The cell uses redox-active species in fluid (liquid or gas) media. Redox flow batteries are rechargeable () cells. Because they employ rather than or they are more similar to fuel cells than to conventional batteries. The main reason fuel cells are not considered to be batteries, is because originally (in the 1800s) fuel cells emerged as a means to produce electricity directly from fuels (and air) via a non-comb.
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Other flow-type batteries include the , the , and the . A membraneless battery relies on in which two liquids are pumped through a channel, where they undergo electrochemical reactions to store or release energy. The solutions pass in parallel, with little mixing. The flow naturally separates the liquids, without requiring a membrane.
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