Fundamental measure theory for the electric double layer: implications for blue-energy harvesting and water desalination

TL;DR

This paper develops a density functional theory model for the electric double layer in porous electrodes, revealing improved predictions for energy harvesting and desalination efficiencies in blue-energy and water treatment technologies.

Contribution

It introduces a novel DFT approach combining fundamental measure theory with Coulombic corrections to accurately model dense ionic packing near electrode surfaces.

Findings

Higher work output in blue-energy cycles compared to simpler models

Increased energy demand for desalination cycles

Good agreement with molecular dynamics simulations

Abstract

Capacitive mixing (CAPMIX) and capacitive deionization (CDI) are promising candidates for harvesting clean, renewable energy and for the energy efficient production of potable water, respectively. Both CAPMIX and CDI involve water-immersed porous carbon (supercapacitors) electrodes at voltages of the order of hundreds of millivolts, such that counter-ionic packing is important for the electric double layer (EDL) which forms near the surface of these porous materials. Thus, we propose a density functional theory (DFT) to model the EDL, where the White-Bear mark II fundamental measure theory functional is combined with a mean-field Coulombic and a mean spherical approximation-type correction to describe the interplay between dense packing and electrostatics, in good agreement with molecular dynamics simulations. We discuss the concentration-dependent potential rise due to changes in the chemical potential in capacitors in the context of an over-ideal theoretical description and its impact on energy harvesting and water desalination. Compared to less elaborate mean-field models our DFT calculations reveal a higher work output for blue-energy cycles and a higher energy demand for desalination cycles.