Mixed-Biomass Wastes Derived Hierarchically Porous Carbons for

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Mixed-Biomass Wastes Derived Hierarchically porous Carbons for High-Performance Electrochemical Energy Storage Lin Peng, Yeru Liang, Jianyu Huang, Linlin Xing, Hang Hu, Yong Xiao, Hanwu Dong, Yingliang Liu, and Mingtao Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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ACS Sustainable Chemistry & Engineering

Mixed-Biomass Porous

Wastes

Carbons

Derived for

Hierarchically

High-Performance

Electrochemical Energy Storage Lin Peng, Yeru Liang, Jianyu Huang, Linlin Xing, Hang Hu, Yong Xiao, Hanwu Dong, Yingliang Liu,* Mingtao Zheng* College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China. Corresponding Authors *E-mail: [email protected] (Y.L. Liu), [email protected] (M.T. Zheng).

1 Environment ACS Paragon Plus

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: A cost-effective route is developed to fabricate hierarchically porous carbons (HPCs) from renewable mixed-biomass wastes of crab shells and rice husks by hydrothermal carbonization followed by KOH activation. Benefiting from the inorganic collaboration between crab shells and rice husks, the resultant HPCs deliver well-developed hierarchical porosity and large specific surface area. Employed as electrode materials for supercapacitors, the as-prepared HPCs exhibit a high capacitance of 474 F g-1 and remarkable cyclic stability with capacitance retention of 95.6 % over 20 000 charge/discharge cycles employing 6 M KOH as electrolyte. Moreover, the HPC-based symmetric supercapacitor displays a high energy density of 30.5 Wh Kg-1 using 1.0 M Na2SO4 as electrolyte. Additionally, the HPCs-based anode for lithium-ion battery presents a high specific capacity of 541 mAh g-1 and still retains 266 mAh g-1 after 800 cycles (1 A g-1). The attractive electrochemical performances of HPCs indicate that this work provides new opportunities to convert wasted biomass into high valued carbon micro/nanomaterials for electrochemical energy storage and conversion.

KEYWORDS: Mixed biomass wastes; crab shells; rice husks; hierarchically porous carbons; electrochemical energy storage.


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INTRODUCTION Due to the fossil fuel exhaustion and environmental problems, there has been a desperately

need for developing efficient, green and outstanding performance energy storage systems and devices. Supercapacitors and lithium-ion batteries (LIBs), as the state-of-the-art electrochemical energy storage devices, have attracted growing attention for storage and conversion of sustainable energy sources, and worked as electrochemical power sources.1,2 However, the low energy density for supercapacitors, weak cycling stability and poor rate capability for LIBs seriously restrict their practical applications.3 Therefore, exploring the energy storage system combined high capacities with the remarkable rate capabilities has been a long-pursued goal. No matter what supercapacitors or LIBs, their overall electrochemical performances are strongly determined by the microstructure of electrode materials. Owing to the good electrical conductivity, cheapness and high stability, carbon-based materials are the dominant choice for energy storage applications.4-6 Compared with variety of carbon materials including carbon aerogel,7 graphene,8 carbide-derived carbons,9 and nanotubes,10 a cost-effective strategy for production of porous carbons with advantages of large specific surface area and appropriate porosity is urgent to be explored. More recently, tremendous efforts have been made to the utilization of single biomass and its wastes to fabricate porous carbons with conventional method (carbonization and chemical activation). On one hand, low-ash biomasses, such as Moringa oleifera branches,11 silk cocoon,12 tobacco,13 and willow catkin,14 derived porous carbons exhibiting high surface area mainly contributed by micropores, but the dominant microporosity always hinders electrolyte ion transport into the inner pores, leading to a great capacity fading at high current density.15-17 On the other hand, high-ash biomasses such as sludge sewage,18 shrimp shells,19,20 and cattle



bone,21,22 have also been employed as precursor to prepare hierarchically porous carbons (HPCs) with well-developed porosity. Although the self-template effect of the ash contents in the biomasses can greatly improve the porosity, the specific surface area of the as-resulted HPCs can hardly break through the bottleneck of 2200 m2 g-1.18-22 Moreover, HPCs always suffer from some drawbacks such as complicated preparing process, limited production of raw materials, and environment unfriendly and so on.23-25 The surface area and porosity play significant roles in determining the charge storage capacity and rate capability for electrochemical energy storage.26, 27

