Surface Properties of Graphite for Efficient Electrocatalysis of Vanadium Redox Reactions

Die Ergebnisse dieser Arbeit tragen zu einem tieferen Verständnis der kinetischen Prozesse in Vanadium-Redox-Flow-Batterien bei. Dafür wurden die elektrokatalytischen und physikochemischen Eigenschaften von industriell relevantem Graphitfilz und verwandten kohlenstoffbasierten Materialien untersucht. Der Einsatz von mehrere spektroskopischen und mikroskopischen Methoden ermöglichte die Abbildung verschiedener Skalen der Mikrostruktur, die chemische Zusammensetzung, die elektronische Struktur und schließlich die kinetischen Vorgänge mittels theoretischen Berechnungen und spektro-elektrochemischen Untersuchungen. Aufgrund widersprüchlicher Ergebnisse in der Literatur im Themenkomplex funktionelle Sauerstoffgruppen hat sich die Arbeit zum Ziel gesetzt, eindeutige Zusammenhänge zwischen Oberflächenstruktur und Elektrokatalyse herzustellen. Die Analyse von Eigenschaften chemisch und strukturell modifizierter Elektroden vor und nach elektrochemischen Tests führte zur Identifizierung graphitischer Defekte als aktive Zentren. Nach der vollständigen Entfernung von Sauerstoff ergaben sich wasserstoffterminierte Defekte anstelle von Oberflächengruppen als Ursache für die katalytische Aktivität. Die Anbringung von Pyrenen mit unterschiedlichen Funktionalitäten auf der Graphitoberfläche diente zum Nachweis der Effekte einzelner Sauerstoffgruppen. Um die Auswirkung der verschiedenen Defekt-Geometrien zu spezifizieren, wurden durch thermochemische Aktivierungen Kohlenstoffatome in Modell- und realen Elektroden durch Phosphor substituiert. Unter gezielter Berücksichtigung aller dabei veränderten Elektrodeneigenschaften konnte so die elektrochemische Aktivität deutlich gesteigert werden. Die Untersuchung der elektronischen Struktur von polyzyklischen aromatischen Kohlenwasserstoffen verband die elektrokatalytische Aktivität mit der Elektrodenarchitektur und der Austrittsarbeit. Theoretische Modellierungen definierten Wechselwirkungen zwischen den solvatisierten Vanadium-Ionen und der Substrate durch verschiedene wasserstoff- und sauerstoffterminierte Kanten. Schließlich formuliert die Arbeit durch die Untersuchung der Elektrode–Elektrolyt-Grenzfläche bei angelegtem Potential einen experimentell und rechnerisch gestützten, innovativen Reaktionsmechanismus

2021: A Surface Odyssey. Role of Oxygen Functional Groups on Activated Carbon‐Based Electrodes in Vanadium Flow Batteries

The market breakthrough of vanadium flow batteries is hampered by their low power density, which depends heavily on the catalytic activity of the graphite-based electrodes used. Researchers try to increase their performance by thermal, chemical, or electrochemical treatments but find no common activity descriptors. No consistent results exist for the so-called oxygen functional groups, which seem to catalyze mainly the V$^{III}$/V$^{II}$ but rarely the V$^{V}$O$_{2}$$^{+}$/V$^{IV}$O$^{2+}$ redox reaction. Some studies suggest that the activity is related to graphitic lattice defects which often contain oxygen and are therefore held responsible for inconsistent conclusions. Activation of electrodes does not change one property at a time, but rather surface chemistry and microstructure simultaneously, and the choice of starting material is crucial for subsequent observations. In this contribution, the literature on the catalytic and physicochemical properties of activated carbon-based electrodes is analyzed and evaluated. In addition, an outlook on possible future investigations is given to avoid the propagation of contradictions

Edge Site Catalyzed Vanadyl Oxidation Elucidated by Operando Raman Spectroscopy

The kinetic processes responsible for the efficient oxidation of dissolved vanadyl oxide species in the positive half-cell of a vanadium flow battery are far from being understood. Despite recent evidence that the reaction is most strongly favored at hydrogen-terminated graphite edge sites, a mechanism involving oxygen-containing surface groups has still been frequently reproduced to date. In this work, operando Raman spectroscopy follows the reaction at the interface between graphite-based model electrodes and vanadium-containing sulfuric acid as the electrolyte. The potential-dependent growth of different vibrational modes is related to the electrocatalytic activity of the sample and allows to track the oxidation of the electrolyte species. Moreover, the results express vanadium reaction intermediates of dimeric origin only on the edge-exposed surface of graphite, which exhibits significantly higher electrochemical activity. No interaction with surface oxygen postulated before could be observed for the active electrodes at potentials relevant to the reaction. Instead, a new growing graphite-related feature shows direct electronic interactions between vanadium ions and carbon atoms during charge transfer

Work Function Describes the Electrocatalytic Activity of Graphite for Vanadium Oxidation

In many applications such as vanadium flow batteries, graphite acts as an electrocatalyst and its surface structure therefore determines the efficiency of energy conversion. Due to the heterogeneity of the material, activity descriptors cannot always be evaluated with certainty because the introduction of defects is accompanied by a change in surface chemistry. Moreover, surface defects occur in multiple dimensions, and their occurrence and influence on catalysis must be separated. In this work, we have studied the surface of graphite felt electrodes by different methods in terms of morphology and chemistry to understand the electrocatalytic activity. We then defined the interaction between the surface and the electronic structure with particular emphasis on the work function and valence band. Using model catalysts with different architectures, we established correlations between the electrocatalytic activity and the size of the conjugation and the orientation of the edges. Finally, it was possible to link the level of the work function to the electrocatalytic activity

