Design, Fabrication and Characterization of Electrochemical Energy Conversion and Storage Devices

The development of human society strongly relies on the utilization of energy. While fossil fuels are still the main energy source of current human activity, concerns about the environment and the greenhouse effect brought by combustion of fossil fuels have led to tremendous research interest on developing renewable energy conversion and storage techniques. Electrochemical energy technologies represent a promising solution to overcome the current energy dilemma. For energy conversion, photoelectrochemical (PEC) water splitting directly converts water and solar energy into hydrogen and oxygen. The generated hydrogen can be used as a clean, sustainable and efficient fuel and recycled as water after combustion. Currently, photoelectrochemical water splitting devices are either expensive, low performance or unstable. Developing new materials and new architectures with improved PEC performance is in high demand. The first half of this dissertation explores a new earth-abundant chalcogenide material Cu2BaSn(S,Se)4 as a promising photocathode for efficient hydrogen evolution.

For energy storage, supercapacitors are indispensable energy sources for portable electronics and electric vehicles. The rapid development of wearable devices, biomedical implants and electronic skin have raised new mechanical challenges for conventional supercapacitors. Large mechanical deformability is required for supercapacitors to integrate with these stretchable electronics. The second half of the dissertation studied novel stretchable supercapacitors based on various carbon nanomaterials that can be used for the applications of wearable and stretchable electronics.

In chapter 2 and 3, a new solar water splitting photocathode, Cu2BaSn(S,Se)4, was systematically studied. The PEC performance of Cu2BaSn(S,Se)4 was found to increase with increasing Se concentration. However, low light absorption, poor electrolyte/semiconductor junction and poor stability limited the performance of the Cu2BaSn(S,Se)4 photocathode. To improve the photoelectrochemical performance, a Pt/TiO2/CdS/Cu2BaSn(S,Se)4 architecture with ~75% Se concentration was designed. With improved light absorption, enhanced charge separation and charge transfer, a world-record-high photocurrent density of 12.08 mA/cm2 at 0 V/RHE was obtained. The Pt/TiO2/CdS/Cu2BaSn(S,Se)4 photocathode also delivered a consistent photocurrent for more than 10 hours demonstrating superior stability at 0 V/RHE. The same architecture was applied to a solution-processed Cu2BaSn(S,Se)4 absorber and yielded similar PEC performance, demonstrating the feasibility of a high performance, low cost and stable Cu2BaSn(S,Se)4 based photocathode.

From chapter 4 to chapter 7, stretchable supercapacitors based on various carbon nanomaterials were designed, fabricated and characterized. A new stretchable supercapacitor based on crumpled carbon nanotube (CNT) forest was developed. The vertically aligned CNT forest structure was well preserved during the transfer process. Intertwining of neighboring tubes provided the electrical integrity across the whole forest. With large surface area and easily accessible pore structure, a crumpled CNT forest supercapacitor with high electrochemical performance and large mechanical deformability was successfully fabricated. To further reduce the resistance of crumpled CNT forest, an Au-CNT network was introduced at the base. A resistance decrease of an order magnitude was obtained using the Au-CNT network. As a result, the electrochemical performance of the crumpled CNT forest was significantly improved especially at high charge/discharge rate where conductivity is more important.

MXene, a new 2-Dimentional Metal Carbide has also been utilized as a stretchable supercapacitor but was found to crack during the stretchable electrode fabrication process, mainly because of its high mechanical stiffness, weak intersheet interaction and small flake size. Thus, reduced graphene oxide (RGO) was incorporated to overcome these issues. The as-prepared stretchable MXene/RGO composite supercapacitor maintained its structural integrity under various mechanical strains and demonstrated good electrochemical performance.

Finally, inkjet printing was introduced to fabricate a carbon nanotube-reduced graphene oxide-poly(ethylenedioxythiophene) (CNT-RGO-PEDOT) stretchable supercapacitor. The high electrochemical performance (20 F/g, 85% rate capability from 0.5 A/g to 5 A/g) and high mechanical robustness of printed CNT-RGO-PEDOT stretchable supercapacitor demonstrates the possibility of fabricating stretchable supercapacitor in a more scalable approach.