Supporting Information

Transcription

1 Supporting Information Scalable High-Performance Ultraminiature Graphene Micro-Supercapacitors by a Hybrid Technique Combining Direct Writing and Controllable Microdroplet Transfer Daozhi Shen, Guisheng Zou, Lei Liu,*, Wenzheng Zhao, Aiping Wu, Walter W. Duley, and Y. Norman Zhou Department of Mechanical Engineering, State Key Laboratory of Tribology, Tsinghua University, Beijing 00084, P. R. China Department of Physics and Astronomy and Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, Ontario N2L 3G, Canada * address: liulei@tsinghua.edu.cn (L. L.) S-

2 Femtosecond laser setup In these experiments, we used a commercial femtosecond (fs) laser system (Coherent Systems) delivering laser pulses at a central wavelength of 800 nm, with a maximum pulse energy of 4 mj, and a pulse duration of 50 fs at the maximum repetition rate of khz. The laser pulse is obtained from an electronic/mechanical shutter (EM shutter). The pulse energy is varied using a neutral density (ND) filter together with a l/2 plate and a polarizer filter. The laser fluence can be continuously adjusted by this setup. The laser and light from a halogen lamp entering the objective are coaxial, so the charge-coupled device (CCD) camera can be used to check whether the sample is at the focal plane of the objective. All processes were carried out in air at atmospheric pressure. Micro-supercapacitors fabrication A silicon wafer with top layer of silicon oxide coated with graphene oxide (GO) film was placed on a XYZ stage. The stage moved at a velocity of 0 µm s -. Fs laser pulses with an energy of 0.0 µj were tightly focused using a microscopy objective (Olymbus) with 00 magnification and a numerial aperture (NA) of 0.4 on the GO film to achieve reduced GO (rgo) electrodes array. After the writing of rgo electrodes, a glass coated with poly(vinyl alcohol)-sulfuric acid (PVA-H 2 SO 4 ) gel with thickness of 00 µm was placed above asprepared rgo electrodes at a distance of 500 µm. Fs laser pulses with an energy of 4 µj were focused using objective (Olymbus) with 0 magnification and a NA of 0.3 through the glass onto the electrolyte gel to generate single electrolyte microdrops. The location of drop transferred on receiver substrate was checked by CCD camera to make sure an accurate coverage on the rgo electrodes. All graphene reisitor-capcacitorfilter fabrication An aqueous GO suspension was drop-cast on glass patterned with gold electrodes as current probes. After drying, the GO film was irradiated by fs laser pulses to form a serpentine resistor circuit and rgo electrodes for micro-supercapacitors (MSCs). These two components were connected by a Au electrode. Electrolyte droplets were then transferred on the rgo electrodes using the method described above. Laser pulse energies of 4 µj and 0 µj were used to transfer droplets with diameter of 75 µm and 200 µm, respectively. The small droplet exactly covered the interlaced electrodes while the area of the large droplet exceeded that of the interlaced electrode and partially covered the rgo resistor. S-2

3 Electrochemical performance calculations Capacitance values were calculated from the cyclic voltammetry (CV) data according to the following equation []: C = ν(v f V i ) V i V I(V) dv () f Where C is the capacitance contribution from rgo electrodes, ν is the scan rate (in V s - ), V f and V i are the potential limits of the voltammetric curve, I(V) is the discharge current (in A) and V is the applied voltage (in V). The capacitance was also calculated from the galvanostatic charge/discharge curves using [2] C = I dv/dt (2) Where dv/dt is the slope of the discharge curve (in V s - ). Specific capacitances were calculated based on the area or volume of the device stack as follows C areal =C/A area (3) C volumetric =C/V volume (4) Where C areal and C volumetric refers to the area and volume capacitance and colume capacitance of the device, respectively. A areal and V volumetric are the total area and volume of the device, respectively. Set the CV test of MSC at the scan rate of mv s - for example: The capacitance of as-prepared fsrgo is C = ν(v f V i ) V i V I(V) dv = f 0.00 (0.5 0) V i V I(V) dv = f 0.00 (0.5 0) = F (5) As the area and thickness of umtraminiature MSC is about 0-4 cm 2 and 0.6 µm respecitvely, the areal and volumetric sepcific capcitance is C areal = C volumetric = C = F = 6.3 mf A areal 0 4 cm 2 cm 2 (6) C A volumetric = F cm 3 = 05 F cm 3 (7) The electrochemical performance shown in the Ragone plot was based on the volume capacitance. For a given scan rate ν (V s - ), the discharge power P (in W) was calculated using the methodology of Pech et al [3] V i P = I(V) VdV (8) V f S-3

