Ultrafine nanoporous intermetallic catalysts by high-temperature liquid metal dealloying for electrochemical hydrogen production

Song, Ruirui; Han, Jiuhui; Okugawa, Masayuki; Belosludov, Rodion; Wada, Takeshi; Jiang, Jing; Wei, Daixiu; Kudo, Akira; Tian, Yuan; Chen, Mingwei; Kato, Hidemi

doi:10.1038/s41467-022-32768-1

Download PDF

Article
Open access
Published: 02 September 2022

Ultrafine nanoporous intermetallic catalysts by high-temperature liquid metal dealloying for electrochemical hydrogen production

Nature Communications volume 13, Article number: 5157 (2022) Cite this article

9732 Accesses
46 Citations
78 Altmetric
Metrics details

Subjects

Abstract

Intermetallic compounds formed from non-precious transition metals are promising cost-effective and robust catalysts for electrochemical hydrogen production. However, the development of monolithic nanoporous intermetallics, with ample active sites and sufficient electrocatalytic activity, remains a challenge. Here we report the fabrication of nanoporous Co₇Mo₆ and Fe₇Mo₆ intermetallic compounds via liquid metal dealloying. Along with the development of three-dimensional bicontinuous open porosity, high-temperature dealloying overcomes the kinetic energy barrier, enabling the direct formation of chemically ordered intermetallic phases. Unprecedented small characteristic lengths are observed for the nanoporous intermetallic compounds, resulting from an intermetallic effect whereby the chemical ordering during nanopore formation lowers surface diffusivity and significantly suppresses the thermal coarsening of dealloyed nanostructure. The resulting ultrafine nanoporous Co₇Mo₆ exhibits high catalytic activity and durability in electrochemical hydrogen evolution reactions. This study sheds light on the previously unexplored intermetallic effect in dealloying and facilitates the development of advanced intermetallic catalysts for energy applications.

Metal bond strength regulation enables large-scale synthesis of intermetallic nanocrystals for practical fuel cells

Article 20 May 2024

Edge sites dominate the hydrogen evolution reaction on platinum nanocatalysts

Article 22 May 2024

Intermetallic synergy in platinum–cobalt electrocatalysts for selective C–O bond cleavage

Article 28 May 2024

Introduction

Electrochemical hydrogen production through water splitting has emerged as a promising strategy for the effective utilization and storage of intermittent renewable energy. Crucial to this technology is the realization of high-efficiency electrocatalytic hydrogen evolution reactions (HERs), particularly in alkaline media¹. Pt and its alloys are currently the most active HER catalysts^2,3,4. However, the scarcity and high cost of Pt limit their large-scale practical applications in electrolysers. Tremendous research efforts have been devoted to the development of low-cost, yet efficient, electrocatalysts based on earth-abundant transition metals, as alternatives to noble Pt, for HERs^5,6,7. Monometallic transition metals (Fe, Co, Ni, Ti, W, Mo, Cu, etc.) show moderate catalytic activity, while alloys of these elements can substantially improve HER catalysis by modifying both active site morphology (geometric effect)^8,9,10 and electronic structure (electronic effect)^11,12,13 to generate desirable catalytic transition states with minimum energy barriers. Compared to random solid-solution alloys, intermetallic compounds with ordered atomic structures and well-defined stoichiometry possess the merits of homogeneous and intensified active sites as well as the capability to form unique crystal structures not commonly demonstrated by random solid-solutions to enhance the geometry effect^10,14,15. The combination of electron localization and directional covalent bonding in intermetallic catalysts can also strengthen the electronic effect¹⁴. Moreover, the strong ionic/covalent interactions between the metal constituents are expected to enhance the chemical/electrochemical stability of intermetallic catalysts during catalytic processes. Intermetallic compounds, such as Ni₄Mo¹⁶, Co₃Mo¹⁰, NiZn¹⁷, and Ni₃Al¹⁸, are active HER electrocatalysts with appreciable energy efficiencies. Apart from the selection of intermetallic compounds with high intrinsic HER activity, a large specific surface area facilitating reactant adsorption, reaction, and desorption is essential for effectively utilizing the high catalytic activity of an intermetallic catalyst. In addition to the extensively studied particulate catalysts, bicontinuous porous nanostructures, with large surface areas, open pore channels, and high electrical conductivities, effectively maximize the density and accessibility of active sites and have emerged as a new type of electrocatalyst^{19,20,21,22,23,24}. Three-dimensional (3D) bicontinuous nanoporous structures fabricated by dealloying²⁵, the selective removal of the less stable component(s) from an alloy, are very attractive owing to their large specific surface area with ample surface defects, high electric conductivity, and rapid mass transport pathways^19,26. Several intermetallic compounds have been fabricated or integrated into 3D porous structures by the chemical etching of heterogeneous precursors (such as Mg₂Cu²⁷, PtSi²⁸, (Pt_1-xM_x)₃Al (M = Ni, Co, Fe)^29,30,31, SbSn³², and Pd₃Bi²⁴) or by segregation as nanoparticulate phases anchored on 3D nanoporous metal frameworks during dealloying (such as Cu₃Sn/Cu³³, Co₃Mo/Cu¹⁰, Al₇Cu₄Ni/Cu³⁴, and Nb₅Si₃/NbTi³⁵). However, these porous intermetallic materials exhibit low flexibility in microstructural optimization, high electric resistance due to copious grain boundaries and interfaces, and poor mechanical stability. Although monolithic nanoporous intermetallic compounds with tunable bicontinuous nanoporosity are desirable for efficient electrocatalysis in energy-related reactions, fabricating bicontinuous nanoporous intermetallic catalysts from single-phase precursors remains a challenge because of the difficulty in forming chemically ordered intermetallic phases during low-temperature chemical dealloying. Accordingly, advanced HER electrocatalysts, based on non-noble nanoporous intermetallics with sufficiently high HER activity and robustness, are yet to be developed.

In this work, we report a liquid metal dealloying (LMD) strategy, conducted at elevated temperatures, to fabricate 3D nanoporous Mo-based intermetallic compounds as high-performance HER electrocatalysts. Coupled with the simultaneous evolution of 3D bicontinuous open porosity during the dealloying of single-phase precursors, the high-temperature LMD process overcomes the energy barrier, driving the direct formation of chemically ordered intermetallic phases. Beyond the conventional wisdom that porous metals produced by LMD typically have large feature sizes and therefore small specific surface areas for catalytic applications, here, the intermetallic Co₇Mo₆ and Fe₇Mo₆ exhibit fine nanoporous structures with ligaments/pores that are ~30 nm in size. Combined experimental analyses and molecular dynamics simulations uncovered an intermetallic effect, whereby high-temperature chemical ordering during nanopore formation suppresses dealloyed-nanostructure coarsening. The ultrafine 3D nanoporous μ-Co₇Mo₆, with a large accessible surface area and abundant active sites, exhibits high HER catalysis in 1 M KOH alkaline electrolyte.

Results and discussion

Design, fabrication, and structural characterization

LMD is a processing technique utilizing the difference in the miscibility of alloy components in a molten metal bath to corrode selected component(s) while retaining the others, self-organizing into a 3D porous structure^36,37,38,39. According to the design principle (Fig. 1a), in the precursor alloy (AB), the pore-forming metal (A) and sacrificial component (B) should have a positive and negative enthalpy of mixing with the melt bath (C), respectively. LMD systems were designed accordingly to fabricate nanoporous Mo-based intermetallic compounds. A mixture of Mo with Co, Fe, or Cr, with designated compositions, was used as the pore-forming component A, while Ni and Mg were chosen as the sacrificial component B and melt bath C, respectively^36,37. As shown in Fig. 1b, Mo, Fe, Cr, and Co have a positive enthalpy of mixing with Mg, whereas the enthalpy of mixing is negative between Ni and Mg, satisfying the principle of LMD⁴⁰. The ternary alloy precursors for LMD were designed to have compositions of Ni₇₀(Co_0.55Mo_0.45)₃₀, Ni₇₀(Fe_0.58Mo_0.42)₃₀, and Ni₇₀(Cr_0.50Mo_0.50)₃₀ (atomic ratios) (Supplementary Table 1), with the objective of fabricating nanoporous intermetallic Co₇Mo₆ and Fe₇Mo₆ and solid-solution Cr₅₀Mo₅₀ phases. Binary alloys of Ni₇₀Mo₃₀ and Ni₇₀Fe₃₀ were also prepared for fabricating nanoporous Mo and Fe, as references. All the precursors are single-phase face-centered cubic (fcc) alloys, as verified by X-ray diffraction (XRD) (Supplementary Fig. 1).

