TECHNOLOGYCreating Renewable Hydrogen
Electrolysis- Splitting H2O into H2 and O
Hydrogen can be produced by breaking water into its constituent parts through a process called electrolysis. Splitting water into hydrogen and oxygen with electricity was discovered a few weeks after the discovery of the battery more than 200 years ago. The introduction of low-cost electricity from solar and wind raises the possibility of creating hydrogen from low-carbon resources. Today, commercially available hydrogen is primarily derived from processing natural gas, leaving a significant carbon footprint. Transitioning to creating hydrogen from water and renewable electricity is a vital part of realizing a low-carbon future.
There are three main electrolyzer technologies today: Alkaline, PEM, and SOE. These are described briefly below.
Alkaline electrolysis is the oldest and simplest of the three electrolyzer technologies. Alkaline electrolyzers are the least expensive, most time-tested, and currently more efficient than the other commercial electrolysis technologies. Alkaline electrolyzers introduce an electric current through water containing an alkaline chemical catalyst. The basic arrangement is shown at right.
Most of the large-scale applications of electrolysis today are of the alkaline electrolyzer type. The Norwegian state utility Norsk Hydro had a 135 MW alkaline electrolyzer in service from 1953 to 1991. Its purpose was to use excess hydropower to produce hydrogen that was used in the production of ammonia-based fertilizer
Proton Exchange Membrane (PEM) Electrolysis
Another technology gaining in importance is PEM electrolysis, based on special polymer materials that can pass protons. The membrane separates the produced oxygen and hydrogen, allowing higher pressures to develop without dangerous mixing of hydrogen and oxygen within the cell. Importantly, this technology is virtually identical to PEM fuel cells that produce electricity from hydrogen and oxygen—the basic configuration, shown at right, is virtually reversible. Fuel cells are the power source for most hydrogen-fueled vehicles, and the association with fuel cells makes PEM a target of research and development efforts.
Solid Oxide Electrolysis
Another potential advantage of SOE is the ability to produce either hydrogen gas or a mixture of hydrogen and carbon monoxide with the addition of a carbon dioxide feedstock. The mixed gas can, in turn, be used to synthesize methane or other hydrocarbon transportation fuels. There is much optimism that SOE will proceed to commercialization in the next dozen years or so.