A cooperative degradation pathway for organic phenoxazine catholytes in aqueous redox flow batteries
Xiaoting Fang,
Lifan Zeng,
Zhiguang Li,
Lily A. Robertson,
Ilya A. Shkrob,
Lu Zhang,
Xiaoliang Wei
Affiliations
Xiaoting Fang
Indiana University-Purdue University Indianapolis, 425 University Boulevard, Indianapolis, IN 46202, USA; Purdue University, 585 Purdue Mall, West Lafayette, IN 47907, USA; Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
Lifan Zeng
Indiana University-Purdue University Indianapolis, 425 University Boulevard, Indianapolis, IN 46202, USA
Zhiguang Li
Indiana University-Purdue University Indianapolis, 425 University Boulevard, Indianapolis, IN 46202, USA; Purdue University, 585 Purdue Mall, West Lafayette, IN 47907, USA; Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA; Joint Center for Energy Storage Research (JCESR), 9700 South Cass Avenue, Lemont, IL 60439, USA
Lily A. Robertson
Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA; Joint Center for Energy Storage Research (JCESR), 9700 South Cass Avenue, Lemont, IL 60439, USA
Ilya A. Shkrob
Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA; Joint Center for Energy Storage Research (JCESR), 9700 South Cass Avenue, Lemont, IL 60439, USA; Corresponding authors at: Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
Lu Zhang
Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA; Joint Center for Energy Storage Research (JCESR), 9700 South Cass Avenue, Lemont, IL 60439, USA; Corresponding authors at: Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
Xiaoliang Wei
Indiana University-Purdue University Indianapolis, 425 University Boulevard, Indianapolis, IN 46202, USA; Corresponding author.
Redox-active organic molecules that store positive charge in aqueous redox flow cells (catholyte redoxmers) frequently exhibit poor chemical stability for reasons that are not entirely understood. While for some catholyte molecules, deprotonation in their charged state is resposible for shortening the lifetime, for well designed molecules that avoid this common fate, it is seldom known what causes their eventual decomposition as it appears to be energetically prohibitive. Here, a highly soluble (1.6 M) phenoxazine molecule with a redox potential of 0.48 V vs. Ag/AgCl has been examined in flow cells. While this molecule has highly reversible redox chemistry, during cycling the capacity fades in a matter of hours. Our analyses suggest a cooperative decomposition pathway involving disproportionation of two charged molecules followed by anion substitution and deprotonation. This example suggests that cooperative reactions can be responsible for unexpectedly low chemical instability in the catholyte redoxmers and that researchers need to be keenly aware of such reactions and methods for their mitigation.
