Capacitive density functional theory for structure and screening in ionic fluids and electric double layers with applications in sustainability

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The sustainable treatment of water and our demand of energy are challenges that we are facing at the beginning of the 21st century. Promising technologies addressing these challenges exploit the properties of ionic fluids and electric double layers (EDLs). The latter develop at the electrode-electrolyte interface when ionic electrolytes and supercapacitive (large surface) electrodes are combined. The physics of EDLs is crucial for the large capacitance of supercapacitive electrodes and for their utilization in energy harvesting, energy conversion, and deionization. In our proposed project we will develop a new theory in order to complete our knowledge of the properties of electrolytes and EDLs. Using this theory we will study structure and electrostatic screening which are relevant, not only for modeling and describing electrolytes and capacitive technologies, but also for related areas like chemistry (colloids) and biology (ion channels, DNA). To reach our goal, we will apply classical density functional theory (DFT), which is a microscopic framework in statistical physics. In particular, we will (i) give a solution for the long-standing problem of combining point-like Coulombic charges and steric hard-core repulsions in the primitive model of ions. The new approach is crucial to describe structural transitions in EDLs and, consequently, to understand the resulting anomalous capacitance of EDL-based electrodes. Furthermore, we will (ii) develop a new approach for explicit solvents in electrolytes and for modeling effective interactions in DFT. Based on this theoretical approach, we will study ionic hydration and the effect of dipolar solvent molecules, which are assumed to be the key ingredients for the description of an unexplained increase in the screening length that has been reported in experiments recently. Our research will be based on extensive numerical calculations. To this end, we will develop the state-of-the-art software package capDFT, which we will share with the public. Though, capDFT can serve as modeling software, especially in the applied sciences. Collaborations with leading scientists and joining conferences will help to disseminate our research and improve our scientific network.