Sulfur Doping: Unique Strategy To Improve the Supercapacitive

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Sulfur Doping: Unique Strategy to Improve Supercapacitive Performance of Carbon Nano-Onions Debananda Mohapatra, Ganesh Dhakal, Mostafa Saad Sayed, Badrayyana Subramanya, Jae-Jin Shim, and Smrutiranjan Parida ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21534 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

Sulfur Doping: Unique Strategy to Improve Supercapacitive Performance of Carbon Nano-Onions

Debananda Mohapatra,1 Ganesh Dhakal,1 Mostafa Saad Sayed,1,3 Badrayyana Subramanya,2 JaeJin Shim,1,* and Smrutiranjan Parida2,*

1School

of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk

38541, Republic of Korea. E-mail: [email protected]; Tel: +82-53-810-2587 2Department

of Metallurgical Engineering and Materials Science, IIT Bombay, Powai,

Mumbai 400-076, India. E-mail: [email protected]; Tel: +91-22-2576-7643 3Egyptian

Petroleum Research Institute, Nasr City, Cairo 11727, Egypt

Abstract Recently, enhancement of the energy density of supercapacitor is restricted by the inferior capacitance of the negative electrodes, which impedes the commercial development of highperformance symmetric and asymmetric supercapacitors. This paper introduces the in-situ bulkquantity synthesis of hydrophilic, porous, graphitic sulfur-doped carbon nano-onions (S-CNO) using a facile flame-pyrolysis technique and evaluated its potential applications as highperformance supercapacitor electrode in a symmetric device configuration. The high surface wettability in the as-prepared state enables the formation of the highly suspended active conducting material S-CNO ink, which eliminates the routine use of binders for electrode preparation. The asprepared S-CNO displayed encouraging features for electrochemical energy storage applications with a high specific surface area (950 m2 g−1), ordered mesoporous structure (~3.9 nm), high Scontent (~3.6 at.%), and substantial electronic conductivity, as indicated by the ~80% sp2 graphitic carbon content. The in-situ sulfur incorporation into the carbon framework of the CNO resulted in a high-polarized surface with well-distributed reversible pseudo-sites, increasing the electrodeelectrolyte interaction and improving the overall conductivity. The S-CNOs showed a specific capacitance of 305 F g−1, an energy density of 10.6 Wh kg−1, and a power density of 1,004 W kg−1 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

at an applied current density of 2 A g−1 in a symmetrical two-electrode cell configuration, which is approximately three times higher than that of the pristine CNO-based device in a similar electrochemical testing environment. Even at 11 A g−1, the S-CNO||S-CNO device rendered an energy density (6.1 Wh kg−1) at a deliverable power density of 5.5 kW kg−1, indicating a very good rate capability and power management during peak power delivery application. Furthermore, it showed a high degree of electrochemical reversibility with excellent cycling stability, retaining ~95% of its initial capacitance after more than 10,000 repetitive charge-discharge cycles at an applied current density of 5 A g−1. Keywords: carbon nano-onion; in-situ sulfur doping; supercapacitor; exohedral; graphitization; mesoporous; thiophene 1. Introduction The sustainability of large-scale clean electrochemical energy-storage (EES) systems has prompted enormous research efforts aimed at increasing the supercapacitor performance metrics, such as energy density, power density, and long-term cycling stability, with the aim of reaching the level of batteries.1 Various carbon nanoforms and their allotropes have been the major focal point in the field of EES, particularly electrical double layer capacitors (EDLCs). EDLCs are recognized as one of the most prominent supercapacitor groups, arising from the development of electric double layers by an electrosorption mechanism at the electrode-electrolyte interface. The constituted electrode materials are composed mainly of porous carbonaceous materials with high surface area, such as activated carbon (AC), carbon nanotubes (CNTs), graphene, and recently, fullerene-like nanoscopic carbon particles (carbon nano-onions).2 Although carbon-based EDLCs can manage a high specific power demand, their drawback compared to batteries is a low specific energy. Carbon nano-onion spheres (CNOs) are a new addition to the fullerene family of carbon nanoallotropes from a morphological characteristics perspective and by structural dimensionality; they are zero-dimensional carbon nanostructures.2 This category of carbon nanostructure (CNS) features internally empty spaces, enabling hollow internal nanostructures, which can accommodate metal/molecules/atoms easily. Generally, they consist of spherical concentric graphene layers with predominantly sp2 hybridization. Recently, CNOs have attracted considerable attention for EES (ultrafast charge/discharge rate capacity) applications.3,4 The presence of exo- and endohedral 2 ACS Paragon Plus Environment

