Alumina substrate-supported electrochemical device for potential application as a diesel particulate matter sensor

doi:10.1016/j.snb.2010.01.030

Sensors and Actuators B: Chemical

Volume 145, Issue 2, 19 March 2010, Pages 708-712

https://doi.org/10.1016/j.snb.2010.01.030 Get rights and content

Abstract

The carbon oxidation activity and sensing performance of an alumina substrate-supported sensor comprised of Sn_0.9In_0.1P₂O₇ as a proton conductor and Pt as an electrocatalyst were investigated. Electrochemically formed active oxygen species exhibited high activity for carbon oxidation, where the current efficiency was estimated from the four-electron reaction (C + 2H₂O → CO₂ + 4H⁺ + 4e⁻) and reached approximately 90%. An operating temperature of 225 °C was found to be the most effective to achieve the highest possible electrochemical carbon oxidation and the lowest possible non-electrochemical carbon oxidation. When carbon introduced to the Sn_0.9In_0.1P₂O₇–Pt interface was oxidized by active oxygen, a large potential jump was observed due to a significant increase in the polarization resistance, which was strongly dependent on the carbon content in the working electrode. Two types of carbon sensors, amperometric and potentiometric, were tested in a feed mixture of 3 vol% water vapor and 10 vol% O₂. In the case of the amperometric sensor, the current increased linearly with increasing carbon content, which enabled the determination of a wide range of carbon content from the current signal. In the case of the potentiometric sensor, a threshold quantity of carbon could be recognized by selection of the current and subsequent monitoring of the sudden potential increase.

Introduction

Diesel particulate filters (DPF) are widely reviewed as the most effective technology for achieving low particulate matter (PM) emission from diesel cars. The system is based on the trapping and subsequent combustion of PM [1], [2]. However, increasingly stringent air pollutant regulations will require the further development of DPF technology to ensure near-zero emission of PM from diesel cars. One promising approach to meet such a requirement is to establish feedback control of the DPF system and diagnosis of its performance, which are accomplished by monitoring trace quantities of PM in the upstream and downstream of the filter, respectively [3]. Thus, the automobile industry needs an innovative on-board PM sensor.

We have recently proposed the selective electrochemical oxidation of carbon by active oxygen for potential application as a PM sensor [4]. An electrochemical cell consisting of a proton conducting Sn_0.9In_0.1P₂O₇ electrolyte and a catalytically active Pt working electrode realized the dissociation of water vapor into protons and electrons at the working electrode, and as a consequence, the production of active oxygen at the electrode surface. This oxygen species exhibited high activity for carbon oxidation at temperatures of 50 °C or higher. More interestingly, the formation rate of CO₂ increased with temperature and reached the theoretical value calculated from the following four-electron reaction at 200 °C.C + 2H₂O → CO₂ + 4H⁺ + 4e⁻

Such a high faradic efficiency provided high sensitivity to the change in resistance corresponding to the quantity of carbon in the working electrode. In addition, Reaction (1) self-regenerated the working electrode, which enables the PM concentration in diesel exhaust to be easily monitored in real-time. In particular, the latter property is advantageous over other PM sensors that employ oxide ion conductors (yttria-stabilized zirconia: YSZ) [5], [6] or electrical insulators (aluminum oxide) [7], [8], where deposited carbon is accumulated. However, the solid electrolyte used in our previous study was prepared by merely pressing Sn_0.9In_0.1P₂O₇ powder into pellets, due to the difficulty in preparing sintered compacts of this material. Thus, the synthesis of an electrolyte with high mechanical resistance is a crucial requirement for practical application of the proposed sensor.

In this study, we attempted to improve the mechanical strength of the sensor by supporting it on a ceramic substrate, which has been widely applied to various poorly sinterable resistance- or capacitor-type sensor materials [9], [10], [11]. This technique could also result in simplification of the sensor structure from dual-chamber to single-chamber type. We demonstrated that such a robust and simple sensor device has high potential for application as both an amperometric and potentiometric PM sensor, which extends its potential applications through various calibration processes.

