Effect of sintering temperature on NO decomposition by solid electrolyte cells with LSM-SDC composite cathodes

Accepted Manuscript Effect of sintering temperature on NO decomposition by solid electrolyte cells with LSM-SDC composite cathodes Wenjie Li, Zhenguo Xu, Han Yu, Zhuang Zhang, Ouwen He, Hongbing Yu PII:

S0925-8388(18)34159-8

DOI:

https://doi.org/10.1016/j.jallcom.2018.11.042

Reference:

JALCOM 48268

To appear in:

Journal of Alloys and Compounds

Received Date: 15 July 2018 Revised Date:

29 October 2018

Accepted Date: 3 November 2018

Please cite this article as: W. Li, Z. Xu, H. Yu, Z. Zhang, O. He, H. Yu, Effect of sintering temperature on NO decomposition by solid electrolyte cells with LSM-SDC composite cathodes, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.11.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effect of Sintering Temperature on NO Decomposition by Solid

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Electrolyte Cells with LSM-SDC Composite Cathodes

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Wenjie Li, Zhenguo Xu, Han Yu, Zhuang Zhang, Ouwen He, Hongbing Yu*

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College of Environmental Science and Engineering, Nankai University, No. 38 Tongyan Road,

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Jinnan District, Tianjin 300350, PR China

Corresponding author: Hongbing Yu

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Cell: +86-13920683244

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E-mail: [email protected]

12 First author: Wenjie Li

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E-mail: [email protected]

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Abstract

A series of solid electrolyte cells with La0.8Sr0.2MnO3 (LSM)-Ce0.8Sm0.2O1.9 (SDC) composite

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cathodes was fabricated for the electrochemical decomposition of nitric oxide (NO). The LSM and

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SDC powders were synthesized by a combined EDTA-citrate method. Thermogravimetry with

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differential scanning calorimetry, X-ray diffraction analysis, scanning and transmission electron

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microscope with electron diffraction were performed to characterize the synthesized powders. The

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NO conversions and power consumptions of the solid electrolyte cells with the cathodes sintered

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at 900-1200 °C were evaluated. The electrochemical properties, microstructures, and crystalline

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phases of the cathodes were further studied by electrochemical impedance spectrum, scanning

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electron microscope and X-ray diffraction, respectively. The NO conversions and electrochemical

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performances of the cells at different operating temperature were also investigated. It was

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concluded that the LSM and SDC powders calcined at 900 °C both showed good crystal structures

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with high purity. The cell with the cathode sintered at 1100 °C had the highest NO conversion of

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65.4% and the lowest power consumption of 0.2024 W under 80 mA applied current. The total

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polarization resistances of the cells with the cathodes sintered at 900 to 1200 °C were 63.32, 41.75,

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38.11 and 97.57 Ω cm2 in 800 ppm NO, respectively. The cathode sintered at 900 °C had an

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incompact structure and connected with the electrolyte loosely, thereby impeding the NO

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adsorption on the cathode surface and the transfer of O2- from three phase boundary (TPB) to

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electrolyte. The excess sintering temperature of 1200 °C resulted in a dense structure of cathode

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and La2Zr2O7 formation at the interface between the cathode and electrolyte, thereby leading to a

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poor NO decomposition performance. This study also found that the NO conversion increased

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with the rise of operating temperature and 600 °C was not suitable for operation because of the

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electrode degradation caused by the overvoltage.

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Keywords: Composite cathode, Solid electrolyte cell, Sintering temperature, NO decomposition

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1 Introduction

Nitrogen oxides (NOx) are one of the major air pollutants which are harmful to human health

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and environment [1]. The combustion of fossil fuels and the exhaust of vehicles are the main

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source of NOx discharges [2]. Selective catalytic decomposition (SCR) is one of the most mature

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technologies for the removal of NOx and it has been widely used in industry [3]. However, this

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technology requires the additional reducing agents such as ammonia and urea, causing the high

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cost of operation. Besides, the additional reducing agents should be handled properly and carefully

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otherwise some problems such as ammonia slip can be introduced, resulting in secondary

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pollution [4, 5].

