Hydrophilic Organic Redox-Active Polymer Nanoparticles for Higher

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Hydrophilic Organic Redox-active Polymer Nanoparticles for Higher Energy Density Flow Batteries Kan Hatakeyama-Sato, Takashi Nagano, Shiori Noguchi, Yota Sugai, Jie Du, Hiroyuki Nishide, and Kenichi Oyaizu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00074 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Applied Polymer Materials

Hydrophilic Organic Redox-active Polymer Nanoparticles for Higher Energy Density Flow Batteries Kan Hatakeyama-Sato, Takashi Nagano, Shiori Noguchi, Yota Sugai, Jie Du, Hiroyuki Nishide, and Kenichi Oyaizu* Department of Applied Chemistry and Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan. *[email protected] KEYWORDS organic battery, redox flow battery, redox-active polymer, radical polymer, energy storage

ABSTRACT

Hydrophilic redox-active polymer nanoparticles with different redox potentials and radii were synthesized via the dispersion polymerization to yield their stable dispersion in aqueous electrolyte media as promising catholytes and anolytes in redox flow batteries. Despite the small physical diffusion coefficient (10-9 cm2/s) of the nanosized particles, the sufficiently large coefficient for charge transfer within the polymer particle dispersion (10-7 cm2/s) was observed as a result of the

1 ACS Paragon Plus Environment

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fast electron propagation throughout the polymer particles. Redox flow cells were fabricated using TEMPO- viologen-, or diazaanthraquinone-substituted polymer nanoparticles as active materials. The reversible charge/discharge over 50 cycles was achieved even at a high concentration of the redox units (1.5 M), which exceeded the limitation of the solubility of the corresponding dissolved species.

Introduction Organic redox flow batteries (RFBs), using the organic redox-active molecules for charge storage, have been attracting substantial attention for stationary electricity storage of renewable energy, due to their environmental compatibility, scalability, low cost, and promising electrochemical performances.1-5 The representative organic redox-active molecules, such as anthraquinone, viologen, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and their derivatives are known to provide the reversible charge/discharge over 103 cycles, high Coulombic efficiency (> 99%), and moderate energy density (101 Wh/L).1-2,

6-9

The compounds consist of abundant

elements (H, C, N, O, and S) while the conventional vanadium RFBs and lithium ion batteries (LIBs) require a large number of expensive, resource-limited elements (e.g., V, Li, Co, and Ni).2 Further, the hydrophilic design of the organic molecules allows the use of safe aqueous electrolytes (including brine), not toxic or flammable electrolytes for LIBs or other conventional batteries.6, 8 One drawback of the organic redox-active species could be their limited solubility in the electrolytes. The redox-active units often consist of aromatic (or bulky alkyl) rings to stabilize the charged/discharged states,1, 10 and those low-polarity groups are not sufficiently soluble especially in aqueous polar electrolytes.1, 11 Although the energy density is not the first priority for RFBs, their limited concentration in the aqueous media (typically 5 M),10, 12 leads to the large-scale flowing system to decrease the overall energy-conversion efficiencies. Another important challenge for the organic material-based RFBs should be the appropriate selection of the separators, which prevent the unfavorable crossover of the active materials (causing self-discharging) while helping the facile movement of electrolyte molecules.5,

13-14

Normally, two types of separators, ion-exchange and porous membranes, are available for the RFBs.1-2 For the low-molecular weight molecules and ions, the ion-exchange membranes are introduced to prevent the crossover reactions.1-2 In this configuration, the molecules are functionalized by some cationic or anionic groups to induce the electrostatic repulsion between the polyelectrolyte membranes and the ionized molecules.5 Although this strategy is commercially utilized in vanadium-based RFBs,14 the ion-exchange membranes, represented by Nafion, tend to suffer from their reduced conductivity, limited selectivity of the ionic species to cause the crossover, insufficient mechanical and chemical stability, and high cost.1-2, 6 For polymers and some oligomers having larger hydrodynamic radius, porous membranes are used preferentially as the separators.6, 15-16 Porous membranes have a long history on, for instance, conventional batteries.13 The membranes, having feasible processability at low cost, can provide the high conductivity and mechanical strength as the separators by virtue of their porous yet robust polymer structures (cellulose, polypropylene, et al.).13 With the optimal configuration, the separators can pass only electrolyte molecules but block the redox-active species by the sizeexclusion effect.6 Still, even in this configuration, the crossover reactions are allowed because the dissolved polymers or oligomers only have the non-discrete statistical hydrodynamic radius (i.e., random coil structures).



