NANO LETTERS
Template Synthesis of Arrays of Nano Fuel Cells
2006 Vol. 6, No. 2 288-295
Kenneth W. Lux*,†,‡,§ and Karien J. Rodriguez| Department of Engineering Physics and Materials Research Science and Engineering Center on Nanostructured Materials and Interfaces, UniVersity of Wisconsin, Madison, Wisconsin 53706, and Department of Chemical Engineering, UniVersity of Puerto Rico, Mayagu¨ez, Puerto Rico 00681 Received November 1, 2005; Revised Manuscript Received December 14, 2005
ABSTRACT A method for the construction of an array of fuel cells wherein each cell is 200 nm in diameter is presented. Electrodeposition of Pt−Cu nanowires inside the cylindrical pores of an Anodisc filter membrane and the subsequent dealloying of the Cu by soaking the filter in fuming nitric acid for several hours are used to construct an array of porous platinum electrodes. About 109 electrically isolated cylindrical porous electrodes, each 200 nm in diameter, are formed in this manner. Utilizing two arrays of porous electrodes with a polymer electrolyte membrane or an electrolyte support matrix sandwiched between, an array of nano fuel cells is produced. This method of producing an array of coplanar fuel cells allows for the series connection of fuel cells outside the array and eliminates the need for fuel and air manifolds, greatly reducing the overall system complexity. Initial prototypes utilizing an aqueous solution of NaBH4 as a fuel have produced power densities of ca. 1 mW/cm2 based on an estimate of the area of the current collectors in contact with the nano-fuel-cell array and have demonstrated the ability to wire bundles of fuel cells either in parallel or in series.
Introduction. Fuel cells are an attractive alternative to many energy conversion or storage devices. One particularly lucrative market is that of power sources for portable applications such as mobile phones, personal media players, hand-held gaming devices, and laptop computers. Current products utilize lithium-ion batteries to store the electrical energy needed to operate these devices. Unfortunately lithium-ion batteries have a relatively low specific energy exemplified by the fact that the long-term US Advanced Battery Consortium goals for specific energy and energy density of advanced battery systems, such as lithium-ion batteries, are only 200 W h/kg and 300 W h/L, respectively.1 On the other hand, the theoretical specific energy and energy density of methanol are about 6100 W h/kg and 4800 W h/L.2 By use of methanol in a direct methanol fuel cell, it is expected that power consumption and operational times before recharging or refueling will be increased dramatically. One hurdle to realizing the promise of fuel cells for portable-power applications is that the voltage of a single lithium-ion cell is nominally 3.7 V. For comparison, the reversible potential of a hydrogen-oxygen fuel cell is * To whom correspondence may be addressed. E-mail:
[email protected]. † Department of Engineering Physics, University of Wisconsin. ‡ Materials Research Science and Engineering Center, University of Wisconsin. § Current address: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Mail Stop 62R0203, Berkeley, CA 94720-8253. | Department of Chemical Engineering, University of Puerto Rico. 10.1021/nl052150j CCC: $33.50 Published on Web 01/10/2006
© 2006 American Chemical Society
1.23 V at 25 °C, but under operating conditions the voltage of a fuel cell is only about 0.3-0.5 V. As such it is necessary to wire several fuel cells in series to produce system voltages which meet the voltage criteria for existing portable applications (3.7-20 V). Multiwatt and multikilowatt fuel-cell systems typically utilize stacks of cells to build the system voltage to appreciable levels. However, this approach requires the use of fuel and air manifolds to supply reactants to and remove products from the individual cells in the stack. Arrays of electrically isolated coplanar cells are an alternate approach which eliminates the need for manifolds by requiring only one fuel reservoir and only one air reservoir. All of the cells in the coplanar design share the common reservoirs, but because they are electrically isolated from each other, they behave as independent fuel cells and can be wired together outside of the reservoirs in any desired arrangement of parallel and series connections. Additionally, current collectors which are larger than the fuel cells themselves can act not only as current collectors but also as “ties” to bundle multiple fuel cells in parallel. The ties themselves can be wired in series or parallel as desired. The smaller the fuel cells are, the smaller the individual ties can be. Previous investigations of the construction of micro fuel cells can be grossly separated into two approaches: (1) a top-down approach involving miniaturizing existing designs for polymer-electrolyte-membrane fuel cells3 and (2) a
Figure 1. Fabrication of an array of porous Pt electrodes by dealloying of electrodeposited Pt-Cu nanowires.
