Impedance Measurements of Silver Gas Diffusion Electrodes during Oxygen Reduction in Alkaline Solution

Kurzfassung

In order to analyze in detail the kinetics of the oxygen reduction reaction (ORR) in alkaline solution (25 wt% KOH and 10 N NaOH) of PTFE-bonded Gas Diffusion silver Electrodes (GDEs) electrochemical impedance measurements and current-voltage curves have been performed. GDE with dispersed silver catalyst were produced by electrochemical reduction of silver oxide-GDE. The silver GDEs were produced by the Reactive Mixing and Rolling (RMR) production process and multilayer electrodes by combination of RMR with dry spraying of a silver catalyst layer on inert graphite GDE. The electrochemical impedance spectra (EIS) were recorded in the frequency range 1 mHz to 64 kHz and 1 mHz to 100 kHz respectively. Silver is a known electrocatalyst for the reduction of oxygen in alkaline fuel cell cathodes. To enhance its catalytic activity, silver is used in high surface area forms such as porous electrodes where the silver particles are dispersed in the porous matrix. In order to evaluate the measured impedance spectra and to obtain kinetic data of the oxygen reduction reaction, the reaction steps can be translated into an appropriate equivalent circuit which contains various impedance elements representing the involved reaction steps. These elements are generally represented as ohmic, capacitive or inductive components with particular dependencies of their complex impedance upon the frequency of the ac signal. The particular linking of these impedance elements described by an equivalent circuit is based upon the relationship between the processes represented by these elements. Subsequently occurring steps are represented by a series connection of the elements while steps occurring simultaneously are represented by a connection in parallel. In the case of porous electrodes the connection of the elements is more complicated. On overview of different porous electrodes models will be given and in detail the homogeneous cylindrical pore model proposed by Göhr. This model is integrated in the simulation and evaluation program Thales (Zahner Elektrik). In this model the cylindrical pore is considered as a transmission line made of a large number of infinitesimally thin sections, with impedance elements of pore’s ground surface (Zn), pores electrolyte (Zp), pore’s wall surface (Zq), porous layer (Zs) and surface layer (Zo).