Intermediate-Temperature NOx Sensor Based on an In[sup 3+]-Doped SnP[sub 2]O[sub 7] Proton Conductor

Abstract

A potentiometric NOx sensor using a proton-conducting Sn 0.9 In 0.1 P 2 O 7 electrolyte with a PtRh/C working electrode was fabricated to study sensing properties for NO and NO 2 at intermediate temperatures. The sensor showed electromotive force ͑emf͒ responses to changes in NO and NO 2 concentrations. Interestingly, the emf values for both NO and NO 2 increased in the positive direction with increasing gas concentration. The sensing mechanism was shown to be based on a mixed potential at the working electrode through measurements of the polarization curves of NO or NO 2 and water vapor.There has been considerable recent demand for the direct detection of NOx in combustion exhausts from automobiles to establish control systems for the reduction of the NOx emissions. At present, conventional NOx sensors signal a limiting current associated with electrochemical NO decomposition, 1,2 which requires using an expensive two-chamber cell structure connected in series. In addition, the limiting current of a few microamperes or lower is not sufficiently high to assure a high S/N ratio. Thus, the automobile industry needs alternate inexpensive and innovative NOx sensors. Potentiometric gas sensors inherently have low manufacturing costs because of their simple cell structure. Moreover, the voltage signal can be more easily amplified when compared to the current signal. Mixed-potential-type gas sensors are promising sensing devices for monitoring low concentrations of pollutants, such as H 2 , 3-5 CO, 6,7 H 2 S, 8 and hydrocarbons, 9-14 in exhausts. Miura et al. were the first to report this type of NOx sensor using yttria-stabilized zirconia ͑YSZ͒ as the electrolyte. 15 According to the sensing mechanism proposed by them, 16,17 the mixed potential is obtained by coupling the following electrochemical reactions at the working electrodeAs a result, the potential of the working electrode is shifted toward the positive direction with increasing NO 2 concentration and toward the negative direction with increasing NO concentration. Indeed, some research groups have reported opposite signals for the potential based on NO 2 and NO. [15][16][17][18][19] However, it should be noted that such signals complicate the interpretation of a mixed potential for NO/NO 2 mixtures in actual applications.We recently reported that an anhydrous proton conductor, 10 mol % In 3+ -doped SnP 2 O 7 ͑Sn 0.9 In 0.1 P 2 O 7 ͒, shows high proton conductivities Ͼ10 −1 S cm −1 between 150 and 350°C. 20 Attempts to apply this material as the electrolyte in some electrochemical devices were also described. A H 2 -air fuel cell using 0.35 mm thick Sn 0.9 In 0.1 P 2 O 7 electrolyte yielded a power density of 264 mW cm −2 at 250°C. 21 In addition, an electrochemical reactor using the Sn 0.9 In 0.1 P 2 O 7 electrolyte with a PtBa/C cathode reduced NOx to N 2 with a current efficiency of 5.81% at 250°C in oxidizing atmospheres. 22 This material also has the potential to be used for NOx sensor applications because the following electrochemical reactions can be exp...

Intermediate-temperature proton conductors and their applications to energy and environmental devices

“…14, wherein the concentrations of water vapor and O 2 were maintained at constant values of 2.7 and 4%, respectively. 58) As expected, the EMF value increased in the positive direction with increasing NO or NO 2 concentration, wherein an approximately linear relationship between the EMF value and the logarithm of NO or NO 2 concentration was obtained. These results suggest the possibility that the present sensor could be used to determine the total NO x concentration, although the low sensitivity to NO needs to be improved.…”

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