Universal FeCl3-Activating Strategy for Green and Scalable

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A Universal FeCl3-Activating Strategy for Green and Scalable Fabrication of Sustainable Biomass-Derived Hierarchical Porous N-Doped Carbons for Electrochemical Supercapacitors Linrui Hou, Zhiyi Chen, Zhiwei Zhao, Xuan Sun, Jinyang Zhang, and Changzhou Yuan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01589 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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ACS Applied Energy Materials

A Universal FeCl3-Activating Strategy for Green and Scalable Fabrication of Sustainable Biomass-Derived Hierarchical Porous N-Doped Carbons for Electrochemical Supercapacitors

Linrui Hou

†, ‡ *,

Zhiyi Chen ‡, Zhiwei Zhao ‡, Xuan Sun †, Jinyang Zhang †, and

Changzhou Yuan †, ‡ * †

School of Materials Science & Engineering, University of Jinan, Jinan, 250022, P.

R. China ‡

School of Materials Science & Engineering, Anhui University of Technology,

Ma’anshan, 243002, P. R. China

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 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 Cost-efficiency utilization of value-added biomass-based carbon materials is essential for versatile electrochemical energy-related applications. Herein, we smartly devised a universal and eco-friendly FeCl3-activating methodology for scalable high-yield fabrication of biomass-derived hierarchical porous N-doped carbons for supercapacitors. The underlying FeCl3-involved activating mechanism was reasonably put forward. The mushroom-derived porous N-doped carbon, as a typical FeCl3-activating example, was endowed with large surface area of ~1143.6 m2 g-1, high-fraction (~66.3%) mesopores, high-content (~3.1 at.%) N doping and good wettability, and exhibited even better electrochemical behaviors when compared to those activated by KOH and/or without activation, thanks to its innately structural/componental/surfacial superiorities. The resultant carbon electrode with a high loading of 5 mg cm-2 displayed competitive gravimetric/volumetric capacitances of ~307.4 F g-1 (~212.1 F cm-3) at 1 A g-1 in 1 M H2SO4, much better than those in KOH, mainly owing to extra pseudo-capacitance. Moreover, the assembled symmetric devices yielded high energy densities of ~6.6 (1 M H2SO4) and ~62.6 Wh kg-1 (1 M TEABF4/PC) at a high rate of 1.5 kW kg-1, and ultra-long cycling performance. More significantly, the FeCl3-involved synthetic protocol is highly promising and general for other biomass-derived porous carbon materials towards advanced supercapacitors.

KEYWORDS: Biomass-derived carbon; FeCl3-activating strategy; Hierarchical porosity; N-doping; Electrochemical supercapacitors 2 ACS Paragon Plus Environment

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INTRODUCTION Recently, porous carbon materials have been extensively utilized in various applications as catalyst supports,1 adsorbents,2 and electrode materials,3-5 benefiting from their easy availability and tunable phsicochemical properties. During the past decades, porous carbons have been widely utilizied in electrochemical energy storage fields, particularly in electrochemical superpcapacitors (ESs) as electroactive materials,4,

5

owing to their large specific surface area (SSA), superior electronic

conductivity and adjustable pore structure/surface chemistry.3-5 Especially, activated carbon (AC) materials stand out from other carbons, and have been intensively explored as potential electrodes for ESs in view of its high SSA, accepted cost-effectiveness, adjustable pore diameter and effortless functional modification via surface treatment and hetero-atom doping.4 Unfortunately, electrochemical utilization of various AC materials is still restricted seriously by its modest supercapacitive behaviors, since they mainly contain micropores, which is no doubt inconvenient for rapid diffusion and transport of eletrolyte ions in electrodes.4,

6

Hence, the

high-content mesopores, which can accelerate ion diffusion kinetics and enhance the high-rate power property of electrodes, are of equal significance, although micropores are greatly beneficial for high-energy storage.4-7 However, the fine control in pore structures, depending enormously upon the involved precursors and selected activators, is still hugely challengeable. With the potential scalable applications of ESs and scarcity of fossil energy in mind, it would be more worthwhile by recycling abundant and sustainable biomasses 3 ACS Paragon Plus Environment


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and/or biomass wastes as the precursors for large-scale yield of high-performance AC materials for advanced ESs.7,

8-13

More promisingly, almost all the biomasses

themselves connately own various hetero-atoms (S, N, P, etc.),7, 8, 14 rendering the in situ doping of hetero-atoms in the bulk, rather than just on the surface of the resultant carbon electrodes. As is well known, the hetero-atom doping can provide extra pseudo-capacitive effect, and meanwhile enhance surface wettability and electrical conductivity of AC electrodes.4, 7, 8 Notably, it is still an ineluctable yet key issue how to efficiently convert biomasses to high-performance AC materials of desired pore structure for high-performance ESs with both low cost and modest environmental impact. Up till now, the majority of AC electrodes for ESs in earlier reports are always prepared by alkali hydroxides (KOH or NaOH)-assisted activation routes.7, 15 Whereas these corrosive alkalies, as activating agents, are highly prone to leading to severe disadvantages including strong corrosion of instruments/equipments, high toxicity and cost, as well as bothersome pollutions, which thus restrains the widespread use and increases the production cost.16,

17

Whatʼs more, these alkalies can decrease the

doping content of hetero-atoms, but enhance the surface oxygen-based functionalities with the negative influences upon the ultimate electrochemical behaviors of the AC.4, 7

In contrast, ZnCl2 emerges as another relatively mild activator. While, it is still not

an ideal activating agent as its pore-forming efficiency is low, and the concentrated ZnCl2 solution commonly dissolves the cellulose in biomasses.18,

19

Therefore, the

activation efficiency, security, cost and environmental benefits must be finely 4 ACS Paragon Plus Environment

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integrated on account of the significance in choosing appropriate activators for high-temperature activation to construct advanced carbons with suitable pore structure. On the basis of the concerns above, in this study, we reported a green yet general activating strategy, where even cheaper and more eco-friendly FeCl3 was utilized as efficient activating agent for mass production of low-cost biomass-derived porous N-doped carbon materials for high-performance ESs. Furthermore, the intrinsic FeCl3-involved activating mechanism was rationally proposed. Impressively, the crude mushroom-derived hierarchical porous N-doped carbon, as a typical example, possessed a large SSA (~1143.6 m2 g-1), high-content mesopores (~66.3%), and high N-doping (~3.1 at.%), and exhibited robust electrochemical performances including large capacitances and long-span cycling properties, compared to those activated with KOH

