Biomass-derived fuels enable low-carbon power generation through solid oxide fuel cells (SOFCs); however, sulphur species present in biogas can severely degrade Ni-based anodes. Although H2S poisoning has been extensively studied in single cells, the impact of other sulphur compounds at stack level and realistic load conditions remains poorly understood.This study exposes a six-cell Ni-anode SOFC short stack to dimethyl sulphide (DMS) under biogas-relevant conditions, probing degradation through IV curves, electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT) analysis, and equivalent circuit modelling (ECM). Tests were performed at 750°C, 0.4 Acm−2 and 65% fuel utilisation.Under load, DMS poisoning (0.5, 1, 2.5 and 5 ppm) reveals voltage degradation trends consistent with concentration- and time-dependence, though not fully separable from exposure history. Electrochemical analysis shows that the anode charge-transfer process is selectively and substantially impaired, while gas-diffusion impedance evolves more gradually. Recovery upon contaminant removal confirms this degradation remains largely reversible within the investigated time window. Sulphur surface coverage, estimated using a Temkin-type adsorption model, correlates linearly with voltage loss, suggesting a coverage threshold of almost 0.7, below which degradation remains negligible. Together, these operando descriptors provide a preliminary, model-dependent framework for interpreting DMS poisoning trends at stack scale.

As the world shift towards sustainable energy solutions, solid oxide fuel cells (SOFCs) using non-carbon fuels like ammonia and hydrogen emerge as promising pathways to produce clean energy and enhance conversion efficiency. However, current implementations encounter challenges such as nitriding effects from direct ammonia injection to the stack, overestimated benefits of anode off-gas (AOG) recirculation, and a sole focus on electrical efficiency that overlooks the thermal advantages of SOFCs. This study addresses these gaps through a comprehensive multi-objective optimization of SOFC systems fueled by ammonia and hydrogen, assessing their efficiency, fuel utilization, and heat exergy. The research translates material phenomena into mathematical constraints and quantifies the effects of control variables through systematic parameter variation. Results indicate that ammonia-fueled SOFC systems slightly outperform hydrogen, achieving an electrical efficiency of about 65% compared to 62% for hydrogen systems, although hydrogen demonstrates superior fuel utilization and exergy efficiency. Optimal AOG recirculation and NH3 cracking fraction that do not compromise stack lifetime and stay in the safe operating zone of nitriding are identified. It also challenges the assumptions that a higher AOG recirculation can benefit performance, suggesting that more extensive AOG recirculation might not always enhance it. Soft sensors are provided to predict system's performance and enable proactive adjustments to facilitate industrial applications where some parameters, such as high-temperature stack's pressure drop, are costly or difficult to measure. This study significantly advances the practical deployment of SOFC technologies, enhancing their feasibility for sustainable energy development.

Introduction Recent demonstrations of commercial stationary natural gas-fed Solid Oxide Fuel Cell (SOFC) Systems have successfully demonstrated LHV to AC electrical efficiency values well above 60% [1,2]. This represents a highly attractive alternative to conventional Diesel- or LNG-fueled gensets of seagoing vessels, as the utilization of SOFC technology can both significantly reduce CO 2 emissions and almost entirely avoid combustion byproducts such as nitrogen oxides (NO x ), carbon monoxide (CO), particulate matter (PM) as well as methane slip. However, there are presently no such gensets demonstrated in a relevant power class and overall uncertainty regarding applicability, transient capability, genset sizing and emission reduction potential remains. The recently completed project Nautilus set its objective to close this research gap by developing and testing a novel maritime genset, consisting of LNG-fueled SOFC modules coupled with battery sets to enable reliable load-following while operating at overall high conversion efficiencies. Apart from marine component qualifications such as ship motion inclination testing of SOFC modules [3] and laboratory validation of energy management systems (EMS) [4], one of the core objectives of the Nautilus project consisted of a land-based demonstration of the proposed hybrid SOFC-battery genset to emulate ship-like operating conditions. This contribution aims to give an overview on the core demonstrator results, which include steady-state and transient testing of the SOFC module itself as well as battery-coupled operation