Molecular Insights into Redox-Active Polymer Interfaces: Solvation and Ion Valency Effects on Metal Oxyanion Selectivity

Abstract

Chemical separations are responsible for 10-15% of the world’s energy consumption. Minimizing energy and materials inputs in selective separations is imperative for a sustainable future. Ion-electrosorption mediated by redox-active metallopolymer interfaces has the unique advantage of selectively capturing and releasing metal oxyanions in a switchable manner by adjusting the applied potential, without any regenerants. Electrosorption addresses the need for selective separation approaches with low chemical and energy inputs. Previous studies on ferrocene metallopolymers have demonstrated the role of polymer structure and applied potential on selectivity—however, the ubiquitous role of solvation in redox-polymers has remained unexplored. Here, we investigate how solvation and ion valency influence selectivity of ReO4 - vs MoO4 2- for two redox-metallopolymers, poly(vinyl ferrocene) (PVFc) and poly(3-ferrocenylpropyl methacrylamide) (PFPMAm). Both polymers display time-dependent Re/Mo selectivity, with PVFc having higher selectivity compared to PFPMAm. Operando neutron reflectometry, ellipsometry, and electrochemical 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 MoO4 2-. We find that the less solvated anion (ReO4 -) is preferably adsorbed by the more hydrophobic redox-polymer (PVFc). We expect that a deeper understanding of solvation and valency effects on selectivity mechanisms in redox interfaces will expedite the development of targeted selective ion-electrosorption systems. Figure 1