Electrolyte/Membrane Design and Engineering for Durable Zinc-Iodine Redox Flow Batteries

Integrating renewable energy sources into the electricity grid has caused an essential need for large-scale energy storage systems. To fulfill this purpose, redox flow batteries (RFBs) are considered one of the best options to be employed in medium- to large-scale applications. As novel and rapidly growing RFB technologies, zinc-iodine redox flow batteries (ZIFB) exhibit great potential for high energy density large-scale energy storage. However, their practical use has been limited by their poor stability, low efficiency, and high cost. In addition, capacity fade and elusive operational instability over charge-discharge cycling severely hinder their large-scale commercialization of ZIFBs. This thesis focuses on the design and engineering of electrolytes and membranes for durable and low-cost ZIFBs to pave the way for future electrolyte research in high-energy-density storage systems.
\nIn the first study, we implemented a novel strategy to improve the performance and cyclability of ZIFBs, as well as decrease the chemical cost, by utilizing ammonium-based electrolytes. The designed ammonium chloride supported zinc-iodine redox flow battery (AC-ZIFB) achieved a high energy density of 137 Wh/L, Coulombic efficiency of ~99%, energy efficiency of ~80%, and a cycle-life of 2,500 cycles at an 11-times lower chemical cost than conventional ZIFBs. Such improvements were mainly attributed to the multifunctional roles of cost-effective chemicals utilized in a new decoupled electrolyte design, which mitigates the zinc dendrite formation, facilitates the anodic and cathodic reaction kinetics, and unlocks extra capacity with the primary aid of I₂Cl⁻. The new design empowered the AC-ZIFB with excellent potential as a robust and practical redox flow battery and more broadly demonstrates a facile strategy of using multifunctional electrolyte chemistry to achieve a reliable, high-performance, and cost-competitive energy storage system. However, when the costly perfluorinated Nafion membrane was replaced with low-cost porous membranes, the AC-ZIFBs suffered from capacity fade and elusive operational instability over charge-discharge cycling, which hinders their successful penetration into the market. Thus, the next two studies focus on the design and engineering of AC-ZIFBs with low-cost porous polyolefin membranes. 
\nIn the second study, the capacity fade in AC-ZIFBs with porous polyolefin (PE) membranes was investigated by systematically evaluating electrochemical performance and electrolyte properties. It was found that the differential hydraulic pressure at both sides of the porous membrane leads to colossal electrolyte transport from catholyte to anolyte via convection. Consequently, an accumulation of (poly)iodide at the negative side is established as cycling proceeds, leading to substantial capacity fade of the flow cells. To remediate the capacity fade, an effective strategy was proposed by adjusting electrolyte flow rate ratios to regulate the induced convection by balancing the hydraulic pressure. Theoretical calculations and experimental analysis confirmed that an asymmetric flow rate condition drastically inhibits catholyte transport and (poly)iodide crossover. Therefore, a strategically designed AC-ZIFB with an optimal catholyte to anolyte flow rate ratio of 1 to 7 was able to achieve energy efficiency (EE) of 82% and cycle life of 1,100 cycles at a high current density of 80 mA/cm², which is the highest performance of all the reported ZIFBs. The insight gained into the capacity fade mechanism and the proposed methodology to sustain capacity substantially benefit the commercialization of flow batteries, particularly ZIFBs. 
\nIn the last study, to combat the convection and subsequent capacity decay, new negative electrolyte (anolyte) chemistries with organic compounds, namely urea and glucose were designed to balance the hydraulic pressure, thereby restricting pressure-dependent active ion transfer across the membrane. In this new design, the urea-supported anolyte was able to triple the lifetime of AC-ZIFBs, while the glucose-based design inhibited the large electrolyte transport and prolonged their cycle life by 25 times. Besides the positive impact of organic additives in balancing the hydraulic pressure, the Zn/Zn²⁺ half-cell study and AC-ZIFB full cell study indicated that both additives also could facilitate zinc reaction kinetics and decrease the ionic resistance of flow batteries, thus improve the electrochemical performance. The glucose-supported AC-ZIFBs with 1.5 M glucose additive achieved outstanding Coulombic efficiency of ~95% and energy efficiency of ~78% under the current density of 80 mA/cm², at a cost below 150 US$ kW/h with discharge times of 8 h. Such improvements in the performance are mainly attributed to the remarkable ability of the designed organic additive-supported anolyte to alleviate electrolyte transport and mitigate capacity decay, all with minimal effect on the cost of the battery system. This straightforward yet impactful strategy to balance electrolyte pressure with the aid of electrolyte chemistries could enable an economically viable scale-up of long-lasting ZIFBs.

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