Biomass-Based Mechanically Strong and Electrically Conductive

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Biomass-based Mechanically-strong and Electrically-conductive Polymer Aerogels and Their Application for Supercapacitors Hai-Bo Zhao, Lei Yuan, Zhi-Bing Fu, Chao-Yang Wang, Xi Yang, Jiayi Zhu, Jing Qu, Hong-Bing Chen, and David A. Schiraldi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00510 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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ACS Applied Materials & Interfaces

Biomass-based Mechanically-strong and Electrically-conductive Polymer Aerogels and Their Application for Supercapacitors

Hai-Bo Zhao1, Lei Yuan1, Zhi-Bing Fu1, Chao-Yang Wang1, Xi Yang1, Jia-Yi Zhu1, Jing Qu Hong-Bing Chen2* and David A. Schiraldi3* 1. Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang (Sichuan), 621000, China 2. Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang (Sichuan), 621000, China 3. Department of Macromolecular Science & Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States

Abstract: A novel biomass-based mechanically-strong and electrically-conductive polymer aerogel was fabricated from aniline and biodegradable pectin. The strong hydrogen bonding interactions between polyaniline (PANI) and pectin resulted in a defined structure and enhanced properties of the aerogel. All the resultant aerogels exhibited self-surppoted 3D nanoporous network structures with high surface areas (207-331m2/g) and hierarchical pores. The results from electrical conductivity measurements and compressive tests revealed that these aerogels also had favorable conductivities (0.002-0.1 S/m) and good compressive modulus (1.2-1.4 MPa). The aerogel further used as electrode for supercapacitors showed enhanced capacitive performance (184 F/g at 0.5 A/g). Over 74 % of the initial capacitance was maintained after repeating 1000 cycles of the cylic voltammetry test, while the

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capacitance retention of PANI was only 57%. The improved electrochemical performance may be attributed to the combinative properties of good electrical conductivity, BET surface areas, and stable nanoporous structure of the aerogel. Thus this aerogel shows great potential as electrode materials for supercapacitors.

* To whom correspondence should be addressed: [email protected] (Hai-Bo Zhao), [email protected] (Hong-Bing Chen).

Keywords: biomass, aerogel, conductivity, mechanical properties, capacitor

INTRODUCTION Conductive aerogels are highly porous solid nanomaterials with electrically conducting features which integrate characteristics of aerogels (such as large pore volumes, extremely high surface areas, low densities) and conductive materials into a single material system.1-5 As such, conductive aerogels have a wide range of potential applications in the fields of batteries, fuel cells, supercapacitors, and chemical sensors.6-10 Over the past several years, substantial progress has been made in designing conductive aerogels with diverse materials, such as metal, metal oxides, carbon nanotubles, graphene, carbon materials, and conductive polymers.11-16 Among these materials, conductive polymers may offer some benefits for commercial application of conductive aerogels because of their low cost and easy synthesis, as well as uique electronic, electrochemical, and optical properties. Zhang et al. reported the first conductive polymer aerogel based on poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate).17,18 These conductive polymer aerogels showed enhanced electrochemical capacitance and superb

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adsorption ability to heavy metal ions. The reports of other conductive polymer aerogels are still rare, however, due in large part to the lack of suitable sol-gel chemistry of the conductive polymers. Polyaniline (PANI) is a notable conductive polymer that has been widely studied for electronic and optical applications.19-21 Especially in the electrochemical fields, PANI has a great deal of attention, due to its useful redox properties, tunable morphogy, low cost and better environmental stabilty as compared to other conductive polymers.22 One of the major shortcomings of PANI is its poor solution processability, thus leading to the difficult construction of PANI-based polymer aerogels via sol-gel processes. Porous materials containing PANI were recently prepared by growing PANI on the surface of aerogels, but no strong physical or chemical interactions existed between PANI coating and aerogels,23-24 resulting in significant aggregation of PANI particles, thereby degrading material stability and nanostructure. The challenge, therefore, is to fabricate PANI-based conductive polymer aerogels with a stable nanoporous structure, perhaps by incorporation of chemical crosslinks. The use of inexpensive, sustainable and scalable fabrication processes would also be highly desirable. Interest in bio-based materials has been increasing in recent years.25-27 Pectin as one of the natural polysaccharides considered to be biocompatible, nontoxic, abundant, biodegradable and mechanically strong.28 Pectin-based aerogels have gained renewed interest due to potential applications in the fields of thermal superinsulation and drug delivery vehicles during the past decade 29-31. It is also a good candidate for robust 3D architecture, as the structural component of cell wall of green plants. Thus, aerogels based on the cross-linking network between pectin and PANI could

potentially exhibit

useful

properties

(such

biodegradability, stability) and be inexpensive.

