Article pubs.acs.org/JPCC
Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles Young-Hoon Chung,† Dong Young Chung,‡,§ Namgee Jung,† Hee Young Park,† Sung Jong Yoo,† Jong Hyun Jang,† and Yung-Eun Sung*,‡,§ †
Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea § Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The thermal annealing process is a common treatment used after the preparation step to enhance the electrocatalytic properties of the oxygen reduction reaction (ORR). The structure of a Pt-based bimetallic nanoparticle, which is significantly affected by the catalytic properties, is reconstructed by thermal energy. We investigated the effect of structural reconstruction induced by thermal annealing on the improvement of the ORR using various physical and electrochemical methods. We found that the structural evolution of PtNi nanoparticles, i.e., the Pt−Ni ordering with the Pt shell and the surface reorientation into the (111) facet, is the source of the enhanced ORR activity as well as electrochemical stability through the thermal annealing. This result confirms the crucial factors for the ORR properties by the thermal annealing process and proposes a way to design advanced electrocatalysts.
1. INTRODUCTION Proton exchange membrane fuel cells (PEMFC) have been attractive as sustainable energy conversion devices.1,2 Efficient energy conversion has been mainly impeded by the sluggish kinetics of oxygen reduction reaction (ORR).3 Pt-based nanoparticles are generally used as the electrocatalyst for ORR due to their relatively higher surface area. Many studies have investigated the enhancement of ORR activity by Pt-based nanoparticles to commercialize PEMFCs, which are economically feasible. The ORR kinetics is expressed by the equation
necessary to conduct a cleaning process to eliminate organic molecules from the nanoparticles.10,11 Thermal annealing of Ptbased nanoparticles is a conventional of treating nanoparticles after the preparation steps for their application as electrocatalysts.9,11 For this reason, it is important in the design of advanced electrocatalysts to understand the effect of the surface reconstruction induced by thermal energy on the electrocatalytic activity for the ORR, although this effect is still difficult to understand. Recently, Wang et al. showed the ORR properties resulting from the heat treatment of chemically ordered PtCo nanoparticles.12 They posited that the structural ordering led to the improvement of both ORR activity and stability. However, the size effect of electrocatalysts on electrocatalytic properties could not be considered because nanoparticles were severely agglomerated during the heat treatment. Also, van der Vliet et al. reported that the surface reorientation of PtNi thin films into the (111) facet after thermal annealing can be significantly enhanced by the ORR activity,13 in agreement with their previous results.14 However, the effect of this reorientation needs to be clarified for nanosized Pt-based particles. Here, we elucidate that the structural reconstruction of Ptbased nanoparticles including bimetallic ordering and surface reorientation affects the electrochemical properties of ORR. To clarify this effect, we prepared PtNi nanoparticles via the
j = nFKcO2(1 − Θad)x exp( −βFE /RT ) exp(−γ ΔGad /RT )
where n, F, K, cO2, R, x, β, and γ are constants and E, T, Θad, and ΔGad are applied potential, temperature, total coverage of spectator species, and Gibbs free energy of adsorption, respectively.4 According to the equation, the rate of ORR is determined by adsorption properties including Θad and ΔGad. In order to tune these parameters, it has been proposed that the frontier d-band structure, directly related with adsorption behaviors, be tailored to d-group electrocatalysts, including Pt,5−9 or that the surface morphology of Pt be controlled because of the different activities with surface orientation.9,10 Colloidal reduction is one of the most common preparation methods used to obtain Pt-based nanoparticles. Surface-capped organic species, such as solvents and surfactants, are contaminated onto the surfaces of metallic nanoparticles during the synthetic process. With consideration of the electrochemical reaction determined by the surface coverage, it is © 2014 American Chemical Society
Received: February 26, 2014 Revised: April 9, 2014 Published: April 28, 2014 9939
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Figure 1. TEM images of (A) PtNi/C_300 and (B) PtNi/C_700: Particle size distributions (inset), high-resolution images (top right), and their reduced FFT pattern (bottom right).
