The technology with the most potential to reshape the world energy economy in the century ahead may be the fuel cell. The principle behind fuel cells was first discovered in 1829, nearly 50 years before the first internal-combustion engine. Fuel cells are electrochemical devices that combine hydrogen and oxygen in an electrolyte fluid (a solution of ions that conducts an electric current), creating an electrical charge across a membrane. The reaction produces a steady flow of electricity. Unlike most power plants, which use mechanically spinning generators, fuel cells have no moving parts. But the technology was expensive, and interest in the concept withered. Advances in materials and electronics were necessary to make fuel cells useful and practical. In the 1960s fuel cells captured the interest of the U.S. space program, which developed small, efficient fuel cells for use in spacecraft. These orbiting fuel cells were expensive, but by the 1980s—in the wake of the 1970s oil shortages—they had again attracted the interest of government researchers and investors. Fuel cells are roughly twice as efficient as conventional engines in converting fuels to mechanical or electrical power. They require little maintenance, are nearly silent, and emit only water vapor. Along with the solar cell, some experts believe the fuel cell could allow human civilization in the 21st century to step beyond the age of fire (combustion), which has provided the bulk of the world’s energy for more than ten millennia. Unlike most power plants, where larger facilities were long associated with lower costs per unit of energy, fuel cells are nearly as economical on a small scale as on a large one. The first generation of fuel cells will likely obtain hydrogen from natural gas, which can be separated into hydrogen and carbon dioxide when it is heated. But the long-term goal is to use hydrogen directly. Hydrogen is the most abundant element in the universe and is found on earth as a component of water. Hydrogen can be produced from water through electrolysis, which involves splitting water molecules into oxygen and hydrogen by running an electric current between submerged electrodes. Electricity generated from renewable resources can produce hydrogen through electrolysis, but the process is expensive using currently available technologies. Chemists recently developed a solar-powered “water splitter” that nearly doubles the efficiency of converting solar energy to hydrogen. But the procedure is costly, using two different semiconductors. Finding less-expensive semiconductors is one key to making the device economical. Some experts believe that the discovery of an inexpensive and efficient way to electrolyze water would make hydrogen-powered fuel cells the world’s dominant energy carrier within a few years. Until that occurs, natural gas could form a kind of bridge to a hydrogen-based energy system. Natural gas is more abundant than oil, and it has been less heavily exploited, raising the prospect that it will be an important energy source early in the next century. Because the system for transporting natural gas can also be used to carry hydrogen, a separate system for hydrogen could be built up gradually. One approach would be to mix hydrogen with natural gas and carry the fuels in the same pipelines, shifting later to new pipelines that are designed to carry pure hydrogen.