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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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Density functional theory modelling of vacancy-mediated oxygen evolution on La1−xSrxCoO3−δ.(a) Surface configurations of the intermediate after AEM Step 1 (I0) and the one after LOM Step 1 (I1). (b) The free energy change of I1 over I0 versus the O vacancy formation enthalpy in the bulk, for the cubic La1−xSrxCoO3−δ (black mark), where x=0, 0.25, 0.5, 0.75 and 1, with the rhombohedral LaCoO3 and optimized SrCoO2.75 phases; for x=0.25 and 0.75, the most energetic favourable vacancy site is selected; the O vacancy formation energy is calculated at the concentration of 1 per 2 × 2 × 2 unit cell with respect to H2O(g) and H2(g) at standard condition; using O2(g) as the reference will shift the O vacancy formation enthalpy around +2.5 eV larger. (c) The density of states of d-band for the active surface Co and the overall p-band for its ligand O, for LaCoO3 and SrCoO3 before and after the lattice oxygen exchange. (d) The OER free energy changes of LOM and AEM on SrCoO3 at the concentration of ¼ ML, with indicated intermediates structures and potential-determining steps.
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f7: Density functional theory modelling of vacancy-mediated oxygen evolution on La1−xSrxCoO3−δ.(a) Surface configurations of the intermediate after AEM Step 1 (I0) and the one after LOM Step 1 (I1). (b) The free energy change of I1 over I0 versus the O vacancy formation enthalpy in the bulk, for the cubic La1−xSrxCoO3−δ (black mark), where x=0, 0.25, 0.5, 0.75 and 1, with the rhombohedral LaCoO3 and optimized SrCoO2.75 phases; for x=0.25 and 0.75, the most energetic favourable vacancy site is selected; the O vacancy formation energy is calculated at the concentration of 1 per 2 × 2 × 2 unit cell with respect to H2O(g) and H2(g) at standard condition; using O2(g) as the reference will shift the O vacancy formation enthalpy around +2.5 eV larger. (c) The density of states of d-band for the active surface Co and the overall p-band for its ligand O, for LaCoO3 and SrCoO3 before and after the lattice oxygen exchange. (d) The OER free energy changes of LOM and AEM on SrCoO3 at the concentration of ¼ ML, with indicated intermediates structures and potential-determining steps.

Mentions: Our results show that Step 1 differentiates the LOM, involving the intermediate with adsorbed –OO and lattice O vacancies (I1 in Figs 6a and 7a), from the AEM, involving the generally proposed adsorbed –O (I0 in Figs 6a and 7a). Therefore, the relative stabilities (free energy difference, ΔG) between these two isomeric intermediates are key to identifying if OER proceeds via the LOM or AEM for a given LSCO composition. This identification approach has been successfully used to demonstrate the preference of LOM on LaNiO3 (ref. 52). The computed values of ΔG are shown as a function of LSCO composition in Fig. 7b, which illustrates two key points. First, increasing x in La1−xSrxCoO3−δ reduces the O vacancy formation energy and therefore bulk stability. Second, ΔG decreases with the decreased bulk stability, becoming negative between 0.25<x<0.5. Therefore, OER on perovskites with low stability such as La0.5Sr0.5CoO3−δ, La0.25Sr0.75CoO3−δ and SrCoO3−δ is predicted to occur via the LOM, whereas LaCoO3 and La0.75Sr0.25CoO3−δare expected to follow the AEM.


Water electrolysis on La 1 − x Sr x CoO 3 − δ perovskite electrocatalysts
Density functional theory modelling of vacancy-mediated oxygen evolution on La1−xSrxCoO3−δ.(a) Surface configurations of the intermediate after AEM Step 1 (I0) and the one after LOM Step 1 (I1). (b) The free energy change of I1 over I0 versus the O vacancy formation enthalpy in the bulk, for the cubic La1−xSrxCoO3−δ (black mark), where x=0, 0.25, 0.5, 0.75 and 1, with the rhombohedral LaCoO3 and optimized SrCoO2.75 phases; for x=0.25 and 0.75, the most energetic favourable vacancy site is selected; the O vacancy formation energy is calculated at the concentration of 1 per 2 × 2 × 2 unit cell with respect to H2O(g) and H2(g) at standard condition; using O2(g) as the reference will shift the O vacancy formation enthalpy around +2.5 eV larger. (c) The density of states of d-band for the active surface Co and the overall p-band for its ligand O, for LaCoO3 and SrCoO3 before and after the lattice oxygen exchange. (d) The OER free energy changes of LOM and AEM on SrCoO3 at the concentration of ¼ ML, with indicated intermediates structures and potential-determining steps.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4814573&req=5

f7: Density functional theory modelling of vacancy-mediated oxygen evolution on La1−xSrxCoO3−δ.(a) Surface configurations of the intermediate after AEM Step 1 (I0) and the one after LOM Step 1 (I1). (b) The free energy change of I1 over I0 versus the O vacancy formation enthalpy in the bulk, for the cubic La1−xSrxCoO3−δ (black mark), where x=0, 0.25, 0.5, 0.75 and 1, with the rhombohedral LaCoO3 and optimized SrCoO2.75 phases; for x=0.25 and 0.75, the most energetic favourable vacancy site is selected; the O vacancy formation energy is calculated at the concentration of 1 per 2 × 2 × 2 unit cell with respect to H2O(g) and H2(g) at standard condition; using O2(g) as the reference will shift the O vacancy formation enthalpy around +2.5 eV larger. (c) The density of states of d-band for the active surface Co and the overall p-band for its ligand O, for LaCoO3 and SrCoO3 before and after the lattice oxygen exchange. (d) The OER free energy changes of LOM and AEM on SrCoO3 at the concentration of ¼ ML, with indicated intermediates structures and potential-determining steps.
Mentions: Our results show that Step 1 differentiates the LOM, involving the intermediate with adsorbed –OO and lattice O vacancies (I1 in Figs 6a and 7a), from the AEM, involving the generally proposed adsorbed –O (I0 in Figs 6a and 7a). Therefore, the relative stabilities (free energy difference, ΔG) between these two isomeric intermediates are key to identifying if OER proceeds via the LOM or AEM for a given LSCO composition. This identification approach has been successfully used to demonstrate the preference of LOM on LaNiO3 (ref. 52). The computed values of ΔG are shown as a function of LSCO composition in Fig. 7b, which illustrates two key points. First, increasing x in La1−xSrxCoO3−δ reduces the O vacancy formation energy and therefore bulk stability. Second, ΔG decreases with the decreased bulk stability, becoming negative between 0.25<x<0.5. Therefore, OER on perovskites with low stability such as La0.5Sr0.5CoO3−δ, La0.25Sr0.75CoO3−δ and SrCoO3−δ is predicted to occur via the LOM, whereas LaCoO3 and La0.75Sr0.25CoO3−δare expected to follow the AEM.

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&ndash;O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co&ndash;O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1&minus;xSrxCoO3&minus;&delta;. We attempt to rationalize the high activities of La1&minus;xSrxCoO3&minus;&delta; 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