Enhancing the ionic conductivity across the electrolyte separator in nonaqueous redox flow batteries (NRFBs) is essential for improving their performance and enabling their widespread utilization. Separating redox-active species by size exclusion without greatly impeding the transport of supporting electrolyte is a potentially powerful alternative to the use of poorly performing ion-exchange membranes. However, this strategy has not been explored possibly due to the lack of suitable redox-active species that are easily varied in size, remain highly soluble, and exhibit good electrochemical properties. Here we report the synthesis, electrochemical characterization, and transport properties of redox-active poly(vinylbenzyl ethylviologen) (RAPs) with molecular weights between 21 and 318 kDa. The RAPs reported here show very good solubility (up to at least 2.0 M) in acetonitrile and propylene carbonate. Ultramicroelectrode voltammetry reveals facile electron transfer with E1/2 ∼ -0.7 V vs Ag/Ag(+)(0.1 M) for the viologen 2+/+ reduction at concentrations as high as 1.0 M in acetonitrile. Controlled potential bulk electrolysis indicates that 94-99% of the nominal charge on different RAPs is accessible and that the electrolysis products are stable upon cycling. The dependence of the diffusion coefficient on molecular weight suggests the adequacy of the Stokes-Einstein formalism to describe RAPs. The size-selective transport properties of LiBF4 and RAPs across commercial off-the-shelf (COTS) separators such as Celgard 2400 and Celgard 2325 were tested. COTS porous separators show ca. 70 times higher selectivity for charge balancing ions (Li(+)BF4(-)) compared to high molecular weight RAPs. RAPs rejection across these separators showed a strong dependence on polymer molecular weight as well as the pore size; the rejection increased with both increasing polymer molecular weight and reduction in pore size. Significant rejection was observed even for rpoly/rpore (polymer solvodynamic size relative to pore size) values as low as 0.3. The high concentration attainable (>2.0 M) for RAPs in common nonaqueous battery solvents, their electrochemical and chemical reversibility, and their hindered transport across porous separators make them attractive materials for nonaqueous redox flow batteries based on the enabling concept of size-selectivity.

We recently showed that viologen-based redox active polymers (RAPs) with molecular weights between 21 and 318 kDa are attractive charge storage materials as anolytes for size-selective non-aqueous redox flow batteries. Here, we characterize the electron transfer mechanisms of these RAPs, as well as a ferrocene based catholyte RAP, in acetonitrile/Li + electrolyte. We utilized scanning electrochemical microscopy (SECM) and rotating disk electrode (RDE) voltammetry to measure the rate of electron transfer and the rate of charge hopping between neighboring pendants along the insulating backbone of RAPs. The electron transfer kinetics of a 271 kDa ferrocene RAP mimic the facile kinetics of its monomer repeating unit. In contrast, viologen RAPs displayed RDE and SECM signatures that suggest a preceding chemical step to electron transfer. Viologen RAPs adsorb strongly to the electrode surface and create a redox active film that controls the rate of electron transfer via self-exchange. In addition, finite element simulations including a preceding chemical step demonstrated that a purely mass-transfer limited model is insufficient to recreate the viologen RAP feedback SECM response. Non-aqueous redox flow batteries (NRFBs) are emerging technologies for electrical energy storage, and are an attractive alternative to their aqueous counterparts. [1][2][3][4][5][6] The choice of organic solvents with voltage windows larger than that required for the electrolysis of water enables the use of a larger variety of redox molecules and electrolyte systems, where the combination of high solubility and higher reaction potentials leads to increased energy density. A major challenge in NRFBs is to increase the conductivity of the electrolyte through the commonly used ion exchange membrane. In response to this challenge, our groups recently reported on a new size-selective concept in flow batteries where a porous separator replaces poorly performing ion exchange membranes.7-9 These porous separators are coupled to high energy density redox active polymers (RAPs) that replaced small molecules as charge storage media. RAPs consist of an unconjugated polymer backbone densely decorated with redox active pendants. We previously reported on the electrochemical characterization of viologen-based RAPs with molecular weight between 21 and 318 kDa, which displayed hydrodynamic radii between 4 and 7 nm. Increasing the size of RAPs decreased transport through Celgard which was used as a porous separator. However, we are also interested in understanding the impact of size on the rate and mechanisms of charge transfer and transport on RAPs to determine their ultimate performance limit. 10RAPs displayed attractive electrochemical properties such as similar standard reduction potentials than those of the parent monomer, efficient bulk electrolysis with up to +98% of redox groups reversibly accessible, and high solubility of up to 2.8 M with quantitative electrode reactivity even when at 1.0 M. Despite these promising features, little is known about el...

Thermodynamic properties of Ba-Bi alloys, including the activity, partial molar entropy and enthalpy, were determined using the electromotive force (emf) technique for fourteen compositions, x Ba = 0.05-0.80. Emf measurements were performed at ambient pressure using a Ba(s)|CaF 2-BaF 2 |Ba(in Bi) or Ba-Bi(x Ba = 0.05)|CaF 2-BaF 2 |Ba(in Bi) electrochemical cells at 723-1073 K. At 773K, activity values of Ba were as low as 6.6 × 10-16 at mole fraction x Ba = 0.05 and approached unity for mole fractions x Ba ≥ 0.80. Stable emf values were observed at mole fractions x Ba = 0.05-0.25, exhibiting less than a 5 mV difference between the heating and cooling cycles. Mole fractions x Ba ≥ 0.30 exhibited increased hysteresis or had an unexpected emf profile due to the formation of metastable phases such as Bi and Ba 5 Bi 3 , confirmed by X-ray diffraction. The Ba-Bi alloys were further characterized using differential scanning calorimetry over the entire composition range. Based on these data, a revised Ba-Bi binary phase diagram is proposed.

The thermodynamic properties of Sr-Bi alloys were determined by electromotive force (emf) measurements to evaluate the viability of liquid bismuth metal as a medium for separating alkali/alkaline-earth fission products from molten salt electrolyte. A Sr(s)|CaF 2-SrF 2 |Sr(in Bi) cell was used to measure emf values at 748-1023 K for thirteen Sr-Bi alloys at mole fractions 0.05 ≤ x Sr ≤ 0.75. Activity values of strontium in bismuth were determined at 788 K, 888 K, and 988 K as well as the partial molar entropy and enthalpy at each composition. Reproducible emf values within ± 5mV were obtained up to x Sr = 0.35 during cooling-heating cycle. At higher mole fractions (x Sr ≥ 0.40), the emf values exhibited increased hysteresis during the thermal cycles due to the strong tendency of the alloys to form meta-stable phases. The non-equilibrium phase behavior of Sr-Bi alloys was verified by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and microstructural analyses. Compared to the existing equilibrium phase diagram, two additional phases of Sr 2 Bi 3 and Sr 4 Bi 3 were identified and discussed. Liquid-state solubility of Sr was 15-40 mol% at 788-988 K and the activity values were as low as 10-13 at 788 K, implying strong chemical interactions between Sr and Bi.

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