Origin of the catalytic activity at graphite electrodes in vanadium flow batteries

For many electrochemical devices that use carbon-based materials such as electrolyzers, supercapacitors, and batteries, oxygen functional groups (OFGs) are considered essential to facilitate electron transfer. Researchers implement surface-active OFGs to improve the electrocatalytic properties of graphite felt electrodes in vanadium flow batteries. Herein, we show that graphitic defects and not OFGs are responsible for lowering the activation energy barrier and thus enhance the charge transfer properties. This is proven by a thermal deoxygenation procedure, in which specific OFGs are removed before electrochemical cycling. The electronic and microstructural changes associated with deoxygenation are studied by quasi in situ X-ray photoelectron and Raman spectroscopy. The removal of oxygen groups at basal and edge planes improves the activity by introducing new active edge sites and carbon vacancies. OFGs hinder the charge transfer at the graphite–electrolyte interface. This is further proven by modifying the sp2 plane of graphite felt electrodes with oxygen-containing pyrene derivatives. The electrochemical evolution of OFGs and graphitic defects are studied during polarization and long-term cycling conditions. The hypothesis of increased activity caused by OFGs was refuted and hydrogenated graphitic edge sites were identified as the true reason for this increase

Structure–activity correlation of thermally activated graphite electrodes for vanadium flow batteries

Thermal activation of graphite felts has proven to be a valuable technique for electrodes in vanadium flow batteries to improve their sluggish reaction kinetics. In the underlying work, a novel approach is presented to describe the morphological, microstructural, and chemical changes that occur as a result of the activation process. All surface properties were monitored at different stages of thermal activation and correlated with the electrocatalytic activity. The subsequently developed model consists of a combined ablation and damaging process observed by Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. Initially, the outermost layer of adventitious carbon is removed and sp(2) layers of graphite are damaged in the oxidative atmosphere, which enhances the electrocatalytic activity by introducing small pores with sharp edges. In later stages, the concentration of reaction sites does not increase further, but the defect geometry changes significantly, leading to lower activity. This new perspective on thermal activation allows several correlations between structural and functional properties of graphite for the vanadium redox couple, describing the importance of structural defects over surface chemistry

Functionalization of Graphite Electrodes with Aryl Diazonium Salts for Lithium‐Ion Batteries

The functionalization of electrode surfaces is a useful approach to gain a better understanding of solid–electrolyte interphase formation and battery performance in lithium-ion batteries (LIBs). Electrografting and deprotection of alkyl silyl protected ethynyl aryl diazonium salts on graphite electrodes were performed. Furthermore, electrografting of aryl diazonium salts carrying functional groups such as amino, carboxy and nitro, and their influence on the electrochemical performance in LIBs were investigated. The drawbacks of electrografted and especially deprotected samples were evaluated and compared to corresponding in situ grafted samples. While electrografted samples tend to lower the delithiation capacities, in situ grafted samples, except amino groups, reveal higher capacities. Ethynyl (TMS) shows improved capacities at 1 C and better capacity retention compared to the pristine graphite electrode. Additionally, the Coulombic efficiency of the first cycle was enhanced for in situ grafted samples

Understanding efficient phosphorus-functionalization of graphite for vanadium flow batteries

Numerous surface treatment methods are known to enhance the electrochemical activity of graphite felt (GF), such as thermal activation or attachment of nanoparticulate catalysts. The integration of heteroatoms into the graphite lattice at the surface could be a promising technique for reliable and efficient electrode activation. However, these functionalization techniques are based on thermochemical activation, which makes it difficult to distinguish between activity effects other than foreign atom integration, such as defects and other surface groups that must be considered. In this work, we analyzed commercial and synthetic phosphorus-doped graphene and GF using different electrochemical and physicochemical techniques. Despite a high doping concentration, the activity of the commercial powder bonded to GF and coated on glassy carbon remained limited due to the low degree of graphitization and high oxygen content. Instead, a low phosphorus concentration of <1 at% combined with a high degree of graphitization increased the catalytic activity. Building on these findings, GF was rationally modified, resulting in twice the power density compared to the original material in full cell tests

Characterization and Comparative Study of Energy Efficient Mechanochemically Induced NASICON Sodium Solid Electrolyte Synthesis

In recent years, there is growing interest in solid-state electrolytes due to their many promising properties, making them key to the future of battery technology. This future depends among other things on easy processing technologies for the solid electrolyte. The sodium superionic conductor (NASICON) Na3Zr2Si2PO12 is a promising sodium solid electrolyte; however, reported methods of synthesis are time consuming. To this effect, attempt was made to develop a simple time efficient alternative processing route. Firstly, a comparative study between a new method and commonly reported methods was carried out to gain a clear insight into the mechanism of formation of sodium superionic conductors (NASICON). It was observed that through a careful selection of precursors, and the use of high-energy milling (HEM) the NASICON conversion process was enhanced and optimized, this reduces the processing time and required energy, opening up a new alternative route for synthesis. The obtained solid electrolyte was stable during Na cycling vs. Na-metal at 1 mA cm−1, and a room temperature conductivity of 1.8 mS cm−1 was attained