The Effect Of Operation And Design Parameters On The Performance Of The Direct Methanol Fuel Cell

S. F. Simpson, A. Cisar, K. Franaszczuk, H. Moaddel, D. Brejchova, C. Salinas, D. Weng, and O. J. Murphy Lynntech, Inc. 7610 Eastmark Drive Suite 105 College Station, TX 77840

Fuel cell technology continues to receive considerable attention as a potential replacement for fossil fuels as a primary source of terrestrial power. Ideally, such power systems would operate at relatively low temperatures (< 100°C) which suggests strongly the use of cell technology based upon the proton exchange membrane (PEM). Without question, hydrogen is a very desirable fuel choice for these types of systems, because of its high energy density. However, the difficulties associated with the production and routine handling of hydrogen limit severely its commercial use at present. The direct methanol fuel cell (DMFC) is a particularly attractive alternative to the use of the hydrogen/oxygen cell. Although not as high as hydrogen, the energy density of methanol is the highest among the organic fuels. Furthermore, because of the similarity in liquid handling requirements between methanol and gasoline, a significant portion of the infrastructure necessary for the marketing and distribution of the fuel is already in place. Other inherent attributes of the DMFC which include rapid start-up and operation with little or no emission or noise signature have led to an intense DMFC research effort over the past twenty years and, indeed, the DMFC has even been referred to as "the electrochemist's dream".1

Despite these advantages and the overall allure of the DMFC, a number of technical challenges must be overcome before the DMFC can become practical. These challenges pertain primarily to performance limitations of the cell that are the result of a number of factors such as the methanol crossover that occurs across the PEM and relatively poor electrocatalysis at the anode. As might be expected, these limitations and their causes will impose significant design restrictions and requirements on a DMFC stack, and an understanding of these constraints is of paramount importance to DMFC commercial development. Thus, the purpose of this work is twofold. Initially, a series of experiments were performed to characterize DMFC performance as a function of a number of experimental

1 S. Srinivasan, J. Electrochem. Soc., 136,41C (1989).

parameters such as reactant backpressure, concentration, and flow rate. Following this characterization, various experimental techniques and component design approaches were evaluated in an attempt to overcome the aforementioned limitations and improve the overall performance of the DMFC.

To appreciate the unique behavior of the DMFC, one need not look further than the nature of the fuel itself. As an example of how the cell reacts to different fuel conditions, the effect of methanol concentration on DMFC performance is presented in Figure 1. In addition, cell potential data obtained from both high and low current density cell operation are shown plotted as a function of fuel concentration in the inset in the figure. As the data in both the figure and the inset illustrate, cell performance and fuel efficiency at low current density decreases monotonically with increasing methanol concentration, and this effect is undoubtedly the result of methanol crossover through the PEM. In contrast, the cell voltage / fuel concentration profile for cell operation at high current density exhibits a maximum. Here, voltage decreases observed for solutions of low methanol concentration under high current conditions are the result of mass transport limitations within the electrode. From this relatively simple and straightforward experiment, it becomes clear that DMFC stack design must involve consideration of both the desired power output level and fuel efficiency which ultimately wijl be governed by the constraints of the application at hand.

Because the methanol fuel in the low temperature DMFC is supplied and utilized in the liquid state, it is to be anticipated that electrode designs implemented and used successfully in the familiar hydrogen/oxygen fuel cell will not be optimum for the DMFC. To illustrate the effect of electrode structure on DMFC performance, polarization curves were collected for a series of membrane and electrode assemblies in which all experimental and design variables were held constant except for the structure of the anode. The resulting curves are shown in Figure 2 and demonstrate clearly the critical dependence of DMFC performance on anode structure. As the data in the figure show, the cell performance level ranges from the very poor performance of structure 5 to the acceptable performance of structure 4. Significantly, it should be noted that some of the differences in the electrode structures represented by the curves in Figure 2 are the result of small, subtle changes in electrode fabrication technique.

As shown by these two examples, DMFC performance depends critically upon a number of experimental factors, and results will be presented which explore the effects of some of these variables. In addition, recent results will be presented regarding work in the area of catalyst development for use at both the anode and the cathode of the DMFC.

Simpson, et al

Figure 1. The Effect of Methanol Concentration on Fuel Cell Performance. The active electrode area was 25 cm2, and the cell temperature was 110°C. Other experimental conditions follow. Membrane: Nation® 117; anode: 4 mg/cm2 unsupported Pt-Ru; cathode: 4 mg/cm2 Pt black; O2 flow rate: 3.9 L/min; O2 backpressure: 30 psi; MeOH flow rate: 8 mL/min; MeOH backpressure: 30 psi; MeOH concentration: 2 M.

Current Density (mA/cm2)

Figure 1. The Effect of Methanol Concentration on Fuel Cell Performance. The active electrode area was 25 cm2, and the cell temperature was 110°C. Other experimental conditions follow. Membrane: Nation® 117; anode: 4 mg/cm2 unsupported Pt-Ru; cathode: 4 mg/cm2 Pt black; O2 flow rate: 3.9 L/min; O2 backpressure: 30 psi; MeOH flow rate: 8 mL/min; MeOH backpressure: 30 psi; MeOH concentration: 2 M.

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