Conjugated Nitroxide Radical Polymers: Synthesis and Application in

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Conjugated Nitroxide Radical Polymers: Synthesis and Application in Flexible Energy Storage Devices Yuan Xie,† Kai Zhang,† Michael J. Monteiro, and Zhongfan Jia* Australian Institute for Bioengineering and Nanotechnology, University of Queensland, Brisbane, Queensland 4072, Australia

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S Supporting Information *

ABSTRACT: The synthesis and electrochemical behavior of nitroxide radical conjugated polymers (NCPs) have long been an intriguing topic in redox polymer-based energy storage. However, common (electro)chemical oxidation polymerization methods have proved difficult in the synthesis of well-defined NCPs, and many of these polymers have been difficult to process into thin films. In addition to these drawbacks and coupled with the complex charge-transfer and storage mechanisms, the use of NCPs as electrodes has been significantly limited. The aim of this work is to provide mechanistic insights into this complex charge-transfer and storage process using a new and well-defined NCP synthesized using an ultrafast cyclopolymerization with the Grubbs 3rd generation catalyst. The monomer, consisting of a 1,6heptadiyne group and a TEMPO (i.e. 2,2,6,6-tetramethylpiperidine-1-oxy) radical, through the cyclopolymerization produced a well-defined NCP with a five-membered ring-containing polyene backbone. This polymer demonstrated excellent film formation properties, allowing the study of their thin-film electrochemical behavior. We found that the electrochemical oxidation of the conjugated backbone and its internal charge transfer to the nitroxide radicals were strongly affected by the applied potential window, current densities, and cycle numbers. Using these new insights, we successfully utilized our NCPs in a flexible energy storage device by fabricating high-performance NCP-coated carbon cloth-based flexible electrodes. KEYWORDS: nitroxide radical, conjugated polymer, electron transfer, cyclopolymerization, energy storage

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INTRODUCTION Over the past two decades, polymers with pendent redox groups have attracted considerable attention as electroactive materials in flexible energy storage devices.1,2 Among the different redox groups, nitroxide radical polymers are of great interest3,4 primarily because of their higher redox potential5,6 and faster redox kinetics that afford batteries with high energy and power outputs compared to many other organic redox groups.7 In addition, the ability to modify the structure of nitroxide radical monomers8 and the composition of polymer backbones allows the synthesis of nitroxide radical polymers with versatile chemical and physical properties,9−12 providing a way to fine-tune and optimize their electrochemical properties.13−17 However, nitroxide radical polymers with an aliphatic backbone generally produce only a low electric conductivity. To improve their conductivity, the addition of the conductive carbon or the formation of percolating domains through thermal annealing to facilitate charge diffusion is often required.18−20 It is therefore not surprising that one would intuitively synthesize nitroxide radical polymers consisting of a conjugated backbone to overcome these limitations to produce polymers with greater electrical conductivity resulting in more efficient and faster charge transfer between the nitroxide radicals. © XXXX American Chemical Society

In the past decade, NCPs with different conjugated backbones such as polyacetylene (PA),21−23 polythiophene (PT),18,24,25 polypyrrole (PP),26,27 and poly(3,4-ethylenedioxylthiophene) (PEDOT)28 have been reported. NCPs with PT, PP, and PEDOT backbones are mostly synthesized through electrochemical polymerization directly on an electrode surface18,24−29 with little or no control over the molecular weight. Although the copper-catalyzed alkyne−azide “click” reaction has been applied as an alternative method to prepare NCPs with a PT backbone,18 polymers with a high radical content are not fully soluble in many solvents.19 The synthesis of NCPs with a PA backbone through metal-coordinated polymerization using Rh-based catalysts requires prolong polymerization times (i.e., 24 h) and produces polymers with high dispersities, poor to fair yields, and even an insoluble fraction.21,22,30 It seems that current synthetic methods cannot produce well-defined NCPs, which restricts their applications in different types of energy storage. Surprisingly, it has been found that when using these NCPs as cathode materials in traditional lithium-ion coin cells, there Received: December 4, 2018 Accepted: January 28, 2019 Published: January 28, 2019 A

