SURFACE MODIFICATION OF CARBON-BASED ELECTRODES FOR ELECTROCHEMICAL CONVERSION PROCESSES: OXYGEN REDUCTION REACTION AND BICARBONATE CONVERSION

Oxygen reduction reaction (ORR) and conversion of bicarbonate into value-added chemicals are two significant electrochemical processes for energy storage and conversion. ORR is an important electrochemical reaction in fuel cells and metal-air batteries that provide power conversion and storage capacity, respectively, for portable electronics, and electric vehicles. However, the performance of catalysts (e.g., platinum-based) is critically limited by slow kinetics, inefficient four-electron pathway, and surface deactivation. This limited performance of platinum-based catalysts, the scarcity of platinum, and vulnerable supply chains for critical minerals require the development of alternative electrocatalysts now more than ever. Carbon-based materials possess several key properties that are beneficial for catalytic applications such as high electrical conductivity, large surface area, inert electrode surface, and low cost. The catalytic activity of carbon-based electrodes can be promoted by tailoring the surface and structure through the incorporation of heteroatom dopants. Chapters 2 and 3 in this dissertation focus on the synthesis of electrocatalysts and their surface modification to achieve effective ORR performance in alkaline media. Chapter 2 narrates ORR performance of nitrogen (N) and boron (B) co-doped carbon nano onions (CNOs). In this work, annealing temperature was found to be a crucial factor in the synergistic benefit of N and B towards ORR. X-ray photoelectron spectroscopy (XPS), Raman analysis, and high-resolution scanning transmission electron microscopy were conducted to elucidate the role of N-B configurations in promoting ORR activity. Chapter 3 focuses on copper nanoparticles supported by nitrogen-doped CNOs. This chapter discusses the impacts of nitrogen heteroatom and copper nanoparticles on ORR performance. In chapter 4, electrocatalytic carbon dioxide (CO2) reduction (CO2RR) in a membrane electrode assembly was investigated. Atmospheric CO2 has significantly increased in the last two decades. Since CO2 is a primary greenhouse gas emitted on earth, it is imperative to suppress the concentration of emitted CO2. While the regulation of CO2 emissions is critical, CO2 capture and storage (CCS), and biological, chemical, and electrochemical conversions are promising approaches to reduce atmospheric CO2 concentration. In electrochemical conversion, a common method employs the feed of high-purity compressed CO2 gas into an electrolyzer. This method, however, is not economically viable because it requires the release and/or pressurization of CO2 from captured CO2 solution, which is energy-intensive. To resolve this issue, aqueous carbonate/bicarbonate (CO32-/HCO3-) transported from the upstream carbon capture process can be directly fed into an electrolyzer. We demonstrated that a cation exchange membrane coated with a thin copper film can efficiently convert bicarbonate to C1-C2 products such as formic acid and acetic acid. Both liquid and gas products were quantified by using proton nuclear magnetic resonance (H1 NMR) and gas chromatography, respectively. The studies herein highlight the importance of structural modification of catalysts, surface chemistry, and membrane-electrode interface to improve the efficiency and selectivity of ORR and CO2RR processes. KEYWORDS: Oxygen reduction reaction, Heteroatom Doping, Copper, (Bi)carbonate Conversion, XPS, Solvotherma

Jackfruit (Artocapus heterophyllus Lam.) and teak (Tectona grandis L.) leaf extracts as green corrosion inhibitors

The inhibition effect of water and methanol extracts (WE and ME) of raw and ripe jackfruit leaves (JL) and raw teak leaves (TL) on the corrosion of mild steel (MS) in 1M HCl was studied. Weight loss measurements and potentiodynamic polarization techniques were used to investigate the behavior of the above inhibitors. The percentage of inhibition efficiency (% IE) increased with the increasing concentration of inhibitors in 1 M HCl medium. Water extract of ripe JL was found to be the most effective inhibitor and % IE was 73 at the concentration of 400 ppm. The effectiveness of other inhibitors towards corrosion in the descending order is WE of TL, ME of ripe JL, WE of raw JL, ME of TL and ME of raw JL. With the increase of the temperature, even the adsorption of the most efficient inhibitor, WE of ripe JL, decreased and the rate of corrosion increased. According to the estimated adsorption equilibrium constant, Kads and standard Gibbs free energy change, ∆G0ads, adsorption of WEs of ripe JL and TL on mild steel surface mainly observed to be by chemisorption. Potentiodynamic polarization scans have revealed the possibility of mixed type corrosion inhibition by WEs of ripe JL and TL