MICRO FUEL CELLS

(January, 2013)

As portable electronic devices advance, with increases in operational speed, improved screen resolution, and higher functionality, there is an increased need for higher energy density and more efficient portable energy sources. Chemical limitations in currently used nickel-metal-hydride and lithium ion batteries prevent their improvement from surpassing 15-25% of the current state-of-the-art, at best. Thus, more advanced energy storage to supplement or to replace batteries is needed. With a much higher energy density than current batteries, micro fuel cells could be one solution. Experts predict that, in the future, fuel cell batteries will be able to last two or three times longer than current batteries (Grinberg and Skundin, 2010). There are differences in the definition of a micro fuel cell, some scientists considering it a very small fuel cell, while others include microfabricated conventional fuel cells within the definition. Independent of the definition, the electrochemical reactions that occur within are common to them all. Applications for portable energy storage devices are almost endless, but the specific requirements of application along with complications related to materials, structures, and manufacturing limitations lead to different design approaches. Despite this technology not yet being available in the marketplace, there is great hope for availability in the future.

Much focus over the past two decades has been placed on fuel cells as a source for future large scale efficient energy conversion, however not as much attention has been given to micro fuel cells. However, good reviews of the work on micro fuel cells can be found in the literature (see the Bibliography). Micro fuel cells have many similarities to conventionally sized fuel cells. The basic components needed for operation of micro fuel cells, such as electrodes, membranes, current collectors, and gas distribution, parallel those needed for conventional fuel cells, but in a miniaturized form. The material composition of the parts may differ however, because of the way the fuel cell is constructed and its processing conditions. Much like traditional fuel cells, micro fuel cells are often not self-contained, since elements such as fuel tanks lie outside of the cell and are connected to it separately. For this reason, some types of micro fuel cells alone are unable to replace batteries in portable electronic devices.

Micro fuel cells are often fabricated using microfabrication processes. Microfabricated fuel cells, on the other hand, are not even necessarily small. �Microfabricated� describes the process by which the fuel cell is made, which are often applied to create microscopic or miniaturized structures and devices. Thus certain flow field components and other structures of a conventional fuel cell might be microfabricated in order to improve performance or to reduce cost. Examples of microfabricated processes include thin-film processes like those used to make silicon chips (Kelley, ... 2002) and thick-film processes like those used to make printed circuit boards (Wainright, ... 2003).

To completely replace batteries, the micro fuel cell must be self-contained and all components including the fuel must be confined within the device. Other challenging design and materials issues include the elimination of pumps or compressors, and a capability to operate on atmospheric air, at ambient temperature and humidity. While various types of micro fuel cells such as those with solid oxide electrolytes are being developed, this article will focus on proton conducting polymer-based hydrogen-air and methanol-air fuel cells.

Fuel cells - how they work

Fig. 1. The reactions occurring within a hydrogen-air fuel cell.

A proton exchange membrane (PEM) serving as the electrolyte is not electronically conductive, so the electrons that are released from the hydrogen are unable to pass through it, but instead flow through an external circuit from the anode to the cathode. The flow of electrons within the circuit creates an electric current that can be used to power various electronic devices. The reaction will continue until all of the fuel is consumed; thus, the overall reaction equation in a hydrogen-air PEM fuel cell is the formation of water from hydrogen and oxygen.

Another simple fuel for PEM fuel cells is methanol; and the same basic principles apply. In a direct methanol fuel cell, the oxidation of methanol and the reduction of oxygen result in the production of water and carbon dioxide and the water can be recycled to be used at the reaction occurring at the anode (see the Appendix).
 

Components of fuel cells

Fig. 2. The basic structure of a PEM fuel cell.	Fig. 3. A deconstructed fuel cell.

 

Electrolyte

In a PEM fuel cell the electrolyte is a proton exchange membrane or the PEM. In both methanol and hydrogen fueled cells, the PEM is typically made of perfluorinated sulfonic acid polymer like Dupont�s Nafion. If the PEM is too thin, a high amount of crossover of reactants occurs, but if it too thick, there is a high resistance and therefore a lower voltage output. A Nafion membrane requires hydration in order to conduct hydrogen ions, so operating under ambient conditions is a challenge since too low of hydration will increase cell resistance and too high hydration will cause water to flood the pores in the cathode electrode. Also, swelling of the polymer during fuel cell fabrication or during operation is a special challenge with micro fuel cells.

