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Improved thermal stability, properties, and electrocatalytic activity of sol-gel silica modified carbon supported Pt catalysts
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 1 1 1 e2 1 2 0
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Improved thermal stability, properties, and electrocatalytic activity of sol-gel silica modified carbon supported Pt catalysts Olga A. Pinchuk a, Furkan Dundar a,b, Ali Ata b, Kenneth J. Wynne a,* a
Department of Chemical and Life Science Engineering, Virginia Commonwealth University, 601 West Main Street, Richmond, VA 23284-3028, USA b Nanotechnology Research Center, Gebze Institute of Technology, Kocaeli 41420, Turkey
article info
abstract
Article history:
An improvement in the thermal properties of catalysts used in PEM type fuel cells was
Received 23 June 2011
achieved by siliceous additions. A sol-gel method was developed to deposit controlled
Received in revised form
amounts of a siliceous composition on Vulcan XC72 carbon (VC). The catalysts designated
16 September 2011
as Pt/(VCeSiO2) were obtained by reduction of H2PtCl6 in an aqueous solution with NaBH4.
Accepted 24 October 2011
A nanoscale Pt particle formation was observed on the support materials having a range
Available online 29 November 2011
2.7e5.1 nm. The thermal stability study for Pt/(VCeSiO2) catalysts demonstrated that the presence of a siliceous phase conferred an increased resistance to Pt nanoparticle
C. In addition, a decrease in low temperature mass loss was
Keywords:
agglomeration at 600
PEM
observed. Electrochemical properties evaluated by cyclic voltammetry coupled with
Fuel cell
a rotating disk electrode (RDE) showed improvements with moderate SiO2 addition. The
Catalyst
synthesized catalysts performance was as good as the performance of the control catalyst
Composite
(46 wt% Pt/VC, Tanaka). Unfortunately, fuel cell performance experiments showed an
Aerogel
unwanted hydrophillic behavior of carbon-silica composite aerogel supports at high current density values. The CeSiO2 aerogel composite catalyst support seems suitable for
Silica
low current applications. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon-supported platinum catalysts are widely used electrocatalysts in low temperature PEM fuel cells [1e5]. The stability of fuel cell materials including the catalyst and the proton exchange membrane (PEM) is critical for fuel cell durability [6]. Among efforts for performance improvement, many studies are focusing on improving catalyst thermal stability [1,2,6e11]. A commonly employed method for Pt-based nanoparticle synthesis is the reduction of chloroplatinic acid in an aqueous solution. Sodium borohydride is the usual reducing agent [1,12e19]. Other reducing agents such as hydrogen [20],
sodium citrate or citric acid [21], or formic acid have also been employed [17]. Economic considerations have generated an interest in decreasing platinum loading by achieving a higher platinum dispersion [22]. The methods using stabilizers have provided narrow particle size distributions and homogeneous catalyst dispersions [23e31]. Boenneman combined the use of stabilizers with varying thermal pretreatment protocols to optimize Pt/C catalyst performance [24]. Thermodynamically driven sintering or aggregation of Pt nanoparticles was brought into attention by this careful processing study. Pt aggregation reduces nanoparticle dispersion and decreases
* Corresponding author. Tel.: þ1 804 828 9303. E-mail address: [email protected] (K.J. Wynne). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.093
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surface area. This could be avoided by using a Pt alloy with improved resistance to sintering [7,8,10,32]. Another goal of the catalyst research is to achieve a better chemical and electrochemical durability of the carbon support (vs. Vulcan XC72) [7]. Alternative carbon-based materials have been explored including graphitized carbon [33,34], carbon with a nitrogen-doped graphitic layer [6,35], mesoporous carbon [36,37], and carbon nanotubes [38e41]. The formation of siliceous materials has been extensively studied for a broad range of applications including catalysis and catalyst modification. Usually, the sol-gel synthesis of silica is based on the hydrolysis of tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) and subsequent polycondensation [42e48]. Zeng studied Pt-SiO2/C catalysts prepared via deposition of platinum nanoparticles on colloidal silica followed by mixing this suspension with carbon [21]. Electrochemical surface area increased along with electrochemical Pt/C catalyst activity. Guo reported preparation of sol-gel silica for coating carbon nanotubes as a support for gold/platinum hybrid nanoparticles [39]. A satisfactory electrocatalytic activity was observed for the oxygen reduction reaction. Rolison reported an enhanced electrocatalytic activity for Pt/C for methanol oxidation by deposition of SiO2 on Ptcoll/C [49,50]. Similarly Takenaka reported increased durability with surface coating of carbon nanotubes as a support material for PEM fuel cell catalysts [51]. Previously, we reported a study of the thermal stability and impact of thermal pretreatment procedures for 46% Pt/Vulcan XC72 (Tanaka) fuel cell catalyst [52]. Herein, an SiO2 modified carbon catalyst support preparation method with improved thermal stability is described. This study is related to that of Rolison (SiO2 on Pt/C) but based on deposition of platinum nanoparticles on carbon already modified with a siliceous layer. Mediating carbon oxidation by Pt is important at the high Pt loadings employed herein [11]. A chloroplatinic acid solution was reduced by sodium borohydride to synthesize Pt/(VCeSiO2) and Pt/VC catalysts. The preparation does not employ any stabilizers so as to understand only the impact of SiO2 on thermal and electrochemical properties. Thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) were used to determine catalyst composition. Transmission electron microscopy (TEM) was used for examining catalyst morphology and size. The Pt particle size distribution without the catalyst support was determined by dynamic light scattering (DLS) and the results are compared with Pt nanoparticles distributed on the catalyst support examined with TEM. The study shows that the presence of a siliceous phase conferred increased resistance to Pt nanoparticle agglomeration at elevated temperatures. Preliminary catalyst electrochemical properties were evaluated with cyclic voltammetry using the rotating disk electrode (RDE) technique.
2.
Experimental
2.1.
Materials
Vulcan XC72R and 45.5% Pt/Vulcan XC72 (Pt/V, TEC10V50E) carbon were obtained from Cabot Co. and Tanaka K, Japan
respectively. Methanol (99%), ethanol (HPLC), ammonium hydroxide (assay NH3 29.5%, A.C.S.), hydrochloric acid (37%, A.C.S.), Nafion 5% solution in a mixture of lower aliphatic alcohols and water, and tetramethyl orthosilicate (TMOS, 98%) were obtained from Aldrich. Potassium phosphate (pH 7.00) and Potassium biphthalate (pH 4.00) buffer solutions, chloroplatinic acid hydrate was purchased from Fisher. Sodium borohydride and isopropanol (HPLC, 99.9%) were purchased Co. Perchloric acid, double distilled, 70% was from ECROS purchased from GRS Chemicals, Inc. All reagents were used as received without any further purification. NanoPure (NP) water was prepared with Millipore system (resistivity is higher than 18 MU cm).
2.2.
Preparation VCeSiO2
Vulcan XC72 carbon was dried for 24 h at 200 C under vacuum. The VCeSiO2 compositions were prepared in aqueous methanol. In a typical experiment, aqueous methanol (60 mL, VCH3OH:VH2O ¼ 1.5:1) was added to 1.00 g of Vulcan XC72 carbon. Ultrasonic agitation (1 h) was followed by TMOS (0.100, 0.180, or 0.360 mL) addition while stirring for 10 min. Nanopure water (40 mL) was added and the mixture was stirred at ambient temperature. The mixture was filtered after 96 h and the filter cake was washed three times with Nanopure water. The product was dried at 60e70 C at ambient pressure for 2 h and then in a vacuum oven (1e6 Torr) for 2 h at 60 C. The product was ground into powder using a mortar and pestle. The particle size of the powder was adjusted by passing through a 60 mesh sieve.
2.3.
Preparation Pt/(VCeSiO2)
Pt-base catalysts were prepared using Vulcan XC72 carbon or silica modified Vulcan XC72. Pt nanoparticles were generated by the reduction of hexachloroplatinic acid with sodium borohydride [12,13]. Three solutions were prepared for the support material, reducing agent and hexachloroplatinic acid. First, 100 ml of water was added to 50 ml H2PtCl6eH2O (3.9 mmol/L) solution to obtain a concentration of 1.3 mmol/L. Then, the pH of the solution was adjusted to 5 by adding aqueous ammonia. Isopropanol (1 mL) was also added to improve carbon wetting. Alternatively, silica modified carbon or VC carbon alone was pre-wetted by sonication for 10 min in 15 mL CH3OHeH2O (20e30% vol.). An aqueous solution of the reducing agent was prepared in parallel by adding 13 ml of water to 5.2 mg of NaBH4. The NaBH4 solution was premixed with the support material solution using a high speed mixer (SpeedMixer, FlackTek Inc., Landrum, SC) followed by an ultrasonic agitation. This mixture was cooled to 0 C prior to the reduction process. Pt nanoparticles were generated by quick introduction of the entire borohydride/support mixture to the hexachloroplatinic acid solution while stirring. The solution was vigorously stirred and ultrasonically agitated two times for few minutes. The final catalyst solution was filtered, dried in a vacuum oven at 60 C for 2 h, ground into powder and sieved as described for the support preparation procedure.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 1 1 1 e2 1 2 0
2.4.
