Wednesday, January 11
7:45 – 8:30 am breakfast
8:30 am – Introductory remarks: SEAS Dean Cherry Murray, Michael Stopa NNIN/C director.
SESSION I – Organic Photovoltaics
9:00 – 9:45 am – Konstantinos Fostiropoulos (Helmholtz-Zentrum Berlin )– Performance limitations in organic solar cells
In organic optoelectronic devices performance is mainly limited by the poor conductivity of molecular layers. Because of this drawback in organic solar cells (OSC) the active absorber layer thickness is limited to about 100 nm resulting in insufficient light absorption.
In this presentation we discuss a preparation method to optimize optical as well as electrical properties of molecular layers by engineering the hybrid interface between the transparent conductive electrode indium-tin-oxide (ITO) and the adjacent Zn-phthalocyanine absorber layer. The effect is based on specific termination treatments of the ITO surface prior to the absorber deposition. The ITO treatments admit either the passivation of unsaturized surface defects or modification of the surface work function. Moreover it functions as template for oriented growth of the molecular absorber layer. Applying this method we demonstrate an increase of the absorption coefficient by a factor of two as well as a significant increase of the absorber's conductivity.
Corresponding OSC devices on thus treated ITO show more than double power conversion efficiency compared to reference devices on untreated ITO substrates.
9:45 – 10:30 am Shane Yost (MIT MSE) – What can simulations teach us about organic photovoltaics?
The general processes required for an organic photovoltaic to function is first sun light is absorbed and forms an exciton that diffuses to an organic-organic interface. At the interface the exciton breaks apart into an electron and a hole, which must travel to the electrodes. In order to further understand these processes we use a combined quantum mechanical/molecular mechanical method. Using this method we investigate the effects of the disordered environment on the processes important photovoltaic function. For example, we find the charge transfer state is able overcome its binding energy and separate into free charge carriers due to the disordered environment at the interface.
10:30 – 10:45 am coffee break
10:45 – 11:30 am Alan Aspuru-Guzik (Harvard Chemistry) – Finding renewable energy materials, one screensaver at a time
Which material is best for organic photovoltaics? Using the volunteer computer time of half a million donors. In this talk, I will describe his group's efforts related to The Clean Energy Project (http://cleanenergy.harvard.edu), a large-scale volunteer-donor distributed computing project to find the best donor and acceptor materials for organic photovoltaics. I will describe their progress towards the goal of an automated search for high-performance materials. Our group has currently computed the electronic structure of more than 3 million candidate oligomer building blocks, and successfully carried out a related theory to experiment demonstration jointly with the group of Zhenan Bao at Stanford. I will describe the status and plans for the project, including the release of our top candidate list of compounds. Come prepared: Go to the Clean Energy Project website, download and install our client software in your computer to help the environment with your free cycles of computer time.
11:30 – 12:15 pm – Tim Kaxiras (Harvard SEAS) – First-Principles Investigations of Charge Carrier Dynamics in Nanostructured Hybrid Photovoltaics
The design and optimization of low-cost organic photovoltaics (OPV) materials is a key component toward achieving energy sufficiency and reducing green-house gas emissions. Better design of these systems by carefully selecting the appropriate organic molecules, semiconductor substrates and surfaces, through the use of theoretical models with predictive power, are required to speed up the development of successful devices. To this end, we have developed a new theoretical tool in the context of time-dependent density-functional-theory (TD-DFT) that allows us to propagate the coupled ion and electron dynamics in real time. This approach provides an accurate description of the optical properties of relevant systems and gives a realistic description of charge injection mechanisms between the organic molecule and the semiconducting substrate. This capability opens new possibilities for comprehensive studies of the factors that affect the stability and efficiency of organic and of hybrid organic-inorganic composite systems.
