Exploration of Electrochemical Interfaces by Impedance Spectroscopy

The electrification of industrial and transportation processes requires deeper understanding of electrochemical interfaces. This work concerns how charged species move in specific solvation or coordination environments that are shaped by their applications and characteristic length scales, and more specifically, how ions move near interfaces and how electrons are transferred at interfaces. The primary tie-line through this work is electrochemical impedance spectroscopy (EIS). We chose EIS because it is a well-established technique that can provide information about the phenomena that persist at a large variety of length and time scales – EIS is particularly well-suited to understanding how charges accumulate and disperse at interfaces. To understand ion movements in more constrained, dense solvation and coordination environments, such as polymers and ceramics (i.e. 1 Å-1 nm) we use higher frequencies up to 1 MHz. This helps to understand the movement of Li⁺ ions between 6-oxygen solvation cages in ethylene-oxide polymer electrolytes and understand how the effectiveness of the solvation changes as a function of polymer architecture. The other dense solvation environment we probed with EIS was the ceramics; in order to understand the reactivity of the LLTO ceramic with Li at its interface, we first needed to quantify the basic ionic conductivity of the material and whether or not ions were mobile. The iron deposition work occurred in a very different solvation environment and over very different length scales – the Nernst diffusion layer of the rotating disc electrode we used to study the iron deposition is on the order of 100 μm, and the concentration gradients that form in aqueous solution electrochemistry persist over those much longer length and corresponding time scales. In order to understand how complex anion interactions impact these large length scales, we used lower EIS frequencies (20 mHz). In all cases we are measuring the same fundamental phenomena – the movement of charge and its resulting current.