Characterization of Pt/(VCeSiO2) catalyst
2.4.1.
Thermal analysis
Thermogravimetric analyses were performed with Q5000 TGA (TA Instruments-Waters, LLC). Platinum pans were used in all measurements. The balance gas was inert (nitrogen or argon) with a 10 mL/min purge rate. The sample gas was dry air (purge rate 90 mL/min). 2.5 K/min heating rate was used for all specimens. The sample masses were 3 1 mg. Three separate TGA runs were performed at least for each sample. The maximum temperature was selected to achieve a stable final specimen weight.
2.4.2.
Dynamic light scattering
Dynamic Light Scattering (DLS) measurements were carried out on Zetasizer Nano ZS (Malvern Instruments). A reducing agent was added alone to hexachloroplatinic acid. Typically, water (100 mL) was added to hexachloroplatinic acid (50 mL) to provide a concentration of 1.3 mmol/L. Freshly prepared aqueous NaBH4 (13 mL, 11 mmol/L) was quickly added using a needle-less syringe to hexachloroplatinic acid solution while stirring. The concentration of NaBH4 was w8e10 times higher than the concentration of platinic acid. The amount of NaBH4 was w1.4 times greater than the stoichiometric amount, which is in agreement with prior work [13,31]. At this point, 0.5 ml of the mixture was removed and diluted 5e10 times for DLS analysis. Usually, a drop of ethylene glycol was added to the diluted solution to stabilize the dispersion. DLS measurements were performed 1e2 h after preparation of the initial colloidal mixture.
2.4.3.
Transmission electron microscope measurements
The catalysts were examined with a JEOL 1010 transmission electron microscope. Suspensions of the catalyst powders were prepared by sonication in an aqueous ethanol. These suspensions were dropped onto copper grids coated with carbon. The excess solution was removed by absorbent paper, and samples were dried in air.
2.4.4.
X-ray photoelectron spectroscopy measurements
X-Ray Photoelectron Spectroscopy (XPS) measurements were carried out on the Thermo Fisher Scientific ESCALAB 250 “XRay Photoelectron spectrometer”. The instrument uses monochromatized Al Ka X-ray and low energy electron flood gun for charge neutralization. X-ray spot size for these acquisitions was on the order of 500 mm. The pressure in the analytical chamber was less than 2 108 Torr during spectral acquisition. The pass energy for survey spectra was 150 eV. The take-off angle was 90 . The data were analyzed with the Thermo Avantage Software (V4.40). The powder samples were distributed on indium foil to enable carbon-free analysis. Atomic concentrations were calculated by dividing the peak area with atomic sensitivity factor of that peak.
2.4.5.
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rotator and a glass cell (Pine Instruments) were used. The counter electrode was platinum mesh; the reference electrode was the double junction Ag/AgCl electrode (4 mol/L KCl). All data were presented with the potentials referenced to the reversible hydrogen electrode (RHE). The catalyst dispersions or inks were prepared by ultrasonically dispersing 5 or 10 mg of carbon-supported catalyst in a 5 mL of aqueous isopropanol/Nafion solution. This solution was obtained by combining 20e30 mL of isopropanol with 0.4 mL of 5% Nafion solution and diluting with water to 100 mL total volume. The catalyst dispersion (ink) was placed onto a polished glassy-carbon disk and dried in air at room temperature for thin-film RDE measurements. The catalyst loading on the disk electrode was either 14 mgPt/cm2 or 23 mgPt/cm2. The electrolyte was HClO4 (0.1 mol/L). The potential was cycled between 0.05 V and 1.2 V for 20 times at a scan rate 200 mV/s in deaerated perchloric acid to remove any residual impurities on the catalyst surface. The final cyclic voltammogram for the catalyst layer was recorded at a scan rate of 20 mV/s. The IeV curves were obtained in O2-saturated aqueous solution of HClO4 with a rotation speed of 1600 rpm and a scan rate of 20 mV/s. The current density was calculated using the geometric surface area of the glassy-carbon disk (0.196 cm2).
2.4.6.
Performance tests
The synthesized catalysts were mixed with a water, isopropanol, propanediol and Nafion solution to form a catalyst slurry. The slurry was coated on Teflon sheets with the screen printing method. A 120 mesh silk screen was used for a homogeneous coating on a 125 mm thick Teflon sheets. The sheets were weighed before and after the coating process to determine the coating mass. The coated sheets were dried at 60 C before weighing. The same catalysts were used as both anode and cathode catalysts with 0.3 mgPt/cm2 and 25 wt% Nafion loading amounts. A Nafion 212 membrane was placed in between two coatings on Teflon sheets and the sandwich was placed into a hot press for the decal transfer process. The hot press was kept constant at 130 C for 8 min at 200 psi pressure and the sandwich was removed from the hot surfaces very quickly. Finally, the coatings on Teflon sheets were peeled off and attached to the membrane. The membrane assemblies were placed in a test cell with 0.2 mm thick silicone gaskets. A microporous layer coated carbon cloth (ETEK) was used as the gas diffusion layer. A Scribner 850C fuel cell test station was used for the performance tests. The fuel cells were conditioned at 60 C with fully humidified nitrogen gas and multiple performance tests with H2 and O2 until the system was completely stabilized. The performance tests were recorded at 60 C with 0.5 l/min fully humidified hydrogen flow on the anode side and 0.5 l/min fully humidified air flow on the cathode side. Air, instead of oxygen, was used as the cathode purge gas to examine the mass transport losses clearly. The effect of catalyst support material on the MEA performance was investigated.
Electrochemical measurements
The electrocatalytic activity of the oxygen reduction reaction (ORR) was studied at ambient temperature with a rotating disk electrode (RDE) technique described by Gasteiger [2]. A PARSTAT 2263 potentiostat equipped with modulated speed
3.
Results and discussion
The preparation of platinum nanoparticle catalysts supported on carbon modified by a siliceous phase was discussed. The
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Fig. 1 e Yield of SiO2 deposition by method VCeSiO2 for various times (0.413 g TMOS/g carbon).
size analysis for Pt nanoparticles by DLS was followed by TEM analysis of Pt nanoparticles. TGA results were presented including TEM analysis of the residue after carbon combustion. Finally, preliminary RDE experiments and fuel cell performance experiments were conducted to examine the electrocatalytic activity for the selected reference and modified catalysts.
3.1.
Preparation and characterization of VCeSiO2
Silica was deposited by a sol-gel method using a tetramethyl orthosilicate (TMOS) as the SiO2 precursor [42]. An aqueous methanol was used for VCeSiO2 preparations
[42,46]. Preliminary experiments showed that factors including pH, concentration, and temperature affected the sol-gel hydrolysis and polycondensation processes. The rate of silica deposition on carbon was studied by TGA in air to optimize the method described in the Experimental Section [52,53]. TGA is a convenient method for SiO2 determination, as on thermolysis in air, carbon is oxidized to CO2 leaving SiO2 (vida infra). Fig. 1 shows that the minimum deposition time for maximum SiO2 yield is w100 h under the conditions employed. The average silica formation compared to the calculated amount was w75% so as to provide a convenient over-weekend sample. These conditions were employed for SiO2/catalyst compositions described below. Typical mass loss curves (40 C to 800 C) and derivative mass loss (DTG) curves for VCeSiO2 and Vulcan XC72 carbon are shown in Fig. 2. TGA (Fig. 2a) and DTG (Fig. 2b) data exhibit a single mass loss for the carbon combustion. The maximum for the DTG curve (Tmax) corresponding to the highest combustion rate is 635 C. It shows that the deposition of silica on carbon does not affect the temperature for carbon combustion. No additional mass loss attributable to silica was observed below 700 C. This is in agreement with TGA data for silica reported by Zhou [54]. TGA curves for VCeSiO2 samples without drying in vacuum at 60 C show a small mass loss from 130 C to 300 C that is attributed to insufficient cure of SieOH to SieO. TGA data show a single maximum on the derivative curbe. This means that samples have no additives and no OHe. The samples have a constant mass attributed to SiO2 above 700 C.
