Effect of Glycine on a Fe-Based Electrolyte for Redox Flow Batteries
Authors: Groiss Emanuele Maria , Duranti Mattia , Macchi Edoardo Gino 
Published: February 2024

Among the rising stationary energy storage technologies, Redox Flow Batteries (RFBs) emerge as a promising electrochemical device capable of high flexibility in terms of power and energy combined with potential low cost and low environmental impact. Alternative chemistries can employ cheap and earth-abundant materials, possibly alleviating the demand for critical raw materials, which are extensively employed in the field of energy production and storage. Iron-based chemistries are among the best candidates for aqueous pH-neutral catholytes, offering fast reaction kinetics and a well-placed standard reduction potential of 0.771 V, that, coupled with a Zinc-based anolyte, would give a nominal cell voltage of 1.53 V [1]. Unfortunately, the dissolution of Iron salts in a neutral aqueous solution triggers hydrolysis, resulting in the formation of Iron hydroxides in the form of insoluble deposits. This causes the loss of electroactive species from the electrolyte, and consequently decreases the battery capacity. Thus far, the conventional solution is the electrolyte acidification, which, however, significantly increases the costs of materials for stacks and electrolyte handling components and aggravates the already critical problem of Hydrogen evolution at the Zinc side. As an alternative, several complexing agents have been proposed in the literature, with preliminary studies on their complexation mechanisms [2]. Glycine is often considered as one of the most promising candidates, even though its suppressing effect on the Fe(II)/Fe(III) kinetics at high concentrations is widely reported [3].
This research investigates an optimized pH-neutral catholyte for Zinc-Iron RFBs, focusing on the use of Glycine to avoid Iron-induced hydrolysis. The study is carried out on highly concentrated solutions of Iron chlorides, with the addition of Glycine as ligand at various concentrations. The aim is to preserve the performances of Fe(II)/Fe(III) kinetics and to avoid insoluble hydroxides formation by an accurate choice of the Gly-Fe ratio. The produced electrolytes have been characterized by measuring the solutions conductivity, kinematic viscosity and density. Then, an electrochemical characterization is performed evaluating the diffusion coefficients (𝐷) of the dissolved species, the equilibrium potential (𝐸0) and the exchange current density (𝑗0) as well as the rate constant of the reactions (𝑘0) by means of cyclic voltammetries with a glassy carbon rotating disc electrode. The diffusion coefficients are obtained from Randles-Sevcik equation while the equilibrium potential, the exchange current density and the reaction rate constants are evaluated with a Koutecky-Levich analysis. An optimization of the electrolyte preparation method was also performed, evaluating the effects of pH and temperature and monitoring the complexation time, with the aim of accelerating the Gly-Fe complexation reaction. The effects of KCl and CaCl2 on the electrolyte conductivity are also investigated, to identify suitable supporting electrolytes.
The research results show how the addition of Glycine successfully prevents the formation of precipitates in equimolar solutions even at low Gly-Fe ratios. However, Glycine suppresses Fe(II)/Fe(III) kinetics and lowers the electrolyte equilibrium potential, consequently reducing both energy density and the maximum current for its use in RFBs. It is thus possible to identify an optimum for the Gly-Fe ratio by setting a compromise between chemical stability and electrochemical performances. We propose the range 0.5:1 to 1:1 as a convenient Gly-Fe ratio for the 1M FeCl2 and 1M FeCl3 system, reaching a potential higher than 0.721 V vs. SHE, making it a very attractive catholyte with excellent suppression of Iron hydrolysis as no precipitation is observed after more than three months.
This work is funded by the Italian Ministry of Enterprises and Made in Italy in the framework of the Important Project of Common European Interest (IPCEI) European Battery Innovation (project IPCEI Batterie 2 - CUP: B62C22000010001). The IPCEI European Battery Innovation is also funded by public authorities from Austria, Belgium, Croatia, Finland, France, Germany, Greece, Poland, Slovakia, Spain and Sweden.
[1] Zhang, Huan, Chuanyu Sun, and Mingming Ge. "Review of the research status of cost-effective zinc–iron redox flow batteries." Batteries 8.11 (2022): 202.
[2] Hawthorne, Krista L., Jesse S. Wainright, and Robert F. Savinell. "Studies of iron-ligand complexes for an all-iron flow battery application." Journal of The Electrochemical Society 161.10 (2014): A1662.
[3] Xie, Congxin, et al. "A Low‐Cost Neutral Zinc–Iron Flow Battery with High Energy Density for Stationary Energy Storage." Angewandte Chemie International Edition 56.47 (2017): 14953-14957.

Figure 1