High Temperature Co-electrolysis Research Articles

Enhancing Direct Eectrochemical CO2 Electrolysis By Introducing a-Site Deficiency for Dual-Phase Pr(Ca)Fe(Ni)O3-Δ Cathode

High-temperature CO2 electrolysis via solid oxide electrolysis cells (CO2-SOECs) has drawn special attention due to the high energy convention efficiency, fast electrode kinetics, and great potential in carbon cycling. However, the development of cathode materials with high catalytic activity and chemical stability for pure CO2 electrolysis is still a great challenge. In this work, A-site cation deficient dual-phase material, namely (Pr0.4Ca0.6)xFe0.8Ni0.2O3- δ (PCFN, x=1, 0.95, and 0.9), has been designed as the fuel electrode for a pure CO2-SOEC which presents superior electrochemical performance. Among all these compositions, (Pr0.4Ca0.6)0.95Fe0.8Ni0.2O3- δ (PCFN95) exhibited the lowest polarization resistance of 0.458 Ω cm2 at open circuit voltage and 800℃. The application of PCFN95 as the cathode in a single cell yields an impressive electrolysis current density of 1.76 A cm-2 at 1.5 V and 800℃, which is 76% higher than that of single cells with stoichiometric Pr0.4Ca0.6Fe0.8Ni0.2O3- δ (PCFN100) cathode. The effects of A-site deficiency on materials’ phase structure and physicochemical properties are also systematically investigated. Such an enhancement in electrochemical performance is attributed to the promotion of effective CO2 adsorption, as well as the improved electrode kinetics resulting from A-site deficiency.

Spectroscopic Elucidation of Phase Evolution in NiO/YSZ Under Oxidative and Reductive Gas Environments

High temperature co-electrolysis of CO2 and H2O serves a viable, energy efficient, and scalable route to produce syngas (CO+ H2), which can be used to generate a variety of utility hydrocarbons. Solid oxide electrolysis cell (SOEC) has proven to be a promising technology for CO2 utilization and a carbon neutral route to curb CO2 emissions. The design of cathode catalyst is critical as it must facilitate the activation of CO2 and H2O and also exhibit high electronic and oxygen ion conductivity. The state-of-the-art cathode material used in SOECs is Ni/YSZ (8 mol% Yttria Stabilized Zirconia) ceramic composite (known as cermet), whose performance is dependent on various operating factors including temperature, composition of feed gas stream, presence of contaminants like SOx/NOx etc. There is a very limited number of studies on the evolution of the cathode materials under process level conditions. To begin with, effect of different oxidative and reductive gas environments and temperature over Ni/YSZ is studied using in situ Raman spectroscopy.Typical flue gas composition is chosen as the basis for the concentration of different components like 15% CO2, 10% H2O, 5% O2 with argon. Under oxidative conditions (O2, CO2, H2O) with temperature variation range of RT-800 0C, NiO/YSZ vibrational modes revealed that it is composed of NiO and YSZ phases. It is seen that there is thermal broadening in the bands of NiO and YSZ, thermal effects reversible upon cooling down and red shift in the NiO bands exemplified the lattice expansion. Exposing the NiO/YSZ to 5% H2 at high temperature resulted in the formation of metallic Ni, which is corroborated by the absence of the Ni-O vibrational modes in Raman spectrum at 400 0C. YSZ phase is quite stable in H2 at elevated temperature.After reduction of NiO/YSZ to get Ni/YSZ, exposure to oxidative environments (i.e., O2, CO2, H2O) aided in the formation of defect laden NiOx phase on YSZ. Specifically, NiOx formation starts at 350 0C under oxygen and at 450 0C under CO2 and H2O. Under 15% CO2, there is an overlap between the bands associated to the carbonate like species and the NiO phase but there is no formation of coke at these conditions as D and G bands were not observed (Fig. 1). It could be due to the oxidation of Ni that obliterated coke formation. The blue shift at 1046 cm-1 (2P mode of NiO) signifies the shortening of bond length in Ni-O with gradual conversion of Ni to NiOx. Compared to the NiOx phase formed under 5% O2 and 15% CO2 which has a sharp feature in Raman spectra, there is a broad band of NiOx phase formed under 10 % H2O in the region 480-580 cm-1 . This observation demonstrates the variation in the microcrystalline size of the defect laden NiOx phase formed under exposure to different oxidizers.Another set of experiments were done with the dry hydrogen (5 % H2) and humidified hydrogen (5% H2 + 10 % H2O) to study the role of presence of water on the reduction of NiO/YSZ with temperature variation between 120 – 800 0C. It was found that reduction of NiO to Ni is not affected by the presence of water. Under both conditions this transformation occurs between 400-600 0C. The results from the current study provide the key information on the effects of temperature and gas compositions over Ni/YSZ which serves as a guide for the process level design and operation of SOECs. References Juliana Carneiro et al, Industrial & Engineering Chemistry Research 2020 59(36), 15884-15893.John Kirtley et al, The Journal of Physical Chemistry C2013, 117 (49), 25908-25916.A. Naumenko et al, Phys Chem Solid State 2008, 9, 121–125. Figure 1

Techno-economic assessment of electro-synthetic fuel based on solid oxide electrolysis cell coupled with Fischer–Tropsch strategy

