Highly stable Pt monolayer on PdAu nanoparticle electrocatalysts for the oxygen reduction reaction

Support-Free, Connected Core-Shell Nanoparticle Catalysts Synthesized via a Low-Temperature Process for Advanced Oxygen Reduction Performance

Nanostructured Pt-based catalysts have attracted considerable attention for fuel-cell applications. This study introduces a novel one-pot and low-temperature polyol approach for synthesizing support-free, connected nanoparticles with non-Pt metal cores and Pt shells. Unlike conventional heat treatment methods, the developed support-free and Fe-free connected Pdcore@Ptshell (Pd@Pt) nanoparticle catalyst possesses a stable nanonetwork structure with a high surface area. This approach can precisely control the atomic-level structure of the Pt shell on the Pd core at a low deposition temperature. The optimized Pd@Pt catalyst with a Pt/Pd atomic ratio of 0.8 and a Pt shell thickness of 1.1 nm exhibits a threefold improvement in oxygen reduction reaction (ORR) mass activity compared to that of commercial carbon-supported Pt nanoparticle catalyst (Pt/C). Durability evaluation demonstrated 100% retention of specific activity after 10,000 load cycles, owing to the stable nanonetwork and uniform coverage of the Pt shell. In addition, the support-free, connected core-shell nanoparticle catalyst overcomes the carbon corrosion issues commonly associated with conventional carbon-supported catalysts while simultaneously improving both ORR activity and load cycle durability. These findings highlight the potential of this innovative approach to develop support-free catalysts for polymer electrolyte fuel cells and other energy devices.

Continuous-Flow and Scalable Synthesis of Pd@PtnL Core-Shell Nanocrystals with Enhanced Activity toward Oxygen Reduction

We report a scalable method based on continuous-flow reactors for conformally coating the surfaces of facet-controlled Pd nanocrystals with uniform, ultrathin shells made of Pt. The key to the success of such an approach is the identification of a proper polyol to generate the Pt atoms at a relatively slow rate to ensure adequate surface diffusion and thus the formation of uniform shells in a layer-by-layer fashion. We first demonstrate the concept using the production of Pd@PtnL (n = 2-5) core-shell icosahedral nanocrystals and then have the strategy successfully extended to the syntheses of Pd@PtnL cubic and octahedral nanocrystals. All these core-shell nanocrystals showed great enhancement in catalytic activity toward the oxygen reduction reaction. Our results suggest that seed-mediated growth can be combined with a continuous-flow reactor to achieve scalable production of bimetallic and even trimetallic nanocrystals with controlled sizes, shapes, compositions, and properties.

Symmetry breaking enhances the catalytic and electrocatalytic performance of core/shell tetrametallic porous nanoparticles

The performance of functional nanocatalysts can be extended by integrating multiple types of metals into well-designed nanoparticles. A porous multimetallic shell grown around high-quality monometallic seeds significantly enhances the availability of active sites. Here, tetrametallic core/shell nanoparticles (Au@mPdPtIr) featuring micro- and mesoporous shells are synthesized with strict control over the overall particle morphology. To reveal the impact of the core nanoparticle morphology on the optical, structural and electrocatalytic properties, tetrametallic particles are prepared using gold cores with different shapes but identical volumes and surface chemistry. Our general synthetic approach ensures the successful and reliable synthesis of porous trimetallic shells around the cores, keeping the final atomic composition of the different multimetallic particles identical. The results clearly highlight the significance of the core morphology in the catalytic performance and the superior activity of symmetry-broken core/shell particles in heterogeneous as well as electrocatalytic oxidation reactions. These can be attributed to the fine structural details of the deposited trimetallic shells and their influence on the charge carrier transport between the multimetallic particles and the organic test molecules. While all nanocatalysts show excellent morphological robustness, the optimal morphology also depends on the reaction type and conditions of the specific reaction.

Epitaxial Ultrathin Pt Atomic Layers on CrN Nanoparticle Catalysts

The construction of platinum (Pt) atomic layers is an effective strategy to improve the utilization efficiency of Pt atoms in electrocatalysis, thus is important for reducing the capital costs of a wide range of energy storage and conversion devices. However, the substrates used to grow Pt atomic layers are largely limited to noble metals and their alloys, which is not conducive to reducing catalyst costs. Herein, low-cost chromium nitride (CrN) is utilized as a support for the loading of epitaxial ultrathin Pt atomic layers via a simple thermal ammonolysis method. Owing to the strong anchoring and electronic regulation of Pt atomic layers by CrN, the obtained Pt atomic layers catalyst (containing electron-deficient Pt sites) exhibits excellent activity and endurance for the formic acid oxidation reaction, with a mass activity of 5.17 A mgPt -1 that is 13.6 times higher than that of commercial Pt/C catalyst. This novel strategy demonstrates that CrN can replace noble metals as a low-cost substrate for constructing Pt atomic layers catalysts.

