RSCF №25-13-00185

(2025—2027)

Annotation

The aim of the project is the development and study of electrode materials for redox-flow batteries that use redox-active polymer compounds as catholyte and anolyte. Flow batteries are promising power sources, in demand for grid-scale storage owing to their high scalability, durability and installation flexibility. However, conventional flow batteries based on inorganic compounds (for example, vanadium and bromides) face a number of operational and economic limitations. One of the main problems remains the rapid degradation of the ion-exchange membranes required to separate the catholyte and the anolyte, which raises the cost and reduces the reliability of such systems.

The project aims to overcome these limitations through the use of redox-active polymers, which, owing to their large molecular structure, cannot penetrate the pores of non-ion-exchange membranes. This makes it possible to use cheaper and more reliable porous membranes (for example, dialysis membranes) instead of expensive ion-exchange membranes (such as Nafion). Polymeric redox compounds have the potential to be used in both catholytes and anolytes, since they can provide a high concentration of active groups capable of taking part in redox reactions, which is necessary for creating flow batteries with high capacity. However, their practical application is limited by the slow kinetics of the electrode reactions: in order to reduce the viscosity of the solutions of such polymers, it is preferable that the macromolecules exist in solution in a compact conformation or as a globular aggregate. In such a conformation only the surface redox groups remain active, which slows down charge transfer and, as a result, only a small fraction of the theoretical capacity is extracted.

To improve the charge-transfer kinetics, the project involves the study of various approaches to controlling the "polymer–electrode" interaction. Carbon-based electrode materials and the redox-active polymers themselves will be modified with functional groups capable of interacting efficiently with each other. This modification is aimed at "unfolding" the polymer chains and providing access to all redox groups of the polymer, which increases the capacity and the rate of the redox reactions. At the same time it is important to select groups whose interaction with the polymer chains is strong enough to improve the charge-transfer kinetics, yet does not lead to irreversible adsorption of the polymer on the electrode. Optimization of the interaction of the polymers with the electrode surface is the central task of the project, and its successful solution will make it possible to increase the efficiency and durability of flow batteries operating on organic redox polymers.

Expected results

The project is aimed at fundamental research that will make it possible to study the mechanisms of interaction of redox-active polymers with modified carbon electrodes in flow batteries. The work is expected to result in a deep understanding of the processes occurring at the polymer–electrode interface and of the nature of the interactions that determine the rate and efficiency of charge transfer in these systems. The studies will be directed at determining the optimal conditions for stabilizing the polymer molecules in solution, preferably in a globular conformation in order to reduce viscosity, as well as at studying the influence of various functional groups on the surface of carbon materials on the kinetics of the electrode reactions of redox-active polymers.

The main attention will be paid to the analysis of various types of surface groups of carbon materials, including both ionic and non-ionic functional groups, and their influence on the sorption of the oxidized and reduced forms of the polymers and on the charge-transfer kinetics. It is assumed that the studies will demonstrate how functional groups on the electrode surface are able to influence the conformational changes of redox polymers and their ability to transfer charge efficiently. As a result of this fundamental research, the key mechanisms responsible for the unfolding of polymer chains at the electrode surface and for their sorption and desorption will be identified, which will ensure the full involvement of the redox-active groups of the molecules in the oxidation and reduction processes, thereby increasing the efficiency of the redox reactions.

The scientific significance of the project lies in obtaining new data on the interactions between the functional groups of redox polymers and carbon electrodes, as well as in studying the mechanics of charge-transfer processes and conformational changes in polymer chains. The study of such interactions at the molecular level is a poorly explored area, despite its importance for electrochemical systems. The experiments carried out will broaden the understanding of the influence of functional groups on the kinetics of redox transitions and will help to refine the mechanisms underlying key electrochemical processes. The results of the project are expected to make a significant contribution to world science, creating a basis for further research in electrochemistry and materials science, especially in the context of alternative energy-storage systems.

The practical significance of the anticipated results is that they will serve as a basis for future applied research aimed at developing efficient and durable energy-storage systems. The data obtained on the charge-transfer mechanisms and on the interactions at the polymer–electrode interface are expected to help formulate general principles for the creation of new classes of electrode and redox-active materials, which will further advance energy-storage technologies.

Thus, the results of this project will be able to make a significant contribution to electrochemical science, broadening the understanding of the interactions at the electrode–polymer interface. The knowledge obtained may contribute to the development of more reliable and efficient energy-storage systems in the future, capable of improving the stability and energy density of batteries.

Results (2025)

During the reporting period, comprehensive work was carried out on creating and developing methods for the quantitative description of the behaviour of redox-active polymers at carbon electrode surfaces, including adsorption, desorption, formation of oxidation products and possible conformational changes of the macromolecules. The results combine experimental studies, the development of new methodological approaches and the construction of a mathematical model that allows the behaviour of polymers in electrochemical systems to be interpreted using the rotating ring-disk electrode (RRDE).

The main results obtained are:

• Modified multi-walled carbon nanotubes with controlled chemical functionality were prepared; SEM, EDX and XPS showed the formation of stable functional layers with a uniform distribution of oxygen- and sulfur-containing groups that do not disrupt the structural integrity of the parent material.
• Using poly-TEMPO-pyrrole as a model system, the nature of the electrolyte anion (perchlorate, tetrafluoroborate, nitrate, sulfate) was shown to determine the potentials and reversibility of the redox transitions: perchlorate increases the internal resistance and lowers the film conductivity, whereas sulfate promotes more efficient charge transfer, in agreement with impedance spectroscopy data.
• The catalytic activity of poly-TEMPO-pyrrole was studied in the oxidation of glucose: the catalytic current increases sharply in nitrate- and sulfate-containing electrolytes, indicating the decisive role of the ionic environment in the reactivity of nitroxide polymers.
• The redox-active polymer PTMA (methacrylate with grafted TEMPO groups) was synthesized with a molecular weight of about 20 kDa, showing stable electrochemical behaviour and being convenient for RRDE studies.
• RRDE methodologies for analysing soluble redox polymers were developed: the optimal disk and ring potentials for selective detection of the different polymer forms were determined; functionalization of the nanotubes was shown to reduce the RRDE currents not because of conductivity degradation but because of changes in the adsorption properties of the surface.
• A mathematical model of adsorption, desorption and conformational changes of the polymers at the electrode surface was constructed; calculated and experimental RRDE profiles are in qualitative agreement, which confirms the applicability of the model for selecting optimal surface modifiers of electrodes.

Taken together, the results obtained substantially broaden the understanding of the mechanism of interaction of redox polymers with carbon surfaces and of the role of the ionic environment in their electrochemical behaviour. The RRDE methodologies and the mathematical model developed provide a basis for the quantitative description of transport and transformation processes, which is a prerequisite for the further development of redox-polymer materials for flow batteries and other electrochemical applications.