Conjugated Pyridine-Based Polymers Characterized as Conductivity

Article pubs.acs.org/JPCC

Conjugated Pyridine-Based Polymers Characterized as Conductivity Carrying Components in Anode Materials Li Yang,† Viorica-Alina Mihali,‡ Daniel Brandell,‡ Maria Strømme,† and Martin Sjödin*,† †

Department of Engineering Sciences, Division of Nanotechnology and Functional Materials, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden ‡ Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden ABSTRACT: Herein, polypyridine (P25Py) is for the first time evaluated as an anode material for organic matter based electric energy storage devices. P25Py is synthesized both chemically and electrochemically and the influence of electrolyte and solvent on the doping behavior of the material is investigated in propylene carbonate and acetonitrile with LiClO4 and TBAPF6. A battery consisting of P25Py coupled to a lithium metal disc is assembled and the electrochemical performance and cycling stability of the conjugated polymer is analyzed. In all electrolyte combinations P25Py is conductive and shows reversible redox chemistry between −1.0 and −2.0 V vs ferrocene with capacitive response characteristics. The electrochemical impedance spectroscopy response of the material can be described by a Randles equivalent circuit with a finite length Warburg diffusion element in which the diffusion coefficient of the cations increases with increasing doping level of the polymer. In the battery cell configuration the polymer shows reversible cycling with no capacity fading during the first 100 cycles without conducting additives. P25Py thus provides a promising alternative conducting polymer base for electrical energy storage applications which expands both the potential widow as well as the electrolyte compatibility of the flora of known conducting polymers.

1. INTRODUCTION Organic matter based electric devices are attracting great interest in the research community due to their high safety, easy disposability, low energy consumption during processing, and readily available raw material resources. Hence, the application of intrinsically conducting polymers in batteries,1−3 capacitors,4,5 and solar cells6,7 has been widely explored. Especially, for the secondary batteries, conducting polymers, such as polypyrrole,8,9 polythiophene,10,11 polyaniline,12 as well as their copolymers,13 have been studied due to their lightweight, superior rechargeability14 compared to commonly used inorganic electrode materials, insolubility and compatibility with organic solvents. As the cathode materials are limiting the capacity of contemporary batteries and also because several conducting polymers based candidates are known with comparable redox potentials most efforts have been devoted to the replacement of existing cathode materials with conducting polymer materials. In particular, our group has previously shown that all-polymer-based composites of polypyrrole covered nanocellulose fibers are promising cathode candidates for sustainable and fast-charging electric energy storage.15,16 The development of conducting polymer based anode materials, on the other hand, has received much less attention. In general, conducting polymers show charge storage capacities that are too low to compete with traditional © 2014 American Chemical Society

inorganic alternatives. This disadvantage can be overcome by introducing high capacity redox groups to the conducting polymer backbone.17−20 This strategy has been used in Sen et al.17 where a polymer based battery was prepared using viologens covalently attached to polypyrrole, serving as charge carrier, coupled to a cathode consisting of polypyrrole doped with 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid). A relatively small discharge capacity of 16 mAh g−1 was however obtained which is possibly due to the resistivity of the polypyrrole backbone in the potential region where the viologens show redox activity. Thiophene-based polymers are characterized by both intrinsic n- and p-doping properties,10,21,22 and they have been studied extensively both by electrochemical23−26 and optical methods.27 Also this polymer has been used together with redox active functional groups as battery material and in the work done by Rosciano et al.,28 an anode material using this polymer base together with cross-linked naphtalene-bisimide redox centers was presented. In this material, as well, the redox reaction of the pendant group occurred in a region where the polymer backbone is neutral and material conductivity relied on Received: September 23, 2014 Revised: October 21, 2014 Published: October 21, 2014 25956

