Jakub Lach

Keywords: in situ exsolution, nanocatalysts, nanofibers, electron microscopy, solid oxide cells In energy conversion and storage technologies, such as Solid Oxide Cells (SOCs), stable and catalytically active materials are highly needed to reduce the operating temperature and ensure long service life. Nanofibrous materials may provide an effective solution to these requirements. Nanofibers are regarded as promising electrode materials, providing continuous porosity that enhances the transport of reactants to the electrode, along with a high specific surface area [1–2]. Improved catalytic activity and stability can be achieved by in situ exsolution process. In situ exsolution is a segregation of selected metals to the surface and forming metallic nanoparticles. This process involves diffusion, reduction, nucleation, and growth, and is driven primarily by oxygen vacancy formation. Oxygen vacancies not only drive exsolution but also increase ionic conductivity by creating fast oxygen-ion diffusion paths. Exsolved nanoparticles anchored in the parent oxide show improved thermal stability and high resistance against sintering due to strong metal-support interaction. Compared to conventional wet impregnation methods, exsolution offers better nanoparticle distribution, size control, and durability. Reversible exsolution, where nanoparticles can be dissolved and exsolved depending on redox conditions can provide additional benefits. Reversibility of the process may mitigate agglomeration and carbon deposition when SOCs are powered by cheaper and more available hydrocarbons. Therefore, nanofiber electrodes with in situ exsolved nanoparticles are considered highly beneficial for the application in SOC electrodes [1–3]. In this work, the in situ exsolution process was comprehensively investigated by electron microscopy studies, including TEM, SEM, and EDX, along with electron and X-ray diffraction techniques. The materials were synthesized by sol-gel, and electrospinning methods. It was found that metallic nanocatalysts are exsolved in situ on the surface of the Fe-doped Ln _0.9 Ba _0.9 Mn _1.8 (Co/Ni) _0.2 O _6−δ (Ln= Nd, Sm) double perovskites. Co, Ni, and Fe are capable of being reduced to their atomic state under the operating conditions of the fuel electrode in SOCs. Microscopic analysis revealed that nanoparticles are partially anchored in the perovskite oxide (see Fig. 1a). This enhances the stability of the mixed ionic-electronic conductors and improves resistance to carbon poisoning when the fuel cell is powered by cheaper, and more available hydrocarbons. The presence of the perovskite structure was proven by the HR-TEM, with all the observed spots in the Fast Fourier Transform FTT pattern assigned to the P 4/ nmm space group. The FTT performed on the selected areas of the TEM images reveled that exsolved from perovskite nanocatalysts crystallize in Fm-3 m space group. These results are consistent with those from the XRD data. The measurements of interplanar distances in the FTT images allowed phase confirmation and determination of the crystals’ orientation. EDX mapping and point measurements confirmed that the nanocatalysts are enriched with Co, Ni, and Fe (see Fig. 1b), forming a Co–Ni–Fe alloy. During the oxidation process in air, metal nanoparticles are partially dissolved into the perovskite structure, while the remaining metals on the surface form an “empty core–shell” structure, increasing the specific surface area of the nanocatalysts. It was also found that the morphology of the material plays a key role in the in situ exsolution process. Nanofibers obtained via the electrospinning method enable the exsolution of nanocatalysts with smaller diameters (around 30 nm) and up to 20 times higher surface nanoparticle density compared to powder compounds. A reversible in situ exsolution process enables the formation of stable nanocatalysts with a reduced tendency toward sintering compared to traditional methods such as impregnation or deposition. This enhances stability, contributes to the efficient and long-term operation of Solid Oxide Cells. The favorable properties of the studied materials were confirmed in SOCs with nanofibrous electrodes decorated by in situ exsolved nanocatalysts. [1] J. Lach, M. Gogacz, P. Winiarz, Y. Ling, M. Zhou, K. Zheng, Materials 2025, 18(6), 1272 [2] J. Lach, Zheng, M. Gogacz, P. Czaja, J. Luo, A. Brzoza-Kos, ECS Transactions 2023, 111(6), 1025−1034 [3] K. Zheng, J. Lach, P. Czaja, M. Gogacz, P. Czach, A. Brzoza-Kos, P. Winiarz, J. Luo, J. Power Sources 2023, 587, 233707 The work is funded by the National Science Centre Poland (NCN) based on the decision number UMO-2021/43/D/ST5/00824. Jakub Lach acknowledge the financial support of research project supported by the program „Excellence Initiative – Research University” for the AGH University of Krakow. HRTEM, EDS and SAED results were a part of CERIC-ERIC grant no. 20232033 and were conducted in National Institute of Materials Physics, Măgurele in Romania. Figure 1

