Supporting Information. Electrochemical Raman Spectroscopy Investigation

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1 Supporting Information High-Capacitance Mechanism for Ti 3 C 2 T x MXene by In Situ Electrochemical Raman Spectroscopy Investigation Minmin Hu,, Zhaojin Li,, Tao Hu,, Shihao Zhu,, Chao Zhang and Xiaohui Wang *, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China School of Materials Science and Engineering, University of Science and Technology of China, Hefei , China University of Chinese Academy of Sciences, Beijing , China S1

2 Figure S1. XRD patterns of as-prepared Ti 3 AlC 2, exfoliated Ti 3 C 2 T x sheets and assembled thin film. The strong (000l) texture in the XRD pattern (green line) shows that the prepared Ti 3 C 2 T x film is in a nearly face-to-face fashion. S2

3 Figure S2. (a) SEM image of as-etched Ti 3 C 2 T x sheets. (b) Ti 3 C 2 T x sheets in water after ultrasonic irradiation. Figure S3. SEM images of (a) Ti 3 C 2 T x -coated nickel foam and (b) the cross sections of coated nickel foam. Note that the Ti 3 C 2 T x film is adhered to the nickel substrate even at the corner where the curvature is large. In addition, the film thickness uniformity is good. S3

4 Figure S4. CV curves of the capacitor comprising electrodes of Ti 3 C 2 T x thin film at a scan rate of 10 mv/s in the solution of (a) (NH 4 ) 2 SO 4 (ph = 5.22), H 2 SO 4 (ph = 5.22) and (b) MgSO 4 (ph = 6.69), H 2 SO 4 (ph = 6.69) at room temperature. Figure S5. Schematic diagram of in situ electrochemical Raman spectroscopy setup. S4

5 Figure S6. In situ Raman spectra of Ti 3 C 2 T x MXene recorded on (a) negative electrode in MgSO 4 electrolyte, (b) positive electrode in MgSO 4 electrolyte. All Raman bands almost remain intact. S5

6 Figure S7. Selected spectra and Lorentzian fits of the band at 208 and 265 cm 1. The intensity of the band at 208 cm 1 assigned to out-of-plane vibrations of Ti2, C, O atoms in Ti 3 C 2 O 2 gradually declines during discharging, which indicates the quantity of Ti 3 C 2 O 2 in the sample decreases. The mode at 265 cm 1 assigned to in-plane vibrations of Ti2, C, O, H atoms in Ti 3 C 2 (OH) 2 almost remains intact. S6

7 Figure S8. 3D confocal microscopy images of Ti 3 C 2 T x negative electrode in H 2 SO 4 electrolyte at 0 V, 0.2 V and 0.4 V. The state of electrode in coordinate system didn t change when the potential scanned from 0 V to 0.4 V, showing that the electrode didn t tilt during charging and discharging. S7

8 Figure S9. In situ Raman spectra of Ti 3 C 2 Tx MXene coated on carbon fiber bundles recorded on negative electrode in H 2 SO 4 electrolyte, showing the voltage-dependent change in Raman bands. The band at 722 cm 1 red shifts continuously down to 707 cm 1 when the potential sweeps from 0 V to 0.4 V and shifts back completely when scanned reversely. The mode at 722 cm 1 highlighted in pink at 0 V is different from the mode (726 cm 1 ) of Ti 3 C 2 T x on nickel foam. The deviation maybe result from the different batch of original MAX phase for MXene preparation. S8

9 Figure S10. The b-values for the Ti 3 C 2 T x thin film electrode plotted as a function of potential versus Ag/AgCl. Inset: power-law dependence of current on sweep rate shows good linearity at 0.2 V in various electrolytes investigated. S9

10 Figure S11. Voltammetric responses (20 mv/s) for the Ti 3 C 2 T x electrode in (a) 1 mol/l H 2 SO 4 solution, (b) 1 mol/l (NH 4 ) 2 SO 4 solution, (c) 1 mol/l MgSO 4 solution with hatched portions of the contributions of the processes not limited by diffusion, that is, capacitive ( C- ); vertical lines limit the cyclic voltammetry area used in calculations; the total current (solid line) is obtained experimentally. The approach that quantifies the surface capacitive effect and diffusion limited contributions to the overall capacitance follows Ref.1. S10

11 Figure S12. Equivalent circuit adopted in the simulation of electrochemical impedance spectroscopy (EIS) spectra. R s : electrolyte resistance; C d : electrical double layer capacitor; R ct : charge transfer resistance; CPE: constant phase angle element. S11

12 Figure S13. CV curves of Ti 3 C 2 T x thin film electrode at scan rates of 2 mv/s and 20 mv/s in 1 mol/l H 2 SO 4. The capacitance increases faster when the potential scans from 0.35 V to 0.45 V at a slow scan rate (2 mv/s) during discharging process, accounting for that ion accommodated on the deep adsorption sites. 2 Correspondingly, the capacitance also increases faster when the potential scans from 0.15 V to 0.05 V at a slow scan rate (2 mv/s) during charging process, accounting for that ion desorbed from the deep adsorption sites. These phenomena are not evident at a medium scan rate (20 mv/s). S12

13 Table S1. Assignment of Raman active vibration modes of Ti 3 C 2 T x monosheets. Mode Formula Ti 3 C 2 (OH) 2 Ti 3 C 2 O 2 Ti 3 C 2 O(OH) 1 (A 1g ) Ti2, C, O, H; 218 Ti2, C, O; 208 Ti2, C, O; (A 1g ) Ti2, C; 684 Ti2, C; 730 Ti2, C; (E g ) Ti2, C, H; 622 Ti2, C, O; 523 Ti2, C, H; (E g ) Ti2, C, O, H; 278 Ti2, C, O; 347 Ti2, C, O, H; (A 1g ) Ti2, C,O, H; 514 Ti2, C, O; 586 Ti2, C, O; 569 a The main contributing atoms to the vibration mode are underlined. b The number is the wave number (cm 1 ) of the active mode. c The font in red highlights the Raman bands recognized in the spectra. S13

14 Table S2. Portions of the contributions of the processes limited by surface capacitive effect in different electrolytes. Portions of surface capacitive effect (%) Scan rate (mv/s) H 2 SO Electrolytes (NH 4 ) 2 SO MgSO S14

15 Table S3. Simulation results of the EIS spectra in Figure.5b. Element 1 mol/l H 2 SO 4 1 mol/l (NH 4 ) 2 SO 4 1 mol/l MgSO 4 Values Error/% Values Error/% Values Error/% R s /ohm C d /F R ct /ohm CPE-T CPE-P ( ) S15

16 References 1. Liu, T. C.; Pell, W. G.; Conway, B. E.; Roberson, S. L. Behavior of Molybdenum Nitrides as Materials for Electrochemical Capacitors-Comparison with Ruthenium Oxide. J. Electrochem. Soc. 1998, 145, Levi, M. D.; Lukatskaya, M. R.; Sigalov, S.; Beidaghi, M.; Shpigel, N.; Daikhin, L.; Aurbach, D.; Barsoum, M. W.; Gogotsi, Y. Solving the Capacitive Paradox of 2D MXene using Electrochemical Quartz-Crystal Admittance and In Situ Electronic Conductance Measurements. Adv. Energy Mater. 2015, 5, S16