Biomass-inherited Porous Carbon

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A Manganese Monoxide/Biomass-inherited Porous Carbon Nanostructure Composite Based on the high Water-Absorbent Agaric for asymmetric Supercapacitor Hai Zhang, Ze Zhang, Xingtao Qi, Ji Yu, Jianxin Cai, and Zhenyu Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06049 • Publication Date (Web): 13 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Sustainable Chemistry & Engineering

A Manganese Monoxide/Biomass-inherited Porous Carbon Nanostructure Composite Based on the high Water-Absorbent Agaric for asymmetric Supercapacitor Hai Zhang1,#, Ze Zhang1,#, Xingtao Qi1,2, Ji Yu1, Jianxin Cai3, Zhenyu Yang*,1,2 1School

of Chemistry, Nanchang University, No.999, Xuefu Road, Nanchang, Jiangxi,

330031, P. R. China 2Engineering

Research Center of None-food Biomass Efficient Pyrolysis and

Utilization Technology of Guangdong Higher Education Institutes, School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan, Guangdong, 523808, P. R. China 3School

of Resources Environmental and Chemical Engineering, Nanchang Unversity,

No.999, Xuefu Road, Nanchang, Jiangxi, 330031, P. R. China

*

Corresponding author. Tel.: +86-791-83969514 (Z. Y. Yang); E-mail: [email protected] (Z. Y. Yang)

#,

The authors contributed equally to this work. 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract Biomass-inherited metal oxide/carbon composites have been utilized as competitive materials of supercapacitor electrodes owing to the hierarchical structures, fast regeneration rate and easy synthesis. However, the low content and agglomeration of metal oxides are the contradictory issues to be addressed for their practical applications. In this work, manganese monoxide/biomass-inherited porous carbon (MnO/BPC) nanostructure composites with high MnO content (~75%) and uniform distribution have been prepared through a simple immersion-calcination process by high water-absorbent agaric. The super-high Mn2+ solution absorption of agaric ensures the high MnO content in MnO/BPC composite, and the abundant internal chitin with hydrogel and hot-melting property enables the uniform dispersion of MnO in carbon matrix. The carbon nanostructure endows the composite with high specific surface area, efficient electron/ion transportation and better electrolyte wettability. As expected, the MnO/BPC composite materials realizes high capacitance of ~735 mF cm-2 (~637 F g-1) at 3 mA cm-2, good rate performance (~608 mF cm-2 at 10 mA cm-2), and excellent cycling performance ( capacity retention of ~91% at 10 mA cm-2, 5000 cycles). In addition, this work presents a facile and productive strategy to obtain metal-based composites with high metal-oxide content and homogenous distribution by adopting the edible and worldwide abundant agaric.

Keywords: Manganese monoxide, Agaric, Water-absorption ability, Hot-melting, High MnO content, Supercapacitor

2


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Introduction With the rapidly increasing energy-demand, burning fossil fuels has resulted in serious environmental and social problems. Energy storage devices (batteries and supercapacitors) are expected to play essential roles as clean power sources for electric vehicles and consumer electronics in our daily lives.1,2 In fact, the large-scale commercial application of energy-storage devices greatly depends on the development of well-designed electrode materials with abundant resources, low-cost and simple preparation process.3,4 The nature gives biomass abundant output, low-cost and various micro-structures. In particular, biomass carbons can be evolved or inherited the special micro-structures.5-9 Importantly, some biomass carbons can be also recycled from the agricultural or daily wastes, indicating the effective, low cost and eco-friendly preparation for the electrode materials.10 Therefore, biomass carbon and its composites, as an important green and abundant renewable energy materials, have attracted many attentions in the research of super-capacitor and battery materials. 11-20 Recently, many kinds of biomass have been investigated extensively as precursors to prepare biomass carbon and its composites for super-capacitor and battery materials, such as the leaves, stems, seeds, pericarp, fructus, nut shells, fungi, abandoned food, shellfish, animal tissue, polysaccharide, protein.21-25 It suggests that there are many advantages for biomass carbon materials in the energy storage devices. For example, hierarchical structures of the biomass carbon promote the electrolyte infiltration and diminish the ion transport pathways. Besides, the biomass carbon containing N and/or 3



