Fuel cells are the next generation of alternative energy. While standard household or car batteries provide stable reliable power for a very wide range of applications, and combustion engines will provide large amounts of power as long as you continue to provide fuel, there are obvious deficiencies to both. Batteries wear out, and engines are undesirably inefficient. Fuel cells are the solution to both of these problems, and they boast several other advantages too!
Fuel cells are devices that convert chemical energy directly into electricity. The easiest fuel cell system to think about is the H2/O2 polymer electrolyte membrane fuel cell (PEMFC). In this system there are two reactions to consider (just like any battery):
| Oxidation (anode): | H2 (g) → 2H+(aq) + 2e– | |
| Reduction (cathode): | O2 (g) + 4H+(aq) + 4e– → 2H2O(l) |
Thus, with continous inputs of hydrogen and oxygen gases, one can build a system that generates a very scalable source of power, like batteries. The oxidation reaction occurs on the surface of a catalyst (usually Pt) deposited on the anode, while the reduction reaction occurs on a potentially different catalyst (though frequently also Pt) deposited on the cathode. The third and most important component in every electrochemical cell is the electrolyte, which in this case must be proton-conducting, electrically insulating, resistant to strong oxidants and reductants, and thermally and mechanically stable. The electrolyte is almost always a polymer membrane, such as Nafion®. The anode, the polymer electrolyte membrane and the cathode are sandwiched together to make the fuel cell.
The anode reaction produces protons and electrons, which, as can be seen from this diagram, are separated: the electrons (black) are routed through a wire to provide access to the electrical energy and the protons (yellow) are allowed to pass through the electrolyte membrane (here depicited by blue/brown interface). The cathodic reaction is the recombination of the protons and electrons in the presence of oxygen (red) to make water. Because this device does not rely on a thermal cycle, the theoretical efficiency is not limited by the Carnot cycle, and so fuel cells can outpreform combustion engines by a significant amount. One final remark, which will appeal to the environmentalists, is that this energy device has the potential to replace petroleum-powered devices, and thus reduce the consumption of fossil fuels and curb the production of CO2. Unfortunately, all affordable sources of fuel for fuel cells today are, in fact, derived from petroleum.
At typical operating temperatures of around 80°C, problems of CO poisoning (as a result of residual CO from the petroleum reformation) of the platinum anode catalyst and thermal and water management make the development of a simple and reliable fuel cell system more difficult. Hydrocarbons, such as natural gas, gasoline or alcohol are more economical fuel sources and are more technically viable in the near-term than hydrogen, but the problem of CO poisoning is exemplified as the partial oxidation products of such fuels form high concentrations of CO at that anode catalyst. One solution is increasing the temperature of fuel cell operation, as this reduces the adsorption of CO onto the platinum electrocatalyst and improves performance (as long as the temperature remains below the phase transition temperature of the membrane). In addition, the heat loss through radiation of the fuel cell stack is greatly enhanced at high temperatures, and minimizes the need for costly heat exchange devices in large scale applications. Also, at high temperatures, product water can be generated as a vapor at the cathode, alleviating the flooding problem of cell, which causes both inaccessibility of the catalyst particles and loss of voltage. However, increasing the temperature above 100°C has a down-side for PEMFCs because of the reliance of the membrane on interior water for proton conduction. As the temperature increases, the evaporation of water from the membrane leads to a dramatic drop in proton conductivity. In order to maintain adequate membrane hydration, increases in temperature need to be accompained by increases in operating pressure to ensure that sufficient water vapor and reactant gases are available in the reaction compartments. The practical upper limit for the operating pressure is around 3-4 atm.
Current group interest in this project include a focus on the advancement of a Direct Ethanol Fuel Cell, an exploration into various non-fluoronated polymer membranes, and an investigation into Solid Oxide Fuel Cells (SOFCs) using β”-alumina as the electrolyte. Additionally, there are efforts to understand the bulk mechanical properties of the electrolyte membrane materials in order to elucidate the microstructure. The first two of these projects rely heavily on two fuel cell test stations equipped to deliver humidified and non-humidified gases as well as liquids to a single cell setup. Automated data collection using a programable variable electronic load makes experiments far less tasking on the students. The other projects depend on the test stations as well, but also utilize electron microscopy and dynamic mechanical analysis techniques.
Publications on fuel cells:
- Electrocatalyst Design for an Elevated Temperature Proton Exchange Membrane Direct Ethanol Fuel Cell. ECS Trans.2008, 16 (2), 1285-1291.
- Viscoelastic Response of Nafion. Effects of Temperature and Hydration on Tensile Creep. Macromolecules. 2008, 41 (24), 9849-9862.
- Water Permeation Through Nafion Membranes: The Role of Water Activity. J. Phys. Chem. B. 2008, 112 (51), 16280-16289.
- NMR Characterization of Composite Polymer Membranes for Low Humidity PEM Fuel Cells. J. Electrochem. Soc. 2007,154 (5), B466-B473.
- Water Sorption, Desorption and Transport in Nafion Membranes. J. Membr. Sci. 2007, 301 (1-2), 93-106.