| dc.description.abstract | With the emerging interest in large-scale long-duration energy storage for the electricity grid, sulfur redox reaction in aqueous solutions presents an attractive candidate due to its low cost and high abundance. To this end, an air-breathing aqueous polysulfide redox flow battery was demonstrated as a promising candidate, which pairs a similarly low-cost oxygen electrode with aqueous polysulfide, to meet the criteria for grid storage. Techno-economic modeling shows such a battery has one of the lowest chemical and installed costs among energy storage options, economically competitive with mechanical storage such as pumped-hydro and compressed air storage but without the geographic constraints.
Further, studies focusing on the materials properties of aqueous polysulfide electrolytes as well as the kinetic and transport properties under various electrolyte/electrode designs were performed to elucidate the mechanisms of and limitations on aqueous polysulfide reactions. Two materials properties limit aqueous polysulfide electrolyte capacity as a redox flow electrolyte: species chemical stability and solubility limit. Even in highly alkaline environments, aqueous polysulfide is found to only exhibit chemical stability in the confined range of oxidation states between S₄²⁻ and S₂²⁻, corresponding to a quarter of the theoretical sulfur capacity. On the other hand, by allowing reversible precipitation and dissolution during cycling, the effective solubility limits can be increased. Aqueous polysulfide electrolytes cycled beyond the solubility limit are extensively examined in order to understand chemical/electrochemical stability as well as the nucleation and growth mechanism during sodium polysulfide deposition.
The last portion of this thesis focuses on improving reaction kinetics by altering the electrode design. To address the sluggish kinetics, a percolating conductive nano-network was introduced by suspending carbon nanoparticles in the polysulfide electrolyte. Such networks improve reaction kinetics by providing high surface area for reaction. Moreover, nickel sulfide, as an exemplary electrocatalyst, was introduced into the suspension electrode in the form of nickelcoated carbon. This modification has the effect of nearly eliminating the nucleation barrier for sodium polysulfide deposition. | |
