How does PEM electrolysis work?

In this electrolysis process, ultrapure water (H₂O) flows around a proton exchange membrane (PEM). Located between the anode and the cathode, this membrane is ionically conductive. When an electric voltage is applied to both electrodes, the water on the anode side oxidizes to oxygen (O₂), free electrons, and hydrogen ions. The ions pass through the membrane by diffusion. They then combine with the electrons at the cathode to form hydrogen atoms (H₂). The reaction products of PEM electrolysis are therefore hydrogen and oxygen (H₂ and O₂).

PEM water electrolysis uses a solid polymer electrolyte – the polymer electrolyte membrane, which is also called the proton exchange membrane – as an ionically conducting medium between the anode and the cathode. This is what sets it apart from other water electrolysis processes such as alkaline electrolysis (AEL) or high-temperature electrolysis (HTE) with solid oxide electrolyzer cells (SOECs).

Compared to other electrolysis methods, PEM electrolysis boasts a number of key benefits, including higher power density and energy efficiency, as well as high hydrogen output pressure. There are other system benefits of using PEM electrolyzers, too – they offer maximum flexibility, require very little space, and can be scaled with ease. Moreover, PEM electrolyzers are very safe and reliable, making them the ideal complementary technology to renewable energy sources such as wind and solar power for producing green hydrogen.

The Bosch PEM electrolysis stack is the compact powerhouse at the center of the electrolysis system. It consists of more than 100 cells, each of which has an anode, a cathode, and a proton exchange membrane. Each stack weighs around 2850 kg and measures 85 × 100 × 153 cm. The stack produces up to 23 kilograms of hydrogen per hour, which is equivalent to a power input of 1.25 megawatts, so it is ideally suited for use on an industrial scale. The hydrogen production output pressure reaches values in excess of 30 bar. In many applications, this high pressure means that expensive auxiliary units are not needed.

_{The Bosch electrolysis stack is currently in the pilot phase. All technical specifications given are development objectives.}