Hierarchical Nanoporous PtFe Alloy with Multimodal Size Distributions

Dec 23, 2011 - Highly active nanoporous Pt-based alloy as anode and cathode catalyst for direct methanol fuel cells. Xiaoting Chen , Yingying Jiang , ...
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Hierarchical Nanoporous PtFe Alloy with Multimodal Size Distributions and Its Catalytic Performance toward Methanol Electrooxidation Caixia Xu,*,† Qian Li,† Yunqing Liu,† Jinping Wang,† and Haoran Geng‡ †

Key Laboratory of Chemical Sensing and Analysis in Universities of Shandong, University of Jinan, School of Chemistry and Chemical Engineering, and ‡School of Material Science and Engineering, University of Jinan, Jinan 250022, People’s Republic of China ABSTRACT: The hierarchical nanoporous (NP) PtFe alloy with multimodal size distributions is straightforwardly fabricated by means of mild de-alloying of the PtFeAl source alloy. This interesting NP structure consists of interconnected larger ligaments around hundreds of nanometers, in which these ligaments are also composed of the three-dimensional network structure with the typical size at 3 nm. In comparison to NP-Pt and Pt/C catalysts, the as-made alloy nanostructure exhibits superior electrocatalytic activity for the methanol oxidation reaction (MOR) with higher catalytic durability and CO tolerance besides the enhanced specific and mass activity. NP-PtFe also shows improved structure stability with the less loss of the electrochemical surface area of Pt upon long-term potential scan in acidic solution. X-ray photoelectron spectroscopy and density functional theory calculations demonstrate that the incorporation of Fe appropriately modified the electron structure of Pt with the downshift of the Pt d-band center, leading to a decreased CO poisoning and an improved MOR activity.

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INTRODUCTION For direct methanol fuel cells (DMFCs), highly active electrocatalysts for the methanol oxidation reaction (MOR) are essential for their commercialization as alternative power sources in vehicles and portable devices.1,2 Pt catalysts are currently the most investigated for MOR because of their unique electrocatalytic activity.3,4 It is well-known that Pt-based bimetallic nanostructures would be preferable because they can not only reduce the loadings of the noble metal Pt but also distinctly improve its catalytic performance. At present, great efforts have been paid to develop various Pt-based nanomaterials to further enhance the MOR activity of Pt, such as PtRu,5−7 PtAu,8,9 even ternary Pt−Fe−Pd,10 Pt−Rh−Pd,11 etc. However, these assistant metals (Pd, Ru, Au, etc.) are also more precious, which is not desired considering the catalyst cost. Nonprecious Fe, as one of the 3d metals, shows a dramatically synergistic catalytic effect to enhance the oxygen reduction reaction (ORR) activity of Pt catalysts12 in terms of higher half-wave potential, reduced catalyst cost, and improved OH resistance, similar to PtCo and PtNi.13 Recently, great attention has been paid to prepare various PtCo and PtNi alloy nanostructures and explore the alloying effect of Co and Ni on the MOR activity of Pt.14−19 However, fabrication of PtFe alloy nanostructures and detailed studies on their MOR activity are relatively lacking up to now.19,20 Xu et al. fabricated carbonnanotube-supported PtFe nanoparticles by co-reduction of metal salts and found an improved MOR catalytic activity compared to the Pt catalyst.21 Zhao et al. prepared a PtFe/ © 2011 American Chemical Society

polypyrrole-carbon catalyst by co-deposition of Pt and Fe on a polypyrrole-carbon support, also finding an activity enhancement for MOR.22 However, these PtFe catalysts are mainly in the form of dispersed nanoparticles based on the carbon support by a complicated chemical reduction process with the usage of organic agents.19−23 Meanwhile, these nanostructures usually suffer the problems of particle aggregation, the corrosion of carbon support, and the de-adhesion between support and nanoparticles.24 Recently, Yang et al. synthesized ultrathin PtFe alloy nanowire networks using a soft template formed by cetyltrimethylammonium bromide in a two-phase water−chloroform system, which also cannot avoid the complicated operation and usage of organic agents.20 Therefore, the facile preparation to a highly active PtFe alloy nanocatalyst is particularly desirable in terms of its practical application as an anode electrocatalyst in DMFCs. In current work, we fabricated a hierarchical nanoporous (NP) PtFe alloy with multimodal size distributions by dealloying the refined PtFeAl source alloy. Al has more reactive property, which can be easily selectively removed in mild alkaline solution without the corrosion of Pt and Fe. The whole alloy-refining/de-alloying process can achieve a good control on the bimetallic ratio and structure uniformity in terms of simple operation, perfect yielding, and large-scale synthesis, Received: September 30, 2011 Revised: December 21, 2011 Published: December 23, 2011 1886

