Department of Chemistry, Graduate School of Science, Tohoku University, Sendai , Japan

Transcription

1 Supporting Information Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-defined Graphene Electrode Probed by in situ Sum Frequency Generation Spectroscopy Qiling Peng, Jiafeng Chen, Hengxing Ji, * Akihiro Morita, and Shen Ye * Institute for Catalysis, Hokkaido University, Sapporo , Japan University of Science and Technology of China, Hefei , China Department of Chemistry, Graduate School of Science, Tohoku University, Sendai , Japan Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto , Japan (1) The Preparation of the Graphene Monolayer The graphene monolayer with large dimension was prepared by chemical vapor deposition (CVD) method on a polycrystalline copper (Cu) surface. 1 Before the CVD process, the Cu substrate was pretreated to ensure high quality graphene deposition. The as-received Cu foil with thickness of 25 m (Alfa Aesar) was soaked into the HCl solution (8 wt%) for about 5 hours to remove the Cr 2 O 3 protective coating above. Then, the Cu foil was repetitively rinsed in Milli-Q water in an ultrasonication bath. Prior to introduction of Cu foil into the quartz tube of the furnace (Fig. A1), the furnace was firstly pretreated in the H 2 atmosphere (~0.13 torr) for 1 hour at After the temperature was cooled to room temperature, the Cu foil was placed into the quartz tube and kept at 1030 in the H 2 atmosphere. The graphene monolayer grew on the copper foil in the CH 4 and H 2 flow at the same temperature for 15 mins (1030 ). Graphene grown on the Cu foil was transferred onto the flat surface of a CaF 2 prism, which is used for the subsequent electrochemical and SFG characterizations (see Fig. A2). Firstly, 1 ml of polymethylmethacrylate (PMMA, MW: 550,000) solution (in chlorobenzene, 50 mg/ml) was covered on surface of the graphene/cu sample by spin-coating method for 40 seconds (rotation speed: 3000 rpm). The sample was placed in the ambient environment for 1 hour until the chlorobenzene solvent was evaporated. Then, the underlying Cu foil was etched 1

2 in a 0.5 M ammonium peroxodisulfate ((NH 4 ) 2 S 2 O 8 ) solution to form a free-standing graphene/pmma membrane floating on the solution surface. On the other hand, to realize electric contact with the graphene which is required for electrochemical potential control, a gold circuit pattern was evaporated on the CaF 2 surface before the graphene transfer (see Fig. A2). Then, the graphene/pmma composite film was transferred to the CaF 2 substrate with the gold circuit pattern by vertically lifting the substrate from the solution. After naturally dried and heated at 170 in the ambient environment to increase its adhesion on the substrate, the PMMA on the graphene was removed by immersion in acetone for 8 hours and further rinsed by isopropanol. The graphene on the CaF 2 surface was further annealed in the Ar/H 2 mixture atmosphere (<10 1 Torr) in the tube furnace at 350 to fully remove the residual PMMA. (2) Characterization of the graphene monolayer Raman spectroscopy was used to confirm the quality of the graphene monolayer before electrochemical and SFG measurements. The Raman spectra were recorded by a Renishaw invia microscope. The excitation laser (532 nm, 50% of intensity) was focused on the graphene surface through a 20 objective and the acquisition time was 15 s. Figure A3 shows typical Raman spectra of a graphene monolayer at different spots in ambient air after the fabrication process. A symmetric peak was observed at 2679 cm 1 for the 2D band, whose peak intensity was significantly higher than the G peak at 1587 cm 1. The full width at half maximum (FWHM) was found to be smaller than 30 cm 1. No D-band (typically at 1341 cm 1 ) was observed, indicating that the graphene monolayer was defect-free. These results confirm that the graphene monolayer electrodes used in this study are in high-quality. 2 The sheet resistance of the graphene monolayer was determined as 1100 Ω 1 by a manual four-point probe apparatus (Keithley 2450 SourceMeter), slightly higher than those previously reported 3 but was 2

