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Unusual switching of ionic conductivity in ionogels enabled by water‐induced phase separation

Unusual switching of ionic conductivity in ionogels enabled by water‐induced phase separation INTRODUCTIONIontronics[1–3] have drawn great interest in recent years for their promising applications in broad fields, including wearable electronics,[4–6] biointerfacing,[7–9] and flexible energy devices.[10–12] In pursuit of intellectualized performance under various application conditions, intensive efforts have been devoted to endowing the iontronic materials with switchable ionic conductivity to realize the stimuli responsiveness of iontronic devices. For instance, photo‐responsive iontronics have recently been developed and applied as ion switches, whereas complex logic circuit design is needed due to the inadequate change in magnitude of ionic conductivity.[13,14] Compared with an external light source, temperature could be a better trigger for inducing a larger change in ionic conductivity, but such a system usually involves undesirable passive energy input.[15,16] An ideal ion switch is that it can be triggered by an environmentally benign stimulus and accompanied by low energy consumption. Such ionic switches are highly desired and have high potential to meet the required sustainability of iontronics‐related areas; unfortunately, they are still not available.As the most robust and ubiquitous resource on Earth, water can significantly affect the solvation and hence ion mobility of a material system[17,18] and serves as an optimal trigger for ion switches. To our knowledge, humidity‐responsive materials reported by far usually show gradual change and positive correlations between relative humidity (RH) and ionic conductivity.[19–24] Nevertheless, due to such a positive correlation, moisture is likely to cause short circuits and subsequent safety problems during electronic equipment operation. To eradicate this potential hazard, there is an urgent need for the iontronic material to have an atypical and sharp change in its ionic conductivity when the ambient humidity exceeds a safety threshold.Ionogels,[25–28] as one of the most promising soft semisolid ionic conductors, are composed of ionic liquids (ILs) solidified by three‐dimensional polymer networks. Homogeneous ionogels usually exhibit good ionic conductivity due to the good compatibility between the polymer networks and the ILs. On the other hand, phase separation[29] within the heterogeneous ionogels would create an additional barrier and restrict the ions from moving freely, leading to a lower ionic conductivity. Based on the above analysis, we hypothesize that an anomalous switching of ionic conductivity within humidity‐responsive ionogels (HRIGs) is achievable if the water molecules can diffuse into the ionogel material and cause significant phase separation. Luckily, a water‐induced stiffening ionogel enabled by phase separation has recently been developed,[30] where water acts as a dynamic modulator in regulating the subtle dissolution equilibrium within the polymer–salt–IL ternary system,[31] providing a dramatic and reversible modulus change upon the alternation between low and high humidity. Inspired by this finding, we realize that this HRIG could be a promising candidate for constructing smart iontronic materials to eliminate safety concerns at high ambient humidity, since ion movement could be successfully restricted by the formation of large‐scale polymer chain aggregates caused by water addition.Herein, an unprecedented HRIG composed of a hydrophobic poly(benzyl methacrylate) (PBzMA) polymer matrix swollen by hygroscopic lithium salt and hydrophobic IL is designed and prepared. The ionogel shows a considerable decrease in ionic conductivity of over two orders of magnitude when exposed to higher humidity, and such conductivity switching is reversible and repeatable. Systematic investigations on the influence of components were performed; meanwhile, molecular dynamic simulations were applied to fundamentally understand the switching processes triggered by humidity. For demonstration, this unique humidity‐responsive property is successfully integrated into a supercapacitor, where the ionogel acts as a smart electrolyte, providing a progressing application for possible scenarios. The HRIG in this work is believed to provide a new pathway for the development of smart iontronic materials.RESULTS AND DISCUSSIONIonogel design and preparationThe working mechanism of the HRIG is illustrated in Figure 1A. At low humidity, the HRIG exhibits a high ionic conductivity due to the high mobility of the freely moving ionic carriers within the swollen polymer networks. When the ambient humidity exceeds a critical value, the water molecules can induce the phase separation of the HRIGs, and the ionic carriers are restricted in dispersed and isolated hydrophilic domains, which could further cause a decline in ionic conductivity. Moreover, this process is highly reversible.1FIGURE(A) Schematic illustration of the humidity‐responsive ionogel (HRIG) with anomalous switching of ionic conductivity. (B) Chemical structures of the precursors and fabrication process of the HRIG. (C) Images of as‐prepared HRIGs (sample composition by weight: poly(benzyl methacrylate) [PBzMA]/1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) [EMITFSI]/bis(trifluorosufonylmethane imide) lithium salt [LiTFSI] = 1:1:0.5) before and after exposure to humidity (scale bar = 10 mm)The HRIGs were fabricated through in situ photo‐initiated polymerization of the benzyl methacrylate (BzMA) monomer in the presence of 1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) (EMITFSI) and bis(trifluorosufonylmethane imide) lithium salt (LiTFSI) using ethylene glycol dimethacrylate (EGDMA) and 1‐hydroxycyclohexyl phenyl ketone as the crosslinker and photo‐initiator, respectively. The chemical structures of the components mentioned above and the preparation routes are shown in Figure 1B. The as‐prepared HRIG was transparent, and it turned opaque when exposed to a humid environment (Figure 1C). Moreover, compared to the swelling behavior of hygroscopic polymer gels,[32,33] this water‐induced transition occurred without obvious changes in the size and shape since the water uptake after equilibrium at an RH of 90% was only 6.6 wt%.