This is the accepted version of the journal article: Santiago, Sara; Muñoz Berbel, Xavier; Guirado López, Gonzalo. «Study of P(VDF-co-HFP)-ionic liquid based ionogels for designing flexible displays». Journal of Molecular Liquids, Vol. 318 (November 2020), art. 114033. DOI 10.1016/j.molliq.2020.114033 This version is available at https://ddd.uab.cat/record/274570 under the terms of the license 1 STUDY OF P(VDF-CO-HFP)-IONIC LIQUID BASED IONOGELS 2 FOR DESIGNING FLEXIBLE DISPLAYS 3 Sara Santiago,[a] Xavier Muñoz-Berbel,[b] and Gonzalo Guirado*[a] 4 [a] 5 Departament de Química, Universitat Autònoma de Barcelona, Bellaterra 6 (Barcelona), 08193, Spain. E-mail: Gonzalo.Guirado@uab.cat [b] 7 Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), Bellaterra 8 (Barcelona), 08193, Spain 9 10 ABSTRACT 11 In flexible displays, solid electrolytes promote electron-transfer without compromising 12 system integrity. Ionogels (IGs) are now preferred among other solid electrolytes, e.g. 13 inorganic and hydrogel electrolytes, for being easier to produce than the former and 14 showing excellent properties, including high ionic conductivities, broad electrochemical 15 window, transparency, flexibility, and non-solvent volatility than the latter. IG properties 16 widely depend on the composition and concentration of the ionic liquid (IL) in the 17 polymeric matrix, affecting its transparency, integrity and flexibility. Here, we have 18 investigated the loading and composition of the IL to produce P(VDF-co-HFP)-based IGs 19 to be employed as substrates in the development of flexible photo-electrochromic 20 displays. Single IG layers of P(VDF-co-HFP) loaded with 79% wt. of N1114 TFSI were 21 optimal among the IGs formulations, reaching conductivity values up to 1.06 mS·cm-1, 22 showing high transparency (83 % transmittance) and a large electrochemical window (3.2 23 V), while maintaining flexibility and integrity. The doping of the IG with the photo- 24 electrochromic molecule spiropyran resulted in flexible displays with fast and reversible 1 25 electrochromic and photochromic dynamics, not affecting the conformational changes of 26 the molecule. These flexible photo-electrochromic displays, due to its simplicity, high- 27 performance and low cost, are foreseen to impact in several fields, including smart 28 windows, smart clothing or gas sensors, among others. 29 KEYWORDS 30 Ionic liquids • Ionogel • Flexible device •Photoelectrochromic devices • 31 32 INTRODUCTION 33 Ionogels (IGs) are gel electrolytes with a liquid phase consisting of an ionic liquid (IL) 34 that percolates the solid polymeric phase. These materials are now a first choice in the 35 development of smart electrochemical devices for combining mechanical strength, 36 durability, flexibility, elasticity and high ionic conductivities, something impossible to 37 attain with stiff and brittle inorganic solid-state electrolytes or hydrogels susceptible to 38 dehydration. [1–12] Especially remarkable is the application of IGs in electro-optic 39 devices, e.g. advanced flexible displays, where their high transparency and ionic 13–16 40 conductivities is combined with their excellent mechanical features, i.e. flexibility 17–20 41 and elasticity . For this reason, the formulation of new IGs is still a reason for the 42 continuous study, as their electrochemical and mechanical properties are entirely related 43 to the chemical nature of the polymer matrix and the IL. In such formulation, the choice 44 of a suitable polymer composition with the desired properties is crucial in the gel polymer 2,9,21–25 45 electrolyte development . Poly(vinylidene fluoride-co-hexafluoropropylene) 46 (P(VDF-co-HFP)) polymers are now preferred due to their high thermal and chemical 26– 47 stability, flexibility, and large dielectric constants, which improve ionic dissociation 2 28 48 . Additionally, HFP units reduce polymer crystallinity while enhancing ionic 29–31 49 conductivity . However, ionic conductivities of such polymers are normally well 50 below the 10-3 and 10-4 S/cm 32–35 required in conventional high-voltage electrochromic 51 devices 36–38. The introduction of IL as ionic charge in the P(VDF-co-HFP) formulation 52 increases its ionic conductivity and expands the electrochemical windows of the material, 53 much beyond traditional solid-gel electrolytes. 