Exploring Conductive Polymer Coatings to Target Reaction Selectivity in Aqueous All-Iron Redox Flow Batteries
Authors: Boz Emre Burak , de Bruijne Ronald , Forner-Cuenca Antoni 
Published: February 2024

An energy grid that relies on wind and solar farms requires the installation of large-scale energy storage systems to balance their fluctuating output. Especially for developing countries, wide adoption of renewables is coupled to progress in sustainable and cost-effective storage technologies.1 From this perspective, aqueous all-iron redox flow batteries (AIRFBs) stand out as a promising technology, owing to the earth-abundance and low-cost of iron, along with its environmental and operational safety. In this battery, iron is utilized in its three oxidation states (Fe0, Fe2+ and Fe3+) and the negative electrode leverages the iron plating/stripping reactions to sustain charge and discharge. However, the standard potential of iron plating is 0.44 V below the hydrogen evolution reaction (HER), which causes faradaic losses during the charging stage and increases the pH of the solution.2 The catalytic activity of iron towards hydrogen evolution and the precipitation of iron hydroxide at pH above 3.5 further complicates the operation of AIRFBs. Thus, the reaction selectivity of the negative electrode should be targeted to improve the operational time and efficiency. So far, research has focused on electrolyte engineering to hinder adsorption of hydrogen on surfaces or excluding water from the solvent shell of iron.2,3 Alternatively, interfacial engineering of electrode surfaces can be a promising strategy to tune the reaction selectivity, given that hydrogen adsorption and iron nucleation are interfacial phenomena. The challenge here is to prevent a new interface from being created by the plated iron as that would render the modified interface obsolete. We hypothesize that polymeric layers where iron deposition can take place within or underneath the polymer (akin to a solid electrolyte interphase of a Li-ion battery anode) would preserve the HER-inhibiting characteristics of the surface.
Here, we propose conductive polymer interfaces to target reaction selectivity during the iron plating reaction. Conductive polymers have been used to engineer HER-inhibiting interfaces and as corrosion protection layers for metals.4,5 Furthermore, they can be conformally coated on porous electrodes, enabling their use in flow reactors.6 We selected two conductive polymer systems: poly(pyrrole) (PPy) owing to its iron-coordinating nitrogen moieties and poly(3,4-ethylenedioxythiophene) (PEDOT) owing to its high conductivity and stability.7 Furthermore, we employed three counterions of different size, chlorine (Cl-), p-toluenesulfonate (pTS-) and poly(styrenesulfonate) (PSS-), to direct the morphology of the coating, resulting in six different polymer systems. On glassy carbon electrodes, all polymers significantly hinder HER at potentials as low as -1.5 V (vs Ag/AgCl) where heavy bubble formation is observed on bare electrodes (Figure 1). To assess the selectivity of the coatings on carbon paper substrates, we are developing an electrochemical protocol with successive plating/stripping cycles that is representative of battery operation. Preliminary results show that all polymer systems hinder the HER, but also the Fe-plating reaction, resulting in lower plating currents than the bare carbon paper electrodes. To validate the methodology, we compare the electrochemically obtained reaction selectivity values with the ones from gravimetric analysis. The ideal polymeric interfaces should inhibit the HER without largely impacting the Fe-plating kinetics, which motivates research into the relationship between the coating thickness, conductivity, and the faradaic efficiency. To understand the plating morphology on polymeric coatings, we investigate the spatial distribution of iron on the surface and the cross-section of plated electrodes using microscopic methods. Hydrogen evolution and limited durability due to complex plating reactions hamper the broad implementation of all-iron redox flow batteries and we hope to tackle these challenges through interfacial electrode engineering.

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Figure 1