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New Approaches for Energy Storage with Hyperbranched Polymers Brandon Jon Yik, Meng Guo, Young Kwon, and Theodore Goodson III J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00781 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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The Journal of Physical Chemistry

New Approaches for Energy Storage with Hyperbranched Polymers Brandon J. Yik, Meng Guo, Young Kwon, and Theodore Goodson, III* Department of Chemistry, University of Michigan, 930 N University Ave, Ann Arbor, MI 48109, United States

ABSTRACT New hyperbranched polymers have been investigated to provide new structure-function relationships necessary for electrical and optical applications. We can take advantage of longrange delocalization in these structures for energy storage applications. This is accomplished by obtaining high dielectric constants. In this article, we demonstrate the need for developing high dielectric hyperbranched polymers by first investigating a ceramic/polymer hybrid system and then studying the design criteria for these hyperbranched systems using detailed optical and electronic characterization techniques. Provided in this contribution are the energy storage results with ion-doped polyaniline (PANI) polymers.

An enhancement in the dielectric constant

emerged from strong long-range polaron delocalization and the mechanism of a hyperelectronic polarization in these polymer systems. A copper phthalocyanine (CuPc) core was selected to build novel hyperbranched polymers to investigate their energy storage and optical properties. We report the results of these hyperbranched polymers, which exhibited high dielectric constants

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and low dielectric losses. Detailed structure-function relationships were carried to probe the polaron delocalization mechanism. An outstanding result of a new hyperbranched polymer showed the greatest energy storage capacity of 7.97 J cm-3. These results provide new insights in to the design of new organic macromolecules for energy storage.

1. INTRODUCTION While there is certainly a need for great attention toward creating new sustainable energy sources, there is also a need for new energy storage capabilities. The rapid development of electronic technology has focused on circuit components, but due to the ubiquity of such high frequency circuit components, the need for miniaturized operating devices has become apparent. This development has demonstrated a great need for superior electronic materials in industry, translating into advancements in capacitor technology.1 The critical requirements in crafting these novel capacitor systems include low cost and high energy storage capacity and power density. Dielectric technology has primarily focused on traditional inorganic ceramics, such as barium titanate (BaTiO3).2–8 Popular dielectric materials, such as other perovskite oxides, show high dielectric constants (up to 105) at low operating frequencies (100 Hz).7 However, limitations with ceramic capacitors include high processing temperatures, high leakage currents, low breakdown fields, and a lower dielectric constant at higher frequencies (10 kHz–1 MHz).9,10 At high frequencies, they also exhibit a large dielectric constant dispersion curve and high dielectric loss, which are not suitable for high frequency applications. In approach to alleviate some of the issues associated with ceramics, organic polymeric materials have been studied as alternative dielectric materials.11–18 These materials may be a viable alternative due to their relatively high energy density, high electric breakdown field, and


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low dielectric loss. These materials also satisfy industrial standards by exhibiting fast speed, low cost, high reliability, and exhibit dielectric losses that are significantly lower than their inorganic counterparts. Organic systems also show far less dispersion over high frequencies than ceramics.19–24 Polyaniline (PANI) has attracted much attention due to its facile preparation, good environmental stability, and high degree of conductivity.11,21,23,25 A dielectric constant of greater than 104 can be observed in partially crystalline PANI and hyperbranched PANI (HBPANI) systems.21,26 However, these dielectric materials generally exhibit very high dielectric loss, posing a major problem to incorporating these materials into high power industrial applications. One possible solution is a hybrid system, made of both organic and ceramic materials. Hybrid materials take advantage of both materials’ desirable characteristics and thus it is possible to have a material with a high dielectric constant of an inorganic ceramic with the small dispersion curve of an organic material. Likewise, the dielectric loss would not be compromised due to contributions from the organic material. Studies have shown that the of doping an organic polymer dielectric with ceramics27–33 and metals29,34–40 yielded materials with high breakdown voltage, low dielectric loss, good thermal stability, good processability, and most importantly, low cost. The hybrid materials focused in this contribution are polyaniline- and poly(4vinylpyridine)-doped barium titanate. Polyaniline-doped materials have shown an improvement in dielectric properties when measured at high frequency.41–43 Hybrid nanocomposites consisting of a PANI core and BaTiO3 shell (PANI@BTO) were shown to exhibit better interfacial polarization and temperature stability than pure PANI.41 This was a result of the BaTiO3 shell preventing direct contact of polyaniline molecules, thus restricting electron transfer capabilities.42 The main issue now is how to dramatically lower the dielectric loss and dispersion while maintaining a high dielectric constant. In creating a hybrid dielectric material, several parameters


