Layer Graphene Nanoribbons as High-Capacitance Supercapacitors

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Shedding Light on Pseudocapacitive Active Edges of Single-Layer Graphene Nanoribbons as High-Capacitance Supercapacitors Mohammad Qorbani, Ali Esfandiar, Hamid Mehdipour, Marc Chaigneau, Azam Iraji Zad, and Alireza Z. Zaker Moshfegh ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00375 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Energy Materials

Shedding Light on Pseudocapacitive Active Edges of SingleLayer Graphene Nanoribbons as High-Capacitance Supercapacitors Mohammad Qorbani,† Ali Esfandiar,*,† Hamid Mehdipour,† Marc Chaigneau,‡ Azam Irajizad,

!#

Alireza

Z. Moshfegh

!#

†Department

of Physics, Sharif University of Technology, Tehran 11155-9161, Iran, ‡HORIBA France,

Avenue de la Vauve, Passage Jobin Yvon, 91120, Palaiseau, France, #Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran *Address correspondence to [email protected]

KEYWORDS: two-dimensional materials, quantum lateral confinement, edge effects, quantum capacitance, electric double-layer, faradaic reaction ABSTRACT In the field of energy storage by high-rate supercapacitors, there has been an upper limit for the total interfacial capacitance of carbon-based materials. This upper limit originates from both quantum and electric double-layer capacitances. Surpassing this limit has been the focus of intense researches in this field. Here, we precisely investigate the effect of chemical functional groups and physical confinement on the electrochemical performance of graphene nanoribbons. We present the results of a quasi-one-dimensional single-layer graphene nanoribbon (120 nm in width and ~100 µm in length) microelectrode fabricated by mechanical exfoliation of graphite, followed by electron beam lithography process and oxygen plasma etching treatment. We directly measure the interfacial capacitance as a function of frequency at different potentials in an aqueous electrolyte using a three-electrode electrochemical system. Electrochemical impedance 1 ACS Paragon Plus Environment

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spectroscopy and cyclic voltammetry tests show an average capacitance of 75 µF cm-2 at 100 kHz to 1.16 mF cm-2 at 1 Hz and 0.35±0.04 mF cm-2, respectively, well above the upper capacitance limit of carbon-based electrodes. First principle density functional calculations and cyclic voltammetry (at varies scan rates) illustrate that presence of oxygen functional groups passivating the nanoribbon edges and lateral structural confinement, as well as occurrence of pseudocapacitive reactions, lead to such very large capacitances at low and high frequencies. Our results suggest a new and closer sight on nanoribbons as a potent candidate for energy storage devices and provide a fundamental platform for studying the effect of lateral structural confinements accompanied by the presence of various functional groups on the electrochemical performance of single/few-layer carbon-based materials. INTRODUCTION Electrochemical double-layer or pseudocapacitive capacitors also called as supercapacitors can store charge using reversible electrostatically adsorption/desorption of ion species or fast faradaic reactions at electrode/electrolyte interface.1 Micro-supercapacitors ?K

B as

miniaturized energy-storage devices not only store electrical energy for portable high power delivery and uptake applications into circuits of microdevices but also can act as a highly durable output regulator for photovoltaic systems and hybrid vehicles.2-4 Performance of a supercapacitor strongly depends upon the local dielectric constant of the electrolyte and electrochemically active specific surface area (SSA) of the electrode materials.5 In this regard, there is a potent interest in using various morphologies of carbon-based materials and electrolytes due to their tenability in confinement effects of fields and often-cited physicochemical properties at the interfaces, respectively.6-8 Microporous activated carbon in 1 M H2SO4,9 carbide-derived carbon (CDC) in 1 M TEABF4,10 onion-like carbon 1 M Et4NBF4/anhydrous propylene carbonate,11 templated carbon 2 ACS Paragon Plus Environment

