Document Type

Dissertation

Date of Award

12-31-2021

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Physics

First Advisor

S. Mitra

Second Advisor

Joshua Young

Third Advisor

Trevor Tyson

Fourth Advisor

N. M. Ravindra

Fifth Advisor

Ken Keunhyuk Ahn

Abstract

Electrolytes with good interfacial stability are a crucial component of any electrochemical device. The development of novel gel polymer electrolytes (GEs) with good interface stability and better manufacturability is important for the development of the next generation electrochemical devices. Gel electrolytes are hybrid electrolyte materials, combining benefits of both liquid and solid systems. Compared with liquid and solid electrolytes, GEs open new design opportunities and do not require rigorous encapsulation methods. In this dissertation, studies on functionalized carbon nanotubes (fCNTs) and graphene oxide (GO) doped polyvinyl alcohol (PVA) based gel electrolytes (GEs) are reported. The ionic conductivity and mechanical strength of fCNT doped gel electrolyte (fCNTGE) is significantly improved, when compared to pure GE and graphene oxide doped gel electrolyte (GOGE). The ionic conductivity is significantly improved by introducing fCNTs into the PVA gel and reaches 6.9×10-2 S cm-1, revealing that the diffusion and transport of ions into electrolyte are much better than the GE and GOGE. A significant enhancement in the gel mechanical properties is observed with Young's modulus (E = 2.3) and tensile strength (22.3 kPa) of fCNTGE. Furthermore, the composite Zn-Ag2O batteries are made and tested using the fCNTGE, GE, and GOGE in three dimensional (3D) -printed battery casings.

However, questions remain about the origin of the property enhancement and the interactions between components of GEs. Density functional theory (DFT) calculations are employed to analyze the interactions between fCNT, PVA, and Zn ions. CNTs with increasing numbers of carboxyl (-COOH) functional groups and PVA chains with varying lengths are studied. Increasing the number of -COOH on the CNTs enhances the adsorption energies (Eads) of PVA, and Eads also increase as the number of monomers increase. Strong fCNT-PVA interactions contribute to the enhanced mechanical strength and thermal stability, while the enhanced ionic conductivity is partly due to weak Zn adsorption. Computational modelling is used to understand how fCNT displays better performance in membrane separation and investigate if the same trend could be seen for different pollutants as well. The nature of the interactions between the pollutants and raw and functionalized CNTs are studied on the atomic level by using DFT calculations. By determining the adsorption energies, DFT calculations theoretically confirm that pollutants interact more strongly with fCNTs than unfunctionalized CNTs, likely partly contributing to the observed some properties such as mass transfer coefficient, selectivity, and flux. It is demonstrated that this is due to enhanced charge transfer between the CNT and pollutants as the number of functional groups increases. Trends in the HOMO-LUMO gap and how they are affected by the functionalization of the CNT are also described. These calculations allow for better understanding of the influence of CNT functionalization on the properties of membranes.

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