Modeling of the electrochemical conversion of CO2 in microfluidic reactors.

摘要

Today's 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 CO 2 emissions. Indeed, global energy consumption is expected to double in the next 50 years. This accelerates the depletion of conventional fossil fuels and leads to a steady increase in CO2 emission. Globally, CO2 emission through the combustion of fossil fuels has increased by about 1.6 times between 1990 (the Kyoto Protocol reference year) and 2013, with approximately 9.9 GtC added to the atmosphere in 2013. Taken together, the dual challenges of finding alternative energy sources and curbing CO 2 emissions are very daunting. When it is powered by carbon-neutral electricity sources, the electrochemical conversion of CO2 into value-added chemicals offers an economically viable route to recycle CO 2 towards reducing CO2 emissions and reducing dependence on fossil fuels.;The majority of prior studies on the electrochemical conversion of CO 2 are experimental in nature, focused on unravelling the mechanisms of known catalysts. As an alternative approach to the laborious experiments, first-principles modeling of the electrochemical reactors can complement the current experimental work by elucidating the complex transport and electrochemistry, particularly in the porous electrodes, and help in the design and optimization of such reactors. Currently, there is a lack of detailed modeling for the aqueous electrochemical reduction of CO2 in a microfluidic reactor, which has been demonstrated experimentally to be an effective reactor and a versatile analytical tool.;This thesis focuses on developing a mathematical modeling framework for the electrochemical conversion of CO2 to CO in microfluidic reactors. Conversion of CO2 into CO is attractive due to the versatility of CO (with H2) as a feedstock for the production of a variety of products including liquid hydrocarbon fuels. A full model that takes into account of all the significant physics and electrochemistry in the cell, including the transport of species and charges, momentum and mass conservations, and electrochemical reactions, is first formulated. The full model that comprises of a system of coupled partial differential equations is solved using finite element method. It is then calibrated and validated using experimental data obtained for various inlet flow rates and compositions. Parametric studies for various design and operating variables are subsequently performed using the validated model. To reduce the computational time, yet preserve the geometric resolution and leading order behavior of the cell, narrow-gap approximation and scaling arguments are invoked which allow for significant reduction in the mathematical complexity of the full model and eventually approximate analytical solutions. The unit cell models are then extended to stack models for simulation and analysis of the electrochemical reduction of CO2 in a microfluidic cell stack.