Abstract
Redox flow batteries (RFBs) are energy storage devices designed for grid-scale application. For next generation RFBs it is desirable to develop low cost materials with low ohmic resistance and high transport selectivity. We present a composite membrane for the vanadium redox flow battery (VRFB) consisting of a composite of a porous polypropylene separator laminated with a thin film of polybenzimidazole (PBI). PBI layers are prepared by solution casting to obtain thicknesses in the range of 0.2 to 10 μm. The ohmic resistance of vanadium electrolyte imbibed PBI is ∼50 mOhm·cm2 per micrometer of film thickness at room temperature. In cell tests, composite membranes show higher coulombic efficiency compared to Nafion® 212. Composite membranes with a PBI layer thickness of 1 μm and below outperform Nafion® 212 in terms of energy efficiency and discharge capacity up to a current density of 250 mA cm−2. With thicker PBI films the ohmic cell resistance is excessively high. Over 100 charge-discharge cycles a higher rate of capacity fading is observed for a composite membrane with 0.7 μm PBI compared to Nafion® 212, which is a result of a more pronounced net electrolyte flux from the negative to the positive electrolyte.
Highlights
Worldwide approximately 50% of all installed commercial flow battery systems are based on the vanadium chemistry
The cost of a porous asymmetric PBI membrane of ∼70 μm thickness for vanadium redox flow battery (VRFB) application has been estimated by Yuan et al to be less than $50/m2.15 Considering the lower areal weight of PBI used in our approach, we estimate that the manufacturing cost of the composite membrane could be in the range of $20/m2
For a target maximum ohmic resistance contributed by the PBI layer that is similar or better than that of Nafion® 212, the film thickness should not exceed ∼2 μm
Summary
Worldwide approximately 50% of all installed commercial flow battery systems are based on the vanadium chemistry. Curing of PBI membranes has been reported in the context of their use in high-temperature polymer electrolyte fuel cells to improve their physico-chemical properties, such as higher mechanical toughness and improved radical-oxidative resistance.[24,25] In the work reported here, different curing times were used for films of different thickness (cf Supplementary Information, Section S1 is available online at stacks.iop.org/JES/167/100502/mmedia).
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