In this sub-project, the kinetics and mass transport of redox reactions in polymer electrolytes is studied. Oxygen reduction is of particular importance because it is the rate limiting process in fuel cell electrochemistry. These properties are determined by ultra-microelectrode techniques. We use a custom-built electrochemical cell designed for rapid electrochemical measurements, and to have control over temperature, humidity, the nature of the polyelectrolyte and electrocatalysts. An example of this work appears in “Electrochemical Reduction of Dissolved Oxygen in Alkaline, Solid Polymer Electrolyte Films”, J. Amer. Chem. Soc. 138 (2016) 15465-15472. Mass transport of oxygen through ionomer contained within the cathode catalyst layer in an anion exchange membrane fuel cell is critical for a functioning fuel cell. Moreover, since water is a reactant in the oxygen reduction reaction (ORR) in alkaline media, an adequate supply of water is required. In this work, ORR mass transport behavior is reported for methylated hexamethyl-p-terphenyl polymethylbenzimidazoles (HMT-PMBI), charge balanced by hydroxide ions (IEC from 2.1 meq/g to 2.5 meq/g), and commercial Fumatec FAA-3 membranes. It is found that the oxygen diffusion coefficients decreased by two orders of magnitude upon reducing the water content of the ionomer membrane by lowering the relative humidity.
Solid State electrochemical cell for microelectrode electrochemistry. Oxygen diffusion coefficients in alkaline membranes obtained for HMT-PMBI-OH- ionomer films and FAA-3 membranes RH, 60°C in air. The red points indicate values obtained for HMT-PMBI-OH- 2.1 meq/g using numerical modeling.