanding of electrified solid-liquid interfaces and toward the operando study of the minute fraction of catalytically active, structurally dynamic non-equilibrium Taylor-type surface sites. Meanwhile, despite mounting evidence of acting as structural proxies in some cases, the concept of catalytic structure sensitivity of well-defined nanoscale solid surfaces continues to be a key organizing principle for the science of shape-controlled nanocrystals and, hence, constitutes a central recurring theme in this review. After addressing key aspects and recent progress in the wet-chemical synthesis of shaped nanocatalysts, three areas of electrocatalytic processes on solid shape-controlled nanocrystals of current scientific priority are discussed in more detail: the oxygen electroreduction on shape-controlled Pt-Ni polyhedra with its technological relevance for low temperature fuel cells, the CO₂ electroreduction to hydrocarbons on Cu polyhedra and the puzzling interplay between chemical and structural effects, and the electrocatalytic oxygen evolution reaction from water on shaped transition metal oxides. The review closes with the conclusion that Surface Science and thermal catalysis, honored by Ertl's Nobel prize a decade ago, continue to show major repercussions on the emerging field of Interface Science.

Differentiating the structure of PtNi octahedral nanoparticles through combined ADF-EDX simulations

Advances in catalysis rely on the synthesis and characterisation of nanoparticles that have tailored structures and compositions. Although energy-dispersive X-ray (EDX) spectroscopy can be used to study local variations in the compositions of individual supported nanoparticles on the atomic-scale in the scanning transmission electron microscope, electron beam induced damage and contamination can preclude the use of long exposure times and tomographic approaches. Here, we perform simulations of EDX maps of seven different octahedral PtNi nanoparticles for a selection of crystallographic orientations and tilts, to evaluate which of them can be distinguished from elemental mapping performed in only one orientation.

Pt-Richcore/Sn-Richsubsurface/Ptskin Nanocubes As Highly Active and Stable Electrocatalysts for the Ethanol Oxidation Reaction

Direct ethanol fuel cells are one of the most promising electrochemical energy conversion devices for portable, mobile and stationary power applications. However, more efficient and stable and less expensive electrocatalysts are still required. Interestingly, the electrochemical performance of the electrocatalysts toward the ethanol oxidation reaction can be remarkably enhanced by exploiting the benefits of structural and compositional sensitivity and control. Here, we describe the synthesis, characterization, and electrochemical behavior of cubic Pt-Sn nanoparticles. The electrochemical activity of the cubic Pt-Sn nanoparticles was found to be about three times higher than that obtained with unshaped Pt-Sn nanoparticles and six times higher than that of Pt nanocubes. In addition, stability tests indicated the electrocatalyst preserves its morphology and remains well-dispersed on the carbon support after 5000 potential cycles, while a cubic (pure) Pt catalyst exhibited severe agglomeration of the nanoparticles after a similar stability testing protocol. A detailed analysis of the elemental distribution in the nanoparticles by STEM-EELS indicated that Sn dissolves from the outer part of the shell after potential cycling, forming a ∼0.5 nm Pt skin. This particular atomic composition profile having a Pt-rich core, a Sn-rich subsurface layer, and a Pt-skin surface structure is responsible for the high activity and stability.

Stable High-Index Faceted Pt Skin on Zigzag-Like PtFe Nanowires Enhances Oxygen Reduction Catalysis

Selectively exposing active surfaces and judiciously tuning the near-surface composition of electrode materials represent two prominent means of promoting electrocatalytic performance. Here, a new class of Pt₃ Fe zigzag-like nanowires (Pt-skin Pt₃ Fe z-NWs) with stable high-index facets (HIFs) and nanosegregated Pt-skin structure is reported, which are capable of substantially boosting electrocatalysis in fuel cells. These unique structural features endow the Pt-skin Pt₃ Fe z-NWs with a mass activity of 2.11 A mg^-1 and a specifc activity of 4.34 mA cm^-2 for the oxygen reduction reaction (ORR) at 0.9 V versus reversible hydrogen electrode, which are the highest in all reported PtFe-based ORR catalysts. Density function theory calculations reveal a combination of exposed HIFs and formation of Pt-skin structure, leading to an optimal oxygen adsorption energy due to the ligand and strain effects, which is responsible for the much enhanced ORR activities. In contrast to previously reported HIFs-based catalysts, the Pt-skin Pt₃ Fe z-NWs maintain ultrahigh durability with little activity decay and negligible structure transformation after 50 000 potential cycles. Overcoming a key technical barrier in electrocatalysis, this work successfully extends the nanosegregated Pt-skin structure to nanocatalysts with HIFs, heralding the exciting prospects of high-effcient Pt-based catalysts in fuel cells.

