Batteries have gained large interest in past few decades as energy storage systems because their merits such as relatively high efficiency, good durability of battery and unique power and energy output design. There are many types of batteries which can be used as reversible, or secondary, energy storage systems like redox flow batteries or metal-air batteries. The hybrid of those two types of batteries which is the Zinc slurry air flow battery uses zinc particles suspended in highly alkaline solution as the electrolyte and electrode for the negative compartment, whereas air is flowing in and out of the positive compartment for the oxygen reaction. As this is a relatively new concept of battery, there are two important factors which needs to be investigated. First, the discharge performance of the battery is the primary problem to be solved and the second challenge is the rechargeability of the battery to make it a secondary battery. In order to achieve those two goals, the bipolar plates are one of the key components
to be studied in redox flow batteries as they require not only a good electrical conductivity, but also good mechanical durability with high corrosion resistance. Furthermore, this component is also important as the electrolyte flow can be improved by carving a flow field on the bipolar plate.
Hence, this study aims first to improve the discharge performance of the Zinc Slurry Air Flow Battery. To do this, several types of flow field designs and material compositions have been tested as they play an important role in the performance of the redox flow battery, especially when using highly viscous liquids. To enhance the discharge power density of zinc slurry air flow batteries, an optimum slurry distribution in the cell is key. Hence, several types of flow fields (serpentine, parallel, plastic flow frames) were tested in this study to improve the discharge power density of the battery. The serpentine flow field delivered a power density of 55 mW·cm−2, while parallel and flow frame resulted in 30 mW·cm−2 and 10 mW·cm−2, respectively. Moreover, when the anode bipolar plate material was changed from graphite to copper, the power density of the flow frame increased to 65 mW·cm−2, and further improvement was attained when the bipolar plate material was further changed to copper–nickel. These results show the potential to increase the power density of slurry-based flow batteries by flow field optimization and design of bipolar plate materials.
The second aim of this work is to improve the rechargeability of the battery. In the last section of this study, carbon additives were introduced to achieve a rechargeable zinc slurry flow battery by minimizing the zinc plating on the bipolar plate that occurs during charging. When no carbon additive was present in the zinc slurry, the discharge current density was 24mA·cm−2 at 0.6 V, while the use of carbon additives increased it to up to 38 mA·cm−2. The maximum power density was also increased from 16 mW·cm−2 to 23 mW·cm−2. Moreover, the amount of zinc plated on the bipolar plate during charging decreased with increasing carbon content in the slurry. A rheological investigation revealed that the elastic modulus and yield stress are directly proportional to the carbon content in the slurry, which is beneficial for redox flow battery applications, but comes at the expense of an increase in viscosity (two-fold increase at 100s−1).
These results show how the use of conductive additives can enhance the energy density of slurry-based flow batteries
