Fuel cells with polymer electrolyte membranes are appealing because of their low-temperature operation and relatively simple construction. The polymer electrolyte membrane (PEM) fuel cell consists 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 Fuel Cell.
The Direct Methanol Fuel Cell (DMFC) was developed to tackle the fuel storage problem of hydrogen, and to eliminate the need of a reformer to convert methanol to hydrogen. 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 limitations for direct methanol fuel cells are (1) methanol crossover from the anode to the cathode across the membrane separator, (2) carbon monoxide poisoning, (3) high polarization of the anode for the oxidation of methanol, and (3) systems design. In a liquid-fed direct methanol fuel cell, excess methanol is supplied to the anode side of the membrane-electrode assembly. It is desirable to have all or most of the methanol to diffuse into the anode and react. A phenomenon occurs called “methanol crossover” where some of the methanol diffuses through the polymer-electrolyte membrane from the anode to the cathode. Most methanol that crosses over to the cathode will be oxidized electrochemically. This oxidation reaction lowers the cell performance and consumes some of the cathode reactants. If a reaction intermediate forms, such as carbon monoxide, it sticks to the catalytic surface, and the cathode catalyst will be poisoned. This phenomenon reduces cathode performance and fuel efficiency and requires the use of a dilute methanol feed solution to minimize crossover.
In the existing liquid feed DMFC, dilute methanol (2 to 3 molar, or 6.41% to 9.61% methanol by volume) is supplied to the anode of the electrode membrane assembly. Under these circumstances, the methanol diffuses into the anode and partially reacts. The remaining methanol leaves the electrode and diffuses across the membrane to the cathode. The loss of fuel reduces efficiency and leads to reduced cathode performance. The loss of methanol at the anode causes the concentration at the catalyst surface to decline near the membrane, which introduces a mass transfer resistance.
Since DMFCs are a new technology compared to other fuel cells, several problems need to be addressed. DMFC’s must become more compact, lightweight, and cost-efficient to meet the power demands of portable applications. The main components that need to be developed are the polymer membrane, the electrodes, the bipolar plates, and the methanol feed subsystem.
The polymer electrolyte membrane usually consists of a PTFE-based polymer backbone, to which sulfonic acid groups are attached. The acid molecules are fixed to the polymer, and the protons are free to migrate through the membrane. The polymer membrane is typically 175 microns, and the electrodes are usually 2 millimeters in thickness. Many polymer membranes are commercially available that have a thickness of less than 175 microns, but they have not been as efficient in conducting protons in the DMFC. One methodology of reducing methanol crossover is to improve the polymer membrane by investigating commercial membranes, creating composite membranes of Nafion, or applying a coating to Nafion.
The electrodes are made of a porous mixture of carbon supported platinum or platinum/ruthenium. To catalyze reactions, catalyst particles must have contact with the protonic and electric conductors. There also must be passages for reactants to reach catalyst sites and for reaction products to exit. The contacting point of the reactants, catalyst, and the electrolyte is conventionally referred to as the three-phase interface. To achieve acceptable reaction rates, the effective area of active catalyst sites must be several times higher than the geometrical area of the electrode. Therefore, the electrodes are made porous to form a three-dimensional network, in which the three-phase interfaces are located. The thickness of the electrodes is typically 0.45 mm (before hot-pressing), with a catalyst loading between 0.2 and 0.5 mg/cm2. The catalyst loading is the cost-prohibiting factor for the DMFC. The cost of the DMFC stack would be lower if the amount of platinum were reduced, another (cheaper) element is combined with it, or the platinum was replaced. Alternative catalysts can be examined to reduce carbon monoxide contamination and to lower the cost of the DMFC stack.
In the DMFC, the bipolar plates separate the reactant gases from adjacent cells, connect the cells electrically, and act as a support structure. The bipolar plates have reactant flow channels on both sides, forming the anode and cathode compartments of the unit cells on the opposing sides of the bipolar plate.
Most bipolar plates are made of stainless steel or graphite. Stainless steel plates are heavy components for a portable power system. Solid graphite plates are highly-conductive, chemically inert, and resistant to corrosion, but are expensive and costly to manufacture. Flow channels are machined or electrochemically etched to the graphite or stainless steel bipolar plate surfaces. These materials are not suitable for mass production, and therefore, new bipolar plates are required. Polymer composites are the material of choice because they are cheap, light-weight, and easily manufactured.
The methanol feed subsystem may consist of pumps, sensors, filters, and electronics to monitor and clean the reactants before being fed into the fuel cell. The methanol can be in the liquid or gaseous form to optimize the reactant concentration fed into the fuel cell. An example system is shown in Figure 2.
Figure 2. Direct Methanol Feed System.
A brief description of the components in Figure 2 is as follows:
• Oxidant Air Flow: The oxidant air is filtered for particulates as it is being sucked into the fuel cell from the atmosphere. The air pressure transducer keeps track of the air pressure coming into the fuel cell. The oxidant air is filtered again for particulates before it enters the fuel cell stack.
• Methanol Flow: The methanol/water mixture is stored in a methanol reservoir. Pumps are required to push the methanol out of its holding tank and into circulation so it can travel to the fuel cell stack. A mixer ensures a uniform solution of methanol and water. The methanol sensor will tell the control system the concentration of methanol (molarity) that will be entering the fuel cell. The cold start heater will heat the methanol to the desired temperature to obtain the correct vapor-to-liquid ratio. Thermocouples will be used to measure the temperature of the methanol input.
• Water, Methanol, and CO2 Out: The water, methanol, and CO2 solution exiting the fuel cell stack are separated into their liquid and vapor components via a gas-liquid separator or a packed bed. The gas portion is cooled (to separate the water and excess methanol from the CO2), and then sent to exhaust. The water/methanol solution is held in a sump tank. Along with the liquid from the gas–fluid separator (water/methanol), water/methanol is also pumped from the sump tank. These two flows are combined and fed into a radiator, which cools the stream before it enters the methanol flow stream.
• Air/Water Out: An air/water purge stream exits the fuel cell stack by going through a particulate filter. The pressure transducer records the pressure of this stream. The air is vented, and the remaining water goes to the sump tank. Only a few sensors and pressure transducers are included in Figures 2. A fully developed control system may consist of thermocouples, pressure transducers, methanol, hydrogen, oxygen or altitude sensors, and mass flow controllers, which will measure and record data, and use feedback to control temperature, humidity, pressure or flowrates using a data acquisition and control program.
A lot of development work still needs to be conducted for DMFCs to be viable for commercial applications. The parts of the system that require further development are the methanol feed subsystem, new materials to construct a compact and inexpensive fuel cell stack, and optimization of processes for a more efficient system. Successful development of the direct methanol fuel cell system would lead to small, portable, low-cost power sources for the military.