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as

electrical

conductivity,


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In this manuscript, we present a novel biomass-based mechanically-strong and electricallyconductive pectin-polyaniline aerogel fabricated by a supercritical CO2 drying of the hydrogel precursor. The structure and properties of obtained aerogels were systematically investigated in this paper.

EXPERIMENTAL SECTION 1.1 Materials. Aniline (An, chemically pure) and ammonium persulfate (APS, chemically pure) were manufactured by Chengdu Chemical Industries Co. (Chengdu, China) and were used as received. Pectin with low methoxyl group concentrations (LEPC, degree of esterification: 33%) was provide Yantai Andre Pectin Co. Ltd. (Yantai, China). Other chemicals were of analytical grade reagents.

1.2 Preparation of LEPC/PANI aerogel. To produce an aerogel containing 4 wt% LEPC and 4 wt % PANI (this will be referred to as LEPC4/PANI4 aerogel), for example, 4 g LEPC was first added to 96 mL deionized water with stirring for 5 h to obtain a transparent solution. 4g An was then added into above solution, stirring for an additional 4 h. Subsequently, 10 g oxidant APS was added into the mixture with vigorous stirring. After standing for few minutes, a stable hydrogel comprising 4 wt% LEPC and 4 wt% PANI was formed, and it was kept at 4° С for 24 h, waiting for the complete polymerization of An. The hydrogels were immersed into water for 7 days to remove impurities and oligomers; the water was renewed every 24 h. The resulting hydrogels were exchanged with

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ethanol for 7 days; the ethanol was changed every 24 h. The alcogel intermediates were then dried in supercritical CO2 to remove the ethanol to obtain LEPC4/PANI4 aerogels. Pure PANI powders were also prepared using a similar method. 4 g An was added to 96 mL deionized water with stirring for 4 h to obtain a transparent solution. Then, 10 g oxidant APS was added into the solution with vigorous stirring at 4° С. The precipitate PANI was formed immediately, and it was maintained at 4° С for 24 h. The precipitate was filtrated and washed by water for many times. IR (KBr): 1566, 1494 (s, Phenyl C-C), 1300 (s, C-N), 1246 (w, C=N), 1117 (s, C-H).

1.3 Characterization. The densities of LEPC/PANI aerogels were calculated from the mass and dimension using an analytical balance and digital caliper. X-ray diffraction (XRD) were performed with a PANalytical X′Pert Pro X-ray diffractmometer with nickel-filtered Cu Kα radiation as the X-ray source. Fourier transform infraed (FTIR) spectra were carried out on a Nicolet 6700 spectrometer. The morphological structure of the samples were characterized using a scanning electron microscope (SEM, Nova 600i). Nitrogen physisorption measurements were carried out at 77 K using Quantachrome Autosorb-1 Instrucment. The samples were degassed for 10 h under vaccuum at 80° С prior to any adsorption experiment. The pore-size distributions were determined by the Density Functional Theory (DFT) method using Quantachrome data reduction softwate. According to Brunauer-Emmett-Teller (BET) theory, the surface area analyses were performed in the relative pressure of 0.05-0.3. Total pore volume was defined as the volume of liquid nitrogen corresponding to the amount adsorbed at a relative pressure P/P0=0.99.

