Controllable Growth of Carbon Nano-Onions for Developing High-Performance Supercapacitors Y. Gao, Y. S. Zhou, J. Hudgins, and Y. F. Lu* Department of Electrical Engineering, University of Nebraska, Lincoln E-mail: ylu2@unl.edu website: http://lane.unl.edu What is carbon nano-onion Motivations CNOs are promising electrode material in fabricating high-performance supercapacitors Carbon nano-onions (CNOs) are concentric multilayer giant fullerenes, which consist of multiple concentric graphitic shells to form encapsulated structures Emergency doors on jet planes Hybrid auto Battleship ignition High specific surface area High electrochemical stability High electronic conductivity Carbon Nano-Onion: non-edible Fresh Onion: edible Ragone plot: www.imechanica.org Backup power supply Wind energy storage Experiments and results Capacitive properties of CNOs A simple method was used to activate the primitive CNOs by using KOH solution to achieve the increased specific surface areas of CNOs. In brief, (1) CNOs was firstly impregnated in KOH solution for 24 h; (2) The solution was filtered to get the impregnated CNOs. (3) The obtained CNOs was dried in an oven for 12 h at ~90 ℃; (4) At last, the CNOs were annealed at ~800 ℃ in N2 atmosphere for 1 h. Capacitive properties of CNOs before KOH activation (c) 400 W: 16 F/g 10 0 -10 40 800 W: 25 F/g 30 Capacitance (F/g) Capacitance (F/g) 20 15 0 -15 -30 -0.4 -0.2 0.0 Potential (V) 0.2 0.4 1000 W: 30 F/g -45 -0.4 -0.2 0.0 Potential (V) 0.2 0.4 Photographs of ethylene-oxygen flames under laser excitation (The images below show molecular vibration under the excitation conditions). 0 -20 -60 -0.4 -0.2 0.0 Potential (V) 0.2 0.4 CNOs deposited onto Ni foam The SSAs increase with the increase in laser power. Consequently, the capacitance of CNOs increases. However, CNOs with much larger SSAs are needed to achieve improved capacitances. 10.333 m (c) D1 (d) 10.532 µm-600 W D3 2D 2000 -1 3000 1200 Raman shift (cm ) 1500 -1 Raman shift (cm ) Summary of G-band FWHM and R3 for CNOs grown without laser excitation and with excitation at wavelengths of 10.333 and 10.532 µm at 1000 W. CNOs 0 2 4 6 8 KOH concentration for activation (mol/L) 1 10 Pore size (nm) (c) Before activation (d) (c) 150 FWHM of G-band (cm-1) R3=ID3/( ID3 +ID2 +IG) Without laser 71.4 0.24 10.333 m 64.6 0.23 10.532 m 59.5 0.19 6 M activation: 108 F/g 100 50 Before activation: 25 F/g 0 -50 -100 -150 TEM images of CNOs (c) before activation (d) after 6M activation. 3000 5nm 1500 2000 2500 -1 3000 Raman shift (cm ) TEM images of CNOs grown (a) without laser excitation and with different laser powers of (b) 400, (c) 600, and (d) 1000 W at 10.532 µm. (e) Raman spectra of CNOs grown with different laser powers. -0.2 0.0 Potential (V) 5nm (a) 25 welding torches with 3 mm orifice tips will be used to generate the flames. A wavelength-tunable CO2 laser at a wavelength of 10.532 µm will be used to resonantly couple laser energy to the flame. Another laser at 10.591 µm will be used to control the size of the CNOs. A fume collector will be used to collect CNOs generated from the flames. It is estimated that a production rate of 500 g/h will be achieved. 200 Then it followed by decomposition of K2CO3 and/or reaction of K/K2CO3/CO2, with carbon. (c) Before activation 0 10 mv/s 50 mv/s 100 mv/s 500 mv/s 1000 mv/s -0.3 110 0.3 6 h deposition 3 h deposition 800 1600 -1 Raman shift (cm ) 90 80 0.0 Potential (V) 12 h deposition Without deposition 100 0 200 400 600 800 Scan rate (mv/s) 1000 (a) Cyclic voltammograms and (b) the capacitances of CNOs after 6 M KOH activation at different scan rates. 3 6 9 Deposition time (h) 12 12 h deposition: 313 F/g Before activation: 25 F/g 0 -200 Without deposition 400 800 -1 Raman shift (cm ) SEM images of CNOs (a) without MnO2 deposition and with (b) 3 h, (c) 6 h, (d) 12 h deposition. (e) Raman spectra of CNOs without MnO2 deposition and with (b) 3 h, (c) 6 h, (d) 12 h deposition. (f) Raman spectra of the samples in the spectra range from 200 to 1000 cm-1 . -0.4 -0.2 0.0 0.2 0.4 Potential (V) 0.6 (a) 200 300 Time (sec) 400 0.8 10 mv 100 mv 500 mv 1000 mv 5000 mv 600 6 h deposition 3 h deposition 100 (a) Capacitance of CNOs after MnO2 deposition. (b) Galvanostatic charge/discharge curves of CNOs before and after 12 h MnO2 deposition. (c) Cyclic voltammograms of CNOs before and after 12 h MnO2 deposition. . 200 -600 MnO2 0 (c) 600 (f) 12 h deposition (b) 0 -400 (e) 200 -400 500 nm 500 nm 0.0 -0.4 400 (a) Capacitance of CNOs after KOH activation. (b) Galvanostatic charge/discharge curves of CNOs before and after 6 M KOH activation. (c) Cyclic voltammograms of CNOs before and after 6 M KOH activation. 