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Highly durable Pt cathode catalysts for polymer electrolyte fuel cells; coverage of carbon black-supported Pt catalysts with silica layers
Applied Catalysis A: General 409–410 (2011) 248–256
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Highly durable Pt cathode catalysts for polymer electrolyte fuel cells; coverage of carbon black-supported Pt catalysts with silica layers Sakae Takenaka ∗ , Hiroshi Matsumori, Hideki Matsune, Masahiro Kishida Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 4 August 2011 Received in revised form 28 September 2011 Accepted 8 October 2011 Available online 14 October 2011 Keywords: Carbon black-supported Pt electrocatalysts Polymer electrolyte fuel cell Cathode catalyst durability Silica-coating
a b s t r a c t Carbon black-supported Pt (Pt/CB) catalysts that have been used at the cathode in state of the art polymer electrolyte fuel cell (PEFC) were covered with silica layers to improve the durability of the Pt catalysts under PEFC cathode conditions. The durability of silica-coated Pt/CB to potential cycling between 0.6 and 0.9 V (vs. reversible hydrogen electrode (RHE)) was strongly dependent on the thickness of the silica layers, i.e., the durability of Pt/CB improved after coverage with thick silica layers. However, the coverage of the whole surface of the Pt/CB catalysts with silica layers produced electrochemically inactive Pt catalysts. Silica-coated Pt/CB catalysts with an optimal silica layer thickness showed similar activity for the oxygen reduction reaction compared to Pt/CB catalysts without a silica coating, and they had excellent durability at the cathode in a PEFC single cell. Coverage with silica layers improved the durability of the Pt/CB cathode catalysts without a decrease in the catalytic activity for the oxygen reduction reaction. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymer electrolyte fuel cells (PEFCs) are promising alternative power sources for transportation and portable applications, because of advantages such as low emission and high energy efficiency [1,2]. Pt metal has been used as a catalytically active metal component for the hydrogen oxidation reaction (HOR) at the anode and for the oxygen reduction reaction (ORR) at the cathode in the PEFC. The sluggish rate of the ORR on Pt compared with the HOR requires more Pt loading in the catalysts at the cathode, which impedes the full commercialization of PEFC, because of the high cost of Pt [3,4]. However, it is difficult to reduce the Pt loading at the cathode in the PEFC. Pt catalysts at the cathode work under very severe conditions, such as low pH, high temperatures, oxygen atmosphere, high humidity and highly positive potentials. Thus, Pt catalysts are seriously deactivated under cathode conditions [5]. Pt metal particles at the cathode easily migrate on carbon supports and subsequently agglomerate upon collision [6]. Pt metal particles also grow through Ostwald ripening, that is, the surface Pt atoms in small Pt metal particles dissolve to form cationic Pt species and the dissolved Pt species are subsequently deposited onto large metal particles, which results in the growth of the Pt metal particle size [7–9]. The migration of Pt metal particles in the cathode catalysts can be suppressed to some extent by the modification of the chemical and physical properties of the carbon supports [10–12].
∗ Corresponding author. Tel.: +81 92 802 2752; fax: +81 92 802 2752. E-mail address: [email protected] (S. Takenaka). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.10.012
However, these methods are not effective for the suppression of Pt metal dissolution. Many research groups have reported that alloy formation between Pt and other metal species such as Co, Pd or Au also improves the durability of the catalysts under PEFC cathode conditions [13–16]. The use of Pt-based alloy catalysts in the cathode contributes to a decrease in the Pt loading at the PEFC cathode, because the alloy catalysts have higher activity and durability compared to pure Pt catalysts. However, the metal species added to the Pt catalysts also inevitably dissolve under these cathode conditions, which causes a loss in activity of the Pt-based alloy catalysts during the PEFC operation [17]. The dissolved metal species are also deposited in polymer electrolyte membranes, which results in a decrease in their proton conductivity [18]. Thus, the diffusion of metal species out of cathode catalysts should be suppressed to reduce the Pt loading at the PEFC cathode. We have previously studied the catalytic performance of carbon nanotube (CNT)-supported Pt cathode catalysts covered with silica layers [19,20]. Silica-coated Pt/CNT showed high durability under the PEFC cathode conditions, whereas Pt/CNT without a silicacoating became seriously deactivated under the same conditions. The silica layers that are wrapped around the Pt metal particles prevent the migration of Pt metal particles on the CNT supports and the diffusion of dissolved Pt species out of the silica layers. The application of CNT to the supports of the Pt cathode catalysts is attractive because the CNT has excellent properties, such as a high electronic conductivity, a high chemical stability and a high surface area. It has been reported that Pt/CNT catalysts show high activity and durability at the PEFC cathode [21–24]. However, the CNT is not easily available and is expensive compared to carbon black,
S. Takenaka et al. / Applied Catalysis A: General 409–410 (2011) 248–256
which is a conventional support used for Pt cathode catalysts. Thus, carbon black-supported Pt (Pt/CB) catalysts should be covered with silica layers to improve the durability of the Pt catalysts under PEFC cathode conditions. In this study, Pt/CB was covered with silica layers by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and tetraethoxysilane (TEOS). Silica-coated Pt/CB catalysts with different silica loadings were prepared to determine the effect of silica layer thickness on the activity and durability of the Pt catalysts. We thus report the high activity and excellent durability of silica-coated Pt/CB electrocatalysts under PEFC cathode conditions. 2. Experimental 2.1. Catalyst preparation Pt/CB was prepared by conventional impregnation. The CB support (Vulcan XC-72, supplied by Cabot) was impregnated into an aqueous H2 PtCl6 solution and dried at 333 K in air. The obtained samples were reduced with hydrogen at 473 K for 5 h. Pt/CB was covered with silica layers by the successive hydrolysis of APTES (supplied from Tokyo Chemical Industry) and TEOS (supplied from Kanto Chemical) [25]. Pt/CB was ultrasonically dispersed in a mixed solution of water and ethanol (a volume ratio = 1/1) at 333 K and the pH of this solution was adjusted to ca. 11 by the addition of aqueous NH3 . The hydrolysis of APTES was performed at 333 K over 30 min by the addition of APTES to the solutions. Subsequently, TEOS was added to the solutions to cover the samples with thick silica layers. After the solutions were stirred for 1 h at 333 K they were filtered and washed several times with distilled water. The samples thus obtained were treated with hydrogen at 623 K for 3 h, and then in argon at 873 K for 5 h. Silica-coated Pt/CB is denoted SiO2 /Pt/CB hereafter. To control the silica loading in SiO2 /Pt/CB, the concentration of APTES and TEOS during their hydrolysis was changed while the total concentration of APTES and TEOS was 45 mM [26]. 2.2. Electrochemical measurements Electrochemical measurements were carried out using a threecompartment electrochemical cell with a Pt mesh and saturated Ag/AgCl electrode serving as the counter and reference electrodes, respectively. The saturated Ag/AgCl electrode was separated from the working electrode compartment by a closed electrolyte bridge. All potentials are given relative to the reversible hydrogen electrode (RHE). A glassy carbon disk electrode (5 mm diameter) was used as a substrate for the catalysts and polished to a mirror finish. Catalyst ink was prepared by ultrasonically blending the catalysts and methanol. An aliquot of this ink was deposited on a glassy carbon disk and dried at 333 K. 1 wt% Nafion solution diluted with methanol was dropped onto the catalysts to ensure the attachment of the catalysts to the disk. The potential of the Pt catalysts was repeatedly changed between 0.05 and 1.20 V in N2 -purged 0.5 M H2 SO4 at 303 K until reproducible cyclic voltammograms (CVs) were obtained. The reproducible CVs for all the Pt catalysts were obtained within 50 cycles of potential cycling. CVs for the Pt catalysts were measured in the potential range between 0.05 and 1.20 V at a scan rate of 50 mV s−1 . Square wave potential cycling between 0.6 and 0.9 V were performed for the Pt catalysts in N2 -purged 0.5 M H2 SO4 at 303 K for accelerated durability tests of the Pt catalysts [27,17,28]. A voltammogram for the desorption of underpotentially deposited hydrogen was used to evaluate the electrochemically active surface area (ECSA) of the Pt catalysts on the electrode. The value 210 C cm-Pt−2 was used to determine the ECSA from the adsorbed hydrogen.
