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

Structure evolution and specific effects for the catalysis of atomically ordered intermetallic compounds

Atomically ordered intermetallic compounds (IMCs) have been extensively studied for exploring catalysts with high activity, selectivity, and longevity. Compared to random alloys, IMCs present a more pronounced geometric and electronic effect with desirable catalytic performance. Their well-defined structure makes IMCs ideal model catalysts for studying the catalytic mechanism. This review focuses especially on elemental composition, electron transfer, and structure/phase evolution under high temperature treatment conditions, providing direct evidence for the migration and rearrangement of metal atoms through electron microscopy. We then present the outstanding applications of IMCs in growing single-walled nanotubes, hydrogenation/dehydrogenation reactions, and electrocatalysis from the perspective of electronic, geometric, strain, and bifunctional effects of ordered IMCs. Finally, the current obstacles associated with the use of in situ techniques are proposed, as well as future research possibilities.

Enhancing Surface Strain of Intermetallic Fuel Cell Catalysts by Composition-Induced Phase Transition

The lattice parameter of platinum-based intermetallic compounds (IMCs), which correlates with the intrinsic activity of the oxygen reduction reaction (ORR), can be modulated by crystal phase engineering. However, the controlled preparation of IMCs with unconventional crystal structures remains highly challenging. Here, we demonstrate the synthesis of carbon-supported PtCu-based IMC catalysts with an unconventional L10 structure by a composition-regulated strategy. Experiment and machine learning reveal that the thermodynamically favorable structure changes from L11 to L10 when slight Cu atoms are substituted with Co. Benefiting from crystal-phase-induced strain enhancement, the prepared L10-type PtCu0.8Co0.2 catalyst exhibits much-enhanced mass and specific activities of 1.82 A mgPt-1 and 3.27 mA cmPt-2, which are 1.91 and 1.73 times higher than those of the L11-type PtCu catalyst, respectively. Our work highlights the important role of crystal phase in determining the surface strain of IMCs, and opens a promising avenue for the rational preparation of IMCs with different crystal phases by doping.

Atom-by-atom optimizing the surface termination of Fe-Pt intermetallic catalysts for alkaline hydrogen evolution reaction

Controlling the atomic arrangement of elemental atoms in intermetallic catalysts to govern their surface and subsurface properties is a crucial but challenging endeavor in electrocatalytic reactions. In hydrogen evolution reaction (HER), adjusting the d-band center of the conventional noble-metallic Pt by introducing Fe enables the optimization of catalytic performance. However, a notable gap exists in research on the effective transition from disordered Fe/Pt alloys to highly ordered intermetallic compounds (IMCs) such as FePt3 in the alkaline HER, hampering their broader application. In this study, a series of catalysts FePt3-xH (x = 5, 6, 7, 8 and 9) supported on carbon nanotubes (CNTs) were synthesized via a simple impregnation method, along with a range of heat treatment processes, including annealing in a reductive atmosphere, to regulate the order degree of the arrangement of Fe/Pt atoms within the FePt3 catalyst. By using advanced microscopy and spectroscopy techniques, we systematically explored the impact of the order degree of FePt3 in the HER. The as-prepared FePt3-8H exhibited notable HER catalytic activity with low overpotentials (η = 37 mV in 1.0 mol L-1 KOH) at j = 10 mA cm-2. The surface of the L12 FePt3-8H catalyst was demonstrated to be Pt-rich. The Pt on the surface was not easily oxidized due to the unique Fe/Pt coordination, resulting in significant enhancement of HER performance.

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.

Simultaneously Geometrical and Electronic Modulation of L10-PtZn by Trace Ge Boosts High-performance Oxygen Reduction Reaction

Developing a highly active, durable, and low-platinum-based electrocatalyst for the cathodic oxygen reduction reaction (ORR) is for breaking the bottleneck of large-scale applications of proton exchange membrane fuel cells (PEMFCs). Herein, ultrafine PtZn intermetallic nanoparticles with low Pt-loading and trace germanium (Ge) involvement confined in the nitrogen-doped porous carbon (Ge-L10-PtZn@N-C) are reported. The Ge-L10-PtZn@N-C exhibit superior ORR activity with a mass activity of 3.04 A mg-1 Pt and specific activity of 4.69 mA cm-2, ≈12.2- and 10.2-times improvement compared to the commercial Pt/C (20%) at 0.90 V in 0.1 m KOH. The cathodic catalyst Ge-L10-PtZn@N-C assembled in the PEMFC shows encouraging peak power densities of 316.5 (at 0.86 V) and 417.2 mW cm-2 (at 0.91 V) in alkaline and acidic fuel-cell, respectively. The combination of experiment and density functional theory calculations (DFT) results robustly reveal that the participation of trace Ge can not only trigger a "growth site locking effect" to effectively inhibit nanoparticle growth, bring miniature nanoparticles, enhance dispersion uniformity, and achieve the exposure of the more electrochemical active site, but also effectively modulates the electronic structure, hence optimizing the adsorption/desorption of the oxygen intermediates.

