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Research Papers

Quantifying Individual Losses in a Direct Methanol Fuel Cell

[+] Author and Article Information
Brenda L. García-Díaz1

 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208

Jennifer R. Patterson

 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208

John W. Weidner2

 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208weidner@engr.sc.edu

1

Present address: Savannah River National Laboratory, Aiken, SC 29808.

2

Corresponding author.

J. Fuel Cell Sci. Technol 9(1), 011012 (Dec 22, 2011) (12 pages) doi:10.1115/1.4005394 History: Received April 07, 2011; Revised September 28, 2011; Published December 22, 2011; Online December 22, 2011

Studying the performance of a direct methanol fuel cell (DMFC) is complicated by the complex interactions of kinetic and transport processes. As a result, changes in one aspect of the cell have consequences in other aspects, which are difficult to elucidate from full-cell polarization (i.e., voltage versus current) behavior. This study outlines a strategy to use current and voltage relationships from anode half-cells, cathode half-cells, and hydrogen pump coupled with methanol crossover data and a mathematical model. In this way, all the kinetic and transport processes have been quantified, and the cell voltage was deconstructed (i.e., individual voltage losses were quantified). This data analysis accounts for all of the voltage losses observed during the operation of the full cell. As expected, the anode and cathode overpotentials accounted for most of the losses (i.e., 92% on average). Also, the cathode flow rate has been shown to affect the methanol crossover by diffusion. Cells operated at constant stoichiometry or where the cathode flow rate is small can show a parabolic shape in the methanol crossover because the electroosmotic drag dominates over diffusion as the primary transport mechanism for methanol through the membrane. Decrease in the methanol crossover was observed for cells with high compression and thicker cathode electrodes. The one-dimensional model, developed previously (García , 2004, “Mathematical Model of a Direct Methanol Fuel Cell,” J. Fuel Cell Sci. Technol., 1 (1), pp. 43–48), was improved by: (1) including methanol transport from the anode flow channel to the backing layer using a mass transfer resistance and (2) accounting for the unreacted methanol transport through the cathode. The model was able to reasonably predict the anode, cathode, full-cell polarization, and methanol crossover data for methanol concentrations between 0.05 M and 2 M at all operating currents.

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References

Figures

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Figure 5

Schematic of the cathode half cell

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Figure 6

Methanol crossover at open circuit as a function of the inlet cathode flow rate with model simulations (lines) for different methanol concentrations

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Figure 7

Experimental cell polarization (symbols) and model simulations (lines) for different methanol concentrations

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Figure 8

Experimental anode polarizations (symbols) and model simulations (lines) for different methanol concentrations

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Figure 9

Voltage response as a function of current for the hydrogen pump experiment

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Figure 10

Experimental cathode polarization (symbols) and model simulation (line)

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Figure 11

Methanol crossover at open circuit as a function of methanol concentration with model simulation (line)

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Figure 12

Experimental methanol crossover (symbols) and model simulations (lines) for different methanol concentrations

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Figure 13

Methanol crossover for 1 M methanol at a constant stoichiometry of 20 in the cathode

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Figure 14

Individual voltage losses from cell polarization with 1 M methanol concentration

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Figure 1

Schematic of the DMFC layers considered in the model showing the resistance at the anode flow channel–backing layer interface

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Figure 2

Schematic of a DMFC

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Figure 3

Schematic of the anode half cell

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Figure 4

Schematic of the hydrogen pump cell

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