Ezgi Yılmaza,
Elif Berna Olutaşab,
Gözde Barıma,
Jayasundera Bandarac and
Ömer Dag*a
aBilkent University, Department of Chemistry, 06800, Ankara, Turkey. E-mail: dag@fen.bilkent.edu.tr
bAbant İzzet Baysal University, Department of Chemistry, 14030, Bolu, Turkey
cNational Institute of Fundamental Studies, Hantana Road, Kandy, CP20000, Sri Lanka
First published on 5th October 2016
Lithium salt (LiCl, LiBr, LiI, or LiNO3) and a non-ionic surfactant (such as 10-lauryl ether, C12E10) form lyotropic liquid crystalline (LLC) mesophases in the presence of a small amount of water. The mesophases can be prepared as gels by mixing all the ingredients in one pot or in the solution phase that they can be prepared by coating over any substrate where the LLC phase is formed by evaporating excess solvent. The second method is easier and produces the same mesophase as the first method. A typical composition of the LLC phases consists of 2–3 water per salt species depending on the counter anion. The LiI–C12E10 mesophases can also be prepared by adding I2 to the media to introduce an I−/I3− redox couple that may be used as a gel-electrolyte in a dye-sensitized solar cell. Even though the mesophases contain a large amount of water in the media, this does not affect the cell performance. The water molecules in the mesophase are in the hydration sphere of the ions and do not act like bulk water, which is harmful to the anode of the dye-sensitized solar cells (DSSC). There are two major drawbacks of the salt–surfactant LLC mesophases in the DSSCs; one is the diffusion of the gels into the pores of the anode electrode and the other is the low ionic conductivity. The first issue was partially overcome by introducing the gel content as a solution and the gelation was carried in/over the pores of the dye modified titania films. To increase the ionic conductivity of the gels, other salts (such as LiCl, LiBr, and LiNO3) with better ionic conductivity were added to the media, however, those gels behave less effectively than pure LiI/I2 systems. Overall, the DSSCs constructed using the LLC electrolyte display high short circuit current (Isc of around 10 mA), high open circuit voltage (Voc of 0.81 V) and good fill factor (0.69) and good efficiency (3.3%). There is still room for improvement in addressing the above issues in order to enhance the cell efficiency by developing new methods of introducing the gel-electrolytes into the mesopores of the anode electrode.
The use of salt–surfactant lyotropic liquid crystalline mesophases32–36 with the I−/I3− redox couple as gel-electrolytes could be an excellent solution. However, most lyotropic liquid crystalline (LLC) phases are formed with the help of a solvent that is also volatile, and they have very limited ionic conductivity due to low concentration of the redox couples and other ionic species.25–31 The solvent, in the LLC mesophase, can also create uncertainty in the cells. Therefore, it is important to form stable and highly conductive mesophases as gel electrolytes, where the volatility is not an issue.32–36
Recently, we have discovered a new LLC phase that can be formed by using lithium salts (LiCl, LiBr, LiI, LiNO3, and LiClO4) and water together with non-ionic surfactants.34–36 The salt–surfactant mesophases are stable under ambient conditions, in that the amount of water in the media remains constant.34–36 The driving force in the formation of the salt–surfactant mesophases is the hygroscopicity of the salts, which absorb enough water to get solvated in the hydrophilic domains of the mesophase with a much smaller amount of water.32–36 Note that the confinement effect is effective to enhance the solubility and reduce the melting point of the salts in the LLC mesophase.32–36 Therefore any of these mesophases can be used as a gel electrolyte by adding the redox couple in the media.
In this contribution, we show the phase behavior, electrical and thermal properties of lithium salt (LiI, LiBr, LiCl, and LiNO3)–surfactant LLC mesophases in absence/presence of LiI/I2 under various conditions and their cell performance in a DSSC.
