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Research area 03 - CHEMISTRY AND MATERIAL SCIENCES, 09-403 - Hydrogen power engineering

Keywordsdurability, stability, electrochemical electrode, membrane-electrode blocks, Nafion migration, stabilizing mesh

Annotation
Electrochemical energy is one of the branches of energy-saving technologies due to the high efficiency of energy conversion, high specific energy consumption of energy carriers, in particular hydrogen, low noise, high autonomy. The development of such areas as low-temperature fuel cells, supercapacitors, water electrolyzers has led to high requirements for the efficiency of electrode processes, and the operating time of devices without a significant decrease in performance. Moreover, often authors publish works on achieving high parameters of membrane-electrode assemblies (MEA), without paying attention to the long-term stability of its work. The life of electrochemical devices and the dynamics of deterioration of technical characteristics during operation, as you know, is a very important consumer characteristic. Modern low-temperature solid-polymer fuel cells and electrolyzers are built on the basis of membrane-electrode assemblies, a very large proportion of which is represented by products containing the Nafion proton-conducting polymer. During operation, such MEA under the influence of an electric field and current flowing through them undergo a series of transformations associated with various processes of electrooxidation, dissolution, crystallization, redistribution of the composition, etc. As a result, the local structure and composition of the electrodes are change, which leads, as a rule, to the deterioration of their characteristics, such as open circuit voltage, current density, efficiency. One of the significant factors causing the degradation of the electrodes is the redistribution of the Nafion polymer in the electrode structure due to colloidization and migration. As a result of this, the structure of the electrode is substantially disrupted, its resistance to proton transfer increases, and the gas diffusion resistance changes. MEA degradation issues are considered in many works and occupy a large share of research in modern literature. At the same time, due to the complexity and variety of degradation processes at this time, this issue is still not well understood. The same can be said about measures to combat negative processes in the MEA. Despite some successes, at this time the issue of extending the life of the MEA remains relevant. Thus, the work aimed at studying the mechanisms of degradation and finding solutions that increase the life of the MEA are of great importance and are currently relevant, they are one of the priority areas for the development of electrochemical energy. The current state of the issue and proposed approaches to solving a scientific problem Significant amount of works has been devoted to the study of degradation processes in the MEA. Note that despite the fact that the historical depth of publications is calculated over an impressive period of more than 10 years, the life of modern MEAs is still not long enough (about 5000 hours with a voltage loss of about 10% of the initial one). Aging is associated with conventional electrochemical processes, such as the electrochemical and chemical oxidation of carbon and metal components of an electrode, the recrystallization of metals, the germination of metal dendrites into a proton-conducting membrane, and the integrity of the membrane due to chemical and electrochemical oxidation-reduction processes. An another area of research and related technologies is the stability problems of the Nafion ionomer (domestic MF-4SK) and its modification Aquivion. The focus of attention in published works is aimed at stabilizing the membranes that make up the MEA. In this case, two main directions can be distinguished: reinforcement and filling. The creation of various coatings in the form of polymer networks increases the life of Nafion through various stabilization mechanisms provides a longer service life of membranes based on Nafion and their greater mechanical strength. The use of fillers to create composite (filled) membranes has established itself as a way to increase resistance to water loss and thermal stability. A separate and relatively new direction is the use of carbon materials in Nafion-based composite proton-conducting membranes to improve their characteristics, such as moisture content at elevated temperatures, ion resistance (conductivity), and thermal stability. For this, additives such as carbon nanotubes, graphene materials are used. The thermal stability of fuel cells with a proton-exchange membrane and the long-term performance are important characteristics, which are the subject of many studies. An analysis of the scientific literature shows that practically no attention was paid to the issues of electrophoretic migration of the ionomer during the operation of the MEA. The features of these processes have not been practically studied. In addition, we note that the bulk of the work is aimed at stabilizing the membrane itself and almost ignored the problems of stabilizing the Nafion structure in the electrode. In this project, in order to prevent degradation of the Nafion structure in the MEA (both electrodes and membranes), spatial stabilization will be used by introducing spatial restrictions in the form of a grid of nanostructured materials: perfluorinated polyethylene (PTFE) and carbon nanotubes with different morphologies and, accordingly, sizes cells. A structure with an effective mesh size that prevents the migration of Nafion particles will be selected. Three parts of the work are planned for this: 1. study of the migration of Nafion particles in a working, unstable MEA; 2. the MEA creation containing in the structure of the electrodes and on the membrane surface a stabilizing additive of PTFE and / or CNTs of various morphologies in various quantities; 3. life tests of stabilized MEA and selection of optimal parameters of the composition and structure of the stabilizer. Stabilizing materials will be introduced into the electrode structure by adding to the catalytic dispersion in the required quantities, which will subsequently be applied to the surface of the membrane. To study the Nafion migration processes in the working MEA, electrochemical methods, dynamic light scattering method, microscopic research methods combined with elemental analysis (EDX), atomic force microscopy will be used. The MEA will undergo electrochemical aging in a standard cell in various modes. To study the migration of Nafion, potentiostatic and galvanostatic modes will be used to study migration in a constant electric field and at a constant current density. In the future, in the final assessment of the stability of the MEA, the following generally accepted methodology will be used. Electrode materials will be examined on a disk electrode in a three-electrode cell in various modes of electrochemical exposure. MEA aging will be carried out at room temperature and atmospheric pressure for a given number of cycles (0, 100, 300, 1000, etc.) of a voltage scan in the range of 0.6 - 1.0 V with a potential scan rate of 50 mV s-1. Wet (≈100%) N2 and H2 will be supplied to the electrodes. An electrode that undergoes aging will be supplied with N2. To assess the size and concentration of particles migrating Nafion, the analysis of water produced during the operation of the MEA by dynamic light scattering will be used. Changes in the structure and component composition of the electrodes during a particular electrochemical exposure will be evaluated by microscopic methods and elemental analysis. The structure of Nafion stabilizing networks and particles will be examined by atomic force microscopy. The dynamics of the ion resistance of the electrodes will be investigated by electrochemical impedance spectroscopy. Scientific novelty. For the first time, new knowledge will be gained about the features of the migration of Nafion to the MEA in the process of their functioning and the dependence of these processes on the structure of the electrode. For the first time, the Nafion spatial stabilization technique in the MEA will be used by restricting its electrophoretic migration by the cellular frame of the stabilizing grid.

