K+ Intercalated MnO2 Electrode for High Performance Aqueous

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K+ Intercalated MnO2 Electrode for High Performance Aqueous Supercapacitor Ting Xiong, Wee Siang Vincent Lee, and Jun Min Xue ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01160 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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ACS Applied Energy Materials

K+ Intercalated MnO2 Electrode for High Performance Aqueous Supercapacitor Ting Xiong a,b, Wee Siang Vincent Lee *a, Junmin Xue* a a.

National University of Singapore, Department of Materials Science and Engineering,

Singapore 117573. b.

Centre for Advanced 2D Materials and Graphene Research Centre, National

University of Singapore, Singapore 117546.

ABSTRACT: MnO2 is often considered to be a promising supercapacitor electrode due to its unique electrochemical properties. This is largely due to the various arrangements of the corner and edge sharing MnO6 octahedra that forms a variety of sublattices. The interstitial sites that are generated in this process allows the occupancy of alkali, alkali earth cations, and water which not only stabilise the MnO2 framework, the process itself requires energy which is competitive against probable oxygen evolution reaction. Owning to its higher mobility and higher conductivity as compared to common alkali cations such as Li+ and Na+, K+ was selected as the intercalating cation to form K0.6MnO2 and it was used as positive electrode. When paired with K+ adsorbed holey carbon as the negative electrode, the 2.4 V asymmetric aqueous supercapacitor was able to deliver 52.8 Wh kg-1 and power density of 58.4 kW kg-1. An good cyclic life of ca. 95 % capacitance retention was also demonstrated after cycling for 10 000 cycles at 20 A g−1. KEYWORDS: aqueous supercapacitor, K+ Intercalated MnO2, K+ adsorbed holey carbon, wide potential window, high supercapacitive performance

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INTRODUCTION Supercapacitor is widely considered to be a promising energy storage device for frequency response and regulation application due to its high power capability.1-8 As a trade-off of its high power density, supercapacitor possesses poor energy density which inevitably limits its widespread application. Thus, to tap on the high power capability of the supercapacitor, enhancing its energy density is a necessary step towards the development of high performance supercapacitor.9,10 As energy density is related to the capacitance of the material and potential window of the supercapacitor according to equation E = ½ C•V2, two common strategies are often used to enhance the energy density; (1) enhancing the capacitance of the materials,11,12 and more importantly (2) widening operational potential window due to its quadratic relationship with energy density.13,14 Thus, based on these strategies, high capacitance materials that are able to achieve a wide stable potential range are highly sought after as supercapacitor electrode. Manganese oxide (MnO2) is a promising material for supercapacitor application due to its low cost, environment benignity, and well-established synthesis methods.1518

Due to the arrangement of its corner and edge sharing MnO6 octahedra into various

sublattice with structurally equivalent interstitial sites,19,20 cation and water can be intercalated into the MnO2 framework. This intercalation process may provide several advantages for MnO2 as supercapacitor electrode; (1) cation plays a key role in stabilizing the polymorph structure and hence may contribute to enhanced cyclic stability,21-23 (2) it is reported to be one of the contributors towards the enhanced capacitance of MnO2,24,25 and (3) such intercalation process can aid in increasing the oxygen evolution overpotential due to the energy required for cation intercalation which is a competitive process against oxygen evolution process.26,27 In addition, the incorporation of cations into MnO2 framework was reported to facilitate the ion diffusion process,28 which further accentuates the importance of cation intercalated MnO2. To date, Na+ is commonly reported as the intercalating cation and exceptional performances have been achieved.26,27 However, as the solvent used in this work is water, the salt ions will be hydrated by surrounding water molecules. Even though K+ possesses larger ionic size as compared to Na+, the radius of the hydrated K+ (3.31 Å) is comparable/slightly smaller as compared to that of hydrated Na+ (3.58 Å).29 This is largely due to the strong Na+ – H2O interaction as compared to K+ - H2O interaction.