Solid oxide cells (SOCs) represent a highly promising energy conversion and storage technology, potentially serving as a cornerstone of sustainable development strategies by significantly increasing the share of clean energy. During periods of surplus energy from renewable sources, hydrogen can be generated in the electrolysis mode, and subsequently consumed to produce electricity and heat when the demand is high in fuel cell mode. However, the conventional operating temperature of the cells, typically above 800 °C, poses challenges associated with thermal compatibility and material degradation, substantially inflating costs and hindering widespread commercialization. At the targeted operating temperatures below 600 °C, most electrode materials suffer drastic reductions in electrochemical performance, underscoring the need for innovative electrodes capable of maintaining high efficiencies at lower temperatures. Recent researches have highlighted the potential of a range of new electrodes engineering strategies, including mechanical milling, infiltration, in situ assembling, in situ exsolution of nanoparticles and morphology modification of materials to boost the electrode materials performance. It has been observed that materials possessing the same crystal structure yet an altered morphology can exhibit significantly distinct electrochemical characteristics, thereby enhancing processes such as the surface exsolution of nanocatalysts. A key advantage of these electrodes lies in their elevated catalytic activity, which bolster performance. In this study, we examine how transforming material morphology from a powder-based form to a nanofibrous structure influences electrochemical performance. Additionally, we investigate techniques such as the in situ exsolution of nanoparticles on material surfaces, evaluating whether morphological changes affect this process and assessing the impact of phase transitions on its effectiveness. While our primary focus is on perovskite-based materials and their derivatives for application in solid oxide cells, the insights gained may also be applicable to other domains within materials engineering. Using the example of the composite material GdBa _0.5 Sr _0.5 CoCuO _5+δ (GBSCC) - Ce _0.9 Gd _0.1 O _1.95 (GDC) and Sr _1.8 Fe _1.4 Ti _0.2 Cr _0.2 Ni _0.1 Co _0.1 O _6-δ perovskite, we demonstrate how changes in morphology (from powder to nanofiber) influence the material's performance, for example, the nanofibrous GBSCC-GDC electrode exhibits a polarization resistance (R _p ) over 60% lower compared to the powder-based material electrode at 600 °C (0.186 Ω·cm² vs 0.493 Ω·cm²). Additionally, we show whether morphological modifications can impact the quantity and quality of in situ exsolved nanoparticles, as well as the elemental concentration after reoxidation. Exsolved nanoparticles from nanofiber materials have diameters over 50% smaller compared to powder-based perovskites. Acknowledgements: The work is funded by the National Science Centre Poland (NCN) based on the decision number UMO-2021/43/D/ST5/00824. Michał Gogacz also acknowledges the financial support of research project supported by the program „Excellence Initiative – Research University” for the AGH University of Krakow. HRTEM, EDS and SAED results were a part of CERIC-ERIC grant no. 20242238 and were conducted in National Institute of Materials Physics, Măgurele in Romania. Figure 1

In the constantly evolving world, reliable energy sources are needed for mankind like never before. Regrettably, the intermittent nature of obtaining energy from renewable energy sources requires technologies such as symmetrical Solid Oxide Cells (SOCs), capable of efficiently converting and storing energy . However, materials for both electrodes must fulfill many requirements, such as mixed ionic-electronic conductivity, robust structural stability in both reducing and oxidizing conditions, as well as high electrocatalytic activity for fuel (H ₂ , CH ₄ ) oxidation and oxygen reduction at a lower operating temperature range (500 – 700 °C) [1-3]. Therefore, the design of materials capable of in situ exsolution of nanocatalysts is regarded as highly advantageous [1-4]. The proposed concept enables the fabrication of electrospun Sm _0.9 Ba _0.9 Mn _1.8-x Fe _x Co _0.2-y Ni _y O _{5+ d} (x = 0, 0.45, 0.9; y = 0, 0.1, 0.2) stable perovskites, decorated with in situ exsolved metallic nanocatalysts on the fuel electrode side, and oxide nanoparticles on the air electrode side. All redox-stable materials were found to have a double perovskite structure with the P 4/ nmm space group, as confirmed by XRD measurements (see Fig. 1a). The high-temperature XRD studies revealed a phase transition from the P 4/ nmm to the P 4/ mmm space group at 200 °C. A large specific surface area and porosity which enables rapid gas diffusion, and continuous charge transport pathways were ensured by the cost-effective electrospinning method (see Fig. 1b [2]). The conducted investigation confirmed the beneficial effect of in situ exsolved nanoparticles on the catalytic