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European Training Network to improve materials for high-performance, low-cost next- generation redox-flow batteries

Periodic Reporting for period 1 - FlowCamp (European Training Network to improve materials for high-performance, low-cost next- generation redox-flow batteries)

Reporting period: 2017-09-01 to 2019-08-31

The objective of FlowCamp is to provide interdisciplinary training to 15 ESRs, covering the development of next generation redox flow battery (RFB) technology. Currently the all-vanadium redox flow battery is the dominating flow battery technology for long duration stationary storage. In FlowCamp three systems were chosen as next generation flow battery systems. All the chosen systems have the potential to improve specific battery characteristics beyond the state of the art. These technologies are: 1) zinc slurry air RFBs for higher capacity 2) hydrogen bromine RFBs for higher power density and 3) aqueous organic RFBs for environmentally friendlier systems.

The complex interfaces of the scientific development of battery systems touch various disciplines in engineering. This is the reason why the interdisciplinary approach of the FlowCamp network combines material science with modelling and applied electrochemical engineering and spans the whole development cycle from the modelling of battery cells and basic battery chemistry, the development of specialized battery materials like new membranes or catalysts to the development of complete cells and cell-stacks arrangements for these systems. The integrative training approach of FlowCamp provides an interdisciplinary hub, in which tomorrow’s battery scientists are trained to face a fast changing and very competitive and challenging research environment. FlowCamp’s 15 ESRs will gain the interdisciplinary skills needed for effective innovation in this key development field.
Zinc slurry air RFB:
Simulation techniques help to establish correlations between cell design and particle size of the slurry, to design cells with higher power output. With these results and the help of rheological additives, new formulations of slurries have improved standing time, viscosity as well as power density. This has been validated in test cells with new designs. Innovative synthesis strategies for alkaline membranes have been developed, which exhibit improved stability in highly alkaline media. Bifunctional catalysts based on cobaltates, which can be produced in a scalable synthesis, have been developed and characterized for charging and discharging operation.

Hydrogen bromine RFB:
A new model has been developed based on new thermodynamic data on bromine. New materials for hydrogen-bromine cells like new carbon protected Pt catalysts have been developed, which show nearly no degradation in bromine solutions. Also, potentially cheaper membranes based on electrospinning have been demonstrated and tested. The gasketing concept of cells has been redesigned and a new material class of gaskets has been successfully integrated into the new cell-stack concept. This concept has been set up and a PCR-board for segmented cell measurement is under development.


Aqueous organic RFB:
A new, non-linear membrane model has been implemented in a multi-physical model of a cell. This model integrates the capacitive effects of membranes as well as cross-over phenomena and can significantly improve existing multi-physics models of flow batteries. In the field of material research, new quinone and isoalloxazine molecules with improved solubility have been synthesized and tested in model cells. Also, a new class of aqueous organic polymer electrolytes with improved rheological behavior have been investigated and improved beyond existing polymer electrolytes. A new class of membranes based on block polymers has already shown improved selectivity over PFSA membranes. Also porous separators equipped with ion-exchange functionalities have been tested and show promising results in organic flow batteries.
Progress beyond the state of the art has already been achieved in the following areas:

Simulation:
In simulation, innovative approaches have been investigated to predict the behavior of cells in more detail, especially on membrane transport as well as chemical equilibria of bromine cells. These models have an impact on how electrochemical processes will be modelled in the future, and can provide better predictions. In the modelling of slurries composed of individual particles, new ways of modelling these electrolytes have been implemented.

Material research:
In material development FlowCamp has a strong focus on membrane technology. This includes cheaper membrane synthesis by only partially fluorinated or non-fluorinated block polymers, membranes made by electrospinning out of cheap components or reducing ionomers by applying thin layers of ionomer as a coating on porous separators. What these methods have in common is that they show viable industrially scalable production technology to obtain to cheaper and improved membranes for the different systems.

The industrially scalable production of catalysts by flame spraying has also been investigated, and would result in a relatively cheap production method for alkaline air catalysts. These catalysts should have an impact on the price of all alkaline cells, e.g. alkaline fuel cells or electrolyzers.
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Redox flow battery hall at Fraunhofer ICT