Could you explain the significance of your article to the non-specialist?

In this work we probe the electrochemical impedance response of a mixed conducting electrolyte exposed to a chemical potential gradient, that is, when held under fuel cell conditions with oxygen on one side and hydrogen on the other side. We demonstrate that the resulting impedance spectra, although complex, can be described exactly. The behavior can be mapped to an equivalent circuit that includes, in addition to standard passive elements, active elements not typically present under uniform chemical potential conditions. What all of this means is that impedance spectroscopy, when carried out carefully, can be used to measure a range of physically important quantities.

What has motivated you to conduct this work?

We wished to understand why our impedance spectra, measured from ceria based cells with Pt as the anode and a conducting oxide as cathode, showed the behavior that they did. Existing models could not explain the shape of the spectra.

Where do you see this work developing in the future?

Some of the key insights have been in understanding the relative influence of electrode activity on the open circuit voltage generated across a mixed oxygen ion and electron conductor. It is known that mixed conductors suffer from leakage currents that lower the voltage from the theoretical value in a fuel cell application. It is less widely recognized that the activity of the electrodes also impacts the voltage. Our analysis methodology allows one to separately measure the cathode and anode influences and therefore could serve as a way for identifying the critical (poor-performing) component in a fuel cell system.

Are there any particular challenges facing future research in this area?

Impedance spectroscopy faces a bit of an image challenge. Unfortunately, it is much too easy to develop phenomenological equivalent circuit models that are not physically based. Such models are then prone to over interpretation, leading some dismiss the significance of impedance spectroscopy as a materials characterization tool. The goal here is to show what can be achieved when due care is taken to collect highly accurate data and apply physically derived models.