12:15 pm – 1:15 pm lunch
1:15 – 1:45 pm Svetlana Boriskina (Boston University, Chemistry) – Plasmonically integrated optical tornadoes for efficient light harvesting
I will demonstrate a novel hydrodynamics-inspired approach to engineering nanostructured platforms for solar energy capture and conversion, which is based on molding the optical energy flow into nanoscale optical vortices (areas of circular motion of light flux) ‘pinned’ to nanostructures. The proposed approach offers the promise of broad and fundamental impact on nanophotonic- and nanoplasmonic-based renewable energy applications as it helps to eliminate the mismatch between the electronic and photonic length scales in thin-film photovoltaic devices. Most importantly, the expected increase of the efficiency and spectral bandwidth of light absorption can be achieved with the simultaneous reduction of the Ohmic losses in metals. The light harvesting platforms designed in the frame of the proposed methodology will help to minimize the thickness of semiconductor needed to absorb light completely, will amplify the signal via plasmonic enhancement mechanism, and will be compatible for integration with either silicon electronics or flexible substrates such as those based on organic and polymer materials.
1:45 – 2:15 pm Jiahao Chen (MIT Chemistry) - Calculating the density of states in disordered organic semiconductors using free probability theory
We have developed a computational framework using random matrix theory and free probability to study the problem of calculating the density of states in disordered organic semiconductors. Using free probability theory, we approximate the density of states by the free convolution of two matrix ensembles formed by decomposing the Hamiltonian ensemble of the disordered system. Our framework allows us to calculate the error in this approximation using asymptotic moment expansions, with coefficients that have combinatorial interpretations as weighted paths of closed loops in Hilbert space. We show how the error, like the free convolution itself, can be calculated without explicit diagonalization of the Hamiltonian. We apply our theory to Hamiltonians for one-dimensional tight binding models with Gaussian and semicircular site disorder. We find that the particular choice of decomposition crucially determines the accuracy of the resultant density of states.
From a partitioning of the Hamiltonian into diagonal and off-diagonal components, free convolution produces an approximate density of states which is correct to the eighth moment. This allows us to explain the accuracy of mean field theories such as the coherent potential approximation, as well as the results of isotropic entanglement theory.
SESSION II – Self-Assembly
Wednesday 2:15 – 3:00 pm Alfredo Alexander-Katz (MIT MSE) – Self-assembly of biomimetic light harvesting antennas
The photosynthetic apparatuses of most systems are templated by a protein scaffolds to position the pigments in precise locations. Interestingly, there exists a class of bacteria that contain so-called chlorosomes. These chlorosomes are single layer lipid sacs that contain a large quantitity of chlorophyls that self-assemble into the antenna structures and have the most remarkable photophysical properties. In this talk we will present our work on understanding how such antenna forms and how we can mimic its formation in a synthetic system, highlighting some of the principles behind the robust assembly. Understanding how this process occurs is important for novel materials design in energy applications with tailored properties.
3:00 – 3:15 pm coffee break
3:15 – 4:00 pm Bradley Olsen (MIT ChemE) – Self-Assembled Functional Polymers for Challenges in Energy Conversion
Block copolymer self-assembly represents an elegant, low-cost technique for the fabrication of complex new soft materials of use for a wide variety of next-generation energy technologies. Critical for many of these applications is incorporating functional polymers into the nanostructured material. In contrast to traditional polymers which have Gaussian coil chain shapes, functional systems often have highly anisotropic shapes and specific interactions that can lead to extremely rich self-assembly behavior. This talk will discuss the incorporation of both globular proteins and semiconducting polymers into block copolymer systems, two classes of functional molecules of particular relevance for energy applications. In both cases, ongoing challenges in controlling both the thermodynamics and kinetics of self-assembly will be discussed.
Enzymes are a potentially attractive technology for a wide variety of energy conversion processes, including solar energy harvesting, electrocatalytic carbon reduction, hydrogen production, and as oxidative catalysts in fuel cells. These applications require integrating the protein into a synthetic material, with careful control over the local nanoenvironment of the protein required to specify the correct orientation, achieve a high packing density, and maintain protein activity and stability. We have demonstrated that the self-assembly of globular protein-polymer diblock copolymers may be used as a technique to produce nanostructured, biofunctional materials. Explorations of the thermodynamics in these systems can provide insight into the forces governing self-assembly and the critical length scales for interaction in the block copolymers, guiding our approach to protein-polymer interactions in bioconjugate materials.
