Determining Solubility and Diffusivity by Using a Flow Cell Coupled to a Mass Spectrometer

• Published in: Chemphyschem 2016-06, Vol.17 (11), p.1647-1655
• Main Authors: Khodayari, Mehdi, Reinsberg, Philip, Abd-El-Latif, Abd-El-Aziz A., Merdon, Christian, Fuhrmann, Juergen, Baltruschat, Helmut
• Format: Article
• Language: English
• Subjects: diffusion; electrochemistry; mass spectrometry; solubility; thin layer flow cell
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201600005

One of the main challenges in metal–air batteries is the selection of a suitable electrolyte that is characterized by high oxygen solubility, low viscosity, a liquid state and low vapor pressure across a wide temperature range, and stability across a wide potential window. Herein, a new method based on a thin layer flow through cell coupled to a mass spectrometer through a porous Teflon membrane is described that allows the determination of the solubility of volatile species and their diffusion coefficients in aqueous and nonaqueous solutions. The method makes use of the fact that at low flow rates the rate of species entering the vacuum system, and thus the ion current, is proportional to the concentration times the flow rate (c⋅u) and independent of the diffusion coefficient. The limit at high flow rates is proportional to D2/3·c·u1/3 . Oxygen concentrations and diffusion coefficients in aqueous electrolytes that contain Li+ and K+ and organic solvents that contain Li+, K+, and Mg2+, such as propylene carbonate, dimethyl sulfoxide tetraglyme, and N‐methyl‐2‐pyrrolidone, have been determined by using different flow rates in the range of 0.1 to 80 μL s−1. This method appears to be quite reliable, as can be seen by a comparison of the results obtained herein with available literature data. The solubility and diffusion coefficient values of O2 decrease as the concentration of salt in the electrolyte was increased due to a “salting out” effect. Solubility and diffusion of gases: The solubility of volatile species can be determined from the mass spectrometric ion current at low flow rates, where the collection efficiency is one. At high flow rates, the diffusion coefficient can be determined.