Design of Complex Solid-Solution Electrocatalysts by Correlating Configuration, Adsorption Energy Distribution Patterns, and Activity Curves

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.

Discovering High Entropy Alloy Electrocatalysts in Vast Composition Spaces with Multiobjective Optimization

High entropy alloys (HEAs) are a highly promising class of materials for electrocatalysis as their unique active site distributions break the scaling relations that limit the activity of conventional transition metal catalysts. Existing Bayesian optimization (BO)-based virtual screening approaches focus on catalytic activity as the sole objective and correspondingly tend to identify promising materials that are unlikely to be entropically stabilized. Here, we overcome this limitation with a multiobjective BO framework for HEAs that simultaneously targets activity, cost-effectiveness, and entropic stabilization. With diversity-guided batch selection further boosting its data efficiency, the framework readily identifies numerous promising candidates for the oxygen reduction reaction that strike the balance between all three objectives in hitherto unchartered HEA design spaces comprising up to 10 elements.

Multiple Metal-Nitrogen Bonds Synergistically Boosting the Activity and Durability of High-Entropy Alloy Electrocatalysts

The development of Pt-based catalysts for use in fuel cells that meet performance targets of high activity, maximized stability, and low cost remains a huge challenge. Herein, we report a nitrogen (N)-doped high-entropy alloy (HEA) electrocatalyst that consists of a Pt-rich shell and a N-doped PtCoFeNiCu core on a carbon support (denoted as N-Pt/HEA/C). The N-Pt/HEA/C catalyst showed a high mass activity of 1.34 A mgPt-1 at 0.9 V for the oxygen reduction reaction (ORR) in rotating disk electrode (RDE) testing, which substantially outperformed commercial Pt/C and most of the other binary/ternary Pt-based catalysts. The N-Pt/HEA/C catalyst also demonstrated excellent stability in both RDE and membrane electrode assembly (MEA) testing. Using operando X-ray absorption spectroscopy (XAS) measurements and theoretical calculations, we revealed that the enhanced ORR activity of N-Pt/HEA/C originated from the optimized adsorption energy of intermediates, resulting in the tailored electronic structure formed upon N-doping. Furthermore, we showed that the multiple metal-nitrogen bonds formed synergistically improved the corrosion resistance of the 3d transition metals and enhanced the ORR durability.

High-Entropy Alloy PtCuNiCoMn Nanoparticles on rGO for Electrooxidation of Methanol and Formic Acid

High-entropy alloy (HEA) nanoparticles have attracted great attention due to their excellent electrocatalytic properties. Herein, PtCuNiCoMn HEA nanoparticles supported on reduced graphene oxide (rGO) are synthesized via a solvothermal co-reduction method and are used as an electrocatalyst for the electrooxidation of methanol and formic acid. Owing to the synergistic effect between the component metals, the high-entropy effect, and the sluggish diffusion effect, the PtCuNiCoMn HEA nanoparticles possess significantly improved electrocatalytic activity and stability compared to PtCuNiCo, PtCuNi, PtCu, Pt nanoparticles, and the commercial Pt/C catalyst. The results reveal the unique advantages of HEA nanoparticles in the field of electrocatalysis. The synthesis method is simple and effective, which may be valuable for the preparation of other HEA electrocatalysts.

Tailoring Local Chemical Ordering via Elemental Tuning in High-Entropy Alloys

Due to the large multi-elemental space desired for property screening and optimization, high-entropy alloys (HEAs) hold greater potential over conventional alloys for a range of applications, such as structural materials, energy conversion, and catalysis. However, the relationship between the HEA composition and its local structural/elemental configuration is not well understood, particularly in noble-metal-based HEA nanomaterials, hindering the design and development of nano-HEAs in energy conversion and catalysis applications. Herein, we determined precise atomic-level structural and elemental arrangements in model HEAs composed of RhPtPdFeCo and RuPtPdFeCo to unveil their local characteristics. Notably, by changing just one constituent element in the HEA (Rh to Ru), we found dramatic changes in the elemental arrangement from complete random mixing to a local single elemental ordering feature. Additionally, we demonstrate that the local ordering in RuPtPdFeCo can be further controlled by varying the Ru concentration, allowing us to toggle between local Ru clustering and distinct heterostructures in multicomponent systems. Overall, our study presents a practical approach for manipulating local atomic structures and elemental arrangements in noble-metal-based HEA systems, which could provide in-depth knowledge to mechanistically understand the functionality of noble-metal-based HEA nanomaterials in practical applications.

