Seaweed Biomass-Derived Flame-Retardant Gel Electrolyte

Nov 14, 2018 - Gel polymer electrolytes (GPEs) have received a great deal of attention for use in solid-state supercapacitors (SSCs). However, a major...
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Seaweed Biomass-Derived Flame-Retardant Gel Electrolyte Membrane for Safe Solid-State Supercapacitors Tingting Ye,† Daohao Li,† Hongli Liu,† Xilin She,† Yanzhi Xia,*,† Shuchao Zhang,† Huawei Zhang,‡ and Dongjiang Yang*,†,§

Macromolecules Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/14/18. For personal use only.

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School of Environmental Science and Engineering, State Key Laboratory of Bio-fibers and Eco-textiles, Shandong Collaborative Innovation Center of Marine Biobased Fibers and Ecological textiles, Institute of Marine Biobased Materials, Qingdao University, Qingdao 266071, P. R. China ‡ Shandong University of Science and Technology, Qingdao 266590, P. R. China § Queensland Micro- and Nanotechnology Centre and School of Natural Sciences, Griffith University, Nathan, Brisbane, QLD 4111, Australia S Supporting Information *

ABSTRACT: Gel polymer electrolytes (GPEs) have received a great deal of attention for use in solid-state supercapacitors (SSCs). However, a majority of the reported GPEs, such as petroleumderived poly(vinyl alcohol), suffer from flammability, poor water retention, and low ionic conductivity, resulting in poor safety and low capacitance. Herein, we report a high-performance flameretardant GPE (FRGPE) for SSCs using natural and sustainable marine biomass alginate as a precursor. The obtained lithium alginate/C2H3LiO2 (Li-Alg/LiOAc) FRGPE not only offers excellent flame-retardant performance (high oxygen index value, 35%) but also can effectively retain water to avoid swelling behavior at high temperatures. Therefore, it can completely resolve the safety problems of SSCs. Importantly, the seaweed GPE displays a considerably high ionic conductivity (32.6 mS cm−1) because of the amorphous structure and abundant oxygen of the polymer. Accordingly, the safe SSC fabricated by FRGPE with activated carbon electrodes delivers a high specific capacitance, excellent rate performance, and superior stability. The natural and sustainable seaweed GPEs could be promising electrolytes for developing safe and high-performance SSCs.

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electrolytes (GPEs), which consist of a polymer matrix and an electrolyte salt, have been widely used as electrolytes and separators in SSCs because of their relatively high ionic conductivity, wide electrochemical window, flexibility, and membrane forming ability as compared to those properties of other solid electrolytes.1,15,16 Several promising polymers such as poly(vinyl alcohol) (PVA), 2 poly(ethylene oxide) (PEO),17,18 poly(acrylonitrile) (PAN),19 poly(methyl methacrylate) (PMMA),20 and poly(vinylidene fluoride) (PVDF)21 have been explored as hosts in SSCs. In particular, the PVA host was widely used to generate acidic (PVA/H3PO422,23), alkaline (PVA/KOH24,25), and neutral (PVA/LiCl26,27) GPEs because of its excellent mechanical property, chemical stability, and membrane forming ability.28 However, the commonly used nonrenewable polymers are flammable in air29,30 because of the low limiting oxygen index [LOI, 32%, which is much higher than that of PVA/LiOAc GPE membranes (28% can be defined as a nonflammable material,40 the FRGPE membrane displays an outstanding flame-retardant property. The flame-retardant mechanism of alginate has been illustrated C

DOI: 10.1021/acs.macromol.8b01955 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. (a) Schematic of the sandwich structured SSC device. (b) CV curves of SSCs based on the FRGPE. (c) GCD curves of SSCs based on the Li-Alg/LiOAc-2M FRGPE. (d) CV curves of SSCs based on the Li-Alg/LiOAc-2M FRGPE at various potential windows, with a scan rate of 50 mV s−1.

Figure 5. (a) CV curves of SSCs using different GPEs, with a scan rate of 50 mV s−1. (b) Comparison of areal specific capacitance among different GPEs. (c) EIS of SSCs using different GPEs. The inset shows a magnified diagram for high frequencies. (d) Ionic conductivity of the FRGPE and PVA/LiOAc GPE at different LiOAc concentrations and the PVA/H3PO4 GPE at different H3PO4 concentrations.

by Xia et al.36 First, a large amount of −COOH and −OH groups in the alginate macromolecular chains will produce H2O and CO2 via dehydration and decarboxylation reaction, which can dilute the concentration of combustible gases and further inhibit thermal cracking. Second, the metal ions will convert to metal oxides/carbonates during the combustion

process, which could cover the combustion surface, thus preventing the penetration of oxygen and fire spreading. The flame-retardant property is also evaluated by using cone calorimetry. The total heat release (THR) and the heat release rate (HRR) values of the Li-Alg/LiOAc-2M FRGPE are much lower than those of the PVA/LiOAc-2M GPE (Figure S9). In D

