Nanoscale confinement and surface effects in energy storage and conversion systems: Structural and Dynamical Insight

Jose Banuelos (Chemical Science Division,ORNL)

Understanding the properties of complex fluids at interfaces and under nano-confinement is essential to describing phenomena in systems ranging from molecular biology to energy research, and a host of materials science problems. For example, to develop advanced electrical energy storage devices, such as supercapacitors, a molecular-level understanding of the structure and dynamics of electrolyte molecules at the electrode interface where energy is stored is necessary. Room-temperature ionic liquids (RTILs) have emerged as promising electrolyte materials due to their extremely low vapor pressure and stability at electrode potentials of up to 4 V without electrochemical decomposition. The highly porous, high surface area networks of the electrode materials, however, introduce nano-confinement and surface effects on the properties of RTILs. The properties of the RTILs [C4mim+][Tf2N-] and [C10mpy+][Tf2N-], confined in nanometer-scale carbon and silica pores have been investigated using small angle scattering, neutron spin echo (NSE), and classical molecular dynamics (cMD). For carbon, high RTIL densities obtained from cMD and from scattering data analysis indicate strong RTIL/pore wall interactions mediated by the liquids' intrinsic affinity for the carbon surface and by the surface's rough microporous topology. Consequently, NSE and cMD show a dramatic slowing of the overall collective dynamics, including an effectively immobilized fraction of RTIL at the pore wall. In studies of the enzymatic hydrolysis of cellulose, SANS revealed that cellulose degradation proceeds on the outer surfaces and only in pores that are large enough to accommodate the active enzymes, and that a natural limit on byproduct transport and exchange with new active enzymes in the pores hindered the hydrolysis rate, unless aided by external mixing. These examples highlight the importance of considering the effect of the nanoscale hierarchical structure of host matrices on the dynamic or catalytic properties of molecules within these matrices, and of the usefulness of neutron scattering techniques to obtain new insights into complex systems.

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