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.

Systematic Characterization of Degraded Anion Exchange Membranes Retrieved from Vanadium Redox Flow Battery Field Tests.

Commercially available anion exchange membranes were retrieved from VRFB field tests and their degradation due to the various operation conditions is analyzed by in-situ and ex-situ measurements. Ion exchange capacity, permeability and swelling power are used as direct criteria for irreversible changes. Small-angle X-ray scattering (SAXS) and Differential scanning calorimetry (DSC) analyses are used as fingerprint methods and provide information about the morphology and change of the structural properties. A decrease in crystallinity can be detected due to membrane degradation, and, in addition, an indication of reduced polymer chain length is found. While the proton diffusion either increase or decline significantly, the ion exchange capacity and swelling power both are reduced. The observed extent of changes was in good agreement with in-situ measurements in a test cell, where the coulombic and voltage efficiencies are reduced compared to a pristine reference material due to the degradation process.

Tuning the performance of vanadium redox flow batteries by modifying the structural defects of the carbon felt electrode.

Polyacrylonitrile (PAN)-based carbon felt was subjected to N2-plasma treatment to increase the heteroatom defects and reactive edge sites as a method to increase the performance in vanadium redox flow batteries (VRFBs). N-doping in the felt was mainly in the form of pyrrolic and pyridinic nitrogen. Even though the amount of oxygen functional groups on the N2-plasma-treated sample was very low, the felt showed enhanced electrochemical performance for both V3+/V2+ as well as V5+/V4+ redox reactions. The result is highly significant as the pristine electrode with the same amount of oxygen functional groups showed significantly less activity for the V3+/V2+ redox reaction. Overall, the single-flow cell experiments with N2-plasma-treated felt showed superior performance compared to the pristine sample. Therefore, the enhanced performance observed for the N2-plasma-treated sample should be attributed to the increase in defects and edge sites. Thus, from the present study, it can be concluded that an alternate way to increase the performance of the VRFBs is to introduce specific defects such as N-doping/substitution or to increase the edge sites. In other words, defects induced in the carbon felt such as heteroatom doping are as beneficial as the presence of oxygen functional groups for the improved performance of VRFBs. Therefore, for an optimum performance of VRFBs, defects such as N-substitution as well as oxygen functionality should be tuned.

A Crosslinked Polyethyleneglycol Solid Electrolyte Dissolving Lithium Bis(trifluoromethylsulfonyl)imide for Rechargeable Lithium Batteries.

Replacing liquid electrolytes with solid ones can provide advantages in safety, and all-solid-state batteries with solid electrolytes are proposed to solve the issue of the formation of lithium dendrites. In this study, a crosslinked polymer composite solid electrolyte was presented, which enabled the construction of lithium batteries with outstanding electrochemical behavior over long-term cycling. The crosslinked polymeric host was synthesized through polymerization of the terminal amines of O,O-bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol and terminal epoxy groups of bisphenol A diglycidyl ether at 90 °C and provided an amorphous matrix for Li+ dissolution. This composite solid electrolyte containing Li+ salt and garnet filler exhibited high flexibility, which supported the formation of favorable interfaces with the active materials, and possessed enough mechanical strength to suppress the penetration of lithium dendrites. Ionic conductivities higher than 5.0×10-4  S cm-1 above 45 °C were obtained as well as a wide electrochemical stability window (>4.51 V vs. Li/Li+ ) and a high Li+ diffusion coefficient (≈16.6×10-13  m2 s-1 ). High cycling stability (>500 cycles or 1000 h) was demonstrated.

Surface Functionalization of Silicon, HOPG, and Graphite Electrodes: Toward an Artificial Solid Electrolyte Interface.

Electrografting of diazonium salts containing a protected alkyne moiety was used for the first functionalization of silicon and highly ordered pyrolytic graphite model surfaces. After deprotection with tetrabutylammonium fluoride, further layers were added by the thiol-yne click chemistry. The composition of each layer was characterized via X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. The same approach was then used to functionalize graphite powder electrodes, which are classically used as negative electrode in lithium-ion batteries. The effect of the coating on the formation of the solid electrolyte layer was investigated electrochemically by cyclovoltammetry and galvanostatic measurements. The modified graphite electrodes showed different reduction peaks in the first cycle, indicating reduced and altered decomposition processes of the components. Most importantly, the electrochemical investigations show a remarkable reduction of irreversible capacity loss of the battery.

Limitation of discharge capacity and mechanisms of air-electrode deactivation in silicon-air batteries.

The electrocatalytical process at the air cathode in novel silicon-air batteries using the room-temperature ionic liquid hydrophilic 1-ethyl-3-methylimidazolium oligofluorohydrogenate [EMI⋅2.3 HF⋅F] as electrolyte and highly doped silicon wafers as anodes is investigated by electrochemical means, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. The results obtained by XPS and EPR provide a model to describe the limited discharge capacity by means of a mechanism of air-electrode deactivation. In that respect, upon discharge the silicon-air battery's cathode is not only blocked by silicon oxide reduction products, but also experiences a major modification in the MnO2 catalyst nature. The proposed modification of the MnO2 catalyst by means of a MnF2 surface layer greatly impacts the Si-air performance and describes a mechanism relevant for other metal-air batteries, such as the lithium-air. Moreover, the ability for this deactivation layer to form is greatly impacted by water in the electrolyte.

Using layer-by-layer assembly of polyaniline fibers in the fast preparation of high performance fuel cell nanostructured membrane electrodes.

Membrane electrode assemblies (MEA) for fuel cells require optimization of their nanoscale organization to reach performance parameters, which include enhanced power density, increased catalyst utilization and reduced cost. We applied sprayed layer-by-layer assembly to produce a high activity MEA for H(2)/O(2) fuel cells from polyaniline fibers (PANI-F). This technique produces "fast-prepared" membranes with nanoscale structure, which allows to adequately address specific tuning of their porosity, platinum loading, electronic conductivity, and proton conductivity. Pt nanoparticles were attached to the PANI-F in a reaction of selective heterogeneous nucleation. After functionalization, Pt/PANI-F were assembled with Nafion. Microscopic investigation revealed that functionalized polyaniline fibers formed a highly porous yet tight network of interpenetrating conductors connected to the catalytic Pt particles. The Pt/PANI-F LBL ultrathin MEA demonstrated a power densitiy of 63 mW cm(-2) and yielded a Pt utilization of 437.5 W g(-1) Pt which is comparable to the traditional fuel cell using carbon black as Pt support. Moreover, the amount of Pt used in this work is almost 2 times lower than for usual carbon-supported Pt catalysts.