3D printing flexible supercapacitors based on crosslinked poly (acrylic acid‐co‐vinylimidazole)

Supercapacitors are an attractive technology for energy storage applications as they allow for fast charging of devices. The fabrication of flexible supercapacitors by additive manufacturing is a promising approach to produce energy storage components for applications where material flexibility and complex geometries are desirable. In this work, digital light projection (DLP) additive manufacturing is used to fabricate polymer electrolytes for flexible supercapacitors based on crosslinked poly(acrylic acid‐co‐vinylimidazole) (PAAVim). Ion gels are prepared through equilibration with 4M lithium chloride (LiCl) in ethylene glycol, deionized (DI) water and ethylene glycol/DI water mixtures. Flexibility and stretchability varied depending on the equilibration solvent with prepared PAAVim/LiCl polymer electrolytes exhibiting up to ˜700% elongation at break. Subsequent flexible supercapacitors fabricated by sandwiching the ion gel between carbon cloth electrodes delivered 60 F/g specific capacitance at the scan rate of 0.2 A/g with an energy density of 8.3 Wh/kg and a power density of 99.8 W/kg. Overall, this work demonstrates the fabrication of flexible capacitors through DLP additive manufacturing, where the resulting material physical and electrochemical properties can be varied through control over the resin chemistry.

GPE consists of a biphasic system including ionic polymer and absorbed liquid solution. 13Ion conduction is through the migration of ions in free volumes or segmental rotations of polymer chains.Thus, to realize higher conductivity and excellent mechanical strength, a good GPE should possess prominent ion exchange groups to facilitate the ion conduction, relatively low glass transition temperature (Tg), and nonvolatile solvents for long-time use. 14Polyacrylic acid (PAA) has good potential in polymer electrolyte due to the ion-conductive carboxylic groups.For instance, Khan et al. fabricated a flexible supercapacitor based on a double network PAA hydrogel electrolyte crosslinked by N,N ′ -methylene bisacrylamide (MBA), and metal complexation. 15Similarly, Zhou et al. demonstrated a regenerable hydrogel electrolyte-based supercapacitor by introducing polyallylamine into the PAA network where the ionic bonds between amine and carboxylic groups work as physical crosslinks to increase the mechanical properties of polymer electrolytes, which show excellent stretchability to 1688%. 16Critically, incorporating a complementary monomer that has high-polarity functional groups, dynamic interactions can form between polymer chains. 17or achieving long-term stability of the supercapacitor, liquid electrolyte medium with extremely low volatility, such as ethylene glycol, plays a significant role.EG is a typical anti-freeze and anti-volatile solvent due to its low vapor pressure (7.5 Pa, 20 • C) and low Henry's law constant (0.0061 Pa m 3 mol −1 ). 18EG also prevents over-heating due to the high boiling point (197.6 • C, 101.3 kPa) and low specific heat capacity. 19igital light projection (DLP) is a vat polymerization-based additive manufacturing (AM) technique that produces high-resolution and complex structures by layer-by-layer polymerization of resin.][25] In our previous work, 26 crosslinked PAAVim demonstrated great flexibility, considerable ion exchange capacity, good mechanical strength, and fast 3D printing behavior using DLP.We also demonstrated the utility of these DLP 3D printed PAAVim copolymers in ionic-polymer metal composite (IPMC) actuators where the mechanical properties and actuator performance (maximum deformation, etc.) in air were dependent on the copolymer composition. 26Here, we investigate this system and DLP AM to fabricate PAAVim specimens followed by solvent exchange with 4M LiCl solutions (in ethylene glycol and ethylene glycol/DI water mixtures) to prepare ion gels for flexible supercapacitors.This work in thereby probes the impact of the swollen electrolyte solution on the resulting physical and electrochemical behavior that are critical for materials design for different applications.

