Editors' Choice-Review-Impedance Response of Porous Electrodes: Theoretical Framework, Physical Models and Applications

Porous electrodes are prevalent in electrochemical devices. Electrochemical impedance spectroscopy (EIS) is widely used as a noninvasive, in situ characterization tool to investigate multi-phase (electronic, ionic, gaseous) transport and coupling interfacial reactions in porous electrodes. Interpretation of EIS data needs model and fitting which largely determine the type and amount of information that could possibly be obtained, and thereby the efficacy of the EIS method. This review focuses on physics-based models, as such models, compared to electrical circuit models, are more fundamental in our understanding of the porous electrodes, hence more reliable and more informative. Readers can have a glimpse of the long history of porous electrode theory and in particular its impedance variants, acquaint themselves with the celebrated de Levie model and a general theoretical framework, retrace the journey of extending the de Levie model in three directions, namely, incorporating new physico-chemical processes, treating new structural effects, and considering high orders. Afterwards, a wealth of impedance models developed for lithium-ion batteries and polymer electrolyte fuel cells are introduced. Prospects on remaining and emerging issues on impedance modelling of porous electrodes are presented. When introducing theoretical models, we adopt a "hands-on" approach by providing substantial mathematical details and even computation codes in some cases. Such an approach not only enables readers to understand the assumptions and applicability of the models, but also acquaint them with mathematical techniques involved in impedance modelling, which are instructive for developing their own models.