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pores contribute to the specific surface area and interlinked ion channeling along with excellent electrochemical activity. Therefore, CNOs can be superior to other carbon nanostructures (CNS) when used as an electrode in a supercapacitor.4 Progressive efforts have been made to enhance the specific capacitance and specific energy of CNOs by coupling with metal oxides (MnO2,5 RuO2,6 Ni(OH)2-NiO) 7 or with conducting polymers (polyaniline,8 polypyrrole 9). Some research has been conducted using harsh chemical activation procedures (H2SO4,8 KOH-KNO3,10 HNO311); however, they failed to achieve this characteristic coupling because the use of an aggressive chemical activating agent alters the external carbon surfaces by disrupting the graphitic carbon concentration (sp2 percentage), leading to an increase in resistivity. The increase in defective graphitic carbon introduces multiple surface defects sites and edges (sp3 character), which may be responsible for the reduced overall electrical conductivity of CNO that ultimately affects the device performance. Heteroatom-doped CNO, especially by an in-situ technique, could be a wise strategy for improving the electrochemical performance without the use of complicated chemical activation and composite formation. Importantly, most of these devices were reported in a 3-electrode configuration, which overestimated the intrinsic results.12 On the other hand, for practical applications, they should have been tested in a symmetrical two-electrode device model, which actually mimics the supercapacitors available on the market. A range of heteroatom doping, such as nitrogen,13 boron,14 and sulfur,15 in different allotropes of carbon allows tailoring of the electronic properties and increases the overall electrochemical performance of carbon nanostructures. On the other hand, there are very limited reports on heteroatom-doped CNO,16−20 where the doping procedure was carried out postsynthesis. The post-doping process involves the use of multistage harsh chemical treatments that introduce structural defects and unwanted byproducts that ultimately affect the electronic conductivity and electrochemical activity.16−19 In particular, there are no systematic reports of the direct synthesis of doped CNOs in a single-step process to date. Such a process would eliminate the tedious post-processing doping treatment. For this purpose, this paper reports the one-step, direct, and facile flame synthesis of highly doped CNOs with high quality and quantity. Studies have shown that doping heteroatoms into the sp2-hybridized carbon frameworks can remarkably modify their electronic structures, surface chemical activities, and bandgaps, providing an alternative strategy to enhancing the capacitive performance of nanocarbon materials.16−24 Here, sulfur acts as an electron donor and its doping has been shown to introduce a variable electronic 3 ACS Paragon Plus Environment


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density of states compared to N or B doping.22−24 The changes in electronic structure are due to the sulfur atom that affects the π electrons in the carbon lattice.23−24 In addition, the presence of discrete pairs of electrons on S enhances the local reactivity of S-doped carbon. These effects of S doping have been studied in various CNSs, such as graphene and nanoporous carbon.24−26 To the best of the authors’ knowledge, however, there are no reports of the in-situ bulk synthesis and application of S-doped CNO in supercapacitors. This paper reports the feasibility of in-situ sulfur doping of CNO by revealing its attractive electrochemical properties in a prototype symmetric supercapacitor device. With high sulfur doping (3.6%), the as-synthesized S-CNOs still maintained a high degree of graphitization, with a high sp2 content (80%) and the appearance of a pronounced 2D peak in the Raman spectrum compared to the characteristic features of pristine CNO and some doped CNOs.16,18,20,27 The superior electrochemical performance of S-CNO was demonstrated in a prototype symmetrical supercapacitor device. The assembled device exhibited a high specific capacitance (305 F g−1), also a high energy density (10.6 Wh kg−1), and good rate capability at an applied current density of 2 A g−1. The electrochemical performance of the S-CNO devices was superior to those reported for pristine and doped CNOs (Table 1). 2. Experimental 2.1 Preparation of S-Doped Carbon Nano-onion (S-CNO) S-CNOs were prepared using a simplified flame pyrolysis method. In a typical laboratory-made design, 50 mL of organic precursor (thiophene) was placed in a small beaker and flamed in air in a fume hood to exhaust properly any toxic gases formed during burning. A previous report

