Electroreduction of divanillin to polyvanillin in an electrochemical flow reactor.

The electrochemical conversion of biobased intermediates offers an attractive and sustainable process for the production of green chemicals. One promising synthesis route is the production of the total vanillin-based polymer polyvanillin, which can be produced by electrochemical pinacolization of divanillin (5-5´bisvanillyl). Divanillin can be easily enzymatically generated from vanillin, a renewable intermediate accessible from lignin on an industrial scale. This study investigates systematically the electrochemical production of polyvanillin in a divided plane parallel flow reactor in recirculation mode. Several analytic methods, such as online UV-VIS spectroscopy, size exclusion chromatography (SEC), 2D-NMR (HSQC, 13C/1H), TGA and DSC were used to monitor the reaction progress and to characterize the reaction products under different galvanostatic reaction conditions revealing new insights into the reaction mechanism and structural features of the polymer. Further, by using an electrochemical engineering-based approach determining the limiting current densities, we readily achieved high current densities over 50 mA cm-2 for the polyvanillin synthesis and reached averaged molecular weights up to Mw = 4100 g mol-1 and Mn = 2700 g mol-1. The cathodic polymerization to polyvanillin offers an innovative approach for the electrochemical production of biobased polymers presented on flow cell level.

Pristine and Modified Porous Membranes for Zinc Slurry-Air Flow Battery.

The membrane is a crucial component of Zn slurry-air flow battery since it provides ionic conductivity between the electrodes while avoiding the mixing of the two compartments. Herein, six commercial membranes (Cellophane™ 350PØØ, Zirfon®, Fumatech® PBI, Celgard® 3501, 3401 and 5550) were first characterized in terms of electrolyte uptake, ion conductivity and zincate ion crossover, and tested in Zn slurry-air flow battery. The peak power density of the battery employing the membranes was found to depend on the in-situ cell resistance. Among them, the cell using Celgard® 3501 membrane, with in-situ area resistance of 2 Ω cm2 at room temperature displayed the highest peak power density (90 mW cm-2). However, due to the porous nature of most of these membranes, a significant crossover of zincate ions was observed. To address this issue, an ion-selective ionomer containing modified poly(phenylene oxide) (PPO) and N-spirocyclic quaternary ammonium monomer was coated on a Celgard® 3501 membrane and crosslinked via UV irradiation (PPO-3.45 + 3501). Moreover, commercial FAA-3 solutions (FAA, Fumatech) were coated for comparison purpose. The successful impregnation of the membrane with the anion-exchange polymers was confirmed by SEM, FTIR and Hg porosimetry. The PPO-3.45 + 3501 membrane exhibited 18 times lower zincate ions crossover compared to that of the pristine membrane (5.2 × 10-13 vs. 9.2 × 10-12 m2 s-1). With low zincate ions crossover and a peak power density of 66 mW cm-2, the prepared membrane is a suitable candidate for rechargeable Zn slurry-air flow batteries.

Electrochemical Lithiation/Delithiation of ZnO in 3D-Structured Electrodes: Elucidating the Mechanism and the Solid Electrolyte Interphase Formation.

Conversion/alloy active materials, such as ZnO, are one of the most promising candidates to replace graphite anodes in lithium-ion batteries. Besides a high specific capacity (qZnO = 987 mAh g-1), ZnO offers a high lithium-ion diffusion and fast reaction kinetics, leading to a high-rate capability, which is required for the intended fast charging of battery electric vehicles. However, lithium-ion storage in ZnO is accompanied by the formation of lithium-rich solid electrolyte interphase (SEI) layers, immense volume expansion, and a large voltage hysteresis. Nonetheless, ZnO is appealing as an anode material for lithium-ion batteries and is investigated intensively. Surprisingly, the conclusions reported on the reaction mechanism are contradictory and the formation and composition of the SEI are addressed in only a few works. In this work, we investigate lithiation, delithiation, and SEI formation with ZnO in ether-based electrolytes for the first time reported in the literature. The combination of operando and ex situ experiments (cyclic voltammetry, X-ray photoelectron spectroscopy, X-ray diffraction, coupled gas chromatography and mass spectrometry, differential electrochemical mass spectrometry, and scanning electron microscopy) clarifies the misunderstanding of the reaction mechanism. We evidence that the conversion and alloy reaction take place simultaneously inside the bulk of the electrode. Furthermore, we show that a two-layered SEI is formed on the surface. The SEI is decomposed reversibly upon cycling. In the end, we address the issue of the volume expansion and associated capacity fading by incorporating ZnO into a mesoporous carbon network. This approach reduces the capacity fading and yields cells with a specific capacity of above 500 mAh g-1 after 150 cycles.

