Estimating Activity Coefficients of Target Components in Poorly Specified Mixtures with NMR Spectroscopy and COSMO-RS

Poorly specified mixtures are common in process engineering, especially in bioprocess engineering. The properties of such mixtures of unknown composition cannot be described using conventional thermodynamic models. The NEAT method, which has recently been developed in our group, enables the calculation of activity coefficients of known target components in such poorly specified mixtures. In NEAT, the group composition of the mixture is determined by NMR spectroscopy and a thermodynamic group contribution method is used for calculating the activity coefficients. In all previous studies with NEAT, the UNIFAC group contribution method was used. In the present work, we demonstrate that NEAT can also be applied with another important method for predicting activity coefficients: COSMO-RS. COSMO-RS (OL) developed in Oldenburg together with its group contribution version GC-COSMO-RS (OL) is used here. The new version of NEAT was successfully tested. For a variety of aqueous mixtures excellent agreement of the NEAT predictions, for which only information on the target component was used, with results that were obtained using the full knowledge on the composition of the mixture was found. The results demonstrate the generic nature of the idea of NEAT and the broad applicability of the method.

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

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.