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in both aqueous and non-aqueous electrolytes,12 N-doped carbon nanotube (CNT) in solid-state PVA-H3PO4 electrolyte,13 and reduced graphene oxide (rGO) in different electrolytes are some examples that have been studied,14-16 in the past years. Among them, graphene has received wide spreading attention due to its very high intrinsic electrical conductivity, great chemical stability, superb mechanical strength and extremely high theoretical SSA of 2630 m2 g-1.17 Despite these superior properties, there are three bottlenecks: (i) a small quantum capacitance (QC), due to the low electronic density of states (DOS) near the Dirac point, in series with the electrical doublelayer capacitance (EDLC);18, 19 (ii) reduction of the dielectric constant (to ~2) (due to the saturation effect and decreasing the rotational freedom of electrolyte (mostly water) molecules) at the Sternlayer caused by high applied electric field (>107 V m-1);20-22 and (iii) low electrochemical active sites because carbon material does not easily participate in reversible electrochemical faradaic reactions. These inherent drawbacks result in a low upper limit for the EDLC of ~21 µF cm-2 (or ~550 F g-1), a small value of QC about an order of magnitude lower than the EDLC, and low pseudocapacitance (PC). In order to overcome the abovementioned drawbacks, so far three distinct research strategies have been proposed theoretically and employed experimentally: (i) enhancing the EDLC by modulating the micropore sizes (800) are fabricated by electron beam lithography (EBL) followed by oxygen plasma etching. Raman spectroscopy and tipenhanced Raman scattering (TERS) reveal that the edges of the fabricated GNRs contain defects in the form of oxygen passivation. Based on our detailed density functional theory (DFT) calculations, confinement and oxygenated edges can increase not only the QC but also the

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conductivity of the GNR near the Fermi level. On the other hand, functionalized groups at the edges can store energy due to the fast faradaic reactions taking place at the edges. These phenomena lead to a high IC measured under low- and high-frequency conditions. In a threeelectrode system, the GNR electrode shows electrochemical capacitances about 25 and 113 times higher than the pristine single-layer graphene at 100 kHz and 1 Hz, respectively. To the best of our knowledge, this is the first report on the single-layer graphene nanoribbons as a supercapacitor electrode which presents considerable values for their electrochemical properties due to key roles of the active edges and confinement effects. RESULTS AND DISCUSSIONS Characterization. The optical micrograph images of the prepared GNRs connected to Au electrodes wo/w PMMA layer are shown in Figure 1(A, B). The geometric surface area of the fabricated GNR microelectrodes is about 3000 to 3500 µm2. To ensure the formation of appropriate contacts, GNRs were connected between two Au electrodes. Figure 1(C-G) show the large area and high-resolution AFM and SEM images of the GNRs as well as the height profile obtained. The images display a uniform and contamination-free 2D fabricated pattern of the single-layer GNR microelectrode. The average width and length of the nanoribbons are measure at about 120 nm and 100 µm, respectively. The gap between the nanoribbons is also 120 nm. Moreover, highresolution SEM image of the nanoribbons demonstrate the presence of smaller ribbon-like branched features on the edge of the nanoribbons, which formed due to the EBL process and oxygen plasma etching (arrows in Figure 1(F)). The length and width of such ribbon-like features are estimated in the ~5-30 nm and ~5-15 nm ranges, respectively. The formed oxygenated edges and confined features can act as doping and thus change the electrical properties of the



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Raman spectroscopy is a widely used technique for characterizing functionalized and defective carbon-based products. The Raman spectrum of the pristine graphene exhibits the characteristic signals of single-layer graphene including the G band (at 1591 cm-1 and full width at half maximum (FWHM) ~7 cm-1) with the doubly degenerate (in-plane transverse optical, iTO, and longitudinal optical, LO) phonon mode (E2g symmetry) at the Brillouin zone center, 2D (or RB band (at 2687 cm-1 and FWHM ~28 cm-1) involving two iTO phonons near the K point (Figure 2(A)).46 After EBL and oxygen plasma etching processes, additional D (due to the one iTO phonon and one defect at the edge regions) and shoulder $R (due to the one iLO phonon and one defect at the edge regions) bands at 1345 cm-1 and 1630 cm-1 emerged.47 Moreover, the 2D band broadens (FWHM ~48 cm-1) and shifts to a lower wave number of 2676 cm-1 because of chemical doping due to the presence of oxygen functional group at the edges.48 We also observe that the G band broadens to FWHM ~84 cm-1 (FWHMGNR/FWHMGraphene~12) with an obvious upshift of 7 cm-1. This broadening of FWHM can also be assigned to the presence of oxygen functional group after plasma etching processes (see

48-51).

