Supplementary Information

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1 Supplementary Information Oxygen Evolution Activity and Stability of Ba 6 Mn 5 O 16, Sr 4 Mn 2 CoO 9 and Sr 6 Co 5 O 15 : the influence of transition metals coordination Alexis Grimaud 1, Christopher E. Carlton 1, Marcel Risch 1, Wesley T. Hong 1,2, Kevin J. May 1,3 and Yang Shao-Horn 1,2,3* 1 Electrochemical Energy Laboratory, 2 Department of Materials Science and Engineering, 3 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, United States *shaohorn@mit.edu S1

2 Table S1. Goldschmidt tolerance factor t calculated for different orthorhombic, pseudocubic and hexagonal perovskites using Shannon et al. ionic radius. 1 Note that tolerance factor close to the limit value (i.e. SrMnO3) can lead to two polymorphs: pseudocubic or hexagonal. Orthorhombic Pseudocubic Hexagonal LaFeO LaNiO Ba6Mn5O LaMnO LaCoO Sr4Mn2CoO LaCrO SrCoO Sr6Co5O LaTiO SrFeO BaCoO SrMnO BaMnO CaMnO SrMnO S2

3 Table S2. Specific surface area measured by the Brunauer Emmet and Teller (BET) analysis (single point analysis). Specific surface area (m 2 /g) Ba6Mn5O Sr4Mn2CoO9 0.4 Sr6Co5O LaCoO3 0.7 La0.4Sr0.6CoO3 0.3 LaMnO3+δ 0.6 CaMnO3 0.9 Co3O4 20 MnO2 2.7 S3

4 Table S3. Estimation of the eg orbitals filling for the studied transition metal oxides. Compounds TM oxidation and spin state ref eg parentage orbitals filling Ba6Mn5O16 Mn 4+ LS 2 0 Sr4Mn2CoO9 Mn 4+ LS 0 Co 2+ HS 3 2 Sr6Co5O15 Co 4+ LS 0 Co 2+ HS 4 2 LaCoO3 Co 3+ IS 5 1 La0.4Sr0.6CoO3 Co 3+ IS 0.4 Co 4+ HS LaMnO3+δ Mn 3+ IS 7 1-δ CaMnO3 Mn 4+ LS 8 0 Co3O4 Co 3+ LS 0 Co 2+ LS 9 3 MnO2 Mn 4+ LS 10 0 S4

5 Space Group Lattice Parameters a (Å) b (Å) c (Å) Ba6Mn5O16 Cmca (3) (6) (1) Sr4Mn2CoO9 P (5) (5) Sr6Co5O15 R (5) (1) Figure S1. X-ray diffraction patterns of a) Ba6Mn5O16, b) Sr4Mn2CoO9 and c) Sr6Co5O15. The space group and lattice parameters obtained by LeBail method are listed in the table. They are consistent with values previously reported. 2-4 S5

6 Figure S2. HRTEM images of surface regions of a) Ba 6 Mn 5 O 16, b) Sr 4 Mn 2 CoO 9 and c) Sr 6 Co 5 O 15 with a fast Fourier transformation analysis as well as a perspective view of a schematic representation of their structure along the chain direction. The occupation of the effective d-block levels for a low-spin Mn 4+ in corner-shared octahedra (c-s), a high-spin Co 2+ in a distorted prism (P) site and a low-spin Co 4+ in c-s octahedra. S6

7 Figure S3. X-ray absorption spectra at the O K-edge measured a) in the total electron yield mode (TEY; surface sensitive) and b) in the total fluorescence yield mode (TFY; bulk sensitive) for LaMnO3+, LaCoO3, Ba6Mn5O16, Sr4Mn2CoO9 and Sr6Co5O15. O K-edge involves the transition of one electron from the O-1s band to various unoccupied and partially occupied molecular orbitals; A: Co or Mn-3d orbitals hybridized with O-2p, B: Ba-5d, Sr-4d or La-5d hybridized with O-2p and C: Co or Mn-4sp hybridized with O-2p. S7

8 Figure S4 X-ray absorption spectra at the a) Mn L-edge for LaMnO3.08, Ba6Mn5O16 and Sr4Mn2CoO9 and b) Co L-edge for LaCoO3, Sr4Mn2CoO9 and Sr6Co5O15 measured in the total fluorescence yield mode (TFY; bulk sensitive) compared to the total electron yield mode (TEY; surface sensitive). The peak amplitude observed for TFY mode is typically lower than for TEY mode, most likely due to self-absorption in the TFY measurements. 11,12 S8

9 Figure S5. Tafel plot of the second cycle for LaMnO 3+δ, Ba 6 Mn 5 O 16, CaMnO 3, MnO 2 and Sr 4 Mn 2 CoO 9. All measurements were performed using ink containing AB carbon, Nafion and oxide particles with a loading of 0.25 mg/cm 2 disk supported on a glassy carbon electrode in O 2 - saturated 0.1 M KOH electrolyte. Error bars were obtained by measuring at least 3 identical electrodes. S9

10 Figure S6. Representative cyclic voltammograms (10 mv/s) for a) Ba6Mn5O16, b) Sr4Mn2CoO9 and c) Sr6Co5O15. All measurements were performed using ink containing, AB carbon, Nafion and oxide particles with a loading of 0.25 mg/cm 2 disk supported on a glassy carbon electrode in O2-saturated 0.1 M KOH electrolyte. S10

11 Figure S7. (left) HRTEM images and (right) FFTs of a) Sr4Mn2CoO9 and b) Sr6Co5O15 and c) Ba6Mn5O16, electrodes after 25 cycles between 1.1 and 1.7 V vs. RHE at 10 mv/s. These electrodes contained Nafion and AB carbon and were tested in O2-saturated 0.1 M KOH electrolyte, loading was 0.25 mg/cm 2 of GCE. Possible transition metal coordinations on the near-surface region is given for each compound. S11

12 Figure S8. (a) Schematic representation of the structure of Ca3Co2O6 and b) 1 st and 5 th cyclic voltamograms (10 mv/s) of Ca3Co2O6 showing the oxidation wave occurring during the first cycle. Electrode measurements were performed using ink containing, AB carbon, Nafion and oxide particles with a loading of 0.25 mg/cm 2 disk supported on a glassy carbon electrode in O2-saturated 0.1 M KOH electrolyte S12

13 Figure S9. Representative cyclic voltammograms (10 mv/s) for CaMnO3, MnO2 and Co3O4. All measurements were performed using ink containing, AB carbon, Nafion and oxide particles with a loading of 0.25 mg/cm 2 disk supported on a glassy carbon electrode in O2- saturated 0.1 M KOH electrolyte. S13

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