Redox behavior and surface morphology of polystyrene thermoplastic electrodes

Abstract

Carbon composite electrodes consisting of graphite and a non-conductive binder are widely used due to their low cost and ease of fabrication but often suffer from slow electrode kinetics and high capacitance relative to traditional electrode materials such as gold and glassy carbon. Modifiers including metallic nanoparticles and carbon nanotubes are used to enhance electrode performance, but this complicates electrode preparation, increases cost, and limits reusability. Our group recently reported a new type of carbon composite electrodes called thermoplastic electrodes (TPEs) that use common thermoplastics mixed with graphite in a solvent processing system that give enhanced electrochemical performance. The polymers reported to date include polymethylmethacrylate, cyclic olefine copolymer, polycaprolactone (PCL), and blends of these materials. Polystyrene (PS) is an inexpensive and commonly used thermoplastic substrate, however, there are only several reports incorporating PS as a binder in carbon composite electrodes. Here, we report the first PS TPEs and show their unique electrochemical behavior relative to electrodes made with other binders. These unmodified composite materials surpass the performance of traditional electrode materials and show similar behavior to nanomaterial-modified electrodes. The capacitance and conductivity were measured for PS and PS-polycaprolactone (PCL) blends using PCL TPEs as a reference. Even small additions of PS to PCL electrodes substantially lowered capacitance and increased conductivity. The electrochemistry of PS and PS-PCL TPEs was investigated using cyclic voltammetry with various common and biologically relevant redox probes. The effect of differing molecular weight and the use of waste expanded polystyrene (EPS) were explored as well, and all PS binder TPEs were found to exhibit similar redox behavior. Optical profilometry (OP) and scanning electron microscopy (SEM) were used to examine morphological characteristics. OP showed lower surface roughness for PS electrodes than for PCL ones, explaining the lower capacitance. SEM data suggests the source of this enhanced redox behavior stems from the edge-plane rich PS TPE surfaces.