Abstract This review examines the role of defective carbon‐based electrodes in sodium‐ion and vanadium flow batteries. Methods for introducing defects into carbon structures are explored and their effectiveness in improving electrode performance is demonstrated. In sodium‐based systems, research focuses primarily on various precursor materials and heteroatom doping to optimise hard carbon electrodes. Defect engineering increases interlayer spacing, porosity, and changes the surface chemistry, which improves sodium intercalation and reversible capacities. Heteroatom functionalisation and surface modification affect solid electrolyte interface formation and coulombic efficiencies. For flow batteries, post‐fabrication electrode enhancement methods produce defects to improve electrode kinetics, although these methods often introduce oxygen functional groups as well, making isolation of defect effects difficult. Continued research efforts are key to developing carbon‐based electrodes that can meet the unique challenges of future battery systems.

Lithium-Sulfur (Li-S) batteries are promising energy storage devices due to their high theoretical energy density. However, challenges such as the shuttling effect and volume expansion have significantly hindered their cycle life and capacity retention. Furthermore, the complex kinetic pathways in Li-S batteries call for advanced characterization techniques to unravel underlying mechanisms. In this study, a hollow porous carbon (HPC) is used as a microreactor, where inverse vulcanization occurs between 1,3-diisopropylbenzene (DIB) and sulfur (S8), resulting in the creation of three-dimensionally interconnected and well-distributed S-DIB in carbon frameworks. As a result, the dual confinement strategy imparts Li-S coin cells with remarkable cycling stability and capacity retention, exhibiting an impressive capacity of 866 mAh g-1 when returning to 0.1 C after 100 cycles of rate capability tests. Particularly, the Energy-selective Backscattered (EsB) assisted Scanning Electron Microscope (SEM) technique as a novel approach is introduced to distinguish different lengths of polysulfides. Their distribution is visualized in the cross-section view of the electrode in a micrometer range. These EsB images provide concrete indications of the sulfur evolution process and explain the capacity degradation during cycling. A hollow carbon framework is used as a microreactor for inverse vulcanization between 1,3-diisopropylbenzene and sulfur. This dual confinement strategy enhances Li-S coin cells' cycling stability and capacity retention. (Poly)sulfide distribution is visualized by Energy selective Backscattered electron imaging (EsB), unraveling the sulfur lithiation and capacity degradation process. image

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) Na3 Zr2 Si2 PO12 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.

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.

The redox flow battery (RFB) is one of the most promising technologies for stationary energy storage. It consists of a voltaic cell whose electrolytes are stored in tanks outside the system. Whilst the chemistries of the anodic and cathodic electrolytes define the energy density of a RFB system, its power density is determined primarily by its electrodes. These are typically porous carbon media on the surface of which the redox reactions occur. They are a critical component since they require a detailed engineering of electrocatalytically active surface area and mass transport. Still today, they remain the main cause of RFB low power density. In this work, we are showing a novel deposition method to improve the performance of commercially available carbon paper already used in RFB. The gain is thanks to the deposition of turbostratic carbon nano-onion (TCNO) nanoparticles on the carbon fibers of the paper. This process is performed with a proprietary technology called NanoJeD: a CVD in supersonic flow, which promises easy industrial scalability and a fine tunability of the nanoparticle properties. Moreover, with this system is possible to deposit over any substrate. NanoJeD consists in two chambers separated by a high aspect ratio slit. A precursor gas is injected through a porous plug from the top and a vacuum system is connected from the bottom. The slit allows the establishment of a high-pressure ratio between the two chambers and hence the formation of a supersonic jet. The precursor gas, a mixture of Argon (98.4%) and Acetylene (1.6%) flows between two electrodes where a radio frequency (RF) signal is fed (13.56 MHz). The RF ignites a non-thermal plasma in which the precursor molecules are ionized and dissociated into radicals, which polymerize forming hydrogenated carbon clusters and nanoparticles (NPs) of different sizes. Once formed, NPs are dragged through the slit by the gas stream and finally impact on the substrate. Therefore, it is possible to achieve a high throughput (500 mg h-1) and to deposit on a large area of 100 cm2 while precisely controlling the properties of the film. The resulting film has hundreds of squared meters per gram of surface area. For TCNO formation, the NP film is further annealed in vacuum at 1000°C for 2 hours. Using in situ transmission electron microscopy (TEM) and energy electron loss spectroscopy (EELS), we were able to visualize the progression of graphitization and changes in structural properties during annealing. The graphitization process starts in the outer layer and proceeds into the interior of the particle. The geometrical constrains of the NP (average diameter of 20nm) and shrinkage of the NP, allow the formation of curved, turbostratic, defective graphitic sheets. UV photoelectron spectroscopy (UPS) and Raman spectroscopy performed ex-situ allow the analysis of valence bands and structural defects. The high presence of defects and general disorder structure are accompanied with a remarkable increase in catalytic activity. Moreover, the annealing process leads to the sintering of the nanoparticles and the formation of an interface between the carbon fibre and the nanoparticles, as demonstrated by the increased thermal stability tested by. Both phenomena improve the adhesion between nanoparticles and with the surface, making the electrode suitable to be adopted in fluxed systems. The TCNO deposited on a carbon paper was tested as electrode in a Vanadium RFB. The kinetic activity and electrochemical properties were tested via in-situ Raman and in a three-electrode cell set-up. The results reveal an increased kinetic activity with respect to the untreated paper. Using in situ Raman we were able to show differences in redox mechanism between TCNO and bare carbon paper. A chronoamperometry test was used to obtain the transfer coefficient from a Tafel plot. These studies allow the correlation of the catalytic activity with the defectiveness of the TCNO nanoparticle. Vanadium RFB full cell measurements of the TCNO electrode show a drastic increase in performance, reaching an energy efficiency of 70.8% and 86.2% at current densities of 600 mA cm-2 and 200 mA cm-2, respectively. The electrolyte utilisation for these current densities is 59% for the former and 80% for the latter. To investigate the chemical and mechanical stability a stability test was performed. Results showed a low degradation rate of 0.004% EE per cycle. Moreover, TCNO can be used for other electrochemical applications only by adjusting the deposition and annealing parameters.

