A Surfactant-Free Strategy for Synthesizing and Processing Intermetallic Platinum-Based Nanoparticle Catalysts

Synergistic Modulation of Multiple Sites Boosts Anti-Poisoning Hydrogen Electrooxidation Reaction with Ultrasmall (Pt0.9Rh0.1)3V Ternary Intermetallic Nanoparticles

Promoting the hydrogen oxidation reaction (HOR) activity and poisoning tolerance of electrocatalysts is crucial for the large-scale application of hydrogen-oxygen fuel cell. However, it is severely hindered by the scaling relations among different intermediates. Herein, lattice-contracted Pt-Rh in ultrasmall ternary L12-(Pt0.9Rh0.1)3V intermetallic nanoparticles (~2.2 nm) were fabricated to promote the HOR performances through an oxides self-confined growth strategy. The prepared (Pt0.9Rh0.1)3V displayed 5.5/3.7 times promotion in HOR mass/specific activity than Pt/C in pure H2 and dramatically limited activity attenuation in 1000 ppm CO/H2 mixture. In situ Raman spectra tracked the superior anti-CO* capability as a result of compressive strained Pt, and the adsorption of oxygen-containing species was promoted due to the dual-functional effect. Further assisted by density functional theory calculations, both the adsorption of H* and CO* on (Pt0.9Rh0.1)3V were reduced compared with that of Pt due to lattice contraction, while the adsorption of OH* was enhanced by introducing oxyphilic Rh sites. This work provides an effective tactic to stimulate the electrocatalytic performances by optimizing the adsorption of different intermediates severally.

Gas-Solid Reaction Synthesis and Electrocatalytic Oxygen Reduction of Pt-Skin L10-PtFe/C

Compared to Pt/C, the atomic ordered Pt-based intermetallic compounds can deliver higher efficiency and reliable stability, and they are considered one of the ideal cathode catalysts for the next generation of fuel cells. This work proposed a simple ferrocene atmosphere annealing method to improve commercial Pt/C and convert Pt to L10-PtFe. After further acid etching treatment, the obtained carbon-supported Pt-skin L10-PtFe (Pt-skin L10-PtFe/C) with superfine particle size (∼3.3 nm) not only was highly dispersed on the carbon but possesses a thin Pt skin, like the armor of L10-PtFe. As excepted, the ORR activity of Pt-skin L10-PtFe/C (0.375 A mg-1; 0.921 mA cm-2) is far better than that of commercial Pt/C (0.121 A mg-1; 0.260 mA cm-2), and its stability is also greatly improved. Our proposed gas-solid reaction is straightforward and has great potential in producing Pt-based intermetallic catalysts on a large scale.

VOx Matrix Confinement Approach to Generate Sub-3 nm L10-Pt-Based Intermetallic Catalysts for Fuel Cell Cathode

Pt-based intermetallic compounds (IMCs) are considered as a class of promising fuel cell electrocatalysts, owing to their outstanding intrinsic activity and durability. However, the synthesis of uniformly dispersed IMCs with small sizes presents a formidable challenge during the essential high-temperature annealing process. Herein, a facile and generally applicable VOx matrix confinement strategy is demonstrated for the controllable synthesis of ordered L10-PtM (M = Fe, Co, and Mn) nanoparticles, which not only enhances the dispersion of intermetallic nanocrystals, even at high loading (40 wt%), but also simplifies the oxide removal and acid-washing procedures. Taking intermetallic PtCo as an example, the as-prepared catalyst displays a high-performance oxygen reduction activity (mass activity of 1.52 A mgPt -1) and excellent stability in the membrane electrode assemblies (MEAs) (the ECSA has just 7% decay after durability test). This strategy provides an economical and scalable route for the controlled synthesis of Pt-based intermetallic catalysts, which can pave a way for the commercialization of fuel cell technologies.

Island-in-Sea Structured Pt3Fe Nanoparticles-in-Fe Single Atoms Loaded in Carbon Materials as Superior Electrocatalysts toward Alkaline HER and Acidic ORR

