Author

Yiwei Zheng

Date of Award

Fall 2019

Document Type

Open Access Thesis

Department

Chemistry and Biochemistry

First Advisor

William E. Mustain

Abstract

Anion exchange membrane fuel cells (AEMFCs) have shown significant promise to provide clean, sustainable energy for grid and transportation applications – and at a lower theoretical cost than more established proton exchange membrane fuel cells (PEMFCs). Adding to the excitement around AEMFCs is the extremely high peak power that can now be obtained (> 3 W cm-2) and continuously improving durability (1000+ h), which has made the future deployment of AEMFCs in real-world applications a serious consideration. For some applications (e.g. automotive), the most critical remaining practical issue with AEMFCs is understanding and mitigating the effects of atmospheric CO2 (in the air supply) on cell behavior and performance.

Most literature discussion around AEMFC carbonation has hypothesized: 1) that the effect of carbonation is limited to an increase in the Ohmic resistance because carbonate has lower mobility than hydroxide; and/or 2) that the so-called “self-purging” mechanism could effectively decarbonate the cell and eliminate CO2-related voltage losses during operation at a reasonable operating current density (> 1 A cm-2). However, this study definitively shows that neither of these assertions are correct. This study is the first comprehensive experimental investigation into the effects of CO2 on operating AEMFCs. It is also the first study to be able to quantitatively determine the root causes for performance decline when CO2 is added to the system, where cell behavior is directly linked to cell chemistry and reaction dynamics. This work, the first experimental examination of its kind, studies the dynamics of cell carbonation and its effect on AEMFC performance over a wide range of operating currents (0.2 – 2.0 A cm-2), operating temperatures (60 – 80°C), and CO2 concentrations (5 – 3200 ppm) in the reactant gases. I have also investigated the influence of reactant gas flowrates (0.2 – 1 L/min) and dew points (50 – 57°C at 60°C cell temperature) on cell carbonation. The resulting data provides for new fundamental relationships to be developed and for the root causes of increased polarization in the presence of CO2 to be quantitatively probed and deconvoluted into Ohmic, Nernstian and charge transfer components, with the Nernstian and charge transfer components controlling the cell behavior under conditions of practical interest. In addition to the demonstrated technology, the lessons learned in this work can also provide transformational insights to other air breathing and/or AEM-based electrochemical systems such as metal air batteries, regenerative fuel cells, electrochemical CO2 capture, CO2 separator and concentrator, CO2 reduction reactors and dialyzers.

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