bottom-up approach employing microfabrication techniques developed for the semiconductor industry to fabricate fuelcell structures.4,5 The miniaturization approach generally produces much higher performance than the microfabrication approach (tens to hundreds of mW/cm2 for miniaturization vs hundreds of µW/cm2 for microfabrication). This stems from the ability to use commercial high-performance membrane and electrode assemblies (MEAs) in the top-down approach, while most bottom-up approaches are based on the use of microfabrication tools which were developed to produce two-dimensional structures. To obtain high-performance and low electrocatalyst loadings (a major component of overall system cost), a three-dimensional porous-electrode structure is necessary to obtain roughness factors (i.e., cm2 of area/cm2 of projected area) on the order of 1000 or greater. However, because many top-down approaches rely on prefabricated MEAs, these approaches have an inherent lower size limit due to the need to cut the MEA down to the individual cell size. We present the results from prototype devices constructed employing a new bottom-up approach which does not require the use of expensive microfabrication equipment and allows for the construction of porous electrodes. Additionally, this approach produces individual fuel cells which are cylindrically shaped with a diameter of only 200 nm which is 101000 times smaller than previously reported structures, even those made with microfabrication tools. “Nano fuel cells” (nFCs) allow the use of submicrometer spacing between fuelcell bundles approaching line widths currently in production in the semiconductor industry. The general approach is based on the construction of arrays of porous platinum electrodes as illustrated in Figure 1. Arrays of PtCu nanowires 200 nm in diameter are electrodeposited inside an Anodisc filter membrane. The copper is leached out to obtain an array of porous platinum electrodes which are electronically isolated from one another. It should be noted that the design and construction of these prototype devices were developed to demonstrate that arrays of nFCs can be constructed and wired simply either in series or in parallel to produce either high system voltages or high system currents, respectively. Nano Lett., Vol. 6, No. 2, 2006
Experimental Techniques. (A) Preparation of Arrays of PtCu Nanowires. Anodisc 0.02 µm membrane filters (Fisher Scientific) were utilized as a nonconductive template. These particular Anodisc membranes are 60 µm thick and have 20 nm pores on one face which coalesce into 200 nm pores about 50 nm from the face of the membrane. The 200 nm pores are cylindrical and penetrate completely through the thickness of the membrane. The membranes used had an active area of 20 mm and incorporated a 2.5 mm wide polymer support ring for a total diameter of 25 mm. To construct each array of PtCu nanowires, a thin film of silver was sputtered onto the small-pore face of the membrane for use as a cathode current collector. An AutoDesk II etching-sputtering machine utilizing a silver target was used to sputter the silver at 50 mTorr and 45 mA. Three 90-s sputterings were used with a 2-min rest between successive depositions. The membrane was placed silver-side down on a 4 cm × 4 cm square copper plate that had a small tab protruding from one edge of the square. This plate acted as a support for the membrane and as a terminal for making electrical contact with the thin-film silver current collector. Two polymer support rings which were made by dissolving two supported Anodisc membranes in 6 N NaOH were placed on top of the anodic membrane to serve as gasket material. The copper plate, sputter-coated Anodisc membrane, and support-ring gaskets were clamped in a standard Millipore filter holder (fritted glass base and graduated reservoir, Fisher Scientific) mounted in a 125 mL Erlenmeyer flask. Three milliliters of an electroplating solution consisting of 7 mM H2PtCl6 and 7 mM CuSO4 was added to the filter-holder reservoir, and a piece of platinum wire was utilized as an anode. Electrodeposition of a Pt-Cu alloy was carried out by room-temperature electrolysis of the electroplating solution in a two-electrode arrangement using a single AA alkaline battery as the power source with the negative lead attached to the copper plate and the positive lead attached to the platinum-wire anode. The use of an alkaline battery as the 289
Figure 2. Individual nano fuel cells, array of nano fuel cells, and bundles of nano fuel cells.
