Exploring the Capacitive Behavior of Carbon Functionalized with

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Exploring the Capacitive Behavior of Carbon Functionalized with Cyclic Ethers: A Rational Strategy to Exploit Oxygen Functional Groups for Enhanced Capacitive Performance Junghyun Lee, Muhammad A Abbas, and Jin Ho Bang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00929 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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ACS Applied Materials & Interfaces

Exploring the Capacitive Behavior of Carbon Functionalized with Cyclic Ethers: A Rational Strategy to Exploit Oxygen Functional Groups for Enhanced Capacitive Performance Junghyun Lee,† Muhammad A. Abbas,‡ and Jin Ho Bang†,‡,#,* Department of Bionano Technology, Nanosensor Research Institute, and Department of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea AUTHOR INFORMATION Corresponding Author: Jin Ho Bang *

†

Email: [email protected] Department of Bionano Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu,

Ansan, Gyeonggi-do 15588, Republic of Korea ‡

Nanosensor Research Institute, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan,

Gyeonggi-do 15588, Republic of Korea #

Department of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-

ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT. The presence of oxygen functional groups (OFGs) on a carbon surface is a doubleedged sword in electric double-layer capacitors (EDLCs) because of their mixed influences on capacitance. Critical problems of common OFGs are greatly decreased electrical conductivity, steric hindrance limiting the migration of ions, and promoted self-discharge via faradaic reactions. To explore a new breakthrough to these long-standing problems, carbon electrodes selectively functionalized with cyclic ether groups (CEGs) are investigated with in-depth spectroscopic and electrochemical analyses. The in-plane CEGs embedded in the graphene matrix are greatly advantageous over conventional out-of-plane OFGs for EDLC performance because they can boost capacitance via pseudocapacitance while substantially minimizing all the negative effects of traditional OFGs. This study also reveals that preserving the original sp2 carbon network during surface functionalization is crucial to maximizing the benefits of OFGs. These new insights call for development of elaborate surface engineering strategies that can introduce functionalities with no significant damage to π-conjugation.

KEYWORDS: electric double-layer capacitor, oxygen functional group, cyclic ether group, surface functionalization, capacitive behavior


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INTRODUCTION EDLCs store energy by utilizing the reversible electrostatic charging phenomenon at the carbon/electrolyte interface.1 Given that capacitance is given by C=εrε0A/d, where εr is the electrolyte dielectric constant, ε0 is the vacuum permittivity, A is the accessible surface area, and d is the Debye length, it is apparent that maximizing the specific surface area of carbon is important for EDLCs. In addition, elaborate engineering of pore size and distribution is crucial to fully accommodate electrolyte ions on a carbon surface.2-4 To this end, tremendous effort has been exerted to establish a design principle for nanoporous carbons that can optimize surface utilization.5 In addition to these important physical characteristics of carbon in EDLCs, the chemical characteristics (i.e., surface properties) of carbon also have a profound influence on the capacitive behavior of EDLCs.6 Pseudocapacitance arising from the redox reaction of surface functionalities (e.g., nitrogen- or oxygen-containing functional groups) is a good example of this chemical effect.7-10 In particular, the role of OFGs in capacitive behavior cannot be neglected because their presence on a carbon surface is unavoidable; OFGs are innately inherited from carbon precursors or are introduced during various activation processes.11,12 While the types of OFGs present in different carbons vary, the most common functionalities are carboxyl, epoxide, hydroxyl, and ketonic-type groups (Scheme S1). A comprehensive understanding of the effects of these OFGs on capacitance is still lacking, but their benefits (pseudocapacitance and improved wettability that facilitates adsorption of electrolyte ions) have been judiciously exploited to enhance capacitance. Many articles in the literature have preferentially highlighted the benefits of OFGs;7,13-16 however, these advantages are often significantly counterbalanced by several negative effects.17 For instance, OFGs decrease the electrical conductivity of carbon, which can lead to considerably distorted EDLC behavior and

significant deterioration of rate capability.



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Furthermore, bulky OFGs not only hinder the migration of electrolyte ions into micropores, but also weaken the interaction between the carbon surface and electrolyte ions,18 reducing pore utilization. Another crucial, yet often overlooked problem with OFGs is that they can serve as a channel for self-discharge via parasitic redox reactions in the form of leakage current.19 Given these conflicting features, it is of great importance to devise a rational strategy to fully exploit the benefits of OFGs while minimizing the drawbacks. To this end, we explored carbon electrodes functionalized specifically with CEGs. The exclusive use of these functional groups could be advantageous because, unlike common OFGs, they can be incorporated into graphene layers without significantly damaging carbon’s π-conjugation system.20,21 For instance, Guo and coworkers recently demonstrated that a controlled oxidation could incorporate oxygen atoms onto the edges of vacancy defects formed in the graphene lattice, forming crown ethers (i.e., cyclic poly-ethers) in the graphene matrix.22 Unlike other OFGs that are out of the basal plane of graphene,23 this unique cyclic ether configuration is in-plane and hence would minimize perturbation of the electron cloud (delocalized electrons) residing between graphene layers. Aggregated cyclic edge ethers that can be formed preferentially at the edges of graphene are another example of CEGs that do not impair π-conjugation significantly.24 In addition, unlike common OFGs that are electron-withdrawing groups, CEGs are electron-donating groups, which could facilitate pseudocapacitance.25 Therefore, exploiting CEGs could be a promising remedy to address the issues of common OFGs. RESULTS AND DISCUSSION Understanding the role of CEGs in the capacitive behavior of carbon is a challenging task because introducing OFGs to carbon often alters the porous structure of the carbon significantly; thus, decoupling the effect of CEGs from that of the pores can be a challenging. In addition, when