DOI: 10.1021/acsami.8b21073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Synthesis of 1,6-heptadiyne functional nitroxide radical monomer 1 and the corresponding polymers through cyclopolymerization with the Grubbs 3rd generation catalyst, (b) SEC traces of polymers with [1]/[3rd Grubbs] = 50/1 sampled at different polymerization times, (c) SEC traces of polymers at different [1]/[3rd Grubbs] ratios with monomer concentration [1] of 0.2 M and polymerization time of 15 min at −15 °C, and (d) ATR-IR and (e) ESR spectra of monomers 1 and P1 in solid state, [1]/[3rd Grubbs] = 100/1.

good solubility in organic solvents such as tetrahydrofuran (THF). These NCP polymers could easily form films on different electrode surfaces that had negligible solubility in the common electrolyte solution. NCPs with these properties not only provide an ideal model for electrochemical studies but also have the potential to be applied in flexible energy storage devices via a simply dip-coating method.

has been no improvements in the battery performance, especially in improving their capacity, rate capability, or cycling stability.29,31 The reason for this lack of improvement could stem from the poorly defined NCP microstructures and the complex electrochemical process of conjugated NCPs. Recent studies on NCP film electrodes prepared through electrochemistry oxidation polymerization revealed an internal charge transfer (ICT) between the nitroxide radicals and conjugated backbone because of their redox potential difference.32,33 In fact, the redox potential of the conjugated backbone depends on the degree of (electro)chemical doping which often overlaps with the redox potentials of nitroxide radicals.34−36 This is further complicated by the rapid redox kinetics of nitroxide radicals and the sluggish redox kinetics of the conjugated backbone, a process to the best of our knowledge that is yet well elucidated. On the basis of the current literature results, there seems to be importance for the choice of pendent radicals that could work in synergy with the conjugated backbone in applications for electrochemical energy storage.32,33 The aim of the work here is to gain a mechanistic understanding of the electrochemical process of NCP film electrodes by using a new and well-defined NCP. Cyclopolymerization of 1,6-heptadiyne monomers catalyzed by Grubbs 3rd generation catalyst is an efficient living polymerization technique to synthesize well-defined polymers (i.e., controlled molecular weight and narrow dispersity) with a conjugated polyene backbone.37,38 The polymerization method has a tolerance to a variety of functionalities, and the resulting polymer with the five-membered ring-containing conjugated backbone possesses excellent film-forming properties, an ideal material for film electrode application. The polymerization was rapid, producing polymers with excellent control over both the number-average molecular weight (Mn) and molecular weight distribution (MWD), but more importantly, a polymer with

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RESULTS AND DISCUSSION

Synthesis of Nitroxide Radical Conjugated Poly(1,6Heptadiyne). Polymerization of 1,6-hetadiyne derivatives through metathesis cyclopolymerization is now a wellestablished procedure to produce conjugated polymers with a polyene backbone using a series of different catalysts (i.e., Pd and Mo).39 However, these catalysts usually require elevated temperature and prolonged polymerization time, with the production of broad MWD polymers and in some cases insoluble fractions.22 Recently, the polymerization of 1,6hetadiyne derivatives to produce well-controlled conjugated polymers has been further developed by using the third generation of Grubbs catalyst, resulting in a fast and living polymerization.37,38 This method, therefore, has the potential to be used to prepare new NCPs. We first designed and synthesized a 1,6-hetpadiyne functional TEMPO radical monomer by coupling 1,6-hetptadiyne malonic acid with 4-hydroxyl-TEMPO (Figure 1a). However, the reaction produced exclusive mono-TEMPO functional product 1 rather than expected di-TEMPO product because of the decarboxylation of malonic acid (Figure S3).40 Polymerization of monomer 1 was carried out using the Grubbs 3rd generation catalyst in THF. The first polymerization at 25 °C ([1]/[Grubbs 3rd] ratio of 50/1) instantaneously produced a gel upon addition of the initiator. Decreasing the reaction B


