Solution-based synthesis of efficient WO3 sensing electrodes for high temperature potentiometric NOx sensors

doi:10.1016/j.snb.2008.09.017

Sensors and Actuators B: Chemical

Volume 136, Issue 2, 2 March 2009, Pages 523-529

https://doi.org/10.1016/j.snb.2008.09.017 Get rights and content

Abstract

Electrode nanostructures as well as species at electrode–electrolyte interfaces have substantial influence on the sensitivity, response and recovery times of electrochemical sensors. YSZ-based potentiometric NO_x sensors with WO₃ sensing electrodes have shown considerable promise for enhanced sensitivity. In this study, we present a solution-based method using peroxytungstate solutions to fabricate WO₃ electrodes. UV-ozone treatment of the YSZ was necessary for effective bonding of the WO₃ to the YSZ. The resulting WO₃ electrode was found to exhibit different surface nanostructures, better mechanical stability, faster recovery times, and better sensitivity than devices made from conventional ceramic WO₃ powders. Upon UV-ozone treatment, the YSZ surfaces become more reactive towards the acidic peroxytungstate solution and results in monoclinic ZrO₂ formation at the electrode–electrolyte interface, which, based on earlier studies, we propose to be responsible for the improved sensor sensitivity. Better adhesion of the peroxytungstate-based WO₃ electrode to the YSZ electrolyte is related to the improved recovery times.

Introduction

High temperature NO_x sensors have emerged as one of the key elements in combustion industry. Internal combustion engines operated at high air/fuel ratio are currently in development with the goal of increased fuel efficiency. However, in an environment of excess oxygen, three-way catalysts traditionally used to reduce NO_x, hydrocarbon, and CO emissions are not functional. Possible proposed solutions include using a chemical trap with periodic regeneration or reductants for continuous NO_x reduction [1], [2]. Reliable NO_x sensors are needed for controlling these processes [3]. Applications of NO_x sensors are also expected in the power, chemical, glass and other high-temperature industries, and as a cross cutting technology in medicine for diagnosing lung diseases.

Most high temperature potentiometric NO_x sensors (>500 °C) in development are based on stabilized zirconia electrolytes and metal oxide electrodes [4], [5], [6]. Tungsten trioxide (WO₃), in addition to its applications in electrochromic devices [7] and semiconductor sensors [8], [9], has received considerable attention as the electrode material for potentiometric gas sensing. Several reports have described the exceptional NO_x sensing performance when using WO₃ electrodes with YSZ (yttria-stabilized zirconia), especially at temperatures higher than 600 °C [10], [11], [12]. We have reported that non-Nernstian potentiometric sensing devices composed of WO₃ electrodes, YSZ electrolytes, and Pt-loaded zeolite Y (PtY) filters possess unique sensitivity and selectivity toward NO_x [13]. The Pt nanoclusters stabilized in the high surface area microporous zeolite cages exhibit excellent catalytic properties. PtY can also be used as a reference electrode since it is effective in equilibrating NO_x at high temperatures. Our previous study showed that the sensitivity towards NO_x for WO₃-based sensing electrodes is improved due to interfacial reactions at the electrode-YSZ interface, but the recovery times of the sensor was poor [14].

For electrode materials, the structure as well as interfacial species have substantial influence on sensor performance [14], [15], [16]. Our previous studies used screen-printing techniques that involved mixing metal-oxide powder with organic binders to fabricate porous metal-oxide electrodes. Ceramic film preparation starting with aqueous peroxy-metal solutions has been used for preparation of metal oxides, including TiO₂ and ZrO₂ [17]. In the present study, peroxytungstate solutions were used to prepare WO₃ sensing electrodes. UV-ozone treatment of YSZ was exploited to increase surface wettability, confine the electrode geometry and provide a lower temperature method to generate the WO₃ layer. Sensors with different electrode structures were characterized by SEM, XRD, and Raman spectroscopy, and their NO₂ sensing performance was evaluated.