4 The power density in the device P d (in W cm -3 ) was obtained from the formula : The energy density E d (W cm -3 ) was calculated from Low Pass resistor-capacitor filter analysis P d = P/V volume (9) E d = (V f V i ) ν 3600 P d (0) In an alternation current (AC) circuit, Ohm s law is amended to take into account the behavior of capacitors and inductors, which are characterized by the impedance. Consider the resistor-capacitor (RC) circuit shown in Fig. 5e, the ratio of the output voltage to the input voltage is V out V in = jwc R+ jwc = = +jwrc = () +(wrc) 2 +(2πfRC) 2 Where j =, w is the phase of the input voltage, f is the frequency of the input voltage. For the circuit in Fig. 5f, R <R, the ratio is V out V in = jwc R + jwc + jwc = = + C C +jwr C (+ C C )2 +(wr C) 2 = (+ C C )2 +(2πfR C) 2 (2) When V out = V out, +(2πf p RC) 2 = (+ C C )2 +(2πf p R C) 2 (3) Then and, + (2πf p RC) 2 = ( + C C ) 2 + (2πf p R C) 2 (4) f p = C2 C 2+2C C 2πC R 2 2 R (5) When f=0, V out < V out. Since both V out and V out monotonously decrease as f increases, V out > V out when f > f p. The relationship between V out, V out and f is shown schematically in Fig. S2. S-4

5 When the input signal contains high frequency noise, the output voltage of small size electrolyte is smaller than that of large electrolyte droplet, resulting good filter ability as shown in Fig. 5h. Figure S. (a) Schematic of the fs laser setup for direct writing of rgo circuits. (b) The fs laser beam is transmitted through the objectives and the focal plane was monitored by CCD imaging. Figure S2. The current during electrochemical testing was collected by the two tungsten tips connected to probe stations. S-5

6 Figure S3. The atom force microscopy (AFM) measurement of as-prepared GO and fsrgo. (a) AFM image of GO/rGO film with edge. The dark area is substrate. (b) The magnified AFM image of fsrgo, GO and substrate. Height distribution of places as dash line (c) line and (d) line 2 shown in (b). It shows the fsrgo has some ripples with more rough compared with as-prepared GO. The thickness of fsrgo and GO is 0.6 µm and 0.5 µm, respectively. Knowing the thickness, the density of fsrgo is estimated to be.2 g cm -3. Figure S4. Microstructure of as-prepared GO without irradiation by fs laser. (a)scanning electron microscopy (SEM) image and (b) transmission electronic microscopy image of GO. (c) Magnification image of place marked in (b). S-6

7 Figure S5. The relationship between the pattern width of the fs laser rgo circuit and laser fluence. Figure S6. The diameter of transferred electrolyte drops vs. laser fluence on the surface of electrolyte gel film on donor glass. Optical microscopy images of transferred electrolyte drops on glass are shown at given fluences. S-7

8 Figure S7. Transmission electron microscopy (TEM) images of fs rgo (a) Graphene flakes. (b) and (c) Abundant nanopores with diameter of <0 nm exist in the graphene flakes. (d) Diffraction pattern of rgo. The graphene flakes deformed significantly after exposure to the electron beam resulting in folding and the appearance of several series of diffraction patterns. The hexagonal patterns shown in d indicate the presence of hexagonal graphene structure. Thus GO is not transformed to amorphous material by reduction by fs laser radiation. Figure S8. TEM images of fs rgo randomly selected from different areas. (a) Area, (b) area 2, (c) area 3 and (d) area 4. S-8