DLP additive manufacturing of PAAVim polymer electrolytes
Photocurable ink was composed of AA and Vim monomers, TMTA as a crosslinker, TPO as a photoinitiator, and DMSO as solvent.Briefly, monomers, crosslinker, and 2 wt% photoinitiator (to the mass of monomers) were dissolved in DMSO (1 g/mL) in a dark ambient environment until a clear solution was obtained; mechanical stirring at 1000 rpm for 30 min.The obtained solution was degassed (15 min at 60 Hz) using an ultrasonic speed mixer (Bransonic).These precursor inks were poured into the vat of an Anycubic Photon-S DLP AM 3D printer (ANYCUBIC Technology Co., Ltd).TinkerCAD was used to design the structures of printed samples, where STL files were generated using ANYCUBIC Photon Slice 64 software (provided by ANYCUBIC).The exposure time of the initial five layers was 70 s, followed by 13 s in each layer (0.01 mm thickness).was performed using a commercial 48 W UV lamp with mixed wavelengths (365 and 405 nm), followed by a solvent exchange in DI water for two days.Samples were dried in a vacuum oven at 40 • C for 12 h.The fabricated PAAVim samples are denoted PX, where X is the molar percent of the imidazolium groups in the polymer backbone, varying from 0 to 20 mol%.

Mechanical characterization
Mechanical characterization was performed using a TA Instrumental dynamic mechanical analyzer RSA III using fabricated rectangular samples (30 mm × 10 mm × 1 mm).Static tensile testing was performed with a stretching speed of 0.1 mm/s and a 10 mm initial gauge length in room temperature based on ASTM D882-18 thin film standard. 27Young's modulus was obtained for all the samples analyzing the initial slope of stress-strain curve.

Fabrication of flexible supercapacitors
The surfaces of fabricated samples were wiped dry with a VWR light-duty tissue wiper.The carbon electrode ink was composed of activated carbon, acetylene black, and PTFE at 8:1:1 mixing with pure alcohol in a Bransonic ultrasonic cleaner (60 Hz for 2 h).Carbon cloths were treated with nitric acid for 12 h and rinsed with DI water, vacuum drying for 2 h at 50 • C. Both surfaces of carbon cloths were coated with carbon electrode ink at loading of 1 mg/cm 2 and vacuum dried for 2 h to remove the solvent.The obtained carbon sheets were applied on both sides of ion gels to build a sandwiched structure.

Ionic conductivity
The ionic conductivity of ion gels was evaluated based on electrochemical impedance spectroscopy (EIS) (frequency range: 1 to 10 6 Hz) using a Gamry Interface 1000 potentiostat to test the supercapacitor.Ionic conductivity () was calculated as  = d∕(R × A) where d was the thickness of the polymer electrolyte (d = 1 mm), R is bulk resistance (obtained from the intercept of x axis from the Nyquist plot), and A is the area of the electrolyte (3 cm 2 ).

Electrochemical characterization
Electrochemical behavior of PAAVim supercapacitors was evaluated by measuring cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) based on a two-electrode system using a Gamry Interface 1000 potentiostat.CV undergoes 0 to 1 V potential at varied scanning rates (10 to 200 mV/s).GCD utilized 0.1, 0.2, 0.5, and 1 A/g under 1 V. EIS was performed by increasing the frequency from 10 to 10 6 Hz at 10 mV/s.The specific capacitance (C, F/g) was calculated from the GCD curves based on Equation (1).
The energy density (E, Wh/kg) and power density (P, W/kg) were calculated using Equations ( 2) and (3), respectively, where I (A) is the discharge current, Δt (s) is the discharge time of the GCD curve, m c (g) is the total mass of activated carbon of the supercapacitor, and ΔV (V) is the applied potential window of the supercapacitor.

Material synthesis
Crosslinked poly(acrylic acid-co-vinyl imidazole) PAAVim films were fabricated using DLP additive manufacturing under 405 nm UV light exposure.Figure 1A shows the chemical structures and reaction schematics for free-radical polymerization for this system.
To confirm successful fabrication of copolymers and polymer electrolyte films, FTIR spectroscopy was performed (Figure 2).PAA has clear peaks at 2852 and 2921 cm −1 for the -OH stretching vibration in -COOH and at 1710 cm −1 for the C=O of the carboxylic groups.The PVim spectra has clear peaks at 1499 and 1651 cm −1 for the C=N in-plane stretching and C=N stretching of the imidazolium ring, respectively.In the PAAVim5-4M LiCl/EG spectra, a broad and overlapping peak at 1645 cm −1 provides a reasonable confirmation of the introduction of vinylimidazole into the structure whereas for P0-4M LiCl/EG graph, there is no peak in 1645 cm −1 .At 3410 cm −1 , a weak and broad peak for the -OH stretching vibration of ethylene glycol in P5-4M LiCl/EG is observed, indicating the successful solvent exchange of LiCl solutions into the copolymer networks.
After solvent exchange from DMSO to DI water and vacuum dried, the films were soaked in 4M LiCl in EG to obtain organogel.The relative rigidity of the films was considered manually and judged by the ability to bend the films to approximately 30 • (results shown in Table 1).Notably, all PAAVim films soaked in 4M LiCl EG were too rigid to bend to 30 • and were thereby deemed not flexible enough for fabricating a flexible supercapacitor.A series of mixed 4M LiCl in EG/DI water of varying EG to DI water ratios were then investigated.The addition of water led to more flexible films with increasing water content needed as the imidazolium content increased.As only P0 and P5 showed good flexibility in this screening test across the different EG/DI mixtures, herein we will focus on comparison of the behavior of these two systems (without and with 5% imidazolium).