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the Monkorst−Pack k points.35 The density of state calculations were calculated at the same level, except with a 5 × 5 × 2 Monkorst−Pack k-point mesh. The adsorption energy of ΔECO and ΔEOH is defined as

which has proven to be very effective in producing threedimensional (3D) bicontinuous NP alloy materials.10,25−30 Electrochemical measurements demonstrated that the hierarchical nanoprous PtFe alloy has superior specific and mass activity as well as improved CO tolerance and structure stability compared to NP-Pt and Pt/C catalysts, holding promising application potential as an anode electrocatalyst in DMFCs.

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ΔECO/OH = E(slab) + E(CO/OH) − E(ads/slab) where E(slab), E(CO), and E(OH) are the energies of the free adsorbate and the clean slab and E(ads/slab) is the energy of the adsorbate−slab adsorption system.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION EDS was first used to analyze the composition of the PtFeAl source alloy. As shown in Figure 1a, the alloy component is

Pt11Fe9Al80 (atomic %) ternary alloy foils were prepared by refining pure (>99.9%) Pt, Fe, and Al in an arc furnace, followed by meltspinning under an argon-protected atmosphere. NP-Pt55Fe45 alloy and NP-Pt were prepared by selectively etching the Pt11Fe9Al80 and Pt20Al80 alloys in 2.0 M NaOH solution for 48 h at room temperature, respectively. During the preparation of carbon-supported PtFe/C nanoparticles, Vulcan XC-72 carbon black was pretreated in 6 M HNO3 solution at 100 °C for 4 h21 and a feeding mole ratio between platinum and iron is controlled at 55:45. The other procedure was operated completely according to the work by Xu et al.22 The commercial Johnson Matthey Pt/C catalyst (20 wt.%) was purchased from Alfa Aesar. Powder X-ray diffraction (XRD) data were collected on a Bruker D8 advanced X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) at a scan rate of 0.04° s−1. The microstructures of the NP metals were characterized on a JEM-2100 high-resolution transmission electron microscope (TEM) and a JEOL JSM-6700F scanning electron microscope (SEM) with an energy-dispersive X-ray spectrometer (EDS) for compositional analysis. Surface structural properties of the NP-PtFe alloy were analyzed with ESCALab250 Xray photoelectron spectroscopy (XPS), using a monochromatized Mg Ka X-ray as the excitation source and choosing C 1s (284.60 eV) as the reference line. All electrochemical measurements were performed on a CHI 760D potentiostat in a standard three-electrode cell with Pt foil as counter electrode and mercury sulfate electrode as the reference electrode. All potentials in this work were referred to reversible hydrogen electrode (RHE). The catalyst suspensions were made by sonicating a mixture of 2.0 mg of catalyst powder, 1.0 mg of carbon powder, 300 μL of isopropanol, and 100 μL of Nafion solution (0.5 wt %) for 30 min. The modified electrode was prepared by placing the catalyst suspension on a polished 4 mm diameter glassy carbon electrode. The Pt loadings for Pt/C, NP-Pt, PtFe/C nanoparticles, and NP-PtFe catalysts on a glass carbon electrode were determined to be 8.2, 10.1, 7.6, and 5.0 μg cm−2 during electrocatalytic measurements for methanol oxidation, respectively. Electrolyte solutions were deoxygenated by bubbling with high-purity N2 for 30 min prior to measurements. CO stripping experiments were carried out by first holding the thus-made electrodes at 0.15 V (versus RHE) in a 0.5 M H2SO4 solution with continuous CO bubbling for 20 min. The electrode was then transferred into a 0.5 M N2-purged H2SO4 solution to record the CO-stripping profiles. The electrochemical surface area (ECSA) of Pt was calculated by integrating the charge associated with H desorption on the Pt catalyst surface.24 All density functional theory (DFT) calculations were performed with the CASTEP31 program using the gradient-corrected PBE exchange-correlation functional.32 Electron−core interactions of Pt, Fe, OH, and CO have been described by ultrasoft pseudo-potentials. The electronic wave functions were expanded in a plane wave basis set with an energy cutoff of 300 eV. For all of the cases, a three-layer slab model with a 10 Å vacuum region was employed. Considering that Fe species on the surface would be dissolved during the voltammetric scan in acidic medium before activity tests, the neighboring surface Pt atoms will undergo a reconstruction to form a nearly pure Pt skin on the alloy surface.33,34 Consequently, the 2 × 2 unit cell Pt58Fe42(111) slab with pure Pt in the first layer, 75% Pt in the second layer, and 50% Pt in the third layer was used to model the experimental case. For the geometry optimization, the calculations were performed with medium accuracy and all three layers of the slabs have been fully relaxed. The Brillouin zone integration was performed on a grid of 4 × 4 × 1 using

