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Starch-derived hierarchical porous carbon with controlled porosity for high performance supercapacitors Jinhui Cao, Chunyu Zhu, Yoshitaka Aoki, and Hiroki Habazaki ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04459 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Sustainable Chemistry & Engineering

Starch-derived hierarchical porous carbon with controlled porosity for high performance supercapacitors

Jinhui CAO†, Chunyu ZHU†, ‡, *, Yoshitaka AOKI †, ‡, Hiroki HABAZAKI †, ‡

†

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo,

Hokkaido 060-8628, Japan ‡

Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo,

Hokkaido 060-8628, Japan

*Corresponding author: [email protected]

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Abstract

The development of green and clean synthetic techniques to produce

carbon materials for energy storage and conversion applications has motivated researchers to use sustainable biomass. In this study, hierarchical porous carbon (HPC) with very high specific surface area and controlled porosity is synthesized by a novel and facile method, which employs an exothermic pyrolysis process of starch-magnesium nitrate raw materials with subsequent high-temperature thermal treatment and acid washing. The biomass starch acts as both a reductant and carbon source, while magnesium nitrate is an oxidant and provides MgO template as pore creator. The vigorous exothermic pyrolysis of starch-magnesium nitrate mixture introduces MgO@C precursor with a highly 3D porous network containing meso- and macropores. After removing MgO template, plenty of micro- and mesopores are further created. Experimental parameters including calcination temperature, starch-nitrate ratio, and magnesium salt species are comprehensively evaluated. The HPC shows a very large specific surface area up to about 2300 m2 g-1 and a hierarchical porous architecture composed of interconnected micro-, meso- and macropores. As an electrode material for supercapacitors, the HPC exhibits high specific capacitance (229 F g-1 at 1 A g-1 in a 6 M KOH electrolyte), good rate capability (211 F g-1 at even 10 A g-1) and outstanding cycling stability (94% capacitance retention after 10000 cycles at 2 A g-1). The superior electrochemical performance of the HPC stems from both high surface area and the hierarchical multi-porous structure, which provides an accessible pathway for electrolyte transport. These results demonstrate a very effective and low-cost method for scalable preparation of HPC using green biomass carbon source for supercapacitors, which also has potential applications such as adsorbent for water/gas treatment. Keywords: Supercapacitor, porous carbon, starch, MgO, template


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Introduction Electrical double layer capacitors (EDLCs), also known as supercapacitors, have drawn tremendous attentions for high power electrochemical energy devices owing to their advantages such as fast charge/discharge, high power density, long cycling stability, and environmental friendliness.1-4 So far, a variety of carbonaceous materials, including porous activated carbons (ACs),5-10 carbon nanofibers (CNFs),11-12 and graphene-based materials13-18, have been widely investigated as the most attractive EDLC electrode materials due to their high electrical conductivity, good chemical stability, and wide availability. Generally, the design and fabrication of porous carbon nanomaterials with high specific surface areas and superior porosity have been considered as a straightforward strategy to increase the electrical double-layer capacitance. A high specific surface area can provide more active sites for the charges to form electric double layers, while an optimal pore structure is crucial to the reversible adsorption/desorption of fast ion transport through the porous structure. Recently, hierarchical porous carbon (HPC), which contains interconnected micro-, meso-, and macropores, has been recognized as an effective design to improve their electrochemical performance.19-26 In such hierarchical structures, the abundant micro- and mesopores provide a high accessible surface area and play an essential role in high energy storage resulting in a large capacitance and high energy density, while the interconnected meso- and macropores facilitate the fast ion transport by supplying ion-buffering reservoirs and ion-transport pathways, which ensure the high rate capability and high power density. Several strategies have been used to prepare HPC successfully with enhanced electrochemical performance. For example, a melt vacuum infiltration method was used for the preparation of HPC with highly ordered straight micro-channels, which used



bundled continuous filament glass fibers array as template and glucose as carbon precursor; in the synthetic process, after the templated carbonization, HF etching and KOH activation were also used. 21 Pr. Ramakrishnan et al. reported the preparation of a three-dimensional hierarchical nitrogen-doped arch and hollow nanocarbons by a co-axial electrospinning approach and subsequent leaching and carbonization processes using polyacrylonitrile as carbon source and polyvinyl pyrrolidone as sacrificial template in N,N-dimethylformamide solvent. 27 In another example, the nitrogen and oxygen-doped ordered mesoporous carbons exhibiting hierarchical structure were synthesized by nanocasting, using SBA-15 silica as template and m-aminobenzoic acid or paminobenzoic acid as nitrogen, oxygen and carbon sources. 20 However, these approaches still suffer from some drawbacks such as complicated preparation processes, expensive raw materials and templates, the safety and environmental concerns because of the use of high toxic etching agents (e.g., HF etc.) to remove templates, and the difficulty in regulating the porosity, all of which severely restrain their large-scale production and application. Therefore, the continued efforts should be focused on developing simple, inexpensive and effective synthetic methods for preparing HPC with well-designed porosity by employing green, reproducible and renewable raw materials. Especially, the manufacturing of carbon materials from renewable biological carbon sources has been considered as an effective approach for the inexpensive large-scale fabrication of high performance supercapacitors, due to the abundance of recyclable biomass resources such as cellulose, starch, lignin, gelatin and so on.28-37 Under these circumstances, in this work, we present a novel, facile, sustainable and scalable synthesis strategy to fabricate HPC by using the highly abundant biomass starch as carbon source. As a new synthetic approach, a highly porous MgO@C precursor