activity and stability of materials, leading to increased SOCs efficiency. Based on TEM studies, a mechanism of nanoparticle anchoring in the perovskite was identified, which leads to nanoparticles with higher resistance against sintering, ensuring stable fuel cell operation. Corresponding to the nanoexsolved metallic phase, Selected Area Electron Diffraction data confirmed the presence of metallic crystallites described in the Fm -3 m space group. In the oxidized nanocatalysts, after analyzing high-resolution images has been found the presence of Co ₃ O ₄ -type ( Fd -4 m ), which has been identified in all three oxidized samples. It was also confirmed that the nanofibrous morphology promotes in situ exsolution of smaller nanocatalysts with higher density, enhancing the catalytic activity of the materials, particularly in the lower temperature range (500 – 700 °C). Recorded polarization resistance for the nanofibrous composite air electrode was below 0.08 Ω·cm ² at 750 °C, indicating high potential of obtained compounds in SOCs. Bibliography [1] Y. Zhang, H. Zhao, M. Zhang, Z. Du, W. Guan, S.C. Singhal, K. Świerczek, Chem. Mater. 35, 499−510, 2023 [2] J. Lach, K. Zheng, M. Gogacz, P. Czaja, J. Luo, A. Brzoza-Kos, ECS Transactions, 111(6) 1025-1034, 2023 [3] K. Zheng, J. Lach, P. Czaja, M. Gogacz, P. Czach, A. Brzoza-Kos, P. Winiarz, J. Luo, Journal of Power Sources 587, 233707, 2023 [4] K. Kousi, C. Tang, I. S. Metcalfe, D. Neagu, Small, 17(21), 2006479, 2021 Acknowledgments The work is funded by the National Science Centre Poland (NCN) based on the decision number UMO-2021/43/D/ST5/00824. Jakub Lach acknowledges the financial support of research project supported by the program „Excellence Initiative – Research University” for the AGH University of Krakow. HRTEM, EDS and SAED results were a part of CERIC-ERIC grant no. 20232033 and were conducted in National Institute of Materials Physics, Măgurele in Romania. Figure 1

Solid Oxide Cells (SOCs) are among the most promising green technologies for energy generation and storage. Electricity and heat can be produced using SOCs in fuel cell mode from traditional and renewable energy sources. When electricity consumption is low, the excess electrical energy can be stored in green hydrogen produced by SOCs in electrolysis mode. However, the excellent performance of SOCs is usually recorded rather at high temperatures (≥800 °C), which contributes to high operating costs, limitations in the choice of device materials, making SOCs commercially unviable. To increase economic competitiveness, it is imperative to reduce the working temperature to a range of ≤700 °C while still maintaining a high-power yield. At the desired low operating temperature, most electrode materials lose their excellent electrochemical properties, and the performance of SOCs decreases radically. Therefore, the development of new electrodes with improved electrocatalytic properties at a low temperature range is critical. The design of heterostructured electrodes with dual phases delivering a synergistic effect, significantly benefits the performance of SOCs, including the fabrication of heterostructured electrodes using mechanical milling, self-assembling techniques and in situ exsolution of nanocatalysts [1-3]. In this study, various strategies for engineering heterostructured electrodes have been employed and explored, targeting to boost the electrode performance for SOCs. Mechanically milled composites, one-pot synthesized electrode materials and in situ self-assembled heterostructured nanofibers were fabricated and studied as electrodes for SOCs, allowing to boost the electrochemical performance of electrodes at intermediate temperature range (≤700 °C). The assembled heterostructured electrode GdBa _0.5 Sr _0.5 CoCuO _5+δ and Ce _0.9 Gd _0.1 O _1.95 shows stable and outstanding performance with electrode polarization Rp ≈ 0.05 Ω·cm ^-2 over 480 h at 800 °C, and excellent power outputs (>1000 mW·cm ^-2 at 700 °C) were recorded. In addition, heterostructured electrodes via the formation of A-site deficient Ln _x Sr _2-x Fe _1.4 Ti _0.2 M _0.2 Ni _0.1 Co _0.1 O _6-δ (Ln = selected lanthanides; M = Mo, W, Cr, Mn) perovskites with in situ exsolved nanoparticles were designed. The in situ exsolution of nanoparticles on the surface of nanofibrous electrodes was investigated for improving the performance of SOCs. References: [1] Z. Liu, Y. Bai, H. Sun, D. Guan, W. Li, W. Huang, C. Pao, Z. Hu, G. Yang, Y. Zhu, R. Ran, W. Zhou, Z. Shao, Nature Communications, 15 (2024) 472. [2] K. Kousi, C. Tang, I.S. Metcalfe, D. Neagu, Small, 17 (2021) 2006479. [3] K. Zheng, J. Lach, P. Czaja, M. Gogacz, P. Czach, A. Brzoza-Kos, P. Winiarz, J. Luo, Journal of Power Sources, 587 (2023) 233707. Acknowledgements: The work is funded by the National Science Centre Poland (NCN) based on the decision number UMO-2021/43/D/ST5/00824. Kun Zheng also acknowledges the financial support of research project supported by the program „Excellence Initiative – Research University” for the AGH University of Krakow.