4

demonstrated how a flame could produce an appropriate high-temperature environment (800−1000˚C) naturally and effectively for the preparation of pristine CNO. A similar flame temperature was observed in the case of a thiophene (C4H4S) flame in air. The SCNO was collected from the upper part of the flame using a fixed water-cooled glass collector. Here, thiophene acts as a sulfur and carbon source to produce the desired S-CNO (Scheme 1). The product in the form of black carbon nanoparticles was collected and used without further post-processing or purification. Scheme 1 presents a schematic illustration of the direct synthesis of S-CNO. Scheme S1 outlines the detailed experimental setup used to obtain the optimal results. 4 ACS Paragon Plus Environment

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2.2 Fabrication of S-CNO//S-CNO Symmetric Supercapacitor Device Electrochemical tests of a symmetric S-CNO||S-CNO supercapacitor (SC) device were conducted in two sets of Swagelok cells, one containing 1 M Na2SO4 and the other, 1 M H2SO4 electrolyte. Taking advantage of the hydrophilicity of the as-synthesized S-CNO, S-CNO ink was prepared by dispersing the active material (S-CNO) in isopropanol and sonicating it for 30 min. Detailed discussion on the hydrophilicity, dispersibility and suspension stability of the as-prepared S-CNO particles have been provided in the supplementary information (Figures S3 and S4). The electrodes were prepared by drop casting the S-CNO ink onto a 2 cm x 2 cm piece of Toray carbon paper (Alfa Aesar), and subsequent drying in a vacuum oven at 80 °C for 12 hrs. Two sets of identical active electrode pairs were soaked in each of 1 M Na2SO4 and 1 M H2SO4 electrolytes and assembled in each of two indigenous Swagelok split cell setups using filter paper as a separator. The electrochemical performance of the complete cell was evaluated in neutral (1 M Na2SO4) and acidic (1 M H2SO4) media by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Electrochemical measurements of the asprepared S-doped CNO were conducted in a three-electrode system to understand their individual electrochemical response and activity towards the two different electrolytes using platinum wire as the counter electrode and Ag/AgCl as the reference electrode. 2.3 Device Performance Evaluation The specific capacitance (C in F g−1) of each electrode in a symmetrical two-electrode cell configuration was calculated using the following equation12 (1): 𝐶=

4𝐼 𝛥𝑡

(1)

𝑚 𝛥𝑉

where I, Δt, m, and ΔV represent the discharge current (A), discharge time (s), total active material electrode mass (g), and discharge voltage window (V), respectively. The device energy density E (Wh kg−1) and power density P (W kg−1) were evaluated using eqs (2) and (3), respectively. 1000

𝐸 = 8 ∗ 3600𝐶(∆𝑉)2 𝑃=

3600 𝐸

(2) (3)

𝛥𝑡

5 ACS Paragon Plus Environment


The electrochemical performances of all assembled supercapacitor devices were measured using electrochemical workstations (Bio-Logic, SP-300 and Metrohm, Autolab PGSTAT 302N).

3. Material Characterizations and Modelling The microstructure of S-CNO was examined by high-resolution transmission electron microscopy (HR-TEM, TecnaiG2) with an accelerating voltage of 300 kV. A QuantachromeAutosorbAS-1 Version-1.55 machine was used to obtain the nitrogen adsorption–desorption isotherms measured at 77 K. The specific surface area (SSA) was obtained using the Brunauer-Emmett-Teller (BET) method, and the pore size distribution (PSD) was analyzed using the Barrett-Joyner-Halenda (BJH) model applied to the desorption branch. X-ray photoelectron spectroscopy (XPS KRATOS AXIS Supra) was performed using a monochromatic Al Kα X-ray source (1486.6 eV). The Raman spectra of S-CNO were recorded on a Horiba HR8000 spectroscope with laser excitation at 514.5 nm. AUTODESK 3DS MAX 2017 and Avogadro version 1.2.0 software were used for the atomistic 3D design, modelling, and visualization of S-CNOs. 4. Results and Discussion 4.1 Structural and Compositional Characterizations The morphology and microstructure of the S-CNOs were examined by HR-TEM (Figure 1a). The as-synthesized S-CNOs showed an almost spherical morphology with a diameter in the range of 25 (±5) nm, where 5 nm is the standard deviation. The quasi-spherical nanoparticles were arranged in an interconnected linkage with interpenetrating graphitic layers at the boundary regions of SCNO particles. Intimate graphitic contact among the S-CNO particles (Figure 1a, c) is desired for better electron transport throughout the bulk electrode materials in the device during the electrochemical process. The white dotted circles indicate the nucleation and growth of approximately closed graphene-like shells towards the outer curvature. The outer-most shells of the adjoining particles appeared to be exfoliated from the core, bearing exposed graphene facets, which might contribute to the surface chemical reactivity of the S-CNO nanoparticles during the electrochemical process. The presence of exposed graphene facets with a high local curvature is expected from zero-dimensional CNOs known for exohedral electric double-sphere capacitors 27 (exohedral supercapacitor). It is important to note that the presence of a high sulfur dopant (3.6 %) 6 ACS Paragon Plus Environment