power source eliminated the need for a potentiostat or galvanostat and limited the total electrolysis-cell voltage to a maximum of 1.5 V, which reduced the possibility of hydrogen evolution at the cathode. Electrolysis was carried out for 30 min to 2 h depending on the desired platinum loading. It should be noted that the diameter of the graduated reservoir is 18 mm, slightly less than the active diameter of the filter. This results in a slight decrease in the area over which electroplating occurs. (B) Conversion of the Arrays of PtCu Nanowires to Arrays of Pt Electrodes. After electrodeposition of the PtCu nanowires, the copper was leached out of the PtCu alloy to yield a porous, or at least very rough, Pt electrode. The Anodisc membrane with the silver coating and the array of electrodeposited nanowires was immersed in concentrated nitric acid overnight to dissolve the silver current collector and leach out the copper from the nanowires. Electrolytic dealloying was not attempted to avoid possible dissolution of the copper support-plate/terminal. The morphology of the resulting Pt electrodes was examined utilizing a LEO 1530 scanning electron microscope with an in-lens secondary electron detector (10-15 kV accelerating voltage, 30 µm aperture). (C) Construction of the Array of Nano Fuel Cells. To construct a prototype array of nano fuel cells, two arrays of Pt electrodes were constructed as described above. As illustrated in Figure 2, an electrolyteseither a Nafion 117 polymer electrolyte membrane or a piece of filter paper (Waltham) soaked with 85% H3PO4swas placed between the two arrays of Pt electrodes to which was added a few drops of either 1 N H2SO4 or 85% H3PO4 for use with the Nafion or H3PO4 electrolyte, respectively. The electrode arrays were contacted with the electrolyte such that the largepore faces of the Anodisc membranes were in contact with the bulk electrolyte and the small-pore faces with the electrode array were facing outward. Electrical contact was made with the electrode array by placing two 5 mm wide strips of uncatalyzed, single-sided ELAT gas-diffusion electrodes (deNora) on each side of the 290
fuel cell array to act as gas-permeable current collectors. Strips on the same face of the fuel cell array were separated by about 5 mm, and strips on opposite faces of the fuel cell array were aligned with each other. Contact between the ELAT current collectors and the electrode arrays was promoted by clamping the fuel-cell array/current collector assembly between two glass slides with binder clips for 15 min. A piece of Parafilm with a circular hole in the center was used to prevent the current collectors from contacting each other and shorting out the bundles. The individual strips act as current collectors that essentially bundle in parallel the electrodes in which they are in contact. Therefore, each pair of aligned current collectors (one on each side of the fuel cell array) becomes a set of nano fuel cells bundled together in parallel. The individual current-collector strips can then be connected either in series or in parallel by simply adding jumper wires between appropriate current collectors. (D) Testing of the Nano-Fuel-Cell Array. The performance of the nano-fuel-cell array was measured by mounting the fuel-cell array/current-collector assembly in a standard Millipore filter holder sealed with clear silicone sealant (General Electric). Current-voltage curves were recorded at room temperature utilizing an aqueous NaBH4 solution (5 w/o) as a source of hydrogen fuel and ambient air as the oxidant. Measurements were taken in three separate configurations: individual fuel-cell bundles (i.e., single pairs of aligned, opposite current collectors), two fuel-cell bundles connected in parallel, and two fuel-cell bundles connected in series. Polarization curves were measured in a constantload mode using a variable resistor to vary the external load while a voltmeter and an ammeter were used to monitor the voltage and current, respectively. Because the current collectors span chords of the circular electrode array, rather than the diameter, the lengths of the fuel-cell bundles are less than the 18 mm diameter of the active area. A 9 mm radius and a 5 mm separation between the current collectors results in a chord length of 17 mm. With the width of the current collectors being 5 mm, we estimate the contact area between the current collectors and Nano Lett., Vol. 6, No. 2, 2006
the electrode arrays to be approximately 1.7 cm2. However, we unable to accurately measure what fraction of this area exhibited good contact between the electrodes and the current collectors. As such, we report the current rather than the current density. The use of NaBH4 as a hydrogen source is problematic due to the neutralization of the phosphoric acid and Nafion employed as electrolytes. However, it is very convenient to use in the proof-of-concept type of experiments carried out here. Results and Discussion. (A) Preparation of Arrays of PtCu Nanowires. During electrolysis of the electroplating solution, steady state was achieved in 15-30 s with a steadystate current of about 1 mA. During the first few minutes of electrolysis, the color of the Anodisc membrane changed from its pre-electrolysis reddish-white color (due to the sputtered silver layer on the opposite face) to black indicating the electrodeposition of metal in the pores of the membrane. During electroplating gas bubbles evolved from the anode, but none were noticed evolving from the cathode. The probable reactions occurring at the cathode are the electrodeposition of Pt and Cu: PtCl62- + 4e- 98 Pt + 6Cl-; Cu2+ + 2e- 98Cu;
