Faradaic Rhenium Recovery with Polyvinyl Ferrocene (PVF) Electrodeposited Carbon Electrodes
Authors: Li Yurui , Roman Santiago Anaira , Candeago Riccardo , Su Xiao , Cusick Roland 
Published: September 2023

Abstract
Rhenium, a valuable rare element used in catalysts and superalloys, is difficult to recover in micromolar concentrations (Schulz, 2017). Electrodes coated with the redox active polymer poly vinyl ferrocene (PVF) have shown the ability to selectively adsorb several transition metal oxyanions, but limited adsorption capacity and longevity remain a challenge for these selective electrosorption technologies(Chen et al., 2021; Kim et al., 2020). In this study, we elucidate the impact of electrochemical deposition conditions on PVF coating as measured by capacitance, rhenium uptake, and longevity. PVF films were electrodeposited onto carbon substrate electrode at three different potentials across the oxidation window of the ferrocene-based metallopolymer to understand how the rate of charge transfer influences the resulting electrodeposited polymer coating. It was observed that PVF films electrodeposited at the peak oxidation current potential of 0.45 V (vs. Ag/AgCl) showed both the highest adsorption of Rhenium (458 ± 61 mg Re/g adsorbent) and capacitance (217 F/g). In comparison, the electrodeposition at 0.35 V and 0.6 V leads to a lower average capacitance 355.36 mg Re/g adsorbent and 342.03 mg Re/g adsorbent respectively. The enhanced performance for 0.45 V electro-deposited electrode can likely be attributed to a more stable deposition structure of the polymer (Pater et al., 1998), which enhances adhesion substrate and charge transfer between the substrate and the PVF. The 0.45 V electro-deposited PVF-carbon paper (PVF-CP) electrode illustrates high capacity and stability, indicating the benefits of polymer coating on selective adsorption of rhenium and prevention of electrode corrosive degradation in capacitive deionization systems. The excellent rhenium uptake and longevity demonstrates potential and will be the platform towards a full techno-economic analysis of the hybrid adsorption system in the future.
Method

Materials:

Polyvinyl ferrocene (34801-99-5, produced by Polysciences, U.S.) and tetrabutylammonium perchlorate (1923-70-2, for electrochemical analysis, ≥99%, produced by Sigmar Aldrich Chemistry, U.S.) were used for electrodeposition. Carbon nanotubes (multiwalled, outer diameter of 7−12 nm, length of 0.5 - 10 μm, >99% carbon) were used as conductivity additives in dip coating electrode fabrication. Toray paper (carbon fiber composite carbon paper, produced by FuelCellStore) was used as the substrate for ferrocene polymer deposition under all conditions.

Electrodeposition

Electrodeposition performed on 2 cm x 1 cm rectangles of carbon paper substrate attached to copper wire with copper tape in a three-electrode cell configuration with a quasi-Ag/organic solution reference and CP counter electrode. 0.35 V, 0.45 V and 0.6 V were applied respectively to the working electrode in 2 mg/mL PVF solution until 0.12 C of charge had passed. Dip the electrodes into chloroform twice to rinse and stored the electrodes in the Milli-Q water.

Cyclic Voltammetry (CV)

CV test was performed in a three-electrode cell configuration with PVF deposited carbon paper as working electrode, platinum wire as counter electrode and Ag/AgCl reference. The voltage was swept between 0 V and 0.7 V in 0.5 M sodium perchlorate solution to capture characterize redox peak around 0.35 V.

Rhenium Uptake

The adsorption and desorption experiments were performed in the same three electrode cell but with a solution containing 1 mM NaReO4 or 10 mM NaCl, respectively. The voltage on working electrode maintained 0.8 V for adsorption and -0.8 V for desorption with stirring bar on the bottom stirring at 300 rpm for an hour.
Results
The deposition happens when the positive charges transferred to the PVF that diffuse to the CP surface. The positive charged ferrocene groups combine with perchlorate, which decreases the solubility of PVF and leads to the deposition. According to the gravimetric mass measurement, the deposition mass range for 0.6 V, 0.45 V and 0.35 V are 0.1 – 0.25 mg, 0.2 – 0.35 mg and 0.4 – 0.55 mg, respectively. Comparing with theoretical deposition mass (0.26 mg, calculated from the total charge transfer), higher transition current and shorter deposition time promote a more sufficient charge transfer and a more localized deposition on the electrode interface. The CV plot (Figure 1(a)) elucidates the prominent specific capacitance for 0.45 V deposited electrode (216.50 F/g on average) and an obvious oxidize peak positive movement for 0.35 V deposition, which is corresponding to a higher deposited mass (0.4-0.55 mg) that may lead to a higher resistance. The mass normalized Re uptake reaches 457.87 mg Re/g polymer with a regeneration rate of 64.47%. For next step, the longevity test will be performed to obtain a further understanding to the depletion of regeneration rate along the increase of cycle number.