Hematite is a promising material for many different energy conversion and storage applications due to advantages such as low cost, high abundance, and good chemical stability. However, the low electrical conductivity of hematite has hindered the wide application of hematite in for different applications. Atomic doping is one of the most used approaches to tackle the electrical conductivity problem in hematite. Although many works have been done to understand the effects of atomic doping to the electrical conductivity of hematite, there are still many questions not answered yet. In this dissertation, we couple first-principles density functional theory (DFT) calculations with experimental approaches to dive deep into the system and answer unsolved questions in the community by taking advantages of both approaches. We, first of all, employ first-principles DFT calculations to predict how atomic doping impacts the carrier concentrations, carrier mobility and electrical conductivity of hematite. Then we conduct experiments to verify previous predicted results starting from materials synthesis, then materials characterizations, and, in the end, performance measurement. This series of works deepen people’s understanding about how atomic doping impacts the electrical conductivities of hematite and provide a possible validated collaboration mode between first-principles DFT calculations and experimental approaches.

The growing awareness of the detrimental effects of fossil fuel consumption on the environment has led to an increased demand for sustainable and clean energy sources worldwide. This has driven the development and application of electrochemical energy storage devices (EESDs), including rechargeable batteries and supercapacitors, in order to better utilize intermittent renewable energy sources such as solar and wind power, and to provide continuous and reliable power supply. The cathodes of EESDs have been found to play a critical role in determining their performance, including specific capacity, output voltage, rate capability, energy/power density, and efficiencies, as well as their calendar life, thermal stability, cost, and environmental impact. Unfortunately, cathodes are still one of the major challenges in the advancements of EESDs. Developing advanced cathodes based on different electrochemical energy storage mechanisms, such as electric double-layer capacitance, pseudocapacitance, bulk ion insertion/desertion, and conversion reactions, has been an important task in order to fulfill the demands of EESDs for different application scenarios.The scope of this dissertation covers the research of my past five years in designing and optimizing different types of cathodes for high-performance EESDs. Chapter one presents the background of EESDs and explains the advances of different types of cathodes for EESDs. Chapter two introduces a 3D printed porous carbon cathode templated by microporous metal-organic framework, which serves as EDLC-type cathodes for aqueous zinc-ion hybrid capacitors with high areal capacitance and energy density. Chapter three focuses on the graded design of a 3D printed graphene aerogel substrate, which improves electrodeposition uniformity and electrochemical accessibility of ultrahigh-loading MnO2 cathode based on lithium-ion insertion/desertion for non-aqueous lithium-ion hybrid capacitors with high volumetric energy density. A double-layered cathode dealing with the efficiency of iodide/iodine conversion chemistry for high-performance aqueous zinc-iodine batteries is discussed in Chapter four. Finally, in Chapter five, an outlook targeting the challenges and opportunities of developing advanced cathodes beyond cathode material itself is provided.