A second challenge that has attracted a great deal of interest is the formation of bulk heterojunction structures in polymer photovoltaics. When light is absorbed in an organic photovoltaic, the photon is converted into an exciton which must be dissociated into a separate electron and hole before it thermally decays. Because the exciton only diffuses approximately 10 nm before thermal decay, bulk heterojunction nanostructures containing both p and n type polymers are desired. One particularly attractive method to form nanostructures at thermodynamic equilibrium is through block copolymer self-assembly, and significant effort has been dedicated towards elucidating the thermodynamics of self-assembly in semiconducting polymer systems. Self-assembly of a desired nanostructure requires control over thermodynamics and understanding the kinetics of these complex polymer systems, but little is known about the dynamics in block copolymers containing both rigid and flexible blocks. Through a combination of scaling theories, molecular dynamics simulation, and experiments we have illustrated that polymer diffusion may be hindered in these systems due to curvature mismatch between the rod and coil blocks within the copolymer, hinting at dynamic design rules required to overcome kinetic barriers to self-assembly in semiconducting polymer systems.
4:00 – 3:45 pm Mark Bathe (MIT) – Rational Engineering of DNA-based nanostructures for energy harvesting and conversion
Nucleic acids enable the programmed self-assembly of complex nanometer-scale objects of precise shape and chemical composition for nanotechnology. Applications include scaffolding enzymes to enhance rates of chemical synthesis and spatially organizing photoreceptors to enhance solar energy collection. A major bottleneck in the widespread use of nucleic acid self-assembly for nanotechnology is the ability to predict three-dimensional folded shapes from underlying nucleotide sequence, as well as functional properties of target structures. In this talk I present a new computational technology called CanDo (Computer-aided engineering for DNA origami, http://cando-dna-origami.org) that predicts the complex three-dimensional solution shape and functional properties of scaffolded DNA origami nanostructures [1, 2]. I illustrate the utility of this design tool with application to a range of complex non-linear nanostructures, as well as present target applications and developments related to solar energy harvesting and conversion.
4:45 – 5:15 pm Shenyu Kuang (Notre Dame) – The Effect of Self-Assembled Monolayer on Transport Properties at Metal-Solvent Interfaces: a Molecular Dynamics Study
With our improved approaches to reverse non-equilibrium molecular dynamics
(RNEMD), it is possible to impose an unphysical thermal and momentum flux between
different regions of inhomogeneous systems such as solid/liquid interfaces. We have
applied them to compute the interfacial thermal conductance and friction coefficients at
metal/organic solvent interfaces that have been covered by butanethiol SAM. Our
calculations suggest that coupling between the metal and liquid phases is enhanced by
the SAM, leading to a greatly enhanced conductivity and friction at the interfaces.
Specifically, the chemical bond between the metal and the SAM introduces a vibrational
overlap that is not present without the SAM, and the overlap between the vibrational
spectra (metal to cap, cap to solvent) provides a mechanism for rapid thermal transport
across the interface. Our calculations also suggest that this is a nonmonotonic function
of the fractional coverage of the surface, as moderate coverages allow diffusive heat
transport of solvent molecules that have been in close contact with the SAM.
Furthermore, the correlation between thermal conductance and friction coefficients
supports previous discussions on the relationship of surface wetting and transport
Thursday, January 12
8:00 – 9:00 am breakfast
SESSION III – Catalysis
9:00 – 9:45 am Ted Betley (Harvard Chemistry) – Driving multi-electron chemistry for energy storage using polynuclear reaction sites
Our efforts towards the preparation of polynuclear complexes as functional surrogates for polynuclear metallocofactors in biology will be discussed. We have developed a family of dendritic amide-based ligands to proximally orient multiple transition metal ions in a shared reaction space. The close M-M proximity can greatly influence the redox and reactivity patterns for these complexes, distinguishing the platforms from their monomeric components. Ligand modifications can directly control the M-M interactions to pursue different reactivities. The synthesis and characterization of these novel materials, as well as their reactivities as small-molecule activation platforms, will be described.
