Semi-Interpenetrating Polymer Networks for Enhanced Supercapacitor Electrodes

Conducting polymers show great promise as supercapacitor materials due to their high theoretical specific capacitance, low cost, toughness, and flexibility. Poor ion mobility, however, can render active material more than a few tens of nanometers from the surface inaccessible for charge storage, limiting performance. Here, we use semi-interpenetrating networks (sIPNs) of a pseudocapacitive polymer in an ionically conductive polymer matrix to decrease ion diffusion length scales and make virtually all of the active material accessible for charge storage. Our freestanding poly(3,4-ethylenedioxythiophene)/poly(ethylene oxide) (PEDOT/PEO) sIPN films yield simultaneous improvements in three crucial elements of supercapacitor performance: specific capacitance (182 F/g, a 70% increase over that of neat PEDOT), cycling stability (97.5% capacitance retention after 3000 cycles), and flexibility (the electrodes bend to a <200 μm radius of curvature without breaking). Our simple and controllable sIPN fabrication process presents a framework to develop a range of polymer-based interpenetrated materials for high-performance energy storage technologies.


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Renishaw Ramascope-1000 using a 633 nm laser excitation source. Thermogravimetric analysis (TGA, TA instruments Q500) was performed under nitrogen gas. Samples were heated from approximately 20 ºC to 900 ºC at a rate of 20 ºC/min. Nitrogen adsorption isotherms were undertaken at 77 K using a MicroMeritics TriStar 3000 Porosimeter. Prior to the N2 adsorption test, all samples were evacuated for 1 hour at 100 ºC under nitrogen flow. The electrical conductivity σ (S/cm) of the films was determined using measurements from a custom-built fourpoint probe based on the following equation: 4 = ( k ln (2) x V I x ) −1 (1) where V is the measured voltage (V), I is the applied current (A), t is the film thickness (cm), and k is a correction factor based on the sample dimensions to account for edge effects (k had a value of 1.49 for the samples in this work). Film thickness was measured using a DEKTAK 6M profilometer. Compressive stress-strain curves were obtained using a thermomechanical analyzer (TMA, TA instruments Q400) at room temperature. Compression tests were conducted with a preload force of 0.1 N and force ramp rate of 0.1 N/m. Young's modulus values were determined using a linear regression over the elastic region of the stress-strain curve.
Cyclic voltammetry (CV), charge-discharge tests, and electrochemical impedance spectroscopy (EIS) were performed with a potentiostat/galvanostat (Ivium Stat XRi). All of these tests were carried out in a three-electrode cell in 1 M lithium perchlorate (LiClO4, Alfa Aesar, 98%) using a platinum foil counter electrode and Ag/AgCl reference electrode. The sIPN films were cut into pieces of approximately 5x3 mm 2 and connected to the external circuit using Ti foil clamps then submerged in solution.

PEDOT quantification in sIPN films
The quantity of PEDOT within the sIPN films (M PEDOT,final ) was evaluated by performing a simple mass balance on the reaction: The value of EDOT,initial is the mass of EDOT impregnated in the PEO matrix (Figure 1f), dictated by the initial EDOT content of the reagent mixture ( Figure 1e). TGA was used to determine the precise weight percent of EDOT at this stage, as exemplified in Figure S1. While the majority of this impregnated EDOT remained in the PEO matrix as it polymerized, some diffused into the FeCl3(aq) solution over the course of the 24-hour polymerization, forming PEDOT precipitates in solution. These precipitates were isolated using centrifugation, repeatedly rinsed with methanol to wash away all FeCl3, and dried overnight in air; their final weight gave the value of M PEDOT,precip to be used in the above equation. Note that any S5 loss of precipitate in this centrifugation process would increase the calculated PEDOT concentration in the film; this quantification method should thus give conservative estimates of the specific capacitance of these materials.

