To get Hydrogen, just add some water. Moreover, perhaps you don't need liquid water because air has some water vapor, but I'm too lazy to do the stoichiometric calculation now.

Someone already though it: "Exclusive: Pioneering scientists turn fresh air into petrol in massive boost in fight against energy crisis" http://www.independent.co.uk/news/uk/home-news/exclusive-pio... [spoiler alert: they use electricity too]