Fig. 2 e (a) TGA and (b) DTG data for VC/SiO2 for various masses of SiO2 in air; A- 11.6%, B- 5.4%, C- 2.8%, D- Vulcan XC72 carbon. Heating rate is 2.5 K minL1 (c) TGA (d) DTG data for Pt/(VCeSiO2) for various masses of SiO2 in air; A- 45.2% Pt; B44.4% Pt, 1.4% SiO2; C- 46.3% Pt, 3.0 SiO2; D- 45.5% Pt, 5.6% SiO2. Heating rate is 2.5 K minL1.
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Table 1 e XPS results. Sample type
Vulcan XC72 Purified Vulcan XC72 VulcaneSiO2 Support Pt/VulcaneSiO2, Catalyst
C
O a
Si a
Pt a
SiO2 or Pt þ SiO2 (TGA)
SiO2 a
At%
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
87.9 97.5 90.4 82.1
84.5 96.7 85.5 50.9
12.1 2.5 7.1 10.0
15.5 3.3 9.0 8.3
2.5 4.5
5.5 6.5
3.4
34.3
2.5 4.5
11.8 14.6
a
Wt%
8.2 49.6
a Calculated from At% obtained from Narrow Scan XPS Analysis.
XPS was used to assess the atomic composition of the support surface and catalyst particles. Initially, the oxygen content on VCeSiO2 was quite high. The source of this problem was Vulcan carbon, which had been opened frequently under ambient conditions. Purification was provided by heating to 200 C, under vacuum. Subsequently, the O at% concentration in Vulcan XC72 decreased to acceptable values (Table 1). The expected peaks for C, O, and Si on the VCeSiO2 support material were in agreement with the literature [55e57]. The peak positions and atom percents obtained from the narrow scans around these peak values are: 285 eV (C, 1s, 95.1), 534 eV (O, 1s, 3.9), and 109 eV (Si, 2p, 0.8). The atomic ratio of O/Si was 2.84. The high O/Si ratio could be attributed to strong hydrogen bonding of Si(OH)x and SiO2 structures with water and residual oxygen in Vulcan carbon (Table 1). Solid state 29Si NMR has shown that Si(OH) and Si(OH)2 species are retained after nominal low temperature sol-gel cure [58]. The 90 take off angle employed in XPS analysis corresponds to an analysis depth of w5 nm that should minimize C known to be present in Vulcan carbon (3.3 at%). The high O/Si atomic ratio is not critical for the subsequent materials and electro analytical work. The combination of TGA and XPS results suggest that the SiO2 concentration on the surface (11.8 wt%) may be higher than the SiO2 concentration in bulk (8.2 wt%). A similar surface concentration of silica was reported previously [51].
3.2.
Preparation and characterization of Pt/(VCeSiO2)
combustion. Compared to support materials (peak combustion rate was at w635 C), lower combustion temperatures were observed for all catalyst samples (peak combustion rate was at w450 C). The residue mass, which corresponds to Pt and SiO2, did not change above 600 C.
3.2.2.
XPS analysis
The atomic concentrations were determined from high resolution scans for C, O, Si, and Pt. The XPS spectrum for a catalyst particle is shown in Fig. 3. The results indicate that the Pt concentration on the surface (34.3 wt%) was lower than the Pt concentration measured by TGA (45 wt%), whereas the SiO2 concentration on the surface (14.0 wt%) was higher than that from TGA (8 wt%). The result can be explained by a surface morphology where SiO2 partly covers the surface.
3.2.3.
Comparison of sol and supported Pt nanoparticle size
In order to compare the size distribution of Pt-sol nanoparticles DLS was used while TEM was employed for the supported catalyst. The sol Pt nanoparticles were stabilized with ethylene glycol for DLS analysis. DLS data for a typical composition is shown in Fig. 4 along with TEM images for two catalyst samples prepared from the same Pt nanoparticles. The average particle size from DLS measurements was w7 nm while the average particle size from TEM is w5 nm. The small difference between DLS and TEM particle size measurements is attributed to hydrodynamic diameter and Brownian motions observed during DLS analysis which exceeds the particle size obtained with TEM.
Pt nanoparticles were prepared by the well established reduction of aqueous H2PtCl6 with sodium borohydride. The preparation of high-surface area Pt catalysts requires a careful attention to factors such as the concentration of metal salt, the concentration of reducing agent, water purity, temperature, and pH. Dilute solutions of H2PtCl6 were employed with 8e10 times higher molar concentrations of NaBH4 than H2PtCl6 to minimize nanoparticle size. This approach follows that of Nagao [59] who describes the preparation of bimetalic nanoparticles via 10 higher concentrations of sodium borohydride compared to the total metal concentration.
3.2.1.
Catalyst composition by TGA/DTG
Typical TGA and DTG curves for the synthesized Pt/(VCeSiO2) catalysts are shown in Fig. 2c and d. Both silica free and silica containing catalysts exhibit a one-step mass loss for carbon
Fig. 3 e XPS survey scan for a Pt/VCeSiO2 Catalyst (C5).
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Fig. 4 e Size distribution of Pt nanoparticles; (a) Size distribution in colloidal mixture from DLS before deposition on support, size distribution from TEM for (b) Pt/VC (45% Pt) from TEM; (c) Pt/(VCeSiO2) (46% Pt, 3.0% SiO2).
3.2.4.
Catalyst nanoparticle size distribution
Catalysts with carbon supports prepared with various siliceous weight percents were examined by TEM (Table 2). The platinum particle size range was 2.7e5.1 nm. TEM images for carbon with 2.5e8.2 wt% silica showed that the silica amount did not affect the platinum particle size. Fig. 5 shows two catalyst samples with two different silica concentrations (4.1 and 8.2 wt%) which have similar particle sizes (3 nm). The faint background image which could be clearly seen in Fig. 5 is attributed to the siliceous phase. This background image is more noticeable in Fig. 5b for the higher SiO2 containing catalyst (46 wt% Pt, 4.4 wt% SiO2) that was prepared with 8.2 wt% SiO2 containing support material.
3.3.
Compared with the Pt/VC catalyst slower combustion processes before and after the thermal treatment were observed for Pt/(VCeSiO2) catalyst. Furthermore, a lesser amount of low temperature mass loss was observed with the SiO2 modified catalyst. The mass loss during the thermal treatment (360 h at 180 C) was calculated as 17 wt% for the unmodified catalyst and 10 wt% for the silica modified one. Thus, the addition of aerogel SiO2 minimized carbon combustion or other low temperature mass losses of Vulcan XC72. The residue of the silica modified catalyst after the TGA analysis was examined by TEM to observe the tendency of Pt agglomeration (Fig. 7). The image shows a very limited Platinum agglomeration in the silica modified catalysts indicating
Thermal stability
Thermal stability of Pt/VC and Pt/VCeSiO2 catalysts were studied in order to develop catalyst which could stand possible local overheatings in the catalyst layer. Low temperature (w180 C) mass loss, platinum agglomeration at elevated temperatures (w650 C) and electrochemical activity before and after a thermal treatment were covered in this chapter. Pt/VC and Pt/VCeSiO2 catalysts were treated in air at 180 2 C for 360 h (15 d), followed by a TGA analysis in order to determine low temperature mass losses. TGA analyses before and after thermal treatment of the catalysts are shown in Fig. 6.
Table 2 e TEM and TGA analysis results for synthesized catalysts. Pt þ SiO2 Pt, SiO2, Size, Catalyst SiO2 (wt%) in carbon wt% (SD 0.6)a wt% wt% nm (TEM) ID C1 C2 C3 C4 C5 C6
0 5.2 0 4.1 8.2 10.3
31.0 37.6 47.1 44.9 50.7 47.5
31.0 34.2 47.1 42.6 46.3 41.5
0 3.4 0 2.4 4.4 6.0
3.4 2.7 5.1 3.3 2.8 e
a Three or more TGA runs were used for each sample. The average is reported; the standard deviation was 0.6 or less for each data set.
Fig. 5 e TEM micrographs and size distribution for representative Pt/(VCeSiO2) catalyst compositions: (a) 4.1% SiO2 in Vulcan XC72 (C4, 43% Pt, 2.4% SiO2), (b) 8.2% SiO2 in Vulcan XC72 (C5, 46% Pt, 4.4% SiO2).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 1 1 1 e2 1 2 0
Fig. 6 e TGA in air (2.5 K minL1) for C3, 46% Pt/VC (A, C) and C6, 42% Pt, 6 wt% SiO2/(VCeSiO2) (B, D): A, B, as prepared; C, D after 15 d at 180 C in air.