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must be optimized to maximize the potential of such systems. Some of these parameters are aspect ratio, functionality of the materials, filler load level, solubility, and the dispersion homogeneity, especially for thin-film applications. In considering the optimization of the dielectric response of a hybrid material, one must consider the polarization mechanism. There are five general polarization mechanisms, each one dominating at different frequency domains: electronic (atomic), dipole (orientation), ionic, interface or surface (Maxwell-Wagner), and hyperelectronic. Electronic polarization is produced when the displacement of the charge center of electrons from the central nucleus induces a dipole moment, which has no frequency dependence, and thus is present along a wide frequency range. Orientation/dipole polarization occurs in materials where permanent dipoles can freely rotate and an applied electric field aligns previously existing, randomly-oriented dipoles. Orientation polarization is operative up to microwave frequencies (108 Hz) and contributions from the dipole polarization is larger than that of electronic polarization under normal conditions. Ionic polarization arises when the electric field moves the charge center of positive ions away from that of the negative ions, thus yielding a net dipole and operates through infrared frequencies (1012-1013 Hz). The dielectric loss curve normally shows abrupt losses with large conductivity in materials where ionic polarization dominates. Interface or surface (Maxwell-Wagner) polarization operates in frequencies up to 104 Hz and is present when dipoles orient to a certain degree under an electric field and contribute to the total polarization. The contribution to the total polarization is outstanding at low frequencies (Hz-kHz), explaining why many heterogeneous dielectric materials, such as metal oxides, have high dielectric constants at lower frequencies.12,44–57 Hyperelectronic polarization can be found in polymeric molecules with a large amount of electronic orbital delocalization capability.58,59 This polarization may be attributed to


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the interaction of charge pairs of excitions on long polymer chains under an external electric field. The displacement of charged pairs results in a strong polarization due to the length of a macromolecule. In the past few decades, hyperbranched polymers have been explored for various applications.60–62 Figure 1 summarizes these polarization mechanisms with their frequency response.

Figure 1. Polarization mechanisms and their frequency response. The perspective of the applications of organic materials for energy storage has attracted an enormous interest from the scientific community due to the unique properties as a result of the delocalization of p-electrons.63–69 Examples of these unique properties are: mutual transition between the insulator and the conductor, which depends on the HOMO-LUMO band gap, excellent thermal stability, and mechanical strength.63,66 One important research area is focused on creating organic high dielectric constant materials by utilizing different polarization mechanisms.12,22,70–73 The particularly attractive features of organic high dielectric constant materials have the possibility to yield low dielectric loss, high breakdown strength, as well as relatively low cost.12,22,73


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In this contribution, we performed extensive investigations of the energy storage properties of organic hyperbranched polymers, finding an ultrafast delocalization process in many of these structures. This lead to development of a novel all-organic capacitor material with high energy density. Here, we initially started with ion-doped hyperbranched polyaniline (HBPANI) polymers, finding that the dielectric constant could be twice as large (~200 at 1 MHz) than that of linear polyaniline.21 This enhancement in the dielectric constant emerged from longrange polaron delocalization and hyperelectronic polarization in these hyperbranched systems.58,59 With this knowledge, a copper phthalocyanine core was selected to build novel hyperbranched polymers to investigate their dielectric and optical properties. These hyperbranched polymers exhibited high dielectric constants and low dielectric losses achieved by using polaron delocalization. In this article, we will discuss the dielectric properties and mechanism of hybrid polymer/BaTiO3 materials, the relationship between the structure of these hyperbranched copper phthalocyanine (HBCuPc) polymers, corresponding dielectric properties and energy storage capabilities, and the physics underlying the relationship via a series of electronic and optical characterization measurements.