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dealing with a carbon powder, this task becomes more difficult because of the complexity and irreproducibility entailed in preparing a carbon paste and a thin film. Potential challenges include contact resistance at the interface of carbon and substrate, the binder effect, and inhomogeneity in film composition, thickness, and porosity.4,26 To circumvent these problems, we used a freestanding, nearly OFG-free carbon monolith composed of highly graphitic carbon fibers (referred to as P-carbon (pristine carbon)) and carefully introduced OFGs while minimally altering the porous structure of the resulting functionalized carbon electrodes. This fabrication process allowed us to isolate the influence of CEGs on the capacitance of carbon. P-carbon was treated either by thermal oxidation or electrochemical oxidation to functionalize its surface (Figure 1a).27-29 Thermal treatment in O2 yielded a carbon electrode that was primarily functionalized with CEGs (E-carbon), whereas electrochemical oxidation yielded a carbon electrode decorated with various common OFGs such as carboxyl, epoxide, hydroxyl, and ketonic species (C-carbon). A key to CEG functionalization is the mild thermal oxidation in O2 over time, which could effectively create an epoxy-oxygen intermediate.30 In contrast, electrochemical oxidation is so radical that it randomly attacks the graphene layers, resulting in significant damage to their basal planes. This electrochemical process produced a hydrophilic carbon electrode within minutes (Figure S1, Table S1). It is evident from Figure 1b that, unlike C-carbon where there was a noticeable color change on the bottom of the strip, E-carbon looked as intact as P-carbon even though it became hydrophilic (Figure S2). This implies that the surface of E-carbon was not as damaged as that of C-carbon. The surface functionalities of the E- and C-carbons were examined by Fourier-transform infrared (FT–IR) spectroscopy. The FT–IR spectra of E-carbon (Figure 1c) showed broad peaks centered at 957 and 1089 cm-1 (Table S2),31 corresponding to C‒O‒C stretch modes of CEGs and unzipped epoxides created during CEG formation.20,21,32,33 Along with the signature of the CEGs, the in-



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plane vibration resulting from the presence of sp2-hybridized C=C and C‒OH vibration from adsorbed H2O were also evident at ~1631 and ~3431 cm-1, respectively. The intensities of peaks from CEGs progressively increased with oxidation time, which is in good agreement with the decrease in contact angles over time (Figure S1). The spectral features of C-carbon, however, were quite different from those of E-carbon. Carbonyl and hydroxyl from carboxyl groups, epoxides, and cyclic edge ethers were observed (Figure 1d, Table S2), among which the bulky carboxylic groups and out-of-plane epoxides constituted the major surface components. The rapid increase in peak intensities over short oxidation times reflects the radical change in the carbon’s surface. Xray photoelectron spectra (XPS) in the region of O 1s verified the observations from the FT–IR spectra. Unlike the XPS spectra of P-carbon, which contains a small peak corresponding to the carbonyl group (Figure 1e, S3), those of E- and C-carbons exhibited multiple OFG peaks associated with vacancy defects formed in the basal plane of graphite.23,34 CEGs and unzipped epoxides were major OFG components of E-carbon (Figure 1f), whereas hydroxyl, epoxide, and carbonyl groups comprised a large proportion of the OFGs in C-carbon (Figure 1g).


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Hydroxyl (major) Cyclic ether (minor) (533.3 eV)

Adsorbed water (534.4 eV)

Epoxide (532.1 eV)

Carbonyl (531.2 eV)

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Figure 1. (a) Schematic illustration of the basal plane and edge-site views of P-, E-, and C-carbons. (b) Contact angle measurement images and photographs of P-, E-, and C-carbons. (c) FT–IR spectra of E-carbon showing the evolution of CEGS during thermal oxidation at 450 °C. (d) FT– IR spectra of C-carbon showing the evolution of various OFGs during electrochemical oxidation. XPS O 1s spectra of (e) P-carbon, (f) E-carbon, and (g) C-carbon. Raman spectroscopy was used to examine the crystalline nature of the carbon electrodes. After each oxidation process was complete, the characteristic Raman signals centered at 1356 and 1581 cm-1 (i.e., D and G bands) became broader, more so in the spectra of C-carbon than in that of E-carbon. The peak intensity ratios (ID/IG) of P-, E-, and C-carbons were 0.73, 0.48, 0.24, respectively (Figure S4), revealing that π-conjugation in C-carbon was more interrupted than that in E-carbon. The significantly reduced peak intensities in the X-ray diffraction patterns are in line with this observation (Figure S5). More insight into the structural changes induced by each