[30]Humidity effect on ionic conductivityTo visually demonstrate the change in ionic conductivity, a simple circuit connecting a piece of dry‐state HRIG with an LED was built. As shown in Figure 2A, the LED presented high brightness in the very beginning. After being hydrated with a humidifier, the LED dimmed, indicating that the HRIG became poorly conductive upon humidity triggering. When the HRIG was dehydrated, the LED brightened again, demonstrating good reversibility of the change in ionic conductivity.2FIGUREHumidity effect on ionic conductivity of the humidity‐responsive ionogel (HRIG) (sample composition by weight: poly(benzyl methacrylate) [PBzMA]/1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) [EMITFSI]/bis(trifluorosufonylmethane imide) lithium salt [LiTFSI] = 1:1:0.5). (A) Photographs of LED ON and OFF under humidity regulation (scale bar = 20 mm). (B) Nyquist plots and (C) Bode plots of HRIG in the dry state (relative humidity [RH] = 40%) and wet state (RH = 90%), where the filled squares and unfilled circles represent |Z| and −ϕ, respectively. (D) |Z| and (E) −ϕ as a function of RH (at five test frequencies between 1 and 100 kHz). (F) σ as a function of RH. Error bars show the standard deviation from three independent measurements. (G) |Z| and −ϕ values during 10 cycles between the dry state (RH = 40%) and wet state (RH = 90%)The electrical properties of the HRIG were then recorded by an electrochemical workstation. Unless otherwise specified, gel sheets at a 1:1:0.5 weight ratio of PBzMA/EMITFSI/LiTFSI were chosen throughout the test. The Nyquist plots comparing the dry state (RH = 40%) and wet state (RH = 90%) show that with the alternation of RH, Zʹ increased from 353 Ω to 51 kΩ at −Zʹʹ close to 0 (Figure 2B). In addition, a comparison of the impedance (|Z|) and negative phase angle (−ϕ) at frequencies ranging from 100 kHz to 0.1 Hz is also presented (Figure 2C). In the dry state, |Z| remained nearly constant at high frequencies (100 kHz–100 Hz), while there was a linear increase in |Z| at low frequencies; the negative phase angle −ϕ remained unchanged at higher frequencies until 10 kHz and then increased from 0° to 78°, with the most drastic increase occurring at frequencies ranging from 1 kHz to 10 Hz. In the wet state, as the frequency was decreased during the sweep, |Z| increased from 2.5 × 104 to 3.2 × 105 Ω, while −ϕ showed a downward trend at high frequency, reaching the bottom (−ϕ = 3°) at 363 Hz and then up to 37° at low frequency. It thus can be seen that the HRIG showed typical ionic conductive behavior in the dry state, while it revealed a trend converting from conductor to insulator in the wet state.To study the impedance change during the ambient RH change, we selected a series of humidity (40%–90%, at an interval of 5%) and five typical frequencies for further discussions. Upon the RH increase, |Z| remained at a magnitude of 102 Ω, where a sharp increase was observed at RH = 55%–70% and held at a magnitude of approximately 105 Ω at high humidity (>70%) (Figure 2D). The largest contrast of |Z| occurred at a frequency of 1 kHz. When RH < 60%, the −ϕ of the HRIGs was nearly zero at 5–100 kHz and then increased abruptly at RH = 60%–70%. At frequencies of 50 and 100 kHz, −ϕ reached a maximum at RH = 70%, 47° and 53°, while it showed a smaller contrast of −ϕ at lower frequencies (Figure 2E).The ionic conductivity (σ) of the bulk material was calculated by the following equation:σ=1RdA$$\begin{equation*}\sigma = \frac{1}{R}\frac{d}{A}\end{equation*}$$where d is the thickness of the HRIG in cm, R is the resistance in Ω, and A is the cross‐sectional area in cm2. In the dry state, the HRIG exhibited a high ionic conductivity (σ) of 2.97 × 10–4 S cm–1. Upon the rising of ambient humidity at the test frequencies, σ decreased gradually until RH = 55%, where the impedance decreased abruptly. At RH = 75%, the ionic conductivity reached the lowest point (5.17 × 10–7 S cm–1), and then a slight upward trend ensued, indicating that the phase separation was completed while the HRIG kept absorbing moisture, which could enhance the mobility of the ionic carriers (Figure 2F).To confirm the repeatability of the water‐induced switching of ionic conductivity, 10 cycles of electrical performance testing were conducted between the dry state (RH = 40%) and wet state (RH = 90%) at a frequency of 100 kHz (Figure 2G). It can be observed that |Z| changed by two orders of magnitude without significant changes in values as the RH was switched from a dry state to a wet state in cycles, and −ϕ showed high repeatability during the cycles.Notably, the above results are all on an equilibrium basis, where the samples were placed in the humidity chamber until reaching constant weight prior to testing. To further study the humidity‐responsive dynamic behavior, HRIG sandwiched by porous electrode nickel foams (thickness = 0.3 mm) was used for real‐time monitoring of the impedance change. During hydration, |Z| increased linearly during the first 2 h when the HRIG was placed in the humidity chamber (RH = 90%) and then reached the peak of 4.1 kΩ (approximately 29 times of the initial value) after approximately 8 h. Similarly, there was a drastic increase in −ϕ in the first 3 h, from 1.3° to 41°, and then reached equilibrium at approximately 46°. The dehydration process was investigated by placing the wet‐state sample in the RH = 40% humidity chamber. It took 14 h to return to the dry state, during which time |Z| decreased from 3.2 kΩ to 156 Ω, and −ϕ decreased from 38° to 0.6° (Figure S1A,B). In the cyclic test, there was a hysteresis of the |Z| peak behind the RH peak, 4 min on average, which was considered to result from the delayed mass transfer of water molecules (Figure S2).Composition effect on the switching performanceTo study how each component contributes to the switching performance in the polymer/IL/salt ternary system, a series of samples with different LiTFSI and EMITFSI loadings were employed. The samples are named P1–ILx–Sy (Table S1), where x and y represent the weight ratio of IL and Li salt with respect to polymer. For different LiTFSI loadings, the polymer gels performed differently in the water absorption process (Figure 3A). Upon the rise of ambient humidity, the drop of ionic conductivity happened first on the HRIG with the highest LiTFSI wt% (33%) at RH = 40%, and sequentially, the drop occurred on the gels with lower LiTFSI wt% of 27%, 20%, 11% in ionic conductivity at higher RH (45%, 55%, 65%). Among the test samples, the higher the loadings of EMITFSI are, the higher the ionic conductivity. At RH = 40%, at an IL loading of 25%, σ was relatively low (4.15 × 10–5 S cm–1), while σ increased to 7.30 × 10–5 S cm–1 at an IL loading of 50%, and the trend of σ change during the RH change was similar for each sample at different IL% (Figure 3B). Notably, among the HRIGs, sample P1–IL1–S1 with the highest LiTFSI wt% of 33% showed the largest ionic conductivity contrast (i.e., σmax/σmin = 4781), which is one of the largest among existing polymer‐based ion switches (Table S2), suggesting that