Redox flow batteries are widely investigated toward cost-effective storage of energy generated via intermittent renewable sources. Many redox chemistries have been proposed for flow batteries, possessing various attractive features such as low-cost reactants, fast electrochemical reaction kinetics without precious metal catalysts, negligible thermal runaway risk, and low toxicity. While all flow batteries rely on heterogeneous electrochemical reactions occurring at electrode surfaces, in a subset of chemistries homogeneous chemical reactions occur in the electrolyte. A prominent example are batteries employing halogen-based catholytes, where halogen molecules complex with halide ions in the catholyte, forming redox-active polyhalide ions. However, state-of-the-art models capturing flow battery performance for halogen systems typically neglect the presence of such homogeneous reactions and polyhalide ions. The latter assumption allows for simpler models, but at the cost of accurately predicting battery chemical state and performance. We here present a generalized flow battery theory extended to include fast homogeneous reactions, which employs a technique known as the method of families to simplify the governing equations. We then apply and solve the model for the specific case of a membraneless hydrogen-bromine flow battery, illustrating the predicted effect of the homogeneous complexation reaction in the catholyte on flow battery performance.
Redox flow batteries (RFBs) are an emerging electrochemical technology envisioned towards storage of renewable energy. A promising sub-class of RFBs utilizes single-flow membraneless architectures in an effort to minimize system cost and complexity. To support multiple functions, including reactant separation and fast reactant transport to electrode surfaces, electrolyte flow must be carefully designed and optimized. In this work, we propose adding a secondary channel adjacent to a permeable battery electrode, solving for the flow field and analysing the effects on the reactant concentration boundary layer at the electrode. We find that an adjacent channel with gradually changing thickness leads to a desired nearly uniform flow through the electrode to the adjacent channel. Consequently, the thickness of the concentration boundary layer is significantly reduced, increasing reactant transport to the electrode surface to 140% of the rate of a battery with a constant width adjacent channel, and 350% of the rate with no adjacent channel. Overall, this theory provides insight into the important role of flow physics for this promising sub-class of flow batteries, and can pave the way to improved energy efficiency of such flow batteries.
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