Toward Improved Alkaline Membrane Fuel Cell Performance Using Quaternized Aryl-Ether Free Polyaromatics

Toward Improved Alkaline Membrane Fuel Cell Performance Using Quaternized Aryl-Ether Free Polyaromatic

High Temperature Polymer Electrolyte Membrane Fuel Cells with High Phosphoric Acid Retention

Phosphoric acid loss poses immense hurdles for the durability of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Here we report quaternary ammonium-biphosphate ion-pair HT-PEMFCs that do not lose phosphoric acids under normal and accelerated stress conditions. Our energetics study explains the acid loss behavior of the conventional phosphoric acid-polybenzimidazole (PA-PBI) system by two mechanisms. If PA loss occurs via acid evaporation, the acid loss is constant over time. On the other hand, when water activity in the PA-PBI system is high, exponential decay of PA loss occurs via the water replacement mechanism. Combined 31P NMR and computational studies show that the proposed ion-pair system has six times higher interaction energy, which allows for containing all PAs in the membrane electrode assemblies under a broad range of operating conditions. In addition, polar interactions between the phosphonic acid ionomer and phosphoric acid explain acid retention in the electrodes of the ion-pair HT-PEMFCs

Nitrogen-Deficient ORR Active Sites Formation by Iron-Assisted Water Vapor Activation of Electrospun Carbon Nanofibers

Fe- and N-modified carbon nanofibers (Fe–CNF) were synthesized via electrospinning and pyrolysis as electrocatalysts for oxygen reduction reaction (ORR). In order to increase the exposed surface area with the active sites buried inside Fe–CNF, we attempted water vapor activation for Fe–CNF and observed a substantial improvement of ORR activity up to the comparable level with Pt/C. Unlike what was expected, however, water vapor activation did not significantly increase the specific surface area of Fe–CNF; instead, it induced a depletion of surface N content, which makes it difficult to explain the improved ORR activity with the increase of surface area with N-based active sites. In water vapor activation, the chemical phase of embedded particles is changed from Fe₃C to Fe₃O₄ and nitrogen-free Fe- and C-based ORR active sites were exposed, which seemed to be related with hierarchical macro/mesopore structure and graphitic edge defects. This study demonstrates a facile activation method for better ORR activity of Fe-modified CNF and suggests a potential relationship of surface carbon structure with the catalytic activity toward ORR rather than the type and concentration of N in Fe–CNF, which should be investigated further