Gas crossover

Hydrogen permeating through the separator or membrane into the oxygen stream, and the reverse.

In any water electrolyser the two product gases are kept apart by a diaphragm or a polymer membrane. That barrier is not perfectly impermeable, so a small quantity of hydrogen continuously diffuses into the oxygen side.

The operationally important behaviour is how the crossover fraction changes with load. Hydrogen production scales with current, but permeation is driven largely by concentration and pressure difference, so it does not fall proportionally. As load drops, the same permeation is diluted into a smaller oxygen flow, and the hydrogen content of the oxygen stream rises. This is why the minimum stable load of an electrolyser is usually set by gas quality rather than by electrical capability.

Crossover also rises as the separator ages or thins, which makes its trend at a fixed load a useful condition indicator, distinct from its instantaneous value, which belongs to the safety system.