Therefore, it still exists a great challenge to fabricate high-surface-area porous carbons with

suitable porosity through a cost-effective and efficient route, for further promoting the electrochemical energy storage greatly. Although the electrochemical performance enhancement has been demonstrated in the porous carbon based electrode materials, the control over the porosity and surface area, and the relationship between the ash contents in biomass with the porosity of porous carbons for boosting the electrochemical energy storage have been rarely exploited. Herein, we report on a novel strategy for fabricating hierarchically porous carbons by employing mixed biomass wastes of crab shells (CSs) and rice husks (RHs) as raw materials. Different from the porous carbons synthesized with single biomass via the traditional method, the resultant HPCs exhibit unique three-dimensional (3D) interconnected architecture with large surface area, which can accelerate electrolyte ion transport and provide abundant energy storage sites. The as-prepared HPCs present improved electrochemical performance as electrode material compared with active carbons (ACs) derived from single biomass by traditional method. This new avenue is of great potential for the massive preparation of high-surface-area HPCs as advanced electrode materials for energy storage applications.


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

EXPERIMENTAL SECTION Synthesis of the HPCs. The crab shells (CSs) were collected from Haozou city (Anhui

province, China), and rice husks (RHs) were obtained from Maoming city (Guangdong province, China). In a typical process, the raw materials of CSs and RHs were dried with 105 oC for 6 h and then crushed by electric grinder to obtain power. For synthesis of HPCs, CSs powder (3.0 g) and RHs powder (1.0 g) were mixed with 40 ml of purified water, and then heated at 220 oC for 3 h in a teflon-sealed autoclave (100 ml). After naturally cooling to room temperature, the brown products were vacuum filtrated, washed with HCl (3.0 M) and deionized water, and then dried at 100 oC in an oven. After that, the above pretreated precursor was mixed with KOH with a KOH/precursor ratio of 3:1. The resultant mixture was then calcined in a tubular furnace (N2 atmosphere) at 700 oC for 3 h at a ramp rate of 5 oC min-1. Subsequently, the powder was washed by HCl (1.0 M) and deionized water for sevral times. The finnal products were dried in a vacuum oven at 100 oC for 10 h. The as-obtained samples were denoted as HPCs-x, in which x represents to the mass ration of CSs and RHs of x:1. To highlight our novel strategy in fabricating HPCs with well developed porosity and high surface area, conventional method (carbonization combined with KOH activation) was employed to prepare active carbons (ACs) from single biomass with low ash content (RHs) and high ash content (CSs) under similar conditions to HPCs-3, but without acid treatment before activation, and the products were denoted as RH-ACs and CS-ACs, respectively. All the water used in the whole synthesis process is deionized water. Material Characterization. The phase structure of the samples was first investigated by the X-ray powder diffraction (XRD, Rigaku Smart Lab X-ray diffractometer with Cu K radiation). Raman spectra were recorded on an inVia Reflex Raman spectrometer (5 mW，532.2 nm), and



the spot size is ca. 6 μm. Elemental analyses were carried out with X-ray photoelectron spectroscopy (Thermo scientific Escalab 250Xi), and the total element contents of C, H, O, N, and S were analyzed by EA2400 Ⅱ elemental analyzer. The thermal gravimetric analyses (TGA) were conducted with PerkinElmer TGA 4000. The field emission electron microscopy (FESEM, ZEISS Ultra55), transmission electron microscopy (TEM), and high-resolution TEM were employed to observe the morphologies. Multipoint nitrogen sorption isotherms were recorded on the ASAP 2020 Surface Analyzer. Before measuring, all samples were degassed at 350 oC for 8 h. Multi-point Bruner-Emmett-Teller (BET) method with relative pressure ranging from 0.01 to 0.2 was adopted to calculate the specific surface area, and non-local density functional theory (NLDFT) was used to determine the pore size distribution (PSD). Electrochemical measurements. In three-electrode system, active materials (HPCs, 80 wt%), conductive additives (acetylene black, 10 wt %), and binder (PTFE, 10 wt %) were homogeneously mixed in ethanol solution with ultrasound for 30 minutes. Then, the solution was dropped onto square Ni foam film (1.0 cm×1.0 cm, ~4 mg active materials loaded on each electrode) and dried in oven at 100 oC for 8 h. The used counter and reference electrode is Pt foil and Hg/HgO electrode respectively, and 6.0 M KOH solution is employed as electrolyte. For two electrode test, 1.0 M Na2SO4 were used as electrolyte to investigate the electrochemical performances. Both the three-electrode and two-electrode tests were performed on CHI 660E electrochemical workstation (Shanghai Chenhua, China). The electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 0.01 Hz to 100 kHz with an amplitude of 5 mV (Im6ex electrochemical workstation, Zahnex Co.). The float-voltage endurance of two-electrode cells was carried out on NEWARE test system. The symmetric devices were galvanostatically cycled three times with operation voltage of 0~1.8 V at 10 A g-1.


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After cycling step, the cells were charged to 1.8 V followed by holding the voltage for 10 h. The cells were then cycled again and this step was repeated until an overall holing period for several hundred hours. The specific capacitance was determined from cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves according to Equation 1 and 2: 𝑉

∫𝑉2𝐼(𝑉)𝑑𝑉 1

C = 𝑚𝑣(𝑉2

(1)

― 𝑉1)

where C (F g-1) refers to the specific capacitance, m (g) is the mass loading of active material on the electrode, 𝑣 (V s-1) is the potential scan rate, V1 and V2 represent the low and high potential limits in the CV tests respectively, and I(V) is corresponding to the instant current. 𝐼∆𝑡

𝐶 = 𝑚∆𝑉

(2)

where 𝐶 (F g-1), I (A), Δt (s), 𝑚 (g), and ∆𝑉 (V) refer to the specific capacitance, discharge current, discharge time, weight of active materials on the electrode, and potential window, respectively. For two electrode cell, the specific capacitance of single electrode was determined from the GCD curves by Equation 3: 𝐼∆𝑡

𝐶𝑠 = 2𝑚∆𝑉

(3)

where 𝐶𝑠 (F g-1) is the specific capacitance, 𝐼 (A) is the discharge current, ∆𝑡 (s) is the discharge time, m refers to the mass loading of active materials on each electrode, and ∆𝑉 (V) is the discharge voltage. The energy density (E, Wh Kg-1) and power density (P, W Kg-1) of the symmetrical supercapacitor were calculated by Equation 4 and 5 respectively: 𝐸=

0.5 ∗ 𝐶 ∗ (∆𝑉)2 3.6

(4)



𝑃=

3600 ∗ 𝐸 ∆𝑡

(5) 1

where 𝐶 (F g-1) is the capacitance of the whole cell and 𝐶 = 4 ∗ 𝐶𝑠, ∆𝑉 (V) is the discharge voltage range excluding the IR drop, and ∆𝑡 (s) is the discharge time. For the LIB tests, the electrochemical measurements were tested with a CR2025 coin-type cell. For preparing working electrodes, active materials (80 %), acetylene black (10 wt%), and PVDF (10 wt%) were homogeneously mixed and then coated on Cu foil current collector followed by drying at 80 °C for 6 h. The mass loading of the active material was about 1 mg. 1 M LiPF6 in a mixed solution of ethylene carbonate and diethyl carbonate with a volumetric ratio of 1:1 was employed as electrolyte, the separator and the counter were polyethene and Li foil, respectively. The electrochemical performance of the as-fabricated batteries were carried out on Neware battery test system with voltage ranging from 0.01 to 3 V. 

