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Keywords:

  • ionic liquid;
  • poly(ionic liquid);
  • electrolyte;
  • fuel cells;
  • dye-sensitized solar cells

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

Ionic liquids are organic salts with melting points generally below 100 °C. They are attracting wide attention and are used as electrolytes in electrochemical devices, such as fuel cells, lithium-ion batteries, dye-sensitized solar cells, supercapacitors and light-emitting electrochemical cells, due to their negligible vapor pressure, high ionic conductivity and wide electrochemical window. This perspective article highlights the applications of ionic liquid- or poly(ionic liquid)-based electrolytes in fuel cells, dye-sensitized solar cells and supercapacitors. © 2013 Society of Chemical Industry


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

The major source of world energy consumption is derived from burning of fossil fuels (coal, oil, natural gas, etc.) which results in environmental pollution and global warming due to the emission of sulfur and nitrogen oxides and carbon dioxide.[1] Therefore, many attempts have been made to develop renewable energy sources and energy storage and transformation devices, such as fuel cells, photoelectrochemical solar cells, lithium-ion batteries and supercapacitors. For these energy devices, electrolytes that transport ions or protons from anode to cathode have been considered as one of the key components.

Ionic liquids (ILs) are defined as room-temperature molten salts with melting points generally below 100 °C. They are attracting much attention due to their novel physicochemical properties: wide electrochemical window, negligible vapor pressure, high electrical conductivity, thermal and chemical stability, low toxicity and nonflammability.[2-8] These unique properties mean that ILs are very useful media as solvents or electrolytes. However, devices with room-temperature IL-based electrolytes still suffer leakage problems in practical use. Therefore, doping polymers with ILs has been investigated. The physical properties of IL/polymer composites are affected by species and contents of ILs.[9] Growing attention has also been paid to poly(ionic liquids) (poly(ILs)) because they combine the novel properties of ILs and the improved mechanical durability and dimensional control of polymers.[6] Compared with non-conductive polymers, poly(ILs) can dramatically increase the conductivity of electrolytes to a certain degree. More recently, poly(ILs) have been successfully applied as electrolytes in various energy devices, including lithium batteries, dye-sensitized solar cells (DSSCs), fuel cells, supercapacitors, light-emitting electrochemical cells and field effect transistors.[6, 10] In this perspective article, we highlight the applications of IL-, poly(IL)- and IL/poly(IL)-based electrolytes in fuel cells, DSSCs and supercapacitors.

Anhydrous Polymer Electrolyte Membranes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

Fuel cells, which convert chemical energy into electrical energy, heat and water via a redox reaction, have been recognized as one of the most promising energy source systems that could provide clean and efficient energy for stationary and mobile applications, due to their high energy density, high conversion efficiency and low pollutant emission.[11-13] Proton-exchange membrane fuel cells (PEMFCs) are polymer electrolyte fuel cells containing a polymer membrane as the separator between the fuel and oxidant streams and simultaneously as an electrolyte to transport ions.

A major drawback of the most commonly used humidified perfluorosulfonic acid membranes, represented by Nafion, is that they cannot be used at high temperatures above 100 °C because of the evaporation of water, which results in a rapid loss of conductivity.[14] However, the operation of PEMFCs at temperatures above 100 °C could enhance the reaction kinetics at both electrodes, improve the carbon monoxide tolerance of the platinum catalyst at the anode and simplify heat and water managements of PEMFCs. To partially overcome this shortcoming, two methods have been adopted. One is the development of new poly(ILs) and the other is using protic ILs instead of water.

