(625h) Template-Free Synthesis of Conducting Polymer Microstructures for Supercapacitor Electrodes

Interest in energy storage technologies, especially to support transportation, portable electronics and renewable energy generation, has increased at a tremendous rate over the past decade. The development of new technologies faces many challenges, such as material accessibility, efficiency, charge storage capacity and cycle-ability. Emerging devices, such as supercapacitors, have the potential to provide high energy devices with fast discharge rates to bridge the gap between traditional batteries and high-power capacitors. Materials ranging from high surface area, inert carbon nanomaterials to Faradaic metal oxides and conducting polymers have been used to achieve a range of performance properties in supercapacitors. Porous carbon nanomaterials utilize electrical double layer capacitance, where charges are stored physically at the electrode interface, allowing devices to achieve high discharge rates (high power density) at the cost of lower energy density. Electroactive conductive polymers (ECPs) are promising alternatives, due to their conductivity, increased energy storage capacity through Faradaic charge transfer, and their amenability to low cost, large area synthesis methods. In this work, we demonstrate a simple approach to preparing large quantities of conducting polymer microtubes without the need for a solution or substrate template. We will discuss how to control the polymer assembly and microtube synthesis on stainless steel mesh substrates, and how the microstructure affects the electrochemical performance.

Microtubes of polypyrrole were electrochemically polymerized on stainless steel mesh substrates (variable mesh sizes) using a range of growth conditions, including current density and monomer concentration and overall mass. Electrodes were studied to understand the growth mechanism and microtube formation, and how the electrode structure affects the electrochemical properties. The microtube’s formation and growth mechanism were studied using scanning electron microscopy (SEM) on samples prepared through different stages of growth (1C up to 30C). The tube structures evolve from an initial nucleus, which forms at the intersection of two metal wires in the mesh, to the final tube. We found that changing the mesh size (mesh diameter and spacing) provided an easy tool to manipulate the microtube size (100μm up to 1 mm), shape (cone-like or tube-like structure) and microtube density on the electrode. Finer mesh substrates (200x200 and 400x400) resulted in the highest density (about 350 microtubes/cm²), which also exhibited the highest electrode capacitance (200 F/g). This microstructure synthesis approach also applied to large substrates, which showed similar physical and electrochemical performance. The results show the effects of the substrate, current density and monomer concentration on the template-free assembly and electrochemical performance of polypyrrole, as well as new ways to manipulate the physical structure of redox materials for supercapacitor electrodes. Importantly, this approach is amenable for large-scale syntheses of micro and nanostructured electrode systems.