Formation of a hydrophobic polyiodide complex during cathodic oxidation of iodide in the presence of propylene carbonate in aqueous solutions, and its application to a zinc/iodine redox flow battery

A polyiodide complex has been newly obtained during the cathodic oxidation of iodide ions (I-) in the presence of propylene carbonate (PC). This complex demonstrates features of both ionicity and hydro-phobicity. The properties and structure of this complex were investigated and discussed in the light of electrochemical investigations, instrumental analyses, and theoretical calculations using quantum chemistry. The oxygen atom of the C=O bond in PC was considered to play an essential role in forming this complex, and a large dipole moment structure between I-2 and PC was suggested by the theoretical calculations. This PC/I-2 structure was considered to play an essential role in enhancing the dissolution rate of I-2 films on a cathode and in accelerating the cathodic oxidation of I-. A model for the structure of this complex is demonstrated. A zinc/iodine redox flow battery of an aqueous-based single electrolyte-type was examined as an application of this complex to a battery without a cation-exchange membrane. Over 90% Coulombic efficiency was observed through charge and discharge cycles. (C) 2019 Elsevier Ltd. All rights reserved.