Bionic Fe-N-C catalyst with abundant exposed Fe-Nx sites and enhanced mass transfer properties for efficient oxygen reduction

Transition metal and nitrogen co-doped carbon electrocatalysts are promising candidates to replace the precious metal platinum (Pt) in oxygen reduction reactions (ORR). Unfortunately, the electrochemical performance of existing electrocatalysts is restricted due to limited accessibility of active sites. Inspired by jellyfish tentacles, we design an efficient ORR micro-reactor called Fe-Nx/HC@NWs. It features abundant exposed Fe-Nx active sites dispersed on nitrogen-doped cubic carbon cages, which have a hierarchically porous and hairy structure. The accessible, atomically dispersed Fe-Nx sites and the elaborate substrate architecture synergize to provide the catalyst withremarkable ORR catalytic activity, extraordinary long-term stability, and favorable methanol tolerance in an alkaline electrolyte; overall, its performance is comparable to that of commercial carbon-supported Pt. Our synthesis is facile and controllable, paving a new avenue toward advanced non-precious metal-based electrocatalysts for energy storage and conversion.

The role of high-resolution transmission electron microscopy and aberration corrected scanning transmission electron microscopy in unraveling the structure-property relationships of Pt-based fuel cells electrocatalysts

Platinum-based fuel cell electrocatalysts are structured on a nano level in order to extend their active surface area and maximize the utilization of precious and scarce platinum. Their performance is dictated by the atomic arrangement of their surface layers atoms via structure-property relationships. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are the preferred methods for characterizing these catalysts, due to their capacity to achieve local atomic-level resolutions. Size, morphology, strain and local composition are just some of the properties of Pt-based nanostructures that can be obtained by (S)TEM. Furthermore, advanced methods of (S)TEM are able to provide insights into the quasi-in situ, in situ or even operando stability of these nanostructures. In this review, we present state-of-the-art applications of (S)TEM in the investigation and interpretation of structure-activity and structure-stability relationships.

Overcoming the Electrode Challenges of High-Temperature Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are becoming a major part of a greener and more sustainable future. However, the costs of high-purity hydrogen and noble metal catalysts alongside the complexity of the PEMFC system severely hamper their commercialization. Operating PEMFCs at high temperatures (HT-PEMFCs, above 120 °C) brings several advantages, such as increased tolerance to contaminants, more affordable catalysts, and operations without liquid water, hence considerably simplifying the system. While recent progresses in proton exchange membranes for HT-PEMFCs have made this technology more viable, the HT-PEMFC viscous acid electrolyte lowers the active site utilization by unevenly diffusing into the catalyst layer while it acutely poisons the catalytic sites. In recent years, the synthesis of platinum group metal (PGM) and PGM-free catalysts with higher acid tolerance and phosphate-promoted oxygen reduction reaction, in conjunction with the design of catalyst layers with improved acid distribution and more triple-phase boundaries, has provided great opportunities for more efficient HT-PEMFCs. The progress in these two interconnected fields is reviewed here, with recommendations for the most promising routes worthy of further investigation. Using these approaches, the performance and durability of HT-PEMFCs will be significantly improved.

Catalysis Sans Catalyst Loss: The Origins of Prolonged Stability of Graphene-Metal-Graphene Sandwich Architecture for Oxygen Reduction Reactions

Over the past decades, the design of active catalysts has been the subject of intense research efforts. However, there has been significantly less deliberate emphasis on rationally designing a catalyst system with a prolonged stability. A major obstacle comes from the ambiguity behind how catalyst degrades. Several degradation mechanisms are proposed in literature,   but with a lack of systematic studies, the causal relations between degradation and those proposed mechanisms remain ambiguous. Here, a systematic study of a catalyst system comprising of small particles and single atoms of Pt sandwiched between graphene layers, GR/Pt/GR, is studied to  unravel the degradation mechanism of the studied electrocatalyst for oxygen reduction reaction(ORR). Catalyst suffers from atomic dissolution under ORR harsh acidic and oxidizing operation voltages. Single atoms trapped in point defects within the top graphene layer on their way hopping through toward the surface of GR/Pt/GR architecture. Trapping mechanism renders individual Pt atoms as single atom catalyst sites catalyzing ORR for thousands of cycles before washed away in the electrolyte. The GR/Pt/GR catalysts also compare favorably to state-of-the-art commercial Pt/C catalysts and demonstrates a rational design of a hybrid nanoarchitecture with a prolonged stability for thousands of operation cycles.

