Tailoring electrocatalytic performance for ORR of medium-entropy Ruddlesden-Popper type electrode through rational ionic potential design.

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

Incorporating more renewable energy into our economy will require us to address the timing imbalance between renewable energy generation and its use. Reversible solid oxide cells which have the ability to be dynamically load cycled between electrolysis mode when renewable energy availability is plentiful, and fuel cell mode when stored energy needs to be used, offer a potential solution. In this work, we have fabricated solid oxide cells featuring a neodymium nickelate based oxygen electrode, nickel – yttria stabilized zirconia (YSZ) fuel electrodes, and YSZ solid electrolyte. The cells have been electrochemically characterized in electrolysis and fuel cells mode, as well as in dynamic load cycling mode for a period of five hundred hours. The cells were characterized by both DC polarization and AC impedance spectroscopy methods. Distribution of relaxation times (DRT) analysis of the impedance data reveal significant details about the mechanisms underlying degradation in electrolysis and dynamic cycling modes, and are strongly linked to microstructural changes in the fuel electrode. Further, the dynamic cycling experiments clearly reveal that operating the cells in the dynamic cycling mode mitigates against degradation that occurs when the cells are operated in electrolysis mode. These results strongly suggest that storage systems featuring stacks of such reversible solid oxide cells may offer a path forward to solving the timing imbalance of renewable energy generation and use.

A set of long-term tests have been carried out in fuel cell and electrolysis modes on typical Ni-YSZ//YSZ//LSCF-CGO cells. In similar operating conditions, the degradation rates were found to be higher in electrolysis than in fuel cell operation. To explain such results, both Ni agglomeration and LSCF destabilization have been investigated by coupling advanced post-test characterizations and modelling. Thanks to 3D electrode reconstructions, it has been found that Ni coarsening upon operation is not sensitive to the polarization, even if the agglomeration explains a significant part of the cell degradation. At the contrary, post-test analyses have revealed that Sr diffusion and formation of SrZrO3 at YSZ/CGO interface occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode. Therefore, the higher degradation rates measured in electrolysis mode could be related to the higher rate of LSCF demixing.

A Ni/Yttria‐stabilized zirconia (YSZ) cell with a (La0.60Sr0.40)0.95Co0.20Fe0.80O3–δ cathode is tested both in fuel cell and electrolysis modes. In fuel cell mode under dry air and wet H2, the cell is operated between the open circuit voltage (OCV) and 0.4 V and reaches 330 mW cm−2 at 850 °C for 157 mL min−1 H2 supply. The influence of temperature and air or hydrogen flow rate is studied, and impedance measurements show that below 0.8 V the electrolyte becomes the more resistive part of the cell. Nevertheless, fuel utilization yields are higher under oxygen or hydrogen depletion. If it is possible at 750 °C to work at low voltage during several hours in the entire voltage range, the voltage decrease must be limited at 850 °C. The cell can also be operated under wet air. The same cell can be operated in electrolysis mode, and a power density of 340 mW cm−2 can be obtained at 0.3 V/OCV under 100 mL min−1 wet (3% H2O) 5% H2–95% Ar mixture on the fuel side and dry or wet 100 mL min−1 air flow on the air side. Nevertheless, bubbling air providing the air electrode in saltwater has an irreversible detrimental effect on the cell.

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

The increasing penetration of renewable energy sources in the electricity mix requires efficient storage solutions on the seasonal scale. Reversible Solid Oxide Cell (rSOC) systems are receiving increased attention as viable options to fulfil this requirement. In this work, a MW-scale rSOC system capable of working over a large operating window is studied via modelling on Aspen Plus®. To ease the thermal integration, a molten salt thermal storage is coupled to the system, enabling heat recovery in fuel cell mode, which is then exploited for water evaporation in electrolysis mode. The rSOC stack is designed to operate exothermically in the electrolysis mode at nominal load. In both modalities, the air mass flow rate is regulated to control the stack temperature, while limiting the in-out gradients within 100°C. At nominal load, the system achieves an electrical efficiency of 52% in fuel cell mode and of 87% in electrolysis mode. The operation at low partial loads, due to the decrease of the air flow rate, requires an additional high-temperature heat source to guarantee the heat integration. In this regard, the adoption of an electrical resistance in electrolysis mode and a hydrogen-fed combustor in fuel cell mode are selected as viable solutions to amplify the operating range of the system. As a results, the system can be operated down to the 30% of the stack nominal power in both modalities, where the system achieves an electric efficiency of 44% and 80% in fuel cell and electrolysis mode, respectively.

