Platinum dispersed on silica nanoparticle as electrocatalyst for PEM fuel cell

Available online at www.sciencedirect.com Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 621 (2008) 198–204 www.elsevier.com/locate/jelechem

Platinum dispersed on silica nanoparticle as electrocatalyst for PEM fuel cell Brian Seger

a,c

, Anusorn Kongkanand a, K. Vinodgopal

a,d

, Prashant V. Kamat

a,b,c,*

a Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556-0579, United States Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556-0579, United States Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556-0579, United States d Department of Chemistry, Indiana University Northwest, Gary, IN 46408, United States b

c

Received 6 June 2007; received in revised form 17 September 2007; accepted 27 September 2007 Available online 25 October 2007

Abstract Colloidal silica has been employed as a support material to disperse Pt and develop Pt–SiO2 composites with electrocatalytic properties. The Pt–SiO2 particles of ratio 1:1 and 2:1 show superior performance towards oxygen reduction reaction (ORR) as these composite particles form an interconnected particle-network and maximize the available electrochemically active area. Upon increasing the Pt content we observe decreased activity as a result of aggregation of particles. The performance of Pt–SiO2 composite particles in H2-fuel cell are compared with Pt-black catalyst. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Fuel cell; Electrocatalyst; Oxygen reduction; Silica; Nanocomposite catalysts; Oxide–metal nanoparticles

1. Introduction Over the past decade, a variety of different strategies have been considered to minimize the use of precious metal catalysts in proton exchange membrane (PEM) based fuel cells [1–6]. While such a reduction in catalyst usage is an important prerequisite for the successful commercialization of fuel cells, the search for new electrocatalyst formulations is also underway. This latter aspect is driven by the need to improve the poor kinetics and the high overpotential associated with oxygen reduction at a platinum electrode [7]. Three current thrusts in cathode catalyst development are as follows: (a) Porphyrin based oxygen reduction catalysts [8] have been found to favor four-electron reduction of O2 to H2O. Similarly, carbon nanoparticles modified with a cobalt polypyrrole (PPy) film have been found to * Corresponding author. Address: Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556-0579, United States. Tel.: +1 574 631 5411; fax: +1 574 631 8068. E-mail address: [email protected] (P.V. Kamat).

0022-0728/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2007.09.037

be good electrocatalysts for the reduction of O2 [9]. In many of these studies, molecular catalysts are used as composites of carbon nanostructures [10]. Zelenay et al. have recently shown that such composites show excellent stability and durability in the acidic environment of a polymer electrolyte fuel cell [10]. (b) Pt alloys containing transition metals such as Co and Ni show enhanced activity for oxygen reduction as compared to pure Pt alone and several groups have used Pt alloy catalysts for this purpose [11– 14]. The reasons for the enhancement in oxygen reduction efficiency are likely to arise from a number of factors. These include alteration of the surface and electronic properties of the platinum as a result of alloying [11]. (c) Deposition of thin layers of platinum on another metal substrate has also been found useful. Adzic and coworkers have developed techniques by depositing thin monolayer of platinum on metal substrates including palladium and gold [15,16]. They have extended this strategy by designing core/shell structures as electrocatalysts. By depositing a thin layer of Pt on different metal cores, it was possible to design core–shell structures such as Pt/Au/Ni, Pt/Pd/

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199

Scheme 1.

Co, and Pt/Pt/Co and thus minimize the use of precious metal [17]. These platinum shell architectures that allow decreased platinum content show excellent activities for oxygen reduction. Another emerging area in the cathode catalyst development is the use of inorganic oxides as supports. For example, deposition of platinum nanocrystallites on large surface area inorganic oxide supports such as titanium oxide [18] and tungsten oxide [19] has been attempted. Mallouk and coworkers [20] have prepared mixed metal catalysts involving Pt–Ir–Ru on TiO2 and Nb doped TiO2. Sasaki and Adzic [21] found that, in the Pt/NbO2 system, NbO2 suppresses the OH adsorption on Pt surface and hence enhances the oxygen reduction activity. These approaches have the additional benefit of reducing carbon corrosion, which has an adverse impact on the long term operational durability of PEM fuel cells. Watanabe and coworkers have also employed silica in the fuel cell membrane assembly with a different approach. They have investigated self humidifying Nafion 112 membranes containing well dispersed, nanometer sized Pt and SiO2 particles [22]. We report here a simple approach to prepare platinum nanoparticles by reduction using sodium borohydride on a colloidal silica substrate (Scheme 1). By controlling the ratio of platinum to silica, we were able to obtain a well dispersed catalyst surface with minimal conductivity loss. The electrocatalytic properties of SiO2 core and Pt shell particles as cathode catalyst in a membrane electrode assembly are discussed. 2. Experimental section Fifteen percentage of colloidal silica solution (pH 10.4) was obtained from NALCO (Naperville, Illinois). The average particle size of the amorphous colloidal particles was 4.0 nm. Hydrogen hexachloroplatinate and sodium borohydride were obtained from Sigma–Aldrich (St. Louis). Platinum-black (Alfa-Aesar, Hi-Spec 1000) and E-TEK 40% Pt on Vulcan XC-72 (Somerset, NJ) were used

