(General Student Award Winner - 3rd Place) Electropolymerized Poly(3,4-ethylenedioxythiophene) Coatings on Porous Carbon Electrodes for Electrochemical Separation of Metals

Abstract

Advances in decarbonization technologies such as batteries, photovoltaics, and wind turbines are accelerating the energy transition from fossil fuels to renewables. However, these technologies drive the demand for so called “critical metals” (e.g., Co, Ni, Li) that are unequally distributed over the planet and their mining has a significant environmental impact1. Electroplating and mining effluents can be utilized as a secondary source for these metals, but the low concentration of metals in their ionic form is a challenging medium for conventional separation methods2. Electrode-assisted separation methods where ions are immobilized on the electrode surface are well-suited for ionic separations as they can offer selectivity for the target metal ion with low energy consumption, minimal chemical input, and direct integration with the electrical grid3. These technologies can greatly benefit from the infrastructure of flow electrochemical platforms, including the use of porous carbonaceous electrodes to boost convective transport and minimize reactor resistances4. Imparting ion-selectivity on carbon electrodes is possible by immobilizing redox-active layers on their surface3. However, traditional coating techniques used to deposit functional materials (i.e., dip coating, spraying, doctor blading) cannot be realized on macroporous supports without compromising their microstructure. The use of conductive polymers can overcome this challenge as they can be coated on complex morphologies with controllable thickness through electropolymerization reactions, and they can provide a conductive and switchable matrix for ion-exchange. We hypothesize that the electrochemically switchable ion exchange (ESIX) property of conductive polymers, combined with the utility of macroporous carbon scaffolds can unlock convection enhanced and zero-gap reactor architectures for ionic separations.In this work, we investigate the electropolymerization of an industrially relevant polymer poly(3,4-ethylenedioxythiophene) (PEDOT) on commercial carbon fiber-based paper electrodes in terms of its synthetic parameters, morphology and ESIX-activity5. When coordinated to poly(4-styrenesulfonate) anions, PEDOT/PSS blend forms smooth and conformal layers on the carbon fibers with a controllable thickness down to 0.1 µm. We show that the PEDOT/PSS system acts as potential-controlled cation exchange material that can ingress Ni2+ at a low potential of -0.25 V (vs Ag/AgCl) and demonstrates higher affinity towards Ni2+ than Zn2+. The advantage of ESIX is that both adsorption and desorption are potential controlled. To verify this, we performed a complete ESIX-swing on PEDOT/PSS coatings with increasing coating weights and analyzed the desorbate for ion content (Figure 1). We find that by increasing the PEDOT/PSS coating weight, more Ni2+ can be separated from binary Ni2+:Na+ solutions, but the adsorption capacity decreases. The highest adsorption capacity of 227 mg g-1 is reached for the lowest coating weight, which suggests that thicker coatings are not well utilized within the time frame of the ESIX experiments. Although this work focuses on conductive polymers for electrochemical separation of metals, it can inform researchers on electropolymerization-based coatings for various electrochemical technologies (e.g., fuel cells, electrolyzers, redox flow batteries) that utilize porous electrodes. References T. Watari, K. Nansai, and K. Nakajima, Resources, Conservation and Recycling, 155, 104669 (2020).K. Kim et al., iScience, 24, 102374 (2021).X. Su, Current Opinion in Colloid & Interface Science, 46, 77–93 (2020).W. Tang et al., Water Research, 150, 225–251 (2019).E. B. Boz, M. Fritz, and A. Forner-Cuenca, submitted (2022). Figure 1