Texter and co-workers recently synthesized poly(IL)-type PEMs via the photo-crosslinking of AC11C1ImAMPS.[15, 16] The resultant PEMs gave protonic conductivity up to 10−3 S cm−1 above 100 °C after ion exchange with an equivalent of HPF6 in water followed by drying. The conductivity spectra of the membranes doped with butylmethylimidazolium tetrafluoroborate or ethylammonium nitrate were similar to that of HPF6-loaded polymer. Protic ILs, consisting of combinations of Brønsted acids and bases which can form hydrogen bonds and act as proton carriers, have been considered as effective proton-transferring carriers for high-temperature PEMFCs because of their non-volatility, high proton conductivity and excellent chemical and thermal stability. Greenbaum and co-workers prepared PEMs using polybenzimidazole (PBI) membrane doped with 1-methyl-3-propylmethylimidazolium dihydrogen phosphate. The resultant composite membranes showed conductivity up to 2.0 × 10−3 S cm−1 at 150 °C under anhydrous conditions.[17] As an alternative approach to improve the problem of leakage when doped with protic ILs, Yan and co-workers recently reported that incorporation of appropriate amounts of mesoporous silica nanospheres could prevent the release of protic IL components from composite membranes due to the capillary force of the mesoporous silica nanospheres.[18] However, further improvements to retain protic IL components upon cell operation still need to be carefully considered. Design and synthesis of poly(ILs) having a strong interaction with protic IL components might be one of the most effective ways to improve the retention of protic ILs.

Quasi-Solid-State Dye-Sensitized Solar Cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

DSSCs have been considered as a potential alternative to conventional silica cells due to their low cost and simple preparation procedure. DSSCs are usually fabricated by sandwiching either a liquid or polymer electrolyte between a dye-sensitized TiO2 electrode and a platinum counter electrode. Although a power conversion efficiency of about 11% under standard AM 1.5 illumination has been achieved with an organic liquid electrolyte, the leakage and evaporation of the organic solvent may shorten the working life and affect the practical application of DSSCs. Therefore, quasi-solid-state electrolytes have been attracting a great deal of interest because of their good contact with nanocrystalline TiO2 electrodes and counter electrodes, high ionic conductivity, and showing comparable efficiencies to the DSSC devices using liquid electrolytes.[19-25] For example, poly[acrylonitrile-co-(vinyl acetate)] as the gelator of a 3-methoxypropionitrile-based liquid electrolyte was applied in DSSCs.[19] Although an overall power conversion efficiency of 8.34% was achieved under illumination of 100 mW cm−2 (AM 1.5), these types of quasi-solid-state DSSCs still suffer from the volatilization of the liquid organic solvent contained in the electrolytes. To overcome this problem, organic solvent-free IL/poly(IL) electrolytes have been synthesized for quasi-solid-state DSSCs (Fig. 1), and a power conversion efficiency of 4.4% under a simulated air mass 1.5 solar spectrum illumination at 100 mW cm−2 was achieved.[25] The biggest challenge for organic solvent-free IL/poly(IL) electrolyte-based quasi-solid-state DSSCs is their relatively low power conversion efficiency if compared with liquid electrolyte cells due to the relatively low conductivity. Addition of inorganic nanoparticles and designing new poly(ILs) are two feasible methods.

image

Figure 1. Chemical structures of poly(ILs) used in DSSCs.[24, 25]

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Recently, IL-tethered TiO2 nanoparticles and TiO2 nanoparticle have been introduced into poly(IL)-based electrolytes. DSSCs based on the hybrid electrolytes showed power conversion efficiencies of 5.26 and 4.89% under simulated AM 1.5 solar spectrum irradiation at 50 mW cm−2, respectively.[21] Both these values are higher than that of DSSCs without the addition of inorganic nanoparticles, which was 4.33% under the same conditions. This could be interpreted as an increased ion-exchange ability that the adsorption of imidazolium cations on the TiO2 nanoparticles would align the anionic I3/I redox couple by electrostatic force.[23] Yan and co-workers synthesized a bis-imidazolium-type poly(IL)-based electrolyte, poly[1-butyl-3-(1-vinylimidazolium-3-hexyl)imidazolium bis(trifluoromethanesulfonyl)imide], for use as a quasi-solid-state electrolyte for DSSC.[22] The DSSCs showed a superior long-term stability and yielded a higher power conversion efficiency of 5.92% under simulated air mass 1.5 solar spectrum illumination at 100 mW cm−2, due to the charge transport networks formed in the gel electrolytes via the π–π stacked imidazolium rings.

