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Novel Nanoporous Au-Pd Alloy with High Catalytic Activity and Excellent Electrochemical Stability X. Y. Lang,† H. Guo,‡ L. Y. Chen,† A. Kudo,‡ J. S. Yu,† W. Zhang,§ A. Inoue,§ and M. W. Chen*,† WPI AdVanced Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan, Graduate School, Tohoku UniVersity, Sendai 980-8577, Japan, and Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan ReceiVed: August 9, 2009; ReVised Manuscript ReceiVed: January 4, 2010
A novel nanoporous gold-palladium alloy has been successfully fabricated by electrochemically dealloying a multicomponent metallic glass. In comparison with conventional nanoporous gold prepared by dealloying Au-Ag alloys, the nanoporous gold-palladium alloy shows much higher catalytic activity for electrooxidation of methanol as a free-standing electrode. Moreover, a small amount of palladium in the nanoporous alloy is found to dramatically improve the electrochemical stability of nanoporous gold. Introduction 1-3
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Nanoporous metals, such as gold, platinum, palladium, and nickel,6 possess fascinating physicochemical properties that are dramatically different from their bulk counterparts, which has attracted increasing attention for a wide variety of functional applications.7-11 As one of the effective approaches, nanoporous metals can be fabricated by electrochemical dealloying by which less noble components of a alloy are selectively dissolved while remained noble components form a three-dimensional nanoporous structure through surface diffusion.1 The dealloying process gives rise to a unique bicontinuous nanostructure consisting of nanosized metallic ligaments and nanoporous channels,1-3,12 which enables nanoporous metals and alloys to have good electrical conductivity13 and large effective surfaces as ideal electrodes for electrocatalysis.9,14 Recently, nanoporous gold (NPG) as an electrocatalyst toward methanol oxidation in alkaline solutions is of special interest because of the important application in fuel cells. It has been demonstrated that NPG with a small pore/ligament size has high activity of electrocatalysis.9,14 Importantly, NPG is congenitally resistant to the poisoning or inhibiting effect that is suffered by Pt-based electrodes during methanol oxidation reactions.9,14,15 Unfortunately, NPG is prone to coarsening during potential cycling, dramatically reducing its long-term functionality and electrocatalytic endurance.1,9 It is known that the addition of a small amount of dopants with electrocatalytic activation can improve nanostructural stability16,17 and optimize electrocatalytic performance of metal mixtures.16-20 This has led to great efforts to fabricate various Au-based bimetallic nanostructures, such as surface modification of porous nanostructures by plating Pt onto NPG (Au-Pt core-shell structure),9,20,21 and synthesize nanoporous Au/Pt alloys by dealloying ternary alloy precursors.22 In comparison with the limited multicomponent crystalline alloys that can be used as precursors for dealloying nanoporous alloys, more than thousands of multicomponent metallic glasses with changeable component elements and * To whom correspondence should be addressed. E-mail: mwchen@ wpi-aimr.tohoku.ac.jp. † WPI Advanced Institute for Materials Research. ‡ Graduate School. § Institute for Materials Research.
relatively wide composition ranges make it possible to fabricate various nanoporous alloys that cannot be achieved from the conventional crystalline alloy systems.4,5 In this study, we report a novel self-supported nanoporous gold-based alloy containing a small amount of Pd, which was fabricated by electrochemically dealloying a gold-based multicomponent metallic glass. It was found that the small amount of Pd significantly improves the catalytic activity and stability of the nanoporous alloy. Experimental Section Glassy Au30Si20Cu33Ag7Pd10 ribbons with a cross section of ∼0.02 mm × 1 mm were produced by single-roller meltspinning in vacuum.23 The X-ray diffraction (XRD) pattern reveals that the as-spun ribbons are fully amorphous (see Figure 1Sa in the Supporting Information). Using a classical three-electrode setup (Iviumstat electrochemical analyzer, Ivium Technology) with a Ag/AgCl reference electrode (SSCE) and a Pt counter electrode, the Au-based glassy alloy ribbons were electrochemically dealloyed in a 1 mol · L-1 H2SO4 solution at the potential of 1.0 V until the current decreases to zero.5 For comparison, NPG was fabricated by chemically dealloying Ag65Au35 (atom %) alloy in HNO3 for 8 h at room temperature. Nanoporous structures were quenched by pure water (18.2 MΩ · cm), and the residual acid within the nanopore channels was thoroughly removed by water rinsing. KOH solutions (0.5 mol · L-1) without and with 1.0 or 5.0 mol · L-1 CH3OH were used as the electrolyte to characterize the electrocatalytic activity or stability of the dealloyed Au-based glassy ribbon and NPG for methanol oxidation, respectively. The morphology and the chemical composition of the dealloyed samples were characterized by a scanning electron microscope (SEM, JEOL JSM7001F) equipped with an Oxford X-ray energy-dispersive spectroscope (EDS). Results and Discussion Figure 1a shows a top-view SEM image of the dealloyed Au30Si20Cu33Ag7Pd10 ribbons, illustrating a uniform nanoporous structure consisting of quasi-periodic nanoporous channels and gold ligaments with the characteristic length of ∼50 nm. The
10.1021/jp907682x 2010 American Chemical Society Published on Web 01/22/2010
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Figure 1. (a) Top-view and (b) cross-sectional SEM images of the as-prepared NPGP ribbon by electrochemically dealloying Au30Si20Cu33Ag7Pd10 in 1 mol · L-1 H2SO4 at 1.0 V vs Ag/AgCl electrode. (c) SEM EDS spectrum of as-prepared NPGP. The inset is the zoom-in portion of the spectrum, showing the small amount of Pd in the alloy. (d) Representative top-view SEM image of conventional NPG prepared by dealloying Ag65Au35 in concentrated HNO3 for 8 h. (e) Top-view and (f) cross-sectional SEM micrographs of the NPGP ribbon after being immersed in concentrated HNO3 for 3 h.
