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Team I: Molecular Scale Charge Transport
To achieve breakthroughs in renewable energy technologies, researchers in the chemical sciences will have to develop new materials that efficiently convert energy from one form to another. For example, we need better fuel cells for converting chemical energy in fuels such as hydrogen to electricity; such fuel cells rely on better "proton exchange membranes" (PEMs) for conducting positive electricity to balance the flow of the negative kind. This kind of process is called "proton transport," and at a molecular scale consists of individual hops of protons know as "proton transfer" from "proton donor" to "proton acceptor."
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The PEM is the component of a fuel cell responsible for shuttling positive charge to balance the flow of electricity (negative charge), and also responsible for keeping the fuel and oxygen from mixing and exploding. Credit:
http://p2library.nfesc.navy.mil/issues/emergeoct2005/fuelcell.jpg
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(Aside: Although not central to the mission of this Chemical Bonding Center, we also need better materials for converting light energy (photons) from the sun into electricity. Such substances are called "photovoltaics" or PVs because "photo" means light and "voltaic" means voltage or electricity. Finally, we also need better catalyst materials for converting plant energy (glucose) into liquid fuels such as ethanal and gasoline.)
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The problem with today's PEMs is that they rely on hydration for rapid conduction of electricity. Because fuel cells are not perfectly efficient, some of the energy goes to heating the PEM, which drives off water, and slows proton motion. As such, the main driver for new technology development is the need for high conductivity at low relative humidity.
To address this problem, synthetic chemists (the kind who make new molecules) around the world are trying to make new and better PEMs, ones that are more stable, and that rapidly conduct protons. Here at UMass, Profs. Coughlin, Thayumanavan and Venkataraman are part of the CBC effort to synthesize new PEMs.
So why is this so challenging?
Although Profs. Coughlin, Thayumanavan and Venkataraman have good ideas how to make better PEMs, nobody in the world understands the basic properties of proton transfer well enough to help design better PEMs. Let's put that into perspective ...
Imagine trying to bake a cake without a recipe, or worse, build a house without any blueprints. That is what synthetic chemists are trying to do when they make educated guesses about better PEMs. If we could give them the "blueprint" -- that is, a complete understanding of what makes PEMs stable while giving rapid conduction of protons -- then the synthetic chemists would have a much better chance of making
improved PEMs.
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| Schematic of new PEM material with various possible designs, with protons flowing on inside of nanopores ("inverse micelle"), outside of nanopores ("micelle"), or both ("vesicle"). Image by: Akamol Klaikherd
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In Aim 1, we are performing research that will provide fundamental understanding of how to maximize the rate of proton transport by studying how protons hop from donor to acceptor. This begins with "theoretical" studies (no experiments, just computers) performed by Profs. Auerbach and Fermann and their coworkers, where we compute in precise detail how protons hop from donor to acceptor, focusing on systems that are used often in synthesis: carboxylate, sulfonate, ammonia, imidazole, and triazole based molecules. Our calculations solve "quantum chemistry" equations, which involve determining energies and electron densities of molecules. In short, we solve the "Schroedinger equation," which was first published in 1926, and which, as Paul Dirac famously said, "explains all of chemistry
and most of physics." These proton energies tell us how protons hop (mechanism) and how long it takes them to hop (rate).
Our theoretical predictions need to be tested by experiments. This is because we must make approximations when solving the Schroedinger equation, largely because molecules have many interacting electrons. It turns out that computing the quantum interactions among all these electrons is presently beyond all computer power on the planet.
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Molecular system showing proton transfer, the jump of the proton from one molecule to the next. In this case, the jump is from one molecule of "nitro ammonia" to another. Image by: Julia Kumpf
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To do this testing, Aim 1 has a tightly integrated program of synthesis and "spectroscopy" (more on this below). Prof. Venkataraman and his coworkers make new molecules that have proton donors and acceptors connected to the same molecular backbone (aka "scaffold"). Samples of these molecules then get sent to the "spectroscopy" laboratory of Prof. Mark Johnson at Yale University. They shine "infra red" light on Prof. Venkataraman's samples. By detecting the precise frequencies of light that get absorbed by the molecules, Prof. Johnson and his coworkers can deduce the rates of proton transfer, for comparison with Auerbach's theoretical calculations. When agreement is obtained, we have chemical insights into the hopping mechanisms of protons.