Ered° ) 0.72 V6
Ered° ) 0.521 V
The probable reactions occurring at the anode are the evolution of chlorine and oxygen gases: 2Cl- 98 Cl2 + 2e-; 2H2O 98 O2 + 4H+ + 4e-;
Ered° ) 1.358 V Ered° ) 1.229 V
Assuming that these reactions are correct, the minimum applied potential necessary for deposition of both copper and platinum is 1.229 V - 0.521 V ) 0.708 V, which is less than the voltage of the battery employed as the power source. Because a two-electrode apparatus was used, it was not possible to determine the potential of the cathode relative to the standard hydrogen electrode (SHE). However, the absence of visible gas evolution from the cathode and the relatively facile kinetics of hydrogen evolution on Pt and Pt alloys imply that the cathode potential was not driven significantly negative of the SHE. Because the porosity of the platinum electrodes is related to the composition of the PtCu nanowires, it is instructive to determine if the electrodeposition process is contolled by kinetic or mass-transfer limitations. Using values of 7.1 × 10-6 and 9.0 × 10-6 cm2/s for the diffusion coefficient of cupric and hexachloroplatinate ions, respectively, limiting current densities for the 7 mM concentrations employed in this work can be estimated to be 1.6 mA/cm2 for copper electrodeposition and 4.1 mA/cm2 for platinum electrodeposition.7 Because the total current observed during electrodeposition of the alloy was 1 mA (0.32 mA/cm2), the reduction potentials and kinetics of the two reactions control Nano Lett., Vol. 6, No. 2, 2006
Figure 3. Scanning electrom micrographs of platinum electrode array.
the final composition of the nanowires. As such we expect the nanowires to have a large mole fraction of platinum due to the facts that the reduction potential for the hexachloroplatinate ion is about 200 mV more positive of the reduction potential for the cupric ion and that the rate of electrodeposition scales exponentially with cathodic overpotential. (B) Conversion of the Arrays of PtCu Nanowires to Arrays of Pt Electrodes. Scanning electron microscopy (SEM) micrographs of cross sections of the array of PtCu nanowires after leaching out the copper are presented in Figure 3. It is worth noting that the process of fracturing the array of electrodes to obtain the cross-section view results in the loss of some electrodes from the Anodisc membrane. Upon fracture of the array, pores at the face of the cross section are split open and may lack the support to keep the electrode in place. As such, although the micrographs in Figure 3 appear to show unfilled pores, we believe that in an intact array, the vast majority of pores contain electrodes. Figure 3a shows a cross-section view of an electrode array showing multiple electrodes. In this particular sample, the electrodes extend about 9 µm into the Anodisc template which corresponds to a platinum loading of about 11 mg/cm2. Figure 3b shows a close-up of two of the electrodes. It can be seen that the electrodes have a rough 291
texture. It is difficult to determine if the electrode is truly porous because it was necessary to sputter-coat gold onto the cross section of the electrode array to avoid excessive charging of the electronically insulating alumina template. The gold sputtered onto the sample prior to examination with the SEM may fill nanopores in the electrode making the distinction between a rough surface and a porous surface difficult. (C) Testing of the Nano-Fuel-Cell Array. If fuel-cell bundles are electrically isolated from each other, then two bundles can be operated not only as individual bundles but also wired in series or in parallel. In either arrangement, the power output of two identical bundles wired together should be twice that of each individual bundle. Connecting the bundles in series should yield double the voltage of an individual bundle at a given current while connecting the bundles in parallel should yield double the current of an individual bundle at a given voltage. If the two bundles exhibit different performance (i.e., due to different numbers of cells in each bundle or nonuniform plating of the nanowires used to construct the electrode arrays), then a reasonable approach to predicting the performance of series-connected bundles is to add the voltage of each bundle together at a given current. Likewise, for parallel-connected bundles one would add the currents at a given voltage. Because the bundles were tested in a constantload mode, it was difficult to obtain data on two different bundles at the same voltage or the same current. Under these circumstances, series-connected voltage was predicted