Materials Science and Engineering PhD Thesis Defense by Feriha Eylül Öztuna

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MATERIALS SCIENCE AND ENGINEERING

PhD THESIS DEFENSE BY FERİHA EYLÜL ÖZTUNA

Title: Graphene (Oxide) – Metal/Metal (Hydr)oxide Composites: Synthesis and Applications in Electrochemical Energy Storage and Conversion

Speaker: Feriha Eylül Öztuna

Time: August 7th, 2018, 9:00 AM

Place: ENG B29

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. Uğur Ünal (Advisor, Koç University)

Assoc. Prof. Funda Yağcı Acar (Koç University)

Assoc. Prof. Alper Uzun (Koç University)

Assoc. Prof. Fevzi Çakmak Cebeci (Sabancı University)

Asst. Prof. Nuri Solak (Istanbul Technical University)

Abstract:

The massive energy demand in the world will be supplied by renewable energy sources (solar, wind, etc.) with the inevitable depletion of fossil fuels. However, efficient utilization of these sources heavily depends on the development of advanced energy storage and conversion devices due to their intermittent nature.  In this regard, electrochemical capacitors, batteries, and fuel cells are the main electrochemical systems that can assist the continuous operation of the renewable sources, or offset the daily energy need individually. The common component in all the electrochemical energy devices is the electrodes which contain active material and the current collector. Physicochemical properties of the active material determine the overall device performance.

This dissertation is devoted to synthesis of graphene oxide (GO) – metal/metal (hydr)oxide composite electrodes for electrochemical applications. In the first part, production of two-dimensional composite electrodes is presented as electrochemical capacitor electrodes.  First row transition metal cations (Co²⁺, Ni²⁺, Mn²⁺, Fe²⁺) and graphene (oxide) were combined by utilization of simple electrostatic interactions. The highest capacitance was obtained with Fe/GO (38.7 mF cm^-2) followed by rCo/GO (31.6 mF cm^-2) and rFe/GO (29.1 mF cm^-2) at 20 mV s^-1, where r represents electrochemically-reduced composites. The nature of metal was influential in the charge-storage mechanism of the composites: while rCo/GO and rNi/GO was governed by diffusion-limited processes, rMn/GO and rFe/GO showed pseudocapacitive behavior. Furthermore, ultrathin electrodes of Ni(OH)₂ and GO were produced via layer-by-layer assembly (LBL) followed by hydrazine vapor reduction to enhance electrical conductivity of GO. Reduced 9-bilayer [Ni(OH)₂/GO] films exhibited areal capacitance of 8.6 mF cm^-2 at 2 mV s^-1, 3-fold higher than that of LBL-assembled [CoAl LDH/rGO] thin film reported in the literature. LBL-assembly was also utilized to produce superparamagnetic iron oxide nanoparticles (SPION)/GO thin films. State-of-the-art electron paramagnetic resonance (EPR) results revealed the competition between carbon defect centers and Fe-related paramagnetic centers on the electrochemical performance. In fact, reduced 1-bilayer [SPION/GO] had enhanced specific capacitance of 1570 F g^-1 at 5 mV s^-1, outperforming all the LBL-assembled iron oxide/rGO films and most of the transition metal oxide/rGO composites in the literature. In all the LBL-grown composites, as-deposited films exhibited pseudocapacitive properties, whereas hydrazine-reduced films showed more diffusion-limited charge storage, like in batteries.

In the second part of the thesis, synthesis of graphene aerogels (GAs) is presented as the next-generation three-dimensional form of graphene. One-step hydrothermal treatment of GO solutions resulted in formation of high surface area GAs (~700 m² g^-1) with the aid of supercritical CO₂ (scCO₂) drying. Obtained metal-free GA electrodes on nickel foam exhibited superior specific capacitance (390 F g^-1 at 5 mV s^-1) than the counterparts reported in the literature. GAs were later decorated with metal nanoparticles as possible electrocatalysts for oxygen evolution (OER) and oxygen reduction reactions (ORR). Ni/GA composites were synthesized via one-pot hydrothermal reaction coupled with thermal reduction under H₂/He environment as possible OER electrocatalysts. By using urea as the continuous hydroxyl ion supplier during the hydrothermal reaction, Ni loading was tuned from 1.5 to 40 wt% in a highly-controlled fashion. Ni/GA with 20 wt% Ni exhibited low overpotential of 340 mV for the supply of 10 mA cm^-2 which is 90 mV lower than that of 80 wt% Ni/GA and 30 mV lower than that of 20 wt% Ir/C than 20 wt% Ru/C reported in the literature. High TOF value of 0.1 s^-1 was calculated along with low overpotential for the same sample. Lastly, Pt loaded GAs were synthesized by scCO₂ assisted deposition as possible ORR electrocatalysts. Pt nanoparticle size was varied from 1.2 to 2.9 nm by simply increasing the thermal conversion temperature from 400 to 800 °C. Simultaneously, gradual thermal deoxygenation of GA was observed. Obtained Pt/GA converted at 600 °C had enhanced electrochemical surface area of 102 m² g^-1, which is higher than that of commercial Pt/C. Overall, the mass activities of Pt/GA electrocatalysts followed the order: Pt/GA(600) > Pt/GA(400) > Pt/GA(800).