The Gn-LiCl-#, Gn-LiBr-#, and Gn-LiNO3-# samples are in LLC mesophase in broad salt concentrations, where # is 2-10, 2-8, and 2-8, respectively.34,35 Both sets of mesophases display focal conic fan texture, characteristic to the hexagonal phase (Fig. 1(a)) in a broad range of salt concentration and display a highly intense diffraction line at around 1.8°, 2θ, corresponding to the (100) plane of 2D hexagonal mesophase, Fig. 1(b). The Gn-LiI-# samples also form LLC mesophase up to a 5 LiI/C12E10 mole ratio however the phase undergoes crystallization at a mole ratio above 3, see Fig. 1(c).37 The addition of I2 to the media changes the viscosity and structure of the samples but if the LiI/I2 mole ratio is kept above 10, there is almost no detectable changes in the properties of the Gn-LiI–I2-# samples, compared to the I2 free counterparts. Both G1-LiI-# and G1-LiI–I2-# (LiI/I2 mole ratio of 10) samples with a little water form 2D hexagonal phase up to 4 LiI/C12E10 mole ratio, Fig. 2, but at a mole ratio above 4, the Gn-LiI–I2-4 mesophase also slowly transforms to a soft solid mesocrystal.37 The mesocrystal formation becomes visible around a 4 LiI/C12E10 mole ratio and enhances with an increasing amount of LiI, where at mole ratio of 5 it is difficult to keep the mixture in a LLC phase.
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Fig. 1 Sample G1-LiI-3 with 9H2O: (a) POM image of the fresh sample, (b) XRD pattern of the fresh sample, and (c) POM image of the G2-LiI–I2-5, prepared from 60H2O solution. |
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Fig. 2 Small angle XRD patterns of the G2-LiI–I2-# (prepared with 60H2O/C12E10 mole ratio) upon aging, # values are (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5. |
The lithium salt–surfactant mesophases are stable in the presence of a certain amount of water, which is highest in the Gn-LiCl-# and lowest in the Gn-LiNO3-# samples. Note also that the amounts of water in the LLC mesophases are much lower than the solubility limit of these salts in an aqueous saturated solution; it is a fixed quantity and only depends on the salt type, amount, and humidity of the media. To quantify and also to identify the phase behavior of the LLC samples, a set of samples were prepared using excess water and aged under ambient conditions to evaporate the excess water; the water evaporation always stops at a certain water concentration. Usually, this process ends after a few hours, but the samples were followed for days to make sure the excess water is completely removed. Fig. 3 shows the ATR-FT-IR spectra of a series of G2-LiCl-# samples, where # is 2, 3, 4, 5, and 6. A plot of the intensity of the water bending mode versus LiCl concentration is linear, going almost through the origin, see inset in Fig. 3. This behavior clearly shows that the only water remaining in the mesophase is the hydration water. If the appropriate amount of water is added to the media, the spectra do not change in all LiX–C12E10 systems during the aging process; see Fig. 3(b). Moreover, if the amount of water in the sample is lower than the critical amount, the sample absorbs ambient water, but if it is in excess, it desorbs the excess water until it reaches to a constant water/salt mole ratio, Fig. 3(c). Notice that this is an important property for LLC mesophases, which could potentially be used in various electrochemical applications as the gel-electrolytes.