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Electrochemical measurements using a rotating disk electrode (RDE) were carried out using a three-electrode electrochemical cell. Pt foil was used as a counter electrode and Ag/AgCl was used as a reference electrode. The working electrode was composed of a 6-mm diameter glassy carbon core embedded in a Teflon cylinder. Catalyst ink was prepared by ultrasonically blending Pt/CB or SiO2 /Pt/CB and ethanol. This ink was deposited on a glassy carbon disk. The amount of Pt catalyst deposited onto the glassy carbon rod was such that the Pt content was equivalent to 10 g-Pt cm−2 in all the experiments. After drying the catalysts at room temperature, the Nafion solution that was diluted with methanol was dropped onto the catalysts to ensure that the catalysts were attached to the disk. The catalysts were immersed in 0.1 M HClO4 at room temperature. The polarization curves for the ORR on the Pt catalysts were measured in O2 -saturated 0.1 M HClO4 at room temperature by changing the potential of the working electrode from 0.1 to 1.1 V at a scan rate of 10 mV s−1 with an electrode rotation rate of 1600 rpm. The catalytic activity of Pt/CB and SiO2 /Pt/CB for the ORR was examined using PEFC single cells. The membrane-electrode assembly (MEA) for a single cell (Electrochem Co., EFC-05-02) was prepared as follows: SiO2 /Pt/CB or Pt/CB electrocatalysts were used for the cathode and Pt/CB catalyst for the anode. Catalyst ink was prepared by ultrasonically mixing the catalysts, 2-propanol and diluted Nafion solution (5 wt% Nafion). The catalyst ink was painted onto the surface of wet-proofed carbon paper (Toray Co.) as a gas diffusion layer. An MEA with an area of 5 cm2 was fabricated by hot pressing the cathode and anode to Nafion 117 at 403 K and 10 MPa for 3 min. The Pt loading in both the cathode and anode was 0.1 mg-Pt cm−2 . Hydrogen (flow rate = 40 ml min−1 , P = 101.3 kPa) and oxygen (flow rate = 40 ml min−1 , P = 101.3 kPa) were supplied to the anode and cathode, respectively. The gases for the cathode and anode were humidified at 353 K before their introduction into the cells. Before obtaining polarization curves for the single cells, MEA was conditioned by the supply of hydrogen to the anode and oxygen to the cathode at a cell voltage of 0.5 V for 2 h. The cell voltage for the single cells was cycled between 0.6 and 0.9 V at 353 K as an accelerated durability test for the cathode catalysts. During the durability test, humidified hydrogen and nitrogen were supplied to the anode and cathode, respectively. Polarization curves for the single cells were also measured after the introduction of oxygen to the cathode.
3. Results and discussion 3.1. Durability of silica-coated Pt catalysts Pt/CB was covered with silica layers by the successive hydrolysis of APTES and TEOS. We have reported that carbon-supported Pt catalysts could not be uniformly covered with thick silica layers by using only APTES or only TEOS, while the successive hydrolysis of APTES and TEOS resulted in the uniform coverage of the Pt catalysts with thick silica layers [26]. During the hydrolysis of APTES in the presence of the carbon-supported Pt catalysts, APTES adsorbed on the surface of the Pt metal and graphene by an interaction between the amino groups in APTES and the Pt metal or graphene. However, the hydrolysis of APTES and the polycondensation of silica precursors that were formed from APTES did not proceed easily, because the 3-aminopropyl groups in APTES were not hydrolyzed under the present preparation conditions, due to the strong bonding between the Si and C atoms. The silica layers formed from APTES on carbonsupported Pt catalysts were thus very thin. On the other hand, silica precursors formed from TEOS did not adsorb on the surface of Pt metal and graphene during the hydrolysis of only TEOS in the presence of carbon-supported Pt catalysts. Thus, carbon-supported Pt catalysts could not be uniformly covered with silica layers by the
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Fig. 1. TEM images of Pt/CB (a and b), SiO2 /Pt/CB (A3) (c), SiO2 /Pt/CB (A5) (d and e) and SiO2 /Pt/CB (A10) (f).
hydrolysis of only TEOS. In contrast, the carbon-supported Pt catalysts could be uniformly covered with thick silica layers by the successive hydrolysis of APTES and TEOS. Thin silica layers that were formed from APTES worked as nucleation sites of silica from TEOS during the coverage of Pt catalysts with silica layers. Thick silica layers were thus formed during the subsequent hydrolysis of TEOS because TEOS was easily hydrolyzed. SiO2 /Pt/CB was prepared at an APTES concentration of 3, 5 or 10 mM with a total concentration of APTES and TEOS of 45 mM. The SiO2 /Pt/CB prepared at APTES concentrations of 3, 5 and 10 mM were denoted SiO2 /Pt/CB (A3), SiO2 /Pt/CB (A5) and SiO2 /Pt/CB (A10), respectively. Table 1 shows the loading of Pt, SiO2 and CB in the Pt catalysts prepared in this study. The Pt loading for Pt/CB was estimated to be 12 wt% by thermogravimetric analysis in air. The SiO2 loading in SiO2 /Pt/CB was increased from 12.5 to 27.9 wt%
by increasing the APTES concentration from 3 to 5 mM. However, a further increase in the concentration of APTES to 10 mM did not change the SiO2 loading appreciably. As the APTES concentration increased from 3 to 5 mM, the Pt/CB surfaces that were covered with
Table 1 Content of SiO2 , Pt and CB in Pt/CB and SiO2 /Pt/CB. Catalyst
APTES concentration (mM)
TEOS concentration (mM)
Content (wt%) SiO2
Pt
CB
Pt/CB SiO2 /Pt/CB (A3) SiO2 /Pt/CB (A5) SiO2 /Pt/CB (A10)
– 3 5 10
– 42 40 35
0 12.5 27.9 25.6
12.0 11.4 7.6 10.1
88.0 76.1 64.5 64.3
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thin silica layers from APTES became larger. Thus, a large amount of silica was deposited onto Pt/CB during the hydrolysis of TEOS, when the APTES concentration was increased from 3 to 5 mM. A further increase in the concentration of APTES to 10 mM resulted in a decrease in the concentration of TEOS. Therefore, the silica loading in SiO2 /Pt/CB does not increase by an increase in the concentration of APTES from 5 to 10 mM. Fig. 1 shows transmission electron microscope (TEM) images of the Pt/CB and SiO2 /Pt/CB catalysts. In the TEM images of Pt/CB (a and b), Pt metal particles are present on the CB supports. The size of the Pt metal particles in the catalysts ranges from 1 to 4 nm. The Pt/CB was covered with silica layers. Pt metal particles are also observed in the TEM image of SiO2 /Pt/CB (A3) and the size of the Pt metal particles is similar to that in Pt/CB. However, silica layers were not found in the TEM image of SiO2 /Pt/CB (A3). It should be noted that the SiO2 /Pt/CB was reduced with hydrogen at 623 K and subsequently treated at 873 K in Ar. We have reported that the durability of silica-coated Pt/CNT catalysts under the PEFC cathode conditions was improved by the treatment of the catalysts at higher temperatures in Ar [29]. The treatment of the silica-coated Pt catalysts at 873 K in Ar resulted in the formation of dense silica layers, which prevented the diffusion of dissolved Pt species out of silica layers. Therefore, the SiO2 /Pt/CB used in the present study was treated at 873 K in Ar. Aggregated Pt metal particles are scarce in the TEM image of SiO2 /Pt/CB (A3) although the catalysts were treated in Ar at 873 K. We have previously reported that the Pt metal particles in Pt/CB easily aggregate during treatment at high temperatures [30]. This result implies that the Pt metal particles in SiO2 /Pt/CB (A3) are covered with silica layers. The silica layers that are wrapped around the Pt metal particles inhibit the migration of Pt metal particles on the CB support during the treatment of the catalysts at 873 K. Pt metal particles were also observed in the TEM images of SiO2 /Pt/CB (A5) and SiO2 /Pt/CB (A10) and the size of the Pt metal particles ranged from 1 to 4 nm. The Pt metal particles and
