Luciano M. Santino

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.

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

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.

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.

Electrochemical capacitors fabricated with polyaniline nanofibers are cycled 150 000 times with 98% capacitance retention. These devices maintain an energy density of 11.41 Wh/kg at a power density of 4000 W/kg, 64 times greater than that of an identically fabricated device based on activated carbon (0.177 Wh/kg at 4600 W/kg). For applications requiring a higher specific energy, 33.39 Wh/kg at a specific power of 600 W/kg is obtained by widening the voltage window; this device retains 93% capacitance after 10 000 cycles. We achieve a high cycling stability through careful device engineering paired with a renewed focus on the electrochemical processes occurring at the positive and negative electrodes during cycling.