This article is part of an in-depth special report on green hydrogen

Alkaline (ALK)

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.

PEM has several potential advantages, which has led the technology to play a leading role in green hydrogen pilots — it can ramp up quickly and is highly efficient at low loads, meaning it is far better suited to the variable energy input supplied by wind and solar power.