LiTFSI plays a crucial role in phase separation (Figure 3C). Nevertheless, there was no apparent correlation between the EMITFSI content and σmax/σmin, indicating that EMITFSI has a more significant impact on the ionic conductivity of the ionogel than on the water‐induced switching behavior (Figure 3D).3FIGUREσ as a function of relative humidity at different loadings of (A) bis(trifluorosufonylmethane imide) lithium salt (LiTFSI) and (B) 1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) (EMITFSI). σmax/σmin at different (C) LiTFSI% and (D) EMITFSI%. Tg,wet and Tg,dry as a function of (E) LiTFSI% and (F) EMITFSI%To further confirm the influence of LiTFSI and EMITFSI on water‐induced phase separation, the glass transition temperature (Tg) of the HRIG was examined by differential scanning calorimetry (DSC) measurements. Tg,dry, Tg,wet, and ΔTg are defined as Tg of the HRIG in the dry state, wet state, and their difference, respectively. As shown in Figure 3E,F, ΔTg increased with elevated loading of either LiTFSI or EMITFSI. Notably, the Tg,wet values with different compositions are all approximately 51°C, which is very close to the Tg of PBzMA (54°C),[34] suggesting little solvation effect of the aggregated polymer chain induced by high humidity. This result is also in good agreement with the high mechanical stiffness of the samples in the wet state. For Tg,dry, both LiTFSI and EMITFSI have shown obvious plasticization effects toward the polymer matrix,[35] while EMITFSI as a liquid seems to be a stronger plasticizer, resulting in a much lower Tg,dry. The large ΔTg clearly revealed the separation between the polymer matrix and the plasticizers, leading to the drop in ionic conductivity of the HRIG.Study of the phase separation microstructureTo explore the microstructure changes during phase separation, the morphologies of the HRIG sample before and after humidity treatment were first analyzed by scanning electron microscopy (SEM). We were not able to carry out the SEM characterization for the HRIG sample in a dry state, since it is not easy to remove the nonvolatile IL without making any change in the microstructure. On the other hand, for the sample in a wet state, considering the volatility of water under vacuum and the reversibility of the phase separation, it is unlikely to conduct SEM observation directly. To solve this issue, before the SEM test, wet‐state HRIG samples were immersed into ethanol/water (3:1) mixed solvent for 7 days prior to freeze drying. The mixed solvent was a good solvent for both IL and Li salt but a nonsolvent for PBzMA; the phase separation structure could thus be preserved during the solvent extraction of IL and Li salt. It was observed that there was no difference in size between the wet state and freeze‐dried state of the HRIG, indicating no morphological collapse of the polymer network during sample preparation (Figure S3). Micron‐scale cavities were observed under SEM at 500× and 5k× of magnitude, evenly distributed on the cross‐section of the freeze‐dried HRIG (Figure 4A,B). According to the statistics from the SEM images, the cavities in the freeze‐dried sample have a number‐average diameter of 4.2 μm (Figure 4C). Therefore, phase separation in this ionogel is microphase separation since the phase structures are on microscopic length scales, as shown by the SEM image.[36]4FIGUREScanning electron microscopy (SEM) patterns of freeze‐dried humidity‐responsive ionogel (HRIG) at (A) 500× (scale bar = 100 μm) and (B) 5k× (scale bar = 10 μm) magnification. (C) The size distribution of the cavities in the freeze‐dried state of the HRIG, counted and calculated from (A)Molecular dynamics simulationsTo fundamentally understand the aforementioned experimental observations, we perform molecular dynamics (MD) simulations to study the detailed structural and dynamic properties of the PBzMA/EMITFSI/LiTFSI ternary system. In the simulation, we coarse grained the ionic species (including the cations/anions of IL, Li+, and TFSI–) and the crosslinkers as dispersed beads and modeled each polymer as connected monomer beads. The simulations were performed in the canonical (NVT, constant number of particles, volume and temperature) ensemble through the large‐scale atomic/molecular massively parallel simulator.[37] The model was constructed based on our previous simulations and other studies,[38–43] and the details can be found in Supporting Information.We started our analysis from the effects of water content (w) on the structures of the polymer/IL/salt mixtures. Figure 5A shows the typical simulation snapshots, where the water content is varied in accord with w = 0, 5.6, 54.7 mol% in our experiments, corresponding to the water absorption of the HRIG at the ideal dry state (RH = 0%), dry state (RH = 40%), and wet state (RH = 90%), respectively. We calculated the structure factors of polymers S(q) at different water contents (Figure 5B), and we also provided S(q) with water contents of w = 54.7 mol% at different MD simulation time steps (Figure 5C). Our simulations indicate that at w = 0 mol%, the PBzMA/EMITFSI/LiTFSI mixtures exhibit a homogeneous structure, as evidenced by the flat curve of S(q) in Figure 5A. With increasing w, the peak position of S(q) exhibits a significant increase at small q, illustrating the formation of the water‐rich phase and polymer‐rich phase. More specifically, we display the number of neighboring water beads (ni‐w) and the number of monomer beads (ni‐m) around the ionic species (including cations and anions of both EMITFSI and LiTFSI) in Figure 5D and show the radial distribution functions of ionic species around each monomer [gi‐m(r)] in Figure S4. With increasing water content, ni‐w exhibits apparent growth, whereas ni‐m and gi‐m(r) decay, indicating that the increased water components snatch away the ionic species from the polymer‐rich region. Accordingly, we calculate the mean‐square displacements of the ions as ⟨[r⃗(t)−r⃗(0)]2⟩$\langle {[ {\vec r( t ) - \vec r( 0 )} ]^2}\rangle $ in Figure S5 and further obtain the diffusion coefficients of the ions (D) at different water contents in Figure 5E. The results show that the increase in w can cause a decrease in D. We therefore could conclude that the addition of water would cause the phase separation of the mixture and that the water beads can grasp the ions from the polymer‐rich region[44] through strong solvation interactions, which further causes the confinement and slower mobility of ions confined in the water‐rich region. Therefore, compared to water‐free systems, water‐containing polymer/IL/salt mixtures typically exhibit worse interconnectivity for ionic conductivity. Our simulations thus clarify the microscopic mechanisms for the