Optimization of Ammonium Head Groups in Partially Fluorinated Anion Exchange Membranes for Alkaline Fuel Cells

Abstract

Anion exchange membrane fuel cells (AEMFCs) have gained attention due to their possible use of non-precious metal electrocatalysts compared to proton exchange membrane fuel cells.1 However, the chemical/physical instability of anion exchange membranes under operating fuel cell conditions remains an issue.1, 2 We developed previously anion exchange membranes containing perfluoroalkyl and phenylene rings in the main chain (e.g., QPAF-C3-TMA (trimethylammonium)). The polymer main chains were alkaline stable because of the absence of oxygen ether bonds in the main chain, however, the ammonium groups suffered from alkaline degradation.3 In this work, the impact of varying head group structure was investigated through utilizing cyclic piperidine (-Pip), dimethylbutylamine (-DMB), dimethylhexylamine (-DMH), and dibutylmethyl amine (-DB) (Figure 1). All of the membranes showed phase-separated morphology as confirmed from TEM images. The DFT calculations revealed that hydrophilic domain sizes were dependent on polar surface area rather than the molecular size of the head groups. The compact piperidinium-based membrane (QPAF-C3-Pip) exhibited the most balanced properties of low water absorbability, high ionic conductivity, alkaline and mechanical stability. An H2/O2 fuel cell using QPAF-C3-Pip membrane achieved maximum power density of 232 mW/cm2, slightly higher than that of TMA-based membrane (224 mW/cm2). Furthermore, QPAF-C3-Pip membrane showed reasonable durability for 240 h in operating fuel cell at a constant current density of 100 mA/cm2 in which the cell voltage decreased from 0.75V to 0.43V with average voltage decay of 1.24 mV/h, that was more durable than the TMA-based membrane (average decay of 4.8 mV/h at 50 mA/cm2). Other membranes properties will be also discussed.Figure 1. Structure and fuel cell performance of QPAF-C3-Pip membranes