Elsevier

Sensors and Actuators B: Chemical

Volume 178, 1 March 2013, Pages 163-168
Sensors and Actuators B: Chemical

Characteristics and performance of NASICON-based CO2 sensor using Bi8Nb2O17 plus Pt as solid-reference electrode

https://doi.org/10.1016/j.snb.2012.12.084Get rights and content

Abstract

The gas sensing properties of NASICON-based CO2 sensor attached with a composite of Bi8Nb2O17 plus small amount of Pt as a solid-reference electrode has been investigated for application of the sensor at high CO2 concentration. Compared with conventional sensor coated with Pt reference electrode, the sensor attached with solid-reference electrode was found to efficiently improve the detection limit due to the presence of solid-reference electrode prevented NASICON reacting with CO2 and water to form carbonate or bicarbonate. A good bonding interface between the solid-reference electrode and NASICON was formed as revealed by the scanning electron spectroscopy (SEM), which provided a powerful guarantee for the thermal stability of the sensor. Furthermore, the as-fabricated sensor also exhibited fast response time, small cross-sensitivity to humidity and little interference from the coexistent oxygen partial pressure at 500 °C. All the present results suggest that using the composite of Bi8Nb2O17 plus Pt as a solid-reference electrode is a promising candidate for NASICON-based CO2 sensor.

Introduction

Development of compact, inexpensive, low energy consumption and reliable solid state sensors for measuring CO2 concentrations has been in high demand for numerous applications ranging from control of combustion processes to environmental monitoring [1]. So far, many types of solid state CO2 sensors based on optical properties [2], capacitance [3], field effect transistor (FET) [4] and electrochemical potential [5] have been investigated. Among them, the electrochemical-type is more suitable than others because CO2 is not a redox gas but an acid–base active gas, which usually reacts with ions of solid rather than electrons [6]. NASICON (Na3Zr2Si2PO12: Na+-ion conductor)-based potentiometric CO2 sensors covered with a binary carbonate Li2CO3–BaCO3 (1:2 in molar ratio) are one of the most popular candidates due to their excellent performance such as good sensitivity, high selectivity and fast response time [7]. However, such sensors suffer from significant interference from humidity [8], [9]. Thus the long-term stability of the sensors cannot be guaranteed. The existing researches confirm that the stability of the sensor is affected not only by the electrode composition [10], [11], but also by the geometry of sensor [12], [13], [14], thermal treatment history [15], and the presence of humidity [16], [17], [18]. And the alkaline metal carbonates often used as auxiliary electrode was attached on one side of NASICON disk by mechanical press [19], leading to poor chemical stability [20] and low mechanical strength due to the thermal expansivity of auxiliary electrode layer was much different from that of NASICON electrolyte. From this point of view, our previous work demonstrated that the use of Li2CO3, NASICON and Pt as auxiliary electrode enhanced the electrode mechanical strength as well as expanded the amount of triple phase boundaries (TPBs), giving a good performance of the sensor [21]. Sadaoka et al. [22] reported that the degradation of the performance of the sensors due to humidity resulted from the formation of NaHCO3 and NaOH at the grain boundary of the NASICON electrolyte, due to the easy diffusion of H2O and CO2 into NASICON. And both Lee et al. [23] and Aono and Sadaoka [24] pointed out that the following reactions might occur when the reference electrode was not sealed.2Na+ + CO2 + 1/2O2 + 2e = Na2CO3

or2Na+ + H2O + 2CO2 + 1/2O2 + 2e = 2NaHCO3

Thus, the Na2O activity in NASICON may probably be altered by the formation of these new compounds at the reference electrode, which results in the electromotive force (EMF) drift of the sensors. Especially, once the Na2CO3 formed on the surface of reference electrode, the identical electrode reaction will occur at both auxiliary electrode and reference electrode, resulting in the EMF to deviate from Nernst equation.

In an attempt to overcome such a problem, a promising approach seems to introduce a new reference electrode system that works as an oxygen-conduction membrane and physical barrier of hindering diffusion of CO2 and water vapor [25], [26]. Up to now, some oxygen ion conductors such as yttrium stabilized zirconia (YSZ) [27] and BICUVOX [28], [29], [30] have been investigated.

In the present work, we propose another fast oxygen ionic conductor of Bi8Nb2O17 (i.e. x = 0.2 in (1  x)Bi2O3·xNb2O5 system) for reference electrode material to upgrade the performance of the sensor. As for oxygen ion conductors, the high-temperature form of bismuth oxide δ-Bi2O3 has a face-centered cublic (fcc) structure with an average oxygen vacancy rate of 25%, is recognized as one of the best solid state oxygen ionic conductors, and Nb2O5 can be added by 7.6–25.0 mol% to stabilize the cubic phase down to room temperature [31]. Meng et al. [32] reported that the conductivity of Bi8Nb2O17 was 1.9 × 10−1 S cm−1 at 700 °C, which was approximately 1–2 orders of magnitude higher than that of YSZ [33]. In addition, the thermal expansion coefficient of δ-Bi2O3 decreases with increasing Nb2O5 content [34]. Thus, Nb2O5 containing could be tailored to match the thermal expansivity of NASICON electrolyte. The applicability of Bi8Nb2O17 plus Pt as solid-reference electrode materials for the NASICON-based potentiometric CO2 sensor was examined in this work.

Section snippets

Preparation of materials

NASICON solid electrolyte was prepared by a sol–gel method using Na3PO4·12H2O, Si(OC2H5)4 and ZrO(NO2)·2H2O as main reagents, as described in our previous work [35]. The as-prepared NASICON precursor was compacted into a disk (8 mm in diameter and 2 mm in thickness) in a stainless steel die at a pressure of 50 kN. The disk was then sintered at different temperature in air for 2 h to obtain crystalline NASICON. And Bi8Nb2O17 was prepared through a conventional solid-state reaction using a mixture of

Characterization of NASICON and Bi8Nb2O17

The XRD patterns of NASICON and Bi8Nb2O17 powders are presented in Fig. 2, Fig. 3, respectively. As shown in Fig. 2, a broad single peak (1 1 6) ascribable to NASICON phase could be clearly observed at diffracting angle of 30–31° for the sample sintered at 600 °C, indicating the initial formation of crystalline NASICON. After being sintered at 900 °C, all the characteristic peaks are in good agreement with the literature data [36] and without any second phase. Whereas ZrO2 exists as impurity when

Conclusions

In this work, a composite of Bi8Nb2O17 and Pt as a solid-reference electrode material has been prepared for NASICON-based CO2 sensors. The EMF of the as-fabricated sensor is linear to the logarithm of CO2 concentration, obeying Nernst's correction. Moreover, no any obvious interference from oxygen partial pressure is observed. In comparison, there is no the deserved detectability above 60% CO2 for the sensor using Pt as reference electrode, suggesting that the solid-reference electrode can

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China (No. 50974012) and Program Changjiang Scholars and Innovative Research Team in University (No. 0708).

Heng-Yao Dang received the B.S. degree in applied physics from the Inner Mongolia University of Science and Technology, Baitou, China, in 2007. He has been pursuing the Ph.D. degree in physical chemistry of metallurgy with the University of Science and Technology Beijing, China, since 2007. His current research interests include solid electrolyte base CO2 sensors.

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    Xing-Min Guo received the Ph.D. degree from Northeastern University, Shenyang, China, in 1992. He is a professor with the University of Science and Technology Beijing, Beijing, China. His current research interests include iron-minerals utilization, electrochemical metallurgy, and electrochemical gas sensors.

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