High Temperature Solid Oxide Cells for Power-to-X applications: Status, Materials, Challenges and Prospects

High-temperature solid oxide cells (SOCs) is in the center of scientific attention for a while as a promising technology related to sustainable society. In this case, SOCs consist of two closely interlinked technologies: solid oxide fuel cells and solid oxide electrolysis cells. Both technologies share several similar traits such as high electrical efficiency and fast reaction kinetics due to the high operating temperature (600–900 °C) without the need for PGM catalysts. These cells basically consist of two porous composite electrodes and a dense ion conducting electrolyte. In particular, the inner microstructure of the electrodes predetermines the SOCs performance owing to its governing effect on the electrical conductivity and/or electrocatalytic activity. Despite its importance, contemporary understanding of the development of the emerging microstructure during the fabrication of these composite materials is far from sufficient.Lanthanum-strontium-manganite (LSM, La1-xSrxMnO3-δ) paired with yttria stabilized zirconia (YSZ, ZrO2:Y2O3) is the typical example of the state-of-the art porous oxygen electrode exhibiting a mixed ion-electron conductivity suitable for SOCs. The fabrication process of LSM:YSZ electrodes involves a crucial step – the sintering, which principle lies in the thermal treatment of the precursor particles for a defined period of time. The LSM:YSZ composite is usually sintered at 1150 °C. At this temperature, exclusively LSM particles tend to soften and form an interlinked solid framework between individual particles, while the YSZ particles remain unaffected by the temperature treatment due to their high melting point (around 2500 °C).Despite the sintering process belongs among the oldest known fabrication techniques, there is no hard evidence on how the sintering proceeds over time at different temperatures while employing materials with significantly different melting points, such as LSM and YSZ. In addition, the sintering temperature along with the sintering time directly influences the resulting inner microstructure and consequently the electrical conductivity of such electrode. Prediction of the resulting inner microstructure is frequently performed by postmortem analyses of multiple samples sintered at different temperatures. However, such practice results only in indirect proofs of the impact of the sintering temperature and is significantly time consuming. The goal of this study is, therefore, to investigate the sintering process of LSM:YSZ composite in situ and to set into relation the resulting inner microstructure with the applied thermal treatment.To answer the present goal, a novel combination of SEM paired with inbuilt MEMS heating chip was developed. This setup allows for online monitoring of the LSM:YSZ sintering process. The results show that at 900 °C the LSM particle agglomerates begin to shrink as the individual LSM particles start to adhere to each other. At this point, significant porosity emerges in the composite, which is considered essential to ensure a sufficient mass transfer during the SOCs operation. On the other hand, the degree of the emerged porosity can be beyond the threshold of mechanical stability of the electrode body, rendering such an electrode unusable. At 1150 °C the densification of the composite occurs, which is essentially a self-healing process resulting in a well-interlinked and mechanically stable composite. The densification is related to the adhesion of the LSM particles to the YSZ particles, thus forming a solid framework, ensuring ample mixed ionic-electronic conductivity of the fabricated electrode. Based on the results, a slight variation in sintering temperature and/or time leads to a significant difference in the electric conductivity of the electrode due to a different sintering degree or, in other words, due to the different void fraction in the body of the electrode. Within the literature, this outcome is reflected by a major inconsistency in reported electric conductivity of the commonly utilized 50:50 wt.% ratio of LSM:YSZ.An interesting insight into the sintering process of the LSM:YSZ composite has provided the SEM paired with inbuilt MEMS heating chip. This experimental technique is very promising for in situ studies of sintering and/or synthesis processes of various materials related to numerous technologies.This work was supported by the Technology Agency of the Czech Republic under project no. TK04030143.

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Performance characterization and durability testing have been completed on two five-cell high-temperature electrolysis stacks constructed with advanced cell and stack technologies. The solid oxide cells incorporate a negative-electrode-supported multi-layer design with nickel-zirconia cermet negative electrodes, thin-film yttria-stabilized zirconia electrolytes, and multi-layer lanthanum ferrite-based positive electrodes. The per-cell active area is 100 cm2. The stack is internally manifolded with compliant seals. Treated metallic interconnects with integral flow channels separate the cells and electrode gases. Stack compression is accomplished by means of a custom spring-loaded test fixture. Initial stack performance characterization was determined through a series of DC potential sweeps in both fuel cell and electrolysis modes of operation. Results of these sweeps indicated very good initial performance, with area-specific resistance values less than 0.5 Ω.cm2. Long-term durability testing was performed with a test duration of 1000 hours. Overall performance degradation was less than 10% over the 1000-hour period. Final stack performance characterization was again determined by a series of DC potential sweeps at the same flow conditions as the initial sweeps in both electrolysis and fuel cell modes of operation. A final sweep in the fuel cell mode indicated a power density of 0.356 W/cm2, with average per-cell voltage of 0.71 V at a current of 50 A.

