Covalency-reinforced oxygen evolution reaction catalyst

Abstract

The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal-air batteries and direct solar water splitting. Cost-efficient ABO3 perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe(4+)-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) and the gold standard RuO2. The covalent bonding network incorporating multiple Cu(2+) and Fe(4+) transition metal ions significantly enhances the structural stability of CaCu3Fe4O12, which is key to achieving highly active long-life catalysts.

Figure 1. Electronic and crystal structures of SFO and CCFO perovskites.

(a) Schematic illustration of molecular orbitals for regular Mn³⁺O₆ and Fe⁴⁺O₆ octahedra. The Mn³⁺- and Fe⁴⁺- ion 3d-orbital energy levels are higher and lower than those of the O 2p orbitals, respectively. Therefore, the highest occupied molecular orbitals σ* generated from the e_g and 2p orbitals have 3d and 2p characteristics for the Mn³⁺ and Fe⁴⁺ ions. The holes at the σ* orbitals are due to the e_g and O 2p orbitals in the former and latter, respectively, resulting in different representations of d⁴ and d⁵ for Mn³⁺ and Fe⁴⁺, respectively, where denotes a ligand hole at the O 2p orbital. The π-bonds between the t_2g and 2p orbitals are neglected for simplicity. (b) Crystal structures and 3D electron density maps of SFO and CCFO. SFO is crystallized in a cubic ABO₃-type perovskite structure, and CCFO is crystallized in a cubic quadruple AA′₃B₄O₁₂-type structure with a 2a₀ × 2a₀ × 2a₀ unit cell (a₀: a-axis length of a simple ABO₃ perovskite). In these types of perovskites, the A-sites are occupied by alkaline, alkaline-earth or rare-earth metal ions, the A′-sites by Jahn–Teller active ions such as Cu²⁺ and Mn³⁺, and the B-sites by d-block transition metal ions. 3D electron density maps of SFO (equi-density level: 0.4 Å⁻¹) and CCFO (equi-density level: 0.5 Å⁻¹) were obtained from maximum entropy method analysis of synchrotron X-ray powder diffraction data. The shaded cross-sections indicate the (110) and planes of SFO and CCFO, respectively. The widespread covalent network incorporating the Cu, Fe and O ions is exemplified by CCFO. These illustrations were drawn using the VESTA3 program. The synchrotron X-ray powder diffraction patterns and Rietveld refinement results are shown in Supplementary Fig. 1 and Supplementary Note 1.

Figure 2. OER catalytic performance of Fe⁴⁺-perovskites and references.

(a) Linear sweep voltammograms for OER for SFO, CFO, CCFO, LMO, BSCF and RuO₂. The overpotential (η) of each catalyst was determined from the onset potential, E_onset (V versus RHE); E_onset is the potential at 0.5 mA  and η=E_onset−1.23 (V). (b) Specific activities (current density at 1.6 V versus RHE) for SFO, CFO, CCFO, LMO, BSCF and RuO₂. (c) Tafel plots for SFO, CFO, CCFO, LMO and BSCF. The error bars show the s.d. of three independent measurements. All data in (a–c) were obtained from the third cycle. Cyclic voltammograms of (d) SFO, (e) CFO and (f) CCFO for 100 cycles. Cycle dependence of Tafel slopes for (g) SFO, (h) CFO and (i) CCFO. Hundred continuous cycle measurements were performed with a higher disk rotation rate of 3,200 r.p.m. to prevent adhesion of the O₂ bubbles to the electrode. In (b,c) the error bars correspond to the s.d. obtained from three independent measurements.

Figure 3. HRTEM and fast fourier transform (FFT) images of perovskite oxides before and after OER measurements.

The boundaries between the crystalline and amorphous regions are divided by orange dotted lines. All the FFT images were obtained from surface regions of ∼10 × 10 nm². Scale bar: 5 nm.

Figure 4. OH⁻ adsorbed surfaces for SFO and CCFO and corresponding OER mechanism.

(a) OH⁻ adsorbates on Fe-terminated (100) plane of SFO for Fe-mediated route (ER type). The interatomic distance between the nearest neighbouring OH⁻ adsorbates is ∼3.9 Å. (b) OH⁻ adsorbate on (Ca,Cu)O-terminated (100) planes of CCFO for Cu-mediated route (ER type). (c) OH⁻ adsorbates on FeO₂-terminated (100) planes of CCFO for Fe-mediated route (LH type). The interatomic distance between the nearest neighbouring OH adsorbates is ∼2.6 A. In all cases, the Cu²⁺/Cu³⁺ or Fe⁴⁺/Fe⁵⁺ redox couple acts as the reaction mediator under the assumption that the adsorbed OH⁻ ions occupy the original oxygen sites in the crystal structure.