Fabrication of Low-Cost and Ecofriendly Porous Biocarbon Using Konjaku Flour as the Raw Material for High-Performance Supercapacitor Application.

Low-cost and ecofriendly porous biocarbons were fabricated from konjaku flour via precarbonization and potassium hydroxide (KOH) activation. The obtained biocarbon ACK-5 derived from a precarbonized carbon/potassium hydroxide (KOH) mass ratio of 1:5 possessed an ultrahigh specific surface area of 1403 m2 g-1 and hierarchical porous structures with the existence of micro- to macropores. When ACK-5 was employed as a supercapacitor electrode in 6 M KOH, it showed a high specific capacitance of 216 F g-1 and excellent cycling stability with capacitance retention remaining 93.7% after 5000 cycles. Moreover, the ACK-5 sample acquired a supramaximal specific capacitance of 609 F g-1, and the high energy density of ACK-5//ACK-5 symmetrical cells reached up to 9.2 Wh kg-1 when p-phenylenediamine serving as a redox electrolyte was added into KOH electrolyte. The reported simple fabrication strategy would leverage a green biomass precursor for the preparation of supercapacitors.

Introduction

The exhaustion of fossil fuels and the increasing seriousness of environmental issues have drawn great attention toward high-performance energy storage devices.[1,2] Supercapacitors have come to the fore due to their high power, quick charging time, and long service life.[3,4] Carbon materials, such as activated carbons,[5] carbon nanotubes,[6] carbon foams,[7] and graphene,[8] have been largely used as supercapacitor electrodes. Among them, activated carbons have been regarded as the most promising candidates for practical supercapacitor devices due to their simple synthesis processes, low equipment requirements, high surface areas, and low cost.[9,10] Especially, activated carbons possessing hierarchical pore structures would greatly enhance the capacitance of supercapacitors: the micropores (<2 nm) store charge during charging–discharging processes and ensure the large specific surface areas, the mesopores (2–50 nm) provide transfer channels for electrolyte ions, while the macropores (>50 nm) as reservoirs for electrolytes facilitate the contact of electrolytes and electrodes.[11,12]

Recently, abundant sustainable biomass, such as rice straw,[13] pistachio nutshells,[14] eggplant,[15] kapok fiber,[16] elm samara,[17]Perilla frutescens,[18] etc., have been used as fascinating precursors of activated carbon materials for supercapacitor electrodes. Biomass-derived carbons have exceptional advantages like simple preparation, renewability, inherent porosity, and wide availability. Moreover, the doping atoms (e.g., N or B) from the parent biomass usually can improve the capacitive performances of porous biocarbons by changing the electronic properties of the carbon matrix and are accompanied by the additional pseudocapacitance by Faradaic reactions.[19,20]

Konjaku flour is produced from the tuberous roots of fresh konjac, which contains cellulose, vitamin, and polysaccharide (konjac glucomannan).[21] China is the first major producer of konjaku flour in the world. Moreover, konjaku flour is not the main food source of human beings. Furthermore, the porous carbon skeleton can be left behind after the decomposition of konjac glucomannan through anaerobic pyrolysis and activation process, and heteroatoms in the carbon structures originate from high-temperature carbonization of amino acids and crude proteins.[22] Therefore, konjaku flour with a large yield and abundant sources is indeed a superior biological precursor of porous biocarbons for supercapacitor electrodes.

In this work, porous biocarbons were fabricated by using konjaku flour as the precursor through precarbonization and subsequent potassium hydroxide (KOH) activation. The obtained biocarbon materials from konjaku flour achieved high specific surface areas and hierarchical porous structures, which could provide abundant storage sites and facilitate the charge transport. At the same time, due to nitrogen doping in the carbon structures, the biocarbons obtained high wettability and pseudocapacitance as supercapacitor electrodes. Moreover, p-phenylenediamine (PPD) as a redox additive was added to the KOH electrolyte to remarkably improve the specific capacitance through a redox reaction as well as the resultant energy density of the assembled symmetric supercapacitor.

Results and Discussion

Scanning electron microscopy (SEM) image of CK-700 in Figure a shows abundant pores and cavities with a smooth surface, which formed from the deprivation of organic matter and volatilization of some small molecules during the pyrolysis process.[23] Relative to CK-700, the pore walls of the activated porous biocarbon ACk-3, 5, and 7 (Figure b–d) were more plicated and fragmentized due to the etching effect of KOH.[24] Although the three biocarbon samples possess porous network structures, in comparison with ACK-3 and ACK-7, ACK-5 had thicker hole walls and more nanometer porous construction in the hole walls.

