Supporting Information

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1 Supporting Information Polyethylenimine Enhanced Electrocatalytic Reduction of CO 2 to Formate at Nitrogen Doped Carbon Nanomaterials Sheng Zhang, Peng Kang, Stephen Ubnoske, M. Kyle Brennaman, Na Song, Ralph L. House, Jeffrey T. Glass, Thomas J. Meyer *, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA 1 Materials and preparation of electrodes All chemicals were purchased from commercial sources if not mentioned otherwise. Polyethylenimine (average Mw ~25,000) and potassium bicarbonate (99.99% purity) were purchased from Sigma Aldrich. Deionized water was further purified by using a Milli-Q Synthesis A10 Water Purification system. CO 2 (National Welders, research grade) was of % purity with less than 3ppm H 2 O and used as received. Multi-walled carbon nanotubes (CNT) of nm in diameter (Cheap Tubes Inc.) were first dispersed in dimethylformamide (DMF) by sonication for 20 min to yield a homogeneous CNT suspension, and then this solution was drop-casted onto a pre-polished glassy carbon (3 mm in diameter) electrode. Nitrogen doped carbon nanotubes (NCNT) were synthesized by exposing the CNT/GC electrode to an ammonia plasma using plasma enhanced chemical vapor deposition system (PECVD, Advanced Vacuum Vision 310). The plasma treatment is a facile, mild doping method at room temperature with the dopant contents tunable by changing plasma power intensities, chamber pressures, and the exposure time. 1 In the present study, the amount of doped nitrogen was controlled by changing the exposure time and quantified using XPS spectra. In brief, the electrode was placed in the plasma chamber, which was backfilled with an ammonia atmosphere at a pressure of 200 mtorr. Plasma power was 100 W, and exposure time was 0, 10, 15, 20, 40, and 60 min. The relationship between nitrogen content and exposure time was summarized in Table S1: the doped nitrogen content in CNT first increased with exposure time and then leveled off after 40 min of exposure. After ammonia plasma treatment, the as-prepared nitrogen doped carbon nanotubes coated glassy carbon (NCNT/GC) electrodes were used for subsequent electrochemical experiments. The above NCNT/GC electrode was immersed into 5 mass % polyethylenimine (PEI) aqueous solution for 30 min and rinsed with plenty of DI water. The adsorbed amount of PEI on CNT was evaluated to be about 17 mass %. Oxygen doped carbon nanotubes coated glassy carbon (OCNT/GC) electrode was prepared using the same procedure except under oxygen plasma instead of ammonia plasma. The details about the synthesis of graphenated carbon nanotubes (GCNT) can be found in ref. 2,3. In brief, they were grown using a 915 MHz microwave plasma enhanced chemical vapor deposition (MPECVD) system. To prepare the substrates, 5-nm iron films were deposited on Silicon(100) wafers using a CHA electronbeam evaporation system. Prior to growth, the coated S1

2 substrates were heated to 1050 C in 150 sccm of NH 3, followed by striking and stabilizing a plasma at 21 Torr and 2.1 kw of magnetron input power. The pressure of 21 Torr was maintained throughout the pretreatment and growth steps. Substrates were then pretreated for 180 s in the plasma. Following pretreatment, growth of the g-cnts was accomplished by changing the gas flow to 150 sccm CH 4 and 50 sccm NH 3 for 360 s. So silicon supported GCNT film was obtained. Then it was immersed into an aqueous solution of nitric acid (1 M) to peel off the GCNT film followed by rinsing with DI water. The free standing GCNT film was then transferred onto the surface of a GC electrode, followed by fixing with 5 µl of Nafion solution (0.05 mass% in isoproponal). N-doped NGCNT/GC and PEI-NGCNT/GC electrodes were obtained using the same procedure as the case of CNT. 2 Electrochemical measurements Iron catalyst for the growth of carbon nanotubes may still remain inside the carbon nanotube even after chemical purification and impact the electrocatalytic performance of carbon materials. 4 To remove the residual Fe catalyst inside carbon nanotubes, all the above electrodes were subjected to an electrochemical purification process prior to subsequent electrochemical measurements. 5 In brief, the electrodes were purified by electrochemical oxidation in a phosphate buffered solution (ph 6.8) at a potential of 1.7 V (vs. Ag/AgCl) for 300 s, followed by potential sweeping from 0.0 V to 1.4 V in 0.5 M H 2 SO 4 until a stable voltommagram was achieved. After purification, no peak of element Fe was detected by high resolution XPS shown in Figure S1. The electrochemical measurements were carried out in a gas-tight two compartment electrochemical cell system controlled with a CHI601 D station (CH Instruments, Inc., USA) with Pt coil as counter electrode and Saturated Calomel Reference Electrode (SCE) as reference electrode. The working electrodes were prepared by loading sample suspension onto the prepolished glass carbon electrodes. Please see ref. 6 for details. The well-prepared electrodes were dried at room temperature overnight before the electrochemical tests. Linear sweep voltammetric (LSV) scans were recorded in CO 2 saturated 0.1 M KHCO 3, while controlled potential electrolysis was performed in 0.1 M KHCO 3 electrolyte with CO 2 flow. 3 Physical charaterization X-ray photoelectron spectra (XPS) were obtained at the Chapel Hill Analytical and Nanofabrication Lab (CHANL) at UNC. A Kratos Analytical Axis UltraDLD spectrometer with monochromatized X-ray Al Kα radiation ( ev) with an analysis area of 1 mm2 was used. A survey scan was first performed with a step size of 1 ev, a pass energy of 80 ev, and a dwell time of 200 ms. High resolution scans were then taken for each element present with a step size of 0.1 ev and a pass energy of 20 ev. The binding energy for all peaks was referenced to the C 1s peak at ev. The amount of PEI on carbon nanomaterials was evaluated based on XPS results. Using PEI-NCNT as the example, N content was determined to be ~7.6 At.% by XPS translating to ~33 At. % PEI. The nitrogen content in PEI-NCNT was found to be 11.3 At.% by XPS results allowing ratio of PEI to NCNT in PEI- NCNT composites to be calculated giving ~17 mass % in the PEI-NCNT. An attempt was also S2

