Charge Storage in Decyl - American Chemical Society

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Charge Storage in Decyl- and 3,6,9-Trioxadecyl-Substituted Poly(dithieno[3,2‑b:2,3‑d]pyrrole) Electrodes Jared F. Mike,† Lin Shao,‡ Ju-Won Jeon,† and Jodie L. Lutkenhaus*,† †

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States Department of Chemical & Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States

‡

S Supporting Information *

ABSTRACT: Poly(dithieno[3,2-b:2,3-d]pyrrole)s, or poly(DTP)s, are potentially promising electrode materials for electrochemical energy storage (EES) because of their ability to reversibly store charge and achieve relatively high doping levels. Here, we explore charge storage in two different sidechain-substituted poly(DTP)s. Substitution at the nitrogen with alkyl and alkyl ether chains is used to demonstrate the ability to tune electrochemical response through favorable interactions with the electrolyte. PolyDTP electrodes are electropolymerized and analyzed using various electrochemical techniques, spectroscopic measurements including UV−vis and Raman, and electron microscopy. Specific capacities for the poly(DTP)s range between approximately 35 and 70 mAh/g for discharge rates of 1−75 C, corresponding to considerably high doping levels of 0.5−0.9 electrons per repeat unit. After 1000 cycles, the alkyl-substituted polymer retains 75% of its initial capacity while the alkyl ether-substituted polymer only retains 50%. The specific capacitance was as high as 121 F/g. These results suggest that poly(DTP)s could be utilized on their own or in composite electrodes for energy storage.

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performance can rapidly fade under such conditions.21−23 Second, the experimental capacity is often limited to only moderate doping levels of 0.3−0.5 mol e− per repeat unit.1,4 Third, at high discharge rates, the diffusion of dopant ions becomes rate-limiting.24,25 If the aforementioned issues can be suitably addressed, then conjugated polymers (CPs) will present an attractive choice as electrodes for future electrochemical energy storage applications.1,2,4 Of particular interest are a class of CPs based on a dithieno[3,2-b:2,3-d]pyrrole (DTP) backbone, which have never before been explored as materials for electrochemical energy storage.26 DTP is an electron-rich heterocyclic unit with a fused-ring structure, which helps to enhance effective conjugation by increasing planarity in the polyDTP backbone, leading to improved π-orbital overlap, which can be directly linked to a polymer’s optical and electronic properties.27,28 To date, polyDTPs have been explored as components in solar cells, light-emitting diodes, and field-effect transistors.26,27,29−32 One early study indicated that polyDTP can achieve doping levels of 0.6−0.7 electrons per repeat unithigher than that reported for many CPs.4,33 It has been shown that DTP monomer may be easily functionalized at the nitrogen in order to fine-tune the properties of the resulting polymer.30,34−37 For example,

INTRODUCTION Explored as electrode materials in both batteries and electrochemical capacitors, conjugated polymers (CPs) are both conductive and electrochemically active. Through oxidation and reduction (or doping and dedoping), a CP stores charge. Common examples include polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylenedioxythiophene) (PEDOT).1−4 There are many motivating reasons to explore CPs in energy storage. Unlike many inorganic or metal oxide electrodes, CPs are especially promising for applications requiring mechanical flexibility or unconventional geometries. CPs have been utilized in lightweight, flexible devices as part of watches, fabrics, or thin-film displays, for example.3,5 CPs are low cost, processable, and easy to modify.6−8 For example, the physical, chemical, and electrochemical properties of a CP can be modified to suit a particular application via side-chain chemistry.9−12 They can also be potentially derived from domestic feedstock. Relevant to Li-ion batteries, CPs have also been used in conjunction with high-capacity inorganic components such as V2O5,13−15 LiFePO4,16 sulfur,17,18 or silicon19 with the goal of producing a hybrid composite that yields a performance exceeding that of either material alone.20 Ideally, a CP electrode would reversibly switch between doped and dedoped states, achieve high doping levels, and bear electrochemical stability, which would lead to elevated capacities (or capacitances) and energy densities. Unfortunately, of many well-studied CPs, several issues have arisen. First, it is desired to store charge under high potentials, but © 2013 American Chemical Society

Received: October 8, 2013 Revised: December 19, 2013 Published: December 26, 2013 79

