Date of Award

12-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Matthew M. Mench

Committee Members

Jagjit Nanda, Stephen J. Paddison, Thomas A. Zawodzinski

Abstract

The realization of redox flow batteries (RFBs) as a grid-scale energy solution depends on improving the performance and lifetime of the technology to decrease the high capital costs. The electrodes are a key component in the RFB; performance enhancement is often achieved through chemical or thermal treatments of commercially available porous carbon materials.

This dissertation uses impedance spectroscopy-based methods to gain insight into performance and durability in RFBs, enabling intelligent cell design. Initial work focused on understanding the impact of improved electrode and membrane properties on system performance. An accelerated stress test was then developed that can be used to screen materials for durability. Given the significant need for advanced diagnostics to understand the changes in performance, subsequent work improved upon the state of the art in impedance spectroscopy for RFBs. This method enabled measurement of finite diffusion resistance under typical laboratory conditions. A previously developed macrohomogeneous porous electrode impedance model was used to interpret impedance data.

With the improved impedance method, the effects of treatment on beginning-of-life performance and durability were elucidated for a series of treatments applied to SGL GFD3 carbon felt. Kinetic deterioration occurred on the negative side and was primarily due to a decrease in the inherent kinetic activity of the surface. The impedance-resolved method identified the excellent electrochemical stability of an ammonia heat treated carbon felt and a long-term cycling experiment conclusively verified this result. The impedance method was also successfully applied to a non-aqueous flow battery chemistry. Finally, a method to passively improve capacity retention was also developed and successfully demonstrated.

The major outcome of this dissertation is the robust methodology for evaluating various aspects of cell design in RFBs, advancing the state of the art for measuring performance and degradation. We demonstrate the broad applicability of a previously developed macrohomogeneous porous electrode model to quantify and resolve ohmic, charge transfer, and finite diffusion processes. Successful application of these methods represents a pathway toward intelligent cell design to optimize performance and lifetime, ultimately improving the viability of RFBs as a grid-scale energy storage solution.

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