Solid-State Supercapacitors

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

Although renewable energy technologies can provide sufficient energy production, fabrication of efficient energy storage devices is desirable. Supercapacitors or electrochemical capacitors have recently received extensive attention because of their unique advantages including long cycle life, superior reversibility and high energy and power densities.[26, 27] The emergence of printed electronics, including smart cards, electronic paper and wearable electronics, reveals the need for low-weight solid-state printable supercapacitors to replace conventional supercapacitors. Therefore, polymer electrolytes have been considered as promising materials owing to their low-cost, facile synthesis.[28] However, most of the polymer electrolytes reported are non-ionic polymers doped with inorganic or organic electrolytes, including KOH, H2SO4 and room-temperature ILs with a relatively low viscosity and wide electrochemical window, such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]).[29-32]

More recently, poly(styrenesulfonic acid) has been demonstrated for the first time as an effective ion-conductive electrolyte in supercapacitors (Fig. 2).[32] The resultant supercapacitors showed a galvanostatic discharge capacitance as high as 85 F g−1 and an ionic conductivity of 1.5 mS cm−1 at 80% relative humidity. Although the cycle stability of devices still needs to be improved (about 67% of initial capacitance was maintained after a galvanostatic charge–discharge test of 7000 cycles) and the volatile, narrow electrochemical window of water needs to be addressed, these results open a door to the design of flexible polyelectrolyte-based supercapacitors for the power sources in printed electronics and wearable electronics. Unfortunately, poly(IL)-based polyelectrolytes have not been applied in supercapacitors, although the conductivities of poly(ILs) are usually higher than those of non-ionic polymers (e.g. poly(vinyl alcohol)). From a materials perspective, poly(IL)-based (quasi- or all)-solid-state electrolytes will offer a feasible choice for solid-state supercapacitors in future practical applications.

image

Figure 2. Schematic of a supercapacitor: two carbon nanotube (CNT) networks sandwiching a polyelectrolyte film.[32]

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Summary

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

ILs have been demonstrated to be important types of electrolytes for energy devices, due to their wide electrochemical window, negligible vapor pressure and high conductivity. In the case of protic IL-doped PEMs, the progressive release of the protic IL component is still a major obstacle for fuel cell operations. In addition, the liquid nature of ILs limits their use in DSSCs due to leakage problems. Growing attention has therefore been paid to poly(ILs) because they are a class of polymers that combine the properties of both ILs and polymeric materials. Previous studies have demonstrated that IL- or poly(IL)-based electrolytes have certainly potential applications in fuel cells, DSSCs, flexible supercapacitors, printed electronics and wearable electronics.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References

