Short description of the five types of water electrolysers. Modified from IRENA²²⁵.

	AWE	PEMWE	AEMWE	SOEC	PCCEL
Operating temperature	70–90 °C	50–80 °C	40–60 °C	700–850 °C	300–600 °C
Operating pressure	1–30 bar	<70 bar	<35 bar	1 bar	1 bar
Electrolyte	Potassium hydroxide (KOH) 5–7 mol L−1	PFSA membranes	DVB polymer support with KOH or NaHCO3 1 mol L−1	Yttria-stabilised zirconia (YSZ)	(Y,Yb)-Doped-Ba(Ce,Zr)O3−δ
Separator	ZrO2 stabilised with PPS mesh	Solid electrolyte (above)	Solid electrolyte (above)	Solid electrolyte (above)	Solid electrolyte (above)
Electrode/catalyst (oxygen side)	Nickel coated perforated stainless steel	Iridium oxide	High surface area nickel or NiFeCo alloys	Perovskite-type (e.g., LSCF, LSM)	Perovskite-type (e.g., LSCF, LSM
Electrode/catalyst (hydrogen side)	Nickel coated perforated stainless steel	Platinum nanoparticles on carbon black	High surface area nickel	Ni/YSZ	Ni/YSZ, Ni-BZY/LSC, BCFYZ
Porous transport layer anode	Nickel mesh (not always present)	Platinum coated sintered porous titanium	Nickel foam	Coarse nickel-mesh or foam	Coarse nickel-mesh or foam
Porous transport layer cathode	Nickel mesh	Sintered porous titanium or carbon cloth	Nickel foam or carbon cloth	None	None
Bipolar plate anode	Nickel-coated stainless steel	Platinum-coated titanium	Nickel-coated stainless steel	None	None
Bipolar plate cathode	Nickel-coated stainless steel	Gold-coated titanium	Nickel-coated stainless steel	Cobalt-coated stainless steel	Cobalt-coated stainless steel
Frames and sealing	PSU, PTFE, EPDM	PTFE, PSU, ETFE	PTFE, silicon	Ceramic glass	Ceramic glass