Design and characterization of catalysts and electrodes for electrochemical energy conversion applications

The modern world faces immense challenges associated with meeting its energy needs, due to its current dependence on fossil fuels. At the same time, the world faces the threat of global climate change linked to CO2 emissions. Indeed, global energy consumption has risen significantly since the industrial revolution and is expected to double again in the next 50 years. This is accelerating the depletion of conventional fossil fuels and has led to a steady increase in atmospheric CO2 levels. Taken together, the dual challenges of finding alternative energy sources and curbing CO2 emissions are daunting. Multifaceted approaches are needed to produce, store, and utilize energy in more efficient and environmentally sustainable ways. This thesis researches two energy conversion technologies that show promise to help address both challenges: fuel cells for efficient electrical power generation, and electrolysis of carbon dioxide into value-added intermediates for chemical production. Fuel cell technologies are expected to play an important role in many alternative energy conversion strategies, particularly for automotive applications. Detailed investigation of the relationship between the physical structure and electrochemical activity of fuel cell electrodes is a critical, yet often poorly reported or proprietary step in the manufacturing of cheaper and more durable configurations. This thesis employs X-ray micro-computed tomography (MicroCT) to visualize the architecture and buried interfaces of fuel cell electrodes in a non-destructive fashion. By coupling MicroCT-based visualization with microfluidic-based electrochemical characterization, variation in catalyst layer morphology can be directly correlated to electrode performance. Depositing catalyst layers via a fully-automated air-brushing method led to a 56% improvement in fuel cell performance and a significant reduction in electrode-to-electrode variability. Electrochemical reduction of CO2 into value-added chemicals potentially offers an economically viable route to recycle CO2 towards reducing CO2 emissions and dependence on fossil fuels. However, the current performance levels of CO2 electrolyzers are insufficient for commercialization due to the lack iii of catalysts with adequate activity and selectivity. This thesis researches the effects of catalyst layer deposition methodology on electrode performance. Air-brushing catalyst layers for CO2 reduction led to a 3-fold increase in partial CO current density and enhanced product selectivity (94%) and a 10-fold decrease in catalyst loading as compared to previous reports. Furthermore, this thesis reports carbon nanotube-supported gold catalysts for CO2 reduction exhibiting both higher activity and higher Faradaic efficiency for CO production. The 160 mA/cm2 partial current density for CO production achieved for this supported gold catalyst is to date the highest performance reported under ambient conditions. Such high activity can be attributed to enhanced catalyst utilization, evidenced by the high electrochemically-active surface area of gold on this material. Finally, the development of no-metal-added nitrogen-doped carbon catalysts for CO2 reduction is reported. Pyrolyzed carbon nitride supported on carbon nanotubes exhibit excellent selectivity for CO production over H2 production (98% CO and 2% H2) as well as high throughput (90 mA/cm2 CO partial current density). Together, these studies present the framework for developing catalytic materials to help CO2 reduction achieve performance benchmarks for commercialization