Figure 1. EDS spectra of the (a) PtFeAl alloy and (b) de-alloyed sample.

Pt11Fe9Al80 (atomic %), which is consistent with the initial feed ratio. XRD is used to examine the crystal structure of this ternary alloy. For the XRD pattern of the PtFeAl alloy (Figure

Figure 2. XRD patterns of the PtFeAl alloy and the de-alloyed sample. The standard patterns of Pt (JCPDS 65-2868), Al (JCPDS 65-2869), Fe (JCPDS 06-0696), and Fe2O3 (JCPDS 25-1402) are attached for comparison.

2), it is noted that there is little pure Al phase existing in the source alloy. However, there are no diffraction peaks related to pure Pt, Fe, PtFe alloy, or their metal oxides observed, 1887

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indicating the preferential formation of the PtFeAl alloy. Despite the rather complicated diffraction patterns of the source alloy, the formation of a uniform NP single-phase PtFe alloy after de-alloying was not influenced, as discussed below. NaOH solution was selected as the electrolyte to selectively dissolve Al atoms considering the amphoteric property of Al. Figure 3 illustrated the resulted structure after etching PtFeAl

diffraction peaks emerged at 41.0, 47.3, and 68.3 (2θ), which can be assigned to the (111), (200), (220) diffractions of the face-centered cubic (fcc) structure. The three peaks were located just between the projected 2θ values for pure Pt and Fe, indicating the formation of a fcc PtFe alloy structure. The broad diffraction peaks are due to the smaller structure dimensions of the NP-PtFe alloy nanostructure, as observed in SEM and TEM characterizations. It is noted that some weak diffractions (marked with an asterisk) also existed, which can be indexed to the Fe2O3 species.36 XPS analysis further demonstrated this point, as discussed below. The formation of Fe oxides is due to the oxidation of Fe atoms on the NP surface during the dealloying and drying procedure. Considering that XPS is more sensitive to the structure properties of nanostructured alloy materials, Figure 4 presents

Figure 3. (a and b) SEM, (c) TEM, and (d) HRTEM images of the NP-PtFe alloy.

alloy foils in 2.0 M NaOH solution for 48 h at room temperature. As shown in Figure 3a, the resulted structure is composed of many interconnected larger ligaments, with the diameter in the range of 100−150 nm and the length around several hundred nanometers, within which a number of irregular pores formed. From a high-magnification SEM image (Figure 3b), it is interesting to find that these large ligaments themselves have 3D bicontinuous spongy morphology with a narrow ligament size at 3 nm. The dark skeletons and bright regions in the TEM image (Figure 3c) further indicate the formation of NP architecture on the large ligaments.25 It is clear that the resulting sample after dealloying exhibits a hierarchical NP structure with multimodal size distributions. It is considered that the specific phase structure constitution of the PtFeAl source alloy (Figure 2) is responsible for the formation of the hierarchical NP structure, in which the dissolution of segregated Al may generate the larger pores and ligaments, while smaller pores/ligaments result from de-alloying of the residual Al-based alloy structure. As shown in the high-resolution TEM (HRTEM) image (Figure 3d), the clear lattice fringes can be easily observed, with the spacing calculated to be ∼0.215 nm, which can be ascribed to the {111} planes of the PtFe alloy. Meanwhile, other planes with narrower spacing also exist in the crystal. EDS analysis (Figure 1b) shows that the bimetallic composition between Pt and Fe in the resulted sample basically maintained the initial feed ratio, and a majority of Al atoms were removed, with ∼2 atomic % remaining in the resulted sample. The crystal structure of the de-alloyed sample was further examined by XRD (Figure 2). In comparison to the diffraction pattern of the PtFeAl source alloy, there are only three

