MnO2 nanostructures deposited on graphene-like porous carbon

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MnO2 nanostructures deposited on graphene-like porous carbon nanosheets for high-rate performance and high-energy density asymmetric supercapacitors Bei Liu, Yijiang Liu, Hongbiao Chen, Mei Yang, and Huaming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04817 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Sustainable Chemistry & Engineering

MnO2 Nanostructures Deposited on Graphene-Like Porous Carbon Nanosheets for High-Rate Performance and High-Energy Density Asymmetric Supercapacitors

Bei Liu a, Yijiang Liu a, Hongbiao Chen a*, Mei Yang a, Huaming Li a,b*

a College

b Key

of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China

Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key

Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China

*Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264. E-mail address: [email protected] (H. Chen), [email protected] (H. Li)

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Abstract

Graphene-like porous carbon nanosheets (GPCNs) have been proved to have great potential in high-rate performance supercapacitors. However, the relatively low specific capacitance and low working voltage of GPCN-based supercapacitors hinder their practical application in high-energy density devices. Herein, we demonstrate the direct deposition of MnO2 nanostructures onto the salvia splendens-derived GPCNs (GPCN-SS) surface through a hydrothermal method. The as-fabricated MnO2/GPCN-SS composite possesses a relatively high specific surface area (483 m2 g−1) and an appropriate mesopore size (2-5 nm). Thanks to the synergistic effect of GPCN-SS and MnO2 nanostructures, the MnO2/GPCN-SS composite electrode exhibits high specific capacitance of 438 F g−1 at 0.5 A g−1 (almost twice as high as the pristine GPCN-SS) and high rate capability (67.8% capacity retention at 50 A g−1) in Na2SO4 electrolyte. More importantly, an asymmetric supercapacitor assembled with MnO2/GPCN-SS composite cathode and GPCN-SS anode in neutral electrolyte displays excellent rate capability (77.8% capacity retention from 0.5 to 50 A g−1) and high energy density (50.2 Wh kg−1 at 516 W kg−1). Considering the abundance and sustainability of biomass-derived GPCNs along with the low-cost of MnO2, MnO2/GPCN-SS composite can be served as a promising new class of electrode materials for high-rate and high-energy density supercapacitors.

Keywords: MnO2; Nanostructures; Graphene-like carbon nanosheets; Biomass; Asymmetric supercapacitors. 2 ACS Paragon Plus Environment

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Introduction

Energy crisis and environmental deterioration are currently thought to be the pressing challenges for the world, which hamper the sustainable development of human society.1, 2 In order to solve such problems effectively, it is important to develop sustainable and environmentally friendly energy storage and conversion devices.3-5 As one of the most promising energy storage devices, supercapacitors (SCs) that can accumulate charge and release energy electrochemically have attracted wide attention because of their inherent high power density, rapid charge/discharge rate, and environmental friendliness.6-8 However, conventional SCs usually suffer from low energy density and cannot satisfy the requirements of practical application.9-12 Considering that the energy density (E = 1/2CV2) of a SC is depended on its specific capacitance (C) and operation voltage (V), energy density can thus be enhanced either by raising the capacitance or by extending the working voltage or even by improving both of them.10,13,14 Although organic electrolytes such as EMImBF4 can extend the working voltage to as high as 3.6 V,15 organic media yet have the disadvantages of poor conductivity, high-cost, high viscosity, and toxicity, which will inevitably impair the overall performance of the devices.16,17 On the other hand, the range of capacitance improvement is also limited by the barrier of electrode materials.18 The only really effective way to improve energy density while without sacrificing the power density is to employ asymmetrical supercapacitor (ASC) technology.19-21 In general, ASCs can be fabricated by using pseudo-capacitive materials as cathode and electrical double-layer (EDL) capacitive materials as anode, in which pseudo-capacitance at 3 ACS Paragon Plus Environment