High temperature solid oxide cells (SOCs) are used for co-electrolysis of carbon dioxide and steam for the production of synthetic fuels. SOC rate-limiting processes can be investigated using Electrochemical Impedance Spectroscopy (EIS), but first, test rig contributions must be subtracted: inductance in particular. Here we illustrate the effect of inductance correction L on interpretation methods, including: impedance Z* and electric modulus M* formalisms, the Analysis of Differences in Impedance Spectra (ADIS) and the determination of Distribution of Relaxation Times (DRT). For inductances in the range ~10-7 H, results were sensitive to small changes in L, giving a significant change in high frequency Z* and M* data. This causes underestimation of the SOC polarisation resistance. ADIS results are less affected, due to the nature of the subtraction method, whereas DRT is highly sensitive to inductance, hence care must be taken to optimise inductance correction, to obtain accurate results.

Electrolysis is a well-known process to split water (H2O) into hydrogen (H2) and oxygen (O2) by using electrical energy. If the water can be converted into steam by waste heat from other processes or steam is directly available at an industrial facility, it is more efficient to perform a high temperature electrolysis (HTE) and convert the steam directly. Steam is commonly supplied at elevated pressure, and the industrial demand for hydrogen product gas is at high pressure too. Therefore, the overall process efficiency can be increased by operating the HTE at elevated pressure level because steps of expansion and compression of the involved gases are saved. As technique for HTE, solid oxide cells (SOC) are used. So far they are mainly known and used as fuel cells (SOFC). SOC are usually made of different ceramics. They consist of a membrane for gas separation, a hydrogen electrode and an oxygen electrode. The SOC is easy scalable. So called Stacks are a series connection of multiple cells, and these Stacks can as well be connected in series or in parallel to fulfill the requirements of the specific application. Sunfire presents a system approach for a pressurized SOC system. It is shown how Stacks and Hot-BoP components can be operated at elevated pressures. First results of a prototype with a nominal electrolysis DC input power of 10 kW and an operation range of up to 15 bar are shown. Main objectives are the control of the different pressure sections, stack performance and thermal behaviour at varying pressure levels. Acknowledgement The research leading to these results has received funding from the EU’s Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°621210 for the HELMETH project (www.helmeth.eu). Figure 1

Highly efficient perovskite-based fuel electrodes for solid oxide electrochemical cells via in-situ nanoparticle exsolution and electron conduction enhancement

This report provides documentation of experimental research activities performed at the Idaho National Laboratory and at Ceramatec, Inc. during FY07 under the DOE Nuclear Hydrogen Initiative, High Temperature Electrolysis Program. The activities discussed in this report include tests on single (button) cells, short planar stacks and tubular cells. The objectives of these small-scale tests are to evaluate advanced electrode, electrolyte, and interconnect materials, alternate modes of operation (e.g., coelectrolysis), and alternate cell geometries over a broad range of operating conditions, with the aim of identifying the most promising material et, cell and stack geometry, and operating conditions for the high-temperature electrolysis application. Cell performance is characterized in erms of initial area-specific resistance and long-term stability in the electrolysis mode. Some of the tests were run in the coelectrolysis mode. Research into coelectrolysis was funded by Laboratory Directed Research and Development (LDRD). Coelectrolysis simultaneously converts steam to hydrogen and carbon dioxide to carbon monoxide. This process is complicated by the reverse shift reaction. An equilibrium model was developed to predict outlet compositions of steam, hydrogen, carbon dioxide, and carbon monoxide resulting from coelectrolysis. Predicted ompositions were compared to measurements obtained with a precision micro-channel gas chromatograph.