Terrace-Rich Ultrathin PtCu Surface on Earth-Abundant Metal for Oxygen Reduction Reaction

The activity and stability of the platinum electrode toward the oxygen reduction reaction are size-dependent. Although small nanoparticles have high Pt utilization, the undercoordinated Pt sites on their surface are assumed to have too strong oxygen binding strength, thus often leading to compromised activity and surface instability. Herein, we report an extended nanostructured PtCu ultrathin surface to reduce the number of low-coordination sites without sacrificing the electrochemical active surface area (ECSA). The surface shows (111)-oriented characteristics, as proven by electrochemical probe reactions and spectroscopies. The PtCu surface brings over an order of magnitude increase in specific activity relative to commercial Pt/C and nearly 4-fold enhancement in ECSA compared to traditional thin films. Moreover, due to the weak absorption of air impurities (e.g., SO2, NO, CO) on highly coordinated sites, the catalyst displays enhanced contaminant tolerance compared with nanoparticulate Pt/C. This work promises a broad screening of extended nanostructured surface catalysts for electrochemical conversions.

Observing the Structural Evolution of Quasi-Monolayer Pt Shell on Pd Core in the Electrocatalytic Oxygen-Reduction Reaction

The use of a quasi-monolayer Pt shell (Ptqms) on a Pd core (Pdc) can reach cost and activity targets for the electrocatalytic oxygen-reduction reaction (ORR). The structure of PdcPtqms in the ORR will vary; however, direct observation of this issue is scarce. Here, during cyclic staircase voltammetry (ranging from 0.5 to 1.15 VRHE) in 0.1 M O2-saturated HClO4, the structure of PdcPtqms was monitored by in situ X-ray absorption spectroscopy. The qualitative and quantitative structural information clearly exhibits a complete picture that Ptqms will directly restructure to form Pt clusters and holes, while Pdc almost remains stable. These findings identify the initial structural evolution of PdcPtqms in the ORR, highlighting the importance of protecting Pdc in the development of high-performance PdcPtqms electrocatalysts.

Particle Size-Controlled Oxygen Reduction and Evolution Reaction Nanocatalysts Regulate Ru(bpy)32+'s Dual-potential Electrochemiluminescence for Sandwich Immunoassay

Multiple signal strategies remarkably improve the accuracy and efficiency of electrochemiluminescence (ECL) immunoassays, but the lack of potential-resolved luminophore pairs and chemical cross talk hinders their development. In this study, we synthesized a series of gold nanoparticles (AuNPs)/reduced graphene oxide (Au/rGO) composites as adjustable oxygen reduction reaction and oxygen evolution reaction catalysts to promote and modulate tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)₃²⁺)'s multisignal luminescence. With the increase in the diameter of AuNPs (3 to 30 nm), their ability to promote Ru(bpy)₃²⁺'s anodic ECL was first impaired and then strengthened, and cathodic ECL was first enhanced and then weakened. Au/rGOs with medium-small and medium-large AuNP diameters remarkably increased Ru(bpy)₃²⁺'s cathodic and anodic luminescence, respectively. Notably, the stimulation effects of Au/rGOs were superior to those of most existing Ru(bpy)₃²⁺ co-reactants. Moreover, we proposed a novel ratiometric immunosensor construction strategy using Ru(bpy)₃²⁺'s luminescence promoter rather than luminophores as tags of antibodies to achieve signal resolution. This method avoids signal cross talk between luminophores and their respective co-reactants, which achieved a good linear range of 10^-7 to 10^-1 ng/ml and a limit of detection of 0.33 fg/ml for detecting carcinoembryonic antigen. This study addresses the previous scarcity of the macromolecular co-reactants of Ru(bpy)₃²⁺, broadening its application in biomaterial detection. Furthermore, the systematic clarification of the detailed mechanisms for converting the potential-resolved luminescence of Ru(bpy)₃²⁺ could facilitate an in-depth understanding of the ECL process and should inspire new designs of Ru(bpy)₃²⁺ luminescence enhancers or applications of Au/rGOs to other luminophores. This work removes some impediments to the development of multisignal ECL biodetection systems and provides vitality into their widespread applications.

Isometric Covalent Triazine Framework-Derived Porous Carbons as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction

Covalent triazine frameworks (CTFs) and their derivative N-doped carbons have attracted much attention for application in energy conversion and storage. However, previous studies have mainly focused on developing new building blocks and optimizing synthetic conditions. The use of isometric building blocks to control the porous structure and to fundamentally understand structure-property relationships have rarely been reported. In this work, two isometric building blocks are used to produce isometric CTFs with controllable pore geometries. The as-prepared CTF with nonplanar hexagonal rings demonstrates higher surface area, larger pore volume, and richer N content than the planar CTF. After pyrolysis, nonplanar porous CTF-derived N-doped carbons exhibit admirable catalytic activity for oxygen reduction in alkaline media (half-wave potential: 0.86 V; Tafel slope: 65 mV dec^-1 ), owing to their larger pore volume and the abundance of pyridinic and graphitic N species. When assembled into a zinc-air battery, the as-made electrocatalysts show high capacities of up to 651 mAh g^-1 and excellent durability.

