Recent progress in intermetallic nanocrystals for electrocatalysis: From binary to ternary to high‐entropy intermetallics

Developing sustainable and clean energy‐conversion techniques is one of the strategies to simultaneously meet the global energy demand, save fossil fuels and protect the environment, in which nanocatalysts with high activity, selectivity and durability are of great importance. Intermetallic nanocrystals, featuring their ordered atomic arrangements and predictable electronic structures, have been recognized as a type of active and durable catalysts in energy‐related applications. In this minireview, the very recent progress in the syntheses and electrocatalytic applications of noble metal‐based intermetallic nanocrystals is summarized. Various synthetic strategies, including the conventional thermal annealing approach and its diverse modifications, as well as the wet‐chemical synthesis, for the construction of binary, ternary and high‐entropy intermetallic nanocrystals have been discussed with representative examples, highlighting their strengths and limitations. Then, their electrocatalytic applications toward oxygen reduction reaction, small molecule oxidation reactions, hydrogen evolution reaction, CO2/CO reduction reactions, and nitrogen reduction reaction are discussed, with the emphasis on how the ordered intermetallic structures contribute to the enhanced performance. We conclude the minireview by addressing the current challenges and opportunities of intermetallic nanocrystals in terms of syntheses and electrocatalytic applications.


| INTRODUCTION
Concerning the ever-increasing global energy demand, the limited fossil fuel reserves, and the environmental pollution issues such as greenhouse gas emissions caused by the excessive usage of fossil fuels, it is highly imperative to develop sustainable and clean energyconversion techniques. For example, fuel cells, powered by fuels such as hydrogen, methanol and ethanol, can produce clean electricity with high energy-conversion efficiency, high energy density and environmental friendliness. 1 Electrolyzers enable the production of high-purity H 2 under mild conditions through water splitting with renewable electricity. 2 For these clean energy devices, electrochemical hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and small molecule oxidation reactions are of significant importance. Furthermore, greenhouse gas CO 2 and/or earth-abundant N 2 can be converted into value-added chemicals such as methane, formate, ethylene, ammonia and urea through electrochemical CO 2 reduction and/or N 2 reduction under ambient conditions. [3][4][5][6][7][8] In order for all these electrochemical reactions to take place with satisfying kinetics, electrocatalysts with high activity, selectivity and stability are normally required. For most of these reactions, noble metals, especially the platinum-group metals, are the most efficient catalysts among other elements. However, considering the scarcity, high cost and the ambition to further boost the catalytic performance, great research efforts have been devoted to exploring high-performance metal-based or even nonmetal-based nanocatalysts. [9][10][11] The catalytic performance of metal-based nanocatalysts can be modulated by adjusting their electronic structures, which can be achieved via tuning their size, composition, shape, exposed facet, crystal phase, defect, etc. [12][13][14][15] In particular, for binary or even multinary metallic systems, manipulating the degree of ordering of their constituent elements has been proven as an effective approach to tune the catalytic performance. [16][17][18] For disordered alloys, the constituent elements can randomly occupy any crystallographic site without local preference. The probability of occupancy by an element is determined by the stoichiometric ratio of the alloy. In contrast, for ordered intermetallics, every crystallographic site is expected to be occupied by a definite element, showing atomic ordering. Thus, the crystal structure of the intermetallic materials, the orbital bonding of metals, and the stoichiometry of each constituent element are explicit, enabling the predictable control of the electronic structures for optimizing their catalytic performance. 19 To date, intermetallic nanocrystals with long-range atomic ordering have been shown to outperform their disordered counterparts toward many reactions, 17,18 which could be ascribed to several mechanisms: (i) Upon transforming disordered alloys to their intermetallic counterparts, bond length and coordination environment could be altered, leading to change of the electronic structures and thus the position of the d-band center, which could directly affect the adsorption and desorption abilities of reactants and/or intermediates on the catalyst surface, thereby affecting the catalytic performance. 20,21 (ii) Intermetallic structures have a more negative enthalpy of formation and thus stronger interatomic bonding compared to their disordered counterparts with similar composition and morphology, leading to enhanced resistance to corrosion and excellent catalytic durability in harsh chemical environments. In contrast, non-noble metals in disordered alloys could undergo severe oxidation and dissolution after long-term operation, especially in acidic environment, leading to the destruction of catalytic active sites and thus loss of catalytic activity. 19,22 For example, the L1 2 Pt 3 Zr nanoparticles exhibited tremendously enhanced stability than the fcc counterparts, showing a minimal Cr leaching (13.5%) after 4 weeks immersion in acidic electrolyte and a minimal loss of ORR activity (14.7%) after 5000 cycles, both of which were smaller than those of the fcc Pt 3 Zr nanoparticles (22.3% and 23.4%, respectively). 23 (iii) For both ordered intermetallics and disordered alloys, leaching of the less-noble metal from the near-surface region results in the formation of a core-shell structure, in which a thin shell composed of the more noble metal is coated over the intermetallic/alloy core. The lattice mismatch between core and shell, induced by the difference in crystal structure or lattice parameter, could cause compressive or tensile strain, thereby affecting the position of the d-band center and the binding energy between the catalyst surface and the reaction species. 24,25 Due to their structural difference, ordered intermetallics and disordered alloys may exhibit distinct degrees of metal leaching, as well as different underlayer and surface atomic structures, and therefore dissimilar strain effects. For example, when ordered and disordered PtCu 3 nanoparticles were subject to acid leaching in 0.1 mol/L HClO 4 , after dealloying for 12 h, in the ordered sample, the coordination number of Cu around Pt was 15%-30% higher than that in the disordered counterpart, resulting in more pronounced ligand and/or strain effects and 23%-37% higher ORR activities than the disordered one. 26 (iv) Intermetallic nanocrystals with tunable stoichiometries and crystal structures enable the creation of isolated active sites resembling single-atom catalysts, as well as active-site ensembles that are composed of more than one catalytic active metal. Such concept has been widely used in selective heterogeneous catalysis [27][28][29][30] and electrocatalysis. 31,32 For example, by constructing Pd-In intermetallic structures, the contiguous Pd sites could be isolated into single-Pd sites or Pd ensembles. 30 It was theoretically and experimentally shown that on the (110) facet of the intermetallic PdIn, where Pd atoms were present as single-atom sites, the selectivity for acetylene hydrogenation to ethylene was higher (92%) than that on the (111) facet of the intermetallic Pd 3 In, where Pd atoms were present as trimer sites (21%).
Although several review papers on intermetallic nanocatalysts have been reported, [16][17][18][19][33][34][35] a timely review summarizing the very recent progress in this field is still of high significance, considering its fast development. In this minireview, very recent progress in the syntheses and electrocatalytic applications of noble metal-based binary, ternary, and high-entropy intermetallic (HEI) nanocrystals is summarized (Scheme 1). Diverse synthetic strategies are first introduced, including the conventional thermal annealing approach and its various modifications, as well as the wet-chemical synthetic method, with highlight on their strengths and limitations. Then, the electrocatalytic applications of intermetallic nanocrystals toward ORR, small molecule oxidation reactions, HER, CO 2 /CO reduction reactions, and nitrogen reduction reaction are summarized. Finally, current challenges and future opportunities in this research field are discussed.

| SYNTHESIS OF BINARY INTERMETALLIC NANOCRYSTALS
The crystal structures of intermetallic nanocrystals are mainly derived from three types of monometallic structures, i.e., face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp). As shown in Figure 1, typical fcc-derived intermetallic structures include L1 0 (CuAu-type, space group P4/mmm, e.g., PtFe, 36 PtCo, 37 PtZn, 38 PdZn 39 ), L1 1 (PtCu-type, space group R3 m, e.g., PtCu 40 ), and L1 2 (Cu 3 Au-type, space group Pm3m, e.g., Pt 3 Co, 31,41 Pt 3 Zn, 42 Pt 3 Sn, 43 Pd 3 Pb, 44,45 Au 3 M (M = Fe, Co, Ni) 46 ). A typical bcc-derived intermetallic structure is B2 (CsCl-type, space group Pm3 m, e.g., PdCu, 47 FeRh 48 ). A typical hcp-derived intermetallic structure is B8 1 (NiAs-type, space group P6 3 /mmc, e.g., PtBi, 49,50 PtPb 25,51 ). For the reported noble metal-based intermetallic nanocrystals, L1 0 and L1 2 are the most commonly observed structures. The L1 0 structure can be viewed as the tetragonal distortion of the fcc structure, in which the atomic layers of two elements with a stoichiometric ratio of 1:1 are stacked alternately along the c-axis. In the L1 2 structure, two elements with a stoichiometric ratio of 3:1 are located in the sites of an fcc lattice, in which the element with a higher atomic ratio is located at the face-centered sites, while the other is located at vertices. These intermetallic structures are identified mainly by two techniques, i.e., X-ray diffraction (XRD) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). XRD identifies the phase of crystalline materials, in which the characteristic ordering peaks at relatively small angles are normally present for the ordered intermetallic structures, while absent for the disordered counterparts. In HAADF-STEM images, the local ordered arrangement of the constituent elements in an intermetallic structure along a specific zone axis can be directly visualized based on their different atomic numbers which give different Z contrasts. Element with a larger atomic number appears as the brighter spot, while the one with a smaller atomic number appears as the darker spot. Nevertheless, the Z contrasts may become ill-defined when the atomic numbers of both elements are quite close, necessitating other characterizations to confirm the ordered atomic arrangement. morphology, uniformity and architecture via diverse synthetic methods. [16][17][18][19][33][34][35] Among them, thermal annealing at high temperatures to induce the formation of ordered intermetallic structures is the conventional approach. Briefly, metal precursors are first impregnated onto a substrate. After drying, the powders are placed in a furnace for thermal annealing at a specific temperature for a specific period of time. [52][53][54][55] For example, [Fe (bpy) 3 ] 2+ [PtCl 6 ] 2− (bpy = bipyridine) precursors were first prepared and impregnated on graphene oxide (GO). 56 After drying, the mixture was annealed at 700°C under Ar, leading to the formation of intermetallic L1 0 PtFe nanoparticles loaded on reduced graphene oxide (rGO), whose ordered structure was visualized in the HAADF-STEM image, showing alternating Pt and Fe atomic columns with bright and dark intensities, respectively ( Figure 2A). Meanwhile, a thin N-doped carbon layer, derived from the bipyridine ligand, was formed over the nanoparticle surface during annealing, serving as a protective shell to prevent particle aggregation. Alternatively, metal precursors are first reduced at relatively lower temperatures to form disordered alloys, which are then loaded on the substrate and subjected to high-temperature thermal annealing to induce disorderto-order transition. 31,57,58 For example, disordered PtM 3 (M = Co, Fe) nanoparticles were first prepared by reducing Pt(acac) 2 and Co(acac) 2 /Fe(acac) 3 precursors in organic solvents, which were then supported on carbon black. 59 After drying, the supported nanoparticles were annealed at 180°C in air to remove the capping ligands, and then 400°C under H 2 /Ar. The disordered-toordered transition was induced by further annealing at 600°C under H 2 /Ar, resulting in the formation of intermetallic L1 2 PtCo 3 and PtFe 3 nanoparticles, whose F I G U R E 1 Typical intermetallic structures, i.e., L1 0 , L1 1 , and L1 2 derived from fcc, B2 derived from bcc, B8 1 derived from hcp. fcc, facecentered cubic; hcp, hexagonal close-packed. ordered structures were confirmed by the appearance of the (100) and (110) ordering peaks in the XRD patterns ( Figure 2B).
In this method, three parameters are highly important, including the substrate, annealing temperature and annealing period. High-surface-area substrates can enlarge the interparticle distance and provide a support for the migration and diffusion of metal atoms, such that particle aggregation can be mitigated to some extent during annealing. Various two-dimensional (2D) substrates have been used for this purpose, such as carbon black, 54,59 GO,56,60,61 and metal oxides. 62 The annealing temperature and period are determinant parameters for manipulating the degree of ordering of intermetallic nanocrystals. Normally, higher temperature and longer annealing period could overcome the kinetic energy barrier and promote atom diffusion, resulting in the formation of intermetallic nanocrystals with higher degree of ordering. The annealing temperature is set at or above the disorder-to-order transition temperature, which is different for different bimetallic systems. To highlight, due to the extremely reduced size of nanoparticles compared to bulk crystals, quantum-size effect arises, which can influence the thermodynamic and kinetic stabilities of many intermetallic materials. 63 Besides, nanoparticles with a high surface-area-tovolume ratio can facilitate atom diffusion and promote the ordering transformation. 19 Therefore, the disorder-toorder transition temperatures shown in the binary phase diagrams of bulk crystals can only be taken as a guidance for the nano-systems, while the real transition temperatures are typically lower. For example, the disorder-toorder transition temperature for Pt-Co nanoparticles with size of 2.4-3 nm was 175°C-325°C, while it increased to approximately 825°C for the bulk counterpart. 64 In addition to size, the shape of nanocrystals 64,65 and the presence of vacancy 66,67 also affect the disorder-to-order transition temperature. It is generally believed that a higher density of vacancies promotes the atom diffusion process. 66 Despite the effectiveness and widespread applications of the thermal annealing method for synthesizing intermetallic nanocrystals, the disadvantages of this method are also evident. 18 First, high temperature may lead to particle aggregation and sintering, resulting in the formation of large particles with broad size distributions. Second, high temperature disables control over the shape of intermetallic nanocrystals, because particles with anisotropic shapes and high-index facets are prone to undergo shape reconstructions in an effort to minimize the total surface energy. 19 All these lead to the reduction of active sites and surface areas of the nanocatalysts, and therefore the catalytic performance deteriorates. Besides, high temperature also disables the synthesis of intermetallic nanocrystals whose structures are only stable at lower temperatures.

