New Approaches Toward High-power-density Redox Flow Battery


Student thesis: Doctoral Thesis

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Award date12 Aug 2022


The vision of sustainable development is to boost the utilization of renewable energies, which in turn relies on the development of energy storage technologies due to their inherent features of intermittence and capriciousness. Reliable utility-scale energy storage technologies are critical to integrating renewable energy into the grid or in-situ storage. Redox flow battery (RFB) stands out due to the advantages of flexibility, safety and long cycling lifespan. However, high capital cost resulting from limited power and energy densities prevents it from wide deployment, where power density remains the primary challenge for stationary batteries. Nevertheless, there is a trade-off between power and energy densities in practical application to meet diverse end users.

Among various RFBs, only all-vanadium (VRFB) has been successfully commercialized owing to its ultra-long cycling life. However, the high cost of the battery stacks due to low power density hampers its market competitiveness. Intensive efforts have been devoted to elevating the power density of VRFB by developing advanced electrodes. Therefore, many approaches have been proposed for electrode modification, however, most of them require tedious processing that is rather laborious and harsh leading to poor scalability. In this thesis, a simple approach that is compatible with industry-scale production is developed to deposit MoO3 on the graphite felt electrode to confer high catalytic activity toward vanadium redox reaction, thus boosting the operating current density from 150 mA cm-2 (pristine electrode) to 250 mA cm-2 with a significant improvement in energy efficiency. The MoO3 deposited graphite felt electrode is promising for the practical application of high-power-density VRFB due to the scalability of the approach and excellent catalytic activity of MoO3 toward vanadium reactions.

Nevertheless, the power density of VRFB is limited by its low cell voltage. Zinc-cerium RFB possesses the highest cell voltage among aqueous batteries, however, it suffers from unacceptable charge losses and unstable cycling performance due to the irreversible hydrogen side reaction, which stems from the incompatibility of the Zn and Ce electrolytes. Therefore, a dual-membrane configuration is designed for Zn-Ce RFB, which decouples incompatible species, such as H+/Zn, to harmonize Zn and Ce electrolytes. In addition, an advanced Zn electrolyte is developed, which facilitates reversible Zn plating/stripping with high resistance toward HER side reaction. Further, the ion transportation mechanism and an electric field regulation strategy are experimentally investigated and verified theoretically, which proves the electric field regulation strategy can effectively decrease the cell polarization avoiding voltage loss. The synergy of cell engineering and electrolyte engineering enables the cell to achieve unprecedently high efficiency and stable cycling performance. Moreover, the developed Zn-Ce RFB highlights the merit of balancing the trade-off between power density and discharge energy density.

Although the cerium redox reaction (Ce3+/Ce4+) has the advantage of highly positive redox potential favorable for high-power-density RFB, its sluggish reaction kinetics and the highly oxidative cerium electrolyte pose a challenge for carbon-based electrodes. Therefore, we propose a catalyst design strategy to address the poor activity and stability of graphite felt electrode, where a compact and dense NiMoO4 nanorods coating is synthesized serving as the active interface between electrolyte and graphite felt with abundant active sites and extended electrolyte flow path, and providing protection to the carbon substrate by reducing direct exposure of carbon fibers to the oxidative electrolyte. The graphite felt electrode provides 3D porous framework and is responsible for conducting electrons and transporting electrolytes. A lab-scale vanadium-cerium flow cell is assembled to characterize the NiMoO4 deposited graphite felt, in which the power density is elevated to >1027 mW cm-2 benefiting from the increased cell voltage and reduced cell polarization. Further, the developed electrode shows good stability in cerium electrolyte owing to the dense morphology of NiMoO4 coating.

Overall, this research work pursues approaches based on the strategies of elevating cell voltage and/or reducing cell polarization for high-power-density RFB, providing guidance for the design of next-generation high-power-density RFB. Besides, the studies of electrocatalysts provide insights into the catalytic mechanism and demonstrate different catalyst design principle for different purposes. Moreover, the study of cell structure sheds light on the characteristics of ions transpiration, where new strategies are proposed to reduce the cell polarization, which can be extended to other batteries involving multi-interfaces or ionic conductors.