Synthesis and electrochemical properties of ionic liquids

Integrated Master in Chemical Engineering Synthesis and Electrochemical Properties of Ionic Liquids Master Thesis of Amadeu Gomes Rocha Developed under the dissertation curricular unit Accomplished in Technische Universität München Orientation in TUM: Prof. Dr. Fritz E. Kühn Chemical Engineering Department July 2015 This page is intentionally left blank Synthesis and Electrochemical Properties of Ionic Liquids Acknowledgements Firstly, I would like to thank my grandmother, whose will to accompany me throughout my engineer course surpassed all barriers imposed by age. Also, to my parents, for giving me all the mental and affective support necessary for me to become a healthy individual. I would also like to thank Joana Trindade, the fellow adventurer and best friend that everyone should have in life. To Prof. Madeira, for the continuous incentive to students to pursuit an academic experience outside their homeland. I would also like to thank all of my professors at FEUP who were responsible for bringing me to this phase of my engineering degree. To Prof. Dr. Kühn, I would like to thank his intuition for what international relations can do for scientific development and also for trusting and welcoming me into TUM. In a more direct way, I would like to thank Robert Reich, Sara Abbassi and Dr. Iulius Markovits. To Robert, for introducing me to the world of experimental chemistry in the best way, to Sara, for all the technical advices given and to Iulius for the positive attitude always showed to me and my results. I could never forget all the invisible work of the technical support, that provide TUM an organized functioning as a teaching facility and to all the inorganic group for the good mood and availability in helping others, which is fundamental in any work environment. I would like to thank all my friends and colleagues, with their help all the obstacles were easier to surpass, I will be their reflexion as a future engineer. To the Karate group in TS Jahn München club for providing me with my first professional experience, as well as the best possible welcome to Bavaria. Synthesis and Electrochemical Properties of Ionic Liquids Resumo Os líquidos iónicos são sais com ponto de fusão inferior a 100 ºC. As suas propriedades electroquímicas únicas fazem com que estes compostos sejam vistos como uma promessa em imensas areas energéticas, nomeadamente na aplicação como eletrólitos em baterias de iãolítio. Nesta tese foram sintetizados e caracterizados sete líquidos iónicos, [BMIm][TFSI], [BnMIm][TFSI], [BnFMIm][TFSI], [BMIm][PF6], [BnMIm][PF6], [BnFMIm][PF6] e [BnMIm][BF4]. Estes líquidos iónicos foram caracterizados com recurso a espectroscopia 1H NMR,13C NMR, análise elementar e janela electroquímica. Dois dos líquidos iónicos foram purificados até altos níveis de pureza, os restantes atingiram também um nível não muito afastado do ideal. Os limites catódicos e anódicos obtidos permitiram sequenciar os catiões e os aniões por ordem de dimensão da janela electroquímica. A sequência de catiões obtida foi BnFMim+ > BnMIm+ > BMIm+ enquanto que a sequencia de aniões obtida foi PF6- > BF4- > TFSI-. A janela mais ampla foi obtida para uma solução 0.1 M de [BnMIm][PF6] com um valor de 7.05 V. Palavras-chave: Líquidos iónicos, janela elecroquímica, baterias ião lítio, catião, anião. Synthesis and Electrochemical Properties of Ionic Liquids Abstract Ionic liquids are salts with a fusion point below 100 ˚C. Their unique electrochemical properties provide these compounds a view of promise in several energetic areas, namely in the application as electrolytes in lithium-ion batteries. In this thesis seven ionic liquids were synthesised and characterized, [BMIm][TFSI], [BnMIm][TFSI], [BnFMIm][TFSI], [BMIm][PF6], [BnMIm][PF6], [BnFMIm][PF6] and [BnMIm][BF4]. These ionic liquids were characterized with the help of 1H NMR and 13C NMR spectroscopy as well as elemental analysis and electrochemical window. Two of the ionic liquids were purified to a high degree of purity, and the remaining also achieved a level close to ideal. The cathodic and anodic limits allowed the sequence of the anions and cations by dimension order of the electrochemical window. The obtained cations sequence was BnFMim+ > BnMIm+ > BMIm+ as the anion sequence was PF6- > BF4- > TFSI-. The wider window was obtained for a 0.1 M solution of [BnMIm][PF6] with a value of 7.05 V. Key words: Ionic liquids, electrochemical window, li-ion batteries, cation, anion. Synthesis and Electrochemical Properties of Ionic Liquids Declaration I hereby declare under commitment of honour, that this work is original and that all its non-original contributions are duly referenced with source identification. Amadeu Gomes Rocha, July 2015 Synthesis and Electrochemical Properties of Ionic Liquids vii List of abbreviations δ chemical shift A Ampere ACM active charcoal method AL anodic limit CE counter electrode CL cathodic limit CV cyclic voltammetry DCM dichloromethane e.g for example EVs electric vehicles EW electrochemical window Fig. figure GC glassy carbon HEVs hybrid-electric vehicles IL ionic liquid LSV linear sweep voltammetry M molar mbar milibar MHz megahertz mL millilitre m.p melting point n.a not analysed NiCd Nickel-Cadmium battery ppm parts per million NMR nuclear magnetic resonance NTU Nanyang Technological University Synthesis and Electrochemical Properties of Ionic Liquids viii QRE quasi reference electrode R organic rest RE reference electrode r.t. room temperature Tab. table TUM Technical University of Munich U.S United States WE working electrode V Volts Synthesis and Electrochemical Properties of Ionic Liquids ix List of ionic liquids and other chemicals [BMIm][Br] 1-Butyl-3-methylimidazoliumbromide [BnMIm][Br] 1-Benzyl-3-methylimidazoliumbromide [BnFMIm][Br] 2,3,4,5,6-Pentafluorobenzyl-3-methylimidazoliumbromide [BMIm][PF6] 1-Butyl-3-methylimidazolium hexafluorophosphate [BnMIm][PF6] 1-Benzyl-3-methylimidazolium hexafluorophosphate [BnFMIm][PF6] 2,3,4,5,6-Pentafluorobenzyl-3-methylimidazolium hexafluorophosphate [BMIm][TFSI] 1-Butyl-3-methylimidazolium bis(trifluormethansulfonyl)imide [BnMIm][TFSI] 1-Benzyl-3-methylimidazolium bis(trifluormethansulfonyl)imide [BnFMIm][TFSI] 2,3,4,5,6-Pentafluorobenzyl-3-methylimidazolium bis(trifluormethansulfonyl)imide [BMIm][BF4] 1-Butyl-3-methylimidazolium tetrafluoroborate [BnMIm][BF4] 1-Benzyl-3-methylimidazolium tetrafluoroborate [BnFMIm][BF4] 2,3,4,5,6-Pentafluorobenzyl-3-methylimidazolium tetrafluoroborate [PMIm][TFSI] Methylimidazoliumbis(trifluoromethylsulfonyl)imide BnBr Benzyl bromide FBnBr 2,3,4,5,6-Pentafluorobenzyl bromide BuBr Butyl bromide MIm 1-Methylimidazole MImBr 1-Methylimidazoliumbromide BuBr Butyl bromide LiPF6 Lithium hexafluorophosphate [EtNH3][NO3] Ethylammoninum nitrate BF4 Tetrafluoroborate PF6 Hexafluorophosphate Synthesis and Electrochemical Properties of Ionic Liquids x TFSI Bis(trifluoromethansulfonyl)imide LiTFSI Lithium bis(trifluoromethansulfonyl)imide BMIm 1-Butyl-3-methylimidazole BnMIm 1-Benzyl-3-methylimidazole BnFMIm 2,3,4,5,6-Pentafluorobenzyl-3-methylimidazole Fe Ferrocene NH4PF6 Ammonium hexafluorophosphate NH4BF4 Ammonium tetrafluoroborate DCM Dichloromethane MgSO4 Magnesium sulfate Synthesis and Electrochemical Properties of Ionic Liquids 1. Introduction 1 1 Introduction 1.1 Theoretical introduction 1.1.1 Battery, a brief history Electricity can be viewed as one of the most important discoveries of Mankind since fire. It is easy to assume that humans were aware of its existence for a long time, since it is observable in natural phenomena such as thunderstorms or electric fishes, but there is some disagreement about when humans were capable of producing electricity. Some believe that the called Parthian Battery, dated around 2000 years ago, was indeed a battery. After all, the main components of a battery were found nearby; a clay jar once filled with some acid, like vinegar (electrolyte), an iron rod (positive terminal) covered by a copper cylinder (negative terminal) is known to be capable of producing from 1.1 to 2.0 Volts (V) [1]. It is not fully accepted that the Parthians were already using the Parthian Jar as a battery, perhaps it was only an hermetical vessel to store important scrolls, where the paper could decompose leaving a trace of acidic organic residue inside [2]. In the beginning of the 19th century, Alessandro Volta gave one of the first public demonstrations of electricity. Volta figured out that two different metals in the presence of some fluids would create a continuous electric effect. By feeling that effect with his own tong, and trying different combinations of metals, Volta sorted them by electric potential. After Volta discovered that the presence of an acid would increase the so called “electric effect” he constructed the “first official device to create an electric current for a length of time”, the battery. It consisted of a column of copper coins (anode) alternated with plates of zinc (cathode) separated by leather soaked with salt (electrolyte). Volta’s work awarded him a Copley Medal from the Royal Society of London, and the interest of Napoleon Bonaparte, which boosted the interest of other scientists in trying to improve the Volta’s battery [1, 3]. In the same beginning of century, in the year 1800, Sir Humphry Davy discovered the principles of electrolysis. Two years later William Cruickshank designed a squared sealed version of the battery similar to the flooded battery that we use today. With this design it was possible to provide more energy than the Volta’s discs battery and did not suffer from drying out. The drying out effect refers to the hydrolysis that comes from the normal use of Synthesis and Electrochemical Properties of Ionic Liquids 1. Introduction 2 batteries or an overcharge that splits the water into hydrogen and oxygen [1] [4, 5]. Later in 1859 the physicist Gaston Planté invented the first rechargeable battery, based in lead-acid. This device has a low energy density but very high current intensity, which does not suffer from memory effects and can recharge itself with accumulators. Due to the advantageous factors, this kind of battery is still used in starting, lighting and ignition batteries in cars [6, 7]. The nickel-cadmium battery (NiCd) was later invented in 1899 by the Swedish Waldemar Jungner as an alternative for the lead-acid batteries. However, it was only later in 1932 that Shlecht and Ackerman achieved satisfactory load currents and improved longevity of NiCd with the invention of the sintered pole plate. This new kind of battery was only available to commercial usage in 1947 when Georg Neumann sealed the cell. This made it possible to apply rechargeable batteries in portable applications due to the fast charging, high number of charge/discharge cycles and the good low temperature performance. With the NiCd growing in the market, there was the environmental problem associated with this new kind of batteries toxicity. This undesirable characteristic soon urged the necessity to change to Nickel-metal hydride batteries, which was the first step to the introduction of Li-ion batteries in the market in 1991 [1]. The Li-ion batteries were being studied since 1976 [8], although they were only introduced in the market after it was proven that the use of an anode with intercalated carbon/graphite could eliminate the problems of poor lithium metal rechargeability; this greatly improved the safety of the battery system. For this major step in the history of batteries the work of J.O. Besenhard at the Technical University of Munich (TUM) was very important. In the 70s, while there were being done numerous researches about lithium batteries, J.O Besenhard was studying the reversible