In this work, Pt3Fe nanoparticles (Pt3Fe NPs) with the ordered internal structure and Pt-rich shells surrounded by plenty of Fe single atoms (Fe SAs) as active species (Pt3Fe NP-in-Fe SA) loaded in the carbon materials are successfully fabricated, which are abbreviated as island-in-sea structured (IISS) Pt3Fe NP-in-Fe SA catalysts. Moreover, the synergistic effect of O-bridging between Pt3Fe NPs and Fe SAs, and the ordered internal structured Pt3Fe NPs with Pt-rich shells of an optimal thickness contributes to the achievement of the local acidic environments on the surfaces of Pt3Fe NPs in the alkaline hydrogen evolution reaction (HER) and the enhancement of the desorption rate of *OH intermediate in the acidic oxygen reduction reaction (ORR). In addition, the electronic interactions between Pt3Fe NPs and dispersed Fe SAs cannot only provide efficient electrons transfer, but also prevent the aggregation and dissolution of Pt3Fe NPs. Furthermore, the overpotential and the half wave potential of the as-prepared IISS Pt3Fe NP-in-Fe SA catalysts toward the alkaline HER and toward the acidic ORR are 8 mV at a current density of 10 mA cm-2 and 0.933 V, respectively, which is 29 lower and 86 mV higher than those (37 mV and 0.847 V) of commercial Pt/C catalysts.

Gram-scale synthesis of small-sized PtM intermetallics as high-performance catalysts for the hydrogen evolution reaction

Pt-based intermetallics exhibit excellent activity in electrocatalysis. However, their controlled syntheses remain difficult. Herein, carbon-supported PtM (M = Fe, Co, Ni, Zn and Mn) intermetallics with small size (3 nm) were prepared at the gramscale and applied as a highly effective electrocatalyst for the hydrogen evolution reaction.

Site-selective sulfur anchoring produces sintering-resistant intermetallic ORR electrocatalysts for membrane electrode assemblies

Intermetallic compounds are emerging as promising oxygen reduction reaction (ORR) catalysts for fuel cells due to their typically higher activity and durability compared to disordered alloys. However, the preparation of intermetallic catalysts often requires high-temperature annealing, which unfortunately leads to adverse sintering of the metal nanoparticles. Herein, we develop a scalable site-selective sulfur anchoring strategy that effectively suppresses alloy sintering, ensuring the formation of efficient intermetallic electrocatalysts with small sizes and high ordering degrees. The alloy-support interactions are precisely modulated by selectively modifying the alloy-support interfaces with oxidized sulfur species, thus simultaneously blocking both the nanoparticle migration and Oswald ripening pathways for sintering. Using this strategy, sub-5 nm PtCo intermetallic electrocatalysts enclosed by two atomic layers of Pt shells have been successfully prepared even at a metal loading higher than 30 wt%. The intermetallic catalysts exhibit excellent ORR performances in both rotating disk electrode and membrane electrode assembly conditions with a mass activity of 1.28 A mgPt-1 at 0.9 V (vs. RHE) and a power density of 1.0 W cm-2 at a current density of 1.5 A cm-2. The improved performances result from the enhanced Pt-Co electronic interactions and compressive surface strain generated by the highly ordering structure, while the atomic Pt shells prevent the dissolution of Co under highly acidic conditions. This work provides new insights to inhibit the sintering of nanoalloys and would promote the scalable synthesis and applications of platinum-based intermetallic catalysts.

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.

Emerging Pt-based intermetallic nanoparticles for the oxygen reduction reaction

The advancement of highly efficient and enduring platinum (Pt)-based electrocatalysts for the oxygen reduction reaction (ORR) is a critical determinant to enable broad utilization of clean energy conversion technologies. Pt-based intermetallic electrocatalysts offer durability and superior ORR activity over their traditional analogues due to their definite stoichiometry, ordered and extended structures, and favourable enthalpy of formation. With the advent in new synthetic methods, Pt-based intermetallic nanoparticles as a new class of advanced electrocatalysts have been studied extensively in recent years. This review discusses the preparation principles, representative preparation methods of Pt-based intermetallics and their applications in the ORR. Our review is focused on L10 Pt-based intermetallics which have gained tremendous interest recently due to their larger surface strain and enhanced M(3d)-Pt(5d) orbital coupling, particularly in the crystallographic c-axis direction. Additionally, we discuss future research directions to further improve the efficiency of Pt-based intermetallic electrocatalysts with the intention of stimulating increased research ventures in this domain.

Machine-learning-accelerated design of high-performance platinum intermetallic nanoparticle fuel cell catalysts

Carbon supported PtCo intermetallic alloys are known to be one of the most promising candidates as low-platinum oxygen reduction reaction electrocatalysts for proton-exchange-membrane fuel cells. Nevertheless, the intrinsic trade-off between particle size and ordering degree of PtCo makes it challenging to simultaneously achieve a high specific activity and a large active surface area. Here, by machine-learning-accelerated screenings from the immense configuration space, we are able to statistically quantify the impact of chemical ordering on thermodynamic stability. We find that introducing of Cu/Ni into PtCo can provide additional stabilization energy by inducing Co-Cu/Ni disorder, thus facilitating the ordering process and achieveing an improved tradeoff between specific activity and active surface area. Guided by the theoretical prediction, the small sized and highly ordered ternary Pt2CoCu and Pt2CoNi catalysts are experimentally prepared, showing a large electrochemically active surface area of ~90 m2 gPt‒1 and a high specific activity of ~3.5 mA cm‒2.