experimentally observed variations in ionic conductivity and phase separation of the polymer/IL/salt mixtures induced by water addition.5FIGURE(A) Typical simulation snapshots of water‐containing poly(benzyl methacrylate) [PBzMA]/1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) [EMITFSI]/bis(trifluorosufonylmethane imide) lithium salt [LiTFSI] mixtures at water contents of w = 0, 5.6, and 54.7 mol%, in which the gray, dark blue, yellow, red, and light blue beads denote the monomer, crosslinker, cation, anion, and water molecule, respectively. (B) Structure factors of polymers S(q) at different water contents. (C) Structure factors of polymers with water contents of w = 54.7 mol% at different simulation time steps. (D) Number of neighboring water molecules (ni‐w) and number of neighboring polymer monomers (ni‐m) around each single ion. (E) Diffusion coefficients of ions (D) at different water contentsHumidity‐responsive supercapacitorsFor demonstration, HRIGs were applied to fabricate humidity‐responsive supercapacitors by being sandwiched between carbon cloth electrodes (Figure 6A,B). Figure 6C shows the change in |Z| and ‐ϕ at frequencies ranging from 100 kHz to 0.1 Hz in both dry and wet states. To evaluate the capacitance and electrochemical performance of the HRIG, the cyclic voltammatry (CV) and the galvanostatic charge‐discharge (GCD) curves were recorded (Figure S6A,B). At low humidity (RH = 40%–55%), the CV curves outline a larger area, which suggests rational ion kinetics and supercapacitive behavior, and become linear at higher humidity over 65%, indicating that the HRIG almost turned into an insulator (Figure 6D). As shown in Figure 6E, the areal capacitance computed by CV curves decreased sharply by 2253 times (from 18.7 to 8.3 μF cm–2) when the RH increases from 55% to 70%, which is in good agreement with the σ–RH curve in Figure 2E.6FIGURE(A) Schematic illustration of the humidity‐responsive supercapacitor and (B) real photographs of the device (scale bar = 20 mm). (C) Bode plots of the supercapacitor comparing the dry state (relative humidity [RH] = 40%) to the wet state (RH = 90%), where the filled and unfilled circles represent |Z| and −ϕ, respectively. (D) CV curves of the supercapacitor in the range of 40%–90% RH. (E) Areal capacitance change regulated by RHCONCLUSIONSIn summary, we have successfully designed and fabricated an unprecedently smart HRIG with anomalous switching in ionic conductivity. The HRIG was composed of hygroscopic lithium salt and hydrophobic IL, supported by a hydrophobic PBzMA polymer matrix. The switching process was driven by ambient humidity, which is accessible, low energy consumption, and pollution free; more importantly, the change in ionic conductivity can reach as high as two orders of magnitude. The composition effect on the switching of ionic conductivity was comprehensively investigated through both experimental characterizations and MD simulations. Furthermore, the HRIG material was successfully applied as a smart electrolyte in a humidity‐responsive supercapacitor. Our work provides new insights toward developing next‐generation smart iontronics through the subtle design and adjustment of phase properties within polymer gels.EXPERIMENTALMaterialsBzMA (98%, Energy Chemical), EMITFSI (99%, Lanzhou Greenchem), LiTFSI (99%, Shangfluoro), EGDMA (99%, Energy Chemical), and 1‐hydroxycyclohexyl phenyl ketone (photo‐initiator 184, 98%, Aladdin) were used as received.Synthesis of ionogelsAll the ionogel samples were prepared through free radical polymerization. BzMA, EMITFSI, LiTFSI, EGDMA, and photo‐initiator 184 were mixed to form a transparent precursor solution. The precursor solution was then degassed and injected into a clean glass mold separated by a polydimethylsiloxane spacer (thickness: 1 mm). The polymerization was carried out under ultraviolet light for 1 h. After polymerization, the ionogel was placed under vacuum at 75°C for 3 h to remove the residual monomers.Electrochemical measurementsThe impedance tests were performed using an electrochemical workstation (AutoLab M204, Metrohm). At each humidity, we placed the sample in a humidity chamber (GHS‐24, Espec) until reaching a constant weight. In the real‐time monitoring processes, an LCR (inductance‐capacitance‐resistance) meter (TH2832, Tonghui) and a 10 mm × 10 mm × 1 mm HRIG sample were employed, and nickel foams were used as electrodes. For the humidity‐responsive supercapacitors, all electrochemical measurements were performed using an electrochemical workstation (CHI660E, Chenhua).Material characterizationThe morphology of the HRIG was characterized by SEM (MAIA 3, accelerating voltage: 10 kV). DSC measurements were performed using a calorimeter (DSC 2500, TA Instruments) under a nitrogen flow, where the samples were placed in alumina crucibles (flow rate: 50 mL min−1; temperature range: −80°C to 110°C; heating rate: 10°C min−1).ACKNOWLEDGMENTSLe Yao and Xiaoqing Ming contributed equally to this work. This work was supported by the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (grant no. 2017ZT07C291), the National Natural Science Foundation of China (grant nos. 22078276 and 22005260), the Shenzhen Science and Technology Program (grant no. KQTD20170810141424366), and Shenzhen Key Laboratory of Advanced Materials Product Engineering (grant no. ZDSYS20190911164401990). Qi Zhang thanks the Presidential Fund (grant no. PF01000949) for supporting his research at CUHK‐Shenzhen. The MD simulations of this work are financially supported by the National Natural Science Foundation of China (grant no. 22073094), the Science and Technology Development Program of Jilin Province (grant no. 20210402059GH), and the Science and Technology Plan Projects of Yunnan Province (grant no. 202101BC070001‐007) of China. We are grateful for the essential support of the Network and Computing Center, CIAC, CAS, and the Computing Center of Jilin Province.ETHICS STATEMENTThis article does not involve any human investigation and animal experiment.CONFLICT OF INTERESTThe authors declare they have no conflicts of interest.DATA AVAILABILITY STATEMENTThe data that support the findings of this study are available from the corresponding author upon reasonable request.REFERENCESS. Z. Bisri, S. Shimizu, M. Nakano, Y. Iwasa, Adv. Mater. 2017, 29, 1607054.Y. Chang, L. Wang, R. Li, Z. 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Unusual switching of ionic conductivity in ionogels enabled by water‐induced phase separation

Yao, Le; Ming, Xiaoqing; Lin, Chengjiang; Duan, Xiaozheng; Zhu, He; Zhu, Shiping; Zhang, Qi
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© 2023 The Authors. Aggregate published by SCUT, AIEI, and John Wiley & Sons Australia, Ltd.
