Nanoscale high-entropy alloy for electrocatalysis

Abstract

High-entropy alloys (HEAs) represent a new paradigm in alloy design, with at least five elements incorporated in near-equal proportions, deviating from traditional alloying strategies. The incorporation of multiple elements offers HEAs with diverse tunability and potential for novel properties, making them a subject of widespread attention. Advances in synthesis techniques have enabled the preparation of nanoscale HEAs, which possess a high specific surface area and quantum size effect, further enhancing their catalytic potential. However, the complexity of the elemental composition in HEAs has hindered the full understanding of their structure. Despite this challenge, recent research efforts have yielded inspiring results in recognizing the structure of HEAs. Additionally, advanced synthetic strategies for nanoscale HEAs have been explored, facilitating significant progress in high-throughput screening and electrochemical applications related to their constituent elements. This review provides a comprehensive summary of the current understanding of the structure of HEAs and corresponding characterization methods, as well as the recent advances in the preparation, high-throughput screening, and electrochemical applications of HEAs. High-entropy alloys (HEAs) are a new kind of alloy with five or more alloy elements at equal or near-equal ratios. The tunable multicomponent structure and potential novel properties of HEAs have attracted widespread attention. Benefiting from advances in synthesis technology, nanoscale HEAs were acquired successfully, the high specific surface area and quantum size effect of which further improved their capacity for catalysis. Nevertheless, the complex composition of elements makes it challenging to clarify the atomic structure of HEAs, and their catalytic studies are still in their infancy. A comprehensive review summarizing the current understanding of the structure of HEAs and their catalytic performance is urgently needed. This review first presents basic insights into HEAs, including concept definition, structural features, and progressive synthesis and characterization technologies. Then, the electrocatalytic performance of HEAs is discussed based on the structural diversity and synergistic role of each component element. Considering the limitation of trial-and-error methods in dealing with this complex system, high-throughput screening technologies are also introduced. This review aims to promote the understanding of HEAs and therefore achieve the design of high-efficiency catalysts relying on HEAs. High-entropy alloys (HEAs) are a new kind of alloy with five or more alloy elements at equal or near-equal ratios. The tunable multicomponent structure and potential novel properties of HEAs have attracted widespread attention. Benefiting from advances in synthesis technology, nanoscale HEAs were acquired successfully, the high specific surface area and quantum size effect of which further improved their capacity for catalysis. Nevertheless, the complex composition of elements makes it challenging to clarify the atomic structure of HEAs, and their catalytic studies are still in their infancy. A comprehensive review summarizing the current understanding of the structure of HEAs and their catalytic performance is urgently needed. This review first presents basic insights into HEAs, including concept definition, structural features, and progressive synthesis and characterization technologies. Then, the electrocatalytic performance of HEAs is discussed based on the structural diversity and synergistic role of each component element. Considering the limitation of trial-and-error methods in dealing with this complex system, high-throughput screening technologies are also introduced. This review aims to promote the understanding of HEAs and therefore achieve the design of high-efficiency catalysts relying on HEAs. Alloys have shown a long history during human development since the invention of bronze. According to historical approaches, the realization of desirable alloy properties mainly relies on containing selected elements at low concentrations to form dilute alloys.1Hummel R.E. Alloys and compounds.in: Understanding Materials Science. Springer, 2004: 74-101Crossref Google Scholar,2He Q.F. Ding Z.Y. Ye Y.F. Yang Y. Design of high-entropy alloy: a perspective from nonideal mixing.JOM. 2017; 69: 2092-2098Crossref Scopus (51) Google Scholar In these alloys, the major element is regarded as the solvent, and the doped element is seen as the solute. The traditional alloying strategy has built the foundation of our knowledge about alloy design and achieved acceptable performance improvement. The compositions of alloys uncovered by the traditional design strategy account for a small portion of the whole area in the multicomponent phase diagram.3Zhang Y. Zuo T.T. Tang Z. Gao M.C. Dahmen K.A. Liaw P.K. Lu Z.P. Microstructures and properties of high-entropy alloys.Prog. Mater. Sci. 2014; 61: 1-93Crossref Scopus (4259) Google Scholar,4Ye Y.F. Wang Q. Lu J. Liu C.T. Yang Y. High-entropy alloy: challenges and prospects.Mater. Today. 2016; 19: 349-362Crossref Scopus (1351) Google Scholar There are still large spaces of composition that are less well known and are difficult to acquire with the traditional approach because of their tendency for phase separation when increasing the element types and contents. Inspiringly, a brand-new alloy system, high-entropy alloys (HEAs), with five or more alloy elements at equal or near-equal ratios, was independently reported by Yeh’s and Cantor’s groups in 2004.5Cantor B. Chang I.T.H. Knight P. Vincent A.J.B. 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Benefiting from the diverse tunability and novel atomic structure brought by multiple-element composition, many unique and exciting properties in HEAs have been continuously discovered, such as excellent superconductivity, exceptional strength and toughness, superparamagnetism, thermoelectricity, high stability, and outstanding hydrogen storage.7Von Rohr F. Winiarski M.J. Tao J. Klimczuk T. Cava R.J. Effect of electron count and chemical complexity in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor.Proc. Natl. Acad. Sci. USA. 2016; 113: E7144-E7150Crossref PubMed Scopus (90) Google Scholar,8Jung S.-G. Han Y. Kim J.H. Hidayati R. Rhyee J.-S. Lee J.M. Kang W.N. Choi W.S. Jeon H.-r. Suk J. Park T. High critical current density and high-tolerance superconductivity in high-entropy alloy thin films.Nat. Commun. 2022; 13: 3373Crossref PubMed Scopus (6) Google Scholar,9George E.P. Curtin W. Tasan C.C. 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The reduced size brings a new dimension of regulation in HEAs and further expands their applications in the field of catalysis, including electrocatalysis, electrode materials, and thermocatalysis, due to the higher surface area and diverse active sites.17Yu Y. Xia F. Wang C. Wu J. Fu X. Ma D. Lin B. Wang J. Yue Q. Kang Y. High-entropy alloy nanoparticles as a promising electrocatalyst to enhance activity and durability for oxygen reduction.Nano Res. 2022; 15: 7868-7876Crossref Scopus (10) Google Scholar,18Sharma L. Katiyar N.K. Parui A. Das R. Kumar R. Tiwary C.S. Singh A.K. Halder A. Biswas K. Low-cost high entropy alloy (HEA) for high-efficiency oxygen evolution reaction (OER).Nano Res. 2022; 15: 4799-4806Crossref Scopus (44) Google Scholar,19Xie P. Yao Y. Huang Z. Liu Z. Zhang J. Li T. Wang G. Shahbazian-Yassar R. Hu L. Wang C. Highly efficient decomposition of ammonia using high-entropy alloy catalysts.Nat. 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Design of complex solid-solution electrocatalysts by correlating configuration, adsorption energy distribution patterns, and activity curves.Angew. Chem. Int. Ed. 2020; 59: 5844-5850Crossref PubMed Scopus (0) Google Scholar,24Zhang R. Wang C. Zou P. Lin R. Ma L. Yin L. Li T. Xu W. Jia H. Li Q. et al.Compositionally complex doping for zero-strain zero-cobalt layered cathodes.Nature. 2022; 610: 67-73Crossref PubMed Scopus (37) Google Scholar Despite the conspicuous potential of HEAs in various applications, the complex multielement compositions bring an incalculable number of possible atomic configurations, which hinders the identification of their atomic structure and realization of guided design. In addition, the entropy-related discussions of catalytic performance induced by HEAs are just budding compared with the enthalpy-centered discussions and need further understanding. To date, with much effort put into this field, progress has been made in the fundamental comprehension of HEAs involving microstructures, synthesis, and characterization. The catalytic behavior of HEAs is highly correlated with their local structures. To bridge the understanding of the structure of HEAs and high-efficiency catalyst design, a comprehensive summary