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containing numerous meso- and macropores is firstly produced by an effective and vigorous exothermic pyrolysis process of starch-magnesium nitrate. In this process, starch acts as both a reductant and carbon source, while magnesium nitrate is an oxidant and provide MgO template as pore creator. Subsequently, after the high-temperature carbonization and acid washing to remove MgO template, a lot of micro- and mesopores are further created. In this manner, a hierarchical porous network consisting of interconnected micro-, meso- and macropores is formed, which also have a very high specific surface area. The influence of preparation parameters on the porous structure and electrochemical performance for supercapacitor are investigated, including the calcination temperature, starch-nitrate ratio, and magnesium salt species (a comparison to the use of magnesium acetate or magnesium oxide, which induces a conventional endothermic pyrolysis process). Based on this facile and effective synthesis process, this work may bring a feasible solution for large-scale production of high-performance carbon-based electrode materials for supercapacitors using abundant biomass resources in future.



Experimental section Preparation of HPC: In the experiment, first, magnesium nitrate hexahydrate (Mg(NO3)2・6H2O, 10 mmol, 2.564 g) and a series amount of commercial soluable starch (st10: 1.621 g, st08: 1.297 g, st06: 0.973 g and st05: 0.811 g) were dissolved in 6 ml hot distilled water to form homogenous mixed solutions. Here, the determination of the amount of starch is based on the following assumption. Starch has a molecular formula of (C6H10O5)n, which can be considered as n units of (C6H10O5) monomer. The n value of 1.0, 0.8, 0.6 and 0.5 were used in this study, named with st10, st08, st06, and st05, respectively. Subsequently, the hot solutions were evaporated on a hot plate with magnetic stirring to form dried gels. The gels were firstly heated to 500 oC at a heating rate of 10 oC min-1 under Ar flow to preliminarily decompose the nitrate and organic material. Subsequently, the pyrolyzed samples were pulverized using a mortar and pestle, and were further calcined at 800, 900 and 1000 oC for 2 h under Ar flow. Finally, the calcined samples were washed with HCl aqueous solution, filtered, washed with distilled water/ethanol for several times, and dried to obtain the final products. The samples were named based on n value, calcination temperature, and washing with HCl or not. After calcination, the MgO@C composites were denoted as st10-900C, st08-900C, st06-900C, st05-900C, st06-800C and st06-1000C, while after HCl washing, the samples were referred as st10-900C-HCl, st08-900C-HCl, st06-900C-HCl, st05-900C-HCl, st06-800CHCl and st06-1000C-HCl, respectively. As a comparison, magnesium acetrate tetrahydrate (Mg(CH3COO)2 ・4H2O, 10 mmol, 2.145 g) was also used to substitue magnesium nitrate, and the sample was prepared with a starch amount of 0.973 g (st06) with a calcination temperature of 900 oC, which were named as MgAC-st06-900 and MgAC-st06-900C-HCl. In addition, comparative samples were also prepared with raw


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materials of commercial mangeiusm oxide (MgO, 10 mmol, 0.403 g) and 0.973 g starch (st06) under a calcination temperature of 900 oC, which were referred as MgO10-st06900 and MgO10-st06-900C-HCl. Material characterization: The obtained samples were characterized by scanning electron microscopy (SEM, JEOL, JSM-7400F), transmission electron microscopy (TEM, 200 kV, JEM-2010F and JEM-2010), and X-ray diffraction (XRD, Rigaku Miniflex, CuKα) for their morphology and crystalline structure observation. Raman spectra of the carbon samples were acquired using a RENISHAW Raman spectrometer using an excitation wavelength of 532 nm. Surface functional groups and bonding characterization of the samples were confirmed by an X-ray photoelectron spectroscopy (XPS, JEOL, JPS9200) system using Mg-Kα X-ray source. A thermogravimeter (TG, STA 2500 Regulus) was used to determine the carbon ratio in the calcined MgO@C composites. The thermogravimeter combined with a differential thermal analyzer (TG-DTA) was also used to evaluate the pyrolysis behavior of the raw gel mixtures. The specific surface area, pore volume and pore size distribution of the samples were characterized by nitrogen adsorption using a BELSORP-mini surface area analyzer. Electrochemical measurement: For the electrochemical performance, both a twoelectrode cell and a three-electrode configuration were assembled, respectively. The working electrode consisted of the obtained HPC, acetylene black conductive carbon, and polytetrafluroethylene (PTFE)-coated teflonized acetylene black (TAB-2®) binder in a weight ratio of 80:10:10. The above electrode materials were mixed in ethanol to form a dough-like paste, which was pressed onto the nickel foam current collector in a disk shape with a diameter of 9 mm. The mass load is about 3.5 mg/cm2. The two-electrode symmetric cell was assembled in a Swagelok-type



