Sensors and Actuators B 177 (2013) 111–115 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Short communication Impedance spectroscopy based characterization of an electrochemical propylene sensor Praveen K. Sekhar a,∗ , Hamid Sarraf a , Hanna Mekonen a , Rangachary Mukundan b , Eric. L. Brosha b , Fernando H. Garzon b a Nanomaterials and Sensors Laboratory, School of Engineering and Computer Science, Washington State University Vancouver, Vancouver, WA 98686, United States b Los Alamos National Laboratory, Sensors and Electrochemical Devices Group, Los Alamos, NM 87545, United States a r t i c l e i n f o Article history: Received 7 August 2012 Received in revised form 23 October 2012 Accepted 31 October 2012 Available online 9 November 2012 Keywords: Impedance spectroscopy Mixed potential Propylene sensor Activation energy Yttria-stabilized zirconia a b s t r a c t In this investigation, an electrochemical mixed potential type gas sensor was characterized using impedance spectroscopy. Specifically, the effect of operating temperature (435–610 ◦ C) on sen- sor response and response time was studied. Propylene was used as the analyte to test the ‘La 0.8 Sr 0.2 CrO 3 /YSZ/Pt’ sensor configuration. Two-electrode AC impedance measurement was performed with a frequency sweep from 13 MHz down to 10 mHz and excitation voltage of 10 mV. For a fixed propylene concentration, the bulk and interfacial resistances was seen to decrease with increase in the sensor operating temperature. An Arrhenius behavior of the bulk and interfacial resis- tance was observed. The activation energy for O 2 ion conduction and charge transfer was found to be 0.94 eV and 1.54 eV respectively. For a 150 ◦ C rise in operating temperature from 485 to 585 ◦ C, a 26- fold improvement in response rise time was observed while an 82% reduction in sensor response was recorded. It is postulated that the increase in operating temperature results in faster reaction kinetics, faster oxygen reduction and greater heterogeneous catalysis. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Impedance spectroscopy (IS) is the study of electrical impedance of a system or material as a function of frequency [1–3]. It has been routinely used as an investigative and diagnostic probe in a wide variety of applications including corrosion, battery research, fuel cells, materials characterization, biosensors and solid-state devices [4–7]. In the case of electrochemical systems, Bauerle [8] in 1969 reported the polarization behavior of zirconia-yttria solid electrolyte specimen with Pt electrode by a complex admittance method. According to the author, the complex admittance spec- trum was able to resolve three types of polarization that includes electrode polarization, capacitive–resistive electrolyte polariza- tion, and ohmic electrolyte polarization. Further, Matsui [9] in 1981 examined the complex impedance of Pt, Au and Ag electrodes in contact with an yttria-stabilized zirconia (YSZ) electrolyte for developing oxygen sensors. The complex-impedance analysis was used to investigate separately the effects of YSZ and metal elec- trode on the properties of the sensor. Progressing over time, IS has become a critical tool in the characterization and optimization of ∗ Corresponding author at: 14202 NE Salom Creek Avennue, Vancouver, WA 98686, United States. Tel.: +1 360 546 9186; fax: +1 360 546 9438. E-mail address: praveen.sekhar@vancouver.wsu.edu (P.K. Sekhar). electrochemical sensor systems. A review article by Pejcic et al. [10] summarizes over 35 years (1970–2006) of research investigation on the use of IS in electrochemical sensor optimization. According to the reviewed literature, IS has been used to provide information on various fundamental processes (i.e., adsorption/film formation, rate of charge transfer, ion exchange, diffusion, etc.) that occur at the electrode–electrolyte interface. In the next section, the application of IS to optimize mixed potential sensor (a subset of electrochemical device) performance has been detailed. Mixed potential sensors are a class of electrochemical devices that develop an open-circuit non-equilibrium potential in the pres- ence of oxygen and an oxidizing/reducing gas [11]. The magnitude of this potential is determined by the rates of oxidation and reduc- tion reactions occurring at each electrode\electrolyte interface. Mortimer et al. [12] used AC IS to investigate a mixed poten- tial carbon monoxide (CO) sensor designed with screen printed electrodes (platinum as the working and counter electrodes, gold as the reference electrode) and a recast film of a sulfonated styrene/ethylene–butylene/styrene triblock copolymer as the pro- ton conducting solid polymer electrolyte. The authors report that chemisorption of CO onto the catalytic platinum electrode surface resulted in a decrease in charge-transfer resistance with an increase in gas concentration. Further, White et al. [13] observed three phenomena in the impedance spectra of a La 2 CuO 4 /YSZ/Pt NO x sen- sor. They were O 2 ion conduction in the YSZ electrolyte, surface 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.137