in load-following mode of scaled-down real cruise ship profiles for both hotel and propulsive load. During all relevant operating conditions, CO 2 and non-CO 2 emissions of the SOFC module exhaust were measured and are compared to conventional genset data. Functional Demonstrator Layout The land-based demonstrator with a total rated power of 80 kW el,AC was commissioned on DLR premises in Stuttgart and is connected both to the local natural gas grid and the electricity grid. The arrangement and overall layout of the system is shown in Figure 1), depicting the (i) SOFC System, (ii) Battery, Controls and EMS and (iii) emission analysis subunits. The SOFC System with a rated electrical power of 60 kW el consists of a set of 40 G8 SolydEra stacks assembled in a system environment including anode off-gas recirculation with pre-reforming, air supply, fuel gas pre-cleaning, internal heat recuperation as well as a heat recovery unit for combined heat and power (CHP) application, the latter one not being used in the testing period shown here. The natural gas is provided by means of a compressor to ensure the required fuel supply pressure of 4 bar, which is meant to emulate the fuel supply on a ship after LNG evaporation. Thus, the compressor power is not accounted for in the efficiency calculation of the genset. In the second subunit, a marine certified 40 kWh Li-Ion battery along with a bidirectional DC-DC converter is installed. Both the battery and the DC output of the SOFC module are connected to a 600 V busbar, which feeds electrical power into the local 3-phase 400 V/50 Hz electricity grid by means of a frequency inverter and a transformer. From an external PC, the instantaneous power demand value, either set manually or coming from a timeseries of the cruise ship profiles, is fed to the EMS. Depending on the employed power distribution algorithm, the EMS sends power demands to the power electronics of SOFC and battery (charge/discharge) subsystems. The performance of EMS strategies was tested in a prior laboratory environment by coupling a single-pass multi-stack arrangement with a Li-Ion battery [4]. Finally, the exhaust emission analysis subunit consists of a Fourier transform infrared spectroscopy (FTIR) gas analyzer (ppm range), ppb range analyzers for NO x and SO 2 , as well as array particle monitors to measure several properties including particle number, mass concentration, and black carbon. Demonstrator Testing and Selected Results At the time of submission, the demonstrator has been operated for a total time of 1600 h and provided a total amount of electrical energy of 62.6 MWh. The testing campaign consisted of several distinct measurement tasks, which are briefly described: SOFC module steady-state electrical efficiencies along the load range , recorded in regular intervals throughout the testing campaign. Each operating point, defined by a constant electrical current, was held for 3 hours, while only the last 30 min were utilized to determine the steady-state performance. The corresponding LHV to AC electrical efficiency measurement points and a respective curve fit are depicted in Figure 2a) in red and are compared to a recently published field demonstration study by Gandiglio et al. [2] (blue). A peak efficiency of 63.4 % LHV to AC could be observed at 73 % of rated electrical power, with the characteristic drop towards lower part load operation. Both qualitatively and quantitatively, the obtained data is in close agreement with Gandiglio et al.. In addition, the potential thermal and combined CHP efficiency achievable with the built-in heat exchanger was estimated assuming exhaust recuperation down to 100°C, reaching values of up to 85 %. SOFC module load ramp tests to identify the transient performance without battery assist, at varying ramp rates (0.5, 2 and 4 % of rated module power per minute). The highest chosen ramp rate was given as a constraint from the SOFC system integrator due to balance of plant performance limitations. All load ramps tested on the demonstrator could be followed precisely (see Figure 2b) and without signs of negative impact of fuel and air supply dynamics on the stack, such as drops in stack voltage due to short-term fuel starvation, thus confirming the internal SOFC system control functionality for the depicted ramp tests. Higher ramp rates were tested in a dedicated SOFC single-pass lab test environment with identical stacks, exemplarily shown by a 10 %/min ramp as green line in Figure 2b). While downward ramps could be performed as intended, the upwards ramping capability was constrained due to minimum voltage constraints for cases where the stacks had remained at low part load for an extended period, resulting in lower stack temperatures. As longer time periods at low part load are to be expected in maritime applications, this characteristic represents a possible system