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did neither alter the evolution of the microstructure nor its characteristic local curvature.27 In particular, the supercapacitor electrode composed of positively curved CNOs (S-CNOs) utilizes the completely exposed outer surfaces instead of the deep pores (pore wall constriction) for the counter electrolyte ions to form electric double layers exohedrally, enabling a high charging– discharging rate capacity (Figure 5g). Moreover, all the available graphitic layers were not arranged in a particular manner (Figure 1b), but with varying intergraphitic layers distance from 0.33-0.35 nm, indicating the presence of defects in the graphitic carbon, as discussed further in the Raman spectroscopy section. EDX and the corresponding elemental mapping of the as-synthesized S-CNO (Figures 1e and 1f) confirmed the even distribution of carbon, oxygen, and sulfur. The coexistence of dopant sulfur could also be observed in the EDX spectra of the carbon majority. Raman spectroscopy is used to monitor the level of defects and the graphitic structure of carbonaceous materials. Figure 2 presents the Raman spectrum of the as-synthesized S-CNOs by flame pyrolysis. The Raman spectra of sulfur-doped CNO exhibited three main peaks, D-band (1,337 cm−1), G-band (1,578 cm−1), and 2D-band (2,702 cm−1). The D-band is sensitive to structural defects and disorder associated with doping while the G-band is due to the in-plane stretching vibration of ordered sp2-bonded carbon atoms.28 The 2D-band is due to a second-order, two-phonon defect-free Raman scattering process that is activated by the simultaneous excitation of two phonons.28,29

Scheme 1. In-situ bulk quantity synthesis of hydrophilic, multigraphene layered S-CNOs.



Figure 1. TEM, HR-TEM, SEM, and EDX spectroscopy images of the electrode materials: (a) TEM image of the as-prepared S-CNOs showing the interconnected networks of spherical nanoparticles, (b) HR-TEM images of the as-prepared S-CNOs showing interpenetrating graphite planes with distinguishable cores (yellow dotted circles), (c) schematic diagram of S-CNO particles, (d) core with almost concentric graphitic layers of a single S-CNO particle, and (e, f) SEM-EDX spectroscopy of different elements (C, O, and S) in an as-synthesized S-CNO particle and their corresponding elemental maps.


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The sharper and narrower G-mode was shifted slightly downward (1,578 cm−1) compared to the planar graphite (usually at 1,582 cm−1)28 possibly due to the spherical carbon shell curvature. Downshifting of the obtained Raman mode in S-CNOs was comparable to previously reported pristine CNOs (at 1,584 cm−1).4 This may be due to the tensile strain induced in the graphene planes by the curvature during the doping process. This process also corroborates the modification of the carbon atomic density during the doping process, resulting in peak downshifting. The D to G intensity ratio is used to measure the extent of disorder with respect to a perfect graphitic structure. An ID/IG value of 0.72 indicates some structural defects in the graphitic planes of SCNOs, which is comparatively less than that of pristine CNO (0.87-0.92).4 In addition, the presence of defects and disorder may be due to the presence of amorphous carbon in the planes of the aromatic rings. A slightly incomplete carbonization process during synthesis can induce graphitic defect sites, resulting in the generation of a larger number of capacitive sites. The local retention of electrolyte ions could facilitate the charge storage density with the support of defectinduced capacitive sites, 16 endowing the S-CNOs with improved supercapacitive performance. The sharp 2D band at 2,705 cm−1 confirmed the ordering of the neighboring multi-stacked graphitic cages during curling and closing of the carbonshells.29 This also suggests a high degree of crystalline lattice perfection in the spherical graphitic carbon shell. The appearance of a sharp 2D band indicates that the stacking

