Developing energy storage devices to be utilized within a rapidly advancing energy market requires a multipronged approach whereby material synthesis and engineering fundamentals combine to enable technological advances. These devices should be able to store a large amount of energy in a small, lightweight package, and should be able to distribute that energy quickly for high rate applications. Pseudocapacitors made from conducting polymers, which store charge via rapid reduction and oxidation reactions, are a particularly promising candidate. This perspective explores conductivity and charge storage mechanisms in conducting polymers and describes how synthetic strategies can affect these properties. We further develop chemical correlations that have been shown to enhance the performance of pseudocapacitive electrochemical capacitors fabricated from conducting polymers. Important device engineering strategies for improving the lifetime and applicability of pseudocapacitors are also discussed.

Iron corrosion, a product from the chemical reaction between iron and oxygen in the presence of water and commonly referred to as rust, is a heterogeneous solid-state material composed of multiple phases that represent an abundant source of chemical waste. Here, we introduce a strategy that advances the state-of-the-art in chemical synthesis by demonstrating the usefulness of this ubiquitous inexpensive inorganic material for developing oxidative radical polymerizations. Rust, when treated with an acid, is an ideal source of Fe3+ ions affording an oxidation potential of 0.77 V for oxidizing thiophene-based moieties and producing conducting polymers characterized by long conjugation lengths. We develop fundamental knowledge and mechanistic understanding that enables the deposition of freestanding nanofibrillar films of the conducting polymer poly­(3,4-ethylenedioxythiophene) (PEDOT) via rust-based vapor-phase polymerization (RVPP). Our process takes place in a single step inside a sealed hydrothermal reactor when monomer vapor makes contact with a solid-state rust coating undergoing dissolutionthis approach is scalable requiring only a rusted steel surface, acid vapor, and monomer vapor. Freestanding nanofibrillar PEDOT films delaminate from a steel substrate characterized by an electronic conductivity of 323 S cm–1 and high electrochemical stability; RVPP enables patterning of a film in situ during synthesis. RVPP–PEDOT films are engineered into supercapacitors resulting in devices that exhibit a state-of-the-art capacitance of 181 F g–1 at a current density of 3.5 A g–1 and retain 80% of their original capacitance after 38 000 cycles.

A low temperature modified vapor phase polymerization affords high-aspect ratio nanofibers of polypyrrole, which conformally coat fibrous substrates.

We introduce a novel condensing vapor phase polymerization (CVPP) strategy for depositing microtubes of the conducting polymer polypyrrole; these serve as one-dimensional hollow microstructures for storing electrochemical energy. In CVPP, water droplets are structure-directing templates for polypyrrole microtubes. Water vapor condensation and polymerization occur simultaneously-conformal coatings of microtubes deposit on porous substrates such as hard carbon fiber paper or glass fiber filter paper. A mechanistic evolution of the microtubular morphology is proposed and tested based on the mass transport of water and monomer vapors as well as on the reaction stoichiometry. A coating of PPy microtubes is characterized by a high reversible capacitance of 342 F g at 5 mV s throughout 5000 cycles of cyclic voltammetry and a low sheet resistance of 70.2 Ω □. The open tubular structure is controlled in situ during synthesis and leads to electrodes that exhibit electrochemical stability at high scanning rates up to 250 mV s retaining all stored charge, even after extensive cycling at 25 mV s.

The design and fabrication of functional scientific instrumentation allows students to forge a link between commonly reported numbers and physical material properties. Here, a two-point and four-point probe station for measuring electrical properties of solid materials is fabricated via 3D printing utilizing an inexpensive benchtop fused-deposition modeling system and designed by standard computer-aided design software. Stainless steel tapestry needles serve as probes for contacting a sample; these are also electroplated in order to study their electrical performance, and provide a framework for discussion of electrical charge transport, contact resistance, and conductivity in materials. A microcontroller board is integrated into the probe and controlled using open-source software. Our robust and simple design provides an instrument that is easily fabricated by students and readily applied to a wide range of classroom settings focused on materials science, mechanical and electrical engineering, as well as solid-state physics and chemistry. This 3D printed probe station costs less than $100 US in materials per unit excluding source meter. We demonstrate that two- and four-point resistance measurements carried out on a solid-state semiconductor differ only by less than 5% in magnitude when compared to data collected using a standard and expensive commercial probe station. Two- and four-point resistance measurements carried out on gold deposited on silicon and on the soft nanostructured organic semiconductor poly­(3,4-ethylenedioxythiophene) result in reproducible and accurate current versus voltage (I–V) curves.

PEDOT nanofibers show unprecedented cycling stability in aqueous supercapacitors, 90% capacitance for 350 000 cycles, and exhibit excellent power and energy density (25 kWkg−1 and 4.3 W h kg−1) at 1 V.

The diversity of nanostructures obtained from organic polymerization is limited when compared to the huge amount of documented inorganic nanostructures. In this paper, we elucidate a synergistic mechanism between in situ inorganic salt hydrolysis and vapor-phase polymerization for the metal oxide-poly­(3,4-ethylenedioxythiophene) (PEDOT) hybrid nanostructure growth. The steady state polymer growth and kinetically controlled hydrolysis enables homogeneous deposition of high-aspect-ratio crystal phases such as β-FeOOH, TeO2, and SnO2 coated by a conducting polymer. By controlling the hydrolysis kinetics, the hybrid material is synthesized in one step with morphologies controlled from 1D nanofibers to 2D nanoflowers and nanostructures from monolithic to core–shell. This fundamental understanding of the connection between hydrolysis and polymerization allows the future development of nanostructured inorganic, polymeric, and inorganic–organic hybrid materials. Enabled by this study, electrodes for energy storage are fabricated with different PEDOT morphologies, and their structure–property relationships are discussed. The 1D fibrillar structure shows a higher capacitance of 185 F/g at 25 mV/s compared to 2D nanoflowers because this morphology enhances electrolyte diffusion kinetics that facilitate PEDOT doping and dedoping, leading to a lower internal resistance.

Horizontally directed nanofibrillar PEDOT mats bearing high impact energy densities are fabricated as electrodes for impact-resistant flexible supercapacitors.