Page 4 of 27

To break through the problems of the dissolved redox-active species, we herein report on the chemically cross-linked polymer nanoparticles as electrode-active materials. Rodríguez-Lopez et al. opened a way to use the cross-linked particles for porous membranes in RFBs.17-18 Crossover reactions were suppressed due to the size-exclusion effects of the cross-linked, ‘rigid’ polymer particles.17 However, the demonstrated concentration of the electrode materials was low (10 mM), certainly due to the concern of the slow diffusion of the large polymer particles and the limited dispersibility in water by the hydrophobic polystyrene backbones, which were designed for the use in organic electrolytes.17 In this paper, we pursued the hydrophilic molecular design of the organic redox-active particles, using acrylamide groups for the higher dispersibility, and studied their detailed charge transport kinetics. The carefully conducted dispersion polymerization yielded the high dispersible, radius-tunable, and suitably swellable organic nanoparticles in aqueous media. Regardless of the small diffusion coefficient (10-9 cm2/s) of the redox-active particles, the apparent coefficient for charge transfer within the polymer dispersion reached over 10-7 cm2/s. The increase in the mobility was rationalized by a newly proposed charge flux model. Also, the fast charge transfer by the nanoparticles enabled the operation of the organic RFBs even at a high concentration (> 1 M) of the redox moieties, exceeding the concentration limit of the corresponding monomers (Figure 1, Table S1).

Experimental methods Materials. Sodium 4-sulfonatooxy-2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO-SO3Na) was synthesized according to the literature.19 Linear poly(TEMPO-substituted acrylamide) was prepared based on our previous report.20 1,3,5-Tris(bromomethyl)benzene, 4,4'-bipyridyl, acryloyl


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chloride, and N,N'-methylenebisacrylamide were purchased from Tokyo Chemical Industry Co. Potassium peroxodisulfate was obtained from FUJIFILM Wako Pure Chemical Co. Polyethyleneblock-poly(ethylene glycol) (PE-b-PEG) (Mn = 2250, 80 wt% of ethylene oxide) was purchased from Sigma-Aldrich Co. Acetylene black was obtained from Strem Chemical, Inc. N,Ndimethylformamide (DMF), methanol , and other chemicals were purchased from Tokyo Chemical Industry Co., FUJIFILM Wako Pure Chemical Co., and Kanto Chemical Co. All reagents were used as received. Measurements. NMR spectra were recorded by JEOL-ECX 500. Electron spin resonance (ESR) spectra were measured by JEOL JES-TE200. Quantum Design MPMS SQUID-VSM magnetometer was used for the magnetization measurements. Scanning electron microscope (SEM) images were obtained by HITACHI FESEM S-4500S. Dynamic light scattering (DLS) measurements were conducted by Malvern ZETASIZER NANO-ZS. Electrochemical measurements. A conventional potentiostat (BAS ALS 660D) was employed for the electrochemical measurements. All measurements were conducted under an inert atmosphere. A conventional three-electrode system was employed using a platinum coil as a counter and an Ag/AgCl wire as a reference. Platinum disk (diameter of 1.6 mm) was selected as a working. During chronoamperometry, a potential pulse from 0 to 0.9 V (vs. Ag/AgCl) was applied to TEMPO derivatives. A pulse from 0 to -0.7 V was applied to viologen particles. Based on the results, Cottrell plots were used to evaluate the apparent diffusion coefficient for charge transfer. For ESR, ultra-fine Ag (reference) and Pt (working and counter) wires were introduced in the ESR tubes. A test flow cell was fabricated using the conventional cell kit by ElectroCell (Micro Flow Cell). Carbon felt (area of 2 cm×2 cm, EC Frontier Co.) was employed as current



