Electrochemical Performance of Pt-Based Catalysts Supported on

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Electrochemical Performance of Pt-Based Catalysts Supported on Different Ordered Mesoporous Carbons (Pt/OMCs) for Oxygen Reduction Reaction Juqin Zeng, Carlotta Francia, Mihaela A. Dumitrescu, Alessandro H. A. Monteverde Videla, Vijaykumar S. Ijeri, Stefania Specchia,* and Paolo Spinelli Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy ABSTRACT: The present work aims to compare and evaluate the behavior of platinum catalysts supported on different ordered mesoporous carbons (Pt/OMCs). OMCs were templated from two kinds of ordered mesoporous silicas (2-D SBA-15 and 3-D SBA16), with sucrose as carbon precursor and boric acid as a pore expanding agent. Structural characterizations revealed that all OMCs inherited the structure from silica templates. OMC synthesized from a mixture of sucrose and boric acid exhibited a larger pore size. According to XRD, TEM, and XPS analysis, particle size and distribution of Pt were influenced by the porosity of OMC supports. In single PEMFC tests, catalysts supported on OMC with a 3-D pore structure (Pt/C16sucr: 0.57 W cm2 at 0.45 V) or with 2-D larger pores (Pt/C15sucr+B: 0.44 W cm2 at 0.45 V) significantly enhanced the FC performance compared to Pt/C15sucr (0.29 W cm2 at 0.45 V), which may be attributed to a better mass transfer and a higher catalytic activity. In particular, it was found that Pt/C16sucr had a catalytic behavior superior to that of the Pt/Vulcan commercial catalyst in the high potential region, suggesting that C16sucr is a promising catalyst support for PEMFC application.

1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have been extensively developed because of their high conversion efficiency, low pollution, light weight, and high power density. However, one of the major barriers to the commercialization of PEMFCs is the high overall manufacturing cost, in which catalysts account for 30% because of the requirement of platinum-based catalysts for both anode and cathode in the FCs at present.1,2 To achieve the imperative cost reduction, a lot of work has been done to enhance Pt utilization efficiency by developing Pt catalysts with small particle size and high surface area. A three-dimensional (3-D) dendritic Pt nanostructure designed by Wang et al.3 showed higher surface area with 6-nm Pt nanoparticles and bulk Pt. Ordered Pt mesostructure with large uniform pores and high surface area was synthesized by Wiesner et al.,4 showing activities toward hydrogen oxidation reaction nearly identical to that of bulk Pt. Nowadays, a porous support is usually used to disperse the active metals, leading to an impressive increase of the active surface area. Carbon materials are usually the preferred catalyst supports due to their high specific surface area, flexible porous structure, and high conductivity as well as low cost.1 Platinum supported on carbon is generally used as electrocatalyst for PEMFC cathodes, having a higher catalytic activity for oxygen reduction reaction than any of the pure metals.5 High specific surface area of the carbon ensures a good dispersion of metal particles. Proper pore structure of carbon support inside the FC electrodes assures a good contact among the electronic conductors (carbon and metal), the ionic conductors (polymer electrolyte), and the reactants.6 Many carbon materials have been developed and tested in FCs, namely carbon gels, carbon nanotubes, nanohorns, nanocoils, activated carbon fibers, and ordered mesoporous carbons r 2011 American Chemical Society

(OMCs); the latter have been recently considered attractive for FC catalyst preparation, due to their periodic and uniform mesopores, high surface areas, tunable pore size, and large pore volumes.710 Since Joo et al.11 first introduced Pt/OMC catalysts into FC applications by using SBA-15 as a template and furfuryl alcohol as a carbon precursor in 2001, many efforts have been directed to improve Pt/OMC catalysts by controlling the morphology and pore size of the OMCs.1214 Particularly, OMC materials with large pore sizes are highly desirable for improving mass transfer in FCs.15 Great efforts have been made to control the pore size of OMC materials, and most of the trials reported were based on the design of silica template.16,17 But in general, the pore size control of OMC materials, based on the design of a mesoporous silica template, has distinct limitations due to the difficult control of the properties of the silica framework.17 To overcome this challenge, some other ways were developed, e.g., Jin et al.18 prepared OMCs with a pore size between 4.7 and 6.8 nm by mixing P123 to F127 triblock copolymer; Joo et al.19 reported mesoporous carbons with tunable pore size using commercial silica particles and a formaldehyderesorcinol resin as a template and carbon precursor, respectively; Chai et al.20 synthesized ordered uniform porous carbons with pore sizes in the range of 10100 nm against removable colloidal silica crystalline templates by carbonization of phenol and formaldehyde as a carbon precursor. These carbon materials exhibited Special Issue: Russo Issue Received: July 29, 2011 Accepted: September 30, 2011 Revised: September 30, 2011 Published: September 30, 2011 7500

