Redox-active Macromolecules Research Articles

A perspective on organic electrode materials and technologies for next generation batteries

In this perspective article, we review some of the most recent advances in the emerging field of organic materials as the electroactive component in solid electrodes for batteries. These comprise, but are not limited to, organometallic salts, small molecular systems, redox-active macromolecules, as well as hybrid formulations with inorganic electrode constituents. The materials are first scrutinized in terms of their general electrochemical performance and most apparent challenges, while an outlook is then made into how to best utilize them in battery electrodes and in all-organic cells. An insight into the fundamental structural-dynamic properties of these compounds, not least explored through a range of modelling and characterization techniques, is also given to complement the experimental advances. The major advantages of these materials as compared to competing technologies are most likely their potentially low environmental impact and general sustainability, which forms the context of this summary of the research field and corresponding technology area.

Octaboraneyl Complexes of Nickel: Monomers for Redox-Active Coordination Polymers.

Herein, we establish the preparation, characterization, and reactivity of a new diphosphine ligand, 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P2 BCy 4 ), a scaffold that contains four pendant boranes. An entryway into the coordination chemistry of P2 BCy 4 is established by using nickel, providing the octaboraneyl complex [Ni(P2 BCy 4 )2 ]-this species contains a boron-rich secondary coordination sphere that reacts readily with Lewis bases. In the case of 4,4'-bipyridine, an air-sensitive coordination polymer is obtained. Characterization of this material by solid-state NMR and EPR spectroscopy reveals the presence of a charge-transfer polymer, which forms as a function of intramolecular Ni→4,4'-bpy electron transfer (ET), providing an array of oxidized nickel sites and reduced 4,4'-bpy radical anion sites. Notably, the related intermolecular reaction between the model fragments [Ni(dnppe)2 ] (dnppe=1,2-bis(di-n-propylphosphino)ethane) and a bis(boraneyl)-protected 4,4'-bpy, provides no ET. Overall, the P2 BCy 4 fragment provides a unique opportunity for Lewis base activation, in one case allowing for the facile construction of monomers for incorporation into redox-active macromolecules.

Investigating the factors that influence resistance rise of PIM-1 membranes in nonaqueous electrolytes

As redox active macromolecules are introduced to the materials repertoire of redox flow batteries (RFBs), nanoporous membranes, such as polymers of intrinsic microporosity (PIMs), are emerging as a viable separation strategy. Although their selectivity has been demonstrated, PIM-based membranes suffer from time-dependent resistance rise in nonaqueous electrolytes. Here, we study this phenomenon as a function of membrane thickness, electrolyte flow rate, and solvent washing using a diagnostic flow cell configuration. We find that the rate and magnitude of resistance rise can be significantly reduced through the combination of low electrolyte flow rate and solvent prewash. Further, our results indicate that, since the increase is not associated with irreversible chemical and structural changes, the membrane performance can be recovered via ex-situ or in-situ solvent washes.

New Cu(I) complexes with biquinolyl-containing polymer ligands as electrocatalysts for O 2 activation in the oxidation of alcohols

Electrochemical approach to a new electroactive catalytic system based on polyamic acids with biquinolyl (biQ) fragments in the polymer backbone capable of coordination to Cu(I)-ions was elaborated using sacrificial Cu anode. The redox properties of the resultant Cu(I) polymer complex were characterized by voltammetric methods. Polynuclear Cu(I) complex with only one biQ ligand per Cu(I) center showed high oxidase activity in air oxidation of aliphatic and aromatic alcohols to corresponding carbonyl derivatives with concomitant reduction of molecular oxygen to water. The reactions proceed in acetonitrile or N-methylpyrrolidone at a potential of Cu II/Cu I electroreduction (−0.55 V versus Ag/AgCl/KCl) with the high preparative yield and current efficiency. The electrocatalytic efficiency is enhanced when the Cu-containing polymer is immobilized on the graphite surface. The stoichiometric composition of the adsorbed polymer complex was determined by means of coulometric assays of the quantities of electroactive biQ fragments and Cu(I) present on the surface. The possibility of the obtained redox-active macromolecules to mimic the active site of certain copper-containing enzymes capable of reversible binding and activation of dioxygen is discussed, providing some insight into the mechanism of the catalytic process.

Probing Diffusional Transport in Redox-Active Dendrimers

The diffusion coefficients of polyamidoamine (PAMAM) dendrimers (generations 0−4) surface-modified with terpyridyl and bipyridyl ruthenium complexes were determined using cyclic voltammetry (CV), voltammetry at a rotating disk electrode (RDE), and pulsed field gradient spin−echo nuclear magnetic resonance (PFGSE-NMR). In all experiments, the magnitude of the diffusion coefficient decreased with increasing dendrimer size, but each experimental technique led to dramatically different experimental results in terms of the value for a particular molecule. The diffusion coefficients determined by PFGSE-NMR were larger than those determined by RDE, and the diffusion coefficients determined by CV were smaller still. The Stokes−Einstein equation was used to estimate the size of the dendrimers from the measured diffusion coefficients and the results from CV and RDE experiments were found to predict unphysically large diameters for the dendrimer molecules. On the other hand, the diffusion coefficients determined by PFGSE-NMR led to physically reasonable estimations of the dendrimers' sizes. It is proposed that, for macromolecules containing multiple redox centers, electrochemical methods may not provide an accurate measure of diffusion coefficients due to incomplete sampling of all redox sites. Unlike the electrochemical methods, PFGSE-NMR does not rely on electron-transfer occurring, and it appears to be superior to those electrochemical methods for measuring the diffusion coefficients of redox-active macromolecules.