High-Density and Robust Charge Storage with Poly(anthraquinone-substituted norbornene) for Organic Electrode-Active Materials in Polymer–Air Secondary Batteries

An excellent functional group tolerance of ruthenium complex catalysts for olefin metathesis gave rise to ring-opening polymerization of norbornene functionalized with redox-active anthraquinone (AQ) pendants, yielding a high-molecular-weight and processable polynorbornene with large redox capacity. A thin layer of the polymer cast on current collectors showed reversible redox reaction at −0.85 V vs Ag/AgCl when immersed in basic aqueous electrolyte solutions. Good cycle performance was observed with a capacity comparable to the formula weight-based theoretical density of 212 mAh/g, which was the largest among those for the previously reported redox-active polynorbornenes. This suggested that all of the AQ units in the layer were redox-active, that electroneutralization was accomplished by successive compensation of counterions throughout the layer, and that the mechanical strength of the polymer layer prevented dissolution or exfoliation from the current collector surface. A robust polymer–air secondary battery with the high capacity was fabricated by using the polymer layer as the anode-active material. The battery showed a discharge voltage of 0.68 V and long life of over 300 cycles of charging/discharging, maintaining the moderate energy density of 143 mWh/g

Ionic Liquid-Triggered Redox Molecule Placement in Block Copolymer Nanotemplates toward an Organic Resistive Memory

The integration of functional components such as metal nanoparticles, metal salts, or ionic liquids with well-defined block copolymer (BCP) nanotemplates via noncovalent bond interactions has afforded hybrid functional materials. Here, we designed an ionic liquid (IL)-functionalized redox-active TEMPO (2,2,6,6-tetramethylpiperidine-<i>N</i>-oxy) radical (<i>guest</i>), investigated phase-selective incorporation/placement into <i>host</i> BCP nanostructured matrices, and established a rational approach to functionalize BCP templates. On-demand domain functionalization of poly­(styrene-<i>b</i>-ethylene oxide) (PS-<i>b</i>-PEO) was triggered by ion–ionophore interaction, as verified by the suppression of PEO melting transition in DSC, and the swelling behavior of the PEO spherical domain in AFM, TEM, and X-ray scattering characterizations. The obtained BCP layer containing the redox-active TEMPO and IL was utilized as an active layer in the diode-structured memory device, which exhibited on/off resistive switching (on/off ratio >10³). Systematic placement of TEMPO and IL in the BCP spherical domain allowed for tuning of the switching characteristics and revealed that the formation of a discontinuous redox-active domain was critical for rewritable resistive switching

Expanding the Dimensionality of Polymers Populated with Organic Robust Radicals toward Flow Cell Application: Synthesis of TEMPO-Crowded Bottlebrush Polymers Using Anionic Polymerization and ROMP

Poly­(norbornene)-<i>g</i>-poly­(4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl) (PNB-<i>g</i>-PTMA) was prepared by a grafting-through approach based on anionic polymerization of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl using a norbornene-substituted diphenylhexyllithium to yield a norbornene-functionalized macromonomer (NB-PTMA) and subsequent ring-opening metathesis polymerization of NB-PTMA using a Grubbs third-generation catalyst, which avoided critical side reactions involving the nitroxide radical of TEMPO moiety. The anionic polymerization resulted in high yields (>94%), narrow polydispersity indices (<1.20), and radical concentrations (0.95 radicals per monomer unit). The ROMP also resulted in high yields (>98%) and high radical concentrations (0.95 radicals per monomer unit), by virtue of the functional group tolerance of these reactions. Single molecular dimension of PNB-<i>g</i>-PTMA was measured by dynamic light scattering and by atomic force microscopy (AFM), which precisely reflected the bottlebrush structure to reveal the presence of the TEMPO group crowded at the periphery of the molecule. The lengths of PNB-<i>g</i>-PTMA along the macromolecular side chains and the polynorbornene main chain were both approximately equal to the theoretical lengths estimated by the degree of polymerization for each chain. The number-average diameter of PNB-<i>g</i>-PTMA in THF increased with initial NB-PTMA ratio to the Grubbs catalyst. Photo-cross-linked thin layer electrodes of PNB-<i>g</i>-PTMA demonstrated the reversible redox reaction at 0.80 V vs Ag/AgCl corresponding to the TEMPO/TEMPO⁺ couple and quantitative charging/discharging processes even at 120 C rate (i.e., full charging in 30 s). As a novel application of redox-active polymers, PNB-<i>g</i>-PTMA exhibited 95% efficiency of the theoretical charge capacity in a flow cell system, based on the unique properties of bottlebrush polymers such as the defined molecular dimension and relatively low solution viscosity in comparison with corresponding linear polymers