9:45 – 10:30 am Bart Bartlett (Michigan) – Low-Temperature Synthesis Routes to Chemically Stability Transition Metal Oxide Nanoparticles for High Power Li-Ion Electrodes
The spinel phases having composition LiM2O4 (M = Ti, Ni, Mn) are widely studied as Li-ion electrodes in commercial batteries. Metal oxides are advantageous because of their chemically stability in a wide electrochemical window. The spinel crystal structure allows for insertion/extraction of one mole of Li+ per formula unit without gross changes in the unit cell, thereby maximizing energy storage capacity. Then, crystal growth on the nanoscale opens the possibility to maximize power as the particle size approaches the Li+ diffusion length. However, parasitic reactions arising from surface defects often limit the lifetime of electrochemical cells composed of oxide nanoparticles. Our group is exploring synthesis pathways to provide nanoparticles without detrimental surface defects. A hallmark of this work is identifying and eliminating oxygen vacancies in lithium manganospinel synthesized by hydrothermal methods followed by annealing. This material stores ~ 450 Wh/kg of energy and cycles reversibly at rates up to 5 C (i.e.—the current required to charge and discharge of the full capacity of the cell in 1/5 of an hour) in a laboratory test cell. The synthesis methods are quite general, and are now being applied to the higher voltage cathode LiNi0.5Mn1.5O4, as well as the anode Li[Li0.33Ti1.66]O4.
10:30 am – 10:45 am coffee break
10:45 – 11:30 am Daniel Nocera (MIT) – The Artificial Leaf
It has been said for an ideal solar fuels process that the system requirements are:
- Earth-abundant materials
- No wires
- Direct solar-to fuels process.
We now describe two earth abundant catalysts that promote the oxygen evolving reaction (OER) and hydrogen evolving reaction (HER) with a solar AM 1.5 (1 sun) input mediated by an earth abundant silicon wafer, and all of this is done with no wires. The system captures many of the elements of photosynthesis and it is indeed functionally an artificial leaf. But the system surpasses the prescription from the community. It also does not rely on a membrane and it operates under very simple conditions, thus obviating complicated engineering requirements. The science behind the artificial leaf will be presented.
11:30 – 12:00 pm Joe Yourey (Michigan) – CuWO4 Photoanodes for Solar Driven Water Oxidation
Solar driven water oxidation for large-scale hydrogen production from semiconductor photoelectrodes has the potential to provide energy on large-scales from completely renewable sources. Our research has focused on developing thin film metal tungstates, and our most promising photoelectrode, CuWO4 has been synthesized by electrochemical deposition from aqueous precursors. Our synthesis allows for highly reproducible and robust construction of n-type CuWO4 onto transparent conducting substrates. CuWO4 has an experimentally determined band gap of ~2.4 eV and valence band edge raised ~0.3 eV compared to d0 metal oxides (e.g.—TiO2 and WO3) due to strong Cu(3d)-O(2p) hybridization. These thin films were electrochemically tested under simulated solar (AM 1.5G) illumination to investigate their photoelectrochemistry. At an applied bias (1.0 V, RHE), we observe significant photocurrent densities in pH 7 aqueous solutions, indicating the formation of H2 and O2 from water. The Faradaic efficiency for O2 production is quite high (> 85%). Most important, these thin-film electrodes are stable against photodegradation when illuminated with visible light under neutral pH. The photocatalytic properties of this material were also evaluated by measuring the rate of methanol oxidation when illuminated with visible light, as well as using ferricyanide as a sacrificial electron acceptor. The former result hints that co-catalysts may prevent surface recombination and increase the rate of water oxidation. The latter result shows promise for overall water splitting at no applied bias using a redox mediator with a photocathode for hydrogen evolution.
12:00 noon – 1:00 pm lunch
SESSION III – Catalysis (continued)
1:00 – 1:45 pm Matthew Kanan (Stanford) – Carbon Dioxide Electroreduction Catalysis for Sustainable Fuel Synthesis
Sustainable production of C-based fuel requires using renewable energy to power the reductive fixation of CO2. Coupling a source of renewable electricity to an electrolytic device is an attractive approach to this goal because it enables the use of solar or wind-derived electricity and independent optimization of catalysis. To date, however, a CO2 reduction catalyst that is efficient at high current density and amenable to long-term use in an electrolyzer has not been developed. This talk will describe Sn- and Cu-based heterogeneous catalysts recently developed in our lab that exhibit exceptional CO2 reduction activity. The first catalyst is a Sn/SnOx composite material inspired by the discovery that Sn electrodes require the presence of a surface oxide to effect CO2 reduction. The second catalyst consists of a modified Cu electrode with a large roughness factor and a high faradaic efficiency for CO2 reduction at low overpotentials. The combination of these features enables CO2 reduction geometric current densities near the mass-transport limit at overpotentials less than 0.4 V. Insight into the nature of the active surface for CO2 reduction for both of these catalysts will be discussed, as well as mechanistic information available from electrokinetic studies.