120 -200 (d) 12 h MnO2 deposition 6 h MnO2 deposition 0.4 100 100 0.4 400 It is suggested that the activation of carbon with KOH proceeds as 6KOH + C ↔ 2K +3H2 + 2K2CO3 0.2 50 Time (Sec) 0.8 12 h deposition 500 nm 500 nm Future directions Scalable production of CNOs (b) 300 Capacitance (F/g) Potential (V) 100 After activation 2D 2700 (a) 0 -0.4 2400 6 M activation 0 0 2 4 6 8 KOH concentration for activation (mol/L) 1800 TEM images of CNOs grown (a) without laser excitation and with laser excitations at (b) 10.333 and (c) 10.532 µm. (d) Raman spectra of CNOs grown without laser excitation and with laser excitations at 10.333 and 10.532 µm . (e) Typical curve fitting of a first-order Raman spectrum. -0.2 -0.4 (e) 2500 0.0 40 0.00 D4 5 nm 60 20 The SSAs increase with the increase of KOH concentration. After activation, pores (<= 5 nm) contribute significantly to the total pore volume of CNOs. D2 80 0.02 G Intensity (a.u.) 1500 0.04 (a) SSAs and (b) Pore size distributions of CNOs activated at different KOH concentrations. 5 nm 10.532 m 0.06 3 h MnO2 deposition 0.2 Intensity (a. u) Without laser 500 0.08 100 Capacitance (F/g) Intensity (a.u.) D 5 nm 600 400 (e) G Intensity (a.u.) (d) 5 nm 700 0.10 (b) (a) Before MnO2 deposition Before activation Intensity (a. u) 5 nm 800 Without activation 4M KOH activation 5M KOH activation 6M KOH activation 7M KOH activation (b)0.4 Capacitance (F/g) 10.532 µm-400 W MnO2 deposited onto CNO/Ni foam Capacitive properties of MnO2/CNO hybrid structure (a)120 Capacitance (F/g) (b) Without laser 2 (a) 3 10.532 m (c) Pore volume (cm /g/nm) 10.333 m (b) Without laser (b)0.12 (a) Specific surface area (m /g) (a) Capacitive properties of CNOs after KOH activation Capacitance (F/g) Experiment results • After drying, the CNO coated Ni foams were immersed into the precursor solution ( mixture of 0.1 M Na2SO4 and 0.1 M KMnO4) for MnO2 coating. • After immersing, the samples were rinsed using deionized water and then heated at 120 ℃ for 12 h in air. • CNOs were dispersed in ethanol. Al(NO3)3 was added into the solution to stabilize the CNO particles in the solution; • The suspension was ultrasonicated for 30 min; • Then a layer of CNOs was deposited onto Ni foams by electrophoretic deposition. (a) Specific surface areas (SSAs) of CNOs grown at different laser powers. (b - d) Cyclic voltammograms of CNO electrodes. Illustration of the experimental setup for CNO growth with resonant excitation by a wavelength-tunable CO2 laser. Deposition of MnO2 on CNO/Ni foam Electrophoretic deposition of CNOs on Ni foams 20 -40 -30 -20 Experiment steps: (d) Voltage (V) 10.532 m 30 (b) Capacitance (F/g) 10.333 m Without laser (a) Deposition of MnO2 on CNOs Capacitance (F/g) Experiment set-up Carbon materials have high SSAs, long cycle life, and high conductivity but low capacitance. Metal oxides have high theoretical capacitance but suffer from low SSAs, short cycle life, and low conductivity. Metal oxide/CNO composite is a potential approach to improve both capacitance and conductivity. In this study, capacitive properties of MnO2/CNO hybrid structure were investigated. Capacitance (F/g) A laser-assisted combustion process for growing CNO in open air was developed by using laser irradiations to achieve resonant excitation of precursor molecules. The laser energy was much more effectively coupled into the flame through the resonant excitation of ethylene molecules at 10.532 µm than other non-resonant wavelength. Capacitive properties of MnO2/CNO hybrid structure 300 0 -300 (b) Capacitance (F/g) Synthesis of CNOs 300 200 100 -600 -0.2 0.0 0.2 0.4 Potential (V) 0.6 0.8 0 1000 2000 3000 Potential (V) 4000 5000 (a) Cyclic voltammograms and (b) the capacitances of CNOs after 12 h MnO2 deposition. Acknowledgements High-performance supercapacitors using hierarchical threedimensional micro/nanoelectrodes To achieve increased SSAs and reduced internal resistance at the same time; The Ni foams will serve as conductive scaffolds to house CNTs and CNOs to reduce the matrix resistivity; The CNTs will be grown within the Ni foams to reduce the matrix resistivity and increase the SSA significantly; CNOs will be used to fill the remaining spacing among the Ni forms and to further increase the total SSA; Since all CNTs and CNOs will be filled within the Ni foams, no binder is required. The authors gratefully thank Nebraska Center for Energy Sciences Research (NCESR) and National Science Foundation (NSF) for financial support.