WANG Wen-Yun HOU Shi-Sheng HU Xiu-Li XIE Kui②

a (CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)b (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China)

Perovskite structure La0.75Sr0.25Cr0.5Mn0.5O3-δ(LSCM) cathode with unique structure can electrolyze CO2to CO in solid oxide electrolysers (SOEs).However, the cell performance is restricted by its electro-catalysis activity.In this work, fluorite structure nanoparticles (CeO2-δ) are impregnated on LSCM cathode to improve the electro-catalysis activity.X-ray diffraction (XRD), scanning electron microscope (SEM)and X-ray photoelectron spectroscopy (XPS) together approve that the fluorite structure nanoparticles are uniformly distributed on the perovskite structure LSCM scaffold.Electrochemical measurements illustrate that direct CO2electrolysis with 10%mol CeO2-δimpregnated LSCM cathode exhibits excellent performance for current density(0.5 A×cm-2) and current efficiency (～95%) at 800 under 1.6 V.It is believed that℃the enhanced performance of directed CO2electrolysis may be due to the synergetic effect of fluorite structure CeO2-δnanoparticles and perovskite structure LSCM ceramic electrode.

1 INTRODUCTION

Solid oxide electrolysers (SOEs) were efficient device with the electrochemical reversible ability,which are expected to play a vital role in the conversion of CO2to CO to relieve “Greenhouse effect”.As is known to all, many factors should be paid attention to when designing the SOEs cathode material, such as: high electronic/ionic conductivity,catalytic activity for CO2splitting, stable structure and so on[1].At present, the widely researched cathode material structures are metal-fluorite/fluorite structure (Ni-YSZ and CeO2-δ)[2,3]and perovskite structure (La0.75Sr0.25Cr0.5Mn0.5O3-δ, and SrTiO3-δ)[1,4,5].

The traditional metal-fluorite structure Ni-YSZ composite cathode with mixed conductivity could only be operated in a reducing atmosphere, such as flowing CO or H2to avoid the oxidation of metallic phase when it is used for the CO2electrolysis[6,7].Nickel is easy to agglomerate at high temperature,which reduces the long term performance[8].

Perovskite structure oxide, ABO3-δ, is an innovative inorganic material with unique physical and chemical properties.The A-site atoms are usually rare earth or alkaline earth element and the B-site atoms are the transition metal element.Both the A-and B-site atoms can be replaced by other metal elements with similar ionic radii, while the crystal structure remains basically unchanged.The structures and properties of crystal defects formed after doping can be used in solid oxide electrolysers(SOEs).The introduction of transition metal into B-site of LaCrO3-δis beneficial to the improvement of methane reforming catalytic activity[9].La0.75Sr0.25Cr0.5Mn0.5O3-δ(LSCM), typical perovskite structure materials[10], is a redox-stable efficient anode for CO2electrolysis, reported by John T.S.Irvine[11].John T.S.Irvine et al also developed an alternative cathode material (Pd–GDC co-impregnated LSCM) for CO2reduction at 900 ℃[12].LSCM with perovskite structure is prospective to replace metal-fluorite structure Ni-YSZ electrode material for CO2reduction in SOEs.However, the catalytic activity of LSCM is not as noble as that of Ni-YSZ.

In this work, active fluorite structure CeO2-δnanoparticles is impregnated on the LSCM scaffold to improve the performance of CO2electrolysis at 800℃.The results show that the synergetic effect of ceria and LSCM improves the electrode performance of current density (0.5 A×cm-2) and current efficiency(～95%) at 800 ℃ under 1.6 V.It is an effective strategy to impregnate oxide nanoparticles on the scaffold surface for CO2splitting in solid oxide electrolysers (SOEs).

