As concern regarding environmental issues such as pollution and global climate change escalates and energy independence grows in importance, substantial international research continues to be focused on hydrogen as an alternative to fossil fuels. Hydrogen, which in pure form exists as molecular H₂, is an attractive option for a variety of reasons. First, it occurs in vast amounts on Earth as a component of water (H₂O) and other chemical compounds, and it is the most abundant element in the universe. Second, the combustion of H₂ with oxygen releases energy with only water vapor and heat as by-products; no carbon dioxide (CO₂), a greenhouse gas, is produced. This aspect makes fuel cells operating with H₂ leading candidates to replace gasoline-powered internal combustion engines. Third, on a gravimetric basis (kilogram for kilogram), hydrogen in the form of H₂ contains about three times the energy of gasoline.