S atoms to induce the heteroatom-doping feature of the carbon materials, which might increase the electric conductivity and active sites. Consequently, the biomass carbon and its composites have been considered as very competitive super-capacitor and battery material due to various hierarchical structures, eco-friendly feature, natural abundance and simple preparation.26-32 In fact, metal oxides were usually loaded onto the biomass carbon to obtain the hybrid electrodes due to their pseudocapacitive behaviors for high specific capacitance. However, such an ex-situ loading process often induces only surface attachment between oxides and biomass carbon.33-35 Therefore, the inhomogeneous distribution and low content of metal oxides are the two intractable/contradictory obstacles impeding the development of the biomass carbon/metal oxide hybrid supercapacitor electrode. Solving these problems will greatly promote the efficient utilization and the commercial applications of biomass carbon composite electrodes in supercapacitors. In addition, the simple and practicable design of biomass carbon composite electrode materials with controllable nanostructures is also still challenging and urgently needed. Agaric are known as natural abundant edible fungus.32,36 Specially, it has super high water-absorption and water-retaining ability due to its rich chitin. Typically, fresh agaric has more than 90% water content, and the dry agaric can be obtained by the process of sun light irradiation or dry room. When re-absorbing water, it can be recovered rapidly to its original state with more than 20 times in volumetric swelling. Moreover, the processes can be repeated many times without any damage, showing the outstanding consistency and stability of its fundamental structure. Even if absorbing 4


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metal-salt aqueous solution, it still exhibits excellent absorptive capacity. Agaric contain the uniform chitin-based cells and cavities (Figure S1) to enable the storage and retention capacity of water to form hydrogels. As for metal-salt aqueous solution, the metal ions can also be concomitantly stored within chitin hydrogels with uniform distribution.(Figure S2) Therefore, both the high content and homogeneous distribution of metal oxides in the composites can be achieved by taking full use of agaric. Herein, we demonstrate the manganese monoxide/biomass porous carbon (MnO/BPC) composites with high MnO content (~75 wt%) and uniform distribution based on the excellent solution-absorption capability of agaric by a simple immersioncalcination process. Impressively, the as-prepared MnO/BPC composite shows special agaric-inherited hybrid nanostructures. The hybrid nanostructure endows the MnO/BPC composite electrode with high specific surface-area and efficient electrical conductance. As a result, it exhibits a outstanding areal capacitance of ~735 mF cm-2 (~637 F g-1) at 3 mA cm-2, good rate performance (~608 mF cm-2 at current density of 10 mA cm-2), and high cycling stability (~91% at 10 mA cm-2, 5000 cycles) for the MnO/BPC composite electrode. Moreover, the assembled asymmetric supercapacitor with MnO/BPC composite (positive electrode) and activated carbon (AC, negative electrode) also demonstrates a promising operating-voltage-window of 1.6 V, highspecific-capacitance of 101 F g−1 at 1 A g−1, a remarkable cycling performance of 5000 cycles. More interestingly, it realizes the high specific-energy of 35.9 Wh kg−1 at 800 W kg−1 and 22.22 W h kg−1 at 8000 W kg−1.

Experimental Section 5



Preparation of MnO/BPC composites In detail, 3 g dried agarics were immersed in 50 mL 1 M manganese acetate solution, and the solution was kept at 60 oC for 6 h for adequate absorption. After that, the pink solution turned to colorless, and the soaked agarics were taken out and dried at 80 oC. Finally, the obtained soaked-agaric precursor was calcined at 800 oC for 2 h with the rate of 5 oC min-1 in a flowing Nitrogen atmosphere to prepare the MnO/BPC composite. As a control, other samples were prepared by immersing dried agarics in the solution with different Mn2+ concentrations of 0.2 M and 2 M. The final products were denoted as MnO/BPC-L and MnO/BPC-H, respectively. “L” and “H” means low and high Mn2+ concentration, respectively. Agaric was annealed directly to obtain the biomass carbon, denoted as BC. Structure characterization Transmission electron microscopy (Tecnai) and scanning electron microscopy (FEI) were used to examine the microstructure and morphology of the as-prepared samples and analyze their elements by energy dispersive spectroscopy (EDS). The phase purity of the sample were examined by X-ray diﬀraction (Bruker D8) in the 2θ range of 10°~80°. The MnO contents of the composites were investigated by thermal gravimetric analysis (PE TGA-400) with the temperature increasing from 30 oC to 800 oC

at a rate of 10 ℃ min-1 in air atmosphere. X-ray photoelectron spectra was collected

with Al Kɑ (1063 eV) source. N2 absorption and desorption experiments were obtained by using the Brunauer-Emmett-Teller (BET) method (Quantachrome Autosorb-iQ2MP analyzer). Raman spectra was measured by using the spectrometer with a 532 nm 6


