Investigation of Co decorated N, S doped reduced graphene oxide (Co/NSrGO) catalysts for oxygen reduction reaction (ORR) and nh3 electrosynthesis through nitrate reduction reaction (NO3RR)

One of humanity's most challenging problems in our century is enabling sustainable and carbon-zero energy, chemical, and fuel production and minimizing the use of fossil fuel sources to reduce CO2 emissions and decelerate climate change. Two crucial routes to decarbonize energy consumption are the widespread commercialization of proton exchange membrane fuel cells (PEMFC) for transportation and the environment- friendly NH3 electrosynthesis. It is essential to understand reaction kinetics, adsorption tendencies, and current performance of feasible catalysts to actualize the use of affordable and high-performance catalysts for oxygen reduction reaction (ORR) at PEMFCs' limiting cathode compartment and to provide a pathway for zero-emission NH3 electrosynthesis that compensate for NH3 production of carbon-intensive Haber- Bosch process. Comparably cheap and abundant Co and high-surface-area graphene- based Co decorated N, S Doped Reduced Graphene Oxide (Co/NS-rGO) catalysts with different metal compositions and heat treatments are investigated for NH3 electrosynthesis through nitrate reduction reaction (NO3RR) and ORR for their activity and selectivity. In the first part of the thesis, it is unraveled that the optimum composition of Co enables the formation of ORR-active CoS species and homogenous distribution of Co Single Atoms (SAs) steer the selectivity towards 4epathway that is desired for PEMFC and provide the lowest overpotential with the highest current output. With the help of rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques that eliminate the effect of mass-transfer limitations the number of electrons transferred (ne -) is measured for Co/NS-rGO as 4ethroughout the studied potential range as of industrial PEMFC catalyst of Pt/C while the 2Co/NS-rGO catalyst that has higher Co composition with CoO species, favored more HO2 - production. Furthermore, the origin of the catalysts' kinetic activity and the materials' electronic properties are investigated using electrochemical impedance (EIS) techniques. Co/NS-rGO catalyst that has the highest performance obtained a higher density of states (D(EF)) around the Fermi level for the electroreduction, higher charge carrier density concentration (ND) and the flatband potential. Also, electrochemical surface area (ECSA) normalized RDE and RRDE figures indicate the maximum utilization of electrochemically active sites for the Co/NS-rGO catalyst, which has the highest charge storage properties detected by EIS capacitive measurement. In the second part of the thesis, the electrochemical NO3RR-to-NH3 activity of NS-rGO and its Co incorporated and/or 900 °C pyrolyzed counterparts are investigated. It is observed that NS-rGO is inclined to generate high amounts of NO2 while unable to convert NO3 to NH3 with high efficiency. On the other hand, pyrolysis facilitates hydrogen evolution reaction (HER) for both metal and non-metal catalysts. Co incorporation is claimed to promote NH3 yield rate and Faradaic Efficiency (F.E.) significantly at lower overpotentials with higher current outputs. A mechanistic approach using X-ray Absorption Spectroscopy (XAS), EIS, EIS-driven Distribution of Relaxed Species (DRT), and Tafel analysis enabled us to find that the Co/NS-rGO follows a mass transfer limited mechanism. At the same time, Co/NS-rGO 900 is driven by charge transfer limitations and protonation properties. Furthermore, mechanistic studies show that NO2 desorption plays a significant role in NO3RR. To modulate and increase NH3 yield and F.E., a pulsed electrolysis strategy is inherited at both mass- transfer and charge-transfer limited potential regimes. Pulsed electrolysis results show that Co/NS-rGO catalyst with a very high NH3 yield rate and F.E. for NO2RR can increase its NH3 F.E. by two-fold and yield rate by 3-fold through pulsed electrolysis. In contrast, the pulsed strategy facilitates HER and suppresses NH3 production efficiency for the Co/NS-rGO 900 catalyst.