Investigating the Electrochemical Performance of Biphenyl-Sodium Polysulfide Organic Redox Flow Batteries

With the increased utilization of renewable energy sources, demand for long-duration energy storage systems has increased to moderate the intermittent power generation problem of these sources. Among the various energy storage technologies, those based on electrochemical conversion have received tremendous attention in recent decades. Organic redox flow batteries (ORFBs) are promising candidates for large-scale energy storage. The advantages of ORFBs are flexibility, wide voltage window, high energy density, and low cost. Although recent developments in ORFBs are promising, their future implementation requires further development, especially to understand their electrochemical performance and alleviate system inefficiencies. Most of ORFBs' performance loss comes from low power density caused by the low ionic conductivity of organic solvents and high kinetic loss due to sluggish reactions.

The first step to improve ORFBs' electrochemical performance is to quantitatively investigate the sources of voltage loss. Biphenyl (Bp)|sodium polysulfide (Na₂S_x) is the system to be studied in this work since it is an emerging chemistry of organic batteries with encouraging potential. Various in-situ and ex-situ electrochemical diagnostic tools were used, such as polarization curves, electrochemical impedance spectroscopy, and cyclic voltammetry. These tools were combined with the following experimental setups: 3-electrode cell, symmetric cell, and a flow cell with an in-situ sodium/sodium ion reference electrode. The results from the experimental methods mentioned above showed that in Bp|Na₂S_x ORFBs, Bp contributes more to the cell's voltage loss compared to Na₂S_x. Deeper investigation for both Bp and Na₂S_x followed, revealing kinetic overpotential as the dominant source of voltage loss. The methods demonstrated in this investigation are non-destructive and applicable to various electrochemical systems.

Next, graphene aerogel (GA) was used to synthesize a carbon electrode. The hydrothermal synthesis process was studied to control the electrode's physical and electrochemical properties. Results demonstrate that a GA-based carbon electrode can be synthesized with tunable microstructure, density, electrical conductivity, and catalytic activity. Directional freezing integrated with the hydrothermal synthesis process can be used to effectively tune pore size and directionality. Catalysts, such as copper nanoparticles, can be added to GA-based electrodes, aiding in reducing Na₂S_x redox reaction kinetics.

This is the final submission of this dissertation. Please note that the abstract was revised.