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CB in SiO2 /Pt/CB (A5) seem to be covered with silica layers a few nanometers thick, as shown in the TEM image (e). All the SiO2 /Pt/CB catalysts were calcined in air at 923 K to determine whether these catalysts were covered with silica layers. During the calcination of the catalysts in air, CB was completely removed by combustion. SiO2 /Pt/CB catalysts for the evaluation of the catalytic activity and durability were not calcined in air. TEM images of the SiO2 /Pt/CB calcined at 923 K are shown in Fig. 2. In the TEM images for SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) that were obtained after calcination, many aggregated Pt metal particles were supported on silica. The Pt metal particles were aggregated during the calcination of SiO2 /Pt/CB. The shape of silica in calcined SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) was not uniform. In contrast, many silica particles with hollow structures were found in the TEM image of calcined SiO2 /Pt/CB (A10). The size of the hollow silica particles is similar to the size of the CB supports. These results indicated that whole surface of SiO2 /Pt/CB (A10) was uniformly covered with thick silica layers before calcination, whereas the SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) before calcination had exposed surfaces of CB and Pt metal particles. It is likely that the size of Pt metal particles in the calcined SiO2 /Pt/CB (A10) was smaller than that in the calcined SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5), because whole surface of SiO2 /Pt/CB (A10) was covered with thick silica layers and the Pt metal particles were enclosed in hollow silica particles during calcination in air. It should be noted that the silica loading in SiO2 /Pt/CB (A5) is almost the same as that in SiO2 /Pt/CB (A10). When Pt/CB was covered with silica layers using an APTES concentration of 10 mM the whole surface of Pt/CB was found to be covered with thin silica layers that formed from APTES and, subsequently, silica precursors from TEOS were deposited onto the silica layers from APTES. By contrast, an APTES concentration of 5 mM was not high enough to uniformly cover the Pt/CB surfaces with thin silica layers. Therefore, SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) had exposed CB surfaces.
Fig. 2. TEM images of SiO2 /Pt/CB (A3) (a), SiO2 /Pt/CB (A5) (b) and SiO2 /Pt/CB (A10) (c) after calcination in air at 923 K.
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Fig. 3. CVs of fresh Pt/CB and SiO2 /Pt/CB catalysts in N2 -purged 0.5 M H2 SO4 at 303 K.
Fig. 3 shows CVs for Pt/CB and SiO2 /Pt/CB in N2 -purged 0.5 M H2 SO4 . Two peak couples are present in the CV for the Pt/CB catalyst. One peak couple is in the range of 0.05–0.3 V and is assignable to the adsorption and desorption of hydrogen on Pt metal, and the other couple is in the range of 0.6–1.2 V and comes from the oxidation and reduction of Pt metal. These peak couples were also observed in the CVs of SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5). These SiO2 /Pt/CB catalysts are thus electrochemically active. We have previously studied the catalytic performance of silica-coated Pt/CNT and silica-coated Pd/CNT [19,31,32]. These catalysts show high activity in electrochemical reactions such as the ORR despite the coverage of the metal particles with silica insulators. Reactants such as oxygen and protons are supplied to the metal surfaces through the porous silica layers. Electrons are transferred to the metal through the exposed CNT surfaces. Therefore, the silicacoated Pt and Pd catalysts showed high catalytic activity during the electrochemical reactions. The peak current due to the Pt metal in SiO2 /Pt/CB (A5) was lower than that in SiO2 /Pt/CB (A3) because of the higher silica loading. In contrast, peak couples due to the Pt metal were not present in the CV of SiO2 /Pt/CB (A10) although the silica loading for SiO2 /Pt/CB (A10) was similar to that for SiO2 /Pt/CB (A5). As described earlier, the whole surface of SiO2 /Pt/CB (A10) was uniformly covered with silica layers while SiO2 /Pt/CB (A5) had exposed CB surfaces. Electrons are not supplied to the Pt metal in the electrochemical reaction on SiO2 /Pt/CB (A10) because the catalysts do not have exposed CB surfaces. Thus, SiO2 /Pt/CB (A10) is electrochemically inactive. The durability of Pt/CB, SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) was examined by potential cycling experiments between 0.6 and 0.9 V in N2 -purged 0.5 M H2 SO4 . CVs for Pt/CB, SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) that were obtained during the durability tests are shown in Fig. 4. Two peak couples due to Pt metal are clearly present in the CV of the fresh Pt/CB catalysts, but the peak currents gradually decrease with the number of potential cycles [27,17,28]. The Pt/CB catalysts were deactivated by potential cycling experiments. The peak currents in the CVs for the SiO2 /Pt/CB (A3) also decreased gradually as the number of the potential cycles increased. In contrast, the peak currents for the SiO2 /Pt/CB (A5) increased slightly up to 3000 cycles, and then remained constant during the potential cycling experiments, suggesting that the coverage of Pt/CB with thicker silica layers improved the durability of the Pt metal particles. The change in ECSAs for these Pt catalysts during the durability tests was evaluated based on the CVs. Fig. 5 shows the results. The
Fig. 4. CVs of Pt/CB (a), SiO2 /Pt/CB (A3) (b) and SiO2 /Pt/CB (A5) (c) during the potential cycling experiments in N2 -purged 0.5 M H2 SO4 at 303 K.
ECSA for the fresh Pt/CB was determined to be 65 m2 g-Pt−1 . The ECSA for the Pt/CB catalysts appreciably decreased during the early period of the durability test, and then it decreased slowly with the number of potential cycles. The ECSA of Pt/CB decreased slightly upon coverage with silica layers. The ECSA for the fresh SiO2 /Pt/CB (A3) was determined to be 53 m2 g-Pt−1 and it decreased with the number of potential cycles, which is similar to the ECSA change of Pt/CB. The ECSA for fresh SiO2 /Pt/CB (A5) was 38 m2 g-Pt−1 , which is smaller value than the ECSAs for the fresh Pt/CB and SiO2 /Pt/CB (A3). However, the ECSA for SiO2 /Pt/CB (A5) increased to 42 m2 gPt−1 during the early period of the durability test and it remained
S. Takenaka et al. / Applied Catalysis A: General 409–410 (2011) 248–256
Fig. 5. Change in ECSAs for Pt/CB and SiO2 /Pt/CB during the potential cycling experiments.
high during the potential cycling experiment. It should be noted that the ECSA for SiO2 /Pt/CB (A5) was higher after 20,000 cycles of potential cycling compared to that of Pt/CB. These results indicate that the coverage of Pt/CB with silica layers improves the durability of the Pt metal particles. The durability of SiO2 /Pt/CB strongly depends on the silica loading in the catalysts. SiO2 /Pt/CB (A5) has excellent durability, because the Pt metal particles in the catalysts are covered with thicker silica layers. The thickness of silica layers in SiO2 /Pt/CB (A5) was roughly estimated to be ca. 3 nm from the TEM image. TEM images of the Pt catalysts after the durability tests were measured to determine the change in morphology of the Pt metal particles during the durability tests. The results are shown in Fig. 6.