alkali metal ion intercalation into graphite (anodes) [9] and oxide materials (cathodes)[10]. Soon he proposed his work to be introduced in lithium batteries [11, 12]. Followed by the work of Goodenough, which used lithium cobalt oxide (LiCoO2) as a cathode material for the first time, and the Yazami’s, who discovered the graphite electrode used in commercial lithium it was possible for SonyTM in 1991 to commercialize the first Li-ion battery [13]. This new kind of batteries revolutionized the market of rechargeable batteries. They had a higher energy density, need not periodical maintenance, the self-discharge was less than half of the nickel based batteries and they did not need to be discharged completely before charging. Furthermore, these batteries appeared not in a fully matured state, and its constitution has been improving on a continuing basis since their appearance [13]. Synthesis and Electrochemical Properties of Ionic Liquids 1. Introduction 3 Nowadays the biggest challenge of Li-ion batteries is the entrance in the electric vehicles (EVs) and hybrid-electric vehicles (HEVs) market. For that purpose improvements should be done in terms of safety, cost and performance. One of the ways to achieve this purpose could be the replacement of the State of the art organic liquid electrolytes with solid electrolytes. This would enable high energy and an intrinsically safe cell design. However the already existing electrolytes of this kind cannot be implemented in real-time practical applications due to their narrow electrochemical window or low ionic conductivity [14]. Another possible solution for the safety and performance problems is the substitution of the used carbonate based solvents by ionic liquids (ILs). 1.1.2 The battery functioning To each chemical species there is an associated energy, this energy, entitled as chemical energy, is present in every molecular bond. As with all chemical reactions, there is a reactant turning into a product with a different energy value. It is comprehensible that science developed ways to use this energy present in all molecules for its own use. A battery is precisely one of those devices. In its basic constitution there is a negative and a positive electrode. While discharging, the battery negative electrode is an anode where oxidation occurs. Meanwhile the positive electrode is a cathode, where there is a reduction process. By making the released electrons pass from one electrode to the other by an external circuit electric energy is produced. The separation of two electrodes is gathered with an electrolyte, which acts as electronical insulant as well as an ionical conductant. This way the electric current is separated from the ionic current since only the positive ions formed on the electrodes can go through the electrolyte [15, 16]. When recharging a battery, electric energy is given to the system, making the electrons flow into the opposite direction. This is only possible if the oxidation-reduction reactions are reversible in both electrodes. If so, the chemical species return to their original state, before discharging. A schematic figure can be seen in the Fig. 1. Synthesis and Electrochemical Properties of Ionic Liquids 1. Introduction 4 1.1.3 Ionic liquids: a brief history and electrochemical applications By convention, ionic liquids are labelled as salts with melting points below 100°C. The number of publications about ILs has been increasing year by year since the beginning of the millennium (Fig. 2). The increase in academical interest is correlated with the potential advantages that ILs can offer to different industries [17]. Figure 2. Number of publications dealing with ionic liquids since 2001 [17]. Being the successors of the high temperature molten salts, ILs do not have the inconvenience of needing high operational temperatures which make industrial processes very expensive due to energy necessities. It was in 1914 the first time an IL was synthesized deliberately for its ionic character, in that time Paul Walden prepared ethylammoninum 0 1000 2000 3000 4000 5000 6000 7000 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 Number of Publications year Figure 1 Battery discharging (left) and charging (right). Synthesis and Electrochemical Properties of Ionic Liquids 1. Introduction 5 nitrate (m.p 12ºC) [EtNH3][NO3], which was later studied in extensive detail [18]. This molecule can be seen in Fig.3. Figure 3. Ethylammoninum nitrate molecule. Later in 1963, the U.S. Air Force Academy tried to find more adequate electrolytes for thermal batteries and intensified the research on ILs. Their first result was a chloroaluminate developed from a mixture of alkali halides and aluminium chloride [19]. But it was only 15 years later that the era of ionic liquids really started. In 1978 Gale et al. synthesized the 1-butylpyridinium tetrachloroaluminate, (C9H14AlCl4N) Fig. 4. This point marked an overturn since it stimulated great interest to chemists trying to improve the properties of this new IL [20]. Figure 4. 