In Operando Raman Spectroscopy – a Powerful Tool for Understanding the Chemistry and Electrochemistry of Aqueous Redox Flow Batteries
Authors: Weller Jon Mark , Nambafu Gabriel , Hollas Aaron , Rice Peter , Li Guosheng , Reed David M , Sprenkle Vincent 
Published: December 2024

Of the many emerging technologies for low-cost, grid-scale energy storage, redox flow batteries have been an area of active investigation for decades. More recently, a focus on lower cost alternatives to the foundational Vanadium-based redox flow batteries has been a major focus of the flow battery research community. Specifically, there is great interest in redox active species based on Earth-abundant materials suitable for aqueous systems such that the overall cost per kWh of energy storage can be minimized to enable economical and reliable grid storage to utilize energy generated from intermittent, renewable sources.
The use of aqueous electrolytes, ideally at mild pH, in conjunction with inexpensive and highly soluble active material presents an exciting challenge for grid-scale flow batteries. Many new aqueous-soluble, redox active compounds have garnered interest, including water-soluble organic redox species and low-cost transition-metal complexes based on Fe or other earth-abundant metals. Recently, our group has devoted significant attention to new anolytes based on Fe(II)/Fe(III) complexes with various types of low-cost and easily synthesizable phosphonate ligands. Specifically, the nature and fine-tuning of phosphonate ligands allows a high degree of control over the redox potential, rate capability and kinetics, and stability of Fe-based complexes for flow battery anolytes. The wide variety of potential ligands with different functional moieties sometimes produces complicated electrochemical behavior, which has motivated us to apply in operando analysis to inform interpretation of electrochemical and battery cycling results.
This presentation focuses on one such analytical tool, namely Raman spectroscopy, applied in operando to several flow batteries based on various Fe-phosphonate anolyte complexes to understand the effects of various substituents and functional groups on the Fe(II)/Fe(III) redox behavior, potential, kinetics, and even structural transformations of these complexes during battery cycling. Examples based on recently reported nitrilotri(methylphosphonic acid) (NTMPA) and related phosphonic acid ligands and their resultant Fe-complex-based anolytes will be presented showing that online vibrational spectroscopy can provide clarity on the effects of varied functional groups on both the molecular structure of redox-active Fe-complexes and the effects of ligand binding on kinetics. This experimental tool beneficially compliments and confirms results from density functional theory to elucidate the structure of these Fe-complexes. Further, the multivariate spectral response provided by in operando Raman spectroscopy allows the state of charge of the cell to be monitored readily. Overall, we aim to highlight the utility of Raman spectroscopy in probing the chemical transformations that occur during electrochemical measurements and battery testing in real time, with potential benefits to a much wider variety of promising redox flow battery systems and the flow battery community at large.

Figure 1