Ethane Dehydrogenation over the Core-Shell Pt-Based Alloy Catalysts: Driven by Engineering the Shell Composition and Thickness

Pt-based catalysts as the commercial catalysts in ethane dehydrogenation (EDH) face one of the main challenges of realizing the balance between coke formation and catalytic activity. In this work, a strategy to drive the catalytic performance of EDH on Pt-Sn alloy catalysts is proposed by rationally engineering the shell surface structure and thickness of core-shell Pt@Pt₃Sn and Pt₃Sn@Pt catalysts from a theoretical perspective. Eight types of Pt@Pt₃Sn and Pt₃Sn@Pt catalysts with different Pt and Pt₃Sn shell thicknesses are considered and compared with the industrially used Pt and Pt₃Sn catalysts. Density functional theory (DFT) calculations completely describe the reaction network of EDH, including the side reactions of deep dehydrogenation and C-C bond cracking. Kinetic Monte Carlo (kMC) simulations reveal the influences of the catalyst surface structure, experimentally related temperatures, and reactant partial pressures. The results show that CHCH* is the main precursor for coke formation, and Pt@Pt₃Sn catalysts generally have higher C₂H₄(g) activity and lower selectivity compared to those of Pt₃Sn@Pt catalysts, which is attributed to the unique surface geometrical and electronic properties. 1Pt₃Sn@4Pt and 1Pt@4Pt₃Sn are screened out as catalysts exhibiting excellent performance; especially, the 1Pt₃Sn@4Pt catalyst has much higher C₂H₄(g) activity and 100% C₂H₄(g) selectivity compared to those of 1Pt@4Pt₃Sn and the widely used Pt and Pt₃Sn catalysts. The two descriptors C₂H₅* adsorption energy and reaction energy of its dehydrogenation to C₂H₄* are proposed to qualitatively evaluate the C₂H₄(g) selectivity and activity, respectively. This work facilitates a valuable exploration for optimizing the catalytic performance of core-shell Pt-based catalysts in EDH and reveals the great importance of the fine control of the catalyst shell surface structure and thickness.

Effects of the Pt Shell Thickness on the Oxygen Reduction Reaction on a Well-Defined Pd@Pt Core-Shell Model Surface

The effect of the Pt shell thickness on the oxygen reduction reaction (ORR) of a Pd@Pt core-shell catalyst was studied using surface science technics and computational approaches. We found Pt shells on Pd rods to be negatively charged because of charge transfer from the Pd substrate when the shell thicknesses were 0.5 or 1 monolayer (ML). The activities of the ORR of the model surface with a Pt shell of 0.5 or 1 ML were similar and more than twice the activities of a Pt/C or Pt rod. The relationship between the ORR activity and the thickness of the Pt shell was the exact opposite of the relationship between the Pt binding energy and the Pt shell thickness. The indication was that more negatively charged Pt had higher ORR activity. Density functional theory calculations confirmed that a single layer of Pt atoms located on Pd was negatively charged compared to pure Pt and resulted in a lower barrier to the rate-limiting step of the ORR.

Adsorption Energy in Oxygen Electrocatalysis

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

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.

Pt Atomic Layers with Tensile Strain and Rich Defects Boost Ethanol Electrooxidation

Surface and strain engineering are two effective strategies to improve performance; however, synergetic controls of surface and strain effects remains a grand challenge. Herein, we report a highly efficient and stable electrocatalyst with defect-rich Pt atomic layers coating an ordered Pt₃Sn intermetallic core. Pt atomic layers enable the generation of 4.4% tensile strain along the [001] direction. Benefiting from synergetic controls of surface and strain engineering, Pt atomic-layer catalyst (Pt_atomic-layer) achieves a remarkable enhancement on ethanol electrooxidation performance with excellent specific activity of 5.83 mA cm^-2 and mass activity of 1166.6 mA mg _Pt^-1, which is 10.6 and 3.6 times higher than the commercial Pt/C, respectively. Moreover, the intermetallic core endows Pt_atomic-layer with outstanding durability. In situ infrared reflection-absorption spectroscopy as well as density functional theory calculations reveal that tensile strain and rich defects of Pt_atomci-layer facilitate to break C-C bond for complete ethanol oxidation for enhanced performance.

