Promoting the ORR catalysis of Pt-Fe intermetallic catalysts by increasing atomic utilization and electronic regulation

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.

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.

MOF-Derived Co Nanoparticles Catalyst Assisted by F- and N-Doped Carbon Quantum Dots for Oxygen Reduction

The oxygen reduction reaction is crucial in the cathode of fuel cells and metal-air batteries. Consequently, designing robust and durable ORR catalysts is vital to developing metal-air batteries and fuel cells. Metal-organic frameworks feature an adjustable structure, a periodic porosity, and a large specific surface area, endowing their derivative materials with a unique structure. In this study, F and N co-doped on the carbon support surface (Co/FN-C) via the pyrolysis of ZIF-67 as a sacrificial template while using Co/FN-C as the non-noble metal catalysts. The Co/FN-C displays excellent long-term durability and electrochemical catalytic performance in acidic solutions. These performance improvements are achieved because the CQDs alleviate the structural collapse during the pyrolysis of ZIF-67, which increases the active sites in the Co nanoparticles. Moreover, F- and N-doping improves the catalytic activity of the carbon support by providing additional electrons and active sites. Furthermore, F anions are redox-stable ligands that exhibit long-term operational stability. Therefore, the well-dispersed Co NPs on the surface of the Co/FN-C are promising as the non-noble metal catalysts for ORR.

Recent Advances of Electrocatalyst and Cell Design for Hydrogen Peroxide Production

Electrochemical synthesis of H2O2 via a selective two-electron oxygen reduction reaction has emerged as an attractive alternative to the current energy-consuming anthraquinone process. Herein, the progress on electrocatalysts for H2O2 generation, including noble metal, transition metal-based, and carbon-based materials, is summarized. At first, the design strategies employed to obtain electrocatalysts with high electroactivity and high selectivity are highlighted. Then, the critical roles of the geometry of the electrodes and the type of reactor in striking a balance to boost the H2O2 selectivity and reaction rate are systematically discussed. After that, a potential strategy to combine the complementary properties of the catalysts and the reactor for optimal selectivity and overall yield is illustrated. Finally, the remaining challenges and promising opportunities for high-efficient H2O2 electrochemical production are highlighted for future studies.

Defect-Induced Atomic Arrangement in CoFe Bimetallic Heterostructures with Boosted Oxygen Evolution Activity

Three CoFe-bimetallic oxides with different compositions (termed as CoFeO_x -A/N/H) are prepared by thermally treating metal-organic-framework (MOF) precursors under different atmospheres (air, N_2, and NaBH₄ /N₂ ), respectively. With the aid of vast oxygen vacancies (O_v ), cobalt at tetrahedral sites (Co²⁺ (Th)) in spinel Co₃ O₄ is diffused into interstitial octahedral sites (Oh) to form rocksalt CoO and ternary oxide CoFe₂ O₄ has been induced to give the unique defective CoO/CoFe₂ O₄ heterostructure. The resultant CoFeO_x -H exhibits superb electrocatalytic activity toward water oxidation: overpotential at 10 mA cm^-2 is 192 mV, which is 122 mV smaller than that of CoFeO_x -A. The smaller Tafel slope (42.53 mV dec^-1 ) and higher turnover frequency (785.5 h^-1 ) suggest fast reaction kinetics. X-ray absorption spectroscopy, ex situ characterizations, and theoretical calculations reveal that defect engineering effectively tunes the electronic configuration to a more active state, resulting in the greatly decreased binding energy of oxo intermediates, and consequently much lower catalytic overpotential. Moreover, the construction of hetero-interface in CoFeO_x -H can provide rich active sites and promote efficient electron transfer. This work may shed light on a comprehensive understanding of the modulation of electron configuration of bimetallic oxides and inspire the smart design of high-performance electrocatalysts.

Status Quo on Graphene Electrode Catalysts for Improved Oxygen Reduction and Evolution Reactions in Li-Air Batteries

Reduced global warming is the goal of carbon neutrality. Therefore, batteries are considered to be the best alternatives to current fossil fuels and an icon of the emerging energy industry. Voltaic cells are one of the power sources more frequently employed than photovoltaic cells in vehicles, consumer electronics, energy storage systems, and medical equipment. The most adaptable voltaic cells are lithium-ion batteries, which have the potential to meet the eagerly anticipated demands of the power sector. Working to increase their power generating and storage capability is therefore a challenging area of scientific focus. Apart from typical Li-ion batteries, Li-Air (Li-O₂) batteries are expected to produce high theoretical power densities (3505 W h kg^-1), which are ten times greater than that of Li-ion batteries (387 W h kg^-1). On the other hand, there are many challenges to reaching their maximum power capacity. Due to the oxygen reduction reaction (ORR) and oxygen evolution reaction (OES), the cathode usually faces many problems. Designing robust structured catalytic electrode materials and optimizing the electrolytes to improve their ability is highly challenging. Graphene is a 2D material with a stable hexagonal carbon network with high surface area, electrical, thermal conductivity, and flexibility with excellent chemical stability that could be a robust electrode material for Li-O₂ batteries. In this review, we covered graphene-based Li-O₂ batteries along with their existing problems and updated advantages, with conclusions and future perspectives.

