This method sounds interesting. 
http://www.cie2007.com.br/trabalhos/eduardolopezsandoval.pdf

The standard potential of the water electrolysis cell is 2.06 V at 25 °C
The energy efficiency of water electrolysis varies widely. Some report 50–70%[2], while the theoretical maximum efficiency of the electrolysis of water is between 80–94%.[3] These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency may be closer to 30–45%.[4]
See:
http://en.wikipedia.org/wiki/Electrolysis_of_water