Long-Range SECCM Enables High-Throughput Electrochemical Screening of High Entropy Alloy Electrocatalysts at Up-To-Industrial Current Densities

High-entropy alloys (HEAs), especially in the form of compositional complex solid solutions (CCSS), have gained attention in the field of electrocatalysis. However, exploring their vast composition space concerning their electrocatalytic properties imposes significant challenges. Scanning electrochemical cell microscopy (SECCM) offers high-speed electrochemical analysis on surface areas with a lateral resolution down to tens of nm. However, high-precision piezo positioners often used for the motion of the tip limit the area of SECCM scans to the motion range of the piezo positioners which is typically a few tens of microns. To bridge this experimental gap, the study proposes a long-range SECCM system with a rapid gas-exchange environmental cell for high-throughput electrochemical characterization of 100 mm diameter HEA thin-film material libraries (ML) obtained by combinatorial co-sputtering. Due to the gas-liquid interface at the positioned SECCM droplet on the sample, high-throughput evaluation under industrial current density conditions becomes feasible. This allows the direct correlation between electrocatalytic activity and material composition with high statistical reliability. The multidimensional data obtained accelerates materials discovery, development, and optimization.

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.

Optimization strategies of high-entropy alloys for electrocatalytic applications

High-entropy alloys (HEAs) are expected to become one of the most promising functional materials in the field of electrocatalysis due to their site-occupancy disorder and lattice order. The chemical complexity and component tunability make it possible for them to obtain a nearly continuous distribution of adsorption energy curve, which means that the optimal adsorption strength and maximum activity can be obtained by a multi-alloying strategy. In the last decade, a great deal of research has been performed on the synthesis, element selection and catalytic applications of HEAs. In this review, we focus on the analysis and summary of the advantages, design ideas and optimization strategies of HEAs in electrocatalysis. Combined with experiments and theories, the advantages of high activity and high stability of HEAs are explored in depth. According to the classification of catalytic reactions, how to design high-performance HEA catalysts is proposed. More importantly, efficient strategies for optimizing HEA catalysts are provided, including element regulation, defect regulation and strain engineering. Finally, we point out the challenges that HEAs will face in the future, and put forward some personal proposals. This work provides a deep understanding and important reference for electrocatalytic applications of HEAs.

Multi-metal electrocatalyst with crystalline/amorphous structure for enhanced alkaline water/seawater hydrogen evolution

The development of well-defined nanomaterials as non-noble metal electrocatalysts has broad application prospect for hydrogen generation technology. Recently, multi-metal electrocatalysts for hydrogen evolution reaction (HER) have attracted extensive attention due to their high catalytic performance arising from the synergistic effect of multi-metal interaction. However, most multi-metal catalysts suffer from the limited synergistic effect because of poor interfacial compatibility between different components. Here, a novel multi-metal catalyst (Ni/MoO2@CoFeOx) nanosheet with a crystalline/amorphous structure is demonstrated, which shows high HER activity. Ni/MoO2@CoFeOx exhibits an ultra-low overpotential of 18, 39, and 93 mV at 10 mA cm-2 in alkaline water, alkaline seawater and natural seawater, respectively, which outperformances most of the state-of-the-art non-noble metal compounds. In addition, the catalyst shows exceptional stability under 500 mA cm-2 in alkaline solution. In-situ Raman and other advanced structural characterization confirms the excellent catalytic activity is mainly contributed by: (1) the strong synergistic effect of multi-metal components provides multiple active sites in the catalytic process; (2) the crystalline/amorphous interface in Ni/MoO2@CoFeOx boosts the catalytically active sites and structure stability; (3) the crystalline phase enhances the intrinsic conductivity greatly; and (4) the amorphous phase provides abundant unsaturated sites for improved intrinsic catalytic activity. This work provides a feasible way to design electrocatalyst with high activity and stability for practical applications.