RESULTS AND DISCUSSION

Overall Process towards the Design and Preparation of HPCs. In the present work, two common and low-cost biomass wastes, rice husks (RHs) and crab shells (CSs), were employed as precursor to prepare three typical porous carbons with regulated porosity. In conventional process (Figure S1, Supporting Information), the ACs derived from low-ash-content biomass wastes (i.e., RH-ACs) always exhibit a high surface area with dominant microporosity,28-31 while the ACs derived from high-ash-content biomass wastes (i.e., CS-ACs) usually present a welldeveloped hierarchical porosity but a relatively low surface area.21,32,33 In our novel strategy (Figure 1), HPCs derived from the mixed biowastes of RHs and CSs present hierarchical porosity and high surface area (>3500 m2 g-1 ). As renewable biomass wastes widely spread in earth, RHs is a typical biomass with low ash content that mainly consists of nanoslica,34 and CSs


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is a typical high-ash-content biomass waste composed of highly mineralized chitin-protein nanofibers and bioceramic CaCO3.35 The analyses of the ash content reveal that rice husks present 14 wt % of SiO2 and 4 wt % of CaCO3, and crab shells exhibit 36 wt % of CaCO3 and 2 wt % of SiO2, which is consistent with XRF results of the Ca and Si content (Table S1).

Figure 1. Schematic diagram of the overall fabrication process for hierarchically porous carbons (HPCs) derived from mixed biowastes of CSs and RHs. To take full advantage of the inorganics in biomass wastes, a novel route was developed in our work to prepare porous carbons possessed typically hierarchical porosity and high surface area at the same time, where a novel strategy was introduced to rational design the porosity, as illustrated in Figure 1. The layered CSs (Figure S2a) and granular RHs (Figure S2b) were employed as precursor. Firstly, the CSs and RHs were mixed by hydrothermal carbonization, in which the inorganics (CaCO3, SiO2) can be dispersed in the mixture evenly (Figure S3a-c). Then, 9 Environment ACS Paragon Plus


the acid-soluble inorganic salts (CaCO3) in the brown precursor were removed by HCl soaking (Figure S3d-f). Finally, HPCs with well developed porosity were achieved by chemical activation of the pretreated precursor. TGA (Figure S4) results suggest the low ash contents in the as-prepared HPCs-3, RH-ACs, and CS-ACs, revealing the efficient removal of the inorganic contens in the biomass precursor. The total element contents of the as-prepared HPCs-3, CS-ACs, and RH-ACs is listed in Table S2, from which less ash contents can be seen in these samples, well consistent to the TGA results. Notably, the inorganics of CaCO3 and SiO2 act as a cruical role on forming the hierarchical porosity with high specific surface area. The acid-soluble salts (CaCO3) in the mixture are removed by HCl to greatly improve the porosity of mixture precursor (Figure S5). In addition, the remained acid-insoluble contents (SiO2) with uniform distribution can support the porous structure inherited from CaCO3, which is benefit to enhance the accessibility of activators to the inner surface of the precursor, thus increasing the contact area between the activator and precursor to improve activation efficiency. Structural and Morphological Characterization of the Resultant HPCs. As shown in Figure 2a, the as-prepared HPCs-3 presents unique 3D hierarchically porous texture. Magnified SEM image of slope face (Figure 2b) reveals the existence of layered structure inherited from CSs (Figure S2a), implying the template of CSs. From HRTEM image in Figure 2c, the carbon framework presents numerous worm-like micropores, indicating the feature of welldeveloped porosity in HPCs. From the enlarged HRTEM image (insert in Figure 2c), the interplanar spacing is ~0.39 nm, which is larger than that of (002) crystal plane for graphite (0.34 nm). One can clearly observe the 3D architecture with interconnected framework even with image magnified five times (Figure 2d and the inset). The 3D architectures are mainly composed of the stacked sheet with abundant pores, in which the opened macropores varying from 100 to


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300 nm in the resultant HPCs-3 are well interconnected. For comparison, the samples derived from single biomass (RH-ACs and CS-ACs) were prepared through conventional method. In sharp contrast, it is vividly found that the control samples exhibit a completely different morphology. The resultant RH-ACs display the large bulk morphology without large pores (Figure 2e), and plenty of white spots (the inset of Figure 2e) indicate large number of micropores, while the CS-ACs presents crumpled sheeted morphology (Figure 2f) and smooth surface indicated fewer micropores. Such a significant difference confirms that porosity engineering strategy plays a vital role in fabricating and controlling the porosity of HPCs.

