Membraneless Fuel Cells: An Application of Microfluidics - ACS

Chapter 48

Membraneless Fuel Cells: An Application of Microfluidics

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 13, 2016 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch048

Tzy-Jiun M . Luo, Jiangfeng Fei, Keng G. Lim, and G. Tayhas R. Palmore* Division of Engineering, Brown University, Providence, RI 02912 *Corresponding author: [email protected]

Introduction Research activity in fuel-cell technology has increased remarkably in the past few years primarily because of environmental concerns. For example in the area of transportation, fuel cells promise to increase the efficiency of converting fossil fuels into useful work (i.e., transportation) while reducing the amount of toxins released into the environment. In the area of portable electronics, fuel cells offer an attractive alternative to batteries and the associated environmental impact of their disposal (1). Secondary to reducing the environmental impact of current energy conversion technology, is the growing need to power miniaturized microelectronic, micromechanical and microfluidic devices, such as those used in environmental sensors and medical implants and their transmitters and receivers. Currently, the lower limit in terms of size and weight of these types of miniaturized devices is determined by the volume and weight of its battery. For example, lithium-ion batteries power many medical implants, including pacemakers, atrial defibrillators, neurostimulators, and drug infusion pumps. As the size of a lithium-ion battery is reduced, its energy capacity does not scale with its volume, because the volume fraction of the casing, seals and current collectors increase. Thus, further reduction in the size of medical implants and the emerging "lab-on-a-chip" technology will require the development of alternatives to battery technology. © 2005 American Chemical Society

Karn et al.; Nanotechnology and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Background A fuel cell is an electrochemical device that manages the flow of electrons and charge-compensating positive ions in a redox reaction in a manner that moves the electrons through an external circuit where they can do electrochemical work (Figure 1 ). The fuels used in a typical low-temperature fuel cell are H and 0 ; other reducing fuels such as methanol often are converted to H before entering the fuel cell. Generation of power occurs upon addition of fuel to the fuel cell.


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Figure L A dihydrogen/dioxygen fuel cell illustrating the direction of electron and ion flow. When the circuit is closed, electronsflowfrom the anode to the cathode through the external circuit and the positively charged ions (i.e., protons)flowthrough the membranefromthe anode compartment to the cathode compartment.

A biofuel cell is a combination of two technologies: fuel cells and biotechnology. Similar to conventional fuel cells, biofuel cells consist of an anode and cathode separated by a barrier that is selective for the passage of positively charged ions (2.3). Unlike conventional fuel cells, which use precious metals as catalysts, biofuel cells utilize enzymatic catalysts, either as they occur in microorganisms, or more commonly, as isolated proteins. The use of microorganisms or enzymatic catalysts instead of noble metal catalysts results in a fuel cell that functions under mild working conditions: ambient temperature and pressure, and moderate pH. Consequently, biofuel cells are attractive



355 devices for powering miniaturized microelectronic, micromechanical and microfluidic components in medical implants or environmental sensors. In the case of medical implants, the accompanying miniature fuel cell would be fueled by glucose and oxygen delivered via the circulatory system. Several prototype f u e l cells recently have been reported that use glucose oxidase as the anodic catalyst and laccase or bilirubin oxidase as the cathodic catalyst (49). The generation of electrical power by these prototypes requires either a membrane to separate the fuels or, if absent, selective electrocatalysis. Difficulties in manufacturing a miniature fuel cell based on either of these configurations, however, make alternative designs desirable. One possible design exploits laminar flow of fluids at low Reynolds number. The Reynolds number is a dimensionless parameter that relates the ratio of inertial forces to viscous forces of a specific configuration of fluid flow. Specifically, the Reynolds number (Re) is given by eq 1 : Re = ν ί ρ η "

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(1)

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where ν is velocity in cm s" ,1 is the diameter of the channel, ρ is the density of the liquid in g cm" , and η is the dynamic viscosity in g cm" V . In our case, the value for each parameter in eq 1 is ν = 1 cm s~\ 1 = 50 μπι and is the height of the channel, and approximating the fluid to be pure water, ρ = 1 g cm" and η 8.94 χ 10" gcnïV . Under these conditions, Re = 0.56 and fluid flow is predicted to be laminar. It is this flow behavior that a fuel cell designed around a microfluidic channel exploits. Two fluids such as the anolyte and catholyte in a microfluidic fuel cell can flow next to each other in direct contact (i.e., liquid-liquid interface) and not mix by turbulence. Instead, mixing occurs only by diffusion of molecules across the interface. This type of design removes the need for either a membrane or selective electrocatalysts. Depending on the location of the electrodes relative to the fluid-fluid interface, diffusion of molecules across the interface may affect the cell potential and thus, power output of the fuel cell. 3

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Materials and Methods In our design, the fuel cell consists of a central channel 300- to 800-μιη wide and 50 μιτι in height. The channels are lined with electrode pairs measuring 300 μηι by 4 mm, with the electrodes in each pair separated by 200 μπι. Fuel can be delivered to the electrode array within the channel via the inlet ports and discharged (or recycled) via the outlet ports. Diffusive transport of molecules from one electrode to the other in each electrode pair is approximated to be in the range of milliseconds to seconds depending on molecular size.