constraint relevant for EMS design. CO 2 and non-CO 2 exhaust emissions along the steady-state load range and during transient operation . The non-CO 2 exhaust emissions, such as NO x and CO gas emissions, were found to vary substantially with load. Part load operation below 50% was particularly unfavorable, while 50% and above had low emission factors, especially for NO x . The emissions were not severely impacted by transient load changes other than that of the described effect from the load. The methane slip was practically negligible, with slightly increased emissions at very high load ramp rates. SOFC module continuous load ramp cycling between minimum and maximum permissible load at 4 % rated power per minute for a total repetition of 240 cycles, to stress the system and investigate if these operating regimes lead to degradation of the stacks. During and after completion of cycling, the steady state performance as described in (i) was repeated regularly, indicating that no relevant degradation could be observed, confirming that load ramps up to 4%/min are not significantly stressing the system. Performance during non-normal operation in a genset context , such as load rejection and black start capability. It could be demonstrated, that after a load rejection from the electrical grid, the SOFC module itself was able to switch into a safe electrically isolated hot standby mode at low electrical current by using the generated stack power to run the BoP consumers and electrically heating the inlet gases to sufficient temperatures, thus not requiring forming gas. It further could be shown that only the battery power was required to return the busbar to nominal voltage enabling resumption of energy flow to the grid, confirming the envisioned black start capability concept. Coupled SOFC-battery operation in load-following mode : Load profiles, deducted from real cruise ship power demands, including both propulsive and hotel load, were scaled to the demonstrator’s rated power to emulate the operation on a seagoing vessel under realistic conditions. Successful co-operation of battery and SOFC could be demonstrated, utilizing two rule-based power distribution strategies, investigated in prior laboratory tests [4]. For a given SOFC System load ramp rate of 2 %/min and cruise ship profiles, the required battery capacities were determined and recently published [5]. Conclusion By presenting these results in detail, the authors aim to provide a profound overview and reliable basis to the scientific community for designing and sizing hybrid SOFC battery gensets for MW-powertrain applications on seagoing vessels. Emission analysis has further confirmed the overall positive environmental impact of employing such systems to reduce the maritime carbon footprint. References Åström, K.; Hakala, T.; and Fontell, E. (2020), Product development and experiences of a new Convion SOFC system , Proceedings of the 14 th European SOFC & SOE Forum, A0602, 124-130. Gandiglio, M.; Marocco, P.; Nieminen, A.; Santarelli, M.; and Kiviaho, J. (2024). Energy and environmental performance from field operation of commercial-scale SOFC systems . International Journal of Hydrogen Energy, 85, 997–1009. https://doi.org/10.1016/j.ijhydene.2024.08.332 van Veldhuizen, B. N., Zera, E., van Biert, L., Modena, S., Visser, K., & Aravind, P. V. (2023). Experimental evaluation of a solid oxide fuel cell system exposed to inclinations and accelerations by ship motions . Journal of Power Sources, 585, 233634. https://doi.org/10.1016/j.jpowsour.2023.233634 Ünlübayir, C., Youssfi, H., Khan, R. A., Ventura, S. S., Fortunati, D., Rinner, J., Börner, M. F., Qaude, K. L., Ringbeck, F., and Sauer, D. U. (2024). Comparative analysis and test bench validation of energy management methods for a hybrid marine propulsion system powered by batteries and solid oxide fuel cells . Applied Energy, 376, 124183. https://doi.org/10.1016/j.apenergy.2024.124183 van Veldhuizen, B. N., van Biert, L., Ünlübayir, C., Visser, K., Hopman, J. J., & Aravind, P. V. (2025). Component sizing and dynamic simulation of a low-emission power plant for cruise ships with solid oxide fuel cells. Energy Conversion and Management, 326, 119477. https://doi.org/10.1016/j.enconman.2024.119477 Figure 1

This study examines a novel Solid Oxide Fuel Cell (SOFC) cogeneration system powered by liquefied natural gas integrating realistic and flexible heat management to support efficient maritime applications. With a nominal power capacity of 125 kW, the SOFC-based system comprises stacks and balance-of-plant components, including pre-reformer, afterburner, heat exchangers, and blowers. In addition to generating electricity for propulsion and onboard services, waste heat is recovered to produce saturated steam and hot water. Detailed process model was developed in Aspen Plus for both beginning (BoL) and end-of-life (EoