The present disclosure is directed to generating hydrogen using thermal energy. In some implementations, a method includes concentrating solar energy on an absorption element to heat the absorption element to about 2,000° C. or greater. The absorption element is in thermal contact with a reservoir of water. The water is at a pressure of, for example, approximately 760 Torr or less, and at least a portion of the water disassociates based on heat from the absorption element. The hydrogen and the oxygen are rapidly cooled to substantially avoid recombination. After cooling, the hydrogen gas and oxygen gas are pressurized and then separated using a size-selective membrane.