Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Recombination Strategy for Processable Ambipolar Electroactive Polymers in Pseudocapacitors Yilin Wang,† Weishuo Li,† Yitong Guo,† Jupeng Cao,† Imran Murtaza,‡ Ahmed Shuja Syed,§ Yaowu He,*,† and Hong Meng*,† †
School of Advanced Materials, Peking University Shenzhen Graduate School, Peking University, Shenzhen 518055, China Department of Physics, International Islamic University, Islamabad 44000, Pakistan § Centre for Advanced Electronics & Photovoltaic Engineering (CAEPE), International Islamic University, Islamabad 44000, Pakistan Downloaded via UNIV OF WINNIPEG on July 9, 2018 at 13:50:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: In order to circumvent the contradiction between processability and electrochemical performances, we report a recombination strategy in both unit and segment scales for processable ambipolar electroactive materials starting from parental moieties: classic ambipolar polymer (PEBE) and its soluble analogue (PPBP). On the basis of this strategy, two recombined polymers (PEB-P and PEBE-PBP) are successfully prepared via direct (hetero)arylation copolymerization. Through systematical characterization of properties, both PEB-P and PEBE-PBP not only possess the solubility fostered by PPBP but also exhibit identical electrochemical and optical properties to PEBE, demonstrating the reliability of this recombination strategy. Moreover, film electrodes of these recombined polymers exhibit desirable pseudocapacitance, acceptable stability, and superior processability to PEBE, making them promising candidates for Type III supercapacitors and other multifunctional energy storage devices.
1. INTRODUCTION Nowadays, energy crisis has drawn great attention as an issue demanding prompt solutions. Besides finding sustainable and renewable resources, exploring reliable and low-cost energy storage systems is regarded as a key solution with higher efficiency. In recent years, rechargeable batteries have been considered as one of the promising approaches for energy storage system exploration. However, power density of these batteries is limited by diffusion of cations through the crystalline frameworks of electrodes.1 By contrast, supercapacitors, including electrochemical double-layer capacitors (EDLCs) and pseudocapacitors, have become a potential alternative to fulfill the demands of fast energy delivery owing to rapid surface reactions of electrodes.2−4 Such superior properties endow supercapacitors with great application perspectives in the area of regenerative banking systems, providing peak power for key devices and helping increase reliability and stability to the energy grid.5 Nevertheless, regardless of increasing focus, supercapacitors still cannot be used as stand-alone units currently as energy density is inferior to that of batteries, which urges researchers to exploit further enhanced energy density of supercapacitors. Pseudocapacitors based on transition metal oxides and/or electroactive polymers outweigh EDLCs significantly owing to their superior energy densities via extra rapid redox reaction besides physical charge accumulation.6,7 Thus, pseudocapacitors have been considered as a promising candidate to achieve satisfactory energy density © XXXX American Chemical Society
of supercapacitors, bridging the trade-off gap between traditional capacitors and batteries and serving as nextgeneration energy storage systems. Among all the available options for pseudocapacitor electrode materials, electroactive polymers have long been considered as a more attractive choice for their advantages like low cost, light weight, flexibility, and environmental-friendly processing, providing attractive opportunities in future portable and wearable electronic devices for versatile applications.7,8 More importantly, as the specific energy density (watt hour per kilogram) is expressed by eq 1 (where Cm is the specific capacitance and ΔV is the operating potential range),9 their distinct advantage of tunable voltages and capacitances based on diverse molecular structures offers a possibility to enhance energy density of pseudocapacitors. E=
1 Cm(ΔV )2 2
(1)
With reference to the factors Cm and ΔV, energy density is essentially determined by operating voltage window, indicated by the squared potential range.10 Hence, it is more viable to achieve optimal energy density of pseudocapacitors by enlarging electrodes’ voltage windows. According to device Received: March 21, 2018 Revised: June 20, 2018
A
DOI: 10.1021/acs.macromol.8b00599 Macromolecules XXXX, XXX, XXX−XXX
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improve the solution processability extensively. In this way, electrochemical behaviors will be obviously influenced by the drop in electronic conductivity as interchain interactions decrease with the introduction of side chains.15 Therefore, even though specific energy could be enhanced with ambipolarity increasing the work voltage window (V in eq 1), this conventional strategy to improve processability would impede the preconceived enhancement due to the depreciation of specific capacitance (Cm in eq 1). This leaves behind an urgent challenge to explore novel molecular design strategies circumventing the contradiction between improvement of processability and maintenance of performance. Herein, we report a recombination strategy (Scheme 2) for processable ambipolar electroactive materials starting from the parental structures: classic ambipolar polymer (PEBE) and its soluble analogue (PPBP), in both unit and segment scales. Based on this refreshing strategy, recombined polymers (PEBP and PEBE-PBP) are successfully prepared via direct (hetero)arylation polymerization (DHAP). Through characterizations of basic properties, both PEB-P and PEBE-PBP not only possess the solubility fostered by PPBP but also exhibit identical electrochemical and optical properties to PEBE, demonstrating the reliability of this recombination strategy. Moreover, with comparable psedocapacitive behaviors, these two recombined polymers could be regard as promising candidates as electrodes for Type III supercapacitors.