Figure 1

Field emission (FE)-SEM images of (a) CK-700, (b) ACK-3, (c) ACK-5, and (d) ACK-7. (e, f) Transmission electron microscopy (TEM) images of ACK-5.

The TEM images (Figure e,f) further demonstrate the existence of a rich nanometer porous structure in ACK-5, which corresponds with the SEM observation (Figure c). This was credited to the added amount of KOH in ACK-3 being less, due to which sufficient etching could not be generated. With the increase in KOH amount, thin pore walls have been removed and more nanoporous structure generated on the thick hole walls of ACK-5. On the other hand, the amount of KOH activating agent was too much in ACK-7, and it led to plicated pore walls and fragmentized cavities that resulted from excessive etching. In brief, the ACK-5 biocarbon possessed more small pores, suggesting that it had a large specific area and an excellent electrochemical behavior.

Figure S1 presents Fourier transform infrared (FT-IR) spectra of all of the obtained biocarbons. A broad and feeble peak and an obvious peak that emerged at 3400 and 1.90 cm–1 were attributed to the stretching vibrations of O–H and N–H, and the C=C tensile deformation vibration of benzene rings, respectively.[25,26] Two other characteristic peaks located at 1350 and 1200 cm–1 corresponded to C–N and C–O stretching vibrations, respectively.[27] The nitrogen- and oxygen-containing groups that existed in the biocarbon samples might have resulted from the konjac glucomannan and amino acids in the konjaku flour after annealing.

X-ray diffraction (XRD) analyses in Figure a exhibit two peaks centered at approximately 24 and 43° of CK-700, ACK-3, ACK-5, and ACK-7, respectively, corresponding to the (002) and (100) lattice planes of graphite crystal.[28] Two distinct bands located at approximately 1350 and 1590 cm–1 in the Raman spectra (Figure b) were D band and G band, respectively.[29] The ID/IG values of CK-700, ACK-3, ACK-5, and ACK-7, were 0.96, 0.99, 1.04, and 1.03, respectively, which indicated the generation of the abundant structural defects after KOH etching.[30] Apparently, the largest ID/IG value for ACK-5 could reveal the existence of heteroatom doping and rich micropores.

Figure 2

(a) XRD diffraction curves and (b) Raman spectra of biocarbon samples; (c) micropore size distributions obtained by the Horváth–Kawazoe method and (d) mesopore size distributions obtained by the Barrett–Joyner–Halenda method of the obtained biocarbon samples.

N2 sorption isothermals of all samples (Figure S2) were composed of type-I and type-IV, which implied the existence of micropores and mesopores.[31] In comparison with CK-700, the three ACK samples possessed a faster and greater adsorption when the relative pressure was below 0.1, and an obvious hysteresis loop appeared at a high relative pressure over 0.5, which implied that more micropores and mesopores existed.[32] Additionally, there was a small steep adsorption at the high relative pressure (0.9–1.0) for all of the carbon samples, suggesting the existence of macropores.[28] The distributions of micropore and mesopore sizes of the biocarbons are clearly shown in Figure c–d. Specifically, the pore distribution of CK-700 was mainly centered at 0.4 nm with a weak peak in the range of 1–3 nm, whereas the pore sizes of ACK were highly concentrated in approximately 1 nm with abundant mesopores. This may be resulting from more micropores and mesopores formed by the etching of KOH. Detailed data of the biocarbon samples are listed in Table with the largest SBET values and total pore volume of ACK-5 being 1403 m2 g–1 and 0.87 cm3 g–1, respectively.

Table 1

Characteristics of the Obtained Biocarbon Porous Structures

samples	SBETa (m2 g–1)	Dnb (nm)	Vtotalc (cm3 g–1)	Smicd (m2 g–1)	Smic/SBET (%)
CK-700	486	0.3–20.0	0.48	114	23.5
ACK-3	1326	0.9–6.0	0.68	596	45.0
ACK-5	1403	0.8–10.0	0.87	702	50.1
ACK-7	1236	0.8–10.0	0.72	558	45.1

Specific surface area determined according to Brunauer–Emmett–Teller (BET) equation.

Pore size.

Total pore volume.

Micropore surface area from the t-plot method.

X-ray photoelectron spectroscopy (XPS) characterization in Figure was carried out to confirm the chemical composition of ACK-5, demonstrating the coexistence of C (86.22 atom %), N (1.88 atom %), and O (11.9 atom %) elements (Figure a). Six branches of peaks could be obtained to best fit the high-resolution C 1s spectrum of ACK-5 (Figure b). The strong-intensity C–C peak (284.8 eV) and C=C signal (285.1 eV) suggested that it was dominated by sp3 carbon in the ACK-5, and some amorphous carbons were transformed into a graphite state. The other three peaks located at 286.3, 287.9, and 289.5 eV corresponded to C–O, C=O, and HO–C=O bonds, respectively. These peaks revealed massive oxygen-containing groups’ residues in the ACK-5 after activation. The high-resolution O 1s spectrum in Figure c could be divided into C–O (532.8 eV), C=O (531.4 eV), and −OH (533.4 eV). Another faint peak corresponded to the C–N bond (285.2 eV), proving that a few N atoms were already doped into the ACK-5 carbon frameworks. From the high-resolution N 1s spectrum in Figure d, the pyridinic-N peak (398.7 eV), pyrrolic-N peak (400.4 eV), and graphitic-N peak (402.6 eV) were rooted in the pyrolyzation process of amino acids and crude proteins in the konjaku flour. From the above analyses, we can conclude that ACK-5 was decorated with abundant oxygen-containing groups and a small number of nitrogen-containing groups, and it might exhibit outstanding wettability and capacitance.

Figure 3

(a) XPS survey spectrum; (b) C 1s, (c) O 1s, and (d) N 1s spectra of ACK-5.

The capacitive performances of the as-obtained porous biocarbons were evaluated by a three-electrode system in 6 M KOH solution. The cyclic voltammetry (CV) curves at 50 mV s–1 are illustrated in Figure a. ACK-3, ACK-5, and ACK-7 showed nearly rectangular CV curves, which is the characteristic of a double-layer capacitor.[33] Moreover, the curves formed a small hump due to the pseudofaradaic redox reactions of heteroatoms.[18] Clearly, ACK-5 exhibited enhanced specific capacitance compared with other biocarbons. Figure b illustrates the CV curves of ACK-5 at various scan rates within 5–300 mV s–1. The curve maintained a relatively good quasi-rectangular shape and underwent no change even when the scan rates increased to 300 mV s–1. Figure c shows the galvanostatic charge–discharge (GCD) plots at a 1 A g–1 current density. According to the calculation of discharge time, the specific capacitances (Cs) of CK-700, ACK-3, ACK-5, and ACK-7, were 152, 187, 216, and 190 F g–1, respectively. Figure d depicts the GCD curves of ACK-5 at a current density interval from 1 to 10 A g–1. Apparently, the shapes of the GCD curves were symmetrical and nearly linear with a small curvature, suggesting an excellent capacitive behavior of the electrical double layer with little pseudocapacitance (deriving from heteroatoms) and good characteristics of capacitive reversibility.[34] For comparison, the capacitance retention of CK-700, ACK-3, and ACK-7 are also provided in Figure e, and it was clear that ACK-5 possessed a larger value compared with other biocarbons.

Figure 4

(a) CV curves of the four biocarbons at a scan rate of 50 mV s–1; (b) CV curves of ACK-5 at different scan rates; (c) GCD curves of the four biocarbons at a current density of 1 A g–1; (d) GCD curves of ACK-5; (e) specific capacitances of the four biocarbons at different current densities; (f) cycling stabilities of the biocarbons at 100 mV s–1.

Moreover, the specific capacitance of previously as-published activated biocarbons from biomass is summarized in Table S1, and it can be seen that the ACK-5 in this work exhibits an excellent electrochemical performance at 1 A g–1 current density. The excellent rate performance of ACK-5 was explained as follows: (1) hierarchical porosity, a large specific area, and proper pore size distribution could effectively provide more electrolyte contact and transfer channels for electrolyte ions and a reservoir for electrolytes, facilitating the fast diffusion of ions at high current density, and thus increase the charge storage density; (2) the heteroatomic doping could greatly enhance the electrical wettability or conductivity and thus reduce the diffusion resistance of electrolyte ion transfer. Cycling stabilities of the four biocarbons were measured at 100 mV s–1 (Figure f), and the curves showed that all of the materials maintained ultrahigh stability after 5000 CV cycles, which may have corresponded to the stability of porous biocarbon structures. Especially, ACK-5 obtained high stability in KOH with the specific capacitance maintained at 93.7%, owing to the heteroatom doping and porous structures, which facilitate the effective and steady transfer of ions and electrons on the interface of the electrode. However, there is a capacitance fading of 6.3% after 5000 cycles. The key factor responsible for the capacitance fading might be the increase of charge transfer resistance due to the presence of the surface functionalities or impurities contained in the activated carbon electrodes.[35]

The Nyquist plots of biocarbons are shown in Figure S3. The equivalent series resistance of ACK-5 (0.8 Ω) was smaller than that of other biocarbons. Meanwhile, ACK-5 displayed a lower resistance value (0.6 Ω) and a lower Warburg impedance compared with other biocarbons due to the vast electron transfer pathways offered by micro- and mesopores.[36−38] Additionally, a vertical bar along the y-axis at low frequencies was characteristic of the capacitive behavior of the ideal electrical double layer capacitors.[39]

To explore the effect of the redox mediator PPD, the electrochemical performances were measured in KOH electrolyte with different concentrations of PPD. A pair of intense oxidation reduction peaks in the CV curves was observed in Figure S4a. The two peaks existed at approximately −0.2 and −0.6 V, respectively, which was due to the state conversion of p-phenylenediamine/p-phenylenediimine on the interfaces of electrodes/electrolyte.[31] There was a highly reversible redox reaction with the generation of two protons and charges in PPD during the charge–discharge process, and the CV curves have lost their rectangular shape compared with the KOH electrolyte.[40] Furthermore, the specific capacitance of ACK-5 was obviously heightened by introducing the pseudocapacitance of PPD. Figure S4b further confirmed the above analyses: the GCD curves of ACK-5 possessed a typical symmetrical triangle profile with KOH electrolyte, whereas there were two gentle slopes in the GCD curves in KOH electrolyte with different concentrations of PPD. Clearly, the specific capacitance of ACK-5 reached up to 609 F g–1 when the additional amount of PPD was 0.5 mM, which was 3 times as much as that in 6 M KOH electrolyte. However, the specific capacitance went down with excess PPD concentration in the KOH electrolyte, which may be due to the decrease of the conductivity and capacitive performance in the electrolyte system caused by the electrolyte phase separation and the accumulation of free ions and charge.[41]

To further explore the practical application for supercapacitors, the assembled ACK-5//ACK-5 symmetrical cells were measured in the 6 M KOH and 6 M KOH + 0.5 mM PPD electrolytes, respectively. The GCD curves of ACK-5//ACK-5 symmetrical cells in the KOH electrolyte formed an isosceles triangle corresponding to three-electrode analysis (Figure a). Based on the total mass of the two electrodes, the specific capacitances (Ct) were calculated to be 40.0, 33.0, 27.8, and 22.2 F g–1 at current densities of 0.5, 1.0, 2.0, and 5.0 A g–1, respectively. Figure b validated that there were two obvious humps in two-electrode GCD curves with the addition of 0.5 mM PPD into the 6 M KOH electrolyte, which was due to the introduction of pseudocapacitance.[42] The specific capacitances were overtly improved, which increased to 65.5, 58.3, 52.0, and 40.5 F g–1 at 0.5, 1.0, 2.0, and 5.0 A g–1, respectively.

Figure 5

GCD curves of ACK-5//ACK-5 devices at different current densities in (a) 6 M KOH and (b) 6 M KOH + 0.5 mM PPD electrolytes; (c) ragone plots, and (d) digital image of a yellow-light-emitting diode (LED) lighted by the ACK-5//ACK-5 device.

The ragone plots are shown in Figure c, reflecting the relationship of energy density and power density. At a power density of 0.25 kW kg–1, the ACK-5//ACK-5 symmetrical supercapacitor showed the maximum energy density of 5.6 and 9.2 Wh kg–1 in 6 M KOH and 6 M KOH + 0.5 mM PPD of electrolytes, respectively. Additionally, the value of energy densities still maintained at 3.1 and 5.6 Wh kg–1, respectively, even the two-electrode system has high power density of 2.5 kW kg–1. The high energy density and power density of ACK-5//ACK-5 symmetrical cells have also been confirmed by comparing with various biocarbons (Table S2). The cycling stability tests (Figure S5) of ACK-5//ACK-5 showed that the capacitance was retained 89.7% in 6 M KOH electrolyte after 5000 CV, while the value was 73.4% in 6 M KOH + 0.5 mM PPD electrolyte due to a portion of PPD being decomposed during the 5000 CV processes[43] To demonstrate the practical application of the supercapacitor, two ACK-5//ACK-5 symmetrical cells were assembled in series to light a yellow LED (Figure d). More importantly, the LED still remained bright for 180 s after being charged for 80 s at 2.0 V.