Fig. 1. Photograph illustrating formation of Pt in SiO2 suspension. A mixture of 1:1 by mass of hexachloroplatinic acid and silica was treated with different amounts of NaBH4 (10 mM) solution. (From left to right: 0, 1, 2, 5 and 10 ml of NaBH4 solution was added to vials.)

as standards. Carbon Toray paper and carbon cloth were obtained from http://Fuelcellstore.com. 2.1. Synthesis of Pt–SiO2 particles One milliliter of the colloidal silica solution (diluted to 2 g/l) was added to 5 different vials. Varying quantities (0.5, 1.0, 2.0, 3.1, and 10.2 ml) of a 10 mM aqueous solution of hydrogen hexachloroplatinate Pt(IV) solution was added to the 5 different vials, to give Pt–SiO2 mass ratios of 0.5:1, 1:1, 2:1, 3:1, and 10:1. Each vial was diluted with deionized water to a total volume of 11.2 ml, stirred for 5 min, and then a 10 mM solution of the reducing agent NaBH4 was added drop-wise with stirring. The solution which initially had a light yellow color1 turned gray-black upon addition of the NaBH4 indicating instantaneous reduction of the Pt(IV). Fig. 1 illustrates the progress of the reduction following the addition of NaBH4 to a 1:1 Pt–silica solution. Excess borohydride was added to ensure complete reduction of the Pt(IV). For example, 10 ml of 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.

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10 mM borohydride solution was added in the case of the 1:1 Pt–SiO2 solution to achieve complete reduction.

activity within the Nafion membrane has been presented elsewhere [26].

2.2. Electrode preparation

2.4. Fuel cell evaluation

The Pt–SiO2 species as well as Pt-black particles were centrifuged, washed 3 times and finally dispersed in tetrahydrofuran to obtain a concentration of 50 lg/ml. The suspension was sonicated in an ultrasonic bath for 10 min at 0 °C, to get a good consistent dispersion of particles. The particles from this suspension was electrophoretically deposited (EPD) on to the carbon Toray paper at a dc electric field strength of 75 V/cm. Details of the EPD method are provided elsewhere [5,23]. The amount of Pt–SiO2 deposited on each electrode was typically about 0.2 mg of Pt/cm2 but the exact amount of platinum on each electrode was determined by atomic emission spectroscopy using an inductively coupled plasma (ICP) spectrometer. Since we were unable to deposit E-TEK platinum by the same electrophoretic deposition technique, we employed a spray technique to deposit this E-TEK catalyst on the Toray paper. For rotating disk electrode experiments, 14 lg-Pt/cm2 of each Pt–SiO2 species was deposited on to a glassy carbon electrode (area = 0.159 cm2). A standard three-electrode configuration was used with a Ag/AgCl reference electrode and a 0.1 M perchloric acid solution as the electrolyte. The solution was first purged with nitrogen and the surface area obtained as described later. Oxygen gas was then bubbled in the solution for 30 min. Once the solution was saturated with O2, a Gamry PC-14 potentiostat was used to run a cyclic voltammetry sweep from 0.23 V to 0.94 V while the electrode was rotating at 1600 rpm. This was cycled twice to make sure there were no deviations between cycles. For the figures and analysis, the data was taken from the scan in the positive direction.

The pressed MEA was then tested in a Scribner (Southern Pines, NC) fuel cell testing unit. Hydrogen was passed over both the anode and cathode overnight at a rate of 300 ml/min and a temperature of 50 °C to condition the membrane. A standard break in procedure was used to condition the fuel cell [27]. (Instead of basing our flow rates on a stoichiometric ratio as in the procedure, we used a constant 300 ml/min and 600 ml/min flow rate on the anode and cathode respectively.) The procedure basically consisted of varying the current and potential for approximately a day and then scanning the current at 10 min a point to get a polarization curve and power density graph. A current interrupt procedure was done after each point to determine the ohmic resistance. The temperature of the cell was maintained at 40 °C and 1 atm of back pressure was employed.

