Nanocomposite Anion Exchange Membranes for Aems Electrolyzers: Ion Transport and Carbonation Studies

Abstract

Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different electrochemical energy conversion and storage (EECS) devices. [1] 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]In this work, nanocomposite AEMs based on Polysulfone ionomer (PSU) and two different nanoadditives, Layered Double Hydroxide (LDH) and Nanoscale Ionic Materials (NIM), have been developed and studied. [3] LDHs are mineral anionic clays 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. NIMs have a nanostructure consisting of a spherical silica core functionalized with an ionic oligomeric corona. Due to the extremely high number of functional groups present, these materials are able to significantly increase the ionic conduction capacity of the resulting electrolyte. [4]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 between 60 and 80%, preserving its stability.Nanocomposites AEMs were investigated in both OH- and HCO3 - 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. Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance.