PNNL

Abstract

Solvation and ion-valency effects on selectivity of metal oxyanions at redox-polymer interfaces are explored through in situ spatial-temporal resolved neutron reflectometry, providing sub-nanometer resolution on solvent movement, combined with large scale ab initio molecular dynamics. Previous studies on ferrocene metallopolymers demonstrate the role of polymer structure and applied potential on selectivity—however, the ubiquitous role of solvation in redox-polymers remains unexplored. Here, we investigate how solvation and ion valency influence selectivity of ReO4- vs MoO42- for two redox-metallopolymers, poly(vinyl ferrocene) (PVFc) and poly(3-ferrocenylpropyl methacrylamide) (PFPMAm). PVFc has a higher Re/Mo separation factor compared to PFPMAm at 0.6 V vs Ag/AgCl. In situ neutron reflectometry, ellipsometry, and quartz-crystal microbalance show that both PVFc and PFPMAm swell in the presence of ReO4- (with PFPMAm having higher solvation), but do not swell in contact with MoO42-. We find that the less solvated anion (ReO4-) is preferably adsorbed by the more hydrophobic redox-polymer (PVFc), and we suggest that physical cross-linking with divalent anions could impair film swelling. Furthermore, interface dynamics during the electrochemically-mediated adsorption/desorption transients is monitored in situ. A deeper understanding of solvation and valency effects on selectivity mechanisms at redox-polymer interfaces is envisioned to expedite the development of targeted selective ion-electrosorption systems.