configuration, the most attractive category is Type III supercapacitors composed entirely of the same ambipolar polymer as electrodes with different doping states. As a result, Type III devices can achieve extremely high operating voltage window (∼3 V), leading to high specific energy according to eq 1.11 However, the practical performances of Type III supercapacitors are not as good as theorized ones due to the difficulty of the n-doping process, ascribed to the fact that ndoped potentials (around −2.5 V vs Ag/Ag+ for polythiophenes)12 are very negative, which is highly demanding in terms of the stability of electrolytes and n-doped electrodes. Fortunately, donor−acceptor (D−A) polymers, a unique type of material in which electron donor and acceptor building blocks can be selected at will,4 allow to tune the optical and electrochemical properties. They have been proved as an effective strategy to accesses satisfactory ambipolar materials in the fields like OTFT and OPV.14 Nonetheless, there are limitted D−A polymers have been investigated for energy storage applications,9,13 especially the introduction of ambipolar D−A polymers in Type III supercapacitors. To date, only a few researchers such as John R. Reynolds, Dwight S. Seferos, and Yuguang Ma have reported several supercapacitor-used ambipoar polymers (Scheme 1).6,9,13 All Scheme 1. Representative Ambipolar D−A Polymers Prepared for Supercapacitors6,9,13
2. EXPERIMENTAL SECTION Additional synthetic procedures, characterizations, materials, and instrumentation details can be found in the Supporting Information. 2.1. Materials. All reagents were purchased from Aldrich. Acetonitrile (ACN), ether (Et2O), tetrahydrofuran (THF), and toluene (PhMe) were purified and dried by Vigor solvent purification system. Propylene carbonate (PC), N,N-dimethylacetamide (DMAc), and N,N-dimethyformamide (DMF) were dried over molecular sieve under a nitrogen atmosphere. Four polymers investigated in this work, shown in Scheme 3, were polymerized by electrochemical polymerization (PEBE), oxidative polymerization (PPBP), and DHAP (PEBE-PBP and PEB-P). The polymer structure of two polymers that we designed (PEBE-PBP and PEB-P) was characterized by 1H NMR, GPC, and FT-IR methods (Figures S6, S7, S10, and S11). The electrolyte used for the electrochemical test was prepared as a solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dried PC or ACN. Before being utilized, the electrolyte solution underwent 30 s ultrasonic treatment for full dissolution and purged with N2 for 5 min to remove oxygen. 2.2. Characterization of Polymer Films. Morphological images of polymer films were acquired by a ZEISS Supra 55 scanning electron microscope (SEM) and a Bruker Multimode 8 atomic force microscope (AFM). UV−vis−NIR spectra were recorded using a PerkinElmer Lambda 750 spectrometer. Electrochemical experiments were performed with a Gamry Interface 1000 electrochemical workstation. Spectroelectrochemistry studies were performed through the Lambda 750 spectrometer under control of the Gamry Interface 1000 electrochemical workstation. The electrochemical characterizations of single electrodes (galvanostatic charge−discharge (GCD), stability, and electrochemical impedance spectrometry (EIS)) were conducted using the Gamry Interface 1000 electrochemical workstation. The mass of active material PEBE on platinum was weighed by a Gamry 10 M electrochemical quartz crystal microbalance (EQCM). Electrochemical-related tests were performed using 0.1 M TBAPF6/PC or TBAPF6/ACN electrolyte solution in a sealed threeelectrode cell consisted of a nonaqueous Ag/Ag+ electrode as the reference electrode, a Pt wire as the counter electrode, and a Pt disk as the working electrode under N2 conditions.
