Developing durable electrodes for selective electroreduction of CO2 to value-added intermediates

Nwabara, Uzoma O

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https://hdl.handle.net/2142/113179 Copy

Description

Title

Developing durable electrodes for selective electroreduction of CO2 to value-added intermediates

Author(s)

Nwabara, Uzoma O

Issue Date

2021-07-16

Director of Research (if dissertation) or Advisor (if thesis)

Kenis, Paul J.A.

Doctoral Committee Chair(s)

Kenis, Paul J.A.

Committee Member(s)

• Gewirth, Andrew A.
• Yang, Hong
• Flaherty, David W

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• CO2 utilization
• electrochemical reduction
• electrocatalysis
• stability
• durability
• ionomers

Abstract

Global energy demands continue to rise as the earth’s population booms and countries proceed with to development. With these increases comes higher pollution and greenhouse gas emissions from processes established to meet the needs of and to support the population. Specifically, rising excess carbon dioxide (CO2) emissions (over 14 Gt CO2 per year) has pushed the atmospheric CO2 concentration to over 400 ppm. This high atmospheric CO2 (and other greenhouse gases) concentration has been linked to rising global temperatures and climate change, both of which have already proven to have drastic effects on the earth and its various ecosystems (i.e., extreme weather, melting icecaps). Many technologies have been researched and refined in response to climate change. One example is alternative energy production from solar and wind to replace fossil fuel-based electricity production. Another prominent method for curbing emissions is carbon utilization. Specifically, CO2 can be converted using [renewable] electricity to value-added chemicals and feedstocks. The electrochemical reduction of CO2 (ECO2RR) has been studied over the last 40 years, becoming popular over the last decade as a promising approach to combat CO2 emissions. That being said, the integration and implementation of multiple technologies into current industrial and societal practices is necessary to solve climate change and drive down carbon emissions. This dissertation details efforts aimed to identify and understand the pitfalls of ECO2RR to improve its feasibility. High selectivity (Faradaic efficiency, FE) to products such as carbon monoxide, formate, methane, methanol, ethanol, and ethylene aids downstream separation, thus cutting overall process costs. Lowering the energy required (cell potential) to run ECO2RR reduces both electricity costs and associated emissions. ECO2RR systems with long lifetimes and sufficient stability decrease the need for maintenance and replace of materials, which decreases expenses. To start off, Chapter 2 reviews ECO2RR durability studies and highlights any systems with long lifetimes and/or notable approaches to augment stability. Many reported cell configurations possess insufficient lifetimes (<100 h) at low activities (<50 mA cm–2) when considering technoeconomic benchmarks for ECO2RR at scale (>3000 h, >200 mA cm–2). In addition, Chapter 2 discusses the degradation mechanisms that plague various ECO2RR cells and cause their eventual failure and short durability. Common degradation modes include catalyst agglomeration, carbonate formation, and binder dissolution. This chapter concludes by proposing a standard durability testing protocol applicable to all ECO2RR setups and providing data for a 50 h durability study in a membrane electrode assembly-based electrolyzer. Chapter 3 focuses on designing catalysts for enhanced ECO2RR to C2+ products and formate at low cell potentials in an alkaline flow cell. This chapter also introduces Pt supported on polymer-wrapped carbon nanotubes (Pt/PyPBI/MWNT) as an anode catalyst for glycerol oxidation opposite of ECO2RR and calculates the FE of liquid anodic products. Stable CuAg core-shell nanoparticles promoted higher ethylene formation (FE=27%) at 2.25 V of cell potential compared to commercial Cu (FE~3%) at the same potential. Doping In2O3 with Sn (ITO) reduced the onset potential for formate production; 12% ITO nanocrystals initiated formate production at just –1.75 V (overpotential of ~320 mV) while undoped nanocrystals required 2.00 V. At 57% less Pt loading, the Pt/PyPBI/MWNT exhibited similar activity for glycerol oxidation as Pt nanoparticles. With the insight gained from Chapter 2, Chapter 4 investigates the properties of binders other than the commonly used Nafion in the catalyst layer of gas diffusion electrodes for ECO2RR. Contact angle measurements and stability experiments revealed the role of binder in catalyst layer hydrophobicity. Ag cathodes with Sustainion and Nafion suffered from carbonate formation on the catalyst after 6 h ECO2RR durability testing while those with PTFE eluded this degradation. Surface-enhanced Raman spectroscopy and electrochemical impedance spectroscopy suggested how the electrical conductivity (or lack thereof) of binders assists carbonate formation. Ag cathodes with a ~1.0 µm thick Sustainion binder coating displayed enhanced ECO2RR stability due to hampered carbonation formation. Chapter 5 delves into accelerated durability testing (ADT) methods for ECO2RR as a means to determine a system’s lifetime in a shorter amount of time. First, other, more established electrochemical technologies (water electrolysis, fuel cells, chlor alkali electrolysis) and their ADT strategies are studied to ascertain appropriate acceleration methods for ECO2RR. Elevated temperatures and pressures intensify ECO2RR, leading to faster degradation; gaseous and liquid feed impurities lower cell performance and induce catalyst poisoning. With this, an ECO2RR ADT protocol is proposed and partially validated in an alkaline flow electrolyzer. Passing the same amount of charge in shorter times lowers the FECO at different rates while increasing the electrolyte molarity leads to high amounts of carbonate deposits which quickly demote cathode performance. Ultimately, this dissertation presents data and results for moving ECO2RR towards feasibility at scale. The knowledge offered in this dissertation can be adapted to design newer, better catalysts, electrodes, and systems for ECO2RR.

Graduation Semester

2021-08

Type of Resource

Thesis

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Copyright 2021 Uzoma Nwabara

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