Imaging Morphology, Ion Conductance and Degradation Processes in Energy Materials on the Nanometer Scale Using Tunneling Atomic Force Microscopy

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Proposal Number: 1608914 PI: Buratto, S. Imaging Morphology, Ion Conductance and Degradation Processes in Energy Materials on the Nanometer Scale Using Tunneling Atomic Force Microscopy Polymer electrolyte membranes (PEMs) are key components in electrochemical devices such as fuel cells and redox flow batteries. These devices are capable of producing clean energy with high efficiency and are useful in tandem with intermittent renewable electricity generation technologies using wind or solar when the wind or sun is not available and also to store electricity in the form of chemical energy for energy storage. PEMs are, in general, composed of a polymer with a hydrophobic backbone and side chains terminated with hydrophilic functional groups. When this polymer is cast in films to make the membrane, phase separation between the hydrophobic and hydrophilic segments results in a random nanoscale network of hydrophilic channels through which ions are transported and hydrophobic domains that give the membrane mechanical strength. Novel membrane chemistries are currently under development, especially alkaline electrolyte membranes, but require feedback from characterization efforts, especially on the nanometer scale, in order to understand ion conductance and its dependence on the pore network structure. Another key question is how the membrane degrades during operation and that impacts the performance of the device. This project addresses this need for fundamental understanding on how degradation occurs in these types of electrochemical devices. Knowledge gained will help to close the feedback loop between designing the membrane's structure and its resultant function. Students working on this research will learn about alternative power sources, help optimize membrane function, and provide valuable insight and inspiration into the development of the next-generation membrane materials. This project leverages a suite of experimental methods that utilize tapping (or AC) mode atomic force microscopy (AFM) and conductive probe AFM (cAFM) to probe the pore connectivity and ion transport in proton-conducting membranes such as PEMs. The Principal Investigator and his research group have developed these techniques. In this project, the tools are adapted and applied to the study of alkaline electrolyte membranes, which transport hydroxide ions. Under alkaline conditions, the oxygen reduction half-reaction has significantly improved kinetics, obviating the need for precious metal catalysts. This project's transformative feature includes the characterization of PEM membrane morphology and hydroxide-ion conductance on the nanometer length scale to provide a link between morphology and conductance. The project also includes the investigation of the ion domain morphology and connectivity in vanadium redox flow batteries that utilize proton-conducting membranes under drastically different operating conditions than the alkaline fuel cells. These experiments probe the dependence of the pore network structure on the electrolyte concentration, the degree of vanadium ion penetration into the membrane and the prolonged exposure of the membrane surface to water and heat. The ultimate goal is fundamental understanding of ion conduction, in terms of the size and distribution of the chemical domains responsible for the transport in both alkaline fuel cells and vanadium flow cell batteries. In both systems, ion conductance will be studied systematically as a function of (1) the device operating conditions, (2) the membrane type, and (3) the degree of membrane degradation and decomposition.