An Ultrasmall Ordered High-Entropy Intermetallic with Multiple Active Sites for the Oxygen Reduction Reaction

Controlling multimetallic ensembles at the atomic level is significantly challenging, particularly for high-entropy alloys with more than five elements. Herein, we report an innovative ultrasmall (∼2 nm) PtFeCoNiCuZn high-entropy intermetallic (PFCNCZ-HEI) with a well-ordered structure synthesized by using the space-confined strategy. By exploiting these combined metals, the PFCNCZ-HEI nanoparticles achieve an ultrahigh mass activity of 2.403 A mgPt-1 at 0.90 V vs reversible hydrogen electrode for the oxygen reduction reaction, which is up to 19-fold higher than that of state-of-the-art commercial Pt/C. A proton exchange membrane fuel cell assembled with PFCNCZ-HEI as the cathode (0.03 mgPt cm-2) exhibits a power density of 1.4 W cm-2 and a high mass-normalized rated power of 45 W mgPt-1. Furthermore, theoretical calculations reveal that the outer electrons of the non-noble-metal atoms on the surface of the PFCNCZ-HEI nanoparticle are modulated to show characteristics of multiple active centers. This work offers a promising catalyst design direction for developing highly ordered HEI nanoparticles for electrocatalysis.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Tailored Lattice Compressive Strain of Pt-Skins by the L12 -Pt3 M Intermetallic Core for Highly Efficient Oxygen Reduction

The sluggish kinetics of oxygen reduction reaction (ORR) and unsatisfactory durability of Pt-based catalysts are severely hindering the commercialization of proton-exchange-membrane fuel cells (PEMFCs). In this work, the lattice compressive strain of Pt-skins imposed by Pt-based intermetallic cores is tailored for highly effective ORR through the confinement effect of the activated nitrogen-doped porous carbon (a-NPC). The modulated pores of a-NPC not only promote Pt-based intermetallics with ultrasmall size (average size of <4 nm), but also efficiently stabilizes intermetallic nanoparticles and sufficient exposure of active sites during the ORR process. The optimized catalyst (L12 -Pt3 Co@ML-Pt/NPC10 ) achieves excellent mass activity (1.72 A mgPt -1 ) and specific activity (3.49 mA cmPt -2 ), which are 11- and 15-fold that of commercial Pt/C, respectively. Besides, owing to the confinement effect of a-NPC and protection of Pt-skins, L12 -Pt3 Co@ML-Pt/NPC10 retains 98.1% mass activity after 30 000 cycles, and even 95% for 100 000 cycles, while Pt/C retains only 51.2% for 30 000 cycles. Rationalized by density functional theory, compared with other metals (Cr, Mn, Fe, and Zn), L12 -Pt3 Co closer to the top of "volcano" induces a more suitable compressive strain and electronic structure on Pt-skin, leading to an optimal oxygen adsorption energy and a remarkable ORR performance.

Synthesis of rhodium intermetallic catalysts by enlarging the inter-particle distance on high-surface-area carbon black supports

Here, we report a "critical distance" method for the synthesis of 9 kinds of sub-5 nm rhodium (Rh)-based intermetallic catalysts. Enlarging the distance between intermetallic particles on high-surface-area carbon black supports could significantly suppress the metal sintering in high-temperature annealing. The prepared Rh₂Sn intermetallic catalysts exhibited enhanced activity in catalyzing the hydrogenation of nitrobenzene.

Small molecule-assisted synthesis of carbon supported platinum intermetallic fuel cell catalysts

Supported ordered intermetallic compounds exhibit superior catalytic performance over their disordered alloy counterparts in diverse reactions. But the synthesis of intermetallic compounds catalysts often requires high-temperature annealing that leads to the sintering of metals into larger crystallites. Herein, we report a small molecule-assisted impregnation approach to realize the general synthesis of a family of intermetallic catalysts, consisting of 18 binary platinum intermetallic compounds supported on carbon blacks. The molecular additives containing heteroatoms (that is, O, N, or S) can be coordinated with platinum in impregnation and thermally converted into heteroatom-doped graphene layers in high-temperature annealing, which significantly suppress alloy sintering and insure the formation of small-sized intermetallic catalysts. The prepared optimal PtCo intermetallics as cathodic oxygen-reduction catalysts exhibit a high mass activity of 1.08 A mg_Pt^-1 at 0.9 V in H₂-O₂ fuel cells and a rated power density of 1.17 W cm^-2 in H₂-air fuel cells.