Electro-synthetic fuel is proposed as an approach to achieve net-zero carbon emissions for heavy-duty internal combustion engines and meet carbon neutrality targets. This method is based on high temperature co-electrolysis using a solid oxide electrolysis cell (SOEC) coupled with Fisher-Tropsch (FT) synthesis due to its high efficiency and carbon recycling capacity. To access the techno-economic viability of this pathway and identify the key elements impacting the cost of electro-synthetic fuels, a comprehensive techno-economic model is constructed. This model aims to offer cost cutting guidance and includes SOEC, FT, and techno-economic sub-models, with SOEC and FT validated using literature data. The cost breakdown and sensitivity analysis indicate that heating costs, electricity prices, and stack lifetime are critical factors in reducing the cost of electro-synthetic fuel. Three alternative system layouts that fully utilize thermal and chemical energy are proposed to address the significant heating expense issue. Among these, the optimal system design lowers costs by approximately 5.3 %. Furthermore, this work introduces the contradiction between high performance, high stability, and low-cost accounting for the SOEC lifetime, which is bounded by operating current density. When considering the degradation and replacement of SOEC stacks, the long-term profitability of operating SOEC at a current density of 500 mA/cm2 is superior to that of 850 mA/cm2. Despite the most effective layout being currently unprofitable, it is anticipated that in the future, carbon trade, renewable electricity and technological advancements will drive the cost of electro-synthetic fuel to become competitive with that of diesel.

Carbon utilization in natural gas-based hydrogen production via carbon dioxide electrolysis: Towards cost-competitive clean hydrogen

Conventional blue hydrogen production still leads to direct carbon emissions, as well as significant energy consumption and costs for carbon dioxide (CO2) capture. CO2 electrolysis, which is one of the promising carbon utilization technologies, faces challenges beyond cell performance, including downstream separation. To address these issues, this study proposes a novel hydrogen production process integrated with CO2 electrolysis. Four integrated processes were developed based on two types of CO2 electrolysis in which cathode separation was either included or excluded. These integrated processes eliminate atmospheric carbon emissions from fossil-fuel-based hydrogen production without requiring a carbon capture unit; furthermore, the hydrogen yield increases. These processes were evaluated in terms of energy efficiency (EE), levelized cost of hydrogen (LCOH), and process carbon emission (PCE). Among the developed processes, the high temperature CO2 electrolysis without cathode separation (HT-CO2EL(w/oSep)) process exhibited the lowest LCOH and PCE. These findings demonstrate that proposed process effectively addresses the challenges associated with downstream separation and upstream heating inherent in standalone CO2 electrolysis systems. Compared with conventional blue hydrogen production, the HT-CO2EL(w/oSep) process achieved 22.2% improvement in EE, 12.0% reduction in LCOH, and 82.0% decrease in PCE. Sensitivity analysis shows that the proposed process remains economically viable compared to conventional blue and green hydrogen and is more adaptable to market fluctuations. The improvement in cell performance is expected to contribute to cost savings. Even accounting for methane leakage and carbon taxes, the process shows only a modest increase in total carbon emissions and LCOH.

Enhancing the CO2 Adsorption of the Cobalt-Free Layered Perovskite Cathode for Solid-Oxide Electrolysis Cells Gains Excellent Stability under High Voltages via Oxygen-Defect Adjustment.

Solid-oxide electrolysis cells are a clean energy conversion device with the ability to directly electrolyze the conversion of CO2 to CO efficiently. However, their practical applications are limited due to insufficient CO2 adsorption performance of the cathode materials. To overcome this issue, the A-site cation deficiency strategy has been applied in a layered perovskite PrBaFe1.6Ni0.4O6-δ (PBFN) cathode for direct CO2 electrolysis. The introduction of 5% deficiency at the Pr/Ba site leads to a significant increase in the concentration of oxygen vacancies (nonstoichiometric number δ of oxygen vacancies increased from 0.093 to 0.132), which greatly accelerates the CO2 adsorption performance as well as the O2- transport capacity toward the CO2 reduction reaction (CO2RR). CO2 temperature-programmed desorption indicates that A-site cation-deficient (PrBa)0.95Fe1.6Ni0.4O6-δ (PB95FN) shows a larger desorption peak area and a higher desorption temperature. PB95FN also exhibits a greater presence of carbonate in Fourier transform infrared (FT-IR) spectroscopy. The electrical conductivity relaxation test shows that the introduction of the 5% A-site deficiency effectively improves the surface oxygen exchange and diffusion kinetics of PB95FN. The current density of the electrolysis cell with the (PrBa)0.95Fe1.6Ni0.4O6-δ (PB95FN) cathode reaches 0.876 A·cm-2 under 1.5 V at 800 °C, which is 41% higher than that of PB100FN. Moreover, the PB95FN cathode demonstrates excellent long-term stability over 100 h and better short-term stability than PB100FN under high voltages, which can be ascribed to the enhanced CO2 adsorption performance. The PB95FN cathode maintains a porous structure and tightly binds to the electrolyte after stability testing. This study highlights the potential of regulating oxygen defects in layered perovskite PrBaFe1.6Ni0.4O6-δ cathode materials via incorporation of cation deficiency toward high-temperature CO2 electrolysis.

Self-optimized and stable nanocomposites via one-pot synthesis for high-temperature CO2 electrolysis in solid oxide electrolysis cells

Solid oxide electrolysis cells (SOECs) hold the promise of directly converting CO2 into CO fuels with exceptional cost-effectiveness and efficiency. However, the widespread application of SOECs is impeded by the significant lack of highly active and stable cathodes. Herein, a one-pot synthesis strategy is proposed to introduce Cu into a composite consisted of Sr2Fe1.5Mo0.5O6−δ (SFM) and Gd0.2Ce0.8O1.9 (GDC). This strategy leads to the automatic formation of hetero-structured composites, comprising double perovskite, Ruddlesden-Popper perovskite, fluorite, as well as in-situ exsolved Cu–Fe bimetals at high temperatures and operational conditions. Consequently, this approach greatly enhances CO2 adsorption capability, CO2 catalytic activity, O2−/e− conductivity, and ensures good phase compatibility with electrolyte. As results, the La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte-supported single cell with the composite cathode achieves a high current density of 2.22 A cm−2 at 850 °C and 1.6 V, which is substantial increase of 65.7% compared to the current density achieved with traditional physically hybrid SFM-GDC cathodes. These results underscore the potential of one-pot synthesized nanocomposites as promising cathodes for CO2 electrolysis in SOECs.

