Experimental values of the molar conductivity Λimp/ΛNMR ratio, the
Although ionic liquids (ILs) have been known since the beginning of the last century (1814) , they are materials of recent advent in the field of green energy applications where their recognition as solvents or electrolytes has ever increased during the last decades.
By definition ILs are molecular salts that melt at low temperatures,
Typically, an IL consists of an organic cation, such as an imidazolium, pyridinium, pyrrolidinium, or ammonium derivative  combined with an organic or an inorganic anion, such as BF4-, PF6-, CF3SO3− and (CF3SO2)2N− , see Figure 1 for the corresponding molecular structures. Since the discovery of the first IL (EtNH3+:NO3−, with melting point at 12 °C ), the number of cation–anion combinations has ever increased. In fact, organic cations can essentially be designed with any molecular structure resulting in a huge diversity of possible cation-anion pairs that enables tuning the properties of ILs to suit a particular application.
Roughly, ILs can be divided into two subgroups: protic (suitable for fuel cells) and aprotic (suitable for lithum batteries and supercapacitors). The former can be obtained through the proton transfer from a Brönsted acid to a Brönsted base (also called neutralization procedure) that, by being a one-step synthesis, results in very pure compounds . The latter can be obtained by the quaternization procedure or the metathesis reaction, which are two-step syntheses involving the exchange of an intermediate anion. Since the intermediate reagents are not always easily eliminated, high-purity can become an issue. The two synthesis procedures to obtain protic and aprotic ILs are also schematically shown in Figure 2. There is a difference between the two types of ILs in the reversibility of the reactions. In protic ILs, for instance, the reaction is theoretically reversible and the completeness of proton transfer can a priori only be predicted by the acid-base pair strength, expressed by the ∆pKa value. The ΔpKa is defined as pKabase - pKaacid .
The ΔpKa is defined as pKabase - pKaacid .
It has been empirically observed that small structural variations on the constituting ions of the ILs have an important influence on the macroscopically observed properties. Understanding these structure-property relationship is therefore key to design new ILs for specific applications. For example, provided a fixed anion, the viscosity and the ionic association degree both increase with the length of the aliphatic chain attached to the cation, which has been related to stronger van der Waals interactions . On the other hand, provided a fixed cation, larger anions result in lower glass transition temperatures, lower melting points, lower viscosities and higher ionic conductivities, which is attributed to a more effective delocalization of the negative charge, hence more loosely coordinating ions . These properties are also strongly affected by the symmetry of the cation and the position of the alkylated substituents on the cationic ring . In protic ILs, it has also been found that provided a fixed anion as for instance the bis(trifluoromethanesulfonyl)imide (CF3SO2)2N-), smaller cations result in lower glass transition temperatures.
2. Ionic liquids for energy conversion devices
Because ILs can provide high ionic density, intrinsic ionic conductivity, non-volatility and non- flammability, as well as wide windows of electrochemical stability (up to 5–6 V for certain cation-anion combinations), they represent very interesting materials for applications where transport of ionic species and structural stability are key properties. Concrete examples are electrochemical conversion devices like fuel cells, Li-ion batteries, solar cells and capacitors . In these fields, the ionic conductivity represents a measure of how easily ionic species are transported through the electrolyte. ILs typically display conductivities in the range 10−3–10−2 Scm−1 at room temperature and stay liquid in wide temperature ranges extending to several hundreds degrees. Indeed, many ILs decompose before evaporation occurs.
Indeed, many ILs decompose before evaporation occurs.
As we will see in more detail below (section 5.1) the ionic conductivity in ILs follows a non-Arrhenius dependence on temperature. Compared to the electrolytes conventionally used in for instance commercially available Li-ion batteries, ILs are both safer and greener, which represents an advantage with respect to both environmental and societal issues.
Despite these advantages, the use of ILs in electrochemical devices is limited by their melted state, since leakage can constitute a serious hazard. Loss of the liquid electrolyte can lead to short circuit and dangerous chemical reactions.
