Supporting Information for

Single-Site Copper Electrocatalyst for Reducing CO2 to Methane

Teng Zhang, Sumit Verma, Soojeong Kim, Timothy T. Fister, Paul J. A. Kenis, and Andrew A. Gewirth

1. Preparation of Cu-N catalysts

All chemicals are used as received. In a typical procedure, Cu(ClO4)2·6H2O (74.1 mg, 0.2 mmol) and 3-amino-5-hydroxypyrazole (AHP, 39.6 mg, 0.4 mmol) were dissolved in 2 mL of water and 8 mL of methanol. Carbon black (Vulcan XC72R, 100 mg) was then added to the solution. The mixture was sonicated for 30 minutes and then stirred overnight. After that, the carbon composite was centrifuged out, washed with methanol and dried under vacuum. Pyrolysis was performed in a tube furnace at desired temperature (400 or 600 °C) for one hour with nitrogen passing over at a flow rate between 40-80 mL/min.

The Cu-N cathode was prepared with brush-painting. In a typical procedure, 3 mg of the catalyst was dispersed in a mixture of water (200 µL), isopropanol (200 µL), and 5% Nafion solution (6 µL in ethanol). The mixture was sonicated for 15 minutes and then painted evenly on a piece of gas-diffusion carbon paper (Sigracet® GDL 35 BC, 2.5×1 cm2) with a size #1 brush. The carbon paper cathode was ready to use after air-drying. Loading of copper was calculated to be 0.06-0.16 mg/cm2 on cathode.

Figure S1. PXRD patterns of synthesized Cu-N catalysts with different Cu wt%.

2. Flow Cell Setup and Product Analysis

Electrolysis is performed using an AUTOLAB PGSTAT-30 potentiostat with a 3-electrode setup. A Ag/AgCl reference electrode was used. An IrO2-coated carbon paper electrode (2.0 mg/cm2) was used as the counter electrode.

In the flow cell, the cathode and anode chambers were separated by a fumapem® FAA-PK-75 anion exchange membrane. Catholyte and anolyte (1M KOH, pH 13.6) were collected separately at the outlets. Carbon dioxide was passed over the back side of the gas-diffusion carbon paper cathode with the gas outlet connected to a Thermo Finnegan Trace GC station, (TCD and FID detectors) for the analysis of products. The flow rate was controlled to 7 sc mL/min. Liquid products in catholyte were analyzed by NMR with DMSO added as an internal standard.

3. XPS analysis of the Cu-N catalysts

XPS was performed using a Physical Electronics 5400 spectrometer with a monochromatic Al Kα (1486.6 eV) X-ray source. All binding energies were referenced to graphitic carbon at 284.5 eV.

Figure S2. Cu LMM Auger peak positions of Cu-N-5%-RT, Cu-N-5%-400, and Cu-N-5%-600.

4. XAS analysis of the Cu-N catalysts

X-ray absorption spectroscopy was carried out at beamline 10BM (MRCAT) at the Advanced Photon Source at Argonne National Lab using a double crystal silicon monochromator with 50% detuning for harmonic rejection. A copper metal foil was measured using a reference ion chamber for energy calibration.

Figure S3. XAS spectrum of Cu-N-5%-400.

Figure S4. EXAFS region (k space) of the Cu-N-5%-400 XAS spectrum (k weight 2).

Figure S5. R-space EXAFS spectra of Cu-N-5%-400 (black), Cu2O (blue) and Cu (red).

Table S1. EXAFS Fitting results of Cu-N-5%-400.Fitting range k 3.000-12.844 Å-1

R 1.0-3.0 ÅΔE0 (eV) -1.54±1.34

R (Cu-N) (Å) 1.946±0.009

R-factor 1.7% σ2 (Cu-N) (Å2) 0.0065±0.0012

S02 0.912±0.096 C.N. (Cu-N) 4

5. Surface EDS analysis of the Cu-N catalysts

Figure S6. SEM image and EDS mapping of Cu-N-5%-400 carbon paper electrodes before electrolysis.

Figure S7. EDS quantification of surface elements: before electrolysis (left) and after electrolysis (right).

Table S2. EDS quantification of surface elements.Element line Before electrolysis After electrolysis

weight % atom % weight % atom %

C K 78.35±0.35 91.41±0.41 44.52±0.20 67.61±0.30

O K 1.89±0.22 1.66±0.19 6.39±0.19 7.28±0.21

F K 3.83±0.22 2.82±0.16 4.84±0.14 4.65±0.13

S K 2.75±0.27 1.20±0.12 1.43±0.03 0.81±0.02

K K N/A N/A 41.00±0.22 19.12±0.10

Cu L 13.18±0.29 2.91±0.06 1.83±0.10 0.53±0.03

6. Additional electrochemical data

Figure S8. Current densities (a), Faradaic efficiencies (b: CH4; d: C2H4; f: CO; h: H2) and partial current densities (c: CH4; e: C2H4; g: CO) of CO2 reduction products for catalysts with different Cu wt%.

7. Comparison of CO2 reduction performance

Table S3. Reported CH4-selective CO2 reduction performance of copper-based catalysts/electrodescatalyst/electrode E (V) vs. RHE ICH4 (mA/cm2) FE (CH4) C2H4:CH4 ratio reference

Cu foil -1.3 17 50% 1:20 [1]

Cu (111) -1.12 2.3 46.3% 1:5 [2]

nanoscale Cu -1.35 ~9.5 76% not reported [3]

CuPd alloy -1.0 1.8 36% 0 [4]

Cu-porphyrin -0.976 13.2 27% 1:1.6 [5]

Cu-P-ED -2.1 38 85% 1:65 [6]

Cu-CeO2 -1.8 ~32 58% ~1:3.6 [7]

Cu-ade MOF -1.6 15 50% 1:2 [8]

Cu-Oh NCs -1.25 0.8 55% 1:4.6 [9]

Cu-modified Ag -1.1 4.9 ~55% 1:5.5 [10]

m-Cu NPs -1.3 ~3.7 50% 1:5 [11]

Cu-N-5%-400 -1.0 ~100 42% 1:4 this work

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