Michael Bron

Carbon-based materials are widely used as electrode materials for the all-vanadium redox flow battery (VRFB) because of their superior stability and electrochemical activity over other material classes. As a result of the required high potentials, different degradation effects at the positive side (VIVO2+/VVO2+) of the battery, such as electrochemical oxidation, ion intercalation, and surface blocking by adsorbed vanadium species, have to be considered but are addressed only in a few studies. To understand the degradation mechanisms, a single graphene sheet is probed by cyclic voltammetry (CV) in a battery-like electrolyte using scanning electrochemical cell microscopy (SECCM). Ex situ Raman microscopy shows increased defect formation of the respective spots after passing higher upper vertex potentials (Euv) of the CV. To gain deeper insights into this process, additional in situ Raman probing during SECCM experiments was conducted at discrete potential steps and showed graphene G band splitting to G1 and G2 (1587 and 1620 cm–1) indicating a contribution of electron-deficient carbon (Cx+) to the active sites of the VO2+/VO2+ conversion reaction. Consequently, the graphene defects of the CV experiments can be the result of increased Cx+ formation in the graphene structure, which damages the 6-fold ring structure and finally forms oxygen-containing functional groups.

Anion exchange membrane water electrolysis (AEMWE) has become a promising technology for generating hydrogen in a carbon‐neutral economy. However, its competitiveness is currently limited by the low durability of AEMWE systems. To increase its durability and to advance its industrial application, this study examines the degradation behavior of a 25 cm ² AEMWE cell utilizing non‐precious metal catalysts and pure water feed in a short‐term durability test. Polarization curves and electrochemical impedance spectroscopy, along with scanning electron microscopy and energy‐dispersive X‐ray spectroscopy, are employed to identify factors affecting cell efficiency and stability. The pure‐water‐fed AEMWE cell exhibits high performance losses and low stability that is mainly related to degradation in the anode and at the interface of anode and membrane. Ionomer degradation, reducing hydroxide ion conductivity, and mechanical stability of the catalyst layer, is identified as the key factor decreasing cell efficiency and stability during pure water operation. The findings aim to guide the development of strategies to enhance cell performance and durability of pure‐water‐fed AEMWE.

This publication compares two different methods for measuring the dielectric breakdown strength on three different engineering plastics (poly butylene terephthalate- glass composites). The current standard procedure DIN EN 60243-1 is compared to a new insulation fluid free method based on a novel test specimen design. Material-specific differences between the materials are only detected in the new test design. These results are compared to material characteristics, such as glass fiber orientation, microstructure and ion content. It is shown that the glass fiber orientation in the new test design is much more random in the electrode area compared to the ideal orientation of the DIN EN 60243-1 method. This allows a more realistic estimation of the dielectric breakdown strength taking a unfavorable fiber orientation in real housing materials into account. In addition, differences in the glass fiber adhesion and thus the microstructure are found in nominally identical materials from different manufacturers. In this context ion accumulations in breakdown paths are detected. To verify ion migration, contrast ions are introduced and their movement can be detected, allowing initial assumptions about the migration mechanism.

The influence of high potentials on amorphous nitrogen‐free and nitrogen‐doped hydrogenated carbon thin film electrodes with thicknesses of 9 to 30 nm is probed toward the vanadium(IV/V) redox reaction by scanning electrochemical cell microscopy (SECCM), which mimics the reaction of the positive side of the all‐vanadium redox flow battery (VRFB). Besides the evaluation of the peak separation (E _PP ) from cyclic voltammograms (CV), the localized probing is adapted in a way that the influence of high overpotentials on the stability of the carbon materials, as well as competitive electrochemical processes, can be analyzed. The sulfate anion insertion process is found to be the predominant process in all samples, with its onset appearing in parallel to the vanadium(IV/V) reaction. The presence of pyridine/pyrrole groups can stabilize the insertion compound, which inhibits the vanadium(IV/V) reaction much more strongly. In all cases, the electrochemical redox features of the vanadium(IV/V) reaction, as well as the initial Raman spectra of the carbon thin films, are fully reconstructed by applying reductive potentials in a suitable time frame, even after polarizing to drastically high potentials (2.5 V vs. RHE). Overall, this competing insertion reaction must be given greater consideration when discussing electrochemical data of the vanadium(IV/V) redox reaction.

Anion exchange membrane (AEM) water electrolysis has emerged as a promising technology for producing hydrogen in a carbon‐neutral economy. To advance its industrial application, performance evaluations of non‐precious metal AEM electrolyzers with electrode areas of 25 cm² were conducted. The focus was on pure water operation, achieving a current density of 0.26 A cm⁻² at a voltage of 2.2 V. To gain a better understanding, the AEM electrolyzer was also operated in aqueous KOH, yielding 1.2 A cm⁻² at 2.2 V. By adding a liquid electrolyte and by varying cell components, causes of the occurring performance limitations and ways to improve the AEM electrolyzer were identified. Electrochemical impedance analysis showed that the activation loss at the anode due to sluggish OER kinetics was the limiting factor at low current densities. At higher current densities, which is the operating range of interest for industrial application, the ohmic resistance from the membrane was the dominant factor limiting high performance in pure water operation.