Loss of the liquid electrolyte can lead to short circuit and dangerous chemical reactions.
2.1. Ionic liquids for PEM fuel cells
The operational principles of a low-temperature fuel cell are schematically shown in Figure 3A. The main components in a proton exchange membrane (PEM) fuel cell are the anode, the PEM, and the cathode. The fuel (like H2 or methanol) is fed at the anode where it is electrochemically split by platinum nano-particles into protons (H+) and electrons (e−). The latter follow an external circuit whereas the former diffuse through the PEM towards the cathode. Here, electrons, protons and oxygen recombine to produce water and heat solely. Since the fuel cell does not produce pollutant or in other way hazardous elements, it is considered one of the most promising future devices for clean energy supply.
The archetypical proton conducting material used in low-temperature fuel cells is Nafion, a perfluorinated polymer membrane containing sulfonic acid pending groups (–SO3H). Nafion has outstanding chemical and mechanical properties and, upon hydration, separates into hydrophilic and hydrophobic domains of the nano-meter size (see e.g. figure 2 in reference ), a structural property that results in well defined channels facilitating the transport of the protonic species (H+ and H3O+). A drawback of Nafion, however, is that at temperatures higher than 80 °C the membrane dehydrates (due to water evaporation) and drastically looses its conducting properties, see also the conductivity of hydrated Nafion in Figure 3B. Meanwhile, for a realistic implementation of the fuel cell into the transport sector (in In the Multi-Years Development Program of the U.S. Department of Energy (DOE) for the Fuel Cell Technology the requirement for next-generation proton exchange membrane (PEM) electrolytes is (≥10-1 Scm-1 at temperatures above 120 °C. Achieving this goal will facilitate the implementation of the fuel cell into the transport sector (buses, cars, scooters, etc).
In the Multi-Years Development Program of the U.S. Department of Energy (DOE) for the Fuel Cell Technology the requirement for next-generation proton exchange membrane (PEM) electrolytes is (≥10-1 Scm-1 at temperatures above 120 °C. Achieving this goal will facilitate the implementation of the fuel cell into the transport sector (buses, cars, scooters, etc).
Because ILs have a high ionic density, an intrinsically high ionic conductivity and are non-volatile, they are considered suitable materials to replace the concept of hydrated Nafion membranes in fuel cell applications. One investigated approach in this direction has been the swelling of PVdF-based polymer membranes by aprotic ILs of the methylpyridinium or propylimidazoliumcation and the bis(trifluoromethanesulfonyl)imide (TFSI) anion , see also Figure 3C (top). However, to provide the protonic species a chemically compatible acid had to be included, more specifically HTFSI. Swelling the membrane with a protic IL instead resulted in a simpler system since the protic species were intrinsically provided by the (protonated) cation of the IL . The IL-swelled polymer membrane concept results in reasonable ionic conductivities, however at the expense of poor mechanical stability. Nafion membranes swelled with protic ILs of the triethylammoniumcation and the methane sulfonate (CH3SO3−) or triflate (CF3SO3−) anion is an alternative and more recent concept of proton exchange membranes . This type of electrolytes show a smaller loss in conductivity with respect to the bulk IL and also better mechanical resistance than PVdF based membranes. In addition, these ammonium based ILs display a range of different degrees of cation:anion dissociation and an ability for proton exchange that also contribute to higher conductivities .
The ionic conductivities of these IL swelled polymer membranes are compared on a common Arrhenius plot in Figure 3B, together with the conductivity of hydrated Nafion. The reader may note that the shadowed area, representing the target set by the U.S. DOE (see footnote 4), is still not hit, implying that further scientific efforts are needed to develop new materials able to meet the set requirements.
