C Electrocatalyst for Direct

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Titanium Oxide Nanosheet Modified PtRu/C Electrocatalyst for Direct Methanol Fuel Cell Anodes Takahiro Saida, Naoki Ogiwara, Yoshio Takasu, and Wataru Sugimoto* Faculty of Textile Science and Technology, Shinshu UniVersity, 3-15-1 Tokida, Ueda 386-8567, Japan ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: June 30, 2010

Composite catalysts consisting of conventional PtRu/C electrocatalysts and TiO2 nanosheets (TiO2ns-PtRu/ C) were prepared to explore the effect of nanostructured titanium dioxides as additives toward the electrooxidation of methanol and preadsorbed CO. An increase in the electrochemically active PtRu surface area and methanol oxidation activity was observed for a composite catalyst with a low TiO2ns content, whereas composites with a high content tended to have a lower catalytic activity compared with pristine PtRu/C. After conducting accelerated degradation tests by potential cycling between 50 and 800 mV versus RHE at 60 °C, almost all of the activity of the unmodified PtRu/C catalyst was lost. In contrast, methanol oxidation was still observed after potential cycling for the composite catalysts, with the best catalyst exhibiting an almost 10-fold higher activity compared with PtRu/C. The enhanced durability of the catalyst is discussed based on the ability of the TiO2 nanosheet to suppress the Ru loss from the catalyst. 1. Introduction The PtRu alloy catalyst system is widely accepted as one of the most promising anode catalysts for the direct methanol fuel cell (DMFC), a device anticipated to serve as a power source for mobile applications owing to its high activity toward methanol oxidation and tolerance to carbon monoxide poisoning.1-7 Although the vast majority of research has been devoted to increasing the intrinsic activity of the precious metal catalyst, the instability of Ru in the alloy has been addressed as a concern under long-term operation.8 Ru is known to be liable to dissolution and dealloying from the PtRu catalyst, leading to activity loss.9-18 In addition to a loss in anode activity, migration and crossover of dissolved Ru species can cause membrane failure and cathode activity loss, reducing fuel cell performance.8,14,15,18-23 Studies have attempted to suppress Ru loss by adding a third metal, such as Au, Ir, Mo-W, and Cu or oxides, such as SiO2 and TiO2.23-28 Among the candidates as additives, titanium oxide seems to be promising, owing to its natural abundance, cost, semiconductivity, and stability in acidic environments. The addition of small amounts of TiOx to Pt or PtRu has been reported to enhance the carbon monoxide and methanol oxidation activity.29-37 The use of titanium-based oxides as a noncorrosive support has also been studied.38-42 For the TiO2 additive to act efficiently as a cooperative catalyst, it is important that the oxide and metal phases are in close contact while allowing diffusion of the fuel to the PtRu surface. Various synthetic methods to prepare homogeneous composite catalysts have so far been developed, for example, solution-based processing involving the intimate mixing of Pt, Ru, and TiO2 precursor solutions;28,33 impregnation and colloidal methods using PtRu/C or TiO2 particles;29,30,40,41,43 physical mixing of PtRu/C with TiO2 particles;34 and electrodeposition.38,39 Regardless of the synthetic procedure, it is essential that the optimized PtRu nanoparticle size, surface composition, and fine dispersion state is not altered. This raises a challenge as PtRu/C * To whom correspondence should be addressed. E-mail: wsugi@ shinshu-u.ac.jp.