In order to confirm and visualize the effect of the oxygen

plasma process, high-resolution TERS map (200×100 nm2, 10×5 pixels, pixel step of 20 nm) was taken across a single GNR. Indeed, TERS is extremely sensitive to the presence of local defects in carbon-based materials, especially graphene. This near-field technique is able to probe down to single defect (the Raman intensity of the D band is proportional to the defect density in the Raman probed surface; in TERS, the probed area is down in 10 nm2).52 Figure 2(B) shows the averaged typical TERS spectra from edge and center regions of a single GNR. It shows the emergence of the D band at the edge regions due to local defects as well as a higher signal of the 2D band and slightly broadening of G band. The TERS image obtained, while monitoring the Raman intensity of the D band, is overlaid on the AFM topography image showing three parallel nanoribbons



(Figure 2(C)). In this TERS image taken of the central nanoribbon, a few pixels show an intense Raman activity related to the D band and thus the local presence of defects. The signal of TER spectrum of the D band very rapidly within a few pixels at the edges of the GNR, the typical green TER spectrum from the edge region of nanoribbon and the blue TER spectrum from the interior of the nanoribbon are spatially separated by 40 nm. This confirms the presence of local defects in the graphitic structure of the GNR, especially at the edge, associated with the oxygen plasma treatment.

1200

B

Pristine Graphene GNR-120nm

D

G D'

D

1400

1600

Edge Center

2D

Intensity (a.u.)

Intensity (a. u.)

A

2600

G

2D

1400

2800

Raman shift (cm-1)

1600

2600

2800

-1

Raman shift (cm )

0.5 nm

C D band

2D band

-3 nm

100 nm

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Figure 2. Micro-Raman and TERS spectra of the graphene nanoribbons. (A) Micro-Raman spectra of the pristine graphene and GNRs. (B) Averaged typical TER spectra obtained from the TERS image, from the edge region (green spectrum) and from the center of the GNR (blue spectrum). (C) Overlay of the AFM topography map of GNR sample and TERS images of D band (1340 cm-1,


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green color) and 2D band (2662 cm-1, blue color) intensities. 10×5 pixels; Integration time per pixel: 1 s; Excitation wavelength: 638 nm.

Electrochemical tests. Figure 3(A) shows the schematic experimental setup designed for measuring the capacitance of the GNRs exposed to 1M H2SO4 aqueous electrolyte. Because of large Thomas-Fermi screening length in graphene, 1M H2SO4 can completely neutralize the impurity charges on the SiO2 substrate, leading to very large charge carrier mobility in the atomically thin GNRs.53 Figure 3(B) illustrates the Nyquist plots of the fabricated electrodes at 500 mV bias voltage. All plots exhibit a similar capacitive behavior with a very low series resistance of about 5-10 %T cm2. Figure 3(C) shows clearly the frequency dependence of the real capacitances for the gold, pristine graphene and GNRs microelectrodes. A very large capacitance of 0.98 mF cm-2 is measured for the GNR-120nm which is two orders of magnitude higher than that of the pristine graphene at 1 Hz. This enhancement in capacitance of the GNR microelectrode can be attributed to the electrochemical activities of the edges of the GNR. The capacitance difference between the GNR and the pristine graphene is very large at very low frequencies and it declines with increasing the frequency, though still remain tangible at very high frequencies tested. The capacitance measurements in high-frequency domains can provide more information about the nature of the IC that will be discussed later. Figure 3(D) shows the Nyquist plots of the high-performance sample, i.e. the GNR-120nm, at different applied voltages. The performance of the GNR-120nm electrode depends on the applied voltage, especially at low-frequency domains. The average phase angle and series resistance are about -75° and 100 T! respectively. The real and imaginary capacitances are calculated showing relaxation time, low and high frequencies capacitances (average values) of 18-25 ms (22 ms), 0.87-1.65 mF cm-2 (1.16 mF cm-2) and 72-81 9 ACS Paragon Plus Environment