dx.doi.org/10.1021/ie2016619 | Ind. Eng. Chem. Res. 2012, 51, 7500–7509

Industrial & Engineering Chemistry Research very high pore volumes of over 4 cm3 g1 and high surface areas of about 1000 m2 g1. Lee et al.21 reported for the first time on a facile synthesis strategy for the production of OMC materials with systematically tunable mesopore size in the range of 310 nm, where boric acid is used as a pore expanding agent. At the same time, porous materials with 3-D interconnected ordered structures are also technologically important.22 For some of the potential uses of mesoporous carbons, such as those related to catalysis, adsorption, or double-layer electrical capacitors, the diffusion rate needs to be enhanced. In this case, materials with a 3-D pore arrangement are more appropriate than materials containing well ordered 2-D pore structures.23 Kim et al.24 reported a 3-D interconnected ordered porous carbon supported PtRu alloy catalyst which demonstrated much higher power density toward hydrogen oxidation than the commercial carbon black Vulcan XC-72 supported ones. The present work investigated how the structure of mesoporous carbon supports can affect the catalytic activity of Pt catalysts toward the oxygen reduction reaction (ORR) in PEMFC system. The pore size was tuned by using boric acid as an expanding agent. 2-D hexagonal p6mm SBA-15 silica and 3-D cubic Im3m SBA-16 silica were employed as templates. Three different kinds of OMCs were synthesized using hard templating method. Samples C15sucr and C15sucr+B were templated from SBA-15, with sucrose and sucrose along with boric acid as carbon precursors, respectively. Sample C16sucr was templated from SBA-16 with sucrose as the carbon precursor. Catalyst preparation was achieved by wet impregnation. Characterization by X-ray diffraction (XRD) and nitrogen adsorption revealed that the OMCs presented hexagonal (from SBA-15 template) and cage-like (from SBA-16 template) mesostructures, with BJH values ranging from 3.0 to 7.0 nm and BET specific surface areas varying from 400 to 1300 m2 g1. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to characterize the catalysts. Electrocatalytic performances of the homemade catalysts were tested by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and single PEMFC polarization curves. Pt/ Vulcan XC-72 (10 wt %, E-TEK) was also tested as a reference. The electrochemical analysis and the FC performances demonstrated that Pt electrocatalytic activity and the electrochemical kinetic parameters for the ORR were affected by the pore structure of carbon supports.

2. EXPERIMENTAL SECTION 2.1. Synthesis. 2-D hexagonal mesoporous silica SBA-15 was prepared following the synthesis procedure reported by Zhao et al.25 2-D hexagonal mesoporous silica SBA-15 was prepared using the triblock copolymer, EO20PO70EO20 (Pluronic P123, Sigma Aldrich) as the surfactant and tetraethylorthosilicate (TEOS, 98 wt %, Sigma Aldrich) as the silica source. Specifically, 5 g of P123 was dissolved into a solution containing 170 mL of distilled water and 24.7 mL of HCl (37 wt %, Sigma Aldrich) under stirring at 40 °C. Then, 12 mL of TEOS was added and the solution was stirred for 20 h at 40 °C. The mixture was placed in an oven for 24 h at 100 °C. The product was filtered, dried, washed, and calcined at 550 °C for 10 h under air in order to remove the surfactants. 3-D cage-like mesoporous silica SBA-16 was synthesized following the synthesis procedure reported by Mesa et al.,26 with