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.

Manipulating Pt-skin of porous network Pt-Cu alloy nanospheres toward efficient oxygen reduction

Designing Pt-skin on the catalyst surface is critical to developing efficient and stable electrocatalysts toward oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. In this paper, an acidic reductant is proposed to synchronously manipulate in-situ growth of Pt-skin on the surface of alloyed Pt-Cu nanospheres (PtCuNSs) by a facile one-pot synthesis in an aqueous solution. Ascorbic acid can create a Pt-skin of three atomic layers to make the typical PtCu-alloy@Pt-skin core/shell nanostructure rather than the uniform alloys generated by using alkaline reductants. Surfactant as soft-template can make the alloyed PtCuNSs with a three-dimensional porous network structure. Multiple characterizations of XRD, XPS and XAFS are used to confirm Pt-alloying with Cu and formation of core/shell structure of such a catalyst. This PtCuNSs/C exhibits a half-wave potential of 0.913 V (vs. RHE), with mass activity and specific activity about 3.5 and 6.4 times higher than those of Pt/C, respectively. Fuel cell tests verify the excellent activity of PtCuNSs/C catalyst with a maximum power density of about 1.2 W cm-2. Moreover, this catalyst shows excellent stability, achieving a long-term operation of 40,000 cycles. Furthermore, theoretical calculations reveal the enhancement effect of characteristic PtCu-alloy@Pt-skin nanostructure on both catalytic ORR activity and stability.

Mo-doped PdCu nanoparticles as high-performance catalysts for oxygen reduction reactions

The instability of palladium-based binary alloys hinders their wide application in the oxygen reduction processes. Here, we prepared Mo-doped PdCu nanoparticles with controllable dopant content and valence. Further research has revealed that Mo, particularly Mo5+, may effectively suppress the oxidation of Pd and Cu, optimize the oxygen binding of Pd, and increase catalytic activity and stability. In particular, Mo-PdCu-1/C with the highest Mo5+ content shows the best oxygen reduction reaction (ORR) mass activity (1.20 A mg-1Pd), which is 4.8 times higher than that of PdCu/C. It also exhibits outstanding stability, retaining 80.8% of the original mass activity after 20 000 cycles. This study clearly explains the mechanism by which Mo doping affects the performance and provides a reference for further optimization of catalyst performance for fuel cell industrialization.

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.

Promoting ordering degree of intermetallic fuel cell catalysts by low-melting-point metal doping

Carbon supported intermetallic compound nanoparticles with high activity and stability are promising cathodic catalysts for oxygen reduction reaction in proton-exchange-membrane fuel cells. However, the synthesis of intermetallic catalysts suffers from large diffusion barrier for atom ordering, resulting in low ordering degree and limited performance. We demonstrate a low-melting-point metal doping strategy for the synthesis of highly ordered L10-type M-doped PtCo (M = Ga, Pb, Sb, Cu) intermetallic catalysts. We find that the ordering degree of the M-doped PtCo catalysts increases with the decrease of melting point of M. Theoretic studies reveal that the low-melting-point metal doping can decrease the energy barrier for atom diffusion. The prepared highly ordered Ga-doped PtCo catalyst exhibits a large mass activity of 1.07 A mgPt-1 at 0.9 V in H2-O2 fuel cells and a rated power density of 1.05 W cm-2 in H2-air fuel cells, with a Pt loading of 0.075 mgPt cm-2.

