Flexible Zn 2 SnO 4 /MnO 2 Core/shell Nanocable - Carbon Microfiber Hybrid Composites for High Performance Supercapacitor Electrodes

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1 Supporting Information Flexible Zn 2 SnO 4 /MnO 2 Core/shell Nanocable - Carbon Microfiber Hybrid Composites for High Performance Supercapacitor Electrodes Lihong Bao, Jianfeng Zang, Xiaodong Li Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA Fabrication and characterization of Zn 2 SnO 4 nanowires on carbon microfibers Commercial woven carbon microfibers (CMFs, Fibre Glast Development Corporation) were used directly as templated substrate without further processing to synthesize Zn 2 SnO 4 (ZTO) nanowires. The ZTO nanowires were synthesized via a simple vapor transport method in a horizontal alumina tube furnace (id: 73 mm, length: 1000 mm, GSL X, MTI Corp.). Zn and SnO powders (weight ratio 1:2, purchased from Sigma-Aldrich) were mixed, ground, and then loaded into an alumina boat, which was then placed in the center of the alumina tube mounted on the furnace. The CMFs sputtercoated with Au film were placed at 5 cm downstream from the alumina boat. The tube furnace was sealed and heated to 900 o C at a rate of 5 o C/min, and then held for 1 h to synthesize ZTO nanowires. During the experiment, high purity Ar gas (99.99%) was introduced into the tube at a flow rate of 50 sccm and the pressure of the tube was evacuated to a pressure of 5 Torr. After the furnace was cooled down to the room To whom correspondence should be addressed. lixiao@cec.sc.edu. 1

2 temperature, the resulting product was collected for characterization by scanning electron microscopy (SEM, Zeiss Ultra Plus FESEM), X-ray diffraction (XRD, Rigaku DMax 2200 using Cu K α radiation, λ = Å), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD equipped with a monochromated Al K α X-ray source and hemispherical analyzer capable of an energy resolution of 0.5 ev), transmission electron microscopy (TEM, Hitachi H8000) and high-resolution TEM (HRTEM, JEOL 2010F equipped with an EDX detector from Oxford Instruments). To investigate the improvement of the electrochemical performance by using CMFs as substrate, a control sample with ZTO nanowires grown on stainless steel (SS) substrate was similarly fabricated by replacing the CMFs with SS substrate while keeping other experimental conditions the same. Coating MnO 2 onto ZTO nanowires grown on CMFs Previous studies have shown that spontaneous redox deposition of MnO 2 on carbon materials is ph-dependent. 1-4 In acid solutions, reduction of permanganate ion (MnO - 4 ) to MnO 2 can result in large agglomerated particles of MnO 2. 1, 2 While in neutral solutions, thin films of MnO 2 can be obtained on the surface of carbon. 2, 3 In this study, the precursor solution for the coating process was prepared by mixing 0.1 M Na 2 SO 4 (Sigma- Aldrich) and 0.1 M KMnO 4 (Sigma-Aldrich) solutions. The CMFs grown with ZTO nanowires were immersed into the solution and the typical duration time of the immersion was 60 min. The loading amount of MnO 2 can be easily controlled by adjusting the immersion time. After immersing, the sample was rinsed with deionized water and then heat treated at 120 C for 12 h in air. In addition, the control samples of 2

3 MnO 2 /CMF and MnO 2 /ZTO/SS composites were fabricated similarly by coating MnO 2 onto commercial CMFs and ZTO nanowires on SS substrate, respectively. Electrochemical characterization Electrochemical performance of MnO 2 /ZTO/CMF hybrid composite electrode was carried out using a CHI 760D electrochemical workstation (CH Instruments Inc., Texas, USA). The standard three-electrode cell was composed of Ag/AgCl as reference electrode, Pt mesh as counter electrode and the synthesized composite sample as working electrode, respectively. A 1 M Na 2 SO 4 solution served as electrolyte at room temperature. Cyclic voltammetry (CV) was performed at various scan rates of 2, 5, 10, 20, 50, and 100 mv S -1. Galvanostatic (GV) charge/discharge curves were obtained at various current densities of 1, 2, 5, 10, 20, 30 and 40 A g -1 to evaluate the specific capacitance. A potential window in the range from 0 to 0.8 V was used in all the measurements. Electrochemical impedance spectra (EIS) were measured in the frequency range from to 0.1 Hz with 0 V mean voltage and amplitude 5 mv using the same setup as CV and GV tests. Analysis of the average manganese oxidation state in MnO 2 /ZTO/CMF hybrid composite X-ray photoelectron spectroscopy (XPS) spectra of the MnO 2 /ZTO/CMF hybrid composite were used to determine the oxidation state of as-coated MnO 2 shells on ZTO nanowire cores. Mn 2p spectrum (Figure S1a) shows that the binding energy peaks of Mn 2p 3/2 and Mn 2p 1/2 are centered at ev and ev, respectively, which is in good 3

