Generalized equivalent circuits for mixed conductors: silver sulfide as a model system

Area of Science:

• Electrochemistry
• Materials Science
• Solid State Chemistry

Background:

• The Hebb-Wagner polarization model traditionally neglects space-charge polarization.
• Accurate electrochemical characterization of mixed ionic-electronic conductors is crucial for device applications.

Purpose of the Study:

• To apply a generalized equivalent circuit model incorporating space-charge polarization to silver sulfide.
• To obtain self-consistent electrochemical parameters for silver sulfide under varying silver activity.
• To elucidate the nature of interfacial capacitance in mixed conductors.

Main Methods:

• Utilized a self-coded Fortran program to implement the generalized equivalent circuit.
• Investigated silver sulfide with an AgI electrode in a galvanic cell setup.
• Analyzed electrochemical impedance spectra over a range of silver activities.

Main Results:

• Successfully applied the generalized equivalent circuit to silver sulfide, obtaining geometric capacitance, partial conductivities, and chemical capacitance.
• Identified interfacial adsorption capacitance as significantly larger than diffuse double-layer capacitance.
• Demonstrated that simplified circuits approximate behavior at extreme silver activities.

Conclusions:

• The generalized equivalent circuit accurately models Hebb-Wagner polarization in silver sulfide, including space-charge effects.
• Adsorption capacitance plays a dominant role at the interface, distinct from space-charge double-layer capacitance.
• The methodology allows for self-consistent determination of key electrochemical parameters in mixed conductors.