**Fig. 1: Design and fabrication of the nanoporous intermetallic compounds.**

The Ni₇₀(Co_0.55Mo_0.45)₃₀ system is used as an example to depict the fabrication of nanoporous intermetallic compounds via LMD. LMD was carried out by immersing a 200 μm-thick Ni₇₀(Co_0.55Mo_0.45)₃₀ sheet sample into a 973 K Mg melt. During this process, Ni selectively dissolved into Mg, and Co-Mo spontaneously evolved into a bicontinuous porous solid structure at the alloy/melt interface. Owing to the fast dealloying velocity (in the order of 5–10 μm s⁻¹)^39,41, dealloying was completed in a short time (120 s). After cooling, the Mg melt was frozen in the porous structure, which was subsequently etched away using a dilute acid to expose the Co-Mo open porous structure. The changes of macroscopic morphology, phase, and microstructure are traced using optical photographs, XRD and scanning electron microscopy (SEM) images as shown in Fig. 1c–g, respectively. The fcc precursor Ni₇₀(Co_0.55Mo_0.45)₃₀ exhibits an average grain size of ~15 μm and a uniform distribution of Ni, Co, and Mo (Fig. 1e and Supplementary Fig. 2). Significantly, the intermetallic compound μ-Co₇Mo₆ (JCPDS 29-0489) together with the solidified Mg phase (JCPDS 35-0821) can be detected after LMD (Fig. 1d). The corresponding SEM image shows a bicontinuous nanocomposite consisting of interconnected μ-Co₇Mo₆ (bright contrast) and Mg (dark contrast) phases, which are the characteristic of nanoporous structures generated by dealloying (Fig. 1f). The feature/ligament size is measured to be 30.8 nm, using the Fiji package of the ImageJ software (Supplementary Fig. 3)⁴². After Mg removal, nanoporous μ-Co₇Mo₆ with a well-maintained bicontinuous structure is obtained (Fig. 1g). In agreement with the nanoscale ligament/pore size, the XRD pattern of nanoporous μ-Co₇Mo₆ shows broad diffraction peaks (Fig. 1d). Notably, the final nanoporous μ-Co₇Mo₆ inherits the free-standing sheet morphology of the precursor alloy with a well-preserved 2 cm × 1 cm lateral dimension (Fig. 1c and Supplementary Fig. 4). Due to LMD-induced volume contraction, the thickness of the sheet decreases from 200 μm to 170 μm. The nanoporous μ-Co₇Mo₆ is further characterized by transmission electron microscopy (TEM). The bright-field TEM image shows interconnected ligaments composed of crystallites of ~30 nm in size (Fig. 1h). Intriguingly, this polycrystalline structure with nanosized grain ligaments is distinct from the typical microstructure of nanoporous gold dealloyed from Au-Ag alloys, which often retains the original grain structure of parent alloys^43,44. Such distinction is likely owing to the difference in dealloyed phase formation between two alloy systems. The Au-Ag alloy system can form an ideal solid solution and has very small difference in atomic volumes between Au and Ag, allowing for an essentially coherent transformation from parent alloy to nanoporous product structure. In contrast, the LMD of nanoporous μ-Co₇Mo₆ is fulfilled by the phase transformation from fcc-Ni₇₀(Co_0.55Mo_0.45)₃₀ to trigonal μ-Co₇Mo₆ with distinctly different crystal structures and lattice parameters (Supplementary Table 2). Consequently, the development of μ-Co₇Mo₆ ligaments during the dealloying of fcc-Ni₇₀(Co_0.55Mo_0.45)₃₀ cannot be coherent and epitaxial, but is more likely through an interfacial heterogeneous nucleation and growth process, resulting in the formation of polycrystalline ligaments with nanosized grains. Similar polycrystalline structures had been observed in nanoporous gold produced by chemical dealloying of amorphous Au₄₀Cu₂₈Ag₇Pd₅Si₂₀ alloy⁴⁵, nanoporous Si by LMD of Mg₂Si alloy⁴⁶, and nanoporous Ni by vapor phase dealloying of Ni₂Zn₁₁ alloy⁴⁷. Despite being polycrystalline and inherently brittle, the nanoporous μ-Co₇Mo₆ maintains structural integrity and is mechanically robust due to the interconnections of ligaments in 3D space (Fig. 1c, Supplementary Figs. 4 and 5). The chemical composition of nanoporous μ-Co₇Mo₆ is verified by energy-dispersive X-ray spectroscopy (EDX) mappings, showing the uniform distribution of Co and Mo in the ligaments (Fig. 1h), which is in accordance with the observation of single-phase, as detected by XRD. Figure 1i and j show the atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of nanoporous μ-Co₇Mo₆ and the corresponding fast Fourier transform (FFT) pattern, respectively. The clear lattice fringes with d-spacings of 0.24 and 0.21 nm, highlighted in Fig. 1i, correspond to the (11$\bar{2}$0) and (000 12) crystal planes of the trigonal μ-Co₇Mo₆ (space group 166, R-3m)⁴⁸. The X-ray photoelectron spectroscopy (XPS) spectra collected from the surfaces of nanoporous μ-Co₇Mo₆ show predominantly metallic Co and Mo signals along with some signals corresponding to oxidized Mo⁴⁺, Mo⁵⁺, Mo⁶⁺, and Co²⁺ species, most likely formed by air-exposure (Supplementary Fig. 6). According to the Pourbaix diagrams of Mo and Co^49,50, these oxidized species can be reduced to metallic states during electrolysis on reduction polarization.

The microstructural characterizations confirmed the formation of monolithic 3D nanoporous structure for the intermetallic compound μ-Co₇Mo₆. This is the first report of the direct fabrication of a nanoporous intermetallic compound by LMD, via atomic self-assembly at the alloy/melt dealloying fronts. As a reference, it is found that the Ni₇₀(Co_0.55Mo_0.45)₃₀ alloy cannot be dealloyed chemically in 0.5 M H₂SO₄, HCl, or HNO₃ solutions at room temperature (Supplementary Fig. 7), due to the low ambient-temperature surface-diffusivity of Mo and surface passivation after leaching the top few atomic layers⁵¹. Therefore, the LMD is of vital importance in fabricating nanoporous intermetallic compounds, owing to the elevated dealloying temperatures that accelerate atomic diffusivity for successful dealloying and drive the formation of chemical order for the intermetallic phases.

