Article pubs.acs.org/Macromolecules
Poly[Bis-EDOT-Isoindigo]: An Electroactive Polymer Applied to Electrochemical Supercapacitors Leandro A. Estrada,†,‡ David Y. Liu,‡ Danielle H. Salazar,‡ Aubrey L. Dyer,†,‡ and John R. Reynolds*,†,‡ †
School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ The George and Josephine Butler Polymer Chemistry Laboratories, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *
ABSTRACT: Poly[6,6′-bis(ethylene-3,4-dioxythien-2-yl)]N,N′-dialkylisoindigo (PBEDOT-iI) was synthesized and incorporated as an electroactive material into electrochemical supercapacitors (ESCs) in type I and type III configurations. In type I ESCs, PBEDOT-iI provides a specific power of ∼360 W/kg and specific energy of ∼0.5 Wh/kg, while retaining about 80% of its electroactivity over 10 000 cycles. In addition, we report on the use of PBEDOT-iI in type III supercapacitors where operating voltages as high as 2.5 V were achieved with specific energies of ca. 15 Wh/kg, albeit with limited stability.
1. INTRODUCTION In π-conjugated polymers, control of electrochemical reduction and oxidation processes, as well as optical band gaps, is closely associated with tailoring of chemical structures such that appropriate setting of frontier energy orbitals is achieved.1 This can be manipulated at will through covalent bonding of alternate electron-donor (D) and electron-acceptor (A) building blocks,2 which has proven useful for the design of polymeric materials utilized in solid state devices such as organic photovoltaics (OPVs)3 and polymer light-emitting diodes (PLEDs),4 along with redox-active systems such as electrochromics,5 mechanical actuators,6 chemical sensors,7 and charge storage.8,9 Conjugated electroactive polymers (EAPs) have increasingly become significant materials in these redoxtype devices due to their inherently low redox potential, color tunability (for electrochromics), solution processability, redox bistability, and mechanical flexibility. The aspects of low redox potential, redox bistability, and mechanical flexibility are relevant in charge storage applications as this translates to devices that are formable, lightweight, and redox stable.10 Some of the most common EAPs utilized in charge storage applications are those based on polythiophenes (PT),8 polypyrroles (PPy),9 and polyanilines (PAni),10 to name a few. Given the redox properties of EAPs and the mechanism by which these store charge, the distinction between batteries and capacitors based on these is yet to be clarified.10 The term pseudocapacity is used to classify the charge storage mechanism taking place in EAP-based devices where both faradaic and capacitive processes occur.11 Thus, EAP-based devices are often referred to as electrochemical supercapacitors (ESCs), since they tend to behave as a hybrid of a battery and a capacitor. The Ragone plot is often used to illustrate the differences between charge storage devices in terms of their energy content © 2012 American Chemical Society
(specific energy) and discharge rate (specific power) and can be found in the literature.12 In EAP-based ESCs, both polymer electrodes can be constructed using the same or different polymer at the anode and cathode. Based on the polymers used and their corresponding oxidation states when charged and discharged, the ESC can be categorized as type I, II, III, or IV.10 Unlike type II and type IV ESCs, the electroactive material is the same on both electrodes in type I and type III ESCs. In the type I/II ESCs illustrated in Figure 1A, the charge storage proceeds when the device is in an initial charged state as one of the EAPbased electrodes is fully oxidized and the other neutralized. A discharged state of the type I and II devices results when both EAP films are in the same half-charged state, resulting in a 0 V potential difference. Consequently, the operating voltage of these is limited to within the stable oxidation window of the polymer with the lowest overoxidation potential. In practice, typical values range within 0.5−1.0 V for these types of devices.10 The inherent oxidative (p-doping) and reductive (ndoping) redox processes of D−A EAPs has allowed for their incorporation into type III and type IV devices, thereby extending the accessible voltage window and consequently increasing the overall specific power and energy. In this case, the charged state of the device arises when one EAP is in the fully oxidized (p-doped) state and the other in the fully reduced (n-doped) state while the discharged state of the device arises when both EAPs are in their neutral state and the potential difference is again 0 V, as illustrated in Figure 1B. To date, poly(3-phenylthiophene) derivatives,13 poly(diheteroaryl cyaReceived: August 7, 2012 Revised: September 14, 2012 Published: October 4, 2012 8211
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Figure 1. Device configuration of ESCs studied herein: (A) type I/II; (B) type III/IV. Adapted from ref 18.