The evaporation of excess water is a rapid (in a few seconds a large fraction of water evaporates) process, but it slows when the LLC mesophase forms, Fig. 3(d). Note also that the LLC phase exists over a broad water range, but it always stabilizes at a specific water amount in all salt–surfactant mesophases, where the absorption and desorption rates are equal. The first FT-IR spectra recorded from the samples are not necessarily quantitative in terms of water concentration in the Gn samples unless the samples have been prepared using correct amount of water. It is difficult to predict the amount of water in the samples using only FT-IR spectroscopy. Therefore, the water concentrations in the samples were also determined using solutions of salt–surfactant–water and a 4 digit balance. Accordingly, we found out that around 2–3 waters per LiX are the final compositions of the G2-LiX-# samples, where LiX is LiI, LiBr, LiCl, or LiNO3, similar to G1 samples. The FTIR spectra of the aged samples of G1 and G2 are identical. Note also that the water concentration in the stabilized LLC samples is similar in the Gn-LiX-# (where X is Cl−, Br−, and I−), but relatively low in the LiNO3 case, Fig. S1.†
The water bending mode also provides insight into the nature of the water molecules in the mesophase, for instance it shifts from 1645 to 1630 cm−1 going from Cl− to I−, indicating that the hydroscopic anions, such as I− ion has hydrated water with less hydrogen-bonding and acts as structure breakers and the lyotropic anions, such as Cl− ion acts as structure makers (which cause the formation of bulk-like water with more hydrogen-bonding). The same behavior is also visible in the water stretching region in that the water peak is quite broad and more red-shifted in the Gn-LiCl-# samples, but sharp and blue shifted in the Gn-LiI-# samples, see Fig. 4. These observations are in accord with the Hofmeister effect in that I− is a hydrotropic ion and break the water structure and hydrates the surfactant domains, however, the Cl− ion is a lyotropic anion and water-structure maker, dehydrates the surfactant domains.38–40 As a result the ν-CO stretching are red-shifted in the FTIR spectra, compare the spectra in Fig. 4(b). Therefore, it is reasonable to say that the water in the Gn-LiCl LLC mesophases is bulk-like, but it is mostly hydration water in the Gn-LiI LLC phases. This is a very critical property in the use of LiI–C12E10 LLC mesophases as gel-electrolytes, especially, if the bulk-water is an issue for the application, such as in solar cells. For instance, bulk-water, in the DSSCs, negatively affects the performance of the cell; it leaches the dye molecules from the electrode surface and drops the current generated from the cell, also affects the open circuit voltage (Voc) value by increasing reduction half-cell potential of the redox couple.24 Therefore, it is undesired to have bulk-like water in the gel-electrolytes in DSSCs.
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Fig. 4 Different region of the ATR-FT-IR spectra of G1-LiCl-3 and G1-LiI-3 (a) all mid-IR, (b) ν-CO stretching, (c) ν-CH (surfactant) and ν-OH (water) stretching, and (d) δ-H2O bending, regions. |
I−/I3− couple has been extensively used as a redox couple in efficient DSSCs. Therefore, we also incorporated these couples into the salt–surfactant LLC mesophases. Unfortunately, the LLC mesophases of the Gn-LiI-# are stable up to 3 LiI/C12E10 mole ratios and above which they undergo to a mesocrystallization.37 Therefore only the Gn-LiI–I2-# samples, where # is 1, 2, and 3 were investigated in detail. However, to demonstrate the other compositions, the Gn-LiI–I2-4 and Gn-LiI–I2-5 samples were also prepared and analyzed. Fig. 2 shows a set of small angle diffraction patterns of these samples. The XRD patterns clearly indicate that the samples made using Gn-LiI–I2-2 to Gn-LiI–I2-4 have sharp diffraction lines, indicating a meso-order; but the samples made using Gn-LiI–I2-1 and Gn-LiI–I2-5 (low and very high LiI) are quite disordered. Note also that the diffraction patterns of the samples of Gn-LiI-4 and Gn-LiI-5 slowly change, where the small angle diffraction lines shift to smaller angles and many new wide angle lines appear overtime, due to mesocrystallization.37 The FTIR spectroscopy is even more sensitive in determining the mesocrystallization, in that even the early stage of the process can be captured. Many IR peaks of the surfactant and water become sharp and shifts upon mesocrystallization, see Fig. S2.† For instance, the water bending mode red shifts down to 1605 cm−1 and becomes sharper and the water stretching frequency also show some shifts and becomes sharper and more intense, in addition to the sharpening of many surfactant peaks. It is likely that the hydration sphere plays a critical role in the mesocrystallization process. The assembly of the hydrated ions in the hydrophilic domains enforces the surfactant molecules to expand through conformational changes and interact with each other more strongly to form the soft mesocrystals.37 We consider the mesocrystallization to be a problem in the DSSCs and addition of I2 to the media does not stop this process, therefore, we limited our investigation to gel-electrolytes between 1 and 3 LiI/C12E10 mole ratios with a LiI/I2 mole ratio of 10 in DSSCs.
