Yiwei Zheng

Date of Award

Fall 2021

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

William E. Mustain

Abstract

Anion exchange membrane fuel cells (AEMFCs) have recently received significant attention as a future high efficiency, environmentally friendly energy conversion device. This attention is due to the potential advantages that AEMFCs can offer compared the much more common, and commercialized, proton exchange membrane fuel cells (PEMFCs) – most notably lower cost. However, there are several remaining roadblocks for the AEMFC technology to be widely adopted, such as: i) the stability of the anion exchange membranes (AEMs) and anion exchange ionomer (AEIs); ii) the development of highly active catalysts with either low platinum group metal (PGM) loading or catalysts that are completely PGM-free; iii) the discovery of water management strategies to prevent electrodes from flooding or drying out; and iv) reducing the negative effect of CO2 on performance.

This last issue, CO2 poisoning in AEMFCs, is considered by many to be the most serious hurdle to overcome. In an AEMFC operating on ambient air, CO2 reacts with the OHanions created from the oxygen reduction reaction at the cathode, forming HCO3 - and CO3 2- . These carbonates are transported from the cathode to the anode during operation. As shown in Chapter 1 of this thesis, the presence of carbonate anions has multiple impacts on the operating AEMFC; carbonates decrease the conductivity and water uptake of AEM, introduce additional charge transfer resistance at the hydrogen oxidation anode and change the anode pH (resulting in a thermodynamic decrease in the cell operating voltage). In total, the CO2-related overpotential can be up to 400 mV, which is unacceptable from a practical perspective.

This work will present an extensive array of experiments that deconvolutes the fundamental electrochemical mechanism for carbonate “poisoning” in AEMFCs. The dynamics of CO2 uptake and removal and dynamics in these systems – with a particular focus on the impact of CO2 concentration in the reacting gas, gas flowrates, backpressure, fuel cell hydration level and temperature, AEM thickness and AEM chemistry, which are the focus of Chapters 2 - 4. With this new understanding strategies to reduce the CO2 related overpotential below 100 mV will be shown. Finally, as shown in Chapter 5, the chemical mechanisms for how carbonation leads to voltage loss in operating AEMFCs have even been used to design systems that minimize the exposure of operating AEMFCs to CO2. Such a device can be called an anion exchange CO2 separator (AECS) which i) is able to generate power; and ii) takes advantage of the carbonation phenomena that harms AEMFCs. In this work, the effectiveness of an AECS in lowering the CO2 concentration of an incoming stream of 400ppm air is investigated. In addition to showing significant CO2 removal, an AECS that operates with a stable output for over 150 h is shown. AEMFC operation on AECS-purified industrial air is successfully demonstrated.

Chapters 6 is a summary of all the fundamental findings in this work. Lastly, Chapter 7 of this thesis is meant to provide some perspective on the state of the technology and where it is going. It also proposes future work that can be done to achieve a AEMFCs with high CO2 resistance in the near future.

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© 2021, Yiwei Zheng

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