English

• 1. INTRODUCTION

  Solid oxide electrolysers have demonstrated tremendous benefits of using renewable electricity to efficiently convert CO₂ into fuel^[1-3]. The high operation temperature delivers enhanced dynamic and thermodynamic advantages. By applying an external potential, CO₂ can be directly electrolyzed into CO and O₂ in an oxygen-ion-conducting solid oxide electrolyser^[4-7].

  Traditional Ni-YSZ composite electrode has been widely used for CO₂ electrolysis; however, the inherent oxidation-reduction instability of Ni–YSZ limits its application when performing direct CO₂ electrolysis without reducing gas atmosphere^[8-12]. It is reported that CO₂ electrolysis using perovskite ceramics electrodes would be highly promising^[13]. Perovskite oxide electrode materials have been therefore extensively studied^{[14, 15]}, which is due to that perovskite oxides are very good parent materials for tailoring chemical composition through the control of element doping in A and B sites. And some of them have shown long-term stability, carbon deposition resistance, sulfur poisoning resistance and excellent transport properties.

  The ABO₃-type perovskite oxide could provide layered lattice structure that has huge potential for the cation doping strategy for the development of new electrode materials^[16]. Perovskite SrFeO_3-δ oxide has been intensively studied as the cathode of solid oxide fuel cells and they have shown good electrochemical performance^[17-19]. SrFeO_3-δ oxide is a typical conductor while it still demonstrates high conductivity in reducing atmosphere^[20]. As a cathode, it has been proven to be effective for steam electrolysis in a solid oxide electrolyser. Enhancing the concentration of oxygen vacancy with dopant would be highly favourable to improve electrode activity. In this case, oxygen defects would not only improve the transport properties but also facilitate the chemisorption of CO₂ on oxide surfaces at high temperature.

  In this work, we dope Mn into the B-site of SrFeO_3-δ lattice to increase oxygen vacancy concentration towards CO₂ electrolysis. The electrical properties and surface oxygen exchange coefficient of the materials are investigated. We perform CO₂ electrolysis with Mn-doped cathode.

  2. EXPERIMENTAL

  2.1 Materials and apparatus

  SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ are synthesized by a solid state reaction method with heat treatment at 1300 ℃^{[21, 22]}. Reduced samples are prepared by treating the samples in 5%H₂/Ar at 800~1000 ℃ for 3~5 hours. La_0.9Sr_0.1Ga_0.8-Mg_0.2O_3-δ (LSGM) electrolyte powders are prepared using a solid state reaction method with heat treatment at 1000 ℃. (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM) and Ce_0.8Sm_0.2O_2-δ (SDC) powders are synthesized using a glycine-nitrate combustion method^{[23, 24]}. The phase formation of powder samples is analyzed using X-ray diffraction (D/MAX2500V). About 2.0 g of SrFeO_3-δ or SrFe_0.85Mn_0.15O_3-δ powder is pressed into a bar and sintered at 1300 ℃ for 6 h in air for conductivity test using a DC four-terminal method (Keithley 2000)^[25]. Electrical conductivity relaxation (ECR) method is used to test the surface oxygen exchange coefficient with oxygen partial pressure shifting between 10^-18 and 10^-12 atm at 800 ℃^{[26, 27]}. In our work, we use the atmospheres of CO/CO₂ by changing the ratio between CO and CO₂ to get the two different oxygen partial pressures.

  2.2 Electrolysis cells preparation

  LSGM electrolyte is prepared by pressing the powder sample and sintered at 1500 ℃ for 10 hours in air. SrFeO_3-δ/SDC and SrFe_0.85Mn_0.15O_3-δ/SDC slurries are prepared by milling the SrFeO_3-δ or SrFe_0.85Mn_0.15O_3-δ with SDC at a weight ratio of 65:35 in alpha-terpineol with suitable cellulose additive. Single cells are assembled using screen printing method and then heat-treated at 1100 ℃. The microstructures of single cells are observed using a scanning electron microscope (SEM, JEOL Ltd). Electrochemical measurement of CO₂ electrolysis is recorded using an electrochemical station (IM6, Zahner, Germany). Pure CO₂ (50 mL·min^-1) is fed to cathode while the anode is exposed to air. The generation of CO is analyzed using an online gas chromatograph (GC2014, Shimazu).