Under identical LMD conditions (973 K for 120 s), nanoporous Fe₇Mo₆, Cr₅₀Mo₅₀, Mo, and Fe were also fabricated from their ternary or binary alloy precursors (Fig. 2 and Supplementary Fig. 8). Nanoporous Fe₇Mo₆ is a μ-phase intermetallic compound (JCPDS 31-0641) with the same trigonal lattice structure as that of nanoporous μ-Co₇Mo₆⁵². Nanoporous Cr₅₀Mo₅₀ consists of two body-centered cubic (bcc) phases of Cr-Mo solid-solutions, with the compositions of Cr₂₅Mo₇₅ and Cr₈₈Mo₁₂ as estimated by the Vegard’s law⁵³. Both nanoporous Mo and Fe are bcc phases. The chemical compositions of these nanoporous materials, measured by SEM-EDX, are consistent with those of their corresponding phases (Supplementary Table 1). For all porous samples, the residual Ni and Mg contents remain below 6 at.%, a reasonable value for dealloyed materials. All five samples have uniform and well-defined 3D bicontinuous nanoporous structures (Fig. 2a–d and Supplementary Fig. 8). The average ligament/pore sizes are 30.8, 31.8, 50.3, 35.5, and 652.0 nm for nanoporous μ-Co₇Mo₆, μ-Fe₇Mo₆, Cr₅₀Mo₅₀, Mo, and Fe, respectively. The large pore size of porous Fe is consistent with the general fact that LMD-generated porous structures usually have feature sizes on the scale of hundreds of nanometer, and even microns, owing to severe coarsening during high-temperature processing³⁹. In contrast, the feature sizes of nanoporous μ-Co₇Mo₆, μ-Fe₇Mo₆, Cr₅₀Mo₅₀, and Mo are small. While the small ligament sizes of nanoporous Mo and Cr₅₀Mo₅₀ could be explained by their high melting points (2896 K for Mo and ~2464 K for Cr₅₀Mo₅₀) and hence low atomic diffusivities, nanoporous μ-Co₇Mo₆ and μ-Fe₇Mo₆ are unique materials, as they have significantly low melting points (1892 and 1920 K for μ-Co₇Mo₆ and μ-Fe₇Mo₆, respectively) comparable to that of Fe (1811 K) but ligament sizes comparable to those of Mo and Cr₅₀Mo₅₀. The ligament sizes are temperature-dependent and can be controlled in the range of 20–43 nm, by varying the dealloying temperature from 873 to 1073 K, for nanoporous μ-Co₇Mo₆ (Supplementary Fig. 9).

**Fig. 2: Structural characterization of the nanoporous intermetallic compounds (or alloys).**

The intermetallic effect on LMD

The thermal coarsening of dealloyed nanoporous materials is a surface diffusion-controlled process^54,55. Sieradzki et al.⁵¹ proposed an empirical correlation describing the scaling relation between the ligament size and inverse homologous temperature, 1/T_H = T_m/T_dealloy, where T_m is the melting point of the pore-forming components (metals and alloys), and T_dealloy is the dealloying temperature, as shown in Fig. 3a. A survey of the literature data indicates a general trend that electrochemical dealloying (ECD) occupies the high 1/T_H region, with ligament sizes mostly below 100 nm, while high-temperature LMD corresponds to the low 1/T_H region, with ligaments of hundreds of nanometer in size^39,56. The scaling correlation between the ligament size and 1/T_H applies to both ECD and LMD. Plotting our LMD results at various temperatures in this diagram (Fig. 3a and Supplementary Figs. 10–14), a reasonable fit of the data to the conventional trend for nanoporous pure metals bcc-Mo, bcc-Fe, and solid-solution bcc-Cr₅₀Mo₅₀ is observed, even though their ligament sizes are relatively smaller than the literature data because of the different dealloying times (120 s for this study and 10-20 min in the literature). Significantly, a distinct deviation from the conventional scaling relation is observed for nanoporous intermetallic compounds of both μ-Co₇Mo₆ and μ-Fe₇Mo₆ (Fig. 3a). To eliminate the effect of the dealloying time, nanoporous μ-Co₇Mo₆ and μ-Fe₇Mo₆ were also fabricated with 15 min of dealloying time (Supplementary Figs. 10 and 11). As shown in Fig. 3a (as the open symbols), the ligament/pore sizes remain small and continue to deviate significantly from the typical scaling relationship. This indicates a unique and previously unexplored effect of intermetallic compounds in hindering the thermal coarsening of dealloyed nanoporous structures at high temperatures. Note that although the formation of surface oxides could also suppress surface diffusion⁵⁷, this effect could be excluded here as the LMD and pore formation are conducted in a highly reducing molten Mg environment.

**Fig. 3: Characteristic lengths of the nanoporous intermetallic compounds (or alloys) and molecular dynamics simulations of the interfacial diffusivities.**

In contrast to pure metals and solid-solution alloys, μ-Co₇Mo₆ and μ-Fe₇Mo₆ intermetallic compounds have chemically ordered crystal structures with covalent bonding, rather than pure metallic bonding^58,59. The μ-Co₇Mo₆ and μ-Fe₇Mo₆ have topologically close-packed (TCP) structures, with alternating Zr₄Al₃ and MgCu₂-type layers^48,52 (Supplementary Fig. 15). Such an ordered lattice structure, with solely tetrahedral interstices⁶⁰, renders much slower atomic mobility than simple fcc or bcc metals. Additionally, covalent bonding with a high bond energy makes it difficult for atoms to hop^52,58,59, forming another barrier for atomic diffusion. The suppressed atomic diffusivity of the intermetallic compounds is confirmed by molecular dynamics simulations. The 14 nm × 15 nm × 8 nm models were built by sandwiching an 8 nm-thick slat of μ-Co₇Mo₆, bcc-Mo, or hexagonal close packed (hcp)-Co, with designated facets ((11$\bar{2}$0), (1$\bar{1}$00), and (0001) for μ-Co₇Mo₆; (100) and (111) for bcc-Mo; (10$\bar{1}$0) and (0001) for hcp-Co), between two 3 nm-thick molten Mg layers (Fig. 3c–e). The models were relaxed in the 900–1200 K temperature range, and the mean square displacement over a duration of 20 ns was calculated for the three atomic layers at the metal/Mg melt interfaces to derive interfacial diffusivity, according to the Einstein-Smoluchowski relation⁶¹ (Supplementary Fig. 16). The obtained average diffusivities are plotted against 1/T_H, as shown in Fig. 3b. The surface diffusivities of bcc-Mo, hcp-Co, and Co in μ-Co₇Mo₆ follow a unified linear trend, but Mo in μ-Co₇Mo₆ shows an apparent downward variation. Considering that the ligament size (d(t)) is related to surface diffusivity (D_s) according to ${d(t)}^{n}={Kt}{D}_{{{{{{\rm{s}}}}}}}$ (where K is a constant; t is the dealloying time; and n, approximately 4, is the coarsening exponent)^47,62,63, the result is in good agreement with the trend of ligament size versus 1/T_H, as shown in Fig. 3a. Therefore, the ultrafine pore/ligament sizes of the nanoporous intermetallic compounds originate from suppressed Mo atomic diffusivity, particularly at the surfaces and interfaces of the intermetallic compounds. Based on the calculated diffusivities, activation energies of surface diffusion are determined to be 59.8, 84.2, 84.7, and 102.5 kJ mol⁻¹ for hcp-Co, bcc-Mo, Co in μ-Co₇Mo₆, and Mo in μ-Co₇Mo₆, respectively (Supplementary Table 3). Therefore, the intermetallic phase provides a large energy barrier for the surface diffusion of Co and Mo atoms.