Preparation of Solid-State Device
Full solid-state supercapacitor devices were fabricated using a PEO-based gel electrolyte.
The synthesis of this gel was directly analogous to that of the sIPN electrodes: PEGM and PEDGM were combined in a 3:1 ratio with approximately 2 wt. % BME. This mixture was polymerized between two glass slides (as described above) for ten minutes under exposure to 365 nm ultraviolet light, producing freestanding gel films. To assemble the supercapacitor, a gel film was placed between two sIPN electrodes; these three components were all briefly submerged in 1 M LiClO4(aq) immediately before assembly. This trilayer structure was then sandwiched between two Ti foil current collectors and clamped together to ensure good contact between the electrodes and electrolyte. The calculated capacitance and energy/power densities of these devices were based on the mass of the PEDOT in the electrodes, excluding the mass of the current collector and electrolyte.

Calculations for Electrochemical Characterization
In the three-electrode configuration, specific capacitance (C, F/g) was calculated from cyclic voltammetry data based on the following equation: 5 where Q is the total voltammetric charge passed in the CV scan, E1 and E2 are the lower and upper potential bounds (V), and m is the mass of the PEDOT in the electrode (g). The total charge Q is given by , where i is the instantaneous current and v is the scan rate (V/s). The factor of two S6 in the denominator takes into account the charging that occurs in both the cathodic and anodic sweeps of the CV.
Charge-discharge test data was used to calculate specific capacitance according to: I (A) is the constant charge/discharge current, Δ (s) is the discharge time, and Δ = 2 − 1 is the electrochemical potential window. Both Δ and Δ are analyzed after the initial voltage drop.
The capacitance of the full symmetric device, CD, was determined from CV tests based on: where M is the combined mass of the PEDOT in both electrodes. The energy density (E, Wh/g) and power density (P, W/g) of the devices are calculated using: = 3600 x Δ / (7)

Effect of PEDOT loading on sIPN performance
To demonstrate the tunability of the electrode fabrication process, sIPNs were fabricated with a variety of PEDOT concentrations ranging from 4 wt. % to 61 wt. % (the samples described in the main text have 61 wt. % PEDOT). As illustrated by the data in Figure S2, the specific capacitance of the sIPNs increases as PEDOT loading increases. Based on these trends, we would in theory expect sIPN electrodes with even greater PEDOT content (above the 61 wt. % fabricated here) to have even greater specific capacitance; we might predict a PEDOT loading which yields maximum performance, above which we observe negligible improvements in ionic conductivity due to the low PEO content. However, samples with greater PEDOT concentration could not be fabricated due to the mechanical instability of the PEO matrix, which is required to produce the freestanding films. This issue appears in the first step of our synthesis, forming an EDOT-impregnated PEO matrix (Figure 1f). If the EDOT content in the original reagent mixture is too high, the PEO-based oligomers are too dilute to create a stable gel film. The resulting product is too mechanically weak to be transferred to the iron chloride solution used to polymerize the EDOT.
S9 Figure S4. Nitrogen adsorption isotherm for a representative sIPN sample. The sIPN films exhibit a Type II nitrogen adsorption isotherm, indicating that the surface is nonporous. 7 Note that while the overall shape of this isotherm gives insight into the sIPNs' lack of porosity, the film surface area could not be accurately calculated due to the very low overall quantity of gas adsorbed. Inevitably, after some threshold thickness, we observe decreases in specific capacitance due to the slow kinetics of ion diffusion in the PEO gel relative to liquid electrolyte. However, the fact that we do not observe these diffusion limitations in films as thick as 130 μm is quite impressive given that many electrode films reported in the literature are one to two orders of S12 magnitude thinner. [8][9][10][11] The formation of supercapacitor electrodes which can maintain both their specific capacitance and flexibility even when very thick is a crucial challenge in developing devices for practical applications. 12 S13 Figure S7. Coulombic efficiency (discharge time / charge time) of the sIPN over long-term cycling.