The more the better: on the formation of single-phase high entropy alloy nanoparticles as catalysts for the oxygen reduction reaction

High entropy alloys (HEAs) are an important new material class with significant application potential in catalysis and electrocatalysis. The entropy-driven formation of HEA materials requires high temperatures and controlled cooling rates. However, catalysts in general also require highly dispersed materials, i.e., nanoparticles. Only then a favorable utilization of the expensive raw materials can be achieved. Several recently reported HEA nanoparticle synthesis strategies, therefore, avoid the high-temperature regime to prevent particle growth. In our work, we investigate a system of five noble metal single-source precursors with superior catalytic activity for the oxygen reduction reaction. Combining in situ X-ray powder diffraction with multi-edge X-ray absorption spectroscopy, we address the fundamental question of how single-phase HEA nanoparticles can form at low temperatures. It is demonstrated that the formation of HEA nanoparticles is governed by stochastic principles and the inhibition of precursor mobility during the formation process favors the formation of a single phase. The proposed formation principle is supported by simulations of the nanoparticle formation in a randomized process, rationalizing the experimentally found differences between two-element and multi-element metal precursor mixtures.

Quinary RuRhPdPtAu high-entropy alloy as an efficient electrocatalyst for the hydrogen evolution reaction

Quinary RuRhPdPtAu high-entropy alloy nanoparticles (HEA-NPs) were prepared for the first time from a deep eutectic solvent by an electrochemical method. Owing to the benefits of high entropy and abundant surface active sites, the RuRhPdPtAu HEA-NPs exhibit outstanding electrocatalytic performance for the hydrogen evolution reaction.

Strong Metal-Support Interaction Facilitated Multicomponent Alloy Formation on Metal Oxide Support

Multicomponent alloy (MA) contains a nearly infinite number of unprecedented active sites through entropy stabilization, which is a desired platform for exploring high-performance catalysts. However, MA catalysts are usually synthesized under severe conditions, which induce support structure collapse and further deteriorate the synergy between MA and support. We propose that a strong metal-support interaction (SMSI) could facilitate the formation of MA by establishing a tunnel of oxygen vacancy for metal atom transport under low reduction temperature (400-600 °C), which exemplifies the holistic design of MA catalysts without deactivating supports. PtPdCoFe MA is readily synthesized on anatase TiO2 with the help of SMSI, which exhibits good catalytic activity and stability for methane combustion. This strategy demonstrates excellent universality on various supports and multicomponent alloy compositions. Our work not only reports a holistic synthesis strategy for MA synthesis by synergizing unique properties of reducible oxides and the mixing entropy of alloy but also offers a new insight that SMSI plays a vigorous role in the formation of alloy NPs on reducible oxides.

Scalable Synthesis of Multi-Metal Electrocatalyst Powders and Electrodes and their Application for Oxygen Evolution and Water Splitting

Multi-metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi-metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam-supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow-through electrolysis cells with electrolyte gaps and in zero-gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA-type electrolyzers reached up to 3 A cm^-2 , and 24 h measurements demonstrated no loss of current density of 1 A cm^-2 .

Designing strategies and enhancing mechanism for multicomponent high-entropy catalysts

High-entropy materials (HEMs) are new-fashioned functional materials in the field of catalysis owing to their large designing space, tunable electronic structure, interesting "cocktail effect", and entropy stabilization effect. Many effective strategies have been developed to design advanced catalysts for various important reactions. Herein, we firstly review effective strategies developed so far for optimizing HEM-based catalysts and the underlying mechanism revealed by both theoretical simulations and experimental aspects. In light of this overview, we subsequently present some perspectives about the development of HEM-based catalysts and provide some serviceable guidelines and/or inspiration for further studying multicomponent catalysts.

