Electroactive nanofluids with high solid loading and low viscosity for rechargeable redox flow batteries

A typical dispersion of iron oxide nanoparticles in alkali electrolyte is limited to 20 wt% solids, above which it has a paste-like consistency (> 370 cP), incompatible with flow applications. The formulation of stable electrochemically active colloids of nanoscale iron oxide (30-50 nm) with up to 70 wt% solids, low viscosity (< 30 cP) with minimal shear dependence (+/- 2 cP), excellent colloidal stability (> 2 weeks at rest), and 55% enhancement in thermal conductivity is reported herein. A thin surface coating allows for good particle dispersion, which is shown to be crucial for these enhanced features. These suspensions of electrochemically active nanoparticles (nanoelectrofuels) can undergo electrochemical charge and discharge in fluidized format through particle-electrode impact events with potential for application in redox flow batteries. The surface coating is found to partially suppress electrochemical access to the nanoparticle in a fluidized format but has no detrimental effects on discharge capacity in the solid state. This approach is also shown to suppress a parasitic nanoparticle agglomeration process, which is otherwise dominant during electrochemical cycling of pristine iron oxide. A dissolution re-precipitation mechanism is proposed to offer insight into this auxiliary benefit. This study provides the first insight into the feasibility of adopting electrochemically active nanofluids as high energy density redox flow battery electrolytes. [GRAPHICS]