Roles of Mo Surface Dopants in Enhancing the ORR Performance of Octahedral PtNi Nanoparticles

Doping with a transition metal was recently shown to greatly boost the activity and durability of PtNi/C octahedral nanoparticles (NPs) for the oxygen reduction reaction (ORR), but its specific roles remain unclear. By combining electrochemistry, ex situ and in situ spectroscopic techniques, density functional theory calculations, and a newly developed kinetic Monte Carlo model, we showed that Mo atoms are preferentially located on the vertex and edge sites of Mo-PtNi/C in the form of oxides, which are stable within the wide potential window of the electrochemical cycle. These surface Mo oxides stabilize adjacent Pt sites, hereby stabilizing the octahedral shape enriched with (111) facets, and lead to increased concentration of Ni in subsurface layers where they are protected against acid dissolution. Consequently, the favorable Pt₃Ni(111) structure for the ORR is stabilized on the surface of PtNi/C NPs in acid against voltage cycling. Significantly, the unusual potential-dependent oxygen coverage trend on Mo-doped PtNi/C NPs as revealed by the surface-sensitive Δμ analysis suggests that the Mo dopants may also improve the ORR kinetics by modifying the coordination environments of Pt atoms on the surface. Our studies point out a possible way to stabilize the favorable shape and composition established on conceptual catalytic models in practical nanoscale catalysts.

Mechanism and kinetics of the electrocatalytic reaction responsible for the high cost of hydrogen fuel cells

The sluggish oxygen reduction reaction (ORR) is a major impediment to the economic use of hydrogen fuel cells in transportation. In this work, we report the full ORR reaction mechanism for Pt(111) based on Quantum Mechanics (QM) based Reactive metadynamics (RμD) simulations including explicit water to obtain free energy reaction barriers at 298 K. The lowest energy pathway for 4 e^- water formation is: first, *OOH formation; second, *OOH reduction to H₂O and O*; third, O* hydrolysis using surface water to produce two *OH and finally *OH hydration to water. Water formation is the rate-determining step (RDS) for potentials above 0.87 Volt, the normal operating range. Considering the Eley-Rideal (ER) mechanism involving protons from the solvent, we predict the free energy reaction barrier at 298 K for water formation to be 0.25 eV for an external potential below U = 0.87 V and 0.41 eV at U = 1.23 V, in good agreement with experimental values of 0.22 eV and 0.44 eV, respectively. With the mechanism now fully understood, we can use this now validated methodology to examine the changes upon alloying and surface modifications to increase the rate by reducing the barrier for water formation.

Engineering Ru@Pt Core-Shell Catalysts for Enhanced Electrochemical Oxygen Reduction Mass Activity and Stability

Improving the performance of oxygen reduction reaction (ORR) electrocatalysts is essential for the commercial efficacy of many renewable energy technologies, including low temperature polymer electrolyte fuel cells (PEFCs). Herein, we report highly active and stable carbon-supported Ru@Pt core-shell nanoparticles (Ru@Pt/C) prepared by a wet chemical synthesis technique. Through rotating disc electrode testing, the Ru@Pt/C achieves an ORR Pt mass-based activity of 0.50 A mg_Pt^-1 at 0.9 V versus the reversible hydrogen electrode (RHE), which exceeds the activity of the state-of-the-art commercial Pt/C catalyst as well as the Department of Energy 2020 PEFC electrocatalyst activity targets for transportation applications. The impact of various synthetic parameters, including Pt to Ru ratios and catalyst pretreatments (i.e., annealing) are thoroughly explored. Pt-based mass activity of all prepared Ru@Pt/C catalysts was found to exceed 0.4 mg_Pt^-1 across the range of compositions investigated, with the maximum activity catalyst having a Ru:Pt ratio of 1:1. This optimized composition of Ru@Pt/C catalyst demonstrated remarkable stability after 30,000 accelerated durability cycles (0.6 to 1.0 V vs. RHE at 125 mV s^-1), maintaining 85% of its initial mass activity. Scanning transmission electron microscopy energy dispersive spectroscopy (STEM-EDS) analysis at various stages of electrochemical testing demonstrated that the Pt shell can provide sufficient protection against the dissolution of the otherwise unstable Ru core.