3 good enough for the present electrochemical and in situ SFG spectroscopy measurements. After electrochemical and SFG characterizations in different nonaqueous electrolyte solutions, it was found that the Raman spectra of graphene monolayer electrode were basically the identical to that of as-prepared samples, implying that the graphene monolayer was stable during these measurements. Raman Intensity (a.u.) 18.0k 15.0k 12.0k 9.0k 6.0k 3.0k 2D: 2679cm -1 G: 1587cm -1 G': 2457cm -1 D: 1341cm -1 Spot 3 Spot 2 Spot Raman shift (cm -1 ) Figure A3. Raman spectra of graphene monolayer when transferred onto CaF2 prism. (3) SFG system A femtosecond broadband SFG spectroscopy system was used in this study. Briefly, a 1 khz, 2 W Ti:sapphire regenerative amplifier produces 120 fs pulses at 800 nm. Half of the amplifier s output is directed into an optical parametric amplifier (OPA) followed by difference frequency generation (DFG) to generate of infrared (IR) pulses. The IR pulses are tunable from 2.5 to 10 μm with a spectral width of ca. 200 cm 1. Another half of the amplifier s output is directed into a home-made pulse shaper to generate narrow-band pulses with a spectral width of ~10 cm 1. The SFG signal was recorded by a CCD detector (DU420-BV, Andor Technology) attached to a spectrograph (MS3504, Solar-TII, f = 35 cm, 1200 grooves/mm). 4-6 (4) in situ SFG setup A specially designed cell used for the SFG measurement on the graphene/solution interface. Figure A4 shows a schematic structure for the in situ SFG cell made by KelF on the graphene/solution interface. The visible and infrared beams spatially and temporally overlapped on the graphene and solution interface with incident angles of 70 o and 50 o, respectively, from backside of the CaF 2 prism. By using the present internal reflection geometry, no beam passes through the bulk solution. The energy attenuation of the IR beam is efficiently reduced and the local electric fields for these laser beams on the graphene surface significantly increase. 7-9 Therefore, the present optical setup is better than that of the conventional thin layer geometry. It should be mentioned that the present setup also significantly improves the electrochemical conditions such as mass transfer and potential control for the 3

4 observation compared to the conventional thin layer geometry. All of SFG spectra were normalized by a SFG spectrum recorded on the gold surface with ppp-polarization. This normalization can be carried out immediately after each in situ SFG measurement only by slightly shifting the sample position where the gold film was evaporated. Both electrochemical and SFG measurements were carried out in the same cell (see Fig. A4). A Ni wire and a Ni-wire coated LiFePO 4 were used as the counter and reference electrodes, respectively. The reference electrode were prepared from mixture of LiFePO 4 powder, carbon black (CB) and polyvinylidene fluoride (PVDF) binder dissolved in N-methylpyrrolidone (NMP) (w LiFePO4 /w PVDF /w CB /w NMP :=5:1:5:10). The obtained slurry was ball-milled at 3000 rpm/min for 15 mins, and coated onto a nickel wire. Then, the resulting electrode was dried in oven at 60 o C in a vacuum for 5 hrs to evaporate the solvents. Electrochemical controls were carried out using a potentiostat (HAB-151, Hokuto Denko, Japan). The current and potential outputs from the potentiostat were recorded by a multifunction data acquisition module (USB-6211, National Instruments) controlled by LabVIEW. The cell was constructed in an Ar-filled glove box (Labmaster MB10-C, MBraun) before the experiments. (5) Electrochemical and in situ SFG results Typical electrochemical behaviors of ORR/OER processes were evaluated on a graphene monolayer electrode in the O 2 -free and O 2 -saturated 0.5 M TABClO 4 -DMSO solutions. Figure A5 shows the cyclic voltammograms (CVs) at various sweep rates. Generally, the CVs are similar to those observed on the carbon and gold electrodes reported before No obvious redox current was observed in the O 2 -free solution. A pair of redox peaks were observed at 2.53 V / 2.74 V in the O 2 -saturated solution at a scan rate of 10 mv/s. The redox peaks can be attributed to the one electron reduction of oxygen to the superoxide (O 2 + e O 2 ) and re-oxidation of the superoxide (O 2 O 2 + e ) on the graphene monolayer electrode surface in the O 2 -saturated solution. The redox couple shows a good reversible behavior. As the scan rate ( ) increases, the peak current (i p ) increases and peak separation becomes wider. The inset of Fig. A5 shows a plot of / for the ORR (i.e., 4

5 the negative-going sweep), which exhibits a linear relationship between and /. The process is in good agreement with the Randles-Sevcik equation: 12. / / / Where the peak current ( ) is a function of the oxygen concentration (C) in solution, graphene monolayer electrode area ( ), the number of electrons transferred (n), the diffusion coefficient (D) and the sweep rate ( ). The present results indicate that the ORR on the graphene monolayer electrode in the O 2 -saturated 0.5 M TABClO 4 /DMSO solution is a mass diffusion-controlled process. Based on the linear fitting in the inset of Fig.A5, the slope is determined to be cm 2 /s. The area of working electrode is 0.5 cm 2. In the previous studies, it has reported that the oxygen concentration (C) and its diffusion coefficient (D) were 2.1 mm and cm/s in the TABClO 4 /DMSO. 13 Then, n can be evaluated to be 1 by the Randles-Sevcik equation, thereby indicating the ORR is a one-electron reduction process. The similar relation was also obtained for the OER process. The noticeable symmetry between the cathodic and anodic peak shapes is attributed to the same diffusion rates of O 2 and O 2 in the solution. Figure A6 shows the ORR/OER behaviors on a graphene monolayer electrode in O 2 -saturated and O 2 -free 0.5 M LiClO 4 -DMSO solution in the potential region between 1.7 V and 4.5 V. No electrochemical response was observed in the same potential region in the O 2 -free 0.5 M LiClO 4 -DMSO solution, indicating that the electrochemical currents observed here should be associated with the ORR/OER on the graphene monolayer electrode surface. It is obvious that the ORR/OER behaviors are different from that observed in the Li-free DMSO electrolyte solution (Fig. A5). In the negative-going sweep (ORR), a cathodic peak appears at 2.35 V, followed by 5