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configuration.38-40 The symmetric supercapacitor was tested within a voltage range from 0.0 to 1.0 V in a 6 mol L-1 KOH aqueous electrolyte. The galvanostatic charge-discharge measurements were carried out in a battery tester (Hokudo Denko) at a constant temperature of 25 ºC. Cyclic voltammetry (CV) was

conducted on the

potentiostat/galvanostat electrochemistry workstation (Princeton) at increasing sweep rate from 5 to 100 mV s-1. Electrochemical impedance spectroscopy (EIS) were measured at a frequency range of 0.1 Hz to 100 kHz and a 10 mV AC amplitude. The specific capacitance based on two-electrode measurement was calculated based on the follow equations:

C=

2𝐼𝐼∆𝑡𝑡

𝑚𝑚∆𝑉𝑉

(1)

where C (F g-1) is the specific capacitance, I (A g-1) is current density, ∆t (s) is discharge time, m (g) is the mass of a single electrode, and ∆V (V) is the potential window during the discharge process (excluding the IR drop), respectively. For three-electrode test, Pt foil and Hg/HgO electrode were used as the counter and reference electrode, respectively. CV and galvanostatic charge-discharge measurements were conducted in the range of -1.0 to 0.0 V. The specific capacitance based on threeelectrode measurement was calculated based on the follow equations:

𝐼𝐼∆𝑡𝑡

C = 𝑚𝑚∆𝑉𝑉

(2)


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Results and discussion Exothermic pyrolysis and calcination to produce MgO@C composites Figure 1 shows the schematic diagram of the experimental procedures for producing HPC. After the gelatinization of magnesium nitrate and starch in hot water and the evaporation of water, a dried gel mixture was obtained (Step1). The dried gel was subsequently heated to 500 oC at a heating rate of 10 oC min-1 in a vertical tube reactor under Ar flow to preliminarily pyrolyze the gel raw material (Step2). It is noted that the pyrolysis of the nitrate-starch gel is a well-known solution combustion synthesis (SCS) process, which generates an exothermic and self-sustaining redox reaction with the emission of a large amount of gases in a short time. The SCS reaction condition was evaluated according to the calculation of equivalence ratio, φ, which is the ratio of the total valencies of fuels to nitrate oxidizers.41-45 Here, a ϕ value larger than 1 represents a fuel-rich condition in a conventional SCS process in air/oxygen atmosphere. The ϕ values were calculated to be 1.20, 1.44, 1.92 and 2.40 for starch amount of st05, st06, st08 and st10, respectively. In this study, the SCS was performed in Ar atmosphere, which is possible to remain the carbon residue in the fuel-rich combustion condition. The evaluation of the pyrolysis behavior of the nitrate-starch gel in comparison to the acetate-starch gel was performed by TG-DTA analysis and the monitoring of the temperature history of the reactions by inserting a K-type thermocouple inside the reactants. Figure 2 shows the TG-DTA curves (a, b) and the temperature history curves (c). From the TG-DTA analysis, it is confirmed that the pyrolysis behavior for the acetatestarch gel and nitrate-starch gel are quite different. For the decomposition of the acetatestarch gel, the weight loss below 150 oC is mainly due to the decompositon and evaporation of combined water and the remained uncombined water; with the continuing



heating, the main pyrolysis reaction starts from about 250 oC, which is an endothermic reaction and the complete decompostion is even not finished untill 500 oC. However, for the decompositon of the nitrate-starch gel, a slight weight loss below 100 oC occurs, which is due to the evaporation of water; it is noted that a quick and complete decompositon occurs in the tempeature range from around 130 to 180 oC; at around 140 o

C, there is an endothermic reaction corresponding to the decompositon of combined

water; quickly after removing water, a fast exothermic reaction starts at around 170 oC, and the complete decompositon process is finished untill 180 oC, after which the weight remain almost unchanged. The decompostion behavior was also monitored by measuring the temperature history of the reactants as shown in Figure 2 (c). The decompostion of nitrate-starch shows a steep temperature rise at around 160 oC, indicating the occurance of the fast exothermic reaction, which was also observed with the emission of a large amount of gases in a short time. However, for the pyrolysis of acetate-starch, a greatly delayed temperature increase occurs ranging from about 110 to 200 oC, indicating the decompositon of the acetate-starch mixture is endothermic which requires a lot of energy input. After the SCS process, we obtained puffy and foamy precursors from nitrate-starch gels, while a dense and compact bulky material was collected from acetate-starch raw material. After a rough pulverization using a mortar and pestle, the precursors were subjected to the calcination step at 800, 900 or 1000 oC for 2 h under Ar flow (Step3) to get the MgO@C composites. From the XRD measurement as shown in Figure 3 (a, b), all samples after calcination can be indexed to a MgO phase (JCPDS No: 00-004-0829) with good crystallinity. It is noted that the peaks belonging to carbon is not obviously observed (for example at ~25 o) for the composite materials due to the low crystallinity


