Supporting Information. High Energy Density Asymmetric Quasi-Solid-State Supercapacitor based on Porous Vanadium Nitride Nanowire Anode

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1 Supporting Information High Energy Density Asymmetric Quasi-Solid-State Supercapacitor based on Porous Vanadium Nitride Nanowire Anode Xihong Lu,, Minghao Yu, Teng Zhai, Gongming Wang, Shilei Xie, Tianyu Liu, Chaolun Liang,, Yexiang Tong,*, and Yat Li,* KLGHEI of Environment and Energy Chemistry, MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou , People s Republic of China. Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States of America. Instrumental Analysis and Research Centre, Sun Yat-Sen University, Guangzhou , People s Republic of China. Corresponding Authors. s: chedhx@mail.sysu.edu.cn and yli@chemistry.ucsc.edu 1

2 Experimental Preparation of porous VN nanowires: Vanadium oxide (VO x ) nanowires were synthesized by a hydrothermal method reported elsewhere g NH 4 VO 3 was dissolved in a 25 ml solution mixture of water and ethanol (volume ratio: 9/1). The solution ph was adjusted to ~ 2 by HCl, and then transferred to a 30-ml Teflon-lined autoclave. A piece of clean carbon cloth was immersed into the precursor solution in the autoclave. The autoclave was heated at 160 o C for 12 h, and then let it cool down at room temperature. The carbon cloth was washed with DI water and blow-dried with compressed air. A green VO x NW film was uniformly coated on the carbon cloth. The mass loading of VO x nanowires is about 1.1 mg cm -2. Porous VN nanowires were obtained by annealing the as-prepared VO x nanowires in NH 3 at 600 o C for 1 h. The mass loading of VN nanowires is about 0.71 mg cm -2. Fabrication of aqueous VN-SSCs: Aqueous VN-SSCs were fabricated by two pieces of identical VN nanowire electrodes with a separator (NKK separator, Nippon Kodoshi Corporation) sandwiched in between. LiCl (5 M) aqueous solution was used as electrolyte. Before assembling, all the electrodes and separator were immersed into the LiCl electrolyte for 10 min. Fabrication of Quasi-Solid-State VN-SCs: The quasi-solid-state SCs were assembled by two pieces of identical VN NW electrodes (work area: 1.25 cm 0.8 cm) or a VN NW electrode and a VO x NW electrode with a separator (NKK separator, Nippon Kodoshi Corporation) sandwiched in between. Polyvinyl alcohol LiCl/PVA gel was used as a solid 2

3 electrolyte. LiCl/PVA gel electrolyte was simply prepared as follows: 3 g PVA was mixed with 30 ml LiCl (5 M) aqueous solution and heated at 85 o C for 1 h under vigorous stirring. All the electrodes and separator were soaked with the PVA/LiCl solution and then let them solidified at room temperature for 6 h. They were assembled together and kept at 45 C for 24 h to remove excess water in the electrolyte. The area and thickness of the fabricated qusai-solid-state VN-SC devices are about 1 cm 2 and 0.08 cm. Material Characterization and Electrochemical Measurement: The morphology, structure and composition of electrode materials were characterized by field-emission SEM (FE-SEM, JSM-6330F), TEM (TEM, JEM2010-HR, 200 KV), and XPS (XPS, ESCALab250, Thermo VG). All the electrochemical measurements were conducted using an electrochemical workstation (CHI 660D). The electrochemical studies of the individual electrode were performed in a three-electrode cell, with a Pt counter electrode and an Ag/AgCl reference electrode, in 5 M LiCl aqueous solution Calculations: 1. Single Electrode : Specific capacitances of the VN and VO x electrodes were calculated from their CVs according to the following equation: where C s (F/g) is the specific capacitance, Q (C) is the average charge during the charging and discharging process, V (V) is the potential window and m is the mass loading of VN or (1) VO x NWs. 3

4 2. VN-based Devices: The cell (device) capacitance (C cell ) and volumetric capacitance of the VN-based devices were calculated from their CVs according to the following equation: (2) (3) where Q (C) is the average charge during the charging and discharging process is the applied current, V is the volume (cm 3 ) of the whole device (The area and thickness of our VN-SSC and VO x //VN-ASC devices are about 1 cm 2 and 0.08 cm. Hence, the whole volume of our VN-SSC and VO x //VN-ASC device are about 0.08 cm 3 ), t is the discharging time, V (V) is the voltage window. It is worth mentioning that the volumetric capacitances were calculated taking into account the volume of the device stack. This includes the active material, the flexible substrate and the separator with electrolyte. Alternatively, the cell (device) capacitance (C cell ) and volumetric capacitance of the electrode (C v ) was estimated from the slope of the discharge curve using the following equations: (4) (5) where I is the applied current, V is the volume (cm 3 ) of the whole device (the whole volume of our VN-SSC and VO x //VN-ASC device is about 0.08 cm 3 ), t is the discharging time, V (V) is the voltage window. Volumetric energy density, equivalent series resistance and power density (P, W cm -3 ) of the devices were obtained from the following equations: 4

