(217an) Material Design and Mechanistic Studies On Temperature-Responsive Polymer Electrolytes for Electrochemical Systems

Smart materials, such as stimuli-responsive polymers, have attracted considerable interest with goals of designing novel systems with properties or functionality that can respond to an external stimulus. A new application of smart materials is presented, which provides the ability to control electrolyte properties, such as pH, ionic strength and conductivity, with temperature. Macromolecular ionomer solutions exhibiting properties that change in response to temperature are referred to here as thermally-responsive polymer electrolytes (RPEs). These systems fall within the realm of “smart materials” as they provide a means to reversibly control, manipulate, or actuate electrochemical systems or processes, such as batteries, supercapacitors or sensors, with an external stimulus.

RPEs are prepared using N-isopropylacrylamide (NIPAM), which governs the thermal properties, and ionic groups, such as acrylic acid, to supply the ions. As the aqueous polymer solution is heated or cooled through a thermally-activated lower critical solution temperature (LCST), the local environment around the ionic groups is reversibly switched and the ionic strength, conductivity and mobility of the electrolyte are altered. We investigate how the molecular properties of RPEs, such as molecular weight and ionic composition, influence the temperature-dependent electrolyte properties using pH and conductivity measurements and the extent to which redox active electrode systems can be controlled. Materials with the highest ionic content and lowest molecular weight provide the highest room temperature ionic conductivity and redox activity; however, RPEs with low ionic content and higher molecular weight provide the highest “on-off” ratio in electrochemical activity as temperature increases. Electrical impedance spectroscopy was used to determine the role the polymer electrolyte plays in the rate of electron transfer in electrochemical systems (i.e. whether solution resistance, charge transfer resistance, or diffusion limitation is the controlling factor in the polymer electrolyte’s regulation of electrochemical activity with temperature).   Both inert metal and redox active electrodes were used to explore the mechanism by which RPEs are capable of altering electrochemical activity as a demonstration of responsive electrolytes in batteries, supercapacitors, and sensors.  These materials provide a novel, reversible approach to thermal control in electrochemical systems and further development of these ideas will lead to tremendous opportunities in electrical energy storage and chemical sensors.