Synthesis of High-Entropy-Alloy Nanoparticles by a Step-Alloying Strategy as a Superior Multifunctional Electrocatalyst

High-entropy alloy electrocatalysts go to (sub-)nanoscale

Alloying has proven power to upgrade metallic electrocatalysts, while the traditional alloys encounter limitation for optimizing electronic structures of surface metallic sites in a continuous manner. High-entropy alloys (HEAs) overcome this limitation by manageably tuning the adsorption/desorption energies of reaction intermediates. Recently, the marriage of nanotechnology and HEAs has made considerable progresses for renewable energy technologies, showing two important trends of size diminishment and multidimensionality. This review is dedicated to summarizing recent advances of HEAs that are rationally designed for energy electrocatalysis. We first explain the advantages of HEAs as electrocatalysts from three aspects: high entropy, nanometer, and multidimension. Then, several structural regulation methods are proposed to promote the electrocatalysis of HEAs, involving the thermodynamically nonequilibrium synthesis, regulating the (sub-)nanosize and anisotropic morphologies, as well as engineering the atomic ordering. The general relationship between the electronic structures and electrocatalytic properties of HEAs is further discussed. Finally, we outline remaining challenges of this field, aiming to inspire more sophisticated HEA-based nanocatalysts.

Nanostructured High Entropy Alloys as Structural and Functional Materials

Since their introduction in 2004, high entropy alloys (HEAs) have attracted significant attention due to their exceptional mechanical and functional properties. Advances in our understanding of atomic-scale ordering and phase formation in HEAs have facilitated the development of fabrication techniques for synthesizing nanostructured HEAs. These materials hold immense potential for applications in various fields including automobile industries, aerospace engineering, microelectronics, and clean energy, where they serve as either structural or functional materials. In this comprehensive Review, we conduct an in-depth analysis of the mechanical and functional properties of nanostructured HEAs, with a particular emphasis on the roles of different nanostructures in modulating these properties. To begin, we explore the intrinsic and extrinsic factors that influence the formation and stability of nanostructures in HEAs. Subsequently, we delve into an examination of the mechanical and electrocatalytic properties exhibited by bulk or three-dimensional (3D) nanostructured HEAs, as well as nanosized HEAs in the form of zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanowires, or two-dimensional (2D) nanosheets. Finally, we present an outlook on the current research landscape, highlighting the challenges and opportunities associated with nanostructure design and the understanding of structure-property relationships in nanostructured HEAs.

Dendritic boron and nitrogen doped high-entropy alloy porous carbon fibers for high-efficiency hydrogen evolution reaction

Among various electrocatalysts, high-entropy alloys (HEAs) have gained significant attention for their unique properties and excellent catalytic activity in the hydrogen evolution reaction (HER). However, the precise synthesis of HEA catalysts in small sizes remains challenging, which limits further improvement in their catalytic performance. In this study, boron- and nitrogen-doped HEA porous carbon nanofibers (HE-BN/PCNF) with an in situ-grown dendritic structure were successfully prepared, inspired by the germination and growth of tree branches. Furthermore, the dendritic fibers constrained the growth of HEA particles, leading to the synthesis of quantum dot-sized (1.67 nm) HEA particles, which also provide a pathway for designing HEA quantum dots in the future. This work provides design ideas and guiding suggestions for the preparation of borated HEA fibers with different elemental combinations and for the application of dendritic nanofibers in various fields.

Controllable synthesis of high-entropy alloys

High-entropy alloys (HEAs) involving more than four elements, as emerging alloys, have brought about a paradigm shift in material design. The unprecedented compositional diversities and structural complexities of HEAs endow multidimensional exploration space and great potential for practical benefits, as well as a formidable challenge for synthesis. To further optimize performance and promote advanced applications, it is essential to synthesize HEAs with desired characteristics to satisfy the requirements in the application scenarios. The properties of HEAs are highly related to their chemical compositions, microstructure, and morphology. In this review, a comprehensive overview of the controllable synthesis of HEAs is provided, ranging from composition design to morphology control, structure construction, and surface/interface engineering. The fundamental parameters and advanced characterization related to HEAs are introduced. We also propose several critical directions for future development. This review can provide insight and an in-depth understanding of HEAs, accelerating the synthesis of the desired HEAs.