The defossilization of the energy sector requires the transfer of sustainable, carbon-neutral technologies and processes into application. Along with the development of a global hydrogen economy, technologies that generate, store, distribute and use hydrogen and derivatives are particularly relevant. Considerable potential in this sense is offered by the solid oxide cell (SOC), which can be operated as a fuel cell (SOFC), as an electrolysis cell (SOEC) and reversible (rSOC). Forschungszentrum Jülich has been involved in the research and development of SOCs for more than 30 years. In addition to material and cell development, stack and system development and understanding degradation effects are among the main topics today.Recently, an rSOC system with an output power of 10kW in fuel cell mode and input power of 40kW in electrolysis mode was developed. Four SOC stacks, separated and surrounded by a total of five heating plates plus an air preheater at one end and a fuel preheater at the other end, form the Integrated Module of the system; each stack has 20 layers with an active cell area of 19x19 cm². A compact and optimized design could be realized, which achieves a system efficiency of 63.3 % and 71.1 % in fuel cell mode and electrolysis mode, respectively. The system has already been tested in stationary operation modes. Current developments focus on the operating strategy, in particular on the temperature control of the stack in fuel cell mode and during the transient operation of the system.With a focus on the SOC stack, progress was made both in the area of actual stack development and in the area of clarification and optimization of performance and lifetime relevant processes. The role of contaminants, foremost silicon species and sulfur dioxide in feed gases, was investigated to support technical applications. Headway was also made in applying advanced measuring technology like fibre-optic sensors for temperature measurements in air channels. Degradation processes were investigated both experimentally and simulatively in fuel cells as well as in steam and co-electrolysis operation. On the one hand, machine learning approaches were pursued to analyze degradational patterns in SOC stacks, utilizing a specifically consolidated and curated set of long-term experiments and EIS measurements. On the other hand, a multiphysical stack model was developed that allows the relevant physical processes within the stack to be analyzed individually and coupled and thus to optimize the overall operation of the stack.In the area of the development and investigation of cells and materials, the performance of the SOC in the fuel cell mode as well as in the electrolysis mode was in the focus. In addition to operation in steam and co-electrolysis modes, operation in pure CO2 electrolysis was also researched. On single cell level the degradation behavior in the different modes of electrolysis operation was investigated. Different alternative materials were examined both on the fuel side and on the air side as well. A hierarchical degradation model framework was developed that relates changes at the level of electrode particles to changes in electrode structure, resulting materials properties and overall lifetime-performance. Model-based diagnostic allows the extraction of model parameters from experimental data, model verification as well as identification and quantification of different degradation mechanisms.Overall, therefore, significant progress can be observed in the field of cell as well as in the field of stack and system development of SOCs in fuel cell, electrolysis and reversible operation at Forschungszentrum Jülich.

A numerical and experimental comparison of a single reversible molten carbonate cell operating in fuel cell mode and electrolysis mode

Comparison of gas phase transport effects between fuel cell and electrolysis cell modes of a 100 cm2 class molten carbonate cell

