Improvements to the Coulombic Efficiency of the Iron Electrode for an All-Iron Redox-Flow Battery

Abstract

The all-iron redox flow battery is an attractive solution for large-scale energy storage because of the low cost and eco-friendliness of iron-based materials. A major challenge to realizing a continuously operable battery is the parasitic evolution of hydrogen at the iron electrode during battery charging. We found that the adsorption of ascorbic acid added to the electrolyte inhibited hydrogen evolution at pH = 0. Elevation of pH near the surface of the electrode during electrodeposition also raised the coulombic efficiency. Thus, electrolyte flow rates significantly influence the coulombic efficiency. Ascorbic acid also served to regulate the pH near the surface of the negative electrode by buffering action. We found that increasing the operating temperature enhanced the kinetics of iron deposition relative to the kinetics of hydrogen evolution, leading to a net rise of coulombic efficiency. Thus, by operating at 60 • C and a pH of 3 with ascorbic acid and ammonium chloride, we achieved a coulombic efficiency of 97.9%. While this value of coulombic efficiency is among the highest values reported for the iron electrode in the context of the all-iron flow battery, further improvement in efficiency is needed for supporting repeated cycling. The results presented here provide insights for further improvements.

A Rechargeable Battery with an Iron Metal Anode

“…The Fe plating occurs at the potential of −0.64 V versus SHE (−0.20 V vs Fe 2+ /Fe), which is −0.32 V lower than the HER thermodynamic potential at pH = 5.5. Undoubtedly, efforts are still needed to further improve the CE, perhaps with the new formula of electrolytes and tailor‐designed current collectors …”

The Influence of Some Electrolyte Additives on the Electrochemical Performance of Fe/Fe2+ Redox Reactions for Iron/Iron Redox Flow Batteries

“…Two recent papers on the hybrid Fe/Fe-RFB by the Narayanan group involved the co-deposition of a secondary metal with Fe to improve coulombic efficiency. 22,23 The authors postulate that these metals (In, Bi and Cd) will remain at the electrode surface and will not diffuse into the bulk of the plated iron because of their theoretical immiscibility with iron. Therefore, only a small amount of secondary metal is necessary to nearly completely inhibit the HER, since there always remain a few monolayers of secondary metal at the surface.…”

Efficient Hydrogen Delivery for Microbial Electrosynthesis via 3D-Printed Cathodes

“…In circumneutral microbial growth medium the concentration of H + and OH – ions is low at 0.1 μM (pH = 7). Therefore, electrochemical proton reduction to H 2 at even moderate current density can lead to high local pH in the diffusive boundary layer ( Jayathilake et al, 2018 ). While the presence of a buffer in mM concentration can significantly mitigate this effect ( Auinger et al, 2011 ; Shinagawa et al, 2019 ), the rate of diffusion of the protonated buffer into the boundary layer from the bulk liquid becomes limiting for a sustained formation of dissolved, bubble-free H 2 at medium or high current densities ( Figure 6B ).…”