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

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.

Figure 1. Schematic illustration of engineering the surface of SC CoO NRs.

(a) SC CoO NRs fabricated directly on carbon fibre substrate. (b) Numerous nanopores present on the surface and across SC NRs. (c) The surface of SC CoO NRs covered with textured nanopyramids. (d) The dominant exposed facets of nanopyramids are electrochemically active vacancy-rich O-terminated {111} facets.

Figure 2. Structural characterization of SC CoO NRs.

(a) Top-view SEM image of SC CoO NRs fabricated directly on CFP. Scale bar, 10 μm. The inset in a shows morphology of SC CoO NRs. Scale bar, 1 μm. (b) High magnification TEM image of an individual SC CoO NR with saw-like edges. Scale bar, 20 nm. (c) High-resolution HAADF-STEM image taken from the outermost surface of a single SC CoO NR revealing the exposed {111} nanofacets (indicated by orange and green arrows), with inset showing the corresponding selected area electron diffraction pattern taken from [110] zone axis. Scale bar, 2 nm. (d) Atomic model of a nanopyramid enclosed with {100} and {111} facets, and (e) the projection of this pyramidal structure along [110] zone axis. (f,g) Experimental and simulated HADDF-STEM images of the pyramidal structure, respectively. Scale bar in f, 1 nm. Note that inelastic and neutron scatterings were not considered in the simulation, which contribute to the background in h. (h,i) The intensity profiles taken from orange and grey lines in f and g, respectively.

Figure 3. Analysis of O-vacancies on the pyramidal nanofaceted surface of SC CoO NRs.

(a,b) O-K edge and Co-L_2,3 edge XANES spectra of SC CoO NRs and reference CoO, respectively. In a the peak at ∼536 eV of SC CoO NRs is assigned to O deficiency. In b the peaks at 781.5 and ∼800 eV of SC CoO NRs shift towards low photon energy relative to the reference CoO, indicating the transfer of electrons from O-vacancies to Co d band. (c) O-vacancy formation energies on {100}, {110} and {111}-O facets of CoO showing a significant reduction in the vacancy formation energy on {111}-O facets.

Figure 4. Bifunctional ORR/OER performance of SC CoO NRs.

(a) ORR linear-sweep voltammograms (LSVs) of SC, PC CoO NRs and Pt catalysts directly deposited on CFP in O₂-saturated 1 M KOH solution at scan rate of 0.5 mV s⁻¹ without iR correction, with ORR chronoamperometric response to methanol addition shown in inset. (b) ORR Tafel plots of SC, PC CoO NRs and Pt catalysts. (c) ORR chronoamperometric response of SC CoO NRs and Pt catalysts at a constant voltage of 0.60 V_RHE. (d) OER LSVs of SC, PC CoO NRs and commercial RuO₂ catalysts directly deposited on CFP in O₂-saturated 1 M KOH solution at scan rate of 0.5 mV s⁻¹ without iR correction, with the corresponding OER Tafel plots shown in inset.

Figure 5. Intrinsic ORR/OER activity of SC CoO NCs in comparison with the activity of PC CoO NCs.

(a,b) J_k and J_k,specific for ORR at 0.6 V_RHE, respectively. (c,d) J and J_specific for OER at 1.65 V_RHE, respectively.

Figure 6. Origin of ORR/OER activity on various facets of CoO.

(a) Atomic configurations of O₂ molecules on {100}, {110}, {111}-O and {111}-O_V 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.