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DOI
10.1002/agt2.249
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Abstract

INTRODUCTIONIontronics[1–3] have drawn great interest in recent years for their promising applications in broad fields, including wearable electronics,[4–6] biointerfacing,[7–9] and flexible energy devices.[10–12] In pursuit of intellectualized performance under various application conditions, intensive efforts have been devoted to endowing the iontronic materials with switchable ionic conductivity to realize the stimuli responsiveness of iontronic devices. For instance, photo‐responsive iontronics have recently been developed and applied as ion switches, whereas complex logic circuit design is needed due to the inadequate change in magnitude of ionic conductivity.[13,14] Compared with an external light source, temperature could be a better trigger for inducing a larger change in ionic conductivity, but such a system usually involves undesirable passive energy input.[15,16] An ideal ion switch is that it can be triggered by an environmentally benign stimulus and accompanied by low energy consumption. Such ionic switches are highly desired and have high potential to meet the required sustainability of iontronics‐related areas; unfortunately, they are still not available.As the most robust and ubiquitous resource on Earth, water can significantly affect the solvation and hence ion mobility of a material system[17,18] and serves as an optimal trigger for ion switches. To our knowledge, humidity‐responsive materials reported by far usually show gradual change and positive correlations between relative humidity (RH) and ionic conductivity.[19–24] Nevertheless, due to such a positive correlation, moisture is likely to cause short circuits and subsequent safety problems during electronic equipment operation. To eradicate this potential hazard, there is an urgent need for the iontronic material to have an atypical and sharp change in its ionic conductivity when the ambient humidity exceeds a safety threshold.Ionogels,[25–28] as one of the most promising soft semisolid ionic conductors, are composed of ionic liquids (ILs) solidified by three‐dimensional polymer networks. Homogeneous ionogels usually exhibit good ionic conductivity due to the good compatibility between the polymer networks and the ILs. On the other hand, phase separation[29] within the heterogeneous ionogels would create an additional barrier and restrict the ions from moving freely, leading to a lower ionic conductivity. Based on the above analysis, we hypothesize that an anomalous switching of ionic conductivity within humidity‐responsive ionogels (HRIGs) is achievable if the water molecules can diffuse into the ionogel material and cause significant phase separation. Luckily, a water‐induced stiffening ionogel enabled by phase separation has recently been developed,[30] where water acts as a dynamic modulator in regulating the subtle dissolution equilibrium within the polymer–salt–IL ternary system,[31] providing a dramatic and reversible modulus change upon the alternation between low and high humidity. Inspired by this finding, we realize that this HRIG could be a promising candidate for constructing smart iontronic materials to eliminate safety concerns at high ambient humidity, since ion movement could be successfully restricted by the formation of large‐scale polymer chain aggregates caused by water addition.Herein, an unprecedented HRIG composed of a hydrophobic poly(benzyl methacrylate) (PBzMA) polymer matrix swollen by hygroscopic lithium salt and hydrophobic IL is designed and prepared. The ionogel shows a considerable decrease in ionic conductivity of over two orders of magnitude when exposed to higher humidity, and such conductivity switching is reversible and repeatable. Systematic investigations on the influence of components were performed; meanwhile, molecular dynamic simulations were applied to fundamentally understand the switching processes triggered by humidity. For demonstration, this unique humidity‐responsive property is successfully integrated into a supercapacitor, where the ionogel acts as a smart electrolyte, providing a progressing application for possible scenarios. The HRIG in this work is believed to provide a new pathway for the development of smart iontronic materials.RESULTS AND DISCUSSIONIonogel design and preparationThe working mechanism of the HRIG is illustrated in Figure 1A. At low humidity, the HRIG exhibits a high ionic conductivity due to the high mobility of the freely moving ionic carriers within the swollen polymer networks. When the ambient humidity exceeds a critical value, the water molecules can induce the phase separation of the HRIGs, and the ionic carriers are restricted in dispersed and isolated hydrophilic domains, which could further cause a decline in ionic conductivity. Moreover, this process is highly reversible.1FIGURE(A) Schematic illustration of the humidity‐responsive ionogel (HRIG) with anomalous switching of ionic conductivity. (B) Chemical structures of the precursors and fabrication process of the HRIG. (C) Images of as‐prepared HRIGs (sample composition by weight: poly(benzyl methacrylate) [PBzMA]/1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) [EMITFSI]/bis(trifluorosufonylmethane imide) lithium salt [LiTFSI] = 1:1:0.5) before and after exposure to humidity (scale bar = 10 mm)The HRIGs were fabricated through in situ photo‐initiated polymerization of the benzyl methacrylate (BzMA) monomer in the presence of 1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) (EMITFSI) and bis(trifluorosufonylmethane imide) lithium salt (LiTFSI) using ethylene glycol dimethacrylate (EGDMA) and 1‐hydroxycyclohexyl phenyl ketone as the crosslinker and photo‐initiator, respectively. The chemical structures of the components mentioned above and the preparation routes are shown in Figure 1B. The as‐prepared HRIG was transparent, and it turned opaque when exposed to a humid environment (Figure 1C). Moreover, compared to the swelling behavior of hygroscopic polymer gels,[32,33] this water‐induced transition occurred without obvious changes in the size and shape since the water uptake after equilibrium at an RH of 90% was only 6.6 wt%.[30]Humidity effect on ionic conductivityTo visually demonstrate the change in ionic conductivity, a simple circuit connecting a piece of dry‐state HRIG with an LED was built. As shown in Figure 2A, the LED presented high brightness in the very beginning. After being hydrated with a humidifier, the LED dimmed, indicating that the HRIG became poorly conductive upon humidity triggering. When the HRIG was dehydrated, the LED brightened again, demonstrating good reversibility of the change in ionic conductivity.2FIGUREHumidity effect on ionic conductivity of the humidity‐responsive ionogel (HRIG) (sample composition by weight: poly(benzyl methacrylate) [PBzMA]/1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) [EMITFSI]/bis(trifluorosufonylmethane imide) lithium salt [LiTFSI] = 1:1:0.5). (A) Photographs of LED ON and OFF under humidity regulation (scale bar = 20 mm). (B) Nyquist plots and (C) Bode plots of HRIG in the dry state (relative humidity [RH] = 40%) and wet state (RH = 90%), where the filled squares and unfilled circles represent |Z| and −ϕ, respectively. (D) |Z| and (E) −ϕ as a function of RH (at five test frequencies between 1 and 100 kHz). (F) σ as a function of RH. Error bars show the standard deviation from three independent measurements. (G) |Z| and −ϕ values during 10 cycles between the dry state (RH = 40%) and wet state (RH = 90%)The electrical properties of the HRIG were then recorded by an electrochemical workstation. Unless otherwise specified, gel sheets at a 1:1:0.5 weight ratio of PBzMA/EMITFSI/LiTFSI were chosen throughout the test. The Nyquist plots comparing the dry state (RH = 40%) and wet state (RH = 90%) show that with the alternation of RH, Zʹ increased from 353 Ω to 51 kΩ at −Zʹʹ close to 0 (Figure 2B). In addition, a comparison of the impedance (|Z|) and negative phase angle (−ϕ) at frequencies ranging from 100 kHz to 0.1 Hz is also presented (Figure 2C). In the dry state, |Z| remained nearly constant at high frequencies (100 kHz–100 Hz), while there was a linear increase in |Z| at low frequencies; the negative phase angle −ϕ remained unchanged at higher frequencies until 10 kHz and then increased from 0° to 78°, with the most drastic increase occurring at frequencies ranging from 1 kHz to 10 Hz. In the wet state, as the frequency was decreased during the sweep, |Z| increased from 2.5 × 104 to 3.2 × 105 Ω, while −ϕ showed a downward trend at high frequency, reaching the bottom (−ϕ = 3°) at 363 Hz and then up to 37° at low frequency. It thus can be seen that the HRIG showed typical ionic conductive behavior in the dry state, while it revealed a trend converting from conductor to insulator in the wet state.To study the impedance change during the ambient RH change, we selected a series of humidity (40%–90%, at an interval of 5%) and five typical frequencies for further discussions. Upon the RH increase, |Z| remained at a magnitude of 102 Ω, where a sharp increase was observed at RH = 55%–70% and held at a magnitude of approximately 105 Ω at high humidity (>70%) (Figure 2D). The largest contrast of |Z| occurred at a frequency of 1 kHz. When RH < 60%, the −ϕ of the HRIGs was nearly zero at 5–100 kHz and then increased abruptly at RH = 60%–70%. At frequencies of 50 and 100 kHz, −ϕ reached a maximum at RH = 70%, 47° and 53°, while it showed a smaller contrast of −ϕ at lower frequencies (Figure 2E).The ionic conductivity (σ) of the bulk material was calculated by the following equation:σ=1RdA$$\begin{equation*}\sigma = \frac{1}{R}\frac{d}{A}\end{equation*}$$where d is the thickness of the HRIG in cm, R is the resistance in Ω, and A is the cross‐sectional area in cm2. In the dry state, the HRIG exhibited a high ionic conductivity (σ) of 2.97 × 10–4 S cm–1. Upon the rising of ambient humidity at the test frequencies, σ decreased gradually until RH = 55%, where the impedance decreased abruptly. At RH = 75%, the ionic conductivity reached the lowest point (5.17 × 10–7 S cm–1), and then a slight upward trend ensued, indicating that the phase separation was completed while the HRIG kept absorbing moisture, which could enhance the mobility of the ionic carriers (Figure 2F).To confirm the repeatability of the water‐induced switching of ionic conductivity, 10 cycles of electrical performance testing were conducted between the dry state (RH = 40%) and wet state (RH = 90%) at a frequency of 100 kHz (Figure 2G). It can be observed that |Z| changed by two orders of magnitude without significant changes in values as the RH was switched from a dry state to a wet state in cycles, and −ϕ showed high repeatability during the cycles.Notably, the above results are all on an equilibrium basis, where the samples were placed in the humidity chamber until reaching constant weight prior to testing. To further study the humidity‐responsive dynamic behavior, HRIG sandwiched by porous electrode nickel foams (thickness = 0.3 mm) was used for real‐time monitoring of the impedance change. During hydration, |Z| increased linearly during the first 2 h when the HRIG was placed in the humidity chamber (RH = 90%) and then reached the peak of 4.1 kΩ (approximately 29 times of the initial value) after approximately 8 h. Similarly, there was a drastic increase in −ϕ in the first 3 h, from 1.3° to 41°, and then reached equilibrium at approximately 46°. The dehydration process was investigated by placing the wet‐state sample in the RH = 40% humidity chamber. It took 14 h to return to the dry state, during which time |Z| decreased from 3.2 kΩ to 156 Ω, and −ϕ decreased from 38° to 0.6° (Figure S1A,B). In the cyclic test, there was a hysteresis of the |Z| peak behind the RH peak, 4 min on average, which was considered to result from the delayed mass transfer of water molecules (Figure S2).Composition effect on the switching performanceTo study how each component contributes to the switching performance in the polymer/IL/salt ternary system, a series of samples with different LiTFSI and EMITFSI loadings were employed. The samples are named P1–ILx–Sy (Table S1), where x and y represent the weight ratio of IL and Li salt with respect to polymer. For different LiTFSI loadings, the polymer gels performed differently in the water absorption process (Figure 3A). Upon the rise of ambient humidity, the drop of ionic conductivity happened first on the HRIG with the highest LiTFSI wt% (33%) at RH = 40%, and sequentially, the drop occurred on the gels with lower LiTFSI wt% of 27%, 20%, 11% in ionic conductivity at higher RH (45%, 55%, 65%). Among the test samples, the higher the loadings of EMITFSI are, the higher the ionic conductivity. At RH = 40%, at an IL loading of 25%, σ was relatively low (4.15 × 10–5 S cm–1), while σ increased to 7.30 × 10–5 S cm–1 at an IL loading of 50%, and the trend of σ change during the RH change was similar for each sample at different IL% (Figure 3B). Notably, among the HRIGs, sample P1–IL1–S1 with the highest LiTFSI wt% of 33% showed the largest ionic conductivity contrast (i.e., σmax/σmin = 4781), which is one of the largest among existing polymer‐based ion switches (Table S2), suggesting