Question 10: What is the difference between hydrogen and hydrogen for fuel cells?

Answer: There is very little free hydrogen on Earth – only about 10% in comparison with helium. Typically, hydrogen as a gas produced by a suitable manufacturing process is a mixture of substances that participate in or influence the process due to boundary conditions. Further, we consider such a parameter as the degree of purity, which expresses the proportion of residual components in the composition of hydrogen. We can distinguish, for example, the degree of purity 3.0, which means that the substance contains 99.9% hydrogen (3 digits “9”), and 0.1% is other substances. We also distinguish these “other substances”, and therefore we can indicate that it is, for example, water vapour, nitrogen, oxygen, hydrocarbons, sulphur compounds and so on.

Some residual impurities in the composition of hydrogen can adversely affect, in particular, the membrane of the fuel cell. This, in turn, reduces the performance of the fuel cell and shortens its life. The list of such substances includes (but is not limited to): carbon monoxide, sulphur compounds, formaldehyde, ammonia, formic acid, and halides. The maximum concentration of these and other impurities in hydrogen intended for fuel cells is determined by the ČSN ISO 14687-2 standard (see the table at the end of this article). The above impurities enter hydrogen during its production. It is the method of producing hydrogen that significantly affects the amount and composition of undesirable residual impurities. For example, the production of hydrogen by steam reforming from natural gas carries an increased risk of CO2, CH4 or halides. Hydrogen obtained from cracking petroleum may contain increased amounts of sulphur compounds. Hydrogen produced by electrolysis of water can contain more water or oxygen. Special filtering equipment can remove unwanted substances and prepare hydrogen for use in fuel cells. In technical practice, achieving a hydrogen purity of 5.0, that is, a purity of 99.999%, is not a problem. However, even in such a clean gas, there should be no substance that restricts or degrades the fuel cell.

Hydrogen quality requirements for hydrogen fuel cells

Total proportion of hydrogen and polluting gases

Minimum content of pure hydrogen 99,97 %
Total amount of gases excluding hydrogen 300 μmol/mol

Maximum concentration of selected polluting gases

Water (H2O) 5 μmol/mol
Total Carbohydrates (Methane) 2 μmol/mol
Oxygen (O2) 5 μmol/mol
Helium (He) 300 μmol/mol
Combined nitrogen (N2) a Argon (Ar) 100 μmol/mol
Carbon dioxide (CO2) 2 μmol/mol
Carbon monoxide (CO) 0,2 μmol/mol
Total sulphur compounds (H2S) 0,004 μmol/mol
Formaldehyde (HCHO) 0,01 μmol/mol
Formic acid (HCOOH) 0,2 μmol/mol
Ammonia (NH3) 0,1 μmol/mol
Total halides 0,05 μmol/mol
Maximum concentration of solids 1 mg/kg