Mechanisms for the evolution of ultrafine nanoporous intermetallic compounds

Based on the above experimental analyses and molecular dynamics simulations, a scheme for fabricating ultrafine nanoporous intermetallic compounds by LMD is outlined in Fig. 4. Taking nanoporous μ-Co₇Mo₆ as an example, immersing the single-phase Ni₇₀(Co_0.55Mo_0.45)₃₀ alloy into the Mg melt selectively dissolves the Ni atoms, leaving behind Co and Mo atoms at the alloy/melt interfaces, which diffuse and self-assemble into a 3D nanoporous solid structure. Despite the thermodynamic advantages, the formation of an ordered intermetallic phase is kinetically hampered by a high energy barrier¹⁴. The LMD overcomes this energy barrier by virtue of its high temperatures, enabling the concurrent formation of the chemically ordered μ-Co₇Mo₆ phase and bicontinuous nanoporosity, which facilitate the direct formation of nanoporous intermetallic compounds. In the dealloying process, nanopore formation is followed by thermal coarsening, which is a surface-diffusion-controlled process driven by thermodynamic surface-energy reduction^39,64,65. For conventional metals and random solid-solution alloys with moderate energy barriers of surface diffusion, high-temperature LMD drives severe thermal coarsening because of the significantly accelerated surface diffusivity, forming very coarse porous structures with pore/ligament sizes larger than hundreds of nanometer. In contrast, intermetallic compounds have a much larger energy barrier for surface diffusion, which regulates slow surface diffusivity even under high-temperature LMD conditions, significantly suppressing dealloyed-nanostructure coarsening and forming nanoporous intermetallic compounds with an ultrafine feature size. In the typical low 1/T_H region of LMD, nanoporous μ-Co₇Mo₆ and μ-Fe₇Mo₆ show very small ligament sizes of 20–100 nm, which are up to two orders of magnitude smaller than those of conventional nanoporous metals processed by LMD (Fig. 3a). Therefore, the intermetallic effect enables the utilization of high-temperature LMD for ordered-intermetallic-phase formation, while maintaining a fine nanoporous structure by circumventing the severe thermal coarsening. This is particularly advantageous for fabricating 3D nanoporous intermetallic compounds for electrocatalytic reactions, as fine structures with large surface areas and abundant under-coordinated surface defects contribute to high catalytic activity^66,67.

**Fig. 4: Schematic illustrations of the fabrication of ultrafine nanoporous μ-Co₇Mo₆ intermetallic compound by LMD.**

Electrochemical characterizations

The electrocatalytic performance of the nanoporous intermetallic compounds for the HER was evaluated in a 1 M KOH aqueous solution at room temperature. Two forms of the intermetallic catalysts were investigated: powder samples and self-supported monolithic sheets. The powder samples were prepared from as-dealloyed nanoporous sheets by mechanical grinding and high-power ultrasonication (Supplementary Fig. 17). As a mixture with conductive carbon and nafion binder, they were cast onto a glassy carbon disk electrode to make thin films with a 2 mg cm⁻² catalyst loading. Such thin film electrodes can reduce the ion diffusion constraints that thick nanoporous sheet electrodes may suffer, allowing for a more precise assessment of the intrinsic activity of nanoporous electrocatalysts. Figure 5a–d illustrate the electrochemical HER performance of nanoporous Mo-based intermetallic compounds and the reference materials. All nanoporous catalysts were fabricated at the temperature of 973 K. Based on the multiple criteria of onset potential (Fig. 5a), Tafel slope (Fig. 5b, c), operating potential at 10 mA cm⁻²_geo current density (Fig. 5a, c), and charge transfer resistance (R_ct) from electrochemical impedance spectroscopy (EIS) measurements (Fig. 5d), the catalytic activities exhibit the following trend: Pt/C > μ-Co₇Mo₆ > μ-Fe₇Mo₆ > Mo > Cr₅₀Mo₅₀. Pure carbon is essentially inert in the tested potential range. Among the nanoporous electrocatalysts, μ-Co₇Mo₆ demonstrats the lowest overpotential of 76 mV at 10 mA cm⁻²_geo current density, the lowest Tafel slope of 82 mV dec⁻¹, and the smallest R_ct of 5.1 Ω, indicating the highest HER kinetics and catalytic efficiency. The catalytic activity of nanoporous μ-Co₇Mo₆ is close, though yet inferior, to that of Pt/C.

**Fig. 5: Electrochemical HER performance of the nanoporous Mo-based intermetallic compounds and the reference materials tested as powder samples cast on a disk electrode.**

To assess the intrinsic activity of the catalysts, the electrochemical surface areas (ECSAs) of the nanoporous electrocatalysts were calculated by measuring their double layer capacitances (C_dl) (Supplementary Fig. 18). The ECSAs for nanoporous μ-Co₇Mo₆, μ-Fe₇Mo_6, Cr₅₀Mo₅₀, and Mo are determined to be 44.6, 54.9, 9.3, and 16.9 m² g⁻¹, respectively, clearly demonstrating an abundance of active sites in the fine-structured nanoporous intermetallic compounds. The ECSA-normalized HER polarization curves, in Supplementary Fig. 18h, indicate an intrinsic catalytic activity trend of the order of μ-Co₇Mo₆ > Mo > Cr₅₀Mo₅₀ > μ-Fe₇Mo₆. The activity trend for μ-Co₇Mo₆, Mo, and Cr₅₀Mo₅₀ remains unchanged compared to the geometric-surface-area-normalized results. However, nanoporous μ-Fe₇Mo₆ exhibits lower intrinsic activity but higher apparent activity (on a geometric surface area basis) than nanoporous Mo and Cr₅₀Mo₅₀, indicating that the large surface area and associated abundant active sites originating from a small ligament size of nanoporous μ-Fe₇Mo₆ contribute to the enhanced catalytic performance. In contrast to μ-Fe₇Mo₆, the nanoporous μ-Co₇Mo₆ has both the highest intrinsic activity and a fine porous structure for a large surface area, resulting in the highest catalytic activity among the nanoporous electrocatalysts. The superior HER activity of μ-Co₇Mo₆ over μ-Fe₇Mo₆ could be attributed to the electronic effect that the stronger d-electron interactions between Co and Mo in this hypo–hyper-d-electronic combinations modulate the optimal adsorption/desorption of reaction intermediates^68,69.

The nanoporous μ-Co₇Mo₆ fabricated by LMD at various temperatures were also examined as powder samples to determine the effect of pore/ligament size on the electrochemical properties. The three samples prepared at 873 K, 973 K, and 1073 K exhibit ligament diameters of 20.7, 30.8, and 43.1 nm, respectively (Supplementary Fig. 9 and Supplementary Table 4). As depicted in Fig. 5e and Supplementary Fig. 19, lower dealloying temperatures and smaller ligament sizes result in larger HER current densities on a geometric surface area basis for a given overpotential, indicating higher catalytic activity. The measured ECSAs for these electrocatalysts are 53.5, 44.6, and 24.1 m² g⁻¹, respectively (Supplementary Fig. 20). Remarkably, the ECSA-normalized HER polarization curves of the three electrocatalysts overlap when the current density is less than 0.037 mA cm⁻²_ECSA (equivalent to 40 mA cm⁻¹_geo for the 873 K sample), suggesting that they all possess the same intrinsic catalytic activity (Fig. 5f). Therefore, the smaller-ligament-induced increment in apparent catalytic efficiency is due to a structural effect in which a larger specific surface area provides a larger number of catalytically active sites for electrochemical reactions. This structural effect highlights the significance of the intermetallic effect in realizing the nanoscale porosity structure of Mo-based intermetallic compounds for superior electrochemical performance. Note that when the current density exceeds 40 mA cm⁻¹_geo, the 873 K sample’s activity decreases and becomes lower than that of the 973 K sample (Fig. 5e, f). This is probable because the ultrafine porous structure (20.7 nm pore size) of the 873 K sample limits ion/gas transport at high current densities⁷⁰. Under the current testing conditions, the 973 K sample with a pore size of 30.8 nm appears to be the optimal electrocatalyst capable of balancing the intrinsic reaction kinetics and accessibility of active sites.

