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In situ formation of oxygen vacancy in perovskite Sr 0.95 Ti 0.8 Nb 0.1 M 0.1 O 3 (M = Mn, Cr) toward efficient carbon dioxide electrolysis

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ABSTRACT

In this work, redox-active Mn or Cr is introduced to the B site of redox stable perovskite Sr0.95Ti0.9Nb0.1O3.00 to create oxygen vacancies in situ after reduction for high-temperature CO2 electrolysis. Combined analysis using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and thermogravimetric analysis confirms the change of the chemical formula from oxidized Sr0.95Ti0.9Nb0.1O3.00 to reduced Sr0.95Ti0.9Nb0.1O2.90 for the bare sample. By contrast, a significant concentration of oxygen vacancy is additionally formed in situ for Mn- or Cr-doped samples by reducing the oxidized Sr0.95Ti0.8Nb0.1M0.1O3.00 (M = Mn, Cr) to Sr0.95Ti0.8Nb0.1M0.1O2.85. The ionic conductivities of the Mn- and Cr-doped titanate improve by approximately 2 times higher than bare titanate in an oxidizing atmosphere and 3–6 times higher in a reducing atmosphere at intermediate temperatures. A remarkable chemical accommodation of CO2 molecules is achieved on the surface of the reduced and doped titanate, and the chemical desorption temperature reaches a common carbonate decomposition temperature. The electrical properties of the cathode materials are investigated and correlated with the electrochemical performance of the composite electrodes. Direct CO2 electrolysis at composite cathodes is investigated in solid-oxide electrolyzers. The electrode polarizations and current efficiencies are observed to be significantly improved with the Mn- or Cr-doped titanate cathodes.

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TGA of the samples in reducing gas from 1000°C to room temperature: (a) STNO and (b) STNMO.
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f6: TGA of the samples in reducing gas from 1000°C to room temperature: (a) STNO and (b) STNMO.

Mentions: To confirm the elemental valence change, XPS analysis was performed to examine the oxidized and reduced samples. As observed in Fig. 3, a strong signal corresponding to Ti4+ and Nb5+ is observed in the oxidized STNO sample. However, a portion of the Ti4+ and Nb5+ is chemically reduced to Ti3+ and Nb4+, respectively, by treating the STNO samples in a reducing atmosphere, as confirmed by the Ti3+ and Nb4+ signal, which is expected to significantly contribute to the electronic conductivity. Similar chemical state changes of the Ti and Nb elements were also observed in the STNMO sample before and after reduction at high temperature, as illustrated in Figs. 4 (a), (b), (c) and (d). Mn3+ (2p1/2), Mn4+ (2p3/2) and Mn3+ (2p3/2) peaks are observed at 653.40, 640.50 and 641.70 eV, respectively (Fig. 4 (f)), demonstrating the redox activity of the Mn element in oxides and that a small amount of Ti3+ remains present even in the oxidized sample. In addition, the XPS data indicates that Mn4+ is also chemically reduced to Mn3+ by treating the STNO samples in a reducing atmosphere. A similar chemical state change of Ti3+/4+ and Nb4+/5+ was also observed in the STNCO samples before and after reduction at high temperature, as shown in Figs. 5 (a, b, c and d). In addition, Cr3+ (575.8 and 585.9 eV) and Cr6+ (577, 580.5 and 588.7 eV) peaks are observed in Fig. 5 (e), indicating the presence of two elemental valences of Cr3+/6+ in the oxidized samples. According to the fitting results, the Cr3+/Cr6+ ratio is ~45.96:54.04 for oxidized STNCO, which indicates that Cr6+ is the main chemical state in the oxidized sample. By contrast, Cr3+ is the main chemical state for reduced STNCO, and all the Cr6+ is reduced to Cr3+ (576.1, 576.9, 579.2 and 586.1 eV) in a strong reducing atmosphere, as observed in Fig. 5 (f). The XPS data reveals that Cr6+ is chemically reduced to Cr3+ by treating the STNCO samples in a reducing atmosphere, which is expected to create oxygen vacancies, and the oxygen vacancy concentration is strongly related to the amount of low-valence ions at the B-sites. Thermogravimetry is common employed to analyze oxygen nonstoichiometry. As observed in Fig. 6, the oxidized STNO sample exhibits a 0.87% weight loss when heated at 10°C·min−1 from 1000°C to room temperature in a reducing gas, indicating a chemical formula of Sr0.95Ti0.9Nb0.1O2.90 for the reduced sample. By contrast, the reduced STNMO sample possesses a chemical formula of Sr0.95Ti0.8Nb0.1Mn0.1O2.85, which suggests that the Mn4+ in the oxidized sample has been completely reduced to Mn3+ accompanied by the generation of oxygen vacancies and a change in the coordination number of the Mn ion in the B-sites. In addition, the oxidized STNO and STNCO samples are tested from room temperature to 1000°C at a heating rate of 10°C·min−1 in 5%H2/Ar. Fig. S2 (a) presents the weight change percentage of the oxidized STNO as a function of temperature upon heating in the reducing atmosphere. The weight loss reaches ~0.8% for the STNO sample due to the loss of oxygen caused by Ti4+/Nb5+ being reduced 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 observed in Fig. S2 (b) (i.e., a chemical formula of Sr0.95Ti0.8Nb0.1Cr0.1O2.85), which is also consistent with the loss of oxygen in a reducing atmosphere. However, the major part of the weight loss is due to the Cr-doped STNO, which suggests that the Cr6+ in the oxidized sample has been completely reduced into Cr3+ accompanied by the generation of oxygen vacancies, further confirming the XPS results for reduced STNCO presented in Fig. 5 (f).