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feature of obtained carbon. SEM was used for the morpholgy and microstructure observation of the MgO@C composites, as shown in Figure S1 (supporting information). The nitrate-starch derived composites show the tpyical porous powders of a SCS product. The composites are composed of irregular particles with size of several tens of nanometers to several microns. Many pores in the range of several nanometers to around 100-200 nanometers are observed in these particles. Quite differently, the acetate-starch derived composite (MgAC-st06-900C) shows large pulverized bulky particles of several microns to several hundreds of microns. Importantly, a porous feature is not observed as compared to the nitrate-starch derived samples. These numerous pores for the nitrate-starch derived samples were formed during the SCS process with the emission of a large amount of gases in a short time of the self-sustainingly exothermic reaction, which is the feature of such a process. The highly porous feature of the composites was further analyzed by nitrogen adsorption measurement in terms of specific surface area, pore volume and pore size distribution. Total pore volumes were obtained at P/P0 = 0.99 and 0.95, corresponding to the pores with diameters up to around 200 and 40 nm, respectively. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. The pore size distributions were evaluated by both Barrett-Joyner-Halenda (BJH) and non-local density functional theory (NLDFT) methods. Figure 4 shows the nitrogen adsorption isotherms (a, b), BJH pore size distribution and NLDFT pore size distribution of the obtained composites. All samples indicate the typical adsorption isotherm with hysteresis loop. The nitrogen absorption at relative pressure less than 0.1 is limited, indicating the shortage of micropores (< 2 nm) for the composite materials. The large hysteresis loops between the



adsorption and desorption branches at ~0.5–0.9 P/P0 indicate the existence of mesopores (2 nm~50 nm). The steep adsorption at relative pressure of ~0.9–1.0 demonstrate the presence of macropores (> 50 nm). It is note that hysteresis loop for sample MgAC-st06900C is not obvious compared with the nitrate-starch derived samples, and the steep absorption at relative pressure of ~0.9–1.0 is almost neglectable, indicating that this sample possesses limited mesopores and is absence of macropores. The BJH pore size distribution as shown in Figure 4 (c, d) confirms that MgAC-st06-900C possesses limited mesopores with size less than around 20 nm, while the nitrate-starch derived composites have wide pore size distributions from mesopores to macropores. For the nitrate-starch derived composites, the samples obtained with different starch amount indicate increased pore size from st10, st08 to st06 and slightly decreased pore size for st06 to st05. The pore size distribution remains almost the same for the samples obtained at different calcination temperature with the same starch amount of st06. The NLDFT pore size distribution as shown in Figure 4 (e, f) further confirms the above conclusions. The summary of the BET specific surface area and pore volume of the composites is shown in Table 1. The above results indicate that numerous meso- and macropores were formed in the SCS process for the nitrate-starch samples. The composites of MgO@C were obtained after calcination, and with an additional step of removing MgO-template by HCl washing (Step4), plenty of micro- and mesopores can be further created. In such a manner, the highly porous carbon with designed hierarchical pores can be produced. The carbon content in the MgO@C composites was measured by their oxygencombustion in a TG analyzer under oxygen flow, as shown in Figure 5. The carbon percentages were determined to be 15.19%, 23.89%, 34.39%, 46.98%, 26.00%, 20.41% and 39.01% for samples st05-900C, st06-900C, st08-900C, st10-900C, st06-800C, st06-


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1000C, and MgAC-st06-900C, respectively. Based on these ratios, the theoretical yields were calculated to be 0.089, 0.130, 0.163, 0.220, 0.146, 0.106, and 0.265 g-carbon/gstarch for these samples, correspondingly.

Characterization of HPCs HPCs were obtained by acid washing of the MgO@C composites to remove MgOtemplate. Figure 6 (a, b) presents the XRD patterns of the final HPC products. All samples indicate the typical lowly graphitized amorphous carbon, with greatly broadened peaks at approximately 25 ° for each sample, corresponding to the (002) plane of graphitic carbon. Raman spectroscopy was further used to characterize the C-C bonds of the carbon materials. As presented in Figures 6 (c) and S2, two peaks appeared at approximately 1350 and 1590 cm-1, referring to the typical D-band and G-band, respectively. The Dband of carbon is associated with a defective or disordered carbon structure, while Gband refers to the ordered sp2 bonded graphitic carbon. Generally, the intensity ratio of D-band to G-band (ID/IG) represents the disorder degree of carbon materials. The ID/IG ratios of the obtained HPC were determined to be larger than 1, representing their great disordering (amorphous) feature. The functional groups and bonding characteristics of the samples were further evaluated using XPS. Figure 6 (d) shows the typical survey spectra of the obtained HPC, which displays two peaks at 285.1 and 534.2 eV, corresponding to C 1s and O 1s, respectively. Figure 7 shows the SEM micrographs of the obtained HPC products in comparison with the typical MgO@C composite precursor. All HPC samples, excluding st05-900CHCl, present similar porous morphology as compared with their MgO@C precursors (see