mental analysis (EA, LECO Corp. US/CHNS-932) were conducted. A three-electrode cell was used to characterize the electrochemical properties of carbon-supported PtNi nanoparticles; the cell had working (catalyst ink on a glassy carbon electrode), reference (saturated calomel electrode, SCE), and counter electrodes (Pt wire). All measurements were performed using an AUTOLAB potentiostat (Eco Chemie, PGSTAT) at 20 °C. The experimental details are described in the Supporting Information. Polarization curves for the oxygen reduction reaction (ORR) were obtained using a rotating disk electrode (RDE) in 0.1 M HClO4 saturated with O2 (99.995%) at a scan rate and rotating speed of 5 mV s−1 and 1600 rpm.
colloidal reduction method and treated them by thermal annealing at different levels of thermal energy without inducing serious agglomeration. Also, we characterized the relationship between the structural changes and the electrochemical activity of the ORR. This understanding can help us to design advanced nanosized bimetallic electrocatalysts.
2. EXPERIMENTAL SECTION Carbon-supported PtNi nanoparticles were prepared via the one-pot colloidal reduction method. The procedure is summarized as follows: 0.071 mL of oleic acid was added in 200 mL of anhydrous ethanol and sonicated. Subsequently, 0.0504 g of PtCl4 and 0.0357 g of NiCl2·6H2O were mixed by vigorous stirring in a carbon dispersed solution. After stirring for 2 h, 0.1500 g of NaBH4 was immediately added into the solution to reduce metal ions and stirred overnight. The solution was filtered with ethanol and dried in a vacuum. All process was conducted under room temperature at ambient atmosphere. The obtained powder was thermally annealed in a flow of H2/Ar (5/95 vol %) for 1 h at the different temperatures, 300 and 700 °C; the two powders were denoted as PtNi/C_300 and PtNi/C_700, respectively. High-resolution transmission electron microscopy (HRTEM) was carried out using a Titan 80-300 (300 kV, FEI) and Tecnai F20 (200 kV, FEI). Cs-corrected scanning transmission electron microscopy (STEM) was carried out by using a JEM-ARM200F (200 kV, JEOL) equipped with an energy dispersed spectrometer (EDS, BrukerQuantax 400). High-resolution powder diffraction (HRPD) was carried out under a 9B beamline of Pohang Light Source-II (PLS-II, 3 GeV). The incident beam had a wavelength of 1.5490 Å, which was obtained using a double-crystal Si(111) monochromator. The scan range of 2θ was 15° < 2θ < 135° with 0.02° increments and 0.5° overlaps to the next detector. X-ray absorption fine structure (XAFS) measurements were performed under the 8C beamline of PLS-II. The Pt L3 edge, transition from Pt 2p3/2 to 5d5/2, was detected at E0 = 11 564 eV and calibrated by Pt foil. The analysis of the Pt L3 absorption edge was carried out using ATHENA and ARITEMIS software of IFEFFIT programs. The experimental details were described in our previous study.15 Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) was measured by a Theta Probe AR-XPS System (Thermo Fisher Scientific, UK). Normal XPS was carried out using the Thermo Sigma Probe with Al Kα radiation. All spectra were calibrated with the C−C peak of C 1s orbitals as 284.6 eV. The peaks were fitted by the XPSPEAK 4.1 software package. To analyze the quantitative information, inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Shimadzu, JP/ICPS-7500) and ele-
3. RESULTS AND DISCUSSION Carbon-supported PtNi nanoparticles were prepared via the colloidal reduction method (Figure S1). Our synthetic method featured the formation of Ni-rich surface due to the difference of standard reduction potential and preferential interaction of surfactant (Ni−OOC). Because of its lower surface energy compared to Ni, Pt species tend to diffuse out onto the surface of nanoparticles during the thermal annealing process at the high temperature.16,17 Therefore, the as-prepared PtNi nanoparticles might be an efficient system to monitor structural reconstruction of Pt and Ni species by