Abstract
We study ionic liquid–solvent mixtures in slit-shaped nanopores wider than a few ion diameters. Using a continuum theory and generic thermodynamic reasoning, we reveal that such systems can undergo a capillary ionization transition. At this transition, the pores spontaneously ionize or deionize upon infinitesimal changes of temperature, slit width, or voltage. Our calculations show that a voltage applied to a pore may induce a capillary ionization, which—counterintuitively—is followed by a re-entrant deionization as the voltage increases. We find that such ionization transitions produce sharp jumps in the accumulated charge and stored energy, which may find useful applications in energy storage and heat-to-energy conversion.
Highlights
Ionic liquids (ILs) under confinement play a key role in science and technology, exhibiting remarkable properties and finding applications in energy storage,[1−4] capacitive deionisation,[5−7] heat-to-energy conversion,[8−10] etc
We show that such capillary ionization goes along with abrupt changes in charge and energy storage, which may find practical applications in electrochemical energy storage and generation
We have studied ionic liquid−solvent mixtures in slit mesopores
Summary
Ionic liquids (ILs) under confinement play a key role in science and technology, exhibiting remarkable properties and finding applications in energy storage,[1−4] capacitive deionisation,[5−7] heat-to-energy conversion,[8−10] etc. We consider a mixture of a room-temperature ionic liquid and solvent confined into a slit mesopore of a supercapacitor’s electrode, with the potential difference U applied to the pore walls with respect to the bulk electrolyte (Figure 1). Applying a voltage to a mesopore can induce a capillary ionization transition (Figure 3a,b) This transition is accompanied by a sharp increase of the charge accumulated in the pore (Figure 3c), which has important consequences for capacitance and energy storage. Where Qrich(U) and Qpoor(U) are the charge accumulated in the pore in the IL-rich and IL-poor phases, respectively, Uci is the transition voltage, and θ(x) is the Heaviside step function, equal to unity for x > 0 and zero otherwise. The magnitudes of ΔQci and ΔEci increase steeply with voltage at high voltages
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