Novel carbon aerogel-supported catalysts for PEM fuel cell application

International Journal of Hydrogen Energy 30 (2005) 149 – 158

www.elsevier.com/locate/ijhydene

Novel carbon aerogel-supported catalysts for PEM fuel cell application A. Smirnovaa;∗ , X. Dongb , H. Harab , A. Vasilievc , N. Sammesa a Connecticut

Global Fuel Cell Center, University of Connecticut, UCONN, 44 Weaver Road, Storrs, CT 06269-5233, USA Composite, LLC c/o ICA, Inc., 102-R Filley Street, Unit H, Bloom/eld CT 06002-1853, USA c Institute of Materials Science, UCONN, 97 North Eagleville Road, Storrs, CT 06269-3136, USA

b Aerogel

Received 9 January 2004; received in revised form 7 April 2004; accepted 7 April 2004

Abstract Novel carbon aerogel supported Pt catalysts with di5erent pore size distributions and Pt content have been synthesized and tested in a proton exchange membrane fuel cell (PEMFC) operation. Characterization of the aerogel supported Pt catalyst has been performed in respect to the total surface area of Pt using HRTEM and BET methods, and was compared to the electrochemical surface area of Pt in a cathode layer of the PEMFC by means of cyclic voltammetry. The e5ect of pore size distribution of the novel aerogel supported Pt catalyst on the performance of the PEMFCs, and kinetic parameters of the catalysts at di5erent temperatures, is discussed in terms of the microstructure of the support and per=uorosulfonate-ionomer distribution. The PEMFC with a low Pt loading (0:1 mg=cm2 ) of a new porous aerogel catalyst has shown high power densities up to 0:8 mW=cm2 in fuel cell operation conditions in air and at ambient pressure. ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Carbon aerogel; Aerogel supported catalyst; Pore size distribution; MEA manufacturing; Screen-printing; PEMFCs

1. Introduction Tremendous e5ort has been made in the past 10 years to develop catalysts for proton-exchange membrane fuel cells (PEMFCs). It has been shown [1], that PEMFCs can be used as very eBcient chemical energy converters in vehicles. They particularly suite automotive applications primarily because of their relatively low operating temperature, high eBciency, and high power density, which is superior to existing internal combustion technology [2]. A remaining problem is the high production cost for the fuel cell units, which, in comparison to the internal combustion engine, is far more expensive per kW [3]. Cost reduction can be achieved by using more e5ective unsupported or supported catalysts with low precious metal loadings [1,4].

∗ Corresponding author. Tel.: +1-860-486-8762; fax: +1-860486-8378. E-mail address: [email protected] (A. Smirnova).

Fuel cell reactions on the surface of the catalyst involve both electrons and protons, which require high electrical and proton conductivity respectively. The incorporation of the proton conductor, NaHonTM , signiHcantly increases the membrane conductivity in the catalyst layer and improves the catalyst utilization [5]. It is well known that the catalytic activity of the catalyst depends upon the contact area between the catalyst and the electrolyte, and that only the catalyst in contact with the electrolyte is active. Therefore, the pore size of the supported catalyst available for electrolyte molecules could be very signiHcant in achieving high electrochemical catalytic activity. The platinum catalyst supported on high surface area carbon, e.g. Vulcan XC 72, contains primary pores, which cannot be Hlled with the polymer electrolyte because of their small size in comparison to the size of a single polymer particle [6]. This does not allow for the eBcient use of the precious metal catalyst, thus decreasing the cell performance, and increasing its cost.