thermal energy. To control the intermetallic ordering of the bimetallic phase, the nanoparticles were thermally annealed at 300 and 700 °C under a flow of H2/Ar (5/95 vol %). The prepared samples were designated according to annealing temperature as PtNi/C_300 and PtNi/C_700. Because individual metallic atoms have sufficient mobility to move around on the surfaces of nanoparticles, the intermetallic structure reconstructs toward the stable phase by bimetallic ordering and surface facet reorientation when thermal energy is applied. According to previous calculations, the formation of the Pt−Ni bulk alloy was energetically favorable compared to the formation of Pt.5,8 It can be expected that the bimetallic ordering of Pt−Ni takes place during the heat treatment process.18,19 Within the thermodynamic correlation, the surface facets of face-centered cubic (fcc) structures including Pt would be stabilized in the order {111} > {100} > {110}, in accordance with the sequence of surface energies.20,21 Specific facets such as {111} can easily develop during thermal annealing.22 As expected, PtNi-based mesostructured thin films were obtained with the enrichment of {111} facets after thermal annealing at 400 °C in a hydrogenrich atmosphere.13 After thermal annealing, well-defined PtNi nanoparticles were formed, as shown in Figure 1A,B. The particle size distribution showed that the average radius of the PtNi 9940
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nanoparticles slightly varied with respect to the thermal energy from low-magnification TEM images: ca. 2.5 and 2.7 nm for PtNi/C_300 and PtNi/C_700, respectively. The nanoparticle was not significantly agglomerated after the heat treatment. This is due to the formation of the sodium oleate, which is derived from the oleic acids by the addition of sodium borohydries.23 High-resolution TEM (HR-TEM) images and reduced fast Fourier transform (FFT) spectra of an individual nanoparticle showed typical fcc structures. Each nanoparticle maintained the bulk atomic composition during the thermal annealing process, Pt1Ni1.1 and Pt1Ni1.1 for PtNi/C_300 and PtNi/C_700, respectively, within the error range of ICP-AES. To clarify this structural information, crystallographic analyses were performed using the high-resolution powder diffraction (HRPD) based on the synchrotron-based X-ray source. (Figure 2) The diffraction patterns of a typical fcc
Figure 3. (A) X-ray absorption fine structure (XAFS) of Pt L3 edge at E0 = 11 564 eV using 8C beamline of PLS-II (3 GeV), (B) coordination number and (111) facet ratio, and (C) Pt/Ni ratio with respect to incident angle using angle-resolved X-ray photoelectron spectroscopy (AR-XPS) of PtNi/C_300 and PtNi/C_700.
Figure 2. Spectra of high-resolution powder diffraction (HRPD) using 9B beamline of Pohang Light Source-II (3 GeV).
structure of Pt (space group, Fm-3m) at 40°, 46°, and 68°,24 corresponding to (111), (200), and (220), were positively shifted by the incorporation of Ni atoms into the Pt structure. This means that the Pt−Pt lattice distance in the fcc structure of Pt was decreased by the Ni phase, relatively smaller size, due to the structural ordering during the heat treatment. The peaks shifted more when the thermal energy was increased. The quantitative analyses of the bimetallic ordering between Pt and Ni species are shown in Table S1. As discussed above, the degree of alloying was varied with temperature, that is, 78 and 94% for PtNi/C_300 and PtNi/C_700, respectively. Also, the average crystallite size, calculated by the Scherrer equation for the (220) peak,25 matched well with the particle size from TEM measurements. Note that the effect of crystalline and particle sizes will be negligible because the size of the reference nanoparticle, Pt/C, was similar to that of the prepared samples.26 The X-ray absorption fine structure (XAFS) of the Pt L3 edge was measured to obtain electronic and structural information on PtNi nanoparticles. (Figure 3A) In the X-ray