Solid oxide cells (SOCs) can operate efficiently in solid oxide fuel cell (SOFC) and/or solid oxide electrolysis cell (SOEC) modes, and are one of the most promising electrochemical devices for energy conversion and storage, facilitating the integration of renewable energies with the electric grid. However, the SOC electrodes suffer performance and stability issues, especially in the case of fuel electrodes when SOCs are fueled by cheaper and more available fuels such as methane and natural gas. Typical Ni-YSZ cermet fuel electrodes suffer problems of coarsening, carbon deposition, and sulfur poisoning. Therefore, developing new electrodes using novel design strategies for SOCs is crucial. In this review work, the fuel electrode development strategies including the in situ exsolution of nanoparticles, multi-elemental nanocatalysts, and nanofiber materials have been reviewed and summarized for the design of new electrodes for SOCs. Nanofiber electrodes with in situ exsolved nanoparticles, which combine the advantages of a unique nanofiber microstructure and stable and active exsolved nanoparticles, are of great interest and significantly contribute to the development of high-performance fuel electrodes for SOCs.

Solid Oxide Fuel Cells (SOFCs) are one of the most attractive energy conversion and storage devices, which can be applied for decentralized energy applications. However, the high operating temperature making SOFCs still unmarketable, which indicates that lowering the working temperature of SOFCs while still maintaining a high-power output is very crucial. For Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs), new cathode materials with high electrocatalytic activity and stability are necessary to provide a stable and high performance of cells. Perovskite-structured oxide (ABO_3−δ) is one group of the most interesting material candidates for IT-SOFCs, presenting great potential in chemical composition modifications, yielding the design and gain of desired physicochemical and electrochemical properties [1]. The simple perovskite LaCuO₃ is one of the well-studied Cu-content oxides with a high conductivity (106 S·cm⁻¹), but it can be hardly obtained and suffers with stability issues in air [1, 2]. To stabilize the perovskite structure and to improve electrochemical properties of Cu-content oxides A-site and B-site modification strategy can be applied. It has been noted that Cu- and Ni-containing LaNi_0.5Cu_0.5O_3−δ simple perovskite possesses low cathodic polarization of 0.056 Ω·cm² at 800 °C, and a high power output of 870 mW·cm⁻² at 900 °C [1, 3]. The generation of oxygen vacancies by strontium doping contributes to the increase of ionic conductivity component, and the Sr dopant can reduce the oxygen vacancy formation energy and increase the ionic conductivity of La_1−xSr_xMO_3−δ (M = Fe, Mn) perovskites [1,4]. Therefore, in this work, Cu-content La_1−xSr_xNi_1−yCu_yO_3−δ oxides with strontium doping at the A-site were evaluated as very promising Co-free cathode material candidates for IT-SOFCs. In this work, all La_1−xSr_xNi_1−yCu_yO_3−δ compounds have been successfully obtained using soft chemistry. The room-temperature crystal structure of all oxides was assigned to the R-3c trigonal system. The substitution of La with Sr lowers the phase transition temperature from the R-3c space group to Pm-3m simple perovskite while increases the oxygen non-stoichiometry and the thermal expansion coefficient of studied materials. All investigated materials are chemically compatible with GDC-10, while unstable in contact with LSGM and 8YSZ. The manufactured anode-supported IT-SOFC with the La_0.95Sr_0.05Ni_0.5Cu_0.5O_3−δ cathode presents an excellent power density of 445 mW·cm⁻² at 650 °C in humidified H₂. The results confirmed that La_1−xSr_xNi_1−yCu_yO_3−δ perovskites can be potentially qualified as promising cathode candidates for IT-SOFCs, yielding promising electrochemical performance in the intermediate-temperature range [1]. References: [1] J. Lach, K. Zheng, R. Kluczowski, A. Niemczyk, H. Zhao, M. Chen, Materials, 15(24), 8737, (2022). [2] J. S. Zhou, L. G. Marshall, J. B. Goodenough, Phys. Rev. B, 89, 245138, (2014). [3] A. Niemczyk, K. Zheng, K. Cichy, K. Berent, K. Kuster, U. Starke, B. Poudel, B. Dabrowski, K. Swierczek, Int. J. Hydrog. Energy, 45, 29449–29464, (2020). [4] A. B. Muñoz-García, A. M. Ritzmann, M. Pavone, J. A. Keith, E. A. Carter, Acc. Chem. Res. 47, 3340–3348 (2014).