The monolithic nanoporous intermetallic compounds are mechanically robust and can therefore be used as self-supported HER electrodes. Figure 6a shows the HER polarization curve of nanoporous μ-Co₇Mo₆ sheet electrodes with a dimension of 1 cm × 0.5 cm × 170 μm (Supplementary Fig. 21), in comparison to a Pt/C electrode drop-cast on carbon paper with a similarly high catalyst loading of 24.4 mg cm⁻². Due to intensified active sites from the bulk electrode, the nanoporous μ-Co₇Mo₆ sheet delivers improved HER performance compared to its powder counterpart, exhibiting a ~0 V onset potential, a 46 mV dec⁻¹ Tafel slope, and a 14 mV overpotential at 10 mA cm⁻²_geo current density (Fig. 6a, b and Supplementary Fig. 22). The property is among the best performances reported for non-precious Mo-based HER electrocatalysts (Fig. 6c and Supplementary Table 5). Due to the inferior intrinsic activity of nanoporous μ-Co₇Mo₆ compared to that of Pt/C, the μ-Co₇Mo₆ sheet exhibits larger overpotentials at current densities below 55 mA cm⁻²_geo than Pt/C (Fig. 6a). In contrast, μ-Co₇Mo₆ outperforms Pt/C at high current densities exceeding 55 mA cm⁻²_geo. For instance, an overpotential of 115 mV is required to obtain a current density of 150 mA cm⁻²_geo for nanoporous μ-Co₇Mo₆, which is significantly lower than the 180 mV necessary for Pt/C (Fig. 6b). While the low-current-density property of the electrocatalysts is mainly controlled by kinetics, at higher current densities the density and accessibility of active sites can dominate the performance, particularly in the porous electrodes. Therefore, the comparison between nanoporous μ-Co₇Mo₆ and Pt/C demonstrates that the 3D bicontinuous porous structure facilitates the electrochemical reactions of the intermetallic compounds by readily exposing more active sites, rendering the nanoporous μ-Co₇Mo₆ particularly suitable for high-current applications. Moreover, unlike Pt/C electrodes, which are prone to exfoliation due to the severe production of H₂ bubbles at high current densities, the robust self-supported μ-Co₇Mo₆ sheet with 3D interconnected ligaments has a far better structural stability. Along with excellent electrocatalytic activity, the nanoporous μ-Co₇Mo₆ electrodes exhibit excellent durability. After 5000 cycles of accelerated degradation testing by cyclic voltammetry, the nanoporous electrode shows a negligible shift in the HER polarization curves and negligible change in its porous structure (Fig. 6d). Furthermore, the nanoporous μ-Co₇Mo₆ delivers a high current density of ~170 mA cm⁻² at 150 mV overpotential, without significant current decay over a long-term chronoamperometry test for 48 h (Fig. 6e). It is necessary to note that despite the enhanced catalytic efficiency of self-supported nanoporous μ-Co₇Mo₆, the sheet electrodes have a much lower material utilization efficiency than powder samples (Supplementary Fig. 23) because only the near surface layers of the thick electrodes may contribute to electrochemical reactions, particularly at a high current density. High material efficiency would necessitate further optimization of the sheet thickness and the nanoporous structure. In addition, advanced catalyst integration techniques must be developed in order to implement the intrinsically brittle nanoporous μ-Co₇Mo₆ sheet as large-sized electrodes in commercial electrolyzers. By contrast, the powder form of the nanoporous intermetallic electrocatalysts, which can be spray-coated on substrates to make scalable electrodes with high material efficiency⁷¹, might be readily applicable for practical hydrogen production applications.

**Fig. 6: HER catalytic performance and durability of the self-supported np-Co₇Mo₆ sheet electrocatalysts (fabricated at 973 K).**

In summary, we presented a high-temperature LMD strategy for fabricating 3D nanoporous intermetallic catalysts of μ-Co₇Mo₆ and μ-Fe₇Mo₆. The formation of pore-forming intermetallic compounds leads to slow atomic diffusivity and retards thermal coarsening of nanoporous structures at high temperatures. The characteristic ligament sizes of nanoporous μ-Co₇Mo₆ and μ-Fe₇Mo₆ are only tens of nanometer, notably departing from the conventional scaling relationship between the ligament size and dealloying temperature. The nanoporous μ-Co₇Mo₆ with ultrafine porous structure exhibits high electrocatalytic activity and durability, indicating its potential use as a commercial HER catalyst for high-current applications. This study sheds light on the intermetallic effect on tailoring nanostructures of dealloyed materials and could assist the development of advanced nanoporous intermetallic electrocatalysts for a broad range of energy applications.

Methods

Fabrication of nanoporous Mo-M (M = Co, Fe, Cr)

The master alloys with nominal compositions of Ni₇₀(Co_0.55Mo_0.45)₃₀, Ni₇₀(Fe_0.58Mo_0.42)₃₀, Ni₇₀(Cr_0.50Mo_0.50)₃₀, Ni₇₀Mo₃₀, and Ni₇₀Fe₃₀ (atomic ratios) were prepared from pure metals (Ni (99.9%) from FUJIFILM Wako Pure Chemical Co., Osaka, Japan; Co (99.99%) from Rare Metallic Co., Ltd., Tokyo, Japan; Fe (>99.95%) from Toho Zinc Co., Ltd., Tokyo, Japan; Cr (99.9%) from Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan; Mo (99.95%) from Furuuchi Chemical Co., Tokyo, Japan) by arc melting in the argon atmosphere. The as-prepared ingots were re-melted in a tilt casting machine with argon protection and casted into thick plates with a thickness of 5 mm. The plates were further cold rolled into a thickness of 200 μm and then thermally treated at 1273 K for 8 h for homogenization and relieving the internal stress. The liquid metal dealloying (LMD) was carried out in a helium atmosphere using a magnesium melt (Mg (99.9%) from Rare Metallic Co., Ltd., Tokyo, Japan). The magnesium was melted and heated to temperatures of 973 K, 1073 K, or 1173 K by induction heating. The precursor alloy sheets of Ni₇₀(Co_0.55Mo_0.45)₃₀, Ni₇₀(Fe_0.58Mo_0.42)₃₀, Ni₇₀(Cr_0.50Mo_0.50)₃₀, Ni₇₀Mo₃₀, and Ni₇₀Fe₃₀ were immersed into the magnesium melt for a duration of 120 s. For LMD at 873 K, the eutectic Mg₉₀Ca₁₀ alloy melt was used in place of the Mg melt and a longer immersion time of 1800 s was used to ensure the complete dealloying of the entire sheet. After LMD, the obtained samples were etched in 0.05 M HCl in ethanol solution for 24 h to remove the solidified magnesium.

Materials characterization

The X-ray diffraction was carried out using a Bruker D8 Discover diffractometer with Co-Kα radiation (Yokohama, Japan, λ = 1.7902 Å) at a scan rate of 0.5° per min. The microstructure and chemical composition were explored by a field-emission scanning electron microscope (Ultra 55, ZEISS, Oberkochen, Germany) coupled with an energy dispersive X-ray spectrometer (Quantax, Bruker, Billerica, MA, USA). The sample after LMD for cross-section imaging was prepared by mechanical polishing and ion milling (E-3500, Hitachi, Tokyo, Japan). Further investigations of the microstructure and chemical compositions were performed using a field-emission transmission electron microscope (JEOL JEM-2100F, 200 keV) with double spherical aberration correctors for both the probe-forming and image-forming objective lenses. Surface compositions of the samples were characterized by an X-ray photoelectron spectroscopy (XPS, AXIS ultra DLD, Shimazu) in a vacuum pressure of 10⁻⁷ Pa with an Al Kα (mono) anode. The specific surface area was measured by N₂ adsorption–desorption method using BELSORP-mini II (BEL. JAPAN. INC) at 77.0 K.