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EO106PO70EO106 (Pluronic F127, Sigma Aldrich) as the surfactant, cetyltrimethylammonium bromide (CTMABr, 99 wt %, Sigma Aldrich) as cationic cosurfactant, and tetraethylorthosilicate (TEOS, 98 wt %, Sigma Aldrich) as the silica source, except for some adjustments in the ratio of reactants. Specifically, 2.8 g of F127 and 0.325 g of CTMABr were dissolved into a solution containing 400 mL of H2O and 13 mL of HCl (37 wt %, Aldrich) under mild stirring at room temperature. Then, 10 mL of TEOS was added into the solution at room temperature under strong stirring and kept under stirring for 10 additional min. The mixture was heated at 95 °C for 40 h under quiescent conditions. The product was filtered, dried, washed, and calcined at 550 °C for 10 h under air in order to remove the surfactants. Three types of ordered mesoporous carbons were synthesized using hard templating method. The synthesis of C15sucr followed the procedure by Jun et al.27 One gram of SBA-15 was added to a solution containing 1.25 g of sucrose, 0.14 g of H2SO4, and 5 g of H2O. The mixture was kept in an oven for 6 h at 100 °C, and subsequently the temperature was increased to 160 °C and maintained for another 6 h. The heating procedure was repeated after the addition of 0.8 g of sucrose, 0.09 g of H2SO4 (9798 wt %, Sigma Aldrich), and 5 g of H2O. The carbonsilica composite was obtained after pyrolysis at 900 °C for 6 h and then it was washed in 5 wt % HF solution to remove silica template. The procedure for synthesis of C15sucr+B was the same as that for C15sucr except that a mixture of sucrose and boric acid was used as the precursor solution.21 The boric acid content was 8 mol. % [boric acid/(boric acid + sucrose)]. The synthesis of C16sucr was also similar to that of C15sucr except that 70 wt % of the sucrose solution was used for impregnating the same amount of SBA-16. Dispersion of the catalytic metal (Pt) on the OMCs was achieved by wet impregnation. A solution of chloroplatinic acid hexahydrated (H2PtCl6 3 6H2O, Sigma Aldrich) in acetone was added drop by drop to the carbon powder and stirred for 3 h. The slurry was then heated at approximately 60 °C, with continuous stirring, in order to evaporate excess acetone. The amount of acid in the solution depended on the desired platinum content in the catalyst,11 theoretically equal to 10 wt % in the present work. The sample was then treated under H2 flow at 300 °C for 2 h in order to reduce the PtCl62 ions to metallic Pt. In this work, C15sucr, C15sucr+B, and C16sucr supported catalysts were prepared and denoted as Pt/C15sucr, Pt/C15sucr+B, and Pt/C16sucr. For comparison, a standard catalyst, made of 10 wt % Pt on Vulcan carbon XC-72R was prepared, characterized, and tested, too. 2.2. Physical Characterization. The small- and wide-angle XRD patterns were collected using a Philips X-Pert MPD X-ray diffractometer equipped with a Cu Kα radiation, with 0.01° step size (20 s step time) at 40 kV and 40 mA for small-angle measurements (0.8° < 2θ < 3°), while with 0.02° step size (6 s step time) at 40 kV and 30 mA for wide-angle measurements (15° < 2θ < 85°). Nitrogen adsorption isotherms at 77 K were recorded with an ASAP 2010 C instrument (Micromeritics). The specific surface area of the samples was calculated through the BET method within the relative pressure range of 0.050.2. The pore size distributions were determined by the BJH method calibrated for cylindrical pores according to the improved KJS method. The platinum-to-carbon weight percentage in the catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a Varian Liberty 100 instrument. Prior to analysis, the samples were digested in hot concentrated HCl/HNO3 3:1 mixture. 7501

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Industrial & Engineering Chemistry Research Scanning electron microscopy (SEM) was performed by a SEM FEI Quanta Inspect LV 30 kV instrument to assess the morphology of the prepared templates and their replica. Transmission electron microscopy (TEM) was performed by a JEOL JEM 2010 operated at 200 kV. The catalyst was deposited on copper grids covered by a holey carbon film with circular holes of about 1.2-mm diameter. The Image-Pro Plus software was used to calculate the Pt particle size distribution on TEM pictures and the related statistic values (minimum/mean/ maximum particle size and standard deviation). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Physical Electronics PHI 5800 (USA) multitechnique ESCA system using monochromatic AlKα X-ray radiation with a beam size of about 400 400 μm. The survey and narrow spectra were obtained with energy of 187.85 and 23.5 eV, respectively. Binding energies calibrated with respect to C1s at 284.6 eV were accurate within (0.2 eV. The samples were placed in an ultra high vacuum chamber at 2 1010 Torr. 2.3. Electrochemical Characterization. Electrochemical measurements were carried out with an electrochemical workstation (CHI 600D series) in a three electrode cell configuration. Ag/AgCl/Cl (1 M) and Pt wire were used as the reference and the counter electrode, respectively. The working electrode was a glassy carbon (with an area of 0.076 cm2 for cyclic voltammetry and 0.126 cm2 for linear sweep voltammetry) covered with a thin layer of Nafion-impregnated catalyst and was prepared as follows: 5 mg of the catalyst was mixed with 0.1 mL of ethanol, 0.1 mL of isopropanol, and 30 μL of Nafion (5 wt.% Electrochem Inc.), followed by sonication for 30 min to form a homogeneous suspension. Than 5 μL of the ink was deposited on the glassy carbon surface and kept at room temperature for 1 h. Cyclic voltammetry (CV) was carried out in N2-saturated 0.5 M H2SO4 at room temperature. The hydrogen desorption peaks were used to quantify the electrochemical active surface area (EASA). The linear sweep voltammetry (LSV) for oxygen reduction reaction was carried out in O2-saturated 0.1 M HClO4. The rotation rate was varied between 100 and 2500 rpm. 2.4. Characterization of Pt/OMC Catalysts in Single Cell PEMFC. All the tests were carried out in 5-cm2 single PEMFCs (Electrochem Inc.) with serpentine patterns. The procedure for the membrane electrode assembly (MEA) preparation was developed in our laboratory.28,29 At the anode E-TEK 20 wt % Pt on Vulcan XC-72 was used, while the cathode contained the synthesized Pt/OMC catalysts (or E-TEK Pt 10 wt % on Vulcan XC-72). The platinum loading was fixed at 0.5 mg cm2 on both the electrodes. LT 1200-W ELAT (E-TEK) gas diffusion layers were used in the MEAs. The polymer electrolyte was Nafion 112 (DuPont delivered by Ion Power Inc.). Electrodes were hot pressed to the membrane at 80 °C, 5 bar, 3 min; relatively low temperature was employed for hot-pressing, compared to similar works available in literature,30 to avoid the breakage of the thin Nafion 112 membranes. Two hard gaskets, made of teflonized glass fibers, were placed between the MEA and the graphite plates. Sealing was checked with a gas detector and operating the cell at 2.0 atm. The pressure distribution on the MEA surface in the assembled cell configuration was checked by observing the pressure film behavior (Fujifilm, LLw, 0.52.5 MPa) interposed between the plate and the MEA.31 A good pressure distribution was obtained when the red dots on the pressure film surface were homogeneous and of the established color intensity. Tests were carried out with a purposely designed bench for single PEMFCs.32 The MEAs underwent the same conditioning