Scientific Approach Within the last years, the development work on reversible solid oxide cell (rSOC) systems has been intensified. This is mainly because this technology can deliver a valuable contribution to carbon-neutral energy supply by storing surplus electrical power into hydrogen and converting it again if necessary. Motivated by this application, in 2018 research by Frank et al. [1] suggests that a round trip efficiency of 50% is possible for a pressurized storage at 70 bars. Based on these results, Forschungszentrum Jülich developed an rSOC demonstration system whose design point is 10 kWAC in fuel cell mode and 40 kWAC in electrolysis mode. The system layout and the evaluation of balance of plant components (BoP) are described by Peters et al. [2].Figure below shows on the left side the core components of the demonstrator system including the Integrated Module (IM). This IM consists of four 20-layer sub-stacks in the mark H20-design by Forschungszentrum Jülich. The fuel and air heaters are located at the top and bottom of the module. The system can be heated up by in total five heating plates which are arranged on top and below each sub-stack. These plates are also used to maintain the temperature management of the stacks during the endothermic electrolysis operation. In order to ensure the most compact system design, the BoP components are suitably arranged in the vicinity of the IM. The right side of Figure shows the core system installed in the laboratory test environment including the full thermal insulation. This environment provides the necessary media supply and disposal as well as the supporting safety equipment and control units.The system was set into operation on the first of June in 2021. The operation started with stationary operating points in fuel cell and electrolysis mode, after passing the commissioning phase. The performance and efficiency data achieved during this operating phase are shown below.An overall power range from 1.7 kWAC to 13 kWAC was achieved in fuel cell mode. At an output power of 10.4 kWAC and a fuel utilization of 98 %, a system efficiency of 63.3 % could be achieved. During the electrolysis mode, an efficiency of 71.1 % could be achieved with an input power of -49.6 kWel and a steam utilization of 80 %. An analysis of the loss mechanisms showed that in the fuel cell mode about 75 % of the losses are caused by the heat production of the stack itself. In the electrolysis mode, the largest share of about 65 % is caused by the power consumption of the steam generator. The system efficiency can be further increased by a skillful heat recovery from the fuel side off-gas into the steam generation process. Outlook In future work new methods of stack temperature control based on artificial neural networks will be investigated on basis of the presented system. Afterwards it is planned to apply realistic load profiles to the system while investigating the performance, temperature and degradation behavior. Furthermore, electricity and gas storage as well as heat decoupling for district heating application will be studied. Acknowledgement The authors would like to thank their colleagues at Forschungszentrum Jülich GmbH for their great support and the Helmholtz Society, the German Federal Ministry of Education and Research as well as the Ministry of Culture and Science of the Federal State of North Rhine-Westphalia for financing these activities as part of the Living Lab Energy Campus.

