High entropy alloy nanoparticles encapsulated in graphitised hollow carbon tubes for oxygen reduction electrocatalysis

Surface Nanostructures of Pt-Compositionally Complex Alloy Single-Crystal Model Catalyst Surfaces for Improved Oxygen Reduction Reaction: Machine-Learning-Assisted Exploration

We investigated oxygen reduction reaction (ORR) properties of Pt-containing compositionally complex alloy (Pt-CCA) single-crystal model catalyst surfaces to optimize dry-process synthesis conditions, that is, CCA compositions of less-noble alloying elements and their synthesis (annealing) temperatures. Using a machine-learning approach, we effectively navigated the large space of possible synthesis conditions to minimize the experimental workload. The ORR activity and durability of the Pt/CCA/Pt(111) model catalyst surfaces (synthesized through vacuum deposition on a Pt(111) substrate of nonequiatomic Cr-Mn-Fe-Co-Ni or Mn-Fe-Co-Ni alloy (111) lattice stacking layers, followed by a surface Pt(111) layer) depend upon the alloy composition and synthesis temperature: the model catalyst surfaces synthesized with specific combinations of these two parameters outperformed benchmark surfaces such as Pt/equiatomic Cr-Mn-Fe-Co-Ni/Pt(111) in terms of the ORR durability during potential-cycle loading. The outstanding ORR properties are attributed to the use of machine learning to predict synthesis conditions that are closely linked to the atomic-level surface microstructures that favor enhanced ORR properties. These microstructures enable the formation of a so-called "pseudo-core-shell-like structure", i.e., surface Pt(111) underlaid with CCA(111) lattice stacking layers with atomically distributed active elements (Co and/or Ni) close to the surface that are beneficial for ORR property enhancements. This study demonstrates that not only the "high-entropy" effect of charged less-noble CCA elements but also the precise control of elemental distributions in the near-surface vicinity in the pristine state, resulting from optimized CCA compositions and synthesis temperatures, are the key factors to improve Pt-CCA catalyst material systems.

Multifunctional High-Entropy Alloys and Oxides for Self-Powered Electrocatalytic Nitrate Reduction to Ammonia

High-entropy alloys (HEAs) show high activities toward oxygen reduction reaction (ORR), Zn-air batteries (ZABs) and nitrate reduction reaction (NO3 -RR). In this work, FeNiCoMnRh HEA supported by N-doped carbon frameworks is prepared and showed excellent ORR performance with a half-wave potential (E1/2) of 0.89 V versus RHE, limiting diffusion current (jL) of 5.6 mA cm-2 and better current stability. The HEA-assembled ZAB exhibited a high-power density of 103.8 mW cm-2 with a specific capacity of 790 mAh gZn -1. Also, its oxides presented 77% Faraday efficiency (FE) for ammonia production at -0.3 V versus RHE. Accordingly, our designed ZAB was employed to drive NO3 -RR to construct a self-powered system, which provides an attractive route for low-energy sewage treatment and environmentally friendly preparation of ammonia.

Synthesis Strategies for High Entropy Nanoparticles

Nanoparticles (NPs) of high entropy materials (HEMs) have attracted significant attention due to their versatility and wide range of applications. HEM NPs can be synthesized by fragmenting bulk HEMs or disintegrating and recrystallizing them. Alternatively, directly producing HEMs in NP form from atomic/ionic/molecular precursors presents a significant challenge. A widely adopted strategy involves thermodynamically driving HEM NP formation by leveraging the entropic contribution but incorporating strategies to limit NP growth at the elevated temperatures used for maximizing entropy. A second approach is to kinetically drive HEM NP formation by promoting rapid reactions of homogeneous reactant mixtures or using highly diluted precursor dissolutions. Additionally, experimental evidence suggests that enthalpy plays a significant role in driving HEM NP formation processes at moderate temperatures, with the high energy cost of generating additional surfaces and interfaces at the nanoscale stabilizing the HEM phase. This review critically assesses the various synthesis strategies developed for HEM NP preparation, highlighting key illustrative examples and offering insights into the underlying formation mechanisms. Such insights are critical for fine-tuning experimental conditions to achieve specific outcomes, ultimately enabling the effective synthesis of optimized generations of these advanced materials for both current and emerging applications across various scientific and technological fields.

Future prospects of high-entropy alloys as next-generation industrial electrode materials

The rapid advancement of electrochemical processes in industrial applications has increased the demand for high-performance electrode materials. High-entropy alloys (HEAs), a class of multicomponent alloys with unique properties, have emerged as potential electrode materials owing to their enhanced catalytic activity, superior stability, and tunable electronic structures. This review explores contemporary developments in HEA-based electrode materials for industrial applications and identifies their advantages and challenges as compared to conventional commercial electrode materials in industrial aspects. The importance of tuning the composition, crystal structure, different phase formations, thermodynamic and kinetic parameters, and surface morphology of HEAs and their derivatives to achieve the predicted electrochemical performance is emphasized in this review. Synthetic procedures for producing potential HEA electrode materials are outlined, and theoretical discussions provide a roadmap for recognizing the ideal electrode materials for specific electrochemical processes in an industrial setting. A comprehensive discussion and analysis of various electrochemical processes (HER, OER, ORR, CO2RR, MOR, AOR, and NRR) and electrochemical applications (batteries, supercapacitors, etc.) is included to appraise the potential ability of HEAs as an electrode material in the near future. Overall, the design and development of HEAs offer a promising pathway for advancing industrial electrode materials with improved performance, selectivity, and stability, potentially paving the way for the next generation of electrochemical technology.

Synthesis of metallic high-entropy alloy nanoparticles

The theoretically infinite compositional space of high-entropy alloys (HEAs) and their novel properties and applications have attracted significant attention from a broader research community. The successful synthesis of high-quality single-phase HEA nanoparticles represents a crucial step in fully unlocking the potential of this new class of materials to drive innovations. This review analyzes the various methods reported in the literature to identify their commonalities and dissimilarities, which allows categorizing these methods into five general strategies. Physical minimization of HEA metals into HEA nanoparticles through cryo-milling represents the typical top-down strategy. The counter bottom-up strategy requires the simultaneous generation and precipitation of metal atoms of different elements on growing nanoparticles. Depending on the metal atom generation process, there are four synthesis strategies: vaporization of metals, burst reduction of metal precursors, thermal shock-induced reduction of metal precursors, and solvothermal reduction of metal precursors. Comparisons among the methods within each strategy, along with discussions, provide insights and guidance for achieving the robust synthesis of HEA nanoparticles.

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.

High entropy alloy nanoparticles as efficient catalysts for alkaline overall seawater splitting and Zn-air batteries

High entropy alloys (HEAs) are those metallic materials that consist of five or more elements. Compared with conventional alloys, they have much more catalytic active sites due to unique structural characteristics such as high entropy effect and lattice distortion, endowing them with promising applications in the region of hydrolysis catalysts. Herein, we successfully loaded high-entropy alloys onto carbon nanotubes (FeNiCoMnRu@CNT) by hydrothermal means. It exhibits excellent HER and OER properties in alkaline seawater. To accomplish two-electrode total water splitting when constructed into Zn air cells, it only needed 1.6 V, and the timing voltage curve showed a steady current density of 10 mA cm^-2 during constant electrolysis for more than 30 h in alkaline seawater. The remarkably high HER and OER activity of FeNiCoMnRu@CNT HEAs NPS indicates the potentially broad application prospect of HEAs for Zn air battery.