Underscreening, Overscreening, and Blue Energy

The efficient storage of energy and the reuse of waste heat are important processes to tackle climate change. Interestingly, supercapacitive energy-storage devices not only allow to store electric energy, but their physical functionality also allows to exploit the charging process itself within novel thermodynamic cycles. These cycles allow to harvest so-called Blue Energy from salinity steps and from waste heat. I will demonstrate how such cycles can be designed and that classical density functional theory allows to explicitly calculate them. In particular, I will discuss the physics of socalled electric double layers (see Figure 1), those regions next to the electrodes of a capacitor where the mobile charges in the electrolyte arrange themselves to screen the electrode's electrostatic field. At sufficiently low ionic concentration, electric double layers are well described by Poisson-Boltzmann theories, which approximate ions by point-like particles. At higher concentrations, however, ionic sizes are non negligible.