Page 6 of 27

collectors. As a separator, dialysis membrane (cellulose, pore size of 5 nm) was selected. 10 mL of catholyte and anolyte were filled in vials, and flowed at 12 mL/min. Synthesis of TEMPO nanoparticles (P1, P1’, and C-P1). Acrylamide monomer was prepared according to our previous report.21 The monomer (20g, 95 mmol), PE-g-PEG (6 g), and N,N'methylenebisacrylamide (1 g, 6.5 mmol) were added into 630 mL water. The dispersion was prepared by stirring the mixture (300 rpm and 80 oC) for 15 minutes under a nitrogen atmosphere. The mixture was stirred for 12 hours after adding potassium peroxodisulfate as an initiator (0.2 g, 0.74 mmol). The polymerized particles were purified by dialysis in methanol for 7 days and then collected by centrifugation (94% yield). For the oxidation of the piperidines, the polymer particles (5 g) and m-chloroperoxybenzoic acid (10 g) were added into tetrahydrofuran (150 mL) and mixed for 1 hour. The mixture was poured into diethyl ether/hexane (1/2 in vol/vol) for precipitation. Orange powder was obtained after centrifugation and drying under reduced pressure (5.1 g yield). Unpaired electron density by TEMPO was estimated by SQUID (typically 2.41×1021 spin/g, corresponding to the specific capacity of 107 mAh/g, Figure S1).

Similar procedures were

conducted for the particles with difference radii and carbon-containing particles. Elemental analysis indicated the 9.6% introduction of the conductive carbons in the particles. Synthesis of viologen nanoparticles (P2 and C-P2). 1,3,5-Tris(bromomethyl)benzene (0.71 g, 2.0 mmol), 4,4’-bipyridine (0.47 g, 3.0 mmol), and PE-b-PEG (2.0 g) were added to 40 mL DMF. The reaction proceeded by stirring the mixture (250 rpm and 80 oC) for three hours. The particles were dialyzed in methanol (3 days) and water (3 days). Yellowish brown powder was obtained after centrifugation and drying under reduced pressure (94% yield). 1H NMR (500 MHz, D2O, soluble part):  9.11 (d, J = 6.80 Hz, 6H, pyridinium), 8.30 (d, J = 7.37 Hz, 6H, pyridinium), 8.01 (s, 3H, Ph), 5.89 (s, 6H, CH2). A similar procedure was conducted for the synthesis of carbon-


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containing particles, C-P2. Elemental analysis indicated the 9.5% introduction of the conductive carbons in the particles. Synthesis of diazaanthraquinone nanoparticles (P3). To the solution of 5-amino-1,4diazaanthraquinone (0.7 g, 2.5 mmol)22 in DMF (60 mL), acryloyl chloride (0.5 mL, 7.5 mmol) was added dropwisely at 0 oC under a nitrogen atmosphere. The mixture was stirred overnight at room temperature. The monomer was obtained as the brown powder after washing the byproducts by water and acetone (64% yield). 1H NMR (500 MHz, DMSO-d6):  8.45 (s, 2H, Ar-H), 8.09 (d, J = 8.5 Hz, 1H, Ar-H), 8.04 (s, J = 8.5 Hz, 1H, N-H), 7.71−7.63(m, 2H, Ar-H), 6.42 (dd, J = 17.8, 11.0 Hz, 1H, COCH), 6.31 (dd, J = 17.8, 1.1 Hz, 1H, CH=CH), 5.82 (dd, J = 11.0, 1.1 Hz, 1H, CH=CH). 13C NMR (500 MHz, DMSO-d6) 184.9, 180.8, 165.2, 149.7, 149.5, 145.3, 143.1, 137.1, 132.1, 128.8, 127.3, 123.6, 117.3. FAB-MS m/z: 278.16 (calcd), 278.25 (found). For

polymerization,

the

acrylamide

monomer

(2.0

g,

8.9

mmol),

N,N'-

methylenebisacrylamide (54 mg, 0.41 mmol), and PE-b-PEG (240 mg) were added into water (30 mL). The mixture was degassed and stirred vigorously (300 rpm) at 80 oC. Polymerization was initiated by adding potassium peroxodisulfate (12.5 mg, 0.046 mmol) dissolved in 3 mL water. After reacting 3 hours, the mixture was cooled to room temperatures. Low molecular weight fractions were removed by dialysis in methanol for one week. Brown powder was obtained as a product (20% yield). The specific capacity was ca. 95 mAh/g for one-electron redox reaction of diazaanthraquinone.