novinylene),14 poly(cyclopenta[2,1-b;3,4-b′]dithiophene-4one),15 poly(5-amino-1,4-naphthoquinone),16 and polyimides17 represent a few of the known examples of D−A systems applied to ESCs. For highly electroactive and redox switchable materials, electrochemical polymerization can provide an attractive route to form electrode-supported films of high electrochemical activity. In many cases, electropolymerization can provide more open film morphology as a result of direct incorporation of the electrolyte during the electropolymerization, which permits the creation of high surface area films. By capitalizing on this porous and open film morphology, formation of threedimensional polymeric structures onto conductive electrodes of various geometries, sizes, and surface structures is attractive as it can create a template that favors charge transport within the film. This can then translate into improved capacitance in the film, implying that direct control over morphology of the polymer is especially important for electrochemical charge storage.19 Bisheterocyle arylenes provide an effective way to allow electropolymerization while including property tuning via internal aryl groups.20 Of all the reported electron-rich heterocycles, 3,4-ethylenedioxythiophene (EDOT) stands out as one of the most used electropolymerizable moieties given that its electropolymerization is well-understood, lacks undesirable defects (viz. α−β and β−β couplings), and the resulting polymers exhibit high conductivity and stability in their oxidized state.21 The use of BisEDOT-arylenes, where the arylene is an acceptor, has traditionally been one of the more desirable routes to obtain D−A polymers through electropolymerization.22 Additionally, of the number of acceptor materials investigated for organic electronics, isoindigo (iI) has recently become of interest for a variety of applications, including photovoltaics and charge transport. We introduced iI in the open scientific literature as a novel acceptor in D−A systems for molecular solar cells where power conversion efficiencies of ca. 1.8% were realized,23 though it has been reported in the patent literature previously.24 Properties such as straightforward synthesis on a substantial scale, ambient stability, ease of derivatization, and deep frontier orbital energies have stimulated further research efforts by our group,25 and others,26 into the fundamental understanding of the electronic properties of this molecule. It is worth mentioning that incorporation of solubilizing alkyl chains onto the iI structure is relatively straightforward compared to other well-known acceptors such as quinolines,27 2,1,3benzothiadiazole (BTD),28 5-di(2-alkyloxy)phenyl)thieno[3,4b]pyrazine, 29 and N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD).30 For example, it requires two steps to attain alkylated iI from commercially available building blocks, as opposed to
5−6 steps for TPD. When considering applications that use solution-deposition techniques such as spin-coating, spraycasting, or roll-to-roll printing (OPVs, PLEDs), this property is desirable as it increases the solubility of the polymer. Furthermore, inclusion of aryl groups is facile via Pd-catalyzed cross-couplings. Based on these advantages, incorporation of iI into D−A type polymers, as shown in Scheme 1, is presently Scheme 1. Representation of Poly-BEDOT-iI with pDopable and n-Dopable Sites
being explored by many.23−25 With this in mind, we designed bis(3,4-ethylenedioxythien-2-yl)isoindigo (BEDOT-iI, 1) to probe the electronic properties of iI as part of a stable D−A system. In this work, we introduce poly-BEDOT-iI (PBEDOT-iI): first, as an electroactive material for type I ESCs where we utilize the polymer as a p-type dopable material as shown schematically in Figure 1A and, second, as a probe for type III ESCs where we utilize the polymer as both a p-type and n-type dopable material, shown in Figure 1B. As the electroactive material in type I ESCs, PBEDOT-iI provides a specific power of 362 W/kg and specific energy of 0.49 Wh/kg, in the range of typical ESCs previously reported in the literature,10,12 while retaining about 80% of its electroactivity over 10 000 cycles performed at a scan rate of 200 mV/s. In addition, we applied PBEDOT-iI to type III devices where operating voltages as high as 2.25 V were achieved with specific energy values of ca. 15 Wh/kg; however, PBEDOT-iI was shown to not be as stable as desired for its utilization as an EAP in this ESC configuration. 8212