dx.doi.org/10.1021/jp509606c | J. Phys. Chem. C 2014, 118, 25956−25963

The Journal of Physical Chemistry C

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70 °C for 16 h. Then 300 mL of acetone was added to the flask to precipitate the product. The brown precipitate was filtered off and washed consequently by hot toluene, hot aqueous ethylenediaminetetraacetic acid (EDTA) (pH 3), hot aqueous EDTA (pH 9), warm aqueous sodium hydroxide (pH 9), water, acetone, and finally dried in a vacuum oven at 40 °C. A yellow powder was isolated with a yield of 24%. The P25Py used for electrochemical impedance spectroscopy and scanning electron microscopy measurement was prepared on a glassy carbon electrode and a platinum foil, respectively, by electrochemical polymerization as previously reported.41 FTIR (neat): 3424, 1585, 1454, 1074, 1011, 826 cm−1. UV−vis (formic acid): λmax = 365 nm. TGA: Td = 520 °C. 2.2. Preparation of Electrodes. A polypyridine composite electrode was prepared for electrochemical studies. P25Py (60 wt %), Super P carbon black (30 wt %, Csp, Alfa Aesar), and poly(vinylidene fluoride) (10 wt %, 1 wt % in N-methyl-2pyrrolidone) were mixed and then ultrasonically stirred for 10 min to form a uniform suspension. The slurry was evenly dropped on nickel foam (current collector). The electrode was dried in vacuum at 40 °C for 20 h and subsequently compressed to form a compact electrode with a thickness of about 0.6 mm. The active material in the composite was about 3 mg for each electrode. All electrodes were used immediately after drying. 2.3. Fabrication and Characterization of Batteries. Batteries consisting of P25Py coupled to a lithium metal disc were assembled as a Swagelok-type battery in an argon filled glovebox. P25Py was used without any additives, employing glass fiber as a separator and 1 M lithium perchlorate (LiClO4) in PC as electrolyte. Although the experimental data presented in this paper is for polymer without any additives, different combinations of polymer and Csp were also tested and the resulting data shows that the addition of additives does not improve the performance of the polymer in the cell. The electrochemical characterization of batteries was carried out on VMP 2 BioLogic and Arbin BT 2043 systems, respectively. Cyclic voltammetry and galvanostatic techniques were used to analyze the Li-polymer batteries. 2.4. Scanning Electron Microscopy Analysis. The morphology of P25Py from both chemical and electrochemical polymerization was characterized by scanning electron microscopy (SEM) with a LEO1550 field emission instrument (Zeiss, Germany). The chemically prepared polymer was spread evenly on aluminum stubs using a conductive adhesive tape. The P25Py powder and P25Py film on the platinum substrate were dried and Au-coated before testing. 2.5. Electrochemical Characterization. An Autolab PGSTAT302N potentiostat (Ecochemie, Utrecht, The Netherlands) was used for all electrochemical measurements of nickel foam electrodes. The electrochemistry of P25Py was investigated by cyclic voltammetry in a three-electrode cell. A platinum wire and an Ag0/Ag+ (10 mM silver nitrate) were used as counter and reference electrode, respectively. The reference electrode was calibrated against the ferrocene/ ferrocenium (Fc0/Fc+) redox couple and, if not stated otherwise, potentials are reported against Fc0/Fc+ throughout the report. The electrochemistry of P25Py was compared in four different electrolyte solutions, namely 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6)/MeCN, 0.1 M TBAPF6/PC, 0.1 M LiClO4/MeCN and 0.1 M LiClO4/PC. The cell was purged with solution saturated N2 for 20 min prior

residual polymer doping in the potential region where polythiophene is considered nonconducting. It is thus clear that in order to enable redox matching between pendant groups and conducting polymer base an expansion of the number of possible polymer bases is instrumental. In this report we investigate a new category of heterocyclic conjugated polymers for energy storage applications based on pyridine. Polypyridine has been extensively used in light-emitting diodes29−31 and for redox catalytic reactions32,33 to date. However, the application of polypyridine as anode material for energy storage has, to the best of our knowledge, not been investigated. Due to the π-deficient nature of pyridine,34,35 pyridine-based conducting polymers show ndoping properties, offering a possible utilization of pyridinebased conducting polymers as anode battery materials. Reversible electrochemically driven n-doping behavior of polypyridine, is accompanied by a color change from blue (reductive) to yellow (neutral).36 Some copolymers based on pyridine also indicate reversible n-doping properties.37−39 In this report, polypyrdine has been synthesized both by chemical and electrochemical methods and the influence of electrolyte and solvent on the doping behavior of polypyridine has been studied. Additionally, a battery consisting of polypyridine coupled to a lithium metal disc was assembled and the electrochemical performance and cycling stability of polypyridine is analyzed.