Mechanical characterization
To investigate the impact of the different solvents on the copolymer mechanical properties static tensile analysis was used; water content within each chemistry also led to increased modulus and elongation at break.For instance, P5-1:2/EG:DI exhibited the steepest stress-strain slope at low strain (largest Young's modulus) and elongation at break (∼700%) whereas P5-LiCl/EG can only be stretched to 60% with a shallower stress-strain slope.The improved performance in the P5 compared to the P0 is attributed to the presence of the hydrogen bonding network between the acrylic acid and imidazolium functionalities. 26,28

Electrochemical characterization
Flexible supercapacitors based on PAAVim copolymers equilibrated with 4M LiCl solutions (in EG and EG/DI) were prepared to evaluate the impact of solvent on electrochemical behavior.GCD is an effective technique for studying the electrochemical capacitance of PAA-Vim supercapacitors.Figure 4A shows the GCD behavior of the fabricated supercapacitors at a current density of 0.2 A g −1 .The P5-EG-based supercapacitor has a longer discharging time than P0-EG, indicating a higher capacitance.P0-EG also exhibits a smaller iR drop (Table 2) than P5-EG such that overall P5 has a better electrochemical capacitance behavior which could be due to faster ion transport in the P5 copolymers.P5 with varied EG/DI water content was also prepared to evaluate the impact of the different solvents on electrochemical behavior.Overall, P5-EG/DI shows a shorter discharging time than P5-EG.However, the iR drop of P5-EG/DI has an apparent decrease by adding DI water to the bi-solvent systems.
Extracted specific capacitance values for each are shown in Table 2. Overall, all samples have specific capacitances within a relatively narrow range (53.2-60.4F/g) with only P5-1:0/EG/DI outside of the 57-60.4F/g range.And so, the DI water has a minor effect on the electrochemical capacitance of these PAAVim polymer electrolytes with the greatest impact seen for the highest DI water content (P5-1:3/EG:DI).Figure 4B shows the CV curves of the supercapacitors based on P5 with different solvents in the 0 to 1 V potential range at a 20 mV/s scanning rate.The P5-EG trace has a  F I G U R E 5 (A) EIS results of P5 with different solvents and (B) Ionic conductivity of P5 with different solvents and same LiCl concentrations quasi-rectangular shape without redox peaks, indicating typical double-layer capacitance behavior.However, the rectangular shapes are more slanted and blunter with increasing DI water content.P5-2:1/EG:DI still retains quasi-rectangular shapes whereas for P5-1:2/EG:DI the curve exhibits a narrow integrated area and blunt shapes.Thus, the electrochemical capacitance behavior is impaired by the addition of DI water into the polymer electrolyte.From the CV and GCD comparison graphs, P5-EG shows slightly better overall capacitor behavior.The Nyquist plots of supercapacitors based on P5 in EG/DI systems are presented in Figure 5A.The interception at the x-axis of the P5-1:2/EG:DI means a low charge transfer resistance (7 Ω), indicating a good contact between the hydrogel electrolyte and the electrodes.The ionic conductivity (using 1 mm thick films) was calculated based on the bulk resistance and summarized in Figure 5B.P5-1:2/ EG:DI exhibits the most considerable ionic conductivity, decreasing with the increasing EG content in the bi-solvents.Moreover, the high and inclined slop in the Nyquist plots indicated a good pseudo-capacitive behavior of the solid-state supercapacitor with P5 ionic gels, which could be likely due to the interception and retention of ions into the porous carbon electrodes.
A series of electrochemical tests were designed to evaluate further the electrochemical performance of 3D printed flexible P5-2:1/EG:DI supercapacitors.Figure 6A exhibited the CV curves of supercapacitors based on 3D printed P5-2:1/EG:DI at different scanning rates in the potential range of 0 to 1 V.The CV curves presented quasi-rectangular shapes at all scan rates, indicating excellent rate capability and remarkable capacitive behavior.Moreover, GCD curves (Figure 6B) show symmetric triangle shapes at different current densities from 0.2 to 1 A/g.The 3D printed flexible P5-2:1/EG:DI supercapacitors deliver 60 F/g specific capacitance at the scan rate of 0.2 A/g with an energy density of 8.3 Wh/kg and power density of 99.8 W/kg.The specific capacitance is comparable to other works (see Table 3).
In order to evaluate the relationship between the flexibility of supercapacitors and electrochemical stability, the P5-2:1/EG:DI star-shape supercapacitor was bent to different angles (0 • , 60    scanning rate of 20 mV/s. Figure 7A shows no significant differences between the various angles from the CV curves. The CV curve maintains a quasi-rectangular shape under a 120 • bending angle, indicating excellent flexibility and electrochemical stability under bending.CV was used to investigate the impact of temperature (−23, 20, and 100 • C) on the electrochemical performance of the P5-2:1/EG:DI supercapacitor.Figure 7B shows that the CV curves maintain quasi-rectangular shapes in different temperatures, indicating excellent electrochemical stability from −23 to 100 • C.However, the integrated area of the P5-2:1/EG:DI supercapacitor shows an increasing trend from −23 to 100 • C due to the higher diffusivity of Li ions at high temperatures.