Abstract Polycyclic aromatic hydrocarbons, such as pyrenes, are a well‐known material class for non‐covalent modification of carbon surfaces in many applications. In electrochemical energy storage, pyrenes are mostly used in large polymeric structures. This work addresses the use of carboxy‐ and amino‐functionalized pyrenes for graphite electrodes for lithium‐ion batteries (LIBs). Pyrenes are explored as adsorbed species on graphite prior to electrode fabrication and as additives to the electrode composition. Thereby, 1‐pyrenecarboxylic acid, 1‐pyrenebutyric acid, 1‐aminopyrene, and 1‐pyrenebutylamine were under investigation. As additives, pyrenes do not influence the cycling performance of the electrode at low current but deteriorate the performance at high current, regardless of the functional group. However, when the pyrenes are adsorbed to the graphite surface, the influence of the different functional groups becomes clearly visible, revealing that an additional butyl group has a positive impact on the cycling performance and lithium‐ion transport of the electrodes. Electrodes with 1‐pyrenebutyric acid even enhanced the performance compared to the pristine electrode.

One of the main reasons for battery aging in lithium-ion batteries (LIBs) is the formation of a solid electrolyte interphase (SEI). SEI formation results in capacity loss and power fading due to irreversible electrochemical decomposition of electrolyte. However, the SEI protects the negative electrode from further parasitic surface reactions with the electrolyte. Therefore, it is of great interest to learn more about factors influencing the SEI and its formation. One approach is to modify the surface of the active material. Since graphite is commonly used as negative electrode material in LIBs, we modified graphite with anchor molecules featuring functional groups. In particular, we use aryl diazonium salts and pyrenes as anchor molecules to cover a covalent and a non-covalent approach. The influence of different functional groups on the electrochemical properties of graphite electrodes is investigated. Furthermore, the importance of the modification method and the influence of the anchor molecule is addressed. The functional groups of modified graphite electrodes enable further layer growth, which is an important aspect for the design of an artificial SEI. For surface and electrochemical characterisation of the modified graphite samples, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and galvanostatic cycling are used. Financial support of the German Research Foundation (DFG) within the project B2 of the Collaborative Research Centre (SFB 1176) is kindly acknowledged.

The interfacial processes controlling charge transfer in vanadium redox reactions at graphite electrodes are still poorly understood. Accurate knowledge of the underlying mechanism is mandatory to improve the efficiency of conversion from electrical to chemical energy and vice versa. Various surface modifications are known to improve the catalytic activity of graphite by introducing active sites. Differentiating between influences on structural properties by activation processes is difficult due to the heterogeneity of the material and the interplay of microstructural changes and surface chemistry. This requires the application of multiple analytical tools and the systematic correlation with electrochemical results. In this way, our group has recently demonstrated that the 30-year-old mechanism involving surface oxygen does not stand up to thorough investigation. Instead, we primarily identified lattice defects such as edges and carbon vacancies as the actual origin of enhanced electron transfer kinetics. We have studied these defects on various scales and introduced an onion-like model to explain why thermal treatment is effective but care must be taken to remove the material from the oxidizing atmosphere at the right time. Otherwise abundant oxygen groups and dull pore edges will counteract the intended activation. Special attention was subsequently paid to the interaction of microstructure and electronic states, establishing a relationship of catalytic activity and the schematic band structure. Whereas edges decrease the work function of graphite, vacancies increase the density of states close to the Fermi level. Using real and model electrodes, we distinguished between the architecture of structural disorder and found zigzag edges to provide superior kinetics over armchair edges. Our experimental studies were complemented by theoretical density functional theory calculations to learn about the adsorption properties of electrolyte species with regard to the orientation of graphite. The potential-dependent structural changes and coordination spheres of the electrode and the near-electrode electrolyte were investigated by operando Raman spectroscopy to elucidate the interaction between vanadium and carbon.

