European Journal of Scientific Research - EuroJournals

Operating Characteristics of Proton-Exchange-Membrane (PEM) Fuel Cells 404
fact, it may be of interest that Al/xxx/Cu cell did not clearly exhibit mass transport overpotential, while
it was clearly observable for Al/xxx/Al cell. Finally, the collector may undergo galvanic reaction,
oxidizing itself to provide external current. Compared to PtxPt fuel cell, the platinum electrodes are
also acting as both catalyst and conductor, shown is Fig. 7.
As an experimental variable, any gas leakage would lower the partial pressure of the gas at the
electrode/electrolyte interface, hence lowering the current density. Actually, the current-voltage curves
for the two fuel cell current collector combinations do not vary all that much; it is only at high currents
that the Al/xxx/Al cell seemed to mass transport limit sooner. Corrosion of the anode current collector
is a fundamental problem of fuel cell technology, and so was observed in this designed too. Corrosion
of some materials may occur even without producing an external current, undergoing a redox reaction
directly with water, O2, or the combination of the two. The anode assumes the potential given by the
partial pressure of O2 over it, which at room temperature, open circuit, acidic standard state, and one
atmosphere of pressure, is 1.23 V. Very few electronically conductive substances are indefinitely stable
under those conditions.
The reason for the initial drop (Fig. 7), or activation polarization, is probably due to effects of
internal resistance of conducting metals as there is no change in the electrode characteristics. This
effect is also depicted by the dominant factor for the linear drop in the operating range adding to the
ohmic resistance of the electrolyte.
Table 3: Variation of efficiency with internal resistance
Conductor Int Res/ohm Efficiency Efficiency %
AlxxxSteel 0.016 0.061 6.0
PtxxxPt 0.035 0.473 47.3
AlxxxNi/Sn 0.043 0.216 21.6
AlxxxCu 0.074 0.557 55.7
AlxxxAl 0.076 0.554 55.4
AlxxxSn 0.089 0.041 4.1
Efficiency
0.60
0.50
0.40
0.30
0.20
0.10
Figure 8: Variation of efficiency with internal resistance
Al/xxx/Steel
Resistive-Efficiency Characteristics
PtxPt
Al/xxx/Ni/S
0.00
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
Internal Resistance / ohm
The current-voltage characteristics show that conductor characteristic play significant role in
the fabrication of the fuel cell. Obviously electronically conductive materials contribute less ohmic
resistance to the cell. In this type of cell design in particular, the current collectors are not porous, they
Al/xxx/Cu
Al/xxx/A
l
Al/xxx/Sn