Bipolar membranes (BPMs) are composed of a cation exchange layer (CEL), an anion exchange layer (AEL), and a nanometer scale bipolar junction (Fig 1). At reverse bias polarization (electrolysis mode) the potential drop within both the AEL and CEL are almost negligible when compared to the potential drop within the junction. This large potential drop within the bipolar junction results in a high electric field, which, in combination with water dissociation catalyst, allow BPMs to facilitate water dissociation (into H+ and OH- ions) at a rate seven orders of magnitude greater than the rate in bulk water [1]. At forward bias polarization (fuel cell mode), H+ and OH- produced on the electrodes transport through the membrane and meet at bipolar junction forming H2O. BPMs have been studied at the laboratory and pilot scale for a wide variety of applications, including: electrodialysis for water treatment [2], CO2 reduction [3], solar water splitting [4], and fuel cells [5]. Despite the commercial availability of BPMs, significant hurdles exist both in the fundamental understanding of limiting processes and their subsequent incorporation into electrochemical devices. This is particularly true for BPM reversible fuel cell applications where the requirements for an optimized bipolar junction are specialized for each case. The main technological challenges to enable BPM reversible fuel cell operation include: i)Optimizing bipolar junction to perform in both fuel cell mode and electrolysis mode; ii)Achieving high bipolar junction stability at high current densities; iii)Achieving high membrane durability when cycling between each mode; IV)Optimizing ion exchange layers to decrease membrane resistance Here we will discuss the optimization of BPM junctions to enable reversible fuel cells. Both 2-D and 3-D approaches with and without water dissociation promoters will be applied to fabricate BPM junctions. An example of the improved interfacial charge transfer for BPMs fabricated using 3-D electrospun junctions is shown in Fig 2. Both membranes show high enough water transport into bipolar junction at current densities up to 500 mA/cm2. BPM#2 with 3-D junction shows lower activation overpotential and 0.8 V less at 500 mA/cm2 than BPM#1 with 2-D junction. Different junction designs are preferred for fuel cell mode and electrolysis mode. Therefore, it is important to find a balance in between that could work in both modes and achieve reversible operation. BPM stability of recycling between fuel cell mode and electrolysis mode will also be shown. Reference [1] Strathmann, H., Krol, J. J., Rapp, H. J., & Eigenberger, G. (1997). Limiting current density and water dissociation in bipolar membranes. Journal of Membrane Science, 125(1), 123-142. [2] Reig, M., Valderrama, C., Gibert, O., & Cortina, J. L. (2016). Selectrodialysis and bipolar membrane electrodialysis combination for industrial process brines treatment: Monovalent-divalent ions separation and acid and base production. Desalination, 399, 88-95. [3] Li, Y. C., Zhou, D., Yan, Z., Gonçalves, R. H., Salvatore, D. A., Berlinguette, C. P., & Mallouk, T. E. (2016). Electrolysis of CO2 to syngas in bipolar membrane-based electrochemical cells. ACS Energy Letters, 1(6), 1149-1153. [4] Luo, J., Vermaas, D. A., Bi, D., Hagfeldt, A., Smith, W. A., & Grätzel, M. (2016). Bipolar Membrane‐Assisted Solar Water Splitting in Optimal pH. Advanced Energy Materials, 6(13), 1600100. [5] Peng, S., Xu, X., Lu, S., Sui, P. C., Djilali, N., & Xiang, Y. (2015). A self-humidifying acidic–alkaline bipolar membrane fuel cell. Journal of Power Sources, 299, 273-279. Figure 1

A dynamic physically based model has been developed to unravel the complex relationships between the overall Solid Oxide Cells (SOCs) response and the reaction mechanisms taking place in the electrodes. This numerical tool combines modules for different length-scales from the electrode microstructure up to the single repeat unit. First, models have been developed to emulate digital twins of electrode microstructures made of the classical SOCs materials [1]. The microstructural properties of the active layers and the current collectors are then computed on the synthetic volumes and used as inputs in the electrochemical models. The microscale models for the active layers take into account the mass and the charge transport phenomena for the solid and gas phases. For each electrode, the proposed reaction mechanism is split in a sequence of elementary steps with different pathways that can be activated depending on the polarization [2, 3]. The electrode models are finally coupled with a macroscopic description including the cell geometry and the design of the gas channels [4]. This model allows computing the cell polarization curves, the overpotentials and the EIS together with the distribution of the local current density and gas composition in the single repeat unit. This multiscale model has been used to study a typical cell made of the classical SOCs materials: the electrolyte was in Yttria Stabilized Zirconia (YSZ) whereas the hydrogen and oxygen electrodes were respectively composed by a cermet of Nickel and YSZ (Ni-YSZ) and a composite of Lanthanum doped Strontium Cobaltite Ferrite and Ceria doped Gadolinium Oxide (LSCF-CGO). For this cell, the representativeness of the electrodes synthetic microstructures have been checked on reconstructions obtained by tomographic techniques. It has been shown that the synthetic electrodes reproduce accurately the real microstructures. Moreover, the models at the electrode level have been carefully validated on polarizations curves and Electrochemical Impedance Spectra (EIS) measured on symmetrical cells using a three-electrode setup. It is worth noting that the electrodes of the tested symmetrical cells were manufactured using the same materials and composition than the complete cell. Finally, a specific attention has been paid in this work to validate the approach with an original experimental setup allowing the measurement of the local current densities on the full cell without electrode segmentation. This setup is composed of nine symmetrical-spaced pins that perform the measurement of local polarization curves from the inlet to the cell outlet. A campaign of tests has been carried out in fuel cell and electrolysis mode with different temperatures and flow rates. A good agreement has been found between the experimental and simulated data. It has been shown that the model is able to reproduce the evolution of the local polarization curves with a higher current density at the cell inlet. Furthermore, the simulated EIS for the cell have been found to be consistent with the experimental ones. The fully validated model has been used to analyze the cell operation in electrolysis and fuel cell mode. The reaction pathways associated to the elementary steps in the active layers have been discussed depending on the position within the electrodes and the polarizations. This model will then be used to implement the degradation laws such as the Ni agglomeration and compute the loss of cell performances and the remaining useful lifetime [5]. Moreover, the model could be applied to interpret the evolution of the EIS upon operation.[1] H. Moussaoui, J. Laurencin, Y. Gavet, G. Delette, M. Hubert, P. Cloetens, T. Le Bihan, J. Debayle, Stochastic geometrical modeling of solid oxide cells electrodes validated on 3D reconstructions, Computational Materials Science. 143 (2018) 262-276.[2] F. Monaco, E. Effori, M. Hubert, E. Siebert, G. Geneste, B. Morel, E. Djurado, D. Montinaro, J. Laurencin, Electrode Kinetics of Porous Ni-3YSZ Cermet Operated in Fuel Cell and Electrolysis Modes for Solid Oxide Cell Application. Under review. [3] E. Effori, J. Laurencin, E. Da Rosa Silva, M. Hubert, T. David, M. Petitjean, L. Dessemond, E. Siebert, An elementary kinetic model for the LSCF and LSCF-CGO electrodes for solid oxide cells: impact of oxygen partial pressure and degradation on the electrode response. Under review. [4] J. Laurencin, D. Kane, G. Delette, J. Deseure, F. Lefebvre-Joud, Modelling of solid oxide steam electrolyser: Impact of the operating conditions on hydrogen production. Journal of Power Sources. 196 (2011) 2080-2093.[5] M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, F. Lefebvre-Joud, Impact of Nickel agglomeration on Solid Oxide Cell operated in fuel cell and electrolysis modes. Journal of Power Sources. 397 (2018) 240-251. Figure 1

