(42y) Preparation and Stability of Imidazolium-Functionalized Alkaline Electrolyte Membrane Incorporating ZrO2 Nanoparticles

As a potential next-generation clean energy technology, alkaline anion exchange membrane fuel cells (AAEMFC) has attracted enormous attention in the past few decades. They have several advantanges over proton exchange membrane fuel cells(PEMFCs) and conventional alkaline fuel cells, including fast oxygen reduction reaction, free of carbonate crystallization at the electrode, more options to use inexpensive catalysts such as nickel, and low fuel crossover. Anion exchange membrane (AEM), one of the core components of AAEMFCs, is a key factor that determines the fuel cells’ power density and life. The desirable AEM should possess high ionic conductivity, good mechanical and thermal stability, and long-term alkaline stability. So far, commercial AEMs still cannot meet the requirements of AAEMFCs due to their low ionic conductivity and poor chemical stability.

The reason for poor alkaline stability of AAEM lies in two aspects: vulnerableness of the ion exchange groups and the strong attack of hydroxide ions under fuel cell operating conditions. In this work, we address the low stability issue by alleviating the intensity of hydroxide attack.

We will make hybrid AAEMs with poly(ether ether ketone) (PEEK) and zirconium dioxide (ZrO₂) nanoparticles, and investigate how the nanoparticle content influence the membranes’ water uptake, water retention, alkaline stability and other properties.  PEEK is a semicrystalline high-performance polymer with excellent chemical stability, thermal stability and mechanical property. It has been widely used for making ion exchange membranes. ZrO₂ nanoparticle has large specific surface area, good hydrophilicity and mechanical properties. We synthesize imidazolium-functionalized PEEK first, and then disperse nano-ZrO₂ fillers into it to make hybrid AAEMs.

The introduction of hydrophilic nano-ZrO₂ fillers may probably produce a solvation effect on the OH^- ions, thus enhancing the alkaline stabilities. The possible mechanism is that the nano-ZrO₂ enhances water uptake of the membrane, thus “diluting” OH^- ions inside the membrane. In other words, there will be a lower OH^- concentration at the vicinity of the imiazolium groups and the OH^- ion is surrounded by large amount of water. The high water uptake also may also have a protection effect to the ion exchange group, thus weakening the OH^- ion’s attack and improving the alkaline stabilities. The main purpose of this work is verify the above hypotheses.

Using the concentrated sulphuric acid as solvent and catalyst, chloromethyl octyl ether (CMOE) as chloromethylation reagent, we successfully synthesized CMPEEK. A low reaction temperature, e.g. -10 °C, is required for the good controllability of chloromethylation degree (DC) of CMPEEK. By finely tuning reactant ratio and reaction time, CMPEEKs with different DC were successfully obtained, and the solubility test manifested that all the CMPEEKs have good solubility in high-boiling-point membrane casting solvents.

The Im-PEEK was synthesized by reacting CMPEEK with 1,2- dimethylimidazole. Differed amounts (2.5%, 5%, 7.5% and 10%) of nano-ZrO₂ was then incorporated into Im-PEEK by a simple blending method, and the resultant membraes are designated as CMPEEK-x%ZrO₂. A good compatibility between ImPEEK and the nanoparticle was comfirmed by the uniform morphology of the composite membranes as seen from a scanning electron microscopy study. The X-ray diffraction was used to determine the crystallinity of the composite membranes. It is seen in the TGA curves that the incorporation of ZrO₂ improves the thermal stability of the membrane. More importantly, the water uptake of the composite membranes increases with the amount of ZrO₂. The composite membrane containing 7.5% ZrO₂has the highest water uptake (25.27% at 20°C), and also exhibites the highest conductivity (53.96 ms/cm at 80°C), but with the lowest swelling ratio(18.52%, 20°C) and the best alkaline stability (3M NaOH treated 24h, the conductivity decreased by 4.27% only).

This work provides some new insight into the AAEM’s stability issue. It may open a new possibility of and a simple route toward making high-performance AAEM with good alkaline stability and mechanical strength.