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Development of modified graphite felt electrodes for the vanadium redox flow battery [article]

Sang Jun Yoon, Universität Des Saarlandes
2020
Hiermit versichere ich an Eides statt, dass ich die vorliegende Arbeit selbstständig und ohne Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe. Die aus anderen Quellen oder indirekt übernommenen Daten und Konzepte sind unter Angabe der Quelle gekennzeichnet. Die Arbeit wurde bisher weder im In-noch im Ausland in gleicher oder ähnlicher Form in einem Verfahren zur Erlangung eines akademischen Grades vorgelegt. Ort, Datum Unterschrift v vi Abstract Redox flow batteries (RFBs)
more » ... batteries (RFBs) are one of the most promising candidates for stationary largescale energy storage systems with regard to cost, cycle life, design flexibility, and safety. Among various RFBs, vanadium redox flow batteries (VRFBs) have the advantage of overcoming the cross-contamination of both electrolytes because VRFBs use the same V ion as the active species in both the anolyte and the catholyte. In spite of the various advantages of VRFBs, the performance of VRFBs needs to be further improved for the commercialization. The performance of VRFBs is significantly affected by the electrochemical activity of the electrode because the vanadium ion redox reactions take place at the graphite felt electrode surface during the charge-discharge process. To improve the electrochemical activity of graphite felt electrodes, facile methods for preparing nitrogen-doped carbon coated graphite felt electrodes using nitrogen-containing materials were developed. First, 1-ethyl-3methylimidazolium dicyanamide (EMIM dca), an ionic liquid containing a high content of nitrogen, was used as an effective precursor for nitrogen doping on graphite felt surfaces. The effect of EMIM dca derived nitrogen doped graphite felt on the performance of VRFBs was investigated by various physical and electrochemical analyses. When employed in chargedischarge tests, the single cells with the nitrogen doped graphite felts showed outstanding performance. The improved performance is attributed to the high nitrogen content on the graphite felt, which increased the electrocatalytic activity of vanadium redox reactions. Second, polyacrylonitrile (PAN), which contains 26 wt% of nitrogen atoms, was employed to fabricate nitrogen-doped carbon materials on graphite felt surfaces. PAN was coated on graphite felt to improve the performance of VRFB single cell electrodes by utilizing the mechanism of the process of manufacturing carbon fiber from PAN. After optimizing the amount of PAN coated, a single cell with PAN derived nitrogen-doped graphite felt showed higher performance than a single cell with electrodes prepared by conventional treatment methods. In addition to improving the performance of the electrode by chemical methods, we examined the effect of local porosity of the electrodes on the electrolyte flow field in VRFBs at high current densities. The optimization of local porosity of the graphite felt electrode was carried out to improve the performance of VRFBs at high current density region. vii viii Zusammenfassung Redox-Fluss-Batterien (RFBs) gehören zu den vielversprechendsten Kandidaten für stationäre Energiespeichersysteme im großen Maßstab in Hinblick auf Kosten, Lebensdauer, Designflexibilität und Sicherheit. Vanadium-Redox-Fluss-Batterien (VRFBs) haben gegenüber anderen RFBs den Vorteil, dass sie keine Kreuzkontamination der Elektrolyte aufweisen, da VRFBs das gleiche V-Ion als aktive Spezies sowohl im Anolyten als auch im Katholyten verwenden. Trotz der verschiedenen Vorteile von VRFBs muss die Leistung von VRFBs für die Kommerzialisierung weiter verbessert werden. Die Leistung von VRFBs wird erheblich von der elektrochemischen Aktivität der Elektrode beeinflusst, da die Vanadiumionen-Redoxreaktionen an der Oberfläche der Graphitfilzelektrode während des Lade-Entlade-
doi:10.22028/d291-31337 fatcat:w43dustw7nd4raonwew2bi7kfy