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In considering of the above problems of SCR, electrochemical NOx decomposition used by

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solid electrolyte cell has attracted more attention [6, 7]. Besides no additional reducing agent, this

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technology also has some advantages such as all solid structure with no electrolyte leak, simplicity

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of operation and fast reaction rate. The schematic of electrochemical NOx decomposition is shown

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in Fig. 1(a) [8, 9]. The external voltage is applied between the cathode and anode to result in the

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polarization of solid electrolyte, thereby creating many oxygen vacancies. The NO is reduced into

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N2 at the three-phase boundary (TPB) in cathode side and O2- is generated simultaneously. The

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generated O2- is transported through electrolyte to anode and then loses electrons to form O2.

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Electrochemical NOx decomposition was firstly demonstrated by Pancharatnam et al. using Pt as

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electrode and yttrium stabilized zirconia (YSZ) as electrolyte in 1975 [10]. Since then, the

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electrochemical decomposition of NOx using the noble metal electrodes has been studied

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extensively [11, 12]. Bredikhin et al. proposed the concept of multilayer functional electrode to

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promote the NO decomposition performance of solid electrolyte cells [9, 13, 14]. Unfortunately,

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noble metal was still not avoided in this kind of structure, thereby preventing the practical

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application because of the expensive cost.

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Strontium doped lanthanum manganate (LSM) is one of the most popular materials for the

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cathodes of solid oxide fuel cells (SOFCs) because it has many advantages such as low cost, good

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chemical stability, high catalytic activity and high electrical conductivity [15, 16]. Additionally, it

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has a thermal expansion coefficient similar to that of solid electrolyte YSZ [17, 18]. However, 3

ACCEPTED MANUSCRIPT LSM shows low ionic conductivity and high activation energy when the operating temperature is

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below 800 °C, limiting its application at intermediate temperature [19]. Samarium doped ceria

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(SDC) is an electrolyte material for intermediate temperate SOFCs which has high ionic

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conductivity because of the large amount of oxygen vacancies [20, 21]. It has been reported that

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the composite LSM-SDC cathode can extend the electrode-electrolyte interface, thereby

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promoting the electrochemical performance of cathode [22]. Therefore, the solid electrolyte cell

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with LSM-SDC composite cathode is promising to be applied in electrochemical NOx

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decomposition. The impedance spectroscopy of (La1-xSrx)MnO3 and doped ceria composite

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electrodes in NOx containing atmosphere were investigated by Werchmeister et al. in 2010 [23]. In

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recent years, Shao et al. has reported the NOx decomposition properties of LSM-CGO (gadolinia

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doped ceria) composite cathodes modified with NOx absorbents [8, 24]. However, there is limited

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publication about the electrochemical NO decomposition using the solid electrolyte cells with the

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structure of LSM-SDC/YSZ/Ag-SDC.

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Sintering temperature is an essential factor which can influence the microstructure and phase

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composition of solid electrolyte cells. Though high sintering temperature can improve the

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connection strength between electrode and electrolyte [25], the dense structure with low specific

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surface area may also be caused [26]. This variation on microstructure may result in the different

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performance of solid electrolyte cells. Therefore, a suitable sintering temperature is required for

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the optimum NO decomposition performance of solid electrolyte cells. However, to our best

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knowledge, the effect of sintering temperature on NO decomposition by solid electrolyte cells

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with LSM-SDC composite cathodes has not been reported.

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The aim of this study is to investigate the effect of sintering temperature on electrochemical

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NO decomposition performance by solid electrolyte cells with LSM-SDC composite cathodes. In

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this study, both SDC and LSM powders were synthesized in laboratory and characterized

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systematically firstly. Then the NO conversions and power consumptions of the solid electrolyte

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cells with the cathodes sintered at different temperature were investigated. To interpret the

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different NO decomposition performances of the solid electrolyte cells, the electrochemical

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properties, microstructures and crystalline phases of the cathodes were further analyzed. Finally,

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the NO decomposition performances of the solid electrolyte cells at different operating 4

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2 Experimental

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2.1 Powders synthesis

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La0.8Sr0.2MnO3 powders were synthesized by a combined EDTA-citrate method. In brief, the

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stoichiometric amount of La(NO3)3·6H2O, Sr(NO3)2 and Mn(NO3)2 were introduced into a breaker