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Pt metal particles are present in the TEM image of the used Pt/CB catalysts. The Pt metal particles in the used Pt/CB are slightly larger than the particles in fresh Pt/CB, as shown in Fig. 1. Pt metal particles are also present in the TEM images for both the SiO2 /Pt/CB catalysts after the durability tests. The Pt metal particles in the used SiO2 /Pt/CB are smaller than the Pt particles in the used Pt/CB although the Pt metal particle sizes in all the Pt catalysts before the durability tests are similar to one another. Based on the TEM images shown in Figs. 1 and 6, the distribution of the Pt metal particle size was determined. The results are shown in Fig. 7. The size of Pt metal particles ranged from 1 to 4 nm in all fresh Pt catalysts. The distribution of Pt metal particle sizes in Pt/CB shifted toward larger sizes after the potential cycling. The average size of the Pt metal particles in fresh and used Pt/CB was determined to be 2.0 and 3.5 nm, respectively. The decrease in the ECSA for the Pt/CB catalysts during the durability tests thus resulted from the increase in the Pt metal particle size. The size of Pt metal particles in the Pt/CB increased by the migration and agglomeration of Pt metal particles and/or the dissolution and redeposition of Pt metal. The size distribution of the Pt metal particles in SiO2 /Pt/CB (A3) also shifted toward larger sizes during the durability tests. The average Pt metal particle size in SiO2 /Pt/CB (A3) increased from 2.4 to 3.7 nm during the durability test. The average particle size of the Pt metal in the used SiO2 /Pt/CB (A3) was almost the same as that in the used Pt/CB. In contrast, the increase in the Pt particle size for SiO2 /Pt/CB (A5) during the durability test was negligible compared to that for Pt/CB and SiO2 /Pt/CB (A3). Pt metal particles larger than 5 nm were seldom present in the used SiO2 /Pt/CB (A5), while some Pt metal particles in Pt/CB and SiO2 /Pt/CB (A3) were aggregated to the particles larger than 5 nm during the durability test. The used SiO2 /Pt/CB (A5) had the smallest average particle size (2.6 nm) among the all the Pt catalysts after the durability test. The silica loading of SiO2 /Pt/CB (A5) was higher than that of SiO2 /Pt/CB (A3). Many Pt metal particles in SiO2 /Pt/CB (A3) were not covered with silica layers or were covered with
Fig. 6. TEM images of Pt/CB (a), SiO2 /Pt/CB (A3) (b) and SiO2 /Pt/CB (A5) (c) after the potential cycling experiments.
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Fig. 8. Polarization curves obtained with RDE for the ORR on Pt/CB and SiO2 /Pt/CB (A5) at room temperature.
SiO2 /Pt/CB (A5). Thus, the catalytic activity of Pt/CB and SiO2 /Pt/CB (A5) for the ORR was examined using a RDE. Fig. 8 shows polarization curves of the ORR on Pt/CB and SiO2 /Pt/CB (A5). The current due to the ORR on Pt/CB significantly increased at less than 1 V. Interestingly, SiO2 /Pt/CB (A5) also showed catalytic activity for the ORR at less than 1 V. The decrease in the current due to the ORR by the coverage of Pt/CB with silica layers was small. Thus, highly durable SiO2 /Pt/CB (A5) has high activity for the ORR, despite the coverage of the Pt metal particles with silica layers. The catalytic activity of SiO2 /Pt/CB (A5) for the ORR was also examined using a PEFC single cell. Pt/CB and SiO2 /Pt/CB (A5) were used as cathode catalysts. The durability of these Pt cathode catalysts was also evaluated using the single cell. The cell voltage of the single cells was repeatedly changed between 0.6 and 0.9 V. Fig. 9 shows polarization curves for the single cells with Pt/CB or SiO2 /Pt/CB (A5) cathode catalysts before the durability tests. The catalytic activity of fresh Pt/CB was slightly higher than that of SiO2 /Pt/CB (A5). This result is consistent with the results obtained using the RDE, as shown in Fig. 8. Polarization curves for the single cells with Pt/CB or SiO2 /Pt/CB (A5) cathode catalysts during the durability tests are shown in Fig. 10. The fresh Pt/CB catalysts showed high activity for the ORR, but its activity seriously decreased with the number of potential cycles. The deactivation
Fig. 7. Particle size distribution of Pt metal in Pt/CB (a), SiO2 /Pt/CB (A3) (b) and SiO2 /Pt/CB (A5) (c) before and after the potential cycling experiments.
thinner silica layers. In contrast, most Pt metal particles in SiO2 /Pt/CB (A5) were covered with silica layers of ca. 3 nm thickness. The silica layers that were wrapped around the Pt metal particles prevented the migration of Pt metal particles as well as the diffusion of dissolved Pt species from the Pt metal out of the silica layers. Thus, SiO2 /Pt/CB (A5) showed high durability during the potential cycling experiment. 3.2. ORR activity of the SiO2 /Pt/CB catalysts As described earlier, SiO2 /Pt/CB (A5) electrocatalysts had higher durability than Pt/CB. The Pt metal particles in the SiO2 /Pt/CB (A5) catalysts are covered with silica layers. Oxygen molecules and protons are supplied to the Pt metal surfaces through porous silica layers that are wrapped around the Pt metal particles during the ORR on SiO2 /Pt/CB (A5). The silica layers would impede the ORR on
Fig. 9. Polarization curves for single cells with fresh Pt/CB or fresh SiO2 /Pt/CB (A5) cathode catalysts.
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during the durability tests. The catalytic activity of the fresh Pt/CB was slightly higher compared to that of fresh SiO2 /Pt/CB (A5). However, the catalytic activity of Pt/CB for the ORR decreased with the number of potential cycles. The deterioration in the catalytic activity of Pt/CB was around 60% after 20,000 cycles of potential cycling. By contrast, the decrease in the catalytic activity of SiO2 /Pt/CB (A5) during the durability test was negligible compared to that of Pt/CB. SiO2 /Pt/CB (A5) showed higher activity for the ORR after 1000 cycles of potential cycling compared to that of Pt/CB. Therefore, we conclude that SiO2 /Pt/CB (A5) is an attractive catalyst for the PEFC cathode from the viewpoint of catalytic activity and durability. Some Pt metal particles in the fresh SiO2 /Pt/CB (A5) would have been electrochemically inactive. The surfaces of the inactive Pt metal particles were covered with impurities such as carbon, which had been deposited during the coverage of Pt/CB with silica layers. The impurities on the Pt metal surface in SiO2 /Pt/CB (A5) were removed during potential cycling [33]. In addition, some Pt metal particles in the fresh SiO2 /Pt/CB (A5) would not have been in contact with the CB support although the Pt metal particles were covered with silica layers. The Pt metal particles also dissolved under severe cathode conditions to form cationic Pt species during the durability test and subsequently the dissolved Pt species were deposited on the CB surface to form Pt metal particles. The Pt metal particles that were newly deposited on CB were electrochemically active. These are reasons for the catalytic activity of SiO2 /Pt/CB (A5) being slightly improved during the durability test. 4. Conclusion
Fig. 10. Polarization curves for single cells with Pt/CB (a) or SiO2 /Pt/CB (A5) cathode catalysts (b) during the potential cycling experiments.
of Pt/CB should result from an increase in the Pt metal particle size during the durability tests. In contrast, the catalytic activity of SiO2 /Pt/CB (A5) improved slightly after 5000 cycles of potential cycling and it remained high up to 20,000 cycles. Fig. 11 shows the change in current density at a cell voltage of 0.7 V for each single cell
Pt/CB catalysts for use in a PEFC cathode were covered with silica layers by successive hydrolysis with APTES and TEOS to improve the durability of the Pt catalysts under the severe cathode conditions of the PEFC. The SiO2 /Pt/CB catalysts showed excellent durability to potential cycling experiments. In addition, the highly durable SiO2 /Pt/CB catalysts had similar activity for the ORR to Pt/CB without a silica-coating. Therefore, coverage with silica layers is effective for the improvement of the durability of Pt cathode catalysts for use in the PEFC. Acknowledgement This study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 11. Change in current density at a cell voltage of 0.7 V for single cells with Pt/CB or SiO2 /Pt/CB (A5) cathode catalysts during the potential cycling experiments.