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Figure 1. XRD spectra of the calcined SBA-15 (A), SBA-16 (B), and their respective carbon replicas.

procedure in order to equilibrate the water content into the Nafion membrane. The single PEMFC was fed with humidified pure hydrogen and oxygen (100 mL min1) at the temperature of 80 °C. FC conditioning was carried out at the constant voltage of 0.5 V for 8 h at 80 °C, at atmospheric pressure. A potentiostat (AMEL MOD 7050) was used to trace the FC polarization curves. The current intensity was recorded by varying the cell voltage at a scan rate of 2 mV s1 from the OCV to 0.40.3 V, with the cell temperature of 80 °C and atmospheric pressure.

3. RESULTS AND DISCUSSION 3.1. Physical Characterization. Figure 1 shows the typical X-ray diffractograms of the SBA-15 (Figure 1A) and of the SBA-16 (Figure 1B) templates. The calcined SBA-15 XRD pattern showed three well-resolved peaks, i.e., the (100), (110), and (200) reflections associated with the p6mm hexagonal symmetry. The calcined SBA-16 was confirmed to have the Im3m cubic ordered structure for the presence of the (110), (200), and (211) peaks.33 Similar XRD patterns were observed for the carbons (Figure 1A and B), indicating that the OMCs replicated the structure of the pristine silica materials. The weakening of peaks at higher angles for the OMCs suggested a decrease of the structure order. The well-resolved peak (100) and peak (110) were found to systematically shift toward the high angle after pyrolysis, suggesting a reduced unit cell constant of the carbon replica. SEM micrographs revealed a good correspondence between the templates and their replica. Figure 2 reports as an example the obtained SBA-16 (Figure 2A) and its replica C16sucr (Figure 2B). Materials SBA-16 of crystalline type with very regular structure and polyhedral (rhomb-dodecahedral) morphology were obtained with copolymer triblock surfactant F127 and presence of CTMABr. 7502


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Figure 2. SEM micrograph of SBA-16 template (A) and of its carbon replica (B).

Figure 4. XRD spectra of the Pt/OMC catalysts.

Figure 3. N2 adsorption/desorption isotherms of the OMCs.

From N2 adsorptiondesorption isotherms analysis (Figure 3), C15sucr possessed a pore volume of 0.82 cm3 g1, a BET specific surface area of 1081 m2 g1 and an average pore size of 3.0 nm, whereas C15sucr+B had a slightly smaller pore volume of 0.72 cm3 g1, a much lower BET specific surface area of 425 m2 g1, and a larger pore size of 7.3 nm. C16sucr was found to have a pore volume of 1.19 cm3 g1, a BET specific surface area of 1325 m2 g1, and an average pore size of 3.5 nm. The BJH pore sizes for C15sucr and C16sucr calculated from the adsorption part of the isotherms were centered about 33.5 nm, whereas pores in C15sucr+B enlarged to about 7 nm due to the pore expanding effect of the boric acid. The XRD patterns of the catalysts are shown in Figure 4. All the catalysts exhibited the characteristics of a crystallite Pt face centered cubic (fcc) structure due to the presence of the Pt(111), Pt(200), Pt(220), and Pt(311) peaks located at 39.58°, 46.36°, 67.84°, and 81.20°, respectively, in the 2θ axes.3436 The first broad peak at 23.5° was attributed to the graphitic carbon support.3436 The average size of the Pt crystallites was estimated using the DebyeScherrer equation from the Pt (220) peak at 67.84°:34,35,3739 d ¼

