Application of ionic liquids, innovative polymer electrolytes and novel carbonaceous materials in supercapacitors

Resumen

The burgeoning world energy crisis and concerns about climate change have recently promoted the use of renewable energy resources (for e.g., sun and wind). Nevertheless, energy from such renewable energy resources is weighed down by fluctuations (e.g., due to intermittent availability of sun and wind). This intermittence nature results in a mismatch between energy supply and energy demand if the energy supplied from such resources is used. An important solution to this problem is coupling renewable energy resources with electrochemical energy storage devices (i.e., batteries and supercapacitors). Currently, the dominant electrochemical energy storage technologies are batteries despite they suffer from slow power delivery and uptake. Similarly, the shortcoming of the current supercapacitors is unable to store high energy. Yet, a device that could concurrently store high energy and power densities is required for different applications such as portable electronic devices, electric vehicles, and different energy storage industries. Supercapacitors are complementary to batteries, since they present higher power density, higher charge-discharge rates, better cycle stability, and safer operation than batteries. Nevertheless, the energy density of supercapacitors is lower than the one in batteries. Consequently, it has raised the question of how to increase the energy density of supercapacitors without deteriorating its power density and safety. To tackle this problem, two main strategies are considered in the scientific community. The first one is the development of novel electrode materials with high intrinsic capacitance and second, the use of electrochemically stable electrolytes such as ionic liquids that allow supercapacitors to operate at higher voltages. However, using liquid electrolytes in conventional supercapacitors is usually accompanied by additional heavy encapsulation materials to prevent liquid leakage, and separator to avoid short circuit between electrodes. These construction restrictions prevent the development of lightweight and flexible supercapacitors for application in emerging market niche such as flexible electronics or textile integration. To overcome this issue, the development of solid or quasi-solid polymer electrolytes is being investigated to replace both liquid electrolytes and separator that are present in conventional supercapacitors. In an attempt to circumvent all the above mentioned drawbacks of supercapacitors technology, this PhD Thesis is aimed at the development of high performance supercapacitors based on novel carbon based materials and electrochemically stable electrolytes including innovative polymer electrolytes. This PhD Thesis is structured into six chapters. Chapter I includes a general introduction to fundamental principles of supercapacitors technology, electrode materials and electrolytes for supercapacitors. Additionally, the current limitations, and challenges of supercapacitors and the main motivation and objectives of this PhD Thesis are briefly presented at the end of Chapter I. Then, the principal part of the Thesis, devoted to the description of the results, is divided into four chapters. Chapter II and Chapter III are devoted to the synthesis of novel carbon based composite materials and their application as electrode for supercapacitors using ionic liquid (IL) electrolytes. Chapter IV and Chapter V are focused on the development of innovative polymer electrolytes based on ionic liquids (IL-b-PE) and their application in solid state supercapacitors. A final chapter, Chapter VI, summarizes the main conclusions of this Thesis and future perspectives of supercapacitors technology. A short summary of those chapters is described below. Chapter II presents the synthesis of nitrogen-doped carbons (N-dC) and their application in supercapacitors. By using a simple “salt templating method”, highly microporous N-doped carbons (N-dC) were synthesized by heat-treatment of mixtures of precursors and eutectic salts. A sustainable and cheap tannic acid (TA) as carbon precursor, urea as nitrogen precursor and different eutectic salts (NaCl/ZnCl2, LiCl/ZnCl2, or KCl/ZnCl2) as porogens were used for the synthesis of N-doped carbons. Physicochemical properties of N-dC were determined by adsorption-desorption isotherms, X-Ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, etc. It was shown that properties of these materials (textural properties, nitrogen content, or electrical conductivity) can be easily controlled by the variation of composition of precursor mixtures, the salt amount, as well as the type of salts that were used as porogens. For instance, nitrogen content in N-doped carbons increases with the amount of urea. It reaches up to 8.8 % N with mixtures of 17:1 molar ratio of urea to TA (urea:TA) and 1:3 ratio of precursors (urea+TA) to NaCl/ZnCl2 (urea+TA:NaCl/ZnCl2) (GT_17_NaZ). Unexpectedly, the maximum electrical conductivity was not obtained for the carbon synthesized with 17:1 ratio of urea to TA (GT_17_NaZ) that exhibits maximum nitrogen content compared with other N-doped carbons. The highest electrical conductivity (76 Scm-1) was obtained from the carbon synthesized with 9:1 molar ratio of urea to TA (GT_9_NaZ) that exhibited 8.3 % N due to its higher content of structural nitrogen functionalities compared with others. Moreover, textural analysis reveals that addition of urea to the precursor mixture decreases the specific surface area and total pore volume of N-doped carbons compared with the non-doped carbon. This is due to the incorporation of nitrogen functionalities into the carbon matrix which resulted in collapse of the carbon walls and blocks some micro pores within the walls. Similar studies were also carried out by varying the amount and type of eutectic salts. It shows that the amount and the type of eutectic salts significantly influence the textural properties of N-doped carbons. Chapter II also includes the electrochemical performance of N-doped carbons in aqueous electrolyte (1 M H2SO4) and in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl) imide (PYR14FSI). The synergetic effect of physico-electrochemical properties of N-doped carbons and intrinsic properties of electrolytes significantly influence the performance