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As a result of its slightly smaller hydration radius, hydrated K+ possesses higher mobility as compared to hydrated Na+ and thus leading to a higher conductivity for K+.30 Hence, using K+ intercalated MnO2 may show advantages due to its higher mobility and higher conductivity as compared to other alkali cations such as Li+ and Na+. To complement the cation intercalated MnO2 positive electrode, carbon-based material was selected as the negative electrode due to its low hydrogen evolution activity. To further enhance its stability in wide operating potential window, it has been shown that cation adsorbed carbon can aid in further damping its hydrogen evolution activity due to the cations physically blocking the adsorption of H+.27 Herein, K+ was demonstrated as the intercalating cation for MnO2 framework, and also as the cation for the adsorption onto the carbon material. The asymmetric aqueous supercapacitor, comprising of K0.6MnO2 nanosheets as the positive electrode, with K+ adsorbed carbon as negative electrode, was able to show stable operation in a potential window of 2.4 V. The supercapacitor is able to deliver 52.8 Wh kg-1 of 0.5 kW kg-1. Even at power density of 58.4 kW kg-1, a respectable energy density of 7 Wh kg-1 was achieved. In addition to the high supercapacitive performance, excellent cyclic stability of 95% capacitance retention was achieved after cycling for 10 000 cycles. This method proposed in this paper provides new insight into cation intercalated MnO2 as a potential electrode material for wide potential window supercapacitor.

MATERIAL AND METHODS Chemicals MnCl2·4H2O (99.99%), sodium hydroxide (NaOH), potassium chloride (KCl), potassium bis(trifluoromethanesulfonyl)imide (KTFSI), Starch, sodium hydrogen carbonate (NaHCO3) were purchased from Sigma-Aldrich. 0.18 mm thick carbon paper with porosity of 77 % was purchased from CeTech Co. Ltd. Synthesis of Mn3O4 and K0.6MnO2 Mn3O4 nanoparticles were prepared according to our previous method.31 The asobtained Mn3O4 was then prepared into electrode to synthesis the K0.6MnO2 nanosheets. In brief, the as-obtained Mn3O4 was grinded thoroughly with



polyvinylidene fluoride (PVDF) as the binder, and carbon black as the conductive medium with a mass ratio of 8:1:1 (Mn3O4: PVDF: Carbon black). The solvent used for the grinding process is N-methyl-2-pyrrolidone (NMP). Eventually, a uniform slurry was obtained. The uniform slurry was then pasted onto carbon paper (0.18 mm, with 77 % porosity). The carbon paper with coated slurry was heated in a conventional oven overnight at 80 oC. Finally, the carbon paper with the active material (Mn3O4/carbon paper) was cut into a square with size of 1 cm × 1 cm. A 3 electrodes electrochemical cell was set up with Mn3O4/carbon paper (working electrode), graphite paper (counter electrode), and Ag/AgCl reference electrode, in saturated KCl electrolyte. 100 cyclic voltammetry (CV) was performed at 25 mV s-1 between 0 – 1.3 V (vs. Ag/AgCl) on the Mn3O4/carbon paper to obtain the final product, K0.6MnO2. Synthesis of K+ adsorbed holey carbon Holey carbon was prepared by pyrolyzing the starch/NaHCO3 mixture. 2 g of starch and 2 g of NaHCO3 were transferred to a 50ml capacity annealing crucible with a lid. The mixture was later pyrolyzed at 800 oC for 4 hours under nitrogen atmosphere. After pyrolysis, the as-obtained black powder was washed repeatedly with various solvent (in the sequence of 1 M HCl, distilled water and finally absolute ethanol). Finally, the sample was dried in conventional oven for 24 hours at 80 ℃. To prepare the K+ adsorbed holey carbon, the dried powder was prepared into electrode as described in the earlier section. Using a 3 electrodes configuration, 100 CV scans between -1.8 – 0 V (vs. Ag/AgCl) was performed at scan rate of 25 mV s-1 in saturated KCl electrolyte to finally obtain the K+ adsorbed holey carbon (denoted as KHC). Electrochemical measurements Cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) measurement were recorded with Bio-logic VMP 3, at room temperature. K0.6MnO2 and KHC can be directly used as electrodes, respectively. In the three-electrode system, Ag/AgCl electrode (reference electrode), platinum foil (counter electrode), and 1 M KTFSI (electrolyte) were used. To test the electrochemical performance of K0.6MnO2 nanosheets and KHC, the carbon papers coated with active materials were directly cut into circular discs of 1 cm diameter. The electrochemical performances of the asymmetric supercapacitors were