2. EXPERIMENTAL SECTION All solvents and compounds were purchased from SigmaAldrich and used without further purification unless otherwise specified. N,N-Dimethylformamide (DMF), acetonitrile (MeCN), and propylene carbonate (PC) were dried using molecular sieves, 3 Å, before use. Nickel foam with a thickness of 1.6 mm and surface density of 346 g m−2 was cut into strips with an area of 1 × 3 cm2, subsequently cleaned with acetone, 0.1 M HCl, water in a BRANSON 3510 ultrasonic cleaning instrument, and dried in an VO-16020r vacuum drying oven (for the drying of polymer and electrodes). An AtlasTM Manual Hydraulic press was used to compress nickel foam electrodes. The thickness of the electrodes for the electrochemical studies was determined by a Mitutoyo tester, IDC150XB. FTIR spectrum of the polymer (KBr pellets) was recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer. UV−vis spectra were acquired using an Agilent 8453 UV− visible Spectroscopy System. TGA curves were obtained using a Mettler Toledo, TGA/SDTA851e analyzer. 2.1. Synthesis of Poly(2,5-pyridine). Polypyridine can be chemically prepared by different methods,40 and a head-to-tail polymer is favored in both electrochemical and chemical polymerization.41 In this study, P25Py was synthesized by dehalogenation polycondensation of 2,5-dibromopyridine (Scheme 1) through an easily controlled reaction42,43 with nickel(II) chloride (NiCl2) reduced by zinc in situ. In a typical preparation, stoichiometric amounts of zinc, NiCl2, triphenylphosphine, and monomer were weighed into a flask containing nitrogen-purged DMF (30 mL), and the mixture was stirred at Scheme 1. Chemical Polymerization Reaction of the Synthesis of Head-to-Tail P25Py

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chemically. As seen from the images, different morphologies of P 25 Py are obtained from the different polymerization techniques. The electrochemically synthesized polymer shows small and relatively uniformly sized particles with a diameter of approximately 400 nm, which are closely packed and evenly distributed on the surface. P25Py prepared chemically, on the other hand, displays agglomeration and several interparticle pores that are randomly distributed throughout the material. Some surface charging of both samples was observed in line with earlier observations in other conducting polymers.44,45 Although the P25Py polymers from chemical and electrochemical polymerization show various surface morphologies, we did not find the electrochemical behavior to differ detectably. 3.2. Electrochemistry of Poly(2,5-pyridine). It has previously been shown that both n- and p-doping behavior of conducting polymers strongly depends on the nature of solvents and the size of counterions.46,47 In order to investigate the possible use of P25Py as a component in rechargeable battery anodes, the electrochemical n-doing properties of P25Py were investigated in common battery electrolyte solutions. The cyclic voltammograms of P25Py in MeCN and PC are shown in Figure 2a,b, respectively, and all electrochemical data are collected in Table 1. A reproducible n-doping/n-dedoping

to each measurement and the solution was kept under an N2 blanket throughout the electrochemical measurements. Electrochemical ac impedance spectroscopy (EIS) measurements were performed on the P25Py in 0.1 M TBAPF6/MeCN with a CH instruments 660D potentiostat (CH Instruments, Inc., USA) using an ac amplitude of 5 mV. The frequency was scanned from 100 kHz to 0.01 Hz. The measurements were performed at four different potentials starting with −1.06 V vs Fc0/Fc+ at which the as-synthesized electrode material is nonconductive and thereafter, stepwise, shifting the potential toward more reductive potentials. In order to experimentally establish the relation between the Fc0/Fc+ reference potential and the lithium scale commonly used in battery research the Fc0/Fc+ potential was determined to be 3.25 V vs Li0/Li+ by cyclic voltammetry in a three electrode setup. The reference electrode was made up by immersing a lithium metal foil in a separate compartment filled with 1 M LiClO4/PC. A glassy carbon disc electrode was used as working electrode and a platinum wire served as counter electrode, and they were both directly immersed into the 1 M LiClO4/PC electrolyte solution containing ferrocene.