cathode is based on faradic reaction and EDL capacitance at anode is formed by electrolyte ions adsorption.14,20-24 As expected, ASCs have both advantages of batteries and SCs and therefore can provide working voltage as high as the sum of potential windows of the two electrodes (cathode and anode), leading to a significant enhancement in both energy and power densities. In this regard, the rational design of cathode and anode materials can better control the overall performance of the ASCs, such as energy density, power density, and rate capability.23-26 At present, carbon-based porous materials (i.e., graphene,20,23,24 carbon nanotubes,25,26 carbon fibers,24,27 and activated carbons28,29) are the most commonly employed anode materials, while the cathode materials mainly include transition metal-based oxides (i.e., RuO2, Co3O4, NiO, Fe2O3 and MnO2),22,25-33 hydroxides (i.e., Cu(OH)2, Ni(OH)2, layered double hydroxides),34-36 chalcogenides (i.e., NiCo2S4, Ni3S2, MoS2),37-39 together with conducting polymers.26,40 Compared with anode materials, pseudo-capacitive cathode materials, especially transition metal oxides, have recently become of more importance because of their much high specific capacitance that stemmed from faradaic charge transfer.25-33 Among these transition metal oxides, MnO2 has emerged as the most competitive cathode material owing to its ultrahigh theoretical capacitance (1370 F g−1), ideal capacitive behavior, resource abundance, and environmental compatibility.41,42 But unfortunately, such theoretical capacitance can only be achieved by using nanostructured MnO2 due to its poor conductivity.41 To achieve better capacitive performance, MnO2 nanostructures should be composited with highly conductive and hierarchically porous materials, such as carbon-based porous materials as mentioned above. Up to now, diverse MnO2 nanostructures have been successfully incorporated into 4 ACS Paragon Plus Environment

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graphene,23,24 carbon nanotubes,25,26 carbon fibers,24,27 carbon aerogel,43 and activated carbons28,29 by electrodeposition or chemical deposition. Although the energy densities of MnO2-based ASCs can be effectively enhanced by combining with porous carbon materials, their rate capability is still unsatisfying.23-29 In many cases, the rate capability has deteriorated seriously owing to their poor porosity and the resulted high-resistance for ion-transfer as well as long-pathway for ion-diffusion.44,45 Rate capability seems to be the major problem with MnO2-based ASCs, which needs the structural design of MnO2/carbon composites to balance the energy density and rate capability. In order to satisfy the demand for high-rate performance ASCs, great efforts have recently been made to combine MnO2 with graphene or graphene-like porous carbon nanosheets (GPCNs) because their ultrathin 2D structure can ensure high electrical conductivity for rapid charge transfer, and their intra- and inter-planar pores can enable fast ionic transport as well as diffusion.24,25 But after all, pristine graphene is still expensive and scarce, which severely hinder its practical application.20,46 In this regard, biomass-derived GPCNs should be a wise choice from the views of sustainability and environmental friendliness. Keeping the above considerations in mind, we herein present a novel hybrid structure that consists of MnO2 nanostructures and salvia splendens (SS)-derived GPCNs (GPCN-SS). The GPCN-SS with a nanosheet-like structure was fabricated by sealing the SS petals in a salt crystal followed by pyrolysis as demonstrated in our recent work.47 The as-fabricated GPCN-SS possesses a high electrical conductivity, a high specific surface area (SSA) and a hierarchical porosity, which make it a suitable support for the MnO2/carbon composites. The MnO2/GPCN-SS composite was therefore fabricated by the deposition of MnO2 5 ACS Paragon Plus Environment


nanostructures on the GPCN-SS surface under hydrothermal conditions. The resultant MnO2/GPCN-SS composite possessed a relatively high SSA (483 m2 g−1) and an appropriate mesopore size (2‒5 nm). Benefiting from such features, the MnO2/GPCN-SS//GPCN-SS ASC exhibited high-rate capability and high energy density in 1.0 M Na2SO4 aqueous electrolyte.

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Experiment Section

Preparation of MnO2/GPCN-SS composite

The washed SS petals were firstly crushed into tiny pieces (100 mesh) followed by immersing such pieces (5 g) in a saturated NaCl aqueous solution (100 mL) for 2 h by stirring vigorously. Then the NaCl crystal-sealed SS petals were prepared by freeze drying. After pyrolyzing the NaCl-sealed petals at 800 °C for 2 h in nitrogen, the GPCN-SS was obtained by successively washing with 1.0 M HCl aqueous solution and ultrapure water followed by oven-drying (for synthesis details, see Supporting Information). The as-fabricated GPCN-SS was used herein as the support to fabricate MnO2/GPCN-SS composites. In a typical synthesis, GPCN-SS (50 mg) was added into 80 mL of KMnO4 aqueous solution (5.0 mM) and stirred for 30 min at room temperature. Then the mixture was transferred into a 100 mL Teflonlined stainless steel autoclave and kept at 120 °C for 2 h. The MnO2/GPCN-SS composite was obtained after filtration, washing several times with ultrapure water, and drying at 100 °C overnight. For comparison, pure MnO2 was also prepared by directly pyrolyzing MnO2/GPCN-SS at 600 °C for 2 h in air. 6 ACS Paragon Plus Environment

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Electrochemical Measurements

The working electrode was made by mixing the active material, carbon black, and PTFE (polytetrafluoroethylene) at a mass ratio of 8/1/1 in ethanol, followed by pasting on nickel foam (1 cm × 1 cm) and drying at 100 °C for 12 h in an oven. All electrochemical tests were carried out on a CHI760D electrochemical workstation (CH Instruments Inc., Shanghai, China) in 1 M Na2SO4 electrolyte at 25 °C. The electrochemical performance was evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements in a three-electrode system using platinum wire as the counter electrode and Ag/AgCl electrode as the reference electrode. The mass loading of active materials was around 3.0 mg. The ASC was fabricated in a stainless steel CR2032 coin cell using MnO2/GPCN-SS cathode and GPCN-SS anode with a glassy fibrous separator. The stability tests were carried out on a battery test system (NEWARE CT-4008).