Figure 4. XPS spectra of (a) Fe 2p and (b) Pt 4f core levels for the NP-PtFe alloy.

the Fe 2p and Pt 4f core level spectra for NP-PtFe. As shown in Figure 4a, the Fe 2p core levels split into 2p1/2 and 2p3/2 components because of spin−orbit coupling, showing satellitelike peaks. The binding energy at 711 eV is corresponding to the Fe 2p3/2 peak for Fe2O3,37 which is in agreement with the XRD result. No obvious peaks from metallic Fe (708 eV) were observed,38 indicating that oxidized Fe was the predominant species on the PtFe alloy surface. These surface Fe oxides will be dissolved during the voltammetric scan in sulfuric solution to form a nearly pure Pt skin on the alloy surface.33,34 The binding energy of the Pt 4f7/2 region in the NP-PtFe alloy is located at 71.6 eV, which is slightly higher than that of a bulk Pt sample (71.2 eV).39 The small doublet around 73 and 77 eV can be assigned to Pt 4f7/2 and 4f5/2 peaks of Pt2+ species.40,41 The upshift of the binding energy for Pt, which is also observed in the previous report,42 can be ascribed to the electron transfer from Fe to Pt because of the formation of a single-phase alloy as well the lower electronegativity of Fe.43 The alloy 1888

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catalysts. The peak current density on the NP-PtFe alloy is over 1.5 times that on PtFe/C, which is ∼4 times and over 2 times compared to NP-Pt and Pt/C, respectively. At lower potential, such as 0.6 V, the current density of NP-PtFe is also more than 2 times those on PtFe/C and Pt/C catalysts. As shown in Figure 5b, it is clear that NP-PtFe exhibits superior mass activity of Pt among all catalysts. It should be noted that the asprepared PtFe/C exhibits similar mass activity to PtFe/ MWNTs.22 It is conclusive that the hierarchical NP-PtFe alloy has remarkable structure advantages compared to PtFe/C, with greatly enhanced MOR activity. In addition, alloying with Fe generates a great synergistic catalytic effect on Pt for MOR. To obtain more insight into the MOR activity of the NPPtFe alloy upon combing with Fe, CO electro-stripping experiments were carried out against NP-Pt and Pt/C catalysts. As shown in Figure 6a, the peak potential for CO electrostripping is significantly different between the three catalysts. CO electrooxidation on the NP-PtFe alloy split into two adjacent peaks at 0.64 and 0.77 V, which probably arose from the oxidation of CO molecules adsorbed on different adsorption sites. This phenomenon has often been observed in other Pt catalysts.47 The CO oxidation peaks on NP-Pt and Pt/C catalysts are located at 0.81 and 0.90 V, respectively. In comparison, both the CO-stripping peaks on the NP-PtFe alloy dramatically shifted to more negative potential, indicating that the CO adsorption is much weaker on NP-PtFe than on NP-Pt and Pt/C. In other words, alloying Fe with Pt greatly decreased the CO poisoning on the catalyst surface, which should mainly be attributed to the electronic effect between Pt and Fe. As observed by XPS, the upshift of the binding energy of Pt after alloying with Fe suggests a lowering of the Fermi level or an increase of the valence electron (5d) vacancy, where an electron back-donation from the Pt 5d orbital to the 2p* orbital of CO would be reduced, thus resulting in the weakened CO adsorption on Pt.42,43,48 Chronoamperometric experiments were also carried out in a 0.5 M H2SO4 + 1.0 M methanol solution to evaluate the longterm catalytic durability of the NP-PtFe alloy (Figure 6b). The potentiostatic process usually includes adsorption/decomposition/oxidation of methanol molecules, adsorption and oxidation of CO-like species, anion adsorption on the catalyst/electrolyte surface, etc.49 The current decay mainly resulted from the inhibition of surface reaction active sites by accumulated COads poisoning species and SO42− adsorption on the catalyst surface.50 From Figure 6c, the amperometric current on NP-PtFe decreased much slower, with more than 45% of the initial current remaining after running for 4000 s, while those of NP-Pt and Pt/C catalysts degraded to 25 and 22%, respectively. This result suggests that the NP-PtFe alloy has a much higher long-term catalytic durability for MOR, with a slow activity decay among the three catalysts. It is considered that the decreased CO poisoning should be responsible for the enhanced catalytic durability of the NP-PtFe alloy. It is known that the structure stability is also very important for the practical application of electrocatalysts.24 Structure stabilities of the three catalysts are measured by continuous potential cycling from 0.6 to 1.0 V in 0.5 M H2SO4 solution. From Figure 6d, after 5000 potential cycles, the ECSA of Pt/C drops to ∼60% of its initial value. The more severe ECSA loss of the Pt/C catalyst was also observed in a previous report,24 which can be ascribed to the aggregation and ripening of Pt nanoparticles. The possible loss of electrical contact to the carbon support may also contribute to the apparent decrease of