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and then dissolved by distilled water. Then a certain amount of EDTA and citrate were added into

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the nitrate solution and the molar ratio of total metal ions: EDTA: citrate was controlled to be

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1:1:1.5. The solution was stirred and heated continuously at 80 °C until a transparent and viscous

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gel was formed. During the process above, NH3·H2O was added carefully to control the pH value

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of solution around 7. The gel was moved into a drying oven at 140 °C to evaporate the residual

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water and then the solid precursors were formed. The obtained precursors were calcined at 700 to

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1000 °C for 4h. Then the LSM powders were obtained, denoted as LSM700, LSM800, LSM900

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and LSM1000, respectively. The Ce0.8Sm0.2O1.9 (SDC) powders were also synthesized by the

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combined EDTA-citrate method and the process was similar to that of La0.8Sr0.2MnO3 described

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above. The SDC powders calcined at 700 to 1000 °C were denoted as SDC700, SDC800, SDC900

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and SDC1000, respectively.

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2.2 Cells fabrication and reaction unit arrangement

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The commercial yttrium stabilized zirconia (YSZ) powders were pressed into a pellet with

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the diameter of 13 mm and then sintered at 1400 °C for 4 h to obtain the dense electrolyte disks.

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The equal amount of synthesized LSM and SDC powders were mixed with terpineol and then

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milled for 1 h to get the cathode slurry. The slurry was screen printed onto the electrolyte disk with

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the area about 0.5 cm2 and then sintered at 900 to 1200 °C for 3h. The cells with the cathodes

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sintered at different temperatures were named as LSM-SDC900, LSM-SDC1000, LSM-SDC1100

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and LSM-SDC1200, respectively. All the anodes in this study were fabricated by a consistent

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method and same material. The anode slurry was prepared by the mixture of SDC powders and 5

ACCEPTED MANUSCRIPT silver paste with the mass ratio around 7:3. Then the anode slurry was screen printed onto the

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opposite electrolyte side and sintered at 850°C for 3h. The SEM image of the cross section of the

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fabricated solid electrolyte cell is shown in Fig. 1(b). It can be observed from the picture that the

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fabricated solid electrolyte cell has the consistent structure with that depicted by Fig. 1(a). The

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cathode and anode were porous and a dense electrolyte was sandwiched between them. The

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thickness of the electrolyte was about 400 µm, and the thicknesses of cathode and anode were both

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about 40 µm.

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Fig. 2 shows the schematic illustration of the reaction unit. In brief, a solid electrolyte cell

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with two silver wires was set into an alundum tube equipped with furnace. The current through the

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cells was applied by a programmable power supply. A temperature control cabinet with a

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connected thermocouple was used to detect and control the temperature in furnace. The NO gas

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was fed into one end of the alundum tube with Ar as the balance gas and the total flow rate was

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controlled to 40 ml/min by gas controller. The outlet gas flowed out at the other end of the

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alundum tube and the NO concentration was measured by a flue gas analyzer.

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2.3 Characterization and measurement

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Thermogravimetry with differential scanning calorimetry analyses (TG/DSC) of the wet gels

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were performed by a TG/DSC analyzer (Mettler Toledo TGA/DSC1) in air from 25 to 1100 °C

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with the heating rate of 10 °C/min. The crystalline phase compositions of the powders synthesized

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and the cathodes sintered at different temperatures were determined by X-ray diffraction (XRD,

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Rigaku Ulitma IV). The micromorphology properties of the synthesized powders were observed

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by a transmission electron microscope (TEM, FEI GF20) with electronic diffraction and scanning

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electron microscope (SEM, S4800). The microstructures of the cells were investigated by a

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scanning electron microscope (SEM, Hitachi S-3500N). The electrochemical impedance spectra of

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the cells were examined by an electrochemical work station (Ametek VersaSTAT3) with the

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amplitude of 10 mV in the frequency from 0.1 to 105 Hz. The inlet and outlet concentrations of

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NO were measured by a flue gas analyzer (Seitron Chemist 600). The NO conversion (∆NO) was

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calculated as:

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∆NO = ([NO]in − [NO]out )/[NO]in

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Where [NO]in is the NO concentration of inlet gas; and [NO]out is the NO concentration of outlet

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gas. The power consumption (P) of the solid electrolyte cell was calculated as:

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P = UI

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Where U is the voltage applied between the cathode and anode of the cell; and I is the current

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flowing through the cell.

