UC Santa Cruz

Sustainable energy is advocated over the past few decades when the traditional fossil fuels are running out and the environmental pollution is getting worse. In response to the call for greener energy supply, various energy conversion methods have been established, such as solar energy, wind energy, hydro energy, geothermal energy and so on. The intermittent and regional nature of these clean energy further drive the development of high-performance energy storage devices. Batteries and electrostatic capacitors are two most investigated power sources. Batteries own good energy densities while their power performance is mediocre. Electrostatic capacitors are advantageous in their high power densities, while their energy densities are much inferior. Supercapacitors represent a new and promising energy storage technique that can bridge the gap between these two power sources. They have higher power densities than batteries and higher energy densities than traditional electrostatic capacitors. The key to achieve sound electrochemical performance of supercapacitor lies in the advanced electrodes, which are required to rapidly store/deliver a good amount of charges and retain their structural integrity after long-time operation.

This dissertation covers the research of my past five years at UC Santa Cruz in design and synthesis of high-performance supercapacitor electrodes. Some nanomaterials have been studied in this dissertation, including metal oxide, metal nitride, and carbonaceous materials. Different electrode structures have been fabricated, such as paper-based electrodes and 3D-printed electrodes. Both the intrinsic electrochemical properties of electrode materials and practical considerations of device design will be analyzed.

The contents in this dissertation are mainly composed of two parts. The first parts are the investigation on the two-dimensional (2D) film-based electrodes for supercapacitors. The second parts are the exploration of three-dimensional (3D) printed electrodes for supercapacitors. To be more specific, Chapter 1 introduces the background of the sustainable energy storage, mechanism of supercapacitors, and the recent advancement of 2D film and 3D-printed supercapacitor electrodes. Chapter 2 reports the fabrication and the electrochemical performance of a flexible and transparent MoO3 nanopaper, a new form of freestanding electrodes for supercapacitors. Chapter 3 demonstrates a freestanding and flexible TiN nanopaper that shows ultrahigh power performance and ultralong electrochemical stability. Chapter 4 to chapter 7 are mainly focused on the development of 3D printed electrodes. Chapter 4 presents a 3D printed carbon aerogel with very high specific surface area via a thixotropic R-F based ink. Chapter 5 describes a new five-scale pore carbon network achieved via additive manufacturing. By adding a new dimension of 3D printed pores, the new carbon network displays superior charge storage capability even under ultralow temperature of -70 ºC. Chapter 6 successfully addresses a long-term challenge for supercapacitors, that is the electrochemical performance significantly gets deteriorated with the increase of active loading. In this chapter, a 3D printed macroporous graphene aerogels has been fabricated to support the record-high loading of MnO2, which is two to three orders higher than the current reports. More importantly, this 3D printed graphene/MnO2 can simultaneously achieve excellent capacitance normalized to area, gravimetry and volume, which is the trade-off for most electrodes. This work also successfully validates the feasibility of printing practical pseudocapacitive electrodes, which might revolutionize the pseudocapacitor fabrication. Chapter 7 exhibits a new surface-functionalized 3D printed graphene aerogel electrode that achieves not only a benchmark areal capacitance at high current densities but also an ultrahigh intrinsic capacitance even at high mass loadings. This work also clearly demonstrates the role of 3D printed structure in boosting the kinetics and intrinsic capacitance of pseudocapacitive graphene aerogels. Chapter 8 is a summary of the current problems and an outlook of further directions in the supercapacitor research. Fundamental scientific questions and practical issues associated with the 2D films and 3D printed electrodes for supercapacitors will also be discussed.