Figure S1). Although plenty of micro- and mesopores were expected to be created after MgO removal, it is difficult to be observed by SEM, and the pore structure changes will be carefully discussed by nitrogen adsorption analysis. The morphology for sample st05900C-HCl is quite different to its MgO@C precursor. The MgO@C precursor (st05-900C) is highly porous with plenty of large meso- and macropores as can be observed by SEM, however, after MgO removal, the st05-900C-HCl sample shows the agglomerate of fine carbon powders. This indicates that at this condition (st05) excess amount of MgO template was introduced, therefore, after acid washing, the original meso-macroporous structure collapsed, which is further confirmed by following nitrogen adsorption analysis. The morphology of sample MgAC-st06-900C-HCl is also quite different to the nitratederived samples, in which a highly meso-macroporous structure is not observed for the acetate-derived sample because of the weak and slowly proceeded endothermic reaction. The porous feature of the HPC products was carefully characterized by nitrogen adsorption measurement. Figure 8 shows the adsorption isotherms and pore size distributions. The isotherms illustrate the hysteresis type loops with strong adsorption at relative pressure < 0.1 and ~0.9–1.0, which indicates the coexisting of micro-, meso- and macropores. It is noted that the strong adsorption at relative pressure < 0.1 was not observed for the isotherms of MgO@C precursors (see Figure 4), and the isotherms for the samples after acid washing also present increased hysteresis loops, indicating the creation of micro- and mesopores by removing MgO template. Here, the strong nitrogen adsorption at relative pressure less than 0.1 is caused by the presence of micropores; the obvious hysteresis loop between the adsorption and desorption branches at ~0.5–0.9 P/P0 indicates the existence of mesopores; the steep adsorption at relative pressure of ~0.9–1.0 demonstrates the presence of macropores. It is also noted that samples MgAC-st06-900C-


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HCl and st05-900C-HCl present limited adsorption at relative pressure of ~0.9–1.0, representing the absence of large macropores for these two samples. Sample MgAC-st06900C-HCl does not include large macropores since that such large pores were not formed in the pyrolysis step by using magnesium acetate as MgO precursor. However, although many macropores were presented in the MgO@C composite for st05-900C (see Figure 4), after removing MgO, the original meso-macroporous structure collapsed due to the introduction of excess amount of MgO template, which was also confirmed by previous SEM observation. The specific surface areas, including BET specific surface area (aBET), total specific surface area (atotal), external specific surface area (aex) and the micro specific surface area (amicro) of the samples were calculated by both BET and t method. The total pore volumes were obtained at adsorption pressure of P/P0=0.99 and 0.95, corresponding to pore diameters up to around 200 and 40 nm, respectively. The micropore volumes, Vmicro, were evaluated by t method, and the mesopore volume, Vmeso, were determined by V0.95-Vmicro. These results are summarized in Table 2. The pore size distributions of the samples were evaluated by both BJH and NLDFT method as shown in Figure 8 (c, d) and (e, f), respectively. Following are the discussion about these changes with the relationship with calcination temperature, starch amount, and magnesium precursor species. With the increase of calcination temperature for the samples obtained at the same starch amount st06, the samples show increased specific surface areas (both aBET and atotal) and increased pore volumes. The pore size distribution for these samples also shift to larger pore size with the increase of calcination temperature. This is caused by the crystal growth of MgO template at higher calcination temperature. It is noted that the increase of specific surface area and pore volume with calcination temperature are mainly derived



from their growth in aex and Vmeso, respectively, since the corresponding mesopores are mainly created by removing MgO template. It is also noted that the micropore volume decreases with increasing calcination temperature. For the samples obtained with different starch amount at the same calcination temperature, with the increase of starch amount from st06 to st10, the samples show decreased specific surface areas and pore volumes, which is caused by the decreased ratio of MgO template. Sample st05-900C-HCl shows decreased specific surface areas and pore volumes as-compared with sample st06-900C-HCl, especially for the external surface area and mesopore volume, which is caused by the collapse of the original mesomacroporous structure due to the introduction of too much MgO template. Sample MgAC-900C-HCl also shows low ratios of aex and Vmeso, since the highly mesomacropore structure was not formed in its endothermic pyrolysis step. As a comparison, the commercial MgO templated sample (MgO10-st06-900C-HCl) was also prepared, as shown in Figure S3 and S4. Even after a long-time acid washing at elevated temperature, it is difficult to completely remove MgO template as confirmed by XRD measurement in Figure S3-(a) (even that the repeated two time of acid washing was applied for this sample). This is quite different to the nitrate or acetate-derived samples. The nitrate and acetate-derived MgO template can be extremely easily removed with acid washing. This is properly caused by the bad surface wetting of the commercial MgOtemplated carbon and that some of the MgO particles are completely covered by carbon. However, the nitrate and acetate-derived carbons are hydrophilic with open pore structures, which is easy for the infiltration of solutions. From the nitrogen adsorption measurement results in Figure S3-(b, c, d), it is confirmed that both sample MgO10-st06900C and MgO10-st06-900C-HCl show very low specific surface area and pore volumes.