| Modifications to the thermal annealing method
To solve the above-mentioned problems, various strategies have been developed as modifications to the conventional thermal annealing method. First, protective shells, such as silica, 38,40 MgO, 22,36 and carbon, 68 can be coated over the disordered alloy nanoparticles before the disorder-to-order transition, functioning to protect the nanoparticles from aggregation during annealing at high temperatures. For example, silica-protected intermetallic L1 0 PtZn nanoparticles loaded on multiwalled carbon nanotubes exhibited size of approximately 3.2 nm, while those without the silica protection had larger size of approximately 27 nm, demonstrating the protective function of silica in constraining the particle size during annealing. 38 Polydopamine, as a polymer, was coated over the caron-supported disordered fcc PtFe nanoparticles. 68 After thermal annealing at 700°C, intermetallic L1 0 PtFe nanoparticles coated with thin layers of Ndoped carbon shell derived from polydopamine were formed ( Figure 3A). The resultant intermetallic nanoparticles had smaller size of only approximately 6.5 nm, which was similar to the size of disordered nanoparticles before annealing, thanks to the protective effect of the carbon shell. However, these protective shells may limit the atom mobility to some extent, thus impeding the complete disorder-to-order transition. Besides, most of the protective shells could block the catalytic active sites and limit the mass transport of reactant molecules. Thus, an addition step is normally required to remove these protective shells before catalytic testing, which may add complexity, increase cost, and limit the practical applications. Nevertheless, thin and porous protective shells that are permeable to the reactant molecules do not have such problem, such as the in-situ formed N-doped carbon shell with thickness smaller than 1 nm that is permeable to the O 2 molecule during the ORR test, 68 and thus shell removal is unnecessary.
Second, substrates having a strong interaction with the supported nanoparticles can be used. Such a strong metal-support interaction can effectively immobilize the particles and prevent them from aggregation during high-temperature annealing, enabling the synthesis of size-controlled intermetallic nanoparticles. 72 S-or Ncontaining substrates are normally used for this purpose, due to the strong interactions between metals and S/N elements. 69,73 For example, S-doped carbon substrate was first prepared, onto which H 2 PtCl 6 and other metal salts were wet-impregnated. 69 Upon annealing under H 2 at 800°C-1100°C, a library of intermetallic Pt-based nanoparticles with size of sub-5 nm were prepared, including Pt alloyed with the early transition metals (Pt 3 Sc, Pt 3 Ti, Pt 3 V, Pt 3 Cr, and Pt 3 Zr), late transition metals (Pt 3 Mn, Pt 3 Fe, PtFe, Pt 3 Co, PtCo, PtNi, PtCu, PtCu 3 , Pt 3 Zn, and PtZn), and p-block metals (Pt 3 Al, Pt 3 Ga, Pt 3 Ge, Pt 3 In, and Pt 3 Sn). Size control was achieved due to the strong chemical interaction between Pt and S, as demonstrated by the presence of Pt-S bond in the S L-edge X-ray absorption near edge structure spectra ( Figure 3B). As another example, the strong interaction between Mo and C was utilized to synthesize carbon-supported intermetallic twinned Pt 2 Mo nanoparticles with size of 3-5 nm. 74 It was experimentally shown that H 2 PtCl 6 was preferentially reduced to Pt nanoparticles at 800°C, followed by the diffusion of Mo into Pt nuclei at 1000°C. The key to the formation of intermetallic structure was the slow diffusion rate of Mo into Pt, which was induced by the strong Mo-C interaction. If the carbon substrate was replaced with MgO, due to the weak interaction between MgO and Mo, Mo tended to undergo selfnucleation, forming the undesired Mo particles. Furthermore, some substrates can also react with the impregnated metal precursors for the formation of intermetallic nanoparticles. For example, through impregnating Pt on two types of MXenes, i.e., Ti 3 C 2 T x and Nb 2 CT x , followed by reduction at 550°C under H 2 , intermetallic L1 2 Pt 3 Ti and Pt 3 Nb nanoparticles loaded on MXenes were formed, respectively, via the reaction between Pt and the surface Ti (or Nb) atoms. 21 The reaction was driven by the relatively weak metal-carbon bonds in MXenes and the thermodynamic stability of the Pt 3 M (M = Ti, Nb) compounds.
Third, instead of using 2D sheets as substrates, threedimensional (3D) frameworks can be used to confine the growth of nanoparticles inside the pores, thereby preventing particle aggregation and enabling size control. Mesoporous carbon, 24,75 silica, 70,76-80 zeolite, 81 and salt matrix 23,71,82,83 have been reported as the 3D frameworks. For example, mesoporous silica (SBA-15) was used as the substrate, onto which Rh and Zn precursors were impregnated. 70 Upon thermal annealing under H 2 at 500°C, intermetallic RhZn nanoparticles with size of approximately 9.6 nm were formed, which were evenly distributed within the pores of SBA-15 frameworks ( Figure 3C). During the synthesis of Pt 3 Fe nanoparticles, the by-product KCl, in situ formed from the metal chloride precursors and the reducing agent KEt 3 BH, served as a protective matrix to confine the nanoparticles and prevent them from aggregation during the subsequent thermal annealing process at approximately 600°C, leading to the formation of intermetallic Pt 3 Fe nanoparticles with size of approximately 4 nm ( Figure 3D). 71 The as-obtained Pt 3 Fe nanoparticles were then released from the KCl matrix and supported on carbon black by dissolving KCl in the mixture of ethylene glycol and water.
Furthermore, metal-containing 3D nanostructures can also be used as sacrificial templates, onto which another metal is impregnated. Upon thermal annealing, metals from the 3D nanostructures will form chemical bond with the loaded metals, leading to the formation of intermetallic nanoparticles. 84,85 For example, ZnO nanorods coated with polydopamine were used as the sacrificial template, onto which the Pt precursor was loaded. 85 Upon reduction under H 2 at 800°C, intermetallic L1 0 PtZn nanoparticles supported on hollow N-doped carbon nanotubes were formed, where Zn came from the sacrificial template ZnO, and the N-doped carbon nanotubes came from the decomposition of polydopamine, functioning to restrict the migration of Pt atoms during high-temperature annealing. The remaining ZnO template was subsequently removed through acid wash ( Figure 4A). Metal-organic frameworks, due to the presence of metal cations and cavities in their structures, have also been widely used as the sacrificial templates. 86,87 For example, ZIF-8, with Zn being its skeleton metal, was used as the sacrificial template, onto which metals such as Pt 86 or Pd 87 were incorporated. Upon annealing, Zn diffused inside the Pt or Pd nanoparticles, leading to the formation of intermetallic L1 0 PtZn nanoparticles with size of approximately 3.2 nm ( Figure 4B) and PdZn nanoparticles with size of sub-2 nm, respectively.
Forth, instead of directly conducting thermal treatment on substrate-supported metal precursors or disordered alloys, heterostructures, such as core-shell [90][91][92][93][94] and dimer 95 structures, can be prepared first via wet-chemical methods, which are subsequently transformed into intermetallic nanoparticles via thermal annealing. Metal oxides are usually synthesized as the shell, such that during thermal annealing in the reductive environment, rich oxygen vacancies can be generated, promoting the inter-diffusion between core and shell. 91,92,95 For example, intermetallic L1 0 PdFe nanoparticles were prepared by synthesizing Pd@FeO x core-shell structures first, followed by annealing under H 2 at 600°C. 91 In the reductive environment, the FeO x shell was reduced to metallic Fe, with the simultaneous generation of rich oxygen vacancies. The vacancies facilitated the inter-diffusion between Pd core and Fe shell. As another example, intermetallic L1 0 PtFe nanoparticles were prepared by annealing the pre-synthesized dumbbelllike PtFe-Fe 3 O 4 nanoparticles under H 2 at 700°C. 95 The oxygen vacancies created during the Fe 3 O 4 reduction process could facilitate the inter-diffusion between Fe and Pt.
Other modifications to the conventional thermal annealing method have also been developed to controllably synthesize intermetallic nanoparticles with desired size and structure. For example, instead of the conventional oven drying process, a hydrogel freeze drying approach was developed before thermal annealing, for the synthesis of a series of intermetallic Pt 3 M (M = Mn, Cr, Fe, Co, etc.) nanoparticles supported on rGO. 96 The freeze drying process could retain the 3D porous structure of the GO hydrogel, such that metal precursors could be confined and immobilized on GO, enabling the formation of intermetallic nanoparticles with small size of approximately 3 nm. Besides, the heating and cooling rates in the thermal annealing process can be controlled. Rapid heating rate (on the order of 10 5 K/s) achieved by Joule heating has been demonstrated to promote the generation of rich vacancies in the initially formed disordered structures, thereby facilitating atomic ordering and the formation of carbon nanofiber-supported L1 2 Pd 3 Pb nanoparticles with small size of approximately 6 nm within 60 s. 88 In contrast, traditional furnace heating with a slower heating rate and longer annealing time (3 h) resulted in large intermetallic nanoparticles with size of approximately 85 nm ( Figure 4C). On the other hand, rapid quenching process has been shown to enable the capture of metastable phases, such as PdSn intermetallic nanoparticles with a metastable FeAs-type orthorhombic phase, whereas the thermodynamically stable monoclinic Pd 3 Sn 2 phase was obtained through the natural cooling process ( Figure 4D). 89 Very recently, a general small molecule-assisted impregnation strategy was reported to prepare 18 intermetallic Pt-based nanoparticles supported on carbon black. 97 These small molecules contain heteroatoms, i.e., O, N, or S, such that they were coordinated with Pt during the impregnation process and thermally converted into heteroatom-doped graphene layers coated over nanoparticles during hightemperature annealing, thereby suppressing particle aggregations. By utilizing the sodium thioglycolate additive, several intermetallic nanoparticles with small sizes of approximately 5 nm were obtained, while those without sodium thioglycolate exhibited larger sizes of 8.2-12.2 nm.

| Wet-chemical synthesis
Alternative to the conventional thermal annealing method, wet-chemical synthesis, featuring its strong ability to prepare nanocrystals with highly controlled size, shape, composition, phase, etc., has been widely explored to directly synthesize intermetallic nanocrystals in solution. 35 For wet-chemical synthesis of noble metalbased intermetallic nanocrystals, those containing lowmelting-point metals (such as Bi-271°C, Pb-327°C, Sn-232°C, and Zn-419°C) are more accessible, as compared to those containing high-meting-point metals (such as Fe-1538°C, Co-1495°C, and Cu-1083°C). 98 This is partially because the mobility of metallic atoms is generally high at temperatures close to their melting points. As a result, when low-melting-point metals are incorporated, atomic diffusion is sufficient to induce ordering transformation at the reaction temperatures. In contrast, for high-melting-point metals, atomic diffusion may be limited by the reaction temperatures that are severely restricted by the solvent boiling points, thereby impeding the formation of intermetallic nanocrystals with high degree of ordering. 98 Besides, differences in crystals structure between noble metals (fcc or hcp) and metals such as Bi (rhombohedral), Sn (body-centered tetragonal), Sb (rhombohedral) suppress the formation of solidsolutions, while enable the direct synthesis of ordered structures according to the phase diagrams. 98 Wet-chemical synthesis of intermetallic nanocrystals can be categorized into two types, i.e., co-reduction of metal precursors and seed-mediated growth. Coreduction refers to the reduction of the constituent metal precursors in a one-pot reaction. 99,100 Since each metal has its own standard reduction potential, their reduction rates will be different. If the reduction potentials of metal ions are similar or a strong reducing agent is used, they will be reduced simultaneously. For example, NaBH 4 , as a strong reducing agent, has been applied to synthesize several intermetallic nanocrystals via co-reduction, such as Pd-M (M = Cu, Zn, Ga, Ge, Sn, Pb, Cd, and In), 101 AuCu, 102 and PtBi 2 . 103 Despite its effectiveness, the usage of strong reducing agents hinders the shape control of intermetallic nanocrystals, due to the rapid nucleation and growth rates. On the other hand, if the difference between their reduction potentials is large, and a relatively mild reducing agent is used, metal ions with a more positive reduction potential will be reduced first, serving as the seeds, onto which the slow-reducing metals are reduced, followed by their inter-diffusion to form intermetallic nanocrystals. For example, L1 2 crenellike Pt 3 Co hierarchical nanowires with high-index facets and Pt-rich surfaces were synthesized by co-reducing Pt (acac) 2 and Co(acac) 3 , where Pt nanowires were formed first followed by inter-diffusion between the newly formed Co atoms and Pt. 41 In addition, the reduction rates of the constituent metal ions can be further manipulated by a wide choice of solvents, capping agents, halide ions, and additives. For example, with the assistance of Br − ions, three types of noble metalbased intermetallic nanocrystals, including hcp PtBi nanoplates, fcc Pd 3 Pb nanocubes and hcp Pd 2.5 Bi 1.5 nanoparticles, were prepared through a one-pot coreduction method ( Figure 5A). 104 Here, Br − ions could not only slow down the reaction rate by coordinating with the metal ions, but also induce oxidative etching with the presence of oxygen to facilitate atom rearrangement, contributing to the formation of intermetallic nanocrystals.
Alternatively, seed-mediated growth is another effective approach to synthesize intermetallic nanocrystals in solution. Briefly, monometallic nanocrystals are prepared first, which are then used as seeds for the diffusion of the second metal to form intermetallics. [106][107][108] Normally, seeds with small size, ultrathin thickness and rich defects can promote the diffusion process. Seed-mediated growth enables control over size and shape of intermetallic nanocrystals, because the overgrowth of secondary metals would inherit the structural features of seeds. For example, monodisperse Au nanoparticles were used as seeds. Upon reduction and diffusion of Cu, intermetallic AuCu and AuCu 3 nanoparticles with narrow size distributions were formed. 106 Ultrathin Pd nanosheets were used as seeds for the formation of Pd-M (M = Pb, Sn, and Cd) ultrathin porous intermetallic nanosheets. 108 As another example, a general seeded diffusion method was developed to prepare a library of intermetallic nanocrystals consisting of transition metals (i.e., Au, Ag, Cu, Pd, and Ni) and low-melting-point metals (i.e., Ga, In, and Zn). 109 Briefly, transition metal nanocrystals were prepared first, followed by the injection of low-melting-point metal precursors. Upon annealing in oleylamine at temperatures >260°C for 10 min, the low-melting-point metals were diffused into the transition metal seeds, leading to the formation of intermetallic nanocrystals with precisely controlled size and composition.
In addition to monometallic seeds, bimetallic nanocrystals can also function as seeds. Compared to the transformation from monometallic seeds, the chemical conversion between bimetallic nanocrystals can be carried out more easily in solution owing to the shorter average diffusion length and the relatively lower diffusion barrier. For example, intermetallic PtSn nanoparticles were converted into intermetallic PtSn 2 and Pt 3 Sn nanoparticles by reacting with SnCl 2 and K 2 PtCl 6 , respectively, at temperature of approximately 280°C. 110 Disordered PdCu nanoparticles were converted into intermetallic B2 PdCu nanoparticles by reacting with Pd and Cu precursors in oleylamine at 270°C. 47 To highlight, a significant strength of wet-chemical synthesis is its ability to prepare intermetallic nanocrystals with highly controlled size, composition, shape and crystal phase, thanks to its great flexibility in tunning a wide range of parameters such as the precursor type, solvent, temperature, capping agent, additive and seed. In this regard, phase-controlled synthesis of bimetallic nanocrystals with either disordered alloy or ordered intermetallic structures has been achieved. For example, by using different types of Sn precursors and solvents, i.e., Sn(acac) 2 in oleylamine and SnCl 2 in oleylamine/ octadecene, intermetallic L1 2 and disordered fcc Pd 3 Sn nanorods with similar composition and shape, but different crystal phases were synthesized, respectively ( Figure 5B). 105 Besides, by tuning the metal precursor ratios and reaction temperatures, Pd-Sn nanoparticles with various structures were obtained, including the intermetallic hexagonal Pd 3 Sn 2 , intermetallic orthorhombic Pd 2 Sn and disordered fcc Pd 3 Sn nanoparticles. 111 As another example, simultaneous control over crystal phase and size of Pt-Bi intermetallic nanoparticles was achieved in microfluidic reactors. 112 Specifically, the employment of reaction temperatures of 260°C and 350°C resulted in the formation of hexagonal Pt 1 Bi 1 and trigonal Pt 1 Bi 2 intermetallic nanoparticles, respectively. By tuning the solvents (i.e., ethylene glycol, PEG400, PEG600) and the length of the reaction channel (i.e., 10-120 cm), the size of the Pt 1 Bi 2 nanoparticles was controlled in the range of 4-33.5 nm. Besides, for the Pt-Sn intermetallic nanoparticles synthesized via the seeded growth method, composition control was realized by tuning the size of the Pt seeds and the amount of Sn precursor, leading to the formation of PtSn, PtSn 2 , and PtSn 4 intermetallic nanoparticles. 113 Shape control has been achieved as well, with the assistant of capping agents that can selectively adsorb onto specific facets. So far, noble metal-based intermetallic nanocrystals with diverse shapes, including onedimensional (1D) nanorods/nanowires, 41,105,114,115 2D nanosheets/nanoplates, 25,39,44,49,116,117 and 3D polyhedra, 43,[118][119][120] have been prepared via wet-chemical synthesis. For example, L1 2 intermetallic Pt 3 Co hierarchical nanowires were prepared by reducing Pt(acac) 2 and Co(acac) 3 in oleylamine, where the formation of nanowires was ascribed to the usage of the cetyltrimethylammonium chloride (CTAC) shape-directing agent ( Figure 6A). 41 Orthorhombic intermetallic Pd 2 Sn nanorods were prepared by reducing Pd(acac) 2 and Sn (acac) 2 in oleylamine, where the usage of trioctylphosphine (TOP) and Cl − ions together contributed to the formation of nanorods. 114 The aspect ratio of nanorods was tuned by changing the concentration of TOP and Cl − ions, where higher concentration of TOP resulted in thicker rod, while higher concentration of Cl − ions resulted in longer rod ( Figure 6B). L1 0 intermetallic PdZn nanosheets with thickness less than 5 nm were synthesized by reducing Pd(acac) 2 and Zn(acac) 2 in oleylamine, where the formation of sheets was due to the usage of Mo (CO) 6 that could release the shape-directing molecule CO under high temperature ( Figure 6C). 39 Rhombohedral intermetallic Pd 8 Sb 3 hexagonal nanoplates with thickness of approximately 15.3 nm were prepared by reducing Pd(acac) 2 and SbCl 3 in benzyl alcohol via the solvothermal reaction, where polyvinyl pyrrolidone and NH 4 Br functioned as the shape-directing agents ( Figure 6D). 116 Interestingly, with the addition of W (CO) 6 while maintaining other reaction parameters unchanged, rhombohedral intermetallic Pd 20 Sb 7 rhombohedra were obtained, which were bounded by six rhombic faces that were obliquely intersected ( Figure 6E). 118 Three types of L1 2 intermetallic Pt 3 Sn nanocubes with cubic, concave cubic ( Figure 6F), and defect-rich cubic shapes were synthesized via the solvothermal method. 43 The large difference in electronegativity between Pt and Sn, as well as the chemical etching process induced by Cl − ions and oxygen enabled the shape control.
Nevertheless, there are still several limitations associated with the wet-chemical synthetic method. First, the boiling point of solvents greatly limits the range of reaction temperatures, which could cause incomplete disorder-to-order transition. Besides, when applying the as-obtained intermetallic nanocrystals as electrocatalysts, additional steps, such as loading of the nanocrystals on substrate and removal of capping ligands to expose active sites via thermal annealing or UV irradiation, are normally required, which may add complexity and increase cost. 121,122