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laser (Renishaw). Electrochemical experiment A mixture of 80 wt % the composites, 10 wt % Super P, and 10 wt % PVDF were ground with the presence of NMP solvent to yield a thick slurry. Then, the slurry was pressed on the dried foamed nickel with a size of 1 x 1 cm2. After dried, another identical foamed nickel was pressed on the top to form a sandwich-like structure. The composite loading in the electrode was about 1.2 mg cm-2. Charge-discharge tests (CC), cyclic voltammetry (CV), and impedance spectroscopy (EIS) were conducted on the workstation (Prinston P4000). A three-electrode system was employed to verify the electrochemical performance in an aqueous solution (3 M KOH). Hg/HgO and Pt foil are involved as the reference and counter electrodes. The asymmetric supercapacitor was assembled using the MnO/BPC composites as positive electrode, active carbon as negative electrode, a cellulose paper as separator and KOH (3M) as electrolyte, respectively. The specific capacitance, power density and energy density of the device were measured based on the total mass of positive and negative active materials including MnO/BPC and AC. The specific capacitance (Cs) and areal specific capacitance (Ca) is determined from the following relation:

It mV It Ca  AV Cs 

(1) (2)

Where I means the discharging current (A), t means the discharging time (s), m represents the mass (g) of the active material , A means the active area of electrode, and 7



ΔV represents the operating voltage window (V). The power density (P) and energy density (E) of the asymmetric super-capacitor is estimated using the following relations：

1 E  CV 2 2 E P t

(3) (4)

Where C represents the specific capacitance estimated by charge and discharge tests (F g-1), ΔV means the discharge voltage window (V), t means the discharg-time (s).

Results and Discussion

Figure 1 (a) Schematic preparation procedure for MnO/BPC composite, (b) complexation between chitin and Mn2+, and (c) The optical microscope showing the structure changes of the agaric before and after immersed in Mn2+ solution.

The MnO/BPC composites are prepared through a simple immersion-calcination process, as illustrated in Figure 1a. Dried agaric are immersed into the pink solution 8


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containing 1 M of Mn2+, and the solution show dramatic change in color from pink to colorless after complete absorption. Figure 1b shows the complexation between Mn2+ and the hydroxyl groups of chitin in agaric cells, which ensures the superior Mn2+ absorbing ability of agaric and the uniform dispersion of Mn2+. The optical micrograph in Figure 1c clearly shows the significantly expanded sectional width of soused agaric compared to its original counterpart, and the soused agaric presents a more obvious sandwich-like structure, which consists of internal chitin and two thin layers of cellulose outer-shells. It suggests that the captured solution are mainly absorbed within the internal chitin to form hydrogels, and partially absorbed on the surface of cellulose thin layers.26,37 Then, the MnO/BPC composite with high MnO content (75%) is obtained after a simple carbonization process, and it shows special hybrid nanostructures, i.e. the dominating structure with MnO nanoparticles embedded in porous carbon from chitin hydrogels and the secondary structure with MnO nanoparticles deposited on carbon layer from cellulose outer-shell, as shown in Figure 1a. SEM and TEM are performed to characterize the morphologies of the MnO/BPC composite, as shown in Figure 2. According to the different absorbing ability of internal chitin and cellulose outer-shells, two different MnO/BPC nanostructures can be obtained (Figure 2a-f). In Figure 2a and the inset, MnO nanoparticles are wrapped and distributed evenly in the carbon substrate derived from hot-melting chitin component. More structural details can be acquired from TEM images in Figure 2b. These MnO nanoparticles shows diameters of 10~20 nm and are uniformly embedded in carbon 9



substrate. As shown in the high-resolution TEM image (Figure 2c), MnO nanoparticles are well surrounded by amorphous carbon substrate, instead of aggregation on the carbon surface. The uniform distribution of MnO nanoparticles can be ascribed to the complexation with Mn2+ ions of chitin, which forms a natural barrier to prevent the aggregation of MnO nanoparticles during carbonization.38,39 Moreover, the conductive biomass-inherited carbon not only provides high conductivity for the redox reaction of MnO, but also serve as a protective cover for MnO from the alkaline electrolyte erosion.

