A promising nonaqueous redox flow battery electrolyte has been developed by leveraging natural selection to elucidate stable, redox-active molecules.
The side reactions during long-term operation of vanadium redox flow batteries (VRFBs) increase the average oxidation state of the electrolyte and associated equilibrium potential of the positive half-cell. This consequently initiates the corrosion reactions at the positive side as the half-cell potential passes the critical limit. In this study, an ex-situ accelerated electrochemical corrosion protocol is performed on the carbon paper electrode to investigate the effects of corrosion conditions induced by extended cycling on electrode morphology and VRFB system performance. In terms of morphological changes of corroded electrodes, only minor mechanical degradation is observed, including disappearance of binder material and reduced mechanical properties. With regards to the cell performance, the flow cell with the corroded electrode demonstrates notably higher charge and discharge capacities which can be attributed to the enhanced active surface area of the electrode. Furthermore, the corroded case exhibits an improved capacity retention during extended cycling which can be related to the improved redox activity caused by the increased carboxyl groups and wettability. This study shows that even at these fairly aggressive corrosion conditions, the process behaves as a treatment method, which oxidizes surface functional groups, increases active surface area, and hydrophilicity, and subsequently enhances VRFB performance.
Long-term performance and lifetime of vanadium redox flow batteries (VRFBs) are critical metrics in widespread implementation of this technology. One challenging issue that negatively affects these parameters is the faradaic imbalance, which is not comprehensively investigated in the literature. Faradaic imbalance is known as the shift in the average oxidation state (AOS) of the electrolyte due to side reactions. This type of imbalance requires chemical/electrochemical mitigation rather than simple electrolyte remixing. Herein, we investigate faradaic imbalance by preparing unbalanced electrolytes with different AOS values. The performance characteristics of the flow cell utilizing electrolytes with different AOS values are reported. Based on the results of charge-discharge cycling, polarization testing, and electrochemical impedance spectroscopy measurements, faradaic imbalance is found to significantly affect discharge capacity, maximum power density, cell resistances, and efficiency values. While the ratio of discharge capacity to theoretical capacity is 83% for the ideally balanced case (AOS 3.5+), it drops to 53.4% for the AOS 3.9+ case. Additionally, there is a substantial decrease of 44% in the maximum available power density for the most unbalanced case. This noticeable performance degradation highlights the importance of faradaic imbalance as a critical factor which requires further attention especially during extended cycling.
Recently, a significant emphasis has been placed on non-aqueous electrolytes for use in redox flow batteries due to their wider electrochemical potential windows, offering higher energy and power densities. To date, several non-aqueous electrolytes using organic molecules [1-2] and metal-ligand complexes [3] have been evaluated for use as electrolytes in non-aqueous redox flow battery (NRFB) systems. With these efforts, substantial performance enhancements, including a several-fold increase in energy density and improved operating temperature range, are possible compared to state-of-the- art vanadium/sulfuric acid flow batteries; however, NRFB progress has been hampered by poor electrolyte stability [4]. Thus far, development has been limited to systems with short cycle-life that exhibit capacity-fade and low current density. This underscores a key challenge currently limiting the advancement of these technologies – the stability. We employ a bio-inspired approach to address the problem of redox-couple instability that impedes commercialization of NRFB. Our strategy of molecular design is based on naturally occurring chelating molecules that have evolved to bind metal ions extraordinarily tightly and with high-specificity. With this approach we have targeted Amavadin, a vanadium compound found in mushrooms of the Amanita genus. This molecule, and its analog, calcium vanadium(iv)bis-hydroxyiminodiacetate (CVBH) (Figure 1 inset) are among the most stable vanadium chelates ever elucidated. Initial, static-cell investigations have demonstrated that CVBH is stable to exhaustive, deep redox cycling (Figure 1), making it an excellent candidate for implementation in an NRFB system. In this presentation we will demonstrate the performance of such an NRFB system, using this mushroom-based (CVBH) electrolyte. Results include battery cycling as well as capacity fade and efficiency analyses. The results of these analyses with respect to various operating conditions and flow cell components will also be reported. References: [1] Milshtein, J. D., Kaur, A. P., Casselman, M. D., Kowalski, J. A., Modekrutti, S., Zhang, P. L., Harsha Attanayake, N., Elliott, C. F., Parkin, S. R., Risko, C., Brushett, F. R., Odom, S. A. Energy Environ. Sci. 2016, 9 (11), 3531-3541. [2] Wei, X., Xu, W., Vijayakumar, M., Cosimbescu, L., Liu, T., Sprenkle, V., Wang, W. Adv. Mater. 2014, 26 (45), 7649-7653. [3] Suttil, J. A., Kucharyson, J. F., Escalante-Garcia, I. L., Cabrera, P. J., James, B. R., Savinell, R. F., Sanford, M. S., Thompson, L. T. J. Mater. Chem. A 2015, 3 (15), 7929-7938. [4] Carino, E. V., Staszak-Jirkovsky, J., Assary, R. S., Curtiss, L. A., Markovic, N. M., Brushett, F. R. Chem. Mater. 2016, 28 (8), 2529-2539. Figure 1
Redox flow batteries (RFBs) are a promising grid-scale energy storage technology for the integration of intermittent renewable sources, such as wind and solar, into the electrical grid due to their modularity, flexible design, and cost-effectiveness in long-duration storage [1]. Unlike traditional batteries, for RFBs, electrolytes are stored in external tanks and are circulated through the flow cell. In the flow cell, electrical energy is stored via the electrochemical reactions of the redox active species dissolved in liquid electrolytes. While this new archetype provides unique benefits, the flow-assisted nature of RFBs presents many challenging issues, including but not limited to significant transport losses due to the poor electrode and cell design, and the related low power density [2-3]. It can be hypothesized that many of these challenges are primarily related to the concept of electrolyte utilization [4]. In this regard, porous electrode is the key component, which is responsible for multiple critical functions including delivery of liquid electrolytes as well as facilitating ion/charge transfer and providing sites for electrochemical reactions [4]. Despite its importance, the required fundamental knowledge on how to systematically design effective electrodes specifically tailored for RFB applications has not received much attention [5]. In this talk, the recent efforts on establishing the structure-function-performance linkages for commercially available carbon electrodes will be presented. The results of the experiments utilizing an electrochemical protocol that combines symmetric flow cell cycling with electrochemical impedance spectroscopy (EIS) and polarization curve (PC) measurements will be discussed. The impedance data is fitted into a mathematical model, and key physicochemical properties of carbon electrodes and the correlated performance losses are quantified. Additionally, we will share our perspective on manufacturing high-performance carbon electrodes using a high-throughput screening platform. References: X. Ke, J. M Prahl, J. I. D. Alexander, J. S. Wainright, T. A. Zawodzisnki, R. F. Savinell, Chem. Soc. Rev., 2018, 47 (23), 8721-8743. P. A. Garcia-Salaberri, T. C. Gokoglan, S. E. Ibanez, E. Agar, M. Vera. Processes, 2020, 8, 775. M. Nourani, B. I. Zackin, D. C. Sabarirajan, R. Taspinar, K. Artyushkova, F. Liu, I. V. Zenyuk, E. Agar. J. Electrochem. Soc., 2019, 166 (2), A353-A363. B. Akuzum, Y. C. Alparslan, N. C. Robinson, E. Agar, E. C. Kumbur, J. Appl. Electrochem., 2019, 49 (6), 551-561. A. Forner-Cuenca, F. R. Brushett, Curr. Opin. Electrochem., 2019, 18, 113-122.
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