Non-Solvent Induced Phase Separation: A Versatile Synthetic Method for High Performance Redox Flow Battery Electrodes

Abstract

Carbonaceous porous electrodes are ubiquitous to advanced electrochemical systems where they are responsible for multiple critical functions in the cell related to thermodynamics, kinetics, and transport, including providing surfaces for electrochemical reactions, conducting electrons and heat, and distributing fluids. Thus, their design governs the performance, durability, and consequently, the cost of these systems. However, there is limited knowledge about how to deterministically design these electrode materials which forces the repurposing of materials that have not been tailored for the specific application (e.g. the use of polymer electrolyte fuel cell gas diffusion layer as redox flow battery porous electrode1,2). Our current arsenal of materials is limited to fibrous electrodes which are manufactured using various mechanical methods (e.g. paper making, weaving, hydro-entangling) resulting in idiosyncratic structures such as papers, cloths, and felts. These fabrication methods involve multiple complex subprocessing steps impacting the final manufacturing cost and offering limited versatility to control the electrode microstructure and surface composition, which ultimately limits the performance of the electrochemical cell. Thus, there is a need to develop novel material sets with precise control over microstructure and composition while employing synthetic methods that are compatible with large scale manufacturing. In this work, we focus our efforts on designing tailored electrodes for redox flow batteries (RFBs), which are promising for grid-scale energy storage3 if their costs can be significantly reduced4.Here, we introduce the non-solvent induced phase separation (NIPS) as a simple and versatile fabrication method for carbonaceous porous electrodes for redox flow batteries5. Drawing inspiration from membrane technology, the NIPS method has been leveraged to synthesize morphologically-diverse microstructures (e.g., isoporous, macrovoids, porosity gradient) which are appealing to electrode manufacturing6. A polymer solution, containing polyacrylonitrile (PAN, carbon-containing) and polyvinylpyrrolidone (PVP, pore-forming agent) dissolved in N,N-dimethylformamide (solvent) was casted in a mold and subsequently immersed in water (non-solvent). Finally, the polymeric scaffold is carbonized under inert conditions to form a conductive network. Easily adjustable parameters, such as solvent type, polymer concentration and temperature enable control of the final electrode microstructure. To elucidate material synthesis-property-performance relationships, we vary the polymer content, polymer ratio, and solvent type and perform microscopic, spectroscopic, and electrochemical characterization techniques over the synthetized electrodes. Microstructural characterization revealed a multimodal pore size distribution composed of fine, interconnected microvoids (pore diameter 2-15μm) coupled with through plane, finger-like macrovoid channels (throat diameter > 50 μm) forming honeycomb networks. The unique microstructure, not attainable with traditional carbon-fiber manufacturing techniques, enables large surface area at the membrane-electrode interface and fast electrolyte replenishing which reduces mass transfer resistance within the electrode. We performed flow cell tests with an aqueous iron single electrolyte which revealed a considerable reduction of the charge transfer and mass transfer overpotentials of the novel electrodes compared to the commercial baseline at the expense of a slight increase in pressure drop. In the final part, we demonstrate the use of NIPS-electrodes in a full all-vanadium RFB which results in a significant improvement in power density compared to the baseline material, which can be attributed to reductions in the charge transfer and mass transport overpotentials. Although nascent, NIPS emerges as a promising platform to engineer porous electrodes for RFBs and other convection-enhanced electrochemical systems. References Mathias, M. F., Roth, J., Fleming, J. & Lehnert, W. Diffusion media materials and characterisation. Handb. Fuel Cells 3, 517–537 (2010).Forner-Cuenca, A. & Brushett, F. R. Engineering porous electrodes for next-generation redox flow batteries: recent progress and opportunities. Curr. Opin. Electrochem. 18, (2019).Weber, A. Z., Mench, M. M., Meyers, J. P., Ross, P. N., Gostick, J. T. & Liu, Q. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).Darling, R. M., Gallagher, K. G., Kowalski, J. A., Ha, S. & Brushett, F. R. 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