Section snippets

Fabrication of basic sensor platforms

The substrate was prepared from YSZ green sheets (3 mol% tetragonal YSZ, NexTech Materials). The 15 mm × 5 mm YSZ green sheets were sintered in air at 1450 °C for 2 h to form dense bodies. Two Pt lead wires (99.95%, 0.13 mm in diameter, Fischer Scientific) were attached to YSZ with a small amount of commercial Pt ink (Englehard, A4731). The end attaching to YSZ was shaped into a disc of 2 mm diameter in order to increase the mechanical stability. The Pt ink was cured at 1200 °C for 2 h to secure bonding

Sensor fabrication and characterization

The four electrochemical sensors used in this study are illustrated in Fig. 1. All sensors are based on the same platform with different sensing electrodes, but the same reference electrode (Pt-zeolite Y/Pt). Of particular interest are sensors C and D prepared with the peroxytungstate solutions, the procedure depicted schematically in Fig. 2.

Fig. 3a shows the morphology of the Pt electrode (Sensor A) after sintering at 1200 °C. The Pt ink used in this work resulted in a dense structure on the

Discussion

In an earlier publication, we investigated in detail the use of commercial WO₃ powder as a sensing electrode and concluded that its superior performance was related to the formation of interfacial zirconia and yttrium tungstates that minimized the heterogeneous equilibration of the NO_x species [14]. We also noted that the recovery times of the sensors were poor, which motivated us to do the present study. Clearly, an improvement in sensor performance was noted with the electrodes prepared via

Conclusions

Aqueous peroxytungstate solutions were used to fabricate WO₃ sensing electrodes for high temperature potentiometric NO_x sensing. WO₃ films can be deposited selectively on Pt electrodes only or on both Pt electrodes and UV-ozone treated YSZ by immersion coating or drop coating. The WO₃/YSZ sensing electrode fabricated by this method has better mechanical stability, higher sensitivity, and better response/recovery times than devices fabricated from commercial WO₃ powder. From the XRD and Raman

Acknowledgements

This work was supported by the Department of Energy (DE-FC26-03NT41615). The authors thank Dr. Jing-Jong Shyue for his assistance on FIB and Professor Umit Ozkan for access to Raman instrumentation.

Jiun-Chan Yang completed his undergraduate studies in Taiwan and received his PhD in chemistry in 2007 from the Ohio State University. He is currently a postdoctoral fellow at Northwestern University.

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Prabir K. Dutta received his PhD degree in chemistry from Princeton University. After four years of industrial research at Exxon Research and Engineering Company, he joined The Ohio State University, where currently he is professor of chemistry. His research interests are in the area of microporous materials, including their synthesis, structural analysis and as hosts for chemical and photochemical reactions.

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Solution-based synthesis of efficient WO3 sensing electrodes for high temperature potentiometric NOx sensors

Abstract

Introduction

Section snippets

Fabrication of basic sensor platforms

Sensor fabrication and characterization

Discussion

Conclusions

Acknowledgements

Prog. Energy Combust. Sci.

Sens. Actuators, B

Solid State Ionics

Solid State Ionics

Sol. Energy Mater. Sol. Cells

Sens. Actuators, B

Sens. Actuators, B

Sens. Actuators, B

Sens. Actuators, B

Solid State Ionics

Selective Catalytic Reduction of NOx with N-Free Reductants

Chem. Rev.

Kinetic study of NO oxidation and NOx storage on Pt/Al2O3 and Pt/BaO/Al2O3

J. Phys. Chem. B

Solution-based synthesis of efficient WO₃ sensing electrodes for high temperature potentiometric NO_x sensors

Selective Catalytic Reduction of NO_x with N-Free Reductants

Kinetic study of NO oxidation and NO_x storage on Pt/Al₂O₃ and Pt/BaO/Al₂O₃