| Other methods
In addition to the above-mentioned methods, other approaches, such as physical vapor deposition, 48 electrochemical method, 98,123,124 microwave-assisted method, 125 and sputtering, 126,127 have also been reported to synthesize binary intermetallic nanocrystals. For example, electrochemical deposition was used to directly synthesize the metastable intermetallic Pd 31 Bi 12 nanocrystals at room temperature from a solution containing Bi(C 2 H 3 O 2 ) 3 , Na 2 PdCl 4 , acetic acid and ethylenediaminetetraacetic acid. 98 Since electrodeposition can be performed at large overpotentials, corresponding to conditions far from equilibrium, the metastable Pd 31 Bi 12 structure that is difficult to be obtained via the conventional methods can be accessed. As another example, through an electrochemical dealloying process, monoclinic PdBi 2 was converted into orthorhombic Pd 3 Bi. 124,128 Its formation could be attributed to the low melting point of PdBi 2 (~480°C), indicating a low vacancy formation energy, such that the dealloying of Bi and atom inter-diffusion could be promoted via the vacancy-promoted diffusion mechanism. 128

| SYNTHESIS OF BINARY INTERMETALLIC NANOCRYSTAL-BASED HETEROSTRUCTURES
It has been reported that, for many noble metal-based binary intermetallic nanocrystals, a thin noble metal shell is observed coating over the binary intermetallic core, forming a core-shell structure. 129 The formation of the thin noble metal shell arises from the dealloying of non-noble metals from the binary intermetallic structures. In some cases, the noble metal shell forms concurrently with the growth of intermetallic nanocrystals or in-situ during the electrocatalytic test. In other cases, an additional step is performed to induce the formation of noble metal-rich surfaces, such as electrochemical dealloying, 59,130,131 acid leaching, 95,132,133 and high-temperature annealing. 31,134 For example, L1 0 PtFe nanoparticles were immersed in 0.1 mol/L HClO 4 solution at 60°C for 12 h, such that Fe atoms in surface layers were removed and a defective Pt shell was formed over the L1 0 PtFe core. 95 Subsequently, the defective Pt layer was smoothed by annealing under Ar/H 2 at 400°C, leading to the formation of L1 0 PtFe@Pt core-shell nanoparticles with the presence of an atomically thin (5 Å) Pt shell ( Figure 7A).
Since electrocatalytic reactions take place on the catalyst surface, such noble metal shells have two significant effects. First, the noble metal-rich surface endows the catalyst with enhanced durability, such that the dissolution of non-noble metals during the electrochemical process can be largely alleviated. Second, because of the lattice mismatch between the binary intermetallic core and monometallic shell, compressive or tensile strain will be developed in the overlayer. As a result of the strain effect, the electronic structure of the catalytic active metal will be changed, such that the binding energy of reaction species could be manipulated to improve the catalytic activity. For example, coating of an ultrathin Pt skin over the intermetallic Pt 3 Co nanoparticles could introduce a 4.4% compressive strain in the Pt shell. 24 This led to a downshift of the antibonding state between O-Pt and a weaker binding energy between the oxide intermediates and Pt, which improved the intrinsic ORR activity of Pt. On the other hand, coating of an ultrathin Pd skin over the intermetallic Pd 3 Pb nanosheets could introduce a 5.4% tensile strain in the Pd shell, because the lattice parameter of Pd was smaller than that of the intermetallic Pd 3 Pb ( Figure 7B). 135 To highlight, such strain effect will decrease with the increase of shell thickness, because strain can be relaxed when atoms have large distance. 137 Therefore, synthesis of thin metal shells with controllable thickness enables the precise control over strain. For example, by means of temperature-controlled Fe etching in acetic acid, three types of PdFe@Pd core-shell nanoparticles were prepared, in which PdFe core had size of approximately 8 nm and Pd shells exhibited controllable thickness of 0.27, 0.65, or 0.81 nm. It was found that the ORR activity was optimal when the Pd shell thickness was 0.65 nm. 138 Monometallic shells having compositions different from the intermetallic cores can also be coated via the seedmediated growth method. 48,139,140 For example, to maximize the utilization efficiency of the expensive noble metal Pt, L1 0 intermetallic AuCu nanoparticles were used as seed, onto which a thin Pt skin, consisting of 1.5 monolayers of Pt, was grown through the galvanic replacement reaction between Pt precursors and Cu atoms in surface layers. 139 As another example, to suppress the CO poisoning effect in methanol oxidation reaction (MOR), sub-monolayer Pb was grown on hexagonal PtBi intermetallic nanoplates via the seedmediated growth. 141 In addition to monometallic overlayers, bimetallic shells have been grown over the intermetallic cores as well, providing more room for strain modulation. [142][143][144][145][146] For example, PtFe@PtBi core-shell nanoparticles were synthesized by using L1 0 PtFe nanoparticles as seeds, followed by the leaching of Fe and the incorporation of Bi in acidic solutions. 145 Subsequently, atom rearrangement was triggered by thermal annealing under NH 3 at 500°C, leading to the formation of PtFe@PtBi core-shell nanoparticles consisting of the intermetallic PtFe core and 2-3 atomic layers of PtBi shell. Impressively, in such a PtFe@PtBi core-shell structure, both the core and shell endowed the surface Pt with a compressive strain. For the core, an interlayer compressive strain was induced due to the smaller atomic radius of Fe (0.127 nm) than that of Pt (0.139 nm). For the shell, an additional surface compression was induced by inplane shearing due to the presence of large Bi atoms (0.155 nm). As another example, PdCu@PtCu core-shell nanoparticles, comprising of the B2 intermetallic PdCu core and the fcc disordered PtCu shell, were prepared via the seed-mediated growth. 136 TEM analyses showed that an orientation-dependent surface strain was developed in the PtCu overlayer, i.e., the compressive strain on PtCu (200) was greater than that on PtCu (111) facets, due to their different degrees of lattice mismatch ( Figure 7C). PtBi@PtRh 1 core-shell nanoplates, comprising of the intermetallic PtBi core and Pt shell alloyed with discrete Rh atoms, were prepared. 147 Briefly, Rh single atoms were incorporated first via galvanic replacement between RhCl 3 and the surface Bi in PtBi nanoparticles, which were subsequently alloyed with Pt upon electrochemical dealloying of Bi atoms.

| SYNTHESIS OF TERNARY INTERMETALLIC NANOCRYSTALS
With the tremendous progress that has been made in preparing a library of binary intermetallic nanocrystals via diverse synthetic strategies, attention is being paid to incorporate more than two elements to form ternary or even multinary intermetallic nanocrystals. The synthetic methods of ternary noble metal-based intermetallic nanocrystals are similar to those for preparing their binary counterparts, except for the addition of the third metal precursor into the reaction system. 51,92,148,149 The conventional thermal annealing method and their modifications, [150][151][152][153][154] wet-chemical synthesis method, 39,155 and other strategies 156 have been reported to prepare ternary intermetallic nanocrystals. For example, L1 0 PtCoNi nanoparticles were prepared by impregnating the metal precursors (H 2 PtCl 6 ·6H 2 O, CoCl 2 ·6H 2 O, and NiCl 2 ·6H 2 O) with controlled molar ratios onto the carbon support, followed by thermal annealing under H 2 at 700°C. 157 Alternatively, the L1 0 PtNiCo nanoparticles with sub-6 nm were also prepared by annealing the presynthesized Pt@NiCoO x core-shell nanoparticles under Ar/H 2 at 600°C, in which the shell structures with rich oxygen vacancies not only promoted the atomic diffusion and ordering process but also protected the particles against sintering during annealing. 150 Intermetallic Pd 2-x Ni x Ge nanoparticles with controllable Pd/Ni ratios were prepared via a wet-chemical solvothermal metho, in which the metal precursors (K 2 PdCl 4 , GeCl 4 , and NiCl 2 ) were coreduced in the tetra-ethylene glycol solvent by the superhydride (Li(Et 3 BH)) reducing agent. 155 Besides, the disordered ternary alloys can also be prepared first via the wetchemical method, followed by transformation to the ordered ternary intermetallics via high-temperature thermal annealing, such as the cases of intermetallic PdCuM (M = Ni and Cu), 158 PtFeCu, 159 and PtCoM (M = Mn, Fe, Ni, Cu, and Zn). 160 In addition to simply extending the synthetic strategies of binary intermetallic nanocrystals, ternary intermetallic nanocrystals can also be prepared by using the binary counterparts as seeds, followed by the introduction of the third metal through galvanic replacement. For example, intermetallic PdZnAu nanoparticles were prepared by incorporating Au into intermetallic PdZn nanoparticles via a galvanic replacement method, which could be realized due to the more positive redox potential of AuCl 4  55 In most cases, the third metal can be viewed as a substitution of either element in the binary system and shares the crystallographic position with that element. As a result, the XRD pattern of the ternary intermetallic nanocrystals is consistent with their corresponding binary one, while shift in peak positions can be observed and the peak shifting direction depends on the atomic radius of the third metal. For example, hcp intermetallic Pt 45 Sn 25 Bi 30 nanoplates were prepared by co-reducing Pt(acac) 2 , SnCl 2 and Bi(act) 3 in oleylamine/octadecene via a one-pot wetchemical method. 161 The XRD pattern of Pt 45 Sn 25 Bi 30 was consistent with that of the standard intermetallic PtSn, while all peaks shifted to lower angles, due to the integration of Bi with a large atomic radius into the PtSn lattice ( Figure 8A).
As another example, L1 0 intermetallic PtZnCu nanoparticles were prepared by using ZIF-8 as the sacrificial template, onto which H 2 PtCl 6 and Cu(NO 3 ) 2 were impregnated, followed by thermal annealing under H 2 at 750°C. 162 The XRD pattern of PtZnCu was consistent with that of the L1 0 PtZn, while all peaks shifted to higher angles because of the incorporation of Cu with a small atomic radius.
Interestingly, unconventional phases that do not match any phases in the ternary phase diagrams or the constituent unary and binary systems can also be accessed in ternary intermetallic structures. A typical example is the synthesis of AuCuSn 2 intermetallic nanoparticles via the co-reduction of HAuCl 4 , Cu (C 2 H 3 O 2 ) 2 and SnCl 2 in tetraethylene glycol using NaBH 4 as the reducing agent. 164,165 Based on the XRD pattern, it exhibited an unconventional NiAs-type ordered structure, which was different from either its bulk counterpart or the constituent binary systems. Such observation has also been extended to synthesize the unconventional AuNiSn 2 intermetallic nanoparticles.
It is generally believed that with the incorporation of more elements, the electronic structure of the catalytic active metals can be further optimized to enhance the catalytic performance. 150,157,158,161,162,166,167 Apart from it, the introduction of a third metal also has several merits in terms of the synthesis of intermetallic structures. First, in some cases, the third metal could facilitate the disorder-to-order transition. 66,[167][168][169] Such third metals can be those with lower surface energy and/or lower solubility relative to the binary alloys, such that they tend to migrate to the surface during thermal annealing, causing the creation of rich lattice vacancies and thereby promoting the ordering process. For example, by partially replacing Fe in PtFe nanoparticles with Cu, a lower annealing temperature of 500°C was sufficient to transform the disordered fcc PtFe x Cu 1-x to the ordered bct PtFe x Cu 1-x , while 700°C was needed for transforming the binary fcc PtFe nanoparticles. 167 A much lower annealing temperature of approximately 300°C was adopted for the partial ordering transformation of Sb-doped PtFe (Fe 32 Pt 45 Sb 23 ) nanoparticles. 66 Such Sb-promoted disorderto-order transition could be attributed to the incorporation of Sb atoms with low surface energy and low solubility, which were prone to escape from the PtFe lattice, leaving behind rich lattice vacancies, thereby promoting the ordering transformation via enhancing the atom mobility of Pt and Fe. Similar effect has also been observed for Ag 168 or Audoped PtFe nanoparticles. 169 Besides, the third metal can also help to preserve the morphology of the intermetallic nanocrystals during high-temperature annealing. For example, fct Pt 0.4 Ir 0.1 Fe 0.5 nanowires with an average diameter of 2.6 nm were prepared via high-temperature annealing of the disordered counterpart at 690°C ( Figure 8B). 163 The introduction of the high-melting-point metal Ir into the PtFe nanowires and the coating of SiO 2 protective shell before annealing were crucial for maintaining the 1D morphology. Otherwise, the thin nanowires would break into fragments and nanoparticles upon annealing.

