Gabriel G. Rodríguez-Calero

Rapid and Efficient Redox Processes within 2D Covalent Organic Framework Thin Films

Two-dimensional covalent organic frameworks (2D COFs) are ideally suited for organizing redox-active subunits into periodic, permanently porous polymer networks of interest for pseudocapacitive energy storage. Here we describe a method for synthesizing crystalline, oriented thin films of a redox-active 2D COF on Au working electrodes. The thickness of the COF film was controlled by varying the initial monomer concentration. A large percentage (80-99%) of the anthraquinone groups are electrochemically accessible in films thinner than 200 nm, an order of magnitude improvement over the same COF prepared as a randomly oriented microcrystalline powder. As a result, electrodes functionalized with oriented COF films exhibit a 400% increase in capacitance scaled to electrode area as compared to those functionalized with the randomly oriented COF powder. These results demonstrate the promise of redox-active COFs for electrical energy storage and highlight the importance of controlling morphology for optimal performance.

Tailored redox functionality of small organics for pseudocapacitive electrodes

Recently, there has been an explosion of literature dedicated to organic electrodes for energy storage applications. While inorganic materials, especially oxides, have generally being explored for these applications, the guiding principles for successful electrical energy storage—maximizing capacity and energy density per unit mass and cost—have naturally led to the pursuit of organic materials. However, there has only been a modest focus on methods for systematic exploration, which could help establish rational design principles for their enhanced properties and performance. Here we focus on a specific class of pseudocapacitive cathodes based on conducting polymers with pendant redox sites. We have previously demonstrated that the addition of such a pendant charge storage component provides a significant increase in the capacity, in addition to well-defined voltage plateaus, all the while maintaining the superior rate capability of these materials. In this report, we present a computational screening study for downselection of pendant candidates and a systematic study of structure–electrochemical property relationships. From this study, a generalized approach for defining the formal potential of oxidation of “violene” organic pendants is presented. Surprisingly, many of the structural parameters with which the oxidations can be tuned are independently addressable. While the methods described here have only been applied to the violene system, it should be emphasized that similar formalisms can be applied to other systems where the ability to rationally tune redox active components is desirable. Notable examples include organic energy storage electrodes based on oxygen and sulfur redox couples, charge shuttles for Li-ion batteries, organic photovoltaics, synthetic metals and organic light-emitting diodes.

Increasing the Gravimetric Energy Density of Organic Based Secondary Battery Cathodes Using Small Radius Cations (Li+ and Mg2+)

One of the major challenges in electrochemical energy storage (EES) is increasing the gravimetric capacity and energy density of the cathode material. Here we demonstrate how to increase the gravimetric energy density of electrical energy storage devices based on the use of organic materials through exploitation of the strong ionic coupling between a reduced carbonyl functionality and small cations such as lithium (Li(+)) and magnesium (Mg(2+)). Binding of the cation to the reduced carbonyl results in a positive shift of the formal reduction potential of the carbonyl couple. This has the effect of increasing the cell voltage which, in turn, results in an increase in the energy density. We show how this interaction can be used to dramatically increase, by up to a factor of 2, the energy density for a selected case study using 1,2-di(thiophen-2-yl)ethane-1,2-dione (DTED). We have carried out electrochemical and computational studies in order to understand the thermodynamic (positive shift of 250 mV and 1 V in the formal potential for the first and second reductions, respectively, of the carbonyl groups of DTED) and kinetic effects between small radii cations (Li(+) and Mg(2+)) and the reduced carbonyl functionality of carbonyl-based organic molecules (C-bOMs).

Development of Highly Active and Regioselective Catalysts for the Copolymerization of Epoxides with Cyclic Anhydrides: An Unanticipated Effect of Electronic Variation

Recent developments in polyester synthesis have established several systems based on zinc, chromium, cobalt, and aluminum catalysts for the ring-opening alternating copolymerization of epoxides with cyclic anhydrides. However, to date, regioselective processes for this copolymerization have remained relatively unexplored. Herein we report the development of a highly active, regioselective system for the copolymerization of a variety of terminal epoxides and cyclic anhydrides. Unexpectedly, electron withdrawing substituents on the salen framework resulted in a more redox stable Co(III) species and longer catalyst lifetime. Using enantiopure propylene oxide, we synthesized semicrystalline polyesters via the copolymerization of a range of epoxide/anhydride monomer pairs.