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investigated with a Swagelok cell in 1 M KTFSI aqueous electrolyte. A normal filter paper was used as the separator. A voltage range of 0 – 2.4 V was used in both CV and galvanostatic charge/discharge of full cell test. Various scan rates (10, 25, 50, 100 and 200 mV s−1) and various current densities (0.5, 1, 2, 5, 10, 20, 30, 40, 50, and 100 A g−1) were investigated for CV and galvanostatic charge/discharge respectively. 0.01 to 105 Hz was measured in EIS. In a typical asymmetric supercapacitor electrochemical investigation, the mass of the electrodes for the K0.6MnO2 and K+adsorbed carbon are approximately ca. 0.6 and 1.08 mg, respectively. Based on the galvanostatic discharge profile, the capacitance of a supercapacitor, i.e. gravimetric capacitance, can be using the following equation, assuming a linear discharge profile: F = I ∆t / ∆V

(1)

where F = gravimetric capacitance (F g-1), ∆V = potential difference (V), ∆t = discharge time (s), and I = current density (A g-1). A integration of the area under the galvanostatic discharge profile was performed for non-linear discharge profile. Energy density and power density can be calculated by assuming the total mass of the positive and negative electrodes, i.e. active materials.

E = V ∙ I ∙ t dt / 3.6

(2)

P = 3600 E / ∆t

(3)

Where t1 = time to fully discharge, t2 = time to fully charge, ∆t = discharge time (s), t = time (s), I = current density (A g-1), and V = cell potential (V)



RESULT AND DISCUSSION Morphology and Chemical Investigation of As-Prepared K0.6MnO2

Figure 1. Investigation of Mn3O4 nanoparticles and K0.6MnO2 nanosheets. a) Raman spectra of both Mn3O4 and K0.6MnO2, b) high resolution Mn 2p spectra, c) high resolution K 2p spectrum, d) TEM image of Mn3O4 (inset showing high resolution TEM of the interlayer spacing), e) survey SEM image of K0.6MnO2, f) high magnification SEM image of K0.6MnO2, g) EDX elemental mappings of the K0.6MnO2, h) TEM image of K0.6MnO2 (inset showing high resolution TEM of the interlayer spacing and SAED pattern). K+ intercalated MnO2 nanosheets were fabricated by the conversion of Mn3O4 nanoparticles via electrochemical oxidation in saturated KCl. To determine the K+ content, inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was performed, and the ratio of K:Mn was determined to be ca. 0.6:1. Thereafter, the K+ intercalated MnO2 prepared from the electrochemical oxidation technique is denoted as K0.6MnO2. In Figure 1a, Raman peaks at ca. 297.7, 353.5 and 638 cm-1 were observed which could be ascribed to Mn3O4.17,32 Interestingly, after the electrochemical oxidation process, four Raman peaks at 285, 505, 566 and 626 cm-1


