Nano-structured perovskite oxide electrodes for planar electrochemical sensors using tape casted YSZ layers

Solid-state NO2 sensors based on yttria stabilized zirconia (YSZ), an O2- conductor, combined either with an n- (WO3), or a p-type semiconducting oxide (LaFeO3), or a mixed electronic and ionic conductor (La0.8Sr0.2FeO3) were investigated. Platinum parallel finger electrodes were applied on the surface of tape-casted YSZ layers and attached with gold wires for current collection. Nanocrystalline perovskite powders were prepared using different chemical methods: LaFeO3 by the thermal decomposition of the LaFe–hexacyanide complex, and La0.8Sr0.2FeO3 by a sol-gel route. A sub-micrometric commercial WO3 powder was used. The oxide powders were mixed with a screen-printing oil and deposited on one Pt finger electrode. The presence of the oxide powder makes one of the electrodes different from the other in terms of catalytic activity, specific surface area, gas adsorption and reaction kinetics. Both electrodes were wholly exposed to the same gas atmosphere, without using reference air. The sensors were investigated at fixed temperature (450–700 C) by measuring the electromotive force (EMF) at different concentrations of NO2 and CO in air in the range 20–1000 ppm. A fast and stable response was measured for all the tested sensors. An EMF of opposite sign was measured for p- and n-type semiconducting based sensors upon exposure to the same gas. After increasing the grain size of the nano-structured La0.8Sr0.2FeO3 powder by a heat treatment at 900 C for 4 h, the response to NOx became small, slow and unstable.