Future storage systems for the energy transition: Polymer-based redox-flow batteries

Project Information

FutureBAT

Grant agreement ID: 101054271

DOI

10.3030/101054271

EC signature date 7 September 2022

Start date 1 January 2023

End date 31 December 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 499 355,00

EU contribution

€ 2 499 355,00

Coordinated by

FRIEDRICH-SCHILLER-UNIVERSITÄT JENA
Germany

Periodic Reporting for period 1 - FutureBAT (Future storage systems for the energy transition: Polymer-based redox-flow batteries)

Reporting period: 2023-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

The efficient storage of electric energy represents a major challenge for a successful energy transition, enabling the utilization of fluctuating renewable resources such as photovoltaics and wind power also as base load. Redox-flow-batteries (RFBs) are the only type of battery where intrinsically power and capacity can be varied independently from each other by adjusting the size of the storage tanks and the size of the electrochemical cell stack, respectively. This unique feature makes this type of battery perfectly suited for scalable stationary applications – for instance, for the storage of renewable energy provided by photovoltaics or wind power.
RFBs based on aqueous electrolytes with organic / polymer active materials have the potential to be suitable alternatives for commercial metal-based RFBs (such as the vanadium RFB). Organic / polymer RBFs can feature a lower CO2 footprint perfectly fitting to the goals of the EU Green Deal.
In particular, polymer-based RFB systems enable the use of cost-efficient dialysis membranes due to the larger size of the active material compared to the ions of the electrolyte. Furthermore, pH neutral table salt solutions can be utilized as electrolytes, which is in stark contrast to the commonly utilized acidic electrolytes of vanadium RFBs. Nevertheless, polymer-based RFB systems still reveal restrictions in terms of capacity, lifetime and temperature-stability.
FutureBAT targets a breakthrough in the development of novel organic active materials for RFBs, by combining the search for new redox-active entities with the improvement of current polymeric materials on the molecular level, by this providing new functions / properties. The key question will be how far polymeric electrolytes can be tuned by adjusting their molecular structure. Advanced polymer structures (incl. (hyper-) branched structures) as well as novel RFB setups will provide access to hitherto unknown properties such as new photo-rechargeable RFBs. Furthermore, new sensor systems (SOC and SOH) will be applied, which also will form the basis for a novel (high-throughput) screening of different electrolytes.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

First, different active materials have been screened. For that purpose, the technology for the screening has been improved. Firstly, new sensors to determine the state-of-charge (SOC) and state-of-health (SOH) of redox flow battery electrolytes have been developed. A novel analytical approach was utilized, which is based on chronoamperometry. In contrast to previous studies, the transient chronoamperometric signal was utilized for the analysis. For this purpose, micro- as well as macroelectrodes could be utilized for ex situ, and more importantly also for in operando SOC measurements. Furthermore, the thermal stability of ferrocene-based copolymers has been studied in detail. Ferrocenes represent very promising active materials. The copolymer revealed still a good thermal stability at elevated temperatures (60 °C). The stability of the ferrocenes is higher compared to the established TEMPO materials.
The study of the active materials has been based on the two “working horses” TEMPO as well as viologen. Both electrolytes are well-known, and these redox moieties are the first active materials, which have been utilized for the assembly of more complex polymer structures. Ferrocenes have been studied intensively as new catholytes. These materials have been proven to be more stable compared to the standard catholyte – TEMPO. Consequently, the stability of different ferrocenes is studied currently in copolymers in order to elucidate the influence of the polymer structure on the stability.
Furthermore, the first advanced polymer structures have been prepared successfully. Hyperbranched TEMPO polymers have been synthesized in a step-growth polymerization. These materials revealed promising electrochemical properties. Furthermore, the hyperbranched structure led to a decreased viscosity compared to linear TEMPO copolymers as well as a faster diffusion and charge transfer rate. Consequently, the major drawbacks of polymeric electrolytes could be decreased.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The new chronoamperometry to determine SOC and SOH is a significant advance of the state-of-the-art. This method enabled also the usage of macroelectrodes, which has not been possible before by the utilization of steady-state currents. Furthermore, the new technique is not limited by the electrolyte flow, which makes this approach ideal for the integration into test cells. The SOC as well as SOH could be determined for TEMPTMA electrolytes with high accuracy (3.1%) and high precision (± 4.5%). Furthermore, the method proved also to be more robust against changes in the electrolytes, which can be caused by various reasons. These include amongst others, different diffusion coefficients of the active materials, cross-over, decomposition and side reactions as well as transfer of solvent. Noteworthy, this method is also insensitive to changes of the temperature.
Futhermore, we could describe the first hyperbranched polymers for the usage in RFB electrolytes. These polymers have very promising properties as these electrolytes featured lower viscosities and showed a higher diffusion coefficient, which is beneficial for achieving later higher current densitities in the operating flow battery.

Redox flow battery test cell (Photo by J. Meyer - FSU Jena)