by adding the voltages of the two bundles obtained at similar (e.g., (15%) currents. Parallel-connected current was predicted by adding similar currents obtained from each bundle and averaging the voltages at which those currents were obtained. (1) System with a Phosphoric Acid Separator. The performance data obtained for fuel-cell bundles utilizing an aqueous NaBH4 solution as a source of hydrogen fuel and phosphoric acid as the separator are presented in Figure 4a (two individual bundles), Figure 4b (same two bundles connected in series), and Figure 4c (same two bundles connected in parallel). As can be seen in Figure 4a, the two individual bundles exhibit slightly different polarization curves but exhibit similar peak power outputs of about 0.12 mW. Also of note is that the polarization curves are approximately linear with voltage and with no activation region or mass-transfer limitation region clearly visible. If the major source of voltage loss in the system is an ohmic resistance, then an estimate of the ohmic resistance from the slope of the polarization curves implies that the bulk resistance of the system is approximately 400-700 Ω. If these bundles are connected in series or parallel, one would expect a peak power output of about 0.24 mW. Series connectivity should yield peak power output at a low current but high voltage while parallel connectivity should yield peak power output at a high current but lower voltage. To a certain extent this is borne out by the data in Figure 4b and Figure 4c. A peak power output of 0.22 mW is observed at 0.4 mA 292
and 0.6-0.7 mA for series-connected and parallel-connected bundles, respectively. The predicted and the observed performance for the two bundles in Figure 4a when they are connected in series are presented Figure 4b. The predicted performance matches the observed performance fairly well, although it underestimates the observed performance at high currents. This is due in part to one bundle exhibiting higher performance than the other at higher currents which leads to the voltage of the system decreasing less rapidly than predicted by the simplistic empirical model used in this work. The predicted and observed performance of the two bundles when they are connected in parallel are presented in Figure 4c. The predicted performance again matches the observed performance fairly well at lower currents. In this instance, the phosphoric acid-soaked filter paper utilized as the separator developed tears in it which allowed the NaBH4 solution to contact the air electrodes resulting in the failure of the system. This can be seen by the rapid drop off in performance above 0.6 mA. From these results we conclude that the fuel cell bundles as constructed operate independently and can be wired in series or parallel to increase either the system voltage or current. Additionally, performance data from two individual bundles can be used to predict the performance of a system where the two bundles are connected in series and in parallel. (2) System with a Nafion Separator. Observed and predicted performances for two bundles in a system utilizing Nafion as the separator are presented in parts d, e, and f of Figure 4. It should be noted that the current and power scales in these graphs are significantly larger than that for the phosphoric acid-based system. The performance of the individual fuel-cell bundles is presented in Figure 4d. The two bundles exhibit rather similar performance with peak power outputs of 0.85 and 0.98 mW. From these results we would expect a series-connected or parallel-connected system to produce a power output of around 1.8 mW. In contrast to the phosphoric acid system, an activation region is clearly visible in the polarization curves. Using the slope of the polarization curve in the linear region as an estimate of bulk resistance yields an estimate of about 50 Ω for the system bulk resistance. It is interesting to note that if our rough estimate of 1.7 cm2 of active array area is correct, then the peak powers obtained above correspond to a power density of ca. 1 mW/cm2. Although this power density obtained in our bottom-up approach falls short of the tens to hundreds of mW/cm2 obtained in top-down approaches, it is about an order of magnitude larger than the hundreds of µW/cm2 obtained with other bottom-up approaches to constructing micro-fuel-cell systems mentioned above. Unfortunately, the large degree of swelling that Nafion exhibits on hydration can rupture the Anodisc membranes which act as the support structure for the electrode arrays. Because of such problems, performance data for seriesconnected and parallel-connected systems utilizing Nafion as the separator were not recorded. However, the performance data for the system utilizing phosphoric acid as the Nano Lett., Vol. 6, No. 2, 2006
Figure 4. Performance data for nano-fuel-cell arrays: individual bundles, two series-connected bundles, and two parallel-connected bundles: phosphoric acid separator (a-c) and Nafion separator (d-f). Note that the current and power scales for the Nafion separator are much larger than those for the H3PO4 separator.
