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 said to be 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. It also uses pure water as its electrolyte, so recovery and recycling of potassium hydroxide is not required; it can output hydrogen at higher pressures, therefore requiring less energy for compression; and it takes up around half the space of comparable ALK systems.

The downsides are that the technology is more expensive, requiring expensive platinum- and iridium-coated electrodes and ultra-thin specialty-plastic membranes — and is said to have a shorter lifespan than ALK electrolysers.

Solid oxide electrolysis (SOE)

This technology uses solid oxides or ceramics as its electrolyte, with high-temperature steam (rather than water) being separated into hydrogen and oxygen.

It operates at higher efficiencies than ALK or PEM, but requires a heat source, so would be perfect for use with clean electricity generation that produces waste heat, such as nuclear, concentrating solar power and geothermal. Low-temperature heat from industrial processes will work equally well.

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