of the basic insight into HEAs is necessary. Herein, we overviewed the structure-related cognition of HEAs acquired currently, including concept definition, structural features, and corresponding synthesis, characterization, simulation technologies, and electrocatalytic performance. In the last 1–2 years, HEAs have undergone significant advancements in synthesis, characterization, and high-throughput screening. This review presents novel insights into the local structure of HEAs, while also providing an overview of the latest characterization techniques and the current state of high-throughput screening technologies. Additionally, the similarities and differences between HEAs, amorphous alloys, and single-atom alloys are discussed. This review focuses on the design principles to obtain local structures with targeted properties and emphasizes clarifying the role of each component element in HEAs toward catalysis to achieve accurate design. Conventionally, traditional alloys consist of a primary element and a small amount of doping with other elements. As a result, the lattice structure of traditional alloys still maintains that of the solvent atom, as displayed in Figure 1A. In contrast, many different elements with different atomic volumes and valence electron counts are incorporated in the same lattice in HEAs (Figure 1B), and, therefore, severe lattice distortions and structural modifications are induced. The concept of HEAs includes two aspects: composition and entropy. In terms of composition, HEAs are defined as alloys in a single phase containing five or more major elements with the ratio of each element ranging from 5% to 35%.10George E.P. Raabe D. Ritchie R.O. High-entropy alloys.Nat. Rev. Mater. 2019; 4: 515-534Crossref Scopus (1540) Google Scholar,25Miracle D.B. Senkov O.N. A critical review of high entropy alloys and related concepts.Acta Mater. 2017; 122: 448-511Crossref Scopus (4258) Google Scholar The restriction of a single phase is introduced to distinguish it from the occurrence of phase separation by several elements (Figure 1C). Another important factor involved in defining HEAs is high entropy. High configuration entropy is the main reason for reducing the free energy and maintaining the stability of HEAs.26Fultz B. Vibrational thermodynamics of materials.Prog. Mater. Sci. 2010; 55: 247-352Crossref Scopus (445) Google Scholar The phase stability of an alloy system is determined by its Gibbs free energy, which can be expressed by the following equations10George E.P. Raabe D. Ritchie R.O. High-entropy alloys.Nat. Rev. Mater. 2019; 4: 515-534Crossref Scopus (1540) Google Scholar:ΔGmix=ΔHmix−TΔSmixΔHmixwhere ΔGmix, ΔHmix and ΔSmix are the Gibbs free energy, mixing enthalpy, and mixing entropy, respectively, and T is the absolute temperature. As simplified in the discussions from Yeh and coworkers,6Yeh J.W. Chen S.K. Lin S.J. Gan J.Y. Chin T.S. Shun T.T. Tsau C.H. Chang S.Y. Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes.Adv. Eng. Mater. 2004; 6: 299-303Crossref Scopus (7947) Google Scholar the single-phase solid solution is considered an ideal solution, and the competitive phase separation is perfectly ordered. As a result, the difference in free energy and therefore stability between solid solution and phase separation depends on the relative value of TΔSmix and the enthalpy of the phase separation structure. The entropy of the system (ΔSmix) includes configurational entropy, phonon vibration entropy, electronic randomness entropy, and magnetic dipole entropy.26Fultz B. Vibrational thermodynamics of materials.Prog. Mater. Sci. 2010; 55: 247-352Crossref Scopus (445) Google Scholar,27Wilson A.G. The use of the concept of entropy in system modelling.Oper. Res. Q. 1970; 21: 247-265Crossref Google Scholar,28Li H. Lai J. Li Z. Wang L. Multi-sites electrocatalysis in high-entropy alloys.Adv. Funct. Mater. 2021; 31: 2106715Crossref Scopus (59) Google Scholar Generally, the configuration entropy contributes dominantly to the entropy of the system. In an ideal solid solution, the configuration entropy can be calculated by the following equation10George E.P. Raabe D. Ritchie R.O. High-entropy alloys.Nat. Rev. Mater. 2019; 4: 515-534Crossref Scopus (1540) Google Scholar,25Miracle D.B. Senkov O.N. A critical review of high entropy alloys and related concepts.Acta Mater. 