Cyclic and square-wave voltammetry for selective simultaneous NO and O₂ gas detection by means of solid electrolyte sensors

Abstract

Solid electrolyte gas sensors (SESs) based on yttria-stabilized zirconia (YSZ) are suitable to detect traces of redox components in inert gases. Usually, their signals are generated as a voltage between two electrodes at open circuit potential or as a current flowing between constantly polarized electrodes. In these rather stationary modes of operation, SESs often lack the desired selectivity. This drawback can be circumvented if SESs are operated in dynamic electrochemical modes that utilize the differences of electrode kinetics for single components to distinguish between them. Accordingly, this contribution is directed to the investigation of cyclic voltammetry and square-wave voltammetry as methods to improve the selectivity of SESs. For this, a commercial SES of the type “sample gas, Pt|YSZ|Pt, air” was exposed to mixtures containing NO and O2 in N2 in the temperature range between 550 and 750 ∘C. On cyclic voltammograms (CVs), NO-related peaks occur in the cathodic direction at polarization voltages between −0.3 and −0.6 V at scan rates between 100 and 2000 mV s−1 and temperatures between 550 and 750 ∘C. Their heights depend on the NO concentration, on the temperature and on the scan rate, providing a lower limit of detection below 10 ppmv, with the highest sensitivity at 700 ∘C. The O2-related peaks, appearing also in the cathodic direction between −0.1 and −0.3 V at scan rates between 100 and 5000 mV s−1, are well separated from the NO-related peaks if the scan rate does not exceed 2000 mV s−1. Square-wave voltammograms (SWVs) obtained at a pulse frequency of 5 Hz, pulses of 0.1 mV and steps of 5 mV in the polarization range from 0 to −0.6 V also exhibit NO-related peaks at polarization voltages between −0.3 and −0.45 V compared to the Pt–air (platinum–air) electrode. In the temperature range between 650 and 750 ∘C the highest NO sensitivity was found at 700 ∘C. O2-related peaks arise in the cathodic direction between −0.12 and −0.16 V, increase with temperature and do not depend on the concentration of NO. Since capacitive currents are suppressed with square-wave voltammetry, this method provides improved selectivity. In contrast to cyclic voltammetry, a third peak was found with square-wave voltammetry at −0.48 V and a temperature of 750 ∘C. This peak does not depend on the NO concentration. It is assumed that this peak is due to the depletion of an oxide layer on the electrode surface. The results prove the selective detection of NO and O2 with SESs operated with both cyclic voltammetry and square-wave voltammetry.