Supporting Information Incorporation of Nitrogen Defects for Efficient Reduction of CO 2 via Two-electron Pathway on Three Dimensional Graphene Foam

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1 Supporting Information Incorporation of Nitrogen Defects for Efficient Reduction of CO 2 via Two-electron Pathway on Three Dimensional Graphene Foam Jingjie Wu 1, Mingjie Liu 1, Pranav P. Sharma 2, Ram Manohar Yadav 1, Lulu Ma 1, Yingchao Yang 1, Xiaolong Zou 1, Xiao-Dong Zhou 2, Robert Vajtai 1, Boris I. Yakobson 1*, Jun Lou 1*, Pulickel M. Ajayan 1* 1 Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005, USA 2 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29201, USA These authors contributed equally to this work Correspondence: biy@rice.edu (B. I. Yakobson), jlou@rice.edu (J. Lou) and ajayan@rice.edu (P. M. Ajayan) Item Page No. Experimental DFT Modeling Figure S1. Raman spectra of pristine graphene and N-doped graphene Figure S2. A representative XPS for NG-800 Figure S3. Partial current density of CO and HCOO - Figure S4. Tafel plots of NG electrodes for CO 2 reduction. Figure S5. CVs of NG and G electrodes. Figure S6. The ground state structure of COOH*. Figure S7. Free energy diagram of HER on NG. Table S1. Apparent exchange current density. Table S2. Gas permeability of gas diffusion electrodes. S2-S6 S6-S7 S3 S4 S4 S5 S5 S8 S10 S7 S7 S1

2 Experimental Materials synthesis: The three-dimensional (3D) graphene foam was synthesized by chemical vapor deposition (CVD) of dissociating methane to deposit on Ni foam at 1000 o C and ambientpressure. Ar/H 2 is purging through the whole synthesis process. The graphene was collected by etching Ni substrate using concentrated HCl solution, followed by vacuum filtering and freezing dry. The as grown graphene foam is free of Ni contamination as evidenced in our previous report. 1 N-doped graphene (NG) was prepared by post-doping the graphene foam using a solid precursor of g-c 3 N 4 at the temperature range of ºC to vary the N atomic concentration. Structural and microstructural characterizations: The microstructures of NG were characterized by FEI Quanta 400 FEG scanning electron microscope and JEOL 2100 FEG transmission electron microscope. The structural disorder related to N doping was studied by the Raman spectra (Renishaw invia Raman microscope, nm laser excitation). The N content and boning state was analyzed by PHI Quantera X-ray photoelectron spectrometer. Electrode preparation and electrochemical measurement: The NG gas diffusion electrodes (NG GDEs) were fabricated from the NG. The details of GDE preparation can be found in our previous work. 2 The GDEs electrode area is 3.8 cm 2 and have a NG loading between 0.3~0.5 mg cm -2. The CO 2 reduction study was performance in a home-designed full electrochemical cell composed of NG GDEs s as the cathode and Pt GDE as the anode, and a Nafion membrane between cathode and anode for separation of gas reactant and product. The cathode was supplied with CO 2 at a rate of 45 SCCM while Pt anode is supplied with H 2 at a rate of 50 SCCM. The Pt GDE has Pt loading of 0.3 mg cm -2. The overpotential of H 2 oxidation on Pt GDE is negligible 3, so the cathode potential is scaled to reversible hydrogen electrode after ir correction, E RHE = E cell + ir pH. The CO 2 reduction occurs in the electrolyte of 0.1 M KHCO 3 at room temperature and ambient pressure. 2 The CO 2 reduction was conducted under the potentiostatic mode for 30 min at each potential controlled by the potentiostat/galvanostat (Solartron 1470E). Electrochemical impedance spectroscopy was carried out to measure the Ohmic resistance of the full electrochemical cell at open circuit voltage, with an amplitude of 5 mv and frequency range from 100 khz to 0.1 Hz. Product quantification: The CO gas concentration was measured by directly venting the outlet of the cathodic compartment into the gas-sampling loop of the gas chromatography (GC, Inficon S2