3. RESULTS AND DISCUSSION 3.1. Morphology of Poly(2,5-pyridine). Figure 1 shows the morphology of P25Py polymerized chemically and electro-

Table 1. Effect of Solvent and Supporting Electrolytes on the Electrochemistry of P25Py solvent

electrolyte

Eoc (V vs Fc0/Fc+)

Eap (V vs Fc0/Fc+)

charge efficiency (%)

MeCN MeCN PC PC

LiClO4 TBAPF6 LiClO4 TBAPF6

−0.79 −0.91 −0.75 −0.89

−1.09 −1.33 −1.03 −1.32

90.5 87.4 94.5 95.3

behavior of P25Py is observed in MeCN and in PC using both Li+ based and TBA+ based electrolyte salts. The onset cathode reduction potential, Eoc, the anode oxidation peak potential, Eap, as well as the charge efficiency do however vary with electrolyte composition (Table 1). The slightly higher charge efficiency of P25Py in PC compared to MeCN is likely due to instability of the electrolyte as cathodic currents were observed when the pure nickel foam substrate was polarized to −2.2 V vs Fc0/Fc+ in MeCN. In both solvents a lower Eoc of P25Py is obtained when using TBA+ salt compared to when Li+ is used as dopant.

Figure 1. SEM images of P25Py (a, b) electrochemically polymerized onto platinum foil and (c, d) chemically polymerized and casted onto a conducting substrate.

Figure 2. Cyclic voltammograms of chemically prepared P25Py in (a) MeCN using 0.1 M LiClO4 (black line) and 0.1 M TBAPF6 (red line) as electrolyte and in (b) PC using 0.1 M LiClO4 (black line) and 0.1 M TBAPF6 (red line) as electrolyte, at a scan rate of 50 mV s−1. 25958



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respect to the scan rate and the weight of active material.) A linear relationship between log Ip and log ν was obtained in both solvents with a slope close to one indicating a surface confined reaction at least at low scan rates. For comparison the voltammogram of electropolymerized poly(3-phenyl-thiophene) recorded under identical conditions is included (red line Figure 3a). (Poly(3-phenyl-thiophene) was chosen for comparison due to its excellent n-doping capability, its relatively wide operative potential range, and its reversibility).50 Evidently the doping onset of P25Py occurs at higher potentials compared to the onset doping of poly(3-phenyl-thiophene). The higher onset doping potential allows for redox matching with several types of known redox active moieties that, with polytyhiophene, have been limited to dicarboxylate type functional groups only. For instance, P25Py redox-matches several of the dithiol based molecules that have been suggested and tested as battery material as well as several of the highly reversible and fast quinone type redox active compounds in addition to matching dicarboxylate redox chemistries (Figure 3b).51 Together with the compatibility with Li-based electrolytes P25Py provides a conducting polymer backbone that expands the possibilities to construct conducting redox polymers with redox matched polymer backbone and pendant group. The charge and discharge capacities of P25Py, corresponding to the n-doping and n-dedoping, respectively, evaluated from cyclic voltammogram at various scan rates are shown in Figure 4, as well as the cycling stability of P25Py at 50 mV s−1. In all cases, the capacity of the polymer decreases gradually with increased scan rate (Figure 4a). The smaller total charge obtained at higher scan rates indicates resistive losses at increased scan rates. This is also reflected by the observed potential shift to lower potentials for cathodic reactions and to higher potentials for anodic processes upon increasing the scan rates (Figure 3a) which gives an increasingly less complete reduction of the polymer with increased scan rates. The capacity fading with scan rate is more pronounced in MeCN. However, the overall specific capacity is more than two times higher in MeCN than in PC despite close to identical onset doping potentials, and hence, the absolute currents are higher. Based on the linear correlation between peak potential and peak current for resistive potential losses resistances were evaluated for the films in PC and in MeCN and were found to be about four times higher in PC than in MeCN possibly reflecting the increased number of charge carriers in the polymer with MeCN as solvent. The increased capacity in MeCN suggests that the interaction between charged species is lower when MeCN is used as solvent something which would enable higher doping levels for a specified potential interval, i.e., a higher specific capacitance. One explanation for a reduced interaction between consecutively introduced charges would be a more pronounced swelling of the polymer when using MeCN as solvent. This would increase the spatial separation between charges and reduce their columbic repulsion. In both solvents a higher capacity of polymer was obtained when using LiClO4 compared to experiments with TBAPF6 as electrolyte salt indicating weaker interaction between polymer charges when using LiClO4. Further investigation is however required to determine the cause of capacity variations in P25Py. Figure 4b shows the charge capacity as a function of cycle number. Evidently only in MeCN with LiClO4 salt appreciable capacity fading is observed over 50 cycles with charge capacity retention of 88%, 98%, 98%, and 100% for the electrolyte