Electrochemical characterization

All the electrochemical measurements were performed in 1.0 M KOH aqueous solution at 293 K using a Bio-Logic VSP 300 electrochemical workstation and a three-electrode electrochemical cell with a Pt wire or graphite rod counter electrode and an Ag/AgCl reference electrode. The powder samples were prepared from as-dealloyed sheet samples using a combination of mechanical grinding with a mortar and pestle and high-power ultrasonication at 130 W. 10 mg of nanoporous sample powder, 2 mg of conductive acetylene black (MTI Corp.), and 50 μL of Nafion solution (Wako Corp., 5 wt%) were dispersed in 1 mL of ethanol and sonicated for one hour to obtain a homogeneous ink. 39.2 μL of the ink was then cast onto a glassy carbon disk electrode (5 mm in diameter). The benchmark Pt/C catalyst was prepared from commercial Pt/C (20 wt%) using the same mixture composition. The catalyst loading of the nanoporous catalysts and Pt/C was 2 mg cm⁻². A pure carbon electrode with a carbon loading of 0.4 mg cm⁻² (equivalent to the loading of carbon components in the mixture electrodes) was also made by the same procedure without adding the nanoporous catalysts. For electrochemical measurements employing self-supported sheet samples, the sheet electrodes had a size of 1 cm × 0.5 cm × 170 μm. The high-load Pt/C catalyst was prepared by casting commercial Pt/C powders on a carbon paper. A total of 610 μL of the ink made by mixing 10 mg of Pt/C (20 wt%) and 125 μL of Nafion solution (5 wt%) in 1 mL of ethanol was drop-cast onto the carbon paper at a loading amount of 24.4 mg cm⁻² and dried at room temperature. The polarization curves were characterized by cyclic voltammetry (CV) with a scan rate of 2 mV s⁻¹. iR-compensation was applied to all polarization curves based on resistance data measured by EIS at −0.9 V vs Ag/AgCl. The potential versus Ag/AgCl was changed into RHE based on E_vs RHE = E_vs Ag/AgCl + E_Ø Ag/AgCl + 0.059 pH. The electrochemical impedance spectroscopy (EIS) was carried out at −0.2 V vs RHE with the frequency from 10⁵ Hz to 0.01 Hz. The accelerated cyclic voltammetry cycling test was conducted at the scan rate of 50 mV s⁻¹ for 5000 cycles. The chronoamperometric responses were tested by holding the potential at −0.15 V vs RHE. The double-layer capacitance (C_dl) was measured by CV in non-Faradaic potential region (−0.9 V to −1.0 V vs Ag/AgCl) with scan rates of 10, 20, 30, 40 and 50 mV s⁻¹. The electrochemical surface area (ECSA) is calculated by subtracting the capacitive contributions from carbon:

$${ECSA}=\left({C}_{{{{{{\rm{dl}}}}}}}-{C}_{{{{{{\rm{dl@carbon}}}}}}}\right)/{C}_{{{{{{\rm{s}}}}}}}$$

(1)

where C_dl is equivalent to the linear slope in the function of charging current density differences (Δj = |j_a-j_c | /2) and scan rate and C_s is the specific capacitance with a moderate value of 0.040 mF cm⁻²_ECSA^10,72.

Molecular dynamics simulation

Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)⁷³ was used to simulate the process of atom diffusion at the interfaces between Co (hcp), Mo (bcc), or Co₇Mo₆ (intermetallic compound, μ-phase) and a magnesium. A 14 nm × 15 nm × 8 nm model was prepared by sandwiching an 8 nm-thick slat of μ-Co₇Mo₆, bcc-Mo or hcp-Co with designated facets ((11$\bar{2}$0), (1$\bar{1}$00), and (0001) for μ-Co₇Mo₆; (100) and (111) for bcc-Mo; (10$\bar{1}$0) and (0001) for hcp-Co) between two 3 nm-thick molten Mg layers. The embedded atomic method (EAM) potential fitted by Zhou et al.⁷⁴ was used to represent interactions between atoms. The crystal/liquid models were relaxed at 1000 K for 20 ns and subsequently equilibrated at 900–1200 K for 200 ps with zero external pressure (NPT). Then, the systems were held at 900–1200 K for 20 ns under the constant volume condition (NVT). The three atomic layers at the interfaces were extracted and diffusivities of these atoms were calculated based on the mean square displacement (MSD) and the Einstein-Smoluchowski relation.

Data availability

The raw/processed data generated in this study are available at Zenodo.org, https://doi.org/10.5281/zenodo.6982712. Source data are provided with this paper.

