Towards Mechanistic Understanding of Carbon-Based Electrocatalysts in Energy Applications

Fossil fuel depletion and environmental pollution are causing major interest in sustainable alternative energy sources. Renewable energy technologies, such as solar cells, fuels cells, metal-air batteries, all rely on few key reactions. Those transformations important to energy storage/conversion devices are oxygen evolution (OER), oxygen reduction (ORR), and carbon dioxide reduction reaction (CO2RR). Sluggish kinetics of those processes raised the need for efficient electrocatalyst to accelerate their performance. Commonly used noble-metal catalysts have many disadvantages such as toxicity, low selectivity, and costly production, making metal-free catalysts attractive alternative. However, graphene-based materials explored for this goal are usually prepared on high temperature pyrolysis yielding poor control over their final structure. As a result, the elucidation of the active sites and the mechanism of actions become very chalanging. This thesis focuses on small well-defined metal-free molecular models that mimic graphene materials for energy related catalytic transformations. They are synthesized via bottom-up approach and studied as electrocatalysts for energy related conversion reactions. Few avenues have been explored starting with purely solution based homogeneous catalysts for OER and ORR. Those small organic cations (oxygen and nitrogen-based heterocycles) have proven to be successful redox mediators for discharge process in lithium-air batteries. On the other hand, they also exhibit electrode assisted OER in synergic action involving electrode and the molecular models. This co-catalysis is attractive approach towards improving the activity of heterogenous catalysts. Second avenue explored in this dissertation focuses on bridging the gap between homogeneous and heterogeneous catalysis. Nanostructured carbon-based electrodes were prepared starting from small well-defined molecules, therefore yielding functional electrodes with well-known structure. Simple model was developed to evaluate the conductivity and electron transfer kinetics of as prepared electrodes using only cyclic voltammetry and simulation of experimental voltammograms. It was also shown that anodic electrochemical treatment could be used to prepare those electrodes. Subsequent study revealed that cathodic treatment could also be used to modify carbon electrodes. Here, graphene nanoribbons deposited onto basal plane of glassy carbon were studied and it was shown that cathodic treatment cause such re-deposition on nanoribbons that they are in strong electronic communication with the carbon support. It was shown for the first time that basal plane could be modified via pi-pi stacking interactions to exhibit strong electronic communication with graphene nanoribbons. Those structures have nitrogen functionalities on the edges and therefore are great platform for catalyst docking. It was illustrated on the example of Ru that catalyst grafting onto conductive nanoribbons is achievable.