| SYNTHESIS OF HEI NANOCRYSTALS
High-entropy materials, owing to their unique structures and exceptional physicochemical properties, have triggered broad interests in diverse research fields. 170,171 As a typical group of high-entropy materials, high-entropy alloys (HEAs) refer to those having at least five principal elements with nearly equal atomic ratio. In contrast to HEAs, which exhibit compositional complexity but disordered distribution of the constituent elements, in HEIs, five or more elements show ordered arrangement while maintaining the compositional complexity. The crystal structure of an HEI normally follows the structure of its "parent" binary intermetallic counterpart. The "parent" lattice provides two sets of crystallographic positions, such that metal atoms with analogous characteristics (e.g., radius and electronegativity) share one sublattice. For example, the crystal structure of (PtCoNi) (SnInGa) HEI follows the structure of its "parent" binary intermetallic PtSn, i.e., B8 1 , while the Pt and Sn atoms are partially substituted by Co/Ni and In/Ga atoms, respectively ( Figure 9A). 172 To date, HEIs have been used as catalysts or electrocatalysts toward various reactions, such as propane dehydrogenation reaction, 172,173 acetylene semihydrogenation reaction, 174 ORR, 175 HER, 176 and ethanol oxidation reaction (EOR). 177,178 So far, intermetallic nanocrystals have been greatly limited to binary or ternary compositions, as discussed above. When five or more elements are included, phase segregation is prone to occur because of the thermodynamic immiscibility of the constituent elements. 179 Therefore, the controlled synthesis of HEIs remains challenging. Methods such as the direct co-reduction of metal precursors, 172,174,175,177 disorder-to-order transition upon thermal annealing, 178,179 and melt-spinning, 176 have been reported. Co-reduction of metal precursors can occur in either liquid or gas phase. For example, a one-pot wet-chemical synthetic method was used to prepare hcp PtRhBiSnSb HEI nanoplates, in which Pt (acac) 2 , Rh(acac) 3 , Bi(act) 3 , SnCl 2 , and SbCl 3 were thermally decomposed in oleylamine/octadecene at 220°C. 177 The successful formation of such HEI derived from the same hcp crystalline structures of PtBi, RhBi, PtSn, and PtSb intermetallics. The crystal structure followed the structure of PtBi, i.e., hcp, while Pt and Bi atoms were partially substituted by Rh and Sn/Sb atoms, respectively. A co-impregnation of metal precursors followed by high-temperature annealing method was developed to prepare (NiFeCu)(GaGe) HEI nanoparticles, whose crystal structure followed the structure of NiGa, i.e., CsCl-type. 174 The co-impregnation method has also been applied to synthesize PtCoNiInGaSn HEIs supported on CeO 2 , whose crystal structure was consistent with that of the "parent" PtSn intermetallic. 172 As another example, PtCoCuGeGaSn HEIs supported on SiO 2 were prepared by impregnating all the metal precursors into the pores of SiO 2 , which were then dried and reduced under H 2 at 700°C. 173 Besides, a 2D organicinorganic superstructure was prepared first, in which metal precursors were homogeneously dispersed. 175 Upon annealing under NH 3 atmosphere, CoFeNiCuPd HEI nanoparticles supported on 2D N-rich mesoporous carbon nanosheets were obtained ( Figure 9B). It exhibited an L1 2 structure, in which Fe and Co atoms were located at the face-centered sites, Pd and Cu atoms were located at the vertex sites, while Ni atoms were randomly distributed over the entire lattice. In contrast, annealing under N 2 resulted in the formation of its disordered HEA counterpart.
Besides, the disorder-to-order transition under hightemperature annealing for the synthesis of binary/ ternary intermetallic nanoparticles can also be extended to HEIs. For example, the disordered HEA nanoparticles containing at least five elements were first prepared by heating metal precursors on the carbon substrate at approximately 1100 K. 179 Then, the corresponding HEI nanoparticles were obtained via annealing HEAs at approximately 1100 K followed by rapid cooling to lock the ordered structures. Such method enabled the preparation of several ultrasmall HEI nanoparticles with size of 4-5 nm, including the L1 0 Pt(Fe 0.7 Co 0.1 Ni 0.1 Cu 0.1 ), L1 2 (Pt 0.8 Au 0.1 Pd 0.1 ) 3 (Fe 0.9 Co 0.1 ), and L1 0 (Pt 0.8 Pd 0.1 Au 0.1 ) (Fe 0.6 Co 0.1 Ni 0.1 Cu 0.1 Sn 0.1 ). As another example, PtRhFeNiCu HEAs supported on carbon were first synthesized by the impregnation-reduction method, which then experienced the disorder-to-order transition under H 2 /Ar at 700°C, leading to the formation of ordered PtRhFeNiCu HEIs. 178 6 | ELECTROCATALYTIC APPLICATIONS OF INTERMETALLIC NANOCRYSTALS