of supercapacitors. Specifically, the carbon synthesized with a mixtures of 9:1 molar ratio of urea to tannic acid (urea:TA) and 1:3 mass ratio of precursors (urea+TA) to eutectic salt NaCl/ZnCl2 (GT_9_NaZ carbon ) presents the highest capacitance of 295 Fg-1 and 119 Fg-1 in 1 M H2SO4 and PYR14FSI, respectively. It was also revealed that the maximum real energy stored in supercapacitors based on N-doped carbons is as high as 6-7 Whkg-1 and 33 Whkg-1 in H2SO4 and PYR14FSI, respectively. This high capacitive performance is due to the combination of high electrical conductivity (76 mScm-1), high specific area (1329 m2g-1), and high nitrogen content presented by GT_9_NaZ N-doped carbon compared with others. The huge difference in the performance of N-doped carbons in 1 M H2SO4 and PYR14FSI electrolytes is probably due to the difference in ionic conductivity of the two electrolytes. Moreover, the variation of effective surface area (Seff) that can be accessed by ions of the respective electrolyte is also significantly influences the performance of supercapacitors. Chapter III presents the synthesis of composites of Vanadium nitride (VN) nanoparticles in N-doped carbons (VN@N-dC) by heating mixtures of Vanadium precursors (VOCl3 or NH4VO3), ionic liquid 1-ethyl-3-methyl-imidazolium dicyanamide (Emim-dca) and salts, i.e., cesium or zinc acetate, as porogens. Chapter III describes the effect of porogen salts and vanadium precursors on physico-chemical and electrochemical properties of composites. For instance, using low concentration of cesium acetate (CsAc) resulted in microporous carbon with small VN nanoparticles (VN@N-dC) and a specific surface area of 1000 m2g-1, while increasing salt amount promotes generation of small meso-pores with bigger nanoparticles and higher specific surface area up to 2400 m2g-1. In addition, utilizing zinc acetate (ZnAc) salt enabled the synthesis of entirely mesoporous composites with very small vanadium nitride nanoparticles (VN@N-dC) and specific surfaces areas of 800 m2g-1. Mixtures of these two salt porogens (CsAc & ZnAc) gave access to independently tuneable pore size, pore volume, particle size, and intermediate surfaces areas. Chapter III also includes the electrochemical performance of VN@N-dC and their applications as electrodes for supercapacitors in two different ionic liquids; PYR14FSI and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI). Better results were obtained in PYR14FSI electrolyte probably due to the higher ionic conductivity of this IL and the smaller size of FSI anion that could easily access the VN nanoparticles. Moreover, considering the synergetic properties of the composite materials, the maximum specific capacitance and real energy was stored by VN@N-dC synthesized with high content of CsAc salt and VOCl3 (VN@N-dc-1000-CsAc-VOCl3), exhibiting 125 Fg-1 and 37 Whkg-1, respectively. This confirms an improvement of specific energy density by 15 % from that of the best performing N-doped carbon (GT_9_NaZ) in PYR14FSI electrolyte. This is due to the high specific surface area of VN@N-dC and the presence of big VN nanoparticles that contribute to additional pseudocapacitance. Chapter IV & V present the synthesis of novel polymer electrolytes and their applications in solid state supercapacitors. In Chapter IV a polymer electrolyte based on a binary mixture of a Polymeric Ionic Liquid, Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PILTFSI) and a ionic liquid (PYR14TFSI) was synthesized and characterized. All-solid state supercapacitors based on this ionic liquid based polymer electrolyte (IL-b-PE), having 60 % wt. of IL, and activated carbon as electrodes were assembled and characterized. Due to the variation of double layer formation at the interfaces of electrolyte and electrodes, impregnation of activated carbon electrodes with a solution of IL-b-PE enhances capacitance of all-solid state supercapacitors compared with supercapacitors assembled by sandwiching an IL-b-PE membrane between two carbon electrodes. Moreover, supercapacitors with different impregnation ratios (electrolyte mass/active material mass) ranging from 5 to 18 were assembled simply by facing two impregnated electrodes without additional separator. All-solid state supercapacitors with impregnation ratio of 7 showed the best performance with specific capacitance and real energy of 100 Fg-1 and 32 Wh kg-1, respectively at 1 mAcm-2 and 3.5 V. Chapter V describes how the nature of ionic liquid in the polymer matrix affects the properties of polymer electrolytes and the final supercapacitor device. In particular, four different polymer electrolytes were prepared by blending the same Polymeric Ionic Liquid, PILTFSI, with four different ionic liquids; (PYR14TFSI) (IL-b-PE1), (PYR14FSI) (IL-b-PE2),1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HEMimTFSI) (IL-b-PE3), and 1-butyl-1-methylpyrrolidinium dicyanamide, (PYR14DCA) (IL-b-PE4). Physicochemical properties of IL-b-PE such as ionic conductivity, thermal and electrochemical stability were found to be dependent on the intrinsic properties of IL embedded in the polymer matrix. Specifically, ionic conductivity was significantly higher for IL-b-PE2 (2.1 mScm-1) and IL-b-PE4 (2.09 mScm-1) containing IL with small size anions (FSI, 0.3 nm and DCA, 0.24 nm) than IL-b-PE1 (0.41 mScm-1) and IL-b-PE3 (0.86 mScm-1) bearing IL with bigger anion (TFSI, 0.8 nm). On the other hand, wider electrochemical stability window (ESW) was found for IL-b-PE1 and IL-b-PE2 (up to 3.5 V) having ILs with electrochemically stable pyrrolidinium cation and FSI and TFSI anions. The best electrochemical performance was obtained by IL-b-PE2, exhibiting maximum specific capacitance and maximum real energy density as high as 150 Fg-1, and 36 Whkg-1, respectively at 1 mAcm-2 and 3.5 V. Those values are higher than the one reported in literature for solid supercapacitors based on conventional polymer electrolytes probably due to the higher operating voltage (up to 3.5 V) of IL-b-PE. It was demonstrated that a conscious selection of stable polymer matrixes such as PILTFSI and compatible ILs as well as integrating and optimizing IL-b-PE with activated carbon electrodes result in high performance all-solid state supercapacitors. For future perspective, this strategy will pave the way for the development of novel solid state, lightweight, and flexible energy storage devices.