Surface Activation by Single Ru Atoms for Enhanced High-Temperature CO2 Electrolysis.

Cathodic CO2 adsorption and activation is essential for high-temperature CO2 electrolysis in solid oxide electrolysis cells (SOECs). However, the component of oxygen ionic conductor in the cathode displays limited electrocatalytic activity. Herein, stable single Ruthenium (Ru) atoms are anchored on the surface of oxygen ionic conductor (Ce0.8 Sm0.2 O2-δ , SDC) via the strong covalent metal-support interaction, which evidently modifies the electronic structure of SDC surface for favorable oxygen vacancy formation and enhanced CO2 adsorption and activation, finally evoking the electrocatalytic activity of SDC for high-temperature CO2 electrolysis. Experimentally, SOEC with the Ru1 /SDC-La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ cathode exhibits a current density as high as 2.39 A cm-2 at 1.6 V and 800 °C. This work expands the application of single atom catalyst to the high-temperature electrocatalytic reaction in SOEC and provides an efficient strategy to tailor the electronic structure and electrocatalytic activity of SOEC cathode at the atomic scale.

Enhancing Electrochemical CO2 Reduction of La(Sr)Fe(Mn)O3 with Metal (Ru, Ni, Co, and Fe) Catalytic Additives for High-Temperature CO2 Electrolysis Cells

Solid oxide electrolysis cells (SOEC) have tremendous potential as highly efficient devices for high-temperature CO2 electrolysis. Commonly used Ni-based fuel electrodes experience inherent redox instabilities under direct CO/CO2 conditions. Therefore, the robust perovskite oxide La(Sr)Fe(Mn)O3 (LSFM) has been suggested as an alternative owing to its excellent coking resistance. However, LSFM electrodes are limited by their poor catalytic activity in CO2 reduction. To alleviate this problem, various metal additives have been added as catalysts to decrease the surface reaction barrier, which accelerates the chemical adsorption of CO2. In this study, the various metal catalysts (Ru, Co, Fe, and Ni) are deposited on the surface of LSFM electrode using the infiltration method. The distribution of the relaxation time (DRT) is investigated to confirm the catalytic activity for CO2 electrolysis. Consequently, when applied Fe catalyst in LSFM electrode, an electrolysis performance of 2.2 A cm-2 at 1.5 V in CO2 electrolysis was obtained at 1123 K.

Enhanced high-temperature CO2 electrolysis on CeO2 by inducing under-coordinated dual-atom active sites

Stable CO2 electrolysis to CO and O2 in solid-oxide electrochemical cells (SOCs) is challenging due to the competitive carbon deposition. CeO2-based electrodes have shown advanced carbon-tolerant ability in SOCs. Here, by conducting density-functional theory calculations, we report highly selective CO2 to CO conversion catalyzed by a novel dual-atom site in CeO2 induced by substituting one Ce with two Pd (Ni or Cu). CO2 is effectively activated in the form of carbonate species (CO2*O) with oxygen strongly adsorbed by the under-coordinated dual-atom site, which then dissociates C–O bond with a 1.00 eV lower barrier compared to clean CeO2. By contrast, a 0.22 eV larger barrier is found for carbon deposition on dual-atom sites than on CeO2, leading to carbon suppression over dual-atom sites. Therefore, 2Pd (2Ni or 2Cu) dual-atom sites in CeO2 can significantly improve kinetics of CO2 reduction into CO because of the crucial role of their under-coordinated local structure in facilitating CO2 activation while preventing carbon deposition during CO2 electrolysis, which sheds light on rational design of dual-atom active sites in metal oxide catalysts.

Performance optimization of solid oxide electrolysis cell for syngas production by high temperature co-electrolysis via differential evolution algorithm with practical constraints

Co-electrolysis of water and carbon dioxide using solid oxide electrolysis cell combined with the catalytic reactor to produce renewable synthesis fuel is considered as a promising strategy for carbon dioxide utilization. However, in order to achieve a target product with a remarkable system-level efficiency, a certain proportion of carbon monoxide and hydrogen from solid oxide electrolysis cell must be sent to the catalytic reactor directly, which imposes practical constraints on the solid oxide electrolysis cell performance. Take this constraint into consideration, we proposed a performance design methodology that enables a rapid and effective optimization of the solid oxide electrolysis cell operating conditions. Firstly, a physical model is developed by integrating chemical equilibrium, electrochemistry, and energy conservation involved in the solid oxide electrolysis cell. Secondly, a physical model is utilized to generate a dataset for the development of neural network model and optimization algorithm. This approach allows for the efficient determination of optimal conditions while considering practical constraints. In the case study, under the constraint that the desired fuel is a long carbon chain fuel with a carbon dioxide conversion rate ranging from 80 % to 90 %, the highest efficiency achieved by the solid oxide electrolysis cell is optimized to 66.75 %. Moreover, this approach is extended to other two target fuels (dimethyl ether and methane) under different carbon dioxide conversion rate, and the optimal performance is designed. According to the results, there is a trade-off between high efficiency and high carbon dioxide conversion rate, as the low carbon dioxide conversion rate often leads to higher efficiency. This method allows for flexible design of operating conditions to meet different fuel requirements and guide practical applications.