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appeared for K0.6MnO2, that is in good agreement with that reported in Li+ and Na+ intercalated birnessite MnO2.33,34 X-ray photoelectron spectroscopy (XPS) was utilized to examine the chemical states for both Mn3O4 and K0.6MnO2. Typically, 2 peaks could be observed in high-resolution Mn 2p spectrum (Figure 1b), and the mean Mn oxidation state can be estimated based on the binding energy difference between these 2 peaks. Based on the XPS result, the binding energy difference for Mn3O4 is ca. 11.8 eV, while K0.6MnO2 revealed a difference of 11.4 eV. This difference hints the higher Mn oxidation state in K0.6MnO2 as compared to Mn3O4.26,27 On top of the smaller binding energy difference in Mn 2p spectrum, 2 peals at 295.3 and 292. eV was observed in the high-resolution K 2p spectrum for K0.6MnO2 (Figure 1c). This further suggests that K+ was successfully intercalated into the manganese oxide framework. Under the investigation with transmission electron microscope (TEM), the as-prepared Mn3O4 possessed nanoparticle-like structure with the size range of 3070 nm (Figure 1d). Based on the high resolution TEM (HRTEM) image of the Mn3O4, a lattice spacing of 0.276 nm was measured which corresponds to the (103) plane. Scanning electron microscope (SEM) was employed to investigate the K0.6MnO2. Based on the SEM images in Figure S1, Figure 1e and 1f, the nanoparticle-like structure

transformed

into

nanosheets-like

structure

for

K0.6MnO2

after

electrochemical oxidation process. Energy-dispersive X-ray (EDX) elemental mappings of the as-prepared K0.6MnO2 showed that the K, Mn, and O elements were evenly distributed across the sample (Figure 1g). TEM image (Figure 1h) revealed that thin nanosheet that can provide large amount of surface for ion adsorption and desorption. HRTEM result clearly revealed the interplanar spacing of 0.716 nm, corresponding to the (001) plane of birnessite manganese oxide.26,27 The as-prepared K0.6MnO2 nanosheets are polycrystalline structure, as revealed by the two rings from the selected area electron diffraction. The possible conversion mechanism from Mn3O4 to K0.6MnO2 is proposed as follows; The first stage involves the dissolution of Mn2+. Next, due to the dissolution of Mn2+, the Mn3+ must be oxidized to Mn4+ so as to maintain charge neutrality in the system. Following this process is the co-insertion of the water molecules and K+ into the manganese oxide framework which eventually completes the conversion of spinel Mn3O4 to birnessite MnO2.26 Thus, based on the collective

results,

K0.6MnO2

nanosheets

was

successfully

electrochemical oxidation of Mn3O4 nanoparticles.


prepared

from


Morphology and Structural Analysis of the K+ adsorbed Holey Carbon Material

Figure 2. a) SEM of KHC, b) TEM of KHC, c) HRTEM of KHC, d) Raman spectrum, e) XPS spectrum of KHC, f) Distribution plot for the pore size (inset showing the nitrogen adsorption–desorption isotherm), g) XPS spectrum of KHC, h) EDX element mapping image of KHC. To complement with the K+ intercalated MnO2, holey carbon denoted as KHC was chosen. KHC was synthesized by a pyrolyzing technique (detailed procedures in experimental section). SEM image in Figure 2a showed that the as-prepared carbon is rich in porous structure in the range 0.5 – 0.02 µm. These pores are mainly derived from the decomposition of NaHCO3 during calcining and the removal of Na2CO3 by HCl. The pore structure was further confirmed by TEM, and these pores are separated by thin carbon wall (Figure 2b). Random crystal lattice and diffusive ring were observed, which indicates the amorphous nature of the as-prepared holey carbon (Figure 2c). The Raman spectrum in Figure 2d showed a peak at 1595 cm-1 (sp2related G band), and another peak at 1336 cm-1 (disorder-induced D band), indicating the presence of limited stacking order.35 The presence of a broad 2D band and the


