Redox-active Pendant Research Articles

Molecular-Level Pore Tuning in 2D Conductive Metal-Organic Frameworks for Advanced Supercapacitor Performance.

Two-dimensional (2D) electrically conductive metal-organic frameworks (MOFs) have emerged as viable candidates for active electrode materials in supercapacitors due to their high electrical conductivity, high specific surface area, and intrinsic redox-active sites. Despite their promising electrochemical performance, their pseudocapacitive behavior via fast and reversible charge transfer reactions remains yet to be fully exploited. Here, we investigate the electrochemical energy storage mechanism of Cu3(HHTATP)2 (HHTATP = 2,3,6,7,10,11-hexahydroxy-1,5,9-triaminotriphenylene), a 2D conductive MOF featuring characteristic redox-active pendant aromatic amines. Cu3(HHTATP)2 exhibited pseudocapacitive charge storage with an average gravimetric capacitance of 340 ± 15 F g-1 at a discharge rate of 0.2 A g-1 and maintained a capacitance retention over 90% after 7000 galvanostatic cycles at 5 A g-1. The polar pendant amines not only enhanced capacitance via additional amine/imine redox activity but also reduced interfacial charge transfer resistance through modified electrode-electrolyte interactions. These results highlight the potential of molecular-level pore environment tuning as a strategic approach in materials design for energy storage applications.

Influence of Backbone on the Performance of Pendant Polymer Electrode Materials in Li-ion Batteries.

Pendant polymers are a promising class of electrode materials due to their synthetic simplicity, derivation from sustainable feedstocks, and potentially benign synthesis. These materials consist of a redox-active pendant tethered to a polymer backbone, which mitigates dissolution during electrode cycling. To date, an extensive number of pendant groups have been studied within the context of metal-ion batteries. However, the choice of the polymer backbone and its impact on the electrode performance have been relatively understudied. In this work, we use a postpolymerization modification approach to synthesize a series of viologen-bearing redox-active pendant polymers with similar molecular weights but three distinct chemical backbones, namely, polyacrylamide, polymethacrylamide, and polystyryl. By evaluating the polymers in lithium-ion batteries, we show that the polymer backbone has a significant influence on electrode performance and behavior. Specifically, the polymethacrylamide displays slower kinetics than the other two polymers, resulting in lower capacities, particularly at high cycling rates. Furthermore, the charge storage mechanism is dependent on the nature of the backbone: the polyacrylamide shows a significant capacitive contribution to charge storage, while the polystyryl does not. The difference in performance between the polymer electrode materials is ascribed to a difference in chain mobility and packing within the electrode films. Overall, this work shows that the fundamental properties of the polymer backbone are critical to the design of high-performance polymer electrodes.

Metal-free polypeptide redox flow batteries

Metal-free redox flow batteries with TEMPO-based polypeptide catholytes and viologen-based polypeptide anolytes were demonstrated. Post-cycling analysis indicated the main source of capacity fade was degradation of the redox-active pendant groups.

Redox-Active Polymeric Ionic Liquids with Pendant N-Substituted Phenothiazine.

Polymers that are elastic while supporting charge transport are desirable for flexible and soft electronics. Many polymers with bulky and conjugated redox-active pendant units have high glass transition temperatures (Tg) in their neutral form that will not lead to elasticity at room temperature. Their behavior in charged form in the solid state without an electrolyte has not been extensively studied. Here, the design strategy of polymeric ionic liquid where two weakly interacting ionic groups are used to maintain a low Tg is shown to lead to flexible redox active polymers. The use of a flexible ethylene backbone and redox-active phenothiazine (PTZ)-based pendant group resulted in polymers with relatively low Tg that are electrically conductive. PTZ that was N-substituted with 2-(2-ethoxyethoxy)ethoxy)ethyl was found to promote solubility of the polymer and lower the Tg of the neutral polymer by ∼150 °C relative to that of the Tg of a variant without the N-substituent. Doping with trifluoromethanesulfonimide leads to an electrically conductive polymer without significantly increasing the Tg. Physical characterization by UV-vis-NIR spectroscopy, electron spin resonance spectroscopy, and impedance spectroscopy verified that the molecular design leads to an efficient charge hopping between the PTZ groups.