2 EXPERIMENTAL

2.1 Syntheses of the materials

All the chemicals (99.9%, AR) were purchased from SINOPHARM Chemical Reagent Co.Ltd(China).La0.75Sr0.25Cr0.5Mn0.5O3-δ(LSCM) powders were synthesized by the glycine-nitrate combustion method in which the appropriate amounts of La2O3,SrCO3, Cr(NO3)3·9H2O and C4H6MnO4·4H2O were mixed evenly and sintered at 1200 ℃ (3 ℃×min-1)for 5 h in air[13].Ce0.8Sm0.2O2-δ(SDC) powders were synthesized though glycine-nitrate combustion method with appropriate amounts of Ce(NO3)3·6H2O and Sm2O3followed by 800 ℃ for 3 h in air[14].(La0.8Sr0.2)0.95MnO3-δ(LSM) powders were synthesized using the glycine combustion method with stoichiometric amounts of La2O3, SrCO3and C4H6MnO4·4H2O, followed by the treatment at 1200℃ for 5 h in air[15].The LSCM cathode loaded with 10 mol% CeO2-δwas prepared through the impregnation method.LSCM and Ce(NO3)3·6H2O were mixed in deionized water according to the ratio of stoichiometry.The aqueous solution was stirred,heated, and dried on magnetic stirrer followed by sintering at 500 ℃ for 3 h to obtain LSCM powders impregnated with CeO2-δ[16]. The La0.9Sr0.1Ga0.8Mg0.2O3-δ(LSGM) electrolyte was prepared by conventional solid state reaction method followed by the treatment at 1500 ℃ for 10 h[17,18].

2.2 Material characterization

X-ray diffraction (XRD) patterns were achieved on a desktop X-ray diffractometer equipped with Cu-Kα tube (Miniflex600, RIGAKU, 20°＜2q＜80°,10 °/min).Scanning electron microscope (SEM)images were collected on Hitachi SU-8010 at 5 KV to observe the microstructures of the powders and the single cell.X-ray photoelectron spectroscopy (XPS)was recorded on an ESCALAB 250Xi spectrometer,with standard Al-Kα X-ray source (300 W) and analyzer pass energy of 20 eV.

2.3 Electrolysis cells preparation

The LSGM electrolyte support was mechanically polished to 3 mm thickness and then used to assemble the single cell[17].LSCM|LSGM|LSM-SDC and LSCM-0.1 CeO2-δ|LSGM|LSM-SDC were the configurations of single cells (effective area: 0.125 cm2) in this work.The cathode slurries (LSCM and LSCM-0.1CeO2) were painted on one side of the LSGM electrolyte, and the anode slurries LSM-SDC(65:35 wt%) were subsequently printed on the other side and then heated at 1100 ℃ for 3 h in air.The ceramic adhesive (JD-767, Jiudian, China) was used to seal the single cells for electrochemical measurements.Before CO2electrochemical reduction, the cathode was pre-reduced at 800 ℃ for 3 h in pure hydrogen atmosphere, whereas the anode was exposed to ambient air.The total flow rate of CO2was sustained at 50 mL×min-1.The current-voltage curves (I–V, 0.006 V×s?1) were recorded using an electrochemical station (IM6, Zahner, Germany).The CO production was collected though online gas chromatograph (GC2014, Shimazu, Japan) with the output gas from cathode[19].

3 RESULTS AND DISCUSSION

3.1 Material structure

The crystal structure schematic diagram of La0.75Sr0.25Cr0.5Mn0.5O3-δis shown in Fig.1.The central ions (Cr and Mn) are six-coordinated to form octahedral geometry.The X-ray diffraction (XRD)patterns of samples (LSCM, LSCM-0.1CeO2-δ) are shown in Fig.2.The peaks of cubic CeO2-δare observed in Fig.2a, indicating that 10%mol CeO2-δis successfully impregnated on the surface of LSCM powders after heat-treatment at 500 ℃ for 3 h.In addition, CeO2-δalso appears in Fig.2b, demonstrating that cubic ceria still exists after treating in pure hydrogen at 800 ℃ for 4 h.The structure of pure LSCM transfers from rhombohedral to orthorhombic structure after heat-treatment in pure hydrogen atmosphere[13], leading to performance decline.No phase transition is observed after high temperature reduction, signifying that impregnating cubic CeO2-δnanoparticles could restrain the phase transition of pure LSCM.

Fig.1.Crystal structure schematic diagram of La0.75Sr0.25Cr0.5Mn0.5O3-δ

Fig.2.(a) XRD patterns for oxidized samples: LSCM, LSCM-0.1CeO2;(b) XRD patterns for reduced samples (LSCM and LSCM-0.1CeO2-δ) heated in pure hydrogen at 800 ℃ for 4 h

3.2 Microstructure

Scanning electron microscope (SEM) is used to further research its micromorphology.As shown in Fig.3, cubic ceria nanoparticles are uniformly distributed on the surface of porous LSCM scaffold with diameters of ～30 nm after reduction in pure hydrogen at 800 ℃ for 4 h, which affirms CeO2-δis successfully impregnated on the perovskite LSCM scaffold as validated by XRD characterization.Cubic CeO2-δnanoparticles are conducive to improve the catalytic activity.

