Optimization of High-Energy-Density Aqueous Zinc-Polyiodide Redox Flow Battery
Here, as shown in Fig. 1, we demonstrate that the zinc-polyiodide electrolyte possesses the desired ambipolar and bifunctional characteristics, as well as high solubility and benign nature, thus enabling a high-energy-density aqueous hybrid RFB. Capitalizing on the high solubility of the I-/Ix- redox species, the zinc-polyiodide flow battery (ZIB) has a theoretical energy density of ~322 Wh L-1 at the solubility limit of ZnI2 in the water (4500 g L-1, 7.0 M). We demonstrate here a discharge energy density of 166.7 Wh L-1 with 5.0 M ZnI2 electrolytes, nearly 7 times that of the current aqueous RFBs (VRB: ~25 Wh L-1) and approaching the energy density of low-end LiFePO4 cathode-based Li-ion batteries. The ZIB system is further optimized in terms of membranes and electrodes, enabling it more competitive in energy storage market. Because of the absence of highly oxidative V5+ and Br2, low-cost hydrocarbon cation exchange membranes are applied in the ZIB thus replacing the expensive Nafion membrane, making the ZIB system more attractive from a cost prospective. In addition, the catalysts or modification of current graphite felts at the cathode side can effective increase the kinetics of I-/Ix- redox couples, which enhances the energy efficiency of ZIB cell run at high current density.
The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE) Office of Electricity Delivery and Energy Reliability (OE) (under Contract No. 57558). The NMR work was carried out at the Environmental and Molecular Science Laboratory at Pacific Northwest National Laboratory (PNNL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research (BER). PNNL is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC05-76RL01830.