entire sample with a uniform nanoporous structure can be observed from the cross-sectional micrograph of a dealloyed ribbon (Figure 1b). After dealloying, the amorphous ribbons turn into the crystalline state, and the separate XRD pattern (see Figure 1Sb in the Supporting Information) confirms that the crystal structure of the metallic ligaments is consistent with that of fcc gold. The SEM EDS spectrum (Figure 1c) demonstrates that ∼3 to 5 atom % Pd remains in the Au ligaments and less noble elements, such as Si and Cu, have been completely dissolved during the dealloying process. The SEM micrograph of the conventional NPG film fabricated by chemical dealloying Ag65Au35, (Figure 1d), reveals the characteristic length of ∼45 nm, close to that of the nanoporous gold-palladium (NPGP) alloy. The electrocatalytic activities of NPGP and NPG were characterized by cyclic voltammetry in an alkaline electrolyte.9,24 Figure 2a shows the cyclic voltammograms (CVs) of as-prepared NPGP in the solutions of 0.5 mol · L-1 KOH without and with 1.0 mol · L-1 CH3OH. The sweep rate is 20 mV · s-1 in the potential range from -0.1 to 0.5 V. The methanol electrooxidation on the surfaces of as-prepared NPGP occurs in two different potential regions, similar to that on the surfaces of NPG or Au nanocrystals.9,14,15 At the low potential region (0.4 V).15,24 As shown in the CV curves (Figure 2a-c), the as-prepared NPGP has the lower current density and an ∼90 mV more positive oxidation peak potential than NPG, indicating less electrocatalytic activity. The electrocatalysis of NPGP can be dramatically improved by immersing the as-prepared NPGP ribbons into a concentrated HNO3 solution for 3 h to remove the surface oxide layer formed during electrochemical dealloying. EDS analysis demonstrates that this simple chemical
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Figure 2. (a) CV curves of as-prepared NPGP in 0.5 mol · L-1 KOH electrolytes without and with 1.0 mol · L-1 CH3OH. CV curves of NPG (i) and HNO3-treated NPGP (ii) in the solutions of (b) 0.5 mol · L-1 KOH and (c) 0.5 mol · L-1 KOH with 1.0 mol · L-1 CH3OH. The potential scan rate was 20 mV · s-1.
treatment does not change the overall chemical composition of NPGP. Figure 1e,f shows the top-view and cross-sectional SEM micrographs of the HNO3-treated NPGP. It can be found that the morphology of the HNO3-treated NPGP does not change too much, except that the surface of the ligaments becomes slightly smoother than that of the as-prepared one (Figure 1a). The typical CV curves of the HNO3-treated NPGP in the KOH solutions without and with CH3OH are shown in Figure 2b,c, respectively. In the methanol-free solution, the CV curve in the positive potential scan has two anodic current peaks located at 0.12 and 0.29 V. The first small current peak is due to the oxidation of bare surface Pd atoms, whereas the second one belongs to the formation of surface gold oxides.14,15 Significantly, the surface HNO3 treatment leads to the gold oxidation peak being more negative than that of as-prepared NPGP (0.34 V) and conventional NPG (0.31 V). According to Assiongbon,15 the OH- anions are strongly adsorbed on the gold electrode in alkaline solutions, assisting the methanol oxidation. Compared to the CV curve from the methanol-absent solution, the methanol oxidation peak can be clearly identified from the CV curve of the mixture of 0.5 mol · L-1 KOH + 1.0 mol · L-1 CH3OH (curve (ii) in Figure 2c). The anodic current remarkably increases in the potential region from -0.1 to 0.4 V, and the broad peak may be due to the overlap of two current peaks from Pd and Au, noticeably different from that of NPG. Moreover, the anodic current peak appears at 0.21 V, which is ∼150 and 40 mV more negative than those of as-prepared NPGP and NPG, respectively. Therefore, the HNO3 treatment gives rise to more active surfaces for OH- chemisorptions and reaction.9,15,24,25 During the negative-going potential sweep, the electrochemical reduction of the gold surface oxides in the HNO3-treated NPGP occurs at 0.08
Lang et al.
