Impedance and its Corresponding Electrochemical Processes

When we begin to investigate an electrochemical system, we normally know little about the processes or mechanisms within the system. Electrochemical impedance spectroscopy (EIS) can be a powerful approach to help us establish a hypothesis using equivalent circuit models. A data-fitted equivalent circuit model will suggest valuable chemical processes or mechanisms for the electrochemical system being studied. From Chapter 1, we know that a fuel cell is actually an electrochemical system involving electrode/electrolyte interfaces, electrode reactions, as well as mass transfer processes. Therefore, EIS can also be a powerful tool to diagnose fuel cell properties and performance. 

The purpose of this chapter is to provide background information on electrochemical interfaces and on the relationship between impedance spectra and interfaces in electrochemical systems, including fuel cells. Two major references for some sections of this chapter are [1] and [2], 

As Schmickler states [3], Electrochemistiy is the study of structures and processes at the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) or at the interface between two electrolytes . The electrode/electrolyte or electrolyte/electrolyte interface is the region whose properties differ from the two adjoining phases, and/or the place where reactant adsorption and electrochemical reactions occur. Commonly, it is recognized as the interface between an electronic conductor (e.g., metals and semiconductors) and an ionic conductor (e.g., electrolyte solutions, molten salts, and solid electrolytes), known as an electrochemical interface. In a narrow region of an electrode/electrolyte interface, an electrical double layer (EDL) exists. The EDL is believed to be extremely thin, and is an important component of the interface. 

Fuel cell researchers deal primarily with interfaces between solid electrolyte materials and solid metallic electrodes. The characterization of electrochemical systems with solid-solid interfaces has become a major issue in the study of fuel cells. It is generally believed that the interface of a solid electrode and solid electrolyte is similar to the electrode/liquid electrolyte interface but more complicated [4], 

Although EIS characterizes many of the electrical properties of materials and their interfaces, to begin with we focus on a typical electrochemical system a metal-solution interface. 

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