0360-3199/$ 30.00 ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.04.014

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The size of the pores cannot be changed in common carbon used as a support for PEMFC catalysts, however it can be changed in aerogels. Aerogel materials possess a wide variety of exceptional properties, which include controlled chemical synthesis and design of structure and porosity of organic and inorganic materials [7]. The porosity of the aerogel can be changed via chemical synthesis modiHcation with further distribution of Pt particles on its surface allowing good contact with the ionically conducting polymer electrolyte. In this paper, we describe the behavior of carbon aerogel-supported Pt catalysts. These supports are produced by aqueous polycondensation of resorcinol with formaldehyde [8] followed by pyrolysis to form the porous carbon network structure. The metallic catalyst is introduced into via organometallic precursors using supercritical carbon dioxide, followed by secondary pyrolysis to form Pt nanoparticles on the surface of aerogel carbon support. Details of these processes and the characteristics of the structures which result have been presented elsewhere [9]. The paper outlines the progress that has been made on “in situ” characterization of novel aerogel supported catalysts in PEMFC operation. The goal of this work was to develop, synthesize, and characterize catalysts with various pore size distributions, Pt content, and Pt surface area, which can be used in PEMFCs. The cell performance for the membrane electrode assemblies (MEAs) under di5erent operating conditions will be discussed in relation to the aerogel structure, composition, and Pt loadings in the cathode catalyst layer ranging from 0.6 to 0:06 mg=cm2 . The results of the electrochemical surface area calculations (ESA) calculations for aerogel-supported catalysts used in this work will be compared with similar data obtained for commercially available supported and unsupported catalysts. 2. Experimental 2.1. Characterization of the aerogel supported Pt catalysts Two aerogel-supported Pt catalysts have been used for the manufacture of cathode catalyst layers. The catalysts di5er by Pt content (37% sample 1 and 20% Pt sample 2) and porosity (16 and 22 nm), estimated by the Brunauer –Emmett–Teller (BET) method. Adsorption and desorption isotherms of nitrogen were measured using a Sorptomatic 1990, a fully automatic Gas Adsorption Analyzer from Horiba. The mesopore size distributions and mesopore volumes were estimated for the pore diameter range 2–50 nm by applying the Dollimore–Heal method [10] to the desorption isotherm of nitrogen. The Pt mean diameter and the Pt surface area in the aerogel-supported Pt catalysts were measured by hydrogen adsorption.

High resolution TEM (HRTEM) was used for the estimation of the size of Pt particles in the aerogel supported Pt samples. The aerogel-supported Pt catalyst with and without NaHon was deposited on QuantifoilJ (Quantifoil Micro Tools GmbH, Jena, Germany) aurum grids covered by a holey carbon Hlm with ∼ 1:2 m circular holes. TEM analyses were performed in a Philips EM-420 TEM equipped with a Kevex-Noran Be-window energy dispersive spectrometer (EDS), and operating at an accelerating voltage of 100 kV. High-resolution TEM (HRTEM) lattice images were obtained in a JEOL JEM-2010 TEM equipped with an ultra high resolution (UHR) objective lens pole-piece (C ∼ = 0:5 mm) and operated at an accelerating voltage of 200 kV. In this conHguration, the point-to-point resolution at Scherzer defocus is less than 0:19 nm. The mean particle size was obtained by measuring the diameter of a suBcient number of particles to ensure good statistics. The surface mean diameter was calculated according to dVA = fi d3i =fi d2i , where fi is the frequency of occurrence of the particles of diameter d in the sample from the particle size distribution [11]. 2.2. Preparation of catalyst pastes and membrane electrode assemblies (MEAs) Anode carbon supported catalyst from Tanaka Inc. (Pt– Ru 53.5%) was mixed under nitrogen with DI water, 5% NaHon 1100 solution, and appropriate solvents. Prepared mixtures were homogenized for few hours using an UltraTerrux homogenizer with further evaporation of the excess amount of the solvent in order to obtain appropriate viscosity. Cathode aerogel supported catalysts with di5erent Pt loadings and pore size distributions received in the form of rods 5 mm in diameter from Aerogel Composite, LCC, were ground Hrst in an agate mortar. After that, the catalyst ink was prepared by the process mentioned above for the anode catalyst. Te=on Hlms used for decals were weighed before the application of the catalyst ink. The ink was screen-printed using a Systematic Automation Model 810 Series Screen Printer. The polyester screen from SAATI Print TM was used for the deposition of ultrathin catalyst layers onto the surface of the Te=on Hlm. To form the MEA, the appropriate decals were placed on either side of the NaHon 117 or 112 membranes purchased from DuPont. After drying and hot pressing at 70 kg=cm2 for 5 min with NaHon 112 or NaHon 117 membrane, the Te=on supported Hlms were peeled o5 from the cathode and anode side of the MEAs. Further experiments were undertaken in the hardware with 6:25 cm2 active area from ElectroChem Inc. The cells included gas di5usion layers from SGL Technologies GmbH and Te=on gaskets with a thickness of 0:25 mm, which corresponded to a total pinch of 0:2 mm.