aforementioned polymers still lag behind the requirements of supercapacitors for practical applications, significantly limited by their unsatisfactory processability.6,11,13 Not to mention the fact that most of them are also facing the problems in stability and the balance of properties between n-doped and p-doped states. Therefore, the challenge with important real-world implications lies in the search of suitable ambipolar D−A polymers equipped with further enhanced energy density and processability, showing the ability to pave the way for flexible multifunctional devices capable of large-scale and highthroughput fabrication. So far, the most well-known and feasible approach of directly loading suitable alkylated side chains has been utilized to B
DOI: 10.1021/acs.macromol.8b00599 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Recombination Strategy Used for Molecular Design
Scheme 3. Polymerization Methods of PEBE, PPBP, PEB-P, and PEBE-PBP
3. RESULTS AND DISCUSSION In order to reconcile the contradiction between performance and processability, a recombination molecular design strategy was introduced to incorporate the classic ambipolar structure PEBE together with its soluble analogue PPBP in both segment and unit scales. Based on this refreshing strategy, PEBE-PBP (Mn: 13.7 kDa; PDI: 1.45) and PEB-P (Mn: 12.8 kDa; PDI: 1.34) were prepared through DHAP (Scheme 3),
which is regarded as a facile and environmental-friendly way to alternate more toxic cross-coupling polymerization methodologies.16 To demonstrate the feasibility of this strategy, electrochemical and optical properties were studied and compared with incorporated parental moieties (PEBE and PPBP). Moreover, capacitive performances as anodes/cathodes were further investigated through galvanostatic charge− discharge (GCD), stability test, and electrochemical impeC
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Figure 1. CV curves of (a) PEBE-PBP and (b) PEB-P polymer films. Potential scan rates differ from 50 to 300 mV s−1 on Pt electrode recorded in a 0.1 M TBAPF6/ACN electrolyte−solvent. Linear relationship between peak current densities and scan rate of (c) PEBE-PBP and (d) PEB-P.
Figure 2. (a) Cyclic voltammograms of PEBE, PEBE-PBP, PEB-P, and PPBP polymer film. (b) HOMO/LUMO energy level of PEBE, PEBE-PBP, PEB-P, and PPBP obtained from the onset potential of oxidation/reduction.