Figure 2 The morphology characterization of MnO/BPC composite with different nanostructures: (a, d) SEM images and (b, c, e, f) TEM images; (g) SEM images and corresponding elemental mapping of Mn, O and C.

Different from the nanostructure in Figure 2a, Figure 2d shows the spherical particles with much larger diameters of about 50~200 nm are agglomerated on the surface of 10


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carbon. Such a different structure should be derived from the cellulose outer-shells due to the surface absorption of Mn2+ ions by cellulose. TEM picture in Figure 2e shows that the spherical particles are agglomerated from numerous nanoparticles, which are interconnected with amorphous carbon. Figure 2f shows the high resolution TEM image with a lattice fringe spacing of 0.25 nm, which is in accord with the (111) plane of cubic MnO.40 The elemental mapping in Figure 2g further verify the presence of manganese, oxygen and carbon elements in the obtained MnO/BPC composite.

(b)

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Figure 3 (a) XRD pattern, (b) TG curve under air atmosphere, (c) Raman spectrum, (d) the pore distribution of the MnO/BPC composite. The inset shows nitrogen adsorption and desorption isotherm.

The phase composition of obtained MnO/BPC composite is confirmed by XRD analysis in Figure 3a. All the diffraction peaks match well with that of cubic MnO (JCPDS No. 07-0230), indicating high purity phase of MnO. The MnO content is estimated using TG analysis under air atmosphere. In detail, as show in the Figure 3b, 11



the initial weight loss (~0.67%) within 200 oC is ascribed to the loss of adsorbed water, and the subsequent increasing mass ranging from 200 oC to 350 oC is induced by the transformation of MnO to Mn3O4. After the complete depletion of carbon with the temperature up to ~500 oC, the following weight raises (~0.58%) between 450-600 oC is attributed to the phase change from MnOx (include MnO and Mn3O4)to Mn2O3. The oxidization of MnO to Mn2O3 exhibits a weight gain of ~11.28% as reported previously.41 Based the above analysis, the weight ratio of MnO in MnO/BPC composite is as high as ~75% and the carbon content is ~25% (WMnO=(1-0.67%16.30%+0.58%)/(1+11.28%)). In addition, Raman spectra (Figure 3c) of the MnO/BPC composite shows typical two peaks of D band (~1358 cm-1) and G band (~1584 cm-1), verifying the existence of carbon component in the composite materials. Besides, the broad peak at ~646 cm-1 is assigned to the Mn-O lattice vibration in Mn3O4, rather than MnO, due to the inevitable under the high energy laser.42,43 The pore structure of the MnO/BPC composite is explored by the Nitrogen adsorption and desorption tests. As displayed in Figure 3d, the MnO/BPC composite demonstrates a typical type-IV isotherm and a hysteresis loop at high relative pressure, indicating the mesoporous characteristics.39,42 The MnO/BPC composite displays a high specific surface area of ~133 m2 g-1, and a main pore size range of 2-5 nm, which should be inherited from the pyrolysis of agaric. Moreover, it is worth emphasizing that high specific area and mesoporous characteristics of the MnO/BPC composite will greatly improve the accessibility of electrolyte to active sites (MnO) and shorten the ion diffusion path,44

12


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which are beneficial to efficient electron/ion transportation and better electrolyte wettability. The surface chemical composition of the MnO/BPC composite sample is explored by XPS. According to the survey spectrum (Figure 4a), the sharp peaks of C 1s, O 1s and Mn 2p peak can be clearly observed, confirming the presence of MnO and carbon. Meanwhile, the slightly weak N 1s peak also is found. The high-resolution N 1s spectrum (Figure 4b) displays three evident peaks at 398.6, 400.1 and 401.4 eV, which can be related with pyridinic, pyrrolic and graphitic nitrogen, respectively.45,46 The doped N-element is derived from the amine groups of chitin in agaric, which will help to improve the conductivity of the composite materials.47 For the refined Mn 2p spectrum (Figure 4c), two distinct peaks at 641.6 and 653.6 eV can be ascribed to Mn 2p1/2 and Mn 2p3/2, of Mn(II) state, respectively, which gives an evidence for the formation of MnO. These results are consistent with the reported literatures previously.48,49 Further, in the refined O 1s spectrum (Figure 4d), the peak at 530.2 eV belong to Mn-O bond in MnO, and other two peaks at 532.5 eV and 533.2 eV are related with C=O and C-O-C/C-OH groups, respectively.50,51

13



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652

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648

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640

636

540

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Binding energy (eV)

528

526

Figure 4 Surface chemical composition for MnO/BPC composite: (a) Survey XPS, (b) N 1s, (c) Mn 2p and (d) O 1s.