2.2. Ionic liquids for Li-ion batteries
The most modern Li-ion battery is based on the rocking chair electrode model and the intercalation of Li ions into and from the electrodes . In this battery concept the anode is typically carbonaceous (
The most investigated family of ILs for Li-ion battery application is that of the pyrrolidinium cation (Pyr), which has shown a better stability in time towards lithium metal electrodes than the previously investigated imidazolium based systems, and a wider electrochemical stability window . Also, TFSI- results in low-melting ILs and high conductivities in combinations with many cationic structures due to the high charge delocalization and is therefore also the most used and investigated anion. To achieve chemical compatibility, the lithium salt most commonly dissolved in these ILs is consequently the LiTFSI. Thus, the typical IL based electrolyte for Li-ion battery applications is LiTFSI-PyrxyTFSI, where
Ionogels constitute a very recent material concept based on the nano-confinement of ILs into silica; see Figure 3C (bottom) for a photo of an ionogel prepared in our laboratories. Compared to IL swelled polymer membranes, ionogels can incorporate a considerably higher volume of liquid (up to 98%) without loosing in mechanical resistance [20, 21]. Ionogels can be prepared through a sol-gel synthesis that follows a non-aqueous route proposed by K. G. Sharp , consisting in the reaction of tetramethylorthosilicate (TMOS) with formic acid (FA). During this reaction, nano-sized particles of silica (SiO2) are formed that first undergo aggregation and then gelation. If the sol-gel reaction is let occurring inside an IL as a co-solvent, the final gel will be three-dimensionally interpenetrated by the IL.
Recent results from magic angle spinning 1H NMR experiments have shown that even at high degrees of nano-confinement Or low ionic liquid contents in the ionogel.
Or low ionic liquid contents in the ionogel.
In our laboratories we are currently investigating the use of the 1-hexyl-3-methyl imidazolium (C6mim) cation, which has a longer alkyl side chain and may thus induce interesting structural features in the gel. We have indeed found by time resolved Raman and 1H NMR spectroscopy that a structural reorientation of the cation may occur in concomitance with the sol-gel transition , which may also be accompanied by a local cation-anion reorganization. From ongoing analysis of confocal µ-Raman/x-ray (small angle x-ray scattering, SAXS) data collected at the ID13 beam line of the ESRF facility in Grenoble .
From ongoing analysis of confocal µ-Raman/x-ray (small angle x-ray scattering, SAXS) data collected at the ID13 beam line of the ESRF facility in Grenoble .
3. Structural investigations
The local structure is one hot issue in the field of ILs, where recent progresses have mainly concerned the understanding of the mesoscopic segregation in the liquid state. With local in this context we mean the nm or molecular scale range, where effects of conformational evolution, crystalline-to-amorphous transitions, ionic clustering
A particular structural feature can also affect the electrochemical performance of the IL. For instance, it has been found that in aprotic ILs based on the 1-ethyl-3-methylimidazolium (C2mim) cation the substitution of the acidic proton on the ring (position C2) by an alkyl chain decreases the reduction potential , which is otherwise too large for practical applications in Li-ion batteries. Also, the addition of a Li-salt to ILs results in ionic clustering and consequently a reduced number of free charges, as also discussed in section 4.1 and reference .
3.1. Raman spectroscopy: Conformational isomerism
Vibrational spectroscopy (including Raman, infrared, neutron and luminescence spectroscopy) is a powerful technique to investigate the structure in materials on a molecular or smaller level. More specifically, issues like dissociation, inter-molecular interactions, crystallinity and conformations can be investigated in both the liquid and solid state. In this section however, only Raman spectroscopy will be treated.
The basic principle of Raman spectroscopy A more thorough description of the Raman spectroscopy technique can be found in reference . Nevertheless, laser light in the UV and near IR range can also be used.
A more thorough description of the Raman spectroscopy technique can be found in reference .
Nevertheless, laser light in the UV and near IR range can also be used.
where k is the bond strength and M the reduced mass of the oscillating system. In a two-atoms system with masses m1and m2the reduced mass M is defined as (m1 m2)/(m1+m2).
In a two-atoms system with masses m1and m2the reduced mass M is defined as (m1 m2)/(m1+m2).
