Abstract
The present Ph.D. dissertation reflects research efforts on two different topics: 1) development and study of all-solid-state mediator supercapacitors (SCs) and 2) atomistic modeling and scanning tunneling microscopy (STM) study of interfaces in Nafion based polymer electrolyte fuel cells (PEFCs). The present research on all-solid-state mediator SCs was focused on two systems: 1) NaI/I2 mediators, poly(ethylene oxide)/LiClO4 electrolyte, and Nafion separator; and 2) NaI/I2 or K3Fe(CN)6/K4Fe(CN)6 mediators, poly(ethylene oxide)/LiClO4 electrolyte, and polyvinylidene fluoride (PVDF)/lithium trifluoromethanesulfonate (LiTFS) separator. The results for the Nafion based mediator SCs are as follows. The cyclic voltammetry results showed that the specific capacitance of the SCs without and with redox mediators were 45 and 210 F g-1 within a voltage window of 0.0-1.0 V and at the scanning rate of 25 mV s-1, respectively, showing a significant enhancement effect for the mediators. The in situ XAS measurements indicate that the reversible redox states during charge/discharge process are I2 (oxidation) and I3- (reduction). PVDF/LiTFS membrane was prepared to replace the Nafion membrane as the separator. With the application of the water-free membrane, the specific energy increased from 20.8 to 49.1 Wh kg-1 within the range of specific energy for batteries. In addition, it was found that the performance of the NaI/I2 mediator SC increased when the mediator concentration increased from 5% to 20%, but decreased slightly when increased from 20% to 30%. This is considered to be a very important characteristic of the mediator SC in contrast to all other types of SCs that can be utilized for their optimization. In the second part of this dissertation, both theoretical modeling and STM imaging were employed to investigate the structure and the properties of the electrode/electrolyte interfaces. In the theoretical study part, an approach of hybrid atomistic simulation was adopted. First, molecular dynamics show that an ordered contact layer is formed on Pt/(H2O+H3O+) and Pt/H2SO4 interface. However, the Pt/Nafion interface is partially ordered. Secondly, ab initio simulation can visualize bond formation and breakdown, representing charge transfer at interface. Thirdly, quantum statistical calculations correctly predict the trends of the electrochemical current density. In the STM imaging study, Nafion film with a thickness of several hundred nanometers was deposited on highly oriented pyrolytic graphite (HOPG) pieces and polycrystalline platinum sheets. STM images indicated that the interface structures resembled to, but were larger than, the surface structures of the blank substrates. Results showed that the ordering structures of the HOPG/Nafion interface were destroyed right after the applied potential was changed, and a new type of ordering was generated. The existing electron tunneling phenomenon theory cannot be used to explain the experiment results. Therefore, a new theory based on principles of quantum mechanics and solid-state physics was developed.