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2.3. Cyclic voltammetry measurements The measurements of Pt surface area in contact with the NaHon layer, and thus available for the electrochemical reaction were made using a cyclic voltammetry (CV) technique in four-point probe conHguration. CV plots were obtained in the range of 0.01–0:8 V at a scan rate of 20 mV=s at room temperature using a Princeton potentiostat Model 273. To avoid the presence of oxygen, and thus obtain the correct data, pure nitrogen was supplied to the cathode side as a working electrode and pure hydrogen to the anode as a counter electrode at 200 cc=min =ow rate. The calculation of the electrochemical surface area of the catalyst was estimated from CV plots using the equation: AESA = A=(K × L × S). In this equation A is the area under reduction part of curve between the current density corresponding to a double layer capacitance at the lowest voltage and the second in=ection point indicating the beginning of hydrogen evolution from the surface of Pt; K = 0:21 mA=cm2 ; S —scan rate (S = 20 mV=s), and L—Pt loading in the cathode catalyst layer.

Fig. 1. The image of the aerogel supported Pt catalyst.

2.4. PEM fuel cell testing procedure The cell performance was evaluated using a Teledyne MedusaTM RD test station. The cell test station was equipped with humidiHcation chambers, mass =ow and temperature controllers, and 50 amp load box 890C from Scribner, which allowed simultaneous correction of the cell potential for the iR-drop. All the experiments were provided at 40◦ C, 60◦ C, and 80◦ C temperature of the cell. In order to avoid water condensation inside the cell, the feed lines from the cathode and anode side were heated 10◦ C higher than the cell temperature, which was equal to the temperature of both saturators. 3. Results and discussion 3.1. Structure and properties of the aerogel supported Pt catalyst Morphological characterization and estimation of nano-scale structure of two aerogel supported Pt samples used in this work have been made after each stage of catalyst ink preparation. The size of the Pt particles after grinding was still too big for the preparation of a catalyst paste and corresponded to ca. 100–150 m. Consequently, this procedure was followed by continuous homogenization with short intermittent stirring to prevent the material from settling. As a result, the size of the catalyst particles decreased to ca. 5–10 nm. The typical image of aerogel supported Pt catalyst (Fig. 1) shows an even distribution of Pt particles on the surface of the carbon support, with the size of the particles ca. 2 nm, in agreement with BET analysis data (2:2 nm).

HRTEM images (Fig. 2) indicate similarity in the structure of the support and in the size of the Pt particles for both samples. The samples show crystalline structure with ca. T spacing between graphite layers [12]. 3:4 A The individual Pt particles range in size from 1 to 2 nm. The corresponding lattice images exhibit fringes that correspond to the (1 1 1) spacing and conHrm the oxidized state of Pt. In both samples no evidence of agglomeration can be seen in comparison to the other carbon supported [6] and unsupported catalysts [13]. As shown previously, the catalysts with a small particle size of ca. 2 nm are the most appropriate for their utilization in PEMFC [14], which could be due to the increase in adsorption energy and the rate of oxygen reduction reaction (ORR) [11]. Therefore, synthesized aerogel catalysts have been considered appropriate for their utilization in PEMFC layers. In addition to the size of Pt particles, another important factor, which signiHcantly in=uences the ORR, is the pore volume distribution in a carbon support. The results of BET analysis gave 16 and 22 nm for samples 1 and 2, respectively (Fig. 3a). In comparison to common carbon-supported catalysts (Fig. 3b) having large pore-size distribution and the presence of small primary pores [6], the pore size distribution in both aerogel samples is very narrow. This is extremely favorable for big polymer electrolyte molecules, which can penetrate the big pores of the aerogel, and signiHcantly increase the electrochemical surface area of the catalyst and ORR kinetics. According to our data the estimated active area of the aerogel supported catalyst (250 cm2 =g) was almost twice higher than the active surface area of one of the best commercial TEC10E40E Tanaka catalyst (150 cm2 =g).

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Fig. 2. The HRTEM images of the aerogel supported catalyst (sample 1 with 25% NaHon and sample 2 in dry state).

