They don't seem to be first off the block. An advanced stage venture (mentioned in the article)..

[ESS] already has a factory pumping out flow batteries just south of Portland, Oregon. Its core product is the Energy Warehouse, which fits a rotund tank and several stacks of battery materials into a shipping container, along with the necessary electronics... During a June visit, I saw a test line in which robots prepared cells to be glued together into the battery stack that the proprietary iron liquid flows through.

https://www.canarymedia.com/articles/ess-is-betting-the-worl...