Conclusions

Hierarchical porous biocarbons were prepared from konjaku flour through carbonization and further KOH activation processes for supercapacitor application. The obtained biocarbons exhibited large specific surface areas with ample porous structures and excellent electrochemical performances. The N and O heteroatoms inherently incorporated into the carbon framework increased the specific capacitances by introducing pseudocapacitances. Especially, ACK-5 acquired a high specific capacitance of 216 F g–1 and a preeminent cycling stability of 93.7% in KOH electrolyte, whereas a greater specific capacitance of 609 F g–1 and a significantly improved energy density of 9.2 Wh kg–1 were obtained when PPD redox electrolyte was added to the KOH electrolyte. Therefore, a cost-efficient and direct strategy has been developed to construct porous biocarbon, which is expected to be commercially produced as high-performance supercapacitor electrode materials.

Experimental Section

Materials

Konjaku flour was directly purchased from local supermarkets (Fujian, China). Air-laid papers and p-phenylenediamine were respectively obtained from Shenzhen Chenyan Trading Co., Ltd. (Guangdong, China) and Shanghai Chemical Reagent Company (Shanghai, China). Hydrochloric acid (HCl) was acquired from Lanxi Xuri Chemical engineering Co., Ltd. (Zhejiang, China) and diluted to a concentration of 1 M. Potassium hydroxide (KOH) was received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Preparation of the Porous Carbons

A typical procedure for the preparation of ACK-x as an electrode is illustrated in Scheme . In detail, konjaku flour was pyrolyzed at 400 °C under nitrogen flow to obtain precarbonized porous carbon with a heating rate of 5 °C min–1 for 1 h. The precarbonized porous carbon (1 g) and appropriate KOH were stirred together for 5 h in 30 mL of deionized water. The mixture solution was dried at 60 °C for 24 h to gain a precursor. Then, the precursor was pyrolyzed at 700 °C under nitrogen for 1 h. Subsequently, the acquired products were washed by 1 M HCl solution and were then dried at 60 °C for 24 h. The obtained porous biocarbons (ACK) are denoted as ACK-3, ACK-5, and ACK-7, which represents that the mass ratio of KOH to the precarbonized porous carbon was 3:1, 5:1, and 7:1, respectively. CK-700 was obtained from the directed carbonization of konjaku flour at 700 °C without adding KOH activation.

Scheme 1

Schematic Illustration of the Preparation of ACK-x for Electrodes

Characterization

The surface microstructures of biocarbon products were represented through scanning electron microscopy (FE-SEM, Carl Zeiss ULTRA 55) and transmission electron microscopy (TEM, FEI TECNAI G2 F30). A Nicolet FT-IR 5700 spectrophotometer was used for characterizing the asymmetric constructions of functional groups of the samples. Phase information was performed on X-ray diffractometer patterns (XRD, ULTIMA III). Raman spectra were characterized by a ThermoFisher DXR2xi Raman spectrometer. The X-ray photoelectron (XPS) spectrum was obtained through an ESCALAB 250 X-ray spectrometer. A Micromeritics 3Flex analyzer was applied to observe the BET specific surface areas and porous size distributions.

Electrochemical Measurements

Electrochemical measurements were performed through a CHI660E electrochemical workstation (Chenhua Instruments Co.). Platinum wire, Ag/AgCl, and working electrodes made up a typical three-electrode system for electrode testing. Two 6 M KOH solutions contained different PPD concentrations as electrolytes. Working electrodes were fabricated by mixing a fully grinded carbon sample (85 wt %), acetylene black conductive additive (10 wt %), and poly(tetrafluoroethylene) binder (5 wt %) to obtain a slurry. The slurry was pressed onto a stainless steel mesh (1 cm × 1 cm), and was finally dried at 60 °C for 12 h. Subsequently, a mass of approximately 4 mg cm–2 was loaded onto each electrode. Specific capacitances (Cs) in the three-electrode system were obtained through calculation from the discharge time of the galvanostatic charge–discharge (GCD) tests, or by computation from the cyclic voltammetry (CV) curve areas, and they were under a current density range of 1–10 A g–1 and a potential window of −1.0–0 V, respectively. Under 0.01–100 kHz scanning frequencies, electrochemical impedance spectroscopy was used for exploring the transfer resistance of the charge and ions.

The symmetrical supercapacitor was assembled by a sandwich method, where two identical electrodes were inserted into an air-laid paper as a separator. Values of specific capacitance (Ct), energy density (E), and power density (P) were obtained by calculating the following equationswhere I (A) represents the discharge current, Δt (s) is the discharge time, m represents the total mass of biocarbon materials in the two electrodes, and ΔV (V) is the discharge voltage.