When the fuel cell is operated in regenerative mode, the anode for the electricity production mode (fuel cell mode) becomes the cathode in the hydrogen generation mode (reverse fuel cell mode), and vice versa. When an external voltage is applied, water at the cathode side will undergo electrolysis to form hydrogen and oxide ions; oxide ions will be transported through the electrolyte to anode where it can be oxidised to form oxygen. In this reverse mode, the polarity of the cell is opposite to that for the fuel cell mode.