Robust high temperature oxygen sensor electrodes

Platinum is the most widely used material in high temperature oxygen sensor electrodes. However, platinum is expensive and the platinum electrode may, under certain conditions, suffer from poisoning, which is detrimental for an
oxygen sensor. The objective of this thesis is to evaluate electrode materials as candidates for robust oxygen sensor electrodes. The present work focuses on characterising the electrochemical properties of a few electrode materials to understand which oxygen electrode processes are limiting for the response time of the sensor electrode. Three types of porous platinum-based electrodes and one porous electrode based on the perovskite-structured strontium and vanadiumdoped lanthanum chromium oxide (LSCV) were investigated. The porous electrodes were applied on yttrium-stabilised zirconium oxide (YSZ) substrates in a collaboration with the company PBI-Dansensor. The electrochemical properties of the electrodes were characterised by electrochemical impedance spectroscopy (EIS), and the structures were characterised by x-ray diffraction and electron microscopy.
At an oxygen partial pressures of 0.2 bar, the response time of the sensor electrode was determined by oxygen reaction kinetics. At oxygen partial pressures below 10^-6 bar at 700 C, the mass transport processes dominated the response
time. The response time increased with decreasing oxygen partial pressure and inlet gas flow rate.
A series of porous platinum electrodes were impregnated with the ionically conducting gadolinium-doped cerium oxide (CGO). The addition of CGO was found to decrease the polarisation resistance of the oxygen reaction by up to
an order of magnitude compared with a single phase platinum electrode by increasing the effective triple phase boundary (TPB) length. It did not have any significant effect on the response time of the electrode at low oxygen partial
pressures. LSCV may take up/release oxygen by changing the oxide ion stoichiometry which gives rise to a large chemical capacitance compared with that of the platinum-based electrodes. The response time of a composite LSCV/YSZ
electrode was around an order of magnitude longer compared with that of the platinum-based electrodes electrode. Even though the response time is longer for a composite LSCV/YSZ electrode it may be applied as electrode for oxygen
sensors which are used in industrial processes. Furthermore, the platinumbased cermet and the composite LSCV/YSZ electrodes have a longer effective TPB length and are therefore more robust compared with a single phase platinum
electrode. The increase in the effective TPB length of the platinum-based electrodes did not have any significant effect on the response time at low oxygen partial pressures, where the response becomes critical.