The conformation adopted by cations and anions in ILs has been one such investigated structural feature. This is of interest since the relative orientation of the cation-anion pair can in turn affect the association degree of the ions and thus the dynamical properties of the IL. The TFSI anion, for instance, can adopt two different conformations, the The
Also the conformational isomerism of cations can be investigated by vibrational spectroscopy, as for instance demonstrated in reference . The number of conformations in cations can be significantly increased when long alkyl side chains are attached, due to a larger degree of rotational freedom around the C-C bonds and the orientation of the chain with respect to the cationic head. For the case of 1-butyl-3-methylimidazolium tetrafluoroborate (C4mimBF4) the coexistence of at least four conformers was found, GG, GA, TA and AA, with the population of the most stable GA and AA increasing as temperature is decreased .
Raman spectroscopy has also greatly contributed to understand the formation of larger ionic aggregates, or [Li(TFSI)n]-(n−1) clusters, upon addition of LiTFSI to ILs of the pyrrolydinium cation. In reference , the authors discuss possible types of [Li(TFSI)n]-(n−1) aggregates in ILs of different cations,
3.2. NMR spectroscopy: Heteronuclear coupling
Nuclear magnetic resonance (NMR) is a phenomenon based on the exchange of electromagnetic radiation when magnetic nuclei are exposed to a magnetic field. A requisite for this phenomenon is that the nuclei have non-zero spins, which applies for all isotopes with an odd number of protons and/or neutrons. A key feature in NMR spectroscopy is that the resonance frequency of a particular nucleus is directly proportional to the strength of the applied field and to the magnetic properties of the nucleus itself. The basic relation is thus:
where ω0 is the precession frequency of the nucleus, B0 is the externally applied magnetic field, and γ0 is the gyromagnetic ratio characteristic of the investigated nucleus. It might appear from this relation that all nuclei having the same γ0 would resonate at the same frequency. This is not the case since the most important perturbation of the NMR frequency is the ’shielding’ effect of the surrounding electrons. The rotation of these electrons produces a spin, which results in a magnetic field that counteracts the magnetic field of the nucleus. In general, this electronic shielding reduces the resonance frequency, whereby same nuclei found in different molecular structures can be resolved by their characteristic chemical shift (∂, expressed in ppm).
By using different types of pulse sequences, where the pulses vary in shape, frequency and duration, dynamical or structural information can be extracted from an NMR experiment. Multi-dimensional NMR spectroscopy is a kind of Fourier Transformed (FT) NMR that allows detecting nuclear-nuclear interactions through magnetization transfer. Through-bond and through-space interactions can be detected, the latter in particular allowing to establish distances between atoms (
One of the exceptions is the study reported in reference , where the structure and local organization in protic ILs of the triethylammonium (TEA) cation have been elucidated by combining 1- and 2D heteronuclear NMR experiments The authors show that the choice of different TEA- anionic species pairs strongly affects the dissociation scheme:
where A− denotes the anion. These experiments also evidence the coupling between the dissociation state of the proton in the IL and the diffusive behaviors of the individual ionic species (see also Table 1 in reference ). For instance, even though in TEA-TFSI the ions are fully dissociated the proton diffuses with the cation and faster than the TFSI anion, whereas in the IL TEA-acetate the proton is fully dissociated from the cation, and is also the fastest diffusing species. These observations have obvious implications for practical use in fuel cells where the transport of the protic species through the electrolyte and its reactivity at the cathode are key properties.
3.3. SAXS: Nano-segregation
Along with a peculiar set of physico-chemical properties, ILs also show a complex local organization with self-aggregating polar and non-polar domains of the nanometer size. This mesoscopic separation was first predicted by molecular dynamic (MD) simulations and later also experimentally confirmed by small (and wide) angle x-ray scattering measurements (SAXS (and WAXS)). Before discussing in detail these results, the basic principles of an x-ray scattering experiment will be briefly explained.