9 Figure S9. Magnified images as marked in (a) Figure S8a, (b) Figure S8b, (c) Figure S8c and (d) Figure S8d. Figure S0. Mesopore size distribution calculated from TEM images of fsrgo sheets. S-9

10 Figure S. Characterization of fsrgo with inter-electrode spacing of ~ 550 µm (a) Optical image of fsrgo pad. The ripple area was reduced by fs laser. (b) CV curves of fsrgo MSC with spacing of 550 µm at a scan rate of 500 mv s -. This curve shows that the operational potential window is.0 V, which is much wider than that of ultraminiature fsrgo MSC with inter-electrode spacing of only 2 µm. Figure S2. The comparison of PVA-H 2 SO 4 gel before and after irradiation by fs laser for min. (a) ph of electrolyte gel and (b) CV curve of fsrgo with electrolyte gel before and after irradiation by fs laser. The irradiation of fs laser does not change neither the H + concentration in electrolyte nor the capacitance of fsrgo MSCs with interspacing of 550 µm. It seems that the irradiation of fs laser does not change the property of electrolyte gel. Figure S3. CV curves of fs rgo MSCs at scan rate of mv s -, 5 mv s -, and 0 mv s -. S-0

11 Figure S4. The IV curve of rgo resistor written by fs laser. The curve has a shape of liner structure, indicating the pure resistor of fsrgo. As the absolute current is very low, it is easily affected by the external fluctuation, resulting the unsmooth curve as illustrate. This fluctuation can be also seen in CV and GCD curves of fsrgo MSCs during the test. It is believed that this affection can be reduced by proper electric shielding tool and by decreasing the vibration of probe station. Figure S5. CV curves of (a) bare GO film and (b) fs rgo interlaced electrodes at a scan rate of 500 mv s -. Note that the current in the GO film is >300 times smaller than that in fs rgo. S-

12 Figure S6. The edge effect of electrodes in determining current density during CV measurements.[4, 5] The schematic shows the relationship between electrode size and the contribution of convergent diffusion to the voltammetry. Figure S7. Image of an rgo circuit used for resistance measurement. rgo circuit was covered with an Au pad for current collection. Table S. The summary of resistance of fs rgo circuit in Figure S7. laser fluence Resistance Length [J cm -2 ] [MΩ] [µm] 0 ~.25x ~.2x S-2

13 Figure S8. RC filter circuits constructed by direct writing before electrolyte transfer. (a) Resistor circuit is collected with MSC electrode in series. (b) Magnified image of resistor circuit marked in (a). Figure S9. Schematic diagram showing the relationship between the output signal and the frequency of the input signal. As analyzed above, the output signal of the RC filter with a small electrolyte droplet is higher than that of the filter with a large electrolyte droplet at low frequency while it is smaller at high frequency. This shows that the filter with a small droplet has higher filter efficiency for white noise. References () Wu, Z. S.; Parvez, K.; Feng, X.; Mullen, K., Graphene-Based In-Plane Micro-Supercapacitors with High Power and Energy Densities. Nat. Commun. 203, 4, (2) Lobo, D. E.; Banerjee, P. C.; Easton, C. D.; Majumder, M., Miniaturized Supercapacitors: Focused Ion Beam Reduced Graphene Oxide Supercapacitors with Enhanced Performance Metrics. Adv. Energy Mater. 205, 5, S-3

14 (3) Pech, D.; Brunet, M.; Durou, H.; Huang, P.; Mochalin, V.; Gogotsi, Y.; Taberna, P. L.; Simon, P., Ultrahigh-Power Micrometre-Sized Supercapacitors Based on Onion-Like Carbon. Nat. Nanotechnol. 200, 5, (4) Marken, F.; Neudeck, A.; Bond, A.; Scholz, F., Electroanalytical Methods: Guide to Experiments and Applications. Scholz, F., ed, Springer-Verlag, Berlin: 200, (5) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G., Electrocatalysis at Graphite and Carbon Nanotube Modified Electrodes: Edge-Plane Sites and Tube Ends Are the Reactive Sites. Chem. Commun. 2005, 7, S-4