1:45 – 2:30 pm Mircea Dinca (MIT) – Crystalline Microporous Metal-Organic Frameworks: Opportunities in Energy Research
Metal-organic frameworks (MOFs) are crystalline solids wherein inorganic nodes are connected by organic ligands to give rise to highly ordered and monodisperse micropores with diameters ranging from 0.5 to ~ 2 nanometers. The micropores are responsible for unprecedented surface areas occasionally exceeding 5000 m2/g, making MOFs popular choices for energy applications in gas storage or separation as well as potentially energy storage. The crystalline nature of these materials also makes them attractive candidates for studying photophysical phenomena in ordered and/or confined organic chromophore aggregates. The various applications of MOFs in energy research will be discussed.
2:30 – 2:45 pm coffee break
SESSION II – Self-Assembly (continued)
2:45 – 3:30 pm Juan Jose de Pablo (Wisconsin) – Directed Assembly of Copolymers for Large-Scale Nanofabrication; Equilibrium and Beyond Equilibrium Considerations
There is considerable interest in devising nanofabrication strategies that rely on the molecular self-assembly of complex fluids and materials. Our efforts over the past several years have been focused on devising strategies to drive and direct that self assembly, largely by developing multiscale modeling models and methods capable of predicting the structure and properties of complex fluids and materials under external fields, including confinement, electric fields, or flow fields. These models and methods can vary considerably in nature and level of resolution, depending on the system and issues of interest. In this presentation I will provide an overview of a modeling strategy that is particularly effective for atomistic to mesoscopic length scales, along with its usefulness and limitations, in the context of a distinct nanofabrication platform that is based on the formation of ordered, defect-free block copolymer structures on nanopatterned substrates. The new formalism has been developed to describe the structure and dynamics of block copolymer blends and composites, and we use it to explain the effects of surfaces and different types of confining walls on the free energy (and the concomitant stability) of a variety of morphologies of interest for lithographic fabrication. Many of these morphologies represent non-equilibrium states that are accessed by specific processing routes, and simulations can be used to discern the boundaries between such states and stable, equilibrium morphologies. More generally, our results indicate that one can increase considerably the palette of structures and morphologies available for nanofabrication by seeking to systematically access and stabilize non-equilibrium states by controlling the dynamic pathways to phase formation.
SESSION IV – Fuel Cells
3:30 – 4:15 pm Sergio Granados-Focil (Clark) – Ion Transport Through Polymer Matrices: Triazole bearing sol-gel and polymer membranes as ion transporting membranes for renewable energy applications
Development and widespread use of new alternative energy sources, such as high temperature proton exchange membrane fuel cells, lithium-ion batteries and dye sensitized solar cells, requires a better fundamental understanding of the nature of ion transport within polymeric matrices. Three substantial challenges face the extensive use of ion conducting membranes: The need to decouple mechanical properties from ionic conductivity, the low concentration of dissociated ions within most polymeric matrices and a relatively low mobility of the macromolecule enclosed ionic species. This talk will summarize the latest results on the preparation of proton and iodide conducting sol-gel membranes where the ion mobility has been decoupled from the polymer matrix mechanical strength. These freestanding membranes show comparable or higher ionic conductivities than their linear, liquid, polysiloxane analogs and fall within an order of magnitude of the target ion mobilities for use in PEMFC’s and DSSC’s. The effects of charge carrier concentration, polymer matrix polarity and crosslink density on ionic conductivity will be discussed. As an expansion of this work a series of linear polysiloxane copolymers containing varying amounts of plasticizing pendant alkyl or ether chains have been synthesized to systematically study the effect of polymer matrix polarity on ion transport. Results from this study show that increasing the polarity of the backbone has a dramatic effect on ion conductivity. These findings are now guiding our synthetic efforts to generate nanophase separated, mechanically robust, ion-conducting membranes. Preliminary results on ion conducting block copolymer synthesis will also be presented.