Another drawback of the water based electrolytes is the reaction between I− and I2, in that a large amount of I2 remains in the water media, which is undesired in a well performing solar cell. However, with many other solvents such as acetonitrile, the I3− formation is favored and it is a good solvent. To elucidate this, UV-vis absorption spectra of LiI/I2 mixture in different media were recorded to compare them to the redox couple in the LLC media, see Fig. S3.† The band at 522 nm is due to I2 in the samples and is clearly visible with water, but absent with other solvents and in all Gn-LiI–I2 mesophases, see Fig. S3.† The sharp and intense peaks, at 362 and 292 nm, originate from the I3− ion.41 The Raman spectra of both Gn-LiI–I2 samples display peaks at 110, 140, 220, and 330 cm−1. The Raman peaks at 110, 220, and 330 cm−1 are due to fundamental, 1st, and 2nd overtones of the symmetric stretching of I3− ion, respectively.41 The shoulder at around 140 cm−1 could be asymmetric stretching of I3− (it is weakly observed in non-linear I3−) or poly-iodide (Ix−, x is 5 or higher) species that form in the presence of I2 at high concentrations.42–44 However, the poly-iodide has also a peak at around 170 cm−1 and it is absent in our spectra, which suggest that the only poly-iodide specie in the LLC phase is triiodide.44 The UV-vis absorption spectra of the LLC phase are also consistent with the above assignment that the poly-iodide species have absorption above 600 nm. The absence of a peak at around 170 or 210 cm−1 in the Raman spectra and 522 nm or higher wavelength peaks in the visible absorption spectra indicate that there is no I2 or poly-iodide species in the LLC media (or below detection limits of Raman and UV-visible absorption spectrometers).44 The same Raman peaks are also weakly observed in the Gn-LiI samples, Fig. S4.† Both UV-vis absorption and Raman spectra collectively show that I−/I3− couple are not exposed to bulk-water in the LLC media. Note also that both Li+ and I− ions are hydrotropic ions and break the water structure and hinders the bulk-water formation and are consistent with the above observations.
Both Gn-LiI–I2-2 samples were tested in DSSCs as gel electrolytes. The gels of G1 samples were directly placed over the anode (FTO–buffer–TiO2–dye) surface, covered and pressed together using the cathode (Pt nanoparticle coated FTO) electrode. The I–V curve of such cell provides 1.74 mA cm−2 short circuits current, 0.42 V open circuit voltage, 0.58 fill factor (FF) and 0.51% cell efficiency. The low efficiency is likely due to bad electrolyte–electrode contact in the anode side. To increase the contact and enhance diffusion of the electrolyte into the mesopores of titania anode electrode, the gels were dissolved in a solvent and dropped over the electrode surface to suck the electrolyte into the pores. Finally, the excess solvent was evaporated to form the gels in and over the pores. This process ensures a better diffusion as well as better contact between the two electrodes. For this purpose water, ethanol, ethanol–water mixture and acetonitrile were tested as a solvent. The use of just water and surprisingly acetonitrile leach out some of the dyes and reduce the collected current. Ethanol works better than both solvents. However the strong performance was found using a mixture of ethanol and water, Fig. 5(a). As noted, not only the performance of the cells is enhanced, the open circuit voltage can also be increased up to 0.72 V, which is unusual for water based electrolytes. In various cells, the best Isc, Voc, and FF values were recorded as 9.26 mA cm−2, 0.72 V, and 0.50, respectively, upon the addition of 4TBP (4-butylpyridine) to the electrolyte solution, Fig. 5(a). 4TBP acts as surface capping agent and bind to the empty titania sites to block the direct contact of the electrolyte with a bared titania surface, where the electron transfer can take place and cause a short-cut. Even, changing the titania source improves the solar performance of the gel-electrolytes, Fig. 5(a). Therefore, there is still room to improve these values by improving the contact between the electrodes and electrolytes.
