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, CO 2 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)CoO 3-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 CO 2 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 La 2-x Sr x Ni 0.8 Co 0.2 O 4+ 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 CO 2 and H 2 O. 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–B217 Figure 1: Histogram of the TPBL from a 400 image sample set of a FIB-SEM cut of a Ni-8YSZ electrode. Figure 1

A continuum mechanistic, real-scale model with the most important reactions of high-temperature electrolysis is proposed. IV-characteristics and impedance spectra serve as comparison to experimental data for steam, CO2 and co-electrolysis. The exchange current-density i0 can be determined to the same order of magnitude as observed in experiments for steam electrolysis. The IV-characteristics for CO2 electrolysis are well described by an i0 one order of magnitude lower as compared to steam electrolysis. Parameter variations identify contributing variables determining the shapes of characteristics. Limitations in prediction are revealed for low current densities up to 0.5 A∙cm-2. EIS calculations demonstrate the dominant behavior of charge transfer or oxygen ion incorporation into the lattice as function of i0 or temperature. The model forms a solid basis for various future investigations like a more detailed reaction system with possible intermediates or degradation and poisoning effects.

In the temperature range of high temperature co-electrolysis of both steam and carbon dioxide, the reverse water-gas shift reaction (RWGS) takes place. Prior studies were conducted with a narrow gas composition range to investigate the role of RWGS during co-electrolysis. The results for steam electrolysis, CO 2 electrolysis, and co-electrolysis caused different conclusions regarding the role of electrochemical CO 2 and H 2 O conversion compared to RWGS during co-electrolysis. This work aims to resolve the role of CO 2 conversion as part of RWGS in co-electrolysis. The boundary is characterized by AC and DC measurements over a broad gas composition range from CO 2 electrolysis towards co-electrolysis with nearly 50% eq H 2 O. Especially, the electrochemical CO 2 reduction and CO 2 conversion in the RWGS are compared to clarify their role during co-electrolysis. The results revealed that gas composition determined the predominant reaction (H 2 O or CO 2 reduction). The cell performance of co-electrolysis in the boundary region up to 5% eq H 2 O was similar to the performance of CO 2 electrolysis. Up to 30% eq H 2 O, the performance increases with H 2 O concentration. Here, both CO 2 and H 2 O electrolysis occur. Above 30% eq H 2 O, steam electrolysis and the RWGS reaction both dominate the co-electrolysis process.

Solid Oxide Electrolysis Cells (SOEC) are on the rise and can usher in the energy transition. Storage of renewably supplied energy using P2X technologies enables on-demand retrieval of energy but also supply to various sectors. High temperature co-electrolysis of H2O and CO2 is a process capable of de-fossilizing the production of syngas, which can be integrated in future renewable P2X-scenarios. To describe the fundamentals of the co-electrolysis process, a theoretical model considering thermodynamics, kinetics, heat transfer, chemistry, and diffusion processes occurring together at the same time in the cell as well as the material properties of the respective layers, was developed. The boundaries are set with only steam and only CO2 electrolysis where the model is verified by matching calculated IV-characteristics with experimental ones. Comparing with experimental data, the activation loss is not dominating at low current densities. The reaction system is kept simple in order to account for the most probable rate-limiting step.

The concept of an integrated power-to-gas (P2G) process was demonstrated for renewable energy storage by converting renewable electrical energy to synthetic fuels. Such a dynamically integrated process enables direct production of synthetic natural gas (SNG) from CO2 and H2O. The produced SNG can be stored or directly injected into the existing natural gas network. To study process integration, operating parameters of the high-temperature solid oxide electrolysis cell (SOEC) producing syngas (H-2+CO) mixtures through co-electrolysis and a fixed bed reactor for syngas methanation of such gas mixtures were first optimized individually. Reactor design, operating conditions, and enhanced SNG selectivity were the main targets of the study. SOEC experiments were performed on state-of-the-art button cells. Varying operating conditions (temperature, flow rate, gas mixture and current density) emphasized the capability of the system to produce tailor-made syngas mixtures for downstream methanation. Catalytic syngas methanation was performed using hydrotalcite-derived 20 %Ni-2 %Fe/(Mg,Al)O-x catalyst and commercial methanation catalyst (Ni/Al2O3) as reference. Despite water in the feed mixture, SNG with high selectivity (>= 90 %) was produced at 300 degrees C and atmospheric pressure. An adequate rate of syngas conversion was obtained with H2O contents up to 30 %, decreasing significantly for 50 % H2O in the feed. Compared to the commercial catalyst, 20 %Ni-2 %Fe/(Mg,Al)O-x enabled a higher rate of COx conversion.

