(Invited) Ions Transport Properties and Carbonation Process Investigation of Nanocomposite Anion Exchange Membranes Containing Layered Double Hydroxide
Authors: Nicotera Isabella , Simari Cataldo , Lufrano Ernestino , Rehman Mohamed Habib Ur , Dekel Dario R , Baglio Vincenzo 
Published: November 2022

Electrochemical energy conversion and storage (EECS) technologies such as fuel cells and electrolyzers are expected to play a pivotal role owing to their compatibility with the environment and high energy conversion efficiency. Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different EECS devices.[1] They have numerous advantages respect to the proton-exchange membranes, such as operation at high pH conditions, significantly lower cost (based on low-cost raw materials), less corrosive environment, and significantly lower fuel crossover (one order of magnitude lower than the acidic counterparts). AEM fuel cells and electrolyzers devices can reach the performance level required by applications with electrocatalysts that do not require a high loading of platinum-group metals (PGMs) due to the alkaline environment at the electrodes.

However, the AEM based EECS development and implementation is significantly hindered by the anion exchange membrane (AEM) stability during cell operation, their low OH- conductivity and the low kinetics of the electrocatalysts. In fact, the electrolytic anion-exchange membrane (AEM) is a crucial component which should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal and mechanical stability and high durability. [2]

We report on an extensive study on nanocomposite AEMs based on tetramethylammonium Polysulfone ionomer (PSU) and Layered Double Hydroxide (LDH) as nanofiller. [3] PSU is a thermoplastic polymer with high thermal stability, good chemical resistance and mechanical properties. However, since it has two activated positions per repetition unit for electrophilic aromatic substitution, it can undergo high degrees of functionalisation, which reduce the mechanical properties. We developed a two steps procedure, with allow a functionalization degree of about 80%, preserving its stability. Concerning the LDHs, these mineral anionic clays are excellent inorganic anionic conductors with elevated ion exchange capacity (IEC). They consist of the positively charged metal hydroxide layers with anions located in the interlayer space. This allows strong hydration of the material together with a large number of hydroxyls on the host layers forming a dense network of hydrogen bonds along the two-dimensional surface, therefore facilitating OH- ion conduction by diffusion mechanism.

PSU/LDH AEMs were investigated in both OH-
and forms, comparing swelling capacity, ionic conductivity and water diffusion. The latter was studied by NMR spectroscopy, measuring the self-diffusion coefficient by the Pulse Field Gradient (PFG) NMR techniques. The nanocomposite membranes are able to maintain good hydration at high temperatures, and to create an adequate nanostructure with the polymer chains, which favour the Grotthuss diffusion mechanism for the OH- ions. Such feature is reflected in the ionic conductivity and in the alkali stability, where they demonstrated the highest conductivity and a reduced membrane degradation rate.

In addition, the 13C-NMR technique was used to investigate carbonation processes in the presence of CO2, showing that the presence of the LDH platelets into the AEM remarkably reduces the conversion rate of hydroxyl groups, as much as slow down the diffusion of carbonate ions.

Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance.

Acknowledgments

This work has been supported by the Italian Ministry for University and Research (MUR) for funding through the FISR 2019 project AMPERE (FISR2019_01294).

References

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Simari, C.,...and Nicotera, I. (2022) Electrochimica Acta, 403, p. 139713.