doping/dedoping process, calculated ipa/ipc values of PEBEPBP get closer to 1 than PEB-P, demonstrating superior redox reversibility. Inferior reversibility of PEB-P also agreed with the worse stability observed during the n-doping/dedoping process. Based on linear regression fitting of the relationship between anodic peak current densities and scan rates, the film conditions were evaluated during doping/dedoping process (Figure 1c,d). For PEBE-PBP and PEB-P, their peak current densities are linear with the scan rates with correlation coefficients (R2) very close to 1, suggesting that both polymers attach very well to the electrodes and the redox processes are non-diffusional-controlled.16,18 In order to correlate electrochemical properties with polymer structures, CV curves and electrochemically derived HOMO and LUMO energy levels of both recombined polymers and their parental D−A polymers have been compared under the same situation. As shown in Figure 2a, PEBE-PBP and PEB-P possess two similar quasi-reversible redox processes with PEBE. By contrast, PPBP exhibits a
dance spectroscopy (EIS). Meanwhile, the morphology of both polymer films was investigated for better understanding of differences in performances. 3.1. Electrochemical Properties. In initial attempts to understand the basic intrinsic properties of two recombined polymers, cyclic voltammetry (CV) measurements, which represent the most commonly reported electrochemical values throughout the D−A polymer literature,17 were examined in a standard three-electrode electrochemical cell with electrolyte solution of 0.1 M TBAPF6/ACN and further compared to those of PEBE and PPBP. PEBE-PBP and PEB-P films were investigated by scanning in both anodic and cathodic regions at rates from 50 to 300 mV s−1 (Figure 1a,b). CV curves of both polymers display a good ambipolarity. Furthermore, in p-doping/dedoping process, the calculated ipa/ipc values (ipa or ipc is calculated as the ratio between anodic or cathodic peak current density under certain potential scan rate) of PEB-P and PEBE-PBP are all almost close to 1, showing a great redox reversibility. While for the nD
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The optical band gaps (Eg,opt) of polymers were calculated from lower band edges (λonset) of thin films and have been listed in Table 1. The optical gaps of two polymers obtained
defined CV curve with more distinct peaks, an irreversible ndoping process, and a narrower voltage window in the pdoping process. Moreover, slight shifts of oxidation/reduction onsets to lower magnitude potentials have been observed in the CV curves of PEBE-PBP and PEB-P compared to PEBE. These differences could be generated from their lower conductivity, owing to the polymer backbone distortion induced by side chains and a larger ProDOT unit.15,19 The HOMO/LUMO energy levels calculated from onset potential of oxidation/reduction agree with the similarities in CV curves (Figure 2b). HOMO/LUMO energy levels of two recombined polymers are almost identical to those of PEBE. Moreover, HOMO−LUMO gaps of PEBE (0.80 eV), PEBEPBP (0.88 eV), and PEB-P (0.99 eV) also share the similar values, which are conspicuously narrower than that of PPBP (1.61 eV). Electrochemically, PEBE-PBP and PEB-P retaining processability of PPBP share significant identity to PEBE, and both of them reveal similar ambipolar CV curves as well as HOMO− LUMO gaps to PEBE, testifying the value of recombination strategy in balancing electrochemical performance and processability. 3.2. Optical Properties. The optical properties of PEBEPBP and PEB-P were investigated as thin films spray-coated on ITO-coated glass slides. Visually similar to PEBE, both polymer films display hues of green while the film of PPBP shows a blue hue. The UV−vis−NIR spectra of PEB-P and PEBE-PBP thin films were further recorded and compared with those of PEBE and PPBP in the form of normalized absorbance. As expected from previous work investigating the spectral distribution of D−A polymers,20 each polymer exhibits a “dual band” of absorption with wide range (Figure 3). Lower energy
Table 1. Optical Properties of the Polymers polymer PEBE PEBE-PBP PEB-P PPBP
λpeaka (nm) 752, 728, 736, 664,
432 424 424 404
λgapb (nm)
Eg,optc (eV)
508 516 508 468
0.96 0.96 0.97 1.19
a
The absorption wavelength of lower energy band (left) and higher energy band (right). bThe absorption wavelength of absorption gap. c Optical band gap calculated from the absorption edges of lower energy band (Eg,opt = 1240/λonset).