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3 Results and discussion

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3.1 Powders characterization

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Fig. 3(a) and (b) shows the TG/DSC curves for the LSM and SDC wet gels, respectively. The

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TG curves of two samples had the similar tends despite some slight differences. The weights of

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LSM and SDC samples decreased drastically from 25 °C to 430 °C and 400 °C, respectively, both

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losing about 90% of their totals. When the temperature rose above 450 °C, the TG curves of both

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samples remained stable with no decline, indicating that the decomposition processes of the two

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gels are finished. The weight losses of the two gels can be further interpreted according to their

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DSC curves. The DSC curves of the two gels also had the similar tends because the both of them

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were synthesized by the same EDTA-citrate method. There were two small endothermic peaks at

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120 and 200 °C in the DSC curves of both samples as a consequence of the removal of the

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physisorbed moisture and hydrated water, respectively [27, 28]. A small exothermic peak appeared

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at 270 °C on both DSC curves, which may be related to the decomposition/oxidation of the metal

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chelates [29, 30]. A strong exothermic peak appeared around 380 °C in both DSC curves, which

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were probably associated to the reaction/ pyrolysis of the metal chelates along with the forming of

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metal oxides [28, 29]. However, the peak of LSM around 380 °C had a larger width and was not

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as sharp as that of SDC. The reason for this phenomenon may be that the contents of EDTA and

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citrate in LSM gel were more than that of SDC gel and the initial weights of the two gels were

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different.

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The XRD patterns of the LSM and SDC powders calcined at different temperatures are

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displayed in Fig. 4 and Fig. 5, respectively. The crystal structures of the two types of powders

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were basically formed at 700 °C and the diffraction peaks of them became sharper with the 7

ACCEPTED MANUSCRIPT increasing calcination temperature. There were two slight differences in the XRD patterns of the

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LSM powders calcined at different temperatures, which can be identified from Fig. 4(b). The

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small peak at 58.5 ° appeared evidently only when the calcination temperature was higher than

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900 °C. Moreover, when the calcination temperature increased to 900 °C, the peaks at 67.7 ° and

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68.4 ° formed evidently and separated from each other clearly. The reason for these phenomena is

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that the crystallinity of LSM increased with the increasing calcination temperature. By comparing

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the XRD patterns of samples with PDF, it is also identified that the crystal structure of LSM was

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more standard when the calcination temperature increased to 900 °C. Therefore, considering the

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more standard structure and lower energy consumption, 900 °C was thought as the appropriate

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calcination temperature for LSM powders. LSM900 and SDC900 were chosen as the powders for

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the following research in this work. It is emphasized that the diffraction peaks of LSM900 and

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SDC900 were highly consistent with the La0.8Sr0.2MnO3 (PDF#53-0058) and Sm0.2Ce0.8O1.9

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(PDF#75-0158), respectively, indicating the powders were synthesized successfully. The

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schematic crystal structures of LSM and SDC are shown in Fig. 4(c) and Fig. 5(b), belonging to

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the space groups of R-3c (167) and Fm-3m (225), respectively.

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Fig. 6 shows the images of TEM, high resolution TEM (HRTEM) and electron diffraction

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patterns of the LSM and SDC powders sintered at 900 °C. As can be seen from Fig. 6(a) and (d),

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there were agglomerations in both powders but LSM900 agglomerated more severely than

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SDC900. The agglomeration of powders could impede the densification and result in an

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inhomogeneous microstructure during the sintering process [31, 32]. Therefore, the process

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parameters of powders preparation should be improved to decrease the agglomeration of powders

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in further study. The HRTEM images of LSM900 and SDC900 are shown in Fig. 6(b) and (e),

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respectively. As is shown in the pictures, the obvious lattice fringes were observed in the two

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samples. Lattice fringes are the black and white lines in HRTEM images which can reflect the

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crystal characteristics. One of the most important purposes of the lattice fringes is to reflect the

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distance of crystal plane by fringe spacing [33, 34]. As is shown in Fig. 6(b) and (e), the fringe

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spacings were about 0.277 and 0.272 nm, which were compatible with the distance between (110)

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plane of LSM and the distance between (200) plane of SDC, respectively. These results were also

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consistent with the results of XRD. The electron diffraction patterns of LSM900 and SDC900 are

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ACCEPTED MANUSCRIPT shown in Fig. 6(c) and (f), respectively. The electron diffraction patterns of both samples were

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arranged orderly and showed the single crystal structures, in accord with the rhombohedral

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structure of LSM and cubic structure of SDC, respectively. The typical reflections shown in Fig.