[15]
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Highly durable Pt cathode catalysts for polymer electrolyte fuel cells; coverage of carbon black-supported Pt catalysts with silica layers Sakae Takenaka ∗ , Hiroshi Matsumori, Hideki Matsune, Masahiro Kishida Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 4 August 2011 Received in revised form 28 September 2011 Accepted 8 October 2011 Available online 14 October 2011 Keywords: Carbon black-supported Pt electrocatalysts Polymer electrolyte fuel cell Cathode catalyst durability Silica-coating
a b s t r a c t Carbon black-supported Pt (Pt/CB) catalysts that have been used at the cathode in state of the art polymer electrolyte fuel cell (PEFC) were covered with silica layers to improve the durability of the Pt catalysts under PEFC cathode conditions. The durability of silica-coated Pt/CB to potential cycling between 0.6 and 0.9 V (vs. reversible hydrogen electrode (RHE)) was strongly dependent on the thickness of the silica layers, i.e., the durability of Pt/CB improved after coverage with thick silica layers. However, the coverage of the whole surface of the Pt/CB catalysts with silica layers produced electrochemically inactive Pt catalysts. Silica-coated Pt/CB catalysts with an optimal silica layer thickness showed similar activity for the oxygen reduction reaction compared to Pt/CB catalysts without a silica coating, and they had excellent durability at the cathode in a PEFC single cell. Coverage with silica layers improved the durability of the Pt/CB cathode catalysts without a decrease in the catalytic activity for the oxygen reduction reaction. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymer electrolyte fuel cells (PEFCs) are promising alternative power sources for transportation and portable applications, because of advantages such as low emission and high energy efficiency [1,2]. Pt metal has been used as a catalytically active metal component for the hydrogen oxidation reaction (HOR) at the anode and for the oxygen reduction reaction (ORR) at the cathode in the PEFC. The sluggish rate of the ORR on Pt compared with the HOR requires more Pt loading in the catalysts at the cathode, which impedes the full commercialization of PEFC, because of the high cost of Pt [3,4]. However, it is difficult to reduce the Pt loading at the cathode in the PEFC. Pt catalysts at the cathode work under very severe conditions, such as low pH, high temperatures, oxygen atmosphere, high humidity and highly positive potentials. Thus, Pt catalysts are seriously deactivated under cathode conditions [5]. Pt metal particles at the cathode easily migrate on carbon supports and subsequently agglomerate upon collision [6]. Pt metal particles also grow through Ostwald ripening, that is, the surface Pt atoms in small Pt metal particles dissolve to form cationic Pt species and the dissolved Pt species are subsequently deposited onto large metal particles, which results in the growth of the Pt metal particle size [7–9]. The migration of Pt metal particles in the cathode catalysts can be suppressed to some extent by the modification of the chemical and physical properties of the carbon supports [10–12].
∗ Corresponding author. Tel.: +81 92 802 2752; fax: +81 92 802 2752. E-mail address: [email protected] (S. Takenaka). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.10.012
However, these methods are not effective for the suppression of Pt metal dissolution. Many research groups have reported that alloy formation between Pt and other metal species such as Co, Pd or Au also improves the durability of the catalysts under PEFC cathode conditions [13–16]. The use of Pt-based alloy catalysts in the cathode contributes to a decrease in the Pt loading at the PEFC cathode, because the alloy catalysts have higher activity and durability compared to pure Pt catalysts. However, the metal species added to the Pt catalysts also inevitably dissolve under these cathode conditions, which causes a loss in activity of the Pt-based alloy catalysts during the PEFC operation [17]. The dissolved metal species are also deposited in polymer electrolyte membranes, which results in a decrease in their proton conductivity [18]. Thus, the diffusion of metal species out of cathode catalysts should be suppressed to reduce the Pt loading at the PEFC cathode. We have previously studied the catalytic performance of carbon nanotube (CNT)-supported Pt cathode catalysts covered with silica layers [19,20]. Silica-coated Pt/CNT showed high durability under the PEFC cathode conditions, whereas Pt/CNT without a silicacoating became seriously deactivated under the same conditions. The silica layers that are wrapped around the Pt metal particles prevent the migration of Pt metal particles on the CNT supports and the diffusion of dissolved Pt species out of the silica layers. The application of CNT to the supports of the Pt cathode catalysts is attractive because the CNT has excellent properties, such as a high electronic conductivity, a high chemical stability and a high surface area. It has been reported that Pt/CNT catalysts show high activity and durability at the PEFC cathode [21–24]. However, the CNT is not easily available and is expensive compared to carbon black,
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which is a conventional support used for Pt cathode catalysts. Thus, carbon black-supported Pt (Pt/CB) catalysts should be covered with silica layers to improve the durability of the Pt catalysts under PEFC cathode conditions. In this study, Pt/CB was covered with silica layers by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and tetraethoxysilane (TEOS). Silica-coated Pt/CB catalysts with different silica loadings were prepared to determine the effect of silica layer thickness on the activity and durability of the Pt catalysts. We thus report the high activity and excellent durability of silica-coated Pt/CB electrocatalysts under PEFC cathode conditions. 2. Experimental 2.1. Catalyst preparation Pt/CB was prepared by conventional impregnation. The CB support (Vulcan XC-72, supplied by Cabot) was impregnated into an aqueous H2 PtCl6 solution and dried at 333 K in air. The obtained samples were reduced with hydrogen at 473 K for 5 h. Pt/CB was covered with silica layers by the successive hydrolysis of APTES (supplied from Tokyo Chemical Industry) and TEOS (supplied from Kanto Chemical) [25]. Pt/CB was ultrasonically dispersed in a mixed solution of water and ethanol (a volume ratio = 1/1) at 333 K and the pH of this solution was adjusted to ca. 11 by the addition of aqueous NH3 . The hydrolysis of APTES was performed at 333 K over 30 min by the addition of APTES to the solutions. Subsequently, TEOS was added to the solutions to cover the samples with thick silica layers. After the solutions were stirred for 1 h at 333 K they were filtered and washed several times with distilled water. The samples thus obtained were treated with hydrogen at 623 K for 3 h, and then in argon at 873 K for 5 h. Silica-coated Pt/CB is denoted SiO2 /Pt/CB hereafter. To control the silica loading in SiO2 /Pt/CB, the concentration of APTES and TEOS during their hydrolysis was changed while the total concentration of APTES and TEOS was 45 mM [26]. 2.2. Electrochemical measurements Electrochemical measurements were carried out using a threecompartment electrochemical cell with a Pt mesh and saturated Ag/AgCl electrode serving as the counter and reference electrodes, respectively. The saturated Ag/AgCl electrode was separated from the working electrode compartment by a closed electrolyte bridge. All potentials are given relative to the reversible hydrogen electrode (RHE). A glassy carbon disk electrode (5 mm diameter) was used as a substrate for the catalysts and polished to a mirror finish. Catalyst ink was prepared by ultrasonically blending the catalysts and methanol. An aliquot of this ink was deposited on a glassy carbon disk and dried at 333 K. 1 wt% Nafion solution diluted with methanol was dropped onto the catalysts to ensure the attachment of the catalysts to the disk. The potential of the Pt catalysts was repeatedly changed between 0.05 and 1.20 V in N2 -purged 0.5 M H2 SO4 at 303 K until reproducible cyclic voltammograms (CVs) were obtained. The reproducible CVs for all the Pt catalysts were obtained within 50 cycles of potential cycling. CVs for the Pt catalysts were measured in the potential range between 0.05 and 1.20 V at a scan rate of 50 mV s−1 . Square wave potential cycling between 0.6 and 0.9 V were performed for the Pt catalysts in N2 -purged 0.5 M H2 SO4 at 303 K for accelerated durability tests of the Pt catalysts [27,17,28]. A voltammogram for the desorption of underpotentially deposited hydrogen was used to evaluate the electrochemically active surface area (ECSA) of the Pt catalysts on the electrode. The value 210 C cm-Pt−2 was used to determine the ECSA from the adsorbed hydrogen.
