(541f) Di-Quaternized DABCO-Based PEEK Hydroxide Exchange Membranes With Improved Ionic Conductivity

Polymer Electrolyte Membrane Fuel cells are considered to be clean and efficient power sources. Among the several kinds, hydroxide exchange membrane fuel cells (HEMFCs) operate in alkaline media with faster kinetics of the oxygen reduction, lower methanol permeability and easier water management, therefore superior to proton exchange membrane fuel cells (PEMFCs) in some aspects such as cost and application on direct methanol fuel cells (DMFCs). Hydroxide exchange membrane (HEM) is a key component of HEMFC, which conducts hydroxide ions and prevents crossover of reactants. Significant research works have been done in recent years to develop new HEMs. However, there are still two main challenges facing with HEMs, i.e. low ionic conductivity and poor stability of basic groups. In this study, di-quaternized 1, 4-diazabicyclo [2,2,2] octane (DABCO)-based PEEK (di-QDPEEK) hydroxide exchange membranes are prepared via the two nitrogen atoms of DABCO to improve ionic conductivity. And the strain free conformation of DABCO provides good resistance to both Hoffman degradation and nucleophilic substitution by OH^-.

We design both mono- and di-quaternized DABCO-based PEEK membranes with similar ionic exchange capacity (IEC) to investigate the influence of distribution of quaternary ammonium functional groups on the microstructures and electrochemical properties of the HEMs. For low IEC around 0.8, di-QDPEEK membrane exhibits much higher ionic conductivity (2.9 times) and swelling ratio (1.7 times) as compared with the mono-QDPEEK membrane. However, for high IEC around 1.2, di-QDPEEK membrane exhibits much higher ionic conductivity (2.1 times) and much lower swelling ratio (0.6 fold) as compared with the di-QDPEEK membranes. Based on our data, a schematic model with scattered or concentrated distributions of the quaternary ammonium groups in the mono- and di-quaternized membranes is proposed. In the di-quaternized membranes, quaternary ammonium functional groups concentrate in a local scale, which enhance the interactions between ionic groups, leading to well hydrophilic-hydrophobic micro-phase separation and better interconnected pathways for ions through the hydrophilic domains. As a result, di-quaternized membranes exhibit much higher ionic conductivity and special swelling behavior as compared with the mono-QDPEEK membranes of similar IEC. More details about the microstructures-properties correlations, such as TEM image, glass transition temperature and mechanical strength of the mono- and di-QDPEEK membranes will be present in this work. For di-QDPEEK of IEC about 1.69, high ionic conductivity of around 35.5 mS cm^-1 at 20 ^oC is achieved, which is within the top values reported in the literatures. Keywords: Separations, Hydroxide exchange membranes, Fuel cells, DABCO, PEEK, Ionic conductivity

The authors thank Program for National Science Fund for Distinguished Young Scholars of China (Grant no.21125628), NSFC (Grant no. 20976027 and 21176044) for supports of this work.

* Corresponding author. Tel: +86 411 84707892 E-mail address: hgaohong@dlut.edu.cn