Stephen E. Burkhardt

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).

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.

Poly(2,5-dimercapto-1,3,4-thiadiazole) as a Cathode for Rechargeable Lithium Batteries with Dramatically Improved Performance

Organosulfur compounds with multiple thiol groups are promising for high gravimetric energy density electrochemical energy storage. We have synthesized a poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT)/poly(3,4-ethylenedioxythiophene) (PEDOT) composite cathode for lithium-ion batteries with a new method and investigated its electrochemical behavior by charge/discharge cycles and cyclic voltammetry (CV) in an ether-based electrolyte. Based on a comparison of the electrochemical performance with a carbonate-based electrolyte, we found a much higher discharge capacity, but also a very attractive cycling performance of PDMcT by using a tetra(ethylene glycol) dimethyl ether (TEGDME)-based electrolyte. The first discharge capacity of the as-synthesized PDMcT/PEDOT composite approached 210 mAh g(-1) in the TEGDME-based electrolyte. CV results clearly show that the redox reactions of PDMcT are highly reversible in this TEGDME-based electrolyte. The reversible capacity remained around 120 mAh g(-1) after 20 charge/discharge cycles. With improved cycling performance and very low cost, PDMcT could become a very promising cathode material when combined with a TEGDME-based electrolyte. The poor capacity in the carbonate-based electrolyte is a consequence of the irreversible reaction of the DMcT monomer and dimer with the solvent, emphasizing the importance of electrolyte chemistry when studying molecular-based battery materials.

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.

Cyclometalated ruthenium oligomers with 2,3-di(2-pyridyl)-5,6-diphenylpyrazine: a combined experimental, computational, and comparison study with noncyclometalated analogous.

Recent investigations on polypyridine transition-metal complexes as potential molecular wires have provided new impetus for these long-studied and well-established systems. Using bridging ligands 2,3-di(2-pyridyl)-5,6-diphenylpyrazine (dpdpz) and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz), a tetrametallic cyclometalated ruthenium complex has been prepared and characterized, with each metal having one Ru-C bond. The electronic properties of this complex and two known monoruthenium and diruthenium complexes with dpdpz (DPDPZ series) were probed by electrochemical and spectroscopic techniques and compared to the previously reported tppz-based noncyclometalated ruthenium complexes (TPPZ series). The frontier orbital energy levels and electronic structures of the two series have been characterized by density functional theory (DFT) calculations. In accordance with the experimental results, these studies suggest that the DPDPZ series oligomers generally have a narrower energy gap relative to the TPPZ series. In addition, the large energy density of states in longer oligomers suggests the possibility of band-type conduction. The DPDPZ series exhibits red-shifted light absorption with enhanced intensity relative to the TPPZ series congeners. Time-dependent DFT computations have been performed to rationalize the electronic absorption of the DPDPZ series. Oxidative spectroelectrochemical measurements of the DPDPZ tetrametallic complex indicate the presence of intervalence charge-transfer transitions among ruthenium sites.