| ORR
ORR is the key reaction occurring at the cathode of the proton exchange membrane fuel cells (PEMFCs). It takes place via either the two-electron (2e − ) pathway, in which O 2 is partially reduced to H 2 O 2 in acidic or HO 2 − in alkaline media, or the four-electron (4e − ) pathway, in which O 2 is completely reduced to H 2 O in acidic or OH − in alkaline media. The latter is desirable for PEMFCs due to its higher current efficiency. One of the bottlenecks for the commercialization of PEMFCs is the sluggish ORR process, which requires high-efficiency electrocatalysts. An optimal adsorption of O* on the catalyst surface is desirable, since too strong O* adsorption could limit the removal of the adsorbed intermediates, while too weak O* adsorption could limit the dissociation of O 2 . 33 So far, platinum-group-metal-based catalysts are considered as the most efficient ORR catalysts among other elements. Nevertheless, considering the scarcity and high cost of Pt, the instability issue arising from metal dissolution during long-term operation, and the ambition to further boost the ORR activity, continuous research efforts are urgently required. An effective strategy is to develop binary or even multinary Pt-based nanocatalysts with intermetallic structures. The introduction of 3d transition metals or p-block metals can not only reduce the usage of expensive Pt, but also enable the modulation of Pt electronic structure through the alloying effect. Besides, through dealloying of the less noble metals, nanocatalysts containing the Pt-based intermetallic core coated with few layers of Pt skin can be formed, exhibiting a compressive strain in the Pt overlayer when the lattice of the metal core is smaller than that of the Pt skin. As a result of the compressive strain, the Pt d-band center shifts downward, leading to the decreased binding energy of the oxygenated intermediates (OH*, O*, OOH*, etc.) on Pt, thereby promoting the ORR activity. 34 The ORR performance of intermetallic binary Pt-M (M = Fe, Co, Ni, Cu, etc) with L1 0 , 56,130,138,150,180 31 In contrast to the conventional Pt-rich intermetallics, such as Pt 3 M and PtM, PtCo 3 @Pt nanoparticles, due to the increased amount of the nonnoble metal Co in the core, exhibited a larger compressive strain in the Pt shell. 59 The L1 2 PtCo 3 @Pt catalyst showed a superior mass activity (0.72 A/mg Pt ) than the L1 0 PtCo catalyst (0.45 A/mg Pt ), and both of them behaved better than their disordered counterparts. Besides, the Pt skins also endowed PtCo 3 with good durability, despite its high content of Co which was prone for leaching during long-term operation. In addition, the compressive strain can be further increased by alloying the Pt overlayer with elements having larger size. For example, by coating PtBi shell over the L1 0 intermetallic PtFe nanoparticles, the resultant PtFe@PtBi core-shell nanoparticles exhibited stronger compressive strain in the Pt overlayer (−4.6%), as compared to the uncoated PtFe (−2.6%). 145 Accordingly, the PtFe@PtBi catalyst showed the best ORR performance among the uncoated ordered PtFe and ordered ternary PtFeBi nanoparticles, with a high mass activity of 0.96 A/mg Pt and specific activity of 2.06 mA/cm 2 ( Figure 10A). Although the compressive strain in Pt skins has been well recognized to enhance the ORR activity of Pt-based nanocatalysts, a few studies also emphasize the importance of tensile strain, which is induced when the lattice of the intermetallic core is larger than that of the Pt shell. A typical example is the B8 1 PtPb@Pt nanoplates, in which Pt skins with (110) planes were coated over the (010) and (001) planes of the intermetallic PtPb core. 25 As a result of the lattice mismatch, a high tensile strain (~7.5%) and a little compressive strain (~1%) were induced in the Pt skins. It was found that the strain effect on the binding energy of the oxygenated intermediates was dependent on the exposed facet of the nanocatalysts. Although the tensile strain on the Pt (111) facet was not favorable, it became favorable when the tensile strain was built up on low-coordination facets, in this case, the (110) facet. As a result of such tensile strain on the Pt (110) planes, the σ* antibonding states shifted downward, leading to a decreased binding energy of the oxygenated species, thereby enhancing the overall ORR performance, with a high specific activity of 7.8 mA/cm 2 and mass activity of 4.3 A/mg Pt ( Figure 10B).
In addition, featuring the more negative formation enthalpy of intermetallic structures, higher durability is normally achieved for intermetallic nanocatalysts compared to their disordered counterparts, especially in acidic media. For example, the Fe atoms in L1 0 PtFe nanoparticles exhibited stronger tolerance to acid leaching compared to those in the disordered fcc PtFe in 0.5 mol/L H 2 SO 4 solution, i.e., only 3.3% loss of Fe in the L1 0 PtFe after immersion in acid for 1 h, in contrast to a heavy loss of Fe (36.5%) in the fcc PtFe ( Figure 10C). 22 After 1000 potential cycles of stability test, the L1 0 PtFe showed a slight compositional change (Fe/Pt = 42:58), while a severe loss of Fe was observed in fcc PtFe (Fe/ Pt = 11:89). As another example, for the nanocatalyst containing the L1 0 intermetallic PtCo core coated with Pt skins, only 5% loss of Co was observed after operating at 60°C in acidic media for 24 h, in contrast to 34% loss of Co after 7 h for the disordered counterpart. 37 By incorporating additional elements, ternary and HEI intermetallic nanocrystals offer more room to tune the ORR performance. 90,169,182 For example, Pt-skincoated L1 0 PtNiCo nanoparticles with controllable Ni/Co ratios, i.e., 9/1, 8/2, 6/4, 4/6, and 2/8, were synthesized by annealing the pre-synthesized Pt@NiCoO x core-shell nanoparticles. 150 Their ORR activities were studied and compared with the binary L1 0 PtNi and L1 0 PtCo nanoparticles. Among them, L1 0 PtNi 0.8 Co 0.2 , with a Ni/Co ratio of 8/2, exhibited the most positive half-wave potential (E 1/2 ), while L1 0 PtNi showed the most negative E 1/2 . DFT calculations suggested that the intermetallic PtNi 0.8 Co 0.2 core could optimize the compressive strain in the Pt shell, thereby optimizing the Pt−O binding energy. L1 2 intermetallic Pt 3 Co 0.6 Ti 0.4 nanoparticles supported on ZIF-8-derived mesoporous carbon exhibited a superior ORR mass activity (1.49 A/mg Pt ) and enhanced durability (20.1% loss of mass activity after 20,000 potential cycles) than the Pt 3 Co counterpart (0.83 A/mg Pt , 42.2% loss of mass activity after 20,000 potential cycles). 151 The improved ORR performance of the ternary Pt 3 Co 0.6 Ti 0.4 nanoparticles could be attributed to the partial substitution of Ti with a lower electronegativity (1.54) for Co (1.88), leading to enhanced electron transfer from Co/Ti to Pt, thereby resulting in an optimized binding energy between Pt and the oxygenated intermediates. The L1 2 CoFeNiCuPd HEI nanoparticles supported on nitrogen-rich mesoporous carbon exhibited a large half-wave potential (0.90 V), a high mass activity (2.037 mA/μg Pd ), and remarkable durability (only 10 mV decay after 10,000 cycles), much superior to the disordered HEA counterpart and the commercial Pt/C. 175 The outstanding activity and stability were attributed to several factors, including the 2D mesoporous nitrogen-doped carbon support, which promoted the mass transfer and electron conductivity, as well as the ordered HEI nanoparticles, which possessed modulated electronic structures and a highly robust structural configuration.
Considering the scarcity and high cost of Pt, lowor non-Pt ORR catalysts have been searched as substitutions. Pd is a typical alternative for ORR in alkaline media. 44,108 Pd has intrinsically lower ORR activity than Pt, because of the insufficient overlap between the t 2g component in Pd-4d 10 orbital and the O-2p orbital of oxygenated species, leading to a slower electron transfer kinetics and a weakened oxygen binding energy. 183 Therefore, various strategies have been developed to enhance the ORR activity of Pd, one of which is to construct Pd-based intermetallic nanocrystals. For example, L1 2 Pd 3 Pb nanosheets and nanocubes coated with Pd skins showed a homogeneous tensile strain in the Pd (100) facets, resulting in an upshift of the Pd d-band center and the optimized oxygen binding energy. 135 As a result, the Pd 3 Pb nanosheets and nanocubes exhibited over 160% and 140% increases in mass activity and over 114% and 98% increases in specific activity compared with their uncoated counterparts, respectively. L1 2 Pd 3 Pb tripods with the preferential exposure of (110) facets exhibited a high mass activity of 0.56 A/mg Pd and specific activity of 1.76 mA/cm 2 in alkaline media, outperforming the commercial Pt/C and Pd/C. 45 DFT calculations suggested that the enhanced ORR activity originated from the strongly coupled Pd-4d and Pb-sp orbitals on the (110) facets, such that the orbital configuration of Pd-Pb became similar to that of Pt. Ternary intermetallic Au 10 Pd 40 Co 50 nanoparticles exhibited comparable activity to the conventional Pt/C in both acidic and alkaline electrolytes, while the durability in alkaline media was superior to that of Pt/ C. 148 After 10,000 potential cycles, the E 1/2 decayed approximately 30 mV for Pt/C, while no obvious change in E 1/2 was observed for the intermetallic Au 10 Pd 40 Co 50 . Both the ordered arrangement of Pd and Co atoms as well as the protective effect of the surface-rich gold atoms contributed to the enhanced durability. As another example, trace amount of Pt substituted Pd, forming intermetallic Pt 0.2 Pd 1.8 Ge nanocrystals. 184 Such a ternary nanocatalyst exhibited an improved E 1/2 of 0.95 V, a high selectivity of H 2 O over H 2 O 2 , and high stability (i.e., mass activity of 320 mA/mg Pt remained unchanged after 50,000 ADT cycles), outperforming the intermetallic Pd 2 Ge counterpart and Pt/C. DFT calculations suggested that upon Pt substitution, the adsorption of OH* became weaker and that of OOH* became stronger, resulting in diminished OH poisoning and thus less H 2 O 2 formation and enhanced stability.
Nevertheless, intermetallic nanocrystals do not always catalyse ORR better than their disordered counterparts. The structure, facet, shape and size are all important factors to be taken into considerations. For example, the ORR activities of Pt 3 Sn nanocubes with different degrees of ordering were compared. 181 Pt 3 Sn nanocubes with 60%-and 30%-ordering exhibited a 2.3fold enhancement in specific activity than their ordered counterpart with 95%-ordering. In this reaction system, the ordered Pt 3 Sn was directly synthesized via the wetchemical method, while the disordered counterparts were obtained by the subsequent thermal annealing. In such a unique order-to-disorder transition process, rich vacancies were developed on the surface of the less ordered particles during thermal annealing, thereby promoting the ORR activity. However, the 95%-ordering Pt 3 Sn catalyst still showed better stability than the disordered counterparts.

