Towards cobalt-containing polyoxometalate-based electrolytes for redox flow batteries operating under mild conditions

Abstract

Sustainability and safety of Redox Flow Batteries (RFBs) are some of the limitations that hinder the further implementation of this technology. To overcome the disadvantages of conventional RFBs, new electrolyte formulations are being proposed. Herein we report on two cobalt(II)-containing Keggin-type polyoxometalates (POMs) working as electroactive species. These are the plenary [α-CoW12O40]6− (CoW12) anion and the cobalt(II)-monosubstituted [α-Co(H2O)SiW11O39]6− (CoSiW11) tungstosilicate. During the battery performance, cobalt-containing POMs play the role of both positive and negative electrolyte, because the ability of Co (CoIIWVI/CoIIIWVI) and W (CoIIWVI/CoIIWV) centres to be oxidized and reduced, respectively. These redox processes are separated by 1.7 V (CoW12) and 1.9 V (CoSiW11) thus, both POMs supply with a suitable operational voltage for RFBs. The reversibility of the reactions was examined by a kinetic study. Among the formulations tested, those accomplished in aqueous 1 M HAc/LiAc and 1 M LiAc revealed high stability while operating under mild pH conditions. Moreover, the nature of the selected supporting electrolytes (SEs) avoids the occurrence of gassing side reactions. In addition, the physicochemical characterization of the electrolytes showed high conductivity (ca. 60 mS cm−1) and low viscosity (ca. 1.4 mPa s) for highly concentrated electrolytes (up to 120 mM). Therefore, the developed electrolytes exhibit promising properties for their direct incorporation in RFBs, including high energy density (21.86 and 24.44 W h L−1 for CoW12 and CoSiW11 respectively) and enhanced safety.