I know what you're thinking: so what? Good question. Once the theoretical calculations are validated, we use them to scan various donor/acceptor systems to find the ones that give the fastest proton transfer. We look for systems that give fast proton transfer even when the donor/acceptor pair are not right next to eachother. We give this information to the synthetic chemists, who use it as a "blueprint" to design their advanced PEMs.
Team 2: Intermolecular Scale Charge Transport
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Helical scaffold for proton bucket brigade
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Today, there are two main hurdles to making better hydrogen fuel cells: better catalysts for generating protons and better “proton exchange membranes.” A proton exchange membrane acts like a microscopic sieve that will allow protons (the smallest atomic entity) to be separated from other atoms. In Fueling the Future we are focused on understanding what properties allow protons to be most efficiently selected and transported through the tiny holes of our molecular sieve. We are interested in how nature transports protons. Because nature can efficiently select and transport protons, we want to mimic the mechanism that is used in plant, animal and even virus proteins. Just like the building blocks for houses are wooden boards or bricks, the building blocks for proteins are called amino acids. Many proteins use an amino acid called histidine to transport protons.
Histidines are special amino acids because they can transport protons, which carry a positive charge. Histidines have the unique characterisitic of being able to exist in two equally stable, but different forms. Histidines are happy both when they have an extra proton, but impressively are equally happy when they have given a proton away. Thus we think that histidines will be able to help us make new and better proton exhange membranes.
On team 2, we made a molecular bucket brigade, where the histidine residues act as buckets for protons. We are able to ask what is the best arrangement of histidines for our proton bucket brigade to work best. To test different arrangements of the proton bucket brigade, we need to have a scaffold that will keep our proton buckets (the histidines) a set distance from each other in a specific orientation. Helices are stable scaffolds that look like the coils of a telephone cord and exist in natural proteins. Helices are ideal for keeping our buckets in defined orientations. Thus, we are making helices, some of which look like those found in the proteins in your body, and some that can only be made in the lab, with histidine “buckets: attached to them. We can then ask whether one arrangement of our bucket brigade is better than the next. By designing various helices we can come to understand which is best for transporting protons and use that arrangement in the next generation of proton exchange membranes in hydrogen fuel cells.
Team 3: Efficient Bulk Scale Charge Transport
CCI’s Team investigates the optimal nanoscale morphology capable of providing optimized charge transport within bulk scale materials. The team also focuses on understanding the correlation between intra-molecular and inter-molecular charge transport. The molecular design guidelines developed within Team 3 are intended to have substantial impact in designing proton exchange membranes for fuel cells, for example.
Team 3’s fundamental research objectives include:
- Designing and synthesizing a range of polymers to provide a systematic variation in nanoscale morphologies of proton conducting functionalities.
- Controlling the density of the charge transport functional groups within these supramolecular assemblies.
- Characterizing bulk nanoassemblies experimentally and through modeling.
- Evaluating the charge transport performance of each assembly to establish structure-property relationships.
One key issue is inter-chain hopping of protons to achieve efficient bulk proton transport. Two factors control inter-chain hopping: the density of functionalities within the nanoscale domains containing the proton transporters in the bulk supramolecular assembly, and the nature of the microphase-separated assembly itself and whether it can provide the appropriate channels for proton transport. A higher density within a domain enhances the opportunities for one or more of the proton transport functionalities in one chain to be in proper orientation with another functionality in the neighboring chain, which should enhance the inter-chain hopping of protons (see figure).
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Schematic of intra- and inter-chain proton transport in polymers
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