ELECTROCHEMICAL IMPEDANCE SPECTRA OF SOLID-OXIDE FUEL-CELLS AND POLYMER MEMBRANE FUEL-CELLS

Electrochemical impedance spectroscopy (EIS) is a very useful method f or the characterization of fuel cells. The anode and cathode transfer functions have been determined independently without a reference elect rode using symmetric gas supply of hydrogen or oxygen on both electrod es of the fuel cell at open circuit potential (OCP). EIS are given for both polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) at current densities up to 0.76 A cm(-2) (PEFC) and 0.22 A cm( -2) (SOFC). With increasing current density the PEFC-impedance decreas es significantly in the low frequency range reaching a minimum at 0.4 A cm(-2) At even higher current densities an increasing contribution o f water diffusion is observed: the cell impedance increases again. Fro m EIS of SOFC a finite diffusion behavior is observed even at OCP, dep ending on water partial pressure of the anodic gas supply. This additi onal element reflects the influence of water partial pressure on the c ell potential. The simulation of the measured EIS with an equivalent c ircuit enables the calculation of the individual voltage losses in the fuel cell. (C) 1998 Published by Elsevier Science Ltd. All rights res erved.