Systematic Study of Quaternary Ammonium Cations for Bromine Sequestering Application in High Energy Density Electrolytes for Hydrogen Bromine Redox Flow Batteries.

Bromine complexing agents (BCAs) are used to reduce the vapor pressure of bromine in the aqueous electrolytes of bromine flow batteries. BCAs bind hazardous, volatile bromine by forming a second, heavy liquid fused salt. The properties of BCAs in a strongly acidic bromine electrolyte are largely unexplored. A total of 38 different quaternary ammonium halides are investigated ex situ regarding their properties and applicability in bromine electrolytes as BCAs. The focus is on the development of safe and performant HBr/Br2/H2O electrolytes with a theoretical capacity of 180 Ah L-1 for hydrogen bromine redox flow batteries (H2/Br2-RFB). Stable liquid fused salts, moderate bromine complexation, large conductivities and large redox potentials in the aqueous phase of the electrolytes are investigated in order to determine the most applicable BCA for this kind of electrolyte. A detailed study on the properties of BCA cations in these parameters is provided for the first time, as well as for electrolyte mixtures at different states of charge of the electrolyte. 1-ethylpyridin-1-ium bromide [C2Py]Br is selected from 38 BCAs based on its properties as a BCA that should be focused on for application in electrolytes for H2/Br2-RFB in the future.

Electrochemical synthesis of biobased polymers and polymer building blocks from vanillin.

Vanillin, one of the few biobased aromatic compounds available on an industrial level, is an attractive candidate for the synthesis of biobased polymers and polymer building blocks. This study presents a detailed investigation of the reductive electrochemical coupling process by pinacolization of vanillin and divanillin in an electrochemical H-type cell setup to the polymer building block hydrovanilloin and to polyvanillin, respectively. Therein, different cathode materials are screened by linear sweep voltammetry for their capability and activity of hydrodimerization of phenolic aromatic aldehydes in alkaline aqueous media. Product distributions and faradaic efficiencies of the electrochemical vanillin reduction are investigated in bulk electrolysis experiments. Dependencies on electrochemical parameters such as current densities, applied charges and cathode materials are studied. Furthermore, the polyvanillin synthesis from divanillin is also investigated by bulk electrolysis experiments. The effects of selected electrochemical parameters (current density, applied charge and electrode material) on yield and structural features (weight-average molecular weight (M W), number-average molecular weight (M N), polydispersity (M W/M N)) measured by size exclusion chromatography of the obtained polyvanillin were evaluated. Structural features of isolated polyvanillin were determined by 2D-NMR (HSQC, 13C/1H) analyses and by 31P-NMR analyses after in situ labeling with Cl-TMDP and possible pathways for their generation are discussed. These two promising electro-synthetic processes studied are free of hazardous materials and reagents and highlight the contributions of preparative electrochemistry to green chemistry and further pave the way toward the application of electrochemistry in the synthesis of biobased building blocks and polymers.

High energy density electrolytes for H2/Br2 redox flow batteries, their polybromide composition and influence on battery cycling limits.

Hydrogen-bromine redox flow batteries (H2/Br2-RFB) are a promising stationary energy storage solution, offering energy storage densities up to 200 W h L-1. In this study, high energy density electrolytes of concentrated hydrobromic acid of up to 7.7 M are investigated. Particular polybromide ion (Br2n+1 -; n = 1-3) concentrations in the electrolyte at different states of charge, their effect on the electrolytic conductivity and cell operation limits are investigated for the first time. The concentrations of individual polybromides in the electrolytes are determined by Raman spectroscopy. Tribromide (Br3 -) and pentabromide (Br5 -) are predominantly present in equal concentrations over the entire concentration range. Besides Br3 - and Br5 -, heptabromide (Br7 -) exists in the electrolyte solution at higher bromine concentrations. It is shown that polybromide equilibria and their constants of Br3 - and Br5 - from literature are not applicable for highly concentrated solutions. The conductivity of the electrolytes depends primarily on the high proton concentration. The presence of higher polybromides leads to lower conductivities. The solubility of bromine increases disproportionately with increasing bromide concentration, since higher polybromides such as Br7 - or Br5 - are preferably formed with increasing bromide concentration. Cycling experiments on electrolyte in a single cell are performed and combined with limitations due to electrolyte conductivity and bromine solubility. Based on these results concentrations of the electrolyte are defined for potential operation in a H2/Br2-RFB in the range 1.0 M < c(HBr) < 7.7 M and c(Br2) < 3.35 M, leading to a theoretical energy density of 196 W h L-1.

