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

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 ReO 4− vs MoO 4 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 ReO 4 − (having higher solvation with PFPMAm), but do not swell in contact with MoO 4 2− . Ab initio molecular simulations suggest that MoO 4 2− maintains a well-defined double solvation shell compared to ReO 4 − . The more loosely solvated anion (ReO 4 − ) 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.

Studying Transient Phenomena in Thin Films with Reinforcement Learning

“…As previously reported by Candeago and co-workers, the PFPMAm film we studied swells during its transition from 0.1 and 0.6 V as it uptakes ReO 4 – . Figure S-1 in the Supporting Information shows the initial 0.1 V steady-state measurement, the following 0.6 V steady-state measurement, and finally the return to 0.1 V. Each reflectivity curve was modeled using a three-layer model, representing the palladium underlayer, a hydration layer, and the polymer layer.…”

“…After that time, we expect the film to start changing as the polymer uptakes ReO 4 – ions. The change in polymer thickness upon ion intake has been shown to happen relatively fast, on the time scale of 60 s . The value of the mixing parameter m as a function of time shows how the film fully transitions from its initial state to its final state in about 5 min.…”

“…The measurements used here to demonstrate our approach were obtained as part of a study of the role of solvation on redox-active metallopolymers for metal oxyanions . Redox-active polymers have shown the capability to selectively bind and separate between oxyanions, which is of importance to water purification and chemical separations. , This study by Candeago and co-workers compares selectivity toward MoO 4 2– and ReO 4 – of both poly­(vinyl ferrocene) (PVFc) and poly­(3-ferrocenylpropyl methacrylamide) (PFPMAm) under potential.…”

Auto-oxidation of redox electrodes for the selective recovery of platinum group metals

“…EQCM can determine the mass change of the electrode by measuring the frequency changes under potential control. 51–53 In Fig. S16 and S17 (ESI†), the mass of the working electrode with the PVF coating increased as the potential of PVF increased during OCP electrosorption, which meant that [PtCl 6 ] 2− and [IrCl 6 ] 2− spontaneously oxidized PVF and were adsorbed.…”

Redox-Mediated Electrodialysis for Desalination, Environmental Remediation, and Resource Recovery

“…The redox-ED system offers unique advantages, which can complement or surpass conventional electrochemical separation techniques such as membrane-integrated systems (e.g., electrodialysis ,, and membrane capacitive deionization , ) and electrosorption-based systems (e.g., redox-based ,, and capacitive-based systems , ). The distinct mechanism and architecture of redox-ED provide significant benefits in terms of energy consumption, sustainability, and economic feasibility.…”