Expected results
1. Fundamental relationships connecting the structure of the electrode and the features of migration of Nafion during the functioning of the MEA. 2. Mechanisms of electrode and MEA degradation associated with Nafion migration and their effect on electrochemical characteristics. 3. Technology of spatially stabilized MEA with extended service life. The development of this topic, which consists in the spatial stabilization of Nafion by creating a certain structure of carbon material, will open a new direction in increasing the stability of fuel cells. This will be an important step in extending the lifespan of such devices.

Annotation of the results obtained in 2023
At the second stage, the following work was completed: 1. Particles of Nafion were studied in a liquid environment using optical methods (dynamic light scattering, etc.) during the fabrication and aging of membrane-electrode assemblies. 2. Dynamics of the component composition and structure of the electrodes in membrane-electrode assemblies during aging were investigated using electron microscopy combined with elemental analysis, and atomic force microscopy. 3. Development of technology and fabrication of membrane-electrode assemblies based on composite electrode materials of a combined composition system Pt/C-polytetrafluoroethylene-carbon material-Nafion. 4. Resource studies of the obtained electrode materials and membrane-electrode assemblies on a rotating disk electrode and in a two-electrode cell (for membrane-electrode assemblies). 5. Adjustment of the electrode composition based on the results of resource studies and fabrication of membrane-electrode assemblies. 6. Generalization of the experimental results and formulation, based on the experimental data obtained for different electrode compositions and structures, requirements for samples, and comparative assessment of degradation times for different systems. 7. Analysis and generalization of data. Recommendations for creating electrode structures depending on the requirements for the fuel cell. 8. Preparation of publications of the obtained results. The following scientific results were obtained at the second stage: 1. Technology of spatially stabilized membrane-electrode assemblies with an extended service life has been developed. A laboratory technology for membrane-electrode assemblies of a hydrogen fuel cell with a proton-conducting membrane with an extended service life has been created by suppressing the migration of the Nafion polymer during operation. The technology involves a specified sequence of technological operations, formulation of conditions for each operation, methods of input, intermediate, and output control. The conditions for preparing the initial components of the electrode material have been formulated. Conditions for preparing catalytic inks, their component composition, liquid-to-solid phase ratio (L:S), dispersing techniques, stages, and times have been determined to obtain a homogeneous dispersion of components in the liquid phase suitable for forming electrodes on the surface of the proton-conducting membrane. Spatially stabilizing functional components of the composite electrode and/or their combination (carbon materials, polytetrafluoroethylene) that provide a clear suppression effect of Nafion migration during electrode operation have been selected. Control of materials for platinum content, carbon materials, Nafion has been carried out. Methods of control in the developed technology have been formulated and tested, including: - control of the component composition of the initial materials using thermogravimetric analysis combined with differential thermal analysis; - control of the component composition of the dispersion of catalytic inks; - control of conditions for preparing and degree of readiness of the dispersion of catalytic inks; - control of loading of electrode material during fabrication of membrane-electrode assemblies; - control of geometric parameters and porosity of electrodes. The technology includes: 1. control of initial components 2. obtaining a dispersion of catalytic inks with a specified composition; 3. application of catalytic inks to the surface of the proton-conducting membrane and formation of symmetrical electrodes on both sides of the membrane; 4. control of composition and loading