DOI
https://doi.org/10.25772/6C5R-Y792
Defense Date
2018
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Dr. Hani M. El-Kaderi
Abstract
Abstract
The electrocatalysis of oxygen plays a significant role in several electrochemical energy storage and conversion systems including metal−air batteries, fuel cells, electrocatalytic and photocatalytic water splitting. The sluggish kinetics and complex reaction mechanism of this cathodic oxygen reduction reaction (ORR) affect the performance and practical application of such renewable energy technologies. To address this limiting factor, a suitable electrocatalyst is required for ORR. In general, platinum or highly dispersed Pt-based nanoparticles on carbon black are considered as the best ORR catalyst. But platinum being very scarce and expensive tends to increase the cost. Moreover, platinum-based catalysts are prone to several serious problems, including declining activity, the fuel-crossover, and poisoning effects. This has initiated overwhelming research attention towards the development of low cost ORR catalysts. Jasinski et al. pioneered in reporting that a N4-chelate complex with a transition metal could be used for electrochemical oxygen reduction. Subsequently many nonprecious ORR catalysts have been investigated so far to replace platinum which include transition metal chalcogenides, nitrogen-doped carbon nanotubes or graphene, carbon nitride, and metal-N4 chelate macrocycles (M-N4-macrocycles). However, most of these current catalysts exhibited insufficient activity and low stability in corrosive environment of fuel cells. Thus, new strategies to develop catalysts which can meet the combined requirements of low cost, high catalytic activity and long-term durability still remains a challenge.
Recently our group has reported synthesis of heteroatom doped porous carbon through chemical activation of simple monomers. The facile synthetic route, high surface areas with abundant micropores, inherent presence of heteroatoms and tunable structure/composition at the molecular level make them potential for high-performance ORR electrocatalysts. To increase the catalytic performance in both acidic and basic media, it is important to incorporate or coordinate the doped heteroatom centers with 3d transition metals such as iron or cobalt. Herein, two different synthetic strategies will be presented to synthesize transition metal-based heteroatom doped porous carbon as ORR catalyst. In the first approach, iron (III) thiocyanate as iron salts was pyrolyzed with benzimidazole to introduce sulfur along with nitrogen and iron in the porous carbon. Another synthetic approach involved hydrothermal synthesis of cobalt oxide on the surface of benzimidazole derived porous carbon. ORR can proceed via either one step four-electron reduction pathway producing water or two step two-electron reduction pathway producing HO2-, OH- and H2O2. Both of these synthesized catalysts favored 4e- reduction pathway which is energetically efficient and do not produce corrosive byproducts. The electrochemical performance of the synthesized catalyst will be analyzed by cyclic voltammetry, linear sweep voltammetry and amperometric i-t technique and compared with commercially available 20 wt% Pt based carbon in both acidic and basic media. The effect of pore size, nitrogen content, bonding configurations of nitrogen and sulfur, influence of cobalt and iron on ORR performance will also be discussed.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
8-10-2018