that LiTFSI plays a crucial role in phase separation (Figure 3C). Nevertheless, there was no apparent correlation between the EMITFSI content and σmax/σmin, indicating that EMITFSI has a more significant impact on the ionic conductivity of the ionogel than on the water‐induced switching behavior (Figure 3D).3FIGUREσ as a function of relative humidity at different loadings of (A) bis(trifluorosufonylmethane imide) lithium salt (LiTFSI) and (B) 1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) (EMITFSI). σmax/σmin at different (C) LiTFSI% and (D) EMITFSI%. Tg,wet and Tg,dry as a function of (E) LiTFSI% and (F) EMITFSI%To further confirm the influence of LiTFSI and EMITFSI on water‐induced phase separation, the glass transition temperature (Tg) of the HRIG was examined by differential scanning calorimetry (DSC) measurements. Tg,dry, Tg,wet, and ΔTg are defined as Tg of the HRIG in the dry state, wet state, and their difference, respectively. As shown in Figure 3E,F, ΔTg increased with elevated loading of either LiTFSI or EMITFSI. Notably, the Tg,wet values with different compositions are all approximately 51°C, which is very close to the Tg of PBzMA (54°C),[34] suggesting little solvation effect of the aggregated polymer chain induced by high humidity. This result is also in good agreement with the high mechanical stiffness of the samples in the wet state. For Tg,dry, both LiTFSI and EMITFSI have shown obvious plasticization effects toward the polymer matrix,[35] while EMITFSI as a liquid seems to be a stronger plasticizer, resulting in a much lower Tg,dry. The large ΔTg clearly revealed the separation between the polymer matrix and the plasticizers, leading to the drop in ionic conductivity of the HRIG.Study of the phase separation microstructureTo explore the microstructure changes during phase separation, the morphologies of the HRIG sample before and after humidity treatment were first analyzed by scanning electron microscopy (SEM). We were not able to carry out the SEM characterization for the HRIG sample in a dry state, since it is not easy to remove the nonvolatile IL without making any change in the microstructure. On the other hand, for the sample in a wet state, considering the volatility of water under vacuum and the reversibility of the phase separation, it is unlikely to conduct SEM observation directly. To solve this issue, before the SEM test, wet‐state HRIG samples were immersed into ethanol/water (3:1) mixed solvent for 7 days prior to freeze drying. The mixed solvent was a good solvent for both IL and Li salt but a nonsolvent for PBzMA; the phase separation structure could thus be preserved during the solvent extraction of IL and Li salt. It was observed that there was no difference in size between the wet state and freeze‐dried state of the HRIG, indicating no morphological collapse of the polymer network during sample preparation (Figure S3). Micron‐scale cavities were observed under SEM at 500× and 5k× of magnitude, evenly distributed on the cross‐section of the freeze‐dried HRIG (Figure 4A,B). According to the statistics from the SEM images, the cavities in the freeze‐dried sample have a number‐average diameter of 4.2 μm (Figure 4C). Therefore, phase separation in this ionogel is microphase separation since the phase structures are on microscopic length scales, as shown by the SEM image.[36]4FIGUREScanning electron microscopy (SEM) patterns of freeze‐dried humidity‐responsive ionogel (HRIG) at (A) 500× (scale bar = 100 μm) and (B) 5k× (scale bar = 10 μm) magnification. (C) The size distribution of the cavities in the freeze‐dried state of the HRIG, counted and calculated from (A)Molecular dynamics simulationsTo fundamentally understand the aforementioned experimental observations, we perform molecular dynamics (MD) simulations to study the detailed structural and dynamic properties of the PBzMA/EMITFSI/LiTFSI ternary system. In the simulation, we coarse grained the ionic species (including the cations/anions of IL, Li+, and TFSI–) and the crosslinkers as dispersed beads and modeled each polymer as connected monomer beads. The simulations were performed in the canonical (NVT, constant number of particles, volume and temperature) ensemble through the large‐scale atomic/molecular massively parallel simulator.[37] The model was constructed based on our previous simulations and other studies,[38–43] and the details can be found in Supporting Information.We started our analysis from the effects of water content (w) on the structures of the polymer/IL/salt mixtures. Figure 5A shows the typical simulation snapshots, where the water content is varied in accord with w = 0, 5.6, 54.7 mol% in our experiments, corresponding to the water absorption of the HRIG at the ideal dry state (RH = 0%), dry state (RH = 40%), and wet state (RH = 90%), respectively. We calculated the structure factors of polymers S(q) at different water contents (Figure 5B), and we also provided S(q) with water contents of w = 54.7 mol% at different MD simulation time steps (Figure 5C). Our simulations indicate that at w = 0 mol%, the PBzMA/EMITFSI/LiTFSI mixtures exhibit a homogeneous structure, as evidenced by the flat curve of S(q) in Figure 5A. With increasing w, the peak position of S(q) exhibits a significant increase at small q, illustrating the formation of the water‐rich phase and polymer‐rich phase. More specifically, we display the number of neighboring water beads (ni‐w) and the number of monomer beads (ni‐m) around the ionic species (including cations and anions of both EMITFSI and LiTFSI) in Figure 5D and show the radial distribution functions of ionic species around each monomer [gi‐m(r)] in Figure S4. With increasing water content, ni‐w exhibits apparent growth, whereas ni‐m and gi‐m(r) decay, indicating that the increased water components snatch away the ionic species from the polymer‐rich region. Accordingly, we calculate the mean‐square displacements of the ions as ⟨[r⃗(t)−r⃗(0)]2⟩$\langle {[ {\vec r( t ) - \vec r( 0 )} ]^2}\rangle $ in Figure S5 and further obtain the diffusion coefficients of the ions (D) at different water contents in Figure 5E. The results show that the increase in w can cause a decrease in D. We therefore could conclude that the addition of water would cause the phase separation of the mixture and that the water beads can grasp the ions from the polymer‐rich region[44] through strong solvation interactions, which further causes the confinement and slower mobility of ions confined in the water‐rich region. Therefore, compared to water‐free systems, water‐containing polymer/IL/salt mixtures typically exhibit worse interconnectivity for ionic conductivity. Our