Multiple-stimuli-responsiveness and conformational inversion of smart supramolecular nanoparticles assembled from spin labeled amphiphilic random copolymers

HypothesisOrganic radical polymers with tailored pendant functionalities have emerged as exciting and promising materials for their application versatility. Moreover, eco-friendly polymer-based organic nanomaterials with redox-active pendant side groups can replace the harmful heavy metal-based inorganic materials. On the other hand, self-assembled nanomaterials are of great interest and attracted more attention recently for their promising application in different advanced fields, but it is yet challenging to predict suitable hydrophilic-lipophilic balance (HLB) for stimuli-responsive random copolymers assembly due to structural irregularity. Among several experimental techniques, electron paramagnetic resonance (EPR) spectroscopy plays a unique and promising role in revealing structural and dynamic information of nanostructured radical containing materials. ExperimentsIn this study, a series of spin labeled amphiphilic random copolymers poly(methyl methacrylate-co-acrylic acid) have been synthesized and characterized by FT-IR, UV–Vis spectroscopies, TGA, DSC and water contact angle (CA) techniques. Their electrochemical properties have been determined by cyclic voltammetry (CV) in different organic solvents. EPR spectroscopy has been applied with other analytical techniques to elucidate the smart supramolecular nanoparticles (SNPs) formation, stimuli-responsiveness and structural changes through the dynamics of different molecular interactions. FindingsThe structural and dynamic information of self-assembled nanoparticles have been observed to be dependent on multiple-stimuli-responsiveness in different microenvironments by applying physiological and chemical parameters such as the different concentration of radicals, pH, temperature, nature of the solvent and reducing agent. The obtained results reveal the knowledge to understand insight into the mechanism for the formation of stimuli-responsive colloidal nanoparticles assembled from amphiphilic random copolymers with apt HLB value. The CV results reveal that the charge transfer process of the nanoparticles in solution was diffusion regulated and depended on the accessibility of radicals. The radical (spin labeled) polymers offer a broad way to develop stimuli-responsive materials in various colloidal nanostructures by changing the microenvironment, appreciating their potential advanced applications in electronic devices, catalysis, stimuli-triggered drug/gene delivery and reactive oxygen species (ROS) scavenger.

Hybrid surfaces active in catalysis based on gold nanoparticles modified with redox-active pendants and polymer brushes

We present hybrid systems bearing both plasmonic colloids and redox-active molecules that have demonstrated excellent performances in the catalytic reduction of nitrophenol (Nip) to aminophenol (Amp) in the presence of sodium borohydride. First, amino-functionalized spherical and triangular gold nanoparticles (AuNSs and AuNTs, respectively) were covalently incorporated onto polyacrylic acid (PAA) brushes attached to silicon surfaces. Next, a redox-active anthraquinone derivative (AQ-COOH) was immobilized onto the AuNPs surface by amidation reactions. The AuNP concentrations were varied to increase the incorporation and distribution of gold onto the PAA brushes. The catalytic activity in the reduction of 3-Nip and 4-Nip to Amp by NaBH4 was evaluated by 1H NMR spectroscopy. Colloidal size and morphology are analyzed by transmission electron microscopy (TEM) and UV–visible spectroscopy. Particle distribution on the silicon wafer was analyzed by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The presence of bonded cysteamine (Cys) and anthraquinone (AQ) derivatives on the gold surface was confirmed by X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman spectroscopy (SERS). The hybrid system could perform seven consecutive catalytic runs without loss of catalytic activity with conversions higher than 80%.

Single-Particle Evaluation of Nano and Micro-Sized Redox-Active Polymers for Energy Storage

