Modeling the Transient Behavior of Ionic Diodes with the Nernst-Planck-Poisson Equations

Ionic circuitry formed from soft charged polymers promises to enable a new kind of analog computing and usher in a new age of human-computer interaction, but devices remain in their infancy. Ionic diodes, created by the interface between two oppositely charged polymers, form one of the most fundamental elements of ionic circuitry. However, it is currently not well understood how the boundary conditions and geometry of a diode influence its transient response and performance. A consistent set of metrics with which to analyze these diodes is first developed, then a continuum model based on the Nernst-Planck-Poisson equations is used to study how device construction and three types of boundary conditions affect performance. This model is solved using the finite volume method and gives insight into transient diode behavior not described by previous analyses. It is demonstrated how different parts of a diode's construction, thickness, and operating voltage act as the limiting factors for different regimes. To conclude, it is demonstrated how blocking electrodes limit both diode lifetime and rectification behavior and how neutral bath boundary conditions play an important role in the performance of some diodes, with recommendations for building higher performance devices in the future. Understanding how ionic devices behave out of equilibrium is critical to improving their performance. This paper describes a detailed analysis of how geometry, material properties, and boundary conditions affect the transient response of ionic interfaces, guided by finite volume simulations and dimensional analysis. image