absorption near-edge region (XANES) of Pt L3, the intensities of the white line, the electron transition from 2p3/2 to 5d orbital, and the d-band vacancy were decreased in the order of PtNi/C_300 < PtNi/C_700. This result is correlated with the extent of structural ordering; i.e., electronic interactions from Ni to Pt species occur more effectively considering the negligible size effect.27 The coordination numbers (CNs) of Pt−Ni and Pt−Pt featured different aspects at each temperature from the extended X-ray absorption fine structure (EXAFS) analysis (Figure 3B). The CN of Pt−Ni is increased in the order of PtNi/C_300 < PtNi/C_700 in the line of our expectation from the HRPD measurement. However, the CN of Pt−Pt showed inverse trends, i.e., 4.6 and 3 for PtNi/C_300 and PtNi/C_700, respectively. According to previous studies, thermal annealing resulted in the formation of Pt-shell by
diffusing out Pt species onto the surface of Pt-based binary metal system.16,17 The EXAFS technique is one of the effective tools to characterize this structural reconstruction.28 Once Ptshell is developed, the CN of Pt−Pt is decreased owing to exposing the large extent of Pt species to the surface of nanoparticles. It is plausible, therefore, that this result is due to the formation of the Pt-shell at the higher temperature, PtNi/ C_700. In order to underpin this structure development, a quantitative analysis considering the atomic ratio of Pt/Ni at the surfaces of PtNi nanoparticles was carried out using angleresolved-X-ray photoelectron spectroscopy (AR-XPS) at various angles, as shown in Figure 3C and Figure S2.5,6 Evidently, the spectra of PtNi/C_700 showed the formation of a Pt-rich surface whereas those of PtNi/C_300 did not at the low angle, 48°, which contains surface information due to a small escaping depth. Bulk compositions were recovered at the high angle, 68°. Ahmadi et al. studied the segregation phenomena of PtNi nanoparticles under different thermal treatment.19 They posited that, at low temperature ( 0.7 V, were retarded with increasing structural ordering, as shown in Figure 4C. Note that the electrochemical active surface area (ECSA) associated with CO stripping and underpotentially deposited hydrogen (Hupd) region (E < 0.4 V) is varied with the surface composition, i.e., ECSAPtNi/C_300 > ECSAPtNi/C_700. This might be originated from the surface roughening by the dissolution of the unstable 3-d metal, Ni, due to the harsh condition of electrochemical measurements.17 It is reported that the dissolution of Ni in PtNi nanoparticles leads to the formation Pt skeleton and a higher ECSA value than the Pt-shell-like structure.22 As discussed above, this feature can be interpreted by the different surface composition, considering enrichment of Pt on the surface of PtNi/C_700 electrocatalyst occurs. One of the evidence to form a Ptenriched surface is that the ECSA values are varied with the electrochemical techniques, i.e., Hupd (17.0 m2 g−1Pt) and CO stripping (32.5 m2 g−1Pt). According to van der Vilet et al., adsorption properties of Hupd are significantly retarded when the Pt-skin surface is formed.39 Therefore, the electrochemical properties also confirmed the previous structural characterization. Additional analyses were performed such as CS-corrected scanning transmission electron microscope (STEM) equipped with the energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) after electrochemical measurements to elucidate the direct correlation between structural reconstructions and catalytic activities (Figure 5A,B). The EDS results for the line scanning of Pt and Ni components clearly supported that a less reconstructed structure severely
Figure 4. Electrochemical measurements of PtNi_300 and PtNi_700: (A) CO stripping curves after fully adsorption of CO molecules on the PtNi surface, (B) polarization curves of the ORR with a saturated O2, and (C) cyclic voltamograms with a saturated Ar. All measurements were carried out in 0.1 M HClO4 at 20 °C.
