Towards Energy Efficient Carbon Dioxide Electrolysis for Sustainable Chemical Production
Creators: name: Nabil, Shariful K
Publisher: name: Graduate Studies
Year: 2022
Subjects: subject: Chemistry--Analytical
subject: Chemistry--General

description: Electrochemical CO2 reduction (eCO2R) reaction is a promising pathway capable of diverting CO2 to valuable low-carbon fuels and chemical feedstocks utilising renewable energy. However, thermodynamic stability of CO2 molecule, sluggish oxygen evolution reaction (OER) and use of purified CO2 feedstock add severe energy penalty to this frontier technology. Therefore, deep investigation into these energetic pitfalls will guide converging on alternative chemistry, optimal operating parameters, and electrolyser design required to ensure economic and industrial viability of eCO2R.I begin this thesis by presenting a comparative cradle-to-gate life cycle assessment (LCA) of eCO2R through mass and energy balances, with thermochemical CO2 conversion and incumbent pathways to reveal their CO2 mitigation potential. This analysis revealed that ~80% of total process energy is attributed to eCO2R to synthesize gaseous products, while 30-85% of the same comes from separation of liquid products. I also found that syngas (CO+H2), ethylene (C2H4), and n-propanol (C3H7OH) are the most viable products providing climate benefits over incumbent routes. Due to inevitable carbonate formation in one-step or direct CO2 electrolysis, two-step route (CO2?CO, CO?products) can be environmentally compelling given its lower energy requirement. Required technological goals for eCO2R were also highlighted- which includes achieving &gt;20 wt% liquid product concentration in eCO2R reactor as a performance metric to compete with incumbent routes. The LCA study thereby set the stage for me to identify and experimentally ameliorate the energetic pitfalls of eCO2R; specially for conversion phase of gaseous products (i.e., C2H4). I then present a co-electrolysis process, wherein I replaced energy intensive oxygen evolution reaction (OER) with glycerol oxidation reaction (a cheap organic biowaste) that offers thermodynamically favorable cell voltage (~0.003 V for glycerol oxidation compared to 1.23 V for OER). I present synthesis of gold nano dendrite catalyst and demonstrate glycerol oxidation in a customized slim flow cell reactor utilising bipolar membrane that limits carbonate formation or CO2 crossover as well. The co-electrolysis of glycerol oxidation and eCO2R reduced overall cell potential by ~0.8 V maintaining high selectivity (~50%) and partial current density for C2H4 (~87-112 mA/cm2) along with glycolic acid formation. I then outline a techno-economic analysis (TEA) that revealed that this co-electrolysis process can attain minimum selling price of ~1.1 $/kgC2H4 (lower than market price) with electricity price of 5 cents/kWh as compared to conventional eCO2R with OER that requires electricity price to plunge &lt;1 cent/kWh to be cost competitive with incumbent process. Finally, to avoid energy intensive capture, regeneration, and transportation phases of CO2 feedstock, I present a novel in-situ capture and conversion process, wherein an eCO2R is able to convert CO2 directly from low concentration streams (~10 vol% CO2). I exploit a polymer-based solid sorbent and polycrystalline copper to demonstrate the concept of in-situ capture and conversion process. Translating the idea of in-situ capture and conversion within gas diffusion electrode, I exemplify selective formation of C2H4 with Faradaic efficiency of ~45% at a current density of 80 mA/cm2 for 11 hrs. A TEA model lastly estimated that the in-situ capture and conversion process can reduce up to ~56% of the production cost for C2H4 by bypassing capture, compression and transportation steps of incumbent or decoupled process.