and/or

without

activation,

benefiting

from

its

uniuqe

structural/componental/surfacial superiorities. EXPERIMENTAL SECTION Synthesis of Porous Carbons. All the chemicals (Sinopharm Chemical Reagent Co., Ltd) were of analytical grade, and directly used without further purification. First, the fresh mushroom purchased in local supermarket was washed repeatedly with de-ionized (DI) water, and then placed in a vacuum oven at 80 °C for 12 h. Then, the dried mushroom was calcinated at 500 °C for 2 h with a ramp of 5 °C min-1 in N2 atmosphere. Afterwards, the obtained sample was mixed with FeCl3∙6H2O (or KOH) with a mass ratio of 3 : 1, and further annealed at 900 °C for 2 h in N2 atmosphere 5 ACS Paragon Plus Environment


with a heating ramp of 5 °C min-1. After mixed well with distilled HCl and HF solution (20 wt.%) in order, then filtrated with DI water until neutral, and finally dried for 12 h, the as-fabricated products activated with FeCl3∙6H2O and KOH were labeled as MCF and MCK, respectively. For comparison, the product obtained without the activation was marked as MCR for convenience, and its synthetic parameters were similar to those for the MCF and MCK, just without the activating agents. In addition, other biomass precursors, such as poplar branch, high mountain tea, Jasminum nudiflorum and bamboo leaf, were also utilized, and corresponding resulted products activated with FeCl3, KOH and without activation were denoted as PCF/PCK/PCR, HCF/HCK/HCR, JCF/JCK/JCR and BCF/BCK/BCR, respectively. Materials Characterization. Crystalline phases of samples were measured by X-ray diffraction (XRD) (Rigaku Ultima IV) using a Cu Ka source (λ = 0.1542 nm) at a scanning speed of 10 ° min-1 over a 2θ range of 10 − 80 °. Raman analysis (514 nm excitation) was recorded by laser Raman spectrometer (T6400, Jobionyzon Corp., France). The morphologies of resulted products were observed by field-emission scanning electron microscopy (FESEM, JEOL-6300F, 15 kV), transmission electron microscope (TEM), scanning TEM (STEM), and high-resolution TEM (HRTEM) (JEOL JEM 2100 system operating at 200 kV). Energy dispersive X-ray (EDX) analysis and corresponding elemental mapping data were performed with the X-ray spectroscopy attached to the TEM instrument. X-ray photoelectron spectroscopy (XPS) measurements were conducted by VGESCALAB MKII X-ray photoelectron spectrometer with Mg Ka excitation source (532.4 eV). Contact angle (CA) test was 6 ACS Paragon Plus Environment

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measured by video contact angle instrument (OCA 15+, Dataphysics). N2 adsorption-desorption tests were determined by Brunauer-Emmett-Teller (BET) on surface area analyzer (TriStar II 3020). The mesopore and micropore size distributions were derived by the Barrett-Joyner-Halenda (BJH) and non-localized density function theory (NLDFT) methods, respectively. Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) were performed in nitrogen atmosphere with a NETZSCH STA 449 PC system TG Analyzer with a heating rate of 10 °C min-1. Electrochemical Evaluation. The as-obtained carbon materials, acetylene black (AB) and polytetrafluoroethylene at a mass ratio of 5 : 2 : 1 were mixed with an appropriate amount of DI water. Subsequently, the resulting slurry was pressed on a piece of fresh nickel foam (1 cm × 1 cm) at a pressure of 15 MPa to make a working electrode for following electrochemical measurements in the three-electrode testing system with 6 M KOH solution as the aqueous electrolyte. Besides, the mixture was also smeared onto the graphite substrate (1 cm2) instead for electrochemical tests in 1 M H2SO4 aqueous electrolyte. The typical loading of the electroactive materials is 5 mg cm-2 for electrochemical evaluations in both the two electrolytes. The saturated calomel electrode (SCE) and a platinum plate (1 cm2) were used as the reference and counter electrodes, respectively. As for two-electrode testing configurations (CR2032) with 6 M KOH and 1 M H2SO4 as the electrolytes, the nickel foam and carbon plate were utilized as current collectors for alkaline and acidic systems, respectively. Two identical electrodes were 7 ACS Paragon Plus Environment


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separated by TF-44 cellulose separator. The total mass loadings of electroactive MCF per cell were 2, 5 and 8 mg, respectively, for comparative evaluation in two-electrode system with 1 M H2SO4 as the electrolyte. For two-electrode organic system, the mixture of the MCF, AB and sodium carboxymethyl cellulose (CMC) at a mass ratio of 7 : 2 : 1 with the DI water was coated on aluminum foil, and utilized as the working electrode. Two same electrodes were assembled together in CR2032 with the Whatman GF/A glass fiber as a separator in argon-shield glove box (MBRAUN, UNILAB Plus) both with the moisture

and

oxygen

content

below

0.5

ppm.

And

1

M

tetraethylammoniumtetrafluoroborate/propylene carbonate (TEABF4/PC, Honeywell Corp.) was applied as the electrolyte for evaluation in organic systems. Electrochemical properties were evaluated by cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) in the frequency range of 105 – 10-2 Hz with an AC signal amplitude of 5 mV on an IVIUM electrochemical workstation (the Netherlands). The charge-discharge cycling behaviors were carried out with a CT2001D tester (Wuhan, China). The gravimetric specific capacitance (GSC) and volumetric specific capacitance (VSC) of electrodes can be calculated based on the charge-discharge plots by the following Equations:20, 21 GSC =

VSC 

It V

1



(1)

 GSC 

1 Vtotal 

1

 GSC

(2)

carbon

where the I is the discharge current (A g-1), ∆t is the discharge time (s), ∆V is the 8 ACS Paragon Plus Environment

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potential window (V), ρ is the density of the MCF (MCK or MCR) (g cm-3), Vtotal is the total pore volume for the MCF (MCK or MCR) (cm3 g-1), and ρcarbon is the true density of the bulk carbon (2 g cm-3). In addition, the energy density (ED) and power density (PD) of the two-electrode devices are calculated with the Equation (3): ED  PD  t 

1 GSC (V ) 2 2

(3)

where GSC, ∆V and t are the mass capacitance (F g-1) based on two electrodes, potential window (V), and discharge time (s) of the symmetric device, respectively. Another important parameter, Coulombic efficiency (CE), can be evaluated from the Equation (4):