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electrode (RDE) technique. Two different Pt weight percent ranges were investigated, namely 31e34 and 43e46 wt%. The activities of these catalysts are shown in Table 2. The mass activity of a commercial catalyst, 46% Pt (Tanaka), which is well known [2], served as a control for the 43e47 wt% experimental compositions. The Pt/VC and Pt/ VCeSiO2 catalysts were prepared with a similar Pt weight percents to validate the performance of the catalyst preparation methods described above. In addition, Pt/VC and Pt/ VCeSiO2 catalysts with 31e34 wt% Pt were examined to assess the effect of lower Pt wt% on performance. The data for 31 wt% Pt/VC, C1 was used also as a reference in assessing the impact of silica content on the performance for the lower Pt wt% range. Interestingly, the mass activity of the two Pt/VC catalysts (C1, 31 wt% Pt and C3, 46 wt% Pt) surpassed the activity of the control (46% Pt e Tanaka). Additionally, the mass current density of C3, the 46 wt% Pt catalyst (0.13 A mg1 Pt , 0.9 V; 0.41 A mg1 Pt , 0.85 V) is higher than that for C1, the 31 wt% Pt catalyst 1 (0.10 A mg1 Pt , 0.9 V; 0.28 A mgPt , 0.85 V). It is known that the
the thermal stability improvement from the presence of silica in the structure, even after carbon combustion. Effect of thermal treatment in air (180 C, 360 h) on electrochemically active surface area (EASA) was investigated with the cyclic voltametry method for Pt/VC and Pt/VCeSiO2 catalysts. Cyclic voltametry experiment results before and after the thermal treatment are shown in Fig. 8. The EASA of Pt/VC (C3; 45% Pt) catalyst decreased significantly (from 34.35 m2/gPt to 17.21 m2/gPt) corresponding to a w50% loss in EASA. However, the EASA of Pt/VCeSiO2 (C6; 41.5% Pt, 6.0% SiO2) catalyst was minimally affected (from 28.58 m2/gPt to 20.62 m2/gPt) from the heat treatment corresponding to only w28% loss in EASA. The EASAs were calculated according to Schmidt [60].
3.4.
Evaluation of electrochemical activity
The electrochemical properties for selected catalysts were examined by cyclic voltammetry using the rotating disk
Fig. 7 e (a) TEM micrograph for as prepared Pt/(VCeSiO2) (C6, 41.5% Pt, 6.0% SiO2), and (b) after TGA to 650 C.
Fig. 8 e Cyclic voltammograms of (a) Pt/VC (C3, 46% Pt; Aas prepared, B- after pretreatment 360 h at 180 C in air) and Pt/(VCeSiO2) (C6, 41.5% Pt, 6.0% SiO2; A- as prepared, Bafter pretreatment 360 h at 180 C in air).
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SiO2 (3.4e3.6 wt%) have some of the highest mass current densities for compositions investigated. This is evident from an inspection of the voltammograms in Fig. 9 for the C2 catalyst (34% Pt, 3.4% SiO2) compared with the control catalyst (46 wt% Pt, Tanaka). The data in Table 3 show that silica containing 31e34 wt% Pt catalysts have better performance than the silica-free 31 wt% Pt catalyst. All of the silica containing catalysts had low active surface 2 areas (23e35 m2∙g1 Pt for 23 mgPt/cm loading) as a result of the insulating behavior of silica. On the other hand some of these catalysts have high mass current densities which can be explained by the structure change and utilization of the catalysts. This can be explained as SiOx sol penetration into pores of Vulcan XC72 followed by the gelation process which breaks down the carbon agglomerates [50] and improves the utilization of the catalyst. A combination of these two features results in catalysts that have high specific current densities.
Fig. 9 e Oxygen reduction current densities of Pt/(VCeSiO2) and Pt/VC; A- 34% Pt, 3.4% SiO2 (C2), B- 46% Pt, 4.4% SiO2 (C5), C- 46% Pt (Tanaka).
3.5.
oxygen reduction reaction activity has a maximum for 3e5 nm Pt nanoparticles [6]. The observed result may be explained due to the slightly higher activity of a 5.1 nm catalyst (C3, 46 wt% Pt) in comparison with a 3.4 nm catalyst (C1, 31 wt% Pt). The RDE results in Table 2 show that performance for the 43e47 wt% catalysts suffers with low silica weight percents (1.4, 2.4 wt% SiO2). However, the performance improves with moderate (4.4 wt% SiO2) silica containing catalysts. The rotating disc electrode experiment for the catalyst with moderate silica content, C5 (46.4 wt% Pt, 4.4 wt% SiO2) resulted in similar activity with the 46 wt% Pt, Tanaka control catalyst (Fig. 9). The mass current density for the C5 catalyst, 1 is 0.05 A mg1 Pt at 0.9 V and 0.15 A mgPt at 0.85 V, whereas 1 values for the control are: 0.08 A mgPt , at 0.9 V and 0.23 A mg1 Pt , at 0.85 V. As noted above, the 31e34 wt% catalysts have lower mass current densities compared to the 43e47 wt% Pt catalysts. However, the 31e34 wt% catalysts with moderate levels of
Fuel cell performance tests
The fuel cell performance test results showed an improvement in fuel cell performance with silica additions up to certain amounts. The silica free (C1) and silica containing catalysts (C2) performed similarly with 31e34 wt% Pt catalyst containing MEAs. (Fig. 10a) Similarly, MEAs prepared with 43e47 wt% Pt catalysts showed better performance with silica addition. Silica added C4 and C5 catalysts performed better than silica free Tanaka and C3 catalysts. (Fig. 10b) The performance improvement can be explained with the continuous mesoporous structure formation with aerogel silica matrix which enables effective transfer of fuel, oxidant and products [50]. Unfortunately, redundant silica addition resulted in hydrophilic behavior and increase in silica mass resulted in poor performance. Mass transport losses with silica added catalysts, were easily observed from fuel cell performance results, especially for the high silica containing C5 catalyst (46.4 wt% Pt, 4.4 wt% SiO2). Higher silica containing C6 catalyst (41.5 wt% Pt, 6.0 wt% SiO2) showed very poor performance, probably due to discontinuety in electron transfer pathway.
Table 3 e Oxygen reduction reaction activity measurement for carbon and Si modified carbon-supported Pt catalysts as well as the commercial Pt/VC Tanaka catalyst.* Catalyst ID
Concentration Pt, wt%
SiO2, wt%
C1
31
e
C2
31
3.6
C3 C4 C5
46 43 46.4
e 2.4 4.4
C6 Tanaka
41.5 46
6.0 e
Lpb mgPt cm2
Apb m2 g1 Pt
im (0.9 V), A mg1 Pt
im (0.85 V), A mg1 Pt
is (0.9 V), mA cm2 Pt
is (0.85 V), mA cm2 Pt
23 14 23 14 23 22 23 14 23 23
59 37 23 38 24 31 35 44 29 58
0.10 0.05 0.13 0.10 0.13 0.02 0.05 0.07 0.06 0.08
0.28 0.18 0.41 0.40 0.41 0.10 0.15 0.22 0.13 0.23
223 142 578 273 558 57 136 152 135 131
624 491 1827 1072 1714 307 417 498 321 399
* All results were recorded during positive-going sweeps at 20 mV/s at 1600 rpm rotation rate in 0.1 M HC1O4.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 1 1 1 e2 1 2 0
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Acknowledgment Support from the NASA Space Science Office (Grant Number NNC04GB13G) is gratefully acknowledged.
references
Fig. 10 e Fuel cell performance results for (a) 31e34 wt% Pt/ VC (C1) or Pt/(VCeSiO2) (C2) catalysts and (b) 43e47 wt% Pt/ VC (C3) or Pt/(VCeSiO2) (C4, C5) catalysts.
4.
Conclusions
A sol-gel method for depositing controlled amounts of siliceous phase on carbon Vulcan XC72 (from 2.5 to 10.3 wt% SiO2) is reported. The Pt/VC and Pt/(VCeSiO2) catalysts with a concentration of Pt from 31 to 47 wt% were obtained by reduction of aqueous H2PtCl6 with NaBH4 without the use of stabilizers. The average Pt particle size ranged from 2.7 nm to 5.1 nm. The TEM images confirmed that platinum nanoparticles have a uniform and narrow distribution on supports. Examination of the catalyst thermal stability (pretreatment at 180 C for 360 h under air environment) showed that the presence of silica decreased the low temperature mass loss. TEM and optical micrographs for the catalyst residue after carbon combustion showed that silica provides a barrier to Pt nanoparticle aggregation. Thus, the presence of silica may enhance catalyst durability. An evaluation of electrochemical performance by cyclic voltammetry showed that all silica containing catalysts have lower active surface areas; however some of them had high mass current densities for oxygen reduction reaction. The combination resulted in high specific current densities. Finally preliminary fuel cell experiments showed similar or improved performances with silica addition. However, flooding was observed with high silica containing catalysts. With the exception of catalysts with low SiO2 content, the performance (mass current density) of silica containing Pt/VC catalysts is similar to the Pt/VC catalysts. A small drop in fuel cell performance could be tolerable for the improvement in catalyst durability. The silica containing catalysts will be tested in real fuel cell conditions for validation of durability as a future work.
