Introduction Solid electrolyte sensors (SES) based on yttria stabilized zirconia (YSZ) can be used to detect traces of other components in inert gases such as Ar or N2. The O2--conducting solid electrolyte YSZ is thermodynamically highly stable and can be operated at high temperatures with stationary (potentiometry, amperometry) or dynamic methods (pulse polarization [1], cyclovoltammetry (CV) [2]). In the stationary mode, the selectivity of such sensors is limited to an extend that it is usually not possible to distinguish between individual gas components. In contrast to that, dynamic methods allow a significantly increased selectivity by using different rates of electrode reactions of the individual components in order to detect them independently in a gas mixture [2]. Dynamic electrochemical methods have been successfully used in liquids for many decades, and preliminary investigations on solid electrolyte gas sensors also show that such methods can be used to measure the concentrations of several redox-active gases, for example, oxygen and carbon monoxide [3]. The commercial solid electrolyte sensor used in this work was previously successfully tested to measure selectively different gases such as hydrogen, oxygen and water vapor [4] in nitrogen by using CV. In this work, these studies will be continued to investigate the ability of CV for selective detection of nitrogen oxides (NOx) and to elucidate the mechanisms of signal formation. Experimental setup The experimental setup shown in Fig. 1 and described in detail in [2] was used to adjust the O2 concentration (SES 1) and the measurement of the concentrations of O2 and NO in nitrogen (SES 2). Both SES of the type "Test gas, Pt|YSZ|Pt, air" contain a YSZ-tube equipped with two cylindrical platinum mesh electrodes at the inner and outer surface. The area of the inner measuring electrode was estimated by SEM to be about 3.5 cm2. A porous YSZ sintered layer was applied to join electrolyte and electrodes. Covering large parts of the reference and measuring electrode this porous layer offers an extended three-phase boundary for the oxygen transfer.SES 1 was continuously polarized during each experiment providing a constant O2 concentration in the purified carrier gas N2. At SES 2, the CVs were obtained at polarization voltages between -0.1 and -0.9 V against the Pt air reference electrode and scan rates in the range 100 ... 5000 mV/s. In each experiment, the second cycle was evaluated to obtain reproducible results. Between two consecutive cyclovoltammograms (CVM), the open circuit potential (OCP) was measured to monitor depolarization of the measuring electrode. Results and discussion The CVMs shown in Fig. 2 exhibit NO-related peaks developing in the cathodic scan direction between -0.2 and -0.7 V at scan rates between 100 and 2000 mV/s. The peak maximum shifts in the cathodic direction with increasing scan rate. The highest sensitivity was achieved at 2000 mV/s for 650 °C. At higher scan rates it decreases again. This is apparently caused by the shift of the oxygen reduction peaks from range -0.1 ... -0.3 V at lower scan rates to the cathodic direction at higher scan rates and leads to their superposition with the NO peaks. The curve shifts to higher electrolysis currents with increasing NO-concentration in the cathodic region, visible for every scan rate at U < -0.7 V correspond to the Faraday currents calculable for the NO reduction. The different peak potentials for O2 and NO enable a highly selective determination NO in O2 containing gases at scan rates between 200 – 2000 mV/s at concentrations c(NO) ≥ 10 vol.-ppm.The height of the NO peak as a suitable NO-related signal was calculated and plotted in Fig. 3 for different scan rates. This peak height rises almost linearly with the NO concentration and the curve slopes increase with the scan rate up to 2000 mV/s. Further investigations revealed that this scan rate-induced slope increase depends also on flow rate.OCP measurements at different NO concentrations and sensor temperatures (Fig. 4) indicate the known tendency of NO to decompose at hot and catalytically active platinum electrodes into N2 and O2. Therefore, in the investigated temperature range, the absolute value of the OCP decreases with increasing NO concentration, coming with rising oxygen partial pressure according to NO decomposition. Another process guiding the OCP in the NO-free carrier gas is the electronic conductivity of the solid electrolyte that increases exponentially with temperature [5].
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Sep 1, 2024
A S L Salazar-Valdez + 8
Open Access
