Recent progress in high-entropy alloys for catalysts: synthesis, applications, and prospects (2023)

High-entropy metallic glass (HEMG) is a new type of metallic material with high-entropy alloy-like composition and amorphous structure, which render HEMGs unusual glass formation behaviors and unique properties. In recent years, fast research progress has been witnessed on the HEMGs, and thus a systematic review is required. In this review, we first introduce the concept of the HEMGs and summarize the developed HEMGs. Then, the glass-forming ability of the HEMGs is discussed, and the general rules are proposed. Focusing on the thermal stability of HEMGs, the effect of entropy on the energy states of HEMGs and the crystallization behavior of HEMGs are discussed. Finally, the mechanical, magnetic, catalytic and other properties of HEMGs are presented, and the advantages and disadvantages of HEMGs are shown. This review can function as a quick guideline for overviewing the HEMG field.

High-entropy alloys (HEAs), also known as multi-principal component alloys, possess a range of attractive properties that cannot be achieved by traditional alloys due to their disordered atomic structure. HEAs are novel materials that have been developed in recent years and exhibit exceptional properties that surpass traditional alloy materials. The HEAs demonstrate characteristics such as the high entropy effect, the slow diffusion effect, the lattice distortion effect, and the "cocktail" effect. These properties have infinite potential for HEAs to become the next generation of electrocatalytic materials. Herein, we review the properties of the basic definition of HEAs, highlight the latest synthesis methods for producing nanoscale HEAs, and explore the research progress of HEAs nanoscale catalysts in various electrochemical reactions.

Laser ablation in liquid (LAL) is a rapid heating and cooling process that enables the formation of a variety of nanomaterials. Extensive LAL studies for metals, semiconductors, insulators, and carbons have been reported. Further, alloys are worthwhile target to investigate the applicability of LAL. We report the LAL of equiatomic FeCoNi medium-entropy alloy (MEA) in water using a high-repetition-rate UV picosecond laser with a galvanometric scanner. The extinction spectra of the colloidal solution, morphology, elemental composition, and size distribution of MEA nanoparticles (NPs) were investigated as a function of laser-irradiation time, laser power, and scanning speed. Electron microscopy revealed that chain-like superstructures longer than 10µm consisting of Ni-rich FeCoNi MEA-core Fe-shell spherical NPs of approximately 50-nm diameter were formed. These superstructures were not originated from the NP agglomeration during sample preparation for microscopy or the successive laser irradiation of colloids, but from the overstriking ablation on the target surface. Generally, the LAL target was irradiated by spatially and temporally separated near-infrared or visible laser pulses to efficiently produce NPs. Overstriking of UV ultrashort laser pulses was shown to be a valuable strategy for modifying the elemental composition of MEA NPs and for forming their superstructures.

High-entropy alloy nanoparticles can significantly enhance catalytic hydrogen production as a low-cost electrocatalyst and may potentially play an important role in the development of hydrogen energy. To obtain array structures with higher width resolutions and more active sites, a dual-tip probe model with different nominal aspect ratios was developed using molecular dynamic methods, and studies were conducted on the nominal height variation of the probe gap, width variation, and machining angle variation. The results show that when the machining aspect ratio limit is less than 0.7 and the nominal aspect ratio is less than 2, a uniform and consistent groove structure and a complete and uninterrupted hump structure can be obtained, and the distribution of defects on both sides of the probe is nearly symmetrical, thereby improving the mechanical properties of the machined structure. It is possible to improve the flow of the material with dual tips and to obtain hump structures with different morphologies by rotating the angular processing. This study helps to obtain continuous hump structures and excellently symmetrical nanogroove arrays, which is significant for promoting the practical application of structured probes.