Performance evolution analysis of a solid oxide cell operated in fuel-cell, electrolysis and cycle modes

The Forschungszentrum Jülich GmbH has successfully developed a 5/15 kW-class reversible Solid Oxide Cell (rSOC) system in recent years, promising a high system efficiency in fuel cell and electrolysis mode [1]. The core element of the system is the well-known Integrated Module developed, which has been adapted from fuel cell to the rSOC operation. This Module consisted of four 10-layer sub stacks, in which each layer had an active cell area of 320 cm². It also contained fuel and air heat exchangers, arranged on top and underneath the stacks, to bring the incoming gases to the operating temperature. In addition, three electrically operated heating plates were implemented to heat up the system at the beginning of the operation and to adjust the stack temperature during the endothermic electrolysis operation. Other components necessary for the system operation such as the evaporator, blowers, condenser and control system are attached near the Integrated Module in a compact and suitable manner. This system was extensively examined after its completion. A maximum power of 5.3 kWDC at a current density of 0.5 A cm-2 and a fuel utilization of 97.3% could be achieved in fuel cell mode. Under these conditions, an electrical efficiency of 62.7% (LHV, DC) was reached. Furthermore, a maximum electrical input power of - 14.3 kWDC could be achieved in electrolysis mode with a current density of -0.89 A cm-2 at a steam conversion rate of 85%. An electrical efficiency of 70% (LHV, DC) was measured for this operating point. In the further course of the tests, the switching between the operating modes was investigated and optimized. The change from fuel cell to electrolysis mode took approximately 13 minutes limited by steam generation and the opposite direction took less than 3 minutes [2]. In the fuel cell mode, a maximum system fuel utilization of more than 99% was achieved through the use of an off-gas recirculation. Such a high utilization could be realized because the steam content has been removed from the recirculated gas stream by condensation before feeding back to the stack inlet. Furthermore, the system behavior during load variation was examined. With this system design, a minimum part load of 0.1 A cm-2 in the fuel cell and - 0.05 A cm-2 in the electrolysis mode could be demonstrated [3]. Based on these experiences, a new rSOC system with a power of 10 kW in fuel cell and 40 kW electrolysis mode will be constructed. For this purpose, the stacks used and the supporting components have to be adapted. In addition, the efficiency in the electrolysis mode will be increased by using a more effective steam generation process. The new developed system as well as its test results will be presented at the conference for the first time.[1] M. Frank, R. Deja, R. Peters, L. Blum, D. Stolten, Applied Energy, 217 (2018) 101-112.[2] R. Peters, M. Frank, W. Tiedemann, I. Hoven, R. Deja, V.N. Nguyen, L. Blum, D. Stolten, ECS Transactions, 91 (2019) 2495-2506.[3] R. Peters, M. Frank, W. Tiedemann, I. Hoven, R. Deja, N. Kruse, Q. Fang, L. Blum, R. Peters, Journal of the Electrochemical Society, (2021).