Redox active polymers (RAPs) and redox active colloids (RACs) are new energy storage materials for size-exclusion redox flow batteries (RFBs).1-3 RAPs show beneficial characteristics for RFBs including high capacity, high solubility, electrochemical reversibility and tailorable properties via synthetic methods.1,2,4 Their crosslinked counterparts, RACs, are well defined spheres that can be nano- or micro-sized and have many similar characteristics to RAPs while showing even higher capacities and greater crossover rejection.3 As with other macromolecular systems, much remains unknown about the applicability and practicality of energy storage systems involving organic macromolecules capable of storing 106-109 electrons per molecule. In this presentation, I want to discuss methodologies we have been using to analyze stability and evolution of the RAC charge/discharge process during cycling with a focus on measuring a single or a limited number of particles. As with other non-conjugated RAPs, the mechanism of charge transfer across a RAC involves electron hopping between redox active pendants and simultaneous counterion movement.3,5 We recently investigated the electron transfer process within viologen-based RACs through various analytical methods based on scanning electrochemical microscopy (SECM) and modeling through COMSOL.6 SECM is a scanning probe technique that can assess local redox reactivity at the micro or nanoscale in a liquid electrolyte amenable to real battery environments.7 With SECM, single RACs could be imaged, sized and directly reduced/oxidized through making contact with an SECM probe. We were also able to evaluate monolayer films of nanometer sized RACs with a spectroelectrochemical setup using a coupled Raman-SECM instrument. This study provided some useful fundamental information and analytical tools/methods to guide future development and refinement for long time-scale experiments on RACs. I will present on recent cycling experiments involving SECM and modified electrodes. Both chronoamperometric steps and cyclic voltammetry (CV) have been used to measure the cycling process and to manipulate capacity access during cycling. Our data indicate that RACs undergo some conditioning before higher charge recovery and the proportion of reduced to oxidized viologen, or state-of-charge (SoC), will affect the amount of charge that can be recollected from the RAC. I will also discuss measurements involving Raman coupled to electrochemistry for single and few RACs. Refining and developing effective analytical methods is the key to understanding electron transfer, storage and failure within RACs and all battery materials. Our methods are pushing the limits of electrochemistry on single macromolecules but hopefully will be extended to other energy storage and electron transfer systems. J. Am. Chem. Soc. 2014, 136 (46), 16309-16316. Nature 2015, 527 (7576), 78-81. J. Am. Chem. Soc. 2016, 138 (40), 13230-13237. Chem. Mater. 2016, 28 (20), 7362-7374. J. Phys. Chem. 1991, 95 (16), 6383-6389. Langmuir 2017, 33 (37), 9455-9463. Scanning electrochemical microscopy. CRC Press: 2012.

Ferrocene-functionalized polyheteroacenes for the use as cathode active material in rechargeable batteries.

Herein we report the synthesis and characterization of new conjugated polymers bearing redox-active pendant groups for applications as cathode active materials in secondary batteries. The polymers comprise a ferrocene moiety immobilized at a poly(cyclopenta[2,1-b:3,4-b′]dithiophene) (pCPDT, P1) or a poly(dithieno[3,2-b:2′,3′-d]pyrrole) (pDTP, P2) backbone via an ester or an amide linker. Electrochemical and oxidative chemical polymerizations were performed in order to investigate the redox behaviour of the obtained polymers P1 and P2 and to synthesize materials on gram-scale for battery tests, respectively. During galvanostatic cycling in a typical battery environment, both polymers showed high reversible capacities of 90% and 87% of their theoretical capacity and excellent capacity retentions of 84% and 97% over 50 cycles.

Hybrid Organic Electrodes: The Rational Design and Synthesis of High-Energy Redox-Active Pendant Functionalized Polypyrroles for Electrochemical Energy Storage

In an effort to dramatically increase the capacity and electronic conductivity of organic-based redox active materials for electrochemical/electrical energy storage applications, polypyrrole (PPy) has been used as an anchoring backbone to form a family of dimethoxybenzene- (DMB-), bis(methylthio)benzene- (BMTB-), N, N, N’, N’-tetramethylphenylenediamine- (TMPD-), and N, N, N’, N’-tetramethylbenzidine- (TMB-) functionalized redox-active polymers. PPy was deliberately modified to provide an electronically conducting backbone with redox active pendants (RAPs) to enable immobilization (via electropolymerization) of the polymers onto current collectors. The RAPs dramatically increase the capacities of the materials by the reversible exchange of multiple electrons per formula unit. The electropolymerization of the pyrrole-anchored RAP monomers was achieved via cyclic voltammetry. The stability of the electropolymerized films was highly dependent upon the redox potential of the RAPs. The effects of charge density, during the electropolymerization process, were studied by varying the structure of the monomers. By the systematic characterization of the electrochemical properties of the electropolymerized films, the electrochemical behavior of these polymers provides validation of our targeted design to maximize the energy density of organic electrode materials for electrical energy storage applications.