One-pot synthesis of Pd@Pt3Ni core–shell nanobranches with ultrathin Pt3Ni{111} skins for efficient ethanol electrooxidation

Pd@Pt₃Ni core–shell nanobranches with ultrathin Pt₃Ni{111} skins were facilely synthesized in one-pot and exhibited outstanding performances for ethanol electrooxidation.

Tuning the decarboxylation selectivity for deoxygenation of vegetable oil over Pt–Ni bimetal catalysts via surface engineering

Alloy and bimetal structures are particularly important in developing highly active catalysts for energy conversion.

Enhanced bifunctional fuel cell catalysis via Pd/PtCu core/shell nanoplates

Depositing the Pt atoms on nanoscale two-dimensional (2D) substrates resulting in the exposure of specific crystal facets is an effective strategy for reducing the Pt content without compromising the catalytic property.

Catalytic CO oxidation on B-doped and BN co-doped penta-graphene: a computational study

The catalytic reaction of carbon monoxide oxidation on boron-doped and boron–nitrogen co-doped penta-graphene materials has been systematically studied by utilizing spin-polarized density functional theory (DFT) calculations.

Amperometric myeloperoxidase immunoassay based on the use of CuPdPt nanowire networks

This research describes a nanowire network-based method for detecting the activity of myeloperoxidase (MPO), a biomarker of acute coronary syndromes (ACS). Trimetallic CuPdPt nanowire networks (CuPdPt NWNWs) were prepared by a one-step chemical reduction method. The metallic precursors quickly form nanowire network structures without the need for additional capping agents or surfactants. This process creates a product with a clean surface. The NWNWs were dropped onto a glassy carbon electrode (GCE) to obtain a sensor with good catalytic activity towards the reduction of hydrogen peroxide (H₂O₂), which was used as an electrochemical probe working at -0.4 V (vs. SCE). It also provided a large surface for further modification. Next, an antibody against MPO was immobilized on the modified GCE via the stable conjunction between Cu, Pt, Pd and amino groups. Upon binding of MPO to the antibody on the GCE, the current response to H₂O₂ was reduced by 35 μA·cm^-2. The immunosensor had a linear response within the 100 fg·mL^-1 to 50 ng·mL^-1 MPO concentration range and a 33 fg·mL^-1 detection limit (at an S/N ratio of 3). The recovery of spiked serum samples ranged from 99.8 to 103.6%. This result suggests that the method can be applied to the quantitation of MPO in human serum samples. Graphical abstract A trimetallic CuPdPt nanowire networks was placed on a glassy cabon electrode (GCE) to design an immunosensor for myeloperoxidase (MPO), a biomarker for the acute coronary syndrome (ACS). Antibody against MPO was immobilized on the network via conjugation between Cu, Pt, Pd and amino groups. Amperometric i-t measurements were conducted to quantify the amount of MPO that binds to the antibody on the surface of the modified GCE.

Radially Phase Segregated PtCu@PtCuNi Dendrite@Frame Nanocatalyst for the Oxygen Reduction Reaction

Pt-based alloy nanoframes have shown great potential as electrocatalysts toward the oxygen reduction reaction (ORR) in fuel cells. However, the intrinsically infirm nanoframes could be severely deformed during extended electro-cyclings, which eventually leads to the loss of the initial catalytic activity. Therefore, the structurally robust nanoframe is a worthy synthetic target. Furthermore, ternary alloy phase electrocatalysts offer more opportunities in optimizing the stability and activity than binary alloy ones. Herein, we report a robust PtCuNi ternary nanoframe, structurally fortified with an inner-lying PtCu dendrite, which shows a highly active and stable catalytic performance toward ORR. Remarkably, the PtCu@PtCuNi catalyst exhibited 11 and 16 times higher mass and specific activities than those of commercial Pt/C.

