Electrolyte Transfer at a Liquid-Solid Polypyrrole Interface Quantified By in Situ X-Ray Reflectometry

Abstract

In situ characterization of the electrochemical (EC) intercalation mechanism of ions in conductive polymer films is one of the most important aspects in the development of novel organic supercapacitors. Compared to atomic force microscopy and quartz microbalance measurements, synchrotron-based in situ X-ray reflectometry (XRR) allows deeper and more detailed insights into electrically controlled transfer processes of electrolytes across liquid-solid interfaces. The low roughness of the layer system to be investigated, which is required for XRR, is a particular challenge: Electrochemically deposited polypyrrole (PPy) films naturally possess a rough surface, the typical cauliflower structure. In the present study, PPy was deposited on a silicon crystal with a fast-varying voltage to obtain a PPy film with a very smooth surface, which led in parallel to a significantly denser film, which also increases the XRR contrast. This 30 nm thin PPy film was measured in situ in a 0.1M HClO4 environment with XRR at different voltage levels between -0.2 and +0.5V vs Ag/AgCl and the film thickness and electron density were determined. A linear and reversible change in thickness and electron density was measured, which is typical for the potential-dependent intercalation process of anions. The number of electrons transferred across the electrolyte-PPy interface was determined from the change in thickness and electron density, from which the transport of matter was derived. The correlation with the electrically measured transferred charge resulted in a ratio that was significantly below the expected value for exclusive anion exchange. Considering all present and available matter, it can be deduced that proton transport partially compensates for the charge and that 2-3 water molecules per incorporated perchlorate anion are removed from the film. Fast XRR measurements on the same PPy film during applied cyclovoltammetry showed two intervals of different behavior of the PPy film: In the higher voltage range anion intercalation dominates, the PPy film shows a very high pseudo-capacity due to its high density, while in the lower voltage range proton exchange starts and reduces the pseudo-capacity. Electrochemical impedance spectroscopy confirmed these results: Above the transition of these two voltage ranges, the conductivity of the PPy film increases dramatically due to the intercalation of perchlorate anions. With the presented synchrotron-based, in situ EC-XRR setup the film thickness, the electron density, and the change of both, as well as the pseudo-capacitance and conductivity as a function of the applied voltage can be measured simultaneously on ultra-thin electrochemical polymer films. Especially for the development of new electrochemical systems this is a highly potent measurement technique for in situ characterization.