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2.3. Imaging. Polymer films on Au−Kapton were prepared as described in the previous section. The electropolymerized films were rinsed with monomer-free electrolyte, followed by dry acetonitrile to remove any electrolyte residing on the polymer surface. Films were then dried under vacuum for 24 h at room temperature to remove any remaining solvent absorbed within the polymer. Optical images were acquired using a stereo microscope accessory as part of a Veeco dInnova atomic force microscope (AFM) and image area capture calibrated using a calibration grid to assess magnification. Scanning electron microscope (SEM) images were acquired using a JEOL 5700. As the electroactive polymer film and supporting electrode are conductive, no additional treatment to the materials (e.g., coating with Au or C powder) was needed. Accelerating voltages of 8−10 kV were used, and beam exposure times were kept to less than 1 min. 2.4. Supercapacitor Assembly. 2.4.1. Type I. An illustrative summary of the ESC assembly is presented in section S4 of the Supporting Information. For each device assembled, two films of the BEDOT-iI derivative were electrochemically polymerized onto two separate Au−Kapton electrodes, utilizing the conditions detailed in the Results and Discussion section. The polymer film was then washed with ACN, followed by monomer-free electrolyte to remove any unpolymerized monomer, low molecular weight oligomers, excess electrolyte salt, and BZN. Without allowing it to air-dry, the film was then contacted with the monomer-free electrolyte indicated and subjected to an electrochemical pretreatment that consisted of a potential hold at 0 V for 30 s followed by repeated cyclic voltammetry for 10 scans in monomer-free electrolyte solution in PC from 0.1 to 0.8 V. The electrode that was to comprise the cathodic material was held at +0.8 V vs Ag|Ag+ for 30 s while the anodic material was held at 0 V vs Ag|Ag+ for the same amount of time. Immediately prior to device assembly, a gel electrolyte, consisting of 1 M LiBTI in 7% m/v poly(methyl methacrylate) (PMMA) in PC, was added to the active polymer surfaces (without allowing them to air-dry) until fully coated. A polypropylene membrane (AN50 prefilter, hydrophobic, pore size 5.0 μM, diameter 47 mm) was sandwiched between both “wet” electrodes and utilized as a separator to eliminate intimate contact between the anode and cathode. Copper Tape (3M, 1131) was attached to each electrode to facilitate easier contact with alligator clips. The device was then sandwiched between two sheets of heat seal laminating plastic, held together by a 50 g weight providing ∼1.45 lb/ in2 pressure, and sealed by heating the assembly using a heat gun. 2.4.2. Type III. Type III device assembly is similar to that utilized for the type I devices, except that the film active area was rectangular and 1.0 cm2 in area with fabrication conducted inside an argon-filled glovebox. The electrodeposited polymer was first rinsed with monomer-free electrolyte and then switched using cyclic voltammetry (CV) in 0.1 M TBAPF6/ACN supporting electrolyte over 10 cycles between −1.2 and 0.7 V vs Ag|Ag+. Prior to device assembly, both cathodic and anodic materials were potentiostatically held at −0.35 V vs Ag|Ag+ for 30 s in order to attain the charge neutral material. The remainder of the device assembly was conducted in the same manner as detailed above, except that the device lamination step was not carried out and the devices were tested under inert conditions as detailed below. 2.5. Supercapacitor Testing. All device measurements were performed under ambient conditions for type I devices and under argon atmosphere for type III devices, using a two-electrode mode with the potentiostat (reference and counter electrodes shorted together). For specific energy (Wh/kg) values, the mass utilized is that estimated from films prepared under the exact same conditions, electrochemically reduced, and dried, followed by mass measurements, as described in the previous section. The type I devices were first held at +0.5 V for 60 s prior to cycling from 0.5 to 0 V. These devices were again charged at +0.5 V for 60 s and then repeatedly cycled (3 scans) at various scan rates (10, 25, 50, 100, 200, 300, 500, 1000, 2000, and 3000 mV/s). Stability measurements on selected devices were performed at 200 mV/s through the same voltage range (0.5−0 V) over 10 000 cycles after device charging.
