Supporting Information

In situ formation of oxygen vacancy in perovskite Sr0.95Ti0.8Nb0.1M0.1O3 (M=Mn, Cr) toward efficient carbon dioxide electrolysis

Jun Zhang1, Kui Xie1,2 *, Haoshan Wei1, Qingqing Qin1, Wentao Qi1, Liming Yang1, Cong Ruan1, Yucheng Wu1,2 *

1Department of Energy Materials, School of Materials Science and Engineering, Hefei University of Technology, No.193 Tunxi Road, Hefei, Anhui 230009, China.

2Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, No.193 Tunxi Road, Hefei, Anhui 230009, China.

*Corresponding author: ;

Captions

Fig. S1: XRD patterns of the oxidized Sr0.95Ti0.9-xNb0.1CrxO3 powder samples with x = 0.1, 0.2 and 0.3.

Fig. S2: TGA of oxidized samples in 5%H2/Ar with temperature ranging from room temperature to 1000 oC, (a) STNO; (b) STNCO.

Fig. S3: The electrical conductivity of oxidized STNO and STNMO as a function of temperature in air from 400 to 800 oC.

Fig. S4: Impedance spectroscopy data for the ionic conductivities of STNO, STNMO and STNCO at 800 oC, (a) oxidized STNO in air; (b) reduced STNO in 5%H2/Ar; (c) oxidized STNMO in air; (d) reduced STNMO in 5%H2/Ar; (e) oxidized STNCO in air and (f) reduced STNCO in 5%H2/Ar.

Fig. S5: TGA of reduced samples in CO2 atmospheres: (a) STNO; (b) STNMO.

Fig. S6: Microstructure of the composite electrodes: (a) Ag-STNO/SDC-YSZ; (b) Ag-STNCO/SDC-YSZ.

Fig. S7: (a) The in situ AC impendence plots for the electrolyzers based on STNMO/SDC cathode at 1.4 V under different flow rates of CO2; The equivalent circuit for the impedance spectra in different CO2 flow rates: (b) 10 mlmin-1; (c) 20 mlmin-1; (d) 30 mlmin-1.

Fig. S8: (a) Short-term performance for the electrolyzer based on STNMO/SDC cathode with the flow of CO2 at 800 oC under 1.6 V applied voltage for direct CO2 electrolysis; (b) SEM and EDS of STNMO/SDC cathode surface after test.

Fig. S9: (a) Short-term performance for the electrolyzer based on STNCO/SDC cathode with the flow of CO2 at 800 oC under 1.6 V applied voltage for direct CO2 electrolysis; (b) SEM and EDS of STNCO/SDC cathode surface after test.

Fig. S10: XRD results for STNMO/SDC cathode before and after CO2 electrolysis test at 800 oC.

Fig. S11: XRD results for STNCO/SDC cathode before and after CO2 electrolysis test at 800 oC.

Fig. S12: Long-term performance for the electrolyzer based on STNMO/SDC cathode with the flow of CO2 at 800 oC under 1.4 V applied voltage for direct CO2 electrolysis.

Fig. S1

Fig. S1 shows the XRD patterns of Sr0.95Ti0.9-xNb0.1CrxO3 (x = 0.1, 0.2 and 0.3) powders after calcining in air atmosphere at 1300 oC for 10 h; however, a single phase material is only achieved with x=0.1 (Sr0.95Ti0.8Nb0.1Cr0.1O3, STNCO). When x equals to 0.2 or 0.3, secondary phase, Sr(CrO4), can be found.

Fig. S2

Fig. S2 shows that the oxidized STNO and STNCO samples are conducted from room temperature to 1000 oC with a heating rate of 10 oCmin-1 in 5% H2/Ar. Fig. S2 (a) presents the weight change percentage of the oxidized STNO as a function of temperature when heated under the reducing atmosphere. The weight loss reaches ~0.8% for the STNO sample due to the loss of oxygen caused by the reduction of Ti4+/Nb5+ to Ti3+/Nb4+ under the reducing conditions, indicating a chemical formula of Sr0.95Ti0.9Nb0.1O2.90 for the reduced sample. In comparison, the weight loss for the STNCO reaches ~1.3% as seen in Fig. S2 (b) (Sr0.95Ti0.8Nb0.1Cr0.1O2.85), which is also consistent with the loss of oxygen in a reducing atmosphere.