Intermetallic PtFe Electrocatalysts for the Oxygen Reduction Reaction: Ordering Degree-Dependent Performance

Platinum-based atomically ordered alloys (i.e., intermetallic compounds) have distinct advantages over disordered solid solution counterparts in boosting the cathodic oxygen-reduction reaction (ORR) in proton-exchange-membrane fuel cells. Nevertheless, the pivotal role of ordering degree of intermetallic catalysts in promoting ORR performance has been ignored heavily so far, probably owing to the lack of synthetic routes for controlling the ordering degree, especially for preparing highly ordered intermetallic catalysts. Herein, a family of intermetallic PtFe catalysts with similar particle size of 3-4 nm but varied ordering degree in a wide range of 10-70% are prepared. After constructing the PtFe/Pt core/shell structure with around 3 Pt-layer skin, a positive correlation between the ordering degree of the intermetallic catalysts and their ORR activity and durability is identified. Notably, the highly ordered PtFe/Pt catalyst exhibits a high mass activity of 0.92 A mg_Pt ^-1 at 0.9 V_iR-corrected as cathode catalyst in H₂ -O₂ fuel cell, with only 24% loss after accelerated durability tests. The ordering degree-dependent performance can be ascribed to the compressive strain effect induced by the intermetallic PtFe core with smaller lattice parameters, and the more thermodynamically stable intermetallic structure compared to disordered alloys.

Engineering Platinum Catalysts via a Site-Isolation Strategy with Enhanced Chlorine Resistance for the Elimination of Multicomponent VOCs

Pt-based catalysts can be poisoned by the chlorine formed during the oxidation of multicomponent volatile organic compounds (VOCs) containing chlorinated VOCs. Improving the low-temperature chlorine resistance of catalysts is important for industrial applications, although it is yet challenging. We hereby demonstrate the essential catalytic roles of a bifunctional catalyst with an atomic-scale metal/oxide interface constructed by an intermetallic compound nanocrystal. Introducing trichloroethylene (TCE) exhibits a less negative effect on the catalytic activity of the bimetallic catalyst for o-xylene oxidation, and the partial deactivation caused by TCE addition is reversible, suggesting that the bimetallic, HCl-etched Pt₃Sn(E)/CeO₂ catalyst possesses much stronger chlorine resistance than the conventional Pt/CeO₂ catalyst. On the site-isolated Pt-Sn catalyst, the presence of aromatic hydrocarbon significantly inhibits the adsorption strength of TCE, resulting in excellent catalytic stability in the oxidation of the VOC mixture. Furthermore, the large amount of surface-adsorbed oxygen species generated on the electronegative Pt is highly effective for low-temperature C-Cl bond dissociation. The adjacent promoter (Sn-O) possesses the functionality of acid sites to provide sufficient protons for HCl formation over the bifunctional catalyst, which is considered critical to maintaining the reactivity of Pt by removing Cl and decreasing the polychlorinated byproducts.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

A General Concurrent Template Strategy for Ordered Mesoporous Intermetallic Nanoparticles with Controllable Catalytic Performance

We report a general concurrent template strategy for precise synthesis of mesoporous Pt-/Pd-based intermetallic nanoparticles with desired morphology and ordered mesostructure. The concurrent template not only supplies a mesoporous metal seed for re-crystallization growth of atomically ordered intermetallic phases with unique atomic stoichiometry but also provides a nanoconfinement environment for nanocasting synthesis of mesoporous nanoparticles with ordered mesostructure and rhombic dodecahedral morphology under elevated temperature. Using the selective hydrogenation of 3-nitrophenylacetylene as a proof-of-concept catalytic reaction, mesoporous intermetallic PtSn nanoparticles exhibited remarkably controllable intermetallic phase-dependent catalytic selectivity and excellent catalytic stability. This work provides a very powerful strategy for precise preparation of ordered mesoporous intermetallic nanocrystals for application in selective catalysis and fuel cell electrocatalysis.

Achievements in Pt nanoalloy oxygen reduction reaction catalysts: strain engineering, stability and atom utilization efficiency

The Pt nanoalloy surfaces often show unique electronic and physicochemical properties that are distinct from those of their parent metals, which provide significant room for manipulating their oxygen reduction reaction (ORR) behaviour. In this Feature Article, we present the progress of our recent research and that of other groups in Pt nanoalloy catalysts for ORR from three aspects, namely, strain engineering, stability and atom utilization efficiency. Some new insights into Pt surface strain engineering will be firstly introduced, with a focus on discussing the effect of compressive and tensile strain on the chemisorption properties. Secondly, the design concepts and synthetic methodologies to intensify the inherent stability of Pt nanoalloys will be summarized. Then, the exciting research push in developing nanostructured alloys with high atom utilization efficiency of Pt will be presented. Finally, a brief illumination of challenges and future developing perspectives of Pt nanoalloy catalysts will be provided.