Results and discussion Synthesis of redox-active polymer nanoparticles P1−P3



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As redox-active polymers, chemically cross-linked poly(TEMPO-substituted acrylamide) (P1, 0.6 V vs. Ag/AgCl),20 poly(tripyridiniomesitylene) (P2, -0.4 V vs. Ag/AgCl),23 and poly(diazaanthraquinone-substituted acrylamide) (P3, -0.7 V vs. Ag/AgCl) nanoparticles were newly synthesized (structures shown in Figure 1). We have reported a series of non-conjugated redox-active polymers, including P1 (as a linear chain) and P2 (electrochemically polymerized layers), to provide the fast redox reactions as promising electrode-active materials.20,

23-25

Diazaanthraquinone-substituted polymer (P3) was also studied in this paper to provide a high voltage (1.3 V with TEMPO as a cathode), which exceeded the potential window of water. While increasing energy density is important, few researches have been reported for the high voltages in the area of organic-based flow batteries (especially with safe aqueous solution such as brine).1, 5 The hydrophilic acrylamide (or ionic pyridinium) design of the polymers P1−P3 with appropriate crosslinking degree resulted in the suitable swellability in aqueous electrolytes. In the aqueous gels, the migration of the compensation ions in the gels proceeds quickly along with the charge/discharge of the introduced redox-active moieties.24, 26-29 The rapid electron self-exchange reactions among the highly populated redox units24 was also found to be crucially important to achieve the fast response as electrode-active materials. The strategy of just combining the hydrophilic polymer backbones and the redox-active pendant groups, conducted in this research, was much simpler than the former approaches for the polymer-based RFBs: the previous reports6, 17

utilized the hydrophobic (methacrylate or styrene) backbones for radical polymerization and

then additionally introduced the ionic groups for hydrophilicity, which simultaneously reduced their redox capacity as active materials. The polymers P1−P3 were synthesized by dispersion (or emulsion) polymerization. Radical polymerization was employed to obtain P1 and P3 from the acrylamide monomers. For


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crosslinking, the hydrophilic bifunctional crosslinker, N,N'-methylenebisacrylamide, was selected. Menschutkin

reaction

between

bipyridine

and

the

trifunctional

monomer,

1,3,5-

tris(bromomethyl)benzene, yielded the cross-linked polyviologen particles, P2. The diameters of the particles were carefully tuned by changing the polymerization conditions (types of surfactants and concentration ratio, Table 1). As surfactants, PE-b-PEG and tetrabutylammonium chloride were found to be suitable to stabilize the dispersion. Dynamic light scattering (DLS) measurements indicated the unimodal size distribution of the particles, supporting the successful completion of the polymerization (Figure 2a). Scanning electron microscope (SEM) images of the particles also supported the formation of the sphere-shaped particles without significant defects (Figure 2b, S2). To support the intra/interparticular charge propagation, electrically conductive carbon-containing particles, C-P1 and C-P2 were also prepared by the polymerization with the existence of acetylene black particles as conductive agents (Table 1, entry 3 and 5). Nanosized particles such as acetylene black were favored to composite with the polymer particles due to their large surface area (> 80 m2/g), small size (40 nm), low cost, sphered shape, and sufficient dispersibility in solvents and polymers.30 Elemental analysis suggested the introduction of the carbons to the particles at a concentration of ca. 10 wt% (the value was higher than the typical percolation limit for conductivity with polymer composites.30-31 By virtue of the hydrophilic design, the stable dispersion in water was observed for all the particles, which was indispensable for the application to RFBs (stable over several days, Figure 2c, 2b). The polymer particles displayed the reversible charge/discharge reactions in brine as an electrolyte. During cyclic voltammetry, clear redox waves were observed at 0.6 and -0.4 V vs. Ag/AgCl for P1 and P2, respectively (Figure S3, results for P3 is described in the last section). The potentials were consistent with the one-electron redox reactions of TEMPO and viologen units.20-21, 23



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The synthesized polymer nanoparticles displayed the suitable size-exclusion advantages as the RFB components. Crossover tests were conducted for the three types of TEMPO derivatives: linear or cross-linked poly(TEMPO-substituted acrylamide) and TEMPO (see Supporting Information for the experiments). The compounds were dispersed or dissolved in the electrolytes, and the permeation against a porous separator was examined. Crossover proceeded quickly with the monomeric species because the molecule was too small in the size of < 1 nm, assuming a spherical approximation. In the case of a linear polymer (Mn =

