Fuel cells employ membranes to keep the fuel and oxidant separate from the anode and cathode. What happens when there is mixing between the fuel and the oxidant?
It depends on the nature of the fuel and oxidant. If they react spontaneously, you'd get combustion inside the the fuel cell, typically generating a lot of heat and causing the cell to burst or even explode. If they don't react spontaneously, they will just mix, causing the cell potential in the two half cells to equalize and rendering the cell incapable of producing electricity.
