Hydrogen purity


Hydrogen purity or hydrogen quality is a term to describe the lack of impurities in hydrogen as a fuel gas. The purity requirement varies with the application, for example a H2 ICE can tolerate low hydrogen purity where a hydrogen fuel cell requires high hydrogen purity to prevent catalyst poisoning.

Hydrogen purity requirements for fuel cells

In the first generation of fuel cells catalysts like palladium, ruthenium and platinum are used in combination with hydrogen production from hydrocarbons which results in performance degradation.
The catalyst poisoning induced by some impurities like carbon monoxide, formic acid, or formaldehyde can be reversed with a high purity hydrogen stream. Presence of other impurities like sulfurs may lead to permanent degradation of the fuel cells. The kind of impurities and their level depends on the H2 production process but impurities can also be introduced due to transport, cleaning of the refueling station, leakages or maintenance.
In Europe, the Directive 2014/94/EU on the deployment of alternative fuels infrastructure states that the hydrogen purity dispensed by hydrogen refuelling points shall comply with the technical specifications included in the ISO 14687-2 standard. ISO 14687-2 specifies maximum impurities levels for particles and several gaseous impurities. For many compounds the limit values are very low including total sulfur or carbon monoxide. The least stringent limits are for helium and total nitrogen/argon.  The sum of the impurities should be less than 300 µmol/mol.

Analytical methods for hydrogen purity analysis

As the limit values for many impurities are very low this sets stringent demands on the sensitivity of the analytical methods. Moreover, the high reactivity of some impurities requires use of a properly passivated sampling and analytical systems. A combination of different instruments is now needed to cover all components listed in ISO 14687-2. Currently, several research efforts are directed to address the existing problems focusing on new multi-component analytical methods, risk assessments to limit the number of impurities to be analyzed and lay the metrological foundation for H2 purity analysis.