Effect of Dimensionality and Doping in Quasi-"One-Dimensional (1-D)" Nitrogen-Doped Graphene Nanoribbons on the Oxygen Reduction Reaction

Designing an efficient metal-free electrocatalyst for the oxygen reduction reaction (ORR) is a challenging research theme having enormous practical importance in several renewable energy technologies like fuel cell and metal-air batteries. Here we discuss a cost-effective and commercially viable strategy to develop high-performance nitrogen-doped graphene nanoribbon (N-GNR), which is a quasi-"one-dimensional" analogue of graphene. We have selected the N-GNR system to identify the doping-induced variation in the distribution of active catalytic sites experimentally in graphene-based electrocatalysts. N-GNR exhibits a comparable exchange current density (1.65 × 10^-9 vs 2.25 × 10^-9 A cm^-2), thermodynamic potential (0.80 vs 0.83 V), and smaller Tafel slope (55 vs 60 mV dec^-1) with respect to the benchmarking platinum/carbon (Pt/C), and also, more precisely, it goes through a four-electron pathway with low hydrogen peroxide yield. Although the exact mechanism is still not clear, the theme of the work is based on the identification of the possible active sites with the help of experimental evidence like X-ray photoelectron spectroscopy. These results support the assumption that an edge N (pyridinic N)-bonded adjacent C lowers the activation energy barriers of O₂ adsorption, predominantly to kinetically facilitate the ORR activity. We hope these results will be helpful in developing more efficient ORR catalysts.

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt-Ni Nanoparticles by Thermal Annealing - Elucidating the Surface Atomic Structural and Compositional Changes

Shape-controlled octahedral Pt-Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), and in situ electrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mg_Pt^-1 at 0.9 V_RHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.

One-Pot Synthesis of Concave Platinum-Cobalt Nanocrystals and Their Superior Catalytic Performances for Methanol Electrochemical Oxidation and Oxygen Electrochemical Reduction

Exploring highly efficient electro-catalysts is of significant urgency for the widespread uptake of the direct methanol fuel cells (DMFCs). Pt-Co nanocrystals have attracted considerable attentions because of their superior catalytic performance toward both methanol oxidation and oxygen reduction in the preliminary assessments. This Research Article presents a Pt-Co bimetal catalyst that is synthesized through a facile coreduction strategy. The Pt-Co nanocrystals have concave cubic shape with a high uniform size of 7-9 nm and Pt-rich surfaces. The catalysis of the concave cubic Pt-Co nanoparticles toward both methanol electrochemical oxidation reaction (MOR) and oxygen electrochemical reduction reaction (ORR) is evaluated. In comparison with the commercial Pt/C catalyst (Johnson Matthey), the present concave cubic Pt-Co catalyst displays superior performances in not only catalytic activity but also durability. The concave Pt-Co catalyst also shows higher activities than spherical and cubic Pt-Co nanoparticles. The dramatic enhancement is mainly attributed to its alloyed composition, Pt-rich surface and the concave nanostructure. The results of our research indicate that the concave Pt-Co nanocrystal could be a promising catalyst for both MOR and ORR. The present work might also raise more concerns on exploiting morphology and composition of nanocrystal catalysts, which are expected to provide high catalytic performance in electrochemical reactions.

3D Analysis of Fuel Cell Electrocatalyst Degradation on Alternate Carbon Supports

Understanding the mechanisms associated with Pt/C electrocatalyst degradation in proton exchange membrane fuel cell (PEMFC) cathodes is critical for the future development of higher-performing materials; however, there is a lack of information regarding Pt coarsening under PEMFC operating conditions within the cathode catalyst layer. We report a direct and quantitative 3D study of Pt dispersions on carbon supports (high surface area carbon (HSAC), Vulcan XC-72, and graphitized carbon) with varied surface areas, graphitic character, and Pt loadings ranging from 5 to 40 wt %. This is accomplished both before and after catalyst-cycling accelerated stress tests (ASTs) through observations of the cathode catalyst layer of membrane electrode assemblies. Electron tomography results show Pt nanoparticle agglomeration occurs predominantly at junctions and edges of aggregated graphitized carbon particles, leading to poor Pt dispersion in the as-prepared catalysts and increased coalescence during ASTs. Tomographic reconstructions of Pt/HSAC show much better initial Pt dispersions, less agglomeration, and less coarsening during ASTs in the cathode. However, a large loss of the electrochemically active surface area (ECSA) is still observed and is attributed to accelerated Pt dissolution and nanoparticle coalescence. Furthermore, a strong correlation between Pt particle/agglomerate size and measured ECSA is established and is proposed as a more useful metric than average crystallite size in predicting degradation behavior across different catalyst systems.

