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 VIII /VII but rarely the VV O2 + /VIV O2+ 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.

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) 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.

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 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.