Supporting Information. Electrochemical Reduction of CO 2 Catalyzed by Fe-N-C Materials: a Structure-Selectivity Study

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1 Supporting Information Electrochemical Reduction of CO 2 Catalyzed by Fe-N-C Materials: a Structure-Selectivity Study Tran Ngoc Huan, 1 Nastaran Ranjbar, 2 Gwenaëlle Rousse, 3 Moulay Sougrati, 2 Andrea Zitolo, 4 Victor Mougel, 1 Frédéric Jaouen, 2 Marc Fontecave 1 1 Laboratoire de Chimie des Processus Biologiques, UMR CNRS 8229, Collège de France, Université Pierre et Marie Curie, 11 Place Marcelin Berthelot, Paris, France. 2 Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Université Montpellier, 2 place Eugène Bataillon Montpellier, France 3 Laboratoire Chimie du Solide et Energie, CNRS FRE 3677, Collège de France, Université Pierre et Marie Curie, 11 Place Marcelin Berthelot, Paris, France. 4 Synchrotron SOLEIL, L'Orme des Merisiers Saint-Aubin - BP 48, Gif-sur-Yvette, France Corresponding authors: * frederic.jaouen@univ-montp2.fr, marc.fontecave@college-de-france.fr Table of contents General considerations... S2 Material syntheses and electrode preparation... S3 Fe 1.0 w... S3 Fe 0.5 d... S3 Fe 0.5 d S4 Fe 0.0 d (metal free)... S4 Deposition of Fe-N-C materials on porous carbon paper (gas diffusion layer-gdl)... S4 Electrocatalytic activity... S4 Faradaic yields... S4 Long term electrolysis... S4 Figures and Table... S5 S1

2 General considerations Electrochemical measurements were performed in a three-electrode two-compartment cell using a Bio-logic SP300 potentiostat. Ag/AgCl/3M KCl (hereafter abbreviated as Ag/AgCl) was used as the reference electrode and placed in the same compartment as the working electrode. A platinum counter electrode was placed in a separate compartment connected by a glass-frit and filled with the electrolytic solution. The change of potential scale vs. RHE was done according to Eq. (1): E(V vs. RHE) = E(V vs. Ref.) + E(V of Ref. vs. NHE) ph (1) where E(V of Ref. vs. NHE)=0.205 V and the ph of 0.5 M NaHCO 3 -saturated CO 2 is 7.2. H 2 measurements were performed by gas chromatography on a Shimadzu GC-2014 equipped with a Quadrex column, a Thermal Conductivity Detector and using N 2 as a carrier gas. Carbon monoxide, methane and other volatile hydrocarbons from the gas phase were analyzed using a gas chromatograph (Shimadzu GC-2010) equipped with a methanizer, a flame induction detector (FID) and a shincarbon ST (Restek) column. Methanol was assayed by gas chromatography (Shimadzu GC 2010) using an Rtx-1 column (Restek) and a flame induction detector (FID). Formate, oxalate and glyoxylate concentrations were determined by ionic exchange chromatography (883 Basic IC, Metrohm). SEM images were acquired using a Hitachi S-4800 scanning electron microscope. TEM and HRTEM images were obtained on a JEM-2100F transmission electron microscope (JEOL, Japan) with an accelerating voltage of 200 kv. The X-ray powder diffraction (XRD) patterns were recorded using an X'Pert Pro Panalytical diffractometer equipped with either a Cu Kα radiation source (λ Kα1 = Å, λ Kα2 = Å) or a Co Kα radiation source (λ Kα1 = Å, λ Kα2 = Å) with an X Celerator detector. Rietveld refinements were performed with the Full-Prof suite of programs. Fe K-edge X-ray absorption spectra were collected in transmission mode at room temperature at SAMBA beamline (Synchrotron SOLEIL). The beamline is equipped with a sagittaly focusing Si 220 monochromator and two Pd-coated mirrors that were used to remove X-rays harmonics. The samples were pelletized as disks of 10 mm diameter using Teflon powder (1 µm particle size) as a binder, and using a mass ratio 50/50 for Teflon and the catalyst sample. S2

3 For each spectrum acquisition, the energy was calibrated with an iron foil to correct any change in beam energy during the experiments. Between three and four EXAFS spectra were acquired for each sample, and then averaged to increase signal/noise ratio. All experimental spectra were normalized and analyzed identically with Athena software in order to derive the Fourier-transform EXAFS spectra. Material synthesis and electrode preparation All catalyst precursors prepared as described below were pyrolyzed at 1050 C in flowing Ar for 1 h, in flash pyrolysis mode (oven pre-heated at set temperature), yielding the different catalysts. Due to a mass loss of wt.% during pyrolysis in Ar, caused by volatile products formed from ZIF-8 and phen, the iron content in final catalysts is circa three times the iron content in the catalyst precursors, e.g. Fe 1.0 wet contains ca 3 wt % Fe. Fe 1.0 w The catalyst precursor was prepared from a Zn(II) zeolitic imidazolate framework (Basolite Z1200 from BASF, labelled ZIF-8), Fe(II) acetate (Fe(II)Ac), and 1,10- phenanthroline (phen) and consisted in the consecutive addition of Fe(II)Ac (31.45 mg), phen (200 mg) and ZIF-8 (800 mg) in a 1:2 ethanol:water solution where the Fe(phen) 3 complex formed. After evaporation of the solvents, the catalyst precursor was collected. This dry powder (1 g) was then ball-milled in a zirconium oxide crucible (45 ml) filled with 100 zirconium-oxide balls of 5 mm diameter, the crucible was then sealed under air and placed in a planetary ball-miller (FRITSCH Pulverisette 7 Premium) to undergo 4 cycles of 30 min of ball-milling at 400 rpm milling speed. The resulting catalyst precursor was pyrolyzed in Ar at 1050 o C for 1 h. Last, the catalyst powder was acid-washed with a ph 1 H 2 SO 4 solution, yielding Fe 1.0 w. Fe 0.5 d The catalyst Fe 0.5 d was synthesized similarly to Fe 1.0 w except for the omission of i) the initial wet impregnation step and ii) the final acid-washing step. Weighed amounts of the powders of Fe(II)Ac (15.7 mg), phen (200 mg) and ZIF-8 (800 mg), all previously dried overnight at 80 C, were ball-milled identically to Fe 1.0 wet and the resulting powder pyrolyzed at 1050 C in Ar for 1 h. The pyrolysis was terminated by opening the split hinge oven and removing the quartz tube. The catalyst powder was collected and gently ground with a marble mortar and pestle. No other post-treatment was applied to the catalyst. S3