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Results and Discussion

Preparation and characterization of MnO2/GPCN-SS composite

In this work, biomass-derived GPCN-SS was chosen as the support for MnO2/GPCN-SS composite fabrication mainly because of its crumpled nanosheet-like structure, large SSA (1051 m2 g−1), hierarchical porosity (micro-, meso-, and macropores, 0.71 cm3 g−1), and O/N-codoping. Such unique characteristics can endow GPCN-SS with not only high electrical 7 ACS Paragon Plus Environment


conductivity (711 S m−1 as determined by a standard four-probe method) and high specific capacitance (235 F g−1 in 1.0 M Na2SO4 aqueous electrolyte at 0.5 A g−1), but also high rate-performance as proved in our recent work.47 Moreover, O-doping (10.26 at%) in GPCN-SS can facilitate the formation of Mn−O−C bond and thus strengthen interfacial contact between GPCN-SS and MnO2,48 while N-doping (2.32 at%) can increase the reactivity of GPCN-SS to KMnO4 and hence shorten the synthesis time.49 As expected, the redox reaction between GPCN-SS and KMnO4 can be finished within 2 h under hydrothermal conditions, giving desired MnO2/GPCN-SS composite. The major advantage of hydrothermal deposition method lies in its strong tendency to produce δ-MnO2, which has been proved to have the second largest specific capacitance among various MnO2 structures.50 In addition, the deposition of MnO2 on and between the GPCN-SS sheets can help to exfoliate the stacked nanosheets and simultaneously to prevent composite sheets from restacking, which is benefit to further improve the electrochemical performance of this composite.20 In the current case, the MnO2/GPCN-SS composite with a MnO2 content of 64.7 wt% (determined by TGA analysis, Fig. S1a, b) displays the highest specific capacitance as proved by our condition experiments (Fig. S1c, d). Therefore, this composite was chosen for detailed characterization. For comparison, pure MnO2 was also prepared by directly pyrolyzing MnO2/GPCN-SS composite at 600 °C in an air atmosphere. The morphologies of MnO2/GPCN-SS and GPCN-SS were firstly characterized by SEM and TEM observations. The GPCN-SS support exhibits typically crumpled graphene-like architecture with lateral sizes up to dozens of microns (Fig. 1a). Moreover, the porous structure along with the localized graphitic structure of the GPCN-SS can also be seen in the 8 ACS Paragon Plus Environment

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HRTEM image (inset of Fig. 1a). The MnO2/GPCN-SS composite has inherited the sheet-like structure from its support, showing the overall architecture of curly 2D nanosheets with uniformly deposited MnO2 layers (Fig. 1b). The layered structure of MnO2 can be clearly seen in high-magnification SEM image (Fig. 1c), in which the layers are composed of a large number of nanoparticles and worm-like nanorods with diameter of around 10 nm (Fig. 1d). Notably, the layer-by-layer stacked MnO2 structure can enable MnO2/GPCN-SS composite to have a high SSA for charge storage and in the meantime to have more pathways for ion-diffusion and ion-transport. The TEM image of MnO2/GPCN-SS further demonstrates the layered structure of MnO2 as well as mesoporous structure that are believed to be derived from the disorderly stacked nanoparticles and worm-like nanorods (Fig. 1e). The rod-like architecture of MnO2 can also be observed in the high-magnification TEM image (Fig. 1f), which are densely deposited on the support surface. Such MnO2 nanostructures in close contact with GPCN-SS can enable the composite to possess a high electrical conductivity.

Fig. 1. SEM images of GPCN-SS (a) and MnO2/GPCN-SS (b-d) at different magnifications, inset of a shows HRTEM image of GPCN-SS. TEM images (e, f) of MnO2/GPCN-SS at different magnifications.



