POTENTIOMETRIC NOX (X=1, 2) SENSORS WITH AG+-BETA''-ALUMINA AS SOLID ELECTROLYTE AND AG METAL AS SOLID REFERENCE

A new Nernst-type NOx (x = 1, 2) sensor was designed with Ag+-beta"-alumina as the solid electrolyte, porous platinum together with AgNO3 as the ps-sensing working electrode, and silver metal as the solid reference electrode. Both planar and tubular Ag+-beta"-alumina based NOx sensors have been prepared and tested in a simple configuration which allows these sensors to be commercially developed. The planar NOx sensors with a thin sputtered platinum electrode are sensitive to a variation in the NO concentration at a fixed temperature around 200-degrees-C, but a difference between the experimental and theoretical EMF response was observed, due to a poor catalytic activity of the working electrode. However, the tubular sensor with a thicker painted platinum electrode revealed experimental EMF values close to the theoretical values in the NO concentration range of 10(3) to 10(4) ppm. The EMF response of the tubular sensors to lower NO concentrations of 50 to 1000 ppm is affected by the thickness of the silver nitrate formed at the platinum working electrode. The EMF response of a tubular sensor shows a temperature dependence in agreement with the theoretical dependence for temperatures at the working electrode. SEM pictures of the platinum working electrode reveal before testing a porous structure, while after testing several AgNO3 crystalline particles were observed, due to the formation of the electrochemically active AgNO3 solid electrolyte layer at the working electrode.