This work investigates the microstructure formed in friction stir welds of FCC alloys, focused on two multi-principal alloys: a CoCrFeMnNi high-entropy alloy (HEA) and a CoCrNi medium-entropy alloy (MEA). A commercial stainless steel AISI 304 is used for comparison. The largest nugget was formed in the MEA, while the smallest was formed in the HEA. Grain refinement occurs in the stirred zone in all welds. Discontinuous dynamic recrystallisation is the predominant restoration mechanism during friction stir welding of the three investigated alloys. A sharp decrement in the Σ3 boundary fraction occurs in the stirred zone of the AISI 304 and HEA welds, while comparable values with the base metal are found for the MEA weld. The peak in the maximum index of crystallographic texture is observed on the advancing side of the stirred zone of the AISI 304 weld. A strong <001>θ-fibre texture is formed in the advancing side of the nugget in the AISI 304 from a well-established {123}<634>S-type texture in the base metal. Multiple crystallographic texture components without specific fibres are identified in most regions of the welds, indicating the complex shear path history during friction stir welding.

The need for electrocatalysts that possess multiple surface sites to facilitate ∗O, ∗OH and ∗OOH intermediates in the complex reactions steps of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) has led to the emergence of the high-entropy materials (HEMs) as promising bifunctional electrocatalysts for rechargeable zinc–air batteries (RZABs). This mini-review presents inherent physico-chemical properties of HEMs that have endowed great interest as electrocatalysts for RZABs. Although the HEMs exhibit excellent electrocatalysis, to date, none of them (just like the conventional catalysts) has achieved the nominal voltage of 1.66V for RZAB. Thus, in our opinion, effort should be devoted to the design, engineering and fabrication to mitigate this key disadvantage.

Nano Energy, Volume 88, 2021, Article 106261

The near equimolar and non-equimolar high entropy alloys (HEAs) having five or more major components along with their mingled sites over the surface have made them unique materials for various catalytic reactions involving renewable energies. HEAs provide a platform to tune the surface microstructure and chemistry by selecting and controlling the elements, opening up vistas to design new materials for catalysis. The present perspective aims to provide the correlation between HEAs' structure and catalytic performance in various applications with views on challenges and unique opportunities. The scientific and technological curiosity needs to dig deep into the multicomponent phase space to discover various new materials with unique catalytic properties.

Scripta Materialia, Volume 138, 2017, pp. 22-27

High-entropy alloys containing 5 and 6 platinum group metals have been prepared by thermal decomposition of single-source precursors non requiring high temperature or mechanical alloying. The prepared Ir_0.19Os_0.22Re_0.21Rh_0.20Ru_0.19 alloy is the first example of a single-phase hexagonal high-entropy alloy. Heat treatment up to 1500K and compression up to 45GPa do not result in phase changes, a record temperature and pressure stability for a single-phase high-entropy alloy. The alloys show pronounced electrocatalytic activity in methanol oxidation, which opens a route for the use of high-entropy alloys as materials for sustainable energy conversion.

Journal of Materials Science & Technology, Volume 93, 2021, pp. 110-118

The properties of high entropy alloys (HEAs) depend on their phase structures and compositions. However, it is difficult to control the composition of the HEAs that contain highly volatile metals by the conventional arc melting method. In this paper, homogeneous powdery face centered cubic (FCC) phase Fe_0.5CoNiCuZn_x HEAs were prepared by the electrolysis of metal oxides in molten Na₂CO₃-K₂CO₃ using a stable Ni11Fe10Cu inert oxygen-evolution anode. The use of oxide precursors and relatively low synthetic temperature are beneficial to efficiently preparing HEAs that contain highly volatile elements such as Zn. Moreover, the microstructures and compositions of the electrolytic HEAs can be easily tailored by adjusting the components of oxide precursors, then further regulating its properties. Thus, the electrocatalytic activity of Fe_0.5CoNiCuZn_x HEAs towards oxygen evolution reactions (OER) was investigated in 1M KOH. The results show that Zn promotes the OER activity of Fe_0.5CoNiCuZn_x HEAs, i.e., the HEA(Zn_0.8) shows the best OER activity exhibiting a low overpotential of 340mV at 10mA/cm² and excellent stability of 24h. Hence, molten salt electrolysis not only provides a green approach to prepare Fe_0.5CoNiCuZn_x HEAs but also offers an effective way to regulate the structure of the alloys and thereby optimizes the electrocatalytic activities for water electrolysis.