The phase structure of MnO2 in the composite was next characterized by XRD and Raman technologies. As illustrated in Fig. 2a, the broad and weak peaks at 23.6° and 43.5° are obviously resulted from the GPCN-SS support,51 while the weak peaks at 12.1°, 26.1°, 37.1°, and 66.5° can be assigned to the (001), (002), (111), and (020) reflections of δ-MnO2 phase (JCPDS 42-1317),24,52 just as in the case of the pure MnO2. In addition, the appearance of weak peaks in the range of 520–640 cm−1 in the Raman spectrum of MnO2/GPCN-SS composite further demonstrates the δ-MnO2 crystallographic structure (Fig. S2). The simultaneous appearance of the characteristic peaks for both MnO2 and GPCN-SS in the XRD pattern and Raman spectrum of MnO2/GPCN-SS composite indicates that the MnO2 layer deposited on GPCN-SS surface is quite thin. The chemical composition and oxidation state of the MnO2/GPCN-SS composite was further studied by XPS analysis. As shown in Fig. S3, the characteristic peaks for Mn, O, C and N elements appear together in the survey XPS spectrum of MnO2/GPCN-SS composite, which also confirms the thin feature of MnO2 layer. The surface elemental composition of this composite was found to be C 25.21 at%, N 0.92 at%, O 41.33 at%, and Mn 31.54 at% (Table S1). In addition, two peaks at 642.6 and 654.1 eV corresponding to Mn 2p3/2 and Mn 2p1/2, respectively, are clearly observed in the high-resolution Mn 2p XPS spectrum (Fig. 2b) with a spin energy separation of 11.6 eV, implying that the oxidation state of Mn element is around +4.29,48 The high-resolution O 1s XPS spectrum in Fig. 2c reveals the coexistence of Mn−O−Mn (529.6 eV), Mn−O−C (530.7 eV), Mn−O−H (531.6 eV), and H−O−H (532.7eV) bonds,53 in which the relative content of Mn−O−C bond is found to be 30.2% (Table S1). This result is in good agreement with the high proportion of C−O/O−C=O configurations (42.9%) in the deconvoluted C 1s XPS 10 ACS Paragon Plus Environment

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spectrum (Fig. 2d and Table S1). Notably, the as-formed Mn−O−C bond has been proved to be beneficial for the enhancement of both composite conductivity and composite stability,48 thus leading to high-rate performance as well as excellent cycling stability of the composite-based electrode.

Fig. 2. XRD patterns of GPCN-SS, MnO2, and MnO2/GPCN-SS (a). High-resolution Mn 2p (b), O 1s (c), C 1s (d) XPS spectra of MnO2/GPCN-SS.

The porosity and SSA of MnO2/GPCN-SS composite were finally determined by the N2 sorption isotherms (Fig. 3a, b). Just as in the case of the GPCN-SS support, the MnO2/GPCN-SS composite also exhibits a type-IV isotherms along with type-H4 hysteresis loops, suggesting the coexistence of micropores and mesopores.54 The SSA and pore volume 11 ACS Paragon Plus Environment


are found to be 483 m2 g−1 and 0.31 cm3 g−1 (Table S2), respectively, for the MnO2/GPCN-SS composite, which are lower than those of the GPCN-SS support (SSA = 1051m2 g−1, pore volume = 0.71 cm3 g−1), but are significantly higher than those of recently reported MnO2/graphene composites (Table S3). Besides the contribution of GPCN-SS support, the high SSA and high porosity of this composite are thought to be connected with the unique nanostructure of MnO2 layer, which is supposed to have a relatively high SSA as evidenced by pure MnO2 (167 m2 g−1). The pore-size distribution (PSD) of MnO2/GPCN-SS composite is in the range of 2 to 5 nm based on density functional theory (DFT) model (Fig. 3b), which is also close to that of GPCN-SS support (2−10 nm). Notably, the high SSA together with an appropriate mesopores (32.7%) with sizes in the range of 2 to 5 nm guarantee not only high specific capacitance but also high-rate performance for the MnO2/GPCN-SS composite electrode.55

Fig. 3. Nitrogen adsorption/desorption isotherms (a) and pore size distributions calculated using DFT method slit pore model, differential pore volume and pore width (b) of GPCN-SS, MnO2, and MnO2/GPCN-SS.


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Capacitive behavior of MnO2/GPCN-SS composite