at ca. 0.62 V to confirm the reorientation of the surface direction33−36 (Figure S4C). These results from two electrochemical techniques indicated a similar ratio of (111) facet within 5% range. Concerning the difference of surface energy for facets, the ratio of (111) facet was significantly increased after the heat treatment, PtNi/C_700 > PtNi/C_300, as expected. Those surface reconstructions including the surface ordering and the reorientation of surface facet are summarized in Figure 3B. It is posited that the thermal energy induces bimetallic ordering with Pt shell and the (111) direction reorientation in proportion to the annealing temperature. Electrocatalytic activity was measured in an acid electrolyte of 0.1 M HClO4, which has weak anion adsorption because of the hydration energy of ClO4− 37 (Figure 4). With respect to ORR for Pt-based electrocatalysts, intrinsic kinetics was decisively determined by adsorption properties on the surface of the metal. For the Pt species, it is favorable that the adsorption strength of reactants and intermediates should be lowered to achieve facile kinetics of the ORR.5,38 Alloying Pt with a 3d metal including Ni can modify the adsorption properties
Figure 5. Dark-field images of CS-corrected STEM and EDS analysis after electrochemical reactions of (A) PtNi/C_300 and (B) PtNi/C_700. 9942
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suffered from the dissolution of Ni species. Moreover, the quantitative investigation of the Pt 4f and Ni 2p XPS spectra revealed that the ratios of Pt and Ni contents were Pt3.8Ni1 and Pt1.8Ni1 for PtNi/C_300 and PtNi/C_700, respectively. Since the Pt-shell like structure can protect the Ni species located in the core part of PtNi nanoparticles, PtNi/C_700 is expected to be more stable in an electrochemical environment, consistent with EXAFS and AR-XPS results as discussed above. It has been reported that the amount of residual Ni in PtNi nanoparticles after the dissolution is proportional to the ORR activity.40,41 Moreover, we investigated the binding energy of Pt 4f7/2, the only active center for the ORR, to elucidate how structural stability is induced from the intermetallic reconstruction, which is affected by the tuning of the electronic structure. The shift of the core-level of the electronic structure for the d-group metal is associated with the frontier d-band structure, resulting in alteration of the adsorption properties.42,43 According to theory, the energy of the center of d-band would be tuned negatively to weaken the adsorption strength as a consequence of the improvement of the ORR kinetics.44 If the downshift of the d-band states occurred, the core states are suppressed downward, resulting in high binding energy.7 Investigated after electrochemical measurements, the measured electronic structure directly reflected environments where the reaction took place. The binding energy of Pt 4f7/2, 71.6, 71.7, and 71.9 eV for Pt/C, PtNi/C_300, and PtNi/C_700, respectively, corresponded to the activity for the ORR. This means that the interaction between Pt and Ni species is more active for highly reconstructed nanoparticles. We depicted the relationship between activity, binding energy, and residual Ni in Figure 6.
Scheme 1. Schematic Illustration of the Surface Reconstruction of PtNi Nanoparticles by Thermal Annealing
nanoparticles. Also, based on the characterization of PtNi nanoparticles after the dissolution of Ni species during the electrochemical reaction, it was found that the bimetallic reconstruction induced the formation of an electrochemically stable structure and increased intermetallic interaction. This result elucidated a point of contention on the source of the enhanced ORR activity for Pt-based nanoparticles through the thermal annealing process.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and supplementary data: TEM image of as-prepared PtNi/C, structural information from HRPD, ARXPS spectra, EXAFS fitting results, supplementary electrochemical data, XPS spectra after electrochemical reactions, and complete lists for refs 9 and 22. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +82-2-880-1889; Fax +82-2-880-1664; e-mail ysung@snu. ac.kr (Y.-E.S.). Notes
The authors declare no competing financial interest.
Figure 6. Correlation between binding energy of Pt 4f7/2 and specific activity with residual Ni contents after electrochemical reactions.
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ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS) in Korea and the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & FuturePlanning) (No. 2013M1A8A1038315).
As a result, it can be carefully concluded that the enhancement and stability of the ORR induced by thermal annealing is originated from the structural reconstruction, i.e., intermetallic ordering of Pt−Ni with Pt shell and surface reorientation into (111) facet as shown in the schematic illustration (Scheme 1).
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4. CONCLUSION We investigated the enhancement of the electrocatalytic activity of Pt-based bimetallic nanoparticles after thermal annealing. As thermal energy was introduced to the PtNi nanoparticles, structural reconstruction occurred in two forms, i.e., increased the degree of alloying with the Pt-shell and surface reorientation toward the (111) facet. This reconstruction significantly improved the adsorption properties on the surfaces of the nanoparticles, resulting in ca. 3.8 times higher electrocatalytic activity of the ORR compared to that for Pt
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