54 In previous reports, electrochromic molecules, such as bipyridinium salts, have been 14,15,32 55 incorporated in IG electrolytes to produce flexible displays in a single-step 39 56 fabrication process, enabling large-scale production . However, the influence of IGs 57 composition in the design, fabrication, and performance of such devices is still 58 controversial, and there are not general tips for a function-specific design. 59 This work presents an exhaustive study on P(VDF-co-HFP)-ionic liquid-based IG 60 composition, and its influence on opto-electrochemical performance and mechanical 61 properties. Several IG membranes at different IL loadings and modifications of the cation 62 and/or the anion components were prepared and studied for optimization. Finally, the 63 optimal material was employed in the development of an photo-electrochromic display, 64 as proof-of-concept, namely a single electrolyte layer (IG) doped with 65 photoelectrochromic spiropyran derivatives with a wide range of applications from smart 66 windows to gas sensors 40–48 67 68 69 70 3 71 EXPERIMENTAL 72 Materials 73 Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP), Mw 300000 g·mol- 1 74 ), 1’,3’-Dihydro-1’,3’,3’-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2’-(2H)-indole] 75 (spiropyran, 98 %), and anhydrous acetone for HPLC (>99.9%) were purchased from 76 Sigma-Aldrich, and used as received. 77 Ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide 78 (BMIM TFSI), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6), 1- 79 butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4), butyl-methyl-ammonium 80 bis(trifluoromethanesulfonyl)imide (N1114 TFSI), 1-methyl-1-propylpiperidinium 81 bis(trifluoromethanesulfonyl)imide (PP13 TFSI), and 1-butyl-1-methylpyrrolidinium 82 bis(trifluoromethanesulfonyl)imide (BMPyr TFSI) were purchased and dried with 83 molecular sieves of 3 Å size under vacuum for 24h to ensure a water content of < 0.001%. 84 ITO-coated PET sheets, 60 Ω/sq resistance, were acquired from Sigma-Aldrich. Screen- 85 Printed electrodes for electrochemical and spectroelectrochemical measurements were 86 purchased from DropSens. 87 IGs preparation 88 IGs were prepared by stirring the desired amount of IL, P(VDF-co-HFP), and acetone for 89 24h in acetone under N2 atmosphere at room temperature. For all IG compositions, the 90 weight ratio P(VDF-co-HFP) : Acetone of 1:20 was maintained. Once the solution was 91 completely homogeneous, the same volume was casted in a mold and dried at room 92 temperature for 24h to ensure the total evaporation of acetone. 93 Photo-electrochromic display fabrication 4 94 The photo-electrochromic display was fabricated by preparing a solution containing 95 spiropyran 2 mM and TEMPO 4 mM in BMIM TFSI. After this step, the desired volume 96 of solution was weighed and mixed with P(VDF-co-HFP) (previously dissolved in 97 acetone in 1:20 weight ratio), in order to obtain 83.3% BMIM TFSI loading. The resulting 98 solution was casted on a glass ¿slide? and dried for 24h. A 1.5x2 cm2 IG area was then 99 cut and placed between two ITO-coated PET electrodes and connected to a potentiostat. 100 Characterization Techniques 101 Potentiostat CHInstrument 660E model was used to perform electrochemical impedance 102 spectroscopy (EIS) to provide further insights into the electrochemical properties of IGs 103 membranes. An A.C. impedance technique was used, and a small amplitude potential 104 oscillation of 5 mV was applied in order to guarantee the measurements in the steady 105 state, where no significant chemical changes were observed at a frequency range between 106 1 MHz and 1 Hz. The D.C. potential was set at open circuit potential (OCP). The resulting 107 impedance spectra were fitted with Chi660e software, modeling the equivalent circuit 108 model according to the physico-chemical processes that took place on the electrode 109 surface. 110 Cyclic voltammetry (CV) measurements were made with the same potentiostat model in 111 order to determine the electrochemical windows for each IG membrane. All 112 electrochemical studies were performed using screen printed electrodes, where carbon 113 electrodes were used as working and counter, and Ag/AgCl were used as pseudo- 114 reference electrodes. 