Designing conducting polymer films for electrochemical energy storage technologies

The search for new materials for electrical energy storage (EES) is one of the most active research areas today. In terms of materials for electrochemical (super)capacitors, most work has focused on high surface area carbons (HSAC) (surface areas in excess of 2000 m2 g−1) and metal oxides (e.g. RuO2). These materials offer high charge/discharge rates but suffer from high cost in the case of metal oxides (RuO2) and/or low capacities (less than 10–20 mAh/g) for HSAC. Our work seeks to design materials with high specific capacity and high gravimetric energy and power densities. Conducting polymers are attractive as electrode materials for electrochemical capacitors due to their ability to store charge in a pseudo-capacitive manner and their insolubility in most organic solvents used as electrolytes. In fact, some devices based on conducting polymer materials have shown promise due to their fast doping/de-doping processes (e.g. polythiophenes). However the capacities of these polymeric materials are low when compared to inorganic battery materials, because the electrochemical reactions involve a fraction of an electron per monomer unit and the need for significant amounts of supporting electrolyte solution which lowers the gravimetric capacity and energy density. We have synthesized and thoroughly characterized hybrid organic-pendant/conducting-polymer composite materials (e.g. poly-3,4-ethylenedioxythiophene (PEDOT)/N,N,N′,N′-tetraalkylated-p-phenylenediamine composite) which can exchange up to 2.6 electrons per monomer unit. The materials can be electropolymerized, ostensibly through the EDOT units, to yield films capable of storing charge with contributions from both the conducting polymer (pseudocapacitor) and the organic pendant (faradaic). Early studies indicate good electrochemical cyclability.

Electrocatalysis of 2,5-Dimercapto-1,3,5-thiadiazole by 3,4-Ethylenedioxy-Substituted Conducting Polymers

The electronic properties of electropolymerized films of the 3,4-ethylenedioxy-substituted conducting polymers (CP) poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,4-ethylenedioxyselenophene) (PEDOS) have been investigated, along with their electrocatalytic activity toward 2,5-dimercapto-1,3,4-thiadiazole (DMcT). For the electropolymerized films, the conductivity onset potential was most negative for PEDOP (-1.50 V), followed by PEDOS (-1.35 V) and with PEDOT possessing the most positive onset (-1.15 V). The heterogeneous charge transfer rate constant for DMcT in solution at polymer-film-modified glassy carbon electrodes (GCEs) was studied. It was found that compared to PEDOP, both PEDOS and PEDOT performed better as electrocatalysts, with PEDOS having a heterogeneous charge transfer rate constant of 1.8 × 10(-3) cm/s. The film morphology of the electropolymerized films was investigated via SEM, and some film characteristics could be correlated with electrocatalytic activity. The potential use of CP/DMcT composites for lithium ion batteries (LIBs) is discussed.

Towards organic energy storage: characterization of 2,5-bis(methylthio)thieno[3,2-b]thiophene

Electrical energy storage devices will play a key role in the development of sustainable energy production and usage, and for integrating intermittent and renewable sources into the energy landscape. One strategy for developing improved energy storage materials and devices is to take advantage of capacitive and pseudocapacitive electrodes such as activated carbons or conducting polymers. However, these materials generally suffer from low energy densities. Functionalization of these materials with pendant redox units has been proposed as a method to improve the energy densities while maintaining the high rate capability. In this report, we present the synthesis and thorough characterization of one such candidate pendant molecule, 2,5-bis(methylthio)thieno[3,2-b]thiophene, and assess its potential use as a cathode material. Electrochemical, spectroelectrochemical and computational data suggest that bis(methylthio)thieno[3,2-b]thiophene is a lightweight molecule, capable of undergoing multiple reversible redox processes, and a good candidate for improving the energy density of cathode materials while still offering high rate (power) capability.

Block copolymer derived 3-D interpenetrating multifunctional gyroidal nanohybrids for electrical energy storage

Electrical energy storage systems such as batteries would benefit enormously from integrating all device components in three-dimensional (3-D) architectures on the nanoscale to improve their power capability without negatively impacting the device-scale energy density. However, the lack of large scale synthesis methods of 3-D architectures with precise spatial control of multiple, functional energy materials at the nanoscale remains a key issue holding back the development of such intricate device designs. To achieve fully integrated, multi-material nano-3-D architectures, next-generation nanofabrication requires departure from the traditional top-down patterning methods. Here, we present an approach to such systems based on the bottom-up synthesis of co-continuous nanohybrids with all necessary functional battery components rationally integrated in a triblock terpolymer derived core–shell double gyroid architecture. In our design three-dimensional periodically ordered, functional anode and cathode nanonetworks are separated by an ultrathin electrolyte phase within a single 3-D nanostructure. All materials are less than 20 nm in their layer dimensions, co-continuous and interpenetrating in 3-D, and extended throughout a macroscopic monolith. The electrochemical analysis of our solid-state nano-3-D Li-ion/sulfur system demonstrated battery-like characteristics with stable open circuit voltage, reversible discharge voltage and capacity, and orders of magnitude decreases in footprint area compared to two-dimensional thin layer designs.