is generally synthesized at low temperatures under a reducing environment, whereas titanium oxide is prepared in oxidizing conditions. Solution-based methods (colloidal method, impregnation, sol-gel) seem desirable, as nanoscale mixing of the metal and metal oxide phases can be achieved. However, pyrolysis conditions must be carefully controlled as oxidative conditions and heat treatment can lead to redundant damage to PtRu/C. Physical mixing is much less demanding; however, intimate mixing is more difficult to obtain than solution-based techniques. These earlier studies are the motive for this study, to develop a simple, solution-based method without any need of post-treatment to make an intimately mixed TiO2-PtRu/C nanocomposite with high activity and durability. In this study, we explored the possibility of using TiO2 nanosheets (TiO2ns, [Ti4O9]2-) as an additive to a PtRu/C anode catalyst for methanol oxidation with an emphasis on the stability of the catalysts. The TiO2ns used here is a two-dimensional nanocrystallite with a thickness of ∼1 nm derived via exfoliation of layered H2Ti4O9.44 As the TiO2ns is obtained in the form of an aqueous colloid, it can be anticipated that a simple combination of a PtRu/C dispersion and TiO2ns colloid will allow easy manipulation of the nanostructured TiO2 to intimately mix with a carbon-supported nanoparticulate PtRu alloy without affecting the optimized nanostructure of PtRu/C. In an earlier study, we have reported the synthesis of an active and durable RuO2 nanosheets-Pt/C composite catalyst by a similar procedure.45,46 The TiO2 nanosheets used in this study can be considered as a less expensive and abundant alternative to RuO2 nanosheets. 2. Experimental Section Carbon-supported PtRu (30 mass % PtRu) was prepared by an impregnation method reported previously.47 Briefly, appropriate amounts of Vulcan XC-72R and Pt(NH3)2(NO2)2 and Ru(NO)(NO3)x dissolved in ethanol were thoroughly mixed and then dried to a powder at 60 °C. The dried powder was then heat-treated in a tube furnace under flowing H2(10%)-N2(90%) gas for 2 h at 200 °C. The titanium oxide nanosheet (TiO2ns, [Ti4O9]2-) was prepared by exfoliation of a tetrabutylammonium-Ti4O9 inter-

10.1021/jp103049x  2010 American Chemical Society Published on Web 07/21/2010

TiO2 Nanosheet Modified PtRu/C

Figure 1. XRD patterns of (a) PtRu/C, (b) TiO2ns(0.05)-PtRu/C, (c) TiO2ns(0.25)-PtRu/C, (d) TiO2ns(0.50)-PtRu/C, and (e) restacked TiO2ns.

J. Phys. Chem. C, Vol. 114, No. 31, 2010 13391 calation compound, which was prepared by a series of ionexchange reactions, as reported previously.44 Briefly, layered tetra-titanic acid hydrate (H2Ti4O9 · xH2O) was obtained by acid treatment of K2Ti4O9. An ethylammonium-Ti4O9 intercalation compound was prepared by reaction of the H2Ti4O9 · xH2O with an aqueous 50% ethlyammonium solution at room temperature for 24 h. Guest-exchange of the interlayer ethylammonium was conducted by reaction with aqueous tetrabutylammonium hydroxide. The solid product was collected by centrifugal separation at 15 000 rpm. The tetrabutylammonium-Ti4O9 intercalation compound was dispersed in distilled water and subject to ultrasonification for 30 min to obtain a white suspension. The suspension was centrifuged at 2000 rpm to remove nonexfoliated material. PtRu/C powder was added to the colloidal TiO2ns and thoroughly mixed at room temperature. Coagulation of PtRu/C with TiO2ns occurs once the two separately stable suspensions are mixed, resulting in sedimentation of the composite catalyst. The mixture was then dried at 80 °C to obtain the final product, a TiO2ns-modified carbon-supported platinum-ruthenium alloy (TiO2ns-PtRu/C). The amount of PtRu/C added to the TiO2ns colloid was set so that the atomic ratio in the catalyst would be Ti/Pt/Ru ) x/1/1, where x ) 0.05, 0.25, and 0.50. These samples will be denoted TiO2ns(x)-PtRu/C according to the amount of TiO2 in the catalyst.

Figure 2. Typical TEM images of (a) PtRu/C, (b) TiO2ns(0.05)-PtRu/C, (c) TiO2ns(0.25)-PtRu/C, and (d) TiO2ns(0.50)-PtRu/C.

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Figure 3. Histograms of particle size distributions for (a) PtRu/C, (b) TiO2ns(0.05)-PtRu/C, (c) TiO2ns(0.25)-PtRu/C, and (d) TiO2ns(0.50)-PtRu/C.

