Redox flow batteries (RFBs) have a great potential for grid-scale renewable energy storage because of their striking features, including site independence, excellent scalability, high efficiency, and long lifetime. However, the commercialization of this technology is hindered by the low power density and high capital cost. The electrode is a key component that largely determines the battery performance and system cost, as it not only provides active surface area for redox reactions but also offers pathways for electron, ion, and mass transport. Unfortunately, conventional carbon electrodes with fixed geometric structures leave limited room for simultaneously maximizing the specific area and hydraulic permeability, posing a great challenge in boosting the performance and reducing the cos...[ Read more ]

Redox flow batteries (RFBs) have a great potential for grid-scale renewable energy storage because of their striking features, including site independence, excellent scalability, high efficiency, and long lifetime. However, the commercialization of this technology is hindered by the low power density and high capital cost. The electrode is a key component that largely determines the battery performance and system cost, as it not only provides active surface area for redox reactions but also offers pathways for electron, ion, and mass transport. Unfortunately, conventional carbon electrodes with fixed geometric structures leave limited room for simultaneously maximizing the specific area and hydraulic permeability, posing a great challenge in boosting the performance and reducing the cost. The primary objective of the thesis is to tackle this fundamental challenge via bottom-up design and fabrication of high-performance electrodes for RFBs.

We begin with addressing the low permeability of the conventional electrospun carbon electrodes by synthesizing self-assembled electrospun fiber bundles. Instead of forming single nanoscale fibers, individual fibers are self-assembled into fiber bundles with a diameter reaching several microns by properly managing the viscosity of the precursor solution, resulting in a substantial increase of hydraulic permeability. Consequently, a vanadium redox flow battery (VRFB) with electrospun carbon fiber bundles shows a 15.2% increase in energy efficiency than that with conventional electrospun carbon fibers at 100 mA cm^-2. To increase the reactive surface area of the electrode, we further develop a dual-diameter electrode formed with ~1 and 10 μm fibers. The large fibers create large macropores for the electrolyte to transport, while small fibers offer plenty of active surface areas. As a result, the dual-diameter electrodes enable a VRFB to achieve an energy efficiency of 74.2% at 200 mA cm^-2. To boost the utilization of carbon fiber, a highly porous microscale electrospun fiber electrode is fabricated by rationally designing the precursor ingredients and adjusting the electrospinning conditions. The as-synthesized electrodes with porous micro-scale fibers offer abundant reactive sites without sacrificing the hydraulic permeability, thereby allowing the VRFB to achieve an energy efficiency of 79.1% at 400 mA cm^-2.

To further enhance the permeability, we develop ordered electrodes with uniaxially-aligned electrospun carbon fibers (AECFs), which enables a much more uniform in-plane distribution of reactants and current when placing the aligned fibers vertical to the directions of flow channels. As a result, the VRFB exhibits an energy efficiency of 84.4% at 100 mA cm^-2, which is 13.2% higher than that with conventional electrospun fiber electrodes. Based on the highly permeable AECFs, we further develop a composite electrode formed with porous electrospun carbon nanofibers dispersed within an aligned fiber bundle skeleton. Owing to the increase of surface area, the VRFB with the composite electrodes is able to achieve an energy efficiency of 79.3% at the current density of 400 mA cm^-2. To improve the integration of nanofiber and the AECF skeleton, carbon nanofibers (CNFs) are in-situ grown on the AECFs, forming CNF/AECF electrodes. The CNF/AECF electrodes enable the VRFB to achieve an energy efficiency of 80.1% at 300 mA cm^-2. Finally, a holey-aligned hierarchical structure is fabricated with an in-situ zeolitic imidazolate framework-8 (ZIF-8)-assisted etching method. The nanoscale holes on the aligned fiber surface render a large surface area for redox reactions, while the large pores between adjacent aligned fibers enable a small electrolyte transport resistance. Remarkably, the holey-aligned fiber electrodes enable the VRFB to achieve an energy efficiency of 79.3% at 400 mA cm^-2 and a peak power density of 1.6 W cm^-2, representing one of the highest performance in the open literature.

Keywords: Redox flow battery; electrodes; electrospun carbon fiber; specific surface area; hydraulic permeability; battery performance.