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Compression testing was conducted on the cylindrical specimens (~10 mm in diameter and height), using a SANS CMT7000 testing machine, fitted with a 500 N load cell. The initial compressive modulus was calculated from the slope of the linear portion of the stress-strain curve. The electrical conductivity of aerogels was measured using the four-point-probe measurement (Guangzhou Four Probes Tech Co). All electrochemical experiments were performed using a three-electrode system at room temperature. For the production of a working electrode, as-prepared materials (LEPC/PANI aerogels and PANI), acetylene black and polyvinylidene difluoride (PVDF) with a weight ratio of 8:1:1 were pasted on a platinum substrate to form a homogeneous film with a surface density of 3 mg/cm2. An aqueous solution of H2SO4 (1 mol L-1) was used as the electrolyte. An Ag/AgCl electrode and a platinum foil and were used as reference electrodes and the counter, respectively. All galvanostatic charge/discharge and cyclic voltammetry (CV) were performed on a CHI 660H electrochemical workstation.

RESULTS AND DISCUSSION 2.1 Synthesis and Structure Characterizations. Neither LEPC nor PANI are able to form hydrogels by themselves. In order to obtain a prospective aerogel, a LEPC/PANI hydrogel was first synthesized via adding oxidant APS into the solution of LEPC and An, as shown in Figure 1. The gelation mechanism of the LEPC/PANI hydrogel is similar to those of PANI/polyanion reported elsewhere.32 The strong physical crosslinking network driven by intermolecular hydrogen bonds between LEPC and PANI plays an important role (Figure 1). The number of hydroxyl and carboxyl groups in the pectin, reflected

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by the degree of esterification (DE), directly determined the formation of the hydrogel. In our work, we found that LEPC with a large number of carboxyl and hydroxyl groups (DE: 33%) could easily crosslink with PANI, but the pectin with high methoxyl groups (DE: 70%) failed to form the hydrogel; the FTIR spectra and XRD pattern of the resulting LEPC4/PANI4 aerogel support this conclusion. From Figure 2a, the shift of the carboxyl peak (1724 cm-1) in the FTIR spectra of the aerogel indicated complex formation between the protonated amino groups of PANI and carboxyl groups of LEPC. The other shifts of peaks in the LEPC4/PANI4 aerogel also indicated the complex formation between PANI and LEPC. In the XRD spectrum (Figure 2b), all characteristic peaks of LEPC4/PANI4 aerogel were broadened in comparison with LEPC and PANI, and the significant shift of the peaks also could be observed. Also, the total loss of LEPC crystallinity can be observed for PANI/LEPC aerogels. These changes were probably attributed to the intermolecular interactions between LEPC and PANI. The densities of the resultant aerogels were found to be in the 0.08-0.1 g/cm3 range (Table 1), typical of polymer aerogels.17,18 The morphology and porous structures of LEPC/PANI aerogels were investigated by nitrogen adsorption/desorption tests and scanning electron microscopy (SEM). Figure 3 shows the typical SEM images of LEPC/PANI aerogels with varying content of PANI. All of these aerogels exhibited porous 3D network structures formed by PANI and LEPC, and were rich in the hierarchical pores with wide distributions, typical of self-supported nanoporous structures enabled by hydrogen bonds. It is noteworthy that many hollow microspheres of ca. 400 nm in diameter could be formed and uniformly dispersed in the matrix of the aerogel upon increasing the content of PANI (Figure 3). With increasing PANI content, PANI chains were entangled with LEPC chains and a residual part of PANI would aggregate into particles, thus leading to the formation of the hollow and honeycomb-like nano-spheres. This

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special self-supported porous structure would play a positive effect on improving the mechanical and conducting properties of the LEPC/PANI aerogels; such a biopolymer aerogel with a porous structure could be favorable in supercapacitor applications.31,33 The porous properties of the obtained aerogels were also evaluated by the nitrogen sorption tests. Figure 4 shows the nitrogen adsorption and desorption isotherms of LEPC4/PANI4 aerogels, and the detailed data are listed in the Table 1. A typical Type IV isotherm characteristic with a small adsorption hysteresis, as shown in Figure 4a, indicates that a large number of mesopores existed within the aerogel. Based on the pore size distribution calculated by DFT (Figure 4b), these aerogels contained mainly mesopores and some micropores. But some macropores observed by SEM (Figure 3b) cannot be evaluated by this approach, limited by the testing method. Combined with SEM observation and nitrogen adsorption-desorption analysis, the aerogel contained mainly mesopores, some micropores and macropores, which provided great potential for broad applications. More importantly, the LEPC4/PANI4 aerogels also exhibited high total pore volumes (Vt, 1.5 cm3/g) and large Brunauer–Emmett–Teller (BET) surface areas (SBET, 248 m2/g), remarkably high for conducting polymer composites. From Table 1, it can be seen that the values of total pore volumes and surface areas decreased with increasing PANI content, as a result of the aggregation of PANI. The high surface areas of the aerogels would provide sufficient active sites for the faradaic reaction of PANI, thus increasing the electrochemical utilization of PANI.