115 FTIR spectra of IG membranes were recorded using a FTIR Tensor 27 Bruker 116 spectrophotometer, and the Specac Golden Gate accessory for the attenuated reflectance 5 117 modality (ATR) in the range of 3300 to 650 cm-1. OPUS 5.5 software was used for 118 monitoring the spectra. 119 Spectroelectrochemistry studies were performed using a VSP100 model potentiostat and 120 controlled by EC-Lab V9.51 software. The potentiostat was coupled to an LV10290 121 Hamamatsu spectrophotometer and controlled using BioKine 32 V4.46 software. 122 IG film thicknesses were examined using confocal imaging as a non-contact optical 3D 123 profiling technique using Sensofar PLuNeox Microscopy and SensoSCAN software for 124 monitoring. 125 Morphological studies of IG membranes at different percentages of BMIM TFSI were 126 carried out using a MERLIN FE-SEM Scanning Electron Microscopy at a voltage of 1.0 127 kV. The samples were pre-treated by coating them with gold using Precision Etching 128 Coating System (PECSTM) sputtering. 129 130 131 132 133 134 135 136 137 138 6 139 RESULTS 140 1. Influence of IL loading on optical and mechanical properties of IGs 141 In this study, the IG membranes presented in Figure 1 were prepared and studied. They 142 contained different cation-anion compositions, IL loading, and doping agents. 143 144 Fig. 1. Molecular structures of cation and anions of IL compositions, polymer matrix, and 145 photoelectrochrome used in this study for IG film forming. 146 The effect of the IL content in the IG (weight percentage) was evaluated by loading 147 different IL amounts in P(VDF-co-HFP) polymer matrices and comparing their optical, 148 electrical and mechanical properties. The combination of BMIM+ cation and TFSI- anion 149 were used as model IL. Resulting IGs membranes presented clear aspect differences 150 depending on the IL load (Fig. 2 a)), which corroborated by spectral analysis. At 50 % 151 (weight) of BMIM TFSI or below, IG membranes were opaque, with transmittance 152 percentages (∆T%) in the visible range below 25% (Fig. 2 b)) for 40 µm-thick membranes 153 (thickness determined by optical profilometry) and too rigid to be used in flexible 154 displays. The optical (i.e. transparency) and rheological properties (i.e. 155 flexibility/elasticity) of the IGs improved when increasing the percentage of IL. 156 Maximum light transmittance was obtained when incorporating 90% of IL (∆T% = 83%), 157 although IG membranes lost mechanical resistance when IL concentration exceeded 7 158 86.7%. Hence, optimal IL concentration in the IG was set at 83.3% for combining 159 excellent optical and mechanical performances. 160 a) b) 161 162 163 164 165 166 Fig. 2. a) IGs prepared at different weight ratios of BMIM TFSI, showing the transparency change upon 167 the addition of BMIM TFSI. b) Collected data for the change in UV-vis transmittance in the range of 400- 168 700 nm at different loadings of BMIM TFSI and conductivity values obtained by a.c. impedance 169 measurements. 170 Although IL almost completely masked the polymer network and the factual membranes 171 morphology, SEM images revealed evident morphological changes in the IGs at high IL 172 concentrations that influenced the optical and mechanical properties. (Fig. 3). a) b) c) 173 174 Fig. 3. Cross-sectional SEM images of a) pure P(VDF-co-HFP) (0 % BMIM TFSI) and IGs at different 175 weight loadings of BMIM TFSI b) 70 % c) and 83.3 %. 176 IL apparently reduced the porosity of the membrane, which may be partially responsible 177 for its opacity, while contributing to polymer swelling, leading to less fragile and more 178 flexible materials. More information was obtained from FTIR spectra (Fig. 4) where 179 pristine IL and polymer were compared to IGs. Bond vibrations corresponding to BMIM 180 TFSI IL were observed at 1465 cm-1 (CH2 scissoring bending), 1350 cm-1 (SO2 181 antisymmetric stretching + C-SO2-N bonding), 1229 cm-1 (CF vibrational stretching), 8 182 1133 cm-1 (SO2 symmetric stretching), 1048 cm-1 (S-N-S asymmetric stretching), 792 cm- 1 183 (C-S + S-N stretching), and 739 cm-1 (CF3 symmetric bending + C-S stretching). On the 184 other hand, and due to its semi-crystalline nature resulting from the folding of polymer 185 monomers on crystalline spherulite structures 49,50, P(VDF-co-HFP) presented two types 186 of vibrational modes: amorphous polar β-phase vibrations at 1404 cm-1 (CF3 187 antisymmetric stretching), 876 cm-1 (CF2 and C-C symmetric stretching) and 838 cm-1 188 (CH2 bonding), and crystalline non-polar α-phase signals at the wavelength of 796 cm-1 189 (CF3 stretching) and 760 cm-1 (CH2 bonding). 