References

Kibsgaard, J. & Chorkendorff, I. Considerations for the scaling-up of water splitting catalysts. Nat. Energy 4, 430–433 (2019).
Article ADS Google Scholar
He, X. et al. A versatile route to fabricate single atom catalysts with high chemoselectivity and regioselectivity in hydrogenation. Nat. Commun. 10, 1–9 (2019).
Article ADS CAS Google Scholar
Pang, F. et al. Bimodal nanoporous Pd₃Cu₁ alloy with restrained hydrogen evolution for stable and high yield electrochemical nitrogen reduction. Nano Energy 58, 834–841 (2019).
Article CAS Google Scholar
Li, G. et al. Dirac nodal arc semimetal PtSn₄: an ideal platform for understanding surface properties and catalysis for hydrogen evolution. Angew. Chem. 131, 13241–13246 (2019).
Article ADS Google Scholar
Wang, J. et al. Non-precious-metal catalysts for alkaline water electrolysis: operando characterizations, theoretical calculations, and recent advances. Chem. Soc. Rev. 49, 9154–9196 (2020).
Article CAS PubMed Google Scholar
Gao, Q., Zhang, W., Shi, Z., Yang, L. & Tang, Y. Structural design and electronic modulation of transition-metal-carbide electrocatalysts toward efficient hydrogen evolution. Adv. Mater. 31, 1802880 (2019).
Article CAS Google Scholar
Hu, K. et al. Graphene layer encapsulation of non-noble metal nanoparticles as acid-stable hydrogen evolution catalysts. ACS Energy Lett. 3, 1539–1544 (2018).
Article CAS Google Scholar
Lu, Q. et al. Highly porous non-precious bimetallic electrocatalysts for efficient hydrogen evolution. Nat. Commun. 6, 1–8 (2015).
Article ADS CAS Google Scholar
Chen, J. et al. Nesting Co₃Mo binary alloy nanoparticles onto molybdenum oxide nanosheet arrays for superior hydrogen evolution reaction. ACS Appl. Mater. Interfaces 11, 9002–9010 (2019).
Article CAS PubMed Google Scholar
Shi, H. et al. Spontaneously separated intermetallic Co₃Mo from nanoporous copper as versatile electrocatalysts for highly efficient water splitting. Nat. Commun. 11, 1–10 (2020).
ADS CAS Google Scholar
Jakšić, M. M. Electrocatalysis of hydrogen evolution in the light of the brewer-engel theory for bonding in metals and intermetallic phases. Electrochim. Acta 29, 1539–1550 (1984).
Article Google Scholar
Xin, H., Vojvodic, A., Voss, J., Nørskov, J. K. & Abild-Pedersen, F. Effects of d-band shape on the surface reactivity of transition-metal alloys. Phys. Rev. B 89, 115114 (2014).
Article ADS CAS Google Scholar
Jiang, W. J., Tang, T., Zhang, Y. & Hu, J. S. Synergistic modulation of non-precious-metal electrocatalysts for advanced water splitting. Acc. Chem. Res. 53, 1111–1123 (2020).
Article CAS PubMed Google Scholar
Yan, Y. et al. Intermetallic Nanocrystals: Syntheses and Catalytic Applications. Adv. Mater. 29, 1605997 (2017).
Article Google Scholar
Yang, C.-L. et al. Sulfur-anchoring synthesis of platinum intermetallic nanoparticle catalysts for fuel cells. Science 374, 459–464 (2021).
Article ADS CAS PubMed Google Scholar
Zhang, J. et al. Efficient hydrogen production on MoNi₄ electrocatalysts with fast water dissociation kinetics. Nat. Commun. 8, 1–8 (2017).
ADS CAS Google Scholar
Zhou, Q. et al. Free-standing trimodal porous NiZn intermetallic and Ni heterojunction as highly efficient hydrogen evolution electrocatalyst in the alkaline electrolyte. Nano Energy 89, 106402 (2021).
Article CAS Google Scholar
Dong, H. et al. Electrochemical performance of porous Ni₃Al electrodes for hydrogen evolution reaction. Int. J. Hydrog. Energy 36, 12112–12120 (2011).
Article CAS Google Scholar
Chen, Q., Ding, Y. & Chen, M. Nanoporous metal by dealloying for electrochemical energy conversion and storage. MRS Bull. 43, 43–48 (2018).
Article ADS CAS Google Scholar
Ding, Y. & Chen, M. Nanoporous metals for catalytic and optical applications. MRS Bull. 34, 569–576 (2009).
Article CAS Google Scholar
Sang, Q., Hao, S., Han, J. & Ding, Y. Dealloyed nanoporous materials for electrochemical energy conversion and storage. EnergyChem 4, 100069 (2022).
Article CAS Google Scholar
Zhu, C. et al. Toward digitally controlled catalyst architectures: hierarchical nanoporous gold via 3D printing. Sci. Adv. 4, eaas9459 (2018).
Article ADS CAS PubMed PubMed Central Google Scholar
Lan, J. et al. Scalable synthesis of nanoporous boron for high efficiency ammonia electrosynthesis. Mater. Today 38, 58–66 (2020).
Article ADS CAS Google Scholar
Wang, X., Luo, M., Lan, J., Peng, M. & Tan, Y. Nanoporous intermetallic Pd₃Bi for efficient electrochemical nitrogen reduction. Adv. Mater. 33, e2007733 (2021).
Article PubMed CAS Google Scholar
Erlebacher, J., Aziz, M. J., Karma, A., Dimitrov, N. & Sieradzki, K. Evolution of nanoporosity in dealloying. Nature 410, 450–453 (2001).
Article ADS CAS PubMed Google Scholar
Weissmüller, J. & Sieradzki, K. Dealloyed nanoporous materials with interface-controlled behavior. MRS Bull. 43, 27–34 (2018).
Article CAS Google Scholar
Liu, W. B. et al. A general dealloying strategy to nanoporous intermetallics, nanoporous metals with bimodal, and unimodal pore size distributions. Corros. Sci. 58, 133–138 (2012).
Article CAS Google Scholar
Yu, J., Zhou, X. & Zhang, C. Synthesis of nanoporous PtSi compound by selected dissolution of a Pt₂₀Si₈₀ alloy precursor. Mater. Lett. 69, 13–15 (2012).
Article CAS Google Scholar
Han, G.-F. et al. Scalable nanoporous (Pt_1-x Ni_x)₃Al intermetallic compounds as highly active and stable catalysts for oxygen electroreduction. ACS Appl. Mater. Interfaces 8, 32910–32917 (2016).
Article CAS PubMed Google Scholar
Williams, B. P., Qi, Z., Huang, W. & Tsung, C. K. The impact of synthetic method on the catalytic application of intermetallic nanoparticles. Nanoscale 12, 18545–18562 (2020).
Article CAS PubMed Google Scholar
Cheng, T. et al. Nanoporous (Pt_1-xFe_x)₃Al intermetallic compounds for greatly enhanced oxygen electroreduction catalysis. J. Mater. Chem. A 4, 18878–18884 (2016).
Article CAS Google Scholar
Lin, T. C. et al. Understanding stabilization in nanoporous intermetallic alloy anodes for Li-ion batteries using operando transmission X-ray microscopy. ACS Nano 14, 14820–14830 (2020).
Article PubMed CAS Google Scholar
Wan, W. B. et al. Nanoporous intermetallic Cu₃Sn/Cu hybrid electrodes as efficient electrocatalysts for carbon dioxide reduction. Small 17, 2100683 (2021).
Article CAS Google Scholar
Sun, J.-S. et al. Nonprecious intermetallic Al₇Cu₄Ni nanocrystals seamlessly integrated in freestanding bimodal nanoporous copper for efficient hydrogen evolution catalysis. Adv. Funct. Mater. 28, 1706127 (2018).
Article ADS CAS Google Scholar
Gaskey, B., McCue, I., Chuang, A. & Erlebacher, J. Self-assembled porous metal-intermetallic nanocomposites via liquid metal dealloying. Acta Mater. 164, 293–300 (2019).
Article ADS CAS Google Scholar
Wada, T., Yubuta, K., Inoue, A. & Kato, H. Dealloying by metallic melt. Mater. Lett. 65, 1076–1078 (2011).
Article CAS Google Scholar
Geslin, P. A., McCue, I., Gaskey, B., Erlebacher, J. & Karma, A. Topology-generating interfacial pattern formation during liquid metal dealloying. Nat. Commun. 6, 1–8 (2015).
Article CAS Google Scholar
Tsuda, M., Wada, T. & Kato, H. Kinetics of formation and coarsening of nanoporous α-titanium dealloyed with Mg melt. J. Appl. Phys. 114, 113503 (2013).
Article ADS CAS Google Scholar
McCue, I., Karma, A. & Erlebacher, J. Pattern formation during electrochemical and liquid metal dealloying. MRS Bull. 43, 27–34 (2018).
Article ADS CAS Google Scholar
Takeuchi, A. & Inoue, A. Classification of bulk metallic glasses by atomic size difference, heat of mixing and period of constituent elements and its application to characterization of the main alloying element. Mater. Trans. 46, 2817–2829 (2005).
Article CAS Google Scholar
McCue, I., Gaskey, B., Geslin, P.-A., Karma, A. & Erlebacher, J. Kinetics and morphological evolution of liquid metal dealloying. Acta Mater. 115, 10–23 (2016).
Article ADS CAS Google Scholar
Schindelin, J. et al. Fiji: an open-source platform for biological-image analysis. Nat. Methods 9, 676–682 (2012).
Article CAS PubMed Google Scholar
Petegem, S. V. et al. On the microstructure of nanoporous gold: an X-ray diffraction study. Nano Lett. 9, 1158–1163 (2009).
Article ADS PubMed CAS Google Scholar
Gwak, E. J. & Kim, J. Y. Weakened Flexural Strength of Nanocrystalline Nanoporous Gold by Grain Refinement. Nano Lett. 16, 2497–2502 (2016).
Article ADS CAS PubMed Google Scholar
Paschalidou, E. M. et al. The mechanism of generating nanoporous Au by de-alloying amorphous alloys. Acta Mater. 119, 177–183 (2016).
Article ADS CAS Google Scholar
Wada, T. et al. Bulk-nanoporous-silicon negative electrode with extremely high cyclability for lithium-ion batteries prepared using a top-down process. Nano Lett. 14, 4505–4510 (2014).
Article ADS CAS PubMed Google Scholar
Han, J. et al. Vapor phase dealloying: a versatile approach for fabricating 3D porous materials. Acta Mater. 163, 161–172 (2019).
Article ADS CAS Google Scholar
Forsyth JB, D. A. d. V. L. The structure of the μ-phase Co₇Mo₆. Acta Crystallogr. 15, 543–546 (1962).
Chen, P. C., Hsieh, S. J., Chen, C. C. & Zou, J. The microstructure and capacitance characterizations of anodic titanium based alloy oxide nanotube. J. Nanomater. 2013, 73 (2013).
Google Scholar
Zeng, Z. et al. Towards first principles-based prediction of highly accurate electrochemical pourbaix diagrams. J. Phys. Chem. C. 119, 18177–18187 (2015).
Article CAS Google Scholar
Chen, Q. & Sieradzki, K. Spontaneous evolution of bicontinuous nanostructures in dealloyed Li-based systems. Nat. Mater. 12, 1102–1106 (2013).
Article ADS CAS PubMed Google Scholar
Schröders, S., Sandlöbes, S., Berkels, B. & Korte-Kerzel, S. On the structure of defects in the Fe₇Mo₆ μ-Phase. Acta Mater. 167, 257–266 (2019).
Article ADS Google Scholar
Vegard, L. Die konstitution der mischkristalle und die raumfüllung der atome. Z. f.ür. Phys. 5, 17–26 (1921).
Article ADS CAS Google Scholar
Geslin, P.-A., Buchet, M., Wada, T. & Kato, H. Phase-field investigation of the coarsening of porous structures by surface diffusion. Phys. Rev. Mater. 3, 083401 (2019).
Article CAS Google Scholar
Flynn, C. P. Why is diffusion in metals and on metal surfaces universal? J. Phys.: Condens. Matter 18, S439–S445 (2006).
ADS CAS Google Scholar
Joo, S. H. et al. Beating thermal coarsening in nanoporous materials via high-entropy design. Adv. Mater. 32, 1906160 (2019).
Article CAS Google Scholar
Biener, M. M. et al. ALD functionalized nanoporous gold: thermal stability, mechanical properties, and catalytic activity. Nano Lett. 11, 3085–3090 (2011).
Article ADS CAS PubMed Google Scholar
Li, P. et al. First-principles investigations on structural stability, elastic and electronic properties of Co₇M₆ (M= W, Mo, Nb) µ phases. Mol. Simul. 45, 752–758 (2019).
Article CAS Google Scholar
Hirata, A., Iwai, A. & Koyama, Y. Characteristic features of the Fe₇Mo₆-type structure in a transition-metal alloy examined using transmission electron microscopy. Phys. Rev. B 74, 054204 (2006).
Article ADS CAS Google Scholar
Wilson, A. S. Formation and effect of topologically close-packed phases in nickel-base superalloys. Mater. Sci. Technol. 33, 1108–1118 (2017).
Article CAS Google Scholar
Mehrer, H. Diffusion in solids: fundamentals, methods, materials, diffusion-controlled processes. Vol. 155. (Springer Science & Business Media, 2007).
Qian, L. H. & Chen, M. W. Ultrafine nanoporous gold by low-temperature dealloying and kinetics of nanopore formation. Appl. Phys. Lett. 91, 083105 (2007).
Article ADS CAS Google Scholar
Lu, Z. et al. Three-dimensional bicontinuous nanoporous materials by vapor phase dealloying. Nat. Commun. 9, 1–7 (2018).
Article ADS CAS Google Scholar
Erlebacher, J. Mechanism of coarsening and bubble formation in high-genus nanoporous metals. Phys. Rev. Lett. 106, 225504 (2011).
Article ADS CAS PubMed Google Scholar
Chen-Wiegart, Y.-cK. et al. Structural evolution of nanoporous gold during thermal coarsening. Acta Mater. 60, 4972–4981 (2012).
Article ADS CAS Google Scholar
Fujita, T. et al. Atomic origins of the high catalytic activity of nanoporous gold. Nat. Mater. 11, 775–780 (2012).
Article ADS CAS PubMed Google Scholar
Liu, P. et al. Visualizing under-coordinated surface atoms on 3D nanoporous gold catalysts. Adv. Mater. 28, 1753–1759 (2016).
Article ADS CAS PubMed Google Scholar
Jakšić, M. M. Brewer intermetallic phases as synergetic electrocatalysts for hydrogen evolution. Mater. Chem. Phys. 22, 1–26 (1989).
Article Google Scholar
Jakšić, M. M. Hypo–hyper-d-electronic interactive nature of synergism in catalysis and electrocatalysis for hydrogen reactions. Electrochim. Acta 45, 4085–4099 (2000).
Article Google Scholar
Falcucci, G. et al. Mapping reactive flow patterns in monolithic nanoporous catalysts. Microfluid. Nanofluid. 20, 1–13 (2016).
Article CAS Google Scholar
Qi, Z. et al. Scalable fabrication of high activity nanoporous copper powders for electrochemical CO2 reduction via ball milling and dealloying. J. CO2 Util. 45, 101454 (2021).
Article CAS Google Scholar
Sun, Q., Dong, Y., Wang, Z., Yin, S. & Zhao, C. Synergistic nanotubular copper-doped nickel catalysts for hydrogen evolution reactions. Small 14, 1704137 (2018).
Article CAS Google Scholar
Plimpton, S. Fast parallel algorithms for short-range molecular dynamics. J. Comput. Phys. 117, 1–19 (1995).
Article ADS CAS MATH Google Scholar
Zhou, X. W., Johnson, R. A. & Wadley, H. N. G. Misfit-energy-increasing dislocations in vapor-deposited CoFe/NiFe multilayers. Phys. Rev. B 69, 144113 (2004).
Article ADS CAS Google Scholar