Elcogen is a manufacturer of core solid oxide technology components comprising solid oxide cells (SOC), stacks, and stack modules. Elcogen also provides engineering services for its customers. Elcogen fuel electrode supported unit cells are optimized for high performance, while the stacks adopt a unique combination of best-in-class performance and low-cost product structure, and the stack modules are designed around the Elcogen stacks to provide safe operation, enabling larger installations and allowing the highest system efficiency. Elcogen solid oxide products can be used in electrolysis, fuel cell and reversible applications. In electrolysis mode, steam or carbon dioxide or combinations thereof are converted with electricity into hydrogen, carbon monoxide or combinations thereof, and a variety of fuels can be converted into electricity in the fuel cell mode. The typical power output of Elcogen products starts from a few watts for the unit cells and extends to hundreds of multi-kilo watts for stack modules. This contribution summarizes Elcogen’s recent unit cell, stack and stack module development activities, public projects involving Elcogen and field data from selected projects.Elcogen cell technology is optimized for performance around fuel electrode supported structure. The cell material system from the fuel to air side is in its reduced state is: Ni (current collector) | Ni-YSZ (support) | Ni-YSZ (active fuel electrode) | YSZ (electrolyte) | GDC (barrier layer) | LSC (active oxygen electrode) | LSC (current collector). Cells are produced with two support layer thicknesses (300 and 400 mm). Currently the development work around the cells is focused on upscaling the manufacturing processes.The main stack product for Elcogen E3000 stack. During 2025, a new version of the product will be launched. The new version has been developed especially to decrease stack cost, and in addition it has been designed with improved flow distribution characteristics and mechanical stability. E3000 produces nominally 3 kW in fuel cell mode and 3 Nm3/h hydrogen in electrolysis mode. It has been tested with various feedstocks (e.g. ammonia, biogases, natural gas, hydrogen, methanol, steam and carbon dioxide), in different operation modes (e.g. fuel cell, steam electrolysis, co-electrolysis), cycling conditions (e.g. current and thermal cycles in both fuel cell mode and in electrolysis mode), as well as trip scenarios including emergency situations (e.g. redox cycles, purge gas cycles).Elcogen’s stack modules provide system integrators with an industrial standard interface for gases, electricity, and instrumentation. The stack module includes the solid oxide stacks, a mechanical compression system for the stacks, electrical connections inside the module between the stacks and the connection ports for the system integrator, electrical isolation solutions between the stacks and mechanical structures, gas distribution manifolds using standard pipe interfaces for the inlet and outlet fuel and air, and instrumentation connections for number of voltage, temperature, and pressure measurements. Elcogen is developing these solutions from laboratory scale (incl. one E3000), to small system (2 and 4 stack modules), and up to industrial scale solutions (more than 10 stacks in single module). The development work of the industry scale stack modules is conducted with Elcogen partners including HD Hyundai, KSOE, Fraunhofer IKTS and AVL. The laboratory and small system scale modules are delivered to customer projects, and the testing phase of the large modules is expected to start during 2025.