Energy storage is a critical enabler for large-scale integration of renewable energy onto the electric grid, as well as to improve grid efficiency, reliability, the distributed siting of resources that can help with transmission and distribution, and other applications. Flow batteries have the potential to provide the high energy, durability, and scalability required of grid-scale energy storage systems at an affordable price. A flow battery consists of two tanks of liquids that are pumped past a membrane held between two electrodes. Electrochemical reactions in the flow battery enable it to provide or store electricity, and ion transfer through a membrane is typically a key part of these reactions. However, current membranes do not have extremely high selectivity; over time they allow the undesired movement of liquid reactants from one side of the battery to the other. Advanced membranes and new liquid reactants could prevent this “crossover,” thus opening pathways to increase battery efficiency and enable operation on a daily basis for fifteen years with little degradation.
Project Innovation + Advantages:
The University of Colorado, Boulder (CU-Boulder) will develop a new type of anion-exchange membrane for chloride (Cl-) transport that is based on a nanoporous lyotropic liquid crystal structure that minimizes cation crossover by molecular size-exclusion and charge exclusion. Due to a lack of suitable Cl- conducting membranes, flow batteries often use microporous membranes or cation-exchange membranes (CEM) to separate the two electrode chambers. Microporous membranes are inexpensive, but do not provide perfect barriers to intermixing of the reactants (or “crossover”) that reduces the battery’s efficiency and, in some cases, damages critical components. In contrast, CEMs such as Nafion provide better isolation but are far more expensive, and also permit the migration of water and protons which can change the pH (acidity) and lead to inefficiencies and undesired side reactions in the battery. This project aims to develop a low-cost separator that eliminates crossover in all-iron flow batteries. The membrane allows for ion transport via nanochannels, which are engineered to have sizes below those of common hydrated cations, thus exhibiting perfect cation rejection. In the all-iron battery, key benefits of reduced crossover include increased roundtrip efficiency as well as the reduction of pH swings and water transport, and hence the reduction or elimination of the rebalancing stacks and system management schemes. In the future, the membrane developed in this project could also be used with other lower cost redox couples, including those using two different elements as active species. The low cost, increased efficiency, and long lifetime of these membranes have the potential to significantly increase the economic viability of flow batteries.
If successful, developments made under the IONICS program will significantly reduce battery storage system costs for the grid to about $150/kWh (for a 5-hour discharge time, on a fully installed basis), a cost point that transforms the grid by enabling cost-effective electrical energy storage.
IONICS program innovations could contribute to energy storage solutions for the grid, improving grid resilience by providing widespread electrical storage, a basic capability the grid has largely done without since its creation over one hundred years ago.
Greater integration of renewable resources into the power mix, enabled by improved energy storage, will reduce the need for other more carbon-intensive forms of electricity generation. In addition, energy storage can improve the efficiency of the grid by allowing greater use of the most efficient, cost-effective generators.
IONICS program innovations could further establish U.S. businesses as technical leaders in energy storage, encouraging greater use of readily available renewable energy and helping to reduce costs on the grid.