Discovery of High-Entropy Oxide Electrocatalysts: From Thin-Film Material Libraries to Particles

Discovery of new high-entropy electrocatalysts requires testing of hundreds to thousands of possible compositions, which can be addressed most efficiently by high-throughput experimentation on thin-film material libraries. Since the conditions for high-throughput measurements ("screening") differ from more standardized methods, it is frequently a concern whether the findings from screening can be transferred to the commonly used particulate catalysts. We demonstrate the successful transfer of results from thin-film material libraries to particles of Cantor alloy oxide (Co-Cr-Fe-Mn-Ni)₃O₄. The chemical compositions of the libraries, all single-phase spinels, cover a wide compositional range of (Cr_8.1-28.0Mn_11.6-28.4Fe_10.6-39.0Co_11.4-36.7Ni_13.5-31.4)_37.7±0.6O_62.3±0.6, with composition-dependent lattice constant values ranging from 0.826 to 0.851 nm. Electrochemical screening of the libraries for the oxygen evolution reaction (OER) identifies (Cr_24.6±1.4Mn_15.7±2.0Fe_16.9±1.8Co_26.1±1.9Ni_16.6±1.7)_37.8±0.8O_62.2±1.2 as the most active composition, exhibiting an overpotential of 0.36 V at a current density of 1 mA cm^-2. This "hit" in the library was subsequently synthesized in the form of particles with the same composition and crystal structure using an aerosol-based synthesis strategy. The similar OER activity of the most active thin-film composition and the derived catalyst particles validates the proposed approach of accelerated discovery of novel catalysts by screening of thin-film libraries.

High-entropy nanoparticles: Synthesis-structure-property relationships and data-driven discovery

High-entropy nanoparticles have become a rapidly growing area of research in recent years. Because of their multielemental compositions and unique high-entropy mixing states (i.e., solid-solution) that can lead to tunable activity and enhanced stability, these nanoparticles have received notable attention for catalyst design and exploration. However, this strong potential is also accompanied by grand challenges originating from their vast compositional space and complex atomic structure, which hinder comprehensive exploration and fundamental understanding. Through a multidisciplinary view of synthesis, characterization, catalytic applications, high-throughput screening, and data-driven materials discovery, this review is dedicated to discussing the important progress of high-entropy nanoparticles and unveiling the critical needs for their future development for catalysis, energy, and sustainability applications.

Advanced metal and carbon nanostructures for medical, drug delivery and bio-imaging applications

Nanoparticles (NPs) offer great promise for biomedical, environmental, and clinical applications due to their several unique properties as compared to their bulk counterparts. In this review article, we overview various types of metal NPs and magnetic nanoparticles (MNPs) in monolithic form as well as embedded into polymer matrices for specific drug delivery and bio-imaging fields. The second part of this review covers important carbon nanostructures that have gained tremendous attention recently in such medical applications due to their ease of fabrication, excellent biocompatibility, and biodegradability at both cellular and molecular levels for phototherapy, radio-therapeutics, gene-delivery, and biotherapeutics. Furthermore, various applications and challenges involved in the use of NPs as biomaterials are also discussed following the future perspectives of the use of NPs in biomedicine. This review aims to contribute to the applications of different NPs in medicine and healthcare that may open up new avenues to encourage wider research opportunities across various disciplines.

Noble-Metal High-Entropy-Alloy Nanoparticles: Atomic-Level Insight into the Electronic Structure

The compositional space of high-entropy-alloy nanoparticles (HEA NPs) significantly expands the diversity of the materials library. Every atom in HEA NPs has a different elemental coordination environment, which requires knowledge of the local electronic structure at an atomic level. However, such structure has not been disclosed experimentally or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group elements (NM-HEA) for the first time. Their electronic structure was revealed by hard X-ray photoelectron spectroscopy and density function theory calculations with NP models. The NM-HEA NPs have a lower degeneracy in energy level compared with the monometallic NPs, which is a common feature of HEA NPs. The local density of states (LDOS) of every surface atom was first revealed. Some atoms of the same constituent element in HEA NPs have different LDOS profiles, whereas atoms of other elements have similar LDOS profiles. In other words, one atom in HEA loses its elemental identity and it may be possible to create an ideal LDOS by adjusting the neighboring atoms. The tendency of the electronic structure change was shown by supervised learning. The NM-HEA NPs showed 10.8-times higher intrinsic activity for hydrogen evolution reaction than commercial Pt/C, which is one of the best catalysts.