4 Fe 0.5 d Fe was obtained by subjecting Fe 0.5 d to a pyrolysis in NH 3 for 5 min at 950 C in flash pyrolysis mode. The short duration of pyrolysis was accurately controlled with a previously described method. The mass loss of carbon during NH 3 pyrolysis was %, further increasing the iron content in the final catalyst. Fe 1.0 d and Fe 4.0 d These two catalysts were prepared identically to Fe 0.5 d except for the mass of iron acetate introduced in the ball-milling crucible, mg for Fe 1.0 d and mg for Fe 4.0 d. Fe 0.0 d (metal free) ZIF-8 (800 mg) and phen (200 mg) were weighed, introduced in the crucible as dry powders and ball-milled as described above for other catalysts. The resulting powder was pyrolyzed directly at 1050 C in flowing Ar for 1 h, in flash pyrolysis mode. Deposition of Fe-N-C materials on porous carbon paper (gas diffusion layer- GDL) 1 mg of the Fe-N-C material was dispersed in a solution of 200 µl ethanol and 20 µl of a Nafion perfluorinated resin solution (5 wt. % in mixture of lower aliphatic alcohols and water, containing 5% water, Sigma-Aldrich). After sonication, the suspension was drop casted on a 1 cm 2 carbon paper electrode and dried in air for 1h. Electrocatalytic activity Faradaic yields Faradaic yields in H 2, CO and CH 4 were determined after 5 minutes electrolysis at a given potential and are represented in Figure 4, 6 and S6. Current densities over that period for Fe 0.5 d are represented in Figure S4. Production rates are represented in Figure S5. Long term electrolysis A long-term electrolysis (6h) was performed at -0.55V vs RHE in a constantly saturated CO 2 solution (ensured by continuous CO 2 bubbling during electrolysis) to investigate the stability of the catalyst. The production rate of CO was determined throughout the electrolysis by GC chromatography to determine the CO production rate. The variation of the current density vs. time is presented in Figure S7. S4

5 Figures and Table Figure S1: Powder XRD diffractograms of Fe 1.0 d (black), Fe 4.0 d (red), Fe 1.0 w (blue), Fe 0.5 d (green), Fe 0.5 d-950 (magenta) and Fe 0.0 d (orange). Figure S2: a) TEM images of Fe 4.0 d b) TEM image of Fe 0.5 d. c) SEM image of GDL (left) and Fe 0.5 d deposited on GDL (right). S5

6 Figure S3: Current density variation during electrochemical reduction of CO 2 in 0.5M NaHCO 3 using Fe 0.5 d/gdl at different potentials: -0.4V (black), -0.5V (red), -0.6V (blue), -0.7V (cyan), - 0.8V (magenta) and -0.9V vs. RHE (dark yellow). Figure S4: Production rates of CO (square) and H 2 (circle) using Fe 0.5 d (black-red) and metal-free Fe 0.0 d (blue-magenta) (determined after 5 minutes electrolysis in CO 2 -saturated 0.5M NaHCO 3 solutions, 1 cm 2 electrode). S6

7 Figure S5. Production rates for CO formation after 5 minutes CPE during CO 2 reduction in CO 2 saturated 0.5 M NaHCO 3 using Fe 0.5 d (black), Fe 0.5 d-950 (blue), Fe 1.0 d (red), Fe 1.0 w (cyan) and Fe 4.0 d (green). S7

8 Figure S6: Faradaic yield of CH 4 after 5 minutes electrolysis of CO 2 using Fe 0.5 d (black), Fe 0.5 d- 950 (blue), Fe 1.0 d (red). Figure S7: LSV of metal-free Fe 0.5 d in CO 2 -saturated 0.1 M (black), 0.5 M (red) and 1 M (blue) NaHCO 3 aqueous solutions. Figure S8: Current density variation during long term electrochemical reduction of CO 2 in 0.5M NaHCO 3 using Fe 0.5 d/gdl at -0.6V vs RHE. S8

9 Table S1: Isomer shift (IS) and quadrupole split (QS) values for the doublet components used for the fitting of the experimental Mössbauer spectra seen in figure 2. The IS and QS values are reported in mm s -1 and the IS-values are reported relative to the calibration made with an -Fe foil. Fe 0.5 d Fe 0.5 d-950 Fe 1.0 d Fe 1.0 w IS QS IS QS IS QS IS QS D D D S9