As a scientist, working with researchers in the same field as oneself is often the path of least resistance; however, connecting with researchers in complementary fields empowers all parties to achieve more than they could alone. Energy Frontier Research Centers (EFRCs) embody this gap-bridging philosophy by bringing together scientists from different fields and institutions to tackle grand challenges.
Just as “like attracts like” among people, similar molecules tend to stick together rather than interact with different molecules. Molecules that include two connected segments with different properties present exciting implications for researchers, such as understanding, predicting, and controlling the lipid bilayers that are essential to human life. Phospholipids contain both hydrophilic (water-loving) and hydrophobic (oil-loving) components and, therefore, arrange themselves into a two-layered sheet with the hydrophilic portions pointing outward. Such arrangements of discrete molecules into an organized structure due to interactions between (or within) the molecules rather than external influences is called self-assembly. This process is illustrated in Figure 1. Molecules can self-assemble into more than just lipid bilayers, however. For example, proteins consist of polypeptide chains that fold upon themselves into exquisitely complex structures, each with unique biological functions. Following self-assembly principles, individual proteins are then able to come together into organized superstructures that can carry out even more complex functions. The geometry of self-assembled architectures can be controlled by adjusting the identity and sizes of each component, much like the optimal team make-up for one EFRC might be two chemists and four mechanical engineers, while another EFRC needs one of each plus two physicists to achieve its goals.
Several EFRCs are both using self-assembled materials to address energy challenges and building on our fundamental understanding of this design space. The examples described here span self-assembly in synthetic block copolymers, biopolymers, and small molecules.
Materials for water purification
Scientists in the Center for Materials for Water and Energy Systems (M-WET) leverage self-assembly to design next generation water filtration membranes. Improved water filtration is essential for providing billions of people worldwide with adequate sanitation and drinking water, as well as enabling energy and food production. Membranes remove impurities such as heavy metals, hydrocarbons, and bacteria from water by passing the water through a porous material that serves as a barrier to the larger molecules dissolved in the fluid. This technology has been transformational to water security and human health in many capacities, from reducing arsenic content in well-water to purifying blood by hemodialysis. Traditional membrane technology, however, is limited because increasing pore sizes to increase the water flowrate through the membrane also results in larger, undesired molecules or particles passing through. One source of membrane inefficiency is that pore sizes are non-uniform, but self-assembled block copolymers can yield uniform pore sizes and spacings. Block copolymers are polymer chains that contain multiple monomer types organized into “blocks” of like monomers. Altering the identity and length of these “blocks” tailors the pore geometry to improve water flux while ensuring target molecule removal, as illustrated in Figure 2. M-WET researchers have used simulations to demonstrate that membrane morphology is strongly connected to performance and are following this guidance to inform experimental trials.1,2
Bio-inspired energy materials
Growing global energy demands and continued reliance upon fossil fuels necessitate development of creative energy generation strategies. Some powerful biological systems rely on multiple classes of compounds to manage energy, such as the light harvesting apparatus of a bacterial cell. Taking inspiration from nature, the Center for the Science of Synthesis Across Scales (CSSAS) focuses on building a fundamental understanding of how high information content building blocks self-assemble into hierarchical structures to build hybrid materials that integrate polymers, proteins, and inorganic compounds. Recently, researchers from CSSAS synthesized a tunable, multilayered material with polypeptoid, protein, and nanoparticle layers. Such a material could be modified by adjusting the architecture and chemistry of the components to enable a broad range of biological functionalities, such as photocatalysis.3 The ability to mimic natural energy systems presents a path towards sustainable fuel and chemical production, a necessary part of reducing fossil fuel consumption.
The Center for Bio-Inspired Energy Science (CBES) also draws inspiration from natural energy systems to address increasing global energy demands. CBES structures small molecules with light-harvesting and catalytic capabilities in soft materials. These small molecules reversibly self-assemble based on their interactions with each other and their environment. For example, researchers in CBES added light absorbing molecules called chromophores to a swollen polymer matrix then triggered crystallization of the chromophores by changing the solvent. Crystallizing these light absorbing molecules immobilized them within the matrix. This system shows promise in harvesting light which can be a critical component of sustainable energy efforts.4
As EFRC research illustrates, well designed molecules can arrange themselves into materials with the potential to address many of the energy challenges faced by humanity. By utilizing and further studying self-assembly, several EFRC’s are developing teams of molecules that can build the materials of tomorrow.