51,52 In the IG membranes, the peaks coming 190 from BMIM TFSI signals became more intense when increasing the IL content in the 191 hosting P(VDF-co-HFP). In the case of vibrational modes associated with the polymer, 192 those assigned to the β-phase remained present in the FTIR spectra, while those 193 corresponding to the crystalline α-phase disappeared after addition of IL. This result 194 suggested a change in the morphology of the P(VDF-co-HFP) matrix, losing part of its 195 crystallinity and becoming more amorphous by the incorporation of IL molecules. In this 196 sense, the addition of ILs may provoke voids or cavities among polymer chains so that 197 spherulites were no longer formed, increasing the flexibility and elasticity of the polymer 198 and reducing light scattering associated with these highly dispersive crystalline structures. 9 199 200 Fig. 4. FTIR spectra of pure BMIM TFSI, pure P(VDF-co-HFP), and IGs at different weight ratios of 201 BMIM TFSI. 202 The ionic conductivity of the IGs was determined by EIS for its direct relationship with 203 the electrochemical performance of the system. EIS spectra showed two consecutives 204 semi-circles (Fig. 5a)) which associated to processes taking place in the electrode- 205 membrane interface and membrane, respectively. Accordingly, EIS data were fitted with 206 the equivalent circuit illustrated in Fig. 5 b), containing: parasite resistance associated to 207 the contacts and the electrode material, R: the components of the electrode-IG interface 208 (RI and CI for the interface resistance and capacitance, respectively); and the electrical 209 parameters associated with the IG, namely the IG resistance (RIG), the IG capacitance 210 (CIG) and constant phase element (CPE) associated to mass transport processes in the IG 211 structure. 212 10 a) b) 213 214 215 Fig. 5. a) Impedance spectra of IG at 83 % BMIM TFSI as an example of general interpretation of Nyquist 216 plots. Experimental data in black dots and fitted impedance curve in solid red line; b) Equivalent circuit 217 model for impedance spectra of IGs measured using screen-printed electrodes. 218 Ionic conductivity of IGs with different IL content (%wt. of BMIM TFSI; Fig. 6 a)) was 219 estimated from RIG impedance as detailed in previous report. 53,54 220 Generally, ionic conductivity of gel electrolytes is studied by ESI using typical 221 electrochemical cells, where the samples are sandwiched between the electrodes. 222 However, this experimental setup directly modifies the thickness of the material affecting 223 final conductivity values. For solving this problem, we used screen-printed electrodes, 224 since samples are placed on the top of the electrodes. Prior use, the corresponding cell 225 constant was calculated for this system. 226 Fig. 6 b) shows a linear decay of the inverse of the IG conductivity (resistivity) with the 227 IL content until reaching a value of 1.05 mS/cm at 90% IL in agreement with the expected 228 conductivity of IL solutions (3.60 mS/cm).55 The increase of conductivity was associated 229 to two main factors: i) the increase in the concentration of ionic species from the IL and 230 ii) the decrease in the number of crystalline domains of P(VDF-co-HFP) polymer that 231 hinder ion mobility and conductivity of the IGs. These processes, however, did not 232 explain the high increase in conductivity observed in the samples between 86.7% and 11 233 90% of BMIM TFSI. In this case, an additional factor influenced conductivity which was 234 the impossibility of the polymer matrix to retain more IL inside its cavities. Thus, the 235 conductivity magnitude obtained in this last case could not be associated to the IG but to 236 IL alone. 