Periodic Reporting for period 1 - NEREFLOW (Novel electrode materials for high performing aqueous organic redox flow batteries.)
Berichtszeitraum: 2021-06-01 bis 2023-05-31
In the European Green Deal (2019), a set of policies aiming at ensuring the European Union becomes climate neutral by 2050 was presented. The energy sector and more specifically the electricity sector are strongly targeted. In order to help the EU reach its decarbonisation targets, the deployment and the integration of renewable energy sources into the grid along with the development of innovative and flexible storage solutions are of paramount importance. In this context, redox flow batteries (RFBs) offer a promising alternative for the storage of large amounts of energy. RFBs present several advantages such as the decoupling of power and energy scaling, the scalability, the long operational lifetime and the safety features. Aqueous organic redox flow batteries (AORFBs) are based on water-soluble organic redox molecules that can be used at a wider range of pH and whose production cost is stable. Currently, only a handful of companies are developing such aqueous organic redox flow batteries worldwide, including Kemiwatt in France. Kemiwatt developed its first 20 kW prototype in 2016 and a 30 kW/30 kWh (1 h) containerized and autonomous system in 2017. In order to pursue their technological development, several bottlenecks need to be overcome. For instance, a better understanding of the interactions between the electrode and the electrolyte is part of the challenges identified by Kemiwatt and more generally by the AORFBs community. Most of the research is currently focusing on the development of novel and competitive electrolytes in terms of solubility and stability, but little has been done on electrode materials for AORFBs applications. Electrodes have an influence on the overall electrochemical reaction losses due to charge transfer, ohmic, and mass transport. Focusing on electrode surface engineering may thus lead to significant performance benefits.The main research question of this fellowship was: how to improve the performance of AORFBs in terms of energy and power densities and stability via electrode surface engineering? The objectives were threefold: to develop electrodes modified by electrodeposition, thermal activation and electrooxidation, to study the performance of these electrodes for AORFBs application, to implement and test the corresponding electrodes in pilot-scale reactors.
Nickel electrodeposition, thermal activation and electrooxidation were investigated. Electrode surface metallisation with nickel was successfully implemented for electrodes of 25 cm² surface area using a pulse deposition method. This method could be adapted to Kemiwatt RFBs architecture. Electrooxidation of carbon felts was also carried out in Kemiwatt RFBs and the modification method was implemented for electrode surface areas of 25 cm² and 245 cm². Thermal activation of carbon felts was studied for electrodes up to 2100 cm². Contact angle measurements have shown that the three methods have led to the improvement of the wettability compared to that of pristine carbon felt. Electrochemical analyses were carried out on carbon felt samples and individual carbon felt fibres and showed an increase of the specific surface area with the three methods. The electrocatalytic behaviour of modified felts towards the redox compounds used as active materials was also compared to that of unmodified felts. The electrochemically oxidised and the thermally activated felts led to a significant increase of the electron transfer rate of ferrocyanide used as posolyte. An increase of the electron transfer rate of the active material could lead to a decrease of the area specific resistance of the battery, in turn increasing the accessible discharge capacity of the system. The results from the study focussing on the analysis of individual carbon fibres were published in Molecules 2022, 27(19), 6584. Electrodes were then implemented in symmetric and non-symmetric cells, using an anthraquinone derivative as negolyte and potassium ferrocyanide as posolyte. The utilisation of thermally activated electrodes led to the best performance in terms of accessible discharge capacity, ASR and capacity fade rate compared to cells equipped with non-activated and electro-oxidised electrodes. Further surface analyses are currently ongoing in order to understand the difference between the modified and non-modified electrodes in terms of chemical surface groups. The performance of thermally activated electrodes were also compared to these of non-activated electrodes in 2100 cm² cells, and the battery cycling tests are still ongoing. A publication presenting the results on electrode modification, characterisation and testing is currently in preparation. The results obtained during this project were also disseminated in 3 national and international conferences. In addition, the redox flow battery technology was also presented in an outreach event mainly targeting year 2 to year 6 students and families. Finally, outreach activities were also conducted to promote Marie Sklodowska Curie Fellowships and especially the benefits of a collaboration between the Fellow and an industrial partner.
We have studied three surface modification methods and their relevance for AORFBs. We showed that the architecture of RFBs is suitable for the implementation of pulse electrodeposition and electro-oxidation (while the thermal activation does not require an electrochemical cell), highlighting the versatility of these cells. The key message resulting from this project is that activation methods of the carbon felt used as electrodes can improve the performance of AORFBs. Most importantly both the electrochemical oxidation and the thermal activation of the felt increased the standard rate constant of potassium ferrocyanide used as active material in the posolyte. Similar improvement was not observed for the anthraquinone derivative used as active material in the negolyte. However, future studies could focus on the screening of other molecules with slower electron transfer kinetics on pristine carbon felt. We showed that batteries equipped with thermally activated felts led to better performance in terms of area specific resistance, accessible discharge capacity and also capacity fade rate. To the best of our knowledge, such studies including electrode surface activation, electrochemical activation and battery cycling tests carried out close to real-life conditions were not reported in the past. This project has shown the relevance of electrode activation for AORFB application, paving the way for more in-depth studies on the impact of these methods on the electrode/electrolyte interactions and on the batteries performance. Thermal activation of the carbon felt, which is already widely used for other applications, can be successfully used for AORFBs: the direct impact on the battery performance was demonstrated and explained, and other redox compounds could be investigated in the future. Targeting active molecules with high solubility and chemical stability is paramount; however, we have shown in this project that the technology can also be improved by focusing on electrode surface activation.