In sulfuric acid (H2SO4) aqueous solutions containing manganese (Mn) and titanium (Ti) ions, which are used for the positive electrolyte in redox flow (RF) batteries, the addition of a small quantity of bismuth (Bi) has been found to be very effective in suppressing the MnO2 precipitation in the charging process from Mn2+ to Mn3+. The effect of the addition of Bi on the structural change in these solutions has been analyzed using x-ray absorption fine structure (XAFS) techniques. X-ray absorption near edge structure (XANES) spectra indicated that some Mn4+ ions were generated only when Bi was not added. It was also confirmed that the Mn-O octahedral coordination was maintained at a high state of charge (SOC). On the other hand, no change was observed in the valence of Ti and Bi ions even at high SOCs. Extended x-ray absorption fine structure (EXAFS) measurements combined with fitting analysis clarified that the addition of Bi resulted in the generation of Mn-O-Bi coordination at the SOCs of 70 and 90 %. Bi-O-Mn coordination may suppress the MnO2 precipitation at a high SOC.
All-solid-state fluoride-ion batteries (FIBs) using metal/metal fluorides are expected to be the next generation of storage batteries because they exhibit high volumetric energy densities by utilizing multielectron reactions, compared to the current lithium-ion batteries. However, method of fabricating a composite electrode for all-solid-state fluoride-ion batteries has not yet been established. A fabrication method for a composite electrode that disperses the active material and solid electrolyte is required. To approach this problem, in this study, we employed a high-pressure torsion (HPT) method, in which an active material, solid electrolyte, and conductive agent can be mixed with size reduction, as a new process and prepared Cu (active material)/PbSnF4 (solid electrolyte)/acetylene black (conductive agent) cathode composites. The crystalline sizes of Cu and PbSnF4 were significantly reduced. The apparent grain boundary resistance was also reduced owing to the more homogeneous distribution in the cathode composites after HPT processing. These structural and morphological changes led to high electrochemical performances, compared to a cathode composite without HPT.
Nitrogen-doped carbon nanotubes (NCNTs) have been considered a promising catalyst for the electrochemical reduction of CO2 (CO2ER) to generate CO. Although pyridinic N sites have been suggested to be the active center of NCNTs, their behavior in the reaction remains unclear because of the lack of experimental evidence. Herein we focused on the pH dependence of CO2ER activity of NCNT and investigated the effects of local pH at the electrode surface to estimate the catalytic role of the pyridinic N. The results of the in situ local pH measurements using surface-enhanced Raman spectroscopy (SERS) revealed that CO2ER activity disappears in an acidic environment at pH below 4. SERS detected no CO species at the surface during the reaction in the acidic electrolyte, and ex situ X-ray photoelectron spectroscopy indicated the protonation of the pyridinic N. These results suggest the protonation of pyridinic N, the active site of NCNT, inhibits the CO2 adsorption and the following reduction to define the catalytic activity.