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6(c) are (110), (122) and (012) of LSM, and the typical reflections shown in Fig. 6(f) are (200),

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(420) and (220) of SDC. These results are consistent with the results shown in HRTEM images.

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Moreover, the zone axes of the two SAED patterns were also identified, corresponding to [221] of

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LSM and [004] of SDC, respectively. Fig. 6(g) and (h) show the SEM images of LSM900 and

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SDC900 powders, respectively. It is evident that the particle size of LSM900 was larger than that

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of SDC900. The size distributions of the two powders are shown in Fig. 6(i) and (j), which were

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obtained by using the image process software of ImageJ [35-38] according to the SEM images. It

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can be calculated that the average sizes of LSM900 and SDC900 were 150.54 and 49.68 nm,

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respectively. The grain sizes of the LSM900 and SDC900 can be calculated from XRD by

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Scherrer’s formula [5, 39], corresponding to 31.66 and 18.78 nm respectively. By comparing the

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grain sizes with the average particle sizes, it can be concluded that the LSM powders

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agglomerated more severely than SDC powders, consistent with the results of TEM images.

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3.2 NO decomposition performance

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Fig. 7 shows the NO conversions of the solid electrolyte cells with the cathodes sintered at

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different temperatures measured at the operation temperatures of 700 °C. The NO conversions of

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the four single cells firstly increased with the applied current and then remained relatively stable

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when the current exceeded the certain thresholds. However, the maximum NO conversions and

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current thresholds of four cells were different. The NO conversion of LSM-SDC1100 at 80 mA

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was highest with the value of 65.4%, followed by LSM-SDC1000 (60.0%), LSM-SDC900 (57.9%)

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and LSM-SDC1200 (53.9%). The NO conversions of LSM-SDC900 and LSM-SDC1000

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remained stable when the current was above the threshold of 30 mA. However, for

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LSM-SDC1100 and LSM-SDC1200, the current threshold was higher, which was around 40 mA.

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Fig. 8 displays the power consumptions of the four solid electrolyte cells. As is shown in the

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figure, the power consumptions of all the cells increased with the applied current almost linearly.

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This linear trend is not consistent with the growth trend expressed by the classical formula as 9

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P = I2R

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Where P is the power consumption; I is the current; and R is the total resistance. This phenomenon

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indicates that the total resistance of each cell decreased with the increasing applied current. This

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decreasing trend was also verified by the voltage data. The reason for this phenomenon may be

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that the Mn3+ in LSM was reduced to Mn2+ and more vacancies were created [40], which can be

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expressed as:

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x Oox (LSM) + 2MnMn + VO.. (electrolyte) + 2e− → VO.. (LSM) + 2Mn,Mn + O0x (electrolyte)

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In general, the power consumption of LSM-SDC1100 was minimal among the four samples with

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the value of 0.2024 W at 80 mA. Besides, it is reiterated that the NO conversion of LSM1100 was

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best. On the contrary, it is identified that the power consumption of LSM-SDC1200 was highest in

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general.

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As discussed above, it can be concluded that the sintering temperature of cathodes can

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influence the NO decomposition performance of the solid electrolyte cells visibly. Considering the

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both results of NO conversion and power consumption, the cells with the cathodes sintered at

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1100 °C had the best NO decomposition performance and the NO decomposition performance of

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the cells sintered at 1200 °C was worst in this work. The reason for these phenomena will be

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discussed following in detail.

