Electrokinetic Properties of Zn-Alginate Beads in a Zn-Air Flow Battery

Abstract

This study explores the electrokinetic properties of zinc-alginate beads in the context of a zinc-air solid- mediated flow battery, as there has been a renewed interest in zinc–air batteries due to their potential for high energy density, safety, and environmental benefits [43]. On the contrary, there are several challenges related to both the anode and the cathode of the Zn-air battery. This research focuses on the challenges related to the anode side which consists of zinc dendrite formation, reducing the battery’s lifetime. Also, Zn passivation increases internal resistance and reduces active capacity. Additionally, hydrogen evolution reaction during charging decreases the coulombic efficiency and leads to safety risks due to gas pressure build-up [40][43]. Trying to solve these issues, a European pathfinder project, ReZilient, has proposed a Zinc-Air Solid Mediated Flow Battery. In the anolyte tank, alginate beads are introduced to encapsulate zinc, aiming to prevent these parasitic reactions. Besides, the encapsulation of solid ZnO microparticles overcomes the limited solubility of ZnO and could significantly enhance the system’s energy density. This study explores the electrokinetic and mechanical properties of Zn/ZnO beads and the capability of preventing these parasitic reactions. Experimental research, including thermogravimetric analysis, scanning elec- tron microscopy, cyclic voltammetry and electrical impedance spectroscopy was used to characterize and evaluate the bead’s electrochemical and mechanical behaviour. After synthesizing the beads it was found that beads consisting of 2.5% and 5% w/v sodium alginate demonstrated a strong structure of which the last one was used for further experimental research. The synthesized beads were uniformly sized, with diameters mainly between 3.3 mm and 3.5 mm. Besides, thermogravimetric analysis was performed on beads produced from a 5% w/v sodium alginate, 3% carbon black and 10% ZnO solution. The analysis revealed that the bead consisted of 82.7% water, 7.4% organic alginate and carbon black, and 9.9% metal oxides including ZnO and CaO, although the specific ratio of calcium remains to be determined. Additionally, the diffusion rates for alkali metal hydroxides like NaOH and KOH were measured at 0.0464 s−1 and 0.0435 s−1, respectively. Conductivity assessments via cyclic voltammetry identified a con- ductivity of 0.01 S/m at a 3% KS4 graphite concentration, which aligned with literature studies. Addi- tionally, hydrogen evolution was observed within the beads during voltammetry experiments conducted between 0 and -1.4 V (vs Ag/AgCl), indicating that the alginate’s carboxylic acid groups do not suffi- ciently increase the overpotential for the reduction of water to mitigate hydrogen evolution. Still, cyclic voltammetry of the bead was performed with a technique, called film voltammetry, inspired by the study of Yoon et al. (2019). The measurement showed that the Zn/ZnO could perform a redox reaction while situating in the bead. it exhibited a redox potential of around -1.4 V (vs Ag/AgCl) at a pH of 12, indicating Zn/Zn(OH)−2 4 . However, it suffered from very low zinc utilization of 0.0007% and limited reversibility as the reduction kinetics were unfavourable. The Zn-Ca-crosslinked bead demonstrated faster reaction kinetics and achieved a redox potential of -1.05 V (vs Ag/AgCl), indicating Zn/Zn+2, yet it also experienced zinc loss and declining efficiency over time as it shifted to similar electrochemical behaviour as the Zn/ZnO bead. To improve zinc utilization, ZnO was encapsulated with a carbon micro-shell before being added to the alginate beads to increase ionic and electronic conductivity. However, it only showed slightly enhanced reduction currents, while zinc utilization remained low. Next to the electrochemical behaviour, were the mechanical properties of the beads tested as well, with a Young’s modulus of 0.58 MPa for the 5% w/v Na-Alg with 3% w/v KS4 graphite bead. These findings show that strong beads were synthesised and Zn/ZnO is electrochemically reactive within the bead. Nevertheless, the low zinc utilization and dogged parasitic reactions require further research. This research provides a foundation for improving the performance of Zn beads in zinc-air solid-mediated flow batteries by addressing their electrochemical behaviour and mechanical properties.