Engineering ultrathin PdAu nanoring via a facile process for electrocatalytic ethanol oxidation

Ultrathin nanoframes with more available electrocatalytic active sites on both internal and external surfaces have attracted great attention especially in the field of electrocatalysis. Herein, we report a facile process to prepare PdAu nanorings (NRs) in aqueous solution without adding any organic ligands. The growth mechanism of PdAu NRs was explored in detail. The Au precursors were reduced into Au clusters around the edges of Pd nanosheets (NSs) via galvanic replacement, then the center of Pd NSs was oxidatively etched by Cl^-/O₂, and finally the Pd and Au atoms on the edge sites were rearranged to form uniform PdAu alloy. PdAu NRs with different ratios and ternary PdAuPt NRs could be easily prepared using this strategy. Owing to the synergistically structural and compositional advantages, Pd₇₉Au₂₁ NRs exhibited higher electrocatalytic activity and stability, as well as low activation energy (E_a) for the ethanol electrooxidation reaction.

Efficient Synthesis of Fe/N-Doped Carbon Nanotube as Highly Active Catalysts for Oxygen Reduction Reaction in Alkaline Media

It is of significant implication to fabricate high-performance, durable and low-cost catalysts toward to oxygen reduction reaction (ORR) to drive commercial application of fuel cells. In our work, we synthesize the Fe/N-CNT catalyst via one-pot grinding combined with calcination using a mixture of carbamide, CNTs and iron salts as precursors, the as-synthesized catalysts show the structure that Fe nanoparticles are encapsulated in the tube of intertwined CNTs with abundant active sites. The catalyst is synthesized at 800 °C (Fe/N-CNT-800-20) obtain high graphitization degree and high N doped content, especially the high content and proportion of Fe-N and pyridinic-N, exhibiting outstanding ORR activity. Moreover, too high calcination temperature (850 °C) and high Fe content (25%) lead to the agglomeration of Fe during the calcination, which blocked some catalytic sites, leading to poor ORR activity. This facile synergy route will provide new thoughts for the fabrication and optimization of catalysts.

Engineering gold-platinum core-shell nanoparticles by self-limitation in solution

Core-shell particles with thin noble metal shells represent an attractive material class with potential for various applications ranging from catalysis to biomedical and pharmaceutical applications to optical crystals. The synthesis of well-defined core-shell architectures remains, however, highly challenging. Here, we demonstrate that atomically-thin and homogeneous platinum shells can be grown via a colloidal synthesis method on a variety of gold nanostructures ranging from spherical nanoparticles to nanorods and nanocubes. The synthesis is based on the exchange of low binding citrate ligands on gold, the reduction of platinum and the subsequent kinetically hindered growth by carbon monoxide as strong binding ligand. The prerequisites for homogeneous growth are low core-binding ligands with moderate fast ligand exchange in solution, a mild reducing agent to mitigate homonucleation and a strong affinity of a second ligand system that can bind to the shell's surface. The simplicity of the described synthetic route can potentially be adapted to various other material libraries to obtain atomically smooth core-shell systems.

Advanced Pt-Based Core-Shell Electrocatalysts for Fuel Cell Cathodes

ConspectusProton-exchange membrane fuel cells (PEMFCs) are highly efficient energy storage and conversion devices. Thus, the platinum group metal (PGM)-based catalysts which are the dominant choice for the PEMFCs have received extensive interest during the past couple of decades. However, the drawbacks in the existing PGM-based catalysts (i.e., high cost, slow kinetics, poor stability, etc.) still limit their applications in fuel cells. The Pt-based core-shell catalysts potentially alleviate these issues through the low Pt loading with the associated low cost and the high corrosion resistance and further improve the oxygen reduction reaction's (ORR's) activity and stability. This Account focuses on the synthetic strategies, catalytic mechanisms, factors influencing enhanced ORR performance, and applications in PEMFCs for the Pt-based core-shell catalysts. We first highlight the synthetic strategies for Pt-based core-shell catalysts including the galvanic displacement of an underpotentially deposited non-noble metal monolayer, thermal annealing, and dealloying methods, which can be scaled-up to meet the requirements of fuel cell operations. Subsequently, catalytic mechanisms such as the self-healing mechanism in the Pt monolayer on Pd core catalysts, the pinning effect of nitrogen (N) dopants in N-doped PtNi core-shell catalysts, and the ligand effect of the ordered intermetallic structure in L1₀-Pt/CoPt core-shell catalysts and their synergistic effects in N-doped L1₀-PtNi catalysts are described in detail. The core-shell structure in the Pt-based catalysts have two main effects for enhanced ORR performance: (i) the interaction between Pt shells and core substrates can tune the electronic state of the surface Pt, thus boosting the ORR activity and stability, and (ii) the outer Pt shell with modest thickness can enhance the oxidation and dissolution resistance of the core, resulting in improved durability. We then review the recent attempts to optimize the ORR performance of the Pt-based core-shell catalysts by considering the shape, composition, surface orientation, and shell thickness. The factors influencing the ORR performance can be grouped into two categories: the effect of the core and the effect of the shell. In the former, PtM core-shell catalysts which use different non-PGM element cores (M) are summarized, and in the latter, Pt-based core-shell catalysts with different shell structures and compositions are described. The modifications of the core and/or shell structure can not only optimize the intermediate-binding energetics on the Pt surface through tuning the strain of the surface Pt, which increases the intrinsic activity and stability, but also offer a significantly decreased catalyst cost. Finally, we discuss the membrane electrode assembly performance of Pt-based core-shell catalysts in fuel cell cathodes and evaluate their potential in real PEMFCs for light-duty and heavy-duty vehicle applications. Even though some challenges to the activity and lifetime in the fuel cells remain, the Pt-based core-shell catalysts are expected to be promising for many practical PEMFC applications.