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ACS Applied Materials & Interfaces Table 1. Electrochemistry Data of P1 Thin Film on Glass Carbon Electrode with Different Thicknesses Γ (mol cm−2)a

electrode

−8

1.17 × 10 4.73 × 10−8 9.79 × 10−8

1 2 3

φ (nm)b 34 137 284

Dapp (cm2 s−1)c −10

3.75 × 10 3.45 × 10−10 3.84 × 10−10

a

Surface concentration of P1 calculated based on the Randles−Sevcik equation i p =

through equation φ = equation i =

ΓM w c . Apparent ρ

nFACDapp d . Surface (πt )0.5

Qareal (μA h cm−2)d

Qmass (mA h g−1)e

0.42 1.38 3.64

123 105 128

n2F 2A Γυ b . Thin-film 4RT

thickness on the electrode calculated

diffusion constant Dapp for the homogeneous charge transport throughout the film calculated from Cottrell

capacity from galvanostatic charge/discharge. eCalculated capacity of thin film at different thicknesses according to

equation Q mas = Q areal ΓM w . Where ip is the peak current (mA), n is the number of charge transfer, F is the Faradaic constant, A is the surface area of the electrode (cm2), Γ is the surface concentration (mol cm−2), ν is the scan rate (mV s−1), R is the gas constant, T is the temperature (K), Mw is the molecular weight of monomer 1 (290 g mol−1), ρ is the density of P1 (assuming 1.0 g cm−3), C is the concentration of redox active molecules (mol cm−3) which was calculated from C = Γ/φ, Dapp is the apparent diffusion constant (cm2 s−1), and t is the time (s). The theoretical capacity of P1 is 92 mA h g−1.

Figure 2. Galvanostatic charge/discharge curves (a,c,e) for the P1 thin-film electrode, and the areal capacity and Coulombic efficiency versus the various current densities (b,d,f). The film thickness was 34 nm for (a,b), 137 nm for (c,d), and 284 nm for (e,f). The supporting electrolyte was a solution of 0.1 M Bu4NPF6 in CH3CN.

temperature to 0 °C resulted in a very broad MWD (data not shown), but only when the temperature was decreased to −15 °C did we observe the formation of nitroxide radical functional poly(1,6-heptadiyne) with desired molecular weight and narrow MWD. The data suggest that at higher temperatures, chain transfer to monomer or polymer may be a competing kinetic process, consistent with the findings from previous reports.37,38 The polymerization rapidly reached nearly quantitative conversion because the size exclusion chromatography (SEC) traces in Figure 1b showed no change after 2 min and nearly full loss of monomer peak from SEC (Figure S4). The Mns were relatively constant but lower than the theoretical value because of the SEC calibration using polystyrene standards (Table S1). With longer reaction times, the slight broadening of the SEC traces suggested chain-transfer side

reactions. This can be avoided by stopping the polymerization after 15 min. The molecular weight of the polymer can be easily controlled by varying the monomer to catalyst ratio. Figure 1c shows the SEC traces of the polymerization with the [1]/[Grubbs 3rd] ratios of 25, 50, and 100. The Mns were close to the theoretical values with narrow MWD (D̵ < 1.15). The dispersity increased to 1.35 when the [1]/[Grubbs 3rd] ratio was increased to 200 (Table S1), indicating the influence of chain transfer when targeting a high molecular weight. Because the polymerization with the [1]/[3rd Grubbs] ratio of 100 produced polymer with both a high molecular weight and a narrow MWD (i.e., Mn of 26 900, Đ = 1.13), the following studies were based on this polymer (denoted as P1). The high conversion was also confirmed by ATR-IR (Figure 1d) through the loss of alkyne peaks at 3220 and 3260 cm−1 for the polymer C