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Available online at www.sciencedirect.com
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Improved thermal stability, properties, and electrocatalytic activity of sol-gel silica modified carbon supported Pt catalysts Olga A. Pinchuk a, Furkan Dundar a,b, Ali Ata b, Kenneth J. Wynne a,* a
Department of Chemical and Life Science Engineering, Virginia Commonwealth University, 601 West Main Street, Richmond, VA 23284-3028, USA b Nanotechnology Research Center, Gebze Institute of Technology, Kocaeli 41420, Turkey
article info
abstract
Article history:
An improvement in the thermal properties of catalysts used in PEM type fuel cells was
Received 23 June 2011
achieved by siliceous additions. A sol-gel method was developed to deposit controlled
Received in revised form
amounts of a siliceous composition on Vulcan XC72 carbon (VC). The catalysts designated
16 September 2011
as Pt/(VCeSiO2) were obtained by reduction of H2PtCl6 in an aqueous solution with NaBH4.
Accepted 24 October 2011
A nanoscale Pt particle formation was observed on the support materials having a range
Available online 29 November 2011
2.7e5.1 nm. The thermal stability study for Pt/(VCeSiO2) catalysts demonstrated that the presence of a siliceous phase conferred an increased resistance to Pt nanoparticle
C. In addition, a decrease in low temperature mass loss was
Keywords:
agglomeration at 600
PEM
observed. Electrochemical properties evaluated by cyclic voltammetry coupled with
Fuel cell
a rotating disk electrode (RDE) showed improvements with moderate SiO2 addition. The
Catalyst
synthesized catalysts performance was as good as the performance of the control catalyst
Composite
(46 wt% Pt/VC, Tanaka). Unfortunately, fuel cell performance experiments showed an
Aerogel
unwanted hydrophillic behavior of carbon-silica composite aerogel supports at high current density values. The CeSiO2 aerogel composite catalyst support seems suitable for
Silica
low current applications. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon-supported platinum catalysts are widely used electrocatalysts in low temperature PEM fuel cells [1e5]. The stability of fuel cell materials including the catalyst and the proton exchange membrane (PEM) is critical for fuel cell durability [6]. Among efforts for performance improvement, many studies are focusing on improving catalyst thermal stability [1,2,6e11]. A commonly employed method for Pt-based nanoparticle synthesis is the reduction of chloroplatinic acid in an aqueous solution. Sodium borohydride is the usual reducing agent [1,12e19]. Other reducing agents such as hydrogen [20],
sodium citrate or citric acid [21], or formic acid have also been employed [17]. Economic considerations have generated an interest in decreasing platinum loading by achieving a higher platinum dispersion [22]. The methods using stabilizers have provided narrow particle size distributions and homogeneous catalyst dispersions [23e31]. Boenneman combined the use of stabilizers with varying thermal pretreatment protocols to optimize Pt/C catalyst performance [24]. Thermodynamically driven sintering or aggregation of Pt nanoparticles was brought into attention by this careful processing study. Pt aggregation reduces nanoparticle dispersion and decreases
* Corresponding author. Tel.: þ1 804 828 9303. E-mail address: [email protected] (K.J. Wynne). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.093
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surface area. This could be avoided by using a Pt alloy with improved resistance to sintering [7,8,10,32]. Another goal of the catalyst research is to achieve a better chemical and electrochemical durability of the carbon support (vs. Vulcan XC72) [7]. Alternative carbon-based materials have been explored including graphitized carbon [33,34], carbon with a nitrogen-doped graphitic layer [6,35], mesoporous carbon [36,37], and carbon nanotubes [38e41]. The formation of siliceous materials has been extensively studied for a broad range of applications including catalysis and catalyst modification. Usually, the sol-gel synthesis of silica is based on the hydrolysis of tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) and subsequent polycondensation [42e48]. Zeng studied Pt-SiO2/C catalysts prepared via deposition of platinum nanoparticles on colloidal silica followed by mixing this suspension with carbon [21]. Electrochemical surface area increased along with electrochemical Pt/C catalyst activity. Guo reported preparation of sol-gel silica for coating carbon nanotubes as a support for gold/platinum hybrid nanoparticles [39]. A satisfactory electrocatalytic activity was observed for the oxygen reduction reaction. Rolison reported an enhanced electrocatalytic activity for Pt/C for methanol oxidation by deposition of SiO2 on Ptcoll/C [49,50]. Similarly Takenaka reported increased durability with surface coating of carbon nanotubes as a support material for PEM fuel cell catalysts [51]. Previously, we reported a study of the thermal stability and impact of thermal pretreatment procedures for 46% Pt/Vulcan XC72 (Tanaka) fuel cell catalyst [52]. Herein, an SiO2 modified carbon catalyst support preparation method with improved thermal stability is described. This study is related to that of Rolison (SiO2 on Pt/C) but based on deposition of platinum nanoparticles on carbon already modified with a siliceous layer. Mediating carbon oxidation by Pt is important at the high Pt loadings employed herein [11]. A chloroplatinic acid solution was reduced by sodium borohydride to synthesize Pt/(VCeSiO2) and Pt/VC catalysts. The preparation does not employ any stabilizers so as to understand only the impact of SiO2 on thermal and electrochemical properties. Thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) were used to determine catalyst composition. Transmission electron microscopy (TEM) was used for examining catalyst morphology and size. The Pt particle size distribution without the catalyst support was determined by dynamic light scattering (DLS) and the results are compared with Pt nanoparticles distributed on the catalyst support examined with TEM. The study shows that the presence of a siliceous phase conferred increased resistance to Pt nanoparticle agglomeration at elevated temperatures. Preliminary catalyst electrochemical properties were evaluated with cyclic voltammetry using the rotating disk electrode (RDE) technique.
2.
Experimental
2.1.
Materials
Vulcan XC72R and 45.5% Pt/Vulcan XC72 (Pt/V, TEC10V50E) carbon were obtained from Cabot Co. and Tanaka K, Japan
respectively. Methanol (99%), ethanol (HPLC), ammonium hydroxide (assay NH3 29.5%, A.C.S.), hydrochloric acid (37%, A.C.S.), Nafion 5% solution in a mixture of lower aliphatic alcohols and water, and tetramethyl orthosilicate (TMOS, 98%) were obtained from Aldrich. Potassium phosphate (pH 7.00) and Potassium biphthalate (pH 4.00) buffer solutions, chloroplatinic acid hydrate was purchased from Fisher. Sodium borohydride and isopropanol (HPLC, 99.9%) were purchased Co. Perchloric acid, double distilled, 70% was from ECROS purchased from GRS Chemicals, Inc. All reagents were used as received without any further purification. NanoPure (NP) water was prepared with Millipore system (resistivity is higher than 18 MU cm).
2.2.
Preparation VCeSiO2
Vulcan XC72 carbon was dried for 24 h at 200 C under vacuum. The VCeSiO2 compositions were prepared in aqueous methanol. In a typical experiment, aqueous methanol (60 mL, VCH3OH:VH2O ¼ 1.5:1) was added to 1.00 g of Vulcan XC72 carbon. Ultrasonic agitation (1 h) was followed by TMOS (0.100, 0.180, or 0.360 mL) addition while stirring for 10 min. Nanopure water (40 mL) was added and the mixture was stirred at ambient temperature. The mixture was filtered after 96 h and the filter cake was washed three times with Nanopure water. The product was dried at 60e70 C at ambient pressure for 2 h and then in a vacuum oven (1e6 Torr) for 2 h at 60 C. The product was ground into powder using a mortar and pestle. The particle size of the powder was adjusted by passing through a 60 mesh sieve.
2.3.
Preparation Pt/(VCeSiO2)
Pt-base catalysts were prepared using Vulcan XC72 carbon or silica modified Vulcan XC72. Pt nanoparticles were generated by the reduction of hexachloroplatinic acid with sodium borohydride [12,13]. Three solutions were prepared for the support material, reducing agent and hexachloroplatinic acid. First, 100 ml of water was added to 50 ml H2PtCl6eH2O (3.9 mmol/L) solution to obtain a concentration of 1.3 mmol/L. Then, the pH of the solution was adjusted to 5 by adding aqueous ammonia. Isopropanol (1 mL) was also added to improve carbon wetting. Alternatively, silica modified carbon or VC carbon alone was pre-wetted by sonication for 10 min in 15 mL CH3OHeH2O (20e30% vol.). An aqueous solution of the reducing agent was prepared in parallel by adding 13 ml of water to 5.2 mg of NaBH4. The NaBH4 solution was premixed with the support material solution using a high speed mixer (SpeedMixer, FlackTek Inc., Landrum, SC) followed by an ultrasonic agitation. This mixture was cooled to 0 C prior to the reduction process. Pt nanoparticles were generated by quick introduction of the entire borohydride/support mixture to the hexachloroplatinic acid solution while stirring. The solution was vigorously stirred and ultrasonically agitated two times for few minutes. The final catalyst solution was filtered, dried in a vacuum oven at 60 C for 2 h, ground into powder and sieved as described for the support preparation procedure.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 1 1 1 e2 1 2 0
2.4.