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poly(N-alkyl DTP)s have demonstrated greater processability, stability in the oxidized state, conductivity, cyclability, and lower band gaps than unsubstituted poly(DTP)s.33,35,38−40 Although poly(N-alkyl DTP)s are indeed very promising, there remain other types of side chains that could perhaps be particularly useful for energy storage. Ideally, the side chain would impart not only stability and processability, as shown before with alkyl side chains, it should also impart some affinity toward the electrolyte solvent and salt. Therefore, an alkyl ether side chain is of interest because it could potentially facilitate the transfer of ions during the doping and dedoping process.41,42 In this paper we synthesize N-substituted alkyl and alkyl ether polyDTPs, and we compare their ability to store charge. Specifically, n-decyl and 3,6,9-trioxadecyl are compared because they are similar in length. We hypothesize that this family of polymers will be suitable for energy storage because of their ability to reversibly achieve high doping levels at oxidizing potentials. Also, we hypothesize that the alkyl ether-substituted polymer will have a more facile response to an applied potential when compared to its alkyl counterpart. There is no precedent for N-substituted alkyl ether polyDTPs, so we first examine their synthesis. This allows for the first time a quantitative comparison between alkyl and alkyl ether side chains for polyDTPs. In order to evaluate their abilities to store charge, these polyDTPs are then utilized as cathodes in a half-cell configuration under nonaqueous conditions. We compare the basic electrochemical characteristics of both polymers using cyclic voltammetry and various charge/discharge experiments. Further characterization is carried out using UV−vis and Raman spectroscopy. In addition, theoretical Raman spectra are calculated in order to elucidate vibrational transitions and to draw parallels with the experimental spectra.

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Table 1. Pd-Catalyzed Amination of 3,3′-Dibromo-2,2′bithiophene Using 3,6,9-Trioxadecylaminea solvent

ligand

base

yield (%)

toluene 1,4-dioxane toluene toluene toluene 1,4-dioxane 1,4-dioxane 1,4-dioxane

BINAP BINAP dppf XANTPHOS dtBupf dppf dppf dppf

NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu K3PO4 Cs2CO3

9 0 30 3 0 35 4 3

a

Reactions were run until disappearance of starting material by thin layer chromatography (TLC). BINAP = 2,2′-bis(diphenylphosphino)1,1′-binaphthyl, dppf = 1,1′-bis(diphenylphosphino)ferrocene, XANTPHOS = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, dtBupf = 1,1′-bis(di-tert-butylphosphino)ferrocene, NaOtBu = sodium tertbutoxide.

ether:hexanes:ethyl acetate). Yields are listed in Table 1. 1H NMR (Figure S1, 300 MHz, CDCl3): δ 7.14 (d, 2H, J = 5.3 Hz), 7.07 (d, 2H, J = 5.3 Hz), 4.40 (t, 2H, J = 5.6 Hz), 3.85 (t, 2H, J = 5.6 Hz), 3.56− 3.48 (m, overlapping, 8H), 3.38 (s, 3H) ppm. 13C NMR (Figure S2, 75 MHz, CDCl3): δ 145.27, 122.81, 114.99, 111.48, 71.99, 70.98, 70.71, 70.63, 70.51, 59.12, 47.55 ppm. HRMS (ESI, [M + H]+ for C15H19NO3S2): calcd, 326.0885; found, 326.0873. Materials Characterization. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM 7500F field-emission SEM instrument. UV−vis spectra were collected on a Hitachi U-4100 spectrometer. Raman spectra were collected on a Horiba Jobin-Yvon LabRam IR system using a 633 nm laser with a JY open electrode CCD as the detector. 1H and 13C NMR spectra were acquired on a Varian Mercury 300 MHz NMR spectrometer (1H 300 MHz, 13C 75 MHz). NMR samples were taken using CDCl3 as a solvent and standard. Electrode Preparation. All electrochemical experiments were carried out in an argon-filled glovebox (1200 nm, and coloration changed from purple to blue. Coincidentally, the peak at 540 nm decreased in absorbance, as neutral polymer segments were gradually eliminated.24,85 Slight differences were observed between the poly(decylDTP) and the poly(trioxadecylDTP) UV−vis spectra. Generally speaking, poly(trioxadecylDTP) peaks were blueshifted relative to poly(decylDTP) by about 10 nm. In reference to the 740 nm peak, the absorbance for poly(decylDTP) increased in absorbance at first and then decreased during oxidation, whereas for poly(trioxadecylDTP), the absorbance only steadily increased. As the polymers oxidized to 4.0 V vs Li/Li+, the peak associated with the large, broad absorbance >1200 nm blue-shifted and grew in absorbance; a peak maximum at ∼1480 nm was observed for poly(decylDTP), but a similar peak for poly(trioxadecylDTP) was outside of detection. The strong absorption exhibited in the near-IR at 4.0 V for both polymers suggests that they could possibly be potential candidates for electrochromic “smart” windows or IR-blocking materials.85,86 The onset of absorbance for the reduced polymers was 713 nm for poly(decylDTP) and 687 nm for poly(trioxadecylDTP), corresponding to bandgaps of 1.74 and 1.81 eV, respectively. The difference in bandgaps suggests that poly(decylDTP) benefits from increased conjugation along the polymer backbone. This can also be inferred from spectral changes as the polymers oxidize. The decrease in intensity of the 740 nm peak coupled with the increase in intensity and blue-shift of the peak furthest in the IR is consistent with a change from polaronic (two peaks) to bipolaronic character (one peak).87,88 Indeed, poly(decylDTP) lost much of the intensity of the 740 nm peak at 4.0 V vs Li/Li+ while the peak at ∼1480 nm came to dominate the spectrum. This result indicates that poly(decylDTP) was able to attain a higher doping level at 4.0 V than poly(trioxadecylDTP), which was consistent with cyclic voltammetry data. Raman spectroscopy was performed within the same range of potentials as for UV−vis spectra. In order to properly assign the Raman-active vibrational modes, theoretical Raman spectra were calculated using a geometry-optimized, neutral, N-methylsubstituted DTP trimer (Figure 6 and Experimental Section). As can be seen in Figure 7, the calculated spectrum matched well with the experimental spectra. A list of peaks and assignments is outlined in Table 2. The Raman spectra of the two polymers displayed only minor differences; for example, the peaks exhibited by poly(decylDTP) at 1524, 1400, and 1247 cm−1 were lower in frequency by only 2 cm−1 versus