Figure 2. Raman spectrum of the as-synthesized S-CNOs.



order of the curved graphitic layers was not disturbed even after the incorporation of sulfur into the sp2 carbon backbone.16,29 A similar 2D band structure has not been observed in most studies of CNO, despite being synthesized using nanodiamond under high vacuum and elevated temperature conditions.6−12,16,18−20 Therefore, as-synthesized high quality S-CNO can be obtained simply by selecting a suitable precursor using a process that does not require any highly sophisticated instrumentation and special post-synthesis conditions. For interpretation, the composition and chemical state of S-doped CNOs, X-ray photoelectron spectroscopy (XPS) was employed. As shown in Figure 3a, the survey scan revealed the presence of a graphitic C 1s peak at 285 eV, a pronounced S 2p peak at 165 eV, and a weak O 1s peak at 533 eV. The high-resolution XPS C 1s spectrum was deconvoluted into three components (Figure 3b). The major peak at 284.5 eV was attributed to sp2 hybridized C atoms. The broad asymmetric nature confirmed the incorporation of S into the sp2 CNO carbon network.30,31 The very high concentration of sp2 graphitic carbon (~80%) confirmed the high degree of structural ordering, which is essential to increase the intrinsic electrical conductivity of the as-prepared S-CNOs. The tight interpenetrating graphitic contact layers (Figure 1a) and the high degree of sp2 carbon ordering among the S-CNO particles lead to an increase in the density of states of conductive electrons. This feature is ultimately required for any nanocarbon form in the search and application of ideal EDLC electrode materials. The peak at 285.4 eV corresponds to sp3 carbon in the C−S bond 32,33 and the broad peak at 286.4 eV was assigned the C−O bond.33 The peak at 287.8 eV corresponds to C=O indicating the presence of carbonyl groups.33 High hydrophilicity and dispersibility were attributed to the presence of hydroxyl and carboxyl functional groups during synthesis, which eliminates the requirement for post-synthesis chemical activation for surface functionalization. This can also be observed in core O 1s XPS spectrum (Figure 3d), it clearly shows the oxygen content which may arise during open-air synthesis (Scheme-1). The peak position at 290.5 eV corresponds to the π−π* shake-up satellite peak assigned to π-electrons delocalized in the graphitic network of S-CNO, which is indicative of its conductive nature.31−33 Furthermore, the high-resolution S 2p spectrum, as shown in Figure 3c, has an S 2p3/2 (S1) and S 2p1/2 (S2) doublet detected at 163.5 eV and 164.5 eV, respectively, of thiophene-S, arising due to spin-orbit coupling. These peaks were attributed to C=S and C−S bonds, respectively, confirming the doping of the CNO network by sulfur.


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Figure 3. (a) XPS survey spectra of S-CNOs, High-resolution XPS spectra of (b) C 1s, (c) S 2p, (d) O 1s, and (e) schematic diagram of modelled S-CNO atomistic view of doping. 11 ACS Paragon Plus Environment


In addition, the small hump located between 166−168 eV was assigned to the oxides of sulfur.32,33 Therefore, a high sulfur level (3.57 at.%) calculated from XPS analysis was achieved for S-CNO, in which most of the S atoms were doped directly into the carbon backbone, and existed in two configurations: thiophenic sulfur and sulfur oxides (Figure 3c, e). The SSA and PSD of the S-CNOs were evaluated by analyzing the N2 sorption isotherms at 77 K. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, 34 the obtained S-CNO isotherms demonstrated a type IV adsorption isotherm with a rich capillary condensation step. They also exhibited a type 3 hysteresis loop, indicating the existence of a large number of mesopores in the as-synthesized S-CNOs (Figure 4a). The SSA calculated by the BET method was 950 m2 g−1 with a micropore surface area of 523 m2 g−1 (55 % in total) and a total pore volume of 0.82 cm3 g−1 with a 29.3% of micropore volume. The ascalculated SSA of the S-CNOs was superior to those of most doped and undoped CNOs, and even comparable to graphene and associated composites.1,3,4,6−11,13−19,21−26,30−32 The PSD was calculated using the BJH method applied to the desorption branch (Figure 4b). S-CNOs possessed abundant mesopores peaked at a pore diameter of 3.9 nm with some at 9.2 nm. The presence of both micropores and mesopores in S-CNOs is beneficial because it integrates both high charge accumulation and rapid diffusion of electrolyte ions.3,6,21,22

Figure 4. (a) N2 adsorption-desorption isotherms and (b) pore size distribution curves of S-CNOs calculated by BJH method.