Figure 3. Electrocatalysis of HNO3-treated NPGP and NPG in 0.5 mol · L-1 KOH solution with 5 mol · L-1 methanol for 500 cycles. (a) Normalized current density of NPGP and NPG, as a function of cycle number. The current is normalized by the intensities of their first CV cycles. (b) CV curves of NPGP treated by HNO3: (i) the first and (ii) 500th cycles of the original HNO3-treated NPGP and (iii) the HNO3 reactivation of NPGP with 500 cycles. The potential scan rate was 50 mV · s-1. (c) Current-time curves for NPGP and NPG at 0.1 V vs Ag/ AgCl electrode.
V, about 70 mV more positive than that of the as-prepared NPGP electrode (0.01 V), further demonstrating that the activated NPGP has much higher electrocatalytic activity than NPG and the as-prepared NPGP. The electrochemical stability of the HNO3-treated NPGP and NPG electrodes can be characterized by evolution of the current densities of methanol oxidation peaks during potential cycles in the electrolyte containing 0.5 mol · L-1 KOH and 5.0 mol · L-1 CH3OH. Figure 3a shows the normalized current densities by the first cycles of NPGP and NPG as a function of cycle number. Obviously, the HNO3-treated NPGP is more stable than NPG, particularly after 300 cycles. As shown in the SEM micrographs of NPG before and after cycling (Figure 4a,b), the pore size increases from ∼45 to ∼60 nm, while the ligament size decreases from 45 to ∼35 nm after 500 cycles. Thus, the current decrease of NPG is mainly caused by the coarsening of the nanoporous structure during the electrochemical cycling in alkaline electrolytes. In contrast, although the reactivity of the NPGP specimen also decreases with cycling, the feature size does not change too much (Figure 4c,d), except that the ligament surfaces, similar to that of as-prepared NPGP, become rough (see Figure 2S, the atomic force microscope (AFM) images, in the Supporting Information). The good electrochemical stability of NPGP is also verified by the chronoamperometric investigation, during which the polarization current of HNO3-treated NPGP for methanol electrooxidation at 0.1 V decreases much slower than that of NPG (Figure 3c). Separate SEM observations
Novel Nanoporous Au-Pd Alloy
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2603 and electrochemical stability toward the direct electrooxidation of methanol in alkaline electrolytes, which holds important promise as a refreshable alloy electrode in methanol fuel cells. Acknowledgment. The research was sponsored by “Global COE for Materials Research and Education”; “WPI Initiative for Atoms, Molecules and Materials”; MEXT, Japan; the Iketani Science and Technology Foundation; the Murata Science Foundation; and the Inamori Foundation. X.Y.L thanks the JSPS Postdoctoral Fellowship Program (P07373) for support. Supporting Information Available: XRD patterns of glassy Au30Si20Cu33Ag7Pd10 ribbon and crystalline NPGP, AFM images of NPGP, SEM micrographs of NPGP after current-time scanning, and CV curves of the Pd electrode. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 4. Top-view SEM images and distributions of the sizes of gold ligaments and nanoporous channels of NPG (a, b) and HNO3-treated NPGP (c, d) before and after electrochemical testing in 0.5 mol · L-1 KOH + 5 mol · L-1 methanol solution for 500 cycles, respectively.
reveal that the changes of feature sizes and morphology of NPGP and NPG are analogous to those during potential cycling (Figure 3S in the Supporting Information). The surface roughening of the NPGP may be caused by the accumulation of Pd oxide in the electrocatalytic process due to incomplete reduction of Pd oxide during negative scans. Apparently, the rough oxide layer weakens the electrocatalytic activity of NPGP and may also inhibit the coarsening of the nanoporous structure.26-29 Interestingly, the electrocatalytic activity of the NPGP with 500 cycles can be completely recovered after being immersed in HNO3 for a few minutes (Figure 3b). The refreshed sample has a smooth surface, similar to the original HNO3-treated NPGP. This is distinctly different from NPG, of which the nanoporous structure becomes coarse during potential cycling,9 resulting in the irreversible loss of active surface area and thus decreasing the electroactivity of the NPG electrode. Conclusions In summary, we have successfully fabricated a novel nanoporous Au-Pd alloy by electrochemically dealloying a multicomponent Au-based metallic glass. The self-supported nanoporous Au-Pd electrode shows high electrocatalytic activity
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JP907682X
