text: The most common tool to characterize the performances of a reversible solid oxide cell is the polarization curve. In this chart, the current density is related to operating voltage of the cell. The usual convention is the one of positive current density for the fuel cell operation, and negative current density for the electrolysis operation. When the rSOC electrical circuit is not closed and no current is extracted or supplied to the cell, the operating voltage is the so-called open circuit voltage (OCV). If the composition of the gas in the fuel electrode and the oxygen electrode are the same for both modalities, the polarization curve for the SOEC mode and the SOFC have the same OCV. When some current density is extracted or supplied to the cell, the operating voltage starts to diverge from the OCV. This phenomenon is due to the polarization losses, which depend on three main phenomena:
  * the activation losses, predominant at very low current densities;
  * the ohmic losses, increasing linearly with the current density;
  * the concentration losses, occurring at very high current density, when the reactants inside the electrode get depleted.
  The sum of the polarization losses takes the name of overpotential.
  Other than the open circuit voltage, another fundamental theoretical voltage can be defined. The thermoneutral voltage  depends on the enthalpy of the overall reaction taking place in the rSOC and the number of charges that are transferred within the electrochemical reactions. Its relationship with the operating voltage gives information about the heat demand or generation inside the cell.
  During the electrolysis operation:
  * if , the reaction is endothermic;
  * if , the reaction is exothermic.
  The fuel cell operation, instead, is always exothermic.