A Cyclable Laminar Flow Battery for Large Scale Energy Storage
Authors: Suss Matthew , Braff William , Buie Cullen R. , Bazant Martin Z. 
Published: February 2020

Electrochemical systems, such as flow batteries, have the potential to enable cost-effective and environmentally friendly large-scale energy storage [1]. Membraneless laminar flow batteries leverage the laminar flow of co-flowing fluids to prevent reactant crossover [2,3,4]. These latter batteries are a promising class of electrochemical large-scale energy storage systems, as they do not require the use of ion exchange membranes, typically the single most expensive component of the flow battery stack [5]. Further, the power density of flow batteries can be limited by the presence of a membrane. For example, in hydrogen-bromine flow batteries, operating at hydrobromic acid concentration of order 1 M (desirable for high liquid-phase conductivity) can cause the membrane to dehumidify and increase membrane resistivity [6]. Laminar flow batteries can eliminate this issue, but still significant challenges remain in their practical implementation. One major challenge is the demonstration of high efficiency closed-loop cycling (charging and discharging) of laminar flow batteries. This cycling relies on maintaining pure (unmixed) anolyte and catholyte fluid streams to prevent crossover reactions in subsequent cycles. However, the mixing layer necessarily developed between co-flowing fluids in laminar flow batteries prevents the extraction of pure fluid streams downstream of the battery [2,3,7]. To our knowledge, the highest reported number of closed-loop cycles attained in a laminar flow battery is a single cycle at 20% energy efficiency, and with a maximum power of about 0.3 W/cm2[8].
We here describe our work in the design and development of a unique prototype laminar flow battery. Unlike previous laminar flow batteries, our device is designed for closed-loop cyclability using innovative means of controlling stream mixing within porous media. This is achieved through two novel mechanisms: i) the use of a porous "dispersion blocker" layer to prevent rapid mixing within the porous structure via transverse mechanical dispersion, and ii) a two-dimensional flow field, including a flow component in the direction of the electric field (in addition to the typical flow which is perpendicular to the electric field), to inhibit oxidant crossover. Through the use of hydrogen-bromine chemistry and flow-through porous electrodes, we demonstrate that our battery can achieve an exceptionally high maximum power density of up to 0.66 W/cm2 in addition to, for the first time in a laminar flow battery, multiple closed loop cycles.

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Figure 1: Schematic of the cyclable laminar flow battery using hydrogen-bromine chemistry. Undesirable bromine (oxidant) flux into the electrolyte channel is prevented through use of a dispersion blocker layer and 2D flow (blue arrows). Insets show numerical results of co-flowing fluids within porous media at high Peclet number, a) without a dispersion blocker layer, and b) with a dispersion blocker. The dispersion blocker can strongly inhibit mixing of co-flowing streams within porous structures of a flow battery.