Such effect has previously been reported for n-doped poly(3phenyl-thiophene) and can be explained by a stronger ion pairing between polymer anion radicals and smaller Li+ cations during the doping process.48,49 The interactions between radicals and counterions also push the Eap to a more positive potential when using Li+. An alternative explanation for the shift in potentials, to higher potential using Li+, would be that the free energy of solvated Li-ions is higher than for TBA+. This, however, suggests a strong dependence of solvent polarity and as the potential differences between PC and MeCN are very modest for the same salt this explanation is unlikely. The successful doping of P25Py in both PC and MeCN suggests that the doping behavior of P25Py is less influenced by solvent polarity compared to, e.g., polythiophene which does not show n-doping when using Li+ cations in PC.46,48 The relatively small shift in onset doping potential of P25Py between Li+ and TBA+ salts, i.e., 120 and 140 meV in MeCN and in PC, respectively, compared to the corresponding shift of almost 1.0 V observed in poly(3-phenyl-thiophene)48 also shows an insensitivity toward dopant ion on the electrochemical performance. Taken together these results show that polypyridine may provide a versatile and robust alternative for electric energy storage applications in particular together with Li+ based electrolytes. Figure 3a shows the cyclic voltammograms of P25Py as a function of scan rate in 0.1 M TBAPF6 in MeCN together with the scan rate dependence of the extracted peak currents (at E = Eap) in both MeCN and PC with 0.1 M TBAPF6. (Note that the currents in the voltammograms have been normalized with

Figure 3. (a) Cyclic voltammograms of chemically prepared P25Py at 10, 30, 50, 70, and 100 mV s−1 (black line) and poly(3-phenylthiophene) (red line) in 0.1 M TBAPF6 in MeCN. (The arrows indicate the effect of increased scan rate.) The inset shows log−log presentation of peak current vs scan rate in MeCN (black) and PC (red) with 0.1 M TBAPF6 together with a linear fit to the data (solid lines). (b) The theoretical capacity of a selection of redox active groups, which have been tested as battery materials, as a function of potential. (Theoretical capacities as well as redox potentials have been taken from ref 51.) The redox groups has been divided into three different categories, dicarboxylates (purple), quinones (blue), and dithiols (yellow), and exemplifying structures are included in the figure. 25959



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Figure 4. (a) Capacity of chemically prepared P25Py as a function of scan rate and (b) cycling stability of P25Py at 50 mV s−1, in different conditions: MeCN (black line) with 0.1 M LiClO4 (open symbol) and 0.1 M TBAPF6 (filled symbol); PC (red line) with 0.1 M LiClO4 (open symbol) and 0.1 M TBAPF6 (filled symbol). The capacity values in b were calculated from the reoxidation process.

Figure 5. (a) Cyclic voltammograms of chemically prepared P25Py at 1 mV s−1 in the potential window 1.2−2.0 V vs Li0/Li+, the arrow (red) indicates the increased oxidation current during cycling and (b) cycling stability of a Li/P25Py cycled galvanostatically between 1.4 and 2.0 V at a rate of C/20 in 1 M LiClO4/PC in a two electrode battery configuration. The inset in panel b presents the first six galvanostatic charge−discharge cycles.

in three-electrode setup using electrochemically polymerized material. Figure 6a shows Nyquist plots illustrating the EIS response of the P25Py electrode at different potentials. In Figure 6b,c the response at the different potentials are magnified and selected frequency points are indicated by arrows. The inset in panel a shows a typical voltammogram recorded using the same setup as in the EIS measurements to allow for assessment of the relation between the applied potentials and the doping state of the material. The EIS response at potentials where the material is doped (−1.46, −1.86, and −2.06 V vs Fc0/Fc+) was found to be similar to that found for polypyrrole-based electrodes in an aqueous NaNO3 containing electrolyte54 described by a Randles equivalent circuit.55 From such a circuit the ac response of a system can be described on the basis of a resistance Rhf (evaluated from the high frequency intersect, or the extrapolation thereof, with the real axis of the Nyquist plots) in series with a circuit containing a charge transfer resistance Rct, a diffusion element of finite-length Warburg type, ZFLW,56 stemming from diffusion of the TBA+ ions in the electrode material, and a double layer capacitance (connected in parallel with Rct and ZFLW), represented by a constant phase element (CPE).57 The finite-length diffusion element dominates the response in the low frequency region of the Nyquist plots in which the TBA+ ions diffusing into the electroactive P25Py layer (yielding a 45° slope) become blocked by the current collector at the lowest frequencies (the almost vertical

