The Nernst equation indicates that a cell potential will develop even when the electrode materials are the same, provided that there is a difference in concentration on each side of the electrolyte. This has been widely utilised as an oxygen sensor using calcia-stabilised zirconia as the active electrolyte. Calcia-stabilised zirconia is a non-stoichiometric oxide
prepared by reaction of ZrO2 and CaO (Section3.4.5, Figure 3.12). Doping leads to a high concentration of oxygen vacancies, which means that oxygen ions can diffuse very rapidly through the ceramic. In its simplest form, the sensor is just a slice of stabilised zirconia separating oxygen gas at two different pressures. The high oxygen ion diffusion
coefficient will allow ions to move from the high-pressure side to the low-pressure side to even out the pressure differential. Connecting both sides of the stabilised zirconia via porous platinum electrodes sets up an electrochemical cell in which the voltage is proportional to the difference between the oxygen pressures. The cell can be represented as