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Fig. 3. (a) Pore size distribution of 16 nm aerogel supported Pt catalyst sample. (b) Pore size distribution of commercially available TEC10E40E Tanaka catalyst. Pt/C (40%) Pt.

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Current density, A/cm 2

0.015 0.01 0.005 0 -0.005 0.00

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0.60

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Current density, A/cm2

Fig. 4. Cyclic voltammetry plots for aerogel-supported Pt catalyst with 25% NaHon (sample 1) before ( ) and after (
) humidiHcation. Pt loading: 0:6 mg=cm2 . The calculated value of the ESA before and after humidiHcation corresponds to 41 and 71 m2 =g.

0.015 0.01 0.005 0 0.00 -0.005

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0.60

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Working electrode potential, V

Fig. 5. Cyclic voltammetry plots for unsupported Pt catalyst with 10% NaHon in cathode catalyst layer before ( ) and after (
) humidiHcation. Membrane: NaHon 117.

3.2. Estimation of the electrochemical surface area The electrochemical surface area (ESA) of the catalyst, i.e. the area of the catalyst actually participating in the electrochemical reaction, is considered one of the most important factors in=uencing the PEMFC performance. It is well known that only the catalyst in contact with NaHon is active, and has the ability to participate in the oxidation–reduction reactions [15]. In our study, the estimation of ESA was made before and in between cell measurements. Interestingly, that the ESA in both of the aerogel supported Pt catalysts used in this study increased more than twice after a few hours of cell testing (Fig. 4). However, this e5ect was much less pronounced in the case of the commercial supported and unsupported catalysts (Figs. 5 and 6) and corresponding change in ESA did not exceed 10%. This phenomenon can be explained by the unique structure of the aerogel catalyst having large pores available for NaHon molecules. During the manufacturing process, the electrolyte molecules penetrate the aerogel pores; however only after humidiHcation of the cell at 80◦ C are they able to be in good contact with the electrolyte. Under these conditions, NaHon signiHcantly

153

increases its volume [16] and possibly is able to maintain better contact with the Pt particles distributed on the internal surface of the aerogel pores. Along with electrochemical surface area, another significant factor, which in=uences the fuel cell performance, is the concentration of Pt in cathode layer (Pt loading). In the present work, the in=uence of Pt loading on fuel cell performance has been estimated for the aerogel catalyst with 16 nm pore size. The ESA values calculated from CV plots (Fig. 7) are consistent and equal to 73 + = − 3 m2 =g for all three plots. The comparative data for commercial carbon-supported Pt catalyst under similar Pt loadings, and the same NaHon content (Fig. 8), indicates lower values of ESA, e.g. 50 + = − 10 m2 =g. It should be noted that at catalyst loadings lower than 0:1 mg=cm2 , the accuracy in weighing and thus calculation of ESA is much lower, and the error can reach 50%. The catalyst stability under cell environments has been estimated by comparison of the ESAs after certain periods of cell testing. Fig. 9 represents the data for aerogel-supported catalysts (sample 2) on the Hrst day (1), after humidiHcation (2), and after 60 h. The corresponding data of calculated ESA values equals to 99 + = − 1 m2 =g, which is about 25% higher than for the aerogel sample 1. This result could be explained by approximately 25% bigger pore size volume of the sample 2 in comparison to sample 1, more e5ective penetration of NaHon molecules into the aerogel structure, and thus, better contact of the ionic conductor NaHon with the Pt particles. 3.3. Performance of PEMFCs on hydrogen=air and oxygen and kinetic parameters 3.3.1. In@uence of aerogel-supported Pt loading on PEMFC performance The aerogel supported Pt catalyst for oxygen reduction in the cathode catalyst layer (sample 1) has been tested in PEMFC with similar membrane and anode composition. The performances of PEMFCs with di5erent loadings of aerogel catalyst in oxygen and air are shown in Figs. 10 and 11. In this experiment, relatively thick NaHon 117 membranes have been used, thus along with the performance curves the curves compensated for membrane ohmic resistance are also presented. As expected, the open circuit voltages (OCVs) as shown in the inset of Fig. 11 are the same for the highest concentration of catalyst (991 mV) and about 50 mV lower for the lowest catalyst loading. Examination of Tafel slopes for iR-compensated voltage–current curves in oxygen display a strong dependence of Tafel slope on catalyst layer loading. The cell with the lowest catalyst loading indicates the highest Tafel slope, ca. 130 mV=decade in the range 50– 100 mA=cm2 . High values of Tafel slopes can be possibly explained by uneven coverage of the membrane surface with catalyst, which causes local current gradients and high

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3 Current Density, mA/cm

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Fig. 6. Cyclic voltammetry plots for carbon supported commercial Pt catalyst with 0:24 mg=cm2 catalyst loading and 25% NaHon loading before ( ) and after (
) humidiHcation. Corresponding ESA values: 26 and 30 m2 =g.

