A simple Maxwell-Wagner-Butler-Volmer approach to the impedance of the hydrogen electrode in a nafion fuel cell.
A simple one-dimensional model of the impedance of a hydrogen/Nafion electrode is set up combining the usual Maxwell-Wagner approach for linear, homogeneous, and isotropic media with the linearized Butler-Volmer equation for the interfacial, electrochemical reaction. Only one relaxation semicircle is normally seen in the Nyquist diagram, but a low-frequency arc may appear at high overvoltages. The model is described by only two dimensionless parameters (in addition to the dimensionless frequency). These parameters are related to the double-layer capacitance and to the interfacial electrochemical reaction rate, respectively. With some adjustments, the model can be used to explain the observed equilibrium impedance from 40 to 70 degrees C of a symmetric cell of the type C/Pt/H(2)|Nafion 117/H(2)/Pt/C. The hydrogen electrodes in this cell were built up as a disperse multiphase region (carbon, platinum grains, Nafion 117, and hydrogen gas) as commonly done in solid polymer fuel cells.
Sørensen
Torben Smith
TS
Physical Chemistry, Modeling and Thermodynamics, Nørager Plads 3, Vanløse, DK-2720, Denmark.
Kjelstrup
Signe
S