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105 μA h cm−2 (Figure 2b), suggesting that more charges can be discharged at high density of current flow. The similar but less distinct phenomenon was observed with the film thickness of 137 nm (Figure 2c,d). When the film thickness was increased to 284 nm, charging at low current densities took a very long time and did not reach the targeting potential owing to backbone oxidation (data not shown). Increasing the current densities resulted in a capacity decrease but a Coulombic efficiency increase (Figure 2e,f). Overall, an increase of Coulombic efficiency from 50% to nearly 100% was observed for all three electrodes when the current densities increased from 3.5 to 700 μA h cm−2, suggesting the high rate charge transfer and reversible charge storage from nitroxide radicals.1 The areal capacities for the thin films with thicknesses (φ) of 34, 137, and 284 nm at high current densities (i.e., 35, 70, and 140 μA cm−2) were about 0.42, 1.38, and 3.64 μA h cm−2, respectively, corresponding to specific capacities of 123, 105, and 128 mA h g−1 at the rates of 165, 50, and 20 C (Table 1). For the film with thickness of 137 nm, a stable discharge capacity of 1.3 μA h cm−2 was delivered at various current densities ranging from 14 to 140 μA cm−2 (equivalent to 10 to 100 C) when repeatedly charged at the same current density of 14 μA cm−2. This demonstrated a much higher rate performance of P1 film electrode compared to previous work (Table S2).33 Potential Effect on Charge Transfer and Storage. Given the electrochemical oxidation of polyene taking place over a broad potential range and being pronounced at a potential window higher than that for the TEMPO radical (Figure S9), understanding the redox kinetics of the P1 in a broad potential window could be of great importance not only to the charge transfer and storage mechanism but also for the stability of the thin-film electrode.46 A P1 thin-film electrode replicating the one with thickness of about 34 nm was then used in CV and CP experiments within a broader potential window. We first carried out a CV experiment for a freshprepared P1 thin-film electrode with a potential window of 0− 1.0 V (vs Fc/Fc+) at scan rates from 500 to 0.01 mV s−1. With a decrease in the scan rate, we clearly observed the increase of a broad irreversible oxidation peak from 0.5 to 1.0 V, in which the current became weak in the second cycle (Figure 3a−d). Remarkably, when the scan rate was below 0.1 mV s−1, the redox peak of nitroxide radicals almost disappeared (Figure 3e,f), resulting in a totally irreversible oxidation of polyene backbone. Because the above observations provided evidence of the prominent backbone redox reaction particularly at high potential and low scan rates, we next explored how the varied potential biases impact on the charge/discharge of the film electrode. Figure 4a depicts the first two charge/discharge cycles within different potential ranges at the current density of 35 μA cm−2. At the potential range of 0.25−0.65 V, the curves were symmetrical with only one plateau at 0.4 V versus Fc/Fc+ corresponding to the redox of nitroxide radicals. By gradually increasing the potential cutoff from 0.75 to 1.05 V, we observed a second charge slope in a broad potential range from 0.65 to 1.05 V with no discharge process being observed in this potential range. This led to a decrease in Coulombic efficiency with an increase of potential cutoff for the first cycle and a recovery of the Coulombic efficiency for the second cycle (Figure 4b). When cycling the electrodes in a small potential window of 0.25−0.55 V, the charge/discharge curves were very