237 a) b) 238 239 Fig. 6. a) Nyquist plot of IGs at different BMIM TFSI weight ratios. b) Linear relationship between the 240 inverse of the ionic conductivity and % wt. of BMIM TFSI in IG membranes. Error bars were calculated 241 from the standard deviation of fitted RIG. 242 In summary, optimal IGs properties (i.e. optical, electrochemical and mechanical) were 243 obtained after loading with 83.3% IL. This proportion was used in further optimizations 244 of the IG formulation. 245 2. Influence of IL composition on optical, mechanical and electrochemical 246 properties of IGs 247 The influence of the IL composition on IG properties was evaluated by combining 248 different cation, i.e. BMIM, N1114, BMPyr and PP13, and anion molecules, i.e. TFSI, PF6 249 and BF4 (Fig.7 a), all containing the same molar ratio compared to IG formulated at 83.3% 250 of BMIM TFSI. Results are summarized in Fig. 7. There was a clear correlation between 251 optical and mechanical properties since IGs with more opacity were also qualitatively 12 252 more rigid and brittle. Two formulations showed a transmittance below 40% and some 253 rigidity, i.e. BMPyr TFSI and BMIM BF4. In the first case, BMPyr may have a poorer 254 plasticizer effect than BMIM, used in the model IL, while the other cations presented 255 similar properties. The differences were more evident and reasonable when changing the 256 anion. The transparency increased with the anion size (TFSI < PF6 < BF4), which 257 attributed to a loss of order in the polymer chain. Qualitatively, elasticity also improved 258 along with the anion size. Large molecules incorporated in the polymeric matrix minimize 259 polymer chain interactions and the formation of crystalline structures, reducing its rigidity 260 and avoiding the formation of highly-dispersing particles responsible of polymer opacity. 261 BF4- was too small to produce these structural changes, resulting in opaque and rigid IGs. 262 a) b) 263 264 Fig. 7. a) IGs prepared with IL with different cations and anions; b) Change in UV-vis transmittance in the 265 range of 400-700 nm using different ILs at the same molar concentration. Ionic conductivity values obtained 266 by a.c. Impedance measurements and electrochemical windows recorded by CV. 267 268 The different IL compositions also affected the electrochemical properties of the IG 269 membranes, i.e. the electrochemical window. The electrochemical window was 270 determined as the difference between the reduction and the oxidation potentials from CV 271 at the scan rate of 10 mV/s (see Fig S2 in supplementary information) that determines the 272 maximum potential range of operation of the electrochromic system. 13 273 In IGs, the electrochemical window is limited by the reduction of the cations, and the 274 oxidation of the anions of the ILs. Large electrochemical windows between 2.7 and 3.2 275 V vs Ag/AgCl were obtained in all cases thanks to the electrochemical stability of the 276 used compounds. The oxidation limits similar values than the reduction ones due to the 277 similarly low electron-donor character of fluorinated anions responsible of the oxidation 278 potential (TFSI, BF4-, PF6-). 279 Regarding reduction potentials, when substituting the quaternary amines by alkyl chains 280 (PP13, BMPyr, N1114) the electrochemical reduction significantly shifted to higher values 281 and can be attributed to a poorer electron-withdrawing effect. The combination N1114 282 TFSI provided the largest redox potentials. 283 Conductivity values for the different IGs formulations were obtained from the inverse of 284 the magnitude of the RIG. EIS spectra when changing anion and cation concentrations and 285 the final conductivity magnitude are illustrated in Fig. 8. In general, all presented 286 conductivities in the range between 0.215 mS/cm and 0.846 mS/cm, which suitable for 287 all-solid and flexible electrochemical applications. The influence of the anion in IG 288 conductivity was low, and only BF4- presented a noticeable increase in the conductivity 289 magnitude which associated to its small size and high mobility in the IG matrix. In the 290 case of cations, it was a clear correlation between the IG conductivity and that of the 291 pristine ILs (BMIM > N1114 > BMPyr > PP13). Thus, PP13, the one with the largest size 292 and less motility, presented lower conductivities. 