The Interfacial Processes Group explores in a cross-disciplinary manner the chemistry and structure of reactive solid interfaces in contact with fluids. Interfaces are broadly interesting because they represent the initial reaction site in many environments, including mineral-water interfaces in natural systems and electrode-electrolyte interfaces in electrochemical devices. However, relatively little is known about the physical nature of these processes because of the extreme challenges associated with probing such interfaces in their relevant environments (e.g., in-situ), as they evolve (e.g., in real-time), and with sufficient detail to understand the relevant mechanisms (e.g., with molecular-scale resolution). These studies are enabled by the application and development of various experimental approaches, primarily X-ray based measurements at hard X-ray synchrotron sources and probe microscopies.
Our research program is focused on two broad scientific areas:
1) Mineral-water interfaces: The fate and transport of elements in natural systems are controlled by their interaction with mineral surfaces. Processes such as adsorption, mineral growth, and precipitation can effectively sequester species, while other processes such as desorption and dissolution can release species. These processes are critical to manage many of the ‘tailpipe’ issues associated with energy use, including the use of geological respositories (e.g., for the sequestration of CO2 into carbonate minerals) or the change in porosity of subsurface rock due to energy production. Ultimately, this work seeks to obtain a robust understanding of mineral-fluid interfaces that will provide a scientific foundation for the rational management of our natural resources.
Our work has made a number of significant advances with respect to the understanding of mineral-water interfaces. These include a direct understanding of the mineral-water interface structure, such as the organization of fluid water within a nanometer of the interface and the adsorption of ions to charged mineral-water interfaces in distinct hydration states. Additional studies have demonstrated the ability to observe mineral-water interfaces in real-time during reactions, including mineral growth/dissolution processes, as well as the growth of heterogeneous mineral coatings and precipitates.
2) Electrode-electrolyte interfaces: Electrochemical energy storage systems rely on the ability to control the distribution and transport of ions at applied electrochemical conditions. Interfaces play a critical role in these processes. For example, capacitive processes involve the adsorption of ions to electrode surfaces leading to fast charging rates (high power) but are limited by the number of available surface sites (low stored energy). In contrast, in batteries, ions (e.g., Li+) are transported across the electrode-electrolyte interface to drive compositional and structural changes within the electrode, enabling substantially higher stored energy than capacitive systems, but with lower discharge rates.
Within the context of lithium-ion battery systems, we have explored the reactivity of anode and cathodes under potential control, including formation of solid-electrolyte interface (SEI) phases, alloying of silicon anodes with lithium, insertion into lithium manganese spinel cathodes, and conversion reactions of metal oxides. A general theme in these studies is the development well-defined model electrodes so that the reactions can be observed with sub-nm resolution. One area of success has been the use of multilayer and thin film electrode architectures that allow us to control the reaction process and to improve reversibility.
We also have been exploring multivalent battery systems (for instance, using Mg2+) to explore ways of substantially increasing the stored energy in these systems. Here our work has focused mostly on developing model cathode thin film systems and understanding plating/stripping reactions in metal anodes.
In the context of capacitive storage systems, we have made direct observation of the molecular-scale ordering of room temperature ionic liquids (RTILs) as a function of potential and time at graphene-RTIL interfaces as a function of applied potential. This is a régime where the traditional models of ion adsorption fail due to the unusually high ion concentrations of the RTILs. Key observations include the formation of charge separated cation/anion layers at extreme potentials, and the unexpectedly slow evolution of the RTIL structure as a function of time.
To achieve these goals, we apply and develop a suite of experimental tools to enable unique insights into these interfacial systems. This includes, primarily, high resolution X-ray based approaches enabled by hard X-ray synchrotron sources, such as the Advanced Photon Source at Argonne. We complement these studies using other approaches such as scanning probe microscopy and other ex-situ tools.