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

Periodic Reporting for period 2 - FlowCamp (European Training Network to improve materials for high-performance, low-cost next- generation redox-flow batteries)

Reporting period: 2019-09-01 to 2022-02-28

FlowCamp was an interdisciplinary training network of 15 doctoral students, 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 mid- to long-duration stationary storage. In FlowCamp three systems were chosen as next-generation flow battery systems to improve specific battery characteristics beyond the state of the art. These technologies were: 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. The interdisciplinary approach of the FlowCamp network combined material science, modelling and applied electrochemical engineering and spanned the whole development cycle from the modelling of battery cells and basic battery chemistry, the development of specialized battery materials like new membranes and catalysts to the development of complete cells and mini-stacks for the three chosen systems. The development was accompanied by an integrative training approach providing an interdisciplinary hub in which tomorrow’s battery scientists were trained to face a fast-changing, competitive and challenging research environment.
Zinc slurry air RFB:
Zinc-air systems have the highest energy density of all chosen systems. In a zinc slurry air flow battery, the electrolyte is also the electrode, leading to a complete redesign of typical flow-cells. In FlowCamp project ,a new simulation technique for Zinc slurry air RFB has been established, which includes percolation theory to investigate the momentary available capacity in a cell. With the help of hydraulic models the geometry of cell frames was optimized with better hydraulic design. In addition, the integration of rheological and electrochemical additives led to new formulations of slurries with improved standing time, viscosity and power density. This was validated in test cells with the optimized hydraulic design, and power densities up to 150 mW/cm2 were achieved with slurries with 30% zinc loadings in discharge mode. Innovative synthesis strategies for alkaline membranes were developed, which exhibit improved stability in highly alkaline media. Bifunctional catalysts based on cobaltates, which can be produced in a scalable synthesis, were developed and characterized.
The next steps for this system: The FlowCamp partners are currently exploring future exploitation possibilities and are in contact with international start-ups in this field.

Hydrogen bromine RFB:
The hydrogen bromine RFB has the highest power density of all RFB systems. The FlowCamp team aimed to solve this challenge by creating new membranes, protected catalysts and models to understand electrolyte composition and crossover phenomena. A new model for OCV in HBr-RFB was developed based on new thermodynamic data of highly concentrated bromine in acidic solutions, which provides insight into the composition of the electrolyte at different state of charge. This helps to understand crossover phenomena in the cell, which are the most prominent degrading mechanism. To mitigate the effects of crossover, a new class of carbon or metal-oxide protected Pt catalysts were developed, which show nearly no degradation in bromine solutions. Also, potentially cheaper membranes based on electrospinning of cheap precurser materials were demonstrated and tested. The gasketing concept of cells was redesigned and a new material class of gaskets were successfully integrated into the new cell-stack concept. Mitigation strategies for crossover were investigated with locally resolved current density measurements with segmented electrodes.
Next steps towards commercialization: The company ELESTOR, as a partner in FlowCamp, was actively involved in the developments.The next achievement will be to advance the technology to pilot scale within follow-up projects, included the EU-funded project MELODY.

Aqueous organic RFB:
Organic RFBs are characterized by the absence of metals, leading to more sustainable electrolyte chemistry. The FlowCamp team made exceptional progress in this field with the help of industrial partners JenaBatteries and Amer-Sil.A new, non-linear membrane model for membrane transport was 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 were synthesized and tested in model cells. Also, new approaches of aqueous organic polymer electrolytes with improved rheological behavior were investigated. 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 were tested and show promising results in organic flow batteries.
The next steps for organic RFB: All partners are continuing their work on organic RFBs in follow-up projects, on future organic redox comples (ZHAW in SONAR, HAS in CompBat) and membranes (Amer-Sil, CNRS).
Progress beyond the state of the art was achieved in the following areas:

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 highly concentrated bromine electrolytes. For this, new validation techniques, like measurements of undisturbed bromine equilibrium potentials with glass electrodes as well as measuring water activity in membranes by measuring freezing enthalpy helped to develop these models and can provide better predictions of cell behavior. Also new models for slurry electrolytes composed of individual particles have been developed, and the integration of percolation theory has opened new ways of modelling slurry-type electrolytes.

Material research:
In material development FlowCamp had a strong focus on membrane technology. This included cheaper membrane synthesis using only partially fluorinated or non-fluorinated block polymers, membranes made by electrospinning out of cheap components or the reduction of 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 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 electrolysers. In addition, a new class of carbon-protected catalysts, generated by growing nanoparticles in carbon nanotubes, has been investigated.

Cell and stack development:
In FlowCamp new cell concepts have been developed and tested in mini-stacks. Especially sealing concepts were a focus of the development. Cell concepts for slurry cells have also been designed. Cross-over phenomena in H2-Br2 cells have been investigated in-situ with segmented electrode measurements for the first time and new mitigation strategies have been proposed.
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Redox flow battery hall at Fraunhofer ICT