CE 

tD 100% tC

(4)

where tD and tC are for galvanostatic discharging and charging time (s), respectively. RESULTS AND DISCUSSION Physicochemical and Structural Charateristics. As retrieved in previous reports,18, 19 the metal salt of FeCl3 is commonly used as a graphitization agent coupled with the activation agent of ZnCl2, and seldom applied as a single activator to obtain porous AC materials with large SSA and suitable pore size. Herein, we tentatively used the FeCl3 as a single activator to obtain porous biomass-derived carbon materials for ESs application. As inquired in Sigma-Aldrich, the FeCl3 is the cheapest (just ~55 USD kg-1) when compared to other most widely-used activating agents including KOH, NaOH, ZnCl2, NaHCO3 and KHCO3 (Supporting Information (SI), Figure S1). But one new issue appears that whether the low-cost yet mild activator of FeCl3 can 9 ACS Paragon Plus Environment


achieve the same and/or even better activating performance when compared to the classic KOH activator. In this regard, we first take the natural mushroom as an example for systematical investigations in the following section. Figure 1a shows the panoramic field-emission scanning electron microscopy (FESEM) image of the resultant MCF, from which a mixed morphology, i.e., irregularly bulk and two-dimensional (2D) nanosheets (NSs), can be visually observed. In sharp contrast, the MCR and MCK just demonstrate an anomalous bulk structure (SI Figure S2). Furthermore, even some transparent NSs with rippled silk morphology, as marked by the red circles in Figure 1b, are evidenced in MCF, suggesting their ultrathin feature, which can be fully validated by following TEM examinations (Figure 1c-e). As observed in Figure 1c, a mixed structure of porous irregularly bulk and NSs is indeed presented. Besides this, the 2D ultrathin graphene-like architecture is more clearly visible, as shown in Figure 1d. The brunet strips, as visualized Figure 1d, e, should be the folded edges and/or wrinkles of the NSs. As estimated from the high-resolution TEM (HRTEM) image (SI Figure S3), the thickness of the 2D ultrathin sheets is estimated as just about 2 ‒ 3 nm. Also notably, the discernable spacing between the adjacent fringes is measured as ~0.385 nm (SI Figure S3), corresponding to the (002) plane of graphite carbon, which is somewhat larger than that for the typical graphite (~0.34 nm), due to the heteroatom doping and subsequent chemical activation. The scanning transmission electron microscope (STEM) image (Figure 1f) and corresponding energy dispersive X-ray


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(EDX) elemental mapping images (Figure 1g-i) present the uniform distribution of O and N species in the MCF sample. Figure 2a shows the typical wide-angle X-ray diffraction (XRD) patterns of the resulted MCF as well as the MCK and MCR materials. Clearly, no any Fe/K-based impurities can be discernable both in the MCF and MCK at all, further proving the successful formation of phase-pure carbon material. Two prominent diffraction peaks are observed at 2θ = ~24.4 and ~43.7 ° in the pattern of MCR, consistent well with the graphitic carbon, albeit with disorder.22 Specifically, the broad diffraction pattern at ~24.4 ° is attributed to the (002) reflection of the graphitic-type lattice, and the peak at around 43.7 ° should be related to a superposition of the (100) and (101) reflections for the graphitic-type lattice, i.e., (10) reflection in Figure 2a. In contrast to the MCR, the (002) peaks for the MCK and MCF both shift to 2θ value of approximately 23.4 °, indicating that the (002) plane space increases up to ~0.38 from ~0.36 nm owing to the chemical activation of the FeCl3 and KOH, which will doubtless benefit the electrochemical charge storage. Moreover, the intensity of (002) peak turns out to be even weaker, particularly for the MCK after the activation, confirming the stronger disruption to graphitic structure by the alkaline KOH. Upon closer examination, even stronger peak at ~43.7 ° is observed for the MCF, by contrast with that for the MCK, probably owing to the formation of a high degree of interlayer condensation, which is of huge benefit to fast electronic transport in the MCF.23 The striking feature can be further supported by Raman spectroscopy (Figure 2b). As seen in Figure 2b, the as-fitted peaks at ~1593.5 cm-1 (G-band) are assigned to the E2g phonon of sp2 carbon 11 ACS Paragon Plus Environment


atoms, indicative of the tangential vibration of carbon atoms and characteristic feature of ordered graphitic layers.23 And the peaks located at ~1359 cm-1 (D-band) are corresponding to the disordered graphitic structures.24 The fitted I band (~1185 cm-1) is commonly corresponding to the sp2‒sp3 bonds, disorder in the graphitic lattice or the presence of polyenes, and the D`` band located at ~1495 ‒ ~1545 cm-1 can be attributed to the presence of amorphous carbon.23, 25, 26 In general, the higher values of IG/ID, the higher electrical conductivity can be expected.23-26 As summarized in Table 1, the IG/ID value (~0.93) of the MCF is higher than those for the MCR (~0.88) and MCK (~0.71), which reveals the enhanced graphitic degree of the MCF, unlike the KOH. More detailed elemental compositions and oxidation states of the MCF, MCK and MCR are analyzed in detail by X-ray photoelectron spectroscopy (XPS) measurement. Corresponding fitted profiles (Figure 3, SI Figure S4) are exhaustively obtained by Gaussian fitting method. As discerned from full-scan elemental XPS spectra (Figure 3a) and enlarged grayish N 1s region (Figure 3b), the co-existence of C, N and O species of various contents is evidenced for the three samples. As collected in Table 1, the elemental C, N and O in the MCR are estimated as ~82.7, ~14.0 and ~3.3 at.%, respectively. With the KOH activation, the atomic content of the elemental C in the MCK shows a substantial decline down to ~66.2 at.%, along with the decrease in the N by ~12.1%, that is, just ~2.9 at.% of the N, while the O content increases significantly up to ~30.9 at.%. By contrast, the obvious advance in C (~89.5 at.%) coupled with a slight decrease in N (~3.1 at.%) but a drastic reduction of O 12 ACS Paragon Plus Environment

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(~7.5 at.%) is observed for the MCF. Thus, it is easy to conclude that the FeCl3, differentiating from the usual KOH, can distinctly boost the activation efficiency and yield (Table 1) without the loss of the positive N species, and effectively restrain excessive introduction of negative oxygen-based functionalities meanwhile. It is extraordinarily beneficial to efficient charge storage for advanced ESs. The high-resolution C 1s spectra of the MCF (Figure 3c), MCK (SI Figure S4a) and MCR (SI Figure S4d) reveal the four components of the carbon bond, i.e., C=C (~284.6 eV, C1), C-OH/O-C-O (~286.1 eV, C2), C=O (~287.6 eV, C3) and COOH (~289.1 eV, C4) in the three products. As shown in Table 1, the relative content of sp2 C, i.e., the C1, in the MCF is estimated as ~72.6 at.%, which is the highest among the three. One particularly note that the C1 content in the MCK (~49.1 at.%) is even lower than that the MCR (~64.0 at.%), suggesting the serious damage of KOH in graphitization structure, which is well consistent with aforementioned Raman analysis (Figure 2b). This striking feature, i.e., high-content C1 (~72.0 at.%), would make the MCF better electrochemical performance, thanks to the positive contribution of Sp2-C to rapid electron transport. As for the N 1s spectra (Figure 3d, SI Figure S4b, e) for the three, the four types of N functional groups centered at binding energies (BEs) of ~398.5 eV, ~399.8 eV, ~401.5 eV and ~402.5 eV can be assigned to pyridinic nitrogen (N1), pyrrolic/pyridonic nitrogen (N2), quaternary nitrogen (N3) and oxidized N-oxide species (N4), respectively. As well reported before, the pseudo-capacitive interactions only occur on the negatively charged N2, and the positive charger on N3 and N4 favors rapid electron transfer through the carbon 13 ACS Paragon Plus Environment


