Hydrogen gas as a power source is obtained from gaseous water, including seawater vapor existing abundantly at near-surface levels of the oceans or humid air over land. An integrated system of photovoltaic cells for capturing and harnessing solar energy is combined with a water vapor electrolysis system comprising an electrolyzer with an anode compartment and a cathode compartment separated by a proton exchange membrane. The photovoltaic aspects of the system convert the energy of the sun to drive electrolysis of gaseous water from the environment. The electrolyzer aspects include an anode, a cathode, and a proton exchange membrane. At the anode, oxygen evolution reaction (OER) catalysts oxidize H2O to oxygen gas and protons, the latter being diffused through a membrane (e.g., a solid polymer electrolyte membrane such as Nafion). At the cathode, photogenerated electrons are conducted to hydrogen evolution reaction (HER) catalysts to reduce the protons to hydrogen gas, while concentration gradients drive the generated O2 back to the atmosphere.