DEVELOPMENT OF POLYELECTROLYTES COMPLEX MEMBRANE FOR SUPERCAPACITOR Pisut Wijitsettakun a, Stephan Thierry Dubas a a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand Keywords: supercapacitors, electrodes, polyelectrolyte complexes, activated carbon ABSTRACT Supercapacitors (SCs) are electrochemical energy storage device that consist of two electrodes with a charge separator between them. In this work, polyelectrolyte complexes (PECs) based on graphite (PECs-G) and activated carbon (PECs-AC) were chosen as the flexible electrodes in SCs. PECs are formed by mixing poly(diallyldimethylammonium chloride)(pdadmac) and poly(styrene sulfonate)(pss). The performance of PECs was studied by different techniques such as cyclic voltammetry (CV), two-point probe, and electrical impedance spectroscopy (EIS). The results showed that PECs-AC has higher specific capacitance (C sp ) but lower conductivity than PECs-G. The C sp and conductivity increased with the increasing amount of graphite or activated carbon content. Moreover, the C sp is also increased with increasing concentration of electrolyte and the results varied with different types of electrolyte. PECs containing 2 M sodium chloride (NaCl) and 44.9506% AC shows the highest C sp even though their conductivity is lower than PECs-G. The Csp and the conductivity of the mentioned PECs-AC were 50.522 F/g at scan rate 20 mv/s in 4 M KOH and 0.315 S/cm, respectively. In comparison, the C sp and the conductivity of the mentioned PECs-G were 4.125 F/g at scan rate 20 mv/s in 4 M KOH and 1.012 S/cm, respectively. * Stephan.D@chula.ac.th INTRODUCTION SCs first appeared as devices which store electric charge in farads for computer memory backup power. SCs, also known as ultracapacitors or electrochemical capacitors (ECs), are electrochemical energy storage devices with high energy density and fast charge-discharge cycle. In addition, SCs store the electric energy at the interface between electrode and electrolyte.(miller and Simon 2008, Thounthong, Raël et al. 2009, Staiti and Lufrano 2016, Tiruye, Muñoz-Torrero et al. 2016) SCs consist of two electrodes with a charge separator between them. The electrode materials used in SCs require high electrical conductivity, high surface areas, and acceptable cost. Carbon based materials are commonly used as materials for SCs electrode because of high conductivity, high surface area range, good corrosion resistance, low cost, and used in electrode production technologies.(pandolfo and Hollenkamp 2006) Carbon based materials can be materials such as activated carbon, carbon aerogel, carbon nanotube (CNT), graphene, and graphite. Graphite and activated carbon are well known natural mineral which are consisted of carbon and are widely used Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1
in various structural, functional, chemical and environment applications.(ke and Wang 2016) Polyelectrolyte complexes (PECs) are formed when oppositely charged polyelectrolytes are mixed together then spontaneously aggregates into hydrated complexes. PECs held together by ion pairing interactions between oppositely charged polyelectrolytes(schaaf and Schlenoff 2015), such as electrostatic repulsion, van der waals attraction, and hydrophobic interaction. The resulting structures may either be solid-like (complex solid) or liquid-like (complex coacervate).(zhang, Yildirim et al. 2015) The formation of PECs relates to many factors, including chemical composition, the concentration of the polyelectrolytes, rate and order of mixing, ionic strength, salt type, ph, temperature, and so on.(hariri and Schlenoff 2010) The preparation of PECs is a safe procedure since water is used as a solvent and salt is used to produce charges; therefore, it is suitable as matrixes in the distribution of carbon. In this work, poly(styrene sulfonate)(pss) and poly(diallyldimethylammonium chloride)(pdadmac) were selected as the polyanion and polycation, respectively, for the preparation of PECs. PSS and PDADMAC are inexpensive, highly stable, flexible and light weight which suitable for electrodes for SCs. The electrical properties of each PECs will be compared by their cyclic voltage (CV), conductivity, and electrical impedance (chargedischarge cycle). EXPERIMENTAL A. Preparation of PECs Polyelectrolyte complexes (PECs) were prepared by mixing of equimolar