systems LiClO4/MeCN, TBAPF6/MeCN, LiClO4/PC, and TBAPF6/PC, respectively. The instability in LiClO4 with MeCN solvent is probably caused by degradation of the polymer and/or the instability of the electrolyte when using nickel foam as current collector. 3.3. Battery Cell with Poly(2,5-pyridine). Also in a battery cell configuration with Li-metal as negative electrode and P25Py as positive electrode the polymer shows reversible cycling behavior as judged from galvanostatic cycling (Figure 5a) between 2.0 and 1.2 V vs Li0/Li+. The capacities and redox behavior are comparable to the measurements in the threeelectrode setup, indicating that those results are directly applicable to battery cell configurations. During galvanostatic cycling in battery configuration (Figure 5b), there is a significant initial capacity drop between the first discharge and second cycle which might be due to unreacted oligomers or monomers from the synthesis. This can also explain the initial difference in charge and discharge capacities. The battery then experiences a gradual capacity increase during the first 50 cycles to a stable value of ca. 2 mAh g−1, which might be attributed to a structural relaxation process in the electrode enabling more and more of the polymer accessible for the redox processes during cycling due to increased electrolyte penetration. 3.4. Impedance of Poly(2,5-pyridine). Neutral conjugated polymers become conductive upon doping chemically or electrochemically.52,53 To further characterize the electrochemical behavior of P25Py, impedance spectroscopy was used 25960



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Figure 6. Nyquist plots of electrochemically prepared P25Py at different offset potentials vs Fc0/Fc+. In panels b and c, the high frequency regions in panel a are magnified. A cyclic voltammogram of the P25Py polymer under study recorded at a scan rate of 0.1 V s−1 is displayed in the inset of panel a.

expansion may explain a facilitated ionic charge transport process inside the polymer film upon increased doping.

line there indicates a dominating capacitive behavior). The double layer CPE takes into account the fact that the double layer capacitance is generated a surface that is not perfectly flat. The semicircle observed at high frequencies, which radius equals Rct, is hence centered slightly below the horizontal axis. The response for the as-synthesized, nondoped material (offset potential −1.06 V vs Fc0/Fc+) can be described by the same circuit where only the first part of the semicircle is visible due to the high charge transfer resistance at this potential in line with the EIS response of other ion insertion materials at low conductivity offset potentials.58 Below the observed EIS response is discussed in more detail. First we observe that Rhf is rather independent of offset potentials and estimated to about ∼7 Ω. This is to be expected since it is the potential-independent electrolyte resistance that exerts the major contribution to this resistance.54 We further find that Rct, extracted by estimating the radius of the semicircle, decreases with increasing doping level, in line with the fact that the material becomes more conductive. We finally observe that the onset frequency for the capacitive, low frequency behavior, i.e. the frequency at which the semi-infinite diffusion related response (45° slope) starts to transform into a finite-length Warburg diffusion response (more vertical line), increases with increasing reductive offset potential. This is a very interesting observation indicating that the diffusion coefficient of the TBA+ ions in the P25Py electrode increases with increasing doping of the polymer. This deduction can be made since the onset frequency of the finite length Warburg response is only depending on the thickness of the film and the diffusion coefficient of the moving species inside the film.57 Electroactive, conducting polymers of the type under study are known to expand with increasing doping level.53 Such

4. CONCLUSIONS Pyridine-based conducting polymers have been prepared chemically and electrochemically using dehalogenation polycondensation, and the electrochemical performance of the resulting polymers has been investigated in MeCN and PC using different electrolyte salts. Reversible n-doping of P25Py was found in MeCN and in PC using both Li+- and TBA+-based electrolyte salts that resulted in enhanced conductivity in the material indicative of the introduction of mobile charge carriers in the polymer upon reduction. In both three-electrode setup and in a battery configuration using Li-metal as negative electrode the polymers show good cycling stability and charge retention even without carbon additives, and taken together with the insensitivity to electrolyte system as well as the polymer potential window, P 25Py provides a versatile conducting polymer base for electrical energy storage applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Swedish Foundation for Strategic Research, the Swedish Research Council, the Carl Trygger Foundation, the Swedish Energy Agency, and the European Institute of Innovation and Technology. L.Y. gratefully 25961



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acknowledges the China Scholarship Council (CSC) for a Ph.D. study fellowship.

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Conjugated Pyridine-Based Polymers Characterized as Conductivity

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