Convection Influence on Redox Potential Measurements at Hot Platinum Electrodes

Abstract

Introduction The equilibrium of redox reactions between H2O and H2, CO and CO2 or water gas can be determined potentiometrically at high temperatures with a solid electrolyte cell (SEC) by establishing defined partial pressures of both partners of the redox couples according to Nernst equation [1]. The dependency of the potential on temperature allows to calculate thermodynamic parameters of the reaction from the equilibrium constant K p. The thermodynamic relations reveal that the measured cell voltage depends on partial pressures of the redox active gas components, cell temperature and the oxygen partial pressure in the reference gas. In this work, the results of experimental studies on the impact of gas convection on the equilibrium potential are presented and possible reasons for this behavior are discussed. Experimental The experimental setup is given schematically in Fig. 1. The flow rate of the redox gas is adjusted between 0 and 50 standard cm³ (sccm) by MFC. The purpose-built humidifier for establishing a constant water vapor partial pressure consists of six horizontally positioned glass tubes of around 10 cm length and 3 cm diameter sequentially connected with each other. The tubes are half filled with distilled and sterilized water and thermostated to 22.00 ± 0.01 K in a stirred water bath. The test gas pass the tubes in a laminar flow above the water filling and is saturated with water vapor avoiding aerosol formation. A downstream capacitive dew point sensor was used to verify the accuracy of the humidification. Over a wide temperature and concentration range, the water vapor dissociation is negligible so that the H2/H2O ratio set at room temperature remains practically unchanged in the heated solid electrolyte cell [2]. Different self-developed and commercial YSZ cells equipped with Pt electrodes of different morphologies were investigated by measuring redox potentials in H2/H2O and CO/CO2 mixtures with various dilution. The oxygen partial pressure of the reference gas air was monitored by continuous measurement of dew point and total pressure. Results The redox equilibrium in H2/H2O mixtures is established at catalytically highly active Pt electrodes in YSZ cells already at relatively low temperatures below 600 °C [3]. In Figure 2a the course of the cell voltage measured in such a mixture is given for different temperatures. At the adjusted H2/H2O partial pressure ratio near 1 and absolute values of component partial pressures around 3000 Pa, small oxygen leakages will not influence the measured potential [2]. The most influencing deviations on the measured voltages are related to temperature measurement and control of the YSZ cell. These are assumed to be responsible mainly for the noise visible in the curve in Fig. 2a. The systematic deviation known from thermocouple application probably causes the small difference of the measured cell voltage from values, which are expected from thermodynamic data (NIST). If the complete voltage difference Δ in Fig 2b is attributed to temperature deviation of the used thermocouple (type B), this parameter amounts to around 11 K higher than it is indicated by the set point value of the controller. Other sources of deviations can arise from asymmetric potentials of the YSZ cell resulting in measurable voltages at equal oxygen partial pressures on both sides of the cell. In order to obtain indications of such deviations, the flow rate and the dilution of the redox mixtures were varied. The curves in Fig. 3 show that especially the flow rate has surprisingly high influence on the cell voltage. After switching off the gas flow the cell voltage decreases abruptly to values 6-10 mV lower than measured in the constant gas flow. The following drift continues this decrease. Immediately after switching on the flow, the cell voltage jumps back to values before switch-off. At flow rates below 10 sccm, this switch-off shift decreases significantly. At all investigated cells, the switch-off shift increases with decreasing dilution of the redox mixture and increasing temperature, reaching its maximum in pure H2/H2O mixtures at the investigated temperature maximum. In the contribution, the signal influencing processes like gas flow cooling of the electrode, flow induced pressure increase, convective diffusion in pores and oxygen exchange between electrode/electrolyte and gas [4] will be discussed and different scenarios will be compared. Literature R. Hartung, H.-H. Möbius, Potentiometrische Bestimmung des Wasserdampf Dissoziationsgleichgewichtes zwischen 1000 und 1300 K mit einer Festelektrolytzelle, Chemie Ingenieur Technik 40 (12), 592–600 (1968); DOI: 10.1002/cite.330401209. H.-H. Möbius, Solid-state electrochemical potentiometric sensors for gas analysis, In: Göpel, W. Hesse, J. Zemel, J.N.: Sensors. A Comprehensive Survey, Volume 3: Chemical and Biochemical Sensors Part II. Weinheim, New York: VCH-Wiley, 1104–1154 (1995). R. Hartung, H.-H. Möbius, Zur unteren Begrenzung der Arbeitstemperatur galvanischer Sauerstoff-Meßzellen mit Zirconiumdioxid-Festelektrolyten Z. Chem. 9, 197-198 (1969). M. Schelter, J. Zosel, V. Vashook, U. Guth, M. Mertig, Electrolyte related parameters of coulometric solid state devices, Solid State Ionics 288, S. 266–270 (2016); DOI: 10.1016/j.ssi.2016.01.020. Figure 1