José Pedro WOJEICCHOWSKI1, Catarina M.S.S. NEVES1, Paula NAVALPOTRO2, Rebeca MARCILLA2, João A.P. COUTINHO1
1CICECO - Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Aveiro, Portugal
2IMDEA Energy, Electrochemical Processes Unit, Móstoles, Spain
Renewable energy sources are increasing their share of electricity supply. Most of renewable energy in Europe is supplied by solar and wind power. To enhance the efficiency of these energetic systems, it is mandatory to reduce the mismatch between its continuous production and consumers’ needs.
The best way to overcome this topic is to resort to energy storage devices, such as redox flow batteries (RFB). RFBs are one of the most promising electrochemical energy storage systems, being investigated mainly due to its scale-up possibility. Moreover, the high costs of the ion-selective membranes and the toxicity and availability of metallic compounds hinder their vast application. Among the approaches proposed to solve these problems, Membrane-Free Battery is an innovative concept to overcome those issues. Aqueous biphasic systems (ABS) can be used for that purpose, in which organic redox molecules replaces metallic substances. One of the advantages of using those systems is the possibility of tunning the ABS phases properties to optimize the separation of the species, improving the difference between the redox potential of the immiscible phases.
Therefore, this work aims to predict the partition coefficient (K) of redox-active compounds, used in RFB, in ABS formed by ionic liquids (IL), salt and water, through the Conductor-like Screening Model for Real Solvents (COSMO-RS). Experimental data were used to validate this model as a tool to investigate the partitioning of redox molecules in IL-based ABS.
Firstly, the geometries of ILs, salt and redox-active compounds, such as methyl viologen dichloride hydrate (MV), were optimized using BP-TZVP. Then, the phase composition of each system was used for the calculation of compounds partitioning. Moreover, the sigma-profiles, which reveal how much surface of a compound is in a polarity interval, were also evaluated.
Results indicated MV as the most hydrophilic redox compound, with a great hydrogen bond acceptor capacity. The other molecules and the ILs exhibited most of their surface areas within the ± 0.01 interval, being weakly polar or non-polar. The K results obtained show that COSMO-RS is able to qualitatively reproduce the behavior of redox compounds in the ABS. It was found a good correspondence between predicted and experimental data, with a Pearson correlation equal to 0.89. Moreover, MV was selectively partitioned into the salt-rich phase, due to lower affinity to hydrophobic IL-rich phase, as revealed by the sigma-profiles.
Thus, this work demonstrates COSMO-RS ability to predict the partitioning of electrolytes in complex IL-based ABS, showing that it could be a powerful tool to evaluate other systems and redox compounds, which can help design a total aqueous membrane-free battery.
Acknowledgments: This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MCTES (PIDDAC). This work has received funding from the European Innovation Council (EIC) under grant agreement 101046742. The EIC receives support from the European Union’s Horizon Europe research and innovation programme.
REFERENCE
NAVALPOTRO, P. et al. Pioneering Use of Ionic Liquid-Based Aqueous Biphasic Systems as Membrane-Free Batteries, Advanced Science, 2018.