Fig.3.SEM micrograph of LSCM-0.1CeO2-δ with particle size distribution reduced in pure hydrogen at 800 ℃ for 4 h

3.3 Valence state of elements

In order to further investigate the component of samples, X-ray photoelectron spectroscopy (XPS) is used to characterize the element valence states of oxidized and reduced samples.Fig.4a reveals that the manganese valence is only present in the form of Mn4+, meanwhile the cerium exists with the presence of Ce4+/Ce3+in the oxidized LSCM-0.1 CeO2-δsample.After treating in pure hydrogen atmosphere at 800 ℃ for 4 h, some of Mn4+is reduced to Mn3+while cerium is still in the form of Ce4+/Ce3+.The redox couples (Mn4+/Mn3+and Ce4+/Ce3+) in the reduced sample are positive to CO2reduction reaction.

3.4 Electrochemical properties

The deeper study of CO2electrolysis with perovskite structure LSCM cathodes is performed at 800 ℃.The defect reaction associated with oxygen change can be written in the Kr?ger-Vink notation for CO2electrolysis.

Here, e’ is the free electron, Vo”is the oxygen vacancy from cathode (CeO2-δand LSCM) or the electrolyte LSGM, and Oo×is an oxygen atom in the cathode (CeO2-δand LSCM) or LSGM lattice.Fig.5 displays the microstructure of LSGM electrolyte supported single cell with the configurations of LSM-SDC|LSGM|LSCM-0.1 CeO2-δ, clearly showing a dense electrolyte and porous electrode microstructure.The open circuit voltage (OCV) of the single solid oxide electrolysers (SOEs) is about 1.1 V, which indicates good sealing of single solid oxide electrolysers (SOEs), when cathode is fed with pure hydrogen at 800 ℃.The electrolysis is performed in CO2atmosphere under applied voltages of 0.8～1.6 V at 800 ℃.The current density of LSCM-0.1 CeO2-δcathode (0.5 A×cm-2) is increased by ～50%, and then compared with pure perovskite structure LSCM cathode (0.34 A×cm-2) at applied bias of 1.6 V in Fig.6a and Table 1.The LSCM cathode with impregnated fluorite structure ceria nanoparticles exhibits stable operation in pure CO2at each different applied voltage, as shown in Fig.6b.As expected, LSCM-0.1 CeO2-δcathode shows the exceptionally high CO productivity and the current efficiency (～95%) due to the tailed perovskite structure LSCM cathode (Fig.6c, Fig.6d and Table.1).The improvement of electrochemical properties is attributed to the synergism of impregnated cubic CeO2-δnanoparticles and perovskite structure LSCM skeleton.

Fig.4.(a, b) XPS results for oxidized sample: LSCM-0.1CeO2;(c, d) XPS results for reduced sample: LSCM-0.1CeO2-δ after reduction in pure hydrogen at 800 ℃ for 4 h

Fig.5.SEM image of LSGM electrolyte supported single cell with the configurations of LSM-SDC|LSGM|LSCM-0.1CeO2-δ

Fig.6.(a) Current density of CO2 electrolysis with different cathodes (LSCM and LSCM-0.1CeO2-δ);(b) Short-term performance of cells with different cathodes (LSCM and LSCM-0.1CeO2-δ);(c, d) CO productivity and Faraday efficiency with different cathodes (LSCM and LSCM-0.1CeO2-δ) confirmed at 800 ℃

Table 1.Electrochemical Properties of Different Cathodes (LSCM and LSCM-0.1CeO2-δ)Measured at 1.2 V, 1.4 V, and 1.6 V, respectively

4 CONCLUSION

In conclusion, excellent CO2electrolysis is achieved through impregnating fluorite structure oxide nanoparticles on the perovskite structure LSCM scaffold.The modified cathode is helpful for the adsorption and activation of carbon dioxide.LSCM-0.1 CeO2-δcathode gives a current density of 0.5 A×cm-2and the current efficiency ～95% at 800℃ under 1.6 V for carbon dioxide electrolysis.Fluorite structure nanoparticles impregnated LSCM cathode can keep carbon dioxide electrolysis stable.It is an effective strategy to impregnate fluorite structure nanoparticles on the LSCM scaffold for solid oxide electrolysers (SOEs).

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