k3λ β 3 cos 2θ

ð1Þ

where d is the average particle size (nm), k is the shape-sensitive Scherrer constant (0.9), λ is the wavelength of radiation used (0.154056 nm), β is the integral breadth (in radians) of the peak located at 2θ. The calculated average Pt particle size of Pt/ C15sucr+B (6.0 nm) was larger than that of Pt/C15sucr (4.0 nm) and Pt/C16sucr (2.8 nm).

From TEM analysis, Pt particles appeared uniformly dispersed on C15sucr (Figure 5A) and C16sucr (Figure 5C). However, Pt/ C15sucr+B displayed a broad particle size distribution (Figure 5B). The Pt particle size distribution evaluated from TEM pictures is reported in Figure 6: the mean particle sizes were almost similar for the three samples (2.2 nm for Pt/C15sucr, 2.3 nm for Pt/ C16sucr, and 2.4 nm for Pt/C15sucr+B; see statistical data reported in Figure 6), with relatively similar and small standard deviations for Pt/C15sucr and Pt/C16sucr (denoting thus a uniform Pt distribution), and a slightly higher standard deviation value for Pt/C15sucr+B, which presented some bigger Pt particles, up to 10.5 nm (clearly visible in Figure 5B). The estimated mean Pt particle sizes from TEM pictures were smaller compared to the values calculated by applying the DebyeScherrer equation, especially for the less homogeneous Pt/C15sucr+B sample (2.4 nm compared to 6.0 nm): this was not surprising considering that on XRD patterns large particles predominate over small, and consequently DebyeScherrer’s equation overestimated the Pt particle size.40 When the same synthesis procedure and Pt loading are used for the catalysts, the structure and surface area of the carbon supports become key factors which influence the Pt distribution. OMCs with higher surface area distribute the highly sparse nuclei, avoiding particle agglomeration during synthesis. 41 C15sucr and C16sucr possessed a specific surface area which was three times as large as that of C15sucr+B, thus it is reasonable for Pt/C15sucr and Pt/C16sucr to have better Pt particle distribution. Furthermore, the confining structure of mesopores in OMCs is expected to limit or inhibit the sintering of particles during the preparation of catalysts at high temperature.10 The small pore sizes of C15sucr and C16sucr thus are likely to have enhanced the dispersion of Pt particles on the carbon supports.34 In addition, the ordered monodimensional aligned channels, which were typically for OMCs templated from SBA-15, were observed for Pt/C15sucr and Pt/C15sucr+B (Figure 5A and B), indicating that the insertion of Pt did not modify the pore structures of C15sucr and C15sucr+B.29 XPS was employed to analyze the surface properties of the catalysts. The XPS spectra for all the catalysts are referred to C 1s value of 284.6 eV. The atomic Pt percentage on the surface of Pt/C15sucr was 0.5%, similar to that of Pt/C16sucr (0.6%), which suggests analogue location of the Pt particles in both catalysts. The atomic Pt percentage on the surface of Pt/C15sucr+B was found to be quite high (1.1%) and could be due to large or agglomerated Pt particles on the surface of C15sucr+B. The XPS 7503



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Figure 6. Particle size distribution evaluated from TEM micrographs (Figure 5) of Pt/C15sucr (A), Pt/C15sucr+B (B), and Pt/C16sucr (C).

Figure 5. TEM micrographs of Pt/C15sucr (A), Pt/C15sucr+B (B), and Pt/C16sucr (C).

spectrum of Pt 4f of Pt/C16sucr is shown in Figure 7. To investigate the electronic state of Pt species, the Pt 4f peak was deconvoluted into three doublets from the spin orbital splitting of the 4f7/2 and 4f5/2 states. These doublets consisted of three pairs of platinum peaks, Pt(0), Pt(II), and Pt(IV). The first pair of peaks located at around 71.5 and 74.8 eV was attributed to metallic platinum, Pt(0).42 The second pair of platinum peaks appeared about 1.01.5 eV higher than that of metallic platinum, belonging to PtO or Pt(OH)2, as reported by Kim et al.43