Section snippets

Experimental

Sn_0.9In_0.1P₂O₇ was prepared as follows. SnO₂ and In₂O₃ were mixed with H₃PO₄ and de-ionized water. The amount of H₃PO₄ used was 1.3 times higher than the stoichiometric value for the preparation of Sn_0.9In_0.1P₂O₇, due to the vaporization of a part of H₃PO₄ during the subsequent heat treatments. The mixture was heated to 300 °C and then held with stirring until a high viscosity paste was formed. The paste was subsequently calcined in an alumina pot at 650 °C for 2.5 h. Heating and cooling were

Carbon oxidation activity

Electrochemical carbon oxidation at the working electrode was inspected in water vapor diluted with Ar. The changes in CO₂ concentration and electrode potential with current for a working electrode with a carbon content of 0.3 mg cm⁻² were measured at an operating temperature of 225 °C. The results are shown in Fig. 2, in addition to the theoretical CO₂ concentration calculated from Faraday's law based on Reaction (1). Fig. 2 shows that the CO₂ concentration increased almost linearly with

Conclusions

A carbon sensor device was successfully developed by fabrication of a Pt–carbon mixture|Sn_0.9In_0.1P₂O₇|Pt cell on the surface of an alumina substrate. The electrochemical formation of active oxygen was accomplished at the working electrode by the electrolysis of water vapor, similar to the self-supported carbon sensor developed in our previous study. Active oxygen reacted with carbon introduced to the Sn_0.9In_0.1P₂O₇–Pt interface, resulting in a high current efficiency of approximately 90% at an

Yanbai Shen is a postdoctoral researcher at the Graduate School of Environmental Studies of Nagoya University in Nagoya, Japan. He received his Ph.D. degree from University of Toyama in 2009. His current research interests are focused in the area of sensors and catalysts using intermediate-temperature proton conductors.

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Takashi Takeuchi is a MS candidate at the Graduate School of Environmental Studies of Nagoya University in Nagoya, Japan. His current research interests include sensors and catalysts.

Shinya Teranishi is a Ph.D. candidate at the Graduate School of Environmental Studies of Nagoya University in Nagoya, Japan, and is also a researcher for Nippon Soken, Inc. He received his MS degree in Applied Chemistry from Nagoya University in 2006. His current research interests include sensors and catalysts.

Takashi Hibino serves in the Graduate School of Environmental Studies of Nagoya University in Nagoya, Japan. He received his Ph.D. from Nagoya University in 1991. Prof. Hibino's current research interests are focused in the area of sensors, fuel cells, electrolyzers, and catalysts.

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Sensors and Actuators B: Chemical

Abstract

Introduction

Section snippets

Experimental

Carbon oxidation activity

Conclusions

References (19)

Diesel engine exhaust gas recirculation—a review on advanced and novel concepts

Energy Convers. Manage.

Engines and exhaust after treatment systems for future automotive applications

Solid State Ionics

Selective electrochemical oxidation of carbon by active oxygen for potential application as a sensor for diesel particulates

Sens. Actuators B

Soot sensor based on a porous solid electrolyte cell

Sens. Actuators B

Sensor for on-line in situ measurement of soot concentration in flue gases

Sens. Actuators B

Cordless solid-state hydrogen sensor using proton-conductor thick film

Sens. Actuators B

A practical capacitive type CO₂ sensor using CeO₂/BaCO₃/CuO ceramics

Sens. Actuators B

Dependence of NO₂ gas sensitivity of WO₃ sputtered films on film density

Thin Solid Films

Selective catalytic reduction of NO_x by H₂ using proton conductors as catalyst supports

J. Catal.

Cited by (16)

Enhanced NO<inf>2</inf> sensing performance of ZnO nanowires functionalized with ultra-fine In<inf>2</inf>O<inf>3</inf> nanoparticles

Effect of hydrothermal process for inorganic alumina sol on crystal structure of alumina gel

Interdigitated Pt-GaN Schottky interfaces for high-temperature soot-particulate sensing

A solid-state particulate matter sensor based on electrochemical oxidation of carbon by active oxygen

Preliminary study on catalytic combustion-type sensor for the detection of diesel particulate matter

Design of a highly sensitive and responsive electrode for particulate matter monitoring