Electrocatalyst/electrodes

The electrode structures of the catalyst layer are constructed to efficiently transport gaseous or liquid reactants, ionic products, and electrons. For this reason the structures are typically porous with a high surface area, and have electronically and ionically conductive components. The combination of the membrane with the anode and cathode electrode structures is called the membrane electrode assembly (MEA).

Gas diffusion layer

Ideally, the chemical reaction in the fuel cell should happen fast, efficiently, and effectively and the design and materials of these components of the cell determine these rates and efficiencies.

Design approaches to micro fuel cells

Among the pioneers in micro fuel cells were Kelley, Deluga, and Smyrl (2000, 2002) who created a miniature PEM fuel cell where all of the components were analogous to that of a large fuel cell, but made through thin-film electronic microfabrication techniques. Their fuel cell was not completely self-contained however, and components such as the methanol fuel were heated. Their research and experimentation still profoundly affected the study of fuel cells, as they found that the performance of a micro fuel cell is able to very closely match that of a large fuel cell operating under similar conditions.

Fig. 4. A cross section of three consecutive hydrogen-air fuel cells.

Wainright, Savinell, Liu, and Litt (2003) reported a design for a microfabricated polymer based fuel cell with on-board hydrogen storage which can operate passively as Figure 4 shows. The principle of their design lies in a planar array of edge collected cells where the cathodes would be all exposed directly to air and beneath the anodes would be a common fuel cartridge. The electrodes are interconnected through the edge of the individual cells. This is a special ability of the small size electrodes which would lead to high ohmic losses and non-uniform electrode activity for large electrodes. The main components of the cell such as the current collector, gas diffusion layer (GDL), catalyst layers, and electrolyte were manufactured through a process of thick film printing. They report using a Nafion membrane deposition technique that gives a good quality and stable membrane film. On-board fuel storage was a main feature in the design shown in Figure 4. One way of storing hydrogen is to print a metal hydride ink, which has high volumetric hydrogen storage density, into the anode compartment. The anode can then be charged with hydrogen. For this to be a viable on-board storage, the metal hydride has to be charged with hydrogen relatively fast at low to moderate pressures and hydrogen must be delivered to the fuel cell at adequate rates. Another means of storing hydrogen fuel is through a chemical form, for example, such as stabilized sodium borohydride in aqueous solution because it is stable at room temperature for several months when its pH is about 14. When this solution comes into contact with a catalyst, a reaction occurs to separate out the hydrogen which is fed to fuel the cell (see the Appendix). To design the cartridge with the catalyst included, a small well might be etched into the support structure with the catalyst printed at the bottom of the well (see Figure 4). To activate the fuel cell, sodium borohydride solution is injected into the well. Upon solution contact with the catalyst, hydrogen bubbles up to the anode electrode structure. One drawback of this design is the orientation dependency of the cell.

Methanol has been receiving much attention as a potential fuel source for micro fuel cells because it is cheap, easily stored and handled, and in a fuel cell can oxidize at room temperature with an appropriate catalyst. Problems are still posed, however, including its slow reaction and many design challenges that come when one tries to shrink a conventional fuel cell.

One challenge in developing a methanol micro fuel cell is designing a fuel tank that holds enough fuel to run for a reasonable period of time before needing to be refueled. For methanol fueled cells, there is a small range of concentration of the methanol in water within which it will function. A high methanol concentration level seems to be more desirable because the fuel has a higher energy density. However, typically the methanol concentration must be only between 0.5 and 2.0 M because if it is too concentrated, crossover rates (rate at which methanol permeates across the membrane) are too high for the fuel cell to function properly. When a large amount of cross-over occurs, the methanol begins to self-discharge which provides the cell with additional heat instead of electricity while greatly reducing the voltage of the cathode. For this reason, much effort has been put into finding a more efficient membrane that can handle high concentrations of methanol fuel.

Fig. 5. Design of a cross section of a fuel cell with a composite Nafion and silicon dioxide membrane with fuel microchannels (from Li, ... 2006).

