Industrialization and increasing population have brought the world two major challenges: climate change and increasing energy demands. First, the atmospheric CO2 level has increased to 400 ppm, and already this high level has been associated with undesirable climate effects such as global warming and an increased occurrence of erratic weather. Second, the world faces challenges in meeting its energy needs due to increasing global population as well as the dwindling resources of fossil fuels on the earth. Various strategies such as switching from fossil fuel derived energy to nuclear energy or renewable energy sources (solar, wind, hydro) need to be pursued to curb the increase in atmospheric CO2 levels while decreasing our dependence on fossil fuels. However, due to the lack of efficient ways for large-scale energy storage, significant amounts of renewable energy will be wasted when the supply is higher than the actual demand due to its intermittency. A potential strategy that can be employed to help overcome both challenges is the electroreduction of CO2 into useful feedstock chemicals or fuels such as formic acid, carbon monoxide, hydrocarbons, and alcohols using otherwise wasted large amounts of intermittent excess renewable energy.
Although the electroreduction of CO2 offers the potential to recycle CO2 and store intermittent renewable energy, this process is still not economically viable due to insufficient performance levels, specifically due to high overpotentials which reduce energy efficiency, low current densities and low selectivities. Better catalysts that show high activity as well as electrodes that exhibit excellent mass transfer capabilities and electron conductivities are needed. This dissertation reports the development of active and durable catalysts that exhibit low overpotentials for the electroreduction of CO2 to products such as CO, C2H4 and/or C2H5OH. This dissertation will also discuss the roles electrolytes and electrode structures play in the electroreduction of CO2.
The focus of Chapters 2 and 3 is on developing better cathode and anode catalysts for the electroreduction of CO2 to CO. Chapter 2 reports on the role of support materials on cathode performance. Ag supported on TiO2 exhibited a twofold higher partial current density for CO than Ag supported on C with the same Ag loading and similar performance compared to unsupported Ag nanoparticles, but at a 2.5 times lower Ag loading. The TiO2 support material was also found to stabilize a reaction intermediate and serves as a redox electron carrier to assist the CO2 reduction reaction. Chapter 3 reports how two forms of IrO2, dihydrate and non-hydrate, improve system energy efficiency and production rate when used as the anode catalyst. For example, when IrO2 dihydrate was used as the anode catalyst instead of platinum black, the energy efficiency increased by 40% and the current density showed a 2-fold improvement.
Chapters 4, 5, and 6 focus on developing active Cu-based cathode catalysts for the electroreduction of CO2 to C2 chemicals. Chapter 4 reports the synthesis and application of active Cu nanoparticles with different morphology and composition (amount of surface oxide) for CO2 reduction in an alkaline electrolyzer. The use of catalysts with large surface roughness results in a high Faradaic efficiency of 46% (only ~30% in most prior work) for the conversion of CO2 to C2 chemicals with a total current density of ~200 mA cm-2 for these C2 products, which, compared to prior work, represents a 10-fold increase in conversion rate at much lower overpotential (only < 0.7 V). The effect of N-containing compounds on the products distribution for the electroreduction of CO2 on Cu catalysts is discussed in Chapter 5. Specifically, 3,5-diamino-1,2,4-triazole (DAT) improves the Faradaic efficiency of C2H4 by more than 1.5 fold when used in combinations with a Cu nanoparticle-based electrode. In-situ surface enhanced Raman Spectroscopy (SERS) was applied to elucidate the possible reasons for this improvement. Chapter 6 describes the design, synthesis and application of CuPd nanoalloys with tunable atomic arrangements (atomically ordered, disordered and phase-separated structures) for the electroreduction of CO2 to C2 chemicals. The results of electrochemical measurement as well as structural and compositional characterization indicate that the observed differences in selectivities for different products can be attributed predominantly to structural differences in the different catalysts. Also, this work for the first time shows that active sites with neighboring Cu atoms are required for the efficient conversion of CO2 to C2 chemicals.
Finally, Chapter 7 describes the improvement in throughput levels for the electroreduction of CO2 to CO through the optimization of electrode structure and composition. The electrode that incorporates multi-walled carbon nanotubes (MWCNTs) in the catalyst layer achieves high levels of CO production of up to 350 mA cm-2 at a high Faradaic efficiency (>95% selective for CO) and an energy efficiency of 45%. This level of performance represents a twofold improvement over the performance achieved with electrodes that lack MWCNTs.
In summary the studies reported in this dissertation provide insight regarding the design and synthesis of active catalysts and electrodes that improve current density (conversion), selectivity and energy efficiency for the electroreduction of CO2 to different chemical intermediates of value