polyelectrolyte solution. Firstly, 100 mm PDADMAC precursor was prepared by dissolving PDADMAC in deionized (DI) water. Then, different type of salts were added (NaCl, KCl, CaCl2 or MgCl2) and stirred until dissolved. Afterwards, 100 mm PSS precursor was prepared by dissolving PSS in DI water and then salts were added and stirred until dissolved. B. Addition of additives (graphite and activated carbon) The addition of additives was required before mixing PDADMAC and PSS together. Weight of additives was varied from 0 to 50 wt% and the weighed additive was mixed in either PDADMAC solution, PSS solution, or both of them by using sonicator for 3 minutes (pulse on = 5 second, pulse off = 5 second). PDADMAC precursor and PSS precursor were mixed together and stirred for 30 minutes. Lastly, PECs were pressed into thin films by using compression machine and were dried in the air overnight. C. Preparation of PECs for cyclic voltage The samples of PECs were cut in circular shape and soaked in electrolyte such as potassium hydroxide (KOH) overnight. D. Characterization The morphology of PECs was characterized by using scanning electron microscopy (SEM). The electrical properties of each PECs were detected by using electrical multimeter, twopoint probe meter, cyclic voltammeter, and electrical impedance spectroscopy Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2
RESULTS AND DISCUSSION A. The morphology of PECs Figure 1a) shows the morphology of PECs with 2 M NaCl and without additives. PECs consist of some polyelectrolytes and salt crystals. When graphites were added into PECs, the morphology of PECs-G with 44.9506 wt% graphite consist of some salt crystals and layers of graphite flakes, as shown in figure 1b). Graphites were expected to improve the electrical properties of PECs-G. On the other hand, the morphology of PECs-AC with 44.9506 wt% activated carbon in figure 1c) show pores inside the structure of activated carbon which is vital in term of increasing C sp of SCs because the pores can store charges and release them from electrolyte and salt during charging and discharging, respectively. Therefore, PEC-AC were expected to have outstanding electrical properties such as conductivity, C sp, and charge-discharge time. a) b) c) Figure 1 The SEM image of the morphologies of a) PECs, b) PECs-G and c) PECs-AC B. The effect of additives to prepare PECs on the electrical properties When the additives were added into PECs, the electrical properties were improved. The conductivity and C sp were increased with increased amounts of additive content. Table 1 shows the C sp in 4 M KOH of various additives to prepare PECs. The C sp of PECs-AC is Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3
higher than the C sp of PECs-G and PECs without additive at scan rate 20 mv/s. However, the conductivity of PECs-AC is lower than the conductivity of PECs-G. Table 1 The effect of additives to prepare PECs with 2 M NaCl on the conductivity and C sp Amounts of additives (wt%) Conductivity (S/cm) Specific capacitance (F/g) at scan rate 20 mv/s PECs-G PECs-AC PECs-G PECs-AC 0 0.11236 0.11236 0.43458 0.587148 11.9789 0.06590 0.14046 2.50408 5.02321 28.9911 0.10947 0.10526 2.38816 16.70343 40.4924 0.75251 0.16561 3.20709 28.30588 44.9506 1.01249 0.31531 4.12491 50.52188 Figure 2 shows the galvanostatic charge-discharge curves of PECs-G and PECs-AC. The charge-discharge time of PECs was increased with increased amount of additives. Moreover, the charge-discharge time of PECs-AC (charge time 162.5 s, discharge time 99 s) is significantly higher than PECs-G (charge time 3.5 s, discharge time 3 s) but the charge-discharge time of PECs-G is also higher than PECs without additive (charge time 1.8 s, discharge time 1 s) at current 0.004046 A in 4 M KOH. From the result, the chargedischarge time of PECs-AC is longer than the charge-discharge time of PECs-G because the morphology of PECs-AC is different from PECs-G, shown in figure 1. PECs-AC has pores in their structure so more charges can be stored, this makes C sp and charge-discharge time of PECs-AC higher. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4
