Deconvolution of electrochemical impedance data for the monitoring of electrode degradation in VRFB

Cooperation of Multifunctional Redox Mediator and Separator Modification to Enhance Li-S Batteries Performance under Low Electrolyte/Sulfur Ratios

Sluggish reaction kinetics of sulfur species fundamentally trigger the incomplete conversion of S8↔Li2S and restricted lifespan of lithium-sulfur batteries, especially under high sulfur loading and/or low electrolyte/sulfur (E/S) ratios. Developing redox mediators (RMs) is an effective strategy to boost the battery reaction kinetics, yet their multifunctionality and shuttle inhibition are still not available. Here, a unique ethyl viologen (EtV2+) RM with two highly reversible redox couples (EtV2+/EtV+, EtV+/EtV0) is demonstrated to well match the redox chemistry of sulfur species, in terms of accelerating the electron transfer in S8 reduction, Li2S nucleation and the Li2S oxidation. When coupling with a functionalized separator with electronegative -SO3Li groups, a synergetic chemistry is established to ensure the substantial inhibition of the shuttle effect and the acceleration of charge transfer. As a result, the activation energies during sulfur species conversion (S8→Li2S4→Li2S/Li2S2→Li2S4→S8) are decreased, especially for Li2S nucleation step. The correspond lithium-sulfur batteries achieve a high specific capacity of 1006.9 mAh g-1 (0.1 C; sulfur loading of 5 mg cm-2; E/S ratios of 6 μL mgs -1), and an outstanding cycling stability. This study provides a paradigm of solving complex problems via multifunctional molecule engineering and strategic cooperation towards Li-S batteries and other battery communities.

Acetylacetonate-modified TiO2 nanoparticles coated on the carbon felt as the negative electrode of vanadium redox flow battery for reducing HER and enhancing V3+/V2+ redox reactions

The occurrence of the hydrogen evolution reaction (HER) on the surface of the carbon-based negative electrode of the vanadium redox flow battery (VRFB) causes high charge transfer resistance (RCT) for the desired V3+/V2+ redox reaction leading to irreversible capacity loss. To this effect, we have synthesized acetylacetonate-modified TiO2 (SGTA) and unmodified TiO2 (SGT) coating colloidal solutions as electrocatalysts for enhanced V3+/V2+ redox reaction on the carbon-felt negative electrodes of VRFB. The SGTA particles exhibit significantly higher homogeneity with sizes of ≤15 nm, in comparison to the severely aggregated SGT particles with diameters of ∼23-75 nm in colloidal solution. When coated on the pristine carbon felt (P-CF), the surface morphology of the SGTA@CF electrode exhibits relatively dense, uniformly coated particles, in comparison to the sparse, non-even coating of aggregated particles on the SGT@CF electrode surface. Consequently, the charge transfer for V3+ → V2+ reduction reaction and charge storage capacity are determined to be in the order SGTA@CF > SGT@CF > P-CF, confirming that the competitive and irreversible HER was higher on the surface of non-evenly coated fibers and bare carbon felt, respectively.

Spectroelectrochemical study of carbon structural and functionality characteristics on vanadium redox reactions for flow batteries

Vanadium redox flow batteries have applications for large-scale electricity storage. This paper reports the influence of carbon structural characteristics of sustainable walnut shell-derived carbons in carbon/polyvinylidene fluoride composite electrodes on vanadium redox reactions. Pyrolysis, gasification, and chemical treatment procedures were used to modify the structural characteristics of carbons. Carbon functional groups were modified by chemical treatment with HNO3, heat treatment with K2CO3, and high-temperature NH3 treatment. Carbon porous structures were characterized using gas adsorption studies. Raman spectroscopy and X-ray diffraction were used to characterize the carbon molecular structure. Functional groups were characterized using X-ray photoelectron spectroscopy, acid/base titrations, temperature-programmed desorption, and Fourier transform infrared spectroscopy. The influence of carbon structure, porosity, and surface functional groups on the redox reactions of vanadium was investigated using cyclic voltammetry and electrical impedance spectroscopy. The VO2+/VO2 + and V2+/V3+ couples had well-defined peaks in cyclic voltammetry, with the former being the most intense, but the V3+/VO2+ couple was not observed for samples carbonized under nitrogen. The results show that V2+/V3+ and VO2+/VO2 + couples observed in cyclic voltammograms were enhanced for carbonization temperatures up to 800 °C. Electrical impedance spectroscopy also showed impedance trends. The electrochemistry results are primarily related to changes in carbon structure and the catalysis of V3+ oxidation by surface functional groups in the carbon structure. The V3+/VO2+ couple was limited by slow kinetics, but it occurs on specific oxygen and nitrogen sites in the carbon structure. The oxidation of V(iii) to V(iv) only occurs on a limited number of surface sites, and the outer-sphere electron transfer to oxidize V(iii) takes place at much more positive potentials. The coulombic, voltage, and energy efficiency of the carbon electrodes were suitable for batteries.

Systematic Characterization of Degraded Anion Exchange Membranes Retrieved from Vanadium Redox Flow Battery Field Tests

Commercially available anion exchange membranes were retrieved from VRFB field tests and their degradation due to the various operation conditions is analyzed by in-situ and ex-situ measurements. Ion exchange capacity, permeability and swelling power are used as direct criteria for irreversible changes. Small-angle X-ray scattering (SAXS) and Differential scanning calorimetry (DSC) analyses are used as fingerprint methods and provide information about the morphology and change of the structural properties. A decrease in crystallinity can be detected due to membrane degradation, and, in addition, an indication of reduced polymer chain length is found. While the proton diffusion either increase or decline significantly, the ion exchange capacity and swelling power both are reduced. The observed extent of changes was in good agreement with in-situ measurements in a test cell, where the coulombic and voltage efficiencies are reduced compared to a pristine reference material due to the degradation process.