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high calculated ID/IG intensity ratio of 1.08 further indicates an intensified disorder degree. XPS indicated the presence of carbon and oxygen elements and the amount of oxygen is ca. 9.0 at% (Figure 2e). Brunauer–Emmett–Teller (BET) analysis was also conducted to determine the specific surface area and the pore size distribution of the as-prepared holey carbon. Based on the result, the holey carbon possessed a BET surface area of ca. 300 m2g−1. According to IUPAC classification, the nitrogen adsorption/desorption isotherm shown in Figure 2f inset revealed a type IV isotherm with a large H3 hysteresis, which suggest the sample is comprised of mesopores.36 Pore size distribution < 2nm and around 4 nm was revealed in the Barrett–Joyner– Halenda (BJH) analysis result (Figure 2f), indicating the presence of micropores and mesopores, these pores would supply more favorable pathways for electrolyte penetration. It was reported previously that Na+ adsorbed onto carbon electrode could act as physical barrier towards H+ adsorption, depressing the hydrogen evolution reaction.27 The adsorption of K+ on the surface of holey carbon could hinder the H+ adsorption to reduce the hydrogen evolution activity. Thus, to enhance the stability of the holey carbon in aqueous electrolyte, i.e. to reduce the HER activity, surface of the holey carbon was modified with adsorbed K+ via a simple electrochemical method. The as-prepared K+ adsorbed holey carbon is denoted as KHC. The K+ content was determined to be 7 wt% as revealed by ICP-OES analysis. Based on the XPS spectrum (Figure 2g), the high-resolution K 2p spectrum exhibited two notable peaks with binding energies at 296.3 eV for K 2p1/2 and 293.2 eV for K 2p3/2, suggesting the successful adsorption of K+ on the surface of the holey carbon.37 Besides, the successful integration of K+ onto the surface of holey carbon could be seen in the EDX elemental mapping (Figure 2h). Hence, based on the collective results, K+ adsorbed holey carbon was successfully synthesized, which is expected to be suitable as supercapacitor electrode. Electrochemical Characterizations of the as-prepared K0.6MnO2 and K+ adsorbed Holey Carbon



Figure 3. Electrochemical performance of K0.6MnO2 nanosheets and K+ adsorbed holey carbon. a) CV curve of K0.6MnO2 nanosheets, b) galvanostatic profiles of K0.6MnO2 nanosheets, c) the rate performance of K0.6MnO2 nanosheets, d) CV curve of KHC, e) galvanostatic profiles of KHC, f) the rate performance of KHC. Electrochemical performances of as-synthesized materials were investigated in a 3 electrodes cell using 1 M KTFSI. K0.6MnO2 electrode exhibited well-defined pseudocapacitive feature within a wide potential window of 0 – 1.2 V versus Ag/AgCl, as shown in Figure 3a. At scan rate of 10 mV s−1, K0.6MnO2 electrode showed two pairs of peaks that can be associated with the transformation of Mn3+ to Mn4+, and K+ intercalation into manganese oxide framework (Figure 3a). A hump was observed in the galvanostatic charge–discharge curves, which is consistent with the cyclic voltammetry (CV) curve (Figure 3b). From the discharge curve, the specific capacitance of K0.6MnO2 was calculated to be ca. 254, 220, 154, 123, 100, 85 and 72 F g−1 at 1, 2, 10, 20, 30, 40 and 50 A g-1 as shown in Figure 3c. To investigate the advantages of KHC over the untreated holey carbon, their CV curves were compared in Figure S2. It can be observed from the area and profile of the CV curves that K+adsorbed holey carbon demonstrates enhanced electrochemical performance as compared to the untreated holey carbon. At low scan rate, the KHC is characterized by CV profile with a rectangular shape in potential range of -1.2 – 0 V (Figure 3d). Nearly linear curves, which is typical of classical EDLC capacitive materials, were characterized for the charge and discharge curves, which suggests a good capacitance behavior.38 From the discharge profiles, the specific capacitance of KHC was estimated to be 143 F g−1 at current density of 1 A g-1. Even at high current density (50 A g-1), the carbon electrode delivered a respectable specific capacitance of ca. 50 ACS Paragon Plus Environment

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F g-1 (Figure 3f). Based on the result, the specific capacitance of K0.6MnO2 is ca. 1.8 times of that of KHC. Electrochemical investigation of the Asymmetric Supercapacitor