Modification of Palladium Nanocrystals with Single Atom Platinum via an Electrochemical Self-Catalysis Strategy for Efficient Formic Acid Electrooxidation

Single atom alloys (SAA) have recently drawn increased attention due to their unique structure, high atomic utilization, and fascinating catalytic performance. However, their controllable synthesis still presents a challenge. This study proposes an electrochemical self-catalysis (ESC) strategy to synthesize Pd@Pt/C SAA catalysts, that is, depositing Pt atoms on Pd nanocrystals through in situ decomposition of sodium formate. The relationship between composition and structure of Pd@Pt/C is distinguished through a combination of electrochemical analysis, sphere-corrected scanning transmission electron microscopy, and X-ray adsorption spectra. That relationship evolved from SAA to a sea-island structure and even a core-shell structure with composition-controllable atomic ratios, highlighting the great diversity and convenience of this method in nanostructure construction. The Pd@Pt/C SAA catalyst showed excellent catalytic activity to formic acid oxidation with a peak current density of 5.2 A/mg^metal, which is about 18.6 times that of the commercial Pd/C. density functional theory calculations revealed that the enhanced activity was due to the "passivation" of Pd sites near the Pt single atoms, which attenuated the adsorption of CO. Based on electrochemical principles, this ESC strategy was also expanded to prepare a series of Pd-based SAA, including Pd-Au, Pd-Ir, and Pd-Bi.

Toward the Continuous Production of Multigram Quantities of Highly Uniform Supported Metallic Nanoparticles and Their Application for Synthesis of Superior Intermetallic Pt-Alloy ORR Electrocatalysts

A fast and facile pulse combustion (PC) method that allows for the continuous production of multigram quantities of high-metal-loaded and highly uniform supported metallic nanoparticles (SMNPs) is presented. Namely, various metal on carbon (M/C) composites have been prepared by using only three feedstock components: water, metal-salt, and the supporting material. The present approach can be elegantly utilized also for numerous other applications in electrocatalysis, heterogeneous catalysis, and sensors. In this study, the PC-prepared M/C composites were used as metal precursors for the Pt NPs deposition using double passivation with the galvanic displacement method (DP method). Lastly, by using thin-film rotating disc electrode (TF-RDE) and gas-diffusion electrode (GDE) methodologies, we show that the synergistic effects of combining PC technology with the DP method enable production of superior intermetallic Pt-M electrocatalysts with an improved oxygen reduction reaction (ORR) performance when compared to a commercial Pt-Co electrocatalyst for proton exchange membrane fuel cells (PEMFCs) application.

MOF-Templated Sulfurization of Atomically Dispersed Manganese Catalysts Facilitating Electroreduction of CO2 to CO

To reach a carbon-neutral future, electrochemical CO2 reduction reaction (eCO2RR) has proven to be a strong candidate for the next-generation energy system. Among potential materials, single-atom catalysts (SACs) serve as a model to study the mechanism behind the reduction of CO2 to CO, given their well-defined active metal centers and structural simplicity. Moreover, using metal-organic frameworks (MOFs) as supports to anchor and stabilize central metal atoms, the common concern, metal aggregation, for SACs can be addressed well. Furthermore, with their turnability and designability, MOF-derived SACs can also extend the scope of research on SACs for the eCO2RR. Herein, we synthesize sulfurized MOF-derived Mn SACs to study effects of the S dopant on the eCO2RR. Using complementary characterization techniques, the metal moiety of the sulfurized MOF-derived Mn SACs (MnSA/SNC) is identified as MnN3S1. Compared with its non-sulfur-modified counterpart (MnSA/NC), the MnSA/SNC provides uniformly superior activity to produce CO. Specifically, a nearly 30% enhancement of Faradaic efficiency (F.E.) in CO production is observed, and the highest F.E. of approximately 70% is identified at -0.45 V. Through operando spectroscopic characterization, the probing results reveal that the overall enhancement of CO production on the MnSA/SNC is possibly caused by the S atom in the local MnN3S1 moiety, as the sulfur atom may induce the formation of S-O bonding to stabilize the critical intermediate, *COOH, for CO2-to-CO. Our results provide novel design insights into the field of SACs for the eCO2RR.