2. EXPERIMENTAL SECTION The synthetic procedures, materials characterization, analytical methods, imaging, and details on theoretical calculations are described in detail in the Supporting Information. 2.1. Materials. The compounds 6,6′-dibromoisoindigo,22 isoindigo,31 and 2-trimethylstannyl-3,4-ethylenedioxythiophene (EDOTSnMe3)32 were synthesized using previously reported procedures. 5Bromoisatin, 5-bromooxindole, Pd2(dba)3, P(o-tolyl)3, 1-bromobutane, 1-bromohexane, 1-bromooctane, concentrated HCl, and glacial acetic acid were purchased from commercial sources and used as received. All used solvents were purified using conventional methods.33 2.2. Electropolymerization for Film Formation. All electrochemical measurements were carried out using an EG&G PAR 273 galvanostat/potentiostat PC-controlled using Scribner Associates CorrWare II software program. A three-electrode conventional electrochemical cell was used where the deposition substrate was changed depending on the type of study performed (i.e., Au buttons, Au−Kapton sheets). Gold button electrodes were purchased from BASi, and the surface area was 0.02 cm2. Gold−Kapton sheets, composed of a 500 μm thick Kapton sheet onto which a 300 nm thick layer of gold was sputter-deposited, were purchased from Astral Technology Unlimited. The active area of the Au−Kapton electrodes (2.83 cm2) was defined by the electrochemical cell as shown in Figure S7 of the Supporting Information. A nonaqueous Ag|Ag+ electrode was used as reference ([Ag+] = 10 mM, [TBAPF6] = 0.1 M in anhydrous acetonitrile), and a Pt flag was used as auxiliary electrode. All electrodepositions were carried out by cyclic volammetry over the voltage range indicated for the desired number of cycles. Benzonitrile (BZN) was predried over activated MgSO4 overnight and then over activated 4A molecular sieves for 4−6 h, prior to vacuum distillation. Acetonitrile (ACN) was refluxed over CaH2 for 1−2 h prior to distillation. Both solvents were stored under argon in contact with activated molecular sieves.33 Anhydrous propylene carbonate (PC) of 99.7% purity was used as received from Sigma-Aldrich.34 Lithium bis(trifluoromethanesulfonyl)imide (LiBTI) was dried under vacuum at 120 °C for 36 h, and the tetra-n-butylammonium salts (TBABF4 and TBAPF6) were recrystallized according to literature procedures and dried under vacuum at 80−90 °C for 12 h prior to use.33 All solutions were bubbled with argon (solvent-saturated in the case of acetonitrile) for 10−15 min before measurements were carried out. Unless otherwise noted, all film switching measurements were carried out in monomer-free electrolyte solutions of 1 M concentration. The electrolyte chosen for switching studies was the same chosen for electrodeposition. Prior to film switching, the polymer was first rinsed in ACN and then in monomer-free PC/electrolyte solution. For the case of scan rate dependence measurements, a silver wire was used as a pseudoreference electrode and calibrated using the Fc|Fc+ redox couple standard (+0.21 V vs Ag wire, +0.07 V vs Ag|Ag+ nonaqueous electrode) due to the inherent high impedance resulting from use of the nonaqueous Ag|Ag+ electrode. Finally, the mass of electroactive material deposited onto Au−Kapton was measured in triplicate using a Mettler Toledo semi-microanalytical balance (Δm = ±0.01 mg). Prior to weighing, each film was thoroughly washed with anhydrous acetonitrile, dried overnight at 80−90 °C under vacuum, and then equilibrated until constant weight