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.

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.

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 sp2 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.

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.

Deoxygenated graphite electrodes exhibited an overall higher electrocatalytic activity than the oxygen-rich starting material. Free edge sites were therefore identified as the origin of the charge transfer process for the vanadium redox reactions.

Nickel oxide/hydroxides [NiOx(OH)(y)] are considered to be promising materials to replace noble metals for the oxygen evolution reaction in alkaline media. While several studies showed that iron impurities promote the activity of nickel-based catalysts, the effects of intrinsic nickel defects and the underlying mechanism remain unknown. In this work, X-ray photoelectron spectroscopy is combined with surface-enhanced Raman scattering to understand the reactivity of NiOx thin films, which were prepared at different temperatures and thus varied in their chemical composition and crystalline order. Raman spectroscopy was used to follow the characteristic oxidation of nickel species from Ni-II(OH)(2) to (NiOOH)-O-III and (NiOO-)-O-IV under electrochemical conditions. A stronger oxide-to-hydroxide conversion, consistent with the post-electrochemistry study, was associated with the presence of initial Ni-III impurities and oxygen vacancies and appears beneficial for the electrocatalytic activity.

Manganese oxide (MnOx) is considered a promising material for the oxygen evolution reaction (OER) to replace noble metal catalysts in water splitting. The improvement of MnOx requires mechanistic and kinetic knowledge of the four-electron transfer steps of the OER. X-ray photoelectron spectroscopy, a widely used tool to characterize the electronic structure of thin films, is used in combination with surface-enhanced Raman spectroscopy to gain a deeper knowledge of the different mixed MnOx types and their respective change in chemical composition. Using Raman spectroscopy during electrochemical measurements, all samples were found to reveal Birnessite-type MnO2 motifs in alkaline media at an applied potential. Their activity correlates with two shifting Raman active modes, one of them being assigned to the formation of Mn-III species, and one to the expansion of layers of MnO6 octahedra. A special activation treatment leads independent of the starting material to a highly amorphous mixed-valence oxide, which shows the highest OER activity.

For many electrochemical devices that use carbon-based materials such as electrolyzer, 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.

Abstract 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 VIII/VII but rarely the VVO2 +/VIVO2+ 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.

Metallic spinel-type CuCo$_{2}$S$_{4}$ nanoparticles were deposited on nanocrystalline TiO$_{2}$ (P25®), forming heterostructure nanocomposites. The nanocomposites were characterized in detail by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), nitrogen sorption (BET) and UV/Vis spectroscopy. Variation of the CuCo$_{2}$S$_{4}$:TiO$_{2}$ ratio to an optimum value generated a catalyst which shows a very high photocatalytic H$_{2}$ production rate at neutral pH of 32.3 µmol/h (0.72 mLh$^{–1}$), which is much larger than for pure TiO$_{2}$ (traces of H$_{2}$). The catalyst exhibits an extraordinary long-term stability and after 70 h irradiation time about 2 mmol H$_{2}$ were generated. An increased light absorption and an efficient charge separation for the sample with the optimal CuCo$_{2}$S$_{4}$:TiO$_{2}$ ratio is most probably responsible for the high catalytic activity.

It is widely accepted that surface-active oxygen functional groups (OFGs) effectively catalyze the vanadium redox reactions. Initial graphitic edge sites, OFGs and their electrochemical stability were examined using graphite felts, which were modified with multi-walled carbon nanotubes and activated with KOH. It is demonstrated that OFGs cannot exclusively be responsible for the electrocatalysis since they did not correlate to the electrochemical activity. The surface composition after electrochemical cycling in the positive half-cell was still different for all samples but did not reflect the performance either. However, a correlation was found between the activity and stable edge site defects. There was neither a correlation between the electrocatalytic activity and the amount of oxygen, nor for the kind of OFG in the negative half-cell. The oxygen concentration after electrochemistry was very similar, even more highlighting the importance of edge sites in the V-III/V-II redox reaction. The results of this work indicate that the major electrocatalytic effect for both half-cell redox reactions is related to stable graphitic edge sites in sufficient quantity.