simulations thus clarify the microscopic mechanisms for the experimentally observed variations in ionic conductivity and phase separation of the polymer/IL/salt mixtures induced by water addition.5FIGURE(A) Typical simulation snapshots of water‐containing poly(benzyl methacrylate) [PBzMA]/1‐ethyl‐3‐methyl imidazolium bis(trifluorosufonylmethane imide) [EMITFSI]/bis(trifluorosufonylmethane imide) lithium salt [LiTFSI] mixtures at water contents of w = 0, 5.6, and 54.7 mol%, in which the gray, dark blue, yellow, red, and light blue beads denote the monomer, crosslinker, cation, anion, and water molecule, respectively. (B) Structure factors of polymers S(q) at different water contents. (C) Structure factors of polymers with water contents of w = 54.7 mol% at different simulation time steps. (D) Number of neighboring water molecules (ni‐w) and number of neighboring polymer monomers (ni‐m) around each single ion. (E) Diffusion coefficients of ions (D) at different water contentsHumidity‐responsive supercapacitorsFor demonstration, HRIGs were applied to fabricate humidity‐responsive supercapacitors by being sandwiched between carbon cloth electrodes (Figure 6A,B). Figure 6C shows the change in |Z| and ‐ϕ at frequencies ranging from 100 kHz to 0.1 Hz in both dry and wet states. To evaluate the capacitance and electrochemical performance of the HRIG, the cyclic voltammatry (CV) and the galvanostatic charge‐discharge (GCD) curves were recorded (Figure S6A,B). At low humidity (RH = 40%–55%), the CV curves outline a larger area, which suggests rational ion kinetics and supercapacitive behavior, and become linear at higher humidity over 65%, indicating that the HRIG almost turned into an insulator (Figure 6D). As shown in Figure 6E, the areal capacitance computed by CV curves decreased sharply by 2253 times (from 18.7 to 8.3 μF cm–2) when the RH increases from 55% to 70%, which is in good agreement with the σ–RH curve in Figure 2E.6FIGURE(A) Schematic illustration of the humidity‐responsive supercapacitor and (B) real photographs of the device (scale bar = 20 mm). (C) Bode plots of the supercapacitor comparing the dry state (relative humidity [RH] = 40%) to the wet state (RH = 90%), where the filled and unfilled circles represent |Z| and −ϕ, respectively. (D) CV curves of the supercapacitor in the range of 40%–90% RH. (E) Areal capacitance change regulated by RHCONCLUSIONSIn summary, we have successfully designed and fabricated an unprecedently smart HRIG with anomalous switching in ionic conductivity. The HRIG was composed of hygroscopic lithium salt and hydrophobic IL, supported by a hydrophobic PBzMA polymer matrix. The switching process was driven by ambient humidity, which is accessible, low energy consumption, and pollution free; more importantly, the change in ionic conductivity can reach as high as two orders of magnitude. The composition effect on the switching of ionic conductivity was comprehensively investigated through both experimental characterizations and MD simulations. Furthermore, the HRIG material was successfully applied as a smart electrolyte in a humidity‐responsive supercapacitor. Our work provides new insights toward developing next‐generation smart iontronics through the subtle design and adjustment of phase properties within polymer gels.EXPERIMENTALMaterialsBzMA (98%, Energy Chemical), EMITFSI (99%, Lanzhou Greenchem), LiTFSI (99%, Shangfluoro), EGDMA (99%, Energy Chemical), and 1‐hydroxycyclohexyl phenyl ketone (photo‐initiator 184, 98%, Aladdin) were used as received.Synthesis of ionogelsAll the ionogel samples were prepared through free radical polymerization. BzMA, EMITFSI, LiTFSI, EGDMA, and photo‐initiator 184 were mixed to form a transparent precursor solution. The precursor solution was then degassed and injected into a clean glass mold separated by a polydimethylsiloxane spacer (thickness: 1 mm). The polymerization was carried out under ultraviolet light for 1 h. After polymerization, the ionogel was placed under vacuum at 75°C for 3 h to remove the residual monomers.Electrochemical measurementsThe impedance tests were performed using an electrochemical workstation (AutoLab M204, Metrohm). At each humidity, we placed the sample in a humidity chamber (GHS‐24, Espec) until reaching a constant weight. In the real‐time monitoring processes, an LCR (inductance‐capacitance‐resistance) meter (TH2832, Tonghui) and a 10 mm × 10 mm × 1 mm HRIG sample were employed, and nickel foams were used as electrodes. For the humidity‐responsive supercapacitors, all electrochemical measurements were performed using an electrochemical workstation (CHI660E, Chenhua).Material characterizationThe morphology of the HRIG was characterized by SEM (MAIA 3, accelerating voltage: 10 kV). DSC measurements were performed using a calorimeter (DSC 2500, TA Instruments) under a nitrogen flow, where the samples were placed in alumina crucibles (flow rate: 50 mL min−1; temperature range: −80°C to 110°C; heating rate: 10°C min−1).ACKNOWLEDGMENTSLe Yao and Xiaoqing Ming contributed equally to this work. This work was supported by the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (grant no. 2017ZT07C291), the National Natural Science Foundation of China (grant nos. 22078276 and 22005260), the Shenzhen Science and Technology Program (grant no. KQTD20170810141424366), and Shenzhen Key Laboratory of Advanced Materials Product Engineering (grant no. ZDSYS20190911164401990). Qi Zhang thanks the Presidential Fund (grant no. PF01000949) for supporting his research at CUHK‐Shenzhen. The MD simulations of this work are financially supported by the National Natural Science Foundation of China (grant no. 22073094), the Science and Technology Development Program of Jilin Province (grant no. 20210402059GH), and the Science and Technology Plan Projects of Yunnan Province (grant no. 202101BC070001‐007) of China. We are grateful for the essential support of the Network and Computing Center, CIAC, CAS, and the Computing Center of Jilin Province.ETHICS STATEMENTThis article does not involve any human investigation and animal experiment.CONFLICT OF INTERESTThe authors declare they have no conflicts of interest.DATA AVAILABILITY STATEMENTThe data that support the findings of this study are available from the corresponding author upon reasonable request.REFERENCESS. Z. Bisri, S. Shimizu, M. Nakano, Y. Iwasa, Adv. Mater. 2017, 29, 1607054.Y. Chang, L. Wang, R. Li, Z. 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Journal

Aggregate – Wiley

Published: Feb 1, 2023

Keywords: humidity‐responsive; ionic liquid; polymer gel; ionic conductivity; phase separation

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