Unraveling the Role of Solvation and Ion Valency on Redox-Mediated Electrosorption through In Situ Neutron Reflectometry and Ab Initio Molecular Dynamics

Riccardo Candeago; Hanyu Wang; Manh-Thuong Nguyen; Mathieu Doucet; Vassiliki-Alexandra Glezakou; James F Browning; Xiao Su

doi:10.1021/jacsau.3c00705

Unraveling the Role of Solvation and Ion Valency on Redox-Mediated Electrosorption through In Situ Neutron Reflectometry and Ab Initio Molecular Dynamics

JACS Au. 2024 Feb 1;4(3):919-929. doi: 10.1021/jacsau.3c00705. eCollection 2024 Mar 25.

Authors

Riccardo Candeago¹, Hanyu Wang², Manh-Thuong Nguyen³, Mathieu Doucet⁴, Vassiliki-Alexandra Glezakou⁵, James F Browning⁴, Xiao Su¹

Affiliations

¹ Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
² Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
³ Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States.
⁴ Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
⁵ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.

Abstract

Solvation and ion valency effects on selectivity of metal oxyanions at redox-polymer interfaces are explored through in situ spatial-temporally resolved neutron reflectometry combined with large scale ab initio molecular dynamics. The selectivity of ReO₄^- vs MoO₄^2- for two redox-metallopolymers, poly(vinyl ferrocene) (PVFc) and poly(3-ferrocenylpropyl methacrylamide) (PFPMAm) is evaluated. PVFc has a higher Re/Mo separation factor compared to PFPMAm at 0.6 V vs Ag/AgCl. In situ techniques show that both PVFc and PFPMAm swell in the presence of ReO₄^- (having higher solvation with PFPMAm), but do not swell in contact with MoO₄^2-. Ab initio molecular simulations suggest that MoO₄^2- maintains a well-defined double solvation shell compared to ReO₄^-. The more loosely solvated anion (ReO₄^-) is preferably adsorbed by the more hydrophobic redox polymer (PVFc), and electrostatic cross-linking driven by divalent anionic interactions could impair film swelling. Thus, the in-depth understanding of selectivity mechanisms can accelerate the design of ion-selective redox-mediated separation systems for transition metal recovery and recycling.