Construction and working mechanism of sulfur dioxide sensor utilizing stabilized zirconia and metal sulfate

An exploratory study was carried out to elucidate the fabrication principles of a potentiometric SO2 sensor utilizing stabilized zirconia (solid electrolyte) and a metal sulfate (auxiliary phase). Among the MgO-, CaO-, Y2O3-stabilized zirconia (abbreviated as MSZ, CSZ, or YSZ) tested, only MSZ samples which contained 15 mole percent (m/o) MgO and were partially stabilized gave a stable SO2 sensor by being attached with Li2SO4, whereas almost fully stabilized MSZ (15 m/o MgO), partially stabilized MSZ (9 m/o MgO), CSZ (11 m/o CaO), or YSZ (8 m/o Y2O3) gave a stable device only when attached with a mixed auxiliary phase of Li2SO4-MgO. These facts indicated an active role by MgO, either segregated from the stabilized zirconia or added intentionally, for the devices. It is estimated that, together with Li2O and ZrO2, MgO is an essential component of the interfacial compound which acts as an ionic bridge between stabilized zirconia (O2- conductor) and auxiliary phase (Li+ conductor). The electromotive force (EMF) response of each device to SO2 followed Nernst's equation well for a two-electron reaction per SO2 molecule, while the EMF dependence on oxygen concentration deviated slightly from the Nernstian behavior for the four-electron reduction of O-2. Such behavior is discussed based on the sensing mechanism proposed.