Construction of Sensitive and Selective Zirconia-Based CO Sensors Using ZnCr2O4-Based Sensing Electrodes

The carbon monoxide (CO) sensitivity of a mixed-potential-type yttria-stabilized zirconia (YSZ)-based tubular-type sensor utilizing a ZnCr2O4 sensing electrode (SE) was tuned by the addition of different precious metal nanoparticles (Ag, Au, Ir, Pd, Pt, Ru and Rh; 1 wt % each) into the sensing layer. After measuring the electromotive force (emf) response of the fabricated SEs to 100 ppm of CO against a Pt/air–reference electrode (RE), the ZnCr2O4–Au nanoparticle composite electrode (ZnCr2O4(+Au)–SE) was found to give the highest response to CO. A linear dependence on the logarithm of CO concentration in the range of 20–800 ppm at an operational temperature of 550 °C under humid conditions (5 vol % water vapor) was observed. From the characterization of the ZnCr2O4(+Au)–SE, we can conclude that the engineered high response toward CO originated from the specific properties of submicrometer sized Au particles, formed via the coalescence of nanosized Au particles located on ZnCr2O4 grains, during the calcining process at 1100 °C for 2 h. These particles augmented the catalytic activities of the gas-phase CO oxidation reaction in the SE layer, as well as to the anodic reaction of CO at the interface; while suppressing the cathodic reaction of O2 at the interface. In addition, the response of the ZnCr2O4(+Au)–SE sensor toward 100 ppm of CO gradually increased throughout the 10 days of operation, and plateaued for the remainder of the month that the sensor was examined. Correlations between SEM observations and the CO sensing characteristics of the present sensor were suggestive that the sensitivity was mostly affected by the morphology of the Au particles and their catalytic activities, which were in close proximity to the ZnCr2O4 grains. Furthermore, by measuring the potential difference (emf) between the ZnCr2O4(+Au) and a ZnCr2O4 electrode, sensitivities to typical exhaust component gases other than CO were found to be negligible at 550 °C.