Multi-rail transmission-line model as an equivalent circuit for electrochemical impedance of a porous electrode

A new type of transmission-line model (TLM) as an equivalent circuit for the electrochemical impedance is developed to describe a porous electrode impregnated with an electrolyte solution. Two kinds of TLMs, i.e., one for diffusion migration and the other for a porous electrode, are combined. To describe the double-layer capacitance strictly, the interfacial impedance is explicitly divided into the faradaic impedance and double-layer capacitance, and the latter is distributed into contributions of each ionic species. The obtained model describes diffusion, migration, and concentration changes of all ionic species within a porous electrode. In combination with a TLM for a bulk electrolyte solution, an equivalent circuit for a whole cell is also made. This should be a powerful tool for analyzing the impedances of devices with porous electrodes, such as batteries and electric double-layer capacitors, especially in the low-frequency region.