2.2 ×104), the amount of

permeation was suppressed to 7.4% after 1 day, due to the exclusion effect. Further, the crossover was hardly detectable ( 0.5 M). The observed viscosity of the particles was comparable or even less than that of the dissolved TEMPO- or viologen-substituted polymers in aqueous electrolytes (e.g., 2 mPa∙s at Cbulk = 1 M for P1’ and 5−20 mPa∙s at Cbulk = 0.37 M for the previous report6). The lower viscosity was achieved by the smaller interaction between the electrolytes and the dispersed particles than the homogeneity dissolved polymers (i.e., only the surface of the particles contributes to viscosity while the dissolved, randomly coiled linear polymers induce the strong friction against the surrounding solvents32). Although the small physical diffusion coefficient was observed with the polymer particles (Dphys =

10-7−10-10 cm2/s), much larger coefficient was obtained for the apparent charge transfer

(Dapp = 10-6−10-9 cm2/s, Figure 3d). The ratio of the diffusion coefficients, Dapp/Dphys, was almost unity with TEMPO-SO3Na because the physical diffusion of the molecules dominated the overall charge transport processes. In contrast, Dapp/Dphys was 100−3 times larger for the polymer particles, meaning that the other transport processes dominated in parallel. To explain the boosting effect of the charge transport, kinetic models were proposed for the polymer dispersion and the monomer solution (Figure 3a). For the polymer dispersion, four major processes could contribute to the charge flux: (a) physical diffusion of the particles (dynamic movement of center of gravity, related to Dphys), (b) random rotation of the particles (Dphys,rotate), (c) intraparticular and (d) interparticular electron transfer by electron self-exchange reactions (related to rate constants of kex,int and kex,ext, respectively). For the monomeric species, where point particle approximation could be valid, only (a) and (d) were important, which is normally called as Dahms-Ruff equation (3).24, 33 𝐷

=𝐷

+𝐷 =𝐷

+ (1⁄6)𝑘

,

𝛿 𝐶

(3)


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(Det: diffusion coefficient for electron transfer and : distance for electron transfer) The slight increase of the apparent diffusion coefficient (Dapp) compared to Dphys for the dissolved monomers was also proposed in the previous studies.24, 34 For the polymer particles, the modification of equation (3) will be needed because the apparent bulk concentration (Cbulk) of the redox-active moieties does not coincide with the local concentration (Clocal) in the particles (Figure 3a). The proper treatment is to sum up each contribution by flux level, not by diffusion coefficient. If we assume that rotational diffusion coefficient is 3/4 times as large as Dphys35-36, and ignore the effects interparticular interactions (d) and other geometric factors, the current density for the potential-step response (i.e., Cottrell equation) could be approximated by equation (4). 𝐽

= 𝑛𝐹⁄𝜋

⁄

𝑡

= 𝐽 +𝐽 +𝐽 +𝐽 ≈ 𝐽 +𝐽 +𝐽 ⁄

(7/4𝐷

)

/

𝐶

+ 𝑥𝐷

⁄

𝐶

(4)

(x: correction factor) The interparticular electron transfer (4) was ignored because of the much smaller contribution to flux than the intraparticular transfer (3) (i.e., electron hopping probability decreases exponentially by the longer distance).11, 24 The correction factor x could be understood by the apparent, timeaveraged surface coverage of the electrode by the polymer particles. For the electrochemical reactions, the redox-active particles collide with and stay on the current collector for a while. In the time, the charge propagation by the intraparticular electron self-exchange reactions could contribute to the charge flux. In the extreme case of the maximum concentration, Cbulk = Clocal, the polymers will cover all the area of the substrate (x = 100%) and the physical diffusion will be strictly limited (Dphys ~ 0). The situation is the practically same as the polymer layer uniformly



Page 14 of 27

coated on the current collector, which has been studied extensively as the polymer electrodes for charge storage (𝐷