Efficient solar hydrocarbon fuel production by integrating Fischer-Tropsch synthesis with high-temperature solid oxide co-electrolysis and electrolysis

Solar production of syngas from water and carbon dioxide followed by solar-driven Fischer-Tropsch synthesis is promising technology to produce syngas with zero carbon emission. A solar-driven integrated system by coupling solid oxide electrolysis cells with Fischer-Tropsch process is proposed for green hydrocarbons production. The proposed system is numerically modeled, developed and verified in this study. The effect of the solid oxide electrolysis cell temperature and current density on the system performance, especially the solar-to-fuels efficiency, is investigated parametrically. The results show that the solar-to-fuels efficiency increases with rise in the solid oxide electrolysis cell temperature. In addition, there is an optimum solar-to-fuels efficiency by varying current density. A maximum solar-to-fuels efficiency of 12.8% is obtained, which is more than 2.82% higher than the benchmark efficiency of solar hydrocarbons production system utilizing high-temperature co-electrolysis only. Moreover, a preliminary techno-economic analysis is performed to investigate the techno-economic feasibility of the proposed system. Based on the analysis, the levelized cost of the fuel for the proposed system is 1.944 Euro/kg. This study does not only verify the thermodynamic feasibility of the system but also provides a baseline for further optimization.

High-Temperature Co-Electrolysis and Degradation by Oxidized Trace Contaminants in Feed Gases

The transformation of our energy system will likely require the indirect electrification of the sectors heating, chemical industry and mobility by power-to-X processes, because not all processes can easily use electrical power directly. The high-temperature co-electrolysis using solid-oxide cells (SOCs) can produce syngas compositions desired by the downstream power-to-X process without the need of a reverse water-gas shift reactor and is therefore particularly well suited to produce synthesis gas as feedstock for various power-to-fuel and power-to-chemicals processes. Besides, a coupling of heat-producing downstream processes would increase system efficiency significantly.1 Besides the intrinsic degradation processes of the cells (e.g. nickel migration) and stacks (e.g. contact deterioration) any additional processes triggered by impurities in the feed gases are of relevance for any technical applications. Under certain circumstances, impurities can be introduced by Balance of Plant (BoP) components like the steam generators or preheaters that can release silicon species.2 Also, the feed gases themselves may contain harmful substances. This poisoning effect must be taken into account, because economically gases of lower qualities are preferred as purification steps increase the costs of the syngas. Depending on the exact source of carbon dioxide (CO2) the supplier uses, it contains different secondary components, of which sulfur species are of particular concern. In contrast to fuel-cell mode operation, in electrolysis mode operation hydrogen sulfide (H2S) is not the most likely candidate, but sulfur dioxide (SO2). The reasons for this is that CO2 may be obtained from combustion processes and that hydrogen sulfide is not stable in the atmosphere but has a short half-life time.Therefore, understanding the influence of SO2 contained in the supplied carbon dioxide on the performance of the co-electrolysis is of great importance. There is substantial knowledge about the impact of H2S in feed gases in fuel-cell mode, but the impact of sulfur dioxide in electrolysis mode is not as well investigated. Skafte et al. reported a severe effect in CO2 electrolysis with as low as 20 ppb H2S.3 In the context of this application, the relevant sulfur species present in the feed gases will be SO2. It should be noted, that the heterogenous reduction of SO2 with hydrogen or carbon monoxide is catalyzed by oxides with oxygen vacancies and mobility like CeO2 or YSZ and facilitated by the presence of transition metals.4 Intermediary steps and products beside H2S can include significant amounts of elemental S and COS and the formation of sulfided nickel seems likely. The formation of these products depends on the exact reaction conditions, e.g. temperature, H2/SO2 ratio, steam content, which makes the situation more complex as in fuel-cell mode with hydrogen as fuel. Jeanmonod et al. came to the conclusion, that even concentrations of SO2 as low as 0.5 ppm caused irreversible degradation in Ni/YSZ electrodes in co-electrolysis mode.5 This contribution will report about investigations performed on the degradation caused by sulfur dioxide and silicon species in sub-ppm concentrations in SOCs with Ni/YSZ and Ni/GDC fuel electrodes operated in co-electrolysis mode, using stacks in the Jülich F10 and F20 design. These targeted their impact, potential recovery, possible damage mechanisms and safe thresholds of sulfur dioxide in carbon dioxide. For this the authors performed long-term stationary operation, monitoring of poisoning experiments by electrochemical impedance spectrocopy and post-test analysis of cells, including SEM/EDX and chemical trace analyses. Acknowledgement The authors would like to thank their colleagues at Forschungszentrum Jülich GmbH for their great support and the Helmholtz Society as well as the the German Federal Ministry of Education and Research for financing these activities as part of the Kopernikus P2X-2 project (03SFK2Z0-2). References (1) Peters, R.; Wegener, N.; Samsun, R. C.; Schorn, F.; Riese, J.; Grünewald, M.; Stolten, D. A Techno-Economic Assessment of Fischer–Tropsch Fuels Based on Syngas from Co-Electrolysis. Processes 2022, 10 (4), 699. https://doi.org/10.3390/pr10040699.(2) Schäfer, D.; Queda, L.; Nischwitz, V.; Fang, Q.; Blum, L. Origin of Steam Contaminants and Degradation of Solid-Oxide Electrolysis Stacks. Processes 2022, 10 (3), 598. https://doi.org/10.3390/pr10030598.(3) Skafte, T. L.; Blennow, P.; Hjelm, J.; Graves, C. Carbon Deposition and Sulfur Poisoning during CO2 Electrolysis in Nickel-Based Solid Oxide Cell Electrodes. Journal of Power Sources 2018, 373, 54–60. https://doi.org/10.1016/j.jpowsour.2017.10.097.(4) Liu, W.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Reduction of Sulfur Dioxide by Carbon Monoxide to Elemental Sulfur over Composite Oxide Catalysts. Applied Catalysis B: Environmental 1994, 4 (2–3), 167–186. https://doi.org/10.1016/0926-3373(94)00019-0.(5) Jeanmonod, G.; Diethelm, S.; Van Herle, J. The Effect of SO 2 on the Ni-YSZ Electrode of a Solid Oxide Electrolyzer Cell Operated in Co-Electrolysis. J. Phys. Energy 2020, 2 (3), 034002. https://doi.org/10.1088/2515-7655/ab8b55.