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3.3 Electrochemical property

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Fig. 9(a) and (b) gives the electrochemical impedance spectra of the solid electrolyte cells

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with the cathodes sintered at different temperature measured at 700 °C in 800 ppm NO and air,

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respectively. As is shown in the figure, each curve shows the shape of an arc despite a few

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disturbances. The intercept on the real axis at high frequencies represents the ohmic resistance (R0)

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of the solid electrolyte cell, including the wire resistance, electrode ohm resistance and electrolyte

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resistance. The difference between the intercepts at high frequencies and low frequencies on the

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real axis represents the total polarization resistance (Rp) of the solid electrolyte cell [15]. Because

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the anodes of all the solid electrolyte cells were same, the Rp values were only influenced by the

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cathodes in this discuss. In the atmosphere of NO, it is identified that R0 values of the different

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LSM-SDC900 < LSM-SDC1200. This phenomenon was consistent with the results of NO

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conversion and power consumption discussed above. As is shown in Fig. 9(b), it is identified that

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the order of Rp values in air was same with that in NO, but the difference of the R0 values of the

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four single cells could be distinguished. The order of R0 values of the four single cells is

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LSM-SDC1000 < LSM-SDC1100 < LSM-SDC900 < LSM-SDC1200 and the R0 value of

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LSM-SDC1100 was only slightly higher than that of LSM-SDC1000. Additionally, by comparing

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the Fig. 9(a) with (b), it is identified that the Rp value of the same cell in 800 ppm NO was almost

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one order of magnitude greater than that in air. The reason for this phenomenon may be that the

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NO decomposition to O2- is more complex than the O2 decomposition to O2- on the cathodes at

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700°C, requiring a higher activation energy [39].

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To discuss the reaction process of the NO decomposition in solid electrolyte cells, the

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equivalent circuit shown in Fig. 10 was used to evaluate the results of electrochemical impedance

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spectra in more detail. In the equivalent circuit, L is the inductance; R0 is the ohmic resistance; R1

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and R2 are the resistances for the high frequency and low frequency, respectively; Q1 and Q2 are

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the constant phase elements for the high frequency and low frequency, respectively. The total

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polarization resistance Rp is the sum of R1 and R2. It is generally accepted that R1 is related to the

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oxygen ion transfer from TPB to the electrolyte, and R2 is related to the adsorption and diffusion

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of gas at cathode and the surface diffusion of the O2- [41, 42].

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Fig. 11 shows the values of ohmic resistances R0 and polarization resistances Rp (including

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R1 at high frequency and R2 at low frequency) of the solid electrolyte cells at 700 °C in 800 ppm

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NO. The Rp values of the cells with the cathodes sintered at 900 to 1200 °C were 63.32, 41.75,

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38.11 and 97.57 Ω cm2, respectively. This result was consistent with the result indicated by the

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intercepts in Figure. 9(a). R1 values decreased with the increasing sintering temperature from 900

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to 1100 °C, but then showed an increasing tends when the sintering temperature was above

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1100°C. This phenomenon indicates that the O2- transfer from the TPB to the electrolyte had the

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minimum resistance when the cathode was sintered at 1100 °C. R2 values decreased when the

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sintering temperature rose from 900 °C to 1000 °C, but then increased with the rise of sintering

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temperature. There was a sharp increase of R2 values when the sintering increased from 1100 °C

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ACCEPTED MANUSCRIPT to 1200 °C. This phenomenon indicated that the adsorption and diffusion of NO at cathode or the

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surface diffusion of the O2- were impeded severely when the sintering temperature increased to

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1200 °C. It is also identified that values of R2 was significantly greater than R1, indicating that the

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adsorption and diffusion of NO at cathode or the surface diffusion of the O2- were dominated

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during the process of NO decomposition.

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3.4 Microstructure

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To further investigate the reason for the different polarization resistances of the cells with the

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cathodes sintered at different temperatures, the SEM images of the cells were captured and shown

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in Fig. 12. It is evident that there were many big holes on the cathodes of LSM-SDC900 and the

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arrangement of the particles was very incompact. The relatively low sintering temperature can

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result in the poor connectivity between the cathode particles, thus impeding the diffusion of O2- at

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cathode [43]. This point is consistent with the relatively high R2 value of LSM-SDC900 shown in

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Fig. 11. With the rise of sintering temperature, the number of big holes decreased and the size of

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particles increased, resulting in the smaller surface area. LSM-SDC1100 and LSM-SDC1200 with