4 agreement with the previously reported peak binding energy separation (11.8 ev) between Mn 2p 3/2 and Mn 2p 1/2. 5 As reported previously, the average oxidation state of Mn in manganese oxides can be determined by the separation of peak energies ( E) of the Mn 3s peaks caused by multiplet splitting, where the E data of MnO, Mn 3 O 4, Mn 2 O 3 and MnO 2 are 5.79, 5.50, 5.41 and 4.78 ev, respectively. 6, 7 The as-prepared MnO 2 /ZTO/CMF hybrid composite electrode showed a separated energy of 4.7 ev for the Mn 3s doublet (Figure S1b), which suggests that the oxidation state of the Mn in the composite is ~4.0. FIGURE S1. XPS spectra of the MnO 2 /ZTO/CMF hybrid composite. (a) Mn 2p spectrum. (b) Mn 3s spectrum. Cyclic voltammogrammetry (CV) test performed on CMF control sample To evaluate the capacitance contributed from the CMFs in the MnO 2 /ZTO/CMF hybrid composite, cyclic voltammogrammetry (CV) test was performed on a CMF control sample, as shown in Figure S2. The maximum specific capacitance of the CMF at the scan rate of 2 mv/s derived from CV curve is 0.15 F/g. 4

5 FIGURE S2. (a) Cyclic votalmmetry (CV) curves of CMF electrode at various scan rates. (b) Specific capacitance as a function of scan rate derived from CV curves. Galvanostatic (GV) tests performed on CMF, ZTO/CMF and MnO 2 /CMF composites To demonstrate the electrochemical performance benefits of the MnO 2 /ZTO/CMF hybrid composite, galvanostatic tests were also performed on respective CMF, ZTO/CMF and MnO 2 /CMF composites. Figure S3 shows the GV constant-current charge/discharge curves of CMF, ZTO/CMF and MnO 2 /CMF composites at the current density of 1 A/g. FIGURE S3. Galvanostatic (GV) constant-current charge/discharge curves of (a) CMF, (b) ZTO/CMF and (c) MnO 2 /CMF composites at the current density of 1 A/g. 5

6 Comparison of the cyclic voltammetry (CV) curves between MnO 2 /ZTO/CMF and MnO 2 /ZTO/SS composites FIGURE S4. (a) SEM image of MnO 2 /ZTO nanocables on SS substrate (MnO 2 /ZTO/SS). (b) Cyclic voltammetry (CV) curves of the MnO 2 /ZTO/SS composite as electrode for supercapacitors at different scan rates in 1 M Na 2 SO 4 aqueous solution. (c) Comparison of the specific capacitances between the MnO 2 /ZTO/SS (black) and MnO 2 /ZTO/CMF (red) composites at different scan rates derived from the cyclic voltammetry. (d) Comparison of the CV curves between the MnO 2 /ZTO/SS (black) and MnO 2 /ZTO/CMF (red) composites at scan rate of 2 mv/s, showing the improvement of the electrochemical performance by using CMFs as substrate. 6

7 To demonstrate the electrochemical performance benefits of the higher surface area of CMFs with reference to a flat conductive substrate, we compared the cyclic voltammetry (CV) curves of MnO 2 /ZTO/CMF and MnO 2 /ZTO/SS composites. Figure S4a shows the SEM image of MnO 2 /ZTO nanocables on SS substrate, indicating that high density of MnO 2 /ZTO nanocables have been grown on the SS substrate. Figure S4b shows the cyclic voltammetry (CV) curves of the MnO 2 /ZTO/SS composite at different scan rates in 1 M Na 2 SO 4 aqueous solution. Figure S4c illustrates the comparison of the specific capacitances between the MnO 2 /ZTO/SS (black) and MnO 2 /ZTO/CMF (red) composites at different scan rates, showing that the MnO 2 /ZTO/CMF hybrid composite has a higher specific capacitance than the MnO 2 /ZTO/SS composite. As shown in Figure S4d, the MnO 2 /ZTO/CMF hybrid composite shows an ideal quasi-rectangular shape, and the area covered by the CV curve of the MnO 2 /ZTO/CMF hybrid composite is larger than that of the MnO 2 /ZTO/SS composite, suggesting that the MnO 2 /ZTO/CMF hybrid composite has a better electrochemical performance than the MnO 2 /ZTO/SS composite. Coverage of the ZTO nanowires on CMF substrate 7

