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Water electrolysis on La 1 − x Sr x CoO 3 − δ perovskite electrocatalysts

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ABSTRACT

Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B–O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co–O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1−xSrxCoO3−δ. We attempt to rationalize the high activities of La1−xSrxCoO3−δ through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

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ABF-STEM imaging of oxygen vacancy ordering in La1−xSrxCoO3−δ (x=0.8, 1.0).(a) [001]p ABF-STEM image of LSCO28 showing the cation and anion sublattices. The contrast is inverted in comparison with the HAADF-STEM images. The assignment of the atomic columns is shown in the enlargement at the top right corner. Half of the perovskite (CoO2) layers appear brighter indicating oxygen deficiency (marked with white arrowheads). The complete (CoO2) layers and anion-deficient (CoO2−δ) layer alternate (see the ABF intensity profile below, the anion-deficient layers are marked with black arrowheads) resulting in doubling of the perovskite lattice parameter in the direction perpendicular to the layers. (b) [001]P ABF-STEM image of SCO showing layered anion-vacancy ordering. The (CoO2−δ) layers are marked with the white arrowheads and demonstrate the contrast clearly distinct from that of the (CoO2) layers. The assignment of the atomic columns is shown in the enlarged part at the bottom left.
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f3: ABF-STEM imaging of oxygen vacancy ordering in La1−xSrxCoO3−δ (x=0.8, 1.0).(a) [001]p ABF-STEM image of LSCO28 showing the cation and anion sublattices. The contrast is inverted in comparison with the HAADF-STEM images. The assignment of the atomic columns is shown in the enlargement at the top right corner. Half of the perovskite (CoO2) layers appear brighter indicating oxygen deficiency (marked with white arrowheads). The complete (CoO2) layers and anion-deficient (CoO2−δ) layer alternate (see the ABF intensity profile below, the anion-deficient layers are marked with black arrowheads) resulting in doubling of the perovskite lattice parameter in the direction perpendicular to the layers. (b) [001]P ABF-STEM image of SCO showing layered anion-vacancy ordering. The (CoO2−δ) layers are marked with the white arrowheads and demonstrate the contrast clearly distinct from that of the (CoO2) layers. The assignment of the atomic columns is shown in the enlarged part at the bottom left.

Mentions: LCO, LSCO and SCO samples were synthesized using our previously developed reverse-phase hydrolysis scheme, using a 950 °C calcination temperature instead of 700 °C to ensure that the correct phase was synthesized161727. Figure 2a shows the powder X-ray diffraction patterns for the system, demonstrating the successful synthesis of the perovskite phases across the whole-composition range. The only minor admixture found in the LCO and LSCO samples was Co3O4. The crystal structures of all compositions have been verified using a combination of powder X-ray diffraction and transmission electron microscopy. The unit cell parameters and space groups of the respective materials are given in Supplementary Table 1. The powder X-ray diffraction and selected area electron diffraction (SAED) patterns of the x=0–0.4 compositions are characteristic of the perovskite structure with the a−a−a− tilting distortion of the octahedral framework (Fig. 2b,e). The monoclinic distortion due to orbital ordering reported for this compositional range was not detected being beyond resolution of our powder X-ray diffraction experiment293031. The LSCO46 composition crystallizes in a cubic perovskite structure. In the crystal structures of LSCO28 and SCO ordering of oxygen vacancies becomes obvious from both SAED patterns and high-angle annular dark-field scanning transmission electron microcopy (HAADF-STEM) images (Fig. 2c,d,f,g). Oxygen vacancies reside in the (CoO2-δ) anion-deficient perovskite layers alternating with the complete (CoO2) layers that results in a tetragonal ap × ap × 2ap (ap indicates the parameter of the perovskite subcell) supercell in LSCO28. The anion-deficient layers manifest themselves as faintly darker stripes in the HAADF-STEM images (marked with arrowheads in Fig. 2f,g), which according to Kim et al.32 is related to the structural relaxation in these planes. The anion-deficient layers form nanoscale-twinned patterns in both the LSCO28 and SCO samples (Fig. 2f,g). In general, the crystallographic observations on the LCO and LSCO samples are in agreement with the La1−xSrxCoO3−δ phase diagram33. However, in contrast to the earlier reported Sr2Co2O5 brownmillerite or hexagonal Sr6Co5O15 phases3435, the SCO sample demonstrates another type of oxygen vacancy ordering. The [010]p SAED pattern of SCO (Fig. 2d, top) is strongly reminiscent to that of the Ln1−xSrxCoO3−δ (Ln=Sm-Yb, Y) perovskites with the I4/mmm 2ap × 2ap × 4ap supercell333637. A detailed deconvolution of this SAED pattern into contributions from the twinned domains is presented in Supplementary Fig. 1. This supercell also allows complete indexing of the powder X-ray diffraction pattern of SCO (Supplementary Fig. 2). The layered ordering of the oxygen vacancies in the LSCO28 and SCO samples was directly visualized using annular bright-field STEM (ABF-STEM) imaging (Fig. 3a,b). In both structures the anion-complete (CoO2) and anion-deficient (CoO2−δ) layers can be clearly distinguished, alternating along the c-axis of the tetragonal supercells. However, establishing the exact ordering patterns of the oxygen atoms and vacancies in these (CoO2−δ) layers requires more detailed neutron powder diffraction investigation.