Sulfur-anchoring synthesis of platinum intermetallic nanoparticle catalysts for fuel cells

[Figure: see text].

Pt-Based Intermetallic Nanocrystals in Cathode Catalysts for Proton Exchange Membrane Fuel Cells: From Precise Synthesis to Oxygen Reduction Reaction Strategy

Although oxygen reduction reaction (ORR) catalysts have been extensively investigated and developed, there is a lack of clarity on catalysts that can balance high performance and low cost. Pt-based intermetallic nanocrystals are of special interest in the commercialization of proton exchange membrane fuel cells (PEMFCs) due to their excellent ORR activity and stability. This review summarizes the wide range of applications of Pt-based intermetallic nanocrystals in cathode catalysts for PEMFCs and their unique advantages in the field of ORR. Firstly, we introduce the fundamental understanding of Pt-based intermetallic nanocrystals, and highlight the difficulties and countermeasures in their synthesis. Then, the progress of theoretical and experimental studies related to the ORR activity and stability of Pt-based intermetallic nanocrystals in recent years are reviewed, especially the integrated strategies for enhancing the stability of ORR. Finally, the challenges faced by Pt-based intermetallic nanocrystals are summarized and future research directions are proposed. In addition, numerous design ideas of Pt-based intermetallic nanocrystals as ORR catalysts are summarized, aiming to promote further development of commercialization of PEMFC catalysts while fully understanding Pt-based intermetallic nanocrystals.

Microstructure and Optical Properties of Nanostructural Thin Films Fabricated through Oxidation of Au-Sn Intermetallic Compounds

AuSn and AuSn₂ thin films (5 nm) were used as precursors during the formation of semiconducting metal oxide nanostructures on a silicon substrate. The nanoparticles were produced in the processes of annealing and oxidation of gold–tin intermetallic compounds under ultra-high vacuum conditions. The formation process and morphology of a mixture of SnO₂ and Au@SnO_x (the core–shell structure) nanoparticles or Au nanocrystalites were carefully examined by means of spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX). The annealing and oxidation of the thin film of the AuSn intermetallic compound led to the formation of uniformly distributed structures with a size of ∼20–30 nm. All of the synthesized nanoparticles exhibited a strong absorption band at 520–530 nm, which is typical for pure metallic or metal oxide systems.

Pt-Co@Pt Octahedral Nanocrystals: Enhancing Their Activity and Durability toward Oxygen Reduction with an Intermetallic Core and an Ultrathin Shell

Despite extensive efforts devoted to the synthesis of Pt-Co bimetallic nanocrystals for fuel cell and related applications, it remains a challenge to simultaneously control atomic arrangements in the bulk and on the surface. Here we report a synthesis of Pt-Co@Pt octahedral nanocrystals that feature an intermetallic, face-centered tetragonal Pt-Co core and an ultrathin Pt shell, together with the dominance of {111} facets on the surface. When evaluated as a catalyst toward the oxygen reduction reaction (ORR), the nanocrystals delivered a mass activity of 2.82 A mg^-1 and a specific activity of 9.16 mA cm^-2, which were enhanced by 13.4 and 29.5 times, respectively, relative to the values of a commercial Pt/C catalyst. More significantly, the mass activity of the nanocrystals only dropped 21% after undergoing 30 000 cycles of accelerated durability test, promising an outstanding catalyst with optimal performance for ORR and related reactions.

Low-PGM and PGM-Free Catalysts for Proton Exchange Membrane Fuel Cells: Stability Challenges and Material Solutions

Fuel cells as an attractive clean energy technology have recently regained popularity in academia, government, and industry. In a mainstream proton exchange membrane (PEM) fuel cell, platinum-group-metal (PGM)-based catalysts account for ≈50% of the projected total cost for large-scale production. To lower the cost, two materials-based strategies have been pursued: 1) to decrease PGM catalyst usage (so-called low-PGM catalysts), and 2) to develop alternative PGM-free catalysts. Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly (MEA)-the power generation unit of a PEM fuel cell-or when PGM-free catalysts are integrated into an MEA. More importantly, there is a significant knowledge gap between materials innovation and device integration. For example, high-performance electrocatalysts usually demonstrate undesired quick degradation in MEAs. This issue significantly limits the development of PEM fuel cells. Herein, recent progress in understanding the degradation of low-PGM and PGM-free catalysts in fuel cell MEAs and materials-based solutions to address these issues are reviewed. The key factors that degrade the MEA performance are highlighted. Innovative, emerging material concepts and development of low-PGM and PGM-free catalysts are discussed.