DOI: 10.1021/acs.macromol.8b01955 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. (a) MDS model of the FRGPE before (top) and after (bottom) simulation. (b) MDS model simulation of the PVA/LiOAc GPE before (top) and after (bottom) simulation. (c) MSD of Li+ in GPEs. (d) Illustration of the mechanism of ion transport.

values of other previously reported aqueous GPEs-based SSCs.34,41 The high electrochemical window is attributed to the strong solvation of the neutral electrolyte that effectively decreases the activity of the water.42 The GCD curves at different current densities (Figure 4c) are almost symmetrical, confirming a low electric resistance of the FRGPE. In addition, there is not significant overpotential observed from the CV (Figure 4d) or GCD (Figures S12−S16) curves at the potential windows from 0.8 to 1.8 V. This means the FRGPEs are very stable at the potential window of 1.8 V. To verify the preponderant electrochemical performances of FRGPE, we fabricated devices based on the PVA/LiOAc GPE and PVA/H3PO4 GPE as well for comparison. As shown in Figure 5a, the FRGPE displays the largest current density and, thus, the best electrochemical performance. The longer discharging time of the FRGPE indicates a higher capacitance of the FRGPE than of the PVA-based GPE (Figure S17). The other electrochemical data of PVA-based GPEs are described in Figures S18 and S19. The GCD curves at different current densities are processed to obtain the rate performance of SSCs. As shown in Figure 5b, the specific capacitance of SSCs with the FRGPE can reach 185.7 mF cm−2 (27.3 F g−1) at a current density of 5 mA cm−2 (0.735 A g−1) and remains at 111 mF cm−2 (16.3 F g−1) at a current density of 50 mA cm−2 (7.35 A g−1). These values are much higher than those of PVA/LiOAc and PVA/H3PO4. The EIS curves of SSCs (Figure 5c) indicate that the FRGPE has the lowest bulk resistance and the lowest charge transfer resistance. The ionic conductivity (σ) of GPEs is determined by EIS. Figure 5d shows the σ of the FRGPE at different LiOAc concentrations. The Li-Alg/LiOAc-2M FRGPE has the largest σ (32.6 mS cm−1), which is much larger than those of the PVA/LiOAc GPE (8.7 mS cm−1) and

addition, we performed a combustion experiment on the LiAlg/LiOAc-2M FRGPE membrane and the PVA/LiOAc-2M GPE membrane. Apparently, the FRGPE membrane hardly ignites (Figure 3c). In contrast, the PVA/LiOAc-2M GPE membrane ignites immediately upon the fire and remains in the combustion state after leaving the fire (Figure 3d). The video of combustion is provided as Supportting Information. The results clearly demonstrate that the seaweed membrane has an excellent flame-retardant property and can significantly improve the safety of SSCs. A sandwich structured SSC device (Figure 4a) was fabricated using the FRGPE membrane as an electrolyte and a separator, and two flexible nickel foam electrodes coated with activated carbon with a mass loading of 6.4 mg cm−2 (Figure S10). Via comparison of the cyclic voltammetry (CV) curves of SSCs at different LiOAc concentrations (Figure S11a), the Li-Alg/LiOAc-2M FRGPE displays the largest current density and the highest specific capacitance. The electrochemical performance of the Li-Alg/LiOAc-2M FRGPE is further confirmed by galvanostatic charge−discharge (GCD) (Figure S11b) and rate performance (Figure S11c). The electrochemical impedance spectroscopy (EIS) curves are shown in Figure S11d. At high frequencies, Li-Alg/LiOAc FRGPE-2M has the smallest bulk resistance. At intermediate frequencies, the smallest semicircle diameter of Li-Alg/LiOAc-2M FRGPE reveals the lowest charge transfer resistance. At low frequencies, the small slope of the Li-Alg/LiOAc-2M FRGPE implies an ideal capacitive behavior. The CV curves of SSC based on the Li-Alg/LiOAc-2M FRGPE are shown in Figure 4b. The CV curves show a quasirectangular shape even at high scan rates of 200 mV s−1, illustrating an ideal EDLC behavior. The electrochemical window of the SSC can reach 1.8 V, which is higher than the E

