Nanoscale Hybrid Electrolytes with Viscosity Controlled Using Ionic Stimulus for Electrochemical Energy Conversion and Storage

Abstract

As renewable energy is rapidly integrated into the grid, the challenge has become storing intermittent renewable electricity. Technologies including flow batteries and CO 2 conversion to dense energy carriers are promising storage options for renewable electricity. To achieve this technological advancement, the development of next generation electrolyte materials that can increase the energy density of flow batteries and combine CO 2 capture and conversion is desired. Liquidlike nanoparticle organic hybrid materials (NOHMs) composed of an inorganic core with a tethered polymeric canopy (e.g., polyetheramine (HPE)) have a capability to bind chemical species of interest including CO 2 and redox-active species. In this study, the unique response of NOHM-I-HPE-based electrolytes to salt addition was investigated, including the effects on solution viscosity and structural configurations of the polymeric canopy, impacting transport behaviors. The addition of 0.1 M NaCl drastically lowered the viscosity of NOHM-based electrolytes by up to 90%, reduced the hydrodynamic diameter of NOHM-I-HPE, and increased its self-diffusion coefficient, while the ionic strength did not alter the behaviors of untethered HPE. This study is the first to fundamentally discern the changes in polymer configurations of NOHMs induced by salt addition and provides a comprehensive understanding of the effect of ionic stimulus on their bulk transport properties and local dynamics. These insights could be ultimately employed to tailor transport properties for a range of electrochemical applications.

Impacts of Bond Type and Grafting Density on the Thermal, Structural, and Transport Behaviors of Nanoparticle Organic Hybrid Materials‐Based Electrolytes

“…Our earlier studies have shown that the polymers in NOHM‐based fluids often exist in multiple states such as tethered, interacting, and free HPE chains. [ 34,35,51 ] Those earlier studies were only focused on NOHM‐I‐HPE, and we anticipated that NOHM‐C‐HPE would lead to different binding interactions in the solution phase, with and without salts. Thus, small‐angle neutron scattering (SANS) experiments were completed to identify the effect of the bond type on the structure and organization of the polymer and nanoparticles in NOHM‐based fluids and electrolytes.…”

“…This concentration of salt was selected because this was recently found to be near the experimentally observed transition from the ionic stimuli‐responsive regime to the saturation regime of a higher grafting density liquid‐like NOHM‐I‐HPE. [ 51 ] The specific viscosity is a dimensionless measure of the viscosity and was used in this case to directly compare the effect of polymer, nanoparticle, or NOHMs addition to the overall solution viscosity. The specific viscosity (η sp ) can be determined by $$\[ \begin{array}{*{20}{c}}{{\eta _{{\rm{sp}}}} = \frac{{\eta - {\eta _{\rm{s}}}}}{{{\eta _{\rm{s}}}}}}\end{array} \]$$where η is the viscosity of the solution/electrolyte containing polymer, nanoparticles, or NOHMs and η s is the viscosity of the solvent (e.g., water or 0.1 m KHCO 3 ).…”

“…Across the polymer/nanoparticle/NOHMs concentration range studied (0–10 wt.%), it is apparent that the change in viscosity remained less than ±5% for the HPE, suggesting that the addition of 0.1 m KHCO 3 does not significantly affect the mobility of the untethered HPE chains. [ 35,51 ] Upon the addition of 0.1 m KHCO 3 , the SiO 2 nanoparticles, NOHM‐C‐HPE 0.5 and NOHM‐C‐HPE 0.7 showed a slight reduction in viscosity compared to the untethered HPE. This can be explained by K + ions screening out some of the nanoparticle‐nanoparticle interactions, which have been similarly shown to lead to a smaller hydrodynamic size of bare nanoparticles.…”

“…[ 34 ] Lastly, the addition of KHCO 3 or NaCl to aqueous suspensions of NOHM‐I‐HPE was found to significantly reduce the solution viscosity, suggesting that cations may compete with the HPE chains to occupy the hydroxyl surface sites on the nanoparticle surface and/or alter the conformation of the ionically grafted polymers. [ 35,51 ]…”

“…[9,50] For example, the strategic selection of polymers with specific functional groups can optimize the binding energy for target species including small gaseous (e.g., CO 2 ) or ionic species (e.g., Cu +2 , Zn +2 ). Though NOHMs synthesized with an ionic bond have been shown to be ionically conductive, they are challenged by high viscosity [31,34,51] and thus would need to be incorporated into electrolytes as additives. It has been shown that elucidating the transport [52][53][54][55] and structural [35,52,56] properties of electrolyte additives in solution is crucial to determining the overall electrochemical system performance.…”