Michael A. Lowe

High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with RuO2·xH2O in an acidic electrolyte. However, we recently demonstrated that a pseudocapacitive mechanism occurs when lithium ions are inserted into mesoporous and nanocrystal films of orthorhombic Nb2O5 (T-Nb2O5; refs 1,2). Here, we quantify the kinetics of charge storage in T-Nb2O5: currents that vary inversely with time, charge-storage capacity that is mostly independent of rate, and redox peaks that exhibit small voltage offsets even at high rates. We also define the structural characteristics necessary for this process, termed intercalation pseudocapacitance, which are a crystalline network that offers two-dimensional transport pathways and little structural change on intercalation. The principal benefit realized from intercalation pseudocapacitance is that high levels of charge storage are achieved within short periods of time because there are no limitations from solid-state diffusion. Thick electrodes (up to 40 μm thick) prepared with T-Nb2O5 offer the promise of exploiting intercalation pseudocapacitance to obtain high-rate charge-storage devices.

Electrochemistry of Individual Monolayer Graphene Sheets

We report on the fabrication and measurement of devices designed to study the electrochemical behavior of individual monolayer graphene sheets as electrodes. We have examined both mechanically exfoliated and chemical vapor deposited (CVD) graphene. The effective device areas, determined from cyclic voltammetric measurements, show good agreement with the geometric area of the graphene sheets, indicating that the redox reactions occur on clean graphene surfaces. The electron transfer rates of ferrocenemethanol at both types of graphene electrodes were found to be more than 10-fold faster than at the basal plane of bulk graphite, which we ascribe to corrugations in the graphene sheets. We further describe an electrochemical investigation of adsorptive phenomena on graphene surfaces. Our results show that electrochemistry can provide a powerful means of investigating the interactions between molecules and the surfaces of graphene sheets as electrodes.

Mechanistic insights into operational lithium–sulfur batteries by in situ X-ray diffraction and absorption spectroscopy

The lithium–sulfur battery is an extremely attractive system for electrical energy storage because of its exceptional theoretical capacity and energy density. However, the practical values typically obtained are much lower and inherently determined by the complex chemistry of reduced sulfur species. The lack of methods to probe sulfur species under realistic battery conditions has frustrated chemical understanding and control. We have employed in situ X-ray diffraction (XRD) and sulfur K-edge X-ray absorption near edge spectroscopy (XANES) to probe the sulfur intermediates and products formed in battery electrodes during operation of prototype lithium–sulfur batteries. Correlations between the X-ray and electrochemical data show that the reduction of sulfur to lithium sulfide is mediated through dissociation and disproportionation reactions of a few dominant sulfur species. Deliberate control of these chemical equilibria is essential to approach the theoretical capacity of the lithium–sulfur system.

In operando X-ray studies of the conversion reaction in Mn3O4 lithium battery anodes

The pursuit of energy storage systems with high energy density has revealed several exciting possibilities, including the unexpectedly reversible conversion reactions between metal oxides and lithium for lithium ion battery anodes. The mechanistic complexity of the drastic chemical and structural changes as well as the sensitivity of the reaction intermediates and products to ambient conditions mean that the reaction mechanism is best studied by non-destructive techniques in the native battery environment (in operando). This work applies synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) to directly observe the conversion reaction of a Mn3O4 anode previously shown to have promising electrochemical performance. The results enable the assignment of electrochemical features to specific reactions, including the formation of LiMn3O4, MnO, metallic Mn, and non-metal-centered reactions, and elucidate the difference between the first and subsequent lithiation reactions. In operando XAS clearly shows that a significant fraction of the charge is stored in non-Mn-centered reactions, a result with serious implications for Mn3O4, in particular, and other metal oxide conversion anodes, in general. This study emphasizes the importance of in situ/in operando studies on next-generation electrode materials to confirm that the observed charge transfer is due to the desired electrochemical reactions.

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.

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.

Spontaneous incorporation of gold in palladium-based ternary nanoparticles makes durable electrocatalysts for oxygen reduction reaction

Replacing platinum by a less precious metal such as palladium, is highly desirable for lowering the cost of fuel-cell electrocatalysts. However, the instability of palladium in the harsh environment of fuel-cell cathodes renders its commercial future bleak. Here we show that by incorporating trace amounts of gold in palladium-based ternary (Pd6CoCu) nanocatalysts, the durability of the catalysts improves markedly. Using aberration-corrected analytical transmission electron microscopy in conjunction with synchrotron X-ray absorption spectroscopy, we show that gold not only galvanically replaces cobalt and copper on the surface, but also penetrates through the Pd–Co–Cu lattice and distributes uniformly within the particles. The uniform incorporation of Au provides a stability boost to the entire host particle, from the surface to the interior. The spontaneous replacement method we have developed is scalable and commercially viable. This work may provide new insight for the large-scale production of non-platinum electrocatalysts for fuel-cell applications.

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.

Templated quasicrystalline molecular ordering.

Quasicrystals are materials with long-range ordering but no periodicity. We report scanning tunneling microscopy (STM) observations of quasicrystalline molecular layers on 5-fold quasicrystal surfaces. The molecules adopt positions and orientations on the surface consistent with the quasicrystalline ordering of the substrate. Carbon-60 adsorbs atop sufficiently separated Fe atoms on icosahedral Al-Cu-Fe to form a unique quasicrystalline lattice, whereas further C60 molecules decorate remaining surface Fe atoms in a quasi-degenerate fashion. Pentacene (Pn) adsorbs at 10-fold symmetric points around surface-bisected rhombic triacontahedral clusters in icosahedral Ag-In-Yb. These systems constitute the first demonstrations of quasicrystalline molecular ordering on a template.

Operando X-ray scattering and spectroscopic analysis of germanium nanowire anodes in lithium ion batteries.

X-ray diffraction (XRD) and Fourier transform extended X-ray absorption fine structure (EXAFS) analysis of X-ray absorption spectroscopy (XAS) measurements have been employed to determine structural and bonding changes, as a function of the lithium content/state of charge, of germanium nanowires used as the active anode material within lithium ion batteries (LIBs). Our data, collected throughout the course of battery cycling (operando), indicate that lithium incorporation within the nanostructured germanium occurs heterogeneously, preferentially into amorphous regions over crystalline domains. Maintenance of the molecular structural integrity within the germanium nanowire is dependent on the depth of discharge. Discharging to a shallower cutoff voltage preserves partial crystallinity for several cycles.