2.2 Electrically conductive and mechanical properites. As electrically conductive materials, a specific conductivity value is an essential requirement. Table 1 lists the detailed bulk conductivities of LEPC/PANI aerogels as a function of the PANI

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content. The conductivities of LEPC4/PANI2, LEPC4/PANI4 and LEPC4/PANI6 aerogels were 0.002, 0.02 and 0.1 S/m, respectively, revealing that PANIs form interpenetrated networks within the aerogels, leading to favorable conductivity of the resulting aerogels. The mechanical properties of the obtained aerogels were studied by uniaxial compression. Figure 5 shows the compressive stress-strain dependence of the LEPC/PANI aerogels. All of the aerogels exhibited linear elastic deformations under small compressive strain, and then yield, followed by inelastic hardening and densification. Unlike inorganic aerogels, these aerogels were ductile and turn into dense solids under compressive load. The compressive moduli were calculated from the slope of the initial stress-strain curves, and are summarized in Table 1. Typical compressive moduli were measured in the range of 1.2-1.4 MPa, relatively large for conducting polymer aerogels, presumably due to the strong inherent strength of LEPC and special physical cross-linking network based on the intermolecular hydrogen bonds between LEPC and PANI. The resulting LEPC/PANI aerogels, therefore, exhibited self-suported 3D nano-structures with high surface areas, electrical conductivity and high mechanical strenths, potentially leading to a wide range of applications.

2.3 Electrochemical Properties. Electrochemical energy storage is important for sustainable and clean energy technologies.34 Supercapacitors as novel electrochemical energy storage devices have received considerable attention due to their excellent reversibility, high power density, and long cycle life.35 In previous reports, carbon and conductive polymer aerogels have been designed and prepared for supercapacitor applications.36-38 In this paper, the capacitive performance of the resulting aerogel

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was evaluated by galvanostatic charge/discharge techniques and cyclic voltammetry (CV) in 1 M H2SO4 aqueous solution, in which LEPC4/PANI4 aerogel and PANI were chosen as a representative and a control, respectively. CV is an appropriate tool to characterize the electrochemical property of electrode materials. As shown in Figure 6a, the shapes of the CV curve of PANI and LEPC4/PANA4 aerogel were distinct from that pure LEPC. Similar with PANI, LEPC4/PANA4 aerogel exhibited electrochemical redox peaks in the range of 0-0.8 V, while the contribution of pure LEPC to the capacitance was very small and could be neglected. The two pairs of redox peaks of the aerogel were attributed to the transition between the leucoemeraldine base state to an emeraldine state of PANI, and a further transition to a pernigraniline state,33 suggesting that LEPC4/PANA4 aerogels possess the Faraday capacitance necessary for electrode materials. The scan rate also had a remarkable impact on the electrochemical behavior of the aerogel. From Figure 6b, the peak values of oxidation increased significantly with increasing scan rate, indicating that the aerogel possesses an excellent electrochemical response. The CV curves did not always maintain their shapes when the scan rates were changed; the redox peaks became vague at high scan rates. This behavior may be attributed to that the charging-discharging process of the aerogel in the H2SO4 electrolyte is governed by the insertion of H+ from the electrolyte into the aerogel and its release from the aerogel to the electrolyte.39 The different scan rates would cause different exchange time of the H+, leading to different shapes of CV curves. Meanwhile, significant I-V distortion could be observed at 20 mV/s or higher scan rates, indicating that there is a high iontransport resistance at high rates for the aerogel electrodes.39 To further investigate this phenomenon, average capacitances (Ca) at different scan rates were calculated according to Ca = (∫IdV)/(2mVυ), where V is potential window, I is the response current density, m is the active