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oxidation process was obtained by careful deconvolution of the raw Raman data (Figure 2a). The G1 and D1 peaks originate from the graphitic basal planes with ordered bond angles, whereas the G2 and D2 peaks originate from the sp2 clusters of amorphous carbon with disordered bond angles.35,36 The appearance of Dʹ is attributed to a double resonance Raman feature created by structural disorder,23 and the ratio of D/Dʹ has been proposed as a gauge to characterize the disorder: the critical D/Dʹ ratios of boundary-like defects, vacancy-type defects, and sp3-type defects are ~3.5, ~7, and ~13, respectively.37 The D/Dʹ ratios of E- and C-carbons were 2.52 and 4.03, respectively, revealing that boundary-like defects (that can preserve sp2 clusters better than vacancy-type defects) were primarily formed in E-carbon, whereas boundary-like and vacancytype defects coexisted in C-carbon. The Raman shift and full-width at half-maximum (FWHM) of each peak are closely associated with degree of structural disorientation in carbon. The similarity between P- and E-carbons in the Raman shift and FWHM indicates that, unlike C-carbon, Ecarbon’s π-conjugation system was not significantly damaged by thermal oxidation (Figure 2b). In contrast, the noticeable shift of the G2 and D2 peaks in C-carbon implies that its functionalized surface was much more disordered than that of E-carbon. This observation is in accordance with the trend in the electrical conductivity of P-, E-, and C-carbons, which was determined to be 1.17×104, 8.12×103, and 7.17×102 Sꞏm-1, respectively. The microtextural properties of the carbon electrodes were examined by N2 physisorption analysis. Figure 2c displays the N2 physisorption isotherms, in which type-I isotherms with small hysteresis loops appeared in E- and C-carbons. This indicates that surface functionalization, whether thermally or electrochemically driven, introduced microporosity along with weak mesoporosity via formation of vacancy defects in the basal plane of the graphene layers.23 The specific surface area and pore volume were determined by the t-plot method (Figure 2d). The


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presence of a steep increase followed by a more gradual increase in the t-plot (marked by the extrapolated dashed lines) indicates that both E- and C-carbons contained relatively homogeneous micropores.38 The total surface areas of P-, E-, and C-carbons were 0.62, 137.67, 124.69 m2ꞏg-1, respectively, and the surface areas contributed solely by micropores were 0.08, 134.54, 119.26 m2ꞏg-1, respectively. This reveals that more than 95% of the pores in the E- and C-carbons were composed of micropores. The micropore size distributions were determined by the micropore analysis (MP) method (Figure 2e).39 The distribution curves of E- and C-carbons were similar, with peaks ranging from 0.6 to 1.2 nm. While smaller micropores were formed in C-carbon than in E-carbon, C-carbon exhibited a slightly broader pore distribution than E-carbon. The surface morphologies of P-, E-, and C-carbons were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (see Figure S6 for high-resolution TEM images). Figure 2f displays the electron micrographs of P-carbon, in which a smooth surface with welldefined, intact graphene layers is clearly visible. The mild oxidation process used for E-carbon barely altered the original surface morphology, although carbon scrolls that resulted from the selfrolling of slightly defected graphene layers were sporadically found in TEM analysis (Figure 2g).40 This showed that incorporation of oxygen atoms in the ether configuration could preserve the original graphene layers. In contrast, the surface morphology of C-carbon was noticeably different. The surface became much rougher, and there were carbon aggregates on the surface that resulted from exfoliation of graphene layers via repulsion (Figure 2h). This re-affirmed that the πconjugation system in C-carbon was significantly damaged by the oxidation process.



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Figure 2. (a) Raman spectra of P-, E-, and C-carbons and fitting lines (residuals represent the difference between the raw data and the sum of all fitting lines). (b) A plot of FWHM as a function of Raman shift. The black symbols in each group (G1, G2, Dʹ, D1, and D2) represent P-carbon, the red symbols E-carbon, and the blue symbols C-carbon. (c) N2 physisorption isotherms, (d) tplots, and (e) pore size distributions of P-, E-, and C-carbons (the error bars were obtained from multiple measurements). Electron micrographs of (f) P-carbon, (g) E-carbon, and (h) C-carbon. The capacitive behaviors of E- and C-carbons in aqueous electrolytes were investigated by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements (Figure 3). For the electrochemical measurements, E-carbon obtained from 4 h of oxidation and C-carbon from 10 min of oxidation were chosen because these conditions showed the highest capacitance for the respective oxidation process (Figures S7,8). It is noteworthy that the currents and specific