from recombination strategy and PEBE are almost the same while that of PPBP is much larger. Taken together, both PEB-P and PEBE-PBP show remarkable similarities to PEBE in absorption spectra and optical band gaps, demonstrating the optical identity based on our recombination strategy. 3.3. Spectroelectrochemical Properties. Besides optical properties under neutral state, in situ spectroelectrochemical measurements of polymer films were performed to further elucidate the optical properties under different doped states. Spectra of all polymer films on ITO-coated glass slides were recorded under different applied constant voltage pulses (Figure S13). All the polymers exhibit electrochromic performances when changing across different states (neutral, reduced, and oxidized) and share similar trends of electrochromic changes in spectra (Figure 4a−d). As the state of polymers change from neutral to doped (reduced or oxidized), two absorption bands observed under neutral states begin to decrease simultaneously with the increase of polaron and bipolarn absorption, and obvious broad absorption bands start to intensify in the nearinfrared region.17 These changes in the absorption spectra are in accordance with the color change observed experimentally. Overall, despite the slight difference of color at n/p-doped states, both polymers prepared through recombination inherit similar ambipolar electrochromic properties from PEBE. 3.4. Electrochemical Characterization of the Ambipolar Electrodes at p-Doped and n-Doped States. With comprehensive information on basic properties of recombined polymers in mind, more efforts were ulteriorly made on the exploration of PEB-P and PEBE-PBP as electrodes for Type III supercapacitors. The electrochemical properties as ambipolar electrodes were examined through galvanostatic charge− discharge (GCD) curves, stability test, and electrochemical impedance spectroscopy (EIS) for further investigation. 3.4.1. Galvanostatic Charge−Discharge (GCD) Curves. The p-doped GCD curves of PEBE-PBP and PEB-P exhibit deviation from the triangular geometry with linear charge and discharge process (Figure 5a,b), generally regarded as uniform charge/discharge behavior.10,22 The slight distortion in GCD curves suggests the existence of psedocapactive contribution from redox reaction of electroactive polymers besides doublelayer contribution.1 For the n-doped state, nonlinear GCD curves with two plateaus (Figure 5c,d) are observed, demonstrating most energy was stored through pseudocapacitive behavior.7 The discharge processes of p-doped and ndoped states were monitored to assess capacitance.10 To obtain the mass of polymer on electrodes, the quartz crystal
Figure 3. Normalized UV−vis−NIR absorption spectra of PEBE, PEBE-PBP, PEB-P, and PPBP thin films on ITO-coated glass slides.
bands (600−700 nm) are attributed to intramolecular charge transfer (ICT) of donors and acceptors,21 while the higher energy bands (400−440 nm) can be ascribed to the localized π−π* transition in the conjugated systems. Both PEBE-PBP and PEB-P inherit the spectral nature from PEBE, albeit with subtle blue-shifts of absorption peaks (λpeak). Blue-shifts of maximum absorption indicate weaker interchain interactions resulting from the presence of solubilizing moieties. Additionally, absorption gaps (λgap) of PEBE, PEBE-PBP, and PEB-P were centered at 508, 516, and 508 nm, respectively, which are essential values for green hues to be observed in their neutral states. E
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Figure 4. Spectroelectrochemistry and photos of (a) PEBE, (b) PPBP, (c) PEBE-PBP, and (d) PEB-P polymer films on ITO-coated glass slides in a 0.1 M TBAPF6/PC electrolyte−solvent in their neutral (red line), reduced (black line), and oxidized (blue line) states.
Figure 5. GCD curves at different current densities between −0.5 and 0.8 V of (a) PEBE-PBP and (b) PEB-P polymer films on Pt electrodes. GCD curves at different current densities between −0.5 and −1.8 V of (c) PEBE-PBP and (d) PEB-P polymer films on Pt electrodes. Measured in a 0.1 M TBAPF6/PC electrolyte−solvent.
