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This paper addresses the damaging role of the parasitic hydrogen evolution reaction (HER) in the negative half-cell of a vanadium redox flow battery (VRFB) on state-of-the-art carbon felt electrodes at different temperatures. It was found that increasing the temperature resulted in a better catalytic performance for both the positive and negative half-cell reactions. In addition, increasing the temperature significantly enhanced the undesired HER at the negative side. Operating the VRFB cell at higher temperature led to a decrease in the coulombic efficiency attributed to the higher hydrogen production. More pronounced hydrogen production caused an oxidation on the surface of the carbon fibers and a degradation of the electrode as indicated from scanning electron microscopy and X-ray photoelectron spectroscopy measurements. This observed degradation results in fading of the overall performance of the vanadium redox flow battery over time.

Actin contributes to an exceptionally wide range of cellular processes through the assembly and disassembly of highly dynamic and ordered structures. Visualizing these structures in cells can help us understand how the molecular players of the actin machinery work together to produce force-generating systems. In recent years, cryo-electron tomography (cryo-ET) has become the method of choice for structural analysis of the cell interior at the molecular scale. Here we review advances in cryo-ET workflows that have enabled this transformation, especially the automation of sample preparation procedures, data collection, and processing. We discuss new structural analyses of dynamic actin assemblies in cryo-preserved cells, which have provided mechanistic insights into actin assembly and function at the nanoscale. Finally, we highlight the latest visual proteomics studies of actin filaments and their interactors reaching sub-nanometer resolutions in cells.

The constant phase element (CPE) is a well-established circuit component for the frequency domain simulation of electrochemical reactions at disperse and heterogeneous electrodes. Computing the time-dependent current response of voltammetric experiments with interfacial CPE behaviour is, however, not a straightforward task, owing to the lack of analytical time domain solutions of the convoluted capacitive current. In this paper, a universal framework for the convolutive treatment of cyclic voltammetry (CV), alternating current cyclic voltammetry (ACCV), (large) sine wave voltammetry (LSWV) and electrochemical impedance spectroscopy (EIS) in presence of interfacial CPEs and uncompensated Ohmic resistances is presented. A combination of numerical and analytical inverse Laplace transformation techniques allows the accurate assessment of the non-ideal capacitive behaviour, which can be used in turn for classical convolution algorithms. The convolutive approach bears the advantage that different electrode geometries with any kind of spatial boundary conditions as well as first order homogeneous chemical reactions, coupled to the charge transfer step, can be implemented readily. This offers an exceptionally large degree of flexibility in the simulations for the electrochemical system at hand.

Decorating carbon felt electrodes with bismuth and bismuth oxide nanoparticles is proposed to be a promising strategy for enhancing the sluggish electron transfer kinetics of the negative half-cell reaction in vanadium redox flow batteries. However, regarding the highly corrosive electrolyte solution, major concerns on the stability of the solid bismuth phase and thus on the local catalyst distribution and catalyst functionality emerge. With this study we present a novel in-operando cell design allowing for X-ray imaging during battery tests in laboratory scale dimensions. In this manner, we verify a dissolution of the bismuth/bismuth oxide particles in open circuit potential condition at SOC = 0 (SOC: state of charge) as well as a re-deposition during the charging process of the battery. No dissolution during the discharging process can be detected. With this knowledge, the effect of the bismuth/bismuth oxide distribution on the electrochemical performance of the battery is investigated. By performing the deposition during the charging process at different electrolyte flow rates, different deposition patterns are obtained. However, independent of the deposition pattern, similar cell performances are achieved that put the enhancement effect by bismuth as heterogeneous catalyst into question.

A novel four-step strategy for real-space simulation of cyclic voltammetry (CV) at carbon felt electrodes is presented, circumventing the diffusion domain approximation approach used so far for CV simulation at porous electrodes. At first, the three-dimensional template of the internal electrode structure is constructed from micro X-ray tomography measurements. Subsequently, by exploiting the Douglas–Gunn modification of the three-dimensional Crank–Nicolson algorithm to Cottrellian boundary conditions, the mass transfer controlled current of this ”true” network is obtained. Based on this current, the third step is to compute the mass transfer functions related to the electrode under investigation by an inverse convolution algorithm. In this manner, the spatial dimensionality of the system is reduced from three to one, resulting in significant savings in computation time. The fourth and final step is then to simulate CV experiments via classical convolution methods, featuring the great advantage that any degree of electrochemical reversibility, coupled homogeneous reactions, electrolyte resistances and double layer capacities can be implemented readily. As a proof of concept, the simulations are supported by experimental data acquired for the oxidation of VO2+ in carbon felt electrodes.

Understanding degradation phenomena occurring during the operation of vanadium redox-flow batteries (VRFB) requires a measurement technique which allows for differentiating the overall performance losses into individual performance losses of the cell components. For this purpose, electrochemical impedance spectroscopy (EIS) is a valuable and well established tool. However, the discrimination of processes taking place at similar time scales is challenging since they overlap in the commonly used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by deconvoluting EIS data with respect to the time constants of the individual processes. It hence circumvents the necessity of finding a suitable equivalent circuit model and thus allows for data evaluation without any a-priori knowledge of the system under study. For the first time, we herein present the application of DRT transform to EIS data of a VRFB. By varying experimental conditions and employing full cell as well as double half cell operational modes, we are able to identify the faradaic process of the negative half cell. This enables us to visualize the negative half cell’s contribution to the overall impedance of a VRFB even in a full cell EIS measurement. By an accelerated degradation experiment we finally demonstrate the great potential of DRT analysis for future application in the monitoring of electrode degradation in VRFB.

Thin film rotating-ring disc electrode (RRDE) technique is exploited to quantify the parasitic hydrogen evolution reaction (HER) competing with the desired V3+-reduction at surface modified carbon nanoparticles for application as electrocatalysts in the negative half-cell of vanadium redox-flow batteries (VRFB). Carbon based electrode materials are derived from standard Vulcan XC-72 carbon, treated by chemical surface etching techniques proposed for carbon felt-electrodes in the literature. Additional electrochemical characterization is performed using stationary cyclic voltammetry (CV) followed by fitting of CV data, Fourier-transform alternating-current cyclic voltammetry (FT-ACCV) and electrochemical impedance spectroscopy (EIS) followed by distribution of relaxation times (DRT) analysis. To our knowledge the present paper is the first study using the RRDE technique for separating HER and V3+-reduction reactions. It is demonstrated that the ratio of HER to V3+-reduction significantly depends on the chemical pretreatment of the carbon electrodes and that the V3+-reduction proceeds at an optimum rate at E − ERHE = −0.45 V. Separating the HER from the V3+-reduction also allows us to provide highly accurate values for the diffusion coefficient of the V3+-ion in sulfuric acid solutions.