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capacitances in Figure 3 were normalized by specific surface area to exclude the dependency of capacitance on surface area.2,41 Hasegawa and co-workers reported that micropores that are 20% larger than hydrated ions could effectively serve as a charge reservoir.4 Given the small size of hydrated alkali ions and hydroxide (~4 Å),4 all the micropores developed in E- and C-carbons could make a full contribution to capacitance. Hence, the difference in capacitive behavior between E- and C-carbons can be attributed mostly to the OFG effect. Figure 3a compares the CV curves of E- and C-carbons measured in 1 M H2SO4 electrolyte at 20 mVꞏs-1 (Figure S9 for CVs at different scan rates), in which a noticeable redox feature resulting from proton-mediated faradaic reactions (Eqs. 1 and 2) is combined with the typical rectangular CV response of EDLCs.16,42,43 >O + H3O+ + e- ↔ >OH(H2O) >CxO + H3O+ + e- ↔ >CxOH(H2O)

(1) (2)

where the >O and >CxO represent CEGs and conventional OFGs on the carbon surface, respectively. Compared to C-carbon, E-carbon exhibited well-resolved redox peaks and a lowersloped CV curve that resulted from higher electrical conductivity (stemming from significantly more preserved graphite structure). The galvanostatic charge–discharge curves in Figure 3b echoed the same trend: the IR drop observed for E-carbon is nearly half that for C-carbon (Figure S10 for the charge–discharge profiles at different currents). The normalized capacitances at various current densities are plotted in Figure 3c. At low current densities, the capacitances of E-carbon electrodes were only slightly higher than those of C-carbon electrodes. With increased current densities, however, the differences in capacitances became much more significant. As rate capability is governed by electrical conductivity and ionic accessibility, the poor rate performance of C-carbon can be attributed to energy loss by IR drop and hindrance of ions by bulky OFGs of



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C-carbon (as will be verified by electrochemical impedance spectroscopy (EIS) analysis). From these observations, it is apparent that CEGs are more beneficial than common OFGs in EDLCs. However, these results incorporated both the negative influences of decreased electrical conductivity and the bulky nature of OFGs and the positive pseudocapacitance effect, so it was difficult to ascertain the pure effect of the CEGs on EDLC behavior with these conditions. Our concern about mingled effects is supported by a report that revealed greater influence of OFGs than specific surface area on capacitance in H2SO4 electrolyte.44 To decouple these effects, the capacitive behaviors of E- and C-carbons were explored in a neutral electrolyte (1M Na2SO4) in which pseudocapacitance is greatly minimized because of no protons available for faradaic reactions.43,45 Nearly pure EDLC behavior was observed in the CV curves of both electrodes (Figures S11,12), and the capacitive performance determined by galvanostatic charge–discharge measurements showed that greater capacitances with less IR drop were obtained in E-carbon than C-carbon, confirming the advantage that CEGs can bring in. Figure 3d shows the CV curves of Eand C-carbons measured in 1 M KOH electrolyte. The CV responses are noticeably different from those observed in the acidic electrolyte in that the redox feature is greatly suppressed. This indicates that capacitance was determined mostly by the EDLC behavior of each carbon electrode while a small redox peak was observed in E-carbon that resulted from the following faradaic reactions (Eqs. 3,4):16,46 >O + [K(H2O)n]+ + e- ↔ >OK(H2O)n-y >O + OH- ↔ >OOH

(3)

(4)

where the >O represents CEGs on the carbon surface, n is the number of water molecules coordinated with K+ before charge, and n-y is the number of water molecules coordinated with K+


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after discharge. E-carbon generally retained the ideal rectangular CV response in KOH, but a significant deviation from ideal EDLC behavior was observed for C-carbon. As will be evidenced by EIS analysis, this distortion was due to the decreased electrical conductivity, larger interfacial resistance, and more limited electrolyte ion transport of C-carbon. This difference was also reflected in the galvanostatic charge–discharge profiles of the electrodes (Figure 3e) with the greater IR drop and shorter discharging time for the C-carbon electrode. In particular, the IR drop became critical when the current density was as high as 3.0 μAꞏcm-2 (Figure S13). The normalized capacitances at various current densities in 1 M KOH are presented in Figure 3f, in which a trend similar to the one measured in 1 M H2SO4 was observed. It is noteworthy that the capacitance of E-carbon at 3.5 μAꞏcm-2 was 12 times greater than that of C-carbon, highlighting the advantages of CEGs. The long-term CV cycling tests revealed the stable capacitive response of both electrodes

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(Figure S14), suggesting the good chemical stability of CEGs.


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Figure 3. (a) Cyclic voltammograms of E- and C-carbons measured in 1 M H2SO4 at 20 mVꞏs-1, (b) their galvanostatic charge–discharge curves at 0.7 μAꞏcm-2, and (c) the capacitances normalized by specific surface area as a function of current density. (d) Cyclic voltammograms of