of electrode components; 5. control of thickness and porosity of electrodes; 6. protonation of membrane-electrode assemblies. Thermogravimetric analysis was used as the main method for controlling the composition of the initial components. Control of the composition of platinum-plated carbon black involves burning a sample of Pt/C and calculating their content based on the mass loss of carbon and unburned platinum residue. The water content in an aqueous suspension of polytetrafluoroethylene (PTFE) was calculated based on mass loss during heating to 110C. The composition of the PTFE aqueous suspension was controlled by thermogravimetric analysis: the water content was determined by mass loss in the temperature range of 35-110C, and the PTFE content was calculated by mass loss in the temperature range of 110-1000C. The composition of catalytic inks was controlled using thermogravimetric analysis in two implementation options: - after drying a portion of the inks at a temperature of 80-85C to calculate the component composition of solid components; - TGA analysis of the initial catalytic inks to calculate the ratio of liquid to solid components. 2. Scientific and technological foundations of membrane-electrode assemblies with composite electrodes based on Nafion with extended service life due to suppression of its electrophoretic migration by spatial stabilization using a network of nanostructured materials. Based on the experimental data obtained on the peculiarities of degradation processes in membrane-electrode assemblies with spatially stabilized Nafion polymer and theoretical concepts, scientific foundations for creating hydrogen fuel cell membrane-electrode assemblies with extended service life due to suppression of Nafion migration during operation have been formulated. Basic requirements for the technology of spatially stabilized composite electrodes and membrane-electrode assemblies with extended service life have been formulated. Recommendations for creating electrode structures depending on the requirements for the fuel cell have been formulated. The influence of the composition and structure of the electrode in the membrane-electrode assembly on the migration of the proton-conducting polymer Nafion during operation has been established. The stabilizing effect of Norit carbon black is associated with spatial restriction of Nafion. Analysis of the results of resource tests of membrane-electrode assemblies showed a pronounced stabilizing effect of Norit supra 30 carbon black, which has a highly developed surface and a microporous structure. At the same time, a sample with a relatively high content of Norit (10%) showed greater inertness in the accumulation of NRC and its low value. Activation of the MEA led to an increase in the NRC, but, nevertheless, its value remained ~100 mV less than that of the standard sample. During the aging process, the NRC even increased slightly, linearly, with a slight slope, which indicates the predominance of the activation process over degradation. The sample with a lower Norit content (5%) is closer to the standard one in terms of electrochemical characteristics. The speed of accumulation and the value of the NRC are close to the standard MEA. At the same time, the stability (uniformity of the ionic resistance of the electrodes) during aging is high, close to the sample with 10% Norit. The data obtained indicate the absence of an extremum on the composition-property optimization curve. In this regard, when adjusting the composition, we used a composition close to the sample (with Norit content (5%)), which demonstrated good characteristics and stability of the Nafion structure in the electrode, but shifted towards a decrease in the proportion of Norit. When formulating requirements for samples with increased stability, it is necessary to take into account their purpose and operating conditions. When operating membrane-electrode units under conditions of increased requirements for reliability and duration of uninterrupted operation, it is necessary to be guided by the redundancy rule for the specific loading of platinum and the amount of Norit stabilizing additive. At the same time, it is necessary to ensure appropriate MEA activation procedures and operating conditions to achieve the best performance and ensure stable operation.

