Revealing Interfacial Reactions and Charge Transfer Kinetics in Electrochemical Energy Storage and Conversion

Abstract

Climate change demands the development of clean energy technologies. Renewable energy sources such as solar or wind energy are intermittent, and it is necessary to develop advanced energy storage and conversion devices to complete the sustainable eco-system, where Li-ion batteries and fuel cells promise a bright future. Batteries and fuel cells also play essential roles in electrifying transportation in replacement of internal combustion engines. Central to these electrochemical systems is the electrode-electrolyte interface, where (electro)chemical surface reactions or intercalation reactions occur, and its thermodynamic and kinetic properties determine the energy density, power density, and lifetime of the electrochemical devices. However, the molecular structures at the interface and how they promote or suppress the desired reactions remain unclear. Furthermore, microscopic-level understandings of reaction mechanisms and electrochemical processes are still lacking, hindering the rational design of electrode-electrolyte interfaces to improve the performance of electrochemical devices.

This thesis focuses on the fundamental understanding of the interfacial (electro)chemical reaction mechanisms at the molecular level and charges transfer kinetics at the electrified interfaces. First, an in situ Fourier-transform infrared spectroscopy (FT-IR) method was developed to examine the parasitic reactions between carbonate electrolytes and lithium nickel, manganese and cobalt oxides (NMC) in Li-ion batteries, and unique evidence for dehydrogenation reactions on Ni-rich NMC was revealed, which accounted for interface impedance build-up and battery capacity fading. Based on the proposed mechanism, strategies to suppress battery degradation were further demonstrated and discussed. Next, the kinetic mechanism for Li-ion intercalation was investigated, and experimental evidence from a charge-adjusted electrochemical method showed that ion intercalation occurs by coupled ion-electron transfer (CIET), which governs the current-dependent maximum capacity and power density of intercalation batteries. Further, the thesis extended in situ interface characterization and kinetic models to electrocatalytic reactions central to fuel cell technologies. Protic ionic liquids with different acid dissociation constants in an interfacial layer were found to enhance the oxygen-reduction reaction (ORR), attributed to strengthened hydrogen bonds between ORR products and ionic liquids, revealed by in situsurface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) calculations. Promoting hydrogen bonding between interfacial water molecules also facilitated proton-coupled electron transfer (PCET) kinetics, resulting in favorable hydrogen evolution reaction (HER) in controllable organic confinements. This thesis has laid a solid foundation for the rational design of electrochemical interfaces employing the physical chemistry of electrodes and electrolytes, for next-generation electrochemical storage devices with improved energy and power density and cycle life.