The Interfacial Processes Group explores the chemistry and structure of solid interfaces in contact with fluids. Interfaces are broadly interesting because they represent the initial reaction site in many environments, encompassing mineral-water interfaces in natural systems and electrode-electrolyte interfaces in electrochemical devices. However, relatively little is known about the nature of these processes because of the extreme challenges associated with in-situ studies of interfaces in their relevant environments and operando studies as the interfaces evolve in real time (e.g., under external stimulus), while retaining the molecular-scale sensitivity that reveals the relevant elementary mechanisms. These studies follow a cross-disciplinary approach. They are enabled by the application and development of experimental measurements at X-ray synchrotron sources (with an emphasis on using hard X-ray energies that can penetrate fluids over millimeter to centimeter distances), and are complemented by parallel in-house studies using scanning probe and optical 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-water interfaces. Processes such as adsorption, mineral growth, and precipitation can effectively sequester species, while processes such as desorption and mineral dissolution can release species, enabling environmental transport. Understanding these processes is critical to managing the impact of society’s energy use. For example, energy generation (e.g., geothermal energy or hydrocarbon production) often involves perturbation of the Earth’s sub-surface (e.g., increasing sub-surface porosity by fracking) that can impact water supplies. Similarly, the effective disposition of energy by-products in geological repositories (e.g., sequestration of CO2 in the form of carbonate minerals) relies on maintaining subsurface porosity during mineralization reactions. Ultimately, our work seeks to obtain a robust understanding of mineral-fluid interfacial processes that will provide a scientific foundation for the rational management of our natural resources.
Our work has made significant advances with respect to understanding mineral-water interface structure. We have demonstrated how the organization of water within ~1-2 nanometers of a mineral-water interface can be described as distinct interfacial hydration layers. Meanwhile, we have demonstrated that the adsorption of ions to charged mineral-water interfaces occurs simultaneously in multiple distinct hydration states (e.g., inner- vs. outer-sphere species). Additional studies have demonstrated the ability to observe mineral-water interfacial reactivity in real time during reactions, including ion adsorption/desorption, mineral dissolution/growth/precipitation, and the evolution of heterogeneous mineral coatings. While the emphasis of this work has been on the intrinsic reactivity of well-defined interfaces, the role of extrinsic contributions to mineral-water reactivity (e.g., due to mineral shape and fluid transport) is a new research direction.
Other ongoing activities have extended these capabilities to a wide range of solid-liquid interfacial systems. This includes the role of cation and anion coordination in reactions of sparingly soluble minerals (e.g., barite); the use of X-ray reflectivity data of well-defined solid-liquid interfaces to validate computational simulations; and the interaction of carbonate minerals with natural petroleum and aqueous brines to understand the processes associated with enhanced oil recovery through “smart water” technology.
2) Electrochemical interfaces: Electrochemical energy storage relies on controlling the distribution and transport of ions under applied electrochemical potentials where interfaces play a critical role. For example, capacitive processes involve the adsorption of ions to electrode surfaces leading to fast charging rates (i.e., high power) but are limited by the number of available surface sites (low stored energy). In contrast, Faradaic energy storage (e.g., batteries) drives ions (e.g., Li+) across the electrode-electrolyte interface to achieve compositional and structural changes within the electrode, enabling substantially higher stored energy than capacitive systems, but with lower reaction rates (i.e., lower power).
We have explored the reactivity of lithium-ion battery anodes and cathodes under potential control, including formation of solid-electrolyte interface phases, alloying of silicon anodes, insertion into lithium manganese spinel cathodes, and conversion reactions of metal oxides. A general strategy in our studies is the development of well-defined model electrodes with which the reactions can be observed in operando with sub-nanometer resolution. This has led to new insights into the well-known challenges of metal oxide conversion reactions (e.g., poor reversibility and capacity loss), which we have shown are due to interfacial interactions (using model NiO cathodes). We have also explored multivalent battery systems (e.g., Mg2+) as a route to substantially increase the stored energy in these systems, with model cathode thin films and through plating/stripping reactions of metal anodes.
Studies of capacitive storage systems have allowed direct observation of the molecular-scale ordering of room temperature ionic liquids (RTILs) as a function of applied potential and time at graphene-RTIL interfaces. Here, the traditional models of ion adsorption often 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 an unexpectedly slow evolution of the RTIL structure at the interfaces. Ongoing studies are focusing on the capacitive energy storage in MXene-water based systems.
Our group also has made a significant effort to understand the complex chemistry of lead-acid batteries, the first rechargeable electrochemical energy storage system (discovered in 1859) and a long-established technology. These batteries rely on a complex suite of chemical reactions (e.g., conversion of lead and lead dioxide electrodes into PbSO4) that include both electrochemical and non-electrochemical reactions. However, these processes remain largely unexplored by modern analytical approaches, and there are substantial opportunities for improvements.
For the above studies, we employ a strategy in which we use, apply, and develop a suite of experimental capabilities to enable unique insights into these interfacial systems. A key component of our approach is the use of hard X-ray capabilities that leverage synchrotron sources (e.g., the Advanced Photon Source at Argonne). Through this work, we have demonstrated a number of novel capabilities to by-pass the well-known phase problem of X-ray scattering. This includes the direct imaging, at the molecular scale, of elemental distributions using “model-independent” phase-sensitive approaches (using either resonant elastic reflectivity or X-ray standing waves) and the use of error correction algorithms to image interfacial structures directly from specular X-ray reflectivity data. We also created a novel technique, X-ray reflection interface microscopy, that images elementary surface topography (e.g., sub-nanometer high steps) in a full field imaging modality. Current and ongoing work seeks to leverage the capabilities of X-ray coherence for operando studies of interfacial systems.