Thin films of carbon nanotubes and chemically reduced graphenes for electrochemical micro-capacitors
Graphical abstract
Research highlights
► Carbon-only thin films of chemically reduced graphene (CRG) sheets separated by layer-by-layer assembled multi-walled carbon nanotubes have high packing density. ► These thin films show high volumetric capacitances in acid, which can be attributed to electrochemical double layer capacitance and redox reactions between protons and surface oxygen-containing groups on CRG.
Introduction
There are a growing number of thin-film electrical energy storage applications from portable electronic devices such as smart cards and e-books, to microelectronics and microsystems, which require higher energy densities than that current micro-capacitor technologies can provide [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Electrochemical capacitors store energy by primarily utilizing electrical double layer (EDL) capacitance and/or surface redox reactions (providing pseudocapacitance), which provide high power (10 kW/kg) and long cycle life but low energy (5 Wh/kg) as compared to rechargeable batteries. State-of-the-art electrodes based on activated carbon (AC) provide reasonably high gravimetric and volumetric capacitances (∼200 F/g and ∼120 F/cm3 in aqueous electrolytes and ∼100 F/g and ∼80 F/cm3 in organic electrolytes) [11], [12]. Recent work on surface functionalization of AC by introducing surface oxygen-containing groups can further increase capacitances up to ∼400 F/g and ∼200 F/cm3 in aqueous electrolytes [13]. The processing of conventional powder-based AC electrodes involves roll-processing methods and doctor-blade techniques, which can control thickness from a few to several hundred microns, but faces many challenges to design more optimized nano-structured, submicron-thick electrodes for micro-scale electronic devices. Very recently AC-based thin-film electrodes of 1–2 μm in thickness prepared by an inkjet printing process have been reported but they exhibit significantly lower capacitances than conventional AC electrodes [5].
Considerable research efforts have been focused on making nano-structured porous carbon electrodes based on carbide-derived carbon [1], carbon nanotubes [3], [6], [7], [14], [15], [16], [17] and graphenes [18], [19], [20], [21], [22], [23], which allow control and tuning of electrode nanostructures and carbon surface chemistry for charge storage. Recent electrochemical micro-capacitor studies based on carbide-derived carbon [1] have shown that ∼2-μm-thick electrodes have much higher volumetric capacitance (∼180 F/cm3) than those of ∼200 μm of thickness (∼30 F/cm3) in organic electrolytes. Decreasing electrode thickness can increase the utilization of electrode surface area in electrochemical reactions and decrease the resistance of ionic and electronic conduction. Although high specific capacitances (up to ∼135 F/cm3 for carbon nanotubes [17] and up to ∼87 F/cm3 for graphenes [21] in aqueous electrolytes) comparable to AC have been reported for electrodes processed from powder having tens/hundreds of microns in thickness, electrodes made by novel processes that can produce electrode thicknesses smaller than one micron, have shown lower capacitances [6], [7], especially on the electrode volume basis (see details in Table S1). Increasing electrode packing densities and coverage of redox-active species on carbon surfaces can increase capacities of thin electrodes required for micro-scale electrochemical capacitors. Previously we have reported the making of multi-walled carbon nanotube (MWCNT) thin-film electrodes by layer-by-layer (LBL) assembly (LBL-MWCNT/MWCNT) [3]. These films consist of only carbon (binder-free) having a high packing density of MWCNTs (∼70%), which show high volumetric capacitances in acidic electrolytes (∼130 F/cm3) [3], and in organic electrolytes (∼180 F/cm3) [4].
The new concept we introduce in this work is demonstrating binder-free thin-film electrodes consisting of graphene sheets separated by MWCNTs for electrochemical capacitor applications. Graphene, a single sheet of graphitic layer, has been considered as a promising electrode material for electrochemical capacitors [18], [19], [20], [21], [22] due to its high surface area (theoretically over 2630 m2/g per single layer) [18], and well-developed surface functionalization methods [24], [25], [26]. However, graphene sheets can pile up via the π–π interaction to have the interlayer distance (∼0.34 nm, too small for ions to access) [18], [20], [21], [22] comparable to that in graphite, which prevents full utilization of the surface area of graphene sheets in electrochemical reactions. Very recently LBL assmbly of negatively charged graphene oxide (GO) and positively charged poly(ethyleneimine) (PEI)-MWCNTs have been reported [23]. However, in order to maximize the electrochemically active area of carbon and electrode capacitance, the presence of insulating polymer/binder in the electrode is not desirable. We here describe the alternate assembly of MWCNT-NH2 (amine-functionalized MWCNT) and GO, after which GO is reduced to CRG (chemically reduced graphene). MWCNTs serve as pillars to separate CRG layers in carbon-only electrodes, which makes the surfaces of graphene sheets electrochemically active. These LBL-assembled MWCNT/CRG electrodes have high volumetric capacitances up to 160 F/cm3electrode, which can be attributed to (1) the EDL capacitance and redox reactions between protons and surface oxygen-containing groups on CRGs in an acidic electrolyte; (2) a high packing density of nanostructured MWCNT/CRG surfaces accessible to ions and electrons; and (3) fast ionic and electronic conduction within the electrode nanostructure.
Section snippets
Amine functionalization of multi-walled carbon nanotube (MWCNT)
To introduce amine groups to MWCNT surfaces, the following two steps were used: (1) oxidation of pristine MWCNT (MWCNT-COOH, carboxylic acid groups on MWCNT) and (2) amine-substitution reaction of the oxidized MWCNT [3]. One gram of raw MWCNT (95% purity, NANOLAB) was refluxed in a solution consisting of 150 mL of H2SO4 (96.7%) and 50 mL of HNO3 (68–70%) at 70 °C for 2 h with stirring, which was then washed in 5% of HCl solution. The oxidized MWCNT dispersion was filtrated at least 3 times using a
Results and discussion
The making of MWCNT/CRG electrodes is shown in Fig. 1. LBL-MWCNT/GO films were first assembled by alternatively dipping plasma-treated ITO-coated glass slides into the MWCNT-NH2 and GO colloidal dispersions. Transmission electron microscopy (TEM) images of a GO sheet and an MWCNT-NH2 are shown in Fig. 2a and b, respectively. Zeta potential measurements show that positively charged MWCNT-NH2 and negatively charged GO colloidal dispersions are both very stable in the pH range of 1–4 while the CRG
Summary
In summary, we report the synthesis of carbon-only MWCNT/CRG thin-films that are cross-linked with amide bonds, which has a packing density of ∼70%. Thin-film electrodes (330–350 nm of thickness) were shown to have high specific (up to 175 F/gelectrode) and high volumetric (up to ) capacitances from three-electrode measurements. The porous MWCNT/CRG network creates fast electronic and ionic conduction channels and surface oxygen-containing functional groups on CRGs enhance
Acknowledgment
This work was supported by the MRSEC Program of the National Science Foundation under the award number DMR 08-19762. S.W. Lee acknowledges a Samsung Scholarship from the Samsung Foundation of Culture. Fruitful discussion with Betar M. Gallant is appreciated greatly.
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