Polybenzimidazole Membrane Design Principles for Vanadium Redox Flow Batteries
Energy storage technologies with long storage duration are essential to stabilize electricity grids with a high share of intermittent renewable power. In a redox flow battery, the electrochemical conversion unit, where the charging and discharging reaction takes place, is spatially separated from the energy storage medium. In the all-vanadium redox flow battery (VRFB), a sulfuric acid aqueous electrolyte with dissolved vanadium ions is used as the storage medium. Vanadium is present in 4 different oxidation states, the redox couple vanadium(II) and (III) on the negative side of the cell, and vanadium(IV) and (V) on the positive side. This allows the battery to be repeatedly charged and discharged. A separator or membrane is used between the negative and positive electrode, which should selectively conduct the ions of the supporting electrolyte and minimize the passage of vanadium ions. Fluorinated membranes, such as Nafion™, are often used for this key component, but these ionomers were not originally developed for this application and therefore have functional shortcomings. Furthermore, the production and use of fluorinated materials is to be severely restricted or even banned in Europe. Therefore, the development of hydrocarbon-based membranes for the VRFB is of great importance. The study reported here focuses on polybenzimidazole polymers and membranes, which could be a promising materials class for next generation flow batteries.
Our society is poised to make the Energy Transition happen from the fossil era towards a society based on renewable primary energies. Deep decarbonization scenarios are centered around power-to-X technologies using renewable electricity. To integrate large shares of inherently intermittent power generation technologies, in particular solar and wind power, energy storage technologies are needed for a continuous and dispatchable electricity output. The all-vanadium redox flow battery (VRFB) is a promising grid-scale energy storage technology owing to its unique architecture that enables the decoupling of energy capacity from power output. Currently, VRFBs commonly employ polymer electrolytes containing a perfluorinated backbone, such as the perfluoroalkylsulfonic acid (PFSA) membranes, such as Nafion™, which contribute significantly to the cost of the VRFB stack. An alternative to PFSA membranes is proposed in the fluorine-free polybenzimidazole (PBI) membranes. PBI is well known for its excellent mechanical and chemical properties and has found use in several applications ranging from fire-resistant clothing to high-temperature fuel cells. However, the ionic conductivity of PBI in VRFBs is only a fraction of that of PFSAs, requiring further research and development for a viable membrane technology.
The studies featured in this highlight provide design principles for PBI membranes that can be used in the search for next generation membranes for VRFBs. The developed principles include strategies to reduce the resistive barrier of the PBI membrane. The first strategy involves alkaline pretreatments that allow for tailored membrane properties by adjusting its swollen morphology. Several alkaline pretreatments were explored, ranging from solutions with a base concentration of 2 M to 4 M and bases ranging from lithium hydroxide to cesium hydroxide. The use of 4 M concentrations was shown to be most effective, with the electrolyte volume fraction within the polymer electrolyte increasing with increasing size of the alkali cation. As a result, an optimum could be obtained for PBI membranes treated in a mixture of sodium and potassium hydroxide.
In the second strategy, the changes in properties upon the introduction of pendant groups to the benzimidazole core are explored. The introduction of a small alkyl group enhanced the conductivity of the polymer electrolyte, thereby improving the energy efficiency of the VRFB without negatively affecting its capacity retention. In contrast, the introduction of a benzyl group increased the affinity between polymer chains, decreasing its ionic conductivity and performance in the VRFB. Another approach is the complete methylation of the benzimidazole backbone, thereby forming a polybenzimidazolium membrane. This class of materials showed a substantially higher ionic conductivity than pristine meta-PBI, resulting in a significantly higher energy efficiency compared to the commercial standard Nafion™ NR212, while at the same time showing improved capacity retention (see Figure).
The findings in this study introduce a toolbox that can be used in the search of tailor-made membranes for other types of flow batteries, such as aqueous organic redox flow batteries, based instead of vanadium on organic redox couples that could be produced at low cost and high volume.