Solid oxide fuel cells (SOFCs) excel by high efficiencies in fuel cell as well as electrolysis modes, and by being able to operate in both modes as a reversible cell (solid oxide cell – SOC). This allows for production of electricity and heat from a green fuel, and for storage of electricity as gas or use as fuel. Lifetime and costs are major factors enabling such reversible SOCs to enter green energy systems. Metal supported SOCs (MSCs) provide cost-competitive materials within the cell. Furthermore, targeting the lower operating temperatures around 650 oC, MSCs will allow for cheaper stack and balance of plant components as well. Lowering operating temperatures leads to a reduction of thermally activated degradation processes, thereby prolonging the lifetime. The present study investigates the option to operate MSCs, fabricated at DTU Energy by tape casting, lamination, and screen-printing, in reversible mode between fuel cell (FC) and electrolysis (EC). Emphasis is on the effect of reversible operation on performance and durability of the MSC, compared to steady state operation in either mode, and to the behavior of state-of-the-art (SoA) fuel electrode supported SOC with Ni/YSZ fuel electrode. The MSCs are composed of a FeCr support, a Ni/GDC (gadolinium-doped ceria) infiltrated LSFNT (lanthanum-doped strontium iron nickel titanate) fuel electrode, a YSZ (yttria-stabilized zirconia) electrolyte, a GDC barrier layer, and an in situ sintered LSC (lanthanum-doped strontium cobaltite) air electrode. The reversible operation was carried out by switching between FC and EC modes at current densities of 0.25 and -0.25 A/cm2, respectively, at 650 oC using a gas mixture of 50/50 H2O/H2 to the fuel electrode and air to the oxygen electrode.Figure 1 shows the evolution of the cell voltages for the SoA cell and the MSC. The degradation rate of the SoA cell was larger during operation in EC as compared to FC mode. Similar observations were made previously, even though these tests were typically carried out at temperatures higher than 650 oC as in this work [1]. Furthermore, the degradation rate decreases over time, more particularly in EC mode, which is also a known phenomenon at this type of cells [2, 3]. In the final ca. 200 h, both degradation rates are in the range of 3%/1000 h, which is an interesting observation, i.e., the longer-term degradation rates are similar in both modes (EC and FC). The analysis of electrochemical impedance spectroscopy (EIS) recorded under current allowed to conclude that the main contribution to the degradation is the increase of polarization resistance, i.e., related to electrodes degradation.In the initial ca. 400 h hundred hours, the cell voltage degradation on the MSC is larger in fuel cell mode, while there is nearly no degradation in electrolysis mode. The good stability in EC mode over a few hundred hours confirms the findings of steady-state electrolysis tests with the same type of cells [4]. In the final period from ca. 600 h, both degradation rates increase but stay fairly constant with ca. 4%/1000 h in EC and ca. 16%/1000 h in FC mode, when calculated as linear increase. EIS reveals that both, the serial and the polarization resistances increase in parallel, which indicates a combination of degradation of electrode and probably corrosion and/or interface attachment. Details will be presented, including comprehensive EIS evaluation combined with micro-structural characterization.Figure 1. Cell voltage vs. operating time under current in reversible mode at 650 oC, 0.25 A/cm2 in fuel cell and -0.25 A/cm2 in electrolysis mode, 50/50 H2O/H2 fuel and air to the oxygen electrode, (a) SoA cell, (b) MSC, gaps in the cell voltage are interruptions of operation due to technical issues in the labReferences[1] X. Sun, B.R. Sudireddy, X. Tong, M. Chen, K. Brodersen, A. Hauch, Optimization and Durability of Reversible Solid Oxide Cells, ECS Trans. 91 (2019) 2631.[2] A. Hagen, R. Barfod, P.V. Hendriksen, Y.-L. Liu, S. Ramousse, Degradation of anode supported SOFCs as a function of temperature and current load, J. Electrochem. Soc. 153(6) (2006) A1165.[3] A. Hauch, K. Brodersen, M. Chen, C. R. Graves, S. H. Jensen, P. S. Jørgensen, P. V. Hendriksen, M. B. Mogensen, S. Ovtar, X. Sun, A Decade of Solid Oxide Electrolysis Improvements at DTU Energy, ECS Transactions, 75(42) (2017) 3.[4] A. Hagen, R. Caldogno, F. Capotondo, X. Sun, Metal Supported Electrolysis Cells, Energies 15 (2022) 2045. Figure 1

Fatigue resistance of steels: Boardman, B. Metals Handbook 10th Edn (ASM International, Materials Park, Ohio, 1990) Vol. 1, pp 673–688