Rapid Synthesis of High-Entropy Oxide Microparticles

High-entropy nanoparticles have received notable attention due to their tunable properties and broad material space. However, these nanoparticles are not suitable for certain applications (e.g., battery electrodes), where their microparticle (submicron to micron) counterparts are more preferred. Conventional methods used for synthesizing high-entropy nanoparticles often involve various ultrafast shock processes. To increase the size thereby achieving high-entropy microparticles, longer reaction time (e.g., heating duration) is usually used, which may also lead to undesired particle overgrowth or even densified microstructures. In this work, an approach based on Joule heating for synthesizing high-entropy oxide (HEO) microparticles with uniform elemental distribution is reported. In particular, two key synthesis conditions are identified to achieve high-quality HEO microparticles: 1) the precursors need to be loosely packed to avoid densification; 2) the heating time needs to be accurately controlled to tens of seconds instead of using milliseconds (thermal shock) that leads to nanoparticles or longer heating duration that forms bulk structures. The utility of the synthesized HEO microparticles for a range of applications, including high-performance Li-ion battery anode and water oxidation catalyst. This study opens up a new door toward synthesizing high-entropy microparticles with high quality and broad material space.

Continuous-flow syntheses of alloy nanoparticles

Alloy nanoparticles (NPs), including core-shell, segregated and solid-solution types, show a variety of attractive properties such as catalytic and optical properties and are used in a wide range of applications. Precise control and good reproducibility in the syntheses of alloy NPs are highly demanded because these properties are tunable by controlling alloy structures, compositions, particle sizes, and so on. To improve the efficiency and reproducibility of their syntheses, continuous-flow syntheses with various types of reactors have recently been developed instead of the current mainstream approach, batch syntheses. In this review, we focus on the continuous-flow syntheses of alloy NPs and first overview the flow syntheses of NPs, especially of alloy NPs. Subsequently, the details of flow reactors and their chemistry to synthesize core-shell, segregated, solid-solution types of alloy NPs, and high-entropy alloy NPs are introduced. Finally, the challenges and future perspectives in this field are discussed.

Bayesian Optimization of High-Entropy Alloy Compositions for Electrocatalytic Oxygen Reduction*

Active, selective and stable catalysts are imperative for sustainable energy conversion, and engineering materials with such properties are highly desired. High‐entropy alloys (HEAs) offer a vast compositional space for tuning such properties. Too vast, however, to traverse without the proper tools. Here, we report the use of Bayesian optimization on a model based on density functional theory (DFT) to predict the most active compositions for the electrochemical oxygen reduction reaction (ORR) with the least possible number of sampled compositions for the two HEAs Ag‐Ir‐Pd‐Pt‐Ru and Ir‐Pd‐Pt‐Rh‐Ru. The discovered optima are then scrutinized with DFT and subjected to experimental validation where optimal catalytic activities are verified for Ag–Pd, Ir–Pt, and Pd–Ru binary alloys. This study offers insight into the number of experiments needed for optimizing the vast compositional space of multimetallic alloys which has been determined to be on the order of 50 for ORR on these HEAs.

What Makes High-Entropy Alloys Exceptional Electrocatalysts?

The formation of a vast number of different multielement active sites in compositionally complex solid solution materials, often more generally termed high‐entropy alloys, offers new and unique concepts in catalyst design, which mitigate existing limitations and change the view on structure–activity relations. We discuss these concepts by summarising the currently existing fundamental knowledge and critically assess the chances and limitations of this material class, also highlighting design strategies. A roadmap is proposed, illustrating which of the characteristic concepts could be exploited using which strategy, and which breakthroughs might be possible to guide future research in this highly promising material class for (electro)catalysis.

