Enhanced oxygen evolution over dual corner-shared cobalt tetrahedra

Abstract

Developing efficient catalysts is of paramount importance to oxygen evolution, a sluggish anodic reaction that provides essential electrons and protons for various electrochemical processes, such as hydrogen generation. Here, we report that the oxygen evolution reaction (OER) can be efficiently catalyzed by cobalt tetrahedra, which are stabilized over the surface of a Swedenborgite-type YBCo₄O₇ material. We reveal that the surface of YBaCo₄O₇ possesses strong resilience towards structural amorphization during OER, which originates from its distinctive structural evolution toward electrochemical oxidation. The bulk of YBaCo₄O₇ composes of corner-sharing only CoO₄ tetrahedra, which can flexibly alter their positions to accommodate the insertion of interstitial oxygen ions and mediate the stress during the electrochemical oxidation. The density functional theory calculations demonstrate that the OER is efficiently catalyzed by a binuclear active site of dual corner-shared cobalt tetrahedra, which have a coordination number switching between 3 and 4 during the reaction. We expect that the reported active structural motif of dual corner-shared cobalt tetrahedra in this study could enable further development of compounds for catalyzing the OER.

Fig. 1. Cobalt tetrahedra over YBC4 surface.

a Crystal structure of YBC4 and the arrangements of corner-shared CoO4 tetrahedra in the Triangular layer and the Kagome layer. b The top view of the YBaCo₄O₇ (110) facet with tri-oxygen-coordinated cobalt (marked in orchid). c O 1 s core spectra from temperature-dependent XPS of an as-prepared YBC4 sample. d O K-edge EELS spectra from the sub-surface (~9 nm and ~1.5 nm) and the surface of YBC4. All spectra are aligned to Fermi energy (E_f) with 0 eV. e Extracted intensities of α peaks from sub-surface to surface. A linear background is applied to subtract the background adsorption. f A typical OH-adsorbed site, including adjacent Co from the Kagome layer and the Triangular layer, over the (110) facet. The O from adsorbed OH group is highlighted in green. g PDOS of unoccupied Co 3d states from sub-surface (dot line) and surface (dash line). The solid lines are the corresponding integrated PDOS of unoccupied Co 3d states.

Fig. 2. OER activity and stability.

a iR-corrected and oxides surface area normalized OER currents at a scan rate of 10 mV/s. The inset is the turnover frequency (TOF) for YBC4, BSCF, CoAl₂O₄, and IrO₂ at an overpotential of 300 mV. b A consecutive CV scanning test for YBC4.

Fig. 3. Enhanced stability by the flexible structure.

a HRTEM images of surface structures from the as-prepared YBC4 and 1000-cycled YBC4. b The XRD patterns of the as-prepared YBC4 powder, electrode substrate of graphite paper (gp), and YBC4@gp before and after 1000 cycles of CV cycling. c The details of peak shifts in the XRD patterns of YBC4@gp before and after CV cycling. d An oxygen intercalation process in the YBC4 lattice. The sky-blue spheres and green spheres represent oxygen atoms inserted into the lattice. e Measured Co K-edge XANES of the as-prepared and electrochemically oxidized YBC4. f k³-weighted Co K-edge EXAFS spectra of the as-prepared and electrochemically oxidized YBC4. g The Co-O distance, the mean coordination number of Co, and oxygen nonstoichiometry (δ) in the as-prepared and the thermally/electrochemically treated YBC4. The inset shows the error bar (SD) for the data. The oxygen nonstoichiometry is estimated based on the mean coordination number from a YBaCo₄O₇ and YBaCo₄O₈. The average coordination number in YBaCo₄O₇ and YBaCo₄O₈ is 4 and 4.44, respectively.

Fig. 4. Hydrogen bond.

a The calculated most likely paths in the OER on the YBC4 (110) surface. The free energies of different possible surface statuses in each proton-coupled electron transfer step are also presented. The O involved in the OER is highlighted in green. b The active structural motif of mono-µ-oxo-bridged Co in a tetrahedral geometry. c–e The key intermediate steps in the proposed OER mechanism.