Development of Nanoelectrofuel Electrodes for Flow Batteries : Rheology and Electrochemistry of Fluidized Nanoparticles
Authors: Sen Sujat , Timofeeva Elena V. , Pelliccione Christopher J. , Katsoudas John P. , Singh Dileep , Segre Carlo U 
Published: February 2020

This presentation reports on the development of unique formulation of nanoelectrofuel anodes for flow batteries. Nanoelectrofuel electrodes are essentially nanofluids prepared from the battery active materials – e.g. nanoparticles stably suspended in base electrolyte. This novel technology leverages the properties of conventional solid state batteries, flow batteries and nanofluid technology [1]. Nanoelectrofuels have high concentrations of active materials and exhibit liquid behavior in a wide range of temperatures for good flow and pump performance. This novel class of high energy-density liquid energy storage is used in combination with uniquely designed flow battery cells. By utilizing nano-sized active materials the charge and discharge rates are enhanced due to shorter diffusion paths. The surface of the nanoparticles are engineered to allow for minimal viscosity while maximizing the concentration of active materials in liquid to ensure high energy density. Nanoelectrofuel approach differentiates from semi-solid flow battery systems [2] by utilization of surface modified nanoparticles without the aid of any conductive additives.
The nanoelectrofuel technology also offers a host of benefits, including superior heat transfer capabilities, separation of the power and energy components (size of the flow cell stack defines the power while volume of nanoelectrofuel storage defines the energy capacity), and ability for rapid charge replenishment through replacement of discharged nanoelectrofuels to the charged ones.
Nanofluids have been traditionally studied for their advanced thermal properties [3]. One of the main challenges in nanofluid and nanoelectrofuel engineering is achieving high concentration of nanomaterial in suspension without dramatic increase in viscosity. Most studies on nanofluids for heat transfer have only reported on nanoparticle loadings less than 10 wt. % because of non-linear viscosity increase with increasing particle concentration, making them unfeasible for use as coolants. In this work we have developed nanofluids that are both electrochemically active and can be prepared with high concentration of nanoparticles with manageable increases in viscosity. This presentation will report on a surface modification procedures developed for anatase titania (TiO2) and iron oxide (Fe2O3) nanoparticles that enables nanofluids with relatively low viscosity (<5 cP at 50 wt%) as compared to their unmodified counterparts. Modified nanoparticles were characterized with x-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Modified and un-modified materials were studied electrochemically in both casted-on solid state form and as suspension nanoelectrofuel electrodes in half cell configuration in aqueous electrolytes. Charge/discharge curves show that close to theoretical capacity (334 mAh/g) of iron oxide can be achieved in both casted and nanoelectrofuel forms with faradaic efficiency of ~80%. Casted Fe2O3 anodes were also studied in situ with x-ray absorption spectroscopy (XAS) to gather information on local and electronic environment of Fe atoms. Modelling of XAS spectra (XANES and EXAFS) allowed us better understanding of charge, discharge, and electrode failure mechanisms.
Additional electrochemical tests with varying charge rates, nanoparticle concentrations and geometric area of the current collector are also conducted to assess kinetics and coulombic efficiency of the nanoelectrofuel electrode.
Engineering the surface of nanoparticles to control the rheological and electrochemical properties of the resulting nanofluids is a promising approach towards realizing the high energy density nanoelectrofuel flow batteries. Future endeavors will be directed towards expanding the gallery of available chemistries.

References

E.V. Timofeeva, J.P. Katsoudas, C.U. Segre, D. Singh, “Rechargeable Nanofluid Electrodes for High Energy Density Flow Battery”, Cleantech 2013, Chapter 9, Energy storage, pp. 363-366.

M. Duduta, B. Ho, V.C. Wood, P. Limthongkul, V. Brunini, C.Carter, Y-M. Chiang, Adv. Energy Mater. 2011, 1, 511-516.

E. V. Timofeeva, CHAPTER: Nanofluids for Heat Transfer: Potential and Engineering Strategies, BOOK TITLE: Two Phase Flow, Phase Change and Numerical Modeling, Ed.: A. Ahsan, InTech, September 2011 (ISBN 978-953-307-584-6).