Directly Deposited Solid Polymer Electrolyte for Enhanced Electrochemical Carbon Conversion
Creators: name: Adnan, Muflih Arisa
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: Electrolysis
subject: Renewable Energy
subject: Net zero emission
subject: Chemical industry
subject: Sustainability
subject: Engineering--Chemical

description: It has become evident in recent years that we need to accelerate our transition to a net-zero future. The dwindling price of low-carbon renewable electricity has been an enabler to develop technologies that relies on low-carbon electrons. Electrochemical carbon conversion (i.e., CO2 electroreduction i.e., CO2R or CO electroreduction i.e., COR) is one of such emerging technologies that allows converting CO2 or CO into various chemical and fuel using low-carbon electricity and water. I begin this thesis with a comprehensive technoeconomic and life cycle analysis for the production of methanol using electrochemical route and the conventional one. This study showed that under current market conditions, the levelized cost of methanol from electrolysis routes is 2 to 4 folds higher than the market price due to the low performance of electrochemical CO2 conversion to methanol. I showed that to achieve market competitiveness, some key performance metrics has to be achieved for the CO2R approach, including energy efficiency &gt;40%, stability &gt;8000 hours, and current density &gt;130 mA/cm2 (CO2-to-CH3OH) or &gt;360 mA/cm2 (CO2-to-CO). I then performed experimental analysis to investigate the key challenges in CO2R using membrane electrode assembly (MEA). The key challenge on CO2R comes from the competing carbonate formation reaction in the cathode which directly depletes the CO2 utilization for CO2R. Furthermore, the carbonate crosses over to the anodes when using the anion exchange membrane (AEM). Alternatively, cation exchange membrane (CEM) or bipolar membrane (BPM) can suppress carbonate crossover. However, CEM or BPM leads to either cation crossover and excessive water transport (promotes salt formation) or proton flooding (promotes Hydrogen Evolution Reaction (HER)) to the cathode, respectively. To overcome the challenge in commercial pre-made standalone AEM and CEM, I developed a direct membrane deposition approach using a simple spray-based coating approach. This direct deposition approach eliminates the need for a pre-made standalone membrane and offer improved stability of the catalyst and cathode. Using this patent-pending approach, I then designed a thin (~3 μm as opposed to over 50 μm commercial membrane) cation infused solid polymer electrolyte (CISPE) which enables bidirectional ion transport mechanism. The use of thin CISPE substitutes the use of standalone membrane and consequently suppresses salt formation and cathode flooding. I found that this approach enables high full cell energy efficiency of 28% at 100 mA/cm2 for one step CO2R to C2H4, which results in a record low overall energy cost (i.e., CO2 capture, electrolysis, CO2 separation and carbonate regeneration) for C2H4 production of 290 GJ per ton C2H4. The use of CISPE also allowed 160 hours of stable operation for continuous production of C2H4. While the direct CO2 electrolysis to C2+ products require further development due to carbonate formation, a CO2-to-carbon monoxide (CO) electrolysis has been commercially deployed. Taking advantage of the high technology maturity of CO2-to-CO electrolysis, I investigated the possibility of CO electrolysis (COR) as an intermediate step for converting CO2 into hydrocarbon. While the salt formation issue is absent in COR, the cation crossover still hinders the COR selectivity as it diminishes the CO availability at the cathode surface. To suppress the cation crossover via both electromigration and water diffusion (diffusion of hydrated cation), I implemented and optimized the direct deposition of a thin (~0.7 μm) CISPE on the surface of the Cu cathode catalyst. This thin CISPE suppressed K+ transport to the cathode, which led to improved CO availability and partial current density to ethylene. This approach enables stable operation at 100 mA/cm2 for over 200 hours with an energy efficiency toward C2H4 of 21%, which can be translated into an overall energy consumption of 218 GJ per ton C2H4. I also reported a high energy efficiency toward ethanol (C2H5OH) production of 17%. Another reason for the low CO availability is the low solubility of CO in the aqueous electrolytes. Then, I carried out a theoretical investigation of CO mass transport at different temperatures. I found that low operating temperature facilitates high CO availability on the catalyst surface due to high CO solubility and less cathode flooding which enhances the current density toward COR product. From the experimental studies on COR at different temperatures (10 to 50oC), I observed that the low-temperature (10oC) COR enables high partial current density towards the C2+ products (657 mA/cm2). The combination of CuNP and NiFe layered double hydroxide (LDH) anode showed excellent Faradaic efficiency of ~87% at 450 mA/cm2 towards C2+ products with a CO single pass conversion of ~90% for 150 hours of stable operation. From the brief technoeconomic analysis, I found that the pressurized electrolysis system (e.g., 10 atm) requires 2.4 folds higher capital cost and 1.5 higher operating cost than the ambient pressure electrolysis cell at low temperature (e.g., 10oC). I concluded this thesis with key findings and recommended future works to address the remaining challenges in electrochemical carbon conversion, including carbonate cross-over, stability etc. Successful demonstration of this technology will enable electrification of chemical industries to produce sustainable chemicals and fuels.