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the lower surface area may require the higher value of applied current to reach the maximum NO

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conversion, thus leading to the smaller trend-curve slope in Fig. 7. The cathode sintered at 1100 °

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C had a relatively uniform pore distribution which was favorable the adsorption and diffusion of

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NO and O2-. However, when the sintering temperature rose to 1200 °C, the cathode surface

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become too dense and the porosity decreased sharply. This dense structure impeded the process of

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NO diffusion, which was also consistent with the very high R2 value of LSM-SDC1200 shown in

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Fig. 11. The SEM images of the cross sections of the cathodes sintered at 900 to 1200°C are

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shown in Fig. 12(c), (f), (i) and (l), respectively. As is shown in Fig. 12(c), LSM-SDC900 had a

23

loose connection between the cathode and electrolyte because of the cracks or big holes at the

24

interface. It is inferred that this structure was not favorable for the transfer of O2- from TPB to

25

electrolyte, thereby resulting in the highest R1 value of LSM-SDC900. With the increase of

26

sintering temperature, the cathode connected with the electrolyte more tightly thus the R1 values

27

decreased. However, when the sintering temperature rose to 1200 °C, a new substance with a

28

dense structure seemed to be formed at the interface between the electrolyte and cathode, which

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impede the transfer of O2- from cathode to electrolyte, therefore the R1 values rose again when the

3

sintering temperature rose from 1100 to 1200 °C.

4

3.5 Crystalline phase

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To investigate whether there was a chemical reaction between the cathode and electrolyte in

6

the process of sintering, the XRD patterns of the cathode sides of the solid electrolyte cells

7

sintered at different temperatures were obtained. As is shown in the Fig. 13, besides the peaks of

8

LSM and SDC, the peak of YSZ electrolyte also appeared in the picture clearly. Each component

9

maintained its structure independently and no impurity peak appeared when the sintering

10

temperatures was below 1100 °C, indicating no obvious chemical reaction between each

11

component. However, when the sintering temperature rose to 1200 °C, the weak peaks around 30,

12

35 and 50 ° appeared, indicating a new crystalline phase was formed. This phenomenon was

13

consistent with the result in Figure.12(l). It was reported that the LSM could react with YSZ to

14

form La2Zr2O7 (LaZrO), which was responsible for the degradation of the cell performances of

15

SOFCs [15, 44]. The LaZrO can result in the performance degradation of the cells mainly because

16

its conductivity is significantly lower than that of LSM and YSZ at the operating temperature of

17

cells [45]. Besides, it also has been reported that LaZrO can impede the transfer of O2- at active

18

reaction site (TPB) [46]. These points are consistent with our result shown in Fig.11 that

19

LSM-SDC1200 had the higher values of R0 and R1 than LSM-SDC1100.

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3.6 Effect of operating temperature

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Fig.14(a) gives the NO conversions of LSM-SDC1100 at the operating temperature from 600

22

to 750 °C. The NO conversion increased with the rise of operating temperature under the same

23

applied current. The NO conversion at the operating temperature of 750 °C was largest with 67.52 %

24

at 80 mA. It is also identified that the NO conversions at the operating temperature above 650 °C

25

all went up with the increasing applied current. However, when the operating temperature reduced

26

to 600 °C, the NO conversion decreased distinctly as the applied current increased from 40 to 80

27

mA. 13

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750 °C are shown in Fig. 14(b). It is evident that the ohm resistance and polarization resistance of

3

the cells both decreased with the rise of operating temperature, in accord with the NO conversion

4

of the solid electrolyte cells operating at different temperatures. The reason for this phenomenon is

5

that the electronic conductivity of LSM and the ionic conductivity of SDC and YSZ increased as

6

the operating temperature rose [47, 48]. The polarization resistance of the solid electrolyte cell

7

was largest at 600 °C, indicating the that the highest negative voltage should be applied on the

8

cathode to obtain the same current with others. However, the high applied voltage can cause the

9

degradation of cathode [49], therefore the NO conversion at 600 °C decreased distinctly with the increase of applied current from 40 to 80 mA.