2.3. Membrane electrode assembly (MEA) preparation A 5 cm2 gas diffusion layer for both the anode and the cathode were created using an optimized procedure described by Antolini et al. [24]. In this experiment a total of 2 mg/cm2 of carbon black/Teflon per half layer was used. 0.5 mg/cm2 of Pt-black (with 20% Nafion) was then sprayed on the anode. This relatively large loading of Pt on the anode side allows us to neglect the overpotential due to the anode half reaction allowing the focus to be solely on the cathode [25]. The cathode loading in all cases was 0.2 mg/cm2of Pt–silica. In all cases the Pt particles were covered by 20% Nafion by weight. Nafion 117 was pretreated by boiling in 5% hydrogen peroxide (1 h.) followed by 3 cycles of 1 M sulfuric acid. After each cycle the Nafion was boiled in de-ionized water for 20 min. The prepared anode and cathode were positioned on both sides of the pretreated Nafion membrane and hot pressed at 34 atm and 160 °C for 2 min to form the membrane electrode assembly (MEA). The details on probing proton

3. Results and discussion 3.1. Synthesis and characterization of the Pt–SiO2 particles The surface area of the catalyst is an important factor in heterogeneous reactions. In the case of an electrocatalytic reactions (e.g., oxygen reduction reaction), additional factors such as charge transfer kinetics and availability of the active surface contribute to the overall electrode performance. Dispersion of catalyst particles on the carbon support has been the choice in increasing the surface area of electrodes. Whereas this strategy has proved useful for anodic reactions (e.g., oxidation of hydrogen or methanol), the cathode performance remains sluggish. Oxide particles can serve as support to anchor Pt particles. Such oxide supports also provide additional chemical stability for long term performance of fuel cells. In the present study, the platinum coated silica nanoparticles were synthesized by reduction of Pt (IV) in a colloidal silica solution using sodium borohydride as the reductant. The Pt–SiO2 particles formed in aqueous solutions eventually settle down. The solutions were centrifuged for 10 min at 10,000 rpm. The clear supernatant liquid was discarded and the centrifugate was then resuspended in tetrahydrofuran (THF) by sonication. The procedure was repeated once more to ensure removal of any ionic or unreduced species. Fig. 2a shows a high resolution TEM image of the platinum coated silica particles with a nominal Pt:SiO2ratio of 2:1. The TEM image suggests a beaded network of platinum particles. Since the SiO2 nanoparticles are spheres with an average diameter of 4 nm we observe fairly uniform size distribution of Pt-coated silica particles. Few aggregated particles are also seen. The Pt shell thickness can be varied by changing the ratio of Pt:SiO2. Most of these Pt–SiO2 particles are linked together forming a

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Fig. 2. (a, b) High Resolution TEM images of 2–1 ratio Pt–SiO2 nanoparticles recorded at different magnifications. The lattice fringes on the platinum particles can be clearly seen in (b).

network. Such a morphology further assists in the dispersion of the platinum catalyst on the electrode surface to form a conducting network. The beads appear spherical, but there are some irregularities in their structure, which is likely due to non-uniform growth of the platinum. EDX analysis confirms the presence of both silicon and platinum. A closer examination of the particles (Fig. 2b) at higher magnification reveals lattice fringes from the platinum which is deposited on the amorphous silica particles. These fringes are observed consistently across the network, indicating that the crystalline platinum is dispersed across the silica particle-network. This continuous Pt coverage as an extended particle-network enables to attain both high conductivity and high surface area. The SEM images of the Pt– SiO2 particles (Fig. 3) on carbon paper confirm the presence of a dispersed platinum network, which is preserved on the carbon fiber electrode. It is interesting to note that the networking is most obvious at the 2:1 ratio of Pt:SiO2. When we increase the ratio of Pt to silica, the particles tend

to aggregate and the TEM images do not show the same type of particle-network as observed with Pt:SiO2 of ratio 2:1. The TEM images obtained at other ratios are shown in the Supporting Information (Fig. S1). 3.2. Electrochemically active surface area (ECSA) A key parameter that determines the electrocatalytic performance of these Pt particles dispersed on a silica framework in fuel cell reactions is the accessibility of the platinum surface. The determination of electrochemically active surface area (ECSA) of the electrode provides a quantitative measure of the effectiveness of the electrocatalyst. ECSA value of the electrocatalyst is measured in terms of the number of electrochemically active sites per gram of the catalyst [28]. In order to determine the ECSA values of electrodes containing Pt:SiO2 of different ratios, we subjected them to hydrogen adsorption/desorption process in an acid medium. Typically, the electrode was cycled between 0.27 and 1.0 V (vs. SCE) in a nitrogen saturated 0.1 M H2SO4 solution at a rate of 20 mV/s until a stable hydrogen desorption peak was observed. The integrated area under the desorption peak in the cyclic voltammogram represents QH is inserted in the expression (1) to determine the ECSA value [28]. ECSA½m2Pt =gPt  ¼