The structure of the catalysts was characterized by X-ray diffraction (XRD, Rigaku RINT-2550 with monochromated Cu KR radiation at 40 kV and 40 mA). The morphology of the catalysts was observed by transmission electron microscopy (TEM, JEOL JEM-2010) operated at an accelerating voltage of 200 kV. The composition of the catalyst was measured by energy-dispersive X-ray spectroscopy (EDX). The working electrode was prepared by depositing 40 µg of the electrocatalyst on a mirror-polished glassy carbon rod (φ ) 5 mm) with a micropipet. A 20 µL portion of a 1 wt % Nafion ionomer (resulting in a calculated Nafion thickness of ∼4.5 µm) was dropped onto the electrode surface to immobilize the electrocatalysts. A beaker-type electrochemical cell equipped with the working electrode, a platinum mesh counter electrode, and an Ag/AgCl/KCl (sat.) reference electrode was used. A Luggin capillary faced the working electrode at a distance of 2 mm. All electrode potentials throughout the paper will be referred to the reversible hydrogen electrode (RHE) scale corrected for the temperature effect.46,48 All electrochemical measurements were conducted in a temperature-controlled bath set to 60 °C. Oxidation of preadsorbed carbon monoxide (COad) was measured by COad-stripping voltammetry in 0.5 M H2SO4 at a scan rate of 10 mV s-1. Gaseous CO (100%) was purged into the 0.5 M H2SO4 electrolyte for 40 min to allow complete adsorption of CO onto the catalyst surface while maintaining a constant voltage of 100 mV versus RHE. Excess CO in the electrolyte was then purged out by bubbling N2 gas for 40 min. The electrochemically accessible PtRu surface area (ECSA) of the catalysts was calculated from the COad-stripping voltammograms. The amount of COad was measured by integration of the COad-stripping peak, corrected for the electrical double-layer capacitance, assuming a monolayer of linearly adsorbed CO on the metal surface and the Coulombic charge necessary for

oxidation as 420 µC cm-2. Electrooxidation of CH3OH was characterized by chronoamperometry at 500 mV versus RHE in 0.5 M H2SO4 containing 1 M CH3OH at 60 °C. The mass activity for the methanol oxidation reaction (MOR) was calculated by normalizing the quasi-steady-state current after 30 min of polarization to the mass of PtRu. The specific activity was calculated using the ECSA measured by COad-stripping voltammetry. After the initial MOR activity was measured, the electrolyte was changed to a fresh 0.5 M H2SO4 electrolyte free of methanol. Accelerated degradation tests (ADTs) were then conducted by potential cycling between 0.05 and 0.8 V at 50 mV s-1 for 2000 cycles. COad-stripping voltammetry and MOR activity tests were finally performed in the same procedure as mentioned above. 3. Results and Discussion The XRD patterns of TiO2ns(x)-PtRu/C with x ) 0.05, 0.25, and 0.50 are shown in Figure 1. The XRD patterns of pristine PtRu/C (Figure 1a) and restacked TiO2ns obtained by casting the TiO2ns colloid (Figure 1e) are also shown as a reference. The XRD patterns of TiO2ns(x)-PtRu/C reveals only peaks attributed to PtRu nanoparticles. Peaks due to restacked TiO2ns are not observed, which is strong evidence that TiO2ns is homogeneously distributed in the composite with negligible agglomeration. The XRD reflections indexed as the PtRu alloy in TiO2ns(x)-PtRu/C are observed at the same diffraction angles with the same peak broadness as pristine PtRu/C. The lattice parameter for PtRu was 0.387 nm, in agreement with sputtered bulk PtRu alloy.49 This indicates that the structure of the PtRu alloy nanoparticles in the composites is the same as that of the unmodified PtRu/C; that is, the degree of alloying and particle size and distribution are not affected by the addition of TiO2ns.

TiO2 Nanosheet Modified PtRu/C

J. Phys. Chem. C, Vol. 114, No. 31, 2010 13393 TABLE 1: Electrochemically Active PtRu Surface Area (ECSA) Obtained from COad-Stripping Voltammetry of Fresh Catalysts and Catalysts after Accelerated Degradation Tests (ADTs) ECSA/m2 (g PtRu)-1

Figure 4. COad-stripping voltammograms of fresh catalysts (a-d) and catalysts after accelerated durability tests (a′-d′) for (a, a′) PtRu/C, (b, b′) TiO2ns(0.05)-PtR/C, (c, c′) TiO2ns(0.25)-PtRu/C, and (d, d′) TiO2ns(0.50)-PtRu/C. The solid line is the COad-stripping voltammogram and the broken line is the background voltammogram in 0.5 M H2SO4 (60 °C). The scan rate is 10 mV s-1.

Typical TEM images of the as-prepared samples are shown in Figure 2, and the corresponding particle size distributions are shown in Figure 3. Regardless of the TiO2ns content, PtRu alloy nanoparticles with an average diameter of 3.1 nm were observed finely distributed on the carbon support, which agrees well with the XRD data. TiO2ns is not clearly observed in the TEM images due to the atomic scale thickness. The presence of TiO2ns in the modified catalyst was confirmed by EDX analysis. The observed atomic ratio was consistent with the nominal content; Ti/Pt/Ru ) 0.05/1/1.1, 0.28/1/1.1, and 0.60/ 1/1.1 for TiO2ns(x)-PtRu/C with x ) 0.05, 0.25, and 0.50. COad-stripping voltammograms of the fresh electrodes are shown in Figure 4. A slight negative shift in the COad oxidation peak was observed for TiO2ns(0.05)-PtRu/C compared with PtRu/C. For composites with a higher TiO2ns content, the COad oxidation peak was observed at the same potential as that of PtRu/C. These results suggest that a small amount of TiO2ns

