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Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis

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ABSTRACT

Engineering the surface structure at the atomic level can be used to precisely and effectively manipulate the reactivity and durability of catalysts. Here we report tuning of the atomic structure of one-dimensional single-crystal cobalt (II) oxide (CoO) nanorods by creating oxygen vacancies on pyramidal nanofacets. These CoO nanorods exhibit superior catalytic activity and durability towards oxygen reduction/evolution reactions. The combined experimental studies, microscopic and spectroscopic characterization, and density functional theory calculations reveal that the origins of the electrochemical activity of single-crystal CoO nanorods are in the oxygen vacancies that can be readily created on the oxygen-terminated {111} nanofacets, which favourably affect the electronic structure of CoO, assuring a rapid charge transfer and optimal adsorption energies for intermediates of oxygen reduction/evolution reactions. These results show that the surface atomic structure engineering is important for the fabrication of efficient and durable electrocatalysts.

No MeSH data available.


Origin of ORR/OER activity on various facets of CoO.(a) Atomic configurations of O2 molecules on {100}, {110}, {111}-O and {111}-OV facets. Notably, on {110} and {111}-Ov facets, the O–O bond of adsorbed O2 is remarkably elongated (the O–O distance of O2 is 1.23 Å), suggesting an effective activation of O2 in ORR. (b) The calculated ORR/OER free energy diagram at the equilibrium potential on different facets. (c) The projected density of states (PDOS) on pristine CoO and (d) CoO with O-vacancies. The arrow in d points new electronic states, which appear near the Fermi level in CoO with O-vacancies, responsible for the adsorption of intermediates on the O-vacancies.
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f6: Origin of ORR/OER activity on various facets of CoO.(a) Atomic configurations of O2 molecules on {100}, {110}, {111}-O and {111}-OV facets. Notably, on {110} and {111}-Ov facets, the O–O bond of adsorbed O2 is remarkably elongated (the O–O distance of O2 is 1.23 Å), suggesting an effective activation of O2 in ORR. (b) The calculated ORR/OER free energy diagram at the equilibrium potential on different facets. (c) The projected density of states (PDOS) on pristine CoO and (d) CoO with O-vacancies. The arrow in d points new electronic states, which appear near the Fermi level in CoO with O-vacancies, responsible for the adsorption of intermediates on the O-vacancies.

Mentions: A series of DFT computations was conducted to get a fundamental understanding of the correlation between the surface atomic structure of CoO and the ORR/OER activity. Fig. 6a clearly reveals that the atomic arrangement on different facets significantly affects adsorption sites and configuration of the reactant, that is, O2, in ORR. Moreover, the overall ORR/OER pathway was calculated, and the free energy diagram at the equilibrium potential (URHE0=1.23 V) are shown in Fig. 6b. As reported by Nørskov et al.515253, both ORR and OER involve four elementary reaction steps, in which ORR proceeds through the formation of HOO* from adsorbed O2, followed by its further reduction to O* and HO*, while OER proceeds in the reverse direction. For both ORR and OER, the ideal thermodynamic free energy change of the intermediates should be (refs 51, 52), indicating no energy would be wasted to activate the reactions. As illustrated in Fig. 6b, for ORR a large on the surface of {100} and {111}-O facets indicates that the first electron transfer step to reduce the adsorbed O2 to OOH* is endothermic, which is consistent with observations for the well-developed metal and metal oxide catalysts1753. Besides, the large negative and on {110} and {100} facets indicate that the chemical adsorption of O* and OH*, respectively, is too strong, which is also unfavourable for the subsequent electrocatalytic reactions. However, when O-vacancy is created on the surface of {111}-O (hereafter, referred to as ‘{111}-OV facet'), the formation of OOH* is facilitated, and all , and exhibit the lowest values among the four facets, suggesting the most favourable ORR kinetics on the {111}-OV facets. As regards OER, a similar analysis of the diagram for the reverse reaction shows that the {111}-Ov surface outperforms the other three facets. Overall, the {111}-Ov surface exhibits a mediated adsorption–desorption behaviour (), which is beneficial for the overall ORR/OER. Thus, the theory and experiment are in an excellent agreement, suggesting that the ORR/OER activity is successfully enhanced through atomic structure engineering. Our results demonstrate that there is a strong correlation between activity and atomic structure of CoO; that is, the ORR/OER activity of CoO increases in the following order {100}<{110}<{111}-Ov. To our knowledge this atomic scale structure–function relationship has not been considered for any other TMO surfaces in the analysis of electrocatalysts.


Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis
Origin of ORR/OER activity on various facets of CoO.(a) Atomic configurations of O2 molecules on {100}, {110}, {111}-O and {111}-OV facets. Notably, on {110} and {111}-Ov facets, the O–O bond of adsorbed O2 is remarkably elongated (the O–O distance of O2 is 1.23 Å), suggesting an effective activation of O2 in ORR. (b) The calculated ORR/OER free energy diagram at the equilibrium potential on different facets. (c) The projected density of states (PDOS) on pristine CoO and (d) CoO with O-vacancies. The arrow in d points new electronic states, which appear near the Fermi level in CoO with O-vacancies, responsible for the adsorption of intermediates on the O-vacancies.
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f6: Origin of ORR/OER activity on various facets of CoO.(a) Atomic configurations of O2 molecules on {100}, {110}, {111}-O and {111}-OV facets. Notably, on {110} and {111}-Ov facets, the O–O bond of adsorbed O2 is remarkably elongated (the O–O distance of O2 is 1.23 Å), suggesting an effective activation of O2 in ORR. (b) The calculated ORR/OER free energy diagram at the equilibrium potential on different facets. (c) The projected density of states (PDOS) on pristine CoO and (d) CoO with O-vacancies. The arrow in d points new electronic states, which appear near the Fermi level in CoO with O-vacancies, responsible for the adsorption of intermediates on the O-vacancies.
Mentions: A series of DFT computations was conducted to get a fundamental understanding of the correlation between the surface atomic structure of CoO and the ORR/OER activity. Fig. 6a clearly reveals that the atomic arrangement on different facets significantly affects adsorption sites and configuration of the reactant, that is, O2, in ORR. Moreover, the overall ORR/OER pathway was calculated, and the free energy diagram at the equilibrium potential (URHE0=1.23 V) are shown in Fig. 6b. As reported by Nørskov et al.515253, both ORR and OER involve four elementary reaction steps, in which ORR proceeds through the formation of HOO* from adsorbed O2, followed by its further reduction to O* and HO*, while OER proceeds in the reverse direction. For both ORR and OER, the ideal thermodynamic free energy change of the intermediates should be (refs 51, 52), indicating no energy would be wasted to activate the reactions. As illustrated in Fig. 6b, for ORR a large on the surface of {100} and {111}-O facets indicates that the first electron transfer step to reduce the adsorbed O2 to OOH* is endothermic, which is consistent with observations for the well-developed metal and metal oxide catalysts1753. Besides, the large negative and on {110} and {100} facets indicate that the chemical adsorption of O* and OH*, respectively, is too strong, which is also unfavourable for the subsequent electrocatalytic reactions. However, when O-vacancy is created on the surface of {111}-O (hereafter, referred to as ‘{111}-OV facet'), the formation of OOH* is facilitated, and all , and exhibit the lowest values among the four facets, suggesting the most favourable ORR kinetics on the {111}-OV facets. As regards OER, a similar analysis of the diagram for the reverse reaction shows that the {111}-Ov surface outperforms the other three facets. Overall, the {111}-Ov surface exhibits a mediated adsorption–desorption behaviour (), which is beneficial for the overall ORR/OER. Thus, the theory and experiment are in an excellent agreement, suggesting that the ORR/OER activity is successfully enhanced through atomic structure engineering. Our results demonstrate that there is a strong correlation between activity and atomic structure of CoO; that is, the ORR/OER activity of CoO increases in the following order {100}<{110}<{111}-Ov. To our knowledge this atomic scale structure–function relationship has not been considered for any other TMO surfaces in the analysis of electrocatalysts.

View Article: PubMed Central - PubMed

ABSTRACT

Engineering the surface structure at the atomic level can be used to precisely and effectively manipulate the reactivity and durability of catalysts. Here we report tuning of the atomic structure of one-dimensional single-crystal cobalt (II) oxide (CoO) nanorods by creating oxygen vacancies on pyramidal nanofacets. These CoO nanorods exhibit superior catalytic activity and durability towards oxygen reduction/evolution reactions. The combined experimental studies, microscopic and spectroscopic characterization, and density functional theory calculations reveal that the origins of the electrochemical activity of single-crystal CoO nanorods are in the oxygen vacancies that can be readily created on the oxygen-terminated {111} nanofacets, which favourably affect the electronic structure of CoO, assuring a rapid charge transfer and optimal adsorption energies for intermediates of oxygen reduction/evolution reactions. These results show that the surface atomic structure engineering is important for the fabrication of efficient and durable electrocatalysts.

No MeSH data available.