Using Micro-Transport Phenomena in MEMs Fuel Cells

Written by Colleen Spiegel on Jan 23, 2018. Posted in Fuel Cell Information

A lot of work has been devoted to the development of long-lasting, efficient and portable, power sources for further technology improvements in commercial electronics devices, medical diagnostic equipment, mobile communication and military applications. These systems all require the power source to be lightweight, energy efficient, and able to operate for long periods of time without refueling. Portable power systems often consist of disposable alkaline, zinc-carbon, or lithium sulfur-dioxide batteries, or rechargeable lead-acid, nickel-cadmium, or lithium-ion batteries. The equipment for military personnel requires a heavy load of batteries to sustain power on 72-hour missions. Also, technological improvements in commercial electronics are slowing down due to the lack of small, high energy power sources to sustain the new features for long periods of time. The current options are not viable for these portable applications because they are heavy, impractical, and will not meet the current power requirements.

Fuel cells are known for high energy density, a variety of fuel sources, and ease of scaling for application-specific requirements. Fuel cells with polymer electrolyte membranes have been historically attractive because of their low-temperature operation and relatively simple construction.

Conventional polymer electrolyte membrane (PEM) fuel cells consist of two catalyst electrodes (the anode and cathode) separated by polymer electrolyte. Gaseous fuels are fed continuously to the anode (negative electrode), while an oxidant (oxygen from the air) is supplied continuously to the cathode (positive electrode). Electrochemical reactions take place at the electrodes to produce an electric current. A schematic representation of a direct methanol fuel cell (DMFC), which is a type of PEM fuel cell, is shown in Figure 1.

Figure 1. Direct Methanol Liquid-Feed Fuel Cell (DMFC)

The PEM fuel cell stack is made up of repeating cells separated by bipolar plates. Increasing the number of cells in the stack increases the voltage while increasing the surface area of the cells increases the current. Figure 2 shows a representation of a traditional fuel cell stack.

Figure 2. An Exploded View of a Polymer Electrolyte Membrane Fuel Cell Stack. (Picture: 3M¹)

The DMFC was developed to tackle the fuel storage problems of hydrogen, and to eliminate the need of a reformer to convert methanol to hydrogen for a hydrogen fuel cell to work. The DMFC is classified as a Proton Exchange Membrane Fuel Cell (PEMFC) because it also uses a PEM membrane. However, in addition to platinum, other catalysts like ruthenium (Ru) must be added to break the methanol bond in the anodic reaction.

The major obstacles for direct methanol fuel cells are:

1. Drying out of the membrane (especially at high temperatures)
2. Methanol crossover from the anode to the cathode across the membrane separator
3. Depolarization losses at the cathode due to methanol crossover
4. An efficient method of distributing the methanol and air on a micro-scale
5. High costs of fuel cell stack materials
6. Systems design

Some of these issues can be minimized by creating a microelectromechanical (MEMs) fuel cell system. MEMS devices range from 20 micrometers to a millimeter (i.e., 0.02 to 1.0 mm) in size, but can be between 1 and 100 micrometers. Many micro fuel cells utilize the traditional proton exchange membrane (such as Nafion®) to date, but this factor significantly limits the fuel cell size.

A novel methodology for solving some of these issues is to utilize the unique transport phenomena properties at the micro-scale to create a MEMs membraneless laminar flow direct methanol fuel cell (DMFC) system to provide a lightweight, low-cost power source that will meet the needs of future electronics devices and the modern soldier. The following technical hurdles would need to be thoroughly investigated to create this type of system:

1. Develop a membraneless DMFC fuel cell that will produce a lightweight and cost-efficient stack that will have minimal methanol crossover, decreased fuel cell thickness and size, limited water management issues;
2. Develop a method of distributing methanol and oxygen into the flow channels based upon the capillary force generated by micro pore sizes and;
3. Connect the fuel reservoirs directly to the fluid distribution system, instead of pumping the fuel into the fuel cell from a separate reservoir.

