Thermodynamically Rigorous Description of the Open Circuit Voltage of Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) are considered an outstanding candidate for the integration of renewable energy sources into the existing power grids. A key property of RFBs is the open circuit voltage (OCV) corresponding to the currentless equilibrium state. In the literature, the Nernst equation describing this property is often simplified by neglecting the activity coefficients. In this work, using a thermodynamically rigorous approach, we show that activity coefficients have a significant influence on the OCV of the Iron-Cadmium and All-Vanadium RFBs. Moreover, this influence varies with the state of charge. Therefore, activity coefficients should not be neglected in the Nernst equation. We show that when doing so, the resulting offset in OCV is actually comparable to typical voltage losses occurring during operation. Hence, fitting kinetic parameters to measurement data of voltage losses can lead to ambiguous results if only the idealized OCV, obtained by neglecting the activity coefficients, is used in that evaluation. Therefore, the implementation of a thermodynamically rigorous model has the potential to significantly improve state-of-the-art models for RFBs.

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

“…Here, E (V) is the electrode potential, E 0 (V) is the formal potential, R (8.314 J mol –1 K –1 ) is the universal gas constant, and T (K) is the absolute temperature. This form of the Nernst equation assumes ideal species activities, although activity coefficients may differ at higher electrolyte concentrations . Given this robust understanding of the pseudoreference electrode, we can apply it directly to conventional three-electrode measurements.…”

“…This form of the Nernst equation assumes ideal species activities, although activity coefficients may differ at higher electrolyte concentrations. 29 Given this robust understanding of the pseudoreference electrode, we can apply it directly to conventional threeelectrode measurements.…”

“…For example, in-line electrochemical cells can be used to measure the open circuit potential between the two electrolytes, which can be related to the overall SOC through equilibrium thermodynamic expressions. [27][28][29] Similarly, external reference electrodes can be introduced to measure the open circuit potential at a model electrode surface, which captures the equilibrium potential of each electrolyte independently, allowing determination of individual electrolyte SOCs. 26,[30][31][32] The resultant SOC assessment from these techniques offers a cursory glance into the health of the electrolyte and the balance of charge between the two half-cells.…”

“…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. For example, in-line electrochemical cells can be used to measure the open-circuit potential between the two electrolytes, which can be related to the overall SOC through equilibrium thermodynamic expressions. − Similarly, external reference electrodes can be introduced to measure the open-circuit potential at a model electrode surface, which captures the equilibrium potential of each electrolyte independently, allowing the determination of individual electrolyte SOCs. ,− The resultant SOC assessment from these techniques offers a cursory glance into the health of the electrolyte and the balance of charge between the two half-cells. However, these methods only describe the relative electroactive species concentrations, which presents inherent limitations in cases where performance decay is related to species crossover and decomposition, as these processes can lead to changes in the total concentrations in each half-cell.…”

Estimation of activity coefficients for aqueous organic redox flow batteries: Theoretical basis and equations

“…In category D, authors attempted to manipulate equations in accordance with concentrated solution theory and sometimes advocated against the dilute solution hypothesis. Hayer and Kohns (2020) for example state that “ fitting kinetic parameters to measurement data of voltage losses can lead to ambiguous results if only the idealized OCV, obtained by neglecting the activity coefficients, is used in that evaluation ” and therefore re-introduce the more fundamental form of the Nernst equation for the calculation of the open-circuit voltage where corresponds the chemical activity of species in the electrodes, a quantity central to concentrated solution theory, which will be the focus of the present publication.…”