sample

fresh catalysts

after ADTs

retention/%

PtRu/C TiO2ns(0.05)-PtRu/C TiO2ns(0.25)-PtRu/C TiO2ns(0.50)-PtRu/C

93 101 70 77

38 62 63 63

41 61 90 82

may enhance the COad oxidation in PtRu/C, as has been observed for other TiO2-PtRu systems.32,34 The cause of this synergistic effect is still open for discussion. The electrochemically active metal surface area (ECSA) obtained from COad stripping is summarized in Table 1. The ECSA of TiO2ns(0.05)-PtRu/C was slightly larger (∼10%) than that of PtRu/C, which may be due to an increase in the interphase between the electrolyte and PtRu nanoparticles as a result of the addition of hydrophilic TiO2ns. An increase in the ECSA was observed also when an electroconducting RuO2 nanosheet was used as the additive to Pt/C.46 At higher TiO2ns, the ECSA tended to decrease. Such a volcano-type dependency was not observed in the case of the RuO2 nanosheet modified Pt/C. The dissimilarity is likely due to the difference in the electronic conductivity of the nanosheet additive, where TiO2ns is a poor electronic conductor, whereas the RuO2 nanosheet is a good electronic conductor. After accelerated degradation tests (ADTs), the COad-stripping peak of pristine PtRu/C shifts 130 mV positive in potential and severely broadens (Figure 4). In contrast, the main COad oxidation peak is maintained at 0.45 V versus RHE after the ADTs for the composite catalysts. A second COad oxidation peak emerges at a higher potential (∼0.70 V) for TiO2ns(0.50)-PtRu/C after the ADT. The potential of this second peak is the same as that of Pt/C,48,50 reflecting the inhomogeneous distribution of Pt and Ru in the degraded catalysts. The retention of the ECSA for the composite catalysts was higher compared with that of the unmodified PtRu/C (Table 1). The decrease in the ECSA from 93 to 38 m2 (g PtRu)-1 and the shift in COad oxidation potential suggest a loss of ruthenium from the PtRu alloy and an increase in particle size. In the case of TiO2ns(0.25)-PtRu/C, 90% of the initial ECSA was retained. The ECSA of 63 m2 g-1 is 1.6 times higher compared with degraded PtRu/C. Typical TEM images of the catalysts after ADTs are shown in Figure 5, and the corresponding particle size distributions are shown in Figure 6. The average particle size for PtRu/C increases from 3.7 ( 0.8 to 6.2 ( 2.5 nm. For TiO2ns(0.05)-PtRu/C, the average particle size increased from 3.6 ( 0.8 to 6.3 ( 2.2 nm. Evidently, the TiO2ns content is too low to have a definite effect on the particle size distribution. A clear positive effect against an increase in particle size of the TiO2ns addition was seen for TiO2ns(x)-PtRu/C with x ) 0.25 and 0.50. At least 90% of the particles are less than 6 nm in diameter after ADTs, with an average particle size of ca. 4.3 ( 1.3 and 4.4 ( 1.2 nm for x ) 0.25 and 0.50. The results of the particle size analysis are in good agreement with the change in the ECSA obtained from COad-stripping voltammetry. Chronoamperograms in 0.5 M H2SO4 + 1 M CH3OH are shown in Figure 7, and the mass and specific activities are summarized in Table 2. An enhancement in the initial MOR is observed for TiO2ns(0.05)-PtRu/C, which is attributed to the higher ECSA and enhanced COad oxidation activity. Increasing the TiO2ns content in the composite catalysts resulted in a

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Figure 5. Typical TEM images of (a) PtRu/C, (b) TiO2ns(0.05)-PtRu/C, (c) TiO2ns(0.25)-PtRu/C, and (d) TiO2ns(0.50)-PtRu/C after accelerated durability tests.

