Due to increasing environmental concerns, global energy production is shifting from fossil fuels to sustainable and renewable energy systems such as solar and wind. Despite their advantages, these systems have two major weaknesses: volatile energy production and irregular supply. Therefore, they are complemented by energy storage systems (ESS). Lithium-ion batteries are at the forefront of ESS, but they are prone to fires due to flammable electrolytes and lithium-based materials. The flowless zinc-bromine battery (FLZBB), which uses non-flammable electrolytes, is a promising alternative, offering cost-effectiveness and a simple battery platform.
A FLZBB battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator to separate the electrodes. Unlike conventional zinc-bromine batteries, the electrolyte in FLZBB battery does not need to be pumped and is instead held in a gel-like container. Graphite felt (GF) is widely used as an electrode in many redox batteries due to its stability in acidic electrolytes. However, in FLZBB batteries, bromine and polybromide ions are formed in the GF-positive electrode during charging. These active materials can leak out and diffuse uncontrollably to the negative electrode, causing self-discharge, which seriously affects the performance and cycle life. Many studies have explored approaches to suppress this crossover phenomenon, however, self-discharge remains a major problem for FLZBB batteries.
To address this problem, a team of researchers led by Professor Chanho Pak and including Youngin Cho, a master’s and doctoral student (first author) from the Graduate School of Energy Convergence, Institute of Integrated Technology, Gwangju Institute of Science and Technology, Korea, developed a novel thick GF electrode coated with nitrogen-doped mesoporous carbon (NMC/GF). Their study was conducted available online April 22, 2024 and published in volume 490 of the Chemical Engineering Journal on June 15, 2024.
The researchers fabricated the NMC/GF electrodes using a simple and cost-effective evaporative self-assembly method. In this method, a pristine GF felt was coated with precursor materials and mixed in a solvent, followed by drying and curing. When applied to a FLZBB, the new electrodes effectively suppressed active material crossover and prevented self-discharge. This success was attributed to the mesopores present on the GF fibers in the NMC/GF electrodes.
Professor Pak explains, “The NMC coating on GF electrodes introduced mesopores with strategically embedded nitrogen sites, which served as a stronghold, capturing bromine and bromine complexes in the positive electrode, suppressing bromine crossover and self-discharge phenomena. In addition, this coating rendered the pristine, initially hydrophobic GF electrodes ultrahydrophilic, improving the interfacial contact with the electrolyte in the aqueous electrolyte and enhancing the electrochemical performance. In addition, it allowed the incorporation of abundant oxygen and nitrogen species, which improved the bromine reaction rates, further increasing the performance.”
The FLZBB with NMC/GF electrodes demonstrated excellent Coulomb and energy efficiencies of 96% and 76%, respectively, at a current density of 20 mA cm-2as well as a high-rate surface capacity of 2 mAh cm-2. In addition, the battery has demonstrated unprecedented durability, with charge/discharge cycle stability extended to more than 10,000 cycles. In addition, the thick GF electrode used can potentially reduce the overall price of the battery.
Highlighting the significance of this achievement, Professor Pak said, “The development of the FLZBB positive electrode, which maintains long-term operation over 10,000 cycles with high efficiencies, will accelerate the development of stable energy storage systems and long-term environmentally friendly energy conversion. In addition, the NMC/GF positive electrode can also be used for other aqueous batteries.”
This revolutionary technology can enable practical applications of FLZBB, leading to safer ESS and more stable renewable energy systems.
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