Operating Strategies for Fischer-Tropsch Tail Gas Recirculation on a 100 kW SOEC Reactor

The efficient use of CO2 and its conversion into CO via high temperature electrolysis is considered as a suitable route for the de-fossilization of anthropogenic activities. Within electrochemical solid oxide cells reactors (SOCs), the co-electrolysis of H2O and CO2 at high temperature (above 800 °C) yields syngas (H2 + CO), one of the most important feedstocks for the production of synthetic fuels and chemicals, e. g. via the Fischer-Tropsch process.At the Institute of Engineering Thermodynamics of the German Aerospace Center (DLR) in Stuttgart Germany, a unique test environment (Galactica) for investigating SOC reactors has been designed and constructed with the aim to deepen the research at the 100 kW-scale reactor in commercial systems. State-of-the-Art SOCs have shown promising results that allow the scale-up to larger, industrially relevant systems sizes. With Galactica, DLR was able to demonstrate successfully how 100 kW-scale reactors are a suitable stepping stone towards multi MW-scale process systems. However, operating strategies are required in order to understand how these systems would perform when steady-state operation cannot be guaranteed and unexpected shortages on the feedstocks and electricity could arise.In this work, the coupling of a 100 kW SOC reactor (consisting of 24 single SOC stacks) with the tail gas recirculation of a Fischer-Tropsch reactor will be presented. The main motivation is to improve the overall process efficiency, increase the conversion (since tail gas hydrocarbons will be reformed) and reduce the net material consumption [1] [2]. By emulating the tail gas composition from the Fischer-Tropsch reactor, different syngas ratios (H2/CO) were investigated in Galactica, not only experimentally but also with the support of the in-house simulation framework TEMPEST [3], specialized for transient simulations on electrochemical reactors. Different operating conditions were evaluated by: (i) adding CH4 at open circuit voltage (OCV) and under current, (ii) performing an experimental ramp of the emulated Fischer-Tropsch gas composition into the inlet gases of the SOC reactor and (iii) by simulating a Feed-Forward controller in the case of H2O and CO2 shortages by varying the current and the air flow values in order to keep the operating temperature and reactor conversion in stable conditions. These results will allow to evaluate the performance of the SOC reactor with the temperature profile along the cells and the stacks, as well as with the syngas ratio behaviour.In this regard, operation strategies will be analyzed and discussed with the aim to mitigate failure conditions on SOC reactors, while operating in transient conditions in the frame of syngas production via high temperature co-electrolysis.[1] Herz, G.; Reichelt, E.; Jahn, M., Techno-economic analysis of a co-electrolysis-based synthesis process for the production of hydrocarbons. Applied Energy 2018, 215, 309-320 [2] Cinti, G.; Baldinelli, A.; Di Michele, A.; Desideri, U., Integration of Solid Oxide Electrolyzer and Fischer-Tropsch: A sustainable pathway for synthetic fuel. Applied Energy 2016, 162, 308-320. [3] Tomberg, M.; Heddrich, M. P.; Sedeqi, F.; Ullmer, D.; Ansar, S. A.; Friedrich, K. A., A New Approach to Modeling Solid Oxide Cell Reactors with Multiple Stacks for Process System Simulation. J. Electrochem. Soc 2022, 169, 054530.

An Analysis of Capacitive Contributions of the Cell Impedance during High-Temperature CO2 and Co-Electrolysis

High-temperature electrolysis is often studied by electrochemical impedance spectroscopy (EIS). The EIS spectra are analyzed using equivalent circuit models (ECM). In many cases, these models use basic elements such as resistor, capacitances or inductances. The discussion of the modelling results is often limited to resistances [e.g. 1-4]. Only some rare papers discuss capacitances as well [e.g. 5-7]. A detailed analysis of all capacitive contributions of the overall cell impedance in electrolysis mode in dependence on temperature, current and composition is missing in literature.The present study provides this detailed analysis of the capacitive contributions, which in the models are usually represented by constant phase elements (CPEs). Impedance spectroscopy measurements of high-temperature CO2 and co-electrolysis on conventional fuel electrode supported cells were performed. The cells consisted of a Ni/YSZ fuel electrode, a YSZ electrolyte, a GDC barrier layer and a (La,Sr)CoO3 air electrode. The spectra were analyzed by an ECM consisting of an inductance, a serial resistance and four RQ elements (resistance and constant phase element in parallel). The dependences of the resistances on operating conditions were already reported elsewhere [1,8].The variation of the parameters temperature, current and composition showed different trends for the single elements. For example, the capacitive contribution of processes 1 – 3 (high to middle frequency range) increases with temperature while it decreases for process 4 (low frequency range). The low frequency process had been assigned to a gas phase diffusion process according to the dependence of the corresponding resistance [1]. The CPE element of process 4 also shows a prominent dependence on the current density. This might correlate with the changing ratio of CO2 to CO in the gas phase and thus support the assignment. Further results of the dependencies of the elements will be presented and discussed.[1] S. Foit, L. Dittrich, T. Duyster, I. Vinke, R.-A. Eichel, L.G.J. de Haart, Processes 2020, 8, 1390.[2] S.E. Wolf, L. Dittrich, M. Nohl, T. Duyster, I.C. Vinke, R.-A. Eichel, L.G.J. de Haart, J. Electrochem. Soc. 2022, 169, 034531.[3] S.D. Ebbesen, X. Sun, M. B. Mogensen, Faraday Discuss. 2015, 182, 393–422.[4] A. Leonide, V. Sonn, A. Weber, E. Ivers-Tiffée, J. Electrochem. Soc. 2008, 155, B36.[5] S. Primdahl, M. Mogensen, J. Electrochem. Soc. 1997, 144, 3409.[6] E.-C. Shin, J. Ma, P.-A. Ahn, H.-H. Seo, D. T. Nguyen, J. S. Lee, Electrochim. Acta 2016, 188, 240-253.[7] X. Ge, C. Fu, S. H. Chan, Phys. Chem. Chem. Phys. 2011, 13, 15134-15142.[8] L. Dittrich, Tailoring of the Synthesis Gas Composition during High-Temperature Co-Electrolysis (Aachen, Germany) (2021).