separator indicate that the performance of individual bundles can be combined to predict the performance of systems utilizing both series-connected and parallel-connected bundles. The predicted performances for series-connected and parallel-connected systems are presented in parts e and f of Figure 4, respectively. The predicted peak power output of the multibundle systems is 1.8 mW. The peak power should occur at ca. 4 and 8 mA for series and parallel connectivity, respectively. The large difference in performance of the systems with different separators is due to the large difference in the slope of the polarization curve in the linear region. This large difference in polarization losses is due either to poorer contact of the current collectors with the nano-fuel-cell electrodes and/or poorer conductivity of the bulk electrolyte or to poorer Nano Lett., Vol. 6, No. 2, 2006
electrocatalytic activity in the phosphoric acid system. Because the difference in the ionic conductivity of the electrolytes is not large enough to produce an order of magnitude difference in ohmic losses (κH3PO4 ≈ ca. 0.08 S/cm, κNafion ≈ 0.1 S/cm), we conclude that either poor contact of the current collectors with the nano-fuel-cell arrays or phosphate anion adsorption is responsible for the difference in performance between the systems with the different separators.8-10 (D) System Limitations and Future Modifications. (1) Use of a Common Bulk Electrolyte. One limitation of these prototypes is the use of a bulk electrolyte common to all of the fuel cells in the array. O’Hayre et al. have discussed some of the issues of utilizing a common electrolyte in multiplecell configurations.11 Several strategies exist for alleviating 293
problems relating to cells short circuiting through the common electrolyte. These include utilizing a thinner electrolyte, eliminating the bulk central electrolyte, and producing porous electrode arrays at either end of the filter pores. (2) Porosity of the Platinum Electrodes. Because the electrochemical reactions in a fuel cell are heterogeneous, an increase the specific surface area (cm2 of surface/cm3 of total volume) of the porous electrodes will result in an increase in the superficial current density for a given electrode thickness. As mentioned above, the porosity, and hence the specific surface area, of the platinum electrodes is controlled by the composition of the PtCu alloy in the electrodeposited nanowires. Although only one ratio of cupric ions to hexachloroplatinate ions was utilized in manufacturing the nano-fuelcell arrays in this work, it may be possible to control the composition of the electrodeposited nanowires by operating in a mass-transfer-limited regime. Stirring of the bulk plating solution would establish a well-defined boundary condition for the concentration of metal ions at the solution end of the membrane pores, namely, the bulk concentrations. If the applied voltage is sufficient to reach the limiting current density and the current efficiencies of the Cu and Pt plating reactions are unity, then the mass transfer of hexachloroplatinate and cupric ions to the cathode will limit the current. The ratio of copper to platinum in nanowires electrodeposited under these conditions would be controlled by the ratio of the diffusion coefficients of the metal ions and the ratio of the number of electrons transferred in the reduction reaction iCu xCu NCu cbulk Cu DCu ) ) bulk )2 xPt NPt iPt c D Pt
Pt
where Nj is molar flux of ion j to the electrode surface, Dj is the diffusion coefficient of ion j, and ij is the fraction of the total reduction current attributable to the reduction of ion j at the electrode surface. Initial results from the plating of the PtCu nanowires near the limiting current density are shown in Figure 5. In production of the sample presented in Figure 5, the copper has been leached out of the alloy in the same manner as before. However, the resulting porous platinum exhibits a different morphology and appears to have larger pores than the electrodes shown in Figure 3. It should be kept in mind that the caveats regarding estimating porosity of a sputtercoated sample mentioned in the discussion of Figure 3 hold here as well. (3) Platinum-Alloy Electrocatalysts for Other Fuels. In the present work we have limited the electrode materials to platinum. Platinum was chosen due to its activity toward both the hydrogen-oxidation and the oxygen-reduction reactions. In principle, any alloy which can be electroplated can be used to form the nanowires. The approach presented here can be used to make nFCs with other alloys as electrocatalysts provided that one utilizes an electrodeposited alloy 294
Figure 5. SEM micrograph of the cross section of an array of porous platinum electrodes: plating bath, 0.5 mM H2PtCl6 and 0.5 mM CuSO4; soaked in fuming nitric acid for 12 h.
