The open circuit voltage also decre

The open circuit voltage also decreased substantially, from 0.91 to 0.25V, between 5000 and 10,000 h of operation. During the extended operation we also observed that we no longer had closure on the mass balances. There was substantially more water in the effluents from the anode and cathode than corresponded to the fuel cell current. For example, the data in Fig. 8 indicate that the water removed in the anode and cathode effluents was nearly four times the water formed by the fuel cell current. This suggests that aging leads to increased membrane permeability allowing greater cross-over of the reactants (molecular hydrogen and oxygen pass through the membrane where they react on the Pt catalyst almost instantaneously to form additional water). The increased membrane permeability can account for both the decreased open circuit voltage and the loss of mass balance