Héctor D. Abruña

Coulomb blockade and the Kondo effect in single-atom transistors

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C60 molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.

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.

Structurally ordered intermetallic platinum–cobalt core–shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts

To enhance and optimize nanocatalyst performance and durability for the oxygen reduction reaction in fuel-cell applications, we look beyond Pt-metal disordered alloys and describe a new class of Pt-Co nanocatalysts composed of ordered Pt(3)Co intermetallic cores with a 2-3 atomic-layer-thick platinum shell. These nanocatalysts exhibited over 200% increase in mass activity and over 300% increase in specific activity when compared with the disordered Pt(3)Co alloy nanoparticles as well as Pt/C. So far, this mass activity for the oxygen reduction reaction is the highest among the Pt-Co systems reported in the literature under similar testing conditions. Stability tests showed a minimal loss of activity after 5,000 potential cycles and the ordered core-shell structure was maintained virtually intact, as established by atomic-scale elemental mapping. The high activity and stability are attributed to the Pt-rich shell and the stable intermetallic Pt(3)Co core arrangement. These ordered nanoparticles provide a new direction for catalyst performance optimization for next-generation fuel cells.

Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles

Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.

Activating Pd by Morphology Tailoring for Oxygen Reduction

Pd has been the focus of recent research for Pt-alternative catalysts for the oxygen reduction reaction (ORR). It has been found that upon appropriate modification of its electronic structure, the catalytic activity of Pd can become comparable to that of Pt. However, the structure-activity relationships of Pd catalysts have hitherto not been well studied or understood. In the present work, we report a new finding that there is a strong dependence of the activity of Pd toward the ORR on its morphology. By simply adjusting the precursor concentration in the electrochemical deposition of Pd, we are able to tailor the morphology of the deposited Pd from nanoparticles to nanorods. Surprisingly, the surface-specific activity of Pd nanorods (Pd-NRs) toward the ORR was found to be not only 10-fold higher than that of Pd nanoparticles (Pd-NPs), the conventional shape of electrocatalysts, but also comparable to that of Pt at operating potentials of fuel cell cathodes. The morphology-activity relationships of Pd-NRs were further studied through a combination of electrochemical experiments and density functional theory (DFT) calculations. As revealed by its characteristic profile for CO stripping, the morphology of Pd-NRs features the exposure of Pd(110) facets, which exhibit superior ORR activity. The underlying mechanism, indicated by DFT calculations, could be ascribed to the exceptionally weak interaction between an O adatom and a Pd(110) facet. This finding furthers our understanding of Pd catalysis and casts a new light on the relevant catalyst design criteria.

Tunable High Performance Cross-Linked Alkaline Anion Exchange Membranes for Fuel Cell Applications

Fuel cells are energy conversion devices that show great potential in numerous applications ranging from automobiles to portable electronics. However, further development of fuel cell components is necessary for them to become commercially viable. One component critical to their performance is the polymer electrolyte membrane, which is an ion conductive medium separating the two electrodes. While proton conducting membranes are well established (e.g., Nafion), hydroxide conducting membranes (alkaline anion exchange membranes, AAEMs) have been relatively unexplored by comparison. Operating under alkaline conditions offers significant efficiency benefits, especially for the oxygen reduction reaction; therefore, effective AAEMs could significantly advance fuel cell technologies. Here we demonstrate the use of ring-opening metathesis polymerization to generate new cross-linked membrane materials exhibiting high hydroxide ion conductivity and good mechanical properties. Cross-linking allows for increased ion incorporation, which, in turn supports high conductivities. This facile synthetic approach enables the preparation of cross-linked materials with the potential to meet the demands of hydrogen-powered fuel cells as well as direct methanol fuel cells.

β-Ketoenamine-Linked Covalent Organic Frameworks Capable of Pseudocapacitive Energy Storage

Two-dimensional covalent organic frameworks (2D COFs) are candidate materials for charge storage devices because of their micro- or mesoporosity, high surface area, and ability to predictably organize redox-active groups. The limited chemical and oxidative stability of established COF linkages, such as boroxines and boronate esters, precludes these applications, and no 2D COF has demonstrated reversible redox behavior. Here we describe a β-ketoenamine-linked 2D COF that exhibits reversible electrochemical processes of its anthraquinone subunits, excellent chemical stability to a strongly acidic electrolyte, and one of the highest surface areas of the imine- or enamine-linked 2D COFs. Electrodes modified with the redox-active COF show higher capacitance than those modified with a similar non-redox-active COF, even after 5000 charge-discharge cycles. These findings demonstrate the promise of using 2D COFs for capacitive storage.

Pt-Decorated PdCo@Pd/C Core−Shell Nanoparticles with Enhanced Stability and Electrocatalytic Activity for the Oxygen Reduction Reaction

A simple method for the preparation of PdCo@Pd core-shell nanoparticles supported on carbon based on an adsorbate-induced surface segregation effect has been developed. The stability of these PdCo@Pd nanoparticles and their electrocatalytic activity for the oxygen reduction reaction (ORR) were enhanced by decoration with a small amount of Pt deposited via a spontaneous displacement reaction. The facile method described herein is suitable for large-scale, lower-cost production and significantly lowers the Pt loading and thus the cost. The as-prepared PdCo@Pd and Pd-decorated PdCo@Pd nanocatalysts have a higher methanol tolerance than Pt/C in the ORR and are promising cathode catalysts for fuel cell applications.

Lithium–Sulfur Battery Cathode Enabled by Lithium–Nitrile Interaction

Lithium sulfide is a promising cathode material for high-energy lithium ion batteries because, unlike elemental sulfur, it obviates the need for metallic lithium anodes. Like elemental sulfur, however, a successful lithium sulfide cathode requires an inherent mechanism for preventing lithium polysulfide dissolution and shuttling during electrochemical cycling. A new scheme is proposed to create composites based on lithium sulfide uniformly dispersed in a carbon host, which serve to sequester polysulfides. The synthesis methodology makes use of interactions between lithium ions in solution and nitrile groups uniformly distributed along the chain backbone of a polymer precursor (e.g., polyacrylonitrile), to control the distribution of lithium sulfide in the host material. The Li(2)S-carbon composites obtained by carbonizing the precursor are evaluated as cathode materials in a half-cell lithium battery, and are shown to yield high galvanic charge/discharge capacities and excellent Coulombic efficiency, demonstrating the effectiveness of the architecture in homogeneously distributing Li(2)S and in sequestering lithium polysulfides.

Solid-state electroluminescent devices based on transition metal complexes

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.