Analysis and Design of Electrochemically-Mediated Carbon Dioxide Separation

Large-scale carbon dioxide (CO2) separation is essential in the efforts to curb climate change, through applications in power stations, natural gas purification and enhanced oil recovery. This thesis describes the development of a novel CO2 separation technology that utilizes electrochemical cycling of metal ions to modulate the CO2 affinity of amine sorbents. By eliminating the large steam requirements of thermal scrubbing and improving energy efficiency, the technology has the potential to considerably cut capture costs. The feasibility of this approach was previously demonstrated with a proof-of-concept device, but significant development is still required to improve the technology’s energy efficiency and stability. Here, we develop a framework to explore the vast design space of the technology, understand energy loss mechanisms and identify conditions for robust, efficient operation. First, the ability of electrolytes to influence activation overpotentials was experimentally evaluated. Small-scale, short galvanostatic pulse experiments demonstrated a significant change in electrochemical kinetic behavior for both anodic and cathodic conditions, with halide salts showing the best performance. Prolonged testing of a bench-scale system under halide salts found their enhancement effects to subside quickly, and XPS analysis of the formed metal deposit suggests that the electrolyte is co-deposited, possibly causing an increase in contact resistance. Nitrate salts were found to drive a more stable performance, and were thus utilized in further studies. Mechanistic analysis suggests that electrolytes act to modulate activation overpotentials by acting as ligand-bridges through an inner-sphere electron transfer mechanism. Next, a two-dimensional modeling framework coupling flow and electrochemical reactions was developed to explore the cell performance for a range of designs and operating conditions. The model predicts operation in a mixed mass-transfer/kinetics controlled regime, with significant mass transfer limitations and rapid formation of a thick boundary layer. Surface concentration changes act to reduce the exchange current density and increase the open circuit potential. The introduction of flow field disturbances to induce mixing between the bulk and boundary layer is predicted to improve energy efficiency with minimal loss of utilization. The performance of bench-scale prototypes designs was then evaluated through experimental measurement of their polarization curves and electrochemical impedance spectra. Parametric studies of channel depth and operating flowrate effects suggest operation at the desired current-densities is in a mixed mass-transfer/activation control regime, qualitatively confirming model predictions. Electrochemical Impedance Spectroscopy experiments at the bench-scale provided further evidence on the mixed regime operation of the process, demonstrating a reduction in activation resistance with increased superficial velocities. A multielectrode cell was built to probe the axial distribution of current density, and results showed a drop in current density along the electrode, due to thermodynamic and kinetic effects of surface concentrations. Those findings were used to suggest an improved design for the electrochemical cell that features short remix lengths using serpentine flow fields, operating flowrates sufficient to maintain a low gas/liquid ratio, and higher temperatures to reduce overpotentials. The implementation of these changes resulted in a 60-70% reduction in voltage requirements at moderate current densities. A detailed energy and cost analysis suggests that this technology can meaningfully reduce capture costs, if this performance improvement is sustained for longer operation. Thesis Supervisor: T. Alan Hatton Title: Ralph Landau Professor of Chemical Engineering