High-entropy materials for catalysis: A new frontier

Entropy plays a pivotal role in catalysis, and extensive research efforts have been directed to understanding the enthalpy-entropy relationship that defines the reaction pathways of molecular species. On the other side, surface of the catalysts, entropic effects have been rarely investigated because of the difficulty in deciphering the increased complexities in multicomponent systems. Recent advances in high-entropy materials (HEMs) have triggered broad interests in exploring entropy-stabilized systems for catalysis, where the enhanced configurational entropy affords a virtually unlimited scope for tailoring the structures and properties of HEMs. In this review, we summarize recent progress in the discovery and design of HEMs for catalysis. The correlation between compositional and structural engineering and optimization of the catalytic behaviors is highlighted for high-entropy alloys, oxides, and beyond. Tuning composition and configuration of HEMs introduces untapped opportunities for accessing better catalysts and resolving issues that are considered challenging in conventional, simple systems.

Complex-Solid-Solution Electrocatalyst Discovery by Computational Prediction and High-Throughput Experimentation*

Complex solid solutions ("high entropy alloys"), comprising five or more principal elements, promise a paradigm change in electrocatalysis due to the availability of millions of different active sites with unique arrangements of multiple elements directly neighbouring a binding site. Thus, strong electronic and geometric effects are induced, which are known as effective tools to tune activity. With the example of the oxygen reduction reaction, we show that by utilising a data-driven discovery cycle, the multidimensionality challenge raised by this catalyst class can be mastered. Iteratively refined computational models predict activity trends around which continuous composition-spread thin-film libraries are synthesised. High-throughput characterisation datasets are then used as input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag-Ir-Pd-Pt-Ru. The method can identify optimal complex-solid-solution materials for electrocatalytic reactions in an unprecedented manner.

High-entropy materials for energy-related applications

High-entropy materials (HEMs), including high-entropy alloys (HEAs), high-entropy oxides (HEOs), and other high-entropy compounds, have gained significant interests over the past years. These materials have unique structures with the coexistence of antisite disordering and crystal periodicity, which were originally investigated as structural materials. Recently, they have emerged for energy-related applications, such as catalysis, energy storage, etc. In this work, we review the research progress of energy-related applications of HEMs. After an introduction on the background, theory, and syntheses of HEMs, we survey their applications including electrocatalysis, batteries, and others, aiming to retrieve the correlations between their structures and performances. In the end, we discussed the challenges and future directions for developing HEMs.

Synthesis of monodisperse high entropy alloy nanocatalysts from core@shell nanoparticles

High-entropy alloy (HEA) nanoparticles (NPs) hold great promise in electrocatalysis because of their nearly unlimited compositions, tailorable active sites, and high durability. However, the synthesis of these compositionally complex structures as monodisperse NPs remains a challenge by colloidal routes because the different rates of metal precursor reduction lead to phase separation. Here, we report the conversion of core@shell NPs into HEA NPs through annealing, with conservation of sample monodispersity. This potentially general route for high-quality HEA NPs was demonstrated by preparing PdCu@PtNiCo NPs via seed-mediated co-reduction, wherein Pt, Ni, and Co were co-deposited on PdCu seeds in solution. These multimetallic NPs were then converted to single-crystalline and single-phase PdCuPtNiCo NPs through annealing. On account of their small particle size, highly dispersed Pt/Pd content, and low elemental diffusivity, these HEA NPs were found to be a highly efficient and durable catalyst for the oxygen reduction reaction. They were also highly selective for the four-electron transfer pathway. We expect that this new synthetic strategy will facilitate the synthesis of new HEA NPs for catalysis and other applications.

