Exploring Iron-Containing Electrodes for Use as Negative Electrodes of All-Iron Redox Flow Batteries
Authors: Gimenez-Garcia Inmaculada , Forner-Cuenca Antoni 
Published: December 2024

The transition towards a renewables-based energy economy is motivated by pressing environmental concerns and economic growth rates. This new energy paradigm necessitates efficient, robust and cost-effective, large-scale energy storage systems. Aqueous, all-iron redox flow batteries (AIRFBs) are a promising contender for large-scale energy storage, motivated by their reliance on environmentally friendly and abundant raw materials, their intrinsic safety, and projected high energy density (ca. 135 Ah L-1) [1]. However, the performance and lifetime of state-of-the-art AIRFBs are greatly limited the poor kinetics and uneven plating taking place in the negative half-cell. Traditionally, the electrochemical stack leverages carbon fiber-based electrodes (e.g. felts), which provide moderate surface area for reaction but suffer from sluggish kinetics and plated layers of poor quality [2].
Hereby, we explore the use of iron-based (e.g. pure iron, steel types 302, 316, 420) electrode alternatives to overcome the challenges of prevalent carbon fiber electrodes or pure iron. Steels have been investigated for numerous electrochemical systems, such as (microbial) fuel cells, electrolyzers and various types of batteries, displaying high performance and excellent chemical stability, robust mechanical properties and low production costs [3]. More precisely, in this work we investigate iron-based electrodes as a substitute for conventional carbon felts due to their enhanced kinetic activity in the negative half cell [Fe2+(aq)|Fe(s)]. Firstly, different metal electrodes were tested in flow-by configuration to obtain their polarization performance. Employing a flat electrode of pure alpha-iron as a baseline, we find that it displays balanced behavior between charging and discharging mode, although it exhibits very limited utilization of its projected gravimetric capacity[4]. Following this, we test a set of steel sheets of different nature, that is, ferritic, martensitic and austenitic, which contain various alloying elements (e.g., Cr, Ni, Mn, Mo, etc.) in different ratios, resulting in different electrochemical performance. We find that materials containing low to moderate Ni:Cr ratios result in enhanced iron plating and stripping kinetics, with increases ranging from ~20 to ~60% with respect to the pure alpha-iron electrode. Furthermore, we observe that the crystallographic structure of austenitic materials tend to favor reactions involving iron ions, as opposed to martensitic structures of which feature higher solid-to-gas surface energy, which favor the competing hydrogen evolution reaction [5]. We additionally investigate advanced three-dimensional structures in a symmetric, flow-through configuration, by selecting the best performing steel composition. By screening various fiber arrangements, we aim to elucidate the structural effects on mass transfer and overall cell performance.
Overall, we studied the interplay between the composition and architecture of different metal-based electrodes. We hope that these findings will help shed light on their potential to improve the performance of the challenging iron plating-stripping reaction and ultimately introduce them in a full cell system to enable higher capacity and rate performance of the promising all-iron flow battery

Acknowledgments

IGG gratefully acknowledges financial support by “la Caixa Foundation” (ID 100010434) under the fellowship number LCF/BQ/EU20/11810076. AFC gratefully acknowledges funding by the European Union (ERC, FAIR-RFB, ERC-2021-STG 101042844). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

References

[1]Hruska, L.W. & Savinell, R.F. (1981). Journal of the Electrochemical Society, 128(1), 18
[2] Noack J. et al., (2021). Journal of the Electrochemical Society, 168(4), 040529
[3] Kim, M. et al. (2022) Chemical Engineering Journal¸ 440, 135459
[4] James, B.D et al., (2016). DoE Strategic Analysis Report.

[5] Gatos, H.C. (1962) Science 137(80), 311