Ultrathin Ultralow-Platinum Catalyst Coated Membrane for Proton Exchange Membrane Fuel Cells

Catalyst coated membrane (CCM) is the core component of proton exchange membrane fuel cells and is routinely fabricated by spraying Pt/C slurries onto membrane, resulting in low activity and thick catalyst layer (CL, 5-10 µm) with an unaffordable Pt loading of 0.2-0.4 mg cm^-2 and a large mass transfer resistance at cathode. Highly active ultrathin ultralow-Pt CL (UUCL) is urgently required, but remains rare. Herein, wet-chemical direct growth of UUCLs on both sides of membrane to achieve integrated ultrathin ultralow-Pt catalyst coated membranes (UUCCMs) with a cathodic CL thickness of 79.7 ± 15.0 nm and a Pt loading of 20.2 ± 1.6 µg cm^-2 is reported. The key to this unique fabrication is the release of proton from membrane to regioselectively initiate the growth of interconnected Pd nanoneedle clusters array on membrane, followed by high-density deposition of Pt nanoparticles on Pd (Pt/Pd UUCLs). The single cell of UUCCMs exhibits the highest mass peak power density of 59.9 W mg_Pt,Cathode ^-1 in the literature. The exceptional activity originates from high electrochemically active surface area, remarkable oxygen reduction reaction activity closely correlated with strain, and electronic effect at Pt/Pd interface, as well as improved mass transfer and optimal water management.

A facile strategy to prepare FeNx decorated PtFe intermetallic with excellent acidic oxygen reduction reaction activity and stability

The construction of low-Pt-content intermetallic on carbon supports has been verified as a promising method to promote the activity of the oxygen reduction reaction (ORR). In this study, we have developed a simple and effective strategy to obtain a well-designed CNT-PtFe-PPy precursor. This precursor contains modulated Pt- and Fe-based content dispersed in polypyrrole (PPy) chain segments, which are in-situ generated on the templates of carbon nanotubes (CNTs). Subsequent pyrolysis of the CNT-PtFe-PPy precursor produces a CNT-PtFe@FeNC catalyst, which contains both Fe-N_x and PtFe intermetallic active sites. Due to the highly efficient dispersion of active species, the CNT-PtFe@FeNC electrocatalyst displays a 9.5 times higher specific activity (SA) and 8.5 times higher mass activity (MA) than those of a commercial Pt/C catalyst in a 0.1 M HClO₄ solution. Additionally, these results, combined with excellent durability (the SA and MA maintained 94 % and 91 % of initial activity after a 10-k cycle accelerated durability test), represent among the best performance achieved so far for Pt-based ORR electrocatalysts. Furthermore, density functional theory (DFT) calculations revealed that the presence of Fe-N₄ species reduces the adsorption energy between the PtFe intermetallic compound and OH*, accelerating the ORR process.

Highly Stable Pt-Based Oxygen Reduction Electrocatalysts toward Practical Fuel Cells: Progress and Perspectives

The high cost and poor reliability of cathodic electrocatalysts for the oxygen reduction reaction (ORR), which requires significant amounts of expensive and scarce platinum, obstructs the broad applications of proton exchange membrane fuel cells (PEMFCs). The principles of ORR and the reasons for the poor stability of Pt-based catalysts are reviewed. Moreover, this paper discusses and categorizes the strategies for enhancing the stability of Pt-based catalysts in fuel cells. More importantly, it highlights the recent progress of Pt-based stability toward ORR, including surface-doping, intermetallic structures, 1D/2D structures, rational design of support, etc. Finally, for atomic-level in-depth information on ORR catalysts in fuel cells, potential perspectives are suggested, such as large-scale preparation, advanced interpretation techniques, and advanced simulation. This review aims to provide valuable insights into the fundamental science and technical engineering for practical Pt-based ORR electrocatalysts in fuel cells.

Design strategies of Pt-based electrocatalysts and tolerance strategies in fuel cells: a review

As highly efficient conversion devices, proton-exchange-membrane fuel cells (PEMFCs) can directly convert chemical energy to electrical energy with high efficiencies and lower or even zero emissions compared to combustion engines. However, the practical applications of PEMFCs have been seriously hindered by the intermediates (especially CO) poisoning of anodic Pt catalysts. Hence, how to improve the CO tolerance of the needed Pt catalysts and reveal their anti-CO poisoning mechanism are the key points to developing novel anti-toxic Pt-based electrocatalysts. To date, two main strategies have received increasing attention in improving the CO tolerance of Pt-based electrocatalysts, including alloying Pt with a second element and fabricating composites with geometry and interface engineering. Herein, we will first discuss the latest developments of Pt-based alloys and their anti-CO poisoning mechanism. Subsequently, a detailed description of Pt-based composites with enhanced CO tolerance by utilizing the synergistic effect between Pt and carriers is introduced. Finally, a brief perspective and new insights on the design of Pt-based electrocatalysts to inhibit CO poisoning in PEMFCs are also presented.