The double layer charging current arises because of the capacitive-like nature of the electrode/electrolyte interface. The current (I) through a capacitor is given by the product of the capacitance (C) and the rate of change in voltage (dV/dt): I (Amp) = C (Faraday) x dV/dt (Volt/sec). The double layer charging current will be constant if dV/dt and C are constant.
If the intrinsic capacitance of the interface is a function of voltage, than the double layer charging current would change.
The more likely source for what appears to be a changing double layer charging current is an electrical short in the cell. The region in which the double layer charging current dominates the CV scan (0.4 V to ~0.75 V) will not be constant (flat line on a plot I vs. V) if the fuel cell has an electrical short. In this case, Ohm’s law tells us that there will be increasing current shorting between the electrodes as the voltage between the electrodes increases. The observed current is the sum of the double layer charging current, current shorted through the fuel cell (and any Faradaic current). The short would result in a linearly increasing current observable in what is normally attributed to the “double layer charging” current.
For additional information, refer to the Electrochemical Surface Area article: http://www.scribner.com/forum/index.php?topic=28.0