The Micron-Droplet-Confined Continuous-Flow Synthesis of Freestanding High-Entropy-Alloy Nanoparticles by Flame Spray Pyrolysis

Alloying multiple immiscible elements into a nanoparticle with single-phase solid solution structure (high-entropy-alloy nanoparticles, HEA-NPs) merits great potential. To date, various kinds of synthesis techniques of HEA-NPs are developed; however, a continuous-flow synthesis of freestanding HEA-NPs remains a challenge. Here a micron-droplet-confined strategy by flame spray pyrolysis (FSP) to achieve the continuous-flow synthesis of freestanding HEA-NPs, is proposed. The continuous precursor solution undergoes gas shearing and micro-explosion to form nano droplets which act as the micron-droplet-confined reactors. The ultrafast evolution (<5 ms) from droplets to <10 nm nanoparticles of binary to septenary alloys is achieved through thermodynamic and kinetic control (high temperature and ultrafast colling). Among them, the AuPtPdRuIr HEA-NPs exhibit excellent electrocatalytic performance for alkaline hydrogen evolution reaction with 23 mV overpotential to achieve 10 mA cm-2, which is twofold better than that of the commercial Pt/C. It is anticipated that the continuous-flow synthesis by FSP can introduce a new way for the continuous synthesis of freestanding HEA-NP with a high productivity rate.

FeNiCo|MnGaOx Heterostructure Nanoparticles as Bifunctional Electrocatalyst for Zn-Air Batteries

Driven by the pressing demand for stable energy systems, zinc-air batteries (ZABs) have emerged as crucial energy storage solutions. However, the quest for cost-effective catalysts to enhance vital oxygen evolution and reduction reactions remains challenging. FeNiCo|MnGaOx heterostructure nanoparticles on carbon nanotubes (CNTs) are synthesized using liquid-phase reduction and H2 calcination approach. Compared to its component, such FeNiCo|MnGaOx/CNT shows a high synergistic effect, low impedance, and modulated electronic structure, leading to a superior bifunctional catalytic performance with an overpotential of 255 mV at 10 mA cm-2 and half-wave potential of 0.824 V (ω = 1600 rpm and 0.1 m KOH electrolyte). Moreover, ZABs based on FeNiCo|MnGaOx/CNT demonstrate notable features, including a peak power density of 136.1 mW cm-2, a high specific capacity of 808.3 mAh gZn -1, and outstanding stability throughout >158 h of uninterrupted charge-discharge cycling. Theoretical calculations reveal that the non-homogeneous interface can introduce more carriers and altered electronic structures to refine intermediate adsorption reactions, especially promoting O* formation, thereby enhancing electrocatalytic performance. This work demonstrates the importance of heterostructure interfacial modulation of electronic structure and enhancement of adsorption capacity in promoting the implementation of OER/ORR, ZABs, and related applications.

FeCoCuMnRuB Nanobox with Dual Driving of High-Entropy and Electron-Trap Effects as the Efficient Electrocatalyst for Water Oxidation

High-entropy borides hold potential as electrocatalysts for water oxidation. However, the synthesis of the tailored nanostructures remains a challenge due to the thermodynamic immiscibility of polymetallic components. Herein, a FeCoCuMnRuB nanobox decorated with a nanosheet array was synthesized for the first time by a "coordination-etch-reduction" method. The FeCoCuMnRuB nanobox has various structural characteristics to express the catalytic performance; meanwhile, it combines the high-entropy effect of multiple components with the electron trap effect induced by electron-deficient B, synergistically regulating its electronic structure. As a result, FeCoCuMnRuB nanobox exhibits enhanced OER activity with a low overpotential (η10 = 233 mV), high TOF value (0.0539 s-1), small Tafel slope (61 mV/dec), and a satisfactory stability for 200 h, outperforming the high-entropy alloy and low-entropy borides. This work develops a high entropy and electron-deficient B-driven strategy for motivating the catalytic performance of water oxidation, which broadens the structural diversity and category of high-entropy materials.

Enhancing oxygen evolution reactions in nanoporous high-entropy catalysts using boron and phosphorus additives

High-entropy alloy (HEA) catalysts are a novel area of research in catalysis that shows great potential for more efficient catalyst development. Recent studies have highlighted the promise of HEA catalysts in applications such as water-splitting electrodes, owing to their better stability and ability to improve catalytic activity compared to traditional catalysts. Dealloying, which is a process that removes elements from metallic alloys, is a popular method for creating nanoporous HEA catalysts with large surface areas and interconnected structures. This study focused on the fabrication of nanoporous HEA catalysts with boron and phosphorus additives for the oxygen evolution reaction (OER) in water splitting. Combining B or P with noble metals such as Ir or Ru enhances the OER activity and durability, showing synergistic interactions between metals and light elements. This study used electrochemical evaluations to determine the best-performing catalyst, identifying CoCuFeMoNiIrB as the best catalyst for OERs in alkaline media. X-ray photoemission spectroscopy (XPS) analysis revealed that B effectively shifted the transition elements to higher valence states and induced excess electrons on the Ir-B surface to promote OER catalysis.

