Master of Science, The Ohio State University, 2005, Graduate School

Committee: Not Provided (Other) Subjects:

Doctor of Philosophy, The Ohio State University, 2021, Chemical Engineering

The oxygen reduction (ORR) and evolution reactions (OER) are of utmost importance to emerging renewable energy technologies. Proton exchange membrane (PEM) fuel cells utilize the ORR to convert hydrogen and oxygen into water and electrical power. These devices have shown promise as the primary power source in vehicular applications. Another emerging technology, unitized regenerative fuel cells, make use of both the ORR and OER to convert excess renewable energy into hydrogen and oxygen through electrolysis, which may be stored indefinitely. When energy demand increases past renewable energy production levels, the hydrogen and oxygen may be converted back into electricity through a fuel cell. One of the challenges facing the development of these technologies is the slow kinetics of the ORR and OER which requires large quantities of precious metal catalysts to achieve functional current densities. The scarcity and high cost of these precious metals, namely platinum and iridium, has resulted in an inspired effort to identify highly active and low cost alternative ORR and OER catalysts. This research work investigates the origin of ORR and OER activity of nitrogen-doped and nitrogen-boron co-doped carbon nanostructures (CNx and CNxBy respectively). The first project demonstrated the use of the bicarbonate ion as a poisoning probe for CNx catalysts and combined density functional theory (DFT) modeling with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the impact of bicarbonate poisoning on the ability of surface nitrogen sites to adsorb oxygen. The second project developed a method for co-doping boron and nitrogen into carbon nanostructures and demonstrated the improved ORR iii activity that resulted from their synergistic interactions. Synthesis methods for CNxBy catalysts were optimized to selectively introduce various boron moieties and then studied using NAP-XPS to elucidate their impact on the adsorption of oxygen on the catalyst surfa (open full item for complete abstract)

Committee: Umit Ozkan PhD, PE (Advisor); Aravind Asthagiri PhD (Committee Member); Anne Co PhD (Committee Member) Subjects: Chemical Engineering

Doctor of Philosophy, The Ohio State University, 2014, Chemical and Biomolecular Engineering

Limited availability of fossil fuels have provided a great impetus towards development of efficient energy conversion devices. Proton Exchange Membrane (PEM) fuel cells, that use hydrogen as a fuel and generate energy with water as the only by-product are among such efficient energy conversion devices. Commercial PEM fuel cells use platinum catalyst on the anode and cathode. The Oxygen Reduction Reaction (ORR) on the cathode is a kinetically slow reaction that requires high loadings of platinum on the cathode, thereby significantly increasing the cost of a PEM fuel cell. The associated high cost and limited platinum availability have motivated research towards development of non-precious metal catalysts (NPMCs) for the ORR. NPMCs include transition metal-nitrogen-carbon-coordinated complexes, nitrogen-doped carbon nanostructures (CNx), heat-treated macrocycles, N-graphene, to name a few. A significant challenge has been to develop stable catalysts which can withstand sustained fuel-cell operation. The reason for their degradation is controversial, since there is much debate regarding the active site of these materials. It is unclear if the transition metal, in conjunction with surface nitrogen groups catalyzes the reaction, or merely acts as a catalyst for generation of nitrogen-containing active sites. The studies undertaken in this work are aimed at elucidating the active sites in two specific ORR catalysts, namely, nitrogen-doped carbon nanostructures (CNx) and iron-nitrogen catalysts supported on carbon (Fe-N-C). The growth process of nitrogen-doped carbon nano-structures (CNx) was studied using in-situ and ex-situ characterization techniques. It was found that the Co phase was seen to go through different transformations during the pyrolysis process, depending on the growth substrate used. CNx fibers that formed were acid-washed, and the structure of CNx obtained as well as the nitrogen content was significantly different on the two substrates, which led (open full item for complete abstract)

Committee: Umit Ozkan Professor (Advisor); Aravind Asthagiri Professor (Committee Member); Jeffrey Chalmers Professor (Committee Member) Subjects: Chemical Engineering

Master of Science (MS), Ohio University, 2007, Chemical Engineering (Engineering)

The design and performance of an alkaline ammonia electrolyzer for hydrogen production and its feasibility for fuel cell applications is presented. Pt based C-paper electrodes were used in the ammonia electrolytic cell (AEC). The separator electrode assembly design (SEA) for the AEC comprised of end plates, end electrode plates, separator plates, the Pt-Ir electrodes, gaskets, and the membrane/separator. The operating procedure for the designed electrolyzer was established. The electrochemical performance of the 9-cell AEC was evaluated at 25°C and 55°C. A Faradaic efficiency of 97.55 ± 0.04 % was obtained on the hydrogen gas produced from the designed electrolyzer. Including the prevailing ohmic losses, a net energy of 4.10 ± 0.97 W h g-1H2 and 6.10 ± 0.97 W h g-1H2 with an energy conversion efficiency of 23.62 ± 0.36 % and 39.71 ± 0.69 % at 25°C and 55°C respectively were achieved from the AEC-PEM fuel cell integration. The results prove the scalability and potential of ammonia electrolysis as a novel and alternative hydrogen production process for PEM fuel cell applications, which would help realize the goal of green hydrogen economy in true sense.

Committee: Gerardine Botte (Advisor) Subjects:

Doctor of Philosophy, University of Akron, 2008, Polymer Engineering

In this work, highly conductive carbon-filled epoxy composites were developed for manufacturing bipolar plates in proton exchange membrane (PEM) fuel cells. These composites were prepared by solution intercalation mixing, followed by compression molding and curing. The in-plane and through-plane electrical conductivity, thermal and mechanical properties, gas barrier properties, and hygrothermal characteristics were determined as a function of carbon-filler type and content. For this purpose, expanded graphite and carbon black were used as a synergistic combination. Mixtures of aromatic and aliphatic epoxy resin were used as the polymer matrix to capitalize on the ductility of the aliphatic epoxy and chemical stability of the aromatic epoxy. The composites showed high glass transition temperatures (Tg ~ 180°C), high thermal degradation temperatures (T2 ~ 415°C), and in-plane conductivity of 200-500 S/cm with carbon fillers as low as 50 wt%. These composites also showed strong mechanical properties, such as flexural modulus, flexural strength, and impact strength, which either met or exceeded the targets. In addition, these composites showed excellent thermal conductivity greater than 50 W/m/K, small values of linear coefficient of thermal expansion, and dramatically reduced oxygen permeation rate. The values of mechanical and thermal properties and electrical conductivity of the composites did not change upon exposure to boiling water, aqueous sulfuric acid solution and hydrogen peroxide solution, indicating that the composites provided long-term reliability and durability under PEM fuel cell operating conditions. Experimental data show that the composites developed in this study are suitable for application as bipolar plates in PEM fuel cells.

Committee: Sadhan Jana (Advisor) Subjects: Engineering, Chemical