Self-Strained Platinum Clusters with Finite Size: High-Performance Catalysts with CO Tolerance for PEMFCs

Strained platinum-based materials with high performance have been regarded as the most promising electrocatalysts for proton exchange membrane fuel cells (PEMFCs) recently. Herein, self-strained platinum clusters with finite size (about 1 nm) are prepared by a combining liquid- and solid-phase UV irradiation cycle strategy. It started with a fresh H₂PtCl₆ solution irradiated by UV light and then mixed with a graphitized carbon, followed by the dried mixture being subjected to UV light to generate monodispersed Pt clusters on the carbon surface. The obtained platinum clusters feature narrower size distribution and higher loading on carbon, exhibiting significantly improved activity and durability, much higher than that of the-state-of-art commercial Pt/C for the oxygen reduction reaction. More importantly, the self-strained Pt clusters display a surprising CO tolerance, which can be attributed to the unique adaptive lattice compressive strain that triggers an electron enrichment phenomenon for the Pt clusters. Therefore, this stepwise UV irradiation method solves the long-standing problem of both wide size distribution and low loading of metal clusters fabricated by one-step photochemical reduction, providing a potential route for the synthesis of other metal clusters with strained structures.

Research Progress in ZIF-8 Derived Single Atomic Catalysts for Oxygen Reduction Reaction

Transition metal (TM) single atomic catalysts (MSAC-N-C) derived from doped zeolite imidazolate frameworks (ZIF-8) are considered attractive oxygen reduction reaction (ORR) catalysts for fuel cells and metal-air batteries due to their advantages of high specific surface area, more active catalytic sites, adjustable pore size, and coordination topology features. This review provides an updated overview of the latest advances of MSAC-N-C catalysts derived from ZIF-8 precursors in ORR electrocatalysis. Particularly, some key challenges, including coordination environments regulation of catalysis center in MSAC-N-C, the active sites loading optimization and synergistic effects between TM nanoclusters/nanoparticles and the single atoms on MSAC-N-C catalysis activity, as well as their adaptability in various devices, are summarized for improving future development and application of MSAC-N-C catalysts. In addition, this review puts forward future research directions, making it play a better role in ORR catalysis for fuel cells and metal air batteries.

Co/Co-N/Co-O Rooted on rGO Hybrid BCN Nanotube Arrays as Efficient Oxygen Electrocatalyst for Zn-Air Batteries

Developing high-performance non-noble metal bifunctional oxygen reduction and evolution reaction electrocatalysts is a critical factor for the commercialization of rechargeable Zn-air batteries. Herein, Co/Co-N/Co-O rooted on reduced graphene oxide (rGO) hybrid boron and nitrogen codoped carbon (BCN) nanotube arrays (BCN/rGO-Co) is prepared by facile low-temperature precross-linking and high-temperature pyrolysis treatment. Benefit from the synergistic effect of its B/N codoping, Co/Co-N/Co-O bifunctional active sites, 3D hybrid porous structure of BCN nanotubes, and highly conductive rGO sheets. The obtained BCN/rGO-Co exhibits superior bifunctional oxygen catalytic activity with a positive ORR half-wave potential (0.85 V) and a low OER potential (1.61 V) at 10 mA cm^-2. Additionally, the BCN/rGO-Co-based liquid Zn-air batteries displays a large peak power density of 157 mW cm^-2, and a long charge/discharge cycle stability of 200 h, outdoing the commercial Pt/C+Ru/C catalyst.

An Investigation into the Bulk and Surface Phase Transformations of Bimetallic Pd-In/Al2O3 Catalyst during Reductive and Oxidative Treatments In Situ

A series of oxidative treatments of PdIn-supported intermetallic nanoparticles at different temperatures were performed. The bulk and surface structure of catalyst during phase transformation was investigated by bulk- and surface-sensitive techniques (in situ XAFS, DRIFTS of adsorbed CO). It was found that comparison of palladium and indium fractions in bulk and on the surface suggests the formation of a «core-shell» structure. According to obtained results, the core consists of In-depleted intermetallic compound or inhomogeneous bimetallic phase with the inner core of metallic Pd, when a mixture of indium oxide, metallic palladium and small part of PdIn is present on the surface.

Rare-Earth Incorporated Alloy Catalysts: Synthesis, Properties, and Applications

To improve the performance of metallic catalysts, alloying provides an efficient methodology to design state-of-the-art materials. As emerging functional materials, rare-earth metal compounds can integrate the unique orbital structure and catalytic behavior of rare earth elements into metallic materials. Such rare-earth containing alloy catalysts proffer an opportunity to tailor electronic properties, tune charged carrier transport, and synergize surface reactivity, which are expected to significantly improve the performance and stability of catalysis. Despite its significance, there are only few reviews on rare earth containing alloys or related topics. This review summarizes the composition, synthesis, and applications of rare earth containing alloys in the field of catalysis. Subsequent to comprehensively summarizing and constructively discussing the existing work, the challenges and possibilities of future research on rare-earth metal compound materials are evaluated.