Sound too good to be true? Well, there is an alloy of aluminum and gallium which, when exposed to water, can produce hydrogen, which, in turn, can be used to generate electricity.
For more about the science behind this, you can check out the article at www.sciencedaily.com/releases/2008/02/080219133226.htm. To watch this phenomenon in action, check out the video at www.youtube.com/watch?v=QekysnaBy1w.