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Electrochemical measurements using a rotating disk electrode (RDE) were carried out using a three-electrode electrochemical cell. Pt foil was used as a counter electrode and Ag/AgCl was used as a reference electrode. The working electrode was composed of a 6-mm diameter glassy carbon core embedded in a Teflon cylinder. Catalyst ink was prepared by ultrasonically blending Pt/CB or SiO2 /Pt/CB and ethanol. This ink was deposited on a glassy carbon disk. The amount of Pt catalyst deposited onto the glassy carbon rod was such that the Pt content was equivalent to 10 g-Pt cm−2 in all the experiments. After drying the catalysts at room temperature, the Nafion solution that was diluted with methanol was dropped onto the catalysts to ensure that the catalysts were attached to the disk. The catalysts were immersed in 0.1 M HClO4 at room temperature. The polarization curves for the ORR on the Pt catalysts were measured in O2 -saturated 0.1 M HClO4 at room temperature by changing the potential of the working electrode from 0.1 to 1.1 V at a scan rate of 10 mV s−1 with an electrode rotation rate of 1600 rpm. The catalytic activity of Pt/CB and SiO2 /Pt/CB for the ORR was examined using PEFC single cells. The membrane-electrode assembly (MEA) for a single cell (Electrochem Co., EFC-05-02) was prepared as follows: SiO2 /Pt/CB or Pt/CB electrocatalysts were used for the cathode and Pt/CB catalyst for the anode. Catalyst ink was prepared by ultrasonically mixing the catalysts, 2-propanol and diluted Nafion solution (5 wt% Nafion). The catalyst ink was painted onto the surface of wet-proofed carbon paper (Toray Co.) as a gas diffusion layer. An MEA with an area of 5 cm2 was fabricated by hot pressing the cathode and anode to Nafion 117 at 403 K and 10 MPa for 3 min. The Pt loading in both the cathode and anode was 0.1 mg-Pt cm−2 . Hydrogen (flow rate = 40 ml min−1 , P = 101.3 kPa) and oxygen (flow rate = 40 ml min−1 , P = 101.3 kPa) were supplied to the anode and cathode, respectively. The gases for the cathode and anode were humidified at 353 K before their introduction into the cells. Before obtaining polarization curves for the single cells, MEA was conditioned by the supply of hydrogen to the anode and oxygen to the cathode at a cell voltage of 0.5 V for 2 h. The cell voltage for the single cells was cycled between 0.6 and 0.9 V at 353 K as an accelerated durability test for the cathode catalysts. During the durability test, humidified hydrogen and nitrogen were supplied to the anode and cathode, respectively. Polarization curves for the single cells were also measured after the introduction of oxygen to the cathode.
3. Results and discussion 3.1. Durability of silica-coated Pt catalysts Pt/CB was covered with silica layers by the successive hydrolysis of APTES and TEOS. We have reported that carbon-supported Pt catalysts could not be uniformly covered with thick silica layers by using only APTES or only TEOS, while the successive hydrolysis of APTES and TEOS resulted in the uniform coverage of the Pt catalysts with thick silica layers [26]. During the hydrolysis of APTES in the presence of the carbon-supported Pt catalysts, APTES adsorbed on the surface of the Pt metal and graphene by an interaction between the amino groups in APTES and the Pt metal or graphene. However, the hydrolysis of APTES and the polycondensation of silica precursors that were formed from APTES did not proceed easily, because the 3-aminopropyl groups in APTES were not hydrolyzed under the present preparation conditions, due to the strong bonding between the Si and C atoms. The silica layers formed from APTES on carbonsupported Pt catalysts were thus very thin. On the other hand, silica precursors formed from TEOS did not adsorb on the surface of Pt metal and graphene during the hydrolysis of only TEOS in the presence of carbon-supported Pt catalysts. Thus, carbon-supported Pt catalysts could not be uniformly covered with silica layers by the
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Fig. 1. TEM images of Pt/CB (a and b), SiO2 /Pt/CB (A3) (c), SiO2 /Pt/CB (A5) (d and e) and SiO2 /Pt/CB (A10) (f).
hydrolysis of only TEOS. In contrast, the carbon-supported Pt catalysts could be uniformly covered with thick silica layers by the successive hydrolysis of APTES and TEOS. Thin silica layers that were formed from APTES worked as nucleation sites of silica from TEOS during the coverage of Pt catalysts with silica layers. Thick silica layers were thus formed during the subsequent hydrolysis of TEOS because TEOS was easily hydrolyzed. SiO2 /Pt/CB was prepared at an APTES concentration of 3, 5 or 10 mM with a total concentration of APTES and TEOS of 45 mM. The SiO2 /Pt/CB prepared at APTES concentrations of 3, 5 and 10 mM were denoted SiO2 /Pt/CB (A3), SiO2 /Pt/CB (A5) and SiO2 /Pt/CB (A10), respectively. Table 1 shows the loading of Pt, SiO2 and CB in the Pt catalysts prepared in this study. The Pt loading for Pt/CB was estimated to be 12 wt% by thermogravimetric analysis in air. The SiO2 loading in SiO2 /Pt/CB was increased from 12.5 to 27.9 wt%
by increasing the APTES concentration from 3 to 5 mM. However, a further increase in the concentration of APTES to 10 mM did not change the SiO2 loading appreciably. As the APTES concentration increased from 3 to 5 mM, the Pt/CB surfaces that were covered with
Table 1 Content of SiO2 , Pt and CB in Pt/CB and SiO2 /Pt/CB. Catalyst
APTES concentration (mM)
TEOS concentration (mM)
Content (wt%) SiO2
Pt
CB
Pt/CB SiO2 /Pt/CB (A3) SiO2 /Pt/CB (A5) SiO2 /Pt/CB (A10)
– 3 5 10
– 42 40 35
0 12.5 27.9 25.6
12.0 11.4 7.6 10.1
88.0 76.1 64.5 64.3
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thin silica layers from APTES became larger. Thus, a large amount of silica was deposited onto Pt/CB during the hydrolysis of TEOS, when the APTES concentration was increased from 3 to 5 mM. A further increase in the concentration of APTES to 10 mM resulted in a decrease in the concentration of TEOS. Therefore, the silica loading in SiO2 /Pt/CB does not increase by an increase in the concentration of APTES from 5 to 10 mM. Fig. 1 shows transmission electron microscope (TEM) images of the Pt/CB and SiO2 /Pt/CB catalysts. In the TEM images of Pt/CB (a and b), Pt metal particles are present on the CB supports. The size of the Pt metal particles in the catalysts ranges from 1 to 4 nm. The Pt/CB was covered with silica layers. Pt metal particles are also observed in the TEM image of SiO2 /Pt/CB (A3) and the size of the Pt metal particles is similar to that in Pt/CB. However, silica layers were not found in the TEM image of SiO2 /Pt/CB (A3). It should be noted that the SiO2 /Pt/CB was reduced with hydrogen at 623 K and subsequently treated at 873 K in Ar. We have reported that the durability of silica-coated Pt/CNT catalysts under the PEFC cathode conditions was improved by the treatment of the catalysts at higher temperatures in Ar [29]. The treatment of the silica-coated Pt catalysts at 873 K in Ar resulted in the formation of dense silica layers, which prevented the diffusion of dissolved Pt species out of silica layers. Therefore, the SiO2 /Pt/CB used in the present study was treated at 873 K in Ar. Aggregated Pt metal particles are scarce in the TEM image of SiO2 /Pt/CB (A3) although the catalysts were treated in Ar at 873 K. We have previously reported that the Pt metal particles in Pt/CB easily aggregate during treatment at high temperatures [30]. This result implies that the Pt metal particles in SiO2 /Pt/CB (A3) are covered with silica layers. The silica layers that are wrapped around the Pt metal particles inhibit the migration of Pt metal particles on the CB support during the treatment of the catalysts at 873 K. Pt metal particles were also observed in the TEM images of SiO2 /Pt/CB (A5) and SiO2 /Pt/CB (A10) and the size of the Pt metal particles ranged from 1 to 4 nm. The Pt metal particles and