Synthesis and Charge Transport Properties of Redox-Active Nitroxide Polyethers with Large Site Density

To maximize the theoretical redox capacity of polymers containing cyclic nitroxides as redox-active pendant groups for high-density charge storage application, a compact five-membered ring with the smallest equivalent weight among the robust cyclic nitroxides was directly bound to a poly(ethylene oxide) chain. 2,2,5,5-Tetramethyl-3-oxiranyl-3-pyrrolin-1-oxyl was synthesized and polymerized via anionic coordinated ring-opening polymerization utilizing diethyl zinc/H2O as an initiator. The unpaired electron in the monomer survived during the polymerization, giving rise to a high density redox polymer with a weight-specific theoretical capacity of 147 mA h/g. Cyclic voltammetry of the polymer layer confined at the surface of an electrode revealed a large redox capacity comparable to the theoretical capacity, which was ascribed to the efficient swelling and yet insoluble properties of the polyether in electrolyte solutions by virtue of the high molecular weight of >105 and adhesive properties allowing immobilization of the layer on the electrode surface. The redox capacity also indicated that the ionophoric polyether matrix accommodated electrolyte anions through the polymer/electrolyte interface to neutralize positive charges produced by the oxidation of the neutral radicals at the polymer/electrode interface. The diffusion coefficient for the redox gradient-driven charge hopping process corresponded to a large second-order rate constant in the order of 107 M−1 s−1, which suggested an efficient electron self-exchange reaction throughout the polymer layer due to the large redox site population and hence to the small intersite distance. Test cells fabricated with a polymer/carbon fiber composite layer on an aluminum current collector as the cathode and a Li anode sandwiching an electrolyte layer were capable of charging and discharging as a secondary battery with an output voltage near 3.7 V and were durable for more than 103 charging−discharging cycles without substantial degradation.

Side-chain coordination polymers containing redox-active pendants: complexation of ferrocene-based bidentate ligands with M(hfac)2(hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate, M = Mn, Ni, Cu, Zn) and CuX (X = I, Br)

Coordination polymers with redox active pendants, {5-ferrocenylpyrimidine·M(hfac)2}n [M = Mn (1a), Ni (1b), Cu (1c), Zn (1d)] and {5-ferrocenylpyrimidine·CuX}n [X = I (2a), Br (2b)], have been prepared and characterized. In complex 1c, the Cu(II) ions are linked by the bridging ligands to form one-dimensional chains. This complex was shown to exhibit an antiferromagnetic intra-chain interaction, which was analyzed by the Bonner–Fisher model (2J/kB = −6.3 K). Complex 2a shows a two-dimensional sheet-like structure, in which one-dimensional chains, consisting of the Cu(I) ions and the ligands, are further bridged by iodide anions. These compounds show quasi-reversible redox processes of the ferrocenyl groups in the solid state.

TTF–porphyrin dyads as novel photoinduced electron transfer systems

A TTF-linked porphyrin dyad and its zinc complex have been synthesized as novel photosystems with a redox-active pendant. The two chromophores of these dyads are not interactive in the absorption spectra, but the fluorescence of the porphyrin chromophore is dramatically quenched by intramolecular electron transfer from the TTF pendant.

In situ FTIR study of charge conduction in a poly(3-methylthiophene)–poly({1-[2-(3-thienyl)ethyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) perchlorate) copolymer film

We have used in situ FTIR spectroscopy to examine the effect of incorporating redox-active pendant [Ni(tetraazamacrocycle)]2+ moieties in poly(3-methylthiophene)(PMT). Experiments were performed on Pt electrodes modified with PMT, and PMT-copoly-{1-[2-(3-thienyl)ethyl]-1,4,8,11-tetraazacyclotetradecane}nicke(II) perchlorate (PMT–P1) in acetonitrile–0.2 mol l–1 tetraethylammonium tetrafluoroborate. Inspection of the IR activated vibration (IRAV) region of the spectrum (1000–1600 cm–1) of PMT shows that oxidation occurs with three distinct regions of behaviour with increasing potential. There is a changeover in the carrier type being generated between the first two regions, probably from polaron to bipolaron. The evolution of the electronic band (1600–6000 cm–1), due to a charge carrier transition, suggests a relatively uniform conjugation length in this polymer. That the copolymer backbone posesses mobile charges is shown by the observation of IRAVs in its spectra, at similar energies to those observed for other thiophene-based polymers. However, the electronic band of the copolymer is at lower energy than for PMT, and its position changes significantly with potential. This is related to the increase in static relative permittivity of PMT–P1 compared with PMT owing to the ion content of the neutral copolymer film. Neutral PMT films appear to contain very little solvent, whereas neutral PMT–P1 films contain significant quantities, possibly associated with the [Ni(macrocycle)]2+ moieties.