Current Density, A/cm

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Fig. 7. Cyclic voltammetry plots for aerogel-supported Pt catalyst (sample 1) at di5erent catalyst loadings:
—0:6, U—0:07 mg=cm2 after humidiHcation. Corresponding ESA data: 72, 69, and 77 m2 =g.

—0:2, and

Current Density, mA/cm 2

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Working electrode potential, V Fig. 8. Cyclic voltammetry plots for commercial carbon-supported Pt catalyst at di5erent catalyst loadings: U—0:12 mg=cm2 . Corresponding ESA values: 49, 39, and 58 m2 =g.

 —0:77,

—0:24, and

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Fig. 9. Cyclic voltammetry (CV) data for the aerogel supported catalysts (sample 2) before humidiHcation on the Hrst day ( ), on the second day after humidiHcation (), and after 60 h of testing (U). Scan rate: 20 mV=s. Pt loading: 0:1 mg=cm2 . NaHon loading in cathode catalyst layer: 25%. Corresponding calculated values of ESA: before humidiHcation 62 m2 =g, after humidiHcation 101 m2 =g, and after 60 h 103 m2 =g.

Compensated Cell Voltage, V

Voltage/Compensated Cell Voltage, V

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Fig. 10. The cell voltage (open symbols) and compensated cell voltage (closed symbols) vs. current density for three MEAs made with di5erent Pt loadings in oxygen: U—0:6 mg=cm2 ; —0:2 mg=cm2 ;
—0:06 mg=cm2 . Membrane: NaHon 117. Cell temperature: 80◦ C. Temperature of humidiHers: 80◦ C. The inset shows Tafel plots for compensated voltage–current plots in oxygen.

total overvoltages. Thus, the cell is not able to support high voltages at relatively high current densities. The Tafel slope of the cell with 0:6 mg=cm2 loading in the same range of current densities also showed high

value of about 100 mV=decade, which is probably a result of the cathode layer ohmic resistance due to the high catalyst loading. However, the cell with ca. 0:2 mg=cm2 loading showed only 78 mV=decade Tafel slope, and thus,

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Voltage/Compensated cell voltage, V

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

200

400

600

Current density,

800

1000

mA/cm2

Fig. 11. The cell voltage (open symbols) and compensated cell voltage (closed symbols) vs. current density for three MEAs made with di5erent Pt loadings in air: U—0:6 mg=cm2 ; —0:2 mg=cm2 ;
—0:06 mg=cm2 . Membrane: NaHon 117. Cell temperature: 80◦ C. Temperature of humidiHers: 80◦ C.

much lower overvoltages, which is favorable for fuel cell performance. The graphs in the inset of Fig. 11 indicate that the values of Tafel slopes for the cells with lowest and highest Pt loadings change with current density and gradually increase in the range from 20 to 100 mA=cm2 . However, for the cell with intermediate Pt loading Tafel slope remains constant in the low current density range. At higher current densities up to more than 1000 mA=cm2 , no signiHcant change in current–voltage slopes has been noticed on iR-compensated plots for this cell either, which indicates a negligible in=uence of mass-transport limitations and thus, well optimized structure of both cathode and gas di5usion layers. Comparison of the cell performances in air show 50 mV lower OCVs and higher Tafel slopes due to higher mass transport limitations in the cathode catalyst layers than in the case of O2 . However, the cell with 0:2 mg=cm2 loading still has a close to theoretical Tafel slope in the kinetically controlled region of current densities, and shows no indication of cathode catalyst layer resistance or mass transport limitation in=uence. 3.3.2. PEMFC performance with low Pt content in aerogel support Since a decrease in a cost of an MEA could be achieved by using cathode layers with low precious metal loadings, attention has been paid to catalyst loadings of ca. 0:1 mg=cm2 . The amount of platinum in catalyst layer can be decreased at least in two di5erent ways: either by decreasing the thickness of cathode catalyst layer, or by decreasing the Pt loading in the catalyst itself. The Hrst method is the most well known, though as discussed earlier, it does not demonstrate good fuel cell performance and shows a dramatic decrease