293 Thus, the P(VDF-co-HFP)-based IG doped with 79 % IL containing N1114 TFSI was 294 selected as the optimal IG formulation in the production of flexible displays for presenting 295 the highest transparency and electrochemical window, while maintaining an acceptable 296 conductivity. Note that 79 % of N1114 TFSI membranes contains same molar 297 concentration than 83.3 % of BMIM TFSI in IG. 14 298 a) b) c) 299 Fig. 8. Nyquist plot of the IGs when changing the a) anion or b) cation composition. c) Comparison of the 300 conductivity magnitude from analysis of the previous EIS spectra. Error bars were calculated from the 301 standard deviation of fitted RIG. 302 303 3. Photo-electrochromic display fabrication 304 A low-cost two electrode display was prepared as a proof-of-concept by incorporating the 305 well-known photo-electrochromic spiropyran molecule 41-44 on the previous IG. TEMPO 306 molecule was also added as redox mediator to reduce de magnitude of the applied 307 potential necessary to achieve the color change. To evaluate the photochromic and 308 electrochromic capacity of the IG, it was placed between two flexible ITO-coated PET 309 electrodes. An initial photochromic study demonstrated that spiropyran in the IG matrix 310 responded to optical stimuli (UV light radiation) leading to the photo-isomerization from 311 the spirocyclic form (colorless state, OFF, SP isomer) to its ring-open or merocyanine 312 isomer (pink color, ON, MC isomer), and subsequent recovery after irradiation with 313 visible light. Similarly, the electrochromism of the system was also explored and it was 314 found that the application of a 2V potential in a two electrodes system (based on the 315 standard potential of spiropyran and TEMPO, (i.e. -1.2 V and 0.80 V vs Ag/AgCl, 316 respectively), was needed to produce the change of color in the IG from transparent to 317 pink-orange by the reduction of SP form and the subsequent electroinduced ring-opening 318 to the MC form (Fig 9). Mechanistically, the SP form is reduced on the cathode, leading 15 319 to formation of the open-ring radical anion, [MC]−•. Simultaneously, TEMPO was 320 oxidized on the anode forming the corresponding cationic form, TEMPO+. Both species, 321 [MC]−• and TEMPO+, diffuse from the respective electrodes through the electrolyte layer 322 to the bulk material. Eventually, TEMPO+ was spontaneously regenerated due to the 323 electron transfer reaction with [MC]−•. The oxidation of [MC]−• after the homogenous 324 electron transfer led to the open or merocyanine form MC, which resulted in an intense 325 orange-pink coloration. The MC form was easily reconverted into the transparent state, 326 SP form, upon irradiation of the material at 473 nm. 327 328 329 Fig. 7. a) Scheme of the electrochemical and photochemical reactions that take place in the IG membrane. 330 b) UV-Vis absorbance spectra before and after the external inputs were applied c) Photographs for 331 transparent, when 0 V was applied or after visible light. 332 333 334 16 335 CONCLUSIONS 336 In summary, an exhaustive study is presented on P(VDF-co-HFP)-IL based IGs, in which 337 their improved formulation perfectly overcome the main issues of typical solid 338 electrolytes for electrochemical applications and more especially for opto- 339 electrochemical devices. Firstly, proper formulations of IGs, particularly, based on 340 BMIM TFSI (83.3% wt.) and N1114 TFSI achieved high ionic conductivities, a wide 341 electrochemical window - for higher demanding electrochemical applications- and 342 excellent rheological properties, such as good consistency, flexibility, transparency, and 343 elasticity. On the other hand, we demonstrated that the possibility to easily change the IL 344 composition owing to their good chemical compatibility allows their use for further 345 applications to be expanded. 346 Eventually, we demonstrated the functionality of formulated IGs, showing the 347 performance of an opto-electrochromic device constructed following simple fabrication 348 steps. 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