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The SEM images in Figure S4 indicate that sample MgO10-st06-900C are micron-sized aggolmerate containing MgO particles of several hundreds of nanometers. After acid washing, the exposed MgO particles are removed, creating many macropores of several hundreds of nanometers. Therefore, it is difficult to produce porous carbon with very high specific surface area and controlled pore size using commerical MgO particles. Several reseraches have also employed the MgO-templated method to produce porous carbon using MgO nanoparticles as pore creators, however, the reported specific surface area is greatly lower than our samples.46-47 In conclusion, by controlling the starch-magnesium nitrate ratio and calcination temperature, it is possible to control the state of MgO template as-dispersed in the carbon matrix, so as to control the micro-mesoporous characteristics of the final HPC products. Furthermore, the use of starch-nitrate exothermic SCS reaction also plays an important role in creating the meso-macroporous structure. By combining the use of SCS reaction and MgO-template, the HPC product is designed with well-controlled hierarchical porous structure. Such a hierarchical multi-porous architecture is highly desirable for energy storage and conversion because it allows fast ion diffusion by shortening the diffusion pathways, where macroporous frameworks may be used as ion-buffering reservoirs, mesoporous walls as ion highways for fast ion transmission and microporous textures for charge accommodation.19, 21, 48 Figure 9 shows the typical TEM observation of the HPC product, including samples st05-900C-HCl, st06-900C-HCl, st10-900C-HCl and MgAC-st06-900C-HCl. The highly hierarchical porous structure, with a continuous interconnected 3D porous network and pores ranging from micro, meso to macro sizes, is confirmed for samples st06-900C-HCl and st10-900C-HCl. Similar to the nitrogen adsorption analysis, the large macropores are



not obviously observed for samples st05-900C-HCl and MgAC-st06-900C-HCl. At a larger magnification (b, e, h, k), all samples present a lot of pores in the size range of mesopores. In addition, the high resolution TEM (HRTEM) images (c, f, i, l) illustrate that the porous carbon is highly disordered with large quantities of slit-type micropores and channels as-distributed in the carbon matrix.

Electrochemical performance The electrochemical properties of HPCs were evaluated in both two-electrode and three-electrode systems for CV and galvanostatic charge/discharge measurements in a 6 M KOH electrolyte. Figure 10 shows the results in the two-electrode measurement. The CV curves, as shown in Figures 10 (a) and S5, present symmetrical rectangular shapes from 5 to 100 mV s-1, indicating the ideal double-layer capacitance behavior for the obtained HPC samples. The galvanostatic charge/discharge curves at varying current densities from 1 to 10 A g-1 are shown in Figures 10 (b) and S6. These charge/discharge curves show the general symmetric triangular shapes, illustrating the typical supercapacitive behavior of the HPCs with excellent capacitive reversibility, which is in well agreement with CV measurement. The corresponding gravimetric capacitance at varying current densities for these samples are plotted in Figure 10 (c, d). It is concluded that sample st06-900C-HCl has the highest specific capacitance with a value of 229 F g-1 at 1 A g-1, which can still retain 211 F g-1 even at 10 A g-1. The capacitance changes of the HPC products are mainly related to their specific surface areas and pore structures. For the samples prepared under different calcination temperature from 800 to 1000 oC (Figure 10 (c)), the samples show increased total specific surface area as increasing the calcination temperature (see Table 2). However, it is noted that from 800 to 900 oC, the


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increase of specific surface area is mainly derived from external mesopores due to the crystal growth of MgO template; the micropore specific surface area even decreases when the temperature is increased to 1000 oC, although the external mesopore surface area increases greatly. The decreased micropore specific surface area at 1000 oC account for its decreased specific capacitance as compared to the sample obtained at 900 oC. For the HPC samples obtained with different starch amount or magnesium salt (Figure 10 (d)), a similar relationship between the specific capacitance and the specific surface areas and pore structures are confirmed. In general, because of the high specific surface areas both for micropores and external mesopores, sample st06-900C-HCl presents the optimized structure with the highest specific capacitance. According to the long cycling test to 10,000 cycles at 2 A g-1 (Figure 10 (e, f)), all electrodes, expect for sample st06-800CHCl, show good capacitance retention; for example, a high capacity retention of 94% is obtained for sample st06-900C-HCl. The greatly decreased capacitance for sample st06800C-HCl upon cycling is due to its structure instability with a low-temperature calcination. EIS is further conducted to evaluate the electrochemical behavior of the HPC electrodes. Figure 11 (a-c) show the Nyquist plots in the two-electrode cells. Figure 11 (d) presents the equivalent circuit model used for analysis. The equivalent circuit model shows that the capacitor circuit is constituted by Rs, W, Rct, C and Q.49 Here, Rs is the internal resistance, Rct is charge transfer resistance, W is the Warburg resistance, while C and Q are capacitor layer formed in the charge-discharge process. Generally, the high frequency intersections of the real axis refer to the internal resistance (Rs), which is mainly determined by the intrinsic ionic resistance of the electrolyte, electrical resistance of the electrode, and interfacial resistance between the electrode and electrolyte. All of



