Guangchun Li

Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading

The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell (PEMFC) electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization. determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed (55-76%) cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder.

Ion transport in poly(3,4-ethylenedioxythiophene)–poly(styrene-4-sulfonate) composites

The ion transport properties and electrochemistry of both chemically prepared and electrochemically prepared poly(3,4-ethylenedioxythiophene)–poly(styrene-4-sulfonate) composites have been investigated by cyclic voltammetry and impedance spectroscopy in a variety of electrolyte solutions. The polymers exhibited facile electrochemistry, fast ion transport, and good electrochemical stability. The ionic conductivities of the polymers are strongly dependent on the concentration and conductivity of the electrolyte solution used. At low electrolyte concentrations, the ionic conductivities of these polymers also show some potential dependence. Ion transport in chemically prepared layers was found to be much faster than in electrochemically prepared films. The high ionic conductivities of the polymers are shown to be due to their very porous structure and electrolytes in pores.

Dependence of Electrode Overpotentials in PEM Fuel Cells on the Placement of the Reference Electrode Measurement of Local Overpotentials

Anode overpotentials in a commercial fuel cell have been measured using dynamic hydrogen reference electrodes (DHE) at the top and bottom of the cell. During operation of the cell with dry H 2 and O 2 , which causes dehydration of the top of the anode, the top DHE consistently yielded higher anode overpotentials than the bottom DHE. Poisoning of the top part of the anode with CO greatly enhanced this disparity. Together with the observed time dependences, these results clearly indicate that overpotentials measured by using reference electrodes outside the active area of a fuel cell are local values.