= 𝐷 = (1⁄6)𝑘

,

𝛿 𝐶

).20-21, 23-24

The fitting of the experimental results by equation (3) and (4) explained the experimental responses, at least, quasi-quantitatively (Figure 3b). The diffusion coefficient for electron transfer Det had been determined for P1 and P2 already in our previous report (ca. 10-10 cm2/s).20-21, 23 The surface coverage x, estimated by the fitting, increased monotonically against the bulk concentration Cbulk in many cases (Figure 3d, Table 2). The increase was caused by the more frequent collision (or coverage) of the particles on the substrates. Interestingly, the apparent coverage x exceeded 100% with the carbon composited viologen particles (C-P2) at a higher concentration (> 0.1 M) while smaller x less than 10% was obtained with the pristine particles (P2). The large value for the composites was ascribed to the conductive percolated carbon networks. The carbons fastened the electron transport and also functioned as the three-dimensional current collectors for the polymers, increasing the apparent electrode surface area, which have been preliminarily reported with the organic electrode-active materials.20, 37 By virtue of those additional charge propagation processes, the apparent charge diffusion coefficient Dapp of C-P2 reached 10-6 −10-8 cm2/s despite the large particles size over 1 m. The value was comparable to the dissolved polymeric species and even monomers.1-2, 6-7

The similar tendency was obtained for TEMPO-

substituted particles (Figure S5). The high charge transportability (Dapp) was maintained by the facile electron propagation despite the increased viscosity at high Cbulk. Although the effects were smaller than the case of conductive carbons, the use of smaller particles (i.e., compare P1: 80 nm and P1’: 1100 nm) yielded the much larger Dapp, according to equation (1). At a concentration (1 M for P1), the apparent surface coverage x became slightly less than zero (Dapp < Dphys), indicating the limitation of the solid sphere assumption due to the coalescence of the polymer particles at


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very large Cbulk, which awaits further investigation. The furtherly detailed analysis of the kinetic parameters, including the particle-particle interactions (collision and interparticular electron hopping), dynamic viscosity, electron conduction by carbons, and particle rotation, will help the clearer understanding of the whole charge transfer processes and more accurate parameter prediction. Redox flow batteries with polymer nanoparticles We fabricated redox flow cells using the organic polymer nanoparticles. P1’ and P2, were dispersed in the aqueous brine as a catholyte and an anolyte, respectively (Figure 4a). Although the particles provided the smaller Dapp than P1, C-P1, and C-P2, the use of a porous carbon felt, having a large electrode surface area (1500 cm2 per cubic centimeter), enabled the operation of the flow cells at high Cbulk (= 0.5−1.5 M). A conventional dialysis membrane was enough to separate the catholyte and anolyte particles. The formal capacity density (11 Ah/L) was comparable to the dissolved polymer- or oligomer-based flow cells.7, 15 For the charge/discharge tests, a plateau appeared at 1.1 V, which was ascribed to the redox reactions of TEMPO and viologen (Figure 4b). The cell displayed the experimental volumetric charge capacity of 7.2 Ah/L, which was maintained over 100 cycles (Figure S6). The relatively low Coulomb efficiency around 80% may have caused by the self-discharge by oxygen, evolved by water splitting.38 Also, utilizing the large Dapp particles (P1, C-P1, and C-P2,) as active materials for flow cells is our ongoing challenge: clogging up of the carbon felts occurs more frequently with those particles, certainly because of their higher viscosity as the dispersion and the stronger interaction with the electrode surface. Pursuing their interface chemistry and the optimal molecular structures of the particles will help the more facile transport in the porous electrodes.



Page 16 of 27

For the higher cell voltage and Coulombic efficiency, the new redox-active polymer, poly(diazaanthraquinone-substituted acrylamide) (P3) was also studied as the anolyte. Although the longer synthetic procedures than P2 were required, the robust quinine groups were believed to provide the high stability as the flow batteries.2 The cyclic voltammogram of diazaanthraquinone indicated the two step, two electron redox reactions in both aqueous and organic electrolytes (Figure S3, S7, and Scheme S1). For the polymer particles, the first redox at -0.7 V vs. Ag/AgCl was used. The observed potential was higher than that of anthraquinone, because the nitrogen groups in the aromatic ring functioned as the electron withdrawing group.10 The higher potential was preferred for the higher stability against oxygen, and preventing hydrogen evolution at the electrode surface.1-2 Despite the large concentration of the redox species (1.5 M), the reversible charge/discharge at 1.3 V was observed with the practical charge capacity of 6 Ah/L (Figure 4b). The high Coulombic efficiency over (99% at the initial cycle), energy efficiency (>90%), and capacity retention over 50 cycles supported the successful redox processes of the particles, which was a promising result for pursuing the larger scale studies (Figure 4c). Although the Coulombic efficiency decreased gradually along with the repeated charge/discharge, no change in dQ/dV curves (Q and V are capacity and voltage, respectively) was observed even after 50 cycles, suggesting the absence of the byproducts originated from the side-reactions of the redox-active particles (Figure S8). Evolution of oxygen, as a result of water splitting (thermodynamically, at a voltage of > 1.23 V), should be the major reason for self-discharging.38 However, the problem could be solved by kinetic approaches, such as exploring appropriate current collectors (to increase overpotential of water splitting) and constantly degassing the electrolytes, in the similar way to the vanadium-based RFBs (formal voltages of 1−2.1 V).39 To withdraw the maximum capacity density (e.g., formal capacity of 40 Ah/L at Cbulk = 1.5 M) and enhance energy efficiencies, we are now