8 FIGURE S5 (a,b) Representative SEM images of the ZTO nanowires grown on CMFs. As shown in Figure S5, high density of ZTO nanowires with a full coverage has been grown on the CMFs. The ZTO nanowires covered all around the CMFs, even the space between the CMFs. Flexibility tests performed on the MnO 2 /ZTO/CMF hybrid composite To demonstrate the flexible nature of the MnO 2 /ZTO/CMF hybrid composite as electrode for supercapacitors, the electrochemical performances of the composite under both normal test and bending conditions were compared. Figures S6a and S6b show the optical photographs of the composite under normal test and bending conditions, respectively. As shown in Figure S6c, the cyclic voltammetry curves of the normal test and bending conditions are almost same, and no apparent changes were observed even when the MnO 2 /ZTO/CMF composite was mechanically bent to an angle of 60 degrees. FIGURE S6. (a,b) Optical photographs of the MnO 2 /ZTO/CMF hybrid composite electrode under normal test and bending conditions, respectively. (c) Cyclic voltammetry curves of the composite under normal test and bending conditions, showing no apparent changes for 8

9 the MnO 2 /ZTO/CMF composite electrode that was mechanically bent to an angle of 60 degrees. Calculations 1. Specific capacitances derived from cyclic votalmmetry (CV) tests can be calculated from the equation: 8 C = Vc 1 I (V)dV m υ(v V ) (1) c a Va where C (F/g), m (g), υ (V/s), V c and V a, and I (A) are the specific capacitance, the mass of the active materials in the electrode, potential scan rate, high and low potential limit of the CV tests, and the instant current on CV curves, respectively. 2. Specific capacitances derived from galvanostatic (GV) tests can be calculated from the equation: 9 C = I t m V (2) where C (F/g), I (A), t (s), m (g) and V are the specific capacitance, the discharge current, the discharge time, the mass of the active materials in electrode, and the potential window, respectively. 3. Specific energy (E) and specific power (P) derived from GV tests can be 9, 10 calculated from the following equations: 1 E = C( V) 2 2 (3) 9

10 P = E t (4) where E (Wh/kg), C (F/g), V (V), P (W/kg) and t (s) are the specific energy, specific capacitance, potential window, specific power and discharge time, respectively. References (1) Bordjiba, T.; Belanger, D. Direct redox deposition of manganese oxide on multiscaled carbon nanotube/microfiber carbon electrode for electrochemical capacitor. J. Electrochem. Soc. 2009, 156 (5), A378-A384. (2) Fischer, A. E.; Pettigrew, K. A.; Rolison, D. R.; Stroud, R. M.; Long, J. W. Incorporation of homogeneous, nanoscale MnO2 within ultraporous carbon structures via self-limiting electroless deposition: Implications for electrochemical capacitors. Nano Lett. 2007, 7 (2), (3) Ma, S. B.; Ahn, K. Y.; Lee, E. S.; Oh, K. H.; Kim, K. B. Synthesis and characterization of manganese dioxide spontaneously coated on carbon nanotubes. Carbon 2007, 45 (2), (4) Lee, S. W.; Kim, J.; Chen, S.; Hammond, P. T.; Shao-Horn, Y. Carbon nanotube/manganese oxide ultrathin film electrodes for electrochemical capacitors. ACS Nano 2010, 4 (7), (5) Toupin, M.; Brousse, T.; Belanger, D. Charge storage mechanism of MnO 2 electrode used in aqueous electrochemical capacitor. Chem. Mater. 2004, 16 (16), (6) Chigane, M.; Ishikawa, M. Manganese oxide thin film preparation by potentiostatic electrolyses and electrochromism. J. Electrochem. Soc. 2000, 147 (6), (7) Chigane, M.; Ishikawa, M.; Izaki, M. Preparation of manganese oxide thin films by electrolysis/chemical deposition and electrochromism. J. Electrochem. Soc. 2001, 148 (7), D96-D101. (8) Fan, Z. J.; Yan, J.; Zhi, L. J.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M. L.; Qian, W. Z.; Wei, F. A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors. Adv. Mater. 22 (33), (9) Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors. Nat. Mater. 2008, 7 (11), (10) Yan, J. A.; Khoo, E.; Sumboja, A.; Lee, P. S. Facile coating of manganese oxide on tin oxide nanowires with high-performance capacitive behavior. ACS Nano 2010, 4 (7),