Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
pubs.acs.org/acsapm
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
Downloaded via SWINBURNE UNIV OF TECHNOLOGY on February 3, 2019 at 19:39:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
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 fast electron propagation throughout the polymer particles. Redox flow cells were fabricated using TEMPO-, viologen-, or diazaanthraquinonesubstituted 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. KEYWORDS: organic battery, redox flow battery, redox-active polymer, radical polymer, energy storage
■
to the large-scale flowing system to decrease the overall energyconversion 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 ionexchange 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 a 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
INTRODUCTION Organic redox flow batteries (RFBs), using the organic redoxactive 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-tetramethylpiperidin1-oxyl (TEMPO), and their derivatives are known to provide 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 for 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 ≪1 M), which is much smaller than the inorganic particle-based LIBs and RFBs (>5 M),10,12 leads © XXXX American Chemical Society
Received: October 31, 2018 Accepted: January 10, 2019 Published: January 10, 2019 A
DOI: 10.1021/acsapm.8b00074 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
Figure 1. Concept for the organic nanoparticle-based redox flow batteries.
provide high conductivity and mechanical strength as the separators by virtue of their porous yet robust polymer structures (cellulose, polypropylene, ect.).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 nondiscrete statistical hydrodynamic radius (i.e., random coil structures). 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. RodriguezLopez 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 use in organic electrolytes.17 In this article, 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-1yloxyl (TEMPO-SO 3 Na) 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 chloride, and N,N′-methylenebis(acrylamide) were purchased from Tokyo Chemical Industry Co. Potassium peroxodisulfate was obtained from FUJIFILM Wako Pure Chemical Co. Polyethylene-block-poly(ethylene glycol) (PE-b-PEG) (Mn = 2250, 80 wt % of ethylene oxide) was purchased from SigmaAldrich Co. Acetylene black was obtained from Strem Chemical, Inc. N,N-Dimethylformamide (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 JESTE200. A Quantum Design MPMS SQUID-VSM magnetometer was used for the magnetization measurements. Scanning electron microscope (SEM) images were obtained by HITACHI FESEM S4500S. 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. On the basis of the results, Cottrell plots were used to evaluate the apparent diffusion coefficient for charge transfer. For ESR, ultrafine 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 a current collector. As a separator, dialysis membrane (cellulose, pore size of 5 nm) was selected. The catholyte and anolyte (10 mL of each) were filled in vials, and flowed at 12 mL/min. Synthesis of TEMPO Nanoparticles (P1, P1′, and C−P1). The acrylamide monomer was prepared according to our previous report.21 The monomer (20g, 95 mmol), PE-g-PEG (6 g), and B
DOI: 10.1021/acsapm.8b00074 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials Table 1. Synthesis of P1−P3 by Dispersion Polymerization entry d
1 2 3e 4 5e 6 a
polymer
surfactant
solvent
[surfactant]/[monomer]/[solvent]a
reaction time (h)
particle size (nm)b
std. dev. (−)c
P1 P1′ C−P1 P2 C−P2 P3
TBACl PE-b-PEG
water/hexane water dioxane DMF DMF water
1/0.5/0.6 1/0.3/33 0/1/10 1/0.6/19 0/1/20 1/0.1/15
3 12 5 3 3 3
80 1100 280 250 1100 80
32 280 110 36 250 24
PE-b-PEG PE-b-PEG
b
By weight. Diameter estimated by DLS. cStandard deviation of diameter. dEmulsion polymerization. TBACl, tetrabutylammonium chloride. Water/hexane = 1/1 (v/v). eConductive carbon added (5 wt % to monomer).
■
N,N′-methylenebis(acrylamide) (1 g, 6.5 mmol) were added into 630 mL of water. The dispersion was prepared by stirring the mixture (300 rpm and 80 °C) for 15 min under a nitrogen atmosphere. The mixture was stirred for 12 h 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 h. 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). 1,3,5Tris(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 of DMF. The reaction proceeded by stirring the mixture (250 rpm and 80 °C) for 3 h. 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, δ): 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 carboncontaining 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-1,4-diazaanthraquinone (0.7 g, 2.5 mmol)22 in DMF (60 mL), acryloyl chloride (0.5 mL, 7.5 mmol) was added dropwise at 0 °C under a nitrogen atmosphere. The mixture was stirred overnight at room temperature. The monomer was obtained as the brown powder after washing the byproducts with water and acetone (64% yield). 1H NMR (500 MHz, DMSO-d6, δ): 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, CHCH), 5.82 (dd, J = 11.0, 1.1 Hz, 1H, CHCH). 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′-methylenebis(acrylamide) (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 °C. Polymerization was initiated by adding potassium peroxodisulfate (12.5 mg, 0.046 mmol) dissolved in 3 mL of water. After being reacted for 3 h, the mixture was cooled to room temperature. Low molecular weight fractions were removed by dialysis in methanol for 1 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. 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 nonconjugated 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 article 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 a safe aqueous solution, such as brine).1,5 The hydrophilic acrylamide (or ionic pyridinium) design of the polymers P1−P3 with an appropriate cross-linking 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 cross-linking, the hydrophilic bifunctional cross-linker, N,N′-methylenebis(acrylamide), was selected. The 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). SEM images of the particles also supported the formation of the C
DOI: 10.1021/acsapm.8b00074 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
the permeation was indispensable, especially for the long-term use of the practical RFBs. Charge Transfer Kinetics for the Polymer Nanoparticles. The charge transport processes in the polymer particle dispersion were studied with a quantitative model. The kinetic study is one of the most important issues for the utilization of the particles for RFBs because it determines the rates and efficiency of charge/discharge reactions. The electrochemical responses of the particles become much slower than those of the dissolved monomeric species and polymers according to the Stokes−Einstein eq 117 Dphys = kBT /(6πηr ) ∞ kBT P(r )dr 6πηr
(or ∫ 0
(1)
for polymer particles), where Dphys is the
physical diffusion coefficient, kB is the Boltzmann constant, T is the temperature, η is the viscosity of the solvent, r is the radius of the particle, and P(r) is the probability density function. The smaller coefficient by the larger radius directly induces the lower current density and poorer rate performances, which should have been the main drawback of the polymer particles. However, the problem could be solved by the fast electron propagation within the polymer particles. In the former report, the experimentally observed, apparent diffusion coefficient for charge transfer by the particles (Dapp) was larger than Dphys, although the clear reason had been unclear.17 Here, a new charge transfer model was proposed to rationalize the increased charge flux (Figure 3a). The kinetic parameters of the particles and a monomeric TEMPO derivative (TEMPO-SO3Na as a control) were determined experimentally using electrochemical measurements (for Dapp), DLS (r), and viscosity measurements (η) (Table 2). NaCl (3 M) aqueous solution was selected as an electrolyte. The Cottrell eq 224,27 was employed to analyze the results of chronoamperometry and evaluate Dapp (Figure 3b)
Figure 2. (a) Size distribution of poly(TEMPO-substituted acrylamide) nanoparticles measured by DLS. (b) SEM image for P1′. See the Supporting Information for the results of the other particles. (c) Photographs of the polymer particles dispersed in water.
sphere-shaped particles without significant defects (Figures 2b and 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,b). 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 are described in the last section). The potentials were consistent with the one-electron redox reactions of TEMPO and viologen units.20,21,23 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 the 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
