Investigation of polyoxometalates and nanostructured carbon materials as electrode materials for energy storage applications
Date of Issue2018-09-07
School of Materials Science and Engineering
Technical University of Munich
Electrochemical energy storage devices such as lithium ion batteries (LIBs), sodium-ion batteries (NIBs) and supercapacitors (SCs) have been developed for various potential applications including portable electronics, electro-mobility, and large-scale stationary energy storage. This thesis focuses on developing novel polyoxometalates (POMs) and nanostructured carbon materials as electrode materials for SCs, LIBs, and NIBs, as well as investigating respective their charge storage/transport mechanism. A polyoxovanadate, Na6[V10O28], is synthesized using a simple solution process, and studied as electrode materials for SCs and LIBs in this thesis. The electrochemical properties of Na6[V10O28] electrodes for SCs are investigated in Li-ion containing organic electrolyte utilizing galvanostatic charge/discharge and cyclic voltammetry in a three-electrode configuration. Activated carbon (AC) is employed as the positive electrode to assemble an asymmetric supercapacitor with Na6[V10O28] as the negative electrode in order to demonstrate that Na6[V10O28] is a promising electrode material for practical supercapacitor applications. The electrochemical properties of Na6[V10O28] as a cathode in LIBs is also tested by galvanostatic charge/discharge and cyclic voltammetry in a half-cell configuration, and exhibits a high capacity (up to 213 mAh g−1). In order to figure out the charge storage and transport mechanism, in-situ V K-edge X-ray absorption fine structure (XAFS) measurements and temperature dependent Chronoamperometry measurements are utilized for Na6[V10O28] LIBs. Those techniques can detect the oxidation states and inner structure changes of Na6[V10O28] during charging/discharging, as well as determine the rate of electron transfer and the reorganization energy of Na6[V10O28] electrodes. A POM/Carbon hybrid material is also investigated in this thesis for SC applications. A nanohybrid material which combines (TBA)5[PVV2MoVI10O40] (TBA-PV2Mo10, TBA: [(CH3(CH2)3)4N]+, tetra-n-butyl ammonium) and single-walled carbon nanotubes (SWCNTs) is synthesized by a simple solution method which electrostatically attaches anionic [PV2Mo10O40]5- anions with organic TBA cations on the SWCNTs. SWCNT-TBA-PV2Mo10 electrodes is tested in an acidic aqueous electrolyte by galvanostatic charge/discharge and cyclic voltammetry in three-electrode-configuration in order to figure out the electrochemical performance. In this SWCNT-TBA-PV2Mo10 nanohybrid material, TBA-PV2Mo10 offers redox activity while the TBA organic groups help to preventing dissolution in the aqueous electrolyte in order to increase the cycling stability, and SWCNTs offers high electrical conductivity with high double-layer capacitance that improve both energy and power density. A SWCNT-TBA-PV2Mo10 symmetric SC is fabricated and exhibits a 39 % higher specific capacitance as compared to a SWCNT symmetric SC, and presents high cycling stability up to 6500 cycles. A nanostructured carbon material, core–shell heterostructure with multi-walled carbon nanotubes (MWCNTs) as the core and graphene oxide nanoribbons (GONRs) as the shell (MWCNT@GONR), is also investigated as anode material for NIBs in this thesis. MWCNT@GONR with oxygen-containing functional groups is synthesized by unzipping process of MWCNTs utilizing microwave-assisted process. Annealing process is employed to study the influence of the amount of carboxylic acid groups on the electrochemistry of MWCNT@GONR. The novel core-shell structure prevents the restacking problem of graphene and enables penetration of the electrolyte. Furthermore, MWCNTs offer high electronic conductivity and direct electron transfer path while GONRs offer high surface area and functional groups which adsorb more Na ions on the surface therefore increasing capacity. A full cell which combines MWCNT@GONR as anode and P2-NaxMnO2 as cathode is assembled and exhibits a high energy density.