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

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.

References in corpus (6)

• Density functional theory for hard-sphere mixtures: the White-Bear version Mark II
• Tension and stiffness of the hard sphere crystal-fluid interface
• Molecular Density Functional Theory of Water
• Boosting capacitive blue-energy and desalination devices with waste heat
• Solvent primitive model of an electric double layer in slit-like pores: microscopic structure, adsorption and capacitance from a density functional approach
• Monte Carlo simulation of the electrical properties of electrolytes adsorbed in charged slit-systems

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• Machine-learning free-energy functionals using density profiles from simulations
• Capacitance and Structure of Electric Double Layers: Comparing Brownian Dynamics and Classical Density Functional Theory
• Pseudospectral methods for density functional theory in bounded and unbounded domains
• Learning classical density functionals for ionic fluids
• Dense ionic fluids confined in planar capacitors: in- and out-of-plane structure from classical density functional theory
• Impedance resonance in narrow confinement
• The Primitive Model in Classical Density Functional Theory: Beyond the Standard Mean-Field Approximation
• Anisotropic pair correlations in binary and multicomponent hard-sphere mixtures in the vicinity of a hard wall: A combined density functional theory and simulation study
• Reversible heat production during electric double layer buildup depends sensitively on the electrolyte and its reservoir