3 made to evaluate adsorbed PEI by thermogravimetric analysis (TGA). Unfortunately, the total weight of the PEI-NCNT on the electrodes was too small to be determined by TGA. NMR analysis was used to quantify the yield of formate during controlled potential electrolysis. NMR spectra were recorded on Bruker NMR spectrometers (AVANCE-400). 1H NMR spectra were referenced to residual solvent signals. At the end of electrolysis periods, gaseous samples (0.8 ml) were drawn from the headspace by a gas-tight syringe (Vici) and injected into the GC (Varian 450-GC, pulsed discharge helium ionization detector, PDHID). Calibration curves for H 2 and CO were determined separately. Raman spectra were collected by Raman measurments (Renishaw) with a 514 nm laser, which was coupled with an electrochemical station for the in situ electrochemical Raman study. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) were obtained on on a FEI Helios 600 Nanolab Dual Beam System focused ion beam (FIB) equipped with an Oxford Instruments, INCA PentaFET-x3 X-ray detector with the electron beam set to 20 kev and a beam current of 0.69 na. S3

4 Figure S1 XPS spectra of carbon nanotubes (CNT) before and after ammonia plasma treatment. No obvious N1s peak was found at as received carbon nanotubes. By contrast, after ammonia plasma treatment, N1s peak was clearly observed, indicating the successful nitrogen doing into CNT. The relationship between nitrogen content and exposure time was summarized in Table S1: the doped nitrogen content in CNT first increased with exposure time and then leveled off after 40 min of treatment. S4

5 Figure S2 XPS spectrum (a) and Raman spectra (b) of OCNT. After oxygen plasma, oxygen content increased from 4.6 % to 14.8 % in carbon nanotubes. The G band is related to the inplane bond-stretching motion of pairs of sp 2 -C atoms. The D band ( disordered band) is the breathing mode of the sp 2 -rings of the graphene layer that is related to a series of defects: bondangle disorder, bond-length disorder, and hybridization, which are caused by heteroatom (nitrogen/oxygen) doping and structure defects by plasma treatment. So the ratio of D band over G band increased from to after oxygen plasma treatment, which can be attributed to the oxygen doping into carbon nanotubes. S5

6 Figure S3 XPS spectra of purified carbon nanotubes. No Fe2p peak was observed even in the high resolution XPS spectrum, which indicated Fe particles in carbon nanotubes were efficiently removed by electrochemical purification process. S6

7 Figure S4 Cyclic voltammetry curves of various electrodes in in 5 mm K 3 Fe(CN) 6 /0.1 M KCl solution. Scan rate: 5 mv s -1. The electro-active surface areas of as-made electrodes were evaluated by cyclic voltammetry (CV) with [Fe(CN) 6 ] 3-/4- as a probe. It can be calculated according to the randles-sevcik equation at room temperature: 7 I p =2.69*10 5 *n 3/2 *A*D 1/2 *ν 1/2 *C A is electrochemical active surface area (cm 2 ), I p is peak current (A), and n=1, D=4.34*10-6 cm 2 s -1, ν is scan rate (V s -1 ), C is the concentration of Potassium ferricyanide (5*10-6 mol cm -3 ). Electrochemical active surface area were calculated to be cm 2 for PEI-NCNT, cm 2 for NCNT, cm 2 for O-CNT, for CNT, and cm 2 for GC electrode. This showed electroactive surface areas have increased after ammonia and oxygen plasma treatment. S7

