To ensure that renewable energy succeeds in delivering reliable power to US consumers, the nation needs cost effective and reliable storage at the grid scale. Conventional rechargeable batteries offer a simple and efficient way to store electricity, but development to date has largely focused on transportation systems and smaller systems for portable power or intermittent backup power; metrics relating to size and volume are far less critical for grid storage than in portable or transportation applications. It therefore stands to reason that optimizing battery performance over a different set of variables might result in an implementation that delivers superior performance for reduced cost. Batteries for large-scale grid storage require durability for large numbers of charge/discharge cycles as well as calendar life, high round-trip efficiency, an ability to respond rapidly to changes in load or input, and reasonable capital costs. Redox flow batteries (RFBs) or redox flow cells (RFCs) promise to meet many of these requirements.
As shown in the figure, a key component of RFBs is the ability to separate power and energy. The power is controlled by the stack while the energy is stored within the separated reactants. Thus, one can optimize over a greater range of variables and storage can be increased with relatively ease and minimal cost compared to the stack, which is typically the most expensive system component.
At Berkeley Lab, we are working on various types of flow cells to understand their operation and durability in order to optimize operating conditions and cell components. Thus, the effort focuses on catalyst materials, membrane separators, flowfield bipolar plates, electrolyte composition, and cell assembly. Of particular interest over the last few years, we have focused on the H2/Br2 couple due to its high performance and power density that will allow for less expensive stacks. The power density achieved is almost an order of magnitude greater than traditional RFBs and the performance is very reversible.