Validation of and Simulations using a High-Temperature Electrolysis Model

Efforts for increasing the share of renewable power generation are dramatically boosted since electrical power from fossil fuels has been used as weapon [1]. Therefore, efficient conversion systems are of great importance to conserve most of the delivered power by nature. High-temperature electrolysis is such an efficient type of energy conversion system compared to similar technologies [2-4]. To bridge the gap to an industrial scale application, the electrochemical and chemical processes have to be understood in detail which is enlightened by some publications from our group [5-8]. In this contribution, a multiphysics model for three types of electrolysis processes (steam, CO2 and co-electrolysis) is presented and evaluated. Relevant parameters like the exchange current density or the anodic / cathodic transfer coefficient including parameter variations for the inlet gas stream, flow rate of the gas, temperature and applied current are investigated. For the microscale properties in the functional layer, Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) is used with the “Slice and View” technique [9,10]. Extracting information like the Triple Phase Boundary Length (TPBL), the fraction of electronic and ionic phase in mixed ionic-electronic conducting (MIEC) materials or the porosity and tortuosity of the (La,Sr)CoO3-δ (LSC) air electrode and the Ni / 8 mol% yttria stabilized zirconia (Ni-8YSZ) fuel electrode help in enhancing the modeling quality of high-temperature SOC model. A conversion from the TPBL (s. Figure 1) to an active area to volume ratio for the computation is needed.Calculated iV-characteristics and experimental curves are compared and serve as a validation for (I,V) data points at higher currents. Depending on the degree of agreement, the (missing) main contributions to loss mechanisms can be identified and initiate a feedback loop for material optimization. Sensitivity analyses are carried out for parameters that are difficult to vary in experiments (e.g. porosity). Porosity and tortuosity parametrize the porous structure for a homogeneously distributed property assumption across the material layers. The ratio between electronic and ionic phase fine-tunes the positioning of iV-characteristics within the potential regime of Ohmic and polarization losses. Calculated electrochemical impedance spectra (EIS) enhance the insight into the theoretical system and may be compared to experimental spectroscopy data via Equivalent Circuit Models (ECM). Calculated data from the model are acquired as in the used experimental setup. As an example, calculated EIS data from steam electrolysis show good agreement with a R-(RQ)4 ECM which as well describes most of the corresponding experimental data in our group. Effects of degradation are envisioned to be incorporated into the model as well.[1] Hosp, G.; Höltschi, R.; Keusch, N.; Schürpf, T.: https://www.nzz.ch/wirtschaft/rohstoffe-als-waffen-die-neusten-entwicklungen-ld.1681268 (28th October 2022, 2.13 pm)[2] Foit, S.R.; Dittrich, L.; Duyster, T.; Vinke, I.; Eichel, R.-A.; de Haart, L.G.J. (2020): Direct Solid Oxide Electrolysis of Carbon Dioxide: Analysis of Performance and Processes. In: Processes 8 (11), 1390[3] Foit, S.R.; Vinke, I.C.; de Haart, L.G.J.; Eichel, R.-A. (2017): Power-to-Syngas: An Enabling Technology for the Transition of the Energy System? In: Angew. Chem. Int. Edit. 56 (20), 5402–5411[4] Tanaka, Y.; Hoerlein, M.P.; Schiller, G. (2016): Numerical simulation of steam electrolysis with a solid oxide cell for proper evaluation of cell performances. In: Int. J. Hydrogen Energ. 41, 752–763[5] Wolf, S.E.; Dittrich, L.; Nohl, M.; Duyster, T.; Vinke, I.C.; Eichel, R.-A.; de Haart, L.G.J. (2022): Boundary Investigation of High-Temperature Co-Electrolysis Towards Direct CO2 Electrolysis. In: J. Electrochem. Soc. 169, 034531[6] Unachukwu, I.D.; Vibhu, V.; Vinke, I.C.; Eichel, R.-A.; de Haart, L.G.J. (2022): Sr Substituted La2-xSrxNi0.8Co0.2O4+ d (0 ≤ x ≤ 0.8): Impact on Oxygen Stoichiometry and Electrochemical Properties.In: Energies 15, 2136[7] Mebrahtu, C.+; Nohl, M.+; Dittrich, L.; Foit, S.R.; de Haart, L.G.J.; Eichel, R.-A.; Palkovits, R. (2021): Integrated Co-Electrolysis and Syngas Methanation for the Direct Production of Synthetic Natural Gas from CO2 and H2O. In: ChemSusChem 14, 2295 – 2302[8] Dittrich, L.; Nohl, M.; Jaekel, E.E.; Foit, S.R.; de Haart, L.G.J. (Bert), Eichel, R.-A. (2019): High-Temperature Co-Electrolysis: A Versatile Method to Sustainably Produce Tailored Syngas Compositions. In: J. Electrochem. Soc. 166 (13), F971 – F975[9] Wilson, J.R.; Kobsiriphat, W.; Mendoza, R.; Chen, H.; Hiller, J.M.; Miller, D.J.; Thornton, K.; Voorhees, P.W.; Adler, S.B.; Barnett, S.A. (2006): Three-dimensional reconstruction of a solid-oxide fuel-cell anode. In: Nat. Mater. 5, 541–544[10] Gostovic, D.; Smith, J.R.; Kundinger, D.P.; Jones, K.S.; Wachsman, E.D. (2007): Three-Dimensional Reconstruction of Porous LSCF Cathodes. In: Electrochem. Solid St. 10 (12), B214–B217Figure 1: Histogram of the TPBL from a 400 image sample set of a FIB-SEM cut of a Ni-8YSZ electrode. Figure 1