Charge½QH ; C=cm2geo  2:1½C=m2Pt   catalyst weight½gPt =cm2geo  ð1Þ

Fig. 3. Scanning electron micrograph showing 2–1 ratio Pt–SiO2 nanoparticles deposited on carbon Toray paper.

Fig. 4 shows the bar graph comparing the experimentally determined electrochemically active surface areas for each of the different Pt–SiO2 mass ratios that we have prepared. The highest value for ECSA is obtained for the Pt:SiO2 with a 1:1 ratio suggesting effective dispersion of the Pt on the SiO2 support to maximize the effectiveness of the electrocatalyst. With increasing ratio of Pt:SiO2 we observe a decrease in the ECSA value. These results indicate the necessity to optimize the ratio of Pt:SiO2 for maximizing the electrocatalytic performance. The Pt–SiO2

B. Seger et al. / Journal of Electroanalytical Chemistry 621 (2008) 198–204

15

2

Surface Area (m /g-Pt)

202

10

5

0 1-1

2-1

3-1

Pt E-TEK Black

10-1

Pt-SiO2 Ratio Fig. 4. Bar diagram displaying the electrochemically active surface area of different platinum–silica catalyst samples of different Pt:SiO2 ratio and carbon black. The electrocatalysts were deposited on the carbon paper and ECSA was determined by hydrogen desorption. The voltammograms for the surface area were determined using scan rate: 20 mV/cm; electrolyte: 0.1 M H2SO4; reference electrode: SCE. The area of the electrode in all cases was 0.5 cm2.

particles generally exhibited higher ECSA than the PtBlack. All the samples except 10:1 ratio Pt–SiO2 sample, exhibited a higher ECSA than the commercial E-TEK. As discussed earlier, increasing platinum content in the Pt–SiO2 particles causes agglomeration and thus further contributes to decreased ECSA. 3.3. Catalytic activity in the ORR reaction by Tafel plot analysis

Overpotential (V)

b

c

-0.30

I (A/mg-Pt)

Tafel plots provide a convenient way to evaluate the electrocatalytic activity of the Pt dispersed on silica. Fig. 5 shows the Tafel plots for each of the Pt–SiO2 samples. To obtain these Tafel plots, the Pt–SiO2 samples at

a

-0.48

0.01 0

d

-0.36

-0.42

0.02

a. b. c. d.

1E-3

2

4

6

8

10

1-1 Pt-SiO2 2-1 Pt-SiO2 3-1 Pt-SiO2 10-1 Pt-SiO2

0.01

0.1

Current (A/mg-Pt) Fig. 5. Tafel plots for the oxygen reduction reaction using the platinum coated silica catalyst particles at 24 °C in O2-sat 0.1 M HClO4. The voltammograms correspond to the following ratios: (a) 1:1 Pt:SiO2, (b) 2:1 Pt:SiO2, (c) 3:1 Pt:SiO2 and (d) 10:1 Pt:SiO2. Inset shows the dependence of current at 0.4 V overpotential on the ratio of Pt:SiO2. (ER = 1.23 V).

