Nickel Hydroxide Nanofluid Cathodes with High Solid Loadings and Low Viscosity for Energy Storage Applications

Abstract

Nanofluid electrodes with high loading of active solid materials have significant potential as high energy density flow battery electrolytes; however, two key criteria need to be met: they must have a manageable viscosity for pumping and simultaneously exhibit good electrochemical activity. A typical dispersion of nickel hydroxide nanoparticles (~100 nm) is limited to 5–10 wt.% of solids, above which it has a paste-like consistency, incompatible with flow applications. We report on the successful formulation of stable dispersions of a nano-scale nickel hydroxide cathode (β-Ni(OH)2) with up to 60 wt.% of solids and low viscosity (32 cP at 25 °C), utilizing a surface graft of small organic molecules. The fraction of grafting moiety is less than 3 wt.% of the nanoparticle weight, and its presence is crucial for the colloidal stability and low viscosity of suspensions. Electrochemical testing of the pristine and modified β-Ni(OH)2 nanoparticles in the form of solid casted electrodes were found to be comparable with the latter exhibiting a maximum discharge capacity of ~237 mAh/g over 50 consecutive charge–discharge cycles, close to the theoretical capacity of 289 mAh/g.

Modeling Electrochemical and Rheological Characteristics of Suspension-Based Electrodes for Redox Flow Cells

“…Within the context of RFBs, numerous earlier works have demonstrated marked increases in theoretical energy density when employing FSEs with redox active particles, compared to more conventional configurations with forced convection of electrolytes containing dissolved active species through porous electrodes. [9][10][11][12] However, complex electrochemical, rheological, and transport property trade-offs frustrate efforts to understand and improve these devices. For example, effective FSE cell operation requires high electronic conductivities that are only achievable at sufficiently high particle volume fractions to maintain system-spanning particle networks, but these gains in electrochemical performance compete with substantial increases in viscosity, amplifying pumping requirements.…”

“…Such trends in suspension properties arise from complex microstructural dynamics of the particle networks, which translate into non-Newtonian rheology (e.g., shear-dependent viscosity, thixotropy) and shear-dependent electronic conductivity. [13][14][15][16][17] While some efforts have been made to reduce the viscosity and improve the electronic conductivity of FSEs by modifying the particle interactions in experiments, 12,18 the multifaceted competition between various physical processes in these electrochemical systems necessitates further analysis to better articulate general operating bounds and cell engineering principles.…”