5 (6) (7) (8) where E (Wh cm -3 ) is the energy density, C V is the volumetric capacitance obtained from Equation (5) and V (V) is the voltage window. ESR (Ω) is the internal resistance of the device. P (W cm -3 ) is the power density. 3. Balance the charge of electrodes in ASC device: As for a supercapacitor, the charge balance will follow the relationship q + = q -. The charge stored by each electrode depends on the specific capacitance (C s ), the potential range for the charge/discharge process ( E) and the area of the electrode (A) following the Equation 9: q = C s E m (9) In order to get q+ = q at 10 mv s -1, the mass balancing will follow the Equation 5: Therefore, the calculated mass ration between the VO x electrode and VN electrode is about 1.17 : 0.71, which is very closed to the mass loading of the VO x and VN NWs on carbon cloth substrates. 5

6 Supplementary Figures Figure S1. XRD spectra collected for VO x nanowires before and after annealed in NH 3 at 600 o C for 1 h. Figure S2. Core level N 1s XPS spectra collected for VN nanowires before and after testing for cycles in 5M LiCl aqueous electrolyte (red curve) and LiCl/PVA gel electrolyte (blue curve). 6

7 Figure S3. SEM images of VN nanowires after testing for cycles in 5 M LiCl aqueous electrolyte. The coverage of VN nanowires on carbon cloth substrate was substantially reduced after the measurement. Figure S4. CV curves collected for VN-SSCs at the scan rate of 100 mvs -1 in 5 M LiCl aqueous electrolyte and LiCl/PVA gel electrolyte. 7

8 Figure S5. Galvanostatic charge-discharge curves collected for VN-SSCs at different current densities in 5 M LiCl aqueous electrolyte and LiCl/PVA gel electrolyte. Figure S6. (a) Nyquist plots of VN SSCs collected in gel electrolyte and aqueous electrolyte. Inset: magnified high-frequency region. (b) Impedance phase angle plot as a function of frequency for VN SSCs measured in gel electrolyte and aqueous electrolyte. Nyquist plots recorded for VN SSCs with these two kinds of electrolytes are shown in Figure S6a, and an expanded view is provided in the inset. Both VN SSCs display pure capacitive behaviors. The series resistances of the VN SSCs with gel electrolyte and aqueous electrolyte are estimated to be ~3.8 ohms and ~3.6 ohms, respectively. This value can be attributed to the contact resistance of the device with the external circuit. The dependence of 8

9 the phase angle on the frequency for the VN SSCs with gel electrolyte and aqueous electrolyte is shown in Figure S6b. The characteristic frequencies f 0 for a phase angle of 45 are 3.6 Hz and 2.6 Hz for the VN SSCs with gel electrolyte and aqueous electrolyte, respectively. These frequencies mark the point at which the resistive and capacitive impedances are equal. 1-2 The corresponding time constant t 0 (=1/ f 0 ) for the VN SSCs with gel electrolyte equals 0.27 s compared with 0.38 s for the VN SSCs with aqueous electrolyte. This rapid frequency response of the VN SSCs with gel electrolyte can be attributed to the efficient mass transport. Therefore, electrochemical impedance spectroscopy (EIS) confirmed the VN nanowires have similarly efficient mass transport of VN nanowires within the aqueous electrolyte and gel electrolyte. References: 1. El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Science 2012, 335, (6074), Taberna, P. L.; Simon, P.; Fauvarque, J. F. J. Electrochem. Soc. 2003, 150, (3), A292-A300. Figure S7. (a) CV curves and (b) Specific capacitance collected for VO x nanowire electrode at various scan rates in 5 M LiCl aqueous electrolyte. 9

10 Figure S8. (a) CV curves of the VO x //VN ASC device collected at a scan rate of 100 mv s -1 in a potential window between 0 to 2.2 V, with a step of 0.2 V. An oxidation peak was observed at ~1.8 V, highlighted by dashed line. (b) Galvanostatic discharge-charge curves of the VO x //VN-ASC device collected at 4 ma cm -2 in the corresponding potential windows. The dashed box shows that the oxidation peak appears beyond 1.8 V, which is in good agreement with the CV result. Figure S9. Cycling performance of the quasi-solid-state VOx//VN-ASC device collected at a scan rate of 100 mv s 1 for cycles in the potential windows of 1.8 V, 2 V and 2.2 V. 10

11 Figure S10. CV curves of the quasi-solid-state VOx//VN-ASC device collected at different scan rates. Figure S11. Galvanostatic charge-discharge curves of the quasi-solid-state VO x //VN-ASC device collected at different current densities. 11

12 Figure S12. Ragone plots of the quasi-solid-state VO x //VN-ASC device measured based on the mass of the entire device. 12