  3. RESULTS AND DISCUSSION

  3.1 Analysis of the crystal structure

  Fig. 1a and 1b present the XRD patterns of the powder samples in oxidized and reduced states, respectively. The SrFeO_3-δ is in a tetragonal phase while the Mn-doped SrFe_0.85Mn_0.15O_3-δ in a cubic phase. After reduction, a phase transition to orthorhombic phase is observed both for SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ^[28]. We therefore obtain Sr₂Fe₂O₅ phase which is the active phase for CO₂ electrolysis at high temperature. The doping of Mn changes the cell parameters as confirmed by the shift of diffraction peaks. As shown in Table 1, we use iodometric method^{[24, 29]} to analyze the oxygen nonstoichiometry of SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ in oxidized and reduced states, respectively. The oxygen deficiency is 0.2880 for SrFeO_3-δ while it is enhanced to 0.3386 for SrFe_0.85Mn_0.15O_3-δ through doping of Mn in lattice.

  Figure 1

  

  Figure 1.  XRD patterns of (a) oxidized forms of samples and (b) reduced forms of samples

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  Table 1

  

  Table 1.  Oxygen Deficiency in Oxidized and Reduced Samples

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  Chemical formula	Oxidized (3-δ)	Chemical formula	Reduced (5-δ)/2	Oxygen loss
  SrFeO3-δ	2.7227	Sr2Fe2O5-δ	2.4347	0.2880
  SrFe0.85Mn0.15O3-δ	2.6375	Sr2Fe0.17Mn0.3O5-δ	2.2989	0.3386
  The oxygen nonstoichiometry is determined using iodometric titration.

  3.2 Electrical properties

  Fig. 2a shows the conductivity of sintered samples in the temperature range of 200 to 800 ℃ in 5%H₂/Ar. The SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ samples show typical p-type conduction in air with conductivity reaching ~8 S·cm^-1 at 800 ℃. In reducing atmosphere, the conductivity of the two samples both gradually decreases and finally reaches ~1 S·cm^-1. Fig. 2b shows that the doping of Mn significantly reduces the re-equilibrium time in the ECR tests. The conductivity shifts from ~0.6 to 1.6 S·cm^-1 for SrFe_0.85Mn_0.15O_3-δ while the conductivity shifts from ~1.2 to 2.2 S·cm^-1 for the SrFe_0.85Mn_0.15O_3-δ during the conductivity relaxation test. The doping of Mn leads to the increase of oxygen vacancy, which accordingly decreases the concentration of charge carrier of hole and thereby reduces the mixed conductivity. The surface exchange coefficient, k value, of the SrFe_0.85Mn_0.15O_3-δ samples is enhanced by ~10 times to 3.5 × 10^-3 cm·s^-1 in contrast to 4.7 × 10^-4 cm·s^-1 for SrFeO_3-δ. The increase of oxygen vacancy is favorable to the enhancement of surface oxygen exchange process that is extremely important to electrode activity.

  Figure 2

  

  Figure 2.  (a) Conductivities of different samples in 5%H₂/Ar ranging from 200 to 800 ℃; (b) Surface oxygen exchange coefficients of the two samples with oxygen partial pressure shifting between 10^-12 and 10^-18 atm

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  3.3 CO₂ chemisorption properties

  Fig. 3a shows FT-IR spectrum of the CO₂ chemisorption on reduced SrFe_0.85Mn_0.15O_3-δ sample at 800 ℃. The band at 2270~2400 cm^-1 is associated with CO₂ molecule while the wave number at 1440~1600 cm^-1 represents the presence of CO₃^2- on sample surface^[6]. The chemisorbed CO₂ is therefore supposed to be in an intermediate state between the molecular CO₂ and carbonate ions. Fig. 3b shows the temperature programmed desorption of the samples in CO₂ atmosphere. It is observed that the chemisorption takes place at ~800 ℃ which is normally close to the decomposition temperature of carbonates. And stronger desorption is observed for the Mn-doped sample which further validates the enhancement of CO₂ chemisorption with increased oxygen vacancy concentration on oxide surfaces. We further correlate the desorption volume of CO₂ with the surface areas of materials. The adsorption capacity of CO₂ is 0.06 and 0.04 mL·m_catal² for SrFe_0.85Mn_0.15O_3-δ and SrFeO_3-δ materials, respectively.

  Figure 3

  

  Figure 3.  (a) FT-IR spectroscopy of CO₂ adsorbed on the reduced SrFe_0.85Mn_0.15O_3-δ at 800 ℃. (b) Temperature programmed desorption of the two samples in CO₂ atmosphere

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  3.4 Electrochemical properties

  Direct electrolysis of pure CO₂ is studied based on SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ cathodes at 800 ℃. Fig. 4a shows the typical I-V curves of CO₂ electrolysis with the two cathodes. For SrFe_0.85Mn_0.15O_3-δ cathode, the current density reaches 0.53 A·cm^-2 at 1.6 V which is much higher than 0.44 A·cm^-2 based on the SrFeO_3-δ cathode under same conditions. This indicates that the Mn-doped cathode enhances CO₂ electrolysis through improving electrode activity including surface oxygen exchange process and chemisorption in composite cathode. Fig. 4b shows the short-term performance of CO₂ electrolysis under different applied voltages, which further confirms the enhanced current densities with Mn-doped cathode. Fig. 4c and 4d show the production of CO and the Faradaic efficiency with the two cathodes. The production of CO reaches 2.25 mL·min^-1·cm^-2 for SrFe_0.85Mn_0.15O_3-δ, which is ~20% higher than 1.92 mL·min^-1·cm^-2 for SrFeO_3-δ at 1.5 V. The maximum current efficiency reaches 82.3% and 74.8% for SrFe_0.85Mn_0.15O_3-δ and SrFeO_3-δ cathodes at 1.1~1.5 V, respectively.

