Novel ferrocene-containing amphiphilic block copolymer nanoparticles as high-capacity charge carriers in aqueous redox flow systems

Abstract

Size-exclusion polymer electrolytes are promising charge carriers to diminish the crossover and allowing commercially available low-cost porous membranes in redox flow batteries. Boosting the solubility in water and maximizing the number of redox sites to enhance the capacity of these polymeric systems is challenging. New highly water dispersed amphiphilic diblock copolymers are reported here, with an average concentration value of 1.7 10−3 mmol of Ferrocene (Fc)-linked moieties per mg of polymer, determined by total X-ray reflection fluorescence. These redox amphiphilic block copolymers are stabilized in water as spherical nanoparticles (20 nm) by using a simple phase solvent inversion procedure. We evidence a maximum polymer dispersibility value of 6 g/L in water, for long-term stable polymer nanoparticle suspensions, yielding a theoretical capacity value of 4.78 mAh at 10.5 mM Fc. Further adjustment of the ionic conductivity and pH of these stable redox block copolymer suspensions has rendered a conductivity value of 44.5 mS/cm at pH values close to a neutral one, by adding a variety of salt supports. Studies using a 3-electrode configuration cell reveal an efficient charge transport between each of the Fc motifs in the polymer nanoparticle. A capacity value of 3.1 mAh with no transient of the polymer nanoparticles crosswise the cheap porous membrane is evidenced when cycled as polycatholyte material in a Zn hybrid aqueous redox flow battery. The particle size and electronic changes of these novel amphiphilic redox block copolymer electrolytes during consecutive redox cycles have also been monitored by dynamic light scattering and ultraviolet-visible spectroscopy, respectively. The analysis of the results enables the understanding of the main mechanisms behind their non-fully reversible capacity. Among them, aggregation and sedimentation, along with retention inside the graphite felt electrode acting the latter as a filter. These insights will aid the design of future polymer electrolyte materials and redox flow battery components with better performance and cost.