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CB in SiO2 /Pt/CB (A5) seem to be covered with silica layers a few nanometers thick, as shown in the TEM image (e). All the SiO2 /Pt/CB catalysts were calcined in air at 923 K to determine whether these catalysts were covered with silica layers. During the calcination of the catalysts in air, CB was completely removed by combustion. SiO2 /Pt/CB catalysts for the evaluation of the catalytic activity and durability were not calcined in air. TEM images of the SiO2 /Pt/CB calcined at 923 K are shown in Fig. 2. In the TEM images for SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) that were obtained after calcination, many aggregated Pt metal particles were supported on silica. The Pt metal particles were aggregated during the calcination of SiO2 /Pt/CB. The shape of silica in calcined SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) was not uniform. In contrast, many silica particles with hollow structures were found in the TEM image of calcined SiO2 /Pt/CB (A10). The size of the hollow silica particles is similar to the size of the CB supports. These results indicated that whole surface of SiO2 /Pt/CB (A10) was uniformly covered with thick silica layers before calcination, whereas the SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) before calcination had exposed surfaces of CB and Pt metal particles. It is likely that the size of Pt metal particles in the calcined SiO2 /Pt/CB (A10) was smaller than that in the calcined SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5), because whole surface of SiO2 /Pt/CB (A10) was covered with thick silica layers and the Pt metal particles were enclosed in hollow silica particles during calcination in air. It should be noted that the silica loading in SiO2 /Pt/CB (A5) is almost the same as that in SiO2 /Pt/CB (A10). When Pt/CB was covered with silica layers using an APTES concentration of 10 mM the whole surface of Pt/CB was found to be covered with thin silica layers that formed from APTES and, subsequently, silica precursors from TEOS were deposited onto the silica layers from APTES. By contrast, an APTES concentration of 5 mM was not high enough to uniformly cover the Pt/CB surfaces with thin silica layers. Therefore, SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) had exposed CB surfaces.
Fig. 2. TEM images of SiO2 /Pt/CB (A3) (a), SiO2 /Pt/CB (A5) (b) and SiO2 /Pt/CB (A10) (c) after calcination in air at 923 K.
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Fig. 3. CVs of fresh Pt/CB and SiO2 /Pt/CB catalysts in N2 -purged 0.5 M H2 SO4 at 303 K.
Fig. 3 shows CVs for Pt/CB and SiO2 /Pt/CB in N2 -purged 0.5 M H2 SO4 . Two peak couples are present in the CV for the Pt/CB catalyst. One peak couple is in the range of 0.05–0.3 V and is assignable to the adsorption and desorption of hydrogen on Pt metal, and the other couple is in the range of 0.6–1.2 V and comes from the oxidation and reduction of Pt metal. These peak couples were also observed in the CVs of SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5). These SiO2 /Pt/CB catalysts are thus electrochemically active. We have previously studied the catalytic performance of silica-coated Pt/CNT and silica-coated Pd/CNT [19,31,32]. These catalysts show high activity in electrochemical reactions such as the ORR despite the coverage of the metal particles with silica insulators. Reactants such as oxygen and protons are supplied to the metal surfaces through the porous silica layers. Electrons are transferred to the metal through the exposed CNT surfaces. Therefore, the silicacoated Pt and Pd catalysts showed high catalytic activity during the electrochemical reactions. The peak current due to the Pt metal in SiO2 /Pt/CB (A5) was lower than that in SiO2 /Pt/CB (A3) because of the higher silica loading. In contrast, peak couples due to the Pt metal were not present in the CV of SiO2 /Pt/CB (A10) although the silica loading for SiO2 /Pt/CB (A10) was similar to that for SiO2 /Pt/CB (A5). As described earlier, the whole surface of SiO2 /Pt/CB (A10) was uniformly covered with silica layers while SiO2 /Pt/CB (A5) had exposed CB surfaces. Electrons are not supplied to the Pt metal in the electrochemical reaction on SiO2 /Pt/CB (A10) because the catalysts do not have exposed CB surfaces. Thus, SiO2 /Pt/CB (A10) is electrochemically inactive. The durability of Pt/CB, SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) was examined by potential cycling experiments between 0.6 and 0.9 V in N2 -purged 0.5 M H2 SO4 . CVs for Pt/CB, SiO2 /Pt/CB (A3) and SiO2 /Pt/CB (A5) that were obtained during the durability tests are shown in Fig. 4. Two peak couples due to Pt metal are clearly present in the CV of the fresh Pt/CB catalysts, but the peak currents gradually decrease with the number of potential cycles [27,17,28]. The Pt/CB catalysts were deactivated by potential cycling experiments. The peak currents in the CVs for the SiO2 /Pt/CB (A3) also decreased gradually as the number of the potential cycles increased. In contrast, the peak currents for the SiO2 /Pt/CB (A5) increased slightly up to 3000 cycles, and then remained constant during the potential cycling experiments, suggesting that the coverage of Pt/CB with thicker silica layers improved the durability of the Pt metal particles. The change in ECSAs for these Pt catalysts during the durability tests was evaluated based on the CVs. Fig. 5 shows the results. The
Fig. 4. CVs of Pt/CB (a), SiO2 /Pt/CB (A3) (b) and SiO2 /Pt/CB (A5) (c) during the potential cycling experiments in N2 -purged 0.5 M H2 SO4 at 303 K.
ECSA for the fresh Pt/CB was determined to be 65 m2 g-Pt−1 . The ECSA for the Pt/CB catalysts appreciably decreased during the early period of the durability test, and then it decreased slowly with the number of potential cycles. The ECSA of Pt/CB decreased slightly upon coverage with silica layers. The ECSA for the fresh SiO2 /Pt/CB (A3) was determined to be 53 m2 g-Pt−1 and it decreased with the number of potential cycles, which is similar to the ECSA change of Pt/CB. The ECSA for fresh SiO2 /Pt/CB (A5) was 38 m2 g-Pt−1 , which is smaller value than the ECSAs for the fresh Pt/CB and SiO2 /Pt/CB (A3). However, the ECSA for SiO2 /Pt/CB (A5) increased to 42 m2 gPt−1 during the early period of the durability test and it remained
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Fig. 5. Change in ECSAs for Pt/CB and SiO2 /Pt/CB during the potential cycling experiments.
high during the potential cycling experiment. It should be noted that the ECSA for SiO2 /Pt/CB (A5) was higher after 20,000 cycles of potential cycling compared to that of Pt/CB. These results indicate that the coverage of Pt/CB with silica layers improves the durability of the Pt metal particles. The durability of SiO2 /Pt/CB strongly depends on the silica loading in the catalysts. SiO2 /Pt/CB (A5) has excellent durability, because the Pt metal particles in the catalysts are covered with thicker silica layers. The thickness of silica layers in SiO2 /Pt/CB (A5) was roughly estimated to be ca. 3 nm from the TEM image. TEM images of the Pt catalysts after the durability tests were measured to determine the change in morphology of the Pt metal particles during the durability tests. The results are shown in Fig. 6.