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material mass, and υ is the potential scan rate. It was found that their average capacitances were 130, 101, 70 and 41 F/g at the scan rates of 10, 20, 50, and 100 mV/s, respectively. With the increase of scan rates, Ca of the electrodes decreased dramatically, which further proved the ion transport issue for the aerogel. This ion transport issue may be ascribed to the relatively poor conductivities of the aerogel (0.02 S/m). To further investigate the potential ability of LEPC/PANI aerogels as electrode materials for supercapacitors, galvanostatic charge/discharge measurements were also carried out. Figure 7a illustrates the galvanostatic charge/discharge curves of LEPC4/PANI4 electrodes at various current densities of 0.5, 1, 2, and 5 A/g. During the charging and discharging steps, their curves were rather symmetric, indicating high reversibility of hybrid materials.40 The time of these charging-discharging procedure decreased gradually with the increase of current density, which was similar to the ever reports on supercapacitors.33,39 The specific capacitances (Cs) could be calculated by the following equation: 41 Cs =

I∆t m∆V

where m is the mass of the active material, I is the applied current in amperes, and ∆V/∆t is the slope of the discharge curve. Figure 7b shows values of the specific capacitance for LEPC4/PANI4 aerogel as a function of current densities, and those of pure PANI were also given as a reference. It can be seen that the specific capacitance of the LEPC/PANI aerogel was as high as 184 F/g at a current density of 0.5 A/g, which is comparable to that of conductive aerogels in previous reports, such as PEDOT-S/PEDOT aerogels (83 F/g) 18, graphene aerogels (128 F/g) 42, PANI-SA aerogels (216 F/g) 43. More importantly, only 50 wt% of PANI was loaded in the aerogel. As discussed above, the specific capacitance of LEPC is very small and can be ignored, and the specific capacitance of the LEPC/PANI aerogel derives from PANI

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contribution. The discharge specific capacitance of PANI within this aerogel was about 368 F/g, while that of pure PANI was only 200 F/g, attributable to the high surface areas and special nanoporous 3D network structures in the aerogels, which could provide numerous active sites for the faradaic reaction. However, it was noticed that there is a large IR drop in the discharge curve at higher current density, which is caused by the relatively high transport resistance for the aerogel electrodes. Similar to the results of Ca, the specific capacitance of the electrodes decreased dramatically with the increase of current density (Figure 7b), because of the slow diffusion of electrolytic ions and redox reaction rates which cannot follow the current density increases.33 As mentioned above, the relatively poor conductivities of the aerogel lead to the ion transport issue. Improving the conductivity of the aerogel is the effective way to mitigate distortion, which may be achieved by adding a few conductive carbon nanoparticles into matrix during the fabrication of the aerogel. When the current density was raised to 5 A/g, the specific capacitances of LEPC4/PANI4 aerogel and pure PANI decreased by nearly 52 and 75%, respectively, indicating that the redox reaction rates and charge diffcusion of the aerogel electrode is faster than those pure P ANI aerogel; thus the capacitance of LEPC4/PANI4 aerogels is more stable than pure PANI. This result further support the concept that the porous 3D network structure plays an important role in enhancing the capacitive performances of the aerogel, although half of matrix is biodegradable LEPC with no electrochemical activity of its own. The power density (P) and energy density (E) of the LEPC/PANI aerogel electrode were also calculated by the following equation and shown in Figure 8: 44 CsV ଶ E= 8 × 3.6

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P=

E t

where Cs is specific capacitances calculated from the discharge tests. V is the discharge potential after the IR drop (V). And t is the discharge time (h). From Figure 8, it can be observed that the energy density of the LEPC/PANI aerogel can reach 3.0 W h kg