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E- and C-carbons measured in 1 M KOH at 20 mVꞏs-1, (e) their galvanostatic charge–discharge curves at 0.7 μAꞏcm-2, and (f) normalized capacitances plotted as a function of current density. Error bars for each point were obtained from multiple measurements with more than 10 electrodes. EIS analysis provided more insight into the various resistive components related to OFGs. EIS spectra were measured at various potentials in 1 M KOH electrolyte, and the resulting Nyquist plots of E- and C-carbons are presented in Figures 4a-d. An equivalent circuit (Figure S15) was employed to extract resistance information (representative fitting results in Figures S16,17), and each resistive component—equivalent series resistance (Rs) and interfacial resistance (Ri), the Warburg coefficient (Aw), and leakage resistance (RL)—was plotted as a function of potential. The EIS analysis results of P-carbon are also provided in Figures S18-21. The Rs was primarily dictated by electrical conductivity of the carbon electrodes and was smaller for E-carbon than C-carbon (Figures 4e,f), reflecting the less damaged sp2 carbon network in E-carbon. In particular, the Ri (often linked to the charge transfer resistance associated with pseudocapacitance) of E-carbon was notably smaller than that of C-carbon, suggesting faster kinetics of the proton-mediated electron transfer in the CEGs. The presence of small redox peaks with E-carbon even in the basic electrolyte (Figure 3d), as opposed to absence in C-carbon, was linked to these substantially smaller Ri values. The Aw values related to diffusion of ions were also remarkably smaller for E-carbon (Figure 4g), which implies that electrolyte ion accessibility to the micropores was much less hindered by CEGs than conventional OFGs. All of these more favorable impedance characteristics highlight the advantages of CEGs. In addition, the CEGs were further advantageous for EDLCs in that the RL values of E-carbon were greater than those of C-carbon (Figure 4h). This revealed that CEGs are capable of exploiting the benefits of OFGs better than common OFGs, while suppressing the selfdischarge that commonly accompanies capacitance enhancement by the surface redox reactions.


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Complex

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the

impedance

(Z(f)=Z´(f)+jZʺ(f)) into complex capacitance (C(f)=C´(f)+jCʺ(f)) using the relationship C(f)=1/jwZ(f), is a powerful tool for analysis of leakage current and rate capability of supercapacitors with greater accuracy and reliability than the EIS analysis.47,48 While a qualitative comparison can be quickly made by examining the shape of the imaginary component of capacitance vs. logarithmic frequency plots (i.e., peak area for capacitance and peak frequency at maximum rate capability) (Figure 4i,j), the overlap between the resistive tail arising from the low frequencies and the capacitive peak often complicates separation of these components, particularly when the leakage current is not negligible. Thus, complex, nonlinear, least-squares fitting based on the transmission line model was employed to provide more detailed information on rate capability and leakage current in terms of time constants (𝜏

,

and 𝜏 ).49-51 For a non-ideally

polarized multiple-pore system, the imaginary part of complex capacitance (𝐶

𝑓 ) can be

expressed as

𝐶

𝑓

𝐶

𝑓, 𝜏 , 𝜏 𝐶 𝑝 𝜏 𝑑𝑙𝑜𝑔𝜏

5

where 𝑓 is the frequency; 𝜏 and 𝜏 are the time constants for the rate capability and the leakage current, respectively; 𝐶 is the total capacitance; 𝑝 𝜏 𝜏 ; and 𝐶

𝐶

𝑓, 𝜏 , 𝜏

𝑓, 𝜏 , 𝜏

is the logarithmic distribution function of

describes the interface of a non-ideally polarized system given as50

𝐼𝑚

1 𝑗2𝜋𝜏 𝑓

The logarithmic distribution function 𝑝 𝜏

1

1 tanh 𝑗2𝜋𝜏 𝑓

𝑗2𝜋𝜏 𝑓 1

1 𝑗2𝜋𝜏 𝑓

6

is given as50



𝑝 𝜏

1 √2𝜋𝜎

exp

1 2𝜎

where 𝜎 is the standard deviation of 𝜏 , and 𝜏

𝑙𝑜𝑔𝜏

,

,

7

is the maximum value of the 𝜏 distribution.

,

Figures 4k and l display the fitting results of 𝜏 that a smaller 𝜏

𝑙𝑜𝑔𝜏

Page 16 of 30

,

and 𝜏 , respectively. It is noteworthy

value correlates with faster charge–discharge capability and a larger 𝜏 value

with a smaller leakage current. The values of 𝜏

,

remained roughly constant regardless of

potential, revealing that ionic conductivity inside the pores, which is inversely proportional to 𝜏

,

, was not significantly altered by the applied potentials. The values of 𝜏

,

, which

corresponds to the peak frequency of each Cʺ vs. log f plot (Figures 4i,j), were 1.6 times greater for C-carbon than E-carbon on average, affirming the better rate capability of E-carbon. Unlike 𝜏

,

, 𝜏 was more potential-dependent, but the dependencies of E- and C-carbons were different.

While further investigation is required on this interesting observation, it suggests that the selfdischarge mechanisms via CEGs and common OFGs differ. The larger 𝜏 values of E-carbon for the potentials tested highlights the less susceptible nature of the CEGs to self-discharge.