3 Micro 3000 GC). A GC run repeats every 10 minutes. The average value of three measurements was taken as the gas concentration for Faradaic efficiency calculation. Liquid products were identified using 1D 1 H NMR spectroscopy (Varian Mercury/VX 400 MHz spectrometer) in a self-programmed WET solvent suppression pulse sequence. The sodium 3-(trimethylsilyl) propionate 2, 2, 3, 3-d (4) (TSP) was used as the internal standard to quantify the liquid product concentration. 4 NG1000 Intensity (a.u.) NG900 NG800 NG700 Graphene Raman shift (cm -1 ) Figure S1. Raman spectra of pristine graphene and N-doped graphene. Raman spectra were taken on NG at the doping temperature of ºC and a pristine graphene sample for reference. The pristine graphene spectrum shows only G and 2D bands at 1583 and 2710 cm -1 without a D peak, implying negligible defect of the CVD-grown graphene. Raman spectra of NGs show a small D peak at 1353 cm -1 attributed to the little structural distortion caused by N-doping. The low peak intensity ratio of D band to G band of (Figure S1) suggests that NG maintains very good crystalline structure in agreement with the TEM results. The peak intensity ratio of G band and 2D band reveals that NG has 2-4 layers. S3

4 Figure S2. A representative XPS for NG-800. (a) Survey scan and (b) fine scan of C 1s. The survey spectra of NGs clearly show the N peak at around 400 ev in addition to a dominant graphitic C peak at ev (Figure S2). The high-resolution asymmetric peak of C1s was fitted into a main peak and four additional peaks, which correspond to the C=C bond (284.5 ev), C=N bond (285.2 ev), C-C bond (285.5 ev), C-N bond (286.2 ev) and C-O bond (286.7 ev), indicating successful incorporation of heteroatom N into the graphene hexagonal plane (Figure S2). Figure S3. (a) Partial current density of CO versus potential. (b) Partial current density of HCOO - versus potential. Red curve for NG-700, blue curve for NG-800, green curve for NG-900 and black curve for NG S4

5 log (-j/ma cm -2 ) NG mv/dec NG mv/dec NG mv/dec NG mv/dec Overpotential (V) Figure S4. Tafel plots of NG electrodes for CO 2 reduction. S5

6 Figure S5. Cyclic voltammograms of different electrodes studied in 0.1 M Na 2 SO 4 at various scan rates for estimation of double layer capacitance. (a) Carbon paper substrate. (b) Pristine graphene. (c) NG-700. (d) NG-800. (e) NCNT. (f) Au powder. (g) Ag powder. (h) Double layer capacitance. Electrodes of (e), (f) and (g) are from reference 11 in the main text. The geometric area of all the electrodes is 1 cm 2. S6

7 Table S1. Apparent exchange current density of N-doped graphene, N-doped CNTs, and metal electrodes. Electrode Capacitance* (µf) j 0 normalized to geometric area (A/cm 2 ) j 0 normalized to capacitance (A/F) Carbon paper Pristine graphene NG E NG E NCNT E Au powder E Ag powder E *The capacitance result is extract from the cyclic voltammogram data in Figure S5. Table S2. Gas permeability measured at a compression ratio of 10%. Electrode Gas permeability ( m 2 ) Carbon gas diffusion layer 6.2 Pristine graphene 3.6 NG NG NG NG Au powder 4.2 Ag powder 3.9 S7

8 DFT modeling Computational methods: The structures are relaxed and the electronic energies are computed by density functional theory (DFT) implemented in Vienna Ab-initio Simulation Package (VASP) with Perdew, Burke and Ernzerhof functional 5. The 5 6 graphene doped by N is applied in this model. The K-point mesh is The aperiodic directions have a 20 Å cell spacing to eliminate the interaction of periodic images of the system. Structures optimization: Pure and N-doped graphene with three types N are relaxed before attaching any molecules. The structural relaxation was performed until all forces were less than 0.01 ev/å. The ground state structures of COOH*, CO*, H* and HCOOH* adsorbed on NG are determined by searching all the possible configurations on possible active sites and find the lowest energy one. The active sites which bond to COOH* and H* directly in different type of N-doped graphene are identical for both groups. For graphitic NG, the active site is the carbon atom next to nitrogen. For other types of NG, the active site is the nitrogen atom. CO* is physical adsorbed on NG except for pyrrolic N in which the CO* is chemically bonded. The ground state structures of the intermediate state COOH* on NG are shown in Figure S5. Figure S6. The ground state structure of COOH* adsorbed on (left to right) graphitic N-; triplepyridinic N-; single-pyridinic N-; pyrrolic N-doped graphene. C, N, O and H atoms are represented by grey, blue, red and white spheres. S8