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.

Enhancement of Activity and Development of Low Pt Content Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O₂-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H₂O₂-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO₂, WO₃, Ta₂O₅, Nb₂O₅, and ZrO₂), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt-Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal-support interactions and interfacial structural changes affecting adsorption and activation of O₂-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional.

Integrating a metal framework with Co-confined carbon nanotubes as trifunctional electrocatalysts to boost electron and mass transfer approaching practical applications

A facile and large-scale construction of robust and inexpensive trifunctional self-supporting electrodes for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in metal-air batteries and water splitting is crucial but remains challenging. Herein, we report a direct and up-scalable all-solid-phase strategy for the synthesis of a porous three-dimensional electrode consisting of cobalt nanoparticles wrapped in nitrogen-doped carbon tubes (Co/N-CNTs), which are in situ planted onto the surface of a cobalt foam. The resultant Co/N-CNTs can directly serve as a self-supporting and adhesive-free electrode with excellent and durable catalytic performances for the ORR, OER and HER. The metal framework substrate with an open-pore architecture is favorable for electron and mass transfer and allows fast catalytic kinetics. More importantly, when used in Zn-air batteries and overall water splitting, the as-prepared Co/N-CNT electrode displays a remarkable performance, implying bright perspects for practical application.

Oxygen Reduction Electrocatalysts toward Practical Fuel Cells: Progress and Perspectives

Fuel cells are an incredibly powerful renewable energy technology, but their broad applications remains lagging because of the high cost and poor reliability of cathodic electrocatalysts for the oxygen reduction reaction (ORR). This review focuses on the recent progress of ORR electrocatalysts in fuel cells. More importantly, it highlights the fundamental problems associated with the insufficient activity translation from rotating disk electrode to membrane electrode assembly in the fuel cells. Finally, for the atomic-level in-depth information on ORR catalysts in fuel cells, potential perspectives are suggested, including large-scale preparation, unified assessment criteria, advanced interpretation techniques, advanced simulation and artificial intelligence. This review aims to provide valuable insights into the fundamental science and technical engineering for efficient ORR electrocatalysts in fuel cells.

Alloying-realloying enabled high durability for Pt-Pd-3d-transition metal nanoparticle fuel cell catalysts

Alloying noble metals with non-noble metals enables high activity while reducing the cost of electrocatalysts in fuel cells. However, under fuel cell operating conditions, state-of-the-art oxygen reduction reaction alloy catalysts either feature high atomic percentages of noble metals (>70%) with limited durability or show poor durability when lower percentages of noble metals (<50%) are used. Here, we demonstrate a highly-durable alloy catalyst derived by alloying PtPd (<50%) with 3d-transition metals (Cu, Ni or Co) in ternary compositions. The origin of the high durability is probed by in-situ/operando high-energy synchrotron X-ray diffraction coupled with pair distribution function analysis of atomic phase structures and strains, revealing an important role of realloying in the compressively-strained single-phase alloy state despite the occurrence of dealloying. The implication of the finding, a striking departure from previous perceptions of phase-segregated noble metal skin or complete dealloying of non-noble metals, is the fulfilling of the promise of alloy catalysts for mass commercialization of fuel cells.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

High-Sensitive MEMS Hydrogen Sulfide Sensor made from PdRh Bimetal Hollow Nanoframe Decorated Metal Oxides and Sensitization Mechanism Study

Here we report the fabrication of a high performance metal oxide semiconductor (MOS) sensor for the detection of hydrogen sulfide (H₂S) using PdRh bimetal hollow nanocube (HC) with Rh-rich hollow frame and Pd-rich core frame as sensitizing materials. PdRh bimetal HC with the edge-length about 10 nm was prepared by chemical etching PdRh bimetal solid nanocube (SC) in HNO₃ aqueous solution. The results of gas-sensing tests indicate that the response value order of the MEMS gas sensors based on MOSs (including ZnO, MoO₃ and SnO₂) is as follows: R_PdRh HC/MOS > R_PdRh SC/MOS > R_MOS. First, in the system of ZnO, gas sensor modified by PdRh (PdRh SC/ZnO and PdRh HC/ZnO) possess enhanced H₂S sensing performance with a better response and excellent low-concentration detection capability (down to 15 ppb) comparing to pure ZnO. The improved H₂S sensing performance could be attributed to the good conductivity of Rh-rich frame, the high catalytic activity of PdRh bimetal and formation of Schottky barrier-type junctions and defect. Second, PdRh HC/ZnO sensor shows better response (185-1 ppm of H₂S) compared to PdRh SC/ZnO sensor (108-1 ppm of H₂S), which is due to the higher specific surface area of PdRh HC/ZnO and good gas diffusion of the hollow structure. This work indicate that the sensitization characteristics of PdRh bimetal HC will provide new paradigms for the future development of the high performance sensor.