The properties of liquids at different scales can be used to provide optimal distribution of the reactants throughout the fuel cell. In the literature, laminar flow has been used to develop micro-fuel cells with a diffusive interface, and eliminate the need for a membrane. The use of micro-flow phenomena may significantly improve the characteristics of portable and MEMS fuel cell systems. The concept theoretically tackles the major obstacles associated with fuel cell membranes and allows for the design of smaller MEMS fuel cells with higher energy and power density.

Traditional fuel cells utilize graphite or other metal plates with machined flowfields to direct and distribute the gases or liquids across the platinum to generate as many hydrogen protons and electrons as possible. Most MEMS fuel cells have utilized a scaled-down version of the same approach. The method of distributing methanol and oxygen into the fuel cell can use the capillary force generated by micropores.

The total size of the MEMS fuel cell package can be reduced by connecting the fuel cartridges directly to the porous fuel channels. Many of the balance-of-plant components currently used with MEMS fuel cell devices are regular laboratory-scale fuel tanks, valves, pumps, etc.

To obtain the optimal dimensions of the fuel distribution layer and the membraneless fuel cell layer, a study of the transport phenomena of the gas and liquid flows through the system is required. Mathematical modeling can be conducted to determine the best flow characteristics with 2-phase flow in the flow channels.

To successfully demonstrate this type of system, the work can be broken into phases:

1. Modeling of the flow channel geometry for fluid distribution and laminar flow for a single channel (taking into consideration 2-phase flow)
2. Creating silicon-based MEMS fuel cells that demonstrate the concept of the model with known photolithography and semiconductor processing techniques
3. Converting the silicon-based MEMS fuel cells to polymer-based fuel cells with further experiments with the catalyst system
4. Creating the final polymer fuel cell package with multiple channels for laminar flow

Working on these phases will set the stage for developing a robust manufacturing process for membraneless high energy density micro fuel cells. For further information on this technology, please refer to the following sources:

[1] Electrochemical Engine Center, Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, 2002, M.M. Mench ,Z.H Wang, K.Bhatia, C.Y. Wang, “Experimental Study of a Direct Methanol Fuel Cell”.
[2] Nasa’s Jet Propulsion Laboratory, October 1999, Dr. S. R. Narayanan, Implications of Methanol Crossover in Direct Methanol Fuel Cells.
[3] Gottsfield, S. T. Zawodzinski, 1998, PEFC Chapter in Advances in Electrochemical Science and Engineering, Volume 5, edited by R. Alkire, H. Gerisher, D. Kolb, C. Tobias, pp. 197-301.
[4] K. Tomantshger, F., F. McCluskey, L. Optorto, A. Reid, and K. Kordesch, 1986, J. Power Sources,18,317,1986.
[5] C.Y. Wang, M.M. Mench, S. Thynell, Z.H. Wang and S. Boslet, August 2001, “Computational and Experimental Study of Direct Methanol Fuel Cells.” Int. J. Transport Phenomena, Vol. 3.
[6] Choban, Eric R., Larry J. Markoski, Andrzej Wieckowski, Paul J. A. Kenis. Microfluidic Fuel Cell Based on Laminar Flow. Journal of Power Sources. 128 (2004) 54 - 60.
[7] D.C. Duffy, J.C. McDonald, O.J.A. Schueller, G.M. Whitesides, Anal. Chem. 70 (1998) 4974 – 4984.
[8] Aravamudhan, Shyam, Abdur Rub, Abdur Rahman, and Shekhar Bhansali. A Porous Silicon Based Orientation Independent, Self-Priming Micro Direct Ethanol Fuel Cell. Sensors and Actuators A (Physical). Vol. 123-124 (2005). 497 – 504.
[9] Patent Application No. US 2006/0003217 A1. Planar Membraneless Microchannel Fuel Cell. Cohen, J., Volpe, D., Westly, D., Perchenik, A. and H. Abruna. Pub. Date: Jan. 5, 2006.
[10] Choban, E., Waszczuk, P. and P. Kenis. Characterization of Limiting Factors in a Laminar Flow-Based Membraneless Microfuel Cells. Electrochemical and Solid-State Letters, * (7) A348-A352 (2005).