*2.3.1. Electrocatalysis*

As mentioned earlier, electrocatalysis is one of the main fields within electrochemistry. Many electrocatalysts are built with the aim to electrocatalytic reactions that are involved, for example, in batteries or fuel cells. In these cases, the studies are more complex since it is important to consider the performance of the anode, cathode, and membrane. All these components contribute to the final impedance value in the electrochemical system.

**Figure 20** shows an example of a Nyquist plot resulting from a solid oxide fuel cell.

In this example, three semicircles are observed, meaning that three electron transfers are taking place over the range of measurement. An ideal panorama for the performance of the electrodes is to have the impedance values as low as possible, meaning that the electrodes are highly conductive and the application of the potential is low.

Another example, in fuel cells, is presented in **Figure 21**, where four zones can be identified. From left to right, we observe proton conduction in electrolyte membrane, charge transfer at the electrode-electrolyte interphase, gas diffusion through gas diffusion layer, and water transport across membrane or relaxation of adsorbed intermediates.

In this case, where a polymer electrolyte fuel cell is studied, we can observe a fourth zone that is attributed to the adsorption of species onto the surface of the electrode. When fitting the results of this diagram, an inductor (L) must be added to the equivalent circuit. An inductor considers both a capacitive and a resistive behavior due to the formation of another double layer. Thus, every time the semicircles pass through the *x*-axis, it is considered that the system presents an inductance in its electrochemical performance. An inductive contribution is usually caused by the connecting wires in the high-frequency domain when low impedances are measured (for battery applications) or when a significant noise in the low-frequency range is found when measuring high impedance.