| Small molecule oxidation reactions
Small molecule oxidation reactions are important anodic reactions in direct liquid-feed fuel cells, such as direct methanol fuel cells, direct ethanol fuel cells (DEFCs) and direct formic acid fuel cells. Such reactions are complicated and kinetically sluggish, which involve the transfer of multiple electrons and many reaction intermediates and products. Taking EOR as an example, the C1pathway, which involves the complete oxidation of ethanol and transfer of 12 electrons to form CO 2 , is desired for high-efficiency DEFCs. 185 However, this process is usually suppressed by the C2-pathway, which involves the incomplete oxidation of ethanol and transfer of either 4 electrons to form acetic acid/acetate or 2 electrons to form acetaldehyde. Besides, various intermediates, such as CH 3 CHO* and CO*, are produced during the reactions. These intermediates, especially CO*, would block the catalytic active metals, usually Pt and Pd, leading to undesired poisoning and deteriorated catalytic performance. Therefore, the goal is to develop catalysts that can achieve high atom utilization of noble metals, high mass activity at low overpotentials, high C1pathway selectivity, long-term durability, and superior anti-poisoning ability.
One of the effective strategies to mitigate the CO poisoning is to construct intermetallic nanocatalysts, such that the active Pt or Pd sites can be isolated using non-noble metals. 186 Such non-noble metals are usually oxophilic, including Sn, Pb, Cu, Fe Co, Zn, etc. This is because the oxygenated species, such as the hydroxyl ions from the electrolyte, have a higher affinity to adsorb on the more oxophilic metal sites. The OH* will then spill-over to the adjacent Pt/Pd sites where CO* is strongly adsorbed, and react with CO*, leading to the oxidative removal of CO on the Pt/Pd sites. 17 For example, by constructing the orthorhombic PdBi intermetallic nanocrystals, the catalytically active Pd sites were isolated by Bi atoms, greatly suppressing the adsorption of CO. 187 The remarkable CO tolerance was clearly demonstrated by the in-situ attenuated total reflection-infrared (ATR-IR) spectroscopy, in which peaks corresponding to the adsorbed CO* were observed for Pd/C with continuous Pd sites, while negligible CO* adsorption peaks were present for the intermetallic PdBi nanocrystals. As a result, the orthorhombic PdBi intermetallic nanocrystals exhibited superior activity toward formic acid oxidation reaction than the disordered counterpart and Pd/C ( Figure 11A). In addition to active site isolation, another mechanism for improving the CO tolerance of intermetallic nanocatalysts is through electronic structure modification. Due to the strong bonding between Pt/Pd and non-noble metals, the position of the Pt/Pd d-band center could be optimized to lower the CO binding energy. For example, by synthesizing intermetallic Pd 3 Pb nanoparticles, the dband center of Pd was shifted downward. 188 Accordingly, the CO binding energy on the Pd 3 Pb (111) facet was calculated to be 1.05 eV, which was lower than that on the Pd (111) facet, i.e., 1.29 eV, leading to stronger CO tolerance of Pd 3 Pb than Pd.
For nanocatalysts containing an intermetallic core coated with few atomic layers of Pt/Pd skins, the strain effect also contributes to the enhanced catalytic activity toward small molecule oxidation reactions. For example, intermetallic PtBi nanoplates covered with a tensilestrained Pt shell alloyed with Rh single atoms (PtBi@PtRh 1 ) showed excellent EOR performance in alkaline electrolyte, exhibiting high mass activities of 5417 mA/mg Pt+Rh at 0.6 V and 13,020 mA/mg Pt+Rh at the peak potential. 147 Significantly, such catalyst delivered a high C1-pathway selectivity of 24.6%, superior antipoisoning abilities toward the CO* and CH 3 CHO* intermediates, and excellent durability during 100,000 s operation. DFT calculations suggested that the tensile strain in the PtRh 1 overlayer and the presence of Rh single atoms together resulted in an enhanced adsorption of ethanol and key surface intermediates, as well as a lower activation barrier for the C − C bond cleavage ( Figure 11B). Intermetallic Pt 3 Sn nanocubes covered with defect-rich Pt atomic layers displayed tensile strain of approximately 4.4% along the [001] direction, leading to an upshift of the Pt d-band center, thereby facilitating the C − C bond cleavage for complete ethanol oxidation and suppressing the generation of poisoned CO intermediates. 189 Accordingly, it showed a superior specific activity (5.83 mA/cm 2 ) and mass activity (1166.6 mA/ mg Pt ) for EOR, compared to the unstrained Pt 3 Sn intermetallic nanocubes, Pt nanocubes and the commercial Pt/C. Intermetallic Pt 3 Ga nanoparticles covered with 2-3 atomic layers of Pt skins exhibited tensile strain in the Pt skins, which were calculated to be more energetically favorable for MOR than the unstrained counterpart. 190 The stronger OH* binding on the strained Pt skins also enabled easier removal of CO*. As a result, it showed superior specific activity (7.195 mA/cm 2 ) and mass activity (1.094 mA/μg Pt ) than the unstrained counterpart and the commercial Pt/C. In addition to the tensile strain, some studies also highlighted the contribution from the compressive strain. For example, when Pt 3 Mn nanoparticles coated with Pt skins were used as the catalyst toward MOR, the MOR activity was found to be dependent on the degree of ordering of the Pt 3 Mn core, i.e., a higher degree of ordering corresponded to a better MOR activity. 191 DFT calculations suggested that the compressive strain induced in the Pt skins resulted in a weakened CO binding and a smaller free energy change from CO* to COOH*, thereby promoting the MOR activity.
Very recently, studies have also demonstrated the promising catalytic performance of HEI nanocrystals toward small molecule oxidation reactions. For example, the hcp PtRhBiSnSb HEI nanoplates exhibited outstanding mass activities of 19.529, 15.558, and 7.535 A/mg Pt+Rh toward the methanol, ethanol, and glycerol oxidation reactions, respectively, in alkaline media, superior to the quaternary PtBiSnSb, binary PtBi, and the commercial Pt/C. 177 Remarkably, the PtRhBiSnSb HEI nanoplates possessed outstanding stability, i.e., retaining 70.2% of the initial MOR activity even after 5000 cycles, maintaining stable current densities over 20000 s, and suppressing CO poisoning. DFT calculations showed that the excellent alcohol oxidation performance could be mainly ascribed to two factors, i.e., the presence of Rh atoms, which optimized the electronic structures of the active sites (Pt and Rh), thereby facilitating electron transfer and electroactivity, as well as the synergistic contributions of Bi, Sn and Sb atoms, which enabled the stabilization of valence states on the active sites via p − d orbital coupling. As another example, PtRhFeNiCu HEI nanoparticles also possessed a higher mass activity (914 mA/mg Pt ), better CO tolerance, and enhanced stability than the HEA counterpart and the commercial Pt/C. 178 DFT calculations indicated that compared to the disordered HEA, in the ordered HEI structure, the adsorption configuration of key reaction intermediates was changed, leading to reduced energy barrier and stronger C − C bond-breaking ability.
Nevertheless, intermetallic nanostructures are not always superior to their disordered counterparts toward small molecule oxidation reactions. Especially, when both metals are functional in a reaction, more care should be taken. For example, Pt 3 Sn nanocubes with 60%-ordering exhibited 5.6 times higher activity toward MOR than those with 95%-ordering. 192 This is because both Pt and Sn were active toward MOR. For the 60%ordered Pt 3 Sn, due to its lower compositional stability, oxidation of Sn 0 to Sn 4+ occurred during the electrochemical oxidation reaction. Compared to Sn 0 , the Sn 4+ sites could bind OH* and oxidize the CO* intermediate adsorbed on Pt more efficiently, thereby leading to the enhanced activity of the 60%-ordered Pt 3 Sn.

| HER
HER is the key cathodic reaction for electrocatalytic water splitting. It requires high-performance electrocatalysts with lower overpotentials at the industrial-level current density, fast HER kinetics, high turnover frequency (TOF), and outstanding durability in both acidic and alkaline media. Pt is recognized as the most active metal toward HER, and great research efforts have been devoted to exploring high-efficiency Pt-based nanocatalysts. 193 Through constructing Pt-based binary or multinary intermetallic nanocrystals, the electronic structure of Pt could be modulated to optimize the adsorption and desorption of hydrogen, thereby promoting the HER activity. For example, intermetallic Pt 3 Ge nanocrystals with the exposure of (202) facet exhibited superior catalytic performance toward HER, with low overpotentials of 21.7 and 96 mV at the current density of 10 mA/cm 2 in acidic and alkaline media, respectively. 194 Using an electrochemical flow cell, it exhibited a longterm durability of >75 h in alkaline media under the industrial-level current density of >500 mA/cm 2 . Several factors, such as the electron transfer from Ge to Pt and the optimal hydrogen binding on the Pt 3 Ge (202) facet, together contributed to its superior activity. Intermetallic Pt 3 Ti nanoparticles, in situ formed by reducing Pt loaded Ti 3 C 2 T x MXenes, exhibited superior HER performance in acidic media, with a low overpotential of 32.7 mV at 10 mA/cm 2 and a low Tafel slope of 32.3 mV/dec. 195 DFT calculations suggested that the (111) and (100) 196 XPS analyses and the electrochemical CO stripping experiment together revealed that with the incorporation of Co, the ternary PtZnCo catalyst possessed a more electronrich Pt surface than its binary counterpart, due to the enhanced electron transfer from Zn/Co to Pt atoms, contributing to the desorption of H 2 and thereby accelerating the HER kinetics ( Figure 12A). Considering the scarcity and high cost of Pt, low-and non-Pt HER electrocatalysts have been extensively explored. One of the strategies to maximize the Pt atom utilization efficiency is to grow thin layers of Pt skins over the non-Pt intermetallic nanocrystals. For example, intermetallic Pd 3 Pb nanoplates coated with Pt sub-monolayer exhibited promising HER performance with a low overpotential of 13.8 mV at 10 mA/cm 2 , a high mass activity of 7834 A/g Pd+Pd at −0.05 V and excellent stability in acidic media. 197 DFT calculations suggested that electron transfer from the Pd 3 Pb core to the Pt shell resulted in an upshift of the Pt d-band center, leading to stronger hydrogen binding on the Pt overlayer and thus enhanced HER activity ( Figure 12B).
Besides, non-Pt intermetallic HER nanocatalysts have been explored as substitutions. For example, B2 intermetallic PdCu nanowires exhibited better HER performance in both acidic and alkaline media than the disordered counterpart, arising from the optimal hydrogen binding energy on the B2 PdCu surface. 123 Intermetallic Ir 3 V exhibited a low overpotential of 9 mV at 10 mA/cm 2 and a high mass activity of 1200 A/g Ir at the overpotential of 20 mV in alkaline media, which was 6.7, 9.4, and 3.3 times greater than those of the commercial Pt/C, Ir/C, and disordered Ir 3 V catalysts, respectively. 192 The enhanced HER kinetics was ascribed to the charge redistribution on the ordered Ir 3 V surface with a maximized number of neighboring Ir/V sites compared to the disordered counterpart, thereby promoting water dissociation on the electron-deficient V sites. bcc intermetallic IrGa nanoparticles on N-doped rGO exhibited excellent performance toward both HER and OER, thus having a low cell voltage of 1.51 V to achieve 10 mA/cm 2 for the overall water splitting in alkaline condition. 61 DFT calculations suggested that the high performance could be ascribed to the creation of electron-rich Ir via incorporating Ga with lower electronegativity, such that the electron transfer between Ir and the adsorbed species could be promoted, thereby decreasing the energy barriers of the potential determining steps. As another example, intermetallic IrMo nanoparticles supported on carbon nanotubes were demonstrated to be an efficient HER catalyst in both acidic and alkaline media, with a specific activity of 0.95 mA/cm Ir 2 and a Tafel slope of 38 mV/dec in 1.0 mol/L KOH, which were close to those in 0.5 mol/L H 2 SO 4 (i.e., specific activity of 2 mA/cm Ir 2 and Tafel slope of 14 mV/dec). 198 In contrast, the commercial Ir/C and Pt/C catalysts showed a drastic drop of activities when the electrolyte was changed from acid to base. The small activity difference in acidic and alkaline electrolytes could be ascribed to the existence of Mo sites in the intermetallic IrMo nanocrystals that could stably adsorb OH*, thereby stabilizing the water dissociation product and decreasing the thermodynamics barrier. Besides, the L1 2 FeCoNiAlTi HEI prepared via the physical metallurgical technique exhibited promising HER activity in alkaline media, with an overpotential of 88.2 mV at 10 mA/cm 2 , a Tafel slope of 40.1 mV/dec and outstanding durability, which were comparable to those of noble metal catalysts. 176 Theoretical calculations suggested that the excellent performance could be attributed to the electronic structure modification through incorporating five metal elements within a highly ordered L1 2 structure, as well as the optimization of hydrogen adsorption/desorption through the site-isolation effect induced by the intermetallic L1 2 structure.