Figure 2. Morphology observation of the resultant samples obtained by novel strategy and conventional method: (a-d) HPCs-3, (e) RH-ACs, (f) CS-ACs. Nitrogen adsorption/desorption isotherms were measured to confirm the porosity of the resultant HPCs-3. As shown in Figure 3a, the HPCs-3 and CS-ACs exhibit combined I/IV type sorption isotherm with H4 hysteresis loop, suggesting the presence of macropores and mesopores in the as-prepared HPCs-3 and CS-ACs. But RH-ACs show apparently different N2 adsorption



isotherm, appearing the saturation adsorption at high relative pressure (>0.3), indicating the micropores are dominate in the porosity, and no large mesopore and macropores exist in RHACs. The resultant HPCs-3 exhibits au high specific surface area of 3557 m2 g-1, which is not only much higher than that of RH-ACs (3032 m2 g-1) and CS-ACs (2109 m2 g-1), but also higher than those of porous carbons derived from the carbohydrates,36-40 and other single biowaste precursors.22,

29, 41-43

The carbon yield of the as-resultant HPCs-3 is as high as 17.1 %. The

removal of acid-soluble inorganics (i.e., CaCO3) in the precursor can result in large pores and the remained inorganics (i.e., SiO2) can support the pore structure, which is benefit to the activator of KOH getting into the inner of precursors and thus improve the activation efficiency.42, 44 The PSD curves, cumulative pore volume, and the detailed porosity parameters of the asprepared samples are shown in Figure 3b, S6, and Table S3, respectively. The proportion of different pore size volume is summarized in Figure 3c. The as-resulted HPCs-3 prepared from our strategy presents a wide pore size distribution with certain amount of micropores with diameter smaller than 2 nm (50 %, 1.01 cm3 g-1), abundant of small mesopores with size of 2-5 nm (40.6 %, 0.82 cm3 g-1), some large mesopores with size from 5 to 50 nm (7.9 %, 0.16 cm3 g-1) as well as some macropores with size larger than 50 nm (1.5 %, 0.03 cm3 g-1). As for the control samples obtained from single biomass by traditional method, the CS-ACs derived from high-ashcontent biomass waste possesses less micropore volume (58.3 %, 0.67 cm3 g-1) to result in low surface area and high pore volume of mesopore (40%, 0.46 cm3 g-1), which is contributed to the high ash contents impeded activator into the inner of precursor to decrease active efficiency and inorganic salts acted as template to expand the pore volume. Nevertheless, the RH-ACs synthesis from low-ash-content biomass presents high micropore volume (85 %, 1.06 cm3 g-1) for high surface area and low mesopore volume (13.5 %, 0.18 cm3 g-1), further conformed the porosity


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demerits of porous carbons derived from low-ash-biomass. Based on the above results, it can be safe to conclude that the novel strategy play key role in fabricating hierarchical porosity with high specific surface area for HPCs-3 by the incorporation of RHs and CSs, which also make the high surface area and large pore volume of HPCs-3, outstanding among the HPCs derived from single biomass precursors (Table S4).

Figure 3. (a) N2

adsorption/desorption isotherms, (b) PSD curves, (c) pore volume with

different range of pore size, and (d) XPS spectra of the as-prepared HPCs-3, RH-ACs, and CSACs. (e) N 1s and (f) C 1s spectra for HPCs-3. To further clarify the synergistic effects of the inorganics, the A-RH-ACs and A-CS-ACs prepared from single biomass (RHs or CSs) were treated with HCl solution before activation, and we also fabricate other two HPCs-x (x=1 or 4, x represent the weight ratio of mixture raw materials between CSs and RHs.) As shown Figure S7a, the samples of A-RH-ACs, HPCs-x, ACS-ACs represent strong N2 adsorption at low relative pressure (p/po

Mixed-Biomass Wastes Derived Hierarchically Porous Carbons for

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