electrode.7-9, 27, 28 As collected in Table 1, similar N2 level is found in the MCF (~1.0 at.%) and MCK (~1.2 at.%) samples, but just ~0.8 at.% for MCR, which means more Faradaic redox contributions in MCF and MCK electrodes. Additionally, the total content of N3 and N4 in MCF (~1.3 at.%) is somewhat higher than that in MCK (~1.1 at.%), although both of the two are much lower than that for MCR (~2.1 at.%), which would render the MCF with higher electronic conductivity. Furthermore, the three fitted peaks at BEs of ~531.4, ~533.3 and ~534.7 eV in O 1s regions (Figure 3e, SI Figure S4c, f) are corresponding to three oxygen-based components including the quinine type group (O*=C, O1), ether and/or phenol type group (O*-C, O2) and chemisorbed oxygen and/or water (O3) bonded to the sample surface. Among them, only the O1 is defined to provide the pseudocapacitive contribution.7, 27, 28 Although the O1 content in MCF (~3.2 at.%) is much lower than that of MCR (~10.2 at.%), but still about three times higher than that of MCK (~1.0 at.%). Thus, the oxygen-based pseudo-capacitive contribution from the MCK is even less when compared to the MCF and MCR. Owing to these O/N-based functional groups located on the surface of the MCF, MCK and MCR specimens, contacting angle (CA) measurements are performed both in 1 M H2SO4 and 6 M KOH aqueous electrolytes, and corresponding digital photographs are demonstrated in Figure 4. Evidently, the CAs of the MCK are even smaller than those of the MCF both in KOH and H2SO4 solutions, probably resulting from the higher oxygen functionalities on the MCK surface.29 Besides, the reasons for the largest CAs of the MCR should be related to its lowest roughness and SSA to 14 ACS Paragon Plus Environment

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some extent. Further investigations in the intrinsic reasons are undergoing in our lab. But one note that the same trend of the smaller CAs in acidic electrolyte is evident for all the three products, as seen in Figure 4, suggesting the better wettability in aqueous H2SO4 solution for all the samples. Such unique characteristics will make much larger inner active surfaces accessible by the electrolyte ions for more efficiently electrochemical charge storage in 1 M H2SO4 solution. Furthermore, the large SSA along with suitable pore structure is another vital factor influencing the ultimate electrochemical behaviors of any porous carbon.2-5, 7 Typical N2 sorption isotherms and corresponding pore size distributions of the MCR, MCF and MCK are shown in Figure 5, and specific pore texture parameters are collected in Table 1. Unlike the MCR, a combination style of type I and IV observed in the adsorption-desorption isotherms (Figure 5a) of the MCF and MCK samples suggests the co-existence of micropores and mesopores in the two. This distinct trait can be well authenticated by the mesopore size distribution data (Figure 5b) derived by the Barrett-Joyner-Halenda (BJH) method and micropore size distribution plots (the inset in Figure 5b) obtained by non-localized density function theory (NLDFT). Accordingly, a larger average pore size (APS) of ~6.7 nm is estimated for the MCR, which is much smaller than those for the MCF (~2.7 nm) and MCK (~1.8 nm), due to the microporous contribution. Furthermore, the Brunauer-Emmett-Teller (BET) SSAs of the MCF and MCK can be calculated as ~1373.5 and ~2776.0 m2 g-1, far larger than that of the MCR (~75.3 m2 g-1), from the adsorption branches. The maximal total volume (Vtotal) of ~1.91 m3 g-1, estimated from the adsorption amount at P/P0 = 0.97, 15 ACS Paragon Plus Environment


is obtained for the MCK, which is higher than those for the MCF (~0.95 m3 g-1) and MCR (~0.06 m3 g-1) products. By determining the micro volume (Vmicro)/Vtotal ratio, ~33.7%, ~54.5%, and ~15.0% of the porosity originating from the micropores can be obtained for the MCF, MCK and MCR samples, respectively. According to BJH pore volume and Equation (2), the density of the MCF is calculated as ~0.69 g cm-3, even higher than that of the MCK (~0.41 g cm-3), which guarantees the synchronous achievement in both gravimetric and volumetric capacitances, enormously highlighting the practical commercial interest of the MCF for ESs applications. Activating Mechanism of the FeCl3 Agent. As discussed above, the FeCl3 can be utilized as an effective pore former, however, its specific activating process and underlying mechanism are still unclear so far. To this end, corresponding XRD and thermogravimetric (TG) analysis coupled with differential scanning calorimetry (DSC) in N2 are conducted accordingly. Evidently, there are two significant mass losses coupled with two exothermic peaks at around 200 °C and in the temperature range from 600 to 800 °C, respectively, for the FeCl3/pre-carbonized mushroom mixture, when compared to that for the single pre-carbonized specimen, as plotted in Figure 6a. To figure out what happens over the mass losing processes, representative XRD patterns of the resultant samples obtained at 250, 380, 550 and 850 °C are plotted in Figure 6b. As examined in Figure 6b, typical diffraction peaks related to the FeCl2, FeOCl, C3N4, FeOOH can be easily detected along with the dehydrated FeCl3 after calcinated at 250 °C. The as-detected FeCl2 should be ascribed to the occurrence of chemical decomposition of FeCl3 (2FeCl3 = 2 FeCl2 +Cl2)30, which can 16 ACS Paragon Plus Environment

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be further corroborated by the blue potassium iodide-starch test paper (SI Figure S5). The FeOOH should corresponds to the partial dehydration of the Fe(OH)3 resulted from the hydrolysis of FeCl3·6H2O. The observed FeOCl and C3N4 are probably related to the inter-reactions among the ferric chloride and the N/O species in the pre-carbonized sample. Thus, mass loss occurs accordingly, and the porous structure can be partially derived. Further decomposition of the FeClO would take place at ~376 °C, where an exothermic peak is presented, and the Fe2O3 and FeCl3 are obtained at such temperature (3FeClO = Fe2O3 + FeCl3), which can be fully verified by the stronger diffraction intensity at 2θ = ~40.5 °, as examined in Figure 6b. The formed Fe2O3, as reported previously, greatly favors for the formation of porous structure in the carbon materials.31 With the temperature up to 550 °C, more Fe2O3 and newly-formed iron-carbon alloy phases including Fe2C and CFe2.5 can be discerned besides the FeCl3, C3N4 and the decreased FeCl2. Interestingly, when the temperature further increases up to 850 °C, the iron-carbon alloy (Fe3C, Fe4C and CFe2.5) and Fe phases (SI Figure S6) are mainly inspected, which should be attributed to the decomposition of FeCl2 (FeCl2 = Fe + Cl2), carbothermal reduction of Fe2O3 (2Fe2O3 + 6C = 4Fe + 6CO), and Fe-etching phenomena (3Fe + C = Fe3C, 2Fe + C = Fe2C and 5Fe + 2C = 2CFe2.5) in the high-temperature range from ~500 to 900 °C. Huge mass loss is thus observed over such temperature range, and mainly accounts for the formation of micro-/mesopores (Figure 5). During the following acid pickling procedure, all these formed noncarbon phases observed in Figure 6b can be completely dislodged, and extra mesopores would further come into being. According 17 ACS Paragon Plus Environment