Download references

Acknowledgements

This work was supported by the Collaborative Research Center on Energy Materials in the Institute of Materials Research (E-IMR), Tohoku University, and Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Nos. 21J12719, 20K05126, 19K15389, and 18H05939). M. Chen was supported by the Whiting School of Engineering, Johns Hopkins University, and the National Science Foundation (NSF DMR-1804320). R. Song acknowledges support from the China Scholarship Council. We thank Ms. Omura of the IMR at Tohoku University for the XPS tests.

Author information

Authors and Affiliations

Institute for Materials Research, Tohoku University, Sendai, Japan
Ruirui Song, Rodion Belosludov, Takeshi Wada, Jing Jiang, Daixiu Wei & Hidemi Kato
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, Japan
Ruirui Song
Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Sendai, Japan
Jiuhui Han
WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan
Jiuhui Han & Akira Kudo
Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials and Low-Carbon Technologies, Tianjin University of Technology, Tianjin, China
Jiuhui Han
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan
Masayuki Okugawa
Mathematics for Advanced Materials Open Innovation Laboratory, AIST, Sendai, Japan
Masayuki Okugawa
Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD, USA
Yuan Tian & Mingwei Chen

Authors

Ruirui Song
View author publications
You can also search for this author in PubMed Google Scholar
Jiuhui Han
View author publications
You can also search for this author in PubMed Google Scholar
Masayuki Okugawa
View author publications
You can also search for this author in PubMed Google Scholar
Rodion Belosludov
View author publications
You can also search for this author in PubMed Google Scholar
Takeshi Wada
View author publications
You can also search for this author in PubMed Google Scholar
Jing Jiang
View author publications
You can also search for this author in PubMed Google Scholar
Daixiu Wei
View author publications
You can also search for this author in PubMed Google Scholar
Akira Kudo
View author publications
You can also search for this author in PubMed Google Scholar
Yuan Tian
View author publications
You can also search for this author in PubMed Google Scholar
Mingwei Chen
View author publications
You can also search for this author in PubMed Google Scholar
Hidemi Kato
View author publications
You can also search for this author in PubMed Google Scholar

Contributions

H.K., J.H. and M.C. conceived and supervised this research. R.S. and J.H. conducted materials preparation, characterization, and electrochemical measurements. M.O performed the MD simulations. A.K. performed the mechanical property measurements. R.S., J.H., M.O., R.B., T.W., J.J., D.W., Y.T., M.C. and H.K. analyzed the data. J.H. and R.S wrote the paper. All the authors discussed the results and reviewed the manuscript.

Corresponding authors

Correspondence to Jiuhui Han, Mingwei Chen or Hidemi Kato.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Peer Review File

Description of Additional Supplementary Files

Supplementary Movie 1

Source data

Source Data

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

Reprints and permissions

About this article

Cite this article

Song, R., Han, J., Okugawa, M. et al. Ultrafine nanoporous intermetallic catalysts by high-temperature liquid metal dealloying for electrochemical hydrogen production. Nat Commun 13, 5157 (2022). https://doi.org/10.1038/s41467-022-32768-1

Download citation

Received: 10 January 2022
Accepted: 16 August 2022
Published: 02 September 2022
DOI: https://doi.org/10.1038/s41467-022-32768-1

This article is cited by

Porous TiFe2 intermetallic compound fabricated via elemental powder reactive synthesis
- Qian Zhao
- Zhenli He
- Yao Jiang
International Journal of Minerals, Metallurgy and Materials (2024)
Partial liquid metal dealloying to synthesize nickel-containing porous and composite ferrous and high-entropy alloys
- Takeshi Wada
- Pierre-Antoine Geslin
- Hidemi Kato
Communications Materials (2023)
Modular assembly of metal nanoparticles/mesoporous carbon two-dimensional nanosheets
- Pengpeng Qiu
- Xingmiao Zhang
- Dongyuan Zhao
NPG Asia Materials (2023)

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.