The incident photon to current efficiency (IPCE) was also measured for the optimized solar cell and shown in Fig. S6(a).† The IPCE spectrum of the solar cell is very well matched with the absorption spectrum of the dye N71. The nearly equal Isc (9.5 mA cm−2) from IV and the estimated Isc (10.0 mA cm−2) from IPCE confirms the validity of the measured solar cell performance, Fig. S6(b).†
Another drawback of G2-LiI–I2-2 is low conductivity (typical conductivity is 0.1 mS cm−1). However, other lithium salt–surfactant mesophases have better ionic conductivity values, see Table 1. Therefore, to improve the conductivity of the G2-LiI–I2 mesophases, we also prepared samples by adding various amounts of LiCl, LiBr, or LiNO3 into the G2-LiI–I2-2 samples, see Table 1. Interestingly, 4-tert-butylpyridine (4TBP) is very important in the mixed salt systems. The performance of the G2-LiI–I2-2-LiCl without 4TBP electrolyte was the worst, where the efficiency drops drastically with an increasing LiCl in the media, even though the conductivity increased by 1.7 to 5 times. The presence of LiCl drops the Voc from 0.63 to 0.58 V at low concentrations (0.2LiCl) and to 0.29 V and display a very low efficiency of 0.24% at higher concentrations (1.0LiCl). LiNO3 also behaves similarly; at low concentrations, the Isc drops and at higher concentration, both Isc and Voc drop, see Table 2 and Fig. S5(a).† Notice also that the nitrate samples have the highest ionic conductivity. The effect is similar to that of LiCl system. However, in the case of LiBr, increasing the LiBr in the media improves both Isc and Voc values of the DSSC, see Fig. S5(a)† and Table 2, but at very low LiBr concentrations, the cell is almost dead (with a recorded lowest efficiency of 0.03%), even though the conductivity has been improved. For instance the Voc, FF, and % efficiency drops drastically, in a cell designed using 0.2LiBr without 4TBP, to 0.05 V, 0.25, and 0.03%, respectively. These numbers improve in the 1.0LiBr samples to 0.63 V, 0.54, and 1.68%, respectively. We also measured the IV curves of the cells in a long run. Fig. S6(c)† shows two IV curves of the cell measured right after the construction and 5 months later. Both cells perform as good, showing the stability of the cell with the gel electrolytes.
Salt/C12E10 mole ratio | Conductivity (mS cm−1) | ||
---|---|---|---|
LiCl | LiBr | LiNO3 | |
0.2 | 0.17 | 0.31 | 0.26 |
0.4 | 0.19 | 0.36 | 0.33 |
0.6 | 0.37 | 0.38 | 0.38 |
0.8 | 0.44 | 0.50 | 0.57 |
1.0 | 0.50 | 0.78 | 0.90 |
Solvent | Voc (V) | Jsc (mA cm−2) | Fill factor | % efficiency |
---|---|---|---|---|
a GSolution from gel, Sprepared as a solution.b Prepared as a solution with 4TBP.c Prepared as a solution with another LiX with/without 4TBP, as indicated. | ||||
H2OG | 0.56 | 3.37 | 0.55 | 1.00 |
C2H5OHG | 0.64 | 4.16 | 0.60 | 1.59 |
H2O + C2H5OHG | 0.68 | 4.93 | 0.61 | 1.85 |
C2H5OHS | 0.72 | 3.17 | 0.51 | 1.18 |
H2O + C2H5OHS | 0.63 | 5.86 | 0.60 | 2.20 |
4TBPb | 0.72 | 9.26 | 0.50 | 3.33 |
0.2LiClc | 0.69/0.58 | 2.64/5.13 | 0.56/0.52 | 1.04/1.55 |
1.0LiClc | 0.71/0.29 | 2.03/2.55 | 0.62/0.33 | 0.89/0.24 |
0.2LiNO3c | 0.72/0.62 | 2.46/3.55 | 0.67/0.46 | 1.29/1.00 |
1.0LiNO3c | 0.70/0.40 | 2.46/1.11 | 0.57/0.47 | 0.98/0.21 |
0.2LiBrc | 0.73/0.05 | 3.10/2.58 | 0.54/0.25 | 1.22/0.03 |
1.0LiBrc | 0.72/0.63 | 1.64/4.95 | 0.53/0.54 | 0.63/1.68 |