This work was supported by Natural Science Foundation of China (Nos. 21174102, 21274101), National Basic Research Program of China (973 Program, No. 2012CB825800), The Natural Science Foundation of Jiangsu Province (BK2011274), Research Fund for PhD Programs Foundation of Ministry of Education of China (20103201110003), and Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Anhydrous Polymer Electrolyte Membranes
  5. Quasi-Solid-State Dye-Sensitized Solar Cells
  6. Solid-State Supercapacitors
  7. Summary
  8. Acknowledgements
  9. References
  • 1
    Dillon AC, Chem Rev 110:68566872 (2010).
  • 2
    MacFarlane D, Forsyth M, Howlett P, Pringle JM, Sun J, Annat G et al., Acc Chem Res 40:11651173 (2007).
  • 3
    Sun XQ, Luo HM and Dai S, Chem Rev 112:21002128 (2012).
  • 4
    Atanase L and Riess G, Polym Int 60:15631572 (2011).
  • 5
    Qiu B, Lin BC, Qiu LH and Yan F, J Mater Chem 22:10401045 (2012).
  • 6
    Lu JM, Yan F and Texter J, Prog Polym Sci 34:431448 (2009).
  • 7
    Earle MJ, Esperanca J, Gilea MA, Lopes JNC, Rebelo LPN, Magee JW et al., Nature 439:831834 (2006).
  • 8
    Qiu B, Lin BC, Si ZH, Qiu LH, Chu FQ, Zhao J et al., J Power Sources 217:329335 (2012).
  • 9
    Matsumoto K and Endo T, Macromolecules 41:69816986 (2008).
  • 10
    Mecerreyes D, Prog Polym Sci 36:16291648 (2011).
  • 11
    Steele BCH and Heinzel A, Nature 414:345352 (2001).
  • 12
    Matsumoto K, Fujigaya T, Yanagi H and Nakashima N, Adv Funct Mater 21:10891094 (2011).
  • 13
    Couture G, Alaaeddine A, Boschet F and Ameduri B, Prog Polym Sci 36:15211557 (2011).
  • 14
    Kawaguti CA, Dahmouche K and Gomes AS, Polym Int 61:8292 (2012).
  • 15
    Gu H, England D, Yan F and Texter J, in Proceedings of the 2nd IEEE International Nanoelectronics Conference, 24–27 March 2008, pp. 863868 (2008).
  • 16
    Texter J, Macromol Rapid Commun 33:19962014 (2012).
  • 17
    Ye H, Huang J, Xu J, Kodiweera N, Jayakody J and Greenbaum S, J Power Sources 178:651660 (2008).
  • 18
    Lin BC, Cheng S, Qiu LH, Yan F, Shang SM and Lu JM, Chem Mater 22:18071813 (2010).
  • 19
    Chen CL, Teng H and Lee YL, J Mater Chem 21:628632 (2011).
  • 20
    Zhao J, Yan F, Qiu LH, Zhang YG, Chen XJ and Sun BQ, Chem Commun 47:1151611518 (2011).
  • 21
    Chen XJ, Li Q, Zhao J, Qiu LH, Zhang YG, Sun BQ et al., J Power Sources 207:216221 (2012).
  • 22
    Chen XJ, Zhao J, Zhang JY, Qiu LH, Xu D, Zhang HG et al., J Mater Chem 22:1801818024 (2012).
  • 23
    Huo ZP, Dai SY, Wang KJ, Kong FT, Zhang CN, Pan X et al., Solar Energy Mater Solar Cells 91:19591965 (2007).
  • 24
    Zhao J, Shen XJ, Yan F, Qiu LH, Lee ST and Sun BQ, J Mater Chem 21:73267330 (2011).
  • 25
    Zakeeruddin SM and Gratzel M, Adv Funct Mater 19:21872202 (2009).
  • 26
    Li X, Rong JP and Wei BQ, ACS Nano 4:60396049 (2010).
  • 27
    Simon P and Gogotsi Y, Nature Mater 7:845854 (2008).
  • 28
    Xie K, Qin XT, Wang XZ, Wang YN, Tao HS, Wu Q et al., Adv Mater 24:347352 (2012).
  • 29
    He SJ, Hu XW, Chen SL, Hu H, Hanif M and Hou HQ, J Mater Chem 22:51145120 (2012).
  • 30
    Feng J, Sun X, Wu CZ, Peng LL, Lin CW, Hu SL et al., J Am Chem Soc 133:1783217838 (2011).
  • 31
    Kang YJ, Chun SJ, Lee SS, Kim BY, Kim JH, Chung H et al., ACS Nano 6:64006406 (2012).
  • 32
    Wee G, Larsson O, Srinivasan M, Berggren M, Crispin X and Mhaisalkar S, Adv Funct Mater 20:43444350 (2010).