The last pair observed at 2.52.9 eV higher than that of metallic platinum, showing quite low intensity, was assigned to PtO2 3 xH2O or Pt(OH)4.44 By multiplying the surface atomic percentage of Pt, Pts, and the fraction of surface metallic Pt, Pt(0)s/Pts, which was calculated from the intensity of the Pt(0) peaks, the percentage of metallic state platinum on the catalysts surface, Pt(0)s, was derived and listed in Table 1. Pt/C16sucr and Pt/ C15sucr showed identical fraction of metallic state platinum on the catalysts surface, Pt(0)s, whereas Pt/C15sucr+B displayed much higher value of Pt(0)s because of the highest surface atomic percentage of Pt (1.1%) as well as the larger fraction of surface metallic Pt, Pt(0)s/Pts (0.81). The fraction of metallic state Pt on the surface provides the catalytic sites and higher values should give a positive effect on the catalyst activity.45 7504



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The platinum-to-carbon weight percentage in the prepared catalysts, determined by ICP-AES analysis, revealed a Pt content of about 11.5 wt % for all the prepared samples. 3.2. Electrochemical Characterization. Figure 8 shows the CVs for Pt/C15sucr, Pt/C15sucr+B, Pt/C16sucr, and Pt/Vulcan samples in N2-saturated 0.5 M H2SO4 at the scan rate of 10 mV s1. All catalysts exhibited similar voltammograms with small differences. Changes in peak shape, in the hydrogen adsorption/ desorption region, can be attributed to the development of different crystalline faces with the increase of the Pt particle size.10,46 The capacitive background current observed for all the catalysts is related to the specific surface area of the carbon supports. The broad peaks at around 0.34 V (vs Ag/AgCl) were attributed to the reduction/oxidation of the quinone-like groups on the OMC supports.47 The electrochemical active surface area (EASA) of Pt/OMCs was calculated by integrating the total charge corresponding to the desorption peak of hydrogen, then normalizing with scan rate, Pt loading, and the charge value of 0.21 mC cm2 for Pt surface. The theoretical specific surface area (TSSA, m2 g1) for Pt particles was also calculated from the crystalline size, using the following equation:48 TSSA ¼

6 F3d

ð2Þ

where F is the density of platinum metal (21.4 106 g m3) and d is the average diameter of the particle (m) determined by

analyzing TEM pictures (see Figure 6), which allowed estimating more accurate values compared to the DebyeScherrer equation. As listed in Table 1, the EASA values were quite lower than the corresponding TSSA ones, perhaps due to the fact that some Pt particles were buried deeply inside the small pores of OMCs and not accessible to the electrolyte. The calculated Pt utilization efficiency, as the ratio between the EASA and TSSA, were 25.0% for Pt/C15sucr, 48.1% for Pt/C15sucr+B, and 35.0% for Pt/C16sucr. Similar results were reported by Zeng et al.34 and Joo et al.12 The decrease of EASA compared to TSSA (i.e., the relatively low values of the Pt utilization efficiency) may be attributed to the blocking or anchoring of the Nafion on the surface of Pt particles and to the inaccessibility of the proton to the Pt surfaces present in the interface of the Pt crystallites and the carbon support.12,34,49 The EASA value of Pt/C15sucr+B was much higher than those of Pt/C15sucr and Pt/C16sucr, in line with the XPS results; moreover, it was comparable to that of Pt/Vulcan because of C15sucr+B larger pores (7.3 nm compared to 3.0 and 3.5 nm for C15sucr and C16sucr, respectively) and a higher amount of superficial Pt confirmed by XPS (see Table 1, Pt(0)s = 0.89%). The Pt particles present in the spaces of larger mesopores are in fact expected to expose more fraction of their surface, compared to the particles in smaller mesopores.12,40,50 The catalytic activities toward ORR of the OMCs supported Pt based catalysts were characterized using the thin film RDE technique.50 The LSV were performed for all the four catalysts at the scan rate of 5 mV s1 with various rotation rates. Figure 9A displays the linear scan voltammogram (LSV) of Pt/C16sucr and the corresponding KouteckyLevich plot at 0.55 V (vs Ag/ AgCl) (in the inset). The Pt/C16sucr catalyst exhibited limited current densities (referred to the geometrical surface of the

Figure 8. CVs for Pt/C15sucr, Pt/C15sucr+B, Pt/C16sucr, and Pt/Vulcan catalysts.

Figure 7. XPS Pt 4f spectra for Pt/C16sucr catalyst.

