Supplementary Figure S1 Scheme and description of the experimental setup. UHV-EC experiment includes the sample preparation and STM measurement in

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1 Supplementary Figure S1 Scheme and description of the experimental setup. UHV-EC experiment includes the sample preparation and STM measurement in UHV environment and transfer of the sample to the EC cell in a clean and controlled atmosphere. UHV sample preparation (1) consists of substrate preparation by means of sputtering and annealing the single-crystal metal surfaces to have atomically clean and defined electrode supports. Then, organic molecules are deposited by organic molecular beam epitaxy from a resistively heated quartz crucible at a sublimation temperature of 465 K. Finally the metal centers are incorporated by using an electron-beam heating evaporator. UHV STM characterization (2): is employed to check the network formation and determine both structure and coverage. Using the transfer system (3) the sample is transported from the UHV chamber to an electrochemical (EC) cell in a clean and controlled environment. Our transfer system uses a pre-chamber, which is located 1

2 between the EC cell and the UHV main chamber. Finally, Electrochemical Experiments (4) are performed in the EC cell. Similar to other UHV- EC systems, 47, 48 the sample surface is facing down and the EC measurements are performed using the hanging meniscus configuration. 2

3 Supplementary Figure S2 STM images of TMA-Fe and TMA-Mn networks. STM comparison between (a-c) TMA-Fe and (d-f) TMA-Mn networks on Au(111). (Images sizes: a,d) nm 2, b,e)12 12 nm 2, and c,f) nm 2.) Both metal species form identical network structures with TMA on Au(111). 3

Supplementary Figure S3 STM images and proposed unit cell of TMA-Fe and PBP-Fe networks. a, 3.5 3.5 nm STM topograph of TMA-Fe on Au(111). Rhomboidal unit cell is indicated.

4 Supplementary Figure S3 STM images and proposed unit cell of TMA-Fe and PBP-Fe networks. a, nm STM topograph of TMA-Fe on Au(111). Rhomboidal unit cell is indicated. b, TMA-Fe model with representation of the unit cell. c, nm STM topography of PBP-Fe on Au(111). Rhomboidal unit cell is indicated. d, PBP model with representation of the unit cell.. 4

The system is reversible and stability is reached after 5 cycles. All following cycles show a clear stability of the system.

5 Supplementary Figure S4 Reversibility and stability of the ORR. Polarization curves in oxygen saturated 0.1M NaOH solution at 0.05 V/s for TMA-Fe for cycle 1(red dot line), 2 (red dash line), 3 (red solid line), 5 (purple line) and 10 (grey line). The system is reversible and stability is reached after 5 cycles. All following cycles show a clear stability of the system. There is a small increase in the catalytic current which is associated with oxygen diffusion due to the experimental set-up (meniscus configuration). 5

05 V/s for: a, Au(111) (grey line), PBP (cyan), Fe clusters (purple), PBP-Fe (green) and PBP-Fe in 10 mm H 2 O

6 Supplementary Figure S5 Polarization curves for the ORR blank experiments. Comparison of polarization curves in oxygen saturated 0.1M NaOH solution at 0.05 V/s for: a, Au(111) (grey line), PBP (cyan), Fe clusters (purple), PBP-Fe (green) and PBP-Fe in 10 mm H 2 O 2 (dashed green); b, Au(111) (grey line), TMA (green), Fe clusters (purple), and TMA-Fe (brown); c, Au(111) (grey line), TMA (green), Mn clusters (orange), and TMA-Mn (dark blue). 6

Supplementary Figure S6 Polarization curves on Au(111). Polarization curves for Au(111) at 0.05 V/s in oxygen saturated 0.1 M NaOH solution with (orange line) and without (grey line) 10 mm H 2 O 2.

7 Supplementary Figure S6 Polarization curves on Au(111). Polarization curves for Au(111) at 0.05 V/s in oxygen saturated 0.1 M NaOH solution with (orange line) and without (grey line) 10 mm H 2 O 2. It is worthwhile to mention that H 2 O 2 has a complicated electrochemistry in alkaline media where several processes simultaneously take place at the electrode, i.e., a) disproportionation producing HO 2 and O 2, b) electrochemical oxidation of HO 2 to O 2, c) electrochemical reduction of the O 2 produce to HO 2, and finally d) the electroreduction of HO 2 to produce OH. In all four cases (Au(111), PBP-Fe, TMA-Fe and TMA-Mn) the addition of 10 mm H 2 O 2 increases the current density. This increase could be traced back to a combination of all the above processes due to a local increase of the O 2 concentration (by the reactions in a) and b)) and the subsequent reduction of O 2 to produce HO 2 (point c). In the ORR on Au(111) the small amount of the generated H 2 O 2 does not produce an electrochemical signal in its reduction to OH. Moreover, the latter reduction does not occur in the potential window in which the measurements were carried out for the networks. Compared to the bare Au(111) TMA-Fe is able to reduce the generated HO 2 to OH under the same conditions (O 2 saturated concentration) in the employed potential window. 7

8 Supplementary Table S1 Dimensions of TMA-Fe and PBP-Fe networks. Unit cell dimension, Fe-Fe and Fe-ligand distances and equivalent iron coverage taken from the models in Fig S5. Both distances are in agreement with values found for other carboxylic Fe complexes either in biomolecules 49 17, 35 or other 2D MOCNs 2D-MONCs Unit cell dimensions Fe O/ Fe N distances Fe Fe distance Fe monolayer a=1.91±0.05 nm TMA Fe PBP Fe b=0.95±0.05 nm =110±5 a=1.60±0.05 nm b=2.22±0.05 nm =41±5 2.2±0.5 Å 3.2±0.5 Å ±0.5 Å 5.8±0.5 Å

9 Supplementary References 47. Hoster, H. E., Gasteiger, H. A. Ex-situ surface preparation and analysis: Transfer between UHV and electrochemical cell. In: Wolf V, Arnold L, A. GH (eds). Handbook of Fuel Cells Fundamentals, Technology and Applications, vol. 2. John Wiley & Sons, Ltd: Chichester, 2003, pp (2003) 48. Reniers, F. The development of a transfer mechanism between UHV and electrochemistry environments. J. Phys. D: Appl. Phys. 35, R169-R188 (2002). 49. Hwang, J., Krebs, C., Huynh, B. H., Edmondson, D. E., Theil, E. C., Penner-Hahn, J. E. A Short Fe-Fe Distance in Peroxodiferric Ferritin: Control of Fe Substrate Versus Cofactor Decay? Science 287, (2000). 9

Water clustering on nanostructured iron oxide films

ARTICLE Received 12 May 2013 Accepted 22 May 2014 Published 30 Jun 2014 Water clustering on nanostructured iron oxide films Lindsay R. Merte1,2, Ralf Bechstein1, W. Guowen Peng3, Felix Rieboldt1, Carrie