An ECM comprises electrical circuit elements that are connected in series or in parallel to represent electrochemical processes. Each element is characterized by its frequencydependent complex impedance function with several element-specific parameters. The experimentally probed frequency range of the impedance spectrum can often be subdivided into three sections. The intersection between EIS curve and impedance real axis at the high frequency end can be interpreted as serial Ohmic resistance R 0 , which represents the voltage drop and power loss to heat when an outside voltage is applied. The semicircles in the mid-frequency range reflect charge transfer and double layer capacitance at the electrode and electrolyte interface , and it can be expressed as serially connected ZARC elements. The diffusion process inside the electrodes, which is visible as a straight line with a characteristic phase angle in the low frequency region, can be modeled by Warburg elements.