Impedancemetric-type NO₂ sensor based on non-stoichiometric perovskite type sensing electrode using multiple response signals

Highlights

• Non-stoichiometric perovskite sensing electrode is used for impedancemetric NO₂ sensor.

• Multiple response signals (Z′, Z″, |Z| and Θ) are developed for the NO₂ sensor.

• Non-stoichiometry of sensing electrode effectively improves the sensitivity of sensor.

• Sensor using Z″ or Θ response can operate at higher frequency with better selectivity.

Abstract

Perovskite-type oxides have received considerable attention as sensing electrode (SE) for solid electrolyte type sensors because of their exceptional sensing performance and compositional flexibility. Herein, non-stoichiometric perovskite oxides (La_0.75Sr_0.25)_xCrO₃-δ (x = 0.95, 1.0, 1.05) with nano-structure were prepared in porous YSZ backbone by impregnation method as SE for impedancemetric-type NO₂ sensor. Because of the non-stoichiometry of SE, the NO₂ adsorption capacity and electrochemical activity of SE are effectively enhanced. Consequently, the sensors based on non-stoichiometric SEs exhibit noticeably improved sensitivity to NO₂. Meanwhile, instead of module value (|Z|) as response signal in traditional impedancemetric-type NO₂ sensor, various parameters including resistive component (Z′), capacitive reactance (Z″), |Z| and phase angle (Θ) extracted from impedance spectra were explored as the response signals for the present sensor. The sensors using all the extracted signals show excellent sensing performances in NO₂ concentration range of 100−700 ppm. Compared with sensor using Z′ or |Z| response, the sensor using Z″ and Θ response can operate at higher frequencies and shows much better selectivity against CH₄, CO₂ and H₂. Using Z″ as the response signal, the sensor exhibits a higher sensitivity to NO₂ than that with other parameter as signal, because the response ΔZ′′ values vary linearly with respect to NO₂ concentrations instead of logarithmic relation.

Introduction

As harmful atmospheric pollutants, nitrogen oxides (NO_x) including NO and NO₂ predominantly originate from road transport vehicles and combustion plants [[1], [2], [3]]. In order to reduce NO_x emissions, strict policies and regulations are introduced. Hence, continuously monitoring of NO_x becomes especially important [4,5]. Solid-state electrochemical sensors are suitable for such utilization because of the good sensitivity, selectivity and excellent stability for in-situ operation in harsh, high-temperature automotive exhaust environments [6,7].

The sensing electrode (SE) is responsible for absorbing NO₂ and providing the gas / SE / electrolyte triple-phase boundaries (TPBs), where electrochemical reduction of NO₂ occurs. Therefore, the properties of the SE are crucial to the sensing performances [6,8]. Among the various sensing materials, perovskite oxides (ABO₃) are employed frequently for NO₂ sensors due to the appreciable sensing properties. Meanwhile, the structural stability of the perovskite oxide SE is expected due to the low temperature of phase formation [9]. Another advantage for perovskite oxides is the tunable properties originated from the exceptional structural and compositional flexibility [2,10]. Except for the ion doping in A or B site [11,12], the non-stoichiometric control of A site is an alternative approach for tuning the properties of the perovskite oxides [10,[13], [14], [15], [16], [17], [18], [19]]. On the one hand, the excess of A site can provide more cation and oxygen vacancies, leading to the improvement of the electrical property [20]. On the other hand, the deficiency of A site can increase the valence of B site, resulting in the decrease of the polarization resistance and the improvement of the electrochemical activity [21]. Therefore, non-stoichiometric regulation could enhance the sensing performance of the perovskite oxide SE. To the best of our knowledge, the preparation and investigation of non-stoichiometric perovskite oxide SE for NO₂ sensor have not been reported.

The choice of appropriate operating mode is also essential for the high performance of the sensor. Impedancemetric-type is one of the used operating modes for the solid electrolyte type NO₂ sensors [4,[22], [23], [24]]. The module value (|Z|) at low frequency regime reflecting the electrochemical processes at the TPBs is frequently used as the response signal. However, the low frequency (usually ≤ 1 Hz) means the low sampling rate of the sensor and long response time. By comparison, another response signal, phase angle (Θ), can be used at higher frequency, which provides higher sampling rate and more stable response [7,24,25]. While there are some other parameters extracted from the impedance spectrum, such as resistive component (Z′), capacitive reactance (Z′′) and a series of fitted parameters. Therefore, it is interesting to explore more potential parameters used as the response signal for the NO₂ sensor.

Herein, we prepared (La_0.75Sr_0.25)_xCrO_3-δ (x = 0.95,1.0,1.05) (denoted as (LS)_0.95C, (LS)_1.0C, (LS)_1.05C) SE by the impregnation method in the porous YSZ electrolyte skeleton for impedancemetric-type NO₂. The non-stoichiometric (LS)_xC-SE with nanostructure shows improved sensing performance to NO₂, which derives from the enhancement of electrochemical activity at TPBs. To further improve the sensing performance, we explore new response signals (Θ, Z′ and Z′′). In conjunction with Θ or Z′′ response, the sensor using non-stoichiometric (LS)_xC-SE exhibits superior sensing performances to NO_2.