1-butylpyridinium tetrachloroaluminat. However, one of the disadvantages of the pyridinium and imidazolium based chloroaluminate was the production of hydrochloric acid in presence of water, which was avoided firstly in 1992 when Zaworotko et al. had the idea to synthesize salts with dialkylimidazolium cations and water-stable anions. This lead to the substitution of the water sensitive tetrachloroaluminate by the more stable tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), nitrate (NO3-), sulfate (SO42-) and acetate salts (C2H3O2−) [21]. Although these compounds were first regarded as potential electrolytes for batteries by Zaworotko et al. these new ionic liquids were more useful in other applications [19]. This step ended the first generation of ILs and marked the beginning of the generation of nonaluminate ILs or second generation ILs. The synthesis of the hydrophobic dialkylimidazolium ILs with the bis(trifluoromethansulfonyl)imide (TFSI) (C2F6NO4S2-) anions was described by Bonhôte et al. in 1996 [22]. The third generation of ionic liquids appeared around the 2000s and is comprised of the task-specific ionic liquids and chiral ionic liquids [23]. Synthesis and Electrochemical Properties of Ionic Liquids 1. Introduction 6 Assembling the ILs in generations is only a way to order the chronological research focus of ILs. There are other ionic liquids which do not fit in these groups such as the previous mentioned ethylammoninum nitrate [23]. In Fig. 5 are shown typical cations and anions from each generation of ILs. One of the most important characteristics in the added value of ILs is their highly flexible nature made possible by the diversity of anion-cation combinations and different modes of preparation. This enables a rational design of ILs to fine-tune their physical and chemical properties for a wide range of applications [24]. In fact ILs are viewed as novel and propitious materials for their wide range of useful physical and chemical properties, such as: thermal and electrochemical stability, low vapor pressure, electric conductivity, interesting solvent properties, possible biphasic systems, liquid crystalline structures, high heat capacity, non-flammability and electroelasticity. All these properties illustrate that ILs are suitable compounds for various applications, e.g: electrolytes, catalysis, separation technologies [25], lubricants [26], heat storage [27] and analytic chemistry [28, 29]. ILs were considered for some time as rather green solvents, as they are non-flammable, have a negligible vapour pressure and excellent thermal and chemical stability compared to Figure 5. Typical cations and anions from each generation of ILs. Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 13 3 Technical Description 3.1 Chemicals used in the synthesis of the ionic liquids All the chemicals used for the synthesis of the ILs can be seen on the following Tab. 2: Table 2. Chemicals used in the synthesis of ILs. Substance Manufacturer Purity [%] 1-methylimidazole abcr GmbH 99 1-bromobutane Sigma Aldrich 99 Toluene Acros 99 Magnesium sulfate hydrate Riedel-de Häen 99 Lithiumbis(trifluormethansulfonyl)imide Acros 99 Benzyl bromide Sigma Aldrich 98 2,3,4,5,6-pentafluorobenzyl bromide Sigma Aldrich 99 Ammonium hexafluorophosphate Sigma Aldrich >98 Ammonium tetrafluoroborate Acros 99 Litium hexafluorophosphate solution 1M Sigma Aldrich 50 Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 14 3.2 Methods The 1H (400 MHz) and 13C (100 MHz) spectrums were performed by a 400 MHz Bruker Advance DPX-400 Spectrometer. The solvent signals used as internal standards to evaluate the synthesized chemicals were CDCL3 (1H: δ=7.26 ppm; 13C: δ=77.16 ppm), DMSO-d6 (1H: δ=2.50 ppm; 13C: δ=39.52 ppm) and CD3CN (1H: δ= 1.94 ppm; 13C: δ=118.26 ppm). The elemental analyses were performed by the scientific staff of TUM on a Flash EA 1112. The electrochemical tests were performed with a Reference 600 Potentiostat ZRA controlled by the Gamry Framework software. All measurements were performed at room temperature and the used scan speed was 25 mV/s. The used electrodes were the glassy carbon ETO074 with 1 mm diameter as working electrode, Pt/titanium wire anode ETO074 as counter electrode and the Ag/AgCl as QRE. Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 15 3.3 Procedure The first step in the synthesis of the proposed ILs was the synthesis of the three bromide salts which would be precursors to the synthesis of the proposed ILs by anion substitution. 3.3.1 Bromides synthesis A scheme for the bromides reactions can be seen in Fig. 7. Figure 7. Scheme with the bromides reactions. The syntheses were performed in 50 mmols batches. For each synthesis, 50 mmols of 1-methylimidazole (MIm) (C4H6N2) and 55 mmol of butyl bromide (BuBr) (C4H9Br), benzyl bromide (BnBr) (C6H5Br) and 2,3,4,5,6-pentafluorobenzyl bromide (FBnBr) (C7H2BrF5) were weighed. The respective weights are shown in the Tab. 3. Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 16 Table 3. Weights of used reactants for the bromides synthesis Reactants Weight(g) MIm 4.10 BuBr 7.54 BnBr 9.40 FBnBr 14.34 50 mL of toluene (C7H8) was used as solvent for the reactions which were done in 250 mL round flasks. The reactions proceeded at 40 °C, and were consecutively warmed at the rate of 10 °C per hour in an oil bath. After reaching the desired temperature of 80 °C, the reaction was left overnight under reflux conditions. In the end, two phases were present for all the synthesized bromides and the upper phase that only contained the solvent was decanted. The small amount of toluene left was removed for three hours in the rotary evaporator at 75 mbar and 40 °C. For purification, the products were washed three times with 10 mL of diethyl ether (C4H10O) in order to remove the excess reactant. Each of the washes was done for 10 minutes while stirring. The diethyl ether was then removed for one hour in the rotary evaporator at 850 mbar and 40 °C. The [BMIm][Br] and the [BnFMIm][Br] crystalized as white and light yellow powders, respectively, while the [BnMIm][Br] was obtained as a very viscous light brown liquid. The [BnFMIm][Br] crystallization occurred in the rotary evaporator, the [BMIm][Br] crystalized in the fridge at 4 °C . 3.3.2 Anion Substitution Seven ILs were successfully synthesized. They were the [BMIm][TFSI] (C10H15F6N2O4S2), [BnMIm][TFSI] (C13H13F6N2O4S2), [BnFMIm][TFSI] (C13H8F11N2O4S2), [BMIm][PF6] (C8H15F6N2P), [BnMIm][PF6] (C11H13F6N2P), [BnFMIm][PF6] (C11H8F11N2P) and [BnMIm][BF4] (C11H13BF4N2). The salts used for the anion substitutions were lithiumbis(trifluormethansulfonyl)imide (LiTFSI) (C2F6LiO4S2), ammonium hexafluorophosphate (NH4PF6) and ammonium tetrafluoroborate (NH4BF4) whose anions are shown in Fig. 8. Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 17 Figure 8. Anions of the salts used for the anions substitutions. The substitutions of the bromide salts to TFSI, PF6 and BF4 salts were done in 30 mmol batches. For each substitution reaction, 30 mmol of each synthesized bromide IL, [BMIm][Br], [BnMIm][Br], [BnFMIm][Br] and 36 mmols of each salt, LiTFSI, NH4PF6, NH4BF4 were weighed. The respective weights are shown in Tab. 4. Table 4. Weights of used reactants for the anions substitutions. Reactants Weight (g) [BMIm][Br] 6.57 [BnMIm][Br] 7.59 [BnFMIm][Br] 10.29 LiTFSI 10.35 NH4PF6 5.87 NH4BF4 3.77 For the anion substitution of bromide to the TFSI salts, the bromide was dissolved in 100 mL of water, while for the PF6 and BF4 salts only 20 mL of water was used as solvent. This was the method chosen since these ILs showed to be more soluble in water than the TFSI ILs and to use similar proportions to the ones used in the work of Laszlo and Compton [52]. The excess reactant was added slowly to the solution. The reactions were all stirred at room temperature for 16 hours in 250 mL Erlenmeyer flasks. In the end two phases were seen in all the solutions except for the [BnFMIm][TFSI] and [BnFMIm][PF6], which were milky foamed solutions dissolved in water. For the [BMIm][TFSI] and [BnMIm][TFSI] the water phase was decanted off while the organic phase was washed three times with 100 mL of water, to remove the excess of reactant salt present in the solution. Freshly distilled dichloromethane Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 18 (DCM) (CH2Cl2) was added to dissolve the product and the water left on the ionic liquid was removed with the addition of a small amount of desiccant magnesium sulfate (MgSO4). The mixture was stirred for 30 minutes and afterwards was filtered. The DCM was then removed for half an hour in the rotary evaporator at 750 mbar and 40ºC. For the ILs with the [BnFMIm] cation, the water was extracted with vacuum filtration while the product was washed with sprayed water. As for the [BMIm][PF6], [BnMIm][PF6] and [BnMIm][BF4] as the miscibility in water was considerable, the washing step was made with a volume ratio of 3 DCM to 1 of water. Afterwards the DCM was evaporated for half an hour in the rotary evaporator at 750 mbar and 40 °C, and the water at 55 mbar and 50 °C till no more bubbling was seen exiting the IL. In the Tab. 11 in the Annex 2 is summarized the final reaction appearance, the extraction method as well as the final product appearance for each synthesized IL. Since it is known that the size of the EW is inversely proportional to the water content in the ionic liquids [40], all the products were transferred to Schlenk tubes, and dried over 16 hours under reduced pressure in the Schlenk line, at room temperature, in order to remove the vestigial water. The purity of the ionic liquids was determined by 1H and 13C NMR spectroscopy, followed by elemental analysis. The first results of elemental analysis for the three TFSI salts revealed the presence of the bromide anion. The washing was repeated for these three salts plus three washes with diethyl ether. The ether was then removed for one hour in the rotary evaporator at 850 mbar and 40°C. After this wash, it was attempted to do an extra cleaning with activated charcoal. This method has already been reported as successful in the removal of impurities from ILs [53]. However this treatment with adsorbents launched some controversy due to the possibility of adding extra impurities, even harder to extract [54]. Due to this possibility, only one third of the samples were subject to this method. The samples were dissolved in DCM and 250 mg of activated charcoal was added to the solution and stirred for 2 hours at 30 ºC. Afterwards the charcoal was filtered and the DCM removed in one hour in the rotary evaporator. One sample of each of the TFSI ILs with charcoal treatment and without were examined with elemental analysis for further comparison. By observation the only difference noticed was in the [BnFMIm][TFSI] which previously had a light yellow colour, and after the charcoal purification turned into a white crystal. It has been already reported by Nockemann et al. that ILs tend to be contaminated by coloured impurities that can be removed by treatment with active charcoal [55]. Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 19 3.3.3 Electrochemical tests Since soluble oxygen is electrochemically active, its reduction can be a problem for accurate electrochemical window measurements [56]. To make sure that the electrochemical tests were carried out under inert atmosphere, all solutions for the LSV tests were prepared in a glove box operating with argon. Each of the IL solutions of 0.10 M and 0.25 M were prepared inside electrochemical cells with 1 mL of lithium hexafluorophosphate (LiPF6) as solvent. The respective weights are shown in the Tab. 5. Table 5. Weights for the LSV solutions Ionic Liquid 0.1 M (mg) 0.25 M (mg ) [BMIm][TFSI] 41.9 105.0 [BnMIm][TFSI] 45.3 113.0 [BnFMIm][TFSI] 54.3 136.0 [BMIm][PF6] 28.4 71.0 [BnMIm][PF6] 31.8 79.5 [BnFMIm][PF6] 40.8 102.0 [BnMIm][BF4] 26.0 65.0 Ideally the LSV tests should be done inside the glove box to make sure that no oxygen or water would enter in the solution, however in the present work that was not possible. The electrochemical cells were sealed with parafilm. The transportation from the glovebox to the LSV room was made with the samples inside a bigger flask also sealed with parafilm. The time between this moving was never more than 15 minutes. Before each measurement a washing and polishing procedure was done to the electrodes. In the first trial of each sample, the three electrodes were washed with deionized water, methanol and acetone. Then a polishing step was done with diamond polisher followed by alumina polisher. Between trials only the WE and RE were