−1

in aqueous electrolyte at a

current density of 0.5 A/g, while the energy density was about 41.6 W kg −1. With the increase of current density, the energy density decreased but the power density increased dramatically, which was caused by IR drop and ion transport issue. The electrochemical stabilities of LEPC4/PANI4 aerogel and pure PANI were investigated at a scan rate of 100 mV/s for 1000 cycles. As shown in Figure 9, there was a slight increase in capacitance in the first cycles and, from then, the capacitance decreased with further increasing of the cycle times, for both PANI and the aerogel. The decrease in slope of LEPC4/PANI4 aerogels was much less than that of pure PANI. Pure conducting polymers generally have poor cycling stabilities since they easily expand and shrink during the electrochemical process.33 From Figure 9, the capacitance retention of PANI was only 57% after 1000 cycles. However, LEPC4/PANI4 aerogel showed a good cycling stability of the hybrid materials, and it had 74% of the initial capacitance after 1000 cycles. This good cycling stability of LEPC4/PANI4 aerogel can be assigned to the strong hydrogen bonding interactions between LEPC and PANI and the stable 3D network structure, which can restrict the structural change of PANI during the electrochemical process.

CONCLUSIONS We have successfully fabricated a novel family of biomass-based, mechanically-strong and electrically-conductive polymer aerogels from aniline and biodegradable pectin. Likely due to

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strong hydrogen bonding interactions between polyaniline and LEPC, the resultant pectinpolyaniline aerogels exhibited self-supported 3D nanoporous network structures with high BET surface areas and hierarchical pores, thus leading to a high mechanical strengths and considerable electrical conductivity. Owning to these features, the aerogels were used as electrode materials for scapacitors exhibited fast electrochemical responses, high specific hydrogel capacitances (184 F/g at 0.5 A/g), and good cycling stability. Over 74 % of the initial capacitance was retained after repeating the cylic voltammetry test for 1000 cycles, while the capacitance retention of PANI was 57%. These biomass-based aerogels are also promising to be further introduced to other fields such as sensors, heavy metal ion adsorption and biotechnological applications.

ACKNOWLEDGMENTS The authors of this paper would like to thank National Science Foundation of China (Grant No. 51403192, 51503191, 51502274), National High Technology Research and Development Program of China (863 Program, 2013AA050905)， and Innovation Foundation of Institute of Nuclear Physics and Chemistry (Grant No. 2013CX04) forfinancial support.

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Table 1. Detailed properties of LEPC/PANI aerogels Sample (aerogel) Bulk density (g/cm3) Vt (cm3/g) SBET (m2/g)

Conductivity

E (MPa)

(S/m) LEPC4/PANI2

0.08±0.01

2.6± 0.1

331± 3

0.002± 0.001

1.2± 0.1

LEPC4/PANI4

0.11±0.01

1.5± 0.1

248± 2

0.02± 0.001

1.2± 0.1

LEPC4/PANI6

0.11±0.01

1.0± 0.1

207± 2

0.1± 0.001

1.4± 0.1

Vt, total pore volume; SBET, BET surface area; E, compressive modulus.

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Figure 1. Schematic illustration of LEPC/PANI aerogel formation and its cross-linking structure

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Figure 2. FTIR spectra (a) and XRD patterns (b) of PANI, LEPC, and LEPC4/PANI4 aerogels

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Figure 3. SEM images of LEPC/PANI aerogels: (a) LEPC4/PANI2, (b), (d) LEPC4/PANI4, (c) LEPC4/PANI6

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Figure 4. (a) Typical nitrogen adsorption and desorption isotherms and (b) DFT (Density Function Theory) pore size distribution of LEPC4/PANI4 aerogels

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Figure 5. Plot of compressive stress/strain of LEPC/PANI aerogels

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Figure 6. (a) Cyclic voltammograms of LEPC, PANI, and LEPC4/PANI4 aerogel electrodes at a scan rate of 10 mV/s, (b) Cyclic voltammograms of LEPC4/PANI4 aerogel electrode at various scan rates of 10, 20, 50, and 100 mV/s

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Figure 7. (a) Galvanostatic charge/discharge curves of LEPC4/PANI4 aerogel electrode (b) Specific capacitances of LEPC4/PANI4 aerogel and PANI electrodes at different current densities of 0.5, 1, 2, and 5 A/g

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Figure 8. Energy density (E) and power density (P) of LEPC4/PANI4 aerogel electrodes at different current densities of 0.5, 1, 2, and 5 A/g.

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Figure 9. Cycling stability of PANI and LEPC4/PANI4 aerogel electrodes at 100 mV/s

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146x68mm (220 x 220 DPI)


Biomass-Based Mechanically Strong and Electrically Conductive

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