What is DMFC?
Fuel cells are electrochemical devices in which electricity is produced when fuel (anode electrode) and oxidant (cathode electrode) are supplied. Other by-products such as water and heat may also be produced. The anode is physically separated from the cathode by an ion-conducting material called electrolyte. Fuel cells are normally classified by the different type of electrolyte they use, for example, Alkaline Fuel Cells (AFCs), Molten Carbonate Fuel Cells (MCFCs), Phosphoric Acid Fuel Cells (PAFCs), Polymer Electrolyte Membrane (PEM) Fuel Cells, and Solid Oxide Fuel Cells (SOFCs). Most of the fuel cells use hydrogen as the main fuel. Hydrogen-rich hydrocarbon fuels can also be reformed to produce hydrogen, which can power the fuel cell. The reforming process can occur inside or outside the fuel cell.
Direct methanol fuel cells (DMFCs) do not require fuel reforming and can produce electricity directly from methanol diluted in water. DMFCs are a subset of PEM fuel cells and are primarily targeted towards small scale technologies (smaller than 1,000 W) and relatively low operating temperatures in the range of 60 to 90oC. The low-temperature oxidation of methanol requires a more active catalyst with a larger quantity of expensive platinum (Pt). DMFCs are becoming more attractive when size and weight are important, as in the case of portable devices used in military operations as well as in other civilian applications (laptop computers, cellular phones, toys, etc.). Methanol is also easy to transport and store, requiring infrastructure similar to gasoline, which is currently available. In contrast, it is projected that hydrogen production, storage and distribution systems may require relatively high investments in infrastructure, hindering implementations in the near future.
A schematic of a single cell DMFC is shown in Figure 1. Methanol (CH3OH) is electrochemically oxidized at the anode (negative electrode), producing electrons, which travel through the external circuit to the cathode (positive electrode) where they are consumed together with oxygen in a reduction reaction. The circuit is maintained within the cell by the conduction of protons in the electrolyte (normally a polymer electrolyte membrane, PEM, for example NafionTM). One of the shortcomings of the DMFCs is the unwanted crossover of methanol between the anode and cathode during operation through the electrolyte. This methanol crossover will reduce overall energy efficiency of the fuel cell. The anode reaction is CH3OH + H2O → CO2 + 6H+ + 6e-. The cathode reaction is 3/2 O2 + 6 H+ + 6e- →3 H2O. The overall reaction occurring in the DMFC becomes CH3OH + 3/2O2 → CO2 + 2H2O.
Generally, the challenges facing DMFCs are: 1) Overall costs have to be lowered, 2) Alternative or modified membranes have to be found to reduce the methanol crossover rate from the anode to the cathode while maintaining the power output, 3) Alternatives to Pt cathode (more tolerant to methanol contamination from the crossover) have to be found, and 4) Better electrocatalysts than Pt/Ru to lower activation overpotential for electro-oxidation of methanol have to be found.
Despite the current relatively high costs, the convenience of using a liquid fuel and the ability to perform at low temperatures without a fuel reforming unit make the DMFCs attractive for certain applications. A reduction in the methanol crossover rate could potentially improve the power density of DMFCs, expanding the range of applications.
Although a costly research to find alternative/modified membranes could be performed to minimize the crossover rate from the anode to the cathode, Kordesch et al. (J. Power Sources, 96, pp. 200-203, 2001) proposed a novel DMFC concept by introducing a flowing electrolyte (for example, diluted sulfuric acid, H2SO4 + H2O), reducing the methanol crossover from the electrolyte compartment by means of advection mechanisms (the methanol is carried away without contaminating the cathode). Through several collaborations with Kordesch’s group, the National Research Council of Canada, and the Alberta Research Council, Pure Energy Visions Corporation (PEV, Richmond Hill, ON) conducted R&D related to FE-DMFC. Approximately 4 million dollars were spent in these previous collaborations. See Figure 2 for a schematic of the PPS/PEV (Polygenic Power Systems / Pure Energy Visions Corporation, now PPS/HET, Polygenic Power Systems / Hybrid Energy Technologies Inc.) single-cell FE-DMFC unit. Diluted methanol passes through the channels inside the left graphite plate (highlighted by the square at the left side of the figure) while the air passes through the channels of the graphite plate at the right side as seen in the figure. The electrolyte flows through the channels shown at the middle of the figure. The electrolyte is separated from the electrodes by NafionTM membranes and Carbon paper. All layers are sealed with spacers. Steel endplates and copper current collectors are added into the single cell. It should be pointed out here that the NafionTM membranes may be substituted or even eliminated from the system in the future. PPS/PEV successfully tested a 5-W-stack FE-DMFC unithaving a power density of approximately 80 mW/cm2, with a methanol concentration of 1 M. A unified control system for mass flow rates (controlled valves for fuel, air, and electrolyte), and temperature (heaters and thermocouples) was also developed. All equipment, prototypes, materials, and technology were transferred to Carleton University.
Although previous experimental data from PPS/HET indicate an increase in FE-DMFC performance by approximately 30% when compared against a DMFC, separate work by Schaffer et al. (Journal of Power Sources 153, 217-227, 2006) showed even higher increases (by 100% in maximum power). Schaffer et al. pumped the electrolyte through a porous material instead of channels as in the case of PPS/PEV. Other advantages of FE- DMFC include a better control of temperature distributions in the whole system, as well as a quick shutdown of the system by simple withdrawing of the electrolyte flow.
One of the shortcomings of FE-DMFC is the management of the flowing electrolyte itself. The flowing electrolyte system could be divided into four categories: 1) Non-circulating (open-loop) flowing electrolyte: where fresh electrolyte is used and the methanol- contaminated electrolyte is stored in a container. The used electrolyte would be retrieved and recycled in separate industrial facilities using distillation or membrane separation processes (this container could also contain the fuel to be used in the DMFC and would resemble a cartridge during replacement), 2) Circulating (closed-loop) flowing electrolyte: requiring electrolyte change from time to time, in analogous manner as an oil change in a car (adequate usage has to be determined), 3) Circulating (closed-loop) flowing electrolyte with in-situ purification using distillation or membrane separation of methanol and electrolyte (not viable due to compactness reasons), and 4) Circulating (closed-loop) flowing electrolyte going through additional DMFC unit with consumption of fuel in the methanol-contaminated electrolyte (co-generation concept already tested by PEV).
Besides the flowing electrolyte management, the FE-DMFC unit also includes management systems for the methanol fuel and the air. All systems are integrated using a central control unit. The energy requirements for all three systems should be addressed as well. It is also anticipated that the performance of individual or stacked fuel cells will be different, mainly due to heat transfer and fluid-dynamic effects. The substitution or elimination of PEM (Nafion 117 as seen in Figure 2) will be addressed. The ultimate goal of the long-term collaboration between PPS/HET and Carleton University is the commercialization of FE-DMFCs. Starting from a 5-W prototype, which is currently available, comprehensive studies will be performed and a scale-up to a 50-W-stack FE- DMFC will be developed in this proposal.
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