the samples show small internal resistances of around 1 Ω, indicating their superior conducting characteristic to be used as excellent electrode materials for high power supercapacitors. The internal resistance of sample st06-800C-HCl (1.38 Ω) is higher than the samples obtained at higher temperatures, which is because of its low graphitization property as-calcined at a lower temperature. In addition, the semicircles in the high frequency region are indicative of the electrode/electrolyte charge transfer resistance (Rs), while the linear curves in the low frequency region represent the capacitive performance. By comparing the diameters of the semicircles, it is obvious that the highly porous HPC electrodes, especially for samples st06-900C-HCl and st06-1000C-HCl, have very low charge transfer resistances, representing a fast mass/charge exchange rate in the interface of active material. It is observed that for the samples (st06 with calcination temperature of 800, 900 and 1000 ºC) with different calcination temperatures, sample st06-800CHCl has the highest charge transfer resistances due to its lower specific surface area, lower pore volume and lower graphitization degree. For the HPC samples obtained with increased starch amount from st06 to st10, the electrodes show increased charge transfer resistance due to the decreased specific surface area and pore volume. As for samples st05-900C-HCl and MgAC-st06-900C-HCl which are absence of large pores with diameter larger than about 20 nm, the electrodes also show increased charge transfer resistance, representing the importance of the introduction of multipores from micro-, meso- and macro-size. The highly porous HPC electrodes, including st06-900C-HCl, st06-1000C-HCl, st08-900C-HCl, st10-900C-HCl and st05-900C-HCl, show almost vertical curve in the low frequency region, indicating nearly ideal capacitor behavior and low diffusion resistance of electrolyte ions in the electrode material. Sample st06-800CHCl presents higher diffusion resistance due to its lower pore volume and graphitization


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degree. Sample MgAC-st06-800C-HCl also shows a high diffusion resistance because of its large particle size, non-continuous pores and low specific surface area. The impedance data are summarized in Table S1. Three-electrode measurements were also carried out to further confirm the electrochemical performance in terms of galvanostatic charge/discharge and CV, as summarized in Figure S7. The typical galvanostatic charge/discharge curves at varying current densities illustrate the general symmetric triangular shapes, indicating the typical supercapacitive behavior of the HPCs with excellent capacitive reversibility, which is in well agreement with the two-electrode measurement. The corresponding gravimetric capacitance at varying current densities as-obtained from three-electrode tests are slighter higher than that calculated from two-electrode measurement, which is normally found in three-electrode measurement due to the possible pseudocapacitances.50-51 This is further confirmed by CV measurement, presenting less symmetrical rectangular shapes ascompared with the two-electrode tests. Generally, the capacitances of these samples show the same tendency based on different preparation parameters with the measurement in two-electrode cells. The high capacitance and superior rate capacity performance gained for the HPC samples, especially for sample st06-900C-HCl, are attributed to the unique hierarchical pore structure and high specific surface areas. The very high specific surface area of HPCs provides a large amount of active sites for charge storage. The hierarchical pore structure, especially for the sample with optimized proportion of micropores and mesopores, not only guarantee the high accessibility of electron/ion transport but also offer a path for fast ion transport into interior micropores with a continuous increase of charge accommodation. Here, the micropores enlarge the specific surface area and contribute to



EDL formation, whereas mesopores act as electrolyte tanks that ensure ions are supplied at high sweep rates and provide highly accessible channels for electrolyte access and ion transport. Moreover, the open-structured and interconnected macropores not only function as buffer reservoirs for electrolyte ions but also facilitate the fast collection and transport of electrons in the charge-discharge process. All the aforementioned features of the HPC play cooperative effects on enhancing the electrochemical performance for supercapacitors, which is fabricated by a low-cost, scalable, and environmentally friendly method using biomass carbon resource.


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Conclusions In this work, we demonstrated a novel and facile method to produce HPC with very high specific surface area and hierarchical multi-pores using biomass starch as carbon source. An effective and vigorous exothermic pyrolysis process of starchmagnesium nitrate was employed to produce highly porous MgO@C precursor containing numerous meso- and macropores. In this process, starch was used as both a reductant and carbon source, while magnesium nitrate was an oxidant and provided MgO template as pore creator. After the subsequent high-temperature calcination and acid washing, a lot of micro- and mesopores were further created. The HPC products showed a very large specific surface area up to approximately 2300 m2 g-1 and a hierarchical porous network composed of interconnected micro-, meso- and macropores. Because of the high surface area and hierarchical multi-porous structure, which provided an accessible pathway for electrolyte transport, the HPC product showed superior electrochemical performance as the electrode for supercapacitor. The HPC exhibited high specific capacitance (229 F g-1 at a current density of 1 A g-1 in a 6 M KOH electrolyte), good rate capability (211 F g-1 at a high current density of 10 A g-1) and outstanding cycling stability (94% of the initial capacitance was retained after 10000 cycles at 2 A g1