Figure 6. DTP trimer used in calculations, shown with optimized geometry. The red arrows indicate atomic movement for each of the largest Raman bands: top, 1524 cm−1; middle, 1400 cm−1; bottom, 1247 cm−1.

Figure 7. Raman spectra of (a) poly(decylDTP) and (b) poly(trioxadecylDTP) as compared to the calculated spectrum for the trimer shown in Figure 6.

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PolyDTPs attained relatively high doping levelsup to ∼0.9 electron per repeat unit for poly(decylDTP). Both polymers showed reasonable stability up to 4.0 V vs Li/Li+. Poly(decylDTP) possessed a higher capacity and capacitance, likely because of its enhanced conjugation. On the other hand, poly(trioxadecylDTP) had the advantage of reduced mass transport limitations owing to favorable polymer−solvent interactions. These materials were capable of outperforming other polythiophenes as far as electrochemical energy storage was concerned. The future of these materials lies in the optimization and exploitation of their already high doping levels, stability, and operating potential. In addition, lithium exchange due to the presence of alkyl ether chains could be useful for composites with metal oxides. Further research into side-chain substitutions that enhance the stability of the DTP core, mitigate mass transport limitations, and/or are redox active would lead to further advances in polyDTPs as energy storage materials.

Table 2. Peak List for Raman Spectra of Poly(DTP)s experimental (cm−1)

theoretical (cm−1)

peak assignment

1524 1446 1400 1247 1169 1097 703 683 667 651 622

1524 1436 1402 1249 1180 1089 703 683 665 646 621

CC stretch CH3 deformation Я mode ring C−H bend C−C stretch C−H and CH3 bending ring out-of-plane ring out-of-plane C−S−C out-of-plane C−S−C stretch C−S−C bend

poly(trioxadecylDTP). The only peak that displayed a substantial difference between the two polymers was associated with deformation of the methylene at the DTP nitrogen, which was 1446 cm−1 for poly(decylDTP) and at 1436 cm−1 for poly(trioxadecylDTP). In general, Raman activity for conjugated polymers can be described by the effective conjugated coordinate (ECC) model.89,90 The model describes a particular vibrational coordinate that is strongly associated with the delocalized πelectrons in the polymer backbone. The coordinate is associated with a change from an aromatic (ground state) to a quinoidal (excited state) structure. This gives rise to the most intense normal mode in a conjugated system and is referred to as the “Я mode”. As the polymer chains become longer, any vibrational modes not associated with the Я coordinate, for example those involving side chains or stretching and bending at the ends of polymer chains, weaken, while other vibrational modes are enhanced. Generally, the Я mode grows in intensity, shifts to lower frequencies, and broadens as chain length increases.91 This can clearly be seen in spectra calculated for DTP monomer, dimer, and trimer (Figure S8). As such, the ECC model can be used to draw inferences as to the effective conjugation of polyaromatics.92−94 Because the simulated trimer and the experimental Raman match fairly well, especially in regards to the Я mode, we concluded that the conjugation length for both polyDTPs was at least three DTP units. As the polymers oxidized, the major active Raman modes tended to broaden and red-shift. The three largest peaks shift to lower frequencies by 10−20 cm−1. This trend arises from a “softening” of the chemical bonds as the polymer adopts a more quinoidal structure, a behavior often observed in polythiophenes.93,95 From the similarities between the Raman spectra of the two polymers along with the observations from UV−vis, it can be concluded that poly(trioxadecylDTP) may possess a less conjugated backbone. One might expect this to lead to a higher onset in oxidation for poly(trioxadecylDTP), but this is not the case (Figure 3). As remarked previously, the lowering of poly(trioxadecylDTP)’s oxidation onset potential is more likely caused by favorable solvent− and electrolyte−side chain interactions.

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ASSOCIATED CONTENT

* Supporting Information S

NMR spectra, conditioning cyclic voltammograms, plots of changes in capacity with current density, and SEM images and cyclic voltammograms of the polymer before and after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.L.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by the Welch Foundation (Grant No. A-1766). We thank the Texas A&M University Materials Characterization Facility.

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REFERENCES

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CONCLUSION PolyDTPs have for the first time been explored for their potential to store and deliver charge. A comparison was made between two polymers bearing side chains of equal length, but different chemical propertiesone with an alkyl n-decyl chain and one with an oxygen-containing 3,6,9-trioxadecyl chain. 86

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dx.doi.org/10.1021/ma402071k | Macromolecules 2014, 47, 79−88