in cell voltage with current density at 0:06 mg=cm2 loading of the catalyst. Very little literature relative to decrease of platinum content in PEMFC catalyst layers by the second method is available. However, from our point of view it looks more favorable, because in this case the Pt loading can be decreased without in=uencing the quality and the uniform coverage of the catalyst layer. Thus, in the second part of the work, we focused mostly on the manufacturing and testing of aerogel supported Pt catalyst with half the Pt content (20%). The tested catalyst also had a larger pore size distribution, which could be favorable for the processes of catalyst homogenization and for the cell performance due to better permeability of NaHon into the catalyst aerogel structure. Fig. 12 represents the performance of the PEMFC with the aerogel supported catalyst (sample 2) and Pt loading ca. 0:1 mg=cm2 at ambient pressure in air. Each point is the average of 20 points taken every 15-s during 5-min intervals at di5erent current densities. The cell performance has been tested at 40, 60, and 80◦ C and at di5erent =ow rates corresponding to 1:2.5 and 1:10 stoichiometry. At low current densities, the cell demonstrated close to theoretical values of Tafel slopes ca. 78 mV=decade in the kinetically controlled region of current densities. In this range of current densities, the commercial catalyst demonstrated similar values of OCV and Tafel slope. The highest power density did not increase linearly with the cell temperature as illustrated in Fig. 12. The maximum values of power density were observed at 60◦ C, but not at 80◦ C, which could be expected from the fact that at high relative humidity conductivity of NaHon increases with temperature. Power density values were much improved at high =ow rates and corresponding values were close to

A. Smirnova et al. / International Journal of Hydrogen Energy 30 (2005) 149 – 158

0.9 0.8

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iR-Free Cell Voltage (V)

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Current density, mA/cm2 Fig. 12. PEMFC performance with Aerogel supported Pt cathode (0:1 mg=cm2 ) at di5erent operating conditions on air/hydrogen: —40◦ C (1:2.5 stoic),
—60◦ C (1:2.5 stoic), U—80◦ C (1:2.5 stoic), ◦—80◦ C (1:9.5 stoic), ×—80◦ C (1:2.5 stoic)—commercial catalyst with 0:2 mg=cm2 Pt loading and 25% NaHon in cathode catalyst layer.

0:8 mW=cm2 at 2300 mA=cm2 , thus indicating that additional e5ort should be made for decreasing of mass-transport limitations.

operation, make this catalyst the most promising novel catalyst for PEM and DMFC operation.

4. Conclusions

The authors acknowledge Aerogel Composite, LLC that supported this work through the Grant with Connecticut Global Fuel Center.

Aerogel supported Pt catalysts have been successfully applied in the preparation of cathode catalyst layers using screen-printing technique and tested as cathode catalysts in PEMFCs at ambient pressure, and fully saturated conditions. The novel catalysts demonstrated good catalytic properties, relatively high OCV, close to theoretical Tafel slope, and high electrochemical surface area, which is twice higher than the ESA of the commercial catalysts. The PEMFC performance has been estimated for the 16 nm aerogel at di5erent cathode catalyst loadings in the range 0.06–0:6 mg=cm2 . Increase in pore size of aerogel support to from 16 to 20 nm allowed signiHcantly increase cell performance and maximum power density of the cell. The 20 nm catalyst with low Pt loading (0:1 mg=cm2 ) showed maximum power density in air at ambient pressure close to 0:8 mW=cm2 . This is possibly due to the better penetration of NaHon particles into the pores of the aerogel carbon support and better coverage of the membrane surface with lower Pt content catalyst. Further improvement in the cell performance can be achieved by optimization of the aerogel structure and catalyst layer composition. The unique structure of the aerogel supported Pt catalyst having large pores, even distribution of Pt particles inside the porous carbon structure, and less tendency for agglomeration and sintering during the cell

Acknowledgements

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