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J. Phys. Chem. C 2010, 114, 868–873
Hydrogen Storage Properties of Nanoporous Palladium Fabricated by Dealloying Masataka Hakamada,*,† Hiromi Nakano,‡ Toshiyuki Furukawa,§ Masaki Takahashi,§ and Mamoru Mabuchi§ Materials Research Institute for Sustainable DeVelopment, National Institute of AdVanced Industrial Science and Engineering (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan; CooperatiVe Research Facility Center, Toyohashi UniVersity of Technology, 1-1 Hibarigaoka, Tempakucho, Toyohashi 441-8580, Japan; and Graduate School of Energy Science, Kyoto UniVersity, Yoshidahonmachi, Sakyo, Kyoto 606-8501, Japan ReceiVed: October 2, 2009; ReVised Manuscript ReceiVed: NoVember 12, 2009
The hydrogen storage properties of nanoporous Pd fabricated by dealloying a Pd0.2Co0.8 alloy were investigated by measuring pressure-composition isotherms (PCTs). The hydrogen storage behaviors of nanoporous Pd were obviously different from those of bulk Pd and other nanostructured Pd materials, such as nanocrystalline Pd and nanoparticle Pd. The miscibility gap narrowed similarly to that in nanocrystalline or nanoparticle Pd, but the gap narrowed in a different manner; that is, the hydrogen dissolution in the R-phase of nanoporous Pd was similar to that in the R-phase of bulk Pd, while that in the β-phase of nanoporous Pd was similar to that in the β-phase of the nanostructured Pd. High-resolution transmission electron microscopy showed lattice disorder (both expansion and contraction) at the surface of nanoporous Pd. First-principles calculations within the generalized gradient approximation suggest that the lattice expansion and contraction are responsible for the hydrogen adsorption in the R-phase of nanoporous Pd. 1. Introdution Palladium (Pd) is a classical hydrogen-storage metal, and many studies on hydrogen storage in Pd have been conducted1-18 because hydrogen has been attracting considerable attention as a clean energy carrier.17 It is reported that nanocrystalline or nanoparticle metals or alloys, including Pd, have hydrogen storage properties different from those of their bulk counterparts.1-13,18,19 In general, at low hydrogen pressures, bulk Pd absorbs hydrogen in a solid solution (R-phase) with a low hydrogen concentration. When the hydrogen pressure increases, phase transition occurs at a certain hydrogen pressure (so-called plateau pressure) to form a hydride phase (β-phase), which can contain hydrogen at a much higher concentration than the R-phase. The miscibility gap between the hydrogen solubilities of the R- and β-phases indicates the hydrogen storage capacity of Pd.20 This behavior is also observed in nanocrystalline Pd1-6 and nanoparticle Pd.7-13 However, the miscibility gaps are narrowed, the R-to-β phase transition gradually occurs, and then the distinction between the R- and β-phases becomes unclear in these nanostructured Pd materials, presumably because of the considerable effects of the surface and grain boundary.3,4,9 Nanoporous metals with nanosized pores and ligaments are emerging nanostructured materials and can be readily fabricated by dealloying (selective dissolution of less noble metals from binary alloys).21 The open-cell nanoporous structure of metals offers many interesting properties, such as mechanical,22,23 catalytic,24 electrical,25 piezoelectric,26,27 and magnetic properties.28 Nanoporous Pd, which can be synthesized by dealloying the Pd-Co alloy,29,30 has a nanostructure that can be distin* To whom correspondence should be addressed. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Engineering (AIST). ‡ Toyohashi University of Technology. § Kyoto University.
guished from those of other nanostructured Pd materials. The rigid network of Pd nanoligaments may affect hydrogen storage properties in a specific manner. This is the first report discussing the hydrogen storage properties of nanoporous Pd fabricated by dealloying. 2. Experimental Methods A Pd0.2Co0.8 alloy ingot was fabricated as a precursor of nanoporous Pd by the arc melting of pure palladium (>99.9%, Tanaka Kikinzoku Kogyo K. K., Japan) and cobalt (>99.9%, Kojundo Chemical Lab. Co., Ltd., Japan). This ingot was rolled and electrochemically dealloyed in 0.1 M H2SO4 for 48 h under a potentiostatic condition at +0.5 V vs saturated calomel electrode (SCE) controlled by a potentiostat (HZ-5000, Hokuto Denko Corp., Japan) at room temperature to fabricate nanoporous Pd.29,30 After the 48 h dealloying, the detected current was negligible and the Co dissolution was completed. Some portion of the fabricated nanoporous Pd was annealed under Ar atmosphere at 773 K for 600 s to coarsen the nanoporous structure. The microstructures of the samples were observed by scanning electron microscopy (SEM: S-4300 by Hitachi HighTechnologies Corp.) to determine their ligament sizes. Energydispersive X-ray spectroscopy (EDXS: EMAX by Horiba) was coupled with the SEM for elemental analysis. X-ray diffraction (XRD) patterns were measured by X-ray diffraction (RINT Ultima III by Rigaku Corp.) with Cu KR radiation (wavelength ) 0.154 18 nm) for phase identification. For observation with a high-resolution transmission electron microscope (HR-TEM: JEM-3000F by JEOL), a 3-mm-diameter disk was cut from initial rolled alloys. One side of the disk was polished and the other side was dimpled to a central thickness of 10 µm. The alloy disk was then dealloyed at +0.5 V vs SCE for 1.8 × 102 s in 0.1 mol/L H2SO4 electrolyte, followed by thinning using an Ar ion milling apparatus.