Synthesis of Structurally Stable and Highly Active PtCo3 Ordered Nanoparticles through an Easily Operated Strategy for Enhanced Oxygen Reduction Reaction

Constructing robust and cost-effective Pt-based electrocatalysts with an easily operated strategy remains a crucial obstacle to fuel cell applications. Conventional Pt-based catalysts suffer from high Pt content and an arduous synthetic process. Herein, through the spray dehydration method and annealing treatment, facile producible synthesis of a small-sized (5.2 nm) low-Pt (10.5 wt %) ordered PtCo₃/C catalyst (O-PtCo₃/C) for oxygen reduction reaction is reported. The fast spray evaporation rate contributes to small size and uniform nucleation of nanoparticles (NPs) on carbon support. O-PtCo₃/C-600 exhibits efficient electrocatalytic performance with mass activity (MA) 6.0-fold and specific activity 3.9-fold higher than commercial Pt/C. The ordered chemical structure generates superior stability with merely 3.5% decay in MA after 10,000 potential cycles. Density functional theory calculations reveal that the enhanced catalytic performance originates from rational modification of d-band through strain and ordering effect and accompanying weaker adsorption of intermediate OH. This work highlights the potentials of low-Pt PtM₃-type ordered NPs for prospective fuel cell cathodic catalysis. The proposed facile and practical synthetic strategy also shows promising prospects for preparing effective Pt-based electrocatalysts.

Strategies to enhance the electrochemical performances of Pt-based intermetallic catalysts

The need for improving the energy conversion efficiency of proton exchange membrane fuel cells (PEMFCs) has motivated the development of advanced electrocatalysts with desirable activity and durability. Pt-Based intermetallic compounds, featuring atomically ordered structures, have long been considered to be very promising alternatives to widely employed Pt and Pt alloy (solid solutions) catalysts. To facilitate the practical application of Pt-based intermetallics in PEMFCs, effective strategies have been developed to further improve their catalytic activity and durability over the last decade. This feature article overviews the recent advances on the strategies for enhancing the electrochemical performances of Pt-based intermetallic catalysts, which include size control, surface engineering, and composition tuning. Thermodynamic and kinetic perspectives on the formation of the intermetallic phases are summarized to better design the synthesis conditions and realize the size control. After this, the size-control approaches (e.g. coating protection, matrix protection) are illustrated and discussed. We highlight the positive effect of surface engineering and discuss the recently developed methods for surface engineering. Finally, we discuss the thermodynamic feasibility of composition tuning and recent work based on composition-tunable intermetallic electrocatalysts.

Direct observation of the formation and stabilization of metallic nanoparticles on carbon supports

Direct formation of ultra-small nanoparticles on carbon supports by rapid high temperature synthesis method offers new opportunities for scalable nanomanufacturing and the synthesis of stable multi-elemental nanoparticles. However, the underlying mechanisms affecting the dispersion and stability of nanoparticles on the supports during high temperature processing remain enigmatic. In this work, we report the observation of metallic nanoparticles formation and stabilization on carbon supports through in situ Joule heating method. We find that the formation of metallic nanoparticles is associated with the simultaneous phase transition of amorphous carbon to a highly defective turbostratic graphite (T-graphite). Molecular dynamic (MD) simulations suggest that the defective T-graphite provide numerous nucleation sites for the nanoparticles to form. Furthermore, the nanoparticles partially intercalate and take root on edge planes, leading to high binding energy on support. This interaction between nanoparticles and T-graphite substrate strengthens the anchoring and provides excellent thermal stability to the nanoparticles. These findings provide mechanistic understanding of rapid high temperature synthesis of metal nanoparticles on carbon supports and the origin of their stability.

Metal nanoparticle-decorated carbon supports are vital for many applications, ranging from energy storage and catalysis to filtration and environmental remedies. Here, using real-time electron microscopy of a single carbon nanofiber during Joule heating, the authors report atomistic mechanisms responsible for nucleation and stabilization of nanoparticles on amorphous carbon supports.

