(352a) Rethinking Grid-Level Energy Storage with Minimal Architecture Zinc-Bromine Batteries

One of the major factors limiting a deeper market penetration of intermittent, renewable energy sources (i.e. wind and solar) is a lack of reliable, cost-effective energy storage technologies. Of the existing technologies, electrochemical energy storage in the form of batteries is a promising candidate due to high efficiencies (up to 95%) and modular designs that can be easily installed next to generation sites. Despite this important application for batteries, significant research efforts still focus on maximizing energy and power density for use in portable applications like consumer electronics and automobiles. One major exception are redox flow batteries, which are designed for grid-level applications, where low cost and long lifetime are the key design criteria. Flow batteries are slowly entering the market; however, in most cases, their cost is still prohibitive. The prohibitive cost is largely due to the use of expensive passive components like membranes and pumps needed to manage the flowing, reacting fluid. To address this issue, our lab has recently developed a minimal architecture zinc-bromine battery (MA-ZBB), which utilizes the same chemistry from commercial zinc-bromine flow batteries, but eliminates the need for any passive components, significantly reducing the cost (projection of < $90/kWh cell cost) [1].

The MA-ZBB is composed of two carbon based electrodes, which are spatially separated in the vertical direction, in an aqueous electrolyte containing zinc-bromide salt. At the negative electrode on top of the cell, zinc ions are reduced (Zn) and oxidized (Zn2+) on a carbon cloth during charge and discharge, respectively. At the positive electrode, which is located on the bottom of the cell, bromide ions are oxidized (Br2) and reduced (Br-) within a carbon foam electrode. The electrodes are vertically separated to utilize the variations in density between bromine and the aqueous electrolyte as an advantage to prevent bromine, which may escape the carbon foam electrode, from reaching with the zinc electrode. This design is key for limiting self-discharge associated with the unwanted chemical reaction of bromine with zinc.

To date, we have demonstrated an energy efficiency of over 60% for over 1000 cycles at lab scale. In this work, we present recent efforts at characterizing the efficiency losses in the system to identify methods for improving the electrochemical performance. Galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) are utilized to determine the relative importance of ohmic resistance, charge transfer (reaction) resistance, and mass transport limitations on the performance of the lab-scale cell. In addition, variations in cell and electrode design are investigated to maximize the capacity and energy efficiency of the system. Finally, insights from these studies are used to develop a prototype MA-ZBB cell with improved performance (>75% energy efficiency) that is used to study the technology’s viability for grid-level energy storage applications.

[1] Shaurjo Biswas, Aoi Senju, Robert Mohr, Thomas Hodson, Nivetha Karthikeyan, Kevin. W. Knehr, Andrew G. Hsieh, Xiaofang Yang, Bruce E. Koel, Daniel A. Steingart, Energy Environ. Sci., 10 (2017) 114.