to the comprehensive analysis above, it is easy to conclude that the FeCl3 applied here plays two significant roles: one is the graphitization agent for enhancing the graphitic degree of carbon materials,19 and the other is the efficiently activating agent, i.e., pore former. Electrochemical Performance in Aqueous Systems. As analyzed above, the MCF specimen is endowed with several distinct physicochemical characteristics favoring for electrochemical charge storage, compared to the other two products. Next, we first conduct comparative electrochemical evaluation of the MCF, MCK and MCR in 6 M KOH. Figure 7a, b show the CV curves of the three electrodes all with 5 mg cm-2 loading in 6 M KOH at scanning rates of 10 and 100 mV s-1, respectively. It is evident that the MCF electrode brings about the largest electrochemical current response among the three at both the two sweep rates within the potential range from -1.0 to 0.0 V (vs. SCE), indicating the highest charge-storage capability of the MCF electrode in alkaline KOH electrolyte. Figure 7c plots typical CP profiles of the MCR, MCK and MCF samples at a high rate of 3 A g-1 (i.e., 15 mA cm-2). Still longer charge/discharge time of the MCF, suggesting its largest GSC, can be visually observed when compared to those for the MCR and MCK. As calculated from the CP profiles (Figure 7c), the GSC values of the MCF, MCK and MCR can be obtained as ~212.4, ~189.6 and ~58.8 F g-1, respectively. Furthermore, the MCF exhibits a GSC as high as ~247.8 F g-1 at a current density of 0.5 A g-1, larger than those for the MCK (~235.0 F g-1) and MCR (~78.6 F g-1), as described in Figure 7d. More encouragingly, with the rate up to 10 A g-1, the MCF still can provide a GSC of ~178 F g-1, corresponding to 18 ACS Paragon Plus Environment

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~71.8% of that at 0.5 A g-1, which is much superior to those for the MCK (~66.4%) and MCR (~53.4%) electrodes in the same case. The striking rate capability of the MCF can be reasonably ascribed to its smallest solution resistance (Rs, ~0.39 Ohm) and charge-transfer resistance (Rct, ~0.67 Ohm) (the upper inset in Figure 7e), as fitted from the EIS spectra (Figure 7e) with the equivalent-circuit model (the lower inset in Figure 7e), when compared to those for the MCK and MCR (SI Table S1). Moreover, it is owing to the co-contributions from its large GSC and high density that the MCF also displays a remarkable VSC of ~171 F cm3, still higher than the MCR (~139.9 F cm3) at a rate of 0.5 A g-1. As a sharp contrast, the MCK just renders a VSC as low as ~96.4 F cm3 at the same rate, as shown in Figure 7f. More promisingly, as for the MCF, a remarkable VSC of ~139.9 F cm3 is still provided at a high rate of 10 A g-1. It is worthy of noting that the VSCs observed here for the MCF are even higher than and/or compared to other carbon materials including carbon aerogels (~123.0 F cm3),30 apricot shell-derived AC (~123.0 F cm3),32 CO2-activated ordered porous carbon (~53.0 F cm3),33 and so on. Particularly, the surface capacitance of the MCF can be estimated as ~18.0 μF cm-2 at a current density of 1 A g-1 considering its SSA, which is much larger than the MCK (~8.5 μF cm-2), suggesting the higher electrochemical

surface

utilization

of

the

MCF,

thanks

to

its

unique

micro-/mesoporous architecture and surface wettability. In contrast to the MCR and MCK, the elegant combination of large GSC/VSC and high surface utilization here strongly highlights the enormous potential of the MCF in practical applications as a competitive electrode for advanced ESs. 19 ACS Paragon Plus Environment


To further verify the generality and universality of the FeCl3 activator for other biomasses, four bio-precursors including poplar branch, high mountain tea, Jasminum nudiflorum and bamboo leaf were also applied. Impressively, similar to the mushroom, enhanced electrochemical phenomena are expectedly demonstrated in Figure 8. Evidently, the highest electrochemical currents in three-electrode CV profiles (20 mV s-1, Figure 8a-d) and the longest charge/discharge times in CP plots (0.5 A g-1, Figure 8e-h) can be observed for the FeCl3-activated samples, including the HCF, PCF, JCF and BCF, when compared to other products activated with KOH and/or without activation. As demonstrated in Figure 8i, the HCF, PCF, JCF and BCF electrodes exhibit striking mass capacitances of ~240.0, ~201.8, ~230.6 and ~247.0 F g-1 at a current density of 0.5 A g-1, respectively, which are higher than those for the HCK (~222.7 F g-1)/HCR (~83.7 F g-1), PCK (~172.2 F g-1)/PCR (~114.0 F g-1), JCK (~209.4 F g-1)/JCR (~87.0 F g-1) and BCK (~232.0 F g-1)/BCR (~60.0 F g-1) electrodes at the same current density. Owing to extra Faradaic pseudo-capacitive contributions from the O/N-based functionalities in acidic electrolyte,4, 7, 8, 34 the electrochemical behaviors of the MCF in 1 M H2SO4 are further investigated in detail, and corresponding profiles are collected in Figure 9. Figure 9a shows the CV curves of the MCF with various sweep rates ranged from 10 to 100 mV s-1 in the potential window from -0.2 to 0.8 V (vs. SCE) by using a three-electrode-cell layout. In contrast to those in KOH (Figure 7a, b), the E-I responses of the MCF in 1 M H2SO4, as seen in Figure 9a, obviously present somewhat distorted rectangular shape with the appearance of some broad and 20 ACS Paragon Plus Environment

