Battery and supercapacitor materials in flow cells. Electrochemical energy storage in a LiFePO4/reduced graphene oxide aqueous nanofluid

Abstract

Exploring conceptual frontiers between batteries, supercapacitors, redox flow batteries (RFBs) and fuel cells (FCs), we have used a battery material (i.e. LiFePO4) and a supercapacitor material (i.e. graphene) in the form of nanoparticles dispersed in an aqueous electrolyte to characterize the electrochemical activity of the resulting electroactive nanofluids. X-ray diffraction, TEM, Raman, XPS and AFM analyses were carried out to characterize the solid LiFePO4 and RGO components. The corresponding electroactive nanofluids were prepared by dispersion in an aqueous Li2SO4 electrolyte and stabilized with Diaminobenzoic Acid (DABA). Cyclic voltammetry measurements were used to analyze their electrochemical behavior in three-electrode cells. Charge-discharge tests of the LiFePO4/RGO (positive) vs. RGO (negative) nanofluids were also performed. Effective utilization of dispersed electroactive particles (ca. 100 mAh/g(LFP) at 1C) was demonstrated, which turned out to be superior to the same LFP material used as solid electrode. A charge-transfer percolation effect provided by the RGO dispersion is proposed as the mechanism for the good performance of LiFePO4 (not coated with carbon!) and RGO Nanofluids. Our results constitute a first step and proof of concept of the possible application of electroactive nanofluid electrodes in alternative flow batteries.