ABSTRACT
Aqueous redox flow battery systems that use a zinc negative electrode have a relatively high energy density. However, high current densities can lead to zinc dendrite growth and electrode polarization, which limit the battery's high power density and cyclability. In this study, a perforated copper foil with a high electrical conductivity was used on the negative side, combined with an electrocatalyst on the positive electrode in a zinc iodide flow battery. A significant improvement in the energy efficiency (ca. 10% vs using graphite felt on both sides) and cycling stability at a high current density of 40 mA cm-2 was observed. A long cycling stability with a high areal capacity of 222 mA h cm-2 is obtained in this study, which is the highest reported areal capacity for zinc-iodide aqueous flow batteries operating at high current density, in comparison to previous studies. Additionally, the use of a perforated copper foil anode in combination with a novel flow mode was discovered to achieve consistent cycling at exceedingly high current densities of >100 mA cm-2. In situ and ex situ characterization techniques, including in situ atomic force microscopy coupled with in situ optical microscopy and X-ray diffraction, are applied to clarify the relationship between zinc deposition morphology on the perforated copper foil and battery performance in two different flow field conditions. With a portion of the flow going through the perforations, a significantly more uniform and compact zinc deposition was observed compared to the case where all of the flow passed over the surface of the electrode. Results from modeling and simulation support the conclusion that the flow of a fraction of electrolyte through the electrode enhances mass transport, enabling a more compact deposit.
ABSTRACT
Among the aqueous redox flow battery systems, redox chemistries using a zinc negative electrode have a relatively high energy density, but the potential of achieving high power density and long cycle life is hindered by dendrite growth at the anode. In this study, a new cell design with a narrow gap between electrode and membrane was applied in a zinc-iodide flow battery. In this design, some of the electrolyte flows over the electrode surface and a fraction of the flow passes through the porous felt electrode in the direction of current flow. The flow battery was tested under constant current density over 40 cycles, and the efficiency, discharge energy density, and power density of the battery were significantly improved compared to conventional flow field designs. The power density obtained in this study is one of the highest power densities reported for the zinc-iodide battery. The morphology of the zinc deposition was studied using scanning electron microscopy and optical profilometry. It was found that the flow through the electrode led to a thinner zinc deposit with lower roughness on the surface of the electrode, in comparison to the case where there was no flow through the electrode. In addition, inhibition of dendrite formation enabled operation at a higher range of current density. Ex situ tomographic measurements were used to image the zinc deposited on the surface and inside the porous felt. Volume rendering of graphite felt from X-ray computed tomography images showed that in the presence of flow through the electrode, more zinc deposition occurred inside the porous felt, resulting in a compact and thinner surface deposit, which may enable higher battery capacity and improved performance.