9 Free energy calculation: Based on computational hydrogen electrode (CHE) model 6, 7 the chemical potential of the proton-electron pairs can be calculated as a function of applied voltage + =. The free energy for adsorbates and non-adsorbed gas-phase molecules is calculated as = + +. The E elec is the electronic energy calculated by DFT; E ZPE is the zero point energy estimated under harmonic approximation by taking the vibrational frequencies of adsorbates or molecules as calculated within DFT. The and are small for the adsorbates compared to E elec and E ZPE, and thus neglected in this study for adsorbates. For non-adsorbed gas-phase molecules, the entropies of H 2 (g), CO 2 (g) and CO (g) at 1 atm are used, while the entropy of H 2 O (l) is calculated at atm, which corresponds to the vapor pressure of liquid water at 300 K. Due to the use of PBE functional, the non-adsorbed gas-phase CO molecule has to include a ev correction. 7 The solvation effect has been considered for COOH* and CO* by stabilizing 0.25 ev and 0.1 ev respectively 7. The selectivity towards formation of CO over HCOO - on NG remains unsolved on the basis of free energy barrier since the modeling assumes the formation of HCOO - and CO through the same intermediate state of COOH* (See details in Supporting Information). The complete elucidation of the selectivity requires a deeper understanding of protonation of oxygen over carbon element in the COOH*. Hydrogen Evolution Reactions in NG: We considered the HER in N-doped graphene system since it is a competing reaction with CRR. The active sites for CRR are identical to HER. Figure S6 is the free energy diagram for HER. We firstly calculated the free energy without considering the ph correction, as shown in purple (pristine graphene), black (graphitic N-G), cyan (singlepyridinic N-G), red (pyrrolic N-G) and blue (triple-pyridinic N-G) plots. The pristine graphene and graphitic N-G reveal an energetic uphill for H* while single-pyridinic N-G, pyrrolic N-G and triple-pyridinic N-G show a downhill for H* but require energy to release the H* into H 2. It mean that single-pyridinic N-G, pyrrolic N-G and triple-pyridinic N-G would be very easily passivated by H*. Therefore, to suppress the passivation and simultaneously enhance the CRR, the choice of electrolyte, for example, 0.1 M KHCO 3 in our case, is significant to ensure a local neutral or base environment near the electrode surface. We can include the PH effect by correcting the free energy of H + ions G(pH) = ktln 10 ph. 6 S9

10 Figure S7. Free energy diagram of HER on NG. References: 1. Wu, J.; Ma, L.; Yadav, R. M.; Yang, Y.; Zhang, X.; Vajtai, R.; Lou, J.; Ajayan, P. M. ACS Appl. Mater. Interfaces 2015, 7, Wu, J.; Risalvato, F. G.; Sharma, P. P.; Pellechia, P. J.; Ke, F.-S.; Zhou, X.-D. J. Electrochem. Soc. 2013, 160, F953-F Wu, J.; Yadav, R. M.; Liu, M.; Sharma, P. P.; Tiwary, C. S.; Ma, L.; Zou, X.; Zhou, X.- D.; Yakobson, B. I.; Lou, J. ACS nano 2015, 9, Wu, J.; Risalvato, F. G.; Ke, F.-S.; Pellechia, P.; Zhou, X.-D. J. Electrochem. Soc. 2012, 159, F353-F Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phy. Chem. B 2004, 108, Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. Energy. Environ. Sci. 2010, 3, S10