The advantages of the electrochemical conversion of carbon dioxide to fuels using renewable energy sources are two-fold: (1) it has the potential to accomplish a carbon-neutral energy cycle and (2) it can provide an approach to tackle the environmental challenges caused by anthropogenic carbon dioxide emissions. Although thermodynamically possible, the kinetics of carbon dioxide reduction to fuels remains challenging and therefore, an efficient and robust electrocatalyst is needed to promote the reaction. The ideal catalyst for the electrochemical CO2 reduction must be capable of mediating multiple proton-coupled electron transfer reactions at low overpotentials, suppressing the concurrent hydrogen evolution reaction, converting CO2 to desired chemicals with high selectivity, and achieving long-term stability. Extensive research has been carried out on metallic electrocatalysts during the past three decades; however, none of these materials are simultaneously efficient and stable for practical purposes. This Ph.D. dissertation focuses on the investigation of the electro-reduction of CO2 on carbon nanostructures with a focus on understanding the relationship between defect structures and electrocatalytic activity.
The initial focus of this work was to accomplish active performance and durability for electrosynthesis of fuels from CO2 using cost-effective catalysts. N-doped carbon nanotubes (NCNTs) were demonstrated as highly efficient, selective and more importantly, stable catalysts to achieve CO2 conversion to CO. The catalytic activity of these NCNTs was further benchmarked against other metallic catalysts reported in literature (Chapter 2). Compared to noble metals Ag & Au, these NCNTs exhibited a lower overpotential to achieve similar selectivity towards CO formation. The second part of this work was a study of the dependence of catalytic activity, i.e., the overpotential and selectivity for CO formation on the defect structures (pyridinic, graphitic, pyrrolic-N) inside NCNTs. The presence of both pyridinic and graphitic-N was found to significantly decrease the absolute overpotential and increase the selectivity towards CO formation (Chapter 3). The third part of this thesis work was to investigate CO2 reduction on N-doped graphene, in order to explore morphology effects on catalytic activity of NCNTs towards CO formation (Chapter 4). Overall, pyridinic-N defects exhibited the highest catalytic activity; thereby suggesting the directions for developing carbon nanostructures as metal-free electrocatalysts for CO2 reduction