Mathematical approaches to modelling the mass transfer process in solid oxide fuel cell anode

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 GdBa0.5Sr0.5CoCuO5+δ (GBSCC) - Ce0.9Gd0.1O1.95 (GDC) and Sr1.8Fe1.4Ti0.2Cr0.2Ni0.1Co0.1O6-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 (Rp) 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

The performance degradation of Solid Oxide Cells (SOC) caused by microstructure evolution limits the SOC’s lifetime. Long-term testing combined with post-mortem characterization is one common approach to clarify degradation mechanisms and develop counter-acting measures, but this is associated with extensive amount of experimental work and long research time. Instead, many researchers have devoted their efforts to investigating degradation using computational methods. The phase-field model has been utilized in the SOC research to illustrate the microstructure evolution during long-term operation from micron to millimeter scale, with the possibility of taking into account the mechanical properties as well. In this article, the principle of the phase-field method and different models are introduced first, followed by examples published in literature on phase-field modeling of various degradation phenomena in SOCs. Finally, possible strategies coupling modelling and experimental research in optimizing SOC performance and microstructure are discussed.

Pressurized operation of SOCs increases performance and enables the coupling with a gas turbine in the fuel cell mode and improves the overall process efficiency in producing pressurized hydrogen or syngas in the electrolysis mode. So far, facilities for pressurized operation of solid oxide cells are commonly based on pressure vessels enclosing the entire single cell housing or stack to be operated. This approach results in reduced gravimetric and volumetric power density, enhanced costs and a complex control strategy to equilibrate fuel-, oxidant- and vessel pressure. Thus, limiting the pressurization solely to the stack or the single cell housing is advantageous.In a previous study [1] a pressurized testing set-up without the need for a pressure vessel was presented. This set-up, which is based on existing solid oxide cell testing technology [2], enables testing of incremental planar solid oxide cells at pressures of up to 11 bara. In order to enable pressurization, the cell is sealed in between two metallic housing components by a glass sealant, which has to withstand the pressure difference between the active part of the cell and the ambient pressure in the furnace.This application of metallic components (figure 1 (a)) in single cell testing might affect a reasonable evaluation of the intrinsic cell losses, thus for ambient pressure testing commonly ceramic cell housings and inert contact grids (Au- or Ni-grid) are applied.To overcome these issues, in a first step, an aluminium oxide scale forming steel grade was chosen for the housing body. The cell is contacted with metallic flow fields similar to a stack as shown in Ref. [3]. As Crofer 22 APU/H are extensively studied steel grades for solid oxide cells, in a second step uncoated Crofer 22 H was used for the cell housing, resulting in severe chromium poisoning of the cell. To overcome this issue, a more inert testing environment was realized by a ceramic inlay combined with Au- and Ni- contact grids as shown in figure 1 (b).In this contribution the different concepts will be evaluated regarding their applicability for pressurized testing. The impact of housing and contacting on cell performance and stability will be discussed. Performance losses and poisoning effects are studied by means of electrochemical impedance spectroscopy and the subsequent analysis by the distribution of relaxation times.Figure 1: CAD-drawing cross section of pressurized test bench with metallic cell housing in (a) [1] and ceramic inlay in (b).

Chromium poisoning of the air electrode is a primary degradation mechanism for solid oxide cells (SOCs) operating under fuel cell mode. Recent experimental findings show that reversed pulse operation for SOCs operated as electrolyser cells can reverse this degradation and extend the lifetime. Here, we use a multiphysics model of an SOC to investigate the effects of reversed pulse operation for alleviating chromium poisoning of the air electrode. We study the effects of time fraction of the operation under fuel cell and electrolysis modes, cyclic operation starting after a certain duration, and fuel cell and electrolysis current densities on the cell lifetime, total power, and hydrogen production. Our modeling shows that reversed pulse operation enhances cell lifetime and total power for all different cases considered in this study. Moreover, results suggest that the cell lifetime, total power, and hydrogen production can be increased by reversed pulse operation at longer operation times under electrolysis mode, cyclic operation starting from the beginning, and lower electrolysis current densities. All in all, this paper documents and establishes a computational framework that can serve as a platform to assess and quantify the increased profitability of SOCs operating under a co-production operation through reversed pulse operation.

Advances in High-Temperature Electrocatalytic Reduction of CO2 and H2O