Proton exchange membrane electrolysis makes hydrogen by splitting water alongside electricity across a thin solid polymer membrane. The membrane lets protons through but keeps hydrogen & oxygen apart. The hydrogen produced is extremely pure, up to 99.999%, making it ideal for fuel cell vehicles & power generation. The main problems are cost, because the cells need rare metals such as iridium & platinum, & gradual wear of the membrane. Companies such as Siemens Energy, ITM Power, Plug Power, Cummins & Nel are building large factories & developing cheaper, longer-lasting designs.