DOI: 10.1021/acs.macromol.8b01955 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules PVA/H3PO4 GPE (5 mS cm−1). This value is also higher than those of most of the previously reported GPEs (Figure S20). The energy density of FRGPE is ≤83.5 × 10−3 mW h cm−2 at a power density of 2.25 mW cm−2, which is higher than that of the PVA-based GPE (Figure S21). Meanwhile, the FRGPE shows excellent cycling stability (Figure S22) at different potential windows. The fabricated SSCs can power the lightemitting diode bulbs (Figure S23), confirming the potential application of the FRGPE. To explore the safety of FRGPE, we heated the soft package of SSCs to 120 °C to investigate the swelling behavior. During the heating process, the soft package of the PVA/LiOAc GPE severely swells, while the FRGPE package exhibits no obvious change (Figure S24). The severe swelling behavior of PVA/LiOAc GPE could be attributed to its strong water absorption and poor water retention abilities. On one hand, the PVA/LiOAc GPE contains more water than the FRGPE because of its better hydrophilicity, which is demonstrated by the small contact angle (Figure S25). On the other hand, the poor water retention ability of the PVA/LiOAc GPE leads to more water loss at room temperature, and a hightemperature condition will accelerate the evaporation of water (Tables S1 and S2). Mechanical bending of the device with the FRGPE is used to prove the flexibility of the FRGPE. The bending photo (Figure S26a) of the flexible SSC shows that the electrodes and FRGPE are tightly bonded. The slight difference in electrochemical performances before and after bending shows that the FRGPE has good flexibility (Figure S26b−d). To investigate the superior electrochemical performance of FRGPE, the diffusion coefficients of Li+ (DLi+) in the FRGPE and PVA/LiOAc GPE were studied by a molecular dynamics (MD) simulation.43 A high DLi+ means a high ionic conductivity. First, the Li-Alg and PVA polymer chains were used to establish an amorphous mode. Afterward, a certain amount of Li+ was inserted into the cell to form the simulated initial structure. The parameters of the model and the computational details of the optimized structures are given in the experimental section in the Supporting Information. The optimized structures of the FRGPE and PVA/LiOAc GPE before and after simulations are presented in panels a and b of Figure 6, respectively. The marked Li+ in the model has changed in position after simulations, indicating the random diffusion of Li+ ions. According to the Einstein relation, the DLi+ is obtained by processing the slope of the mean-square displacement (MSD) (Figure 6c). By calculations, the DLi+ in the FRGPE (92.84 × 10−7 cm2 s−1) is higher than that in the PVA/LiOAc GPE (37.68 × 10−7 cm2 s−1), indicating that the FRGPE possesses a higher ionic conductivity. The slopes of the MSD are listed in Table S3. The schematic structure of ion transport in GPE is shown in Figure 6d. The physically cross-linked Li-Alg is used as a backbone to provide mechanical strength for the GPE, where ions can be transported in water around the backbone. A low crystalline content and a high polymer segmental mobility are more conducive for ion transport.1 Therefore, compared to the crystalline structure of PVA, the Li-Alg backbone with an amorphous structure can result in faster ion diffusion (Figure S27). Furthermore, the amorphous Li-Alg can cause the mobility of the chain segment to promote the formation and/ or cleavage of Li−O bonds.44 Abundant oxygen atoms in LiAlg polymer chains can form and break Li−O bonds, resulting in the migration of Li+ and facilitation of Li+ diffusion.

In summary, a flame-retardant and high-ionic conductance (32.6 mS cm−1) seaweed-derived GPE with excellent water retention was developed for safe and high-performance SSCs. The Li-Alg/LiOAc-2M FRGPE exhibits excellent flameretardant performance (high LOI value, 35%) and exhibits no swelling at 120 °C. The SSC fabricated with the resultant Li-Alg/LiOAc-2M FRGPE shows a high areal specific capacitance, excellent rate performance, and excellent stability. Such excellent electrochemical performance stems from the amorphous structure of the seaweed polymer and the abundant oxygen on the molecular chains, which can lead to superior ionic conductivity. This study highlights new opportunities to prepare safe SSCs by using a seaweed-based flame-retardant FRGPE.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01955. Additional data and figures and a detailed description of the experimental methods (PDF) Video of combustion (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Huawei Zhang: 0000-0002-0071-6186 Dongjiang Yang: 0000-0002-9365-3726 Author Contributions

T.Y. and D.L. contributed equally to this work. D.Y. and Y.X. designed the experiments. H.L. performed the theoretical calculations. T.Y. performed experiments. T.Y., D.L., H.L., and D.Y. wrote the manuscript with input from all authors. All authors discussed the obtained results. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51473081, 51672143, and 81502246), the Taishan Scholars Program, Outstanding Youth of Natural Science in Shandong Province (JQ201713), the ARC Discovery Project (170103317), and the Key Research and Development Program of Shandong Province (Project 2017GSF18128).

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