Triggered lattice-oxygen oxidation with active-site generation and self-termination of surface reconstruction during water oxidation

Abstract

To master the activation law and mechanism of surface lattice oxygen for the oxygen evolution reaction (OER) is critical for the development of efficient water electrolysis. Herein, we propose a strategy for triggering lattice-oxygen oxidation and enabling non-concerted proton-electron transfers during OER conditions by substituting Al in La_0.3Sr_0.7CoO_3-δ. According to our experimental data and density functional theory calculations, the substitution of Al can have a dual effect of promoting surface reconstruction into active Co oxyhydroxides and activating deprotonation on the reconstructed oxyhydroxide, inducing negatively charged oxygen as an active site. This leads to a significant improvement in the OER activity. Additionally, Al dopants facilitate the preoxidation of active cobalt metal, which introduces great structural flexibility due to elevated O 2p levels. As OER progresses, the accumulation of oxygen vacancies and lattice-oxygen oxidation on the catalyst surface leads to the termination of Al³⁺ leaching, thereby preventing further reconstruction. We have demonstrated a promising approach to achieving tunable electrochemical reconstruction by optimizing the electronic structure and gained a fundamental understanding of the activation mechanism of surface oxygen sites.

Keywords: lattice-oxygen oxidation; oxygen evolution reaction; perovskite oxide; self-termination; surface reconstruction.

Fig. 1.

Structural characterizations of as-prepared LSCAOs catalysts. (A) Schematic illustration of perovskite structures of LSCO, LSCAO-0.1, and LSCAO-0.15. (B) The XRD patterns for series of LSCAOs. (C) Co L-edge EELS spectra of LSCAOs with different Al substitution. (D) Atomic-resolution HAADF-STEM image of the partially inverse perovskite LSCAO-0.2 structure observed along [001] orientation. (Inset) Theoretical STEM images simulated by the Quantitative TEM/STEM Simulations Package (QSTEM) software projected along [001] orientation. (E) EDS atomic elemental mappings of the as-synthesized LSCAO-0.2.

Fig. 2.

Environments of Al species and electronic structure characterizations of as-prepared LSCAOs catalysts. (A) ²⁷Al MAS NMR spectra recorded at 130.33 MHz using a 30-kHz spinning frequency for LSCAOs with different Al substitution. The signals centered at 0 and 53 ppm were ascribed to octahedrally coordinated (AlO₆ structure unit) and tetracoordinated aluminum (AlO₄ structure unit), respectively. Electronic structure and valence bond information: (B) Co K-edge XANES adsorption edges; (C) R-space Fourier-transformed FT [k⁴ X(k)] of Co K-edge extended X-ray absorption fine structure (EXAFS) for series of LSCAOs. Three dominant peaks were observed at 1.4, 2.7, and 3.3 Å, corresponding Co−O, Co−La/Sr, and Co−Co single scattering, respectively. (D) Calculated oxygen deficiencies and Co−Co EXAFS distance of series of LSCAOs. The K-edge position is determined by an integral method as described in SI Appendix, Fig. S11 (28, 29), and the details about edge positions and nominal valence state of Co are shown in SI Appendix, Table S2. (E) The soft X-ray absorption spectra of O K-edges for series of LSCAOs. The peak at about 527 eV is assigned as the pre-edge peak caused by internal charge transfer between Co⁴⁺ and O²⁻; the peak at about 530 eV represents the overlapping bands between Co 3d and O 2p; the peak at about 532 eV is attributed to s* resonance extinction of O₂⁻ species; and peaks at about 534 eV and 537 eV are recognized as the hybridization between O 2p and La 5d/Sr 4d/Co 4sp in LSCAOs (–34).

Fig. 3.

OER performances of as-prepared LSCAOs catalysts and electronic interpretation of the effect of Al substitution on surface reconstruction. (A) CV curves of LSCAOs (x = 0; 0.05; 0.10; 0.15; 0.20; 0.25) in O₂-saturated 1 M KOH with a scan rate of 10 mV s⁻¹. Inset, Corresponding Tafel plots after oxide surface area normalization, capacitance correction, and iR correction. (B) OER current densities (left axis) of LSCAO at 1.65 V vs. RHE. The O 2p−band centre (right axis) in LSCAOs oxide is plotted to show its correlation with OER activity. (C) Computational models of LSCAOs (x = 0; 0.11; 0.22). (D) The d−band center of Co−3d and p−band center of O 2p PDOS in LSCAOs. (E) Pseudocapacitive behavior in the first and second cycles of LSCO and LSCAO-0.2 during CV cycling. (F) HRTEM images, showing the surface regions for as-prepared LSCAO-0.2 and after two cycles.

Fig. 4.

In situ investigation of pre-OER behaviors of catalysts and schematic of the surface reconstruction and deprotonation process. (A) CVs of LSCAO (x = 0.0, 0.10 and 2.0) scanned in O₂-saturated KOH (pH ≈ 12.5 to 14) at a scan rate of 10 mV s⁻¹. DEMS signal ratios of ³⁴O₂ (¹⁶O¹⁸O) and ³²O₂ (¹⁶O¹⁶O) from the reaction products for ¹⁸O-labeled. (B) LSCO, (C) LSCAO-0.1, and (D) LSCAO-0.2 catalyst tests in H₂¹⁶O aqueous KOH electrolyte. In situ ATR-IR spectra recorded during the multi-potential steps for (E) LSCO (F) LSCAO-0.1 and (G) LSCAO-0.2. Proposed deprotonation mechanism before OER, shown for (H) LSCO and (I) LSCAO-0.2. The absorption band peaked at around 1,212 cm⁻¹ is ascribed to the O−O stretching mode of surface-adsorbed superoxide (OOH_ad), the band at around 1,400 and 1,650 cm⁻¹ are attributed to the O−O stretching mode of weakly adsorbed molecular oxygen (52, 53).

Fig. 5.

Reconstruction terminating mechanism with Al³⁺ leaching. (A) Normalized ex-situ Co K-edge XANES analysis of LSCAO-0.2 under 1.175, 1.375, 1.475, 1.575, and 1.625 V (vs. RHE), as well as the FT k⁴χ(R) Co K-edge EXAFS. (B) Calculated Co−O and Co−Co EXAFS distance of LSCAO-0.2 under different potentials. (C) ICP-MS test of the electrolyte for LSCAO-0.2 under an operation time of 0 to 10 min (in 1 M KOH under 20 μA cm_ox⁻²). The dissolubility of Al in terms of Al(OH)₄⁻ is far beyond the concentration of Al³⁺ in our tested electrolytes. (D) Computational model for LSCAO-0.22 after Al³⁺ leaching and schematic band diagrams of LSCAO-0.22 with and without Al³⁺ vacancies. The perovskite structure beneath the reconstructed surface was confirmed by the atomic-resolution HAADF-STEM image (SI Appendix, Fig. S32).