2017; 122: 448-511Crossref Scopus (4258) Google Scholar:ΔSconf=−RΣxilnxi where R is the gas constant and xi is the mole fraction of the ith component. From the equation, it can be deduced that the maximal configuration entropy can be acquired when the mole ratio of each component is equal:ΔSconf=Rlnnin which n is the number of elements in the alloy. Accordingly, the configuration entropies of equiatomic ternary, quaternary, quinary, senary, and nonary alloys are calculated to be 1.10R, 1.39R, 1.61R, 1.79R, and 2.20R, respectively. From the above discussions, it can be concluded that containing more elements in near-equal ratios could significantly increase the contribution of the entropy part and stabilize the solid-solution structure, which is why the alloys are called HEAs. It is worth noting that some studies indicate that the number of elements in HEAs is not strictly limited to five or more, and concepts proposed earlier, such as multiprincipal element alloys or complex concentrated alloys, also belong to the category of HEAs.5Cantor B. Chang I.T.H. Knight P. Vincent A.J.B. Microstructural development in equiatomic multicomponent alloys.Mater. Sci. Eng., A. 2004; 375-377: 213-218Crossref Scopus (4861) Google Scholar,29Cantor B. Kim K. Warren P.J. Novel Multicomponent Amorphous Alloys. Trans Tech Publ, 2002: 27-32Google Scholar,30Cantor B. Multicomponent and high entropy alloys.Entropy. 2014; 16: 4749-4768Crossref Scopus (345) Google Scholar Nanoscale HEAs exhibit novel high stability compared with other nanomaterials. According to the efforts put into understanding the structure of HEAs in recent years, the stability of HEAs could come from entropy stabilization and sluggish diffusion effect (Figure 2A). The large configuration entropy could compensate for the enthalpy part and reduce the Gibbs free energy. Therefore, the high configuration entropy of HEAs is able to enhance the thermodynamic stability of alloys and avoid structural collapse.3Zhang Y. Zuo T.T. Tang Z. Gao M.C. Dahmen K.A. Liaw P.K. Lu Z.P. Microstructures and properties of high-entropy alloys.Prog. Mater. Sci. 2014; 61: 1-93Crossref Scopus (4259) Google Scholar,6Yeh J.W. Chen S.K. Lin S.J. Gan J.Y. Chin T.S. Shun T.T. Tsau C.H. Chang S.Y. Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes.Adv. Eng. Mater. 2004; 6: 299-303Crossref Scopus (7947) Google Scholar Besides, the diffusion coefficient of atoms in HEAs is much slower than that in single metals or traditional alloys.34Tsai K.-Y. Tsai M.-H. Yeh J.-W. Sluggish diffusion in co–cr–fe–mn–ni high-entropy alloys.Acta Mater. 2013; 61: 4887-4897Crossref Scopus (1328) Google Scholar The origin of sluggish diffusion is ascribed to the severe lattice distortion. The lattice potential energy varies with location in HEAs due to the change in the lattice, which increases the activation energy of atomic diffusion and inhibits diffusion. The calculation research also indicates that the composition-dependent barriers for vacancy migration in HEAs lead to sluggish diffusion and further increased stability.35Osetsky Y.N. Béland L.K. Barashev A.V. Zhang Y. On the existence and origin of sluggish diffusion in chemically disordered concentrated alloys.Curr. Opin. Solid State Mater. Sci. 2018; 22: 65-74Crossref Scopus (80) Google Scholar Lattice distortion could also strengthen the hardness of HEAs.36Tsai M.-H. Yeh J.-W. High-entropy alloys: a critical review.Mater. Res. 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A perspective on high-entropy two-dimensional materials.SusMat. 2022; 2: 65-75Crossref Google Scholar Moreover, the synergistic effect among constituent elements in HEAs gives rise to novel properties that cannot be achieved by any constituent elements such as high electrocatalytic stability.41Zhao R.-F. Ren B. Zhang G.-P. Liu Z.-X. Cai B. Zhang J.-j. CoCrxCuFeMnNi high-entropy alloy powders with superior soft magnetic properties.J. Magn. Magn Mater. 