Unlocking the Potential of High Entropy Alloys in Electrochemical Water Splitting: A Review

The global pursuit of sustainable energy is focused on producing hydrogen through electrocatalysis driven by renewable energy. Recently, High entropy alloys (HEAs) have taken the spotlight in electrolysis due to their intriguing cocktail effect, broad design space, customizable electronic structure, and entropy stabilization effect. The tunability and complexity of HEAs allow a diverse range of active sites, optimizing adsorption strength and activity for electrochemical water splitting. This review comprehensively covers contemporary advancements in synthesis technique, design framework, and physio-chemical evaluation approaches for HEA-based electrocatalysts. Additionally, it explores design principles and strategies aimed at optimizing the catalytic activity, stability, and effectiveness of HEAs in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. Through an in-depth investigation of these aspects, the complexity inherent in constituent element interactions, reaction processes, and active sites associated with HEAs is aimed to unravel. Eventually, an outlook regarding challenges and impending difficulties and an outline of the future direction of HEA in electrocatalysis is provided. The thorough knowledge offered in this review will assist in formulating and designing catalysts based on HEAs for the next generation of electrochemistry-related applications.

Highly Dispersed Ultrasmall High-Entropy Alloys Nanoparticles as Efficient Electrocatalysts for Oxygen Reduction in Acidic Media

High-entropy alloys nanoparticles (HEAs NPs) have gained considerable attention due to their extensive compositional tunability and intriguing catalytic properties. However, the synthesis of highly dispersed ultrasmall HEAs NPs remains a formidable challenge due to their inherent thermodynamic instability. In this study, highly dispersed ultrasmall (ca. 2 nm) PtCuGaFeCo HEAs NPs are synthesized using a one-pot solution-based method at 160 °C and atmospheric pressure. The PtCuGaFeCo NPs exhibit good catalytic activity for the oxygen reduction reaction (ORR). The half-wave potential relative to the reversible hydrogen electrode (RHE) reaches 0.88 V, and the mass activity and specific activity are approximately six times and four times higher than that of the commercial Pt/C catalyst. Based on X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses, the surface strain and optimized coordination environments of PtCuGaFeCo have led to high ORR activities in acidic media. Moreover, the ultrasmall size also plays an important role in enhancing catalytic performance. The work presents a facile and viable synthesis strategy for preparing the ultrasmall HEAs NPs, offering great potential in energy and electrocatalysis applications through entropy engineering.

High-entropy alloys in electrocatalysis: from fundamentals to applications

High-entropy alloys (HEAs) comprising five or more elements in near-equiatomic proportions have attracted ever increasing attention for their distinctive properties, such as exceptional strength, corrosion resistance, high hardness, and excellent ductility. The presence of multiple adjacent elements in HEAs provides unique opportunities for novel and adaptable active sites. By carefully selecting the element configuration and composition, these active sites can be optimized for specific purposes. Recently, HEAs have been shown to exhibit remarkable performance in electrocatalytic reactions. Further activity improvement of HEAs is necessary to determine their active sites, investigate the interactions between constituent elements, and understand the reaction mechanisms. Accordingly, a comprehensive review is imperative to capture the advancements in this burgeoning field. In this review, we provide a detailed account of the recent advances in synthetic methods, design principles, and characterization technologies for HEA-based electrocatalysts. Moreover, we discuss the diverse applications of HEAs in electrocatalytic energy conversion reactions, including the hydrogen evolution reaction, hydrogen oxidation reaction, oxygen reduction reaction, oxygen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and alcohol oxidation reaction. By comprehensively covering these topics, we aim to elucidate the intricacies of active sites, constituent element interactions, and reaction mechanisms associated with HEAs. Finally, we underscore the imminent challenges and emphasize the significance of both experimental and theoretical perspectives, as well as the potential applications of HEAs in catalysis. We anticipate that this review will encourage further exploration and development of HEAs in electrochemistry-related applications.

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.