microbalance (QCM) method and mass-absorbance estimation method were used for electrochemically deposited PEBE and spray-coated recombined polymers, respectively. Specific capacitances were calculated based on above data and have been listed in Table 2. During both p-doping/dedoping and n-doping/dedoping processes, specific capacitances (CmP and CmN) of PEB-P and PEBE-PBP are relatively matched and share the same
magnitude to those of PEBE with acceptable decrease. The decreased capacitances of recombined polymers stem from the introduction of ProDOT moieties with lower mass contribution of conjugated parts, which also distort the polymer backbones and further reduce the electronic conductivity.23,24 Even so, both PEBE-PBP and PEB-P could store a similar amount of charge per donor or acceptor unit compared to PEBE (Table S2). Furthermore, their specific capacitances, F
DOI: 10.1021/acs.macromol.8b00599 Macromolecules XXXX, XXX, XXX−XXX
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To evaluate the p-doped cycling stability, further tests of PEBE and recombined polymers were performed using the GCD method at current density of 1.25 mA cm−2 (Figure S16). PEBE-PBP exhibits 60% retention of initial capacitance after 2000 cycles for the p-doped process, while PEB-P shows 50% retention. Both PEBE-PBP and PEB-P exhibit relatively poorer stabilty in p-doped cycling tests compared to that of PEBE (65% retention after 2000 cycles). To sum up, PEBE-PBP and PEB-P show more obvious differences in n-doped stability compared to PEBE. Interestingly but unintelligibly, the capacitances of our recombined polymers both decay relatively faster than PEBE. Great challenge remains to give a reasonable explanation, since many factors should be taken into comprehensive consideration, including volume change,25 film adherence,26 and charge-trapping effect27 during the stability test. Moreover, the cycling stability is related to backbone structure, morphology, and electrolyte, inspiring us to focus on concepts of molecular design, methods of film formation, and choices of counterion in the future study. 3.4.3. Electrochemical Impedance Spectroscopy (EIS). The charge transfer natures of both recombined polymers and PEBE were probed with the help of the EIS method (Figure 6d) in the frequency range from 100 kHz to 1 Hz at the open circuit potential by applying a small potential of 10 mV sinusoidal signal. The Nyquist plot of all polymers exhibits a straight line at the low-frequency range with a negligible arc at the high-frequency range, indicating very fast charge transfer at the electrode−electrolyte surface.27 In contrast with PEBE, the behaviors of PEBE-PBP and PEB-P at lower frequencies deviate more significantly from the vertical line that would be seen for an ideal supercapacitor. This deviation is caused by the ionic resistance of electrolyte within polymer electrodes.10 Moreover, PEB-P and PEBE-PBP exhibit similar internal resistance with the value larger than that of PEBE, demonstrating lower conductivity for obtained polymers.
Table 2. Specific Capacitances of PEBE-PBP, PEB-P, and PEBE Cma (F g−1) polymer
CmPb
CmNc
PEBE-PBP PEB-P PEBE
156.8 135.9 249.8
107.4 106.6 191.0
a Calculated under 0.05 mA cm−2. bCalculated as the voltage ranging from 0.8 to −0.5 V for PEBE-PBP and PEB-P and from 0.6 to −0.7 V for PEBE. cCalculated as the voltage ranging from −1.8 to −0.5 V for PEBE-PBP and PEB-P and from −2.0 to −0.7 V for PEBE.
gratifyingly, can be regarded as satisfied values among ambipolar D−A polymers reported so far (Table S3).4 Accordingly, both PEBE-PBP and PEB-P are promising candidates to construct the Type III supercapacitors.6 3.4.2. Stability Test. Reliable stability, related to utilization and duration during application, is another important requirement for electrode materials of supercapacitors.10 The lack of the n-doped stability is a critical limiting factor in development of D−A polymer for Type III supercapacitors.22 Hence, ndoped cycling stability was first investigated by CV test for 250 cycles at a scan rate of 50 mV s−1 (Figure 6a−c). In comparison to their ambipolar parent PEBE, the obtained polymers exhibit quite different n-doped stability. PEBE-PBP displays the best stability with 63% retention of the initial capacitance after 250 cycles (Figure 6a), while the capacitance of PEB-P remains only at 25% after 250 cycles (Figure 6b). Unfortunately, the PEBE reveals inferior stability and loses almost all capacitance after 250 cycles (Figure 6c). In general, both obtained polymers showed good redox stability, which is better than that of PEBE and comparable to values reported previously for ambipolar D−A polymers during the n-doping process (Table S3).