inside a desiccating chamber.35 The reported polymer mass per electrode is the average of these three measurements. For the study of reduction processes and type III supercapacitor assembly, electropolymerizations of BEDOT-iI-But2 were performed inside an argon-filled Vac Atmospheres OmniLab glovebox, using similar electrochemical materials and procedures described above. TBAPF6 was chosen as the supporting electrolyte for electrodeposition and switching under this environment. BZN, ACN, and PC were purified as described above and degassed via three to five freeze− pump−thaw cycles prior to introduction to the glovebox. In the case of type III devices fabricated inside the glovebox, the electrode geometry was rectangular (surface area of 1.0 cm2) rather than circular. 8213
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The type III devices were first held at 0 V for 60 s prior to cycling from 0 V to the potential of interest. These devices were again discharged at 0 V for 60 s and then repeatedly cycled (three scans) at various scan rates (10, 25, 50, 100, 200, 300, 500, 1000, 2000, and 3000 mV/s). Stability measurements on selected devices were performed at 200 mV/s through the 0−2.5 V range over 10 000 cycles after device discharging. Capacitance values were calculated from their average current density as a function of sweep rate by using the relation C = j/υ, where j is the current density (mA/cm2) and υ is the scan rate (mV/s). The specific capacitance values were evaluated by dividing by the mass of the total amount of EAP (subtracting out the mass of Au/Kapton current collector/substrate material).
to reach a clean completion, thus permitting isolation of the desired products in high yields after column chromatography. 5,5′-Bis(3,4-ethylenedioxythien-2-yl)-N,N′-dihexylisoindigo was isolated as a green powder (mp 160 °C, dec), while 6,6bis(3,4-ethylenedioxythien-2-yl)-N,N′-dihexylisoindigo was isolated as a deep purple powder (mp >200 °C). All other alkyl versions of the latter compound (i.e., n-butyl, n-octyl) presented the same appearance with melting points higher than 200 °C. The compounds were slightly soluble in common solvents (THF, DMF, DCM, chloroform, toluene), with the exception of the dioctyl version of 6,6-BEDOT-iI, which presented good solubility after gentle heating (>1 mg/mL). All compounds presented good solubility (>1 mg/mL) in aromatic solvents of high boiling points such as benzonitrile, chlorobenzene, and nitrobenzene after heating at 80−90 °C for a brief period of time (2−5 min). 3.2. Comparative Electropolymerization of 5,5′- and 6,6′-BEDOT-iI on Au Button. Attempts at oxidative electrochemical polymerization of both BEDOT-iI-Hex2 isomers were performed in the standard three-electrode system described. Solutions consisting of 1.0 mM monomer in 0.5 M LiBTI/BZN were repeatedly cycled (10 scans) over the range of −0.45 to 0.90 V vs Ag|Ag+ at a rate of 50 mV/s. The resulting cyclic voltammogram responses of both the 5,5′- and 6,6′-BEDOT-iI analogues are provided in Figure 2. Both BEDOT-iI isomers
3. RESULTS AND DISCUSSION 3.1. Synthesis of the Materials. The synthetic route followed for the preparation of BEDOT-iI is outlined in Scheme 2. The 5,5′-iI isomer was synthesized in order to Scheme 2. Synthetic Scheme for Preparation of BEDOT-iI Isomersa
a Reagents and conditions: (a) HCl(cat.), HOAc, reflux, 24 h; (b) (i) NaH, DMF, 30 min (ii) n-AlkBr, 70−80 °C, 3 h; (c) EDOT-SnMe3, Pd2(dba)3, Pd(o-tolyl)3, toluene, 100−120 °C.