When designing a fuel cell, the key idea is to increase the cell�s energy and power density while also maintaining reliability. In a portable fuel cell, each and every individual component of the fuel cell must be reduced in size proportionately. This creates issues with the Nafion membrane because when the membrane becomes thin, more fuel crossover occurs. In addition, a Nafion membrane needs to be humidified, which is difficult to maintain without external humidification and operating in a variety of atmospheric conditions. Kohl and coworkers (Moore, ... 2005, Li, ... 2006) proposed a silicon dioxide membrane as a potential solution to this problem because silicon dioxide does not swell with water and is less susceptible to environmental changes or even drying out. The silicon dioxide membrane, however, is a thin glass membrane that is brittle and can be easily cracked or formed defectively. An ideal membrane may be fabricated by layering Nafion on top of silicon dioxide as a composite structure because this helps to ensure long term reliability. They have focused attention on developing such a membrane for a microfabricated fuel cell, and Figure 5 shows their design approach. They have found that by doping silicon dioxide with phosphorus they were able to increase the hydrogen ion conductivity of the membrane. They could therefore make the membrane thicker in order to increase the mechanical strength of the membrane without sacrificing any of the cell�s performance because of the higher membrane resistance.
 

A look into the future

Hydrogen-oxygen fuel-cell reactions

Direct-methanol fuel-cell reactions

Hydrogen storage in sodium borohydride

Acknowledgements

Related articles

• Progress and Perspectives in Micro Direct Methanol Fuel Cell, S. Sundarrajan, S. Allakhverdiev, and S. Ramakrishna, �International Journal of Hydrogen Energy� Vol. 7, pp 8765-8786, 2012.

• Microfuel Cells: Modern State and Future Development, A Review, V. A. Grinberg and A. M. Skundin, �Russian Journal of Electrochemistry� Vol. 46, pp 963-978, 2010.

• Investigations on Fabrication and Lifetime Performance of Self-Air Breathing Direct Hydrogen Micro Fuel Cells, S. Giddey, S. P. S. Badwal, F. T. Ciacchi, D. Fini, B. A. Sexton, F. Glenn, and P. W.Leech, �International Journal of Hydrogen Energy� Vol. 35, pp 2506-2516, 2010.

• Performance of Micro-PEM Fuel Cells with Different Flow Fields, Y. Lu and R. G. Reddy, �Journal of Power Sources� Vol. 195, pp 503-508, 2010.

• Designing Microfuel Cells for Portable Electronics, B. Flipsen, �Journal of Fuel Cell Science and Technology� Vol. 7, pp 061014-1 - 061014-8, 2010.

• Fuel Cell Engines, M. M. Mench, John Wiley & Sons, Hoboken, NJ, 2008.

• Overview on the Challenges and Developments of Micro-Direct Methanol Fuel Cells (DMFC), S. K. Kamarudin, W. R. W. Daud, S. L. Ho, and U. A. Hasran, �Journal of Power Sources� Vol. 163, pp 743-754, 2007.

• Micro-Fuel Cells � Current Development and Applications, A. Kundu, J. H. Jang, J. H. Gil, C. R. Jung, H. R. Lee, S. H. Kim, B. Ku, and Y. S. Oh, �Journal of Power Sources� Vol. 170, pp 67-78, 2007.

• Microfabricated Fuel Cell with Composite Glass/Nafion Proton Exchange Membrane, J. Li, J. C. W. Moore, D. Bhusari, S. Prakask, and P. A. Kohl, �Journal of The Electrochemical Society� Vol. 153, pp A343-A347, 2006.

• Microfabricated Fuel Cells with Thin-Film Silicon Dioxide Proton Exchange Membranes, C. W. Moore, J. Li, and P. A. Kohl, �Journal of The Electrochemical Society� Vol. 152, pp A1606-A1612, 2005.

• PEM Fuel Cells, F. Barbir, Elsevier-Academic Press, Burlington, MA, 2005.

• Investigation of Acidic Methanol Solution as a Fuel for Microchannel Fuel Cells, J. Li, C. Moore, and P. Kohl, �Journal of Power Sources� Vol. 138, pp 211-216,(2004).

• Microfabricated Fuel Cells, J. S. Wainright, R. F. Savinell, C. C. Liu, and M. Litt, �Electrochimica Acta� Vol. 48, pp 2869-2877, 2003.

• Miniature Fuel Cells Fabricated on Silicon Substrates, S. C. Kelley, G.A. Deluga, and W. H. Smyrl, �Journal of the American Institute of Chemical Engineers� Vol. 48, pp 1071-1081, 2002.

• A Miniature Methanol/Air Polymer Electrolyte Fuel Cell, S. C. Kelley, G. A. Deluga, and W. H. Smyrl, �Electrochemical and Solid-State Letters� Vol. 3, pp 407-409, 2000.

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