No abstract

Hydrogen oxidation (HOR) and oxygen reduction (ORR) reactions at the platinum/alkaline ionomer interface were investigated using two different alkaline polymer electrolytes, i.e., benzyl-trimethyl ammonium tethered poly(phenylene) (ATM-PP) and phenylpentamethyl guanidinium tethered perfluorinated polymer (M-Nafion-FA-TMG). Substantial inhibition of HOR was taking place at the platinum-ATM-PP interface due to the possible cationic group adsorption of ATM-PP, whereas the reaction was virtually unaffected at the platinum-M-Nafion-FA-TMG interface after high anodic potential preconditioning. Moreover, the apparent ORR activity of platinum coated with M-Nafion-FA-TMG was found higher than that in 0.1 M tetra methyl guanidinium solution. In addition, the oxygen permeability of M-Nafion-FA-TMG was found to be ∼2.5 times higher than that of ATM-PP. The above properties of the perfluorinated polymer make it a very promising ionomeric binder for the use in both anode and cathode of alkaline membrane fuel cells. Alkaline membrane fuel cells (AMFCs) have drawn tremendous attention because they have the potential to convert hydrogen fuel to electricity without using precious metal catalysts in the electrodes. 1,2However, the current performances of AMFCs using non-precious or even precious metal catalysts are inferior to those of proton exchange membrane fuel cell (PEMFC) counterparts.3 The inferior performance of AMFCs have been explained by the low hydroxide mobility in anion exchange membranes, 4,5 carbonate/bicarbonate formation, [6][7][8] slow hydrogen oxidation reaction (HOR) kinetics, 9,10 and poor water management.11-13 The differences in performance between AMFCs and PEMFCs are also largely derived from the polymer electrolytes employed in the electrodes that provide different environments for electrochemical reactions and material diffusion.The effects of cation-tethered alkaline polymer electrolytes on AMFC performance have been investigated by several research groups. Varcoe et al. first used a hydroxide conducting ionomer in preparation of alkali metal-cation-free AMFC electrodes.14 There was a substantial increase in the peak power density of AMFCs (55 vs. 1.6 mW cm −2 ) by adding quaternized poly(vinylbenzyl chloride) to the catalyst layers and further improved AMFC performance was reported later using better ionomer dispersion technology, 15 i.e., the peak power density of 185 mW cm −2 . However, the improved AMFC performance was still inferior to the state-of-the-art PEMFC performance. They suggested that relatively low gas permeation by ionomer binder may be the reason for the inferior performance. Researchers from Tokuyama Corporation reported a series of hydroxide conducting ionomers for AMFC applications.16,17 They found that the peak power density of the AMFC performance increased from 22 mW cm −2 to 296 mW cm −2 , as the ion exchange capacity of their hydrocarbon ionomers increased from 0.7 to 1.8 meq. g −1 . They suggested the ionic conductivity of the ionomer is the major factor contributing to the p...

Anion exchange polymer electrolytes play an important role both in separator (membrane) and in electrode binder (ionomer). Depending on their role, polymer electrolytes must possess certain properties which are essential to improve the alkaline membrane fuel cell performance & durability. In this study, we will present molecular design aspects of anion exchange polymer electrolytes for the use of membranes and ionomeric binders. In-depth studies on chemical degradation of polyaromatic anion exchange membranes and importance of perfluoirnated structure in the catalyst layer will be discussed in addition to the effects of cation type on chemical degradation and interaction with electro-catalysis.