Table 1. Characterization of Pt/C Catalysts from XPS, CV, and ORRa XPS

TSSA 1

EASA 1

ORR 2

Pts [%]

Pt(0)s/Pts [-]

Pt(0)s [%]

[m g ]

[m g ]

ikin [mA cm ]

imass [A gPt1]

Pt/C15sucr

0.5

0.69

0.34

127.5

31.9

1.55

19.37

Pt/C15sucr+B

1.1

0.81

0.89

116.8

56.2

1.32

16.50

Pt/C16sucr

0.6

0.57

0.34

121.9

42.7

2.34

29.25

Pt/Vulcan

0.9

0.56

0.50

58.4

2.70

33.75

electrocatalyst

2

2

a

Pts: surface atomic percentage of Pt obtained from XP survey; Pt(0)s/Pts: fraction of metallic Pt obtained from the highest doublet; Pt(0)s: percentage of metallic Pt on the surface; TSSA: calculated from the crystalline size determined by TEM analysis; EASA: calculated from the hydrogen desorption peak of CV; ikin and imass: obtained from KouteckyLevich analysis at 0.55 V. 7505



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Figure 10. FC polarization curves of the prepared catalysts at 80 °C.

Figure 9. LSV of Pt/C16sucr and the corresponding KouteckyLevich plot at 0.55 V (inset) (A); KouteckyLevich plots for all the catalysts in the diffusion-controlled region at 0.20 V (B).

electrode) of 4.4, 6.8, 9.9, and 12.5 mA cm2 at 100, 400, 1000, and 2500 rpm, respectively, similar to those for standard Pt/ Vulcan and higher than those for Pt/C15sucr and Pt/C15sucr+B catalysts, with a trend as follows: Pt/C16sucr > Pt/Vulcan > Pt/ C15sucr+B > Pt/C15sucr. The increase in the value of the limiting current is associated with the increase of molecular oxygen diffusion through the electrode surface.51 The enhancement of ORR on Pt/C16sucr and Pt/C15sucr+B compared to Pt/C15sucr, therefore, could be attributed to the diffusion-favored porosity of carbon supports. In the more positive potential region, the oxygen reduction reaction is kinetically controlled and depends on the electrochemical active surface area (EASA) of the catalysts. The kinetic currents density (ikin) and mass activity (imass) of all supported Pt based catalysts for ORR were calculated from KouteckyLevich analysis at 0.55 V (vs Ag/AgCl), as listed in Table 1. Even though a lower EASA value was found for Pt/ C16sucr, the mass activity of Pt/C16sucr was higher than those of Pt/C15sucr+B and Pt/C15sucr catalysts, which could be attributed to the better distribution of Pt particles and successful utilization of Pt inside the pores of C16sucr. Therefore, Pt particles inside the small pores of C16sucr, which are not involved in the hydrogen adsorption/desorption process, contribute to oxygen reduction.51 The mass activity of Pt/C16sucr at 0.55 V was 29.25 A gPt1, which was similar to the activity of 16.6 wt % Pt/C catalysts reported by Elezovic et al.,52 and also in the range of 2045 A gPt1 for Pt/ OMCs tested in 0.1 M HClO4 at 1 mV s1.10 Another important factor for ORR is the multielectron electroreduction of oxygen, which may include a number of elementary steps and involve different reaction intermediates.53,54 The four-electron pathway is

the most interesting process for the application in fuel cells. To calculate the number of electrons involved in the ORR, Koutecky Levich plots are usually analyzed in the diffusion-controlled region, which is associated with the limiting diffusion current plateau. For the ORR, the potential range between 0.1 and 0.4 V (vs Ag/AgCl) is usually considered for such a calculation, thus avoiding unwanted effects due to hydrogen adsorption and desorption at lower potentials and Pt-oxides reduction at high potentials.55 Figure 9B shows the KouteckyLevich plots for all the catalysts at 0.2 V. The number of electrons n, for Pt/C15sucr+B and Pt/C15sucr catalysts, was calculated to be between 2 and 4, which indicates that the two-electron and four-electron pathways were simultaneously involved in the oxygen reduction, resulting in both H2O2 and H2O formation.54 The number of electrons varied for the investigated catalysts in the decreasing order: Pt/ C16sucr > Pt/Vulcan > Pt/C15sucr+B > Pt/C15sucr. 3.3. Characterization of the Pt/OMC Catalysts in the PEMFC. To investigate the behavior of the different Pt/OMC catalysts, membrane electrode assemblies (MEAs) with the synthesized samples at the cathode were prepared and tested in single 5-cm2 H2/O2 PEMFC. After 8 h of conditioning, polarization curves were traced at the cell temperature of 80 °C. Figure 10 shows the polarization curves for all the Pt/OMCs catalysts, plus the standard catalyst 10% wt. Pt/Vulcan. All the asprepared MEAs were subjected to medium-term duration tests (up to 50 h), by maintaining the PEMFCs at constant voltage of 0.5 V. During these tests, polarization curves were traced every 8 h. The recorded polarization curves versus time on stream (not reported here) were absolutely overlapping those reported in Figure 10, denoting a very stable performance of the MEAs, even if 50 h cannot be considered as a real aging test. The analysis of the polarization curves showed that the enlargement of the average pore diameter from 3.0 to 7.3 nm corresponded to a raise in the FC performance, with the power density increasing from 0.29 W cm2 (Pt/C15sucr) to 7506