Chemical Engineering Journal, Volume 429, 2022, Article 132410

Recently, high entropy alloy (HEA) based materials have been vigorously explored as viable catalysts in water electrolysis for their unique properties. However, the synthesis of efficient and robust high entropy catalysts remains a challenge. Here a facile and scalable approach is reported to synthesize advanced HEA (CoNiCuMnAl)/C nanoparticles from the polymetallic Metal–organic framework (MOF). The novel core-shell nanoarchitectures feature face-centered cubic HEA wrapped in ultra-thin carbon shell. The optimized catalyst (deposited upon Ni foam) boosted alkaline Oxygen evolution reaction (OER) (in 1.0M KOH) with an ultralow overpotential of 215mV at 10mA/cm² (also by a low Tafel slope of 35.6mVdec⁻¹). The enhanced performance is closely tied to surface reconstruction (i.e., formation of oxyhydroxide catalytic active species) and the high entropy effect, revealed by Fourier-transformed alternating current voltammetry (FTACV) and in-situ Raman spectrum analysis, in conjunction with Density Functional Theory (DFT) computations. Furthermore, the new design showed excellent long-term OER stability with negligible decay through 30h testing (under 200mAcm⁻²). The present work demonstrates the feasibility and advantage of utilizing highly efficient and durable high entropy alloys for catalyzing electrochemical water splitting process.

Journal of Power Sources, Volume 430, 2019, pp. 104-111

Efficient oxygen evolution reaction catalysts based on earth-abundant and low-cost elements are urgently required for water splitting devices and metal-air batteries. Herein, for the first time we report a novel and promising MO_x (M = Mn, Fe, Co and Ni) nanosheets catalyst for oxygen evolution reaction based on the MnFeCoNi high entropy alloy. By an electrochemical cyclic voltammetry scan activation, the MO_x nanosheets can grow directly on the MnFeCoNi high entropy alloy particle surfaces forming a core-shell structure. The core-shell structure exhibits a low overpotential of 302 mV to achieve current density of 10 mA cm⁻², a small Tafel slope of 83.7 mV dec⁻¹ and exceptional long-term stability of electrolysis for over 20 h in 1 M KOH alkaline solution, which is comparable to the state-of-the-art oxygen evolution reaction electrocatalyst RuO₂. We make an investigation of the MnFeCoNi high entropy alloy before and after cyclic voltammetry scan activation on their morphologies, chemical states and elements composition to understand the materials evolution. The present work not only provides a promising electrocatalyst but also broadens the application areas of high entropy alloys.

Chemical Engineering Journal, Volume 430, Part 2, 2022, Article 132883

Great interests have been focused on high entropy alloys, a new class of single-phase solid solution materials, because of their unique physical and chemical properties. Herein, CoNiCuMgZn high entropy alloy (HEA) has been homogeneously embedded onto ultrathin two-dimensional (2D) holey graphene conducting substrates via a simple anchoring and alloying strategy. During the growth of the HEA, graphene oxide is employed as both direct template and reductant. By modulating the initial input amounts of metal sources, the particle sizes of HEA can be finely regulated from 40 to 200nm. Benefiting from the synergistic effect between the excellent electronic conductivity of graphene and the abundant catalytic sites of HEA particles, the obtained material displays remarkable catalytic activity and stability for hydrogen evolution reaction compared to previous reported non-noble-metal catalysts. The density functional theory calculation unveils that the unique embedded structure of HEA onto graphene benefits to the H* sorption and promotes the catalytic kinetics. This work is of great importance in the practical utilization of HEA and indeed give a brilliant perspective in fabricating HEA as efficient electrocatalysts for water splitting.