Hydrogen offers the potential to be a key renewable zero-carbon fuel source for the future. Proton exchange membrane (PEM) electrolysis technology can be used to produce hydrogen gas from water, which can then be stored, used in hydrogen fuel cells, or mixed with existing natural gas supplies (HENG). As PEM electrolysis requires energy input to power the reaction, hydrogen gas can therefore be used as a form of energy storage. Other renewable sources, such as wind and solar, often produce a surplus which, at the moment, cannot be sufficiently stored. Hydrogen gas, therefore, could be used to respond to this variability, ‘soaking up’ excess or supplementing grids when supplies are low. However, due to this variability, stored hydrogen can experience long ‘idle’ times, during which the gas quality can degrade. A full understanding of the characteristics of hydrogen and how it behaves under various storage conditions, including within HENG, will therefore be vital for implementing hydrogen as a safe and efficient fuel source.