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trying to improve the polymer design for higher charge transportability and dispersibility, and elucidate the overall transport processes including inter- and intra-particle interactions, fluid convection,

particle

distortion,

effects

of

redox-active

species

(TEMPO,

viologen,

diazaanthraquinone, et al.), and heterogeneous charge transfer between the polymer particles and the porous electrodes.

Conclusions A series of organic redox-active polymer nanoparticles were synthesized for aqueous-based redox flow batteries. The hydrophilic acrylamide (or bipyridinium) design of the polymer backbones and the suitable cross-linked structures provided the sufficient swellability to brine and the reversible charge/discharge as catholytes or anolytes. The design was much simpler and easier than the conventional strategy for the polymers, that is, introducing the additional hydrophilic ammonium groups. Although the physical diffusion was decreased (Dphys =

10-7−10-10 cm2/s) by

the large radius of the polymer particles, the fast redox kinetics (Dapp ~10-7 cm2/s) were maintained because of the electron self-exchange reactions throughout the polymers. The polymer particlebased flow cells, which had a superior volumetric capacity than the dissolved species (1.5 M), operated successfully over 50 cycles without significant capacity decay, due to the size-exclusion effects of the particles against the porous separators. The facile synthesis of the particles and reliable operation of the cells opened a way for cost effective, environmentally friendly, and high energy density redox flow batteries.



Page 18 of 27

Figure 1. Concept for the organic nanoparticle-based redox flow batteries. Table 1. Synthesis of P1−P3 by dispersion polymerization.

Water/hexane

[Surfactant]/ [Monomer]/[ Solvent]a 1/0.5/0.6

Reactio n time (hour) 3

Particle Size (nm)b 80

Std. Dev. (-)c 32

PE-b-PEG

Water

1/0.3/33

12

1100

280

C-P1

-

Dioxane

0/1/10

5

280

110

4

P2

PE-b-PEG

DMF

1/0.6/19

3

250

36

5e

C-P2

-

DMF

0/1/20

3

1100

250

6

P3

PE-b-PEG

Water

1/0.1/15

3

80

24

Entry

Polymer

Surfactant

Solvent

1d

P1

TBACl

2

P1’

3e

a

By weight. b Diameter estimated by DLS. c Standard deviation of diameter. d Emulsion polymerization. TBACl: tetrabutylammonium chloride. Water/hexane =1/1 (v/v). e Conductive carbon added (5 wt% to monomer).


Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c) Figure 2. (a) Size distribution of poly(TEMPO-substituted acrylamide) nanoparticles measured by DLS. (b) SEM image for P1’. See Supporting Information for the results of the other particles. (c) Photographs of the polymer particles dispersed in water.



Page 20 of 27

Table 2. Electrochemical parameters for TEMPO-SO3Na, P1, and P2.

Entry

Species (diameter)

1 2 3

TEMPOSO3Na (0.8

nm)d

Cbulka (M) 0.001 0.01 0.1

Clocal a (M)

-

 (mPa∙s) 0.94

log Dappb (cm2/s) -4.08e

log Dphysc (cm2/s) -5.24

0.92

-5.01

-5.23

1.1

-5.34

-5.30

Surface coverage x (%)

-

4

0.2

1.3

-5.39

-5.38

5

0.001

1.12

-6.27

-7.17

0.98

1.23

-6.60

-7.21

4.4

1.98

-6.87

-7.42

27

7

P1

0.01

8

(80 nm)

0.1

4.0

9

1.0

7.94

-9.35

-8.02

(< 0)

10

0.001

1.35

-7.48

-8.47

0.26

1.45

-10.0

-8.50

(

Hydrophilic Organic Redox-Active Polymer Nanoparticles for Higher

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