In a SAXS experiment the sample is exposed to x-ray radiation with wavelength in the range of a few Å, and the elastically scattered x-rays are recorded at low angles, typically close to 0°, Figure 6A. In the presence of structural inhomogeneities in the nm range, a diffraction pattern is recorded if the following condition is fulfilled (Bragg’s law):
the real space correlation length
The intriguing property of ILs is that even in the liquid state they can display clear scattering peaks in the x-ray diffraction pattern. This feature has been repeatedly reported by several authors and for diverse cation-anion combinations. The first studies focused primarily on ILs of the imidazolium cations , but more recent investigations have extended to those of ammonium and pyrrolidinium derivatives also, associated with PF6−, BF4−, Cl− or TFSI− . SAXS (and WAXS) diffraction patterns show that the low-
As shown in Figure 6C, a typical SAXS diffraction pattern recorded for 1-alkyl-3-methylimidazolium TFSI ILs, and covering the wide
This finding fits well into the vivid debate currently ongoing on the true interpretation of the SAXS patterns: some researchers believing in a real mesoscopic separation as several times evidenced by SAXS and WAXS experiments , and other claiming that the observed diffraction pattern only reflects the internal structural inhomogeneity of the cation with no implications of a long-range ordering . The main point of our study , however, is the correlation experimentally found between the transport properties (from pfg-NMR measurements) and the local ordering (from SAXS data) of the individual ionic species. In particular, we have found that there is a correlation between the dispersion curves of the anion-anion and cation-cation correlations and the measured self-diffusion constants independently measured for anions (D−) and cations (D+) in the ILs (see also Figure 8B and the thorough discussion in reference ).
4. Dynamical investigations
When ILs are used as electrolytes in energy conversion devices like fuel cells or Li-ion batteries, they have the two-fold functionality to separate the electrodes (and thus prevent from short circuit) and to be the conducting medium for the electro-active ionic species (H+ or protic species in PEM fuel cells, and Li+ in Li-ion batteries). It is therefore of great interest to investigate and understand the transport properties in ILs, in particular the ionic conductivity and the self-diffusion of ionic species, but also transport phenomena under operational conditions that could lead to concentration gradients of the electrolyte (
4.1. Dielectric spectroscopy: Ionic conductivity
Dielectric spectroscopy can be used to measure the ionic conductivity in diverse materials, including liquids. In such a dielectric experiment, the sample is sandwiched between two electrodes of surface area
This equation shows that the real part of conductivity contributes to the imaginary part of the dielectric constant. Thus, because of the
In our laboratories we have found that if the ionic conductivity of ILs is plotted as a function of Tg/T, where Tg is the experimentally found glass transition temperature, data fall onto master curves [42, 43]. This universal behavior resulting from Tg-scaling is a strong indication that conductivity is dominated by the viscous properties in the whole temperature range investigated. Since viscosity is a quantity strongly related to the glass transition temperature Tg.
Since viscosity is a quantity strongly related to the glass transition temperature Tg.
4.2. NMR spectroscopy: Self-diffusion
A very powerful tool to study the dynamics of ionic species in liquid materials is by pulsed field gradient nuclear magnetic resonance (pfg-NMR) spectroscopy. This technique measures the translational diffusion of molecules in time scales larger than milliseconds.
The most basic pfg (or Stejskal and Tanner 1967) pulse sequence used to estimate self-diffusion constants is schematically shown in Figure 8A. This consists of a spin-echo experiment with the 180° pulse in between two equal gradient pulses of magnitude
Typically, the magnetic field gradient Where ω is the Larmor frequency (radians s−1), γ the gyromagnetic ratio (rad T−1s−1) and B is the strength of the magnetic field (T).
Where ω is the Larmor frequency (radians s−1), γ the gyromagnetic ratio (rad T−1s−1) and B is the strength of the magnetic field (T).
From this relation the self-diffusion constant
For the few ILs investigated through the fractional form of the Stokes-Einstein equation,
Another interesting aspect of pfg-NMR measurements is that the molar conductivity (ΛNMR) can be calculated from the self-diffusion constants using the Nernst-Einstein equation Where NA is the Avogadro number, e is the electric charge on each ionic carrier, k is the Boltzmann constant and T is the temperature.
