A mini-review on decorating, templating of commercial and electrospinning of new porous carbon electrodes for vanadium redox flow batteries

Carbon-based materials have become indispensable in the field of electrochemical applications, especially for energy storage or conversion purposes. A large diversity of materials has been proposed and investigated in the last years. In this mini-review, we present recent advances in the design of carbon-based materials for application in vanadium redox flow batteries. As main part, different modification and fabrication methods for carbon-based electrodes are described. The decoration of carbon felts and graphite felts with metals or metal compounds to enhance mostly the electrocatalysis of the negative side is illustrated with examples. Furthermore, various options of synthesizing porous C-C composites are discussed, with specific emphasis on graphene-based composites as well as nitrogen doped composites and biomass-derived carbons. Apart from that the method of electrospinning is also examined in detail, a method which not only allows the production of nanofibrous high surface area electrodes, but also allows adaptation of fiber thickness and architecture. In this review the significant strengths of each method are pointed out, but also particular weaknesses are discussed with respect to the later battery performance. Finally, an outlook is given pointing to the remaining challenges that need to be overcome in the future

X‐Ray‐Computed Radiography and Tomography Study of Electrolyte Invasion and Distribution inside Pristine and Heat‐Treated Carbon Felts for Redox Flow Batteries

Porous carbon felts (CFs) are widely used electrode materials for vanadium redox flow batteries (VRFBs). These materials differ in their precursor material, thickness, or graphitization degree and demonstrate broad differences in electrochemical performance. Prior to operation, an activation step, such as acid or heat treatment (HT), is commonly performed to improve their performance. A thermal treatment in air functionalizes the surface of the electrode and improves reaction kinetics as well as the wettability of the electrode. Herein, pristine and heat‐treated CFs are compared regarding their electrolyte wetting behavior for the use in VRFB. Contact angle (CA) measurements are conducted ex situ to investigate the effect of the HT. Furthermore, the porous CFs are examined in situ with an in‐house‐built flow cell regarding their invasion behavior with different types of electrolytes by X‐ray radiography. Additionally, the distribution of the electrolyte inside the felts is investigated by X‐ray tomography. The results demonstrate the effect of the HT and choice of electrolyte on the wetting behavior and electrolyte distribution.DFG, 276655287, FOR 2397: Multiskalen-Analyse komplexer Dreiphasensystem

Strukturierung und Modifizierung kohlenstoffbasierter Elektroden für die Anwendung in Redox-Flow-Batterien