To evaluate the electrochemical property for MnO/BPC composite in supercapacitor, three-electrode system is used to carry out the electrochemical tests in aqueous KOH (3M) electrolyte. Figure 5a displays CV behaviors of MnO/BPC electrode at various scan-rate ranging from 5 to 100 mV s-1. A pair of broad peaks can be found, which is mainly associated with the pseudocapacitive reaction of redox conversion among Mn (III) and Mn (IV) due to the irreversible transformation between Mn (II) and Mn (III) in the alkaline electrolyte.43,45 Actually, the peaks of Mn(II)/Mn(III) redox can be observed at only at a slow scan rate of 1 mV s-1 (Figure S3), and becomes weaker in the subsequent cycles. It indicates that the specific capacities of the electrodes are dominated by faradaic pseudocapacitance of MnO in alkaline electrolyte. The similar shape of the curves at various scan rates indicate the highly reversible pseudocapacitive nature of the MnO/BPC composite in alkaline electrolyte. Figure 5b displays the charge and discharge test of MnO/BPC electrode. All curves show good 14


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symmetry at different current-density range from 3 mA cm-2 to 10 mA cm-2, indicating high Coulombic efficiency.48 Figure 5c presents the areal capacitance and the corresponding gravimetric capacitance of the MnO/BPC electrode. High areal capacitances of ~735, ~700, ~670, ~654, ~624 and ~608 mF cm-2 are investigated at 3, 4, 5, 6, 8 and 10 mA cm-2, respectively, indicating an outstanding rate-capability. Since the mass loading of the MnO/BPC composite in the electrode is 1.2 mg cm-2, the corresponding gravimetric capacitances are ~608, ~583, ~558, ~545, ~520 and ~507 F g-1, respectively. The high capacitances and promising rate capability suggest the advantages of the MnO/BPC composite as a supercapacitor electrode.

(b) MnO/BPC

MnO/BPC

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MnO/BPC 700

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Figure 5 (a) CV curves at different scan rates, (b) charge and discharge curves and (c) gravimetric and areal capacitances estimated by charge and discharge tests of MnO/BPC electrode

To investigate the effect of different MnO content on electrochemical properties of the MnO/BPC composite electrode, the control composite samples have been prepared in the same procedure with lower (0.2 M) or higher (2 M) concentration of 15



Mn2+. The samples are denoted as MnO/BPC-L, MnO/BPC-H and MnO/BPC, respectively. XRD results (Figure S4) confirm the MnO phase in these samples. The lowest value of ID/IG of the MnO/BPC composite in Raman spectra (Figure S5) suggests the highest graphitization degree of carbon, indicating the high electrical conductivity. This phenomenon can be explained that the added manganese acetate decomposes to release numerous gas to ensure the complete crack of biocarbon substrate by preventing the reformation of compact carbon aggregates in the carbonization process. Thus, a unique and loose structure can be obtained with a reasonable amount of the additional inorganic salt. As a contrast, the BC sample annealed directly from agaric shows the bulk structure (Figure S6), further indicating the catalysis and pyrolysis function of MnO content for the complete crack of carbon substrate. The results of both XRD and Raman spectra for BC sample (Figure S7) also confirm the typical properties of carbon materials. Notably, the value of ID/IG of BC sample is higher than that of MnO/BPC composite, suggesting the influence on the graphitization degree of the carbon substrate from the decomposition of manganese acetate. Figure 6a shows the CV curves of the different electrodes at the scan rate of 50 mV s-1. The pseudocapacitive reaction of MnO can be observed for all the samples, MnO/BPC-L, MnO/BPC, and MnO/BPC-H. Compared with the other two samples, the MnO/BPC composite shows the higher specific capacities. The different capacitive performances of the samples are closely related to their microstructures. For the MnO/BPC-H sample, the extremely high MnO content leads to the severe agglomeration of MnO particles (Figure S8), leading to low conductivity and the low 16


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MnO utilization of MnO/BPC-H electrode, while low MnO content in electrode means low active substance concentration, leading to low electrode capacitance of MnO/BPCL electrode. Meanwhile, the CV curves (Figure S9) of BC electrode have been investigated and it present typical double-layer capacitance, and the values of the capacitance is insignificant comparing with the pseudocapacitance of MnO. (a) 40

(b)

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MnO/BPC-H MnO/BPC MnO/BPC-L

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Figure 6 (a) CV curves at scan rate of 50 mV s-1, (b) EIS profile, and (c) cycling performance at current density of 10 mA cm-2 for different MnO-based electrodes.