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Pt metal particles are present in the TEM image of the used Pt/CB catalysts. The Pt metal particles in the used Pt/CB are slightly larger than the particles in fresh Pt/CB, as shown in Fig. 1. Pt metal particles are also present in the TEM images for both the SiO2 /Pt/CB catalysts after the durability tests. The Pt metal particles in the used SiO2 /Pt/CB are smaller than the Pt particles in the used Pt/CB although the Pt metal particle sizes in all the Pt catalysts before the durability tests are similar to one another. Based on the TEM images shown in Figs. 1 and 6, the distribution of the Pt metal particle size was determined. The results are shown in Fig. 7. The size of Pt metal particles ranged from 1 to 4 nm in all fresh Pt catalysts. The distribution of Pt metal particle sizes in Pt/CB shifted toward larger sizes after the potential cycling. The average size of the Pt metal particles in fresh and used Pt/CB was determined to be 2.0 and 3.5 nm, respectively. The decrease in the ECSA for the Pt/CB catalysts during the durability tests thus resulted from the increase in the Pt metal particle size. The size of Pt metal particles in the Pt/CB increased by the migration and agglomeration of Pt metal particles and/or the dissolution and redeposition of Pt metal. The size distribution of the Pt metal particles in SiO2 /Pt/CB (A3) also shifted toward larger sizes during the durability tests. The average Pt metal particle size in SiO2 /Pt/CB (A3) increased from 2.4 to 3.7 nm during the durability test. The average particle size of the Pt metal in the used SiO2 /Pt/CB (A3) was almost the same as that in the used Pt/CB. In contrast, the increase in the Pt particle size for SiO2 /Pt/CB (A5) during the durability test was negligible compared to that for Pt/CB and SiO2 /Pt/CB (A3). Pt metal particles larger than 5 nm were seldom present in the used SiO2 /Pt/CB (A5), while some Pt metal particles in Pt/CB and SiO2 /Pt/CB (A3) were aggregated to the particles larger than 5 nm during the durability test. The used SiO2 /Pt/CB (A5) had the smallest average particle size (2.6 nm) among the all the Pt catalysts after the durability test. The silica loading of SiO2 /Pt/CB (A5) was higher than that of SiO2 /Pt/CB (A3). Many Pt metal particles in SiO2 /Pt/CB (A3) were not covered with silica layers or were covered with
Fig. 6. TEM images of Pt/CB (a), SiO2 /Pt/CB (A3) (b) and SiO2 /Pt/CB (A5) (c) after the potential cycling experiments.
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Fig. 8. Polarization curves obtained with RDE for the ORR on Pt/CB and SiO2 /Pt/CB (A5) at room temperature.
SiO2 /Pt/CB (A5). Thus, the catalytic activity of Pt/CB and SiO2 /Pt/CB (A5) for the ORR was examined using a RDE. Fig. 8 shows polarization curves of the ORR on Pt/CB and SiO2 /Pt/CB (A5). The current due to the ORR on Pt/CB significantly increased at less than 1 V. Interestingly, SiO2 /Pt/CB (A5) also showed catalytic activity for the ORR at less than 1 V. The decrease in the current due to the ORR by the coverage of Pt/CB with silica layers was small. Thus, highly durable SiO2 /Pt/CB (A5) has high activity for the ORR, despite the coverage of the Pt metal particles with silica layers. The catalytic activity of SiO2 /Pt/CB (A5) for the ORR was also examined using a PEFC single cell. Pt/CB and SiO2 /Pt/CB (A5) were used as cathode catalysts. The durability of these Pt cathode catalysts was also evaluated using the single cell. The cell voltage of the single cells was repeatedly changed between 0.6 and 0.9 V. Fig. 9 shows polarization curves for the single cells with Pt/CB or SiO2 /Pt/CB (A5) cathode catalysts before the durability tests. The catalytic activity of fresh Pt/CB was slightly higher than that of SiO2 /Pt/CB (A5). This result is consistent with the results obtained using the RDE, as shown in Fig. 8. Polarization curves for the single cells with Pt/CB or SiO2 /Pt/CB (A5) cathode catalysts during the durability tests are shown in Fig. 10. The fresh Pt/CB catalysts showed high activity for the ORR, but its activity seriously decreased with the number of potential cycles. The deactivation
Fig. 7. Particle size distribution of Pt metal in Pt/CB (a), SiO2 /Pt/CB (A3) (b) and SiO2 /Pt/CB (A5) (c) before and after the potential cycling experiments.
thinner silica layers. In contrast, most Pt metal particles in SiO2 /Pt/CB (A5) were covered with silica layers of ca. 3 nm thickness. The silica layers that were wrapped around the Pt metal particles prevented the migration of Pt metal particles as well as the diffusion of dissolved Pt species from the Pt metal out of the silica layers. Thus, SiO2 /Pt/CB (A5) showed high durability during the potential cycling experiment. 3.2. ORR activity of the SiO2 /Pt/CB catalysts As described earlier, SiO2 /Pt/CB (A5) electrocatalysts had higher durability than Pt/CB. The Pt metal particles in the SiO2 /Pt/CB (A5) catalysts are covered with silica layers. Oxygen molecules and protons are supplied to the Pt metal surfaces through porous silica layers that are wrapped around the Pt metal particles during the ORR on SiO2 /Pt/CB (A5). The silica layers would impede the ORR on
Fig. 9. Polarization curves for single cells with fresh Pt/CB or fresh SiO2 /Pt/CB (A5) cathode catalysts.
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during the durability tests. The catalytic activity of the fresh Pt/CB was slightly higher compared to that of fresh SiO2 /Pt/CB (A5). However, the catalytic activity of Pt/CB for the ORR decreased with the number of potential cycles. The deterioration in the catalytic activity of Pt/CB was around 60% after 20,000 cycles of potential cycling. By contrast, the decrease in the catalytic activity of SiO2 /Pt/CB (A5) during the durability test was negligible compared to that of Pt/CB. SiO2 /Pt/CB (A5) showed higher activity for the ORR after 1000 cycles of potential cycling compared to that of Pt/CB. Therefore, we conclude that SiO2 /Pt/CB (A5) is an attractive catalyst for the PEFC cathode from the viewpoint of catalytic activity and durability. Some Pt metal particles in the fresh SiO2 /Pt/CB (A5) would have been electrochemically inactive. The surfaces of the inactive Pt metal particles were covered with impurities such as carbon, which had been deposited during the coverage of Pt/CB with silica layers. The impurities on the Pt metal surface in SiO2 /Pt/CB (A5) were removed during potential cycling [33]. In addition, some Pt metal particles in the fresh SiO2 /Pt/CB (A5) would not have been in contact with the CB support although the Pt metal particles were covered with silica layers. The Pt metal particles also dissolved under severe cathode conditions to form cationic Pt species during the durability test and subsequently the dissolved Pt species were deposited on the CB surface to form Pt metal particles. The Pt metal particles that were newly deposited on CB were electrochemically active. These are reasons for the catalytic activity of SiO2 /Pt/CB (A5) being slightly improved during the durability test. 4. Conclusion
Fig. 10. Polarization curves for single cells with Pt/CB (a) or SiO2 /Pt/CB (A5) cathode catalysts (b) during the potential cycling experiments.
of Pt/CB should result from an increase in the Pt metal particle size during the durability tests. In contrast, the catalytic activity of SiO2 /Pt/CB (A5) improved slightly after 5000 cycles of potential cycling and it remained high up to 20,000 cycles. Fig. 11 shows the change in current density at a cell voltage of 0.7 V for each single cell
Pt/CB catalysts for use in a PEFC cathode were covered with silica layers by successive hydrolysis with APTES and TEOS to improve the durability of the Pt catalysts under the severe cathode conditions of the PEFC. The SiO2 /Pt/CB catalysts showed excellent durability to potential cycling experiments. In addition, the highly durable SiO2 /Pt/CB catalysts had similar activity for the ORR to Pt/CB without a silica-coating. Therefore, coverage with silica layers is effective for the improvement of the durability of Pt cathode catalysts for use in the PEFC. Acknowledgement This study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] Fig. 11. Change in current density at a cell voltage of 0.7 V for single cells with Pt/CB or SiO2 /Pt/CB (A5) cathode catalysts during the potential cycling experiments.
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