cleaned, and this process was always as it follows: rinsed with water and acetone, polished in the diamond polisher, rinsed again, polished in alumina polisher and followed by a last wash. In the end the parafilm seal was pierced by the three electrodes and the tests began. Two trials were done for each sample. The data was inputted for the LSV tests as it is shown in Tab. 6. Synthesis and Electrochemical Properties of Ionic Liquids 3. Technical Description 20 Table 6. Data inputted in Gamry framework software. Initial potencial Between 4.5 and 3.5 V Final potencial Between -4.5 and -3.5 V Scan rate 25 mV/s Step size 1 mV Max current 0.1 mA Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 21 4 Results and discussion 4.1 Products characterization In all the displayed NMR results it is also presented the respective molecule. The lower case letters are always written first in the brackets and can also be seen in the molecules figures. They represent the correspondent 1H signals, while the numbers written after each peak signal represent the 13C. All the obtained spectrums are presented in the Annex 3. 4.1.1 Bromides spectrums Due to being intermediate products, the synthesized bromides were only characterized with 1H and 13C NMR spectrometry. It is only presented one graph for each synthesis, and they can be seen in the following Fig. 9 to 11. Figure 9. [BMIm][Br] 1H NMR (400MHz, CDCl3, RT, ppm) δ = 10.01 (d, 1H, s), 7.52 (b, 1H, s), 7.41 (a, 1H, s), 4.11 (c, 2H, t), 3.89 (e, 3H, s), 1.67 (f, 2H, quin), 1.15 (g, 2H, sex), 0.71 (h, 3H, t); 13C NMR (100.28 MHz, CDCl3, RT, ppm) δ = 136.53 (3), 123.53 (2), 121.91 (1), 49.29 (5), 36.23 (4), 31.70 (6), 18.95 (7), 13.02 (8). Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 22 Figure 10. [BnMIm][Br] 1H NMR (400MHz, CDCl3, RT, ppm) δ = 10.22 (d, 1H, s), 7.63 (b, 1H, s), 7.51 (a, 1H, s), 7.447.25 (f, 5H, m), 5.53 (c, 2H, s), 3.96 (e, 3H, s); 13C NMR (100.28 MHz, CDCl3, RT, ppm) δ = 136.96 (3), 133.10 (6), 129.38 (7), 129.32 (8), 128.92 (9), 123.81 (2), 122.06 (1), 53.10 (5), 36.73 (4). Figure 11. [BnFMIm][Br] 1H NMR (400MHz, CDCl3, RT, ppm) δ = 10.37 (d, 1H, s), 7.70 (b, 1H, s), 7.57 (a, 1H, s), 5.80 (c, 2H, s), 4.09 (e, 3H, s); 13C NMR (100.28 MHz, CDCl3, RT, ppm) δ =137.91 (3), 124.91 (2), 122.25 (1), 41.02 (5), 37.06 (4). In this 13C NMR spectrum it is not possible to see the atom carbons of the FBn group, this is due to the fluorine atoms that tend to disturb the proper measuring of the carbon atoms. The excess of bromide reactant would be seen in the alkane region (1-2 ppm) and this was never noticed in the analysed spectrums. Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 29 4.2 Electrochemical tests results In the Fig. 19 and 20 it is shown the graphs as they were plotted by the software Gamry Framework. Figure 19. LSV graph of Blank LiPF6. Figure 20. LSV graph of [BnMIM][PF6] 0.10 M (2nd trial). -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10 -5.00 -3.00 -1.00 1.00 3.00 5.00 mA V Blank LiPF6 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10 -5.00 -3.00 -1.00 1.00 3.00 5.00 mA V [BnMIM][PF6] 0.10 M (2nd trial) Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 30 It is possible to notice the slope present around -2 V for the IL and in slight extension, the blank sample. This slope was present in all the Ionic liquids, in greater or lesser extent. However, for the ILs the slopes have always similar proportions to the one in the presented in the Fig. 20. It is important to notice that even with the all the efforts made to try to get an inert atmosphere inside the vial, it is impossible to assure that at the moment of the piercing of the seal some air and humidity from the air did not enter. Silvester and Compton showed that the EWs for imidazolium-based ILs suffer a narrowing in the cathodic area when performed in ambient air [58]. This bigger affinity of imidazolium with water than other moieties can be a reason for the bigger slopes in all the tested ILs. In the work of Hayyan et al. the small slope is also seen in some of the electrochemically tested ILs, the authors relate the issue with the amount of impurities present in the ILs [40, 59]. However in the present work two ILs were synthesized with higher purity than the others, the [BMIm][TFSI] and the [BnMIm][BF4], by observing their graphs and comparing with one of the less pure ILs, the [BnFMIM][TFSI] it is not possible to conclude the same. This comparison can be seen in the Fig. 43 in the Annex 4 This comparison is not the more accurate possible since the compared ionic liquids are different and their interaction with the impurities may be different. After normalizing the y axis with regard to the working electrode area (0.007854 cm2), the graphs showed in Fig. 19 and 20 were obtained as it showed in the Fig. 21. Figure 21. LSV graph of [BnMIM][PF6] 0.1 M (2nd trial) and Blank LiPF6. -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 -5.00 -3.00 -1.00 1.00 3.00 5.00 mA/cm2 V [BnMIM][PF6] 0.10 M (2nd trial) Blank LiPF6 Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 31 With current density (mA/cm2) in the y axis the slope produces a bigger impact in the graph. This way in order to be possible to see the full EW and be able to compare the obtained EWs of the different ILs it was chosen to calculate the EWs with the maximum cutoff current density advisable, 1 mA/cm2 [40]. The CL, AL and EWs obtained for each performed test are shown in Tab. 7. Table 7. LSV EWs results Ionic Liquid EWs 1st trial 2nd trial Blank 6.17 6.16 [BMIm][TFSI] 0.1 M 6.10 6.07 [BMIm][TFSI] 0.25 M 4.72 6.23 [BnMIm][TFSI] 0.1 M 6.05 6.23 [BnMIm][TFSI] 0.25 M 6.18 4.22 [BnFMIm][TFSI] 0.1 M 6.34 6.43 [BnFMIm][TFSI] 0.25 M 6.26 6.26 [BMIm][PF6] 0.1 M 6.07 6.10 [BMIm][PF6] 0.25 M 5.89 6.15 [BnMIm][PF6] 0.10 M 6.25 7.05 [BnMIm][PF6] 0.25 M 6.31 6.10 [BnFMIm][PF6] 0.10 M 6.39 6.50 [BnFMIm][PF6] 0.25 M 6.27 