Electrochemical utilization of carbon dioxide : an investigation into intrinsic thermodynamics, microfluidic network and interface control

Abstract

A microfluidic pH-differential reactor was developed in this study for multiple applications. Relying on a membraneless laminar flow-based design, co-flow fluids in the microchannel could perform distinctive behaviors with high surface-to-volume ratios and super-fast mass transfer rates. pH environments in the cell were optimized independently to ensure the most favorable thermodynamics for different electrode reactions. This design addressed the major obstacles of current regenerative fuel cells, which have been limited by the high cost and poor round-trip efficiency. The cell allowed not only water electrolysis at a significantly lowered voltage, but also rendered an elevated open-circuit voltage in the fuel cell mode and high round-trip efficiency. Inspired by the highest performance to date for laminar flow-based cells, the integration of pH-differential technique onto a microfluidic platform was applied to the electrochemical conversion of CO2 to fuels, which were in need of major improvements regarding the efficiency and reactivity. Present experimental results demonstrated a promising solution to alleviate CO2 emission explosion via a pH-differential microfluidic reactor that could improve the thermodynamic property and raise the electrochemical outcome. Freed from hindrances of the membrane structure and thermodynamic limitations, electrode potentials were drawn closer to the equilibrium status at higher reactivity. During the conversion of CO2, the peak Faradaic and energetic efficiencies were monitored with noteworthy improvement, and hence, facilitating the thirst for a broader exploration. The feasibility study of the pH-differential microfluidic architecture for CO2 reduction was followed by a comprehensive parametric optimization. Experimental analysis was conducted to study the mechanisms and intrinsic correlations of key operation condition parameters. An investigation on the cell durability was also carried out in the way of repetitiveness and long period operation. A mathematical model was established to obtain an in-depth understanding of the intrinsic mechanisms, particularly with regard to the micro-level transport and the acid-base interface. Major limiting factors were considered, including mass transfer constraints, kinetic losses and overpotentials. Computational results were validated by experimental data. Based on the model, an extensive study on the slug flow formation principle was conducted to maximize the interfacial contact area between gaseous reactants and liquid electrolytes. Keeping the acid-base co-flow required continuous electrolyte flowing in the microchannel. Besides, neutralization reaction occurred within the mixing layer between the dual-pH electrolyte streams, demanding for higher flow rates to constrain the layer thickness. These led to considerable electrolyte wastage that would significantly weaken the low cost merit and electrolyte utilization efficiency. To tackle this issue, key parameters of electrolytes were tracked and monitored by mimicking different reactor situations. Results indicated that with appropriately adjusted operating conditions, electrolyte recycling would be feasible in a microfluidic pH-differential network. Accordingly, an electrolyte recycling scheme was proposed. In a word, this thesis experimentally and numerically proved the concept of dual-pH stream as an effective strategy to achieve a superior reactor performance by pairing electrodes with thermodynamically favored pHs. On top of the membraneless structure, the electrolyte recycling scheme consolidated its advantageous economical aspect, revealing the potential for larger-scale applications in future energy conversion systems.