composition within the detection depth of XPS was determined to be Pt32Fe68. In comparison to the EDS results presented above, there is a certain degree of Fe enrichment in the near surface region of the PtFe alloy. This is because the formation of more stable oxides provides the driving force to surface segregation of active components, such as Fe, which is frequently seen in other alloy nanostructures.26,44 The hierarchical NP-PtFe alloy represents an interesting class of particularly desirable bimetallic electrocatalyst with a bimodal structure size distribution, especially in which the interconnected voids extending in the whole structure are favorable for the unlimited transport of medium molecules and the 3D bicontinuous channels are beneficial for electron conductivity.45 Consequently, it is intriguing to explore its MOR activity. To better understand the influence of the morphology and alloy effect with Fe, we also examined the MOR activities of asprepared PtFe/C, NP-Pt, and commercial Pt/C catalysts for comparison. Figure 5a illustrated the cyclic voltammetric (CV)

Figure 5. (a) ECSA-normalized and (b) mass-normalized CVs for methanol electrooxidation on NP-PtFe, PtFe/C, NP-Pt, and Pt/C catalysts in 0.5 M H2SO4 + 1.0 M CH3OH. Scan rate = 50 mV s−1.

curves of all catalysts in the mixed 0.5 M H2SO4 + 1.0 M CH3OH solution. It can be clearly found that the four catalysts exhibited markedly different voltammetric features for peak potential and specific activity. The oxidation peak on NP-PtFe was located at 0.85 V in the forward scan, which is lower than those of NP-Pt and Pt/C (∼0.90 V), with the negative shift of about 50 mV, and even lower than that of the commercial PtRu/C catalyst.46 The peak potential of PtFe/C (0.87 V) is in agreement with the observation by Xu et al. for PtFe/multiwall nanotubes (MWNTs),22 which is also higher than that of NPPtFe. The lower oxidation potential indicated the facilitated reaction kinetics for methanol oxidation to CO2 on the NPPtFe alloy surface.25 More importantly, the specific activity for MOR on the NP-PtFe alloy at a peak potential and lower potentials is much higher than those on the other three 1889

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Figure 6. (a) CO-stripping curves for NP-PtFe, NP-Pt, and Pt/C catalysts in 0.5 M H2SO4 solution. Scan rate = 50 mV s−1. (b) Current−time curves and (c) relative activity loss in 0.5 M H2SO4 + 1.0 M methanol solution at 0.6 V. (d) Relative loss of ECSA after 5000 potential cycles between 0.6 and 1.0 V in 0.5 M H2SO4 solution at room temperature.