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overlapped humps. Such phenomenon should originate from the electric double-layer capacitance (EDLC) coupled with highly reversible pseudo-capacitance related to N/O functionalities, rather than single electric double-layer capacitive contribution in KOH (Figure 7a, b). As a result, much larger electrochemical currents can be identified in these CV curves, indicating the enhanced electrochemical capacitances of the MCF in 1 M H2SO4 when compared to those in 6 M KOH. The nonlinear CP plots at various current densities also reveal the pseudo-capacitive contribution of the MCF in H2SO4 solution, as discerned in Figure 9b. The GSC of the MCF in 1 M H2SO4, as summarized in Figure 9c, can be estimated as ~307.4, ~263.4, ~255.6, ~239.0, ~224.0 and ~214.0 F g-1 from these charge-discharge profiles (Figure 9b), corresponding to ~212.1, ~181.6, ~176.4, ~164.9, ~154.6 and ~147.7 F cm-3 at current rates of 1.0, 2.0, 3.0, 5.0, 8.0 and 10.0 A g-1, respectively. In view of smaller sizes of K+ and OH- ions than those of H+ and SO42- ions, that is OH-< K+≈ H3O+ (3.62 ‒ 4.2 Å) < SO42- (5.33 Å),7, 35, 36 the additional pseudo-capacitance and better wettability of the MCF should be mainly responsible for its higher GSC/VSC values in H2SO4 solution than those in 6 M KOH. Moreover, the GSC values of the MCF in acidic H2SO4 electrolyte are far higher than, and/or comparable to other N-doped porous carbon electrodes reported previously (SI Table S2). To further study its promising application in ESs, we fabricate the MCF-based symmetric cell using two-electrode coin configuration with 1 M H2SO4 as the electrolyte. Figure 9d displays the typical CV curves of the device with the total loading of 5 mg per cell. Appealingly, the cell exhibits quasi-rectangular shape within 21 ACS Paragon Plus Environment


the window of 0.0 ‒ 1.0 V at all these scanning rates. Especially, no obvious change in CV shape over such wide sweep rate span from 50 to 400 mV s-1 reveals the superior electrochemical capacitance of the MCF-based symmetric cell in 1 M H2SO4. It is worth mentioning that the integrated areas under the E-I curves in Figure 9d are all larger than those in KOH at the same scanning rates (SI Figure S7a), further confirming the superior charge-storage ability of the symmetric device in acidic medium. The statement can be solidly supported by the longer charge/discharge time of the MCF-based symmetric capacitor in H2SO4 solution (Figure 9e) than those in alkaline KOH electrolyte (SI Figure S7b). Strikingly, the symmetric cell with 1 M H2SO4 as the electrolyte shows a GSC of ~54.2 F g-1 (i.e., 37.4 F cm-3) at a current of 0.5 A g-1, and even ~37.0 F g-1 (i.e., 25.5 F cm-3) at a high current density of 10 A g-1, as examined in Figure 9f. While the symmetric device only provides a GSC of ~50.1 F g-1 (i.e., 34.6 F cm-3) at 0.5 A g-1, and ~36.0 F g-1 (i.e., 24.8 F cm-3) at 10 A g-1 in KOH solution (SI Figure S7c). Furthermore, the symmetric device shows excellent electrochemical stability, i.e., ~104.3% capacitance retention, in 1 M H2SO4 over 87000 consecutive charge-discharge cycles at a rate of 1 A g-1 (Figure 9g). By contrast, a GSC degradation of ~5.0% is estimated after 60000 continuous cycles in aqueous KOH solution (SI Figure S7d). More significantly, the symmetric device manifests a maximal ED of ~7.5 Wh kg-1 at a PD of 250 W kg-1 in 1 M H2SO4, higher than that in 6 M KOH electrolyte, i.e., ~6.9 Wh kg-1 at the same power rate, as shown in Ragone plots (SI Figure S8). It is well known that the mass loadings greatly influence the ultimate electrochemical behaviors of any electrochemical device. 22 ACS Paragon Plus Environment

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Figure 9h shows the CV curves (10 mV s-1) of the MCF-based full cell in 1 M H2SO4 with various mass loadings of 2, 5 and 8 mg. The increased electrochemical current response is observed with the mass loading increasing. As plotted in Figure 9i, the cell (2 mg mass loading) with 1 M H2SO4 as electrolyte obtains a GSC of ~54.7 F g-1 at 0.5 A g-1, and even 40 F g-1 at 10 A g-1, which are just somewhat larger than those with 5 mg per cell. With the mass loading up to 8 mg, the assembled device delivers a GSC as large as ~47.5 F g-1 at a current density of 0.5 A g-1. More remarkably, the cell still can provide a GSC of ~34.0 F g-1 at a high current rate of 10 A g-1 (i.e., 80 mA cm-2). Of especial note, the MCF-based symmetric device has been vested with impressive supercapacitive behaviors irrespective of electroactive mass loadings. And the capacitive dependence of the whole cell upon mass loading is highly acceptable in acidic electrolyte. Electrochemical Behaviors in Organic Systems. As is known to all, the ED of any supercapacitor is proportional to the square of the electrochemical working voltage window.37 Thus, the higher the operation window of any cell, the larger the ED can be obtained. Nevertheless, owing to thermodynamically narrow working window of theoretically ~1.23 V for the aqueous asymmetric device,7, 8, 38 the high demand of alternative organic electrolytes with higher cell voltage is thus stringent yet effective particularly for high-energy-density applications but not at the expense of high-rate power. Whatʼs more, the high cell voltage would potentially decrease the cell number in series, and reduce the additional burden on external voltage-balance circuits as well as enhance the reliability of the device. Accordingly, we assembled asymmetric cells 23 ACS Paragon Plus Environment


by using the MCF as electroactive material and 1 M TEABF4/PC as non-aqueous electrolyte. Figure 10a illustrates the CV curves of the MCF-based organic cells with various upper voltage limits of 2.5, 2.8, 3.0 and 3.2 V, respectively, at a high sweep rate of 1.0 V s-1. Remarkably, quasi-rectangular shapes can be visualized at such high sweep rate in various electrochemical windows with all the upper limits, indicative of superior electrochemical capacitance of the MCF-based device, which can be manifested by the linear charge-discharge plots at a current density of 1 A g-1 (Figure 10b). But it is worth mentioning that a turnup peak arises above 3.0 V, maybe corresponding to the electrolyte decomposition, which should be responsible for the lower Coulombic efficiency (CE) of 95.4% for the case with the upper potential limit of 3.2 V. In addition, the CE increases up to 97.2% (3.0 V), 98.1% (2.8 V) and 98.9% (2.5 V), respectively, with the upper voltages decreasing, as estimated in Figure 10b. Figure 10c presents the long-term cycling performance of the MCF-based device with different upper potential limits as indicated. Notably, the GSC degradations of the cell are ~14.2% (3.2 V), ~10.7% (3.0 V), ~9.7% (2.8 V) and ~9.1% (2.5 V), respectively, over 10000 consecutive charge-discharge cycles at 1 A g-1. Owing to contiguous capacitance retention of the device at the cases of 2.5, 2.8 and 3.0 V, we thus choose 3.0 V as the suitable voltage upper limit for our symmetric organic device. Figure 10d shows the CV curves of the cell in the electrochemical window of 3.0 V with a wide-span scanning rate range from 0.5 to 2.0 V s-1. Obviously, the device shows symmetric rectangular-shaped voltammetry feature with rapid current responses on voltage reversals at each end voltage along the zero current baseline 24 ACS Paragon Plus Environment