2. EXPERIMENTAL METHODS 2.1. General considerations. All reagents were used as received unless otherwise stated. Polyaniline (emeraldine base) was synthesized using previously reported methods.74 N,Ndimethylacetamide (DMAc) was dried with CaH2 and distilled in vacuo prior to use. The synthesis of the HBCuPc and HBCuPc-TPA-CN polymers has been described in previous methods.22,73


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2.2. Synthesis of HBCuPc-CN. A previously reported similar synthesis was followed to prepare the HBCuPc-CN polymer (Scheme 1A).75 Four equivalents of 1,3-bis(3,4dicyanophenoxy)benzene was mixed with one equivalent CuCl in 40 mL N,N-dimethylacetamide (DMA) at 200 °C for 6 h. The mixture was then poured into 800 mL of water to precipitate the polymer. Consecutive filtration, cold methanol washes, and dying steps were repeated to purify the product, and finally the blue-green product was obtained. 1H-NMR, IR, UV-Vis, TGA, and GPC measurements were carried out to characterize and determine the structure of the HBCuPcCN polymer and matched that as previously reported.75 The molecular weight (Mw) was found to be 13,000. TGA: 471 °C (10% weight loss). Based on the relative ratio as compared to the absorbance of the pure CuPc, the content of CuPc unit in HBCuPc-CN was calculated to be 85%.

Scheme 1. Synthesis of the HBCuPc-CN and HBCuPc-COOH polymers. CN OH CN A) OH

O 2N

CN

O

CN

K 2CO 3

CuCl

DMSO rt, 48 h

DMAc 200 °C, 6 h

O

CN

HBCuPc-CN Polymer

CN CN CN

OH B) OH

O 2N

CN

K 2CO 3 DMSO rt, 48 h

O O

CN

CuCl

CN

DMAc 200 °C, 6 h

HBCuPc Polymer

KOH EtOH/H 2O 60 °C, 24 h

HBCuPc-COOH Polymer

CN

2.3. Synthesis of HBCuPc-COOH. The HBCuPc-COOH polymer (Scheme 1B) was prepared using a modified procedure used to synthesize the HBCuPc polymer.22,76 In short, 500 mg HBCuPc polymer was dissolved in 80 mL of a 1:1 water/ethanol solution with 10 wt% (50 mg) KOH. The mixture was refluxed for 24 h at 60 °C. The solution was then precipitated into


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150 mL of pH-adjusted (pH = 3-4) water. The green product was obtained with collected with consecutive filtration, 0.1 M HCl and water washes, and drying steps to give an 81% yield. The CuPc unit content in HBCuPc-COOH was also calculated to be 85%. The molecular weight (Mw) was 12,800. TGA: 444 °C (10% weight loss). 1H-NMR (400 MHz, DMSO-d6): d = 11.25 (s), 7.70-7.68 (m), 7.41-7.32 (m), 7.16-7.15 (m), 7.04-7.02 (m), 7.69 (br, m).

2.4. Preparation of the Hyperbranched Polymer Films. The hyperbranched polymer films were prepared by the solution cast method. To a vial, 20 mg of the polymer powder was dissolved in 1.0 mL DMAc, then the solution was stirred for 5.5 h at 40 °C, with another 10 h of stirring at r.t., followed by sonication for 10 min. The polymer solution was then drop-casted onto the conductive side of aluminum foil by pipette and dried in air overnight. The film was moved into a vacuum oven and annealed at increasing temperature intervals for 10 h (initially at r.t., then increased 20 °C every 2 h), followed by a final drying process in vacuum (12 psi) at 125 °C for 10 h. Finally, the film was slowly cooled down to r.t. The typical thickness of the hyperbranched polymer film was 15 µm, measured using a Veeco surface profiler. 2.5. Preparation of BaTiO3/Polymer Pellets. Barium titanate was dispersed in ethanol, ball-milled, and dried in vacuo. Pellets were made from weight percent PANI/BaTiO3 (about 0.06 g total) and poly(4-vinylpyridine)/BaTiO3 (about 0.40 g total) nanocomposites. The two samples were thoroughly mixed and ground together into a fine powder before being thoroughly scraped into a die and pressed at 12,500 psi for 10 min. The pellet thickness was measured using an electronic micrometer before being sputter coated with approximately 500 Å of gold with a Denton Vacuum Desk II Sputter Coater. The pellets had a thickness of

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