CONCLUSIONS
In conclusion, gel polymer electrolytes based on PAAVim equilibrated with 4M LiCl (in EG and EG/DI water) were successfully fabricated using DLP additive manufacturing.Increasing imidazolium content limited fabricated specimen flexibility (likely due to increased hydrogen bonding within the structure), however the use of water within the electrolyte improved flexibility such that P5 with 1:2 EG:DI achieved ∼700% elongation at break and exhibited the highest Youngs modulus.The P5-2:1/EG:DI supercapacitor demonstrated the best electrochemical performance based on GCD and CV characterization.The supercapacitors with 3D printed star-shape polymer electrolytes exhibit low charge transfer resistance of 7 Ω, an excellent specific capacitance of 60 F/g at the current density of 0.2 A/g.Moreover, the quasi-rectangular shapes of CV curves show no significant changes and redox peaks at different bending angles (0 • , 60

Figure 3 .
Overall increasing imidazolium content led to increased modulus and elongation at break and increasing DI F I G U R E 1 (A) The fabrication chemistry of PAA-Vim polymer electrolyte.(B) Schematic of the polymer electrolyte and supercapacitor configuration F I G U R E 2 FTIR spectra of the PAA, PVim, P5-4M LiCl/EG, and P0-4M LiCl/EG TA B L E 1 Summary of whether the polymer electrolytes with different compositions and salts are bendable to 30 •Ability to bend to 30•

F I G U R E 3
Static tensile results of (A) P0 and (B) P5 with 4M LiCl in EG:DI mixtures (A) (B) F I G U R E 4 (A) GCD results of P0-4M LiCl EG and P5 4M LiCl with varied EG/DI solvent and (B) CV results of P5 4M LiCl with varied EG/DI solvent , 120 • ) and underwent a CV test at the same F I G U R E 6 (A) CV results of P5-2:1/EG:DI and (B) GCD results of P5-2:1/EG:DI)

F
I G U R E 7 (A) CV results of P5-2:1/EG:DI star-shape supercapacitor at different bending angles and (B) CV results of P5-2:1/EG:DI supercapacitor at different temperatures TA B L E 2 Summary of specific capacitance, energy density, and power density of P5 samples TA B L E 3 Comparison of specific capacitance of poly(acrylic acid) based supercapacitors a AAm: acrylamide.b PAH: polyallylamine.c PANI: polyaniline.