Swelling-Resistant Membranes for Nonaqueous Redox Flow Batteries

The DOE’s mission to address the US’s energy challenges includes facilitating the development of a reliable, modern grid that is compatible with large percentages of intermittent, renewable power generation. This will require large-scale, long-duration storage. Redox flow batteries RFBs) are the most promising technology for long-duration storage, but their performance, lifetime, and cost are limited by the problem of redox species crossover. Crossover can be prevented by a selective membrane, however current membranes are not selective enough, leading to high operating costs. This problem is particularly acute for nonaqueous RFBs, for which swelling and fouling of currently available ion-exchange membranes has been a major barrier to implementation. Nonaqueous RFBs could offer higher energy densities than aqueous RFBs if the membrane problem can be overcome. Triton will develop membranes for RFBs based on 2D covalent-organic frameworks COFs) with tightly defined pore sizes for blocking redox species crossover. The unique advantages of the COF-based membranes we propose would be 1) total rejection of species with molecular diameters greater than the cut-off pore size and 2) inherent resistance to swelling no matter the temperature, degree of solvation, or identity of the nonaqueous solvent, even when thinned to several nanometers. The overall objective of the Phase I is to demonstrate at the 3 cm × 3 cm scale that our COF-based membranes can reduce crossover in an RFB over many charge/discharge cycles. To accomplish the objectives of the Phase I, we will 1) synthesize a COF material chosen rationally for its resistance to the harsh environment of a nonaqueous RFB, 2) manufacture membranes by a scalable method, and 3) test the membranes under full cell conditions in a nonaqueous RFB with a highly stable, novel catholyte and correspondingly stable, commercial anolyte. If the proof of concept is successful, we will have demonstrated a prototype scale membrane that is far more selective than commercial membranes. As the project moves into the next phase of commercialization, the objective will be to scale up the materials and processes to be suitable for a pilot module e.g. 30 kWh). Once validated at that scale, our technology could start to be a replacement for less selective membranes in full storage installations, with flow battery system companies as the customer. The public will benefit from lower cost per installed kWh storage, which could lead to lower energy prices, emissions reductions, and a growing domestic clean energy economy and associated jobs.