Ion Exchange Membranes (IEM)

An ion exchange membrane (IEM) is a semipermeable dense layer that selectively allows ions to pass through. IEMs are broadly divided into two families: anion exchange membranes (AEM) and cation exchange membranes (CEM) — as the names suggest, AEMs and CEMs selectively allow anions and cations to pass through them, respectively. IEMs are widely used in different applications, including the energy, water treatment, pharmaceutical and food industries. With the increasing demand and aspiration of building a greener future, IEMs are abundantly used in green hydrogen production and its utilisation in fuel cells.

However, in hydrogen energy applications the cation to be transported is the proton (H+), hence CEMs are often referred to as proton exchange membranes (PEMs) in water electrolyzers and fuel cells. The mechanisms usually involved in the transport of protons are the vehicular mechanism and the Grotthuss mechanism, also known as the hopping/shuttling mechanism. The vehicular mechanism involves transport of protons in the form of H3O+ ions from one side to the other via diffusion or movement, without transfer of the proton to another water molecule. The Grotthuss mechanism, in contrast, involves continuous transfer of the proton to the next water molecule, creating a continuous chain. This can be achieved if the polymer of the membrane itself supports the mechanism — hence polymers involving anionic functional groups (e.g. –OH) in their structures are used, for example perfluorosulfonic acid (PFSA) and sulphonated poly ether ether ketone (SPEEK).

Meanwhile, AEMs are used in many applications at industrial level, especially in the food industry, where AEMs are used for deacidification of juices, dairy processing and more. AEMs grabbed the attention of researchers and scientists for green hydrogen applications between the mid-2010s and 2020. One of the reasons is the high cost of the catalysts used in PEM water electrolyzers and fuel cells. Similar to the mechanisms explained above, AEMs transport OH- ions where the carriers are water molecules with hydrogen bonds, unlike in PEMs where dative bonds are present. Polymers used in AEM synthesis include cationic functional groups, with quaternary ammonium (e.g. –[N(CH3)3]+), one of the more stable cations, being widely used.

Beyond the chemistry of ion transport, membrane selection also depends on physical and electrochemical properties such as thickness, melting point, swelling ratio, tensile strength, porosity, areal resistance and ionic selectivity. As a rule of thumb: the thinner the membrane, the higher its conductivity and the lower its strength. Likewise, the higher the porosity, the higher the electrolyte-holding capacity — decreasing resistance, but simultaneously increasing water uptake. It is also important to use dense membranes with no or very low pore size to avoid gas transport across the membrane.