10.1021/jp909479m 2010 American Chemical Society Published on Web 12/08/2009
Hydrogen Storage Properties of Nanoporous Pd
Figure 1. H/Pd atomic model for first-principles calculations.
Pressure-composition isotherms (PCTs) were measured by a JIS H 7201 method31 at 298, 323, 423, and 523 K. Before the PCT measurements, the samples were exposed to hydrogen at 10 MPa for 1 h at the measurement temperatures for activation. Unfortunately, the annealed samples required a very long time for complete activation at 298 K. The reason for this is unclear, and thus, the PCT result for the annealed sample measured at 298 K is not included in the present paper. 3. First-Principles Calculation Methods As discussed later, the lattice disorder at the surfaces of nanoporous Pd seems to affect the resulting hydrogen storage properties. To confirm the effect of the lattice disorder on the hydrogen adsorption in nanoporous Pd, first-principles calculations were carried out. All the calculations were performed within the generalized gradient approximation proposed by Perdew et al.32 using the CASTEP code. The electron-ion interaction is described on the basis of ultrasoft pseudopotentials for H and Pd with a cutoff energy of 400 eV. Brillouin-zone integrations were performed with grid of 4 × 4 × 1 special k-points.
J. Phys. Chem. C, Vol. 114, No. 2, 2010 869 The surface is modeled using a periodically repeated slab of three fixed layers separated by a vacuum slab of 10 Å thickness. For the H-adsorbed model, an H atom is embedded at the fcchollow site, where there is no Pd atom just below the H in the second layer, of the Pd (111) surface as shown in Figure 1. The lattice constant a is varied at 0.369 (contracted), 0.389 (normal), and 0.409 nm (expanded), and the H atom is located above the first Pd layer so that the Pd-H distance is 1.81 Å for all three models. This Pd-H distance corresponds to a stable adsorption site for fcc-hollow site.33,34 For these three models, the hydrogen adsorption energy ∆Ead is calculated as
∆Ead ) EH/Pd - EPd - EH
(1)
where EH/Pd, EPd, and EH are the total energies of the H-adsorbed Pd surface, H-free Pd surface, and H atom, respectively. 4. Results Figure 2 shows SEM images of the as-dealloyed and annealed nanoporous Pd samples. Both samples showed open-cell nanoporous structures. The average ligament diameters, which are calculated from the SEM images of more than 100 ligaments, are 20 nm for the as-dealloyed nanoporous Pd and 140 nm for the annealed one. This coarsening trend is similar to that observed in previous studies on nanoporous metals.22,35 Figure 3 shows XRD patterns of the as-dealloyed and annealed nanoporous Pd samples. All the peaks in these patterns are those of face-centered cubic (fcc) Pd. The peak angles for the two samples were almost the same, as exemplified in Figure 3b, andtherefore,themacroscopiclatticeconstants(a)0.3891-0.3892 nm, fairly equal to that ()0.3890 nm) of bulk Pd found in
Figure 2. Scanning electron microscopy images of (a) as-dealloyed and (b) annealed nanoporous Pd samples.
Figure 3. X-ray diffraction patterns for as-dealloyed and annealed nanoporous Pd samples. (b) Magnified 111 peaks for the two samples.
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Figure 4. Energy-dispersive X-ray spectra for (a) as-dealloyed and (b) annealed nanoporous Pd samples.
JCPDS database) were not affected by annealing. The peaks were broader in the XRD pattern of the as-dealloyed nanoporous Pd than in that of the annealed one, reflecting the smaller ligament size of the as-dealloyed nanoporous Pd. EDXS analysis of the samples (Figure 4) showed a trace amount (