Sulfur Impurities in COx Feed Gases of Solid Oxide Cells: Effect on Degradation and Removal Strategies

High-temperature solid oxide cells (SOC) offer unrivaled Faradaic and energetic efficiencies for carbon dioxide electrolysis. However, it is yet unclear which contaminant level in carbon monoxide and carbon dioxide feed gases are tolerable to achieve low degradation rates in commercial applications. In the present work, electrolyte-supported cells (ESC) with Ni/Gadolinia-doped ceria (Ni/CGO) fuel electrodes were exposed to CO/CO2 gas mixtures at open circuit voltage and in electrolysis operation. The resistance evolution over time was monitored by means of electrochemical impedance spectroscopy. With this approach it is shown that a severe reversible degradation over time periods of up to 72 h occurs. X-ray photoemission spectroscopy (XPS) and high-resolution secondary ion mass spectrometry (SIMS) imaging techniques were used to show that this degradation effect is related to sulfur impurities on the Ni surface. Inlet gas composition variations were carried out to show that sulfur impurities most likely in the low ppb level are present in both industrial CO and CO2 feed gases. The efficacy of different gas cleaning units at room temperature was investigated to identify a suitable desulfurization solution. The results demonstrate the importance of feed gas purity for the commercialization of high-temperature CO2 electrolysis.

Enhancing the Catalytic Activity of Ce(Mn, Fe)O2 on a Perovskite-Based Electrode via Spraying for High-Temperature CO2 Electrolysis

Solid oxide electrolysis cells (SOECs) have tremendous potential to be highly efficient devices for high-temperature CO2 electrolysis. In general, the commonly used Ni-based cathodes suffer from inherent redox instability during direct CO/CO2 conditions. Therefore, the robust perovskite oxide La(Sr)Cr(Mn)O3 (LSCM) is suggested as an alternative due to its excellent coking resistance. However, the LSCM cathodes are limited due to their poor catalytic activity on CO2 reduction. [1] To alleviate this problem, the Ce(Mn, Fe)O2 (CMF) was added as a catalyst owing to the surface oxygen vacancy of CMF, which accelerates the chemical adsorption of CO2. [2] In this study, the solid direct injection process by ultrasonic spraying (SDI-USSP) is employed to fabricate the nano-decorated embedded CMF on LSCM surface as enhancing catalytic activity. The highly decorated electrode surface with CMF nano catalyst was embodied by an advanced one-step SDI-USSP without any complicated pretreatment and infiltration. We also analyzed the uptake of the catalytic activity and the ratio of CMF nanocatalysts through comparison with conventional composite electrodes. Consequently, when applied as a cathode for electrolyte-supported cells, the performances are measured at 1123 K, exhibiting a maximum power density of 1.349 W cm-2 in fuel cell mode (H2) and electrolysis performance of 2.756 A cm-2 at 1.5 V in CO2 reduction with high faradaic efficiency of nearly 94 %.

Characterization of a Nickel / Gadolinia Doped Ceria Fuel Electrode for Co-Electrolysis

Modelling of the high temperature co-electrolysis process requires understanding of the underlying reaction pathways. These include the electrochemical steam reduction, the electrochemical CO2 reduction and their coupling via the catalytic (reverse-)water-gas-shift reaction (RWGS). The assumption of a very fast water-gas-shift reaction and therefore neglectable CO electro-oxidation is widely used. The validity of this assumption has been previously investigated and proven to be sufficient by Kromp et al. [1] for an anode supported cell (ASC) with a nickel / yttria-stabilized zirconia (Ni/YSZ) fuel electrode.Within this contribution we aim to extend the approach by Kromp et al. [1] towards an electrolyte-supported cell (ESC) with a nickel / gadolinium-doped ceria (Ni/CGO) fuel electrode. Previously, it has been proposed, that the electro-oxidation /-reduction of CO / CO2 can be present under reformate gas mixtures [2,3] on Ni/CGO fuel electrodes. However, a detailed comparison of pure activation resistance contributions during different operating modes has not been performed yet.We present results from a complex variation of operating parameters (such as various gas compositions, exemplarily shown in fig. 1) for process identification by the use of electrochemical impedance spectroscopy and the subsequent impedance analysis by the distribution of relaxation times (DRT). Visualization and deconvolution of gas diffusion impedance contributions is done via the evaluation of the polarization resistance differences with different inert gases. This has been successfully applied for binary gas mixtures of hydrogen and steam [4] and also CO2 and CO. [5] The applicability of this method towards more complex multicomponent gas mixtures incorporating hydrogen, steam, CO2 and CO will be shown. By subtracting the gas diffusion impedance contributions, it is possible to evaluate reaction kinetics of the underlying activation polarization and a meaningful comparison of operation modes is facilitated. The ratio of direct CO2electro-reduction and CO2-conversion by RWGS will be discussed for different compositions and temperatures to show a non-neglectable influence of the CO2 electroreduction at even lower pCO2 / pH2 ratios than previously reported. Furthermore, we compare our results with ones obtained from Ni/YSZ fuel electrodes, to investigate the influence of fuel electrode structure and material.