investigate the effect of a break in conjugation of this molecule. This goal was stimulated by our previous findings where the conjugation break conferred by substitution of EDOT at the 3 and 6 carbons of carbazole (Cz) benefits formation of somewhat localized radical cation and dication states.36 Preparation of dibrominated versions of iI (viz. 5,5′ and 6,6′) was achieved via condensation under acid conditions of a selected bromoisatin with its corresponding bromooxindole partner in high yields. The dibromoisoindigo species were alkylated following a modification of the original procedure,22 where the K2CO3/DMF mixture was replaced by NaH/DMF. Such modification allowed shortening of the reaction time from 24 h to only 3 h, plus implementation of simpler purification steps without sensitive losses of product.37 Initial incorporation of EDOT units at the iI flanks was pursued by means of Negishi coupling using Pd(PPh3)4 as catalyst.38 However, this method resulted in recovery of starting materials, and subsequent attempts at tuning the catalyst/cocatalyst system resulted in obtaining complex mixtures. This set of drawbacks prompted us to utilize the Stille reaction,39 where EDOT-ZnCl was substituted by EDOT-SnMe3 as coupling partner. The use of Pd2(dba)3/P(o-tolyl)3 as catalytic system enabled the reaction
Figure 2. Electropolymerization of 5,5′- (A) and 6,6′-BEDOT-iI-Hex2 (B). [Monomer] = 1.0 mM, [LiBTI] = 0.5 M in benzonitrile (scan rate = 50 mV/s). Insets: HOMO maps of 5,5′- (A) and 6,6′-BEDOT-iIMe2 (B) modeled at the B3LYP/6-31G* DFT level.
presented similar anodic peaks at 0.67−0.70 V, relatively close to that reported for the easily electropolymerized BisEDOT against the same reference (Ep,a = 0.56 V).36c However, the current response of the 6,6′-BEDOT-iI-Hex2 monomer is clearly higher than that for the 5,5′-analogue (∼5 μA/cm2 for 5,5′-BEDOT-iI-Hex2 versus 300 μA/cm2 for 6,6′-BEDOT-iIHex2; note scale differences in Figure 2). This implies that, while the oxidation of the materials take place at their common 8214
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Figure 3. Cyclic voltammograms as a function of scan rate in the range of 10−300 mV/s (A) and 300−3000 mV/s (B) of poly(6,6′-BEDOT-iI-But2) film switched in monomer free LiBTI solution ([LiBTI] = 0.5 M in PC).
(in a similar way to aniline electropolymerization) and confers electroactivity to the film formed on the electrode surface.36b 3.3. Polymer Electrochemistry on Au Buttons. Polymer switching studies were performed by first rinsing the film (poly(6,6-BEDOT-iI-Alk2 polymerized to 10 cycles as detailed above) twice with dry acetonitrile, followed by monomer-free electrolyte solution (i.e., 0.5 M LiBTI/solvent). After immersing the electrode-supported polymer film in monomer-free electrolyte solution, it was held potentiostatically at 0 V for 30 s prior to voltammetric cycling between 0.1 and 0.8 V. The current responses were stable and comparable when cycling in PC and ACN after 1−2 break-in cycles. This is contrary to BZN, where the current responses decrease dramatically on repeated cycling where the formed polymer is slowly solubilized in BZN and not in ACN or PC. As shown in Figure 3A, poly(6,6′-BEDOT-iI-But2) shows well-behaved capacitive behavior upon cycling in 0.5 M LiBTI/PC solution. This current response is comparable in shape to those previously shown for PBiEDOT,36c PProDOT,41 and PProDOT-Me2,42 although within a smaller voltage window (
