This article is part of an in-depth special report on green hydrogen
This technology has been around since the 1920s and was widely used, particularly in the chemicals industry, before the natural-gas boom of the 1970s made steam methane reforming cheaper.
It is, however, the lowest-cost and most efficient form of electrolyser, particularly when powered by baseload sources. The electrolyte (electricity-conducting medium) is a solution of water and potassium hydroxide.
Proton exchange membrane (PEM)
This relatively new technology is currently more expensive and slightly less efficient than ALK electrolysers, but is rapidly catching up on both fronts.