6.50 [BnMIm][BF4] 0.10 M 6.30 6.33 [BnMIm][BF4] 0.25 M 6.37 6.53 The EWs present in the Tab. 7 cannot be compared with data from literature for the following reasons; literature was not found reporting EWs with the same electrodes for any of Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 32 these ILs and it was not possible to ensure totally inert air conditions. The presented ILs EWs are only comparable with the ones present in this thesis. The CL and AL values present in the Tab. 12 in the Annex 4 cannot be compared with others present in literature for the reasons stated above and because no internal reference compound, like ferrocene, was used to calibrate the measurements. However some trends are analysed in this chapter. The obtained EWs with values below 5 V are crossed out in Tab. 7 and have been discarded from the following analysis. The great voltage difference between the two trials with each ionic liquid and also between the different ILs showed in Tab. 7 caused the discard of the values outside the general pattern. It is thought that in these measurements a problem must have happened, probably an inadequate cleaning step. 4.2.1 EW overview The wider obtained EW was 7.05 V, obtained for the [BnMIm][PF6] 0.10 M, while the narrower EW found was 5.89 for the [BMIm][PF6]. Comparing with the work of Can-can et al, it is possible to see that the value of the [BMIm][PF6] has been reported with a wider EW than the one for [BMIm][TFSI] with experiments done with the same electrodes. The same happened in this work. Comparing the averages obtained for some groups of ILs it is possible to realize some patterns. In Tab. 8 are reported the averages for the group of EWs with the same cation and same concentration. Table 8. Average values for the same cation and concentration Concentration Cation 0.1 M 0.25 M [BMIm]+ 6.09 V 6.09 V [BnMIm]+ 6.36 V 6.30 V [BnFMIm]+ 6.42 V 6.32 V It is possible to establish the following order for the cations EWs for both concentrations: [BMIm]+ < [BnMIm]+ < [BnFMIm] +. It is also possible to see that apart from the BMIm cation the EW tends to get narrower for the higher concentrations. For the anions, the analysis has to be done with respect only to the BnMIm cations since it is the only cation Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 33 coupled with the three anions. In Tab. 9 are reported the averages for the group of EWs with the [BnMIm]+ the same anion and same concentration. Table 9.Average values for BnMim cation, same anion and concentration. Concentration Anion 0.1 M 0.25 M [TFSI]- 6.14 V 6.20 V [PF6]- 6.65 V 6.20 V [BF4]- 6.32 V 6.45V The best result of this thesis was obtained for the [BnMIm][PF6], for lower concentrations. In this scenario, the [PF6] was the anion with the best performance. For 0.1 M the order in width for the EW is as follows: [PF6]+ > [BF4]+ > [TFSI]+. Although the results compiled in the work of Can-can et al. are for the [BMIm], the order of the values discovered in Tab. 1 is the same as in this study. And it is also in accord with the performed work of Ue et al.[60] . In Tab. 10 the averages for the group of EWs with the same cation and same concentration are showed. Table 10.Average values for the same anion and concentration Concentration Anion 0.1 M 0.25 M [TFSI]- 6.20 V 6.23 V [PF6]- 6.39 V 6.20 V A more complete anion analysis can be done between the PF6 and TFSI since they have been synthesised coupled with the three cations. With this analysis it is possible to see that their performance is similar for 0.25 M but the PF6 has better results for 0.1 M. This decrease in the performance for higher concentrations can be explained by the much higher viscosities of the PF6 based ILs compared with the TFSI based ILs [61]. Observing the values regarding the same concentrations in the last three tables it is possible to see that sometimes the EW widens with an increase in the concentration and sometimes it narrows. With a more Synthesis and Electrochemical Properties of Ionic Liquids 4. Results and Discussion 34 attentive look it is possible to notice that the drop or increase is somehow related with the content of fluorine. ILs with more fluorine tend to have a decrease in their EW for the 0.25 molar solutions. Another fact to note is that the bad measurements crossed out in Tab. 7 happened in the CL of TFSI ILs as can be seen in the Tab.12 seen in Annex 4. Besides the possibility of a bad washing step, this occurrence can be due to the lower hydrophobicity of the TFSI anion when compared with the BF4 and PF6. Since these measurements where done under uncontrolled atmosphere conditions it could happen that the TFSI ILs had a bigger interaction with the H2O present in the air affecting the LSV measurement [48]. The fact that this also happened for the higher concentrations of this salt is another event that corroborates this idea. Synthesis and Electrochemical Properties of Ionic Liquids 5. Conclusions 35 5 Conclusions Seven ILs were successfully synthesized and purified, two of them with high purity levels. Their electrochemical windows were characterized making it possible to establish the following relations to their width: BnFMIm+ > BnMIm+ > BMIm. A sequence was also established for the anions PF6- > BF4- > TFSIwhich is in accord to what was found in literature. Both [BMIm][BF4] and [BnFMIm][BF4] were synthesized, but their purification was not completed in the present work. The method used to order the referred sequences was based in a statistical approach done with the results. The failure of two EW characterizations caused that they only had one measurement reported as opposed to every other IL, who had two for each concentration. This fact, along with the absence of the [BMIm][BF4] and [BnFMIm][BF4] ILs from the electrochemical tests, makes the calculation of the average values a less rigorous approach, seen as the same number of