IR Spectroscopy as a Method for Online Electrolyte State Assessment in RFBs

Abstract

viability. However, this shift in energy production has a major drawback: While conventional power plants are designed to produce energy on demand with constant output, the intrinsic intermittency of wind, and solar energy productions calls for suitable energy storage solutions, in which redox flow batteries (RFBs) represent a promising solution. [1] Compared to the current commercially available inorganic systems based on vanadium (VRFB) or zinc-bromine/zinc-iron, the use of organic active materials in RFBs promises a decoupling from commodity prices as well as an environmentally and socially responsible production of the storage materials. [2] However, organics are prone to decomposition reactions on the molecular level that inevitably occur within the pursued decadal lifetimes required for an economic operation of RFBs. [3] Thus, methods to accurately assess the key electrolyte state variables state of charge (SOC) and state of health (SOH) are of paramount importance for these RFB concepts.As recently reviewed by our group, the most established SOC monitoring methods for RFBs are represented by Coulomb counting (CC) and measurements of the electrolytes' redox potential. [4] Both methods require either relatively simple measurement equipment (open circuit voltage (OCV) cells) or are implemented into a battery management system (CC) and can therefore be used independently of the RFB chemistry. Nevertheless, both methods are prone to error accumulation and long-term changes and, thus, require frequent recalibration. Furthermore, the CC method can only provide full-cell resolution (i.e., battery state variables) instead of a more informative half-cell resolution, that is able to provide an electrolyte-based state assessment. Apart from the aforementioned methods, spectroscopic methods have as well been used to determine flow battery state variables, in particular for the VRFB, although other chemistries have been investigated as well. [4,5] Especially UV-vis spectroscopic measurements have been used in this context, due to the relatively simple and reliable setup as well as their ability to provide half-cell resolution. However, due to the nonlinearity of the absorption for optically dense mixtures, as described by the Beer-Lambert law, the hitherto published methods generally rely on empirically-derived calibrations or extremely thin transmittance measurement cells. [4] Furthermore, UV-vis data of molecular compounds usually bear limited structural information, as the The transition from fossil to renewable energy sources requires adequate storage technologies due to the intermittency of the supplied energy. With respect to this, organic redox-flow batteries (ORFBs) represent a promising concept for the storage of electricity on a large scale at economically justifiable costs. However, these storage technologies can only be operated reliably if parameters representing the actual condition of the storage medium (i.e., the electrolyte) can be accurately assessed. These so-called electrolyte state variables are ...

Microelectrode-Based Sensor for Measuring Operando Active Species Concentrations in Redox Flow Cells

“…There are numerous experimental tools that can be used to track SOC and SOH in RFB electrolytes, and the method of choice may vary depending on the chemistry and the desired analytical information. Spectroscopic techniques, including spectrophotometry, , nuclear magnetic resonance spectroscopy, − electron paramagnetic resonance spectroscopy, ,, and infrared spectroscopy, , can enable quantitative measurements of the species concentration and, in some cases, elicit structural information about the identity of possible decomposition products; but these instruments typically require specialized hardware and infrastructure to perform in situ or operando diagnostics . In lieu of more comprehensive chemical characterizations of the electrolyte, one can monitor physicochemical (e.g., density, viscosity) and electrochemical (e.g., conductivity, , open-circuit potential) descriptors that, in many instances, correlate with electrolyte SOC.…”

State of Charge and State of Health Assessment of Viologens in Aqueous‐Organic Redox‐Flow Electrolytes Using In Situ IR Spectroscopy and Multivariate Curve Resolution

“…The recorded IR calibrations show a high linearity, as indicated by the coefficient of determination at each wavenumber visible in Figure S2 (Supporting Information). In accordance with the approach presented in our previous study, [ 9 ] the refractive index of all solutions was also recorded, as it is crucial for the SOH determination (see Supporting Information).…”

“…couples of interest, such as the V IV /V V or hydroquinone/quinone couples, for which species interactions are known as well. [10a,17] As discussed within our previous study, which established the use of in situ IR spectroscopy for the online monitoring of RFBs, [9] any of the recorded IR spectra have been subjected to a pre-processing scheme that was designed to minimize the influences of water vapor and temperature changes, correct for the wavelength-dependent penetration depth in the ATR setup as well as provide a background correction for the solvent (and any supporting electrolyte). Detailed information on the preprocessing steps is available in the Supporting Information.…”

Synthesis and characterization of new 1,3-Oxazepine-4,7-dione compounds from 1,2-diaminobenzene

“…FT-IR spectra for compounds C1-C3 showed in Figures (1, 3) the disappearance of the -NH2 group and on other hand the appearance of a new imine group in the range (1636 -1642 cm -1 ) (Nolte et al, 2021). The bands at the range (3062 -3081cm -1 ) for the C-H aromatic stretching.…”