Template-Free Synthesis of Polypyrrole Microtubes

Interest in new energy storage technologies, especially to support transportation and renewable energy generation, is increasing at a rapid rate. New technologies face many challenges, such as material abundance, efficiency, charge storage capacity and long term cycle-ability. Emerging devices, like supercapacitors, have the potential to provide high energy storage capacity and fast charge-discharge rates to bridge the gap between traditional batteries and high-power capacitors.

In a supercapacitor, many types of electrode material can be used, ranging from high surface area, inert carbon nanomaterials to Faradaic metal oxides and conducting polymers. Porous carbon nanomaterials rely upon electrical double layer capacitance, in which charged are stored physically at the electrode interfaces rather than through charge transfer with the electrode (Faradaic). While EDLC devices can discharge at high rates (high power), they are limited by low energy densities. Electroactive conductive polymers (ECPs) are promising materials, since they are conductive, possess moderate to high energy storage capacity, and can be synthesized using low cost and large scale methods.

In this work, we investigate a simple approach to preparing large quantities of conducting polymer microtubes without the need for a solution or substrate based template. Due to its fairly high specific power and energy, polypyrrole is studied as the electrode material for microtube-based devices. However, since the mechanism for electrochemical synthesis is similar for various monomers, we expect this approach can be applied to other materials relative ease. In this presentation, we will discuss how to control the polymer assembly and microtube synthesis by changing the substrate geometry.

Figure 1. Polypyrrole microtubes on 200x200 stainless steel mesh. Deposition at 10mA/s for 30C.

Electrodes prepared using our template-free synthesis with polypyrrole to achieve microtube structures is shown in Figure 1. Polypyrrole was electrochemically polymerized on various stainless steel mesh substrates (40x40, 200x200, 400x400, among others) with various mass, current density and monomer concentration. Electrodes were studied to understand the growth mechanism and microtube formation, and how the electrode structure affects the charge storage properties.

Variations in the substrate surface structure resulted in an easy tool to manipulate the microtube size (100μm up to 1 mm), shape (cone-like or tube-like structure) and density of microtubes along the electrode. The finer meshes (200x200 and 400x400) gave the highest density (about 350 microtubes/cm²) and the best electrochemical performance (200 F/g) measured by cyclic voltammetry, electrochemical impedance spectroscopy and galvanodynamic cycling.

The microtube’s formation and growth mechanism was studied using scanning electron microscopy (SEM) on samples prepared through different stages of growth (1C up to 30C). Figure 2 shows how the tube structures evolve from an initial nucleus, which forms at the intersection of two metal wires in the mesh, to the final tube.

Figure 2. Microtubes growth mechanism.

Polypyrrole microtubes exhibit similar electrochemical properties compared to thin films. Electrodes prepared on 200x200 and 400x400 mesh displayed a high density of microtubes and good electrochemical properties, as shown in Figure 3. It should be noted that some microtubes exhibited poor electrochemical performance due to the high potentials required during synthesis (e.g. large mesh sizes, 40x40).

Figure 3. Cyclic voltammetry of 8.26mg polypyrrole microtube electrodes.

These results led us to understand the influence of the substrate, current density and concentration in 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 has been used to synthesis tens of milligrams of material per square centimeter, and is amenable for scale to large systems.