ENGINEERING A NOVEL SOLVENT REGENERATOR THAT LEVERAGES ELECTROCHEMICALLY INDUCED PH SWINGS FOR DIRECT AIR CAPTURE AND HYDROGEN GENERATION

Author

Emmanuel Aluhunmile Ohiomoba, University of KentuckyFollow

Author ORCID Identifier

https://orcid.org/0009-0005-3928-4623

Date Available

3-3-2026

Year of Publication

2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Mechanical Engineering

Faculty

Dr Kunlei Liu

Abstract

Direct Air Capture (DAC) is considered to be a key component for achieving global net-negative emissions. However, to capture CO₂ at the low concentrations in the atmosphere requires materials with high affinity for CO₂. This makes the solvent-based chemical approach for DAC more applicable compared to physical approaches. Nonetheless, regenerating the capture solvent presents a significant challenge due to high heat requirements for thermal decomposition of carbonates. Leveraging electrochemical principles provides an opportunity to simplify the regeneration process, thus, this work focuses on engineering an electrochemical solvent regenerator for recovering alkaline solvents for DAC, with co-production of hydrogen to offset cost.

Firstly, a theoretical framework for analyzing the energetics of the DAC regenerator (DACR) compared to low temperature water electrolyzerrs is presented. The simulation study identified the key differences between the DACR and the other electrolyzerrs, highlighting how these differences contribute to the overall energy efficiency of the DACR. Results show that the constraint of pH swing for CO₂ release significantly impacts on the resistances within the regenerator. Furthermore, mitigating the accumulation of gas bubbles within the DACR is shown to provide higher voltage savings compared to other electrolyzerrs, owing to significant CO₂ contribution to transport losses. The hydrodynamic conditions in the DACR was emphasized as key to reducing its overall energy penalty via reduction in transport resistance.

Secondly, the effects of different operating parameters on the CO₂ release behavior in the DACR were evaluated via experimental studies. Two dimensionless parameters; potassium factor’ and ‘buffering factor’ were presented as key to promoting efficient CO₂ release in the DACR. Whereas increasing the potassium factor resulted in efficient release of CO₂,lowering the buffering factor is also key to more efficient CO₂ releases. Nonetheless, the results show that both cases have negative impacts on the voltage penalty of the DACR. Experimental results showed the operability of the DACR regenerator at 450 kJ/mol-CO₂ with high purity H₂ generation and > 70 percent CO₂ released. A simplified theoretical relationship between the buffering factor and potassium factor was also proposed to estimate the CO₂ release efficiency of DAC regenerator at varying conditions.

Furthermore, different methods were considered towards improving the efficiency of CO₂ release in the DACR by reducing the transport of protons instead of potassium ions from the anode to the cathode, especially at higher current densities. The anode-membrane spacing was highlighted as a major contributor to proton slip limiting the application of commercial zero-gap membrane electrode assemblies in the DACR. A minimum gap of ~1 mm between the anode and membrane was required to achieve transport efficiency > 60 percent. Moreover, the modification of the flow pattern at the anode by using different flow field designs and inline mixers, was shown to improve the transport efficiency of potassium ions to ~75- > 90 percent.

Lastly, the utilization of gaseous by-products (i.e. oxygen and hydrogen from water electrolysis) in the DACR to lower its voltage penalty is explored. By adjusting its design configuration, a gas-looping approach is presented with justification for its applicability shown in specific cases. This gas-looping approach was also experimented for applications in unloading CO₂ from ocean water with the aim of promoting CO₂ absorption with the ocean as a natural absorber.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2025.416

Funding Information

Department of Energy (DE-FE0032125)

University of Kentucky’s Institute of Decarbonization and Energy Advancement

Research and Development team at PPL Corporation

Recommended Citation

Ohiomoba, Emmanuel Aluhunmile, "ENGINEERING A NOVEL SOLVENT REGENERATOR THAT LEVERAGES ELECTROCHEMICALLY INDUCED PH SWINGS FOR DIRECT AIR CAPTURE AND HYDROGEN GENERATION" (2025). Theses and Dissertations--Mechanical Engineering. 244.
https://uknowledge.uky.edu/me_etds/244