(Invited) Investigating the Influence of Electrolyte Additives and Anode Structures on Electroplating Performance Towards Use in Iron-Based Batteries
Authors: Gimenez-Garcia Inmaculada , Forner-Cuenca Antoni 
Published: February 2024

Environmental concerns and economic development motivate a shift towards an energy economy based on renewable energy sources. In this new paradigm, large scale energy storage plays a pivotal role in the integration of intermittent and fluctuating renewable energy technologies, such as wind or solar. Among the available solutions, redox flow batteries (RFBs) are promising contenders as they allow independent scaling of power and energy, have a projected long lifetime and lower costs than other battery technologies[1]. However, the most developed RFB systems to date, i.e. all vanadium-RFBs, are still not extensively deployed, due to the elevated and fluctuating costs of vanadium, and the geographical dependency on resource extraction. Iron-based flow batteries, on the contrary, rely on earth-abundant active materials and benign water as solvent, granting potential for cost-effective energy storage systems[2]. However, the efficiency of these electrochemical reactors is limited by challenges such as unoptimized electrolyte compositions, which cause parasitic generation of hydrogen gas or irregular coating distributions[3], and electrode materials that have not been optimized for this specific application. These can severely affect current density distribution, therefore conditioning the plating and stripping or detachment of plated regions (irreversible capacity loss), which limit battery efficiency and operational lifetime. Current cycle life of iron-based batteries has been reported around 2,000 while the US DOE establishes targets comprised between 1,000 to 10,000 cycles[4] Therefore, in this research we aim to understand the underlying mechanisms of iron electroplating and the key parameters which influence the resulting structures, to enhance overall iron-based battery performance.
In this talk, we present our advances to improve iron-RFB efficiency and durability, focusing on the iron plating electrode. In this research, we first leverage planar, model iron surfaces in a Swagelok-type cell and employ a suite of electrochemical techniques (e.g., chronoamperometry, electrochemical impedance spectroscopy) to gain insight into the poorly understood process of iron electroplating. The experimental data is coupled with parameter fitting to pre-existing physical models[5]. Additionally, we study the influence of chelating agents, such as buffering agents, surfactants and levelers (e.g., ascorbate, tetrabutylammonium ion or thiourea, respectively), on plating phenomena and elucidate their effects on the kinetics and structure of the resulting iron coatings, as well as their cycling behavior in symmetric cell configuration. Overall, we find that buffering agents achieved the greatest improvement in terms of faradaic efficiency, resulting in moderate cell currents, while other additives, such as levelers, allow larger plating currents at the expense of limited faradaic efficiencies.
In addition to electrolyte-induced effects, another bottleneck of iron-RFBs are the electrodes employed in the negative half-cell. Common electrodes rely on iron-containing films or sintered iron oxide particles to form 3D structures[6]. However, comprehensive analysis relating anode structure and composition to flow cell performance have not been addressed to date. Accordingly, we hereby study the effects of diverse types of negative electrodes. To minimize complexities emerging in porous electrodes, we first analyze the effects of electrode composition in 2D films (e.g., pure iron, doped iron or iron-carbon hybrid systems) on the cell performance via polarization. Building on these findings, and to further improve iron-RFB performance, we study the impact of electrode architecture, by introducing complex, three-dimensional porous electrodes that enable higher surface areas and mass transport rates. We find that these three-dimensional electrode structures with large pore sizes produce an enhancement in exploitable active species and increased capacity, although there arises a trade-off with the structural integrity of these 3D electrodes. With this research we hope to shed light onto the control of ion-ion, solid-liquid and solid-solid interplay for iron-based batteries to aid the design of tailored electrolytes and advanced electrodes to realize low-cost iron-based battery systems.

References

[1] Dunn, B., Kamath, H., & Tarascon, J. M. (2011). Science, 334(6058), 928-935.
[2] Zhang, H., & Sun, C. (2021). Journal of Power Sources, 493, 229445.
[3] Tucker, M. C., Phillips, A., & Weber, A. Z. (2015). ChemSusChem, 8(23), 3996-4004.
[4] US Department of Energy (2020). Duration Addition to Electricity Storage (DAYS), Program Overview
[5] Weinrich, H., Gehring, M., Tempel, H., Kungl, H., & Eichel, R. A. (2019). Electrochimica acta, 314, 61-71.
[6] Thirumalraj, Balamurugan, et al. (2019). Journal of the American Chemical Society 141.46: 18612-18623.