It is necessary to determine the potential range of the MnO2/GPCN-SS electrode in 1 M Na2SO4 aqueous electrolyte before studying its capacitive behavior. In theory, the negative potential limit of the MnO2/GPCN-SS electrode is dictated by either the hydrogen evolution reaction (HER) or the Mn(IV) to Mn(II) reduction reaction, whereas the positive potential limit depends on the oxygen evolution reaction (OER) or the Mn(IV) to Mn(VII) oxidation reaction. In 1 M Na2SO4 aqueous electrolyte (pH = ~7), the thermodynamical HER and OER start at −0.41 V (vs. Ag/AgCl, the same hereinafter) and 0.817 V,56 respectively, while the thermodynamic Mn(IV) reduction and Mn(IV) oxidation potentials are 0.27 V and 0.99 V at pH = ~6.4,57,58 respectively. That is to say, the theoretical potential range of the MnO2/GPCN-SS electrode is determined by the Mn(IV) reduction and Mn(IV) oxidation reactions, and is only around 0.72 V. Experimentally, the potential range of the MnO2/GPCN-SS electrode can be extended to 0–1.0 V due to the high kinetic barriers of these redox reactions in our case as demonstrated by CV and linear sweep voltammetry (LSV) measurements (Fig. S4, S5). Moreover, the pre-insertion of Na+ ions into the MnO2 lattice can significantly increase the potential range.59 For example, the stable potential window in neutral Na2SO4 electrolyte was demonstrated to be 0–1.3 V for the Na0.5MnO2 electrode,60 −0.1–1.2 V for the rGO@Mn3O4 electrode,59 −0.1–0.9 V for the MnO2@CNT electrode,57 0–1.0 V for the graphene/MnO2 composite network electrode,61 and so forth.



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Fig. 4. CV curves tested at 10 mV s−1 (a), and GCD curves tested at 0.5 A g−1 (b) of GPCN-SS, MnO2, and MnO2/GPCN-SS.

The capacitive behavior of MnO2/GPCN-SS composite was therefore investigated in 1 M Na2SO4 aqueous electrolyte using a three-electrode configuration. For comparison, pure MnO2 and GPCN-SS materials were also tested under the same conditions. As shown in Fig. 4a, the CV curve of GPCN-SS electrode at 10 mV s−1 reveals a perfectly rectangular shape, confirming a purely ideal EDL capacitive behavior that originated from its high SSA (1051 m2 g−1) together with abundant micropores (above 79.6%). As for the MnO2/GPCN-SS and MnO2 electrodes, obvious redox peaks were not appeared in their CV curves even though a slight distortion in rectangle shape was observed, indicating that the specific capacitance of both MnO2 and MnO2/GPCN-SS electrodes was mainly contributed by EDL capacitance and faradic capacitance had less contribution. The powerful EDL capacitive behavior of MnO2 and MnO2-based composites in Na2SO4 electrolyte can be explained by the adsorption of Na+ cations on the surface of MnO2 matrix:62 (MnO2)surface + Na+ + e− ↔ (MnO2−Na+)surface

(1)

Such adsorption-desorption processes predominantly occur on the amorphous MnO2 surface 14 ACS Paragon Plus Environment

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owing to the relatively large SSA. Whereas the pseudo-capacitive behavior is associated with the insertion and desertion of Na+ cations in MnO2 matrix upon reduction and oxidation:63 MnO2 + Na+ + e− ↔ MnOONa

(2)

This faradic capacitive behavior has been proved to occur mainly in crystalline MnO2 materials. In our case, both MnO2 and MnO2/GPCN-SS composite exhibited a highly amorphous feature together with a relatively high SSA, which make them intrinsically more appropriate for EDL capacitance. To further prove the EDL capacitive performance, CV curves of MnO2, GPCN-SS and MnO2/GPCN-SS electrodes at different scanning rates were recorded. As shown in Fig. 5a and Fig. S7a, S8a, the CV profile of pure MnO2 electrode maintains a basically rectangular shape at a relatively high scanning rate of 100 mV s−1 (Fig. S7a). In addition, the CV curves of GPCN-SS electrode at 500 mV s−1 and MnO2/GPCN-SS composite electrode at 300 mV s−1 also remain in a moderately rectangular shape (Fig. S8a, Fig. 5a), confirming the ideal capacitive performance as well as the excellent rate capability, in which the rate capability will be discussed below. The GCD curves of MnO2, GPCN-SS, and MnO2/GPCN-SS electrodes at 0.5 A g−1 are illustrated in Fig. 4b. As can be seen, all GCD curves exhibit a symmetrically triangular shape without obvious IR drops, suggesting a rapid and reversible charge/discharge processes. However, the symmetry of GCD profile is the best for the GPCN-SS electrode, the next is MnO2/GPCN-SS electrode, and MnO2 electrode is the worst, which are in good agreement with the CV results. The GCD curves and specific capacitance (Cs) of the three electrodes at various current loads are depicted in Fig. 5b-c and Fig. S7b-c, S8b-c. As expected, the pure MnO2 electrode exhibits poor rate capability together with poor electrical conductivity as 15 ACS Paragon Plus Environment