Electrolysis is the technology, which provides the key for sector coupling. It enables the conversion of renewable energy to material value generation. Obviously, it is of utmost importance, that the energy is used in the most efficient way. High-temperature electrolysis on solid oxide cells provides high efficiency combined with the possibility to convert both water (H2O) and carbon dioxide (CO2) to hydrogen (H2) and carbon monoxide (CO) at the same time. In this contribution, we show the summary and conclusions of multiple investigations of the high-temperature electrolysis. A series of detailed analysis has been performed using current-voltage characteristics (IV curves), electrochemical impedance spectroscopy (EIS), and theoretical calculations.

This contribution highlights selected current activities of the SOC development at Forschungszentrum Jülich. Continued efforts are being made to gain a better understanding of degradation process in our cells and stacks. New materials are being developed to mitigate known degradation phenomena. Systems development was directed at the improvement of reversible operation, especially in electrolysis mode. On cell and stack level investigation of electrolysis operation was also intensified, focusing on CO 2 -valorization.

High-temperature co-electrolysis of CO2 and H2O at elevated temperatures between 700 °C and 900 °C valorises CO2 to produce a mixture of carbon monoxide (CO) and hydrogen (H2), called syngas. Co-electrolysis has the great advantage over conventional processes, that the desired syngas ratios of downstream processes can be realized by varying process parameters such as temperature and feed gas composition accordingly in a one step process. Co-electrolysis can also play a vital role in counteracting power fluctuations of renewable energy sources by storing temporarily unused electricity through conversion to other energy resources like chemicals or heat for later use. The underlying processes in co-electrolysis for CO production are direct electrochemical CO2 reduction and reverse water gas shift equilibrium (RWGS). Their specific significance has not been clarified in detail yet and was controversially discussed in literature up to this day [1,2]. The impact of the equilibrium partial pressure of H2O on the physical processes in the transition boundary of co-electrolysis towards direct CO2-electrolysis was investigated by AC and DC measurements for various gas compositions. The analysis led to identifying the role of the underlying electrochemical processes during co-electrolysis, in particular the electrochemical CO2 reduction compared to the conversion of CO2 in the reverse water-gas shift reaction and the electrochemical H2O reduction. The area specific resistance (ASR) was, amongst others, taken as an indicator to determine, which of the reduction reactions (H2O or CO2 reduction) is dominant depending on the gas composition. The experiments were conducted using commercially available cathode-supported full cells (Elcogen) made of Ni-8YSZ/8YSZ/CGO/LSC. Results as seen in Figure 1 show that the ASR for an equilibrium concentration of 5 % H2O is considerably larger than for higher H2O contents. Above 15 % H2O, the ASR shows no dependency on the gas composition and is comparable to pure H2O-electrolysis [3]. These observations underline the hypothesis that CO2-electrolysis becomes pre-dominant compared to H2O-electrolysis for low H2O content during co-electrolysis. With increasing H2O content, CO2-electrolysis becomes less significant and carbon dioxide is converted in the reverse water gas shift equilibrium. The origin of the discrepancy in literature was found to be the different operating H2O concentrations. A threshold has been established for the perception of CO2-electrolysis during co-electrolysis experiments. figure caption: Arrhenius plot of ASROCV for different steam concentrations at 6 l·h-1. [1] C. Stoots, J. O'Brien, J. Hartvigsen, Int. J. Hydrogen Energy 2009, 34, 4208. [2] S. D. Ebbesen, R. Knibbe, M. Mogensen, J. Electrochem. Soc. 2012, 159, F482-F489. [3] L. Dittrich, M. Nohl, E. E. Jaekel, S. Foit, L.G.J. (Bert) de Haart, R.-A. Eichel J. Electrochem. Soc. 2019, 166, F971-F975. Figure 1

The Distribution of Relaxation Times (DRT) is an important analytical tool that is capable of giving initial information from Electrochemical Impedance Spectra (EIS) with respect to the number of relaxation processes occurring in the system and their corresponding relaxation frequencies. The DRT transformation with the Tikhonov regularization is used for analysis of EIS data obtained from the characterization Solid Oxide Fuel and Electrolysis Cells (SOFC/SOEC) operating at high temperatures. The effects of this transformation together with occurring pitfalls on the most commonly implemented circuit elements used to describe EIS data was investigated to gain a better understanding. The behavior of the DRT transformation as a function of the individual circuit elements is reported, the regularization parameter λ is taken as a sweep parameter to investigate its influence and optimal ranges for the selection of λ are presented.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. High-temperature co-electrolysis shows comparable performance to steam electrolysis. Current densities above 1 A cm –2 can be reached between 700 (cid:2) C and 800 (cid:2) C. Tailor-made syngas is produced, mainly determined by the reactant ratio. The experimental results are supported by modeling. Durability tests with cathode-supported cells show increased voltage degradation rates during electrolysis compared to fuel cell operation. Nickel depletion is found to be the main cause.