Molten Alkali Metal Borate/Carbonate Salts for High Temperature CO₂ Capture and Electrochemical Conversion

Abstract

In recent years, alkali carbonate molten salts have been developed as a medium for electrochemical conversion of CO2 into value-added carbonaceous materials, including carbon nanotubes (CNTs). While electricity requirements are significant, the high economic value of CNTs make these processes potentially appealing both as a means of carbon sequestration and as an alternative to current greenhouse gas-intensive CNT synthesis pathways. Prior work in this field has primarily focused on the effects of parameters such as alternate chemistries, electrolyte additives, and electrode composition on the achievable products and energetic demands. This research has worked towards commercial operation of electrochemical CNT synthesis.

In this thesis, we present research advancing integration of electrochemical conversion of CO2 in molten salts into real chemical processes at moderate temperatures (500-650°C). First, we examine molten alkali borates as a novel hybrid sorbent for CO2 conversion. Alkali borates have been demonstrated as a promising high-temperature molten salt sorbent for acid gas separations, but prior studies have focused on regeneration through steam sweeping or thermal cycling. Here, we demonstrate that NaxB1-xO1.5-x with x=0.75 can be regenerated electrochemically, achieving CNT synthesis in the process. We determine an optimal mixture of borate/carbonate salts to maximize CO2 uptake and coulombic efficiency. We then examine novel materials for containment of borates and demonstrate the effects of varying cathode materials on electrolysis. We also investigate potential synergies between carbonate electrolysis and the alkaline thermal treatment (ATT) process for conversion of oceanic biomass and plastic wastes into hydrogen. We perform preliminary investigations into the possibility of an all-in-one gasification/electrolysis reactor, determining that the presence of seaweed ash inhibits CNT synthesis, but LDPE can be gasified without affecting the electrochemistry. Finally, we present a technoeconomic analysis of the ATT process, evaluating the relative merits of both the originally proposed slag-regenerated ATT process and an electrochemically mediated alternative. We determine that variable operating expenses are prohibitive in most cases for a slag-regenerated system, making electrochemical regeneration attractive if practical concerns can be addressed.