evidenced by the severely distorted triangle shape as well as the huge IR drop in its GCD profile at 50 A g−1 (Fig. S7c). Conversely, the GCD curve of GPCN-SS electrode still retains a triangular shape and shows a small IR drop at 50 A g−1 (Fig. S8c), implying good electrical conductivity and excellent rate capability that stemmed from its unique sheet-like architecture, high SSA, and appropriate content of mesopores (20.4%). Such mesopores with pore sizes in the range of 2 to 10 nm are believed to be able to shorten the diffusion pathway and simultaneously to reduce the transport resistance for electrolyte ions, therefore leading to a high-rate performance. Benefiting from the good conductivity and the outstanding rate capability of GPCN-SS, MnO2/GPCN-SS electrode also displays high-rate capability, as evidenced by the fairly symmetrical GCD profile as well as low IR drop (227 mV) at 50 A g−1 (Fig. 5c). Besides the contribution of GPCN-SS, the high-rate performance of MnO2/GPCN-SS composite electrode is also closely bound up with the unique structure of MnO2 deposited onto the GPCN-SS surface. As mentioned previously, the deposited MnO2 was mainly composed of worm-like nanorods and nanoparticles, which were stacked on the GPCN-SS surface layer by layer and formed a densely layered structure. Moreover, the O-containing groups on GPCN-SS surface are expected to strengthen the interfacial contact between GPCN-SS and MnO2, i.e., the formation of Mn−O−C bond in the composite as proved by XPS analysis, which is highly beneficial for enhancing electrical conductivity as well as composite stability.48 As a result, the electrical conductivity determined by four-probe method can be as high as 177 S m−1 for MnO2/GPCN-SS composite, only somewhat lower than that of the GPCN-SS support (711 S m−1). Such high conductivity together with relatively high content of Mn−O−C bond in this composite can guarantee rapid and effective 16 ACS Paragon Plus Environment

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charge transfer, which is one of the key factors in raising rate performance. The other reason is that MnO2/GPCN-SS composite possesses a relatively high SSA (483 m2 g−1), a hierarchical porosity with an appropriate content of mesopores (32.7%), which can significantly reduce electrolyte ion-diffusion distance and ion-transport resistance, thus leading to a high-rate performance.55 Due to the synergistic effect of GPCN-SS and MnO2, the Cs value of MnO2/GPCN-SS composite electrode in 1 M Na2SO4 aqueous electrolyte was found to be 438 F g−1 at a current load of 0.5 A g−1, which was much higher than that of pristine GPCN-SS electrode (235 F g−1) as well as pure MnO2 electrode (201 F g−1). The capacitance retentions from 0.5 to 50 A g−1 were found to be 80.8%, 67.8%, and 44.5%, respectively, for the GPCN-SS, MnO2/GPCN-SS, and MnO2 electrodes (Fig. 6).

Fig. 5. CV curves tested at 10–300 mV s−1 (a) and GCD curves tested at 0.5–50 A g−1 (b, c) of MnO2/GPCN-SS.

Fig. 6. The correlations of specific capacitances with current densities for GPCN-SS, MnO2, and MnO2/GPCN-SS.



Electrochemical performance of MnO2/GPCN-SS//GPCN-SS ASC

For the purpose of practical application, MnO2/GPCN-SS//GPCN-SS ASC was therefore assembled with MnO2/GPCN-SS cathode and GPCN-SS anode in 1 M Na2SO4 aqueous electrolyte. The maximum working voltage of this ASC was initially evaluated by CV tests in a three-electrode setup at a scanning rate of 10 mV s−1. As illustrated in Fig. 7a, the stable potential window was between −1.0 and 0 V for GPCN-SS electrode in 1 M Na2SO4 aqueous electrolyte, while for MnO2/GPCN-SS electrode was between 0 and 1.0 V as evidenced by their fairly rectangle-shaped CV profiles, implying that the working voltage of MnO2/GPCN-SS//GPCN-SS ASC in neutral electrolyte can be extended up to 2.0 V. In addition, the specific capacitances were found to be 397 F g−1 and 230 F g−1 at 10 mV s−1, respectively, for the MnO2/GPCN-SS and GPCN-SS electrodes as estimated from their CV curves in neutral electrolyte. Based on the specific capacitances and stable potential ranges of the two electrodes, the mass ratio of MnO2/GPCN-SS cathode to GPCN-SS anode was close to 1/1.7 in ASC based on the charge balance theory.58 On the other hand, according to the equation of capacitors in series, C = (C+C–/(C– + kC+)) × (k/(1 + k)),64 where C+, C− and C are expressed in F g−1 for cathode, anode, and ASC, respectively, and k = m+/m− is the mass ratio of cathode and anode (i.e., 1.7 in our case). From the above equation, the value of the specific capacitance of the ASC is calculated to be 73.1 F g−1, which agrees very well with the measured capacitance (72.4 F g−1) as mentioned below. Obviously, the specific capacitance of the cathode is much higher than that of the anode. Therefore, the ASC capacitance is limited by the GPCN-SS anode and is lower than that of the GPCN-SS anode. 18 ACS Paragon Plus Environment

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Fig. 7. CV curves of GPCN-SS and MnO2/GPCN-SS at 10 mV s−1 in a three-electrode set-up in 1 M Na2SO4 electrolyte (a). CV profiles at 10 mV s−1 (b), GCD profiles at 0.5 A g−1 in different operation voltages (c), and the correlations of specific capacitances with current densities (d) of MnO2/GPCN-SS//GPCN-SS ASC in 1 M Na2SO4.