In the presence of 4TBP, even though the conductivity of the gels were improved with the addition of another lithium salt in the media, the cell performance diminished with increasing concentrations of the second salt (LiCl, LiBr, or LiNO3), see Table 2 and Fig. 5(b). We also monitored the melting point of the mesophases with the addition of a second lithium salt to the media and found out that the melting point of the G2-LiI–I2-2-LiX mesophase increases (from 38 to 48 °C in LiCl, 42 to 60 °C in LiNO3, and 52 to 70 °C in LiBr cases) with increasing the lithium salt concentration (from 0.2 to 1.0) in all cases. The higher melting point may affect the diffusion of the gels into the mesopores of the electrodes. A drastic drop in the collected current from the cells with the addition of the second salt could be due to a bad contact between the gel-electrolyte and anode surface. However, the Voc value of the DSSC can be improved if G2-LiBr-2-LiI–I2 (LiI/C12E10 mole ratio of 0.7 and I2/I− mole ratio of 0.1, the major salt is LiBr) is employed as the gel-electrolyte. A cell, designed using G2-LiBr-2-LiI–I2, provides a better Voc value of 0.81 V with a better fill factor of 0.69, see Fig. S5.† However, the gel with 2 LiCl in place of LiBr drops the Voc drastically, Fig. S5(b).† The presence of 4TBP in these gels levels the Voc values in all gels, Fig. S5(c).† The origin of the above behaviours is not clear at this moment. Therefore, further investigations are necessary to elucidate the effect of ions on the solar performance of the Gn-LiI–I2–X–LiX mesophases.
The infiltration of the gel into mesopores of the anode is in general challenging but the mixture of all ingredients can be introduced as a solution at which point the gelation can be carried in and over mesopores of the anode. This ensures a better contact between the anode and gel electrolyte and the cells perform much better. The best cells using Gn-LiI–I2-2 display high open circuit voltage, as high as 0.75 V, which is unusual in a water-based electrolyte with a decent fill factor of 0.67.
Water molecules, in the Gn-LiCl and Gn-LiNO3, are bulk-like but in the Gn-LiI, Gn-LiI–I2, and Gn-LiBr, are in the hydration sphere of the ions, in which their behavior is consistent with the Hofmeister effect; the Br− ion is a water structure breaker or hydrotropic. The addition of LiBr to the media enhances the conductivity of the mesophase but also increases the melting point of the mesophase. Notice that the Gn-LiBr–LiI–I2-0.7 has the highest recorded Voc value of 0.81 V with a very good fill factor of 0.69 and of low Isc. It is likely that the high melting point of Gn-LiI–I2–LiBr makes it difficult to infiltrate into the mesopores of the anode. The DSSC performs efficiently using the salt–surfactant liquid crystalline gel-electrolytes, but requires further investigations to improve the diffusion and conductivity of the gels, towards a finding promising alternatives to other gel and solution based electrolytes.
Footnote |
† Electronic supplementary information (ESI) available: ATR-FTIR spectra of G2-LiCl-2, G2-LiBr-2, G2-LiI-2, and G2-LiNO3-2, ATR-FTIR spectrum and XRD pattern of mesocrystals of G2-LiI–I2-4, UV-vis absorption spectra of G2-LiI–I2-x (x is 1, 2, 3, 4, and 5) and LiI–I2 mixture in ethanol, water, acetonitrile and glycol Raman spectra of G2-LiI-2 and G2-LiI–I2-2, IPCE and time dependent IV curves. See DOI: 10.1039/c6ra19979h |
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