Hydrogen purity

The specification a hydrogen product must meet, and the impurities that threaten it.

Electrolytic hydrogen leaves the stack saturated with water and containing traces of oxygen, and is then dried and purified to meet a product specification. Fuel cell applications are governed by ISO 14687:2025, which sets limits for water, oxygen, nitrogen and several other species at very low concentrations.

Two distinct failure paths matter operationally. Oxygen carried over from crossover, which points at separator or membrane condition, and moisture breakthrough, which points at the dryer rather than at the stack.

Distinguishing them quickly matters because they have different owners and different remedies. A purity excursion diagnosed as a stack problem when the dryer regeneration cycle has drifted is an expensive misdiagnosis.