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µF cm-2 (75 µF cm-2) (Figure 3(E)). The obatiend capacitances (at high and low frequencies) are larger than measured capacitances reported so far for single layer carbon-based materials. They are also comparable with capacitances obtained for bulk semi-industrial electrodes (see Table 1). High-frequency measurements provide more detailed information about the role of structural confinement and oxygenated edges on the overall capacitance. As shown in Figure 3(F) the average IC value of the pristine graphene is 2.9±0.3 µF cm-2 which is almost constant for all the electrode potential applied. This can be rationalized by noting that the phase angle of pristine graphene is -86° (Figure 3(G) and this capacitance generally comes from the EDLC in series with the QC). The minimum value of the QC for the pristine graphene is calculated of 3.4±0.3 µF cm-2 (see Section S1). This minimum value corresponds to impurity density n imp =9.8×1012 cm-2 by applying a self-consistent random phase approximation (RPA)–Boltzmann formalism which is quite reasonable for graphene on SiO2 substrate.54 Also, the effective dielectric constant of water is calculated

im

=3.2±0.2 for the Stern layer (see Section S2) in agreement with previous reports.20, 22 The low dielectric constant of the electrolyte at the solid/liquid interface is due to the saturation effect (modelled by Booth)20 and decreasing the rotational freedom of electrolyte (mostly water) molecules made by the high local applied field. Therefore, calculated small DOS (see DFT calculation) and the very small dielectric constant of the Stern-layer, the overall capacitance of graphene flake is expected to be very low. Instead, GNR-2000nm, GNR500nm, and GNR-120nm electrodes show minimum ICs of 13.9±0.6, 36.1±1.1 and 72.2±3.4 µF cm-2, respectively (Figure 3(F)), which are more than one order of magnitude larger than that of pristine graphene.


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Since the diffuse layer capacitance ( C GC ) is independent of the geometry, by using

1 (C A C PS ) 1 C q 1 C St 1 C GC , both the quantum and Stern-layer capacitances increases with narrowing the oxygenated edge GNRs. It should be noted that the phase angle of this electrode is about -70° which is smaller than the phase angle of pristine graphene (-86°) measured at the same frequency. This implies the occurrence of some fast pseudocapacitive reactions at the GNR edges in the high-frequency range. Given that, a proposed equivalent circuit, i.e. ESR C EDLC ,QC PR ct ,1 C PS PR ct ,2 (where ESR , C EDLC ,QC , C PS and R ct ,i s are equivalent series resistance, non-faradaic capacitance, pseudocapacitance and chargetransfer resistance elements, respectively) suggest that roughly about 50% of the measured total capacitance is originated from pseudocapacitive reactions. By subtracting the faradaic component, contribution of non-faradaic capacitance is estimated 35-40 µF cm-2 which is still much higher than the pristine graphene capacitance. This enhancement is attributed to the structural confinement accompanied by the presence of the oxygen functional groups (passivating the edge sites) which gives rise to the increase of the C q . Also the reduction of the electric field at the basal plane contributes to such capacitance enhancement by increasing the C St . To investigate the capacitive properties of the most interesting samples (GNR120nm) with frequency-independent measurements, we performed CV tests in a similar electrolyte. Figure 3(H) displays the CV curves of the GNR-120nm electrode at different scan rates from 200 to 2000 mV s-1 in a 1M H2SO4 aqueous electrolyte. It shows a rectangular shape implying an ideal pseudocapacitive response. Since the potential is continuously changing in a CV measurement, current is proportional to the scan rate. By calculating the slope of cathodic current as a function of the scan rate, the total normalized 11 ACS Paragon Plus Environment


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capacitance is obtained at around 0.35±0.04 mF cm-2 (to compare with other reports see Table 1). On the other hand, lower scan rates (

Layer Graphene Nanoribbons as High-Capacitance Supercapacitors

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