Mitigation of Atmospheric Carbon Dioxide on Two Fronts: Improving Efficiency of the Vanadium Diboride Anode Material for Primary Batteries and Direct Electrolysis of CO2 into Carbon Nanotubes
Open AccessIt has been well established that atmospheric carbon dioxide (CO2) poses one of the most imminent threats to our planet, with most industries today still reliant on fossil fuel combustion for their energy demands. The result is ever-increasing atmospheric CO2 levels, which lead to global temperatures rising through the greenhouse effect, ocean acidification, severe weather patterns, and more. Much of the research today on CO2 mitigation centers on reclaiming CO2 from industrial flue, known as carbon sequestration. While this is a vital avenue, additional approaches must be taken up; namely reducing the flow of CO2 emissions (particularly from energy production methods) and recapturing the CO2 that has already been emitted and accumulated in the atmosphere. To reduce our dependence on carbon-based energy sources, this work reports on a novel anode material for metal air batteries (MAB) with a volumetric energy density superior even to that of lithium MABs. Next, for the capture and conversion of existing atmospheric CO2, a molten salt electrochemical reduction of atmospheric CO2 at low voltages is demonstrated, forming a product found to be a value-added material with a wide range of applications.As the energy sector is among the worst emitter of CO2 due to its use of fossil fuels, improvements in battery technologies will allow for diminished dependence on carbon emitting energy sources. In the first part of this dissertation, a highly energy dense material, vanadium diboride (VB2), is developed as an anode material for a metal air battery (MAB). This material undergoes a multiple electron per molecule transfer process that allows it to achieve much higher theoretical capacities than many other materials. Zinc, a traditional battery material, is currently used in commercially available MABs (charge density of 820 mAh/g), and the lithium MAB is being developed due to its high theoretical charge density of greater than 3,860 mAh/g. The active material studied here, VB2, undergoes an impressive 11 electron per molecule charge transfer process (lithium only transfers one electron per molecule and zinc transfers two), resulting in a remarkable theoretical capacity of 4,060 mAh/g. While the theoretical gravimetric charge density of VB2 is greater than that of lithium, the true advantage of VB2 lies in its volumetric energy density. Whereas lithium (density of 0.534 kg/L) contains a volumetric energy density of only ~6.2 kWh/L, VB2 (5.1 kg/L) far surpasses that, achieving a volumetric energy density greater than five times that of lithium, ~32.1 kWh/L. However, the practical capacity of these cells is significantly less than that of the theoretical capacity due in large part to passivation caused by the discharge products that impede the depth of discharge. This hinders the ability to fabricate higher capacity cells with thicker anodes.Here, two solutions are offered: one is the formation of a composite mixture of VB2 with another transition metal boride, TiB2, to mitigate the passivation effects of the VB2 discharge products; the other is an improved conductive matrix allowing for increased current collector area, thereby preventing passivation from affecting the bulk sample. The VB2/TiB2 composite anode material was effective in slightly elevating both the cell voltage and the coulombic discharge efficiency (the experimental capacity versus the theoretical capacity of a cell). However, the improved conductive matrix was more successful in terms of coulombic efficiency, raising the discharge efficiency of a 30 mAh cell from ~50% to 75%. This allowed for the fabrication of larger capacity cells, up to 100 mAh, while maintaining discharge efficiencies of 50%.Until now, CO2 has been viewed as a molecule that is impractical to attempt to break down because of its high stability, allowing it to accumulate in the atmosphere. To the contrary, the second focus of this dissertation is the facile electrochemical reduction of CO2 in molten carbonates at low voltages to form highly valuable products. Utilizing the benefits of a thermally energetic (high temperature) system, CO2 can be split in molten carbonate electrolyte at low voltages, producing solid carbon at the cathode and a pure oxygen product at the anode. Not only does this process absorb and reduce atmospheric CO2, it also generates a valuable nanostructured carbon product, specifically carbon nanotubes (CNTs). Here, advances in the CO2 to CNT process in molten lithium carbonate are presented. Electrolytic conditions are optimized such that the carbon product forms high yields of CNTs, while electrode materials and electrolyte additives are evaluated for their abilities to control the resulting morphologies of the CNT product. It was found that certain substrates cause the growth of straighter, longer CNTs, while others were observed to produce CNTs of smaller diameters that were more tangled. As such, alteration of the electrode material was used to tune the morphology of the resulting carbon product, where CNT diameter, length, and crystallinity can be modified through the use of different substrate materials.The different morphologies of the CNT products can be useful in a range of applications. In this work, the ability to use CO2-derived CNTs as anode materials in the secondary (rechargeable) Li-ion battery system is demonstrated with two morphologies of CNT: straight-CNTs with an average diameter just under 200 nm, and tangled-CNTs with a higher defect (less crystalline) structure and slightly smaller diameter. Whereas the more crystalline morphology of CNT achieved reversible anodic capacities of ~360 mAh/g in Li-ion cells, compared to the theoretical capacity of 372 mAh/g for graphitic anodes, extended cycling revealed that the tangled-CNT morphology tested in these cells far exceeded that, providing approximately 460 mAh/g of capacity, where the experimental capacity exceeds the theoretical capacity likely due to defect-induced modifications to storage processes over the course of cycling.Additionally, in an effort to move towards a more sustainable battery system, these CNTs were also tested in Na-ion cells. Sodium has a much higher natural abundance than lithium, and as a result it is a much less expensive material; however, the Na-ion battery system is far less developed than the Li-ion system due to low storage capacities stemming from difficulties in intercalation of the larger ion. Here, the straight- and tangled-CNTs achieved capacities of ~40 mAh/g and 130 mAh/g, respectively, indicating that the modified storage process mentioned above, analogous to pore-filling, was likely the major mechanism for reversible storage in Na-ion batteries using CO2-derived CNTs.
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