Structure-Dynamics Investigation of Electro-Responsive Redox-Polymer by In Situ Neutron Reflectometry

Heavy metal contamination has been a major health and environmental hazard on the global scale. The development of smart materials for water purification and environmental remediation has received intense attention. In recent years, redox-active materials have been demonstrated to be an attractive platform for separations due to their molecular selectivity and electronic tunability. The main features of these systems are the fast electron transfer and redox processes at moderate potentials, which allow reversible adsorption and desorption of ionic components. Furthermore, due to the selective performance differences among these redox-polymers, a promising platform for electrochemical separations can be built by tailoring polymer structures for targeted metal recovery processes, depending on the stream compositions. Our planned study aimed to combine the electrochemical characterization with a simultaneous, non-destructive <i>in situ</i> or <i>operando</i> characterization technique, so to correlate oxidation states of the redox-active centers with migration of ions and solvent in the polymer film. Neutron reflectometry (NR) is the ideal candidate for investigating thin-film systems in a non-destructive and non-invasive fashion. Liquids Reflectometer (LR) at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) measures the surface and interfacial structures of the thin films in a horizontal sample geometry on length scale of 0.5 nm to 350 nm, and tracks changes of layer thickness, scattering length density (SLD), and roughness as a function of depth. The unique sample environment at LR offers the capability to conduct <i>in situ</i> electrochemistry to probe the morphological and structural changes of redox-polymer interfaces during operation as a function of chemical and electrochemical gradients over time. Electrochemical <i>in situ</i> NR experiments studied the equilibrium of swelling/deswelling of the poly(vinyl ferrocene) (PVFc) in presence of ReO₄^- with H₂O and D₂O (Figure 1 a and b, reflectivity curve; Figure 1 c, corresponding scattering length density profile). The NR data revealed high uptake capacities when applying voltages beyond the oxidation potential for ferrocene, showing promising results for implementation in real conditions. Dynamic data was collected too (Figure 1 d). After data analysis, we were able to track the capture or the release of the target ions (e.g., ReO₄^-) as the electrochemical process happens (Figure 1 e). This study will allow us to answer questions such as: what is the role of solvation in determining selectivity? What roles do uptake kinetics, diffusion limitations in the film play for selectivity? Answering these questions will be critical for creating new, more selective separation systems through the rationally design of the polymer structure and operating conditions.