Characterization and Testing of Exsolution-Based Solid Oxide Cells for Reversible Operations in CO2 Electrolysis

Reversible solid oxide cells (rSOCs) are a promising electrochemical technology able to work both as energy storage devices in solid oxide electrolysis (SOEC) mode, and as power generators in solid oxide fuel cell (SOFC) mode. rSOCs implemented for CO2 electroreduction into valuable CO-rich steams through electrolysis would provide opportunities for CO2 utilization, and hence become useful tools for the reduction of greenhouse gases emissions. High-temperature co-electrolysis of CO2/H2O mixtures has recently become of interest since high conversion and energy efficiency are thermodynamically favored in addition to the reduction in cell area-specific resistance when compared to pure CO2 electrolysis [1]. However, reversible operation with CO2-containing feeds requires developing flexible, high-performing, and long-lasting materials for the rSOCsManaging issues such as coking and low redox tolerance in the state-of-the-art (SoA) Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in the rSOCs is crucial. Ni-YSZ cermet exhibits high electronic conductivity and electrochemical activity in hydrogen. Nonetheless, Ni is prone to oxidation due to operation under high steam concentrations, and rapid variations in the fuel supply. Additionally, Ni is well-known for its coke-formation tendency which becomes detrimental in CO2-rich mixtures for electrolysis [2]. Thus, ceramics with mixed ionic and electronic conductivity (MIEC) have emerged as alternative fuel electrodes thanks to the larger electrochemically active area compared to standard cermets. Among the MIEC electrodes, the SrTi0.3Fe0.7O3 (STF) perovskite has proven to give low polarization resistances especially when modification techniques such as exsolution of metal nanoparticles are employed [3]. This work focuses on the investigation of the electrocatalytic properties of exsolution-based STF electrodes operating with CO/CO2 and H2O/CO2 mixtures. The mixtures used were 50% CO2/50% CO, 25% H2O/25% CO2/25%H2/25%CO, and 45% H2O/45% CO2/10% H2. A comparison between the performance and microstructure of the studied electrode formulation is provided.Electrolyte-supported cells were manufactured via screen-printing of the electrode inks on scandia-stabilized zirconia (ScSZ) electrolytes. STF perovskite-based electrodes were used, namely: SrTi0.3Fe0.7O3 (STF), Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni), and Sr0.95(Ti0.3Fe0.63Ru0.07)O3 (STF-Ru). The latter two formulations allowed the enhancement of the perovskite structure via exsolution of catalytic nanoparticles (Ni, Co alloyed with Fe) on the oxide surface during cell operation. Each cell configuration included a gadolinium-doped ceria (GDC) buffer layer coupled with an LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O1.95) oxygen electrode. Baseline performances of cell mounting the Ni-YSZ cermet electrode were also evaluated for comparison with the perovskite alternatives, both in the short and long term. Electrochemical characterization was done via impedance spectroscopy, polarization experiments, and durability tests in potentiostatic mode. Chemical characterization was performed via X-ray diffraction, scanning electron microscopy, and temperature-programmed reduction analyses.Compared to Ni-YSZ, significant improvement in terms of reversibility and maximum current densities of the current-voltage (I/V) curves in both SOFC and SOEC modes is found on the STF-based cells at 750°C under CO/CO2 mixtures (Figure 1B). The initial difference in the performance of the STF cells with respect to the Ni-YSZ cermet in humidified H2 (SOFC mode, Figure 1A) was overcome by using the exsolved electrode formulations. Aging experiments up to 100 h performed in alternating operation modes from SOFC (6 h) to SOEC (6 h) indicated an initial performance degradation within the first 24 h for both the cermet and perovskite-based fuel electrodes which later stabilized in time. Compared to the undoped STF formulation, improvement in performance stability was observed for the exsolved STF-Ni and STF-Ru fuel electrodes while working under the target CO/CO2 mixture. This was attributed to the combination of the STF structure benefits (low polarization resistance towards CO/CO2 and H2O/CO2 mixtures) with the boost that metallic nanoparticles provide to the electrode’s heterogeneous catalytic reactivity and overall stability over time.

Optimal integration modeling of Co – Electrolysis in a power-to-liquid industrial process

High temperature co-electrolysis using solid-oxide electrolysis cells is a highly efficient pathway for green syngas production owing to the possibility of heat integration with other processes. Therefore, this study described and evaluates a flexible and efficient configuration for producing sustainable synthetic fuels using electricity from renewables and captured CO2 by integrating co-electrolysis in a power-to-liquid industrial plant. Thereafter, novel and efficient technologies were implemented for green syngas production and its subsequent purification, increasing the overall process efficiency and achieving a significant reduction in the carbon footprint compared to mature synthetic crude production processes. Catalytic partial oxidation and dual pressure swing adsorption were integrated with co-electrolysis and Fischer–Tropsch synthesis in a scaled industrial plant, using residual streams from the complex or those of renewable origin as feed, which allowed the continuous operation of the process independent of renewable power generation. The mass and energy balance, performance, and efficiency estimations were also included in this study. A solid-oxide electrolytic cell (SOEC) plant using renewable electricity and heat input from thermal integration with the outlet syngas stream of the catalytic partial oxidation reactor was selected as a case study. Both the performance and efficiency analyses of the co-electrolysis unit demonstrated the benefits of such thermal integration in comparison with current solutions. In this study, both the thermal integration of the process streams, as well as the energy and heat consumed by the syngas purification process were considered.