consisting of one component or components which can be leached out leaving behind the desired electrode alloy. This may be particularly useful for direct methanol fuel cells which, generally, utilize PtRu alloys as anode electrocatalysts.
Conclusions We have demonstrated a very inexpensive means of constructing arrays of fuel cells, each ca. 200 nm in diameter. Because bundles of these “nano fuel cells” operate independently (subject to certain geometrical constraints), they can be wired in series to increase system voltage while eliminating the need for traditional stack components such as fuel and air manifolds. Current work is underway to eliminate the need for a common bulk electrolyte and to increase the amount of platinum which is electrochemically accessible. The materials and processes used to construct these arrays of nano fuel cells are compatible with semiconductor materials and manufacturing techniques, enabling the integration of such power sources in microelectronic and microand nanopower devices. Acknowledgment. The authors thank Professor Wendy Crone, Dr. Anne Bentley, and Mohammed Farhoud for helpful discussions, the UW Materials Research Science and Engineering Center, and the NSF-funded UW-Madison SURE/REU summer research program. This material is based upon work supported by the National Science Foundation under Grant No. DMR 0079983. References (1) USCAR, URL http://www.uscar.org/consortia&teams/ consortiahomepages/con-usabc.htm, (accessed 10/13/05). (2) Xie, C.; Bostaph, J.; Pavio, J. J. Power Sources 2004, 136, 55. (3) Liu, B. H.; Li, Z. P.; Arai, K.; Suda, S. Electrochim. Acta 2005, 50, 3719. Savinell; Smyrl (4) Motokawa, S.; Mohamedi, M.; Momma, T.; Shoji, S.; Osaka, T. Electrochemistry 2005, 73, 346. (5) Seo, Y.-H.; Cho, Y.-H. Sens. Mater. 2004, 16, 277. (6) Calculated from the reduction potentials for PtCl62- + 2e- f PtCl42+ 2Cl- (E° ) 0.68 V) and PtCl62- + 2e- f Pt + 4Cl- (E° ) 0.755 V).
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(7) Ionic Conductivity and Diffusion at Infinite Dilution. In CRC Handbook of Chemistry and Physics, Internet Version, 2005; Lide, D. R., Ed.; http://www.hbcpnetbase.com, CRC Press: Boca Raton, FL, 2005. Cupric ion diffusion coefficient taken from table, diffusion coefficient for hexachloroplatinate ion estimated from values in table for tetracyanoaurate, hexacyanocobaltate, hexacyanoferrate(II), and hexacyanoferrate(III) ions. (8) Chakrabarti, H. J. Phys.: Condens. Matter 1996, 8, 7019.
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(9) Mason, C. M.; Culvern, J. B. J. Am. Chem. Soc. 1949, 71, 2387. (10) Halseid, R.; Vie, P. J. S.; Tunold, R. J. Electrochem. Soc. 2004, 151, A381. (11) O’Hayre, R.; Fabian, T.; Lee, S.; Prinz, F. B. J. Electrochem. Soc. 2003, 150, A430.
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