the different ratios were deposited on a glassy carbon electrode and the oxygen reduction current was determined via rotating disk voltammetry. The slopes obtained for all Pt:SiO2 samples are similar indicating that the energetics of the ORR remains unaffected and is dictated by the electrocatalytic reaction at the Pt surface. However, differences emerge in the electrode kinetics as the onset potential for O2 reduction varies with the Pt:SiO2 ratio. Both 1:1 and 2:1 show lower overpotentials. The current at a given overpotential is a direct measure of electrode kinetics at the catalyst interface. The inset in Fig. 5 displays these currents as a function of the ratio of Pt to silica at overpotential of 0.4 V. The highest current and therefore the composition for the most favorable kinetics for the ORR are obtained for the Pt–SiO2 sample of ratio 2:1. The performance of this sample is consistent with the excellent dispersion of the catalyst observed in the TEM images. As discussed in the previous section, both 1:1 and 2:1 ratio Pt:SiO2 samples exhibited relatively high ECSA values. However, the morphology and conducting network makes the 2:1 ratio Pt–SiO2 sample as the best performer for the ORR reaction. 3.4. Fuel cell analysis In order to determine the effectiveness of these platinum particles dispersed on silica for fuel cell operation, we have prepared MEAs using cathodes with the Pt–silica mixtures at different ratios. All MEAs were prepared with a constant loading of 0.2 mg/cm2 of total Pt–silica, irrespective of the mass ratio between the silica and platinum. Thus, the MEA prepared from the 1:1 ratio Pt–SiO2 samples has a platinum loading of 0.1 mg-Pt/cm2 while the 2:1 ratio Pt–SiO2 based MEA has 0.13 mg-Pt/cm2. For comparison purposes, we have also prepared a MEA using commercial platinum-black with a loading of 0.2 mg-Pt/cm2. The fuel cell polarization curves for the different MEAs are shown in Fig. 6. All these systems exhibit losses due to polarization and further efforts are necessary to overcome these effects. The normalized power density curves shown in Fig. 6 further highlight the beneficial aspects of Pt:SiO2 catalysts. Pt–SiO2 sample at the 2:1 ratio exhibit highest power density values. The superior performance of this sample in delivering maximum power is consistent with the trend observed in Tafel plots. A current interrupt test was also employed to determine any possible ohmic voltage losses arising from the presence of silica in the fuel cell [29–31]. This test is carried out by operating the fuel cell at a constant current and then interrupting the current flow by breaking the circuit. Typically, a quick increase in voltage is seen following the interruption. This quick rise in potential is followed by a gradual increase as it attains the open circuit voltage. This slow rise in potential is attributed to the dissipation of the double layer. The initial potential jump corresponds to the ohmic loss and thus provides a means to evaluate the resistivity of the silica based samples. In other words, any ohmic losses

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a

Voltage (V)

200

b

0.8

c

150

0.6 0.4

100

c a

0.2

50

b

0.0 0

100

200

300

400

500

600

Power Density (mW/cm2 ) @ 0.2 mg-Pt

250 a: 2-1 Pt-SiO2 b: 1-1 Pt-SiO2 c: Pt-Black

1.0

203

K414. Such support does not constitute endorsement by the US Army of the views expressed in this publication. This is contribution no. NDRL 4730 from Notre Dame Radiation Laboratory. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jelechem. 2007.09.037. References

0 700

Current Density (mA/cm2) Fig. 6. Galvanostatic polarization data of the fuel cell at 40 °C. The electrocatalyst on the cathode side of MEA consisted of: (a) 2:1 Pt:SiO2, (b) 1:1 Pt:SiO2 and (c) Pt-black. The loading of the electrocatalyst was 0.2 mg/cm2 (including the mass of silica). The anode side of MEA for these three sets of experiment was Pt-black at 0.5 mg/cm2 anchored on carbon black at 2 mg/cm2. The electrode area was 5 cm2. The right hand Y-axis shows power output normalized to Pt content of 0.2 mg/cm2.

Table 1 Evaluation of MEA resistance based on current interrupt analysis Current density (mA/cm2)

Resistance (mX cm2) 1:1 Pt:SiO2

2:1 Pt:SiO2

Pt-black

280 320 360

54.9 56.4 57.5

55.4 51.2 48.7

52.6 53 53.6

arising from the presence of silica should reflect in the magnitude of the quick recovery of the potential. Table 1 shows the resistance determined from the quick recovery at three different current densities. The Pt–SiO2 samples show the ohmic losses similar to that of the commercial Pt-black sample. These results confirm that the silica support does not contribute to any significant increase in the overall resistivity. The long branching network of platinum as observed in both the SEM and TEM must be helpful enough in transferring the electrons to the electrode. In summary, we have prepared Pt–SiO2 composite using chemical reduction method. At low Pt concentrations (Pt:SiO2 ratio of:1:1 and 2:1) the dispersions are quite uniform and form an interconnected particle-network. Higher electrochemical active area and improved charged transfer kinetics makes these composite catalysts a viable candidate in fuel cell applications. Experiments are underway to maximize the fuel cell performance of oxide support based electrocatalysts and evaluate their corrosion resistant properties during extended period of fuel cell operation. Acknowledgements The research described herein was supported by the US Army CECOM RDEC through Agreement DAAB07-03-3-

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