Abstract
Redox flow batteries (RFBs) have considerable promise in addressing the ever-increasing demands for robust energy storage devices. The nature of these devices makes it possible to create storage with potential applicability at multiple size scales1. However, conventional RFBs require energy to pump the liquid electrolyte, reducing the overall system efficiency, and limiting its viability to mid- and large-scale operations only2. Eliminating the pumping load can improve overall efficiency and create opportunities for mobile applications. In this work, we demonstrate a 3D printed prototype of a pumpless non-aqueous organic RFB that induces flow in the electrolyte from sinusoidal rocking motion. The body of the flow cell was created using 3D printing, allowing us to modify and adapt geometry easily during testing.First, the compatibility of the 3D-printed material was studied by testing the chemical stability of widely available additive manufacturing materials under RFB conditions. Then, we evaluated the performance of this pumpless system using an easily scalable, soluble, and stable one electron donor phenothiazine derivative, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT) which has been widely studied for RFB applications3. We used 0.25 M MEEPT and its bis(trifluoromethanesulfonyl)imide radical cation salt (MEEPT-TFSI) as a redox-active couple in 0.5 M tetraethylammonium bis(trifluoromethanesulfonyl)imide (TEATFSI)/acetonitrile (MeCN). Our results suggest an inversely proportional empirical relationship between limiting current density and the frequency of sinusoidal motion due to varying mass transfer rates. After 100 cycles in this symmetric cell with an area specific resistance of 2.62 Ωcm2, we achieved a capacity retention of 63% and an average Coulombic efficiency of 96%, with minimal sign of decomposition of the redox couple when analyzed post-cell cycling.The results of this study can be used for 3D printing material selection in numerous electrochemical devices that use organic solvents, including flow cells, supercapacitors, and lithium-ion batteries. Using sinusoidal motion to drive electrolytes instead of a pump can increase the theoretical efficiency and decrease the overall size of a flow cell. This makes it attractive in wearable applications where biomechanical motion can be harnessed.1 Wang, W. et al. Recent Progress in Redox Flow Battery Research and Development. Advanced Functional Materials 23, 970-986 (2013). https://doi.org:10.1002/adfm.2012006942 Tian, C. H., Chein, R., Hsueh, K. L., Wu, C. H. & Tsau, F. H. Design and modeling of electrolyte pumping power reduction in redox flow cells. Rare Metals 30, 16-21 (2011). https://doi.org:10.1007/s12598-011-0229-13 Milshtein, J. D. et al. High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries. Energy & Environmental Science 9, 3531-3543 (2016). https://doi.org:10.1039/C6EE02027E
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