356 Microfluidic channels were fabricated using replica-molding techniques (10,11). Briefly, a master channel is made in photoresist (Micochem SU-850, 50-μηι high, 800-μηι wide) by photolithography using a mask fabricated from a high-resolution transparency. This negative-relief master is replicated by molding in PDMS, which is peeled from the substrate to produce a slab of PDMS containing channels -50 μιη in depth and of varying width depending on the configuration of the electrode array and the desired pattern of flow. A metal-lift off procedure was used to fabricate microelectrode arrays with different configurations (12). This procedure is inexpensive and facile, thus enabling the rapid-prototyping of new electrode configurations on the basis of new results. Briefly, the patterns for the electrodes and electrical leads are generated initially with CAD software and printed at high resolution onto an overhead transparency to generate an inexpensive mask for photolithography. Glass slides (or other substrates) are spin-coated with Shipley 1813 photoresist to a desired thickness (i.e., typically 1-2 μπι), exposed to ultraviolet light through the overlaying mask, and developed in tetramethylammonium hydroxide. Subsequent to pattern formation on a substrate, a conductor is evaporated to a thickness of -1000 A on top of the patterned photoresist. Dissolving the underlying photoresist with acetone facilitates the lift-off of the unwanted segments of conductor to obtain the desired microelectrode array. Covering the microelectrode array with the channel-containing PDMS slab completes the fabrication of the microfluidic fuel cell.

Results Shown in Figure 2a is a photograph of a prototype microfluidic biofuel cell displaying the microelectrode array enclosed by the central channel of the PDMS slab. Electrical contact to the enclosed electrodes is made via the gold lines that lead to large gold pads external to the PDMS slab. The dashed box indicates the section of the device magnified and shown in Figure 2b, which reveals the laminar flow of fuel and oxidant. Shown in Figure 3a are the molecular structures of the anodic fuel (1,4dihydrobenzoquinone, BQH ) and cathodic mediator {2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonate), ABTS} used in the microfluidic fuel cell shown in Figure 2. Two electrons and two protons are liberated at the anode upon oxidation of each molecule of BQH2 to BQ (Figure 3b), Electrons are transported via an external circuit to become available at the cathode while protons diffuse across the fluid-fluid interface for charge balance. Bioelectrocatalytic reduction of dioxygen in the catholyte was achieved with the copper oxidase, laccase, and the cathodic mediator, ABTS (13): Shown in Figure 3c is a plot of voltage as a function of current density generated by this device as fueled with 1,4-dihydrobenzoquinone (anodic fuel) and dioxygen as the terminal oxidant. The performance of the fuel cell, reflected by the shape of its current density-voltage curve, is similar to that 2



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Figure 2. (a) A microfluidic biofuel cell consisting of a linear channel that encloses six pairs of electrodes. The location ofports for entry and exit offuel and oxidant are indicated by the arrows, (b) Magnified view of area enclosed in the dashed box in (a).—Arrows point to fuel (dark gray on right inlet) and oxidant (outlined on left inlet as color is indistinguishable from background color). Only thefirstpair and part of the second pair of electrodes are seen in this photo (black lines).



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Figure 3. (a) Molecular structure of anodic fuel and cathodic mediator used in the microfluidic fuel cell, (b) Electron and protonflowduring bioelectrocatalytic reduction of dioxygen. (c) Current density vs. voltage plot of the microfluidic biofuel cell operated at different loads.

observed with fuel cells that contain a membrane separating catholyte and anolyte. The open-circuit voltage of the device was 350 mV, reflecting the potential difference between 1,2-dihydrobenzoquinone and ABTS. Maximum current density for this prototype was 80 μΑ cm' and the maximum power density was 8.3 μ\ναη~ at 150 mV. Two other microfluidic fuel cells recently were reported in the literature. In the first example, few specifics were given as to the eléctrocatalyst or anodic fuel used (14). Dioxygen was the terminal oxidant and both anodic fuel and 2