comparing to the monomer 1 immediately after the polymerization. In addition, the radical content in P1 is also determined from the integration of electron spin resonance (ESR) spectra of both monomers 1 and P1 (Figure 1e), giving a 90% radical content with a very high radical retention. Apparent Diffusion Constant in Thin Film. We first studied the thin-film formation of P1. Scanning electron microscopy (SEM) micrographs of the P1 films formed on a silicon wafer revealed that a uniform polymer film was formed when the concentration of P1 was greater than 1.0 mg/mL, whereas at low concentration (0.1 mg/mL), only scattered nanoparticles were observed (Figure S5). Thin-film electrodes were then prepared from P1 in THF (1.0−2.5 mg/mL) onto the surface of glass carbon electrode by a drop-casting method.41,42 Cyclic voltammogram (CV) of the thin-film electrode in CH3CN demonstrated a symmetrical oxidation peak (Epa) and reduction peak (Epc) with ΔEp (Epa − Epc) less than 30 mV at the scan rates (ν) up to 10 mV s−1 (Figure S6a, inset), and the peak current ip increased linearly versus the scan rate (Figure S6a). Using the Randles−Svecik equation, the actual surface concentration Γ* of nitroxide radicals and film thickness (φ) can be calculated by assuming a density of P1 equal to 1 g/cm3. In this work, three P1 film electrodes with different thicknesses of 34, 137, and 284 nm were prepared on the glass carbon surface (Table 1). A log−log plot of scan rates (ν) versus the peak current (ip) showed slope values of 0.45 for 1 and 0.93 for P1 (Figure S7), respectively, suggesting a semiinfinitive diffusion for monomer 1 in solution and a surface electrochemistry for the P1-coated film. Similar trends were also found on Au and Pt electrodes (Figure S8). Taken together, the results show excellent film formation of P1 on different surfaces with negligible solubility and good swellability of the film in the organic electrolyte, in which the latter allows the electrolyte ions to diffuse close to the radical sites and is of critical importance for the highly efficient redox reaction as recently reported.43 A potential-step chronoamperometry experiment was carried out for all film electrodes (Figure S6), and the apparent diffusion constants (Dapp) were determined using the Cottrell equation.44,45 It was found that for all three film electrodes, the average Dapp values were very similar at 3.7 × 10−10 cm2 s−1 (Table 1), which is close to that found in a cross-linked nonconjugated nitroxide radical polymer film electrode.44 This suggested that a polyene backbone did not significantly compromise the electron selfexchange nor improved the electron transfer in comparison to nonconjugated radical polymers.43 Current Density Effect on Charge Transfer and Storage. The pendant groups on the conjugated polymer can significantly reduce the conductance of the backbone,19,36,46 which could be one of the reasons that the redox kinetics of conjugated polymers becomes very sluggish.47 This raises the question as to how the redox reaction of conjugated backbone and nitroxide radicals respond to different current densities. To study this effect, a chronopotentiometry (CP) experiment was carried out for the P1coated thin-film electrodes with a potential window between 0.25 and 0.65 V (vs Fc/Fc+). Figure 2a depicts the charge− discharge curves with the film thickness of 34 nm. It was noted that the charge capacities decreased with the increasing of current densities from 3.5 to 105 μA cm−2 as found for most of the redox polymer electrodes. Notably, the discharge capacities initially increased and approached close to the charge capacities and then decreased at a high current density of D


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ICT and possibly side reactions. Such a process could be strongly affected by the applied potential window and current density because of their different redox potentials, kinetics, and reversibility of nitroxide radicals and conjugated backbone. From the above experiments, we clearly observed that at high current densities in a narrow potential window (i.e., 0.25−0.65 V vs Fc/Fc+), the high Coulombic efficiency can always be achieved. In this case, a highly reversible redox couple P1/P1+ was mainly contributed by nitroxide radicals owing to their fast redox kinetics, that is, redox process I as illustrated in Scheme 1a. Whereas at low current densities, P1 could be oxidized to P1+-Polaron due to the partial oxidization of polyene backbone (Scheme 1) because the redox potential window of conjugated polymer backbone partially overlaps with that of nitroxide radicals as found in P1 (Figure S9) and other NCPs.18,34,35 Owing to the irreversible redox of polyene and ICT from polyene to oxidized nitroxide radicals, that is, oxoammonium cations, a much less charges can be discharged and a ring-stabilized P1-Soliton may form. Moreover, the carbon-centered radical on the backbone of P1+-Polaron could be rapidly trapped by nitroxide radicals to form an alkoxyamine bond through a known nitroxide radical coupling (NRC) reaction as previously reported in our group.48,49 This hypothesis was further supported by the CV experiments which a fast-slow-fast scan (i.e., 500, 0.1, and 500 mV/s) was carried out at two different potential windows for P1 electrodes (Figure S10). For the potential range of 0.25− 0.65 V, the peak currents were the same at the scan rate of 500 mV/s before and after the slow scan at 0.1 mV/s. Whereas for the potential range of 0−1.0 V, the current at 500 mV/s after the slow scan (0.1 mV/s) was 5.7 × 10−5 A, much lower than that of before the slow scan (1.5 × 10−4 A), indicating a possible side NRC reaction between the nitroxide radicals and backbone. Such processes eventually led to an overall irreversible redox couple P1-Soliton/P1+-Polaron, that is, redox process II as depicted in Scheme 1b. Although the proposed mechanism explained the electrochemical behaviors of the P1 film electrode and is on pair with previous studies,