8 Figure S5 Structure of branched Polyethylenimine (PEI). PEI, a polymer with multiple amine groups, commonly used CO 2 absorbent. In aqueous phase, this polymer is positively charged because portions of amine nitrogens are protonated under neutral ph conditions. It can be attached onto the surface of carbon nanotubes through non-covalent, dispersion interactions based on van der Waals forces, driven largely by elimination of the hydrophobic interface between the carbon nanotubes and water. 8 S8

9 Figure S6 In situ electrochemical Raman spectra of PEI-NCNT at controlled potential electrolyses in 0.1 M KHCO 3 with CO 2 flow. The weak peak at around 1069 cm -1 can be assigned to the C-H bending of HCOO - produced during CO 2 reduction. 9 S9

10 Figure S7 Formate partial current density Tafel plots at PEI-NCNT (a) and NCNT (b) electrodes. Tafel plot data were collected in 0.1 M KHCO 3 /CO 2 aqueous solution. The Tafel relationship for formate production can be derived as 10 : η =E - E 0 = b logi 0 - b logi formate (1) α=2.3rt/bf (2) where E is the applied potential, E 0 is the equilibrium potential ( 0.67 V vs. SCE for the CO 2 /HCOO couple) in ph6.8 aqueous solution, η is the overpotential for CO 2 /HCOO couple, b is the Tafel slope, α is transfer coefficient, i 0 is the exchange current density, and i formate is the partial current density for formate production during CO 2 reduction. S10

11 a b c d e f Figure S8 Different resolution SEM images of graphenated carbon nanotubes ( a and b for cross section; c and d for top view) on silicon wafer and commercial CNT (e and f) on gold coated silicon wafer. It is obvious that three dimensional GCNT have porous structure with much larger pore size than CNT, which may facilitate the mass transport of CO2 through them. The diameter of GCNT is about 200 nm, much larger than that of commercial CNT. S11

12 Figure S9 XPS Spectra of as prepared GCNT, NGCNT prepared by ammonia plasma, and PEI-NGCNT. No obvious N1s peak was observed at as prepared GCNT, while nitrogen contents were evaluated to be about 7.8 mass% for NGCNT and 12.6 % for PEI-NGCNT. S12

13 Figure S10 Raman spectra of GCNT, NGCNT, and PEI-NGCNT. The ratios of D band over G band increased and their peak positions negatively shifted in the following sequence: GCNT, NGCNT, and PEI-NGCNT. This indicates the successful nitrogen doping and PEI overlay coating. S13

14 Figure S11 Cyclic voltammetry (CV) curves of NGCNT/GC and PEI-NGCNT/GC electrodes in 5 mm K 3 Fe(CN) 6 /0.1 M KCl solution. Scan rate: 5 mv s -1. The electroactive surface areas were calculated to be about cm 2 for NGCNT/GC and cm 2 for PEI- NGCNT/GC, which are smaller than those for NCNT/GC and PEI-NCNT/GC electrodes. This can be attributed to much larger diameter of GCNT than CNT, as shown in Figure S8. S14

15 Figure S12 Deconvoluted N1s spectra for NCNT. It elucidates the existence of four main nitrogen species, that is, for pyridinic N (B.E. ~ ev), pyrrolic N (B.E. ~ ev), quaternary N (B.E. ~ ev), and nitrogen oxide (B.E. ~ ev). 11 The quantitative analyses demonstrate that the fraction of the various nitrogen species is approximately, 62.5% for pyrrolic, 23.7 % for pyridinic, 8.2 % for quaternary, and 5.6 % for nitrogen oxide. These doped nitrogen would increase the DOEs of graphene at its Fermi level and open the band gap of carbon nanotubes. The pyridinic-n and pyrrolic-n were the dominant nitrogen states. S15

16 Table S1 The relationship between Faradaic efficiencies for formate, nitrogen contents, and ammonia plasma exposure time. The doped nitrogen content in CNT first increased with exposure time and then leveled off after 40 min of treatment. The maximal Faradaic efficiencies for formate (59 %) were achieved using NCNT with nitrogen content over 7 %, further N-doping did not show any observable improvement in electrolyses. Samples Ammonia plasma exposure time / min Nitrogen content % Faradaic efficiency for formate / % As received # # # # # S16

17 Table S2 Current densities and Faradaic effieiencies during Controlled potential electrolyses at -1.8 verse SCE in 0.1 M KHCO 3 /CO 2 aqueous solution at various electrodes. Samples CNT OCNT NCNT NGCNT PEI-CNT Current density / ma cm -2 geometric PEI- NCNT PEI- NGCNT Current density / ma cm - 2 electroactivie Faradaic efficiency for formate / % Partial current density for formate/ ma cm - 2 electroactivie S17

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