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.

Systematic structural and electrochemical studies on the Mg[TFSA]2-based electrolytes revealed that the coordination state of [TFSA]− predominates the electrochemical magnesium deposition/dissolution activity.

A first-principles molecular dynamics study is presented for the structural, dynamical, vibrational, and dipolar properties of the solvation shell of a nitrate ion in deuterated water. A detailed description of the anisotropic structure of the solvation shell is presented through calculations of various structural distributions in different conical shells around the perpendicular axis of the ion. The nitrate ion-water dimer potential energies are also calculated for many different orientations of water. The average vibrational stretch frequency of OD modes in the solvation shell is found to be higher than that of other OD modes in the bulk, which signifies a weakening of hydrogen bonds in the hydration shell. A splitting of the NO stretch frequencies and an associated fast spectral diffusion of the solute are also observed in the current study. The dynamics of rotation and hydrogen bond relaxation are found to be faster in the hydration shell than that in the bulk water. The residence time of water in the hydration shell is, however, found to be rather long. The nitrate ion is found to have a dipole moment of 0.9 D in water which can be attributed to its fluctuating interactions with the surrounding water.

Understanding the inherent response of water to an external electric ͑E͒-field is useful towards decoupling the role of E-field and surface in several practically encountered situations, such as that near an ion, near a charged surface, or within a biological nanopore. While this problem has been studied in some detail through simulations in the past, it has not been very amenable for theoretical analysis owing to the complexities presented by the hydrogen ͑H͒ bond interactions in water. It is also difficult to perform experiments with water in externally imposed, high E-fields owing to dielectric breakdown problems; it is hence all the more important that theoretical progress in this area complements the progress achieved through simulations. In an attempt to fill this lacuna, we develop a theory based on relatively simple concepts of reaction equilibria and Boltzmann distribution. The results are discussed in three parts: one pertaining to a comparison of the key features of the theory vis a vis published simulation/experimental results; second pertaining to insights into the H-bond stoichiometry and molecular orientations at different field strengths and temperatures; and the third relating to a surprising but explainable finding that H-bonds can stabilize molecules whose dipoles are oriented perpendicular to the direction of field ͑in addition to the E-field and H-bonds both stabilizing molecules with dipoles aligned in the direction of the field͒.

Mechanism of magnesium ion alloying reaction into bismuth electrode in magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA) 2 )/ acetonitrile (AN) and Mg(TFSA) 2 /2-methyltetrahydrofuran (2-MeTHF) electrolyte was examined by a combination of operando soft X-ray absorption spectroscopy (XAS), Raman spectroscopy, and density functional theory (DFT) calculations. In 0.5 M Mg(TFSA) 2 /AN, the magnesium ions alloying reaction occurred, whereas the alloying reaction did not occur in 0.5 M Mg(TFSA) 2 /2-MeTHF. Raman spectroscopy showed that less than 15% of [TFSA] − coordinates with magnesium ions in 0.5 M Mg(TFSA) 2 /AN, while more than 90% of [TFSA] − coordinates with magnesium ions in Mg(TFSA) 2 / 2-MeTHF. Using operando XAS measurements, we observed that electronic and local structure of magnesium ion changed similarly upon cathodic polarization in both electrolytes. These results indicate that the difference of the behavior of alloy formation should be affected by the difference of coordinate structure of [TFSA] − in both electrolytes. Our DFT calculation results indicates [TFSA] − coordinated to magnesium ions undergoes reduction decomposition more easily than [TFSA] − uncoordinated to magnesium ions. In 0.5 M Mg(TFSA) 2 /2-MeTHF, the [TFSA] − coordinating to magnesium ions undergoes reduction decomposition, which inhibits the alloying reaction into the bismuth electrode. On the other hand, in 0.5 M Mg(TFSA) 2 /AN, the [TFSA] − reduction decomposition occurs relatively slowly because of the weak coordination between [TFSA] − and magnesium ions, which allows the magnesium ions alloying into the bismuth electrode in the electrolyte.