Characterization of Pt/(VCeSiO2) catalyst
2.4.1.
Thermal analysis
Thermogravimetric analyses were performed with Q5000 TGA (TA Instruments-Waters, LLC). Platinum pans were used in all measurements. The balance gas was inert (nitrogen or argon) with a 10 mL/min purge rate. The sample gas was dry air (purge rate 90 mL/min). 2.5 K/min heating rate was used for all specimens. The sample masses were 3 1 mg. Three separate TGA runs were performed at least for each sample. The maximum temperature was selected to achieve a stable final specimen weight.
2.4.2.
Dynamic light scattering
Dynamic Light Scattering (DLS) measurements were carried out on Zetasizer Nano ZS (Malvern Instruments). A reducing agent was added alone to hexachloroplatinic acid. Typically, water (100 mL) was added to hexachloroplatinic acid (50 mL) to provide a concentration of 1.3 mmol/L. Freshly prepared aqueous NaBH4 (13 mL, 11 mmol/L) was quickly added using a needle-less syringe to hexachloroplatinic acid solution while stirring. The concentration of NaBH4 was w8e10 times higher than the concentration of platinic acid. The amount of NaBH4 was w1.4 times greater than the stoichiometric amount, which is in agreement with prior work [13,31]. At this point, 0.5 ml of the mixture was removed and diluted 5e10 times for DLS analysis. Usually, a drop of ethylene glycol was added to the diluted solution to stabilize the dispersion. DLS measurements were performed 1e2 h after preparation of the initial colloidal mixture.
2.4.3.
Transmission electron microscope measurements
The catalysts were examined with a JEOL 1010 transmission electron microscope. Suspensions of the catalyst powders were prepared by sonication in an aqueous ethanol. These suspensions were dropped onto copper grids coated with carbon. The excess solution was removed by absorbent paper, and samples were dried in air.
2.4.4.
X-ray photoelectron spectroscopy measurements
X-Ray Photoelectron Spectroscopy (XPS) measurements were carried out on the Thermo Fisher Scientific ESCALAB 250 “XRay Photoelectron spectrometer”. The instrument uses monochromatized Al Ka X-ray and low energy electron flood gun for charge neutralization. X-ray spot size for these acquisitions was on the order of 500 mm. The pressure in the analytical chamber was less than 2 108 Torr during spectral acquisition. The pass energy for survey spectra was 150 eV. The take-off angle was 90 . The data were analyzed with the Thermo Avantage Software (V4.40). The powder samples were distributed on indium foil to enable carbon-free analysis. Atomic concentrations were calculated by dividing the peak area with atomic sensitivity factor of that peak.
2.4.5.
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rotator and a glass cell (Pine Instruments) were used. The counter electrode was platinum mesh; the reference electrode was the double junction Ag/AgCl electrode (4 mol/L KCl). All data were presented with the potentials referenced to the reversible hydrogen electrode (RHE). The catalyst dispersions or inks were prepared by ultrasonically dispersing 5 or 10 mg of carbon-supported catalyst in a 5 mL of aqueous isopropanol/Nafion solution. This solution was obtained by combining 20e30 mL of isopropanol with 0.4 mL of 5% Nafion solution and diluting with water to 100 mL total volume. The catalyst dispersion (ink) was placed onto a polished glassy-carbon disk and dried in air at room temperature for thin-film RDE measurements. The catalyst loading on the disk electrode was either 14 mgPt/cm2 or 23 mgPt/cm2. The electrolyte was HClO4 (0.1 mol/L). The potential was cycled between 0.05 V and 1.2 V for 20 times at a scan rate 200 mV/s in deaerated perchloric acid to remove any residual impurities on the catalyst surface. The final cyclic voltammogram for the catalyst layer was recorded at a scan rate of 20 mV/s. The IeV curves were obtained in O2-saturated aqueous solution of HClO4 with a rotation speed of 1600 rpm and a scan rate of 20 mV/s. The current density was calculated using the geometric surface area of the glassy-carbon disk (0.196 cm2).
2.4.6.
Performance tests
The synthesized catalysts were mixed with a water, isopropanol, propanediol and Nafion solution to form a catalyst slurry. The slurry was coated on Teflon sheets with the screen printing method. A 120 mesh silk screen was used for a homogeneous coating on a 125 mm thick Teflon sheets. The sheets were weighed before and after the coating process to determine the coating mass. The coated sheets were dried at 60 C before weighing. The same catalysts were used as both anode and cathode catalysts with 0.3 mgPt/cm2 and 25 wt% Nafion loading amounts. A Nafion 212 membrane was placed in between two coatings on Teflon sheets and the sandwich was placed into a hot press for the decal transfer process. The hot press was kept constant at 130 C for 8 min at 200 psi pressure and the sandwich was removed from the hot surfaces very quickly. Finally, the coatings on Teflon sheets were peeled off and attached to the membrane. The membrane assemblies were placed in a test cell with 0.2 mm thick silicone gaskets. A microporous layer coated carbon cloth (ETEK) was used as the gas diffusion layer. A Scribner 850C fuel cell test station was used for the performance tests. The fuel cells were conditioned at 60 C with fully humidified nitrogen gas and multiple performance tests with H2 and O2 until the system was completely stabilized. The performance tests were recorded at 60 C with 0.5 l/min fully humidified hydrogen flow on the anode side and 0.5 l/min fully humidified air flow on the cathode side. Air, instead of oxygen, was used as the cathode purge gas to examine the mass transport losses clearly. The effect of catalyst support material on the MEA performance was investigated.
Electrochemical measurements
The electrocatalytic activity of the oxygen reduction reaction (ORR) was studied at ambient temperature with a rotating disk electrode (RDE) technique described by Gasteiger [2]. A PARSTAT 2263 potentiostat equipped with modulated speed
3.
Results and discussion
The preparation of platinum nanoparticle catalysts supported on carbon modified by a siliceous phase was discussed. The
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Fig. 1 e Yield of SiO2 deposition by method VCeSiO2 for various times (0.413 g TMOS/g carbon).
size analysis for Pt nanoparticles by DLS was followed by TEM analysis of Pt nanoparticles. TGA results were presented including TEM analysis of the residue after carbon combustion. Finally, preliminary RDE experiments and fuel cell performance experiments were conducted to examine the electrocatalytic activity for the selected reference and modified catalysts.
3.1.
Preparation and characterization of VCeSiO2
Silica was deposited by a sol-gel method using a tetramethyl orthosilicate (TMOS) as the SiO2 precursor [42]. An aqueous methanol was used for VCeSiO2 preparations
[42,46]. Preliminary experiments showed that factors including pH, concentration, and temperature affected the sol-gel hydrolysis and polycondensation processes. The rate of silica deposition on carbon was studied by TGA in air to optimize the method described in the Experimental Section [52,53]. TGA is a convenient method for SiO2 determination, as on thermolysis in air, carbon is oxidized to CO2 leaving SiO2 (vida infra). Fig. 1 shows that the minimum deposition time for maximum SiO2 yield is w100 h under the conditions employed. The average silica formation compared to the calculated amount was w75% so as to provide a convenient over-weekend sample. These conditions were employed for SiO2/catalyst compositions described below. Typical mass loss curves (40 C to 800 C) and derivative mass loss (DTG) curves for VCeSiO2 and Vulcan XC72 carbon are shown in Fig. 2. TGA (Fig. 2a) and DTG (Fig. 2b) data exhibit a single mass loss for the carbon combustion. The maximum for the DTG curve (Tmax) corresponding to the highest combustion rate is 635 C. It shows that the deposition of silica on carbon does not affect the temperature for carbon combustion. No additional mass loss attributable to silica was observed below 700 C. This is in agreement with TGA data for silica reported by Zhou [54]. TGA curves for VCeSiO2 samples without drying in vacuum at 60 C show a small mass loss from 130 C to 300 C that is attributed to insufficient cure of SieOH to SieO. TGA data show a single maximum on the derivative curbe. This means that samples have no additives and no OHe. The samples have a constant mass attributed to SiO2 above 700 C.
