Lambda probe, solid electrolytes

The lambda sensor, which is found in cars with catalytic converters, is an example of an oxygen probe based on the principle of selective electrodes. This sensor, which looks like a spark plug, has a zirconium sleeve (Zr02) that behaves as a solid electrolyte. The external wall is in contact with emitted gas while the internal wall (the reference) is in contact with air. Two electrodes measure the potential difference between the two walls, which is indicative of the difference in concentration of oxygen. 

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

The Lambda probe consists of a solid-state electrolyte (yttrium-stabilized zirconia) that is covered with porous platinum - electrodes on the inside (1) and outside (2) (see Fig.). 

The Lambda probe is usually shaped like a sparking plug where the solid-state electrolyte forms a closed-end cylinder that it inserted into the exhaust gas. The inside of the cylinder is in contact with ambient air with a constant partial pressure of oxygen, po2,i> and the outside is in contact with the exhaust gas in which the partial pressure of oxygen, po2,2, is monitored by measuring the potential difference between the two platinum electrodes... 

Many - gas sensors based on - solid electrolytes operate under potentiometric conditions [iii]. The sensors for oxygen use oxide -> conductors, such as ZrC>2 -based ceramic, those for halogens use halide conductors (e.g., KAg s), while -> hydrogen sensors use protonic conductors. There are sensors for C02, N02, NH3, S03) H2S, HCN, HF, etc. (see -> lambda probe). 

Galvanic cells can be set up with solid electrolytes rather than electrolytic solutions. Such a cell is the basis for a well-known potentiometric gas sensor, the lambda probe. The latter is designed to determine the oxygen content of combustion gases, e.g. in motor vehicles. The lambda probe can operate in two different modes, either potentiometrically or amperometrically. 

Up to 1995, the year in which he became Professor emeritus, and even thereafter he was active in different fields of solid state electrochemistry (solid electrolytes, sensors, SOFC). His pioneering work encompasses contributions to solid oxide fuel cells, theoretical considerations regarding electrochemical phenomena in solid electrolyte cells, basic discoveries in solid electrolyte gas cells, and their applications as gas sensors in different branches of industry. Without any doubt, the discovery of the fuel-oxygen titration curve by means of solid electrolyte cells on which the lambda probe is based is his most memorable contribution to electrochemistry. At the Second European Solid Oxide Fuel Cell Forum, which took place in Oslo on May 1996, Prof. Dr. Hans-Heinrich Mobius was awarded the Christian Friedrich Schonbein Medal of Honor in recognition of his outstanding contributions to the fundamentals and the technology of solid electrolyte fuel cells [1]. 

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