6 a new cathodic peak at 2.06 V. As is previously reported, the first reduction peak at 2.35 V can be attributed to the one-electron reduction from O 2 to superoxide (LiO 2 ). 10 Similar reaction occurs in the Li-free solution (Fig. 5A). As Li-ion is present in the solution, the superoxide anion (O 2 ) or LiO 2 can be further reduced to form Li 2 O 2 (LiO 2 + Li + + e Li 2 O 2 ). This reaction corresponds to the second reduction peak at 2.06 V. Since Li 2 O 2 has low solubility in DMSO, one expects that Li 2 O 2 is deposited on the graphene monolayer electrode surface. As the potential is swept to the positive direction, an anodic current starts to flow at 3.10 V, followed by wide anode waves with two peaks around 3.27 V and 3.82 V. These anodic peaks appear at much more positive potential region in comparison with that observed in the Li-free solution (2.74 V, Fig. A5). It is still difficult to make exact assignments for each anodic peak at present stage. The inset in Fig. A6 shows the plot of the first cathodic peak current (2.35 V) versus the square root of the scan rate ( / ) determined on a graphene monolayer electrode in O 2 -saturated 0.5 M LiClO 4 /DMSO solution. A linear relationship was also observed, indicating that the first reduction step of O 2 in Li-contained DMSO solution is a diffusion-controlled reaction. The slope is determined to be cm 2 /s. In the same literature, it has reported that the oxygen concentration (C) and its diffusion coefficient (D) were 2.1 mm and cm/s in LiClO 4 /DMSO solution. 13 n was estimated as 1. Hence, the electrochemical reaction at 2.35 V is one-electron process. 6

7 Figure A7 presents the normalized ssp- and sps-polarized SFG spectra on a graphene monolayer surface in DMSO containing 0.5 M TBAClO 4. A sharp peak at 2918 cm 1 could be assigned to the symmetric stretching mode of CH 3 group. The sps-polarized SFG spectra were much weaker than those of ssp-polarized SFG spectra either in DMSO. To verify the effect of ORR/OER on a graphene monolayer electrode, the in situ SFG observation was also carried out in O 2 -free 0.5 M TABClO 4 -DMSO solution at a potential sweep rate of 1 mv/s (Fig. A8(a)) between 1.7 V and 3.5 V (OCP 1.7 V 3.5 V OCP). A SFG peak was observed at 2918 cm 1. Figure A8(b) shows the potential dependence of the peak intensity in Fig. A8(a). No Faradaic current was observed during the potential cycle and the SFG intensity kept almost constant during the potential sweep in O 2 -free TBAClO 4 -DMSO solution. This confirms that the SFG intensity of the methyl group of DMSO in the O 2 -saturated solution comes from the ORR/OER on the graphene electrode surface. 7

8 Figure A9(a) shows in situ ssp-polarized SFG spectra on a graphene monolayer electrode in O 2 -free 0.5 M LiClO 4 -DMSO solution during the potential sweep. Figue1 A9(b) gives the potential dependence of the peak intensity for methyl group of DMSO. No Faradaic current was observed and the SFG intensity of DMSO was constant throughout the potential sweep. Generally, the behaviors observed in Fig.2 should be related to the change of molecular structure on the graphene electrode surface caused by the ORR/OER. On the other hand, no change in the SFG peak position was observed during the potential cycle (Fig. 1), which could be attributed to the relatively weak interaction between the methyl group of DMSO and graphene surface. One expects that DMSO may orient its methyl group to the solution side and its S=O group to the graphene surface. References 1. Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S., Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils, Science 2009, 324, Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K., Raman Spectrum of Graphene and Graphene Layers, Phys. Rev. Lett. 2006, 97, Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S., Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotech. 2010, 5, Ye, S.; Noda, H.; Morita, S.; Uosaki, K.; Osawa, M., Surface molecular structures of Langmuir-Blodgett films of stearic acid on the solid substrate studied by sum frequency generation spectroscopy, Langmuir 2003, 19,

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