Fig. 2 e (a) TGA and (b) DTG data for VC/SiO2 for various masses of SiO2 in air; A- 11.6%, B- 5.4%, C- 2.8%, D- Vulcan XC72 carbon. Heating rate is 2.5 K minL1 (c) TGA (d) DTG data for Pt/(VCeSiO2) for various masses of SiO2 in air; A- 45.2% Pt; B44.4% Pt, 1.4% SiO2; C- 46.3% Pt, 3.0 SiO2; D- 45.5% Pt, 5.6% SiO2. Heating rate is 2.5 K minL1.
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Table 1 e XPS results. Sample type
Vulcan XC72 Purified Vulcan XC72 VulcaneSiO2 Support Pt/VulcaneSiO2, Catalyst
C
O a
Si a
Pt a
SiO2 or Pt þ SiO2 (TGA)
SiO2 a
At%
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
87.9 97.5 90.4 82.1
84.5 96.7 85.5 50.9
12.1 2.5 7.1 10.0
15.5 3.3 9.0 8.3
2.5 4.5
5.5 6.5
3.4
34.3
2.5 4.5
11.8 14.6
a
Wt%
8.2 49.6
a Calculated from At% obtained from Narrow Scan XPS Analysis.
XPS was used to assess the atomic composition of the support surface and catalyst particles. Initially, the oxygen content on VCeSiO2 was quite high. The source of this problem was Vulcan carbon, which had been opened frequently under ambient conditions. Purification was provided by heating to 200 C, under vacuum. Subsequently, the O at% concentration in Vulcan XC72 decreased to acceptable values (Table 1). The expected peaks for C, O, and Si on the VCeSiO2 support material were in agreement with the literature [55e57]. The peak positions and atom percents obtained from the narrow scans around these peak values are: 285 eV (C, 1s, 95.1), 534 eV (O, 1s, 3.9), and 109 eV (Si, 2p, 0.8). The atomic ratio of O/Si was 2.84. The high O/Si ratio could be attributed to strong hydrogen bonding of Si(OH)x and SiO2 structures with water and residual oxygen in Vulcan carbon (Table 1). Solid state 29Si NMR has shown that Si(OH) and Si(OH)2 species are retained after nominal low temperature sol-gel cure [58]. The 90 take off angle employed in XPS analysis corresponds to an analysis depth of w5 nm that should minimize C known to be present in Vulcan carbon (3.3 at%). The high O/Si atomic ratio is not critical for the subsequent materials and electro analytical work. The combination of TGA and XPS results suggest that the SiO2 concentration on the surface (11.8 wt%) may be higher than the SiO2 concentration in bulk (8.2 wt%). A similar surface concentration of silica was reported previously [51].
3.2.
Preparation and characterization of Pt/(VCeSiO2)
combustion. Compared to support materials (peak combustion rate was at w635 C), lower combustion temperatures were observed for all catalyst samples (peak combustion rate was at w450 C). The residue mass, which corresponds to Pt and SiO2, did not change above 600 C.
3.2.2.
XPS analysis
The atomic concentrations were determined from high resolution scans for C, O, Si, and Pt. The XPS spectrum for a catalyst particle is shown in Fig. 3. The results indicate that the Pt concentration on the surface (34.3 wt%) was lower than the Pt concentration measured by TGA (45 wt%), whereas the SiO2 concentration on the surface (14.0 wt%) was higher than that from TGA (8 wt%). The result can be explained by a surface morphology where SiO2 partly covers the surface.
3.2.3.
Comparison of sol and supported Pt nanoparticle size
In order to compare the size distribution of Pt-sol nanoparticles DLS was used while TEM was employed for the supported catalyst. The sol Pt nanoparticles were stabilized with ethylene glycol for DLS analysis. DLS data for a typical composition is shown in Fig. 4 along with TEM images for two catalyst samples prepared from the same Pt nanoparticles. The average particle size from DLS measurements was w7 nm while the average particle size from TEM is w5 nm. The small difference between DLS and TEM particle size measurements is attributed to hydrodynamic diameter and Brownian motions observed during DLS analysis which exceeds the particle size obtained with TEM.
Pt nanoparticles were prepared by the well established reduction of aqueous H2PtCl6 with sodium borohydride. The preparation of high-surface area Pt catalysts requires a careful attention to factors such as the concentration of metal salt, the concentration of reducing agent, water purity, temperature, and pH. Dilute solutions of H2PtCl6 were employed with 8e10 times higher molar concentrations of NaBH4 than H2PtCl6 to minimize nanoparticle size. This approach follows that of Nagao [59] who describes the preparation of bimetalic nanoparticles via 10 higher concentrations of sodium borohydride compared to the total metal concentration.
3.2.1.
Catalyst composition by TGA/DTG
Typical TGA and DTG curves for the synthesized Pt/(VCeSiO2) catalysts are shown in Fig. 2c and d. Both silica free and silica containing catalysts exhibit a one-step mass loss for carbon
Fig. 3 e XPS survey scan for a Pt/VCeSiO2 Catalyst (C5).
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Fig. 4 e Size distribution of Pt nanoparticles; (a) Size distribution in colloidal mixture from DLS before deposition on support, size distribution from TEM for (b) Pt/VC (45% Pt) from TEM; (c) Pt/(VCeSiO2) (46% Pt, 3.0% SiO2).
3.2.4.
Catalyst nanoparticle size distribution
Catalysts with carbon supports prepared with various siliceous weight percents were examined by TEM (Table 2). The platinum particle size range was 2.7e5.1 nm. TEM images for carbon with 2.5e8.2 wt% silica showed that the silica amount did not affect the platinum particle size. Fig. 5 shows two catalyst samples with two different silica concentrations (4.1 and 8.2 wt%) which have similar particle sizes (3 nm). The faint background image which could be clearly seen in Fig. 5 is attributed to the siliceous phase. This background image is more noticeable in Fig. 5b for the higher SiO2 containing catalyst (46 wt% Pt, 4.4 wt% SiO2) that was prepared with 8.2 wt% SiO2 containing support material.
3.3.
Compared with the Pt/VC catalyst slower combustion processes before and after the thermal treatment were observed for Pt/(VCeSiO2) catalyst. Furthermore, a lesser amount of low temperature mass loss was observed with the SiO2 modified catalyst. The mass loss during the thermal treatment (360 h at 180 C) was calculated as 17 wt% for the unmodified catalyst and 10 wt% for the silica modified one. Thus, the addition of aerogel SiO2 minimized carbon combustion or other low temperature mass losses of Vulcan XC72. The residue of the silica modified catalyst after the TGA analysis was examined by TEM to observe the tendency of Pt agglomeration (Fig. 7). The image shows a very limited Platinum agglomeration in the silica modified catalysts indicating
Thermal stability
Thermal stability of Pt/VC and Pt/VCeSiO2 catalysts were studied in order to develop catalyst which could stand possible local overheatings in the catalyst layer. Low temperature (w180 C) mass loss, platinum agglomeration at elevated temperatures (w650 C) and electrochemical activity before and after a thermal treatment were covered in this chapter. Pt/VC and Pt/VCeSiO2 catalysts were treated in air at 180 2 C for 360 h (15 d), followed by a TGA analysis in order to determine low temperature mass losses. TGA analyses before and after thermal treatment of the catalysts are shown in Fig. 6.
Table 2 e TEM and TGA analysis results for synthesized catalysts. Pt þ SiO2 Pt, SiO2, Size, Catalyst SiO2 (wt%) in carbon wt% (SD 0.6)a wt% wt% nm (TEM) ID C1 C2 C3 C4 C5 C6
0 5.2 0 4.1 8.2 10.3
31.0 37.6 47.1 44.9 50.7 47.5
31.0 34.2 47.1 42.6 46.3 41.5
0 3.4 0 2.4 4.4 6.0
3.4 2.7 5.1 3.3 2.8 e
a Three or more TGA runs were used for each sample. The average is reported; the standard deviation was 0.6 or less for each data set.
Fig. 5 e TEM micrographs and size distribution for representative Pt/(VCeSiO2) catalyst compositions: (a) 4.1% SiO2 in Vulcan XC72 (C4, 43% Pt, 2.4% SiO2), (b) 8.2% SiO2 in Vulcan XC72 (C5, 46% Pt, 4.4% SiO2).
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Fig. 6 e TGA in air (2.5 K minL1) for C3, 46% Pt/VC (A, C) and C6, 42% Pt, 6 wt% SiO2/(VCeSiO2) (B, D): A, B, as prepared; C, D after 15 d at 180 C in air.