Publications

1. A. Krasnova, A.A. Nechitailov, A. Pelageikina, N.V. Glebova Differences in the electrochemical behavior of Vulcan XC-72 carbon black and glassy carbon after prolonged potential cycling Electrochemistry Communications, Electrochemistry Communications Volume 155, October 2023, 107578 (year - 2023) https://doi.org/10.1016/j.elecom.2023.107578

2. N.V. Glebova, A.S. Mazur, A.O. Krasnova, I.V. Pleshakov, A.A. Nechitailov ЯМР-исследование интерфейса наноуглеродный материал/Nafion в радиопоглощающем композите Письма в Журнал Технической физики/Technical Physics Letters, Письма в ЖТФ, 2023, том 49, вып. 17 (year - 2023) https://doi.org/10.21883/PJTF.2023.17.56077.19544

3. Nadezhda V. Glebova, Anton S. Mazur, Anna O. Krasnova, Ivan V. Pleshakov, Andrey A. Nechitailov Investigation of stability of composite Nafion/nanocarbon material Nanosystems: Phys. Chem. Math., Phys. Chem. Math., 2023, 14 (2), 202–207 (year - 2023) https://doi.org/10.17586/2220-8054-2023-14-2-202-207

4. Nechitailov A.A., Volovitch P., Glebova N.V., Krasnova A. Features of the Degradation of the Proton-Conducting Polymer Nafion in Highly Porous Electrodes of PEM Fuel Cells Membranes, Membranes 2023, 13(3), 342, Q1 по JCR ( Polymer Science) на момент опубликования статьи (year - 2023) https://doi.org/10.3390/membranes13030342