events were not utilized for all the used anion and cation combinations. However, since the characterized EWs followed clear patterns, it is possible to assume that the obtained results were a success in identifying the most promising ILs in the scope of this work. It is possible to create a relation between the ions with better performances and the higher number of fluorine atoms in the molecule. Nonetheless, if someday it is applied, it is necessary to keep in mind that ILs with the PF6and BF4anions at certain temperatures are hydrolytically unstable and can evolve into HF [62, 63]. The inert air conditions necessary to benefit from their physical properties is another drawback shared by all the ILs synthesized in this work. The high prices of industrial scale inert air can be a prohibitive factor for some industries. The EW values obtained are too wide when comparing with the available literature, this happened because of the maximum cut off current used which was 1 mA/cm2. Most of the reported literature uses 0.1 mA/cm2, however this needs to be adapted for each case. However the sequences made make possible to predict which compounds should be studied more deeply in the future. Beyond the scope of the proposed work was also studied the effect of the charcoal on the purification of ILs, and this method had positive results in two out of three tested cases. The work presented in this thesis was the beginning of a new topic in TUM, and it will be directly continued by a member of the research team. The ILs electrochemical tests should be reproduced with an internal reference, in order obtain reliable Cl and Al values, if possible ensuring inert atmosphere conditions. The ILs for which was not achieved an ideal Synthesis and Electrochemical Properties of Ionic Liquids 5. Conclusions 36 level of purity should be washed again. It would also be advisable to determine the water content of each ionic liquid before the new tests. Afterwards other important physical and chemical properties of the ILs should be tested e.g. viscosity, conductivity. Then it should be studied how the proposed ILs influence the lithium ion transport and the electrode interactions that occur in an operating lithium cell [34]. The biggest difficulties in the accomplishment of this thesis were at level of the purification of the ILs which was much time consuming. The work in a glove box was also a big challenge, namely in the accurate weighing of the ILs to make the solutions for the electrochemical tests. All the machinery inside the glove box room increased the room temperature, making it very hot and a more difficult task. The ILs can turn to be the future of a wide range of applications, to expedite the research in this area some conventions should be made regarding the experimental conditions. The optimal electrodes combinations should be established in way that the CLs and ALs can be recorded with the same conditions. This would enable more accurate comparisons between different studies and consequently a better understanding of the chemical interactions at stake. Synthesis and Electrochemical Properties of Ionic Liquids References 37 References 1. Buchmann, I., Batteries in a Portable World: A Handbook on Rechargeable Batteries for Non-Engineers. Cadex Electronics Inc, 2011. 2. Fitzpatrick-Matthews, K. The ‘batteries of Babylon’. 2009. Retrieved at [28/04/2015]; Available from: http://www.badarchaeology.com/out-of-placeartefacts/anomalously-old-technology/the-%E2%80%98batteries-ofbabylon%E2%80%99/. 3. Volta, A. J. Banks, I. 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NMR spectrum of [BnFMIm][Br] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 46 Figure 29. NMR spectrum of [BMIm][TFSI] 1H. Figure 30. NMR spectrum of [BMIm][TFSI] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 47 Figure 31. NMR spectrum of [BnMIm][TFSI] 1H. Figure 32. NMR spectrum of [BnMIm][ TFSI] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 48 Figure 33. NMR spectrum of [BnFMIm][TFSI] 1H. Figure 34. NMR spectrum of [BnFMIm][TFSI] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 49 Figure 35. NMR spectrum of [BMIm][PF6] 1H. Figure 36. NMR spectrum of [BMIm][PF6] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 50 Figure 37. NMR spectrum of [BnMIm][PF6] 1H. Figure 38. NMR spectrum of [BnMIm][ PF6] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 51 Figure 39. NMR spectrum of [BnFMIm][PF6] 1H. Figure 40. NMR spectrum of [BnFMIm][PF6] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 3 52 Figure 41. NMR spectrum of [BnMIm][BF4]1H. Figure 42. NMR spectrum of [BnMIm][BF4] 13C. Synthesis and Electrochemical Properties of Ionic Liquids Annex 4 53 Annex 4 Figure 43. EWs comparison between [BnMIm][BF4], [BnFMIm][TFSI] and [BMIm][TFSI]. -1.0 -0.5 0.0 0.5 1.0 -5.0 -3.0 -1.0 1.0 3.0 5.0 V mA/cm2 [BnMIm][BF4] 0.10M (1st trial) [BnFMIm][TFSI] 0.10M (1st trial) [BMIm][TFSI] 0.10M (1st trial) Synthesis and Electrochemical Properties of Ionic Liquids Annex 4 54 Table 12.Full LSV results Ionic Liquid CL(V) Vs.Ag QRE ALV) Vs. Ag QRE EWs 1st trial 2nd trial 1st trial 2nd trial 1st trial 2nd trial Blank -3.19 -3.23 2.98 2.93 6.17 6.16 [BMIm][TFSI] 0.1 M -3.04 -3.04 3.06 3.03 6.10 6.07 [BMIm][TFSI] 0.25 M -1.73 -3.21 2.99 3.02 4.72 6.23 [BnMIm][TFSI] 0.1 M -2.86 -3.19 3.19 3.04 6.05 6.23 [BnMIm][TFSI] 0.25 M -3.14 -1.21 3.04 3.01 6.18 4.22 [BnFMIm][TFSI] 0.1 M -3.02 -3.22 3.32 3.21 6.34 6.43 [BnFMIm][TFSI] 0.25 M -3.16 -3.20 3.10 3.06 6.26 6.26 [BMIm][PF6] 0.1 M -3.07 -3.10 3.00 3.00 6.07 6.10 [BMIm][PF6] 0.25 M -2.84 -3.20 3.05 2.95 5.89 6.15 [BnMIm][PF6] 0.10 M -3.09 -3.40 3.16 3.65 6.25 7.05 [BnMIm][PF6] 0.25 M -3.26 -3.09 3.05 3.01 6.31 6.10 [BnFMIm][PF6] 0.10 M -3.20 -3.25 3.19 3.25 6.39 6.50 [BnFMIm][PF6] 0.25 M -3.22 -3.26 3.05 3.24 6.27 6.50 [BnMIm][BF4] 0.10 M -3.12 3.19 3.18 3.14 6.30 6.33 [BnMIm][BF4] 0.25 M -3.21 -3.09 3.16 3.01 6.37 6.53