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(9.68) Flowing electrolyte through porous electrodes presents a number of challenges, both at the single-cell and full-stack level. At the pore scale within each electrode there will be significant differences in the interstitial flow rate in each pore owing to size differences, with flow largely confined to the largest pores in the medium. Such pore-scale-channeling behavior provides convective mass transport at a limited number of surfaces, while dead zones of relatively stagnant flow and localized limiting currents would exist elsewhere throughout the electrode. Fibrous materials are the favored porous-electrode substrate for several reasons because high porosity can be achieved while still maintaining electrical conductivity and percolation in the solid phase due to the bridging between long fibers. As discussed above, high porosity is advantageous since i) there is a strong positive correlation between porosity and permeability308, thereby resulting in reduced pressure drop and associated pumping cost; and ii) the effective ionic conductivity of the electrolyte is directly proportional to porosity314 and inversely proportional to tortuosity which tends to increase with decreasing porosity308. Due to the wide spread use of fibrous electrodes for various applications, a number of studies have looked at mass transfer in carbon-fiber electrodes61,62,315,316. Schmal et al. 117 compared mass transfer at single fibers to fiber assemblies (bundles and felts) and found that per unit length of fiber the mass transfer to a single fiber was significantly higher. This was attributed to channeling within the fiber assemblies causing dead-zones or stagnant regions, effectively reducing the active area for reaction. A porous material with very uniform pore-size distribution would help alleviate this problem, but such materials may be impractical. Saleh317 studied the effectiveness factor in packed bed electrodes and found that ohmic resistance, which is a combination of fluid properties and bed geometry, also played a key role in determining the extent to which porous electrode was utilized. Another cell-scale issue arising from the convective flow in porous electrodes is large scale heterogeneities due to assembly tolerances or uneven thermal expansion, which could lead to bypassing of large sections of a cell. Moreover, flow though porous electrodes presents major manifolding issues at the stack-scale since each cell must have nearly identical permeability. This would be difficult to achieve since stacks may be compressed significantly when assembled. This situation is analogous to interdigitated flow fields proposed for low-temperature fuel cells, which showed very promising performance results in single-cell tests, but the inevitable differences in permeability from cell to cell in a stack created uneven flow distribution among cells318. To enhance flow and electrolyte utilization during deep discharge where high flow rates are required, physical barriers or roughened electrode materials can be used inside cell to promote turbulence and mass transport. Lessner et al. designed a flow-through porous electrode for bromine/polysulphide RFBs96. To ensure uniform flow distribution and prevent channeling, quartz particle, with a diameter of 0.5 to 1.0 mm, were placed 0.5 cm above the inlet. 237PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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