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electrode (RDE) technique. Two different Pt weight percent ranges were investigated, namely 31e34 and 43e46 wt%. The activities of these catalysts are shown in Table 2. The mass activity of a commercial catalyst, 46% Pt (Tanaka), which is well known [2], served as a control for the 43e47 wt% experimental compositions. The Pt/VC and Pt/ VCeSiO2 catalysts were prepared with a similar Pt weight percents to validate the performance of the catalyst preparation methods described above. In addition, Pt/VC and Pt/ VCeSiO2 catalysts with 31e34 wt% Pt were examined to assess the effect of lower Pt wt% on performance. The data for 31 wt% Pt/VC, C1 was used also as a reference in assessing the impact of silica content on the performance for the lower Pt wt% range. Interestingly, the mass activity of the two Pt/VC catalysts (C1, 31 wt% Pt and C3, 46 wt% Pt) surpassed the activity of the control (46% Pt e Tanaka). Additionally, the mass current density of C3, the 46 wt% Pt catalyst (0.13 A mg1 Pt , 0.9 V; 0.41 A mg1 Pt , 0.85 V) is higher than that for C1, the 31 wt% Pt catalyst 1 (0.10 A mg1 Pt , 0.9 V; 0.28 A mgPt , 0.85 V). It is known that the
the thermal stability improvement from the presence of silica in the structure, even after carbon combustion. Effect of thermal treatment in air (180 C, 360 h) on electrochemically active surface area (EASA) was investigated with the cyclic voltametry method for Pt/VC and Pt/VCeSiO2 catalysts. Cyclic voltametry experiment results before and after the thermal treatment are shown in Fig. 8. The EASA of Pt/VC (C3; 45% Pt) catalyst decreased significantly (from 34.35 m2/gPt to 17.21 m2/gPt) corresponding to a w50% loss in EASA. However, the EASA of Pt/VCeSiO2 (C6; 41.5% Pt, 6.0% SiO2) catalyst was minimally affected (from 28.58 m2/gPt to 20.62 m2/gPt) from the heat treatment corresponding to only w28% loss in EASA. The EASAs were calculated according to Schmidt [60].
3.4.
Evaluation of electrochemical activity
The electrochemical properties for selected catalysts were examined by cyclic voltammetry using the rotating disk
Fig. 7 e (a) TEM micrograph for as prepared Pt/(VCeSiO2) (C6, 41.5% Pt, 6.0% SiO2), and (b) after TGA to 650 C.
Fig. 8 e Cyclic voltammograms of (a) Pt/VC (C3, 46% Pt; Aas prepared, B- after pretreatment 360 h at 180 C in air) and Pt/(VCeSiO2) (C6, 41.5% Pt, 6.0% SiO2; A- as prepared, Bafter pretreatment 360 h at 180 C in air).
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SiO2 (3.4e3.6 wt%) have some of the highest mass current densities for compositions investigated. This is evident from an inspection of the voltammograms in Fig. 9 for the C2 catalyst (34% Pt, 3.4% SiO2) compared with the control catalyst (46 wt% Pt, Tanaka). The data in Table 3 show that silica containing 31e34 wt% Pt catalysts have better performance than the silica-free 31 wt% Pt catalyst. All of the silica containing catalysts had low active surface 2 areas (23e35 m2∙g1 Pt for 23 mgPt/cm loading) as a result of the insulating behavior of silica. On the other hand some of these catalysts have high mass current densities which can be explained by the structure change and utilization of the catalysts. This can be explained as SiOx sol penetration into pores of Vulcan XC72 followed by the gelation process which breaks down the carbon agglomerates [50] and improves the utilization of the catalyst. A combination of these two features results in catalysts that have high specific current densities.
Fig. 9 e Oxygen reduction current densities of Pt/(VCeSiO2) and Pt/VC; A- 34% Pt, 3.4% SiO2 (C2), B- 46% Pt, 4.4% SiO2 (C5), C- 46% Pt (Tanaka).
3.5.
oxygen reduction reaction activity has a maximum for 3e5 nm Pt nanoparticles [6]. The observed result may be explained due to the slightly higher activity of a 5.1 nm catalyst (C3, 46 wt% Pt) in comparison with a 3.4 nm catalyst (C1, 31 wt% Pt). The RDE results in Table 2 show that performance for the 43e47 wt% catalysts suffers with low silica weight percents (1.4, 2.4 wt% SiO2). However, the performance improves with moderate (4.4 wt% SiO2) silica containing catalysts. The rotating disc electrode experiment for the catalyst with moderate silica content, C5 (46.4 wt% Pt, 4.4 wt% SiO2) resulted in similar activity with the 46 wt% Pt, Tanaka control catalyst (Fig. 9). The mass current density for the C5 catalyst, 1 is 0.05 A mg1 Pt at 0.9 V and 0.15 A mgPt at 0.85 V, whereas 1 values for the control are: 0.08 A mgPt , at 0.9 V and 0.23 A mg1 Pt , at 0.85 V. As noted above, the 31e34 wt% catalysts have lower mass current densities compared to the 43e47 wt% Pt catalysts. However, the 31e34 wt% catalysts with moderate levels of
Fuel cell performance tests
The fuel cell performance test results showed an improvement in fuel cell performance with silica additions up to certain amounts. The silica free (C1) and silica containing catalysts (C2) performed similarly with 31e34 wt% Pt catalyst containing MEAs. (Fig. 10a) Similarly, MEAs prepared with 43e47 wt% Pt catalysts showed better performance with silica addition. Silica added C4 and C5 catalysts performed better than silica free Tanaka and C3 catalysts. (Fig. 10b) The performance improvement can be explained with the continuous mesoporous structure formation with aerogel silica matrix which enables effective transfer of fuel, oxidant and products [50]. Unfortunately, redundant silica addition resulted in hydrophilic behavior and increase in silica mass resulted in poor performance. Mass transport losses with silica added catalysts, were easily observed from fuel cell performance results, especially for the high silica containing C5 catalyst (46.4 wt% Pt, 4.4 wt% SiO2). Higher silica containing C6 catalyst (41.5 wt% Pt, 6.0 wt% SiO2) showed very poor performance, probably due to discontinuety in electron transfer pathway.
Table 3 e Oxygen reduction reaction activity measurement for carbon and Si modified carbon-supported Pt catalysts as well as the commercial Pt/VC Tanaka catalyst.* Catalyst ID
Concentration Pt, wt%
SiO2, wt%
C1
31
e
C2
31
3.6
C3 C4 C5
46 43 46.4
e 2.4 4.4
C6 Tanaka
41.5 46
6.0 e
Lpb mgPt cm2
Apb m2 g1 Pt
im (0.9 V), A mg1 Pt
im (0.85 V), A mg1 Pt
is (0.9 V), mA cm2 Pt
is (0.85 V), mA cm2 Pt
23 14 23 14 23 22 23 14 23 23
59 37 23 38 24 31 35 44 29 58
0.10 0.05 0.13 0.10 0.13 0.02 0.05 0.07 0.06 0.08
0.28 0.18 0.41 0.40 0.41 0.10 0.15 0.22 0.13 0.23
223 142 578 273 558 57 136 152 135 131
624 491 1827 1072 1714 307 417 498 321 399
* All results were recorded during positive-going sweeps at 20 mV/s at 1600 rpm rotation rate in 0.1 M HC1O4.
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Acknowledgment Support from the NASA Space Science Office (Grant Number NNC04GB13G) is gratefully acknowledged.
references
Fig. 10 e Fuel cell performance results for (a) 31e34 wt% Pt/ VC (C1) or Pt/(VCeSiO2) (C2) catalysts and (b) 43e47 wt% Pt/ VC (C3) or Pt/(VCeSiO2) (C4, C5) catalysts.
4.
Conclusions
A sol-gel method for depositing controlled amounts of siliceous phase on carbon Vulcan XC72 (from 2.5 to 10.3 wt% SiO2) is reported. The Pt/VC and Pt/(VCeSiO2) catalysts with a concentration of Pt from 31 to 47 wt% were obtained by reduction of aqueous H2PtCl6 with NaBH4 without the use of stabilizers. The average Pt particle size ranged from 2.7 nm to 5.1 nm. The TEM images confirmed that platinum nanoparticles have a uniform and narrow distribution on supports. Examination of the catalyst thermal stability (pretreatment at 180 C for 360 h under air environment) showed that the presence of silica decreased the low temperature mass loss. TEM and optical micrographs for the catalyst residue after carbon combustion showed that silica provides a barrier to Pt nanoparticle aggregation. Thus, the presence of silica may enhance catalyst durability. An evaluation of electrochemical performance by cyclic voltammetry showed that all silica containing catalysts have lower active surface areas; however some of them had high mass current densities for oxygen reduction reaction. The combination resulted in high specific current densities. Finally preliminary fuel cell experiments showed similar or improved performances with silica addition. However, flooding was observed with high silica containing catalysts. With the exception of catalysts with low SiO2 content, the performance (mass current density) of silica containing Pt/VC catalysts is similar to the Pt/VC catalysts. A small drop in fuel cell performance could be tolerable for the improvement in catalyst durability. The silica containing catalysts will be tested in real fuel cell conditions for validation of durability as a future work.
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