Figure 2 The effect of additives to prepare PECs with 2 M NaCl in 4 M KOH on the galvanostatic charge-discharge curves D. The effect of salt concentration to prepare PECs on the electrical properties The conductivity of PECs was detected by two-point probe meter. When the concentration of NaCl was increased, the conductivity of PECs-G with 1.5 M NaCl was higher than PECs-G with 1 and 2 M NaCl. Conversely, the conductivity of PECs-AC with 1.5 M NaCl was higher than both PECs-AC with 1 and 2 M NaCl. However, the conductivities of both PECs with 1 and 2 M NaCl did not show significant difference, as shown in table 2. Table 2 shows the C sp of PECs with 40.4904 wt% additives in 4 KOH where the salt concentration (NaCl) during the preparation PECs were varied from 1M, 1.5M, to 2M. The C sp at scan rate 20 mv/s was increased when the concentration of NaCl was increased. The C sp of PECs-AC at scan rate 20 mv/s were increased from 8.6866 to 16.0594 to 28.30588 F/g when the concentrations of NaCl were 1, 1.5 and 2 M, respectively. Likewise, the C sp of PECs-G at scan rate 20 mv/s were increased from 0.76776 to 1.36257 to 2.05647 F/g when the concentrations of NaCl. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5
Table 2 The effect of salt concentration to prepare PECs with 40.4904 wt% additives on the conductivity and C sp salt concentration (M) Conductivity (S/cm) Specific capacitance (F/g) at scan rate 20 mv/s PECs-G PECs-AC PECs-G PECs-AC 1 0.70392 0.16747 0.76776 8.68656 1.5 0.92306 0.13558 1.36257 16.05948 2 0.75251 0.16561 3.20709 28.30588 At current 0.004046 A, the charge-discharge time of PECs-G slightly decreased with increased salt concentration from 1 M to 2 M NaCl while the charge-discharge time of PECs-AC dramatically increased with increased salt concentration as shown in figure 3. The salt concentration did not effect on the charge-discharge time of PECs-G but did effect on the charge-discharge time of PECs-AC due to the different morphology of PECs-AC is and PECs-G which can be seen in figure 1. As mentioned earlier, PECs-AC have pores inside the structure which may absorb NaCl therefore, charges from electrolyte can be stored more in PECs-AC than in PECs-G. Figure 3 The effect of salt concentration to prepare PECs with 40.4904 wt% additives in 4 M KOH on the galvanostatic charge-discharge curves Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6
CONCLUSIONS In summary, the salt concentration affected the formation of PECs and also affected the C sp of PECs. PECs were softer and gave higher C sp when the concentration of salt was increased. When the additives were added into PECs, the electrical properties were improved. In conclusion, the results show that PECs-AC has higher C sp but lower conductivity than PECs-G. PECs containing 2 M NaCl and 44.9506% AC shows the highest C sp and charge-discharge time although their conductivity is lower than PECs-G. ACKNOWLEDGEMENTS This research is financially supported by the Petroleum and Petrochemical College, Chulalongkorn University, Thailand. REFERENCES Hariri, H. H. and J. B. Schlenoff (2010). Saloplastic Macroporous Polyelectrolyte Complexes: Cartilage Mimics. Macromolecules 43(20): 8656-8663. Ke, Q. and J. Wang (2016). Graphene-based materials for supercapacitor electrodes A review. Journal of Materiomics 2(1): 37-54. Miller, J. R. and P. Simon (2008). Electrochemical Capacitors for Energy Management. Science 321(5889): 651-652. Pandolfo, A. G. and A. F. Hollenkamp (2006). Carbon properties and their role in supercapacitors. Journal of Power Sources 157(1): 11-27. Schaaf, P. and J. B. Schlenoff (2015). Saloplastics: processing compact polyelectrolyte complexes. Adv Mater 27(15): 2420-2432. Staiti, P. and F. Lufrano (2016). Nafion and Fumapem polymer electrolytes for the development of advanced solid-state supercapacitors. Electrochimica Acta 206: 432-439. Thounthong, P., et al. (2009). Energy management of fuel cell/battery/supercapacitor hybrid power source for vehicle applications. Journal of Power Sources 193(1): 376-385. Tiruye, G. A., et al. (2016). Performance of solid state supercapacitors based on polymer electrolytes containing different ionic liquids. Journal of Power Sources 326: 560-568. Wang, Q. and J. B. Schlenoff (2014). The Polyelectrolyte Complex/Coacervate Continuum. Macromolecules 47(9): 3108-3116. Zhang, Y., et al. (2015). The influence of ionic strength and mixing ratio on the colloidal stability of PDAC/PSS polyelectrolyte complexes. Soft Matter 11(37): 7392-7401. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 7