| CO 2 /CO reduction reactions
Electrochemical CO 2 /CO reduction reactions refer to the processes that convert CO 2 /CO into value-added chemicals, during which C − H and C − C bonds are built to form hydrocarbons, acids and alcohols. In such processes, electrocatalysts are essential to activate the CO 2 /CO molecules and achieve high production yield with high product selectivity. Capitalizing on the remarkable catalytic performance of Cu toward CO 2 reduction, Cu-based intermetallic nanocrystals have been widely explored. For example, three types of bimetallic Pd-Cu catalysts with ordered, disordered, and phase-separated atomic arrangements (atomic ratio of Pd/Cu = 1:1) were studied toward CO 2 reduction. 199 The ordered PdCu catalyst exhibited the highest selectivity for the C1 products such as CO and CH 4 (>80%), while the phase-separated PdCu catalyst exhibited the highest selectivity for the C2 products such as ethylene and ethanol (>60%), suggesting that C − C coupling was more favorable on surface with continuous Cu atoms. Four types of AuCu nanoparticles with similar size but different degrees of ordering were synthesized by annealing the disordered AuCu nanoparticles, in which the degree of ordering was controlled by tuning the annealing temperature and period. 200  AuCu core during the thermal annealing-induced disorderto-order transition. Interestingly, when the intermetallic core was changed to the ordered AuCu 3 nanoparticles, the Au overlayer exhibited an unconventional face-centered tetragonal (fct) phase, which was stabilized by the large interface strain. 201 Compared to the conventional fcc Au, the fct Au exhibited an upshift of the Au d-band center, thereby decreasing the energy barrier of COOH* formation and facilitating the CO 2 -to-CO reaction kinetics. Accordingly, the AuCu 3 @fct Au nanoparticles exhibited a high CO FE of 94.5% at −0.8 V, superior to the fcc Au nanoparticles (72.2%).
In addition, Au-Cu intermetallic systems with three types of structures, i.e., Au 3 Cu, AuCu and AuCu 3 nanoparticles, were compared toward CO 2 reduction. 202 In general, more types of products were generated with increasing amount of Cu and/or more negative potentials, while Au 3 Cu nanoparticles gave the highest CO production rate compared to the others.
Besides Cu, other intermetallic nanocrystals have also been investigated as catalysts to electrochemically reduce CO 2 into value-added chemicals, such as formate. For example, intermetallic Pd 3 Bi nanocrystals were reported as the efficient catalyst to produce formate, showing a high formate FE of approximately 100% and remarkable stability even at potentials more negative than −0.35 V, superior to the disordered Pd 3 Bi which gave a lower formate FE of <60%. 203 DFT calculations suggested that compared to pure Pd and the disordered Pd 3 Bi, the atomic ordering of Pd and Bi within the intermetallic Pd 3 Bi structure could enhance the *CO 2 adsorption, suppress the *CO binding and promote the *OCHO adsorption, leading to a high formate selectivity ( Figure 13A). Intermetallic Ag 3 Sn coated with an ultrathin SnO x layer (Ag 3 Sn@SnO x ) enabled the production of formate with a high FE of approximately 80% and partial current density of −16 mA/cm 2 at −0.8 V at an optimal SnO x shell thickness of approximately 1.7 nm. 204 DFT calculations suggested that the high electronic conductivity of the intermetallic Ag 3 Sn core and the favorable stabilization of the *OCHO intermediate on the SnO (101) facet with oxygen vacancies together contributed to the good catalytic performance.
As an example for CO reduction, intermetallic B2 PdCu nanoparticles were reported as a promising catalyst, showing a high acetate FE of 70 ± 5% and partial current density of 425 mA/cm 2 at −1.03 V ( Figure 13B). 58 The intermetallic PdCu structure featured alternately aligned rows of Pd and Cu atoms, providing abundant Pd-Cu pairs. These Pd-Cu pairs were the catalytic active sites, promoting the absorption of *CO and stabilization of ethenone as a key intermediate, inhibiting the competing HER, thereby facilitating the formation of acetate.

| Nitrogen reduction reaction
Electrochemical nitrogen reduction at ambient conditions has been developed as a sustainable approach to produce ammonia (NH 3 ), an important raw material in chemical industry and a promising carbon-free energy carrier, alternative to the energy-intensive Haber-Bosch process. Unfortunately, such approach is greatly limited by the weak N 2 adsorption on catalyst and the sluggish cleavage of the intrinsically inactive N ≡ N bond, leading to the production of NH 3 with low yield and selectivity. Therefore, it is essential to develop high-efficiency catalysts with enhanced adsorption and activation of N 2 . Only a few papers have touched on the utilization of intermetallic nanocrystals as the NRR catalyst. A typical example is the nanoporous intermetallic Pd 3 Bi nanocrystals, which were prepared by chemically etching the nanoporous PdBi 2 . 124 When it was used as the NRR catalyst in 0.05 mol/L H 2 SO 4 electrolyte, it exhibited a high NH 3 yield of 59.05 ± 2.27 µg/(h·mg cat ) and FE of 21.52 ± 0.71% at −0.2 V. Operando X-ray absorption spectroscopy studies and DFT calculations together suggested that the strong coupling between the Pd and Bi sites in the intermetallic structure was critical, such that N 2 was preferentially adsorbed and activated on the Bi sites, followed by the hydrogenation with protons adsorbed on the surrounding Pd sites, leading to the formation of NH 3 ( Figure 14A).
Considering the low solubility of N 2 in water and the intrinsically inert N ≡ N bond, other nitrogenous molecules can be used as substitutions, such as nitrate (NO 3 − ) that is abundant in wastewater and NO that can be found in air pollutants from the combustion of fossil fuels. simultaneous production of valuable NH 3 and elimination of wastes, demonstrating a waste-to-valuableresources concept. B2 intermetallic PdCu nanocubes have been used as catalyst for the reduction of nitrate into NH 3 , showing a high FE of 92.5% at −0.5 V and a high yield rate of 6.25 mol/(h·g) at −0.6 V. 20 Machine learning and DFT calculations suggested that on the (100) facet of the B2 PdCu nanocubes, Cu atoms stabilized *NO 3 due to the upshift of the d-band center, while Pd atoms destabilized *N due to the larger interatomic coupling from the subsurface Pd, facilitating NO 3 − addsorption onto the catalyst surface and protonation of N-bonded species toward NH 3 . The bcc intermetallic RuGa nanocrystals have been used as catalyst for the reduction of NO into NH 3 , exhibiting a high NH 3 yield rate of 320.6 μmol/(h·mg Ru ) ad FE of 72.3% at −0.2 V in neutral media. 205 DFT calculations suggested that in the intermetallic bcc RuGa structure, Ru atoms were electron-rich, as a result of the electron donation from Ga to Ru. The electron-rich Ru atoms facilitated the adsorption and activation of the *HNO intermediate, thereby reducing the energy barrier of the potential-determining step in NO reduction( Figure 14B).

| CHALLENGES AND OPPORTUNITIES
In this minireview, the very recent progress of noble metal-based intermetallic nanocrystals, including binary, ternary, and HEI nanocrystals, has been summarized. Various synthetic strategies, including the conventional thermal annealing approach and its modifications, as well as the wet-chemical synthetic method, have been discussed, highlighting their strengths and limitations. Their electrocatalytic applications toward ORR, small molecule oxidation reactions, HER, CO 2 /CO reduction reactions, and nitrogen reduction reaction have been discussed, with emphasis on how the intermetallic structures contribute to the enhanced catalytic performance. Despite these remarkable achievements, challenges and opportunities remain in many aspects. In terms of synthesis, first, both the thermal annealing and wet-chemical synthesis methods for the preparation of intermetallic nanocrystals have their own merits and limitations. The conventional thermal annealing approach enables control over the degree of ordering by tuning the annealing temperature and period for many systems, but most of the products are limited to spherical nanoparticles because the high-index facets tend to reconstruct at high temperatures to minizine surface energy, disabling anisotropy in nanocrystals. The wet-chemical method, on the contrary, enables control over shape and architecture by using shape-directing agents. However, achieving high degree of ordering for some systems is difficult, because high temperature is usually required to overcome the thermodynamic energy barrier for ordering, which is, however, constrained by the solvent boiling point. To tackle these problems, continuous efforts are expected toward improving the existing methods and creating new synthesis protocols. It is essential to develop effective methods to achieve precise control over the nucleation and growth processes, and to facilitate atom diffusion for ordering under milder conditions.
Second, most reported works hitherto have been limited to bimetallic nanocrystals with ordered intermetallic structures, while the syntheses of intermetallic ternary, multinary and HEI nanocrystals remain challenging. Especially, due to the high tendency for phase segregation when more than five elements are integrated within one particle, it remains extremely difficult to construct HEI nanoparticles with high compositional homogeneity. Therefore, it is imperative to develop effective and general protocols for their syntheses, expanding the family of intermetallic nanomaterials.
Third, since the catalytic performance of metal-based nanocrystals can be affected by several structural parameters, such as size, shape, composition, architecture, phase and defect, synthesizing metal-based nanocrystals with different degrees of ordering, while maintaining other structural parameters the same is a prerequisite for investigating the ordering-dependent properties. Nonetheless, the construction of noble metal-based nanocrystals with simultaneous control over the degree of ordering and other structural parameters remains difficult, which calls for further studies. Besides, to further boost the intrinsic catalytic activity of noble metal-based intermetallic nanocrystals, more delicate control over the structural features of intermetallic nanocrystals is expected, such as the introduction of defects and the exposure of reactive facets.
Forth, the formation mechanisms of intermetallic nanocrystals need to be clearly understood, which can in turn guide the design and synthesis of new and unconventional intermetallic nanostructures. In-situ techniques, such as in situ TEM, XRD, and X-ray absorption spectroscopy, have been utilized to study the disorder-toorder transition mechanisms of bimetallic nanocrystals. 67,206 On the other hand, thermodynamic and kinetic studies on the nucleation and growth processes of intermetallic nanocrystals could also offer insights in their formation mechanisms. More studies are expected to unveil the general rules of intermetallic formation mechanisms.
In terms of electrocatalytic applications, first, since the nanocatalyst surface will come into contact with the gas and electrolyte molecules during the electrochemical reactions, surface reconstruction from their pristine state to a more active state would happen. 201 Besides, nanocatalysts may experience structural degradation and performance deterioration during longterm operation. Therefore, understanding the dynamic behavior of the catalyst surface and the degradation mechanism through advanced ex situ and in situ techniques is essential to reveal the real catalytic active sites, structural and electronic changes of catalysts during electrochemical reactions, etc. Coupled this information with theoretical calculations, the catalytic reaction pathways, the adsorption modes of intermediate species, and the rate determining steps can also be unveiled.
Second, so far, most of the noble metal-based intermetallic nanocatalysts are designed for ORR and small molecule oxidation reactions, while their electrocatalytic applications toward CO 2 /CO reduction reactions, nitrogen reduction reaction, and C − N coupling reactions via nitrogen-integrated CO 2 reduction reactions are rarely explored. Considering the abilities to tune the electronic structures of active sites and improve catalytic durability via the construction of intermetallic nanocrystals, it is anticipated that intermetallic nanocrystals with well-controlled compositions and structures could serve as effective catalysts toward these reactions.
Third, to realize industrial commercialization, it is important to examine the as-prepared nanocatalysts in the industrially relevant conditions, e.g., high current densities, high temperatures, harsh acidic or alkaline media, and long-term operation. This is highly important, because the real industrial operation conditions can be quite different compared to the lab-scale conditions. Besides, to be more cost effective in industrial applications, it is necessary to partially or completely replace noble metals with the non-noble ones in intermetallic nanocrystals, while maintaining an acceptable catalytic performance. To this end, designing intermetallic cores covered with thin noble metal shells, intermetallic nanocrystals doped with noble metal single atoms, or even non-noble metal intermetallic nanocrystals could be possible solutions.
Overall, intermetallic nanocrystals, featuring their fascinating ordered structures and catalytic performance, have created considerable opportunities in the fields of material sciences and chemistry. Yet challenges exist in many aspects, which call for more research efforts to tackle the problems. It is envisioned that more breakthroughs in the controlled syntheses of intermetallic nanocrystals and utilization of them toward industriallevel energy-related applications will be achieved in the future.