Page 17 of 30

20

40

60 Z' (Ω)

(c) 80

80

-1 V -0.9 V -0.8 V -0.7 V -0.6 V -0.5 V -0.4 V -0.3 V -0.2 V -0.1 V 0V

-Z'' (Ω)

60 40 20 0 0

20

40

60 Z' (Ω)

(i)

80

0.09 0.06 0.03 E-carbon -1

0

10

10

1

Frequency (Hz)

11

1

12

13 14 Z' (Ω)

15

-1.0 -0.8 -0.6 -0.4 -0.2

(h) 600

90

500

60 C-carbon

0.15 0.10 0.05 C-carbon

0 -2 10

-1

10

0

10

1

10

Frequency (Hz)

300 200

0


9 6

-1.0 -0.8 -0.6 -0.4 -0.2

(l)

C-carbon

5

0

0

Potential (V vs. Hg/HgO) 80 60

E-carbon

40

E-carbon C-carbon

20

3

10

E-carbon

400

12

-1 V -0.9 V -0.8 V -0.7 V -0.6 V -0.5 V -0.4 V -0.3 V -0.2 V -0.1 V 0V

C-carbon

E-carbon

-1.0 -0.8 -0.6 -0.4 -0.2

(k)

0


120

16

E-carbon

0


30

C-carbon

2

0

0 11

3

1 E-carbon

-1.0 -0.8 -0.6 -0.4 -0.2

(g)

-0.4 V -0.3 V -0.2 V -0.1 V 0V

0.20

5

Ri ()

Rs () 10

0.25

10

4

9

7 8 9 Z' (Ω)

-1 V -0.9 V -0.8 V -0.7 V -0.6 V -0.5 V

0 10

100

7

5

C-carbon

8

3

2

-1 V -0.9 V -0.8 V -0.7 V -0.6 V -0.5 V -0.4 V -0.3 V -0.2 V -0.1 V 0V

0.12

10

6

(j)

0.15

0 -2 10

0 5

100

(d)

C-carbon

1

10

RL ()

0 0

2

(f)

 (s)

20

-1 V -0.9 V -0.8 V -0.7 V -0.6 V -0.5 V -0.4 V -0.3 V -0.2 V -0.1 V 0V

)

40

-Z'' (Ω)

-Z'' (Ω)

60

-Z'' (Ω)

-1 V -0.9 V -0.8 V -0.7 V -0.6 V -0.5 V -0.4 V -0.3 V -0.2 V -0.1 V 0V

-0.5

E-carbon

-C'' (F)

80

(e)

3

AW (ꞏs

(b)

max (s)

(a)

-C'' (F)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.0 -0.8 -0.6 -0.4 -0.2

0


0

-1.0 -0.8 -0.6 -0.4 -0.2

0


Figure 4. Nyquist plots of (a,b) E-carbon and (c,d) C-carbon measured at various potentials. Plots of (e) Rs, (f) Ri, (g) Aw, and (h) RL vs. potential. The imaginary part of capacitance (Cʺ) vs. frequency in (i) E-carbon and (j) C-carbon. The plots of (k) τ1,max and (l) τ2 vs. potential. Since thermal treatment can remove OFGs from oxidized graphite, the damaged sp2 carbon network in C-carbon could be restored by thermal reduction (Figure 5a), and the resulting capacitive performance of C-carbon could be comparable to that of E-carbon. Thermogravimetric analysis performed in an inert atmosphere (Figure 5b) shows a significant weight loss in C-carbon during the temperature sweep, indicating removal of OFGs by thermal treatment. Unlike C-carbon, there was no remarkable weight loss in E-carbon up to ~600 °C. FT–IR analysis further confirmed the robustness of the CEGs against heat as no significant spectral changes were observed with increasing temperature (Figure 5c). Figure 5d shows an intense peak in the FT–IR spectra of Ccarbon at 1382 cm-1 that corresponds to various carbonyl and carboxylic groups and a peak at 1230



Page 18 of 30

cm-1 that corresponds to the epoxide group; both of these peaks vanish with increasing temperature, while the peak at 1631 cm-1 that corresponds to sp2-hybridized C=C is intensified. This reveals the recovery of π-conjugation in C-carbon with heat treatment. It is also noteworthy that peaks at 957 and 1089 cm-1 from ether groups that were either initially present or created by thermally driven merging of initial hydroxyl and epoxy groups52 survived even at 600 °C, suggesting that complete retrieval of sp2 carbon network is very difficult due to remnant ether groups.25,52,53 There was also a change in microtextural properties in C-carbon with thermal treatment. While N2 physisorption isotherms remained unchanged for E-carbon, the porous structure of C-carbon was altered after heat treatment; mesoporosity developed at the expense of microporosity (Figure S22, Table S3). In addition to loss of micropores and the resulting surface area decrease, the micropores widened during removal of OFGs (Figure S23), which would be further detrimental to capacitance. CV performed in 1 M H2SO4 supported the spectroscopic evidence of the OFG removal from C-carbon during heating (Figure S24). In contrast to E-carbon, which exhibited no significant change in CV response with thermal treatment, the redox peaks from C-carbon disappeared with temperature. It is noteworthy that this loss was accompanied by a decrease in capacitance (Figure S24), which is attributed to changes in microtextural properties and substantially decreased pseudocapacitance. The cyclic voltammograms obtained in 1 M KOH (Figures 5e,f and Figure S25) echoed the same trend. Less sloped, more rectangular shaped CV responses were observed upon thermal treatment of C-carbon, revealing the improved electrical conductivity after heat treatment. However, the capacitances determined at various current densities (Figures 5g,h and Figure S26) revealed that a compromise between capacitance and rate capability was inevitable when oxidized carbon recovered its partially broken sp2 carbon network. EIS analysis and complex capacitance analysis (Figure S27,28) were performed to provide more insight into the effect of thermal treatment, and