Where NA is the Avogadro number, e is the electric charge on each ionic carrier, k is the Boltzmann constant and T is the temperature.
In electrolytic systems this quantity becomes interesting if compared to the molar conductivity directly measured by impedance spectroscopy (Λimp, see section 5.1). The molar conductivity ratio Λimp/ΛNMR illustrates well the degree of cation-anion aggregation in ILs at equilibrium and represents a measure of the tendency to form higher ionic complexes, as opposed to completely dissociated systems. Indeed, while impedance measurements record the displacement of charged species only, pfg-NMR measurements record the movement of all probed molecules regardless their charged state (or ionic complexation). As a representative case, values for the Λimp/ΛNMR ratio in ILs of the imidazolium cation are given in Table 1. These are all smaller than one indicating that not all the diffusing species in the IL contribute to the ionic conduction,
μ-Raman spectroscopy: in-situ fuel cell diagnostic
A very informative way to investigate the transport properties within a PEM is by
In reference  the authors demonstrate the potentiality of this technique through the study of a Nafion membrane swelled with protic ILs of the triethylammonium cation. In this particular case, the humidification of the H2 gas was varied during fuel cell operation, whereby also the membrane became more or less hydrated. This
The blue-shift of the νSO3 (IL’s anion) in the more hydrated state indicates a stronger association state, most probably to water molecules that are thought to find interstitial positions between cations and anions in H2O/IL mixtures, thus disrupting the original local organization. These features are of high relevance for fuel cell applications and deserve further investigations. The employment of
The authors also demonstrate that if the Raman spectra are correctly interpreted, Through calibrated relative intensity curves.
Through calibrated relative intensity curves.
ILs are materials with an incredible variety of application fields. In this chapter, we have given examples of IL structures that can be used in fuel cells and Li-ion batteries, but use in super-capacitors, solar cells and green chemistry must not be forgotten. Very recently, ILs have also shown to have an important role in the extraction process of heavy metals from waste water. In all these applications, dynamical and structural properties jointly govern the functionality of ILs; yet in the field of ILs local structure and dynamics are rarely investigated in strict relation to each other. The combination of complementary experimental techniques like SAXS and pfg-NMR and the use of
The author acknowledges the financial support from the Chalmers’ Areas of Advance Energy and Materials Science, as well as the Swedish Foundation for Strategic Research (SSF). Dr. M. Maréchal is also acknowledged for fruitful scientific discussions on the use (and interpretation) of SAXS in ionic liquids. A special thank goes also to all my previous and current collaborators.
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- The ΔpKa is defined as pKa - pKaacid .
- Indeed, many ILs decompose before evaporation occurs.
- Loss of the liquid electrolyte can lead to short circuit and dangerous chemical reactions.
- In the Multi-Years Development Program of the U.S. Department of Energy (DOE) for the Fuel Cell Technology the requirement for next-generation proton exchange membrane (PEM) electrolytes is (≥10 Scm-1 at temperatures above 120 °C. Achieving this goal will facilitate the implementation of the fuel cell into the transport sector (buses, cars, scooters, etc).
- Or low ionic liquid contents in the ionogel.
- From ongoing analysis of confocal µ-Raman/x-ray (small angle x-ray scattering, SAXS) data collected at the ID13 beam line of the ESRF facility in Grenoble .
- A more thorough description of the Raman spectroscopy technique can be found in reference .
- Nevertheless, laser light in the UV and near IR range can also be used.
- In a two-atoms system with masses m1and m2the reduced mass M is defined as (m1 m2)/(m1+m2).
- The transoid is indeed the conformation most stable at low temperatures and commonly found in the crystalline phase.
- Since viscosity is a quantity strongly related to the glass transition temperature Tg.
- Where ω is the Larmor frequency (radians s), γ the gyromagnetic ratio (rad T−1s−1) and B is the strength of the magnetic field (T).
- Where NA is the Avogadro number, e is the electric charge on each ionic carrier, k is the Boltzmann constant and T is the temperature.
- Through calibrated relative intensity curves.