Synthesis of Pendant Nitronyl Nitroxide Radical-Containing Poly(norbornene)s as Ambipolar Electrode-Active Materials

Ambipolar redox-active polymers with a reversible charging and discharging capability were synthesized via ring-opening metathesis polymerization (ROMP) of nitronyl nitroxide radical (NN) mono- and disubstituted norbornenes which exhibited p- and n-type redox processes (i.e., one-electron oxidation and reduction per NN group, respectively), using Grubbs catalyst to avoid side reactions of the radical moiety allowing over 95% of radicals to survive after ROMP. ROMP of the NN monomers was accomplished with well-controlled molecular weights of the resulting NN polymers which were coincident with theoretical values in the ratio of [monomer]/[catalyst] = 25–200, narrow polydispersity index (ca. 1.2), and high yields even with [monomer]/[catalyst] > 600. The living character for the ROMP of the NN monomers also allowed block copolymerization. NN-containing block copolymers were synthesized through sequential ROMP with benzyl ether-containing norbornene in high yields. The NN polymer/carbon composite electrode exhibited both p- and n-type charging/discharging with plateau potentials near the redox potentials of the polymer at 0.78 and −0.80 V vs Ag/AgCl, respectively. The spin-coated layer electrode of the NN polymer immobilized on a current collector also demonstrated a fast charging/discharging performance in the range of 10–100 C rates and a cycle stability especially for the p-type reaction. These results made the NN polymer accessible as ambipolar electrode-active materials and also encouraged other organic radicals to be candidates for electroactive polymers

Supramolecular Organic Radical Gels Formed with 2,2,6,6-Tetramethylpiperidin-1-oxyl-Substituted Cyclohexanediamines: A Very Efficient Charge-Transporting and -Storable Soft Material

A supramolecular gelator, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-substituted cyclohexanediamine derivative, was synthesized, and its excellent charge-transporting capability was explored. The gels with organic solvents and electrolytes, or with ionic liquids, were formed via reversible sol–gel phase transition at ca. 50 °C. The organogels displayed electrochemical redox responses at <i>E</i>_1/2 = 0.72 V (vs Ag/AgCl) ascribed to the TEMPO moiety. Charge diffusion coefficient of the gel reached 3.3 × 10^–7 cm²/s even in the quasi-solid state, which was comparable to those of the homogeneous solution (ca. 10^–6). The high charge-transporting capability led to the tremendously large current density (a diffusion limited one) of ca. 1.0 mA/cm² on a current collector and long distance for the charge-transporting beyond the organogel thickness of 50 μm. A half-cell of the organogel performed a plateau output voltage at the <i>E</i>_1/2, very high rate, and almost quantitative charging–discharging, and it had cyclability without any additives such as conductive carbons and binder polymers

Synthesis of Pendant Radical- and Ion-Containing Block Copolymers via Ring-Opening Metathesis Polymerization for Organic Resistive Memory

Ring-opening metathesis polymerization (ROMP) using Grubbs third-generation catalyst directly yielded a norbornene-based polymer bearing robust redox-active radicals without any protection. Successive addition of imidazolium-containing norbornene in a one-pot reaction during ROMP produced pendant radical- and ion-containing block copolymers. The diode-structured thin-film devices fabricated with the obtained block polymers that had morphologies of spheres, lamellae, and inverse spheres exhibited conductive switching (write-once read-many-times, WORM) under a bias voltage, which revealed the dominant effect of the location of radicals and ions in the microphase-segregated domains on memory characteristics

Adsorption of a Carboxylic Acid-Functionalized Aminoxyl Radical onto SiO₂

Silicon wafers both without and with silicon­(IV) oxide surface coverage were covered with benzene solutions of stable organic radical 3-(<i>N-tert</i>-butyl-<i>N</i>-aminoxyl)­benzoic acid (mNBA). X-ray photoelectron spectroscopy supported the presence of the radical on both surface-cleaned (oxide-reduced) and oxide-covered surfaces. Optical waveguide spectroscopy showed that the radical retained its structure while adsorbed to the surface of the wafers, without noticeable decomposition. AFM and MFM imaging showed that the radical formed blocky particles with a change in rms roughness from 0.3 nm premodification to 1.7 nm postmodification on the surface-cleaned silicon. Similar experiments using oxide-coated silicon showed that the radical adsorbed to form much smoother layers, with a small change in rms roughness from 0.2 to 0.3 nm. Contact angle measurements of water on the premodified and postmodified samples showed a large, hydrophobic change in the silicon oxide surface but only a modest change in the surface-cleaned silicon surface. Samples of mNBA adsorbed onto silica gel showed strong electron-spin resonance signals from the aminoxyl spin, even years after production. The results demonstrate the prospects for treating and coating oxide-covered silicon wafers and silicon oxide-coated particles with a paramagnetically active organic substrate, without major chemical modification of the pretreatment surface; the resulting organic spin sites can be stable for years