Fe(II)-Assisted one-pot synthesis of ultra-small core-shell Au-Pt nanoparticles as superior catalysts towards the HER and ORR

In this work, uniform ultra-small core-shell Au-Pt nanoparticles (denoted as USCS Au-Pt NPs) with Au-decorated Pt surfaces are successfully prepared by Fe(ii)-assisted one-pot co-reduction of Au(iii) ions and Pt(ii) ions in a citrate solution. The as-prepared USCS Au38.4@Au9.3Pt52.3 NPs have an average diameter of 2.3 ± 0.5 nm. It is found that the morphology, composition and size of Au-Pt NPs are highly dependent on the reaction conditions including the addition sequence of the precursors, and the concentrations of Fe(ii) ions, Au(iii) ions and Pt(ii) ions. In addition, USCS Au38.4@Au9.3Pt52.3-NP/C catalysts (USCS Au38.4@Au9.3Pt52.3 NPs loaded on the Vulcan XC-72R carbon black) exhibit excellent electrocatalytic performance towards the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) in acidic media due to the higher electrochemically active surface area (ECSA) and electronic effect between Pt and Au. For instance, USCS Au38.4@Au9.3Pt52.3-NP/C catalysts exhibited greatly enhanced HER activity in terms of overpotential (16 mV at a current density of -10 mA cm-2) and are better than commercial Pt/C catalysts (31 mV at a current density of -10 mA cm-2) reported in the literature thus far, to the best of our knowledge. Strikingly, their mass activity is about 13.1-fold higher than that of commercial Pt/C catalysts. Moreover, they also show an improved ORR activity, Eonset = 1.015 V and E1/2 = 0.896 V, which are positively shifted by nearly 28 mV and 21 mV than those of commercial Pt/C catalysts (0.987 V and 0.875 V), respectively. In addition, they also showed a higher kinetic current density (12.85 mA cm-2 at 0.85 V) and a better long-term durability. Our synthetic strategy presented here may be extended to the preparation of ultra-small Au-based bimetallic or multi-metallic NPs.

Site-specific electrodeposition enables self-terminating growth of atomically dispersed metal catalysts

The growth of atomically dispersed metal catalysts (ADMCs) remains a great challenge owing to the thermodynamically driven atom aggregation. Here we report a surface-limited electrodeposition technique that uses site-specific substrates for the rapid and room-temperature synthesis of ADMCs. We obtained ADMCs by the underpotential deposition of a non-noble single-atom metal onto the chalcogen atoms of transition metal dichalcogenides and subsequent galvanic displacement with a more-noble single-atom metal. The site-specific electrodeposition enables the formation of energetically favorable metal-support bonds, and then automatically terminates the sequential formation of metallic bonding. The self-terminating effect restricts the metal deposition to the atomic scale. The modulated ADMCs exhibit remarkable activity and stability in the hydrogen evolution reaction compared to state-of-the-art single-atom electrocatalysts. We demonstrate that this methodology could be extended to the synthesis of a variety of ADMCs (Pt, Pd, Rh, Cu, Pb, Bi, and Sn), showing its general scope for functional ADMCs manufacturing in heterogeneous catalysis.

Atomic-Level Functionalized Graphdiyne for Electrocatalysis Applications

Graphdiyne (GDY) is a two-dimensional (2D) electron-rich full-carbon planar material composed of sp2- and sp-hybridized carbon atoms, which features highly conjugated structures, uniformly distributed pores, tunable electronic characteristics and high specific surface areas. The synthesis strategy of GDY by facile coupling reactions under mild conditions provides more convenience for the functional modification of GDY and offers opportunities for realizing the special preparation of GDY according to the desired structure and unique properties. These structural characteristics and excellent physical and chemical properties of GDY have attracted increasing attention in the field of electrocatalysis. Herein, the research progress in the synthesis of atomic-level functionalized GDYs and their electrocatalytic applications are summarized. Special attention was paid to the research progress of metal-atom-anchored and nonmetallic-atom-doped GDYs for applications in the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) catalytic processes. In addition, several potential development prospects and challenges of these 2D highly conjugated electron-rich full-carbon materials in the field of electrocatalysis are presented.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.