Very highly efficient reduction of CO2 to CH4 using metal-free N-doped carbon electrodes

We report the first work on the electrocatalytic reduction of CO2 to CH4 using metal-free N-doped carbon electrodes.

The method to prepare the Graphene/CP electrode was similar to that reported, 2,3 which is described briefly as follows. 1 mg commercial graphene was suspended in 1 mL ethanol with 10 μL Nafion D-521 dispersion (5 wt%) to form a homogeneous ink assisted by ultrasound. Then, it was spread onto the CP (1 cm  1 cm) surface by a micropipette and dried under room temperature. Here the CP was heated at 1000 °C in argon (Ar) atmosphere in 2 h.

Materials characterization
1 H and 13 C NMR spectra of the precursors of NGMs were recorded on a Bruker Avance III 400 HD spectrometer operating at 400 MHz for 1 H and 100 MHz for 13 C in d 6 -DMSO with TMS as an internal standard. X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromatic Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3×10 -10 mbar. Typically, the hydrocarbon C1s line at 284.8 eV from adventitious carbon is used for energy referencing.
The Raman spectra of the NGMs were obtained at room temperature in flamesealed capillary on a FT Bruker RFS 106/S spectrometer, equipped with a 514 nm laser, in the region from 4000 to 100 cm -1 with a resolution of 2 cm -1 .
Thermogravimetric (TG) measurements were conducted using a Pyris 1 thermogravimetry analyzer from room temperature to 700 °C at a heating rate of 10 °C·min -1 under a N 2 flow of 60 mL·min -1 with open alumina pans.
Differential scanning calorimetry (DSC) (Q-2000 TA Instruments) was used to determine the melting points of the NGMs at a heating rate of 10 °C/min under N 2 atmosphere.
The N 2 adsorption/desorption isotherms of the NGMs were determined using a Quadrasorb SI-MP system.
The morphologies of NGM/CP electrodes were characterized by a HITACHI S-4800 scanning electron microscope (SEM) and a JEOL JEM-2100F high-resolution transmission electron microscopy (HR-TEM).

linear sweep voltammetry (LSV) study
The electrochemical workstation (CHI 6081E, Shanghai CH Instruments Co., China) was used. LSV measurements were carried out in a single compartment cell with three-electrode configuration, which consisted of working electrode, a platinum gauze auxiliary electrode, and an Ag/Ag + (0.01 M AgNO 3 in 0.1 M TBAP-MeCN) reference electrode for IL or IL-based electrolytes. Before each experiment, all the electrodes were sonicated in acetone for 10 min and then washed with water and ethanol, followed by drying in N 2 atmosphere. In a typical experiment, the electrolyte was bubbled with N 2 or CO 2 for at least 30 min to form N 2 or CO 2 saturated solution. LSV measurement in gas-saturated electrolyte was conducted in the potential range of 0.6 to -1.6 V versus the standard hydrogen electrode (SHE) at a sweep rate of 20 mV/s. Slight magnetic stirring was applied in the process.

CO 2 reduction electrolysis and product analysis
The electrolysis experiments were conducted at 25 °C in a typical H-type cell with an Ag/Ag + reference electrode. The apparatus was similar to that used by other researchers, 3 and is schematically shown in Figure S6. In the experiments, the cathode and anode compartments were separated through a Nafion 117 proton exchange membrane. The IL (or IL-water) and H 2 SO 4 aqueous solution (0.5 M) were used as cathodic and anodic electrolytes, respectively. Under the continuous stirring, CO 2 was bubbled through the catholyte (2 mL/min) for 30 min before electrolysis. Then, potentiostatic electrochemical reduction of CO 2 was carried out with CO 2 bubbling (2 mL/min), and the gaseous product was collected using a gas bag and analyzed by gas chromatography (GC, HP 4890D), which was equipped with TCD and FID detectors using helium as the internal standard. The liquid product was analyzed in DMSO-d 6 with TMS as an internal standard by 1 H NMR (Bruker Avance III 400 HD spectrometer). The Faradaic efficiency of the products were calculated from GC analysis data. 4 The experiments were run at different potentials.

Results and discussion
TGA and DSC study: Fig. S1 shows the thermal gravimetric analysis curves of the NGM procedures. The data indicated that the NGM procedures had different thermal stability. However, all of them were stable below 200 o C. Table S2 shows the melting points of the NGM procedures determined by DSC, which show that the NGM procedures were solid at the experimental temperature of the electrochemical investigations. XPS study: The XPS spectra of NGMs are shown in Fig. S2. It can be known that N atoms are integrated into the structure of the materials. Taking NGM-1 as an example, the N1s peak can be resolved into three compositions at binding energies of 398.0 eV (pyridinic N), 400.0 eV (pyridonic and pyrrolic N) and 401.3 eV (quaternary N), respectively (inset in Figure S2A). 5 The extent of the doped N can be controlled by varying precursors.

Raman spectroscopy study:
In Raman spectra of NGMs, D band is a breathing mode with A1g symmetry involving phonons near the K zone boundary. In the perfect graphite structure, it is forbidden. But D band becomes active in the presence of defects such as edges, structural disorders and functional groups. 6 It has been reported that the distortion in the carbon hexagonal lattice may lead to the increase in the intensity of D band. In the meantime, G band respects E2g symmetry that involves the in-plane bond-stretching motions of pairs of sp 2 C, which does not require the presence of hexatomic ring. 2 In fact, the band position, the intensity ratio of D band and G band (I D /I G ), as well as the full width at half maximum (FWHM) of D band or G band are reported to be related to the disorder or defects in the graphitic structure. 6 Fig . S4 shows  XRD study: Fig. S5 gives the XRD patterns of different NGMs. Judging from the Bragg reflection of the (002) peak, the present NGMs with high N content display a more positive shift of the diffraction position. In particular, NGM-1 shows a highintensity (002) diffraction peak centered at 26.2 o , along with clearly observable (100) and (110) reflection characteristics for graphitic structures. The calculated interlayer distance (d 002 ) of 0.343 nm is very close to that of graphite. 7