This failed because you used an aluminum barrier. When you 'charged' it, you generated hydrogen and oxygen on the platinum, but the aluminum also underwent a reaction because you must have ion flow to get a circuit. So one side of the aluminum generated oxygen, which did nothing to the aluminum, and the other generated hydrogen which partially destroys the oxide coating. This basically turns your fuel cell into a low grade electrolytic capacitor. The minute you removed the charging potential there is a reverse potential generated by the aluminum that reforms the oxide layer thereby preventing any serious flow of ions in your device. This lack of ion flow stymies your electric current, but will give you a measurable action potential until the hydrogen and oxygen are resorbed by the water.