Nornjen the active site of hydrogenases is fairly well-studied, due to their potential utility in the energy industry. You can see find the active site structure of all three types of hydrogenase listed in the article here. The change in free energy of this reaction is relatively small under the right conditions, meaning there may not need to be an energy input to "power" the reaction. Due to the mechanism of proton reduction utilized by the enzyme, there would need to be an associated reductase not mentioned by the researchers in their article. Hydrogenases in vivo utilize iron-center oxidoreductase proteins to replenish/sink their electrons - something like cytochrome or ferredoxin. As exciting as this research may sound, it's a long way off from commercial viability.
Though there may not be a direct input of energy into the enzyme-catalyzed reaction, you would need to maintain the conditions under which the preferred direction of reaction would dominate; and this is essentially how you would power the reaction. The question of electron supply is separate, as masstergee says -- in the article the researchers must provide electrons, as at time of writing they did not have a method for in-situ oxidation of a substrate to provide the electrons -- with water as a source of both protons and electrons likely being the end goal.
Ah, that was what I was missing! They have to supply this reaction with electorns.
But why is this useful for cars then, as they mentioned in the article? I mean if I need electrical energy (or some other form of energy) already. Why would i want to turn water into hydrogen to just burn it again in an inefficient combustion engine to power my car? When i could just use an electric engine instead?
Nevetheless, would be a more efficient way of producing rocket fuel, since a rocket can't power itself just by electric energy.