You regenerate the water when H2 and O2 are recombined to produce electricity. Dan Nocera ( http://nocera.harvard.edu http://nocera.harvard.edu ) is very promi

You regenerate the water when H2 and O2 are recombined to produce electricity. Dan Nocera ( http://nocera.harvard.edu http://nocera.harvard.edu ) is very prominent in the water splitting field and has done a calculation in which a volume of an olympic sized pool would need to be split every second to replace all power sources on earth ( https://poptech.org/blog/daniel_nocera_on_personalized_energy https://poptech.org/blog/daniel_nocera_on_personalized_energ... ). It seems like a lot, but it would presumably be decentralized across many many locations. Additionally, you don't need 24 * 60 * 60 * olympic pool volumes -- you can cycle the same amount of water into/out of a fuel cell in a closed system.