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Modelling for the search for new active materials for redox flow batteries

Periodic Reporting for period 2 - SONAR (Modelling for the search for new active materials for redox flow batteries)

Reporting period: 2021-05-01 to 2022-08-31

SONAR will develop a framework for the simulation-based screening of electroactive materials for aqueous and nonaqueous organic redox flow batteries (RFBs). It will adopt a multiscale modelling paradigm, in which simulation methods at different physical scales will be further advanced and linked by combining physics- and data-based modelling. For the iterative traversal of the different scales, exclusion criteria like solubility, standard potentials and kinetics will be defined, and the results for individual candidates will be stored in a database for further processing. To increase the throughput of the screening, SONAR will exploit advanced data integration, analysis and machine-learning techniques, drawing on the growing amount of data produced during the project. The models will be validated e.g. by comparison with measurements of redox potentials for known chemistries, or measurement data of RFB half-cells and lab-sized test cells.
The overall objectives are:
1. to optimize scale models and adapt them to the requirements of organic RFBs
2. to link the models from objective 1 into an automated, high-throughput, multiscale workflow
3. to validate and exploit the developed models
A demonstrator of the high-throughput screening process has been developed for external usage at The work continued to optimise every relevant software component and all parts have achieved a basic functionality.
A computational protocol for calculations of redox potentials using a hybrid cluster-continuum approach has been developed. The developed methodology for a test set of quinones using different electronic structure methods has been benchmarked.
The capability of automated potential energy surfaces (PES) exploration algorithms to find products and reactions paths of degradation reactions in organic flow battery electrolytes has been explored.
Multiple experiments have been conducted to extract the thermodynamic and kinetic parameters for targeted systems, which are TEMPOL (4-hydroxytempo) and Methyl Viologen, and TIRON and AQS. Formal potential E0’, diffusion coefficient D0 from two methods ((Randles-Sevcik and Levich), rate constant k0 and charge transfer coefficient α were obtained.
The upscaling strategies has been advanced following two approaches. The first one is the mean field approach model and the second one is the Lattice Boltzmann Method model. The kinetic Monte Carlo model was simplified and the mathematical expression which simulates the evolution of the Faradaic current and the potential drop through the compact layer in a more efficient way was extracted. A single-phase LBM model to simulate the dynamic fluid properties in fibrous electrode structure was established.
A reduced electrochemical interface model, which is based on a thermodynamically consistent description of interface processes has been developed. The correctness of the implementation was verified using various published results. Furthermore, a small parameter study was carried out by applying the reduced model for the electrochemical interface to different electrolyte systems.
A methodology based on the volume averaging method to determine these effective parameters in terms of pore-scale geometries and transport properties has been developed. Five different geometries were designed in terms of one or several overlapping cylinders representing carbon fibres used in porous felt materials.
A micro structure analysis including computational and experimental investigations of different commercially available carbon-based electrode materials was carried out.
A 3D resolved continuum model was developed and validated on simplified structured electrodes (cylinders) in the previous reporting period. Within the actual period, the reconstruction workflow of the electrode material is developed. A digital twin of the graphite felt microstructure is gained and used as computational domain within the continuum model.
The permeability tensor of a reconstructed digital twin with periodic boundary conditions is determined. The periodicity of the domain is achieved by mirroring techniques.
Topology optimisation calculations were conducted to enhance the design of a flow battery cell. A homogenized 2D cell model with simple chemistry is developed in COMSOL Multiphysics® and the existing topology optimisation framework of the software is used for the calculations.
A hydraulic stack model and an electrochemical stack model has been developed. Both models incorporate an input interface that allows input parameters to be varied for different chemistries and system designs. The hydraulic stack model considers the electrolyte flow and the associated pressure losses through the active cell areas, secondary manifolds feeding the cells and the flow through the stack manifolds into the secondary manifolds.
An optimized techno-economic model was created in order to be able to take into account further factors influencing the costs and technical properties of organic RFBs. Furthermore, an inventory of existing technical components was made based on laboratory cells as a reference for commercial batteries. To obtain experimental data, different RFBs were set up and investigated using different electrochemical methods.
An optimised version of the technoeconomic model was programmed in Python as a stand-alon software, using the extensive input variables determined in the laboratory itself.
A range of dissemination channels have been used to create awareness about the project’s activities and results: a website ( newsletter, Twitter and LinkedIn. On the initiative of SONAR coordinator Fraunhofer ICT, the Network of (EU-funded) Flow Battery Initiatives “FLORES” was founded in 2021. Its goal is scientific exchange, joint outreach and joint dissemination of project results. Several scientific publications are in preparation for submission, and all project partners contributed to a book, edited by the SONAR coordinator.
The developments made in SONAR beyond the state of the art were 1) individual scale models and 2) the development of a high-throughput screening. The approach of SONAR is the linear linking of optimized individual scale models and the transfer of results into the respective other scales to calculate the performance values and costs of organic RFBs. So far, the foundations have been laid by creating a data structure and database and starting to train a machine-learning program based on database values. In addition, individual scale models have been developed with the requirements of aqueous organic RFBs. These include, in particular, the consideration of pH changes in quantum mechanical calculations of redox potentials, the development of a kinetic Monte Carlo model for organic RFBs, the development of a fast 0D cell model and the consideration of the specific properties of felts in the modeling of cells. Furthermore, experimental work was carried out to validate and provide experimental data at practical concentrations of active materials.
This work will be continued and intensified until the end of the project. With the start of the work in the area of stack and system modeling as well as techno-economics, an analysis of the costs will be possible for the first time, in addition to the calculation of additional system performance parameters.