decrease in the initial MOR activity, more than would be expected from the decrease in the ECSA. Nevertheless, the modified catalysts exhibited a 6-10 fold higher activity compared with the unmodified PtRu/C after ADTs. The activity retention rate is higher for the composite catalysts with a higher TiO2ns content (Table 2). Thus, TiO2ns(0.25)-PtRu/C initially has a smaller MOR activity compared with PtRu/C, but the overall activity retention rate and activity after ADTs is improved. The MOR activity of PtRu/C after ADTs is extremely low (97% loss of activity) considering that 40% of the metal ECSA is retained. In fact, the MOR activity of the degraded PtRu/C is close to that of fresh Pt/C. This suggests that the active sites for MOR at 500 mV versus RHE in the degraded PtRu/C catalyst originates mainly from Pt-rich surfaces, as evidenced from the positive shift in COad oxidation potential. Dissolved Ru diffuses into the electrolyte and/or does not alloy with the Pt particles upon redeposition. In the case of the composite catalysts, the main COad oxidation peak is maintained after the ADTs. This suggests that a sufficient amount of Ru is still present in the catalysts, although its alloying state is not as good as the pristine state. Indeed, EDX analysis of the catalyst after ADTs showed a decrease in Ru content; Pt/Ru ) 1/0.18 for PtRu/C and 1/0.29 for TiO2ns(0.25)-PtRu/C. Clearly, the

presence of TiO2ns suppresses the Ru loss. As the TiO2ns is negatively charged ([Ti4O9]2-), dissolved Ru species may be trapped on the TiO2ns surface, inhibiting diffusion into the electrolyte. Once the potential is scanned in the cathodic direction, the dissolved Ru species is subsequently reduced on the PtRu/C surface. Deposition on the TiO2ns surface is probably less favorable due to the lower conductivity of the nanosheet compared with carbon. 4. Conclusion Titanium oxide nanosheets (TiO2ns) derived from layered H2Ti4O9 were mixed with PtRu/C catalyst in different compositions, and the electrooxidation of adsorbed carbon monoxide (COad) and methanol were characterized before and after consecutive potential cycling tests. The modified composite catalyst with TiO2/Pt ) 0.05/1 exhibited enhanced COad and methanol electrooxidation properties, which is tentatively attributed to an increase in the interphase between the electrolyte and PtRu nanoparticles as a result of the addition of hydrophobic TiO2ns. After accelerated durability tests by consecutive cycling between 0.05 and 0.8 V at 50 mV s-1 for 2000 cycles, the ECSA and activity of TiO2ns-modified PtRu/C were higher than those of nonmodified PtRu/C. The ECSA of TiO2ns-modified PtRu/C

TiO2 Nanosheet Modified PtRu/C

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Figure 6. Histograms of particle size distributions of (a) PtRu/C, (b) TiO2ns(0.05)-PtRu/C, (c) TiO2ns(0.25)-PtRu/C, and (d) TiO2ns(0.50)-PtRu/C after accelerated durability tests.

Figure 7. Chronoamperograms for (a) PtRu/C, (b) TiO2ns(0.05)-PtRu/C, (c) TiO2ns(0.25)-PtRu/C, and (d) TiO2ns(0.50)-PtRu/C in 0.5 M H2SO4 + 1 M CH3OH (60 °C) at 500 mV versus RHE. Fresh catalysts (dotted lines) and catalysts after accelerated durability tests (solid lines).

TABLE 2: Mass and Specific Activity Toward Methanol Oxidation of Fresh Catalyst and Catalysts after Accelerated Degradation Tests (ADTs) j/A (g PtRu)-1 sample PtRu/C TiO2ns(0.05)-PtRu/C TiO2ns(0.25)-PtRu/C TiO2ns(0.50)-PtRu/C

j/A (cm2 PtRu)-1

fresh after fresh after catalysts ADT catalysts ADT retention/% 54 65 34 25

1.6 9.3 14.8 9.6

0.58 0.64 0.49 0.32

0.04 0.15 0.23 0.15

3 14 44 39

with TiO2/Pt ) 0.25/1 was 63 m2 g-1, 1.6 times higher than 38 m2 g-1 for PtRu/C. The methanol oxidation activity was 14.8

A (g PtRu)-1, almost 10 times higher than that of PtRu/C. Thus, it is concluded that TiO2ns inhibits the degradation of PtRu/C. Acknowledgment. The authors acknowledge Dr. Yusuke Yamauchi (World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS)) for fruitful discussions and experimental assistance. This work was supported, in part, by the Polymer Electrolyte Fuel Cell Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, a Grant-in-Aid for Scientific Research (18685026), and a Grant-in-Aid for the Global COE Program from the Ministry of Education, Science, Sports, and Culture (MEXT). The authors are grateful to Ishifuku Metal Industry Co. for kindly supplying Pt(NH3)2(NO2)2.

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