Investigation of alkali and alkaline earth solvation structures in tetraglyme solvent.

This study compares molecular calculations performed with molecular and periodic codes through an investigation of the solvation structures of alkali and alkaline earth metal ions in tetraglyme solution. The two codes are able to produce equivalent structural and energetic information at the same level of theory, and in the presence of the implicit solvation model or not. This comparison reveals that molecular optimisations can be performed with periodic codes and used directly as input models for interface or electrochemistry calculations in order to preserve the solvent-solute interaction and the cavitation energy. By a rigorous comparison, we have demonstrated that equivalent energetic values can be obtained with the conventional PBE-D3 and the newly developed SCAN-rVV10 functionals. Nevertheless, as far as the vibrational features are concerned and when the molecule possesses a highly conjugated system, the SCAN-rVV10 functional is required to describe the vibrational modes properly. The computed IR/Raman spectra can thus be used as essential information to determine the first solvation shell of metal ions in glyme-based solutions. In tetraglyme solution, the alkali and alkaline earth metal ions exhibit a diverse solvation structure. Small ions like Li+ and Mg2+ tend to adopt a coordination number of five or six, while larger ions, Na+, K+, and Ca2+, prefer an eight-coordinated environment, and the metal-ligand interaction increases in the order K+-O < Na+-O < Li+-O < Ca2+-O < Mg2+-O. The solvation spheres play a significant role in the stability and the reactivity of the solvated ions, and can thus be used as input models to construct the solvation structure in more sophisticated electrolytes, such as polyethylene oxide, or perform electrochemical calculations.

Cathodic dissolution in the electrocoagulation process using aluminium electrodes.

All the authors working with aluminium electrodes in the electrocoagulation process have shown that a dissolution occurs at the cathode. This result cannot be explained by the electrochemical process in which only the anodes should be dissolved. The most probable reaction is a chemical attack by hydroxyl ions (generated during water reduction) on the aluminium cathode but nobody has proved it in the framework of the electrocoagulation process. So we are interested in determining what kind of reactions occurs at the cathode. For that, we have elaborated a batch pilot apparatus divided into two compartments, allowing measurement of gas formation taking place only in one compartment. The gases measurements were performed by mass spectrometry with helium as carrier gas. To validate our experimental protocol, the first experiments have been done with a stainless steel cathode: in this case, the results have indicated that the amount of created hydrogen is in good agreement with the values calculated using the second Faraday's law. The experiments realised with an aluminium cathode have shown that the hydrogen formation, in these conditions, was higher than those observed with the stainless steel cathode. All our investigations enable us to propose that with an aluminium cathode, hydrogen formation can be separated into two phenomena. The first one is due to an electrochemical reaction (water reduction), and the second one arises from a chemical reaction explaining the dissolution observed at the cathode.