Solid oxide cells (SOCs) offer great promise for efficient energy generation and storage in a variety of applications. Symmetrical SOCs with identical anode and cathode materials configuration favoring an efficient reversible operation of the cells, allow both electricity production from traditional and renewable energy sources in fuel cell mode, and clean, efficient fuel production (in reverse/electrolysis mode). In addition, the symmetrical cell design is particularly advantageous, including: decreased number of cell components, simplification of fabrication process and alleviating the chemical stability issues related with materials, as well as the enhanced resistance to coking and sulphur poisoning, when cells are fuelled by alternative sources (non-H₂) [1]. However, electrode material candidates with excellent redox stability and electrocatalytic activities in both oxidizing and reducing conditions for symmetrical SOCs are limited due to the strict criteria. The design of electrode materials with in situ exsolution of nanoparticles significantly benefits the performance of SOCs [2]. In the anode operation condition, metallic nanoparticles are in situ exsolved from the parent electrode materials decorating on the electrode surface, substantially improving the catalytic activity and electronic conductivity of anode, thus boosting the cell performance. The in situ exsolved nanoparticles can be socketed to the parent oxide, enhancing the stability and hydrocarbon coking tolerance, making the symmetrical SOCs fuelled by cheaper and readily available non-hydrogen alternatives. Moreover, the possible reversible exsolution/dissolution of nanoparticles can potentially resolve the particle agglomeration and coke formation during usage of non-hydrogen fuels [2, 3]. The electrospinning technique has been reported to obtain nanofiber-structured electrodes for SOCs, which efficiently contributes to the enhancement of the electrochemical performance of SOCs [4]. In this work, the A-site deficient (Ln, Ba/Sr)_2-α(Fe,M)_2-x(Ni,Co)_xO_6-δ (Ln = selected lanthanides; M = Mn, Ti, Cr, Mo, W) perovskites were successfully obtained and evaluated as novel electrode materials for SOCs. It has been found that the A-site nonstoichiometry favours the in situ exsolution of metallic nanocatalysts (Ni_xCo) from the parent materials in reducing condition. The selection of A-site cations (lanthanides) significantly affects the crystal structure (simple or double perovskite structure), while the B-site cations determine the stability of materials in air and reducing atmospheres. The reversibility of exsolved nanoparticles is systematically investigated under reducing and oxidizing conditions with different annealing temperatures. The possibility of in situ exsolution of nanooxides on the cathode (Ni_xCoO_y-like nanooxides) is also investigated. Nanofiber-structured electrodes with the ability of in situ exsolution of nanocatalysts are fabricated using the electrospinning technique, which significantly boost the electrochemical properties of constructed SOCs. References: [1] Y. Yang, Y. Wu, H. Bao, W. Song, H. Ni, D. Tian, B. Lin, P. Feng, Y. Ling, Electrochim. Acta, 362, 137171, (2020). [2] C. Su, W. Wang, M. Liu, M. O. Tadé, Z. Shao, Adv. Energy Mater., 5(14), 1–19, (2015). [3] K. Kousi, C. Tang, I. S. Metcalfe, D. Neagu, Small, 17(21), 2006479, (2021). [4] S. T. Aruna, L. S. Balaji, S. S. Kumar, B. S. Prakash, Renew. Sustain. Energy Rev., 67, 673–682, (2017).

In this work, A-site deficient Sm_0.9Ba_0.9Mn_1.8−xFe_xCo_0.1Ni_0.1O_6−δ (x = 0, 0.45 and 0.9) double perovskites with in situ exsolved nanoparticles were successfully obtained and evaluated as electrode materials for symmetrical Solid Oxide Cells (SOCs). All obtained oxides belong to the P4/nmm tetragonal system, and a phase transition from P4/nmm to P4/mmm was recorded by the HT-XRD in the air. The in situ exsolution of nanoparticles has been confirmed by the XRD and SEM analysis. The proposed materials present excellent redox stability and moderate thermal expansion coefficients. Electrospun nanofibers (with 150 nm diameter) were successfully fabricated, indicating an excellent potential application in the electrode. The results show the developed A-site deficient double perovskites with iron doping at Mn-site can be potentially applied as novel electrode materials for symmetrical SOCs.