Low temperature methods

The default large-scale purification of H₂ produced in oil refineries exploits its very low boiling point of −253 °C. Most impurities have boiling points well above this temperature. Low temperature methods can be complemented by scrubbing to remove particular impurities.^[1]

Palladium membrane hydrogen purifiers

Hydrogen can be purified by passing through a membrane composed of palladium and silver. This alloy with a ca. 3:1 ratio for Pd:Ag is more structural robust than pure Pd, which is the active component that allows the selective diffusion of H₂ through it. Diffusion is faster near 300 °C. This method has been used for purification of hydrogen on a laboratory scale, but not in industry. Silver-palladium membranes are unstable toward alkenes and sulfur-containing compounds.^[1]

Dense thin-metal membrane purifiers are compact, relatively inexpensive and simple to use.^[2][3][4]

Pressure swing adsorption

Pressure swing adsorption is used for the removal of carbon dioxide (CO₂) as the final step in the large-scale commercial synthesis of hydrogen. It can also remove methane, carbon monoxide, nitrogen, moisture and in some cases, argon, from hydrogen.

Metalorganic vapour phase epitaxy

Hydrogen purifiers are used in metalorganic vapour phase epitaxy reactors for LED production.^[5]

PEM fuel cells

Fuel cell electric vehicles commonly use polymer electrolyte membrane fuel cells (PEMFC) that are susceptible to a range of impurities. Impurities impact PEMFC using a range of mechanisms, these may include poisoning the anode hydrogen oxidation reaction catalysts, reducing the ionic conductivity of the ionomer and membrane, altering wetting behaviour of components or blocking porosity in diffusion media. The impact of some impurities like carbon monoxide, formic acid, or formaldehyde is reversible with PEMFC performance recovering once the supply of impurity is removed. Other impurities, for example sulphurous compounds, may cause irreversible degradation.^[6] The permissible limits of hydrogen impurities are shown below.

Efforts to assess the compliance of hydrogen supplied by hydrogen refuelling stations against the ISO-14687 standard have been performed.^[8][9][10] While the hydrogen was generally found to be 'good'^[8] violations of the standard have been reported, most frequently for nitrogen, water and oxygen.

Combustion engines and appliances

Combustion applications are generally more tolerant of hydrogen impurities than PEFMC, as such the ISO-14687 standard for permissible impurities is less strict.^[11] This standard has itself been criticised with revisions proposed to make it more lenient and therefore suitable for hydrogen distributed through a repurposed gas network.^[12]