Annotation of the results obtained in 2022
As a result of the work done at the first stage of the project, technologies for electrode materials of a given composition of systems were developed: Pt/C- PTFE-Nafion and Pt/C-CNT-Nafion. The technology has been developed and membrane-electrode assemblies (MEA) based on composite electrode materials of various compositions of the Pt/C-CNT-Nafion system have been manufactured, a number of samples for research have been made, including samples on a disk electrode and as part of MEAs. The samples were characterized by component, elemental composition and structure by differential thermal analysis, electron microscopy combined with elemental analysis (energy dispersive X-ray spectroscopy), atomic force microscopy, adsorption-structural analysis, gravimetry combined with measuring the geometric parameters of the electrode to measure the total porosity, taking into account macropores. The electrochemical behavior and degradation processes of electrodes on a disk rotating electrode and as part of MEAs were studied by electrochemical methods, such as: cyclic voltammetry, potentiometry, and electrochemical impedance spectroscopy. Dynamic light scattering and spectrophotometry were used to study Nafion polymer particles that passed into the electrolyte solution during material aging on a rotating disk electrode. Results 1. Fundamental dependencies relating the structure of the electrode and the features of Nafion migration in the presence of PTFE and CNT during the operation of the MEA in various modes. When studying the absorption spectra of electrolyte solutions after aging of the material on the disk electrode, intense absorption (optical density of about 1.5) was found in the near ultraviolet region with maxima characteristic of Nafion at ~204 and ~307 nm. In this case, in the second half of the potential cycling, the first maximum shifts by several nm to the short-wavelength region of the spectrum, while the second one practically disappears. Such dynamics of the absorption spectra may indicate a slowdown in the washout of Nafion. Moreover, the transition of Nafion into the electrolyte solution occurs in the first half of aging (the first ~2500 cycles). The impossibility of detecting Nafion particles by dynamic light scattering can be associated with both small particle sizes and their insufficient concentration. The data of the experiment on the study of Nafion particles passing into the electrolyte solution allow us to state that in the system with PTFE there is a primary washout of Nafion in the first half of aging, followed by a slowdown in the process. The studied materials and composites based on them contain a significant amount of macropores >100 nm in size, which are not visible in the study by the method of low-temperature nitrogen adsorption (due to the peculiarities of the method). All individual materials (CNT, carbon black) have some peak characterizing the number of pores in the range of 30-50 nm. In this case, the porous structure in the range of micro and mesopores for carbon materials varies greatly. So Vulcan-type carbon black contains much fewer pores than CNTs. CNTs of Taunit MD type have a significant peak in the region of micropores (~1 nm) and a significant amount of mesopores in the range of characteristic sizes of 3–30 nm. Thus, the porous structure of the studied carbon nanomaterials is very different, which determines their pronounced structure-modifying properties. The presence of CNTs in the Pt/C-CNT-Nafion composite system leads to an increase in the proportion of large micron-sized pores. In the sample of the MEA with CNTs, due to the spatial mismatch of the components, in particular, platinum and Nafion, the recrystallization of platinum is slowed down. As a result of the high porosity of the sample with CNTs, due to large pores, the process of Nafion migration occurs more intensively in it and the appearance of inhomogeneity in the proton resistance of the electrodes with its increase. 2. Mechanisms of degradation of electrodes and MEAs associated with the migration of Nafion in the presence of a stabilizing additive PTFE and CNTs, and their effect on electrochemical characteristics. In the presence of PTFE, the mechanisms of its stabilizing action are two processes: stabilization of Nafion on the surface of PTFE due to the energy of formation of a surface compound or absorption; stabilization of Nafion by creating a porous structure with small pores (2-10 mkm) and relatively large agglomerates (grains) of the material separated by macropores. This structure, along with the presence of large transport pores for mass exchange between grains, spatially stabilizes Nafion inside the grain. In the presence of CNTs, the degradation mechanism leading to the deterioration of electrochemical characteristics during aging for a sample with CNTs and high porosity consists in the migration of Nafion in the MEA electric field through large pores, resulting in the creation of inhomogeneity and an increase in the resistance to proton transfer. An increase in the resistance to proton transfer is also facilitated by the disruption of the interfacial region and the precipitation of part of the platinum from the electrode process due to the loss of contact with Nafion. In the sample without CNTs, these processes also occur, however, to a lesser extent. The decrease in the electrochemically active surface area of platinum for the sample with CNTs is largely determined by the loss of contact with Nafion, while for the sample without CNTs, this decrease is primarily due to recrystallization. The combination of Nafion-stabilizing properties of PTFE and high mass-transport characteristics of CNTs should make it possible to create a fractal pore structure in an electrode consisting of microporous grains with stabilized Nafion connected by large transport pores.

Publications

1. Primachenko,ON; Kulvelis,YV; Odinokov,AS; Glebova,NV; Krasnova,AO; Antokolskiy,LA; Nechitailov,AA; Shvidchenko,AV; Gofman,IV; Marinenko,EA; Yevlampieva,NP; Lebedev,VT; Kuklin,AI New Generation of Compositional Aquivion-Type Membranes with Nanodiamonds for Hydrogen Fuel Cells: Design and Performance Membranes, Т. 12, №9, Id827 (year - 2022) https://doi.org/10.3390/membranes12090827

2. Pelageikina A.O., Nechitailov A.A., Volovitch P., Glebova N.V., Krasnova A.O. Деградация МЭБ на основе композитных электродных материалов системы Pt/C-УНТ-Nafion Сборник докладов Школы молодых ученых – М.: ИНЭИ РАН, Возобновляемые источники энергии и приоритеты научно-технологического развития энергетики России. Сборник докладов Школы молодых ученых – М.: ИНЭИ РАН, 2022 – 214 с., ISBN 978-5-91438-034-9 (year - 2022)