Dependence of potentiometric oxygen sensing characteristics on the nature of electrodes

A comparative study of gas sensing response and electrical properties of three electrodes for a potentiometric YSZ-based oxygen sensor is presented. Platinum (Pt), lanthanum strontium iron cobalt oxide (La0.6Sr0.4Fe0.8Co0.2O3: LSFCO) and chromium oxide (Cr2O3) were used as the sensing electrodes. The microstructural features, electrochemical impedance for the charge transfer processes and oxygen sensing characteristics were studied at different temperatures and correlated. Impedance measurements indicate that the LSFCO electrode exhibits lower resistance and higher capacitance by almost two to three orders of magnitude for the chemical exchange and ion transfer processes compared to the other two electrodes. The relaxation time constants of all three electrodes are comparable to each other and found to be in the range of fraction of a second. But, the measured response times for oxygen sensors using these electrodes showed values of the order of minutes at temperatures of 500 °C and below. So, the controlling factor for oxygen response at temperatures below 500 °C seems to be adsorption and/or surface diffusion of oxygen rather than the charge transfer process for a given morphology of the electrode.