Self-Assembled Dendritic Pt Nanostructure with High-Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction

The durability issues of Pt catalyst should be resolved for the commercialization of proton exchange membrane fuel cells. Nanocrystal structures with high-index facets have been recently explored to solve the critical durability problem of fuel cell catalysts as Pt catalysts with high-index facets can preserve the ordered surfaces without change of the original structures. However, it is very difficult to develop effective and practical synthetic methods for Pt-based nanostructures with high-index facets. The current study describes a simple one-pot synthesis of self-assembled dendritic Pt nanostructures with electrochemically active and stable high-index facets. Pt nanodendrites exhibited 2 times higher ORR activity and superior durability (only 3.0 % activity loss after 10 000 potential cycles) than a commercial Pt/C. The enhanced catalytic performance was elucidated by the formation of well-organized dendritic structures with plenty of reactive interfaces among 5 nm-sized Pt particles and the coexistence of low- and high-index facets on the particles.

One-pot synthesis of dendritic Pt3Ni nanoalloys as nonenzymatic electrochemical biosensors with high sensitivity and selectivity for dopamine detection

Preparation of Pt-based nanocatalysts with high catalytic activity and exploration of their novel applications have attracted significant interest in the nanoscale field. Herein, we report a facile synthesis of dendritic Pt₃Ni nanoalloys and their applications for electrochemical nonenzymatic dopamine biosensors. As a result of their unique structure, the dendritic Pt₃Ni nanoalloys show high electrocatalytic activity towards dopamine oxidation. Amperometric dopamine biosensors based on dendritic Pt₃Ni nanoalloy microelectrode exhibit a wide linear detection ranges from 0.5 μM to 250 μM with ultrahigh sensitivity, fast response, and excellent selectivity at a potential of 0.3 V in a 0.1 M phosphate buffered solution (pH = 7.2). The limit of detection on dendritic Pt₃Ni nanoalloy microelectrodes can decrease down to 10 nM, which is the least concentration of dopamine in serum samples with a value of sensitivity up to 4.6 μA mg^-1_Pt cm^-2. This study shows an effective approach for the development of dendritic Pt₃Ni nanoalloys as electrocatalysts for electrochemical nonenzymatic dopamine biosensors.

In Situ Fabrication of Defective CoNx Single Clusters on Reduced Graphene Oxide Sheets with Excellent Electrocatalytic Activity for Oxygen Reduction

A facile one-step strategy for anchoring defective CoN_x single clusters on partly reduced graphene oxide (RGO) is constructed to significantly improve the catalytic performance of non-noble metal complexes toward oxygen reduction reaction (ORR). Sequent loading with trace amounts of metal-free porphyrin and Co²⁺ in RGO can dramatically enhance both the half-wave potential and the peak current density. Intriguingly, the RGO/P/2Co single cluster exhibits the best ORR catalytic performance with the half-wave potential of 0.834 V, extremely approaching that of commercial Pt/C (0.836 V). This half-wave potential surpasses most of the reported half-wave potentials of RGO supported non-noble metal ORR catalysts through low-temperature synthesis. Furthermore, the as-prepared RGO/P/2Co delivers a peak current density of 1.3 times higher than that of Pt/C at the same loading, together with a high mass activity of 2.76 A mg_Co^-1. During the durability test, a cathodic current loss less than 10% is recorded after 8000 continuous potential cycles. Insights into this successful example will be conducive to the development of elegant routes for constructing metal nitrogen (MN)-based ORR catalysts with high efficiency, outstanding stability, and excellent selectivity.

PtPb/PtNi Intermetallic Core/Atomic Layer Shell Octahedra for Efficient Oxygen Reduction Electrocatalysis

Although explosive studies on pursuing high-performance Pt-based nanomaterials for fuel cell reactions have been carried out, the combined controls of surface composition, exposed facet, and interior structure of the catalyst remains a formidable challenge. We demonstrate herein a facile chemical approach to realize a new class of intermetallic Pt-Pb-Ni octahedra for the first time. Those nanostructures with unique intermetallic core, active surface composition, and the exposed facet enhance oxygen reduction electrocatalysis with the optimized PtPb_1.12Ni_0.14 octahedra exhibiting superior specific and mass activities (5.16 mA/cm² and 1.92 A/mg_Pt) for oxygen reduction reaction (ORR) that are ∼20 and ∼11 times higher than the commercial Pt/C, respectively. Moreover, the PtPb_1.12Ni_0.14 octahedra can endure at least 15 000 cycles with negligible activity decay, showing a new class of Pt-based electrocatalysts with enhanced performance for fuel cells and beyond.