The CV profiles of the MnO2/GPCN-SS//GPCN-SS ASC at different operating voltages are illustrated in Fig. 7b. As seen, this ASC exhibits box-like CV profiles even extending the cell voltage up to 2.0 V, indicating an ideal capacitive behavior. The total capacitance of this ASC was estimated to be 72.4 F g−1 from its CV profile in the potential window of 2.0 V at 10 mV s−1. The GCD profiles of this ASC measured from 1.0 to 2.0 V at 0.5 A g−1 are depicted in Fig. 7c. As can be seen, the GCD curves still remain highly linear and symmetrical within all potential ranges, further confirming the high working voltage of this ASC. The GCD curves 19 ACS Paragon Plus Environment


of this ASC at various current loads are shown in Fig. S9a, b. It can be seen that the GCD curve still maintained a nearly triangular shape with an IR drop of 357 mV at a high current load of 50 A g−1 (Fig. S9b), revealing the high-rate performance. The total specific capacitance (Ct) of this ASC was found to be 70 F g−1 at 50 A g−1, at least 77.8% of its Ct value at 0.5 A g−1 (Fig. 7d). It is worth noting that this high-rate capability of the MnO2/GPCN-SS//GPCN-SS ASC in Na2SO4 aqueous electrolyte is significantly higher than those of the MnO2-based ASCs reported recently (Table S4). In order to further demonstrate the high-rate performance of this ASC, the Nyquist plot of the MnO2/GPCN-SS//GPCN-SS device in 1 M Na2SO4 electrolyte was measured in the frequency range from 0.1 Hz to100 kHz at open circuit potentials. As shown in Fig. 8a, the equivalent circuit obtained by fitting EIS data with Zview software consists of Rs (intrinsic resistance), Rct (charge transfer resistance), ZW (Warburg resistance), Cdl (double-layer capacitance), and CL (limit capacitance) (inset of Fig. 8a). Notably, the rather low Rs (0.89 ohm) and Rct (1.21 ohm) values of this ASC demonstrate again the good electrical conductivity and the low resistance for both charge-transfer and ion-diffusion at the electrode/electrolyte interface. Moreover, the characteristic frequency, f0, defined as the frequency at a phase angle of 45°, is found to be 1.1 Hz for this ASC (Fig. 8b), corresponding to a time constant (τ0) of 0.91 s, which also confirms the much fast charge/discharge rate. The phase angle at low frequency is found to be 86.7° for this ASC, further confirming the predominantly EDL capacitive performance as discussed previously. In addition, the EIS was also used to measure the resistance evolution of the MnO2/GPCN-SS//GPCN-SS ASC during 10,000 galvanostatic charge/discharge cycles. After long-term cycling, the Nyquist plot of this device shows subtle 20 ACS Paragon Plus Environment

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changes (Fig. 8a). By comparison of the Nyquist plots and charge/discharge curves before and after 10,000 cycles for each electrode (Fig. S10, S11), it is evident that the MnO2/GPCN-SS electrode is mainly responsible for the performance loss.

Fig. 8. Nyquist plots in the frequency range of 100 kHz to 10 mHz at open circuit potentials before and after cycles tests (a). Bode plot before cycles tests (b), Ragone plot (c), and cycling performance (d) of MnO2/GPCN-SS//GPCN-SS ASC in 1 M Na2SO4 electrolyte.

Benefiting from the high working voltage as well as unique structure of MnO2/GPCN-SS composite, high energy density (50.2 W h kg−1 at 516 W kg−1) and high power density (47.5 kW kg−1 at 26.1 W h kg−1) are achieved concurrently for the MnO2/GPCN-SS//GPCN-SS ASC as evidenced by Ragone plot of this device (Fig. 8c). Notably, such high energy and



power densities are much higher than those of other MnO2-based ASCs in Na2SO4 aqueous electrolyte reported recently (Table S4).24, 64-68 In addition to the high energy and power densities, the MnO2/GPCN-SS//GPCN-SS ASC also displays excellent cycling stability. As depicted in Fig. 8d, the capacitance loss was only 8.9% after 10,000 cycles in the potential window of 2.0 V at a current load of 2.0 A g–1. The excellent electrochemical stability should be attributed to the densely layered structure as well as the formed Mn−O−C bond in the MnO2/GPCN-SS composite.47,48 Self-discharge curve of the MnO2/GPCN-SS//GPCN-SS ASC revealed that the self-discharge time was around 39.6 h (from Vmax to 1/2Vmax, Fig. S12), which was longer than that of other MnO2-based ASCs reported recently.69-71



