Carbon and Fluorinated Carbon Materials for Fuel Cells
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"
C02
+ 4H+ + 4e-
(1)
Water is the product of both the PAFC and PEM fuel cells and for the PEM fuel cell water is also a constituent of the perfluorinated sulfonic acid membrane electrolyte. In addition, reaction (1) is favored at the oxidizing potentials typical of the oxygen reduction electrode of the fuel cell, e.g., 0.6V - 1.2V vs. SHE. Two approaches to increase the stability of the carbon components are used: graphitization and application of Teflon to form a hydrophobic barrier on the surface of the carbon composite. The hydrophobic barrier prevents water, and phosphoric acid in the case of the PAFC, from contacting the carbon component. Only low levels of Teflon can be used in the carbon composite because Teflon is an electronic insulator and a fundamental requirement of the fuel cell component is high electronic conductivity. The unit cell for the PAFC fuel cell is schematically given in Figure 1 and the major carbon containing structures are identified. This unit cell is repeated up to several hundred times in a commercial PAFC cell stack and the cross-sectional area of the stack can be up to one square meter. The anode electrode, cathode electrode, and integral separator plate (ISP) shown in Figure 1 are three components of interest reported here. All three components are carbon-graphite-Teflon composite structures designed to control the distribution of phosphoric acid electrolyte in the PAFC. In particular, the ISP is designed as an impermeable barrier to the transmission of phosphoric acid electrolyte between adjacent cells. 1 The structure of the ISP is schematically given in Figure 2 which shows a graphite sheet with thin films of Teflon bonded to each surface of the graphite sheet. These 139 Mat. Res. Soc. Symp. Proc. Vol. 496 © 1998 Materials Research Society
thin bonded Teflon sheets are discontinuous allowing carbon-carbon contact between the separator graphite sheet and the adjacent respective anode or cathode flow fields as shown in Figure 1. A critical requirement of the ISP is to fill the pores of the up to 25% porous graphite separator to create a hydrophobic barrier to acid penetration while at the same time permitting the carbon - carbon contact to assure optimum electronic conductivity.
Figure 1. PAFC Cell Stack Assembly Configuration.
Teflon
Graphite
{
Teflon iAI'd1
Graphite Sheet
ISP Figure 2. Schematic configuration of an ISP. 140
HS97111 971612
EXPERIMENT and RESULTS Anodic polarization studies of graphitized separator plates identified the time dependence of the corrosion of the separator plates over the potential range 0.9V to 1.2V, vs. a hydrogen electrode in the same solution. Data were obtained using the potentiostatic method in 99% H3 PO 4 at 200'C with the data collected continuously over a 1000 minute interval. Typical data are shown in Figure 3. At potentials below 1000 mV the corrosion current, expressed as pA/mg carbon, continually decreases from 2 pA/mg C to nearly 0.1 pA/mg cm - an order of magnitude decrease in corrosion current. On the other hand, the c
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