Charge–Discharge with Rocking-Chair-Type Li⁺ Migration Characteristics in a Zwitterionic Radical Copolymer Composed of TEMPO and Trifluoromethanesulfonylimide with Carbonate Electrolytes for a High-Rate Li-Ion Battery

Redox-active copolymer containing organic robust radical, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and charge neutralizing anion, trifluoromethanesulfonylimide (TFSI^–) was synthesized as a cathode material of a Li-ion battery. The copolymer, poly­(2,2,6,6-tetramethylpiperidin-1-oxy-4-yl methacrylate-<i>co</i>-styrenesulfonyl­(trifluoromethanesulfonyl)­imide) (P­(TMA-<i>co</i>-TFSI)), was designed to give rise to the Li⁺ migration during its charge–discharge process, based on the self-charge compensation of TEMPO with TFSI^– bound to the polymer chain in a widely used electrolyte system for Li-ion battery, organic carbonate mixtures. Copolymerization was performed to achieve efficient self-charge compensation with uniformly distributed TFSI^– units. The P­(TMA-<i>co</i>-TFSI) layer electrode exhibited reversible redox reaction at 0.73 V vs Ag/AgCl. Electrochemical measurements combined with quartz crystal microbalance analysis evidenced that the redox reaction involved the Li⁺ migration in binary system of ethylene carbonate and diethyl carbonate. A test cell fabricated with the P­(TMA-<i>co</i>-TFSI) cathode exhibited high discharging voltage of 3.7 V and high-rate charge–discharge capability at 30 C (i.e., full charging in 2 min)

White Polymer Light-Emitting Electrochemical Cells Fabricated Using Energy Donor and Acceptor Fluorescent π‑Conjugated Polymers Based on Concepts of Band-Structure Engineering

The authors report on white polymer light-emitting electrochemical cells (PLECs) fabricated with a polymer blend film composed of a blue fluorescent π-conjugated polymer (blue FCP), poly­(9,9-di-<i>n</i>-dodecyl­fluorenyl-2,7-diyl) (PFD), and a red-orange FCP, poly­[2-methoxy-5-(2′-ethyl­hexyloxy)-1,4-phenylene­vinylene] (MEH-PPV), based on concepts of band-structure engineering. Polymer blending is one of the simplest and most promising methods for fabrication of van der Waals interfaces, which convert electricity to light in PLECs. By optimizing the composition of PFD, MEH-PPV, poly­(ethylene oxide) (PEO), and salt (KCF₃SO₃) in the active layer, white-light emission with Commission Internationale de l’Eclairage (CIE) coordinates of (<i>x</i> = 0.33, <i>y</i> = 0.31) can be achieved through light mixing of blue exciton emission from PFD and red-orange exciton emission from MEH-PPV at an applied voltage higher than the threshold voltage, <i>V</i>_th^blue‑FCP, which corresponds to <i>E</i>_g^blue‑FCP/<i>e</i>, where <i>E</i>_g^blue‑FCP is the band gap of PFD and <i>e</i> is the elemental charge. The white light produced by light mixing of PFD and MEH-PPV emissions can be obtained at a low MEH-PPV concentration, while only red-orange emissions from MEH-PPV are obtained at high MEH-PPV concentrations. The emission color of FCP-blend PLECs can be explained by Förster resonance energy transfer (FRET) from the excited PFD to the MEH-PPV because the photoluminescence (PL) spectrum of PFD overlaps with the UV–vis absorption spectrum of MEH-PPV. However, FRET was limited by the presence of PEO in the active layers of the FCP-blend PLECs. This meant it was much easier to tune the emission colors compared to FCP-blend polymer light-emitting diodes (PLEDs), in which FRET occurs predominantly. Utilization of a polymer blend film of blue and red-orange FCPs in PLECs is a very effective and promising method for fabrication of white light-emitting devices

Anionic Polymerization of 4‑Methacryloyloxy-TEMPO Using an MMA-Capped Initiator

Anionic polymerization of 4-methacryloyloxy-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-oxyl) was successfully carried out using methyl methacrylate-capped 1,1-diphenylhexyllithium (DPHLi/MMA), of which nucleophilicity is moderate enough to suppress the side reaction between the nitroxide radical of TEMPO moiety and the carbanion of DPHLi, to yield the radical polymer with well-controlled molecular weight, narrow polydispersity index (PDI < 1.10), high yield (>95%), and almost 1.0 radicals per monomer unit