Figure 3. Evolution of CV curves for P1 thin-film electrode with the thickness of about 34 nm with a potential window of 0−1 V vs Fc/Fc+ at the different scan rates (a) 500, (b) 100, (c) 10, (d) 1, (e) 0.1, and (f) 0.01 mV s−1. Red curves were from the first cycle, and the blue traces were from the second cycle. The supporting electrolyte was a solution of 0.1 M Bu4NPF6 in CH3CN, in which E0 for Fc/Fc+ was 30 mV vs Ag/AgNO3 reference electrode.

symmetrical over 20 cycles with a stable Coulombic efficiency of 95% (Figure 4c), whereas cycling within a potential window of 0.25−1.05 V as shown in Figure 4d, the broad charge slope from 0.65 to 1.05 V disappeared over the cycling with the gradual increase in the symmetry of the charge/discharge curves, resulting in an increase in Coulombic efficiency from 57 to 95%. It seems that the irreversible electrochemical oxidation of conjugated backbone can eventually fade after cycling, a process yet reported. Proposed Charge Transfer and Storage Mechanism. The above electrochemical studies on the film electrode revealed a complicated redox behavior of NCPs, including an

Figure 4. (a) Galvanostatic charge/discharge curves and (b) Coulombic efficiency for the first and second cycles of P1 thin-film electrode with the thickness of about 34 nm at the different potential windows; and cycling profile for the electrode at potential window of (c) 0.25−0.55 V and (d) 0.25−1.05 V vs Fc/Fc+, the cycling number was shown on the top of the curves. The supporting electrolyte was a solution of 0.1 M Bu4NPF6 in CH3CN. E


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Scheme 1. Schematic Illustration of the Charge-Transfer Mechanism in P1 Thin Film (a) Rapid and Reversible Charge Transfer on Nitroxide Radicals with P1/P1+ Redox Couple at the High Current Densities and Narrow Potential Window, and (b) Irreversible Redox of Polyene Backbone and Nitroxide Radicals Resulting from NRC Reaction and ICT Leading to an Overall Irreversible Redox Reaction of P1 at the Low Current Densities and/or Broad Potential Window

Figure 5. Galvanostatic charge/discharge curves (a,c) and rate performance (b,d) of P1 electrode prepared by drop-coating of different amounts of polymer solution on CCC (a,b) and CFC (c,d) substrates, drying under vacuum and rinsing in CH3CN before testing. The current density was 250 μA cm−2. The supporting electrolyte was a solution of 0.1 M Bu4NPF6 in CH3CN.

we further demonstrated that at high current densities with a broad potential range, the charging capacity contributed by the conjugated backbone can gradually disappear after cycling and result in a recover of Coulombic efficiency (Figure 4d). Carbon Cloth@P1 Flexible Electrode. Using NCPs as cathodes in a traditional lithium-ion battery has yet achieved the expected superior battery performance, and in many cases, battery performance is even worse than that for nitroxide radical polymers with a nonconjugated backbone. Nevertheless, the excellent film formation and inherent low solubility may allow the application of NCPs in other types of energy storage devices. To exploit this, P1 solution was then coated on different flexible current collectors, that is, carbon-coated cloth (CCC) and carbon fiber cloth (CFC) to fabricate the flexible electrodes. Without a polymer loading, the blank CCC acts as a capacitor because of the coated carbon layer and

