Development of Functional Materials with Novel Nanoscale Architectures Designed for Applications in Energy Storage and Electrochemical Water Splitting
Developing functional materials for advanced energy applications is a critical step in minimizing our use of fossil fuels for energy. The first part of this dissertation describes pseudocapacitive energy storage materials for fast charge storage applications. In this section, nanostructured transition metal oxides and sulfides are examined as attractive pseudocapacitive materials. These materials discussed in this dissertation show the hallmarks of pseudocapacitors, and consistently delivered more than 100 mAh/g at 100C (~30 seconds) and thousands of (dis)charge cycles. Synchrotron operando X-ray diffraction studies of these materials establish a clear structural charge storage mechanism that is responsible for the extremely fast kinetics and long lifetimes. We specifically focus on the idea that in most battery materials, distinct phase transitions occur between lithiated and non-lithiated states, and phase transition kinetics can dominate the kinetics of Li+ intercalation. We show that a key component of intercalation pseudocapacitance is suppression of such phase transitions in nanostructured materials.
The second part of this work describes Li-alloying negative electrodes, which are attractive candidates to replace graphite in Li-ion batteries because of their high energy densities. However, charge storage in these materials is accompanied by a large volume change which pulverizes the particles, leading to electrode failure To addresses these problems, two scalable solution based synthetic methodologies will be discussed to synthesize nanoporous tin and silicon that show long cycle-lifetimes. This work specifically employs synchrotron operando transmission X-ray microscopy to directly examine structural changes of single tin particles during cycling to show how structure, morphology, and size modify the fundamental storage mechanisms in nanostructured tin.
The third section of this work describes our work on a high performance nanostructured porous NiFe-based oxygen evolution catalyst made by selective alloy corrosion. A major challenge is developing oxygen-evolving catalysts that are inexpensive, highly corrosion-resistant, and able to oxidize water at high current densities and low overpotentials. A practical NiFe-based catalyst was developed with high surface area, and therefore, a high density of oxygen-evolution catalytic sites per unit mass leading to unprecedented performance.