In situ formation of oxygen vacancy in perovskite Sr 0.95 Ti 0.8 Nb 0.1 M 0.1 O 3 (M = Mn, Cr) toward efficient carbon dioxide electrolysis
TGA of the samples in reducing gas from 1000°C to room temperature: (a) STNO and (b) STNMO.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382710&req=5

f6: TGA of the samples in reducing gas from 1000°C to room temperature: (a) STNO and (b) STNMO.
Mentions: To confirm the elemental valence change, XPS analysis was performed to examine the oxidized and reduced samples. As observed in Fig. 3, a strong signal corresponding to Ti4+ and Nb5+ is observed in the oxidized STNO sample. However, a portion of the Ti4+ and Nb5+ is chemically reduced to Ti3+ and Nb4+, respectively, by treating the STNO samples in a reducing atmosphere, as confirmed by the Ti3+ and Nb4+ signal, which is expected to significantly contribute to the electronic conductivity. Similar chemical state changes of the Ti and Nb elements were also observed in the STNMO sample before and after reduction at high temperature, as illustrated in Figs. 4 (a), (b), (c) and (d). Mn3+ (2p1/2), Mn4+ (2p3/2) and Mn3+ (2p3/2) peaks are observed at 653.40, 640.50 and 641.70 eV, respectively (Fig. 4 (f)), demonstrating the redox activity of the Mn element in oxides and that a small amount of Ti3+ remains present even in the oxidized sample. In addition, the XPS data indicates that Mn4+ is also chemically reduced to Mn3+ by treating the STNO samples in a reducing atmosphere. A similar chemical state change of Ti3+/4+ and Nb4+/5+ was also observed in the STNCO samples before and after reduction at high temperature, as shown in Figs. 5 (a, b, c and d). In addition, Cr3+ (575.8 and 585.9 eV) and Cr6+ (577, 580.5 and 588.7 eV) peaks are observed in Fig. 5 (e), indicating the presence of two elemental valences of Cr3+/6+ in the oxidized samples. According to the fitting results, the Cr3+/Cr6+ ratio is ~45.96:54.04 for oxidized STNCO, which indicates that Cr6+ is the main chemical state in the oxidized sample. By contrast, Cr3+ is the main chemical state for reduced STNCO, and all the Cr6+ is reduced to Cr3+ (576.1, 576.9, 579.2 and 586.1 eV) in a strong reducing atmosphere, as observed in Fig. 5 (f). The XPS data reveals that Cr6+ is chemically reduced to Cr3+ by treating the STNCO samples in a reducing atmosphere, which is expected to create oxygen vacancies, and the oxygen vacancy concentration is strongly related to the amount of low-valence ions at the B-sites. Thermogravimetry is common employed to analyze oxygen nonstoichiometry. As observed in Fig. 6, the oxidized STNO sample exhibits a 0.87% weight loss when heated at 10°C·min−1 from 1000°C to room temperature in a reducing gas, indicating a chemical formula of Sr0.95Ti0.9Nb0.1O2.90 for the reduced sample. By contrast, the reduced STNMO sample possesses a chemical formula of Sr0.95Ti0.8Nb0.1Mn0.1O2.85, which suggests that the Mn4+ in the oxidized sample has been completely reduced to Mn3+ accompanied by the generation of oxygen vacancies and a change in the coordination number of the Mn ion in the B-sites. In addition, the oxidized STNO and STNCO samples are tested from room temperature to 1000°C at a heating rate of 10°C·min−1 in 5%H2/Ar. Fig. S2 (a) presents the weight change percentage of the oxidized STNO as a function of temperature upon heating in the reducing atmosphere. The weight loss reaches ~0.8% for the STNO sample due to the loss of oxygen caused by Ti4+/Nb5+ being reduced 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 observed in Fig. S2 (b) (i.e., a chemical formula of Sr0.95Ti0.8Nb0.1Cr0.1O2.85), which is also consistent with the loss of oxygen in a reducing atmosphere. However, the major part of the weight loss is due to the Cr-doped STNO, which suggests that the Cr6+ in the oxidized sample has been completely reduced into Cr3+ accompanied by the generation of oxygen vacancies, further confirming the XPS results for reduced STNCO presented in Fig. 5 (f).

View Article: PubMed Central - PubMed

ABSTRACT

In this work, redox-active Mn or Cr is introduced to the B site of redox stable perovskite Sr0.95Ti0.9Nb0.1O3.00 to create oxygen vacancies in situ after reduction for high-temperature CO2 electrolysis. Combined analysis using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and thermogravimetric analysis confirms the change of the chemical formula from oxidized Sr0.95Ti0.9Nb0.1O3.00 to reduced Sr0.95Ti0.9Nb0.1O2.90 for the bare sample. By contrast, a significant concentration of oxygen vacancy is additionally formed in situ for Mn- or Cr-doped samples by reducing the oxidized Sr0.95Ti0.8Nb0.1M0.1O3.00 (M = Mn, Cr) to Sr0.95Ti0.8Nb0.1M0.1O2.85. The ionic conductivities of the Mn- and Cr-doped titanate improve by approximately 2 times higher than bare titanate in an oxidizing atmosphere and 3–6 times higher in a reducing atmosphere at intermediate temperatures. A remarkable chemical accommodation of CO2 molecules is achieved on the surface of the reduced and doped titanate, and the chemical desorption temperature reaches a common carbonate decomposition temperature. The electrical properties of the cathode materials are investigated and correlated with the electrochemical performance of the composite electrodes. Direct CO2 electrolysis at composite cathodes is investigated in solid-oxide electrolyzers. The electrode polarizations and current efficiencies are observed to be significantly improved with the Mn- or Cr-doped titanate cathodes.

No MeSH data available.