Conclusions

In summary, we have demonstrated a novel MnO2/GPCN-SS composite by the hydrothermal deposition of MnO2 nanostructures on salvia splendens-derived GPCN-SS. The MnO2 with both worm-like nanorod and nanoparticle architectures has been stacked layer-by-layer on the GPCN-SS surface. The as-fabricated MnO2/GPCN-SS composite has a relatively high SSA (483 m2 g−1) and an appropriate mesopores with sizes in the range of 2‒5 nm. Owing to these unique characteristics, the MnO2/GPCN-SS electrode exhibits high specific capacitance (438 F g−1) and high-rate performance (67.8% of capacitance retention at 50 A g−1) in neutral aqueous electrolyte. Moreover, the ASC assembled with MnO2/GPCN-SS composite cathode and GPCN-SS anode in neutral electrolyte displays excellent rate capability (77.8% of capacitance retention at 50 A g−1), high energy density (50.2 Wh kg−1), power energy density 22 ACS Paragon Plus Environment

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(47.5 kW kg−1), and long cycling life (91.1% of capacitance retention after 10000 cycles). Considering the abundance and sustainability of biomass-derived GPCNs along with the low-cost of MnO2, MnO2/GPCN-SS composite can be served as a promising new class of electrode materials for high-rate and high-energy density supercapacitors.



Associated Content

Supporting Information This material is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04817.

Calculation equations of electrochemistry; Experimental section; Material characterizations; Electrochemical performance measurements; Additional characterization and measurement (PDF)



Author Information

Corresponding Authors *E-mail: [email protected] (H. Chen); *E-mail: [email protected] (H. Li).

Notes The authors declare no competing financial interest.





Acknowledgments

Financial support from Program for NSFC (51674219) and the Construct Program of the Key Discipline in Hunan Province is greatly acknowledged.



References

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Figure Captions Fig. 1. SEM images of GPCN-SS (a) and MnO2/GPCN-SS (b-d) at different magnifications, inset of a shows HRTEM image of GPCN-SS. TEM images (e, f) of MnO2/GPCN-SS at different magnifications. Fig. 2. XRD patterns of GPCN-SS, MnO2, and MnO2/GPCN-SS (a). High-resolution Mn 2p (b), O 1s (c), C 1s (d) XPS spectra of MnO2/GPCN-SS. Fig. 3. Nitrogen adsorption/desorption isotherms (a) and pore size distributions calculated using DFT method slit pore model, differential pore volume and pore width (b) of GPCN-SS, MnO2, and MnO2/GPCN-SS. Fig. 4. CV curves tested at 10 mV s−1 (a), and GCD curves tested at 0.5 A g−1 (b) of GPCN-SS, MnO2, and MnO2/GPCN-SS. Fig. 5. CV curves tested at 10–300 mV s−1 (a) and GCD curves tested at 0.5–50 A g−1 (b, c) of MnO2/GPCN-SS. Fig. 6. The correlations of specific capacitances with current densities of GPCN-SS, MnO2, and MnO2/GPCN-SS. Fig. 7. CV curves of GPCN-SS and MnO2/GPCN-SS at 10 mV s−1 in a three-electrode set-up in 1 M Na2SO4 electrolyte (a). CV profiles at 10 mV s−1 (b), GCD profiles at 0.5 A g−1 in different operation voltages (c), and the correlations of specific capacitances with current densities (d) of MnO2/GPCN-SS//GPCN-SS ASC in 1 M Na2SO4. Fig. 8. Nyquist plots in the frequency range of 100 kHz to 10 mHz at open circuit potentials before and after cycles tests (a), Bode plot before cycles tests (b), Ragone plot (c), and cycling performance (d) of MnO2/GPCN-SS//GPCN-SS ASC in 1 M Na2SO4 electrolyte.


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Table of Contents

We demonstrate a novel MnO2/GPCN-SS composite by the hydrothermal deposition of MnO2 nanostructures on biomass-derived GPCN-SS. The as-prepared MnO2/GPCN-SS//GPCN-SS ASC exhibited high-rate capability and high energy density as well as high power density in 1.0 M Na2SO4 aqueous electrolyte.



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MnO2 nanostructures deposited on graphene-like porous carbon

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