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4 Conclusions

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Solid electrolyte cells with LSM-SDC composite cathodes were fabricated for

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electrochemical NO decomposition. The LSM and SDC powders were synthesized successfully

14

when the calcination temperature was above 900°C. The solid electrolyte cell with the cathode

15

sintered at 1100 °C had the highest NO conversion of 65.4% and the lowest power consumption of

16

0.2024 W when the applied current was 80 mA at 700 °C. The total polarization resistances of the

17

cells with the cathodes sintered at 900 to 1200 °C were 63.32, 41.75, 38.11 and 97.57 Ω cm2 in

18

800 ppm NO at 700 °C, respectively. The cathode sintered at 900 °C had an incompact structure

19

and connected with electrolyte loosely, impeding the NO adsorption and the transfer of O2- from

20

TPB to electrolyte. With the increasing sintering temperature, the pore distribution of the cathodes

21

became more uniform and the connection between the electrolyte and cathode became tighter.

22

However, the excess sintering temperature of 1200 °C resulted in a dense cathode structure and

23

the formation of LaZrO at the interface between the cathode and electrolyte, also degrading the

24

NO decomposition performance. As the operating temperature increased from 600 to 750 °C, the

25

NO conversion of the solid electrolyte cells increased in general. 600 °C was not suitable for the

26

operation of solid electrolyte cells because the high applied voltage required can cause the

27

degradation of cathodes.

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Acknowledgements

The authors gratefully acknowledge financial support by the National Key Research and Development Plans of China (2016YFC0209301).

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Fig. 1. (a) Schematic of electrochemical NOx decomposition by solid electrolyte cell, (b) SEM

3

image of the cross section of the solid electrolyte cell fabricated in laboratory

4

Fig. 2. Schematic illustration of the reaction unit.

5

Fig. 3. TG/DSC curves of the two synthesized wet gels. (a) LSM, (b) SDC.

6

Fig. 4. XRD patterns of the LSM powders calcined at 700 to 1000 °C for (a) 2θ from 10 to 90 °

7

and (b) 2θ from 57.5 to 70.0 °, (c) schematic crystal structure of LSM.

8

Fig. 5. (a) XRD patterns of the SDC powders calcined at 700 to 1000°C, (b) schematic crystal

9

structure of SDC.

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Fig. 6. (a) TEM image, (b) HRTEM image, (c) electron diffraction pattern, (g) SEM image and (i)

11

size distribution of LSM900, (d) TEM image, (e) HRTEM image and (f) electron diffraction

12

pattern of SDC900, (h) SEM image and (j) size distribution of SDC900.

13

Fig. 7. NO conversions of the solid electrolyte cells with the cathodes sintered at 900 to 1200 °C

14

measured at the operation temperature of 700 °C.

15

Fig. 8. Power consumptions of the solid electrolyte cells with the cathodes sintered at 900 to

16

1200 °C measured at the operation temperature of 700 °C.

17

Fig. 9. Electrochemical impedance spectra of the solid electrolyte cells in (a) 800 ppm NO, and (b)

18

air measured at 700 °C.

19

Fig. 10. Equivalent circuit used to evaluate the results of electrochemical impedance spectra.

20

Fig. 11. Ohmic resistances R0 and polarization resistances Rp (including R1 in frequency and R2 in

21

low frequency) of the solid electrolyte cells with the cathodes sintered at different temperatures in

22

800 ppm NO measured at 700 °C.

23

Fig. 12. SEM images of the surfaces and cross sections of the cathodes sintered at different

24

temperatures. (a)-(c) LSM-SDC900, (d)-(f) LSM-SDC1000, (g)-(i) LSM-SDC1100, (j)-(l)

25

LSM-SDC1200.

26

Fig. 13. XRD patterns of the cathode sides of the cells sintered at 900 to 1200 °C.

27

Fig. 14. (a) NO conversions and (b) Electrochemical impedance spectra of the solid electrolyte

28

cells at the operating temperature from 600 to 750 °C.

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Highlight

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A series of solid electrolyte cells with LSM-SDC composite cathodes is successfully

•

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fabricated for NO decomposition. Cathode sintered at 900 °C has an incompact structure and a loose connection with electrolyte.

Cathode sintered at 1100 °C has the lowest polarization resistance and the best NO

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Excess sintering temperature results in a poor NO decomposition performance due to dense

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