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even at a sweep rate of 2.0 V s-1, which reveals its super-high rate behaviors. As calculated from the CP data (Figure 10e), the symmetric device produces large capacitances of ~38.0, ~36.9, ~35.1, ~32.4 and 30.5 F g-1 at high current densities of 10.0, 20.0, 30.0, 40.0 and 50.0 A g-1, respectively, that is, a high capacitance retention of ~80.3% even with the rate up to 50.0 A g-1, further verifying its outstanding power behavior. Figure 10f shows the Ragone plot of the device with an upper potential limit of 3.0 V. Competitively, the cell exhibits a large ED of ~62.6 Wh kg-1 at a power rate of 1.5 kW kg-1, and with the rate up to ~75.0 kW kg-1, a remarkable ED of ~38.1 Wh kg-1 still can be provided none the less, in particular, which are apparently higher than and/or comparable to other organic devices.7, 8, 24, 39-49 Furthermore, two organic cells in series can power three light-emitting-diode lights (SI Figure S9) for about 2 min. CONCLUSIONS In conclusion, a general and environmental-friendly FeCl3-involved activating strategy was intensively explored to massively fabricate biomass-based hierarchical porous N-doped carbon materials as low-cost yet high-performance electrodes for advanced ESs. Furthermore, the intrinsic FeCl3-activating mechanism was rationally put

forward.

Especially

with

FeCl3

activation,

the

mushroom-derived

micro-/mesoporous N-doped carbon was endowed with a large SSA of ~1143.6 m2 g-1, ~66.3% of mesopores, ~3.1 at.% of N doping, and good wettability. When evaluated as an electroactive material for ESs, the MCF displayed even better electrochemical capacitances, compared to those activated with the classic KOH and/or without 25 ACS Paragon Plus Environment


activation, benefiting from its innately structural/componential/surfacial advantages. Remarkably, the MCF electrode (5 mg cm-2) exhibited high gravimetric/volumetric capacitances of ~307.4 F g-1 (~212.1 F cm-3) at 1 A g-1 in 1 M H2SO4, which is much better than those in KOH, mainly owing to extra pseudo-capacitances. In addition, the MCF-based symmetric devices yielded a high energy density of ~6.6 Wh kg-1 along with ~104.3% capacitance retention over 87000 cycles in 1 M H2SO4, and even ~62.6 Wh kg-1 in 1 M TEABF4/PC at a high rate of 1.5 kW kg-1. The FeCl3-involved synthetic protocol was proved essentially general to other biomass-derived porous carbons. More significantly, our contribution here plays a vital role in recycling sustainable biomasses even towards more versatile electrochemical energy-storage applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsmi.7b15808. Prices of different activators, FESEM and HRTEM images, digital pictures, XPS data, EIS fitted data, electrochemical comparisons and other electrochemical data of the controlled experiments (PDF)

AUTHOR INFORMATION * Corresponding authors. Tel: +65 531 82769410. 26 ACS Paragon Plus Environment

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E-mail: [email protected] (Prof. Linrui Hou) [email protected]; [email protected] (Prof. Changzhou Yuan)

ORCID Linrui Hou: 0000-0002-3163-3391 Changzhou Yuan: 0000-0002-6484-8970

NOTES The authors declare no competing financial interst.

ACKOWLEDGEMENTS The authors acknowledge the financial support from National Natural Science Foundation of China (No. 51502003, 51572005, 51772127, 51772131), Anhui Province Funds for distinguished Young Scientists (No. 1508085J09), Taishan Scholars (No. ts201712050) and Major Program of Shandong Province Natural Science Foundation (ZR2018ZB0317).



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(33) Xu, K. S.; Gao, Q. M.; Jiang, J. H.; Hu, J. Hierarchical Porous Carbons with Controlled Micropores and Mesopores for Supercapacitor Electrode Materials. Carbon 2008, 46, 1718-1726. (34) Olieira L. C. A.; Pereira, E.; Guimaraes, I. R.; Vallone, A.; Pereira, M.; Mesquita J. P.; Sapag K. Preparation of Activated Carbons from Coffee Husks Utilizing FeCl3 and ZnCl2 as Activating Agents. J. Hazard Mater. 2009, 165, 87-94. (35) Hulicova, D.; Kodama, M.; Hatori. H. Electrochemical Performance of Nitrogen-Enriched Carbons in Aqueous and Non-Aqueous Supercapacitors. Chem. Mater. 2006, 18, 2318-2326. (36) Ue, M. Mobility and Ionic Association of Lithium and Quaternary Ammonium Salts in Propylene Carbonate and γ-butyrolactone. J. Electrochem. Soc. 1994, 141, 3336-3342. (37) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer, 1999. (38) Hulicova, D.; Kodama, M.; Hatori, H. Electrochemical Performance of Nitrogen-Enriched Carbons in Aqueous and Non-Aqueous Supercapacitors. Chem. Mater. 2006, 18, 2318-2326. (39) Tian, W. Q.; Gao, Q. M.; Tan, Y. L.; Yang, K.; Zhu, L. H.; Yang, C. X.; Zhang, H. Bio-Inspired Beehive-Like Hierarchical Nanoporous Carbon Derived from Bamboo-Based Industrial By-Product as A High Performance Supercapacitor Electrode Material. J. Mater. Chem. A 2015, 3, 5656-5664.



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(40) Karthikeyan, K.; Amaresh, S.; Lee, S. N.; Sun, X. L.; Aravindan, V.; Lee, Y. G.; Lee,

Y.

S.

