The simplest and most common element, hydrogen is all around us, but always as a compound with other elements. To make it usable in fuel cells or otherwise provide energy, we must expend energy or modify another energy source to extract it from the fossil fuel, biomass, water, or other compound in which it is found. Nearly all hydrogen production in the United States today is by steam reformation of natural gas. This, however, releases carbon dioxide in the process and trades one relatively clean fuel for another, with associated energy loss, so it does little to meet national energy needs. Hydrogen can also be produced by electrolysis—passing an electrical current through water to break it into hydrogen and oxygen—but electrolysis is inefficient and is only as clean as the energy source used to produce the electrical current. There are, however, many possible ways to produce hydrogen with renewable energy. Some of the most promising are the following:
Heating biomass (or fossil fuels) with limited or no oxygen present can gasify it to a mixture of hydrogen and carbon monoxide known as synthesis gas or "syngas." Syngas can then be catalytically converted to increase the amount of hydrogen with a "water-gas-shift reaction, which reacts the carbon monoxide with water to form carbon dioxide and hydrogen. Another high-temperature process can convert biomass to an oily liquid known as pyrolysis oil, which can be converted to hydrogen using steam reformation and the water-gas-shift reaction.
Electrolysis can electrochemically split water into hydrogen and oxygen in essentially the reverse of the reaction in a fuel cell. To make sense for large-scale use, this process must use an inexpensive source of electricity. Because wind energy is currently the lowest-cost renewable energy, it is the leading candidate. It is also a variable source that would benefit from being able to produce hydrogen when its electricity is not needed and to add fuel-cell generation when electricity demand exceeds what the wind turbines can provide. The combination also benefits because electrolyzers require direct current and wind turbine power is produced as direct current before conversion back to an alternating current suitable for the electric grid.
How about short-circuiting the process to have renewable energy such as solar power produce hydrogen directly? Photoelectrochemical (PEC) hydrogen production replaces one electrode of an electrolyzer with photovoltaic (PV) semiconductor material to generate the electricity needed for the water-splitting reaction. The efficiency loss of separate steps is done away with, as is the cost of the other components of a solar cell. PEC is elegantly simple, but finding PV materials both strong enough to drive the water split and stable in a liquid system presents great challenges for researchers.
Another way to directly tap solar energy for hydrogen production is to take advantage of ways in which nature does so. Certain microalgae and photosynthetic bacteria do sometimes use photosynthesis to make hydrogen instead of sugar and oxygen. Among the challenges here is the fact that the algal enzyme that triggers the hydrogen production is inhibited by oxygen, which of course, the organism also normally produces. Another biological research avenue is to develop microorganisms that will ferment sugars or cellulose to hydrogen instead of alcohol.
Learn more about NREL's hydrogen production research.
Also see the hydrogen production basics from the U.S. Department of Energy's Fuel Cell Technologies Program.