Yu, Hao2025-01-202025-01-202025-01-202025-01-16https://hdl.handle.net/10012/21391The transformation from energy based on fossil fuels to that based on sustainable options such as wind, solar and hydroelectric sources is crucial to reduce air/water pollution and carbon emissions. However, the production of electricity from these sustainable sources is typically intermittent in nature and can perturb the stability of the existing power grid. Redox flow batteries (RFB) have emerged as promising devices for grid-scale energy storage to stabilize power systems and improve their efficiency. Among the different types of RFBs, zinc- and cerium-based RFBs are promising for large-scale applications that require high output power density due to their low cost and high cell voltage. Motivated by its potential for future applications, this work focuses on the performance improvement of Zn-Ce RFBs through both experimental and modeling studies. Many of the findings and general ideas for RFB performance improvement are also applicable to other RFB systems and to commercial scale RFBs in real-life scenarios. In this work, the effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce RFB has been studied. The effectiveness of mixed methanesulfonic/sulfuric acid, mixed methanesulfonic/nitric acid and pure methanesulfonic acid has been assessed and compared. The Ce(III)/Ce(IV) reaction exhibits faster kinetics and the battery exhibits higher coulombic efficiency in the mixed 2 mol/L MSA-0.5 mol/L H2SO4 electrolyte compared to that achieved in the commonly used 4 mol/L MSA electrolyte due to lower H+ crossover and higher Ce(IV) solubility. The rate of the fade in coulombic efficiency in the mixed MSA-H2SO4 electrolyte is 0.55% per cycle over 40 charge-discharge cycles, while the fade rate is 1.26% in the case of 4 mol/L MSA. Furthermore, the positive electrode reaction is no longer the limiting half-cell reaction even at the end of long-term battery charge-discharge operation. The effect of ion crossover on the overall Zn-Ce RFB performance has also been investigated through the measurement of the Zn(II), Ce(III), Ce(IV) and H+ concentrations on both sides of a Nafion 117 membrane during charge-discharge cycles. As much as 36% of the initial Zn(II) ions transfer from the negative to the positive electrolyte and 42.5% of the H+ in the positive electrolyte has crossed over to the negative side after 30 charge-discharge cycles. Both of these phenomena contribute to the steady fade in battery performance over the course of operation. Based on these findings, experiments aimed at reducing the concentration gradient driving crossover by intentionally adding different amounts of Zn(II) to the positive electrolyte at the outset of operation have been conducted. This approach has been shown to reduce the crossover of Zn(II) from the negative side to the positive side, improve both the battery coulombic and voltage efficiencies and reduce the decay of battery performance. Since the ion crossover phenomena is very commonly observed, this strategy to improve battery overall performance and reduce ions crossover by minimizing concentration gradient is not only applicable to similar lab-scale RFB research, but also beneficial for real-life RFB applications. Since the positive electrode reaction becomes the limiting half-cell reaction during the course of battery operation, two strategies have been investigated to regenerate the positive electrolyte by converting the accumulated Ce(IV) ions back to Ce(III) ions. The first strategy which utilizes RuO2 as a catalyst for Ce(IV) reduction improves the voltage efficiency from 71.1% to 77.8% over 16 cycles but reduces the coulombic efficiency from 74.1% to 57.8% due to the leakage of RuO2 catalyst through the porous filter into the positive electrolyte. The method utilizing H2O2 to regenerate the positive electrolyte improves the average coulombic efficiency from 63.7% to 68.3% and the average voltage efficiency from 56.8% to 76.1% over 30 cycles. Similar battery performance and life-cycle improvement can also be expected if these electrolyte regeneration methods are applied on a commercial scale. Furthermore, the implementation of these regeneration methods should also reduce the overall operating costs since it will reduce the frequency with which electrolytes have to be replaced. Finally, a transient 2-D model for the Zn-Ce RFB that accounts for the crossover of different electroactive species through the membrane has been developed. All three modes of transport (migration, diffusion and convection) coupled with electrode kinetics of Zn/Zn(II) and Ce(III)/Ce(IV) redox couples as well as HER and OER side reactions are included in the model. This model has been successfully validated against measurements of the evolution of the cell voltage, negative and positive electrode potentials and ion crossover during the course of 5 charge-discharge carried out in our laboratory. The validated model is then used to simulate the battery behaviour when operated under various operating conditions and using positive electrodes with different geometries. The results obtained provide useful information for the future design of Zn- or Ce-based RFBs with the aim of further improving their performance.enenergy storageredox flow batterymixed-acid electrolytebattery modelingDoctoral Thesis