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Preparation of DABXO Quaternized PEEK Hydroxide Exchange Membrane and Its Modification

Author: WangJunJun
Tutor: WuXueMei; HeGaoHong
School: Dalian University of Technology
Course: Chemical Engineering
Keywords: hyd roxide exchange membranes DABCO iodomethane crosslinking quaternization alkaline fuel cell
CLC: TQ425
Type: Master's thesis
Year: 2013
Downloads: 39
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Abstract

Alkaline anion-exchange membrane fuel cell (AAEMFC) is relatively a new category as compared to proton exchange membrane fuel cell (PEMFC). Faster kinetics of the oxygen reduction, lower methanol permeability, non-precious metals can be used as catalysts and easier water management in alkaline media make the AAEMFC superior to PEMFC in some aspects, such as cost and application on direct methanol fuel cells (DMFCs). Alkaline anion-exchange membrane (AAEM) is a key component of AAEMFC, which conducts hydroxide ions and prevents crossover of reactants.The quaternary ammonium was proved to exhibit a higher thermal and chemical stability than other anion conducting groups such as quaternary phosphonium or tertiary sulfonium groups, and thus was used as ion conducting groups. Commonly, AAEM is prepared by quaternization of chloromethylated polymers. In alkaline medium, quaternary ammoniums formed from aliphatic amine are very sensitive to Hoffman degradation, which will damnify the hydroxide conductivity. In the present work, bifunctional cyclic ammonium1,4-diazabicyclo [2,2,2] octane (DABCO) was used as quaternized reagent to prepare DABCO-based poly(ether ether ketone)(QDPEEKOH) hydroxide exchange membranes, which are not sensitive to Hoffman degradation. And then two different methods were used to improve the conductivity of membranes. The first, simultaneous quartemization-crosslinking reagent DABCO was used to prepare membranes with high concentration of hydroxide conducting groups and suitable swelling ratio. The second, changing the density of hydroxide conducting groups of membranes to perform continuous network of ionic channels which will benefit to conductivity.In the present work, appropriate amount of CMPEEK and DABCO solutions were mixed and stirred at room temperature for12H to make the quaternization compeletelly. At room temperature, the hydroxide conductivity of QDPEEKOH membranes increased from4.11to12.67mS cm-1with DC varied from35%to68%. However, when DC=72%, the membrane swelling excessively and lose its mechanical properties, which reduced the conductivity of membranes.Balancing the electrical and mechanical properties is extremely important for AAEMs. Bifunctional cyclic ammonium1,4-diazabicyclo [2,2,2] octane (DABCO) is designed as simultaneous quartemization-crosslinking reagent to prepare DABCO-based poly(ether ether ketone)(CL-QDPEEKOH) hydroxide exchange membranes (AAEMs). Concentration of hydroxide ion ([OH]) is proposed to detect the combined effect of ion exchange capacity (IEC) and the degree of crosslinking (DCL) on the hydroxide conductivity of the CL-QDPEEKOH membranes. By balancing the degree of chloromethylation (DC) and the theoretical degree of crosslinking (DCL,), high [OH] and hydroxide conductivity were obtained. CL-QDPEEKOH membrane with DC149%and DCLt50%reached high hydroxide conductivity of33.4mS cm"1and low swelling ratio of42.9%at room temperature.Quatemization between CH3I and the free N of DABCO in QDPEEKC1was carried out to prepare membranes with high density of hydroxide conducting groups. CH3I-QDPEEKOH membranes have bigger ionic clusters compared with QDPEEKOH of the same IEC. The highest conductivity is35.5mS cm-1at room temperature and the swelling ratio is39.8%, which is promising candidates for alkaline fuel cell applications.

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