  Figure 4

  

  Figure 4.  (a) I-V curves, (b) Short-term performances, (c) CO production rate and (d) Current efficiency of CO₂ electrolysis with SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ cathodes at 800 ℃

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  Fig. 5 presents the AC impedance of CO₂ electrolysis recorded at 1.2~1.6 V. In general, the values of R_s are generally stable while the R_p values considerably decrease with the voltage increasing to 1.5 V. We observe the R_p values at 0.62 and 0.46 Ω·cm² for SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ cathodes at 1.5 V, respectively, which is comparable to the electrode activity of Sr₂Fe_1.6Mo_0.5O_6-δ for CO₂ electrolysis^[24]. The doping of Mn effectively improves electrode activity and therefore reduces the electrode polarization resistance. Fig. 6a shows the cell microstructure with a configuration of the SrFe_0.85Mn_0.15O_3-δ/LSGM/(La_0.8Sr_0.2)_0.95MnO_3-δ, which presents the uniform porous electrode microstructure and very dense electrolyte support. Fig. 6b shows the long-term performance of CO₂ electrolysis at 1.3 V at 800 ℃. No degradation is observed even after operation of 100 hours, which further indicates the good stability of the Mn-doped cathode.

  Figure 5

  

  Figure 5.  AC impedance for electrolysers based on (a) SrFeO_3-δ and (b) SrFe_0.85Mn_0.15O_3-δ at various voltages

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  Figure 6

  

  Figure 6.  (a) Scanning electron microscopy of the single cell with SrFe_0.85Mn_0.15O_3-δ cathode. (b) Long-term performance of CO₂ electrolysis with SrFe_0.85Mn_0.15O_3-δ at 800 ℃ under 1.3 V

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  4. CONCLUSION

  In this work, enhanced CO₂ electrolysis is achieved with Mn-doped SrFe_0.85Mn_0.15O_3-δ cathode. The doping of Mn increases the concentration of oxygen vacancy and surface oxygen exchange process, which therefore improves the electrode activity toward CO₂ splitting. The increase of oxygen vacancy also facilitates the chemisorption of CO₂ with the desorption temperature at ~800 ℃. We then demonstrate enhanced CO₂ electrolysis with no degradation being observed even after high temperature operation of 100 hours.

  
  
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• 

  Figure 1  XRD patterns of (a) oxidized forms of samples and (b) reduced forms of samples

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  Figure 2  (a) Conductivities of different samples in 5%H₂/Ar ranging from 200 to 800 ℃; (b) Surface oxygen exchange coefficients of the two samples with oxygen partial pressure shifting between 10^-12 and 10^-18 atm

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  Figure 3  (a) FT-IR spectroscopy of CO₂ adsorbed on the reduced SrFe_0.85Mn_0.15O_3-δ at 800 ℃. (b) Temperature programmed desorption of the two samples in CO₂ atmosphere

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  Figure 4  (a) I-V curves, (b) Short-term performances, (c) CO production rate and (d) Current efficiency of CO₂ electrolysis with SrFeO_3-δ and SrFe_0.85Mn_0.15O_3-δ cathodes at 800 ℃

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  Figure 5  AC impedance for electrolysers based on (a) SrFeO_3-δ and (b) SrFe_0.85Mn_0.15O_3-δ at various voltages

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  Figure 6  (a) Scanning electron microscopy of the single cell with SrFe_0.85Mn_0.15O_3-δ cathode. (b) Long-term performance of CO₂ electrolysis with SrFe_0.85Mn_0.15O_3-δ at 800 ℃ under 1.3 V

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  Table 1.  Oxygen Deficiency in Oxidized and Reduced Samples

  Chemical formula	Oxidized (3-δ)	Chemical formula	Reduced (5-δ)/2	Oxygen loss
  SrFeO3-δ	2.7227	Sr2Fe2O5-δ	2.4347	0.2880
  SrFe0.85Mn0.15O3-δ	2.6375	Sr2Fe0.17Mn0.3O5-δ	2.2989	0.3386
  The oxygen nonstoichiometry is determined using iodometric titration.

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