4:15 – 5:00 pm Mark Mathias (General Motors R&D) – Platinum Reduction Pathways for Commercial Fuel Cell Vehicles
Whereas it is clear that we are moving towards less-petroleum-based and lower-carbon-footprint transportation technologies, the nature and the pace of the transformation will be determined by the performance, durability, and cost evolution of several electrification technologies. Polymer-electrolyte fuel-cell technology offers the promise of a long-range fast-refill zero-emissions vehicle, and performance and durability challenges now have appear to have been met by several automakers. Cost reduction is the remaining commercialization hurdle, and platinum used in the electrocatalyst is the largest material cost element. In this presentation, I will describe several material families that offer promise to enable platinum reduction to the same precious-metals cost level of modern catalytic convertors. Success will require fundamental material development, design of the electrode structure at the nanometer scale for efficient mass transport, and careful system control to minimize material degradation.
Friday, January 13
8:00 – 9:00 am breakfast
SESSION IV – Fuel Cells (continued)
9:00 – 9:45 am Yu Morimoto (Toyota Central R&D) – Present Status and Challenges on PEFC development -Roles and Expectation for Nanotechnology & Computation-
Since early 1990s, Toyota has working on PEFCs and FC vehicles and now is propelling their development for wider commercialization targeted in the middle 2010s. In this talk, after briefly describing the present status of overall FCV development, remaining technical issues on PEFCs are presented focusing on material- and structure-related topics, to which state-of-the-art nanotechnology and computation have a potential to contribute for higher efficiency and power, better durability and lower cost of the system.
9:45 – 10:30 am Peter Pintauro (Vanderbilt) – Nanofiber-Based Membranes and Electrodes for Hydrogen/Air Fuel Cells
The heart of a hydrogen/air proton exchange membrane (PEM) fuel cell is the membrane-electrode-assembly, where catalytic precious metal powder electrodes are attached to the opposing surfaces of a cation-exchange membrane. The membrane in such a device has three functions: (1) it physically separates the positive and negative electrodes, (2) it prevents mixing of the fuel and oxidant, and (3) it provides pathways for proton transport between the electrodes. Thus, fuel cell membranes must exhibit a high ionic conductivity, with zero electronic conductivity, controlled water swelling, and good thermal/mechanical/chemical stability in the wet and dry states. The Pt-containing anode and cathode in a PEM fuel cell, where hydrogen is electrochemically oxidized and oxygen is reduced, must have a composition and structure that maximizes catalytic activity and long-term durability/performance. In this talk, the use of nanofiber electrospinning techniques to fabricate proton conducting fuel cell membranes and high performance Pt/C electrodes will be described. Methods for membrane and electrode fabrication will be presented, along with structure/property data and fuel cell power output results.
10:30 – 11:15 am Thomas Zawodzinski (UT Knoxvilee, ORNL) – Interweaving computation and experimentation to build functioning electrolytes and electrodes from the nano-scale
One under-rated aspect of nanoscience is the optimal assembly of nano-objects into larger scale structures to form functional materials. This requires some knowledge of the behavior of the nano-objects as well as an understanding of their interactions. On top of that, the structural motifs that develop must be taken into account. In each of these cases (intrinsic properties, interactions, structure), scientists’ intuition has led to ideas about target properties. In many cases, these intuitive notions embody poor guesses, in some cases surprisingly based on clear misunderstanding of some basic properties.
This talk will be something of a ‘shaggy dog’ story. I will discuss some of our studies of fuel cell ion conductors, especially ‘water replacements’ and other approaches to developing electrolytes for use at T>100oC, and composite electrodes. These studies, which are incomplete, have evolved over a number of years with alternate phases of computational and experimental studies. In the course of that evolution, we have come to identify some critical misapprehensions in the literature and by us and some holes in necessary data and methodology for satisfactory descriptions of mesoscale properties building on nanoscale or molecular properties. We will describe these gaps and thereby suggest some areas in which we need more information. We will conclude with generalization of some of these ideas to materials for other purposes (e.g. batteries).
11:15 am Concluding remarks