Structural Evolution of Sub-10 nm Octahedral Platinum-Nickel Bimetallic Nanocrystals

Octahedral Pt alloy nanocrystals (NCs) have shown excellent activities as electrocatalysts toward oxygen reduction reaction (ORR). As the activity and stability of NCs are highly dependent on their structure and the elemental distribution, it is of great importance to understand the formation mechanism of octahedral NCs and to rationally synthesize shape-controlled alloy catalysts with optimized ORR activity and stability. However, the factors controlling the structural and compositional evolution during the synthesis have not been well understood yet. Here, we systematically investigated the structure and composition evolution pathways of Pt-Ni octahedra synthesized with the assistance of W(CO)₆ and revealed a unique core-shell structure consisting of a Pt core and a Pt-Ni alloy shell. Below 140 °C, sphere-like pure Pt NCs with the diameter of 3-4 nm first nucleated, followed by the isotropic growth of Pt-Ni alloy on the seeds at temperatures between 170 and 230 °C forming Pt@Pt-Ni core-shell octahedra with {111} facets. Owing to its unique structure, the Pt@Pt-Ni octahedra show an unparalleled stability during potential cycling, that is, no activity drop after 10 000 cycles between 0.6 and 1.0 V. This work proposes the Pt@Pt-Ni octahedra as a high profile electrocatalyst for ORR and reveals the structural and composition evolution pathways of Pt-based bimetallic NCs.

Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability

Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal-air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however, carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni-Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni(OH)₂ nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. On the basis of its unique local and extended structure, the ORR specific activity of Ni-Pt 2D nanoframes (5.8 mA cm_Pt^-2) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 V_RHE showed that the metallic Ni-Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni-Pt nanoframes supports that at low overpotentials the first electron transfer is the rate-determining step, and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications.

High-Indexed Pt3Ni Alloy Tetrahexahedral Nanoframes Evolved through Preferential CO Etching

Chemically controlling crystal structures in nanoscale is challenging, yet provides an effective way to improve catalytic performances. Pt-based nanoframes are a new class of nanomaterials that have great potential as high-performance catalysts. To date, these nanoframes are formed through acid etching in aqueous solutions, which demands long reaction time and often yields ill-defined surface structures. Herein we demonstrate a robust and unprecedented protocol for facile development of high-performance nanoframe catalysts using size and crystallographic facet-controlled PtNi₄ tetrahexahedral nanocrystals prepared through a colloidal synthesis approach as precursors. This new protocol employs the Mond process to preferentially dealloy nickel component in the ⟨100⟩ direction through carbon monoxide etching of carbon-supported PtNi₄ tetrahexahedral nanocrystals at an elevated temperature. The resultant Pt₃Ni alloy tetrahexahedral nanoframes possess an open, stable, and high-indexed microstructure, containing a segregated Pt thin layer strained to the Pt-Ni alloy surfaces and featuring a down-shift d-band center as revealed by the density functional theory calculations. These nanoframes exhibit much improved catalytic performance, such as high stability under prolonged electrochemical potential cycles, promoting direct electro-oxidation of formic acid to carbon dioxide and enhancing oxygen reduction reaction activities. Because carbon monoxide can be generated from the carbon support through thermal annealing in air, a common process for pretreating supported catalysts, the developed approach can be easily adopted for preparing industrial scale catalysts that are made of Pt-Ni and other alloy nanoframes.

Intermetallic Nanocrystals: Syntheses and Catalytic Applications

At the forefront of nanochemistry, there exists a research endeavor centered around intermetallic nanocrystals, which are unique in terms of long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure. In contrast to alloy nanocrystals with no elemental ordering, it is challenging to synthesize intermetallic nanocrystals with a tight control over their size and shape. Here, recent progress in the synthesis of intermetallic nanocrystals with controllable sizes and well-defined shapes is highlighted. A simple analysis and some insights key to the selection of experimental conditions for generating intermetallic nanocrystals are presented, followed by examples to highlight the viable use of intermetallic nanocrystals as electrocatalysts or catalysts for various reactions, with a focus on the enhanced performance relative to their alloy counterparts that lack elemental ordering. Within the conclusion, perspectives on future developments in the context of synthetic control, structure-property relationships, and applications are discussed.