Proton and Anion Exchange Membranes for Fuel Cells
U.S. reliance on fossil fuels is increasingly recognized as a substantial threat to national energy security and a source of global climate change. The development of batteries and fuel cells may provide viable clean energy alternatives for replacing internal combustion engines in automobiles and for powering personal electronics. But despite decades of research, these technologies continue to lag behind fossil fuels in performance and cost. Breakthroughs are hindered both by a lack of understanding of transport and catalytic mechanisms as well as the complexity of modeling chemical processes in the individual components of fuel cells, in addition to modeling dynamics at the interfaces between each component. The Voth group develops computational tools based on multiscale theory to examine and predict the behavior of complex systems. These methods use high-resolution information, such as quantum calculations, and rigorous statistical mechanics to systematically construct simple models that retain the essential physics. A key area of research is the study of materials that are relevant to renewable energy technologies, such as proton and anion exchange membranes. Brief descriptions of the fuel cell related projects underway in the group are given below.
Proton Exchange Membranes
Fuel cells form a class of promising “green” energy technologies which generate electricity from simple reactants, i.e. hydrogen and oxygen gas, and produce environmentally benign waste in the forms of water and heat. The basic chemistry responsible for their operation is well understood for proton-exchange systems: the hydrogen molecules are broken apart at one electrode to produce free electrons which then traverse an external circuit before recombining with positively-charged protons and oxygen at a second electrode to form water. The latter reaction depends heavily on the ability for protons to migrate between the two electrodes, which are separated by a proton electrolyte(exchange) membrane. In order to improve the efficiency of fuel cells and overcome issues of cost and durability, novel membrane materials that are highly proton conductive must be developed to facilitate proton transport. While several membranes are currently on the market, their properties are not well understood. Hence, developing new membranes will first require understanding the conductivity of current state of the art materials. The Voth group’s research involves investigating these properties over a range of length scales, from the level of individual atoms to length scales closer to present engineering efforts. To accomplish this task, the group has developed a novel reactive simulation methodology to determine the detailed proton movement within water pores in the membrane in addition to using large-scale atomistic simulations to examine the size and shape of the water pores. Information from these simulations is then used to parameterize very large-scale coarse-grained simulations, which allow analysis of the behavior of the membrane as a whole. This multiscale investigation will provide a wealth of information and predictive capability to drive the development of proton exchange membranes technology.
Figure 1: A multiscale approach to modeling proton exchange membranes from atomistic level reactive simulations to coarse-grained descriptions.
- Shulu Feng and Gregory A. Voth, Proton solvation and transport in hydrated nafion, J. Phys. Chem. B, 115, 5903 (2011)
- Chris Knight and Gregory A. Voth, The Curious Case of the Hydrated Proton, Acc. Chem. Res., 45, 101 (2012)
- Ryan Jorn and Gregory A Voth, Mesoscale Simulations of Proton Transport in Proton Exchange Membranes, J. Phys. Chem. C, 116, 10476 (2012)
- Ryan Jorn, John Savage and Gregory A. Voth, Proton Conduction in Exchange Membranes across Multiple Length Scales, Acc. Chem. Res., Article ASAP DOI: 10.1021/ar200323q
- Shulu Feng, John Savage and Gregory A. Voth, Effects of polymer morphology on proton solvation and transport in proton exchange membranes, J. Phys. Chem. C, 116, 19104-19116 (2012)
Anion Exchange Membranes
Although proton exchange membranes (PEMs) have received a lot of attention, their wide spread use has been hampered by cost, durability and fuel versatility. The Voth group has been working on alternative fuel cell technologies that employ anion exchange membranes (AEMs), which have numerous advantages over PEMs including compatibility with non-precious metal catalysts. Unfortunately, AEMs also show insufficient chemical stability and poor ionic conductivity. The Voth group is working to develop and characterize AEM materials to circumvent these challenges. A key aspect of our effort is to understand the solvation and transport of hydroxide in the highly basic environment of the membrane. Hydroxide can be transported in water via standard vehicular diffusion or by a proton “hopping” from a neighboring water to the hydroxide ion, such that the two species swap identities. Therefore we have been developing a reactive hydroxide model that reproduces accurate forces calculated with quantum chemistry methods, but is efficient enough to generate simulations of the relevant system sizes and time scales. By understanding anion transport and the design characteristics of AEMs that promote the desired membrane properties, we are helping to direct AEM improvement.
Figure 2: On the left, a hydroxide anion is shown (orange) with four-fold coordination by water molecules and interacting with a benzyltrimethyl ammonium cation in an AEM. The way the cation interacts with the hydroxide anion may cause the solvation structure in the membrane to be different than in aqueous solution. On the right, a hydrogen bond network of water molecules is shown spanning a cylindrical cavity in an AEM, which may be important for facilitating anion transport through the membrane.
- Gerrick E. Lindberg, Chris Knight, Ryan Jorn, James F. Dama and Gregory A. Voth, Multiscale simulations of Hydroxide Solvation and Transport in Anion exchange membranes, ECS Trans. 41, 1785 (2011).
