To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis­(trifluoromethanesulfonyl)­amide (Mg­(TFSA)2)/triglyme, magnesium borohydride (Mg­(BH4)2)/tetrahydrofuran (THF), and Mg­(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg­(TFSA)2/triglyme and Mg­(BH4)2/THF, while the reactions do not occur in Mg­(TFSA)2/2-MeTHF. Raman spectroscopy shows that the [TFSA]− in the Mg­(TFSA)2/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]− in Mg­(TFSA)2/2-MeTHF and [BH4]− in Mg­(BH4)2/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg+ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]−, strongly bound to the magnesium ion in the Mg­(TFSA)2/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg­(BH4)2/THF electrolyte, because the [BH4]− coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH4]−. Conversely, because [TFSA]− is weakly bound to the magnesium ion in Mg­(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

Fluoride ion batteries (FIBs) are regarded as promising energy storage devices, and it is important and urgent to develop cathode materials with high energy densities for use in FIBs.

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The Z-scheme CO2 reduction activity of metal complex–semiconductor hybrid photocatalysts was investigated in detail with a focus on the interfacial electron transfer process. Semiconductors of GaN:ZnO solid solutions, TaON, and Ta/N-codoped TiO2 were examined as components of the hybrid photocatalyst in combination with a binuclear Ru­(II) complex. The (photo)­physical properties of the semiconductor part were found to strongly affect the efficiency of interfacial electron transfer from/to the Ru complex photosensitizer unit, which was attached to the semiconductor surface. The photocatalytic activity of the hybrids showed a reasonable relationship with the efficiencies of forward and backward electron transfer. Among the three semiconductors, the highest activity was obtained with GaN:ZnO, which had the most negative conduction band potential among the semiconductors examined. The experimental results clearly demonstrated that analyses of the emission quenching process of the excited photosensitizer moiety of the binuclear Ru­(II) complex allowed visualization of the interfacial electron transfer between the semiconductor and the Ru complex, giving us a rational guideline to improve the efficiency of the hybrid photocatalyst for Z-scheme CO2 reduction.