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4.2 Electrochemical Device Performance Studies The supercapacitor performance of novel S-CNOs was investigated in a symmetric two-electrode cell configuration, which has not been reported previously. For sustainable practical supercapacitor device operation, the electrode should be tested under ambient conditions, ensuring the environmentally benign aqueous electrolyte at high sweep rates (>20 mV s−1). Therefore, the actual electrochemical performance of S-CNO was examined by CV and GCD measurements in an aqueous 1 M Na2SO4 electrolyte in a two-electrode system. Figure 5a shows the CV curves of SCNO at various scan rates (10−200 mV s−1). The obtained CV curves of S-CNO are rectangular and symmetric in shape in the entire potential range from 0 to 1 V, indicating capacitive behavior without any influence of charge transfer kinetic limitations to the small pores. The nature of the CV curves revealed the mechanism of storing energy mainly due to the electric double layers at the high mesoporous S-CNO electrode and electrolyte interface. Even at a higher scan rate of 200 mV s−1, the rectangular shape of the CV curves was retained, indicating a very good rate performance, which was attributed to the mesoporous structure mostly centered at 3.9 nm allowing the easy diffusion of ions in the structure and to the presence of sulfur in the micropores. Moreover, the symmetric and rectangular nature of CV can be correlated with the compatibility between the size of the mesoporous and electrolyte ions. The sulfur present in small pore walls attracts more electrolyte ions by inducing positive polarity on the neighboring carbon atoms, thereby ensuing charge transfer to the small pores of a generally hydrophobic carbon matrix and improving the surface wettability and charge storage density. The evolution of the device discharge current at various scan rates (10−200 mV s−1) in Figure 5b showed a linear dependence relation according to dV

the theoretical equation, C = I( dt ), where

dV dt

is the applied scan rate indicating high power

management capacity of the S-CNO||S-CNO device. The purely double layer capacitive behavior of the device can also be verified from the capacitive current being directly proportional to the sweep rate. In addition, Figure 6a-d shows the capacitive performance of pristine CNO for comparison. The as-obtained CV profile of un-doped CNO depicts the similar rectangular and symmetric voltammogram to that of S-CNO electrode, even at a high scan rate of 200 mVs−1. As we can observe clearly, the capacitive current output and linear discharge time of S-CNO (Figure 5a, c) was relatively higher than that of pristine CNO (Figure 6a, b) under similar experimental conditions. This electrochemical performance highlights the importance of sulfur doping in CNO. 13 ACS Paragon Plus Environment


The discharge curves of S-CNO at current densities ranging from 2 to 11 A g−1 (Figure 5c) were almost linear and highly symmetrical without any redox behavior, indicating high coulombic efficiency (nearly 100% even after 10,000 charge-discharge cycles). This indicates that the fabricated S-CNO electrodes were charged and discharged at a constant rate, even at a high current density of 11 A g−1 over a given potential range from 0 to 1 V, showing no obvious IR drop and suggesting a low contact resistance (between inter-particle, electrode, and current collector). This was verified by EIS (Figure 5g), where the Rs component was as low as 0.3 Ω. From equation (1), the specific capacitance (C) of S-CNO was calculated to be 305 F g−1 at a current density of 2 A g−1, which was three times higher than that of the pristine CNO (101 F g−1) under the same electrochemical conditions, i.e., electrolyte and current density. Table 1 lists the specific capacitance of different CNO samples reported in the literature. The directly doped S-CNO nanoparticles used in this study were used directly in a device electrode without any post-synthesis treatment, such as activation, functionalization, and purification. The as-synthesized S-CNOs show good hydrophilicity and surface wettability (Figure S3a), which forms a highly dispersible conductive ink (Figure S3b) in aqueous solvents without the use of a binder and surfactant. Very good surface wettability also allows enhanced interactions between the electrolyte ions and the exposed surfaces by electrosorption, which enhances the effective formation of electric double layers. This, along with the nano dimension (

Sulfur Doping: Unique Strategy To Improve the Supercapacitive

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