0428同步年報-2021-全

020 ACTIVITY REPORT 2021 (0.15 A mg -1 Ir ), respectively. Sr 2 CoIrO 6−δ possessed the smallest Tafel slope (54 mV dec -1 ), indicating that it had the best reaction kinetics of all other Co- and Ir-containing control electrocatalysts. More importantly, using rapid operando X-ray absorption near-edge spectroscopy (XANES) measured at TPS 44A , 1 the true active sites and the evolution of their valence states in Sr 2 CoIrO 6−δ were examined at the Co-K and Ir-L 3 edges during the OER. It is well known that XANES spectra at the 3 d element K edge are highly sensitive to the valence state: an increased valence state of the metal ion by one unit typically causes an energy shift of the absorption edge by more than 1 eV ( e.g. 2.3 eV from Co 2+ to Co 3+ and 1.5 eV from Co 3+ to Co 4+ ). Figure 1(a) shows the operando Co-K XANES spectra of the Sr 2 CoIrO 6−δ catalyst under OER. The pristine Sr 2 CoIrO 6−δ shows Co ions in Co 3+ state in the Co K-edge XANES spectrum, which was also confirmed by Co L-edge soft X-ray absorption spectra measured at TLS 11A1 . Upon increasing the applied voltage, the Co- K XANES spectra of Sr 2 CoIrO 6−δ gradually shifted to increased energies up to 0.5 eV at 1.8 V. This effect indicates a transition from Co 3+ state to Co 4+ state in Sr 2 CoIrO 6−δ . Compared with the Co 3+ and Co 4+ reference materials EuCo 3+ O 3 and BaCo 4+ O 3 , the average Co valence state of Sr 2 CoIrO 6−δ was estimated to be +3.3 at 1.8 V, and nearly +3.4 in SrCoO 2.7 . Similarly, the XANES Ir-L 3 edge spectra of Sr 2 CoIrO 6−δ were recorded as a function of applied voltage as shown in Fig. 1(b) , and compared with spectra of Ir reference materials SrIrO 3 for Ir 4+ , stoichiometric Sr 2 CoIrO 6−δ for Ir 5+ , and Sr 2 CoIrO 6 for Ir 6+ . The authors then established that the Ir-L 3 edge significantly shifted to higher energy with increasing applied potential up to 1.8 eV, indicating a valency increase from Ir 4+ to Ir 5+ , and further increased to the highest valence state of Ir 6+ under OER conditions. The estimated average valence state of Ir was +5.4 at 1.8 V. The authors thus proved that operando Co-K and Ir-L 3 XANES spectra showed that both Co and Ir ions are OER-active sites in Sr 2 CoIrO 6−δ . This is the first observation of such a high valence state of Ir 6+ under OER condition. Further, the authors studied the local coordination of the OER-active Co and Ir ions using operando extended X-ray absorption fine structure (EXAFS) spectra. 3D Fourier- transform patterns of the Co-K and Ir-L 3 edge spectra as a function of applied potential appear in Figs. 1(c) and 1(d) ; the evolution of Co-O and Ir-O bond lengths was obtained from fitting the EXAFS, as shown in Figs. 1(e) and 1(f) . The Co-O bond length was found to decrease only slightly with increasing applied voltage ( Fig. 1(e) ), but the Ir-O bond length showed a gradual decrease from 2.048(7) Å at 1.0 V to 1.984(8) Å at 1.55 V and became nearly constant for 1.6–1.8 V ( Fig. 1(f) ). Using a known relation 2 between Ir-O bond distance and the corresponding Ir oxidation state that showed the Ir-O bond length decreases by 0.039 Å per oxidation state, the Ir oxidation state under operando condition was independently estimated from ≈5 at 1.4 V to ≈5.4 for 1.6–1.8 V. This result is consistent with the oxidation state obtained from the white line shift of the XANES analysis and supports the presence of a high valence state of Ir 6+ under OER conditions. To understand the origin of the ultrahigh OER in the Sr 2 CoIrO 6−δ system, the authors made calculations with density-functional theory (DFT). They employed supercell calculations of SrCoO 3−δ embedded with Ir to show that the electron density surrounding the Ir-O-Co path increases with respect to the Co-O-Co path. This effect indicates stronger bonding between Ir/Co 5 d /3 d and O 2 p states, thereby supporting the important role of 5 d /3 d electrons in bonding synergistically to enhance OER activity. The authors concluded that the special double perovskite structure with multiple metal-active sites, three- dimensional ordering and a corner (or edge)- sharing network via O atoms is well suited to maximize synergistic effects and is expected to break through the limit of the OER bottleneck for water splitting. (Reported by Ashish Chainani) This report features the work of Zhiwei Hu, Jian-Qiang Wang, Linjuan Zhang and their collaborators published in Adv. Functional Mater. 31 , 2104746 (2021). TPS 44A Quick Scanning X-ray Absorption Spectroscopy TLS 11A1 Dragon MCD, XAS • XANES, EXAFS, XAS • Materials Science, Chemistry, Condensed-matter Physics References 1. L. Li, H. Sun, Z. Hu, J. Zhou, Y.-C. Huang, H. Huang, S. Song, C.-W. Pao, Y.-C. Chang, A. C. Komarek, H.-J. Lin, C.-T. Chen, C.-L. Dong, J.-Q. Wang, L. Zhang, Adv. Functional Mater. 31 , 21047746 (2021). 2. J.-H. Choy, D.-K. Kim, S.-H. Hwang, G. Demazeau, D.-Y. Jung, JACS 117 , 8557 (1995).