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359 dioxygen were delivered to the anode and cathode, respectively, as aqueous solutions. The maximum power density was 0.22 mW cm" at 0.45 V , corresponding to a current density of -0.5 mAcm" . In the second example, 1 M solutions of vanadium ions dissolved in 25 % H S0 were used as anodic [V(III)/V(II)] and cathodic fuels [V(V)/V(IV)] (15). The electrodes in this device were carbon-coated gold and the open-circuit voltage was 1.59 V. The maximum current density was 35 mAcm" at 1.1 V , corresponding to 38 mW cm" . It is difficult to compare the performance of these two reported devices to our device for several reasons. First, different fuels and oxidants were used, which impacts the open-circuit potential of the fuel cell (e.g., 1.1 V vs. 1.59 V vs. 0.35 V), and ultimately the maximum current density and power density. Second, although all examples use aqueous solutions, the pH is different in each case (e.g., unspecified vs. pH 0 vs. pH 5), which impacts the impedance of the device. Third, the height and width of the channels, respectively, are different (e.g., 1 mm 1 mm vs. 0.2 mm * 2 mm vs. 0.050 mm χ 0.8 mm). Fourth, the rate at which fuel flowed through the microfluidic fuel cell is different in each case (e.g., unspecified vs. 12.5 cms" vs. 1 cms' ). Finally, the distance between the anode and cathode is different (e.g., unspecified vs. 1 mm vs. 0.20 mm). Although the performance of these devices cannot be compared directly, power is generated by all three examples and thus provides a basis for a new type of fuel cell configuration that does not require a membrane or selective catalysts. 2

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Conclusions Our results demonstrate that the behavior of fluid flow at low Reynolds number can be exploited to generate small amounts of electrical power when used in combination with an electrode array and bioelectrocatalysts. Further efforts will focus on improving the performance of this type of fuel cell configuration {i.e., membraneless) with other channel/electrode configurations and biocatalysts.

References 1. Dyer, C. K. Fuel cells for portable applications. J. Power Sources 2002, 106, 31-34. 2. Palmore, G. T. R.; Whitesides, G. M . Microbial and enzymatic biofuel cells. In Enzymatic Conversion of Biomass for Fuels Production; Himmel, M . E.,

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Baker, J. O., Overend, P., Eds.; Am. Chem. Soc. Symp. Ser. 566: Washington, DC, 1994; pp 271 -290. Kreysa, G.; Sell, D.; Krämer, P. Bioelectrochemical fuel cells. Ber. Bunsenges. Phys. Chem. 1990, 94, 1042-1045.

4. Katz, E.; Bückmann, A. F.; Willner, I. Self-powered enzyme-based biosensors. J. Am. Chem. Soc. 2001, 123, 10752-10753. Karn et al.; Nanotechnology and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Palmore, G. T. R.; Kim, H.-H. Electro-enzymatic reduction of dioxygen to water in the cathode compartment of a biofuel cell. J. Electroanal. Chem. 1999, 464, 110-117. Katz, E.; Willner, I.; Kotlyar, A. B. A non-compartmentalized glucose |O biofuel cell by bioengineered electrode surfaces. J. Electroanal. Chem. 1999, 479, 64-68. Tsujimura, S.; Fujita, M ; Tatsumi, H.; Kano, K.; Ikeda, T. Bioelectrocatalysis-based dihydrogen/dioxygen fuel cell operating at 2

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Chen, T.; Barton, S. C.; Binyamin, G.; Gao, Α.; Zhang, Y. Kim, H.-H.; Heller, A. A miniature biofuel cell. J. Am. Chem. Soc. 2001, 123, 86308631. 9. Mano, N.; Kim, H.-H.; Zhang, Y.; Heller, A. An oxygen cathode operating in a physiological solution. J. Am. Chem. Soc. 2002, 124, 6480-6486. 10. Xia Y. N.; Whitesides, G. M . Soft lithography. Angew. Chem. Int. Ed. Engl. 1998, 37, 551-575. 11. Duffy, D. C.; McDonald, J. C.; Schueller, O. J. Α.; Whitesides, G. M . Rapid prototyping of microfluidic systems in poly(dimethylsiloxane). Anal. Chem. 1998, 70, 4974-4984. 12. Moreau, W. M . Semiconductor lithography principles, practices, and materials. In MicroDevices Physics and Fabrication Technologies—The

Physics of Micro/Nano-Fabrication; Muray, J. J.; Brodie, I., Eds.; Plenum: New York, 1988, p 607. 13. Gelo-Pujic, M . ; Kim, H.-H.; Butlin, N . G . ; Palmore, G . T . R . Electrochemical studies of a truncated laccase produced in Pichia pastoris. Appl. Environ. Microbiol. 1999, 65, 5515-5521.

14. Choban, E. R.; Markoski, L. J.; Stoltzfus, J.; Moore, J. S.; Kenis, P. Α.; Microfluidic fuel cells that lack a PEM. Power Sources Proc. 2002, 40, 317-320. 15. Ferrigno, R.; Stroock, A. D.; Clark, T. D.; Mayer, M.; Whitesides, G. M., Membraneless vanadium redox fuel cell using laminar flow. J. Am. Chem. Soc. 2002, 124, 12930-12931 (addition: J. Am. Chem. Soc. 2003, 125, 2014).


Membraneless Fuel Cells: An Application of Microfluidics - ACS

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