Ultrafast and surfactant-free synthesis of Sub-3 nm nanoalloys by shear-assisted liquid-metal reduction

The uniform and surfactant-free synthesis of ultrafine alloy nanoparticles (NPs) still remains an ongoing challenge. Here, we developed a general route for synthesizing sub-3 nm nanoalloys on a carbon support via a shear-assisted liquid-metal reduction (SA-LMR) strategy. The utilization of shearing treatment resulted in the rapid release of electrons from the liquid sodium-potassium (NaK) alloy, which enabled the ultrafast reduction of precursor metal ions to metallic nanoalloys at room temperature. As a model system, Pt-Cu NPs with uniform distribution were synthesized and characterized by spectroscopic and microscopic techniques. The size, composition and structure of the Pt-Cu NPs could be tuned by controlling the volume of liquid NaK alloy and the reaction time. To prove the universality and utility of our method, the binary Pt-M (Fe, Co, Ni, and Cu) and ternary Pt-Cu-Fe nanoalloys deposited on a carbon support were prepared as hydrogen evolution reaction (HER) catalysts. This facile, rapid and universal synthesis technique has the potential to be employed in the large-scale production of nanomaterials and suggests a new direction for nanoalloys.

Atomically ordered non-precious Co3Ta intermetallic nanoparticles as high-performance catalysts for hydrazine electrooxidation

Nano-ordered intermetallic compounds have generated great interest in fuel cell applications. However, the synthesis of non-preciousearly transition metal intermetallic nanoparticles remains a formidable challenge owing to the extremely oxyphilic nature and very negative reduction potentials. Here, we have successfully synthesized non-precious Co3Ta intermetallic nanoparticles, with uniform size of 5 nm. Atomic structural characterizations and X-ray absorption fine structure measurements confirm the atomically ordered intermetallic structure. As electrocatalysts for the hydrazine oxidation reaction, Co3Ta nanoparticles exhibit an onset potential of -0.086 V (vs. reversible hydrogen electrode) and two times higher specific activity relative to commercial Pt/C (+0.06 V), demonstrating the top-level performance among reported electrocatalysts. The Co-Ta bridge sites are identified as the location of the most active sites thanks to density functional theory calculations. The activation energy of the hydrogen dissociation step decreases significantly upon N2H4 adsorption on the Co-Ta bridge active sites, contributing to the significantly enhanced activity.

Trimetallic Synergy in Intermetallic PtSnBi Nanoplates Boosts Formic Acid Oxidation

Platinum is the most effective metal for a wide range of catalysis reactions, but it fails in the formic acid electrooxidation test and suffers from severe carbon monoxide poisoning. Developing highly active and stable catalysts that are capable of oxidizing HCOOH directly into CO₂ remains challenging for commercialization of direct liquid fuel cells. A new class of PtSnBi intermetallic nanoplates is synthesized to boost formic acid oxidation, which greatly outperforms binary PtSn and PtBi intermetallic, benefiting from the synergism of chosen three metals. In particular, the best catalyst, atomically ordered Pt₄₅ Sn₂₅ Bi₃₀ nanoplates, exhibits an ultrahigh mass activity of 4394 mA mg^-1 Pt and preserves 78% of the initial activity after 4000 potential cycles, which make it a state-of-the-art catalyst toward formic acid oxidation. Density functional theory calculations reveal that the electronic and geometric effects in PtSnBi intermetallic nanoplates help suppress CO* formation and optimize dehydrogenation steps.

Ordered Intermetallic Pd3Bi Prepared by an Electrochemically Induced Phase Transformation for Oxygen Reduction Electrocatalysis

The synthesis of alloys with long-range atomic-scale ordering (ordered intermetallics) is an emerging field of nanochemistry. Ordered intermetallic nanoparticles are useful for a wide variety of applications such as catalysis, superconductors, and magnetic devices. However, the preparation of nanostructured ordered intermetallics is challenging in comparison to disordered alloys, hindering progress in material development. Herein, we report a process for converting colloidally synthesized ordered intermetallic PdBi₂ to ordered intermetallic Pd₃Bi nanoparticles under ambient conditions by electrochemical dealloying. The low melting point of PdBi₂ corresponds to low vacancy formation energies, which enables the facile removal of the Bi from the surface while simultaneously enabling interdiffusion of the constituent atoms via a vacancy diffusion mechanism under ambient conditions. The resulting phase-converted ordered intermetallic Pd₃Bi exhibits 11 times and 3.5 times higher mass activity and high methanol tolerance for the oxygen reduction reaction compared with Pt/C and Pd/C, respectively, which is the highest reported for a Pd-based catalyst, to the best of our knowledge. These results establish a key development in the synthesis of noble-metal-rich ordered intermetallic phases with high catalytic activity and set forth guidelines for the design of ordered intermetallic compounds under ambient conditions.