Redox Flow Batterien sind vielversprechende Vertreter, wenn es um die flexible und zuverlässige Zwischenspeicherung von erneuerbaren Energien in einem großtechnischen Maßstab geht. Sie bieten viele Vorteile gegenüber anderen Batteriesystemen. Aufgrund der langen Lebensdauer des Systems sowie der unabhängigen Skalierung von Leistung und Kapazität, sind sie im Bereich der Energiespeicher nicht mehr wegzudenken. Andererseits gibt es hinsichtlich der Elektrodenentwicklung Verbesserungspotential, wodurch ihre breite Anwendung in der Industrie bisher ausgebremst wurde. In dieser Arbeit stand deshalb die Entwicklung neuer, poröser Elektroden mit gesteigerter spezifischer Oberfläche im Fokus. Dabei wurde ein breites Spektrum an verschiedenen Syntheserouten eingesetzt. Unter anderem wurde der Einfluss der aktiven Oberfläche auf die elektrokatalytische Aktivität der Elektroden untersucht, aber auch eine Dotierung mit Heteroatomen wie Stickstoff war von Interesse. Die Co Dotierung mit zwei verschiedenen Heteroatomen – in diesem Fall eine Kombination aus Stickstoff und Schwefel – und ihre Wirkung auf die Aktivität fand ebenfalls Anwendung. Es wurden verschiedene Synthesemethoden entwickelt und ihre jeweiligen Vor und Nachteile erforscht. Die Methode der Salztemplatierung erlaubte die Stickstoffdotierung in Kombination mit einer signifikanten Erhöhung der spezifischen Oberfläche der Elektrode. Außerdem konnte eine umweltfreundliche Syntheseroute entwickelt werden, die sich das Verfahren des weichen Templatierens zu Nutze macht, und die Co Dotierung ermöglichte. Abschließend gelang die erstmalige Anwendung der Zwillingspolymerisation zur Herstellung einer porösen Vlieselektrode mit bimodaler Porengrößenverteilung und genau definierter Porosität. Von besonderer Wichtigkeit für zukünftige Untersuchungen war die Feststellung der nicht eindeutigen Zuordnung katalytischer Effekte. Es konnte keine direkte Korrelation zwischen der spezifischen Oberfläche, der Heteroatomdotierung und der katalytischen Performance nachgewiesen werden. Dies verdeutlicht einen Bedarf an neuen, genaueren Analysemethoden, um ein besseres Verständnis bezüglich der elektrokatalytischen Zentren zu erlangen. Die erhaltenen Kohlenstoff Kohlenstoff Komposite wurden nicht nur bezüglich ihrer elektrokatalytischen Aktivität überprüft, sondern sie wurden auch hinsichtlich ihrer strukturellen Eigenschaften im Detail untersucht. Neben der elektromikroskopischen Darstellung der Makrostruktur, erfolgte auch die Ermittlung der spezifischen Oberfläche mit Hilfe der Stickstoffsorption. Im Falle der heteroatomdotierten Elektrodenmaterialien wurde die Umgebung des Kohlenstoffs mittels Röntgenphotoelektronenspektroskopie analysiert, sowie die Elementzusammensetzung der Oberfläche ermittelt. Außerdem wurden kommerziell erhältliche Vliese der Firma SGL Carbon SE mittels bildgebender Verfahren wie der Radiografie und Computertomografie studiert. Dies ermöglichte nicht nur genaue Einblicke in die Transportwege des Elektrolyten, sondern auch dessen Verteilung innerhalb des Vlieses zu erhalten. Besonderes Augenmerk wurde dabei auf die Unterschiede zwischen unbehandelten und wärmebehandelten Vliesen gelegt. Ein abschließendes Mini Review, das die aktuellen Entwicklungen von Elektroden-materialien hinsichtlich neuer Synthesemethoden zusammenfasst und diskutiert, komplettiert diese Arbeit. Hierbei lag der Fokus auf der detaillierten Betrachtung drei vielversprechender Synthesemethoden. Durch kritische Betrachtung der Vor und Nachteile der verschiedenen Ansätze, wurden die noch verbleibenden zukünftigen Heraus-forderungen aufgezählt.  Redox Flow Batteries are promising candidates, when it comes to the flexibel and reliable storage of renewable energies on a large technical scale. Compared to other battery systems, they offer many advantages. They have become indispensable in the field of energy storage, due to their long cycle life. Redox Flow Batteries also benefit from the fact, that their capacity and their power can be adjusted independently. On the other hand, there is still a great need of improvement concerning the development of the electrodes. Up to now this circumstance has hindered the widespread usage in the industry. Therefore, this work focused on the development of new, porous electrodes having an increased specific surface area. A wide range of different synthesis routes were used. During this work, the following topics were of particular interest: the influence of the active area, the effect of doping with heteroatoms and the co doping of surface with nitrogen and sulfur. Various synthesis routes were examined regarding their advantages and disadvantages. With the help of the salt templating approach nitrogen doping in combination with a significant increase in the specific surface area of the electrode was possible. In addition, an environmentally friendly synthesis route could be developed, which allowed the co doping utilizing the soft templating method. Finally the twin polymerization was used for the first time, to produce a porous carbon felt electrode with a bimodal pore size distribution and a defined porosity. Figuring out, that the clear assignment of catalytic centers is not possible, is of particular importance for future investigations. No direct correlation could be demonstrated between the specific surface area, the doping of heteroatoms and the catalytic performance. This insight highlights the need for new and precise analytical methods in order to gain a better understanding of the electrocatalytic centers. Besides the detailed electrochemical characterization, the obtained carbon carbon composites were investigated by means of different structural analyses. Scanning electron microscopy was used for a more detailed insight into the morphology of the electrode, while nitrogen sorption measurements were applied to analyze its porosity. Additional x ray photoelectron spectroscopy measurements were utilized, with respect to the co doped electrode materials, to determine the elemental composition of the respective surfaces as well as the chemical state of the elements. Furthermore commercially available carbon felts from SGL Carbon SE were evaluated using imaging methods like radiography and computed tomography. This enabled precise insights into the invasion and the distribution of the electrolyte inside the felts. Within the analysis, particular attention was paid to the difference between the untreated and the heat treated felts. Finally a mini review was published, which summarizes the current developments of electrode materials in terms of new synthesis techniques. Here, the emphasis was laid on the detailed analysis of the three most promising synthesis routes. By considering the advantages and disadvantages of the different approaches in a critical way, the remaining challenges for future research were enumerated

Degradation Phenomena of Bismuth-Modified Felt Electrodes in VRFB Studied by Electrochemical Impedance Spectroscopy

The performance of all-V redox flow batteries (VRFB) will decrease when they are exposed to dynamic electrochemical cycling, but also when they are in prolonged contact with the acidic electrolyte. These phenomena are especially severe at the negative side, where the parasitic hydrogen evolution reaction (HER) will be increasingly favored over the reduction of V(III) with ongoing degradation of the carbon felt electrode. Bismuth, either added to the electrolyte or deposited onto the felt, has been reported to suppress the HER and therefore to enhance the kinetics of the V(II)/V(III) redox reaction. This study is the first to investigate degradation effects on bismuth-modified electrodes in the negative half-cell of a VRFB. By means of a simple impregnation method, a commercially available carbon felt was decorated with Bi 2 O 3 , which is supposedly present as Bi(0) under the working conditions at the negative side. Modified and unmodified felts were characterized electrochemically using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode setup. Surface morphology of the electrodes and composition of the negative half-cell electrolyte were probed using scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (TXRF), respectively. This was done before and after the electrodes were subjected to 50 charge-discharge cycles in a battery test bench. Our results suggest that not only the bismuth catalyst is dissolved from the electrode during battery operation, but also that the presence of bismuth in the system has a strong accelerating effect on electrode degradation