2019; 491: 165574Crossref Scopus (37) Google Scholar However, the complex element compositions make the synergistic effect quite confusing, and further understanding is needed from both experiments and theory. Given the potential applications of HEAs, it is highly probable for them to encounter oxidizing or reducing environments. Therefore, it is crucial to comprehend the stability and structural changes of HEAs under such conditions. Yassar et al. conducted a study on the oxidation behavior of FeCoNiCuPt HEA nanoparticles in air at 400°C through in situ gas-cell transmission electron microscopy.42Song B. Yang Y. Rabbani M. Yang T.T. He K. Hu X. Yuan Y. Ghildiyal P. Dravid V.P. Zachariah M.R. et al.In situ oxidation studies of high-entropy alloy nanoparticles.ACS Nano. 2020; 14: 15131-15143Crossref PubMed Scopus (41) Google Scholar The results indicate that the oxidation of HEA nanoparticles displays logarithmic rate constants and is governed by Kirkendall effect with surface segregation of Fe, Co, Ni, and Cu, while Pt remains in the core region. Moreover, the HEA nanoparticles exhibit a significantly slower oxidation rate compared with monometallic nanoparticles. Subsequently, the researchers investigated the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles in H2 at 400°C using the same method.43Song B. Yang Y. Yang T.T. He K. Hu X. Yuan Y. Dravid V.P. Zachariah M.R. Saidi W.A. Liu Y. Shahbazian-Yassar R. Revealing high-temperature reduction dynamics of high-entropy alloy nanoparticles via in situ transmission electron microscopy.Nano Lett. 2021; 21: 1742-1748Crossref PubMed Scopus (12) Google Scholar During reduction, the outward diffusion of transition metals results in the expansion of the oxide layer, and Cu was reduced to Cu nanoparticles while Fe, Co, and Ni remained in the oxidized state. Rather than a perfect crystal, George et al. concluded that various structural defects in other metals or alloys are also present in HEAs, such as vacancies (point defects, 0D), dislocations (line defects, 1D), grain boundaries (interface defects, 2D), stacking faults (interface defects, 2D), and precipitates (volume defects, 3D) (Figure 2B).10George E.P. Raabe D. Ritchie R.O. High-entropy alloys.Nat. Rev. Mater. 2019; 4: 515-534Crossref Scopus (1540) Google Scholar The existence of these defects may lead to a decrease in stability. Vacancies are missing atoms in the 3D atomic structure and determine the diffusion of atoms.44Nong Z.-S. Gu Z.-H. Liu Y.-W. Wang Z.-Y. Zhu J.-C. Formation and migration behavior of vacancy in multi-component alloys.Intermetallics. 2022; 151: 107724Crossref Scopus (1) Google Scholar Dislocations include edge dislocations (Burgers vector normal to the dislocation line) and screw dislocations (Burgers vector parallel to the dislocation line), which are highly related to the plastic deformation in metals.45Huang M. He B. Alloy design by dislocation engineering.J. Mater. Sci. Technol. 2018; 34: 417-420Crossref Scopus (43) Google Scholar Grain boundaries refer to the interface separating two domains with different orientations in a polycrystal.46Yu Z. Cantwell P.R. Gao Q. Yin D. Zhang Y. Zhou N. Rohrer G.S. Widom M. Luo J. Harmer M.P. 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It is worth noting that the CSRO also shows periodicity with an interplanar spacing (dCSRO) twice that of {-311} planes (dfcc). The uneven distribution of elements was also confirmed by atomic-resolution chemical mapping. Yu and coworkers mapped the atomic-scale element distributions of CrMnFeCoNi and CrFeCoNiPd HEAs (Figure 2E).33Ding Q. Zhang Y. Chen X. Fu X. Chen D. Chen S. Gu L. Wei F. Bei H. Gao Y. et al.Tuning element distribution, structure and properties by composition in high-entropy alloys.Nature. 2019; 574: 223-227Crossref PubMed Scopus (661) Google Scholar After substituting Mn atoms with Pd atoms that are more different in atomic size and electronegativity, the homogeneity of the elementals decreases considerably and shows apparent atomic segregation with a concentration wave wavelength of approximately 1–3 nm. These defects and short-range order provide more adjustable structural space of HEAs for desirable properties and help identify the structural origin of interesting features. In addition to the chemical-disordered solid-solution structure, the other two structures with high entropy are discovered and classified as HEAs: high-entropy intermetallics (HEIs) and high-entropy single-atom catalysts (HESACs). As displayed in Figure 3A, different from the complete random atomic arrangement, there is also a kind of chemically ordered occupation of atoms in HEAs similar to the well-known intermetallic compounds. The atom sites of components are definite in HEIs, which is beneficial to the study of the role of each element. Quan and coworkers synthesized hcp PtRhBiSnSb HEI nanoplates possessing a similar structure to PtBi intermetallic compounds, where Rh atoms substitute on Pt columns and Sn/Sb atoms occupy Bicolumns (Figures 3B and 3C).50Chen W. Luo S. Sun M. Wu X. Zhou Y. Liao Y. Tang M. 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