Figure 6. Stability of (a) PEBE-PBP, (b) PEB-P, and (c) PEBE polymer films on Pt disk electrodes recorded in 0.1 M TBAPF6/PC. (d) Nyquist plots of PEB-P, PEBE-PBP, and PEBE polymer films on Pt disk electrode measured in 0.1 M TBAPF6/ACN. G
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Figure 7. AFM image of (a) PEBE-PBP (RRMS = 2.13 nm) and (b) PEB-P (RRMS = 3.13 nm) polymer films spray-coated on ITO-coated glass slides with the scan size of 5 × 5 μm2.
possibilities for other ambipolar electroactive polymers. Based on the performance tests, these two recombined polymers reveal competitive pseudocapacitances and superior processability to PEBE, leading us to explore applications in Type III supercapacitors for next-generation energy storage systems in our next research work. Additionally and interestingly, unique electrochromic properties of PEBE-PBP and PEB-P may broaden the potential application fields of smart energy storage as well as more multifunctional devices.
This could be attributed to the fact that soluble moieties with alkyl chains may decrease the degree of backbone conjugation, leading to a lower conductivity, i.e., larger resistance.15 The EIS of both recombined polymers were further studied at different states (Figure S17). Compared to neutral states, their Nyquist plots in oxidized states exhibits straight lines at the low frequency range, indicating double-layer behaviors, and the plots in reduced state indicate the faradaic charge transfers.5,10 3.5. Morphology. To further understand the differences observed during electrochemical processes, surface morphology of PEBE-PBP and PEB-P as polymer films was also probed using AFM (Figure 7) and SEM (Figure S17). Both of our recombined polymers exhibit homogeneous and uniform surfaces, which could be beneficial to increase the electrical conductivity and electron transfer capability of the conjugated polymers.28 From AFM study, porous structures have been observed, which could facilitate solvent/electrolyte diffusion.29,30 Moreover, the lower roughness of PEBE-PBP might imply a more compact surface morphology,31 which could be induced by the different distribution of modified moieties with side chains. With lower roughness, the diffusion of moisture and oxygen would be reduced into the interior of the polymer film,32 which might result in better stability of PEBE-PBP than PEB-P during both p-doping and n-doping tests. It is worth mentioning that the surface morphology would critically influence the electrochemical performances of electroactive polymers,7,11 but a clear and exact mechanism has not been fully understood for our recombined polymers yet. Thus, further investigations are required to correlate the film morphology with the electrochemical performances.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00599. Synthesis of the monomers and their structural characterizations; characterization of the polymers using 1H NMR, GPC, FT-IR, and spectroelectrochemistry of polymer films; specific capacitance calculations; comparison of the capacitance and stability with literatures; the p-doped stability of PEBE-PBP and PEB-P polymer films (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(H.M.) E-mail:
[email protected]. *(Y.H.) E-mail:
[email protected]. ORCID
Yaowu He: 0000-0003-2887-735X Hong Meng: 0000-0001-5877-359X
4. CONCLUSIONS In summary, we report a recombination strategy to circumvent the contradiction between improvement of processability and maintenance electrochemical performance. Based on this refreshing strategy, ambipolar (PEBE) and soluble (PPBP) moieties were recombined in both unit and segment scales, and two donor−acceptor polymers (PEB-P and PEBE-PBP) were successfully developed via the DHAP route. To evaluate the reliability of this novel recombination strategy, both PEB-P and PEBE-PBP films were systematically investigated, including electrochemical, optical, and capacitive behavior. From property characterization, both PEB-P and PEBE-PBP, with desirable processability from soluble PPBP, exhibit optical and electrochemical identity to parental ambipolar PEBE, proving the feasibility of our strategy and opening up more potential
Author Contributions
Y.W. and W.L. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by Shenzhen Science and Technology Research Grant (JCYJ20160331095335232 and JCYJ20160510144254604), National Natural Science Foundation of China (51761145101, 51603003), the Shenzhen Engineering Laboratory (Shenzhen development and re-form commission [2016]1592), the Shenzhen Peacock Plan (KQTD2014062714543296), the National Basic Research Program of China (973 Program, No. 2015CB856500), and the Pakistan Science Foundation under the provision of PSFH
DOI: 10.1021/acs.macromol.8b00599 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
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NSFC grant. Weishuo Li thanks Yilin Wang for her kindness and sincerely wishes she would realize her dream and find her own happiness.
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DOI: 10.1021/acs.macromol.8b00599 Macromolecules XXXX, XXX, XXX−XXX