Rational Development of Structurally Ordered Platinum Ternary Intermetallic Electrocatalysts for Oxygen Reduction Reaction

Structurally ordered intermetallic structure is an efficient catalyst design strategy to significantly improve the catalytic performance of Pt alloy electrocatalysts for oxygen reduction reaction in fuel cells. However, a high structural ordering degree generally relies on high-temperature annealing, which results in detrimental catalyst particle sintering. Herein, we reveal that the incompatibility between high ordering degree and minimum particle sintering during thermal annealing can be resolved through rational development of structurally ordered Pt ternary alloys. Ordering transformation mediated by high-temperature annealing of three representative Pt ternary alloys (Pt–Fe–Co, Pt–Ni–Co and Pt–Fe–Ni) at a similar Pt composition was systematically studied. It was found that Fe can significantly promote the structural ordering due to a faster atomic diffusion, whereas Co can effectively inhibit the particle sintering. As a result of the synergy between Co and Fe, the ordered PtCoFe catalyst exhibited the highest ordering degree after thermal annealing at 600 °C with the minimum nanoparticle growth, leading to the highest catalytic activity (0.65 A/mgPt at 0.9 V, 4 times that of pure Pt catalyst) and best stability (16% drop after 10,000 potential cycles). This study provides important clues for the rational design of high-performance structurally ordered ternary Pt alloys.

Spatially Confined Formation and Transformation of Nanocrystals within Nanometer-Sized Reaction Media

The extensive research performed in the past two decades has enabled the production of a range of colloidal nanocrystals, mostly through solution-based procedures that generate and transform nanostructures in bulk-phase solutions containing precursors and surfactants. However, the understanding and control of each nanocrystal (trans)formation step during the synthesis are still complicated because of the high complexity of this process, in which multiple diverse events such as nucleation, subsequent growth, attachment, and ripening occur simultaneously in bulk suspensions. Unlike well-established solution-based methods, solid-state reactions, which had been at the forefront of traditional inorganic materials chemistry, are quite rarely utilized in the realm of nanomaterials because of the high temperatures required for most solid-state reactions, as a result of which the clusters and NCs are prone to migrate through the bulk reaction medium and sinter together uncontrollably into large particles. We have been pursuing the "nanospace-confined approach" to explore the use of a variety of solid and hollow silica nanoparticles as either solid-state or solution-phase reaction media to carry out the syntheses and transformations of nanocrystals in a unique microenvironment, partitioning the reactants, intermediates, and transition states from the rest of the bulk reaction medium. Such nanoconfined systems have the potential not only to enable efficient and selective nanocrystal conversion chemistries but also to provide fundamental understanding pertaining to the synthetic evolution of nanostructures and transient mechanistic steps. The unique spaces with sizes of a few tens of nanometers inside nanoconfined systems offer the opportunity to observe and elucidate novel deconvoluted chemical phenomena that are impossible to investigate in bulk systems, and comprehensive understanding of nanoconfined chemistry can be implicated in explaining and controlling the macroscopic chemical behaviors. This Account describes our focused research on developing spatially confined platforms for nanocrystal syntheses and transformations, highlighting our diversity-oriented strategy, namely, the "postdecoration approach", which results in the evolution of new nanocatalytic sites in a preformed cavity for diversifying and controlling their morphologies, number, density and combinations. We discuss key examples of the "nanoconfined solid-state conversion approach" that involve novel reactions of nanocrystals within thermally stable solid silica nanospheres to synthesize and transform complex hybrid nanocrystals with increased complexity and functionality. In addition, an enlightening discussion of the examples of nanocrystal syntheses and conversions in nanoconfined solutions inside enclosed and exposed cavities of silica nanospheres is included. Finally, the important applications of nanospace-confined systems in various fields are also briefly discussed.

Highly Durable and Active Pt-Based Nanoscale Design for Fuel-Cell Oxygen-Reduction Electrocatalysts

Fuel cells are one of the promising energy-conversion devices due to their high efficiency and zero emission. Although recent advances in electrocatalysts have been achieved using various material designs such as alloys, core@shell structures, and shape control, many issues still remain to be resolved. Especially, material design issues for high durability and high activity are recently accentuated owing to severe instability of nanoparticles under fuel-cell operating conditions. To address these issues, fundamental understanding of functional links between activity and durability is timely urgent. Here, the activity and durability of nanoscale materials are summarized, focusing on the nanoparticle size effect. In addition to phenomenological observation, two major degradation origins, including atomic dissolution and particle size increase, are discussed related to the activity decrease. Based on the fundamental understanding of nanoparticle degradation, recent promising strategies for durable Pt-based nanoscale electrocatalysts are introduced and the role of each design for durability enhancement is discussed. Finally, short comments related to the future direction of nanoparticle issues are provided in terms of nanoparticle synthesis and analysis.