Scalable, template-free synthesis of conducting polymer microtubes

Abstract

The integration of new materials in commercial energy storage systems faces many challenges, such as scalable manufacturing, charge–discharge efficiency, long term cycle stability, and high power and high energy density. Compared with conventional high-surface area carbon nanomaterials, electroactive conducting polymers (ECPs) exhibit an increase in energy density, which is attractive for next generation supercapacitor electrode materials. When designed with micro- and nanoscale dimensions, ECP electrodes display an increase in power density by decreasing the ion diffusion length in bulk electrodes. Here, we describe a template-free method for synthesizing polypyrrole microtubes on various stainless steel meshes with a process inherently scalable to large-area substrates. Microtube growth is governed by the nucleation of hydrogen gas at the mesh joints as a result of proton reduction at the platinum counter electrode. Depending on the size and spacing of the mesh wires and the substrate proximity to the working electrode, polypyrrole microtubes can be created with cylindrical and conical shapes with diameters ranging from 50–400 μm and heights up to 1400 μm. Polymer electrodes exhibiting cylindrical structures electrochemically grown with electrode potentials below 0.8 V exhibit excellent electrochemical performance comparable to thin polymer films. The process scalability is demonstrated using larger area substrates (up to 4 cm²) by carefully controlling the spacing between the working (substrate) and counter electrodes, which also provides an increase in microtube density from 350 cm⁻² to 560 cm⁻² without any loss in performance.