Mechanistic Insights into the Lithiation and Delithiation of Iron-Doped Zinc Oxide: The Nucleation Site Model.

The detailed mechanistic understanding of the electrochemical reactions occurring in lithium-ion battery electrodes is fundamental for their further improvement. Conversion/alloying materials (CAMs), such as Zn0.9Fe0.1O, one of the most recent alternatives for classic graphite anodes, offer superior specific capacity and rate capability. However, despite fast kinetics, CAMs suffer from a large voltage hysteresis upon de-/lithiation and improvable Coulombic efficiencies when cycled in a large voltage window. Here, we use isothermal microcalorimetry together with operando X-ray diffraction as well as ex situ 7Li NMR and 57Fe Mössbauer spectroscopies to investigate the asymmetric reaction mechanism of the lithiation and delithiation of Zn0.9Fe0.1O during electrochemical cycling. We demonstrate that the measured heat flow is correlated with compositional changes of the electrode material. This combination of highly complementary techniques allows us to propose a new nucleation site model for the initial lithiation of Zn0.9Fe0.1O. Modeling the heat flow provides concrete evidence for the deleterious impact of high anodic cutoff potentials (>2 V), resulting in a continuous quasireversible solid electrolyte interphase formation. The presented methodology is suggested to provide improved insights into the reaction mechanism of conversion- and alloying-type energy-storage materials.

Revisiting the Electrochemical Lithiation Mechanism of Aluminum and the Role of Li-rich Phases (Li1+x Al) on Capacity Fading.

Aluminum is an appealing anode material for high-energy-density lithium-ion batteries (LIBs), owing to its low cost, environmental benignity, high specific capacity, and lower relative volume expansion compared with other alloying materials. However, both, the working and capacity fading processes are not yet consistently and comprehensively understood, which has largely hindered its development. In this study, the electrochemical alloying process of aluminum anodes with lithium is systematically studied by the combination of several in situ and ex situ techniques, providing new insights into phase transitions, electrode dynamics, and surface chemistry. Particular attention is paid to the role of the Li-rich alloys (Li1+x Al). Its existence on the surface of the Al electrode is unexpectedly observed, and its growth in the electrode bulk is found to be strictly correlated with cell failure. Interestingly, cell failure can be delayed by choosing an appropriate electrolyte. This work contributes to a solid and comprehensive understanding of the puzzling Al (de-)lithiation processes, which is fundamental and highly enlightening for future research work on Al and other alloyed anodes.

Size matters! On the way to ionic liquid systems without ion pairing.

Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium-sized [NTf2 ](-) (0.230 nm(3) ; Tf=CF3 SO3 (-) ) and the large [Al(hfip)4 ](-) (0.581 nm(3) ; hfip=OC(H)(CF3 )2 ) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel-Fulcher-Tammann (VFT) behavior. Ion-specific self-diffusion constants were measured at room temperature by pulsed-gradient stimulated-echo (PGSTE) NMR experiments. In general, self-diffusion constants of both cations and anions in [Al(hfip)4 ](-) -based ILs were higher than in [NTf2 ](-) -based ILs. Ionicities were calculated from self-diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4 ](-) -based ILs yield higher ionicities than their [NTf2 ](-) analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4 ](-) -based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4 ](-) . Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.

High capacity vertical aligned carbon nanotube/sulfur composite cathodes for lithium-sulfur batteries.

Binder free vertical aligned (VA) CNT/sulfur composite electrodes with high sulfur loadings up to 70 wt% were synthesized delivering discharge capacities higher than 800 mAh g(-1) of the total composite electrode mass.

Naphthobipyrrole: versatile synthesis and electropolymerization.

A facile synthetic route to a new polycyclic pyrrole derivative, 3,8-diethyl-1,10-dihydro-benzo[e]pyrrolo[3,2-g]indole (1), is reported. This annulated bipyrrole acts as a monomer for electropolymerization and forms an electrochromic conducting polymer (poly1) when electrooxidized at low potentials (0.4 V vs Ag/Ag(+)) in acetonitrile. The presence of alkyl substituents at the 3 and 8 carbons (ß-pyrrolic positions) induces regioselective 2,5'-coupling of the pyrrole repeat units and gives rise to the more uniform polymeric product, poly1. Poly1 exhibits globular morphology, as judged from SEM pictures. Its spectroelectrochemical features can be attributed to the formation of four possible states: neutral, polaron, bipolaron, and transverse bipolaron. The relatively low switching potentials (-0.6 to +0.9 V vs Ag/Ag(+) in MeCN) displayed by poly1 leads us to suggest that 1 has a role to play as a polymerizable moiety for the development of multicolor electrochromic materials.