Fig. S3

As shown in Fig. S3, the conductivities of oxidized STNO and STNMO gradually improve with temperature, which indicates a typical p-type semiconducting behavior. The conductivity only reaches approximately ~10-3 S·cm-1 for oxidized STNO and ~10-2 S·cm-1 for STNMO at 800 oC in air due to an increase of charge carrier generated by the combination of the oxygen vacancy created by the Mn dopant and the atmospheric oxygen.

Fig. S4

Fig. S4 shows the impedance spectroscopy of oxidized/reduced STNO, STNMO and STNCO for the ionic conductivities at 800 oC. The calculated results have the same order of magnitudes as shown in Fig. 8. The ionic conductivity is 2.0×10-4 for oxidized STNO, 7.1×10-4 S·cm-1 for reduced STNO, 4.2×10-4 for oxidized STNMO, 2.0×10-3 S·cm-1 for reduced STNMO, 4.6×10-4 for oxidized STNCO and 1.67×10-3 S·cm-1 for reduced STNCO, respectively. These values obtained by AC impedance spectroscopy are consistent with the values tested by DC method.

Fig. S5

Fig. S5(a) shows the weight loss of the reduced STNO sample after CO2 adsorption above 400 oC. The weight loss reaches approximately 0.15% and the chemical desorption is observed at approximately 600 oC, which implies the presence of chemical significant adsorption of CO2 on the reduced STNO sample. However, the weight loss of the reduced STNMO sample after CO2 adsorption is substantially increased to 1%. The strong chemical desorption has been extended to approximately 800 oC as shown in Fig. S4 (b).

Fig. S6

Fig. S6 shows the microstructures of electrodes with configurations of Ag-STNO/SDC-YSZ and Ag-STNCO/SDC-YSZ after tests, respectively. The YSZ-electrolyte supports are also quite uniform and dense; the porous electrode layers are approximately 10 μm in thickness and adhere to the electrolyte very well.

Fig. S7

Fig. S7 (a) shows the in situ AC impedance spectra for the electrolyzers based on STNMO cathode under different CO2 flow rates at 1.4 V. The Rs is stable at 1.4 V, so it is set at 0 Ωcm2 in the figures. The low frequency process, R2, is related to the flow rates of CO2. R2 decreases with increasing flow rates of CO2. In addition, the equivalent circuit is used for the impedance spectra in Figs. S6 (b, c and d). The results indicate that R2 decreased from 1.48 to 1.32 Ωcm2, with flow rates of CO2 ranging from 10 to 30 mlmin-1, which confirms the relationship between the gaseous mass transfer and the flow rates of CO2 (i.e., the polarization performance was improved by increasing the mass flow of CO2 through the cathode).

Fig. S8

Fig. S9

To validate the short-term stability, direct CO2 electrolysis is performed at a fixed voltage of 1.6 V at 800 oC for 24 h with pure CO2 flowing over the cathode. As shown in Figs. S7 (a) and S8 (a), a slight decrease in the current density is observed within the first few hours. However, the current density is stable, which confirms the excellent short-term stability of the STNMO or STNCO cathode for direct CO2 electrolysis. As shown in Figs. S8 (b) and S9 (b), SEM and EDS mapping are employed to analyze the cathode surface after short-term operation for high temperature carbon dioxide electrolysis. No microstructure cracks are observed, which further confirms the stability of the cathode material.

Fig. S10

Fig. S11

The XRD results for STNMO/SDC and STNCO/SDC are shown in Fig. S10 and Fig. S11. No other phases are present, except for STNMO, STNCO, SDC and YSZ before and after the CO2 electrolysis test at 800 oC. Therefore, STNMO or STNCO is chemically stable against SDC and YSZ at high temperature.

Fig. S12

The long-term performance based on the STNMO cathode for direct CO2 electrolysis is conducted at 800 oC under an applied voltage of 1.4 V. Fig. S11 indicates that the current density is stable (approximately 90 mA·cm-2), which confirms the excellent long-term stability of the STNMO cathode for direct CO2 electrolysis.

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