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the complex nonlinear least-squares fitting results are presented in Figures 5i-l. The 𝜏

,

and 𝜏

of E-carbon were almost invariable even after high-temperature heat treatment (Figures 5i,k), highlighting the excellent thermal stability of the CEGs. Given the significant effect of temperature on capacitance,54 this characteristic may be potentially useful if carbon-based EDLCs were placed in harsh temperature environments. For C-carbon, however, the effect of thermal treatment was dramatic (Figures 5j,l). While annealing at 300 °C suppressed self-discharge (greater 𝜏 ), it seemed insufficient to enhance electrical conductivity (similar 𝜏 improved electrical conductivity (smaller 𝜏

,

,

). Conversely, annealing at 600 °C

) but decreased 𝜏 compared to annealing at

300 °C. The seemingly worsened self-discharge characteristic after annealing at 600 °C was initially puzzling because the parasitic faradaic reaction via surface functionalities was the primary source of self-discharge. However, the changes in charge distribution driven by modification of the porous structure can affect 𝜏 significantly.19,55 Therefore, structural rearrangement that occurred during recovery of the sp2 carbon network at 600 °C, which altered C-carbon’s porous structure, as verified in N2 physisorption analysis, could be responsible for the decreased 𝜏 . This hypothesis was partly corroborated by significantly broader distribution of 𝜏 after annealing of C-carbon at 600 °C (Figure S29), which is attributed to reduced uniformity of pore geometry resulting from high-temperature annealing. This new insight will be very important to establish a design principle for nanoporous carbons developed for high-performance EDLCs. Once the sp2 carbon network is damaged by incorporating oxygen atoms into the graphene layers, it will seldom be fully recovered by thermal reduction. Further, the gains in enhanced electrical conductivity are counterbalanced by deterioration of the porous structure and loss of pseudocapacitance. Given this compromise, it is highly desirable to design a carbon functionalization process that can minimize the damage to the sp2 carbon network.



(c) 95

Weight (%)

85 80

OFG removed

75 0

400

600

800

Temperature (℃)

0 -0.5 E-carbon E-carbon_300 E-carbon_600

-1.5 -1.0

-0.8

-0.6

-0.4

-0.2

0

0 -0.5 -1.0

C-carbon C-carbon_300 C-carbon_600

-1.5 -1.0

-0.8

E-carbon E-carbon_300 E-carbon_600

8 6 4 2 0

-0.4

-0.2

0

-0.8

-0.6

-0.4

-0.2


0

30 20 10 0


8 6 4

0

0.5

1.0

1.5

2.0 -2

(l)

-0.8

-0.6

-0.4

-0.2


40

0

40

0


30 20 10 0

0.5

1.0

80


-1.0

-0.8

-0.6

-0.4

1.5

2.0 -2


60

20

-1.0

600

-1


60

2 -1.0


(k) 80

12

3000 1800 1500 1200 900

(h)


10

max (s)

10

-0.6

3500

Wavenumber (cm )

40


(j)

12

600

-1

-2

-2

0.5


(i)

3000 1800 1500 1200 900


Wavenumber (cm )

 (s)

-1.0

3500


0.5


(g) 1.0

-2

-2

200

(f) 1.0


(e) Current Density (μAꞏcm )


90


OFG preserved

(d)


100


(b)

 (s)

(a)

max (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

40 20

-0.2


0

0

-1.0

-0.8

-0.6

-0.4

-0.2

0


Figure 5. (a) Schematic illustration of thermal treatment and (b) TGA analysis of E- and C-carbon. FT–IR spectra of thermally treated (c) E-carbons and (d) C-carbons. Cyclic voltammograms of thermally treated (e) E-carbons and (f) C-carbons measured in 1 M KOH. The normalized capacitances of thermally treated (g) E-carbons and (h) C-carbons at various current densities. Plots of τ1,max vs. potential of thermally treated (i) E-carbons and (j) C-carbons. Plots of τ2 vs. potential of thermally treated (k) E-carbons and (l) C-carbons.

EXPERIMENTAL SECTION Electrode Preparation. Carbon Fiber Paper, CFP (SGL group, SICRACET, GDL 10 AA, thickness: ~400 μm), was used for a graphitic carbon monolith for surface functionalization. CFP was cut into strips (1 cm × 7 cm) and carefully washed with water, ethyl alcohol, and acetone to remove any possible organic contaminants. After the washing, the CFPs were dried in 40 °C for 24 h. These CFP strips were designated as P-carbon. C-carbon was prepared by the electrochemical oxidation of P-carbon. In this process, P-carbon was dipped into N2-saturated 1 M aqueous solution