Sputter deposition of highly active complex solid solution electrocatalysts into an ionic liquid library: effect of structure and composition on oxygen reduction activity

Complex solid solution electrocatalysts (often called high-entropy alloys) present a new catalyst class with highly promising features due to the interplay of multi-element active sites. One hurdle is the limited knowledge about structure-activity correlations needed for targeted catalyst design. We prepared Cr-Mn-Fe-Co-Ni nanoparticles by magnetron sputtering a high entropy Cantor alloy target simultaneously into an ionic liquid library. The synthesized nanoparticles have a narrow size distribution but different sizes (from 1.3 ± 0.1 nm up to 2.6 ± 0.3 nm), different crystallinity (amorphous, face-centered cubic or body-centered cubic) and composition (i.e. high Mn versus low Mn content). The Cr-Mn-Fe-Co-Ni complex solid solution nanoparticles possess an unprecedented intrinsic electrocatalytic activity for the oxygen reduction reaction in alkaline media, some of them even surpassing that of Pt. The highest intrinsic activity was obtained for body-centered cubic nanoparticles with a low Mn and Fe content which were synthesized using the ionic liquid 1-etyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Emimi][(Tf)2N].

Electrocatalysis as the Nexus for Sustainable Renewable Energy: The Gordian Knot of Activity, Stability, and Selectivity

The use of renewable energy by means of electrochemical techniques by converting H2 O, CO2 and N2 into chemical energy sources and raw materials, is the basis for securing a future sustainable "green" energy supply. Some weaknesses and inconsistencies in the practice of determining the electrocatalytic performance, which prevents a rational bottom-up catalyst design, are discussed. Large discrepancies in material properties as well as in electrocatalytic activity and stability become obvious when materials are tested under the conditions of their intended use as opposed to the usual laboratory conditions. They advocate for uniform activity/stability correlations under application-relevant conditions, and the need for a clear representation of electrocatalytic performance by contextualization in terms of functional investigation or progress towards application is emphasized.

On the electronic structure and hydrogen evolution reaction activity of platinum group metal-based high-entropy-alloy nanoparticles

We report the synthesis of high-entropy-alloy (HEA) nanoparticles (NPs) consisting of five platinum group metals (Ru, Rh, Pd, Ir and Pt) through a facile one-pot polyol process. We investigated the electronic structure of HEA NPs using hard X-ray photoelectron spectroscopy, which is the first direct observation of the electronic structure of HEA NPs. Significantly, the HEA NPs possessed a broad valence band spectrum without any obvious peaks. This implies that the HEA NPs have random atomic configurations leading to a variety of local electronic structures. We examined the hydrogen evolution reaction (HER) and observed a remarkably high HER activity on HEA NPs. At an overpotential of 25 mV, the turnover frequencies of HEA NPs were 9.5 and 7.8 times higher than those of a commercial Pt catalyst in 0.05 M H2SO4 and 1.0 M KOH electrolytes, respectively. Moreover, the HEA NPs showed almost no loss during a cycling test and were much more stable than the commercial Pt catalyst. Our findings on HEA NPs may provide a new paradigm for the design of catalysts.

Design of Complex Solid-Solution Electrocatalysts by Correlating Configuration, Adsorption Energy Distribution Patterns, and Activity Curves

Complex solid‐solution electrocatalysts (also referred to as high‐entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single‐phase solid solutions from five or more constituents, they offer unique capabilities of fine‐tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media.

Synthesis of plasmonic Fe/Al nanoparticles in ionic liquids

Bottom-up and top-down approaches are described for the challenging synthesis of Fe/Al nanoparticles (NPs) in ionic liquids (ILs) under mild conditions. The crystalline phase and morphology of the metal nanoparticles synthesized in three different ionic liquids were identified by powder X-ray diffractometry (PXRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and fast Fourier transform (FFT) of high-resolution TEM images. Characterization was completed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) for the analysis of the element composition of the whole sample consisting of the NPs and the amorphous background. The bottom-up approaches resulted in crystalline FeAl NPs on an amorphous background. The top-down approach revealed small NPs and could be identified as Fe₄Al₁₃ NPs which in the IL [OPy][NTf₂] yield two absorption bands in the green-blue to green spectral region at 475 and 520 nm which give rise to a complementary red color, akin to appropriate Au NPs.

Fe/Al NPs of the right size mimic with their red color the electronic surface structure of Au NPs.