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Table 2. Electrochemical Parameters of the Various Pt/ OMCs and Standard Catalyst Calculated from the Polarization Curves i0c

bO2

iη=0.3

R0

electrocatalyst

[A cm2]

[V dec1]

[mA cm2]

[Ω cm2]

Pt/C15sucr

2.7 107

0.075

1.96

0.28

Pt/C15sucr+B

1.7 106

0.084

6.04

0.15

Pt/C16sucr

1.2 106

0.075

11.91

0.16

Pt/Vulcan

5.5 107

0.070

10.72

0.07

0.44 W cm2 (Pt/C15sucr+B) at 0.45 V. Such effect can be ascribed to the enhanced accessibility of the metal toward both the reactant and the Nafion ionomer because of the bigger pore size of C15sucr+B, which increased the catalyst utilization.12,34,40 The highest power density of Pt/OMCs was recorded for the Pt/C16sucr catalyst both at 0.6 V (0.38 W cm2) and 0.45 V (0.57 W cm2), which was consistent with the previous obtained results in acidic solution, indicating that the 3-D channels of Pt/ C16sucr enhanced the FC performance with respect to the unidirectional 2-D hexagonal pore system typical of the carbons derived from SBA-15. In particular, the performance of the cell containing Pt/C16sucr was slightly higher than that containing commercial standard Pt/Vulcan in the high voltage region (above 0.8 V), whereas the Pt/C16sucr catalyst performed worse in the low voltage region (below 0.75 V). Table 2 lists the most important parameters obtained by analyzing the polarization curves of the tested MEAs according to the method reported by Spinelli et al.32 These parameters included the exchange current density (i0c), the Tafel slope (bO2) for the ORR, the current density at the overpotential |η| = 0.3 V for the ORR (iη=0.3), which has been indicated in literature as an effective parameter for evaluating the catalytic activity of the oxygen electrode, and the ohmic resistance (R0).32 The analysis clearly confirmed the results obtained by the physical and chemical characterization of the Pt/OMC catalysts. The higher i0c for ORR were observed for both Pt/C16sucr (1.2 106 A cm2) and Pt/C15sucr+B (1.7 106 A cm2), compared to the 5.5 107 A cm2 obtained with the commercial standard Pt/Vulcan catalyst, highlighting that the 3-D channel network and the larger pore size positively influenced the FC performances. However, a significant increase in the ohmic resistance R0 was also observed for these Pt/OMCs catalytic materials, leading to a worse FC performance at higher current densities. Because all the MEAs were prepared with the same Nafion membrane and tested in the same conditions (temperature, gas pressure, RH, etc.), differing only in the catalytic cathode materials, the change in the overall ohmic resistance must be attributed to a change of the ohmic resistance of the cathodic layer, which is known to be one of the important issues in using OMCs as carbon supports.

4. CONCLUSIONS Three catalysts Pt/OMCs with different support structures have been successfully synthesized, characterized, and tested in a single PEMFC. FC tests showed that the catalytic activity was notably influenced by the carbon supports. In particular, a 3-D channel network and a 2-D structure with large pore size were important features to be considered. The overall results indicated that carbon support with a large-pore structure (Pt/C15sucr+B) improved the catalytic activity by affecting both the dispersion of

the metal on the surface and the reduction of the platinum ions to metallic Pt during catalyst preparation. Best results were obtained for Pt/C16sucr, indicating that materials with a threedimensional (3-D) cubic mesostructure are excellent candidates for the Pt catalyst supports. The 3-D channel network acted as an open porous host which promoted mass transfer. The particular structure and morphology of this material proved to be important for increasing the amount of the active metal catalyst available for the electrochemical reaction. Future work will be focused on the materials that showed the best performance, by extending the investigation to other important issues such as graphitization of OMC and durability test of Pt/OMCs under operational conditions.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +39.011.0904608; fax: +39.011.0904699; e-mail: stefania. [email protected].

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Electrochemical Performance of Pt-Based Catalysts Supported on

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