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of mixed acids (1:1 (v:v) ratio of 1 M H2SO4 (>98%, DAEJUNG) and 1 M HNO3 (extra pure grade, DUKSAN)), and a potential of 2 V (vs. SCE) was applied to it for 2-20 min. The oxidized carbon electrode was then washed with distilled water, ethyl alcohol, and acetone, and dried at 30 °C for 24 h. On the other hand, E-carbon was prepared by heating P-carbon in air at 450 °C for 1-8 h. After a desired period of oxidation, the thermally treated carbon electrode was taken out of a furnace and rapidly cooled within 5 min in air. Characterization. FT–IR (JASCO, FT/IR-4600) explored the surface functionalities of P-, E-, and C-carbons, and Raman spectroscopy (Reinshaw RM 1000) investigated the crystallinity of the carbon electrodes. The degree of hydrophilicity/hydrophobicity was examined by contact angle measurement (Kruss, K11-MK1) using a water microdroplet (40 5 μm), and surface analysis was performed by XPS (PHI Versa Probe system). The electrical conductivity was determined by fourpoint probe resistivity measurements. The surface morphology of the carbon electrodes was observed by SEM (Hitachi S-4800) and TEM (JEOL 2010F). The porous nature of E- and Ccarbons was characterized by N2 adsorption/desorption (BELSORP-mini II) at 77 K and analyzed using the t-method (MP-method) and the Barrett–Joyner–Halenda (BJH) plot. Thermogravimetric analysis (TGA N-1000, SCINCO) was conducted to explore the thermal stability of oxygen functional groups. For the measurements, temperature was raised from 40 to 800 °C at the rate of 5.5 °Cꞏmin-1 under N2 flow (5 mLꞏmin-1). For the thermal treatment to recover the damaged sp2 carbon network, E- and C-carbons were heated gradually at the rate of 5.5 °Cꞏmin-1 to 300 or 600 °C under N2 atmosphere, and when the target temperature was reached, the temperature was hold for 5 min to complete the treatment. Electrochemical Characterization. The capacitive behavior of the carbon electrodes was investigated using potentiostats (CHI-660D and Gamry reference 600) in a three-electrode cell.



Page 22 of 30

The P-, E-, and C-carbons were used as the working electrodes, a clean platinum wire electrode was used for the counter electrode, and saturated calomel electrode (SCE, 002056 RE-2B, BAS Inc.) in an acid electrolyte (N2-purged 1 M H2SO4) or mercury/mercury oxide (Hg/HgO) electrode (RE-61AP) in a basic electrolyte (N2-purged 1 M KOH) was used for the reference electrode. EIS analysis was carried out in the KOH electrolyte in the frequency range of 10-2 to 105 Hz with the potential perturbation of 5 mV. The distribution of τ1 and τ2 from the EIS analysis were calculated by Matlab routines (the distribution of relaxation times (DRT) tools) as implemented in the code that is based on the Tikhonov regularization with a slight modification.56

CONCLUSIONS Controlled thermal oxidation of a monolithic carbon electrode enabled specific introduction of cyclic ether groups onto the carbon surface, and the influence of these surface functionalities on EDLC performance was investigated by in-depth spectroscopic and electrochemical analyses. Due to the in-plane positioning of oxygen in the graphene matrix, introduction of cyclic ether groups significantly minimizes damage to graphene’s π-conjugation system, while effectively creating microspores for charge storage. This resulted in an electrode with high capacitance, good rate capability, and diminished leakage current. Our study also revealed that preserving the original sp2 carbon network during surface functionalization was the key to maximizing the benefits of oxygen functional groups for EDLCs. This is because, once the conjugated system is disturbed by various oxygen functionalities, it cannot be restored fully to its original state due to residual oxygen surviving on the graphene layers in the form of highly stable ether groups. In addition, an effort to restore the damaged sp2 carbon network was accompanied by a distortion of the electrode pore structure, which greatly compromised the advantages of oxygen functional groups by decreasing


Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


capacitance. Given the versatility of carbon in diverse energy storage applications, these new insights suggest that it is crucial to develop new, practical surface functionalization strategies for carbon-based energy storage devices that can minimize disruption to carbon’s π-conjugation system. ASSOCIATED CONTENT Supporting Information. Additional characterizations and electrochemical analysis results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author: Jin Ho Bang *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by grants from the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT (NRF-2016R1A1A1A05005038, NRF-2018M3A7B8061494) and by the Ministry of Education (NRF-2018R1A6A1A03024231). This work was also supported by the Samsung Research Funding Center of Samsung Electronics, under Project Number SRFCMA1601-03.



Page 24 of 30

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TOC GRAPHICS

- Less hindered (AW ) - Lower leakage (RL , Hydrated ions

-

-

-

-

High conductivity

)

Distribution

Current Density

Cyclic ether functionality

- Functionality preserved - Pore size preserved

600 °C annealed

Voltage

1 (s) - Higher leakage (RL , Hydrated ions

-

-

Low conductivity

Voltage

Cyclic ether functionality

- Limited accessibility (AW )

Carboxyl functionality

)

Distribution

Current Density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

- Functionality removed - Pore size broadened Carboxyl functionality 600 °C annealed

1 (s)


Exploring the Capacitive Behavior of Carbon Functionalized with

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