- Himanshu Sarode, Melissa A. Vandiver, Ashley M. Maes, Benjamin Caire, James L. Horan, Yating Yan, Yifan Li, Gerrick E. Lindberg, James F. Dama, Chris Knight, Ryan Jorn, Martin E. Lenz, Robert Kasper, Shuang Gu, Bingzi Zhang, Sönke Seifert, Tsung-han Tsai, Wen X. Zhang, E. Bryan Coughlin, Daniel M. Knauss, Yushan Yan, Gregory A. Voth and Thomas A. Witten, Matthew W. Liberatore and Andrew M. Herring, Designing Alkaline Exchange Membranes from Scratch, ECS Trans. 41, 1761 (2011)
- Chris Knight, Gerrick E. Lindberg, and Gregory A. Voth, ”Multiscale Reactive Molecular Dynamics,” J. Chem. Phys., (accepted) (Nonadiabatic Dynamics special issue).
Fuel cell projects funding acknowledgements:
Army Research Office via MURI grant (AEM)
Argonne Leadership Computing Facility (PEM and AEM)
Energy storage materials
The advent of Li-ion rechargeable battery technology has revolutionized the personal electronics industry and holds great promise to have a similar impact on the transportation sector via electric vehicles. Given the importance of these applications it is not surprising that in recent years research interest in this area has grown rapidly. However, in spite of this renewed enthusiasm and prior decades of development, there are still many unknowns in optimizing Li-ion systems for application to plug-in vehicles. The standard Li-ion batteries consist of a graphitic carbon material at the anode, a lithiated metal oxide at the cathode, and a non-aqueous solvent containing a dissolved lithium salt. During battery discharge, lithium ions are removed from the graphite and inserted into the metal oxide, making the interfaces between these electrodes and the electrolyte paramount to battery operation. Furthermore, it is also known that reactions take place at these interfaces between the electrodes and the electrolyte components, resulting in the formation of surface films. Films formed by decomposition of the solvent are termed a solid electrolyte interphase (SEI) since they conduct lithium ions (solid electrolyte) and form a layer separating the electrode from the electrolyte (interphase). which likewise impact battery performance. Although the SEI forms a passivating layer, protecting the electrode from further degradation, it adversely impacts battery performance by consuming Li+, thus diminishing battery capacity, and impeding Li-ion transport to the electrodes. A key research focus of the Voth group is to understand the solvation structure and dynamics of ions in the electrolyte and at the various interfaces in battery systems in order to determine the mechanism of ion transport to the electrode through these complex environments.
The large system sizes and long time scales associated with these battery systems make the use of ab initio simulation methods prohibitively expensive, thus our group has focused on the development of empirical force-fields based on ab initio data. The first step in this research effort is the development of accurate yet computationally inexpensive models for the electrolyte. In order to do, so we have utilized the novel force-matching algorithm developed by our group wherein the force-field is parameterized by fitting the atomic forces of the model to forces from ab initio calculations. Together with the structural information on the SEI provided by experimental collaborators at Argonne National Laboratory, we our using this force-field to study the electrolyte structuring at the interface with clean electrodes, the effects of the SEI on this ordering, and finally Li-ion transport into the SEI.
Figure 3: A snapshot of the interfaces between the graphitic anode, a representative SEI, and the electrolyte.
- Jorn R*, Kumar R*, Abraham D, and Voth GA, submitted to J. Phys. Chem. (*contributed equally). Atomistic Modeling of the Electrode-Electrolyte Interface in Li-ion Energy Storage Systems: Electrolyte Structuring.
- Petersen M, Kumar R, White HS, and Voth GA (2012) A computationally efficient treatment of a polarizable metallic electrode held at a constant potential , J. Phys. Chem. C, 116, 4903.
Biology contains a toolbox of processes and functionality that can be exploited in the design of new efficient materials. One particularly vital application of these bio-inspired materials is renewable energy. Work in our group focuses on the study of biological systems that can be used to improve the efficiency of hydrogen fuel cells. Two systems currently under investigation are hydrogenase enzymes that catalyze the production of H2 and proton transport channels that selectively transport H+ across biological membranes.
The study of these biological systems is difficult due to their size and reactivity. Computational techniques such as molecular, reactive and coarse-grained dynamics pioneered in the Voth group are able to provide unique insight into these systems. In particular, we use a combination of classical molecular dynamics (MD), multiscale coarse-graining (MS-CG), multi-state empirical valence bond (MS-EVB) reactive dynamics, and mixed quantum mechanical molecular mechanical (QM/MM) approaches to gain a full understanding of the complex structure and processes composing these biosystems.
Figure 4: [FeFe]-hydrogenase (PDB code 1FEH) with the H2 production reaction designated with arrows. The transparent ribbon representation depicts the atomistic protein while the balls and sticks depict the coarse grained (CG) protein. The coloring of the CG sites shows the number of protonatable amino acids in that CG site