H₂-O₂ Gas Cross-Over Safe Practice

Electrolysis cell membranes and diaphragms are good at keeping generated oxygen and hydrogen separate from each other during generation – but “cross-over” selectivity is not absolute. Analysis has shown that commercial electrolyzer product gases frequently include a very small percentage of H₂ in O₂ and O₂ in H₂ in the part per million range or lower – well below the flammable mixture limit of 4% H₂ in O₂ found in the literature. However, under transitory process conditions such as startup, intermittent power turn down, and cell stack pressure imbalance, or toward the end of life for the electrolysis cell stack, these concentrations may increase significantly. In the worst case, flammable mixtures may be generated and transported downstream into electrolysis gas system and storage volumes, with hazardous outcomes.^[5]

Best practice for electrolysis system integrators includes thorough adherence to electrolyzer manufacturer startup, generation, transient, turn down, and shutdown operation limits. Integrators should also closely adhere to manufacturer-prescribed maintenance of safety controls and devices such as gas mixture detectors and catalytic recombiners.

For electrolysis system developers, a thorough hazard analysis must be conducted at each stage of development to identify and mitigate safety vulnerabilities, of which H₂-O₂ cross-over can have one of the most serious outcomes. During system verification and validation testing, best practice includes frequent testing of safety instrumentation and controls that monitor hazardous process deviations to gas purity, pressures, temperatures, and power. As informed by process hazard analysis, best practice includes interlock of the safety control system with safety shutoff valves to isolate significant system or storage gas volumes upon detection of hazardous process gas mixtures or conditions that may cause them to be generated.