). The influence of preparation parameters on the porous structure and electrochemical

performance was carefully investigated, including calcination temperature, starch-nitrate ratio, and magnesium salt species (a comparison with magnesium acetate and magnesium oxide). The current work has developed a very effective and low-cost method for preparing HPC with controllable hierarchical porous structure, which is not only usable in supercapacitor, but also has potential applications such as adsorbent for water/gas treatment.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1. SEM images of the MgO@C composites obtained after calcination. Figure S2. Raman spectra of the obtained HPC products. Figure S3. XRD and nitrogen adsorption analysis results of the commercial MgO templated sample. Figure S4. SEM observation of the commercial MgO templated sample. Figure S5. CV curves of the HPC electrodes at different scan rates. Figure S6. Charge/discharge curves of the HPC electrodes at varying current densities. Figure S7. Electrochemical measurement results of the HPC electrodes in three-electrode cells. Table S1. Impedance data of the symmetric cells.

AUTHOR INFORMATION * Corresponding Author: Tel.: 81-11-706-6736. Fax: 81-11-706-6736. E-mail: [email protected]


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Table 1. Porous structural properties of the MgO/C composites obtained after calcination. Samples

aBET [m2 g-1]

Samples V0.99

V0.95

[cm3 g-1]

[cm3 g-1]

BET SSA

V0.95

[m2 g-1]

V0.99

[cm3 g-

[cm3 g-1]

1

]

st05-900C

179

0.593

0.306

st06-800C

218

0.668

0.354

st06-900C

228

0.669

0.371

st06-900C

228

0.669

0.371

st08-900C

210

0.637

0.336

st06-1000C

242

0.658

0.374

263

0.217

0.208

MgACst10-900C

184

0.45

0.239

900C

aBET: specific surface area as-calculated from the adsorption data by BET method; V0.99: total pore volume at P/P0=0.99, corresponding to the pores with diameters up to around 200 nm; V0.95: total pore volume at P/P0=0.95, corresponding to the pores with diameters up to around 40 nm.

Table 2. Porous structural properties of the final HPC products. Samples

amicro =

Vmeso =

aBET

atotal

aex

atotal-aex

V0.99

V0.95

Vmicro

V0.95-Vmicro

[m2 g-1]

[m2 g-1]

[m2 g-1]

[m2 g-1]

[cm3 g-1]

[cm3 g-1]

[cm3 g-1]

[cm3 g-1]

MgAC-900C-HCl

1150

1220

64

1156

1.103

1.08

0.973

0.107

st05-900C-HCl

1521

1492

66

1426

1.055

0.995

0.885

0.11

st06-900C-HCl

2200

1980

702

1278

3.417

2.45

1.163

1.287

st08-900C-HCl

1681

1651

729

922

2.889

1.866

0.539

1.327

st10-900C-HCl

1348

1365

408

957

1.625

1.119

0.417

0.702

st06-800C-HCl

1845

1720

425

1295

2.039

1.652

0.917

0.735

st06-900C-HCl

2200

1980

702

1278

3.417

2.45

1.163

1.287

st06-1000C-HCl

2311

2147

1057

1090

4.81

3.102

1.169

1.933

aBET: specific surface area as-calculated from the adsorption data by BET method; V0.99: total pore volume at P/P0=0.99, corresponding to the pores with diameters up to around 200 nm; V0.95: total pore volume at P/P0=0.95, corresponding to the pores with diameters up to around 40 nm; atotal: the total surface area as-determined by t method; aex: the external surface area as-determined by t method; amicro: the micropore surface area as-determined by atotal-aex; Vmicro: the micropore volume as-analyzed by t method; Vmeso: the mesopore volume as-determined by V0.95-Vmicro (here, V0.95 is used to avoid the over-estimation of mesopore volume).


Page 31 of 39

Exothermic pyrolysis

Heat treatment Acid washing

Exothermic reaction Ar atmosphere

Gelatinization in hot water Gel Step 1 mixture Step 2 Evaporation of water by heating

Step 3

Leach with acid

Mesopore Micropore

HPC product

Schematic diagram for the formation process of HPC.

80

100

Exothermic reaction

DTA 6 5

80

3

60

2

40

1

4

60

3 2

40

1

20

0

20

0

-1 50 100 150 200 250 300 350 400 450 500 Temperature [°C]

0

(c)

400 350

Temperature [°C]

(b) TG

Heat flow [uV/mg]

DTA 4

Weight [%]

Mg(CH3COO)2-starch-st06

Figure 2.

Step 4

MgO@C precursor

Heat flow [uV/mg]

TG 100

Carbon

Pyrolysis Heat treatment (

Starch-derived hierarchical porous carbon with ... - ACS Publications

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