From Nanofluids to Nanoelectrofuels: Suspension Electrodes and Application in Flow Batteries
Authors: Timofeeva Elena V. , Sen Sujat , Katsoudas John P. , Singh Dileep , Ramani Vijay K , Segre Carlo U 
Published: February 2020

Electrodes in the form of moving conducting particles are potentially useful in many processes of applied electrochemistry in cases where the supply of reactants to the electrode surfaces is a controlling step. The process can be intensified by the use of such electrodes as the result of improved diffusion conditions and considerable increase of the specific surface. Despite interest in many applied fields, the electrochemistry of particle suspensions in electrolytic solutions has received little attention from the fundamental perspective. The concept of a suspension electrode has been demonstrated more than four decades ago [1-4], but has not been applied to the redox flow battery until a recent publication from MIT [5], where the suspension of 30 micron cathode particles mixed with carbon nanotube conductive filler in a Li-ion electrolyte were successfully charged/discharged during passage though the flow battery half-cell. Challenges that are faced in this proof-of-concept experiment are the extremely high viscosity of the suspensions (>1000 times higher than the electrolyte alone) which incur pumping power penalties, limited power ratings, limited discharge rate, incomplete discharge, random electron conduction path, as well as passivation of the carbon nanotube solid/electrolyte interface, all resulting in extra cost, inactive battery weight, and short life cycle. Since 2011 several other groups have published results on different forms of suspension-based energy storage media, including on electrochemical flow capacitor for fast energy storage and recovery [6]; use of reversible redox ions as mediators for Li intercalation/deintercalation into LiFePO4nanoparticles [7]; demonstration of high charge retention of silicon/carbon dispersions in non-aqueous electrolytes [8]. The emergence of new publications in Li-based high energy density electrolytes for flow batteries confirms worldwide interest in, and demand for, suspension electrode technology.
Engineered nanoparticle suspensions also known as nanofluids, have been extensively studied in the last decades due to their potential applications as advanced heat transfer fluids [9], but have not attracted attention for their energy storage capabilities. On the macroscale, nanofluids are a liquid phase, easy to store, transport and maintain. At the nanoscale they possess a huge area of solid/liquid interface represented by electrical double layers (capacitors), while a rechargeable nanoparticle material is capable of storing/delivering energy through electrochemical (red/ox) reactions similar to solid-state batteries.
Our approach to suspension electrodes (nanoelectrofuels) uses electroactive nanoparticle suspensions. Nanoparticle suspensions have significantly higher stability than micron-sized suspension due to relative balance of Brownian motion and gravitational forces. In rechargeable nanofluid flow batteries energy is stored in nanoparticles and released through a reversible electrochemical reaction between two cathodic and anodic nanoelectrofuels (NEFs) that are stored externally to the cell stack and circulated through the cells of the stack as required. The combination of NEFs and unique flow battery cells increase the system level energy density of flow batteries to and above that of solid-state lithium-ion batteries. More than 50% improvement in energy density of solid batteries with the same chemistry are possible because of a lower fraction of inactive packaging material that is used in a NEF flow battery.
NEFs can be prepared with as high as 60% solid loading while still keeping the viscosity manageable. The low viscosity of nanoelectrofuels increases the efficiency and power ratings of the flow battery and the nanosized battery materials have demonstrated a significantly faster charge/discharge rate than micron sized anode and cathode materials [10].
This presentation will cover historical development in the field of suspension electrodes from theoretical and applied perspective with focus on energy storage applications. The nuances of formulating electrochemically active nanofluids will be discussed and also updated results on the development of nanoelectrofuel flow batteries within our research group will be presented.
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