Water electrolysis on La 1 − x Sr x CoO 3 − δ perovskite electrocatalysts
ABF-STEM imaging of oxygen vacancy ordering in La1−xSrxCoO3−δ (x=0.8, 1.0).(a) [001]p ABF-STEM image of LSCO28 showing the cation and anion sublattices. The contrast is inverted in comparison with the HAADF-STEM images. The assignment of the atomic columns is shown in the enlargement at the top right corner. Half of the perovskite (CoO2) layers appear brighter indicating oxygen deficiency (marked with white arrowheads). The complete (CoO2) layers and anion-deficient (CoO2−δ) layer alternate (see the ABF intensity profile below, the anion-deficient layers are marked with black arrowheads) resulting in doubling of the perovskite lattice parameter in the direction perpendicular to the layers. (b) [001]P ABF-STEM image of SCO showing layered anion-vacancy ordering. The (CoO2−δ) layers are marked with the white arrowheads and demonstrate the contrast clearly distinct from that of the (CoO2) layers. The assignment of the atomic columns is shown in the enlarged part at the bottom left.
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f3: ABF-STEM imaging of oxygen vacancy ordering in La1−xSrxCoO3−δ (x=0.8, 1.0).(a) [001]p ABF-STEM image of LSCO28 showing the cation and anion sublattices. The contrast is inverted in comparison with the HAADF-STEM images. The assignment of the atomic columns is shown in the enlargement at the top right corner. Half of the perovskite (CoO2) layers appear brighter indicating oxygen deficiency (marked with white arrowheads). The complete (CoO2) layers and anion-deficient (CoO2−δ) layer alternate (see the ABF intensity profile below, the anion-deficient layers are marked with black arrowheads) resulting in doubling of the perovskite lattice parameter in the direction perpendicular to the layers. (b) [001]P ABF-STEM image of SCO showing layered anion-vacancy ordering. The (CoO2−δ) layers are marked with the white arrowheads and demonstrate the contrast clearly distinct from that of the (CoO2) layers. The assignment of the atomic columns is shown in the enlarged part at the bottom left.
Mentions: LCO, LSCO and SCO samples were synthesized using our previously developed reverse-phase hydrolysis scheme, using a 950 °C calcination temperature instead of 700 °C to ensure that the correct phase was synthesized161727. Figure 2a shows the powder X-ray diffraction patterns for the system, demonstrating the successful synthesis of the perovskite phases across the whole-composition range. The only minor admixture found in the LCO and LSCO samples was Co3O4. The crystal structures of all compositions have been verified using a combination of powder X-ray diffraction and transmission electron microscopy. The unit cell parameters and space groups of the respective materials are given in Supplementary Table 1. The powder X-ray diffraction and selected area electron diffraction (SAED) patterns of the x=0–0.4 compositions are characteristic of the perovskite structure with the a−a−a− tilting distortion of the octahedral framework (Fig. 2b,e). The monoclinic distortion due to orbital ordering reported for this compositional range was not detected being beyond resolution of our powder X-ray diffraction experiment293031. The LSCO46 composition crystallizes in a cubic perovskite structure. In the crystal structures of LSCO28 and SCO ordering of oxygen vacancies becomes obvious from both SAED patterns and high-angle annular dark-field scanning transmission electron microcopy (HAADF-STEM) images (Fig. 2c,d,f,g). Oxygen vacancies reside in the (CoO2-δ) anion-deficient perovskite layers alternating with the complete (CoO2) layers that results in a tetragonal ap × ap × 2ap (ap indicates the parameter of the perovskite subcell) supercell in LSCO28. The anion-deficient layers manifest themselves as faintly darker stripes in the HAADF-STEM images (marked with arrowheads in Fig. 2f,g), which according to Kim et al.32 is related to the structural relaxation in these planes. The anion-deficient layers form nanoscale-twinned patterns in both the LSCO28 and SCO samples (Fig. 2f,g). In general, the crystallographic observations on the LCO and LSCO samples are in agreement with the La1−xSrxCoO3−δ phase diagram33. However, in contrast to the earlier reported Sr2Co2O5 brownmillerite or hexagonal Sr6Co5O15 phases3435, the SCO sample demonstrates another type of oxygen vacancy ordering. The [010]p SAED pattern of SCO (Fig. 2d, top) is strongly reminiscent to that of the Ln1−xSrxCoO3−δ (Ln=Sm-Yb, Y) perovskites with the I4/mmm 2ap × 2ap × 4ap supercell333637. A detailed deconvolution of this SAED pattern into contributions from the twinned domains is presented in Supplementary Fig. 1. This supercell also allows complete indexing of the powder X-ray diffraction pattern of SCO (Supplementary Fig. 2). The layered ordering of the oxygen vacancies in the LSCO28 and SCO samples was directly visualized using annular bright-field STEM (ABF-STEM) imaging (Fig. 3a,b). In both structures the anion-complete (CoO2) and anion-deficient (CoO2−δ) layers can be clearly distinguished, alternating along the c-axis of the tetragonal supercells. However, establishing the exact ordering patterns of the oxygen atoms and vacancies in these (CoO2−δ) layers requires more detailed neutron powder diffraction investigation.

View Article: PubMed Central - PubMed

ABSTRACT

Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B–O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co–O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1−xSrxCoO3−δ. We attempt to rationalize the high activities of La1−xSrxCoO3−δ through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.

No MeSH data available.


Related in: MedlinePlus