exhibited a typical capacitor triangle charge/discharge profile. After coating with P1 at 125 μg/cm2, the charge/discharge curves consisted of a plateau at about 0.4 V (vs Fc/Fc+) belonging to nitroxide radicals and a triangular capacitance from the CCC, which formed a pseudocapacitor electrode working at the current densities ranging from 100 to 1000 μA cm−2 (Figure S11). Increasing the polymer loading to 250 μg cm−2 produced a plateau capacity that also increased, and the electrode delivered a total discharge capacity of 10.2 to 18.6 μA h cm−2 (Figure 5a). The capacity reached a maximum and remained unchanged when the loading mass was higher than 375 μg cm−2 because of the saturation of polymers on the carbon surface (Figure 5b). Whereas for the P1-coated CFC electrode, the charge/discharge profile only shows a Faradaic energy storage from P1 because of very low capacitance storage from CFC (Figures 5c and S12). This CFC electrode F



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can deliver a capacity of 2.0 to 10.7 μA h cm−2 at current density of 250 μA cm−2 depending on the polymer loading (Figure 5d), where the capacity was nearly 10-fold higher at the current density of 25-fold higher than that of a NCP electrode with PT backbone in an earlier report.33 In addition, when loading with 250 μg cm−2, this P1-coated CCC electrode can deliver 90% of initial capacity after 100 cycles at the current density of 250 μA cm−2 (Figure S13). This facile dropcoating strategy may be applied on different types of flexible conductive substrates as high-performance electrodes for pseudocapacitor or battery.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael J. Monteiro: 0000-0001-5624-7115 Zhongfan Jia: 0000-0001-9690-7288 Author Contributions †

Y.X. and K.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.J. acknowledges the financial support from the ARC Future Fellowship, and the award of Advanced Queensland Fellowship (AQF) from Queensland Government and Foundation Research Excellent Award (FREA) from the University of Queensland. M.J.M. acknowledges the financial support from the ARC Discovery grant. This work was performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers.

EXPERIMENTAL SECTION

All the experiments can be found in the Supporting Information.

CONCLUSIONS In conclusion, we have successfully synthesized and polymerized within minutes a new nitroxide radical functional 1,6heptadiyne monomer 1 through cyclopolymerization using the Grubbs 3rd generation catalyst within minutes. The resultant NCP P1 with well-controlled molecular weights and low dispersities (Đ < 1.15) has demonstrated excellent filmforming capability on different electrode surfaces, and insolubility but good swellability in the electrolyte solution. These properties allow the electrochemical studies on the film electrodes and revealed that the conjugated polyene backbone in P1 did not increase the apparent charge diffusion constant of nitroxide radicals but significantly influenced the charge transfer and storage efficacy of the P1 electrode. A fast potential change within a narrow window and/or large current flux led to a rapid and reversible charge transfer and storage mainly based on nitroxide radical redox couple P1/P1+. This process provided a stable discharge areal capacity of 1.3 to 4.4 μA h cm−2 at current density ranging from 10 to 100 C. Conversely, a slow scan within a broad potential window and/or small current flow resulted in continuous irreversible electrochemical oxidation of polyene backbone and intramolecular charge transfer, that is, a P1-Soliton/P1+-Polaron redox couple, leading to a low Coulombic efficiency (57− 80%). We also found that the low Coulombic efficiency can be recovered to nearly 95% after cycling of the electrode. At an extremely slow electrode process, continuous oxidation of polyene backbone led to the total loss of the charge storage capability. With these new insights, we successfully demonstrated the utilization of NCP to fabricate flexible working electrodes on a CCC and carbon fiber cloth, respectively. Our work not only produced well-defined NCPs but also provided new insights into the electrocehmical properties of these NCP thin-film electrodes through utilizing the excellent filmformation property of conjugated backbone, representing an advance in the applications of NCPs in flexible energy storage.

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REFERENCES

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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/acsami.8b21073. Materials and methods, synthesis procedure, NMR, SEC, SEM, and electrochemical characterization data of monomer 1 and polymer P1 (PDF) G


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Conjugated Nitroxide Radical Polymers: Synthesis and Application in

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