The kinetics of the electrodes are investigated by EIS tests. As shown in Figure 6b, obviously, both the MnO/BPC-L and MnO/BPC electrode exhibit very low resistance due to the appropriate carbon content in comparison with MnO/BPC-H electrode. It is worth noting that the charge-transfer resistance increases along with the increasing MnO content of the three samples (MnO/BPC-H > MnO/BPC > MnO/BPCL), which also illustrates that the biomass-inherited carbon acts a vital role for the reduction of charge-transfer resistance of these composite electrode. The conductive 17



carbon component helps to improve the effective inter-particle contact, shortened the pathway for electron and ion transfer, thus enhancing the redox kinetics of the active MnO during long cycling in alkaline electrolyte. The cycling capabilities of the samples are investigated at a current density of 10 mA cm-2. As shown in Figure 6c, the MnO/BPC electrode exhibits excellent cycling performance with high initial areal capacitance of ~608 mF cm-2 and high capacitance retention of ~91% after 5000 cycles. Interestingly, in the initial stage, the specific capacitance of MnO/BPC continuously increases, which might be ascribed to the activation process due to the electrolyte penetration into the porous network structure of MnO/BPC composite.48 And the subsequent capacitance decrease should result from the consumption of MnO in the alkaline electrolyte. Although MnO/BPC-L electrode also shows outstanding cycling performance with a negligible capacitance fade within 5000 cycles, the initial areal capacitance is extremely low as ~204 mF cm-2 due to the low MnO content. Meanwhile, the MnO/BPC-H electrode with highest MnO content shows rapid decrease of capacitances with a low retention of ~59% after 5000 cycles. It should be attributed to the severe agglomeration of MnO particles and the dissolution into the alkaline electrolyte since the low carbon content in the composite provide less protection of active MnO species. The specific capacitances of the electrodes are shown in Figure S10, and the result clearly demonstrates that the areal capacitance of MnO/BPC is optimal in the MnO-based electrodes studied in this work. Therefore, the biomassinherited carbon from agaric exhibits high specific area, mesoporous characteristic, enhanced electrical conductivity, and a protective layer for the active MnO species from 18


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dissolving into the alkaline electrolyte. The aforementioned superiorities enable the greater accessibility of electrolyte to active sites (MnO), good rate capability and promising cycling stability of the MnO/BPC electrode for supercapacitor. In order to further confirm the application of MnO/BPC, an asymmetric supercapacitor (ASC)

was assembled with a two electrode configuration (Figure S11).

The MnO/BPC composite, active carbon (AC) and a cellulose paper were employed as the positive and negative electrode, and separator. Figure 7a shows the comparative CV curves of MnO/BPC and AC electrode at 20 mV s-1. The CV curve of AC electrode at the potential ranges of −1 and 0 V is shaped like a rectangle, revealing the typical characteristics of the double-layer capacitor. While the CV curve of MnO/BPC electrode present a pair of broad oxidation/reduction peaks at the potential range of −0.6 and 0.4 V, which reveals the typical faradaic property. In addition, the MnO/BPC present the significant stronger current response than that of AC, proving superior capacitance performance of the MnO/BPC electrode. Figure 7b shows CV curve for the MnO/BPC//AC ASC with different operating voltage windows at 50 mV s-1. The features including similar rectangular shapes and no obvious polarization confirm the capacitive behavior and reversibility of the MnO/BPC//AC ASC. The CV curves at various scan rates (Figure 7c) combine the pseudocapacitive behavior and electrical double-layer characteristics are shown in the operating voltage window of 0 and 1.6V. Moreover, the shape of the CV curve is kept well, even ~100 mV s−1, which indicates the rapid ion transportation and outstanding rate performance of the assembled ASC. The specific capacitances of MnO/BPC//AC ASCs are calculated as 101, 91, 80, 71 and 19



62 F g-1 at the current densities of 1, 2, 3, 5 and 10 A g-1, respectively, from the chargedischarge curves (inset of Figure 7d). An excellent cycling stability (Figure 7e) of MnO/BPC//AC ASC can be realized with high capacitance retention of ~87% after 5000 cycles. a

(a)

(b)10

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Figure 7 (a) Cyclic Voltammetry curves of active carbon and MnO/BPC electrode at 20 mV s-1, (b) Cyclic voltametry curves of MnO/BPC//AC supercapacitor under various potential ranges, (c) Cycled voltammetry curves of MnO/BPC//AC supercapacitor at various scan rates, (d) calculated capacitance for the MnO/BPC//AC supercapacitor for various current densities; the inset displays the relevant charge and discharge curves of the supercapacitor, (e) cycling stability tests over 5000 20


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cycles for the MnO/BPC//AC supercapacitor at 10 A g−1, the inset represents the schematic graph of the ASC device, (f) Ragone graph of the MnO/BPC//AC supercapacitor.

Power density and energy density are crucial to evaluate electrical performance of supercapacitor. Figure 7f shows the comparison between the MnO/BPC//AC ASC and typical data in the reported literatures. The relevant energy densities of the MnO/BPC//AC supercapacitor can reach 35.9, 32.35, 28.44, 25.24 and 22.22 Wh kg-1 at the corresponding power densities of 800, 1500, 2400, 3278 and 8000 W Kg-1, respectively. Significantly, the energy densities of MnO/BPC//AC is superior comparing with these reported results.52-58 Therefore, this work puts forward a feasible and productive strategy to prepare MnO/BPC composites with high content and homogenous distribution of MnO by adopting abundant natural agaric, and demonstrates the promising practical applications of MnO/BPC composites in supercapacitors .

Conclusion In summary, the biomass-inherited MnO/BPC composite with high MnO content and uniform MnO distribution has been prepared by a simple immersion-calcination process using natural agaric as the precursor. The high absorption amount of Mn2+ in agaric ensures the high MnO content in MnO/BPC composite, and the abundant internal chitin and hot-melting property of agaric offer complexation with Mn2+ to enable the uniform dispersion of MnO nanoparticles in the MnO/BPC composite and the protective layer for the active MnO species from the electrolyte erosion. As expected, the MnO/BPC electrode displays a promising electric capacity of ~735 mF cm-2 at 3 21



mA cm-2 ( ~637 F g-1), good rate-capability with high capacitance of 608 mF cm-2 at 10 mA cm-2, and outstanding cycling performance with capacity retention rate of ~91% after 5000 cycles. The ACS supercapacitor based on MnO/BPC have achieved a high electric capacity of 101 F g-1 at 1 A g-1 and superior energy-density of 35.9 Wh kg-1. This work puts forward a feasible and productive strategy to prepare metal oxide/biomass-inherited carbon composites with high content and homogenous distribution of metal oxides by adopting abundant natural agaric for supercapacitors and other energy storage devices. Associated content Supporting information SEM photographs of agaric, additional CV curves for MnO/BPC electrode, XRD patterns of the two control samples, Raman spectra of the samples, SEM images of the BC sample, XRD pattern and Raman spectra of BC, SEM images of the two control samples, CV curves of BC electrode, additional cycling performance for the samples, Physical picture of asymmetric supercapacitor device.

Acknowledgment This work is financial supported by the National Natural Science Foundation of China (No. 21363015, 51662029, 21863006), the Engineering Research Center of None-food Biomass Efficient Pyrolysis and Utilization Technology of Guangdong Higher Education Institutes (2016GCZX009), and the Research Project of DGUT High Level Talents (KCYKYQD2017017). 22


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foam for aqueous asymmetric supercapacitors. J. Mater. Chem. A 2014, 2(13), 47954802. DOI: 10.1039/C3TA14647B [58] Wang, D. W.; Li, F.; Cheng, H. M. Hierarchical porous nickel oxide and carbon as electrode materials for asymmetric supercapacitor. J. Power source 2008, 185(2), 15631568. DOI: 10.1016/j.jpowsour.2008.08.032

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Manganese monoxide/biomass-inherited porous carbon (MnO/BPC) composites with high MnO content (~75%) and uniform distribution have been prepared for highperformance supercapacitor through a simple immersion-calcination process by taking the full use of agaric.

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Biomass-inherited Porous Carbon

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