ECSA of Pt/C.24 NP-PtFe remains ∼80% of the initial ECSA, which is higher than that on NP-Pt, indicating that the incorporation of more reactive Fe did not decrease the structure stability of Pt. Among the three catalysts, NP-PtFe shows the higher structure stability, where the unsupported hierarchical NP architecture frees from the particle aggregation and support corrosion, while a nearly pure Pt skin with the PtFe alloy core formed because the dissolution of surface Fe also suppresses the dissolution and redeposition of Pt as a result of a stained surface.27,34 As stated above, the NP-PtFe alloy exhibited superior MOR activity with decreased CO poisoning. During methanol electrooxidation, methanol molecules will adsorb on a catalyst surface and form adsorbed carbonaceous intermediates, such as CO, which will be oxidized at higher potential because of the formation of Pt−OHads species.51 Consequently, CO electrooxidation on Pt-based electrocatalysts is essential to obtain high MOR activity, which includes the (reversible) step of water decomposition to form adsorbed OH and the following COads oxidation via the interaction with OHads. As for the case of the NP-PtFe alloy, its CO-stripping potential is more negative than that of NP-Pt and Pt/C, showing the enhanced CO electrooxidation activity, as well as resulting in higher MOR activity. DFT calculations were further employed to explore the physical origination for these activity enhancements upon alloying with Fe. Pt58M42(111) slabs with a pure Pt surface were used to be close to the experimental case (Figure 7a). CO adsorption energy (ΔECO) on Pt58M42(111) slabs is calculated to be 1.39 eV, corresponding to the most stable top adsorption site, which is markedly lower than that on a pure Pt(111) surface with the most stable fcc site (Figure 7). OH adsorption energy (ΔEOH) on Pt58M42(111) slabs was calculated to be 1.81 eV on the most stable bridge sites, which also decreased in

comparison to the 1.99 eV on pure Pt(111) with the most stable bridge sites. This is also consistent with previous reports in the ORR studies.13 On the basis of these results, it can be concluded that the lower CO-stripping potential on NP-PtFe originates from the decreased CO adsorption energy rather than the stronger OH bonding. In addition, COads and OHads generated on the catalyst surface during MOR are poisoning, which usually occupy the surface active sites and block the adsorption of methanol. Both the decreased CO and OH adsorption energies on PtFe are beneficial for methanol electrooxidation, leading to the lower oxidation peak potential, as shown in Figure 5. From Figure 7b, the d-band center of Pt downshifts relative to the Fermi level based on the density of the state of Pt,13 which means the decrease of an electron backdonation from the Pt 5d orbital to the 2p* orbital of CO and OH,27,48,52 consequently weakening the CO−Pt and OH−Pt bonding. It is conclusive that the modified electron structure of Pt with the downshift of the d-band center by alloying with Fe plays the key role for the decreased CO poisoning and the dramatically improved MOR activity. Beside this factor, the 3D interconnected multimodal nanoprorous architecture is also favorable for the mass and electron transport during the methanol electrooxidation.

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CONCLUSION The hierarchical NP-PtFe alloy with multimodal size distributions was fabricated by an extremely simple de-alloying process, which shows enhanced specific and mass activity for MOR with higher CO tolerance and structure stability. It is concluded that alloying with Fe can enhance the anodic catalytic activity of Pt as a result of the modification of the Pt electron structure with the downshift of the Pt d-band center. Further research is still needed to optimize and correlate the 1890

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Figure 7. (a) Most stable adsorbed schematic model of CO and OH adsorbed on Pt(111) and Pt-skin Pt58Fe42(111) slabs. (b) DFT calculated d-orbital density of states of Pt atoms in Pt(111) and Pt58Fe42(111) slabs, where the first layer of the alloys consists of pure Pt. The inset is the corresponding ΔECO, ΔEOH, and d-band center on Pt(111) and Pt58Fe42(111) slabs.

catalytic performance and the Fe content. The as-made hierarchical NP-PtFe alloy holds great potential as a promising anode catalyst for direct methanol fuel cells in terms of unique catalytic performance and simple preparation.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-531-82767367. Fax: +86-531-82765969. Email: [email protected].

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (51001053, 21107030, and 50871047) and Shandong Province (ZR2010EQ039 and ZR2010ZM071) and the China Postdoctoral Science Foundation (201104617).

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REFERENCES

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dx.doi.org/10.1021/la203835n | Langmuir 2012, 28, 1886−1892