Construction

of

High-Energy-Density

Supercapacitors

from

Pine-Cone-Derived High-Surface-Area Carbons. ChemSusChem 2014, 7, 1435-1442. (41) Wang, D. W.; Min, Y. G.; Yu, Y. H. Facile Synthesis of Wheat Bran-Derived Honeycomb-Like Hierarchical Carbon for Advanced Symmetric Supercapacitor Applications. J. Solid State Electrochem. 2015, 19, 577-584. (42) Xie, L. J.; Sun, G. H.; Su, F. Y.; Guo, X. Q.; Kong, Q. Q.; Li, X. M.; Huang, X. H.; Liu, W.; Song, W.; Li, K. X.; Lv, C. X.; Chen, C. M. Hierarchical Porous Carbon Microtubes Derived from Willow Catkins for Supercapacitor Applications. J. Mater. Chem. A 2016, 4, 1637-1646. (43) Wang, J.; Shen, L. F.; Xu, Y. L.; Dou, H.; Zhang, X. G. Lamellar-Structured Biomass-Derived Phosphorus- and Nitrogen-Co-Doped Porous Carbon for High Performance Supercapacitors. New J. Chem. 2015, 39, 9497-9503. (44) Cheng, P.; Gao, S. Y.; Zang, P. Y; Yang, X. F.; Bai, Y. L.; Xu, H.; Liu, Z. H.; Lei, Z. B. Hierarchically Porous Carbon by Activation of Shiitake Mushroom for Capacitive Energy Storage. Carbon 2015, 93, 315-324. (45) Qian, W. J.; Sun, F. X.; Xu, Y. H.; Qiu, L. H.; Liu, C. H.; Wang, S. D.; Yan, F. Human Hair-Derived Carbon Flakes for Electrochemical Supercapacitors. Energy Environ. Sci. 2014, 7, 379-386. (46) Rufford, T. E.; Hulicova-Jurcakova, D.; Fiset, E.; Zhu, Z. H.; Lu, G. Q. Double-Layer Capacitance of Waste Coffee Ground Activated Carbons in An Organic Electrolyte. Electrochem. Commun. 2009, 11, 974-977. 34 ACS Paragon Plus Environment

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(47) Wang, L.; Mu, G.; Tian, C. G.; Sun, L.; Zhou, W.; Yu, P.; Yin, J. Porous Graphitic Carbon Nanosheets Derived from Cornstalk Biomass for Advanced Supercapacitors. ChemSusChem 2013, 6, 880-889. (48) Luan, Y. T.; Wang, L; Guo, S. E.; Jiang, B. J.; Zhao, D. D.; Yan, H. J.; Tian, C. G.; Fu, H. G. A Hierarchical Porous Carbon Material from A Loofah Sponge Network for High Performance Supercapacitors. RSC Adv. 2015, 5, 42430-42437. (49) Yun, Y. S., Park, M. H.; Hong, S. J.; Lee, M. E.; Park, Y. W.; Jin, H. J. Hierarchically Porous Carbon Nanosheets from Waste Coffee Grounds for Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 3684-3690.



Page 36 of 48

Tables, Figures and Captions

Table 1. Summary of XPS, SSA, APD, Vtotal, ρ and ID/IG data for the MCF, MCK and MCR samples MCF

MCK

MCR

C (at.%)

~89.5

~66.2

~82.7

C1 C2 C3 C4

72.6 14.0 2.7 0.3

49.1 14.2 0.5 2.3

64.0 10.4 2.9 5.4

N (at.%)

~3.1

~2.9

~3.3

N1 N2 N3 N4

0.8 1.0 0.8 0.5

0.6 1.2 0.8 0.3

0.4 0.8 1.3 0.8

O (at.%)

~7.5

~30.9

~14.0

O1 O2 O3

3.2 3.4 0.9

1.0 23.4 5.9

10.2 2.5 1.3

SSA (m2 g-1)

~1373.5

~2776.0

~75.3

APS (nm)

~2.7

~1.8

~6.7

Vtotal (cm3 g-1)

~0.95

~1.91

~0.06

Vmicro (cm3 g-1)

~0.32

~1.04

~0.009

ρ (g cm-3)

~0.69

~0.41

~1.78

IG/ID

~0.93

~0.71

~0.88

Yield (%)

~21.6

~10.9

~30.1

Elemental percentages


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Figure 1. (a, b) FESEM, (c ‒ e) TEM images for the MCF; (f) STEM image and corresponding EDX elemental (g, C; h, O and i, N) mapping images



Figure 2. (a) XRD patterns and (b) Raman spectra for the MCF, MCK and MCR samples as indicated


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Figure 3. (a) Overview XPS survey spectra, and (b) enlarged sections for the gray shaded region in panel (a) for the MCF, MCK and MCR; high-resolution spectra for (c) C 1s, (d) N 1s and (e) O 1s of the MCF



Figure 4. Digital contact angle images for the (a, b) MCR, (c, d) MCK and (e, f) MCF in (a, c, e) 1 M H2SO4 and (b, d, f) 6 M KOH electrolytes


Page 40 of 48

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Figure

5.

(a)

N2

sorption

isotherms

(hollow/solid

rhombuses

for

the

desorption/adsorption data), and (b) mesopore size distribution and micropore size distribution (the inset in panel b) plots for the MCF, MCK and MCR products



Figure 6. (a) TG and DSC curves of the FeCl3/pre-carbonized mushroom mixture mixture (red) and pre-carbonized mushroom (blue) measured in N2 atmosphere, and (b) XRD patterns for the products obtained by calcinating the mixture at various temperatures as indicated


Page 42 of 48

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Figure 7. Electrochemical performance of the MCF (red), MCK (blue) and MCR (green) electrodes in three-electrode systems with 6 M KOH as the electrolyte: CV curves tested at (a) 10 and (b) 100 mV s-1; (c) CP plots at 3.0 A g-1; (d) GSCs at various current densities; (e) Nyquist plots (the upper inset: magnified high-frequency region; the lower inset: fitting equivalent circuit) and (f) VSCs as a function of current densities



Figure 8. Electrochemical performance of various biomass-derived carbon materials obtained by directly carbonizing (green), KOH activating (blue) and FeCl3 activating (red) in three-electrode systems with 6 M KOH as electrolyte: (a ‒ d) CV curves (20 mV s-1); (e ‒ h) CP plots (0.5 A g-1); (i) GSC values for the obtained samples as indicated


Page 44 of 48

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Figure 9. Electrochemical properties of the MCF in (a ‒ c) three-electrode system and (d ‒ i) two-electrode symmetric cells both with 1 M H2SO4 as the electrolyte: (a) CV curves, (b) CP plots, (c) GSCs and VSCs as a function of current densities; (d) CV curves at various sweep rates, (e) CP profiles, (f) GSCs and VSCs vs. current rates and (g) cycling stability (1 A g-1) for symmetric device with total mass loading of 5 mg; (h) CV curves (10 mV s-1) and (i) GSC vs. current rate for symmetric systems with different loadings of electroactive materials as indicated



Figure 10. Electrochemical behaviors of the MCF-based symmetric devices with 1 M TEABF4/PC as the electrolyte: (a) CV curves (1.0 V s-1), (b) CP plots (1 A g-1) and (c) cycling stabilities (1 A g-1) with different upper voltages (2.5, 2.8, 3.0 and 3.2 V); (d) CV curves at various scan rates, (e) CP plots and (f) Ragone plot for the symmetric cell with a upper voltage of 3.0 V. The scattered signals in panel (f) for the ED/PD data of other carbon-based devices with organic electrolytes reported previously.


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Table of Content



ACS Paragon Plus Environment

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Universal FeCl3-Activating Strategy for Green and Scalable

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