Figure 4. a) Individual CV curves of the K0.6MnO2 and the KHC, b) CV of the K0.6MnO2//KHC conducted at scan rate of 25 mV s-1 from 0 – 1 V with 0.2 V step increment to investigate the operating window, c) CV curve of K0.6MnO2//KHC with potential window of 0 – 2.4 V, d) galvanostatic charge/discharge profiles of K0.6MnO2//KHC at various current densities, e) specific capacitance at different current densities, f) the Ragone plot for K0.6MnO2//KHC, g) the cyclic performance investigation. An asymmetric supercapacitor based on K0.6MnO2 positive electrode and K+ adsorbed holey carbon negative electrode was assembled. The K0.6MnO2 electrode was able to operate in the potential window of 0 – 1.2 V while the K+ adsorbed holey carbon showed stable operation in the range of -1.2 – 0 V (Figure 4a). As a result, the assembled supercapacitor was able to perform a 2.4 V potential window as shown in Figure 4b. From the CV curves of Mn3O4//carbon and K0.6MnO2//KHC shown in Figure S3, it can be concluded that the electrochemical performance of K0.6MnO2//KHC is much higher than that of Mn3O4//carbon. The CV curves of



K0.6MnO2//KHC with rectangular shapes were observed at scan rates up to 200 mV s−1 (Figure 4c). Figure S4a showed the CV curves of K0.6MnO2//KHC at different scan rates from 10 to 200 mV s−1. In each curve, 1 reduction peak and 1 oxidation peak were observed. An equation based on their peak currents (i) and scan rates (v) can be derived as follows:39,40 i = avb

(1)

which can be rewritten as log(i) = blog(v) + log(a) (2) where b = slope of log(i) vs. log(v) curve. Typically, the value of b is between 0.5 – 1, and the value can suggest the type of electrochemical process. The electrochemical process is controlled by ionic diffusion as b value is 0.5. On the other hand, surface capacitive effects will become pronounced when the b value reaches 1. As shown in Figure S4b, the b values for peak 1 and 2 are estimated to be 0.88 and 0.89 respectively. These b values suggest the both ionic diffusion and surface capacitive effects influence the electrochemical reaction for K0.6MnO2//KHC. To determine the capacitive contribution, the following equation can be used:41,42 i =k1v + k2v1/2 (3) which can be reformulated as i/v1/2=k1v1/2 + k2 (4) where i = total current response, k1v = current due to surface capacitive effects, and k2v1/2 = current due to ionic diffusion process. Since k1 could be obtained by fitting the i/v1/2 vs. v1/2 plots, the capacitive contribution is calculated to be 58.7% (with scan rate = 10 mV s–1). With the increase in scan rate, the percentage of capacitive contribution raises to 69.2%, 76%, 81.8%, and 86.4% at 25, 50, 100, and 200 mV s-1, respectively (Figure S4c). Figure 4d shows the charge/discharge curves at various current densities had a small “IR drop” that indicated a small internal resistance. The specific capacitance at 0.5 A g−1 estimated from the galvanostatic profile was ca. 65 F g−1, and with 200 folds increment in current density, a modest specific capacitance of 32 F g−1 was achieved (Figure 4e). The energy and power densities of the as-prepared supercapacitor calculated based on the data are shown in Figure 4f. The as-fabricated


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supercapacitor possessed a maximum energy density of 52.8 Wh kg-1 at power density of 0.5 kW kg-1. The high energy density is much higher than most manganese oxide based systems such as carbon nanofiber/MnO2//activated carbon (36.7 Wh kg-1),43 Polypyrrole/MnO2//activated

carbon

(25.8 -1 45

MnO2/polyaniline//activated carbon (40.2 Wh kg ).

Wh

kg-1),44

and

At an energy density of 7 Wh

kg−1, a maximum power density of 58.4 kW kg-1 was achieved, superior to other supercapacitors such as Na0.5MnO2//Fe3O4@Carbon (20 kW kg−1),26 porous carbon//MnO2/carbon (8 kW kg-1),35 and rGO/MnOx//Activated carbon (20.8 kW kg1 46

), As demonstrated in the inset of Figure 4g, the supercapacitor could power a red

round light-emitting diode (LED) indicator brightly. In addition, the assembled supercapacitor showed a capacitance retention of ≈ 95% after 10 000 cycles at 20 A g1

, demonstrating the outstanding stability of the supercapacitor in terms of long-term

cycling. Besides, the kinetics of electrode processes has been studied by electrochemical

impedance

spectroscopy

(EIS)

as

shown

in

Figure

S5.

K0.6MnO2//KHC gave a series resistance (Rs, 43 Ω), a charge-transfer resistance (Rct, 255 Ω) and a Warburg diffusion impedance (Zw, 158 Ω). In comparison, Mn3O4//carbon showed much higher Rs (354 Ω), Rct (2771 Ω), and Zw (1875 Ω). Obviously, supercapacitors based on K0.6MnO2//KHC showed much smaller resistance than that of Mn3O4//carbon, suggesting the fast charge transfer process, high electrochemical activity and then the enhanced power density. In general, insertion of K+ cation in MnO2 has been preferred to modify the manganese oxide. It can aid in increasing the oxygen evolution overpotential to widen the potential window due to the energy required for cation intercalation, and it is one of the contributors towards the enhanced capacitance of MnO2. The adsorption of K+ on the surface of holey carbon could hinder the adsorption of H+ to reduce the hydrogen evolution activity. When the two electrodes materials are assembled to form an asymmetric supercapacitor, enhanced supercapacitive performance could be obtained. Thus, electrodes with K+ modification is a promising method to optimize the interaction between the electrodes and electrolyte. This work presents a novel effective strategy to design high performance aqueous asymmetric supercapacitor.

CONCLUSIONS In summary, an asymmetric supercapacitor with K+ modification was developed. By using K0.6MnO2 (positive electrode) and K+ adsorbed holey carbon (negative ACS Paragon Plus Environment


electrode), 2.4 V aqueous asymmetric supercapacitor with high electrochemical performance was assembled. The asymmetric supercapacitor delivered 52.8 Wh kg-1 at 0.5 W kg-1 and 7 Wh kg-1 at 58.4 kW kg-1. Furthermore, the supercapacitor exhibited excellent cycling performance as it managed to retain 95 % of its initial capacitance retention after cycling for 10 000 cycles. Thus, based on the collective results, K+ intercalated MnO2 and K+ adsorbed holey carbon may be promising electrode materials for the development of high performing, wide potential window aqueous supercapacitor.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images, CV profiles, and Nyquist plots.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Wee Siang Vincent Lee), [email protected] (Junmin Xue). Tel./fax +65 65164655.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Singapore MOE Tier 1 funding R-284-000-162-114.

REFERENCES (1) Zhang, L. L.; Zhao, X. S.; Carbon-based Materials as Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38, 2520–2531. (2) Yu, G.; Xie, X.; Pan, L.; Bao, Z.; Cui, Y. Hybrid Nanostructured Materials for High-performance Electrochemical Capacitors. Nano Energy 2013, 2, 213–234. (3) Wang, X.; Zhang, Y.; Zhi, C.; Wang, X.; Tang, D.; Xu, Y.; Weng, Q.; Jiang, X.; Mitome, M.; Golberg, D.; Bando, Y. Three-dimensional Strutted Graphene Grown by Substrate-free Sugar Blowing for High-power-density Supercapacitors. Nat. Commun. 2013, 4, 2905. (4) Li, C.; Zhang, X.; Wang, K.; Sun, X.; Liu, G.; Li, J.; Tian, H.; Li, J.; Ma, Y. Scalable Self‐ Propagating High‐ Temperature Synthesis of Graphene for


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Kang, F.; Wong, C. P.; Yang, C. A Reduced Graphene Oxide/mixed-valence Manganese Oxide Composite Electrode for Tailorable and Surface Mountable Supercapacitors with High Capacitance and Super-long Life. Energy Environ. Sci. 2017, 10, 941−949.



TOC

K+ was selected as intercalating cation to form K0.6MnO2 nanosheets that was employed as the positive electrode. When paired with K+ adsorbed holey carbon as the negative electrode, the 2.4 V aqueous asymmetric supercapacitor was able to deliver an enhanced supercapacitive performance.


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K+ Intercalated MnO2 Electrode for High Performance Aqueous

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