Influence of Ion-Exchange Capacity on the Solubility, Mechanical Properties, and Mass Transport of Anion-Exchange Ionomers for Alkaline Fuel Cells

We have studied the solubility, mechanical properties, and mass transport behavior of a series of methylpiperidinium-functionalized polyethylene (PEPM) ionomers with different ion-exchange capacities (IECs) for anion-exchange membrane fuel cells (AEMFCs). The influence of IEC on the ionomer viscoelasticity in water and the transport/permeability of charged and uncharged species were investigated by acoustic impedance tests and electrochemical measurements, respectively. We found that the solubility of the PEPM ionomers increased with increasing IEC, while the rigidity of the ionomer films in water was reduced due to the higher content of the hydrophilic cations in the ionomers. In electrochemical tests, N, N, N ′, N ′-tetramethyl- p -phenylenediamine (TMPD) and 1,4-benzoquinone (BQ) were used as redox probes with ionomers serving as inert layers coating the electrodes. Analysis of the cyclic voltammetric profiles indicated that the electrostatic effects from the ionomers exclude cations, such as TMPD +, from the ionomer-coated electrode and attract anions like BQ 2– into the ionomer films. Such effects intensified with an increase in the IEC, which could be screened by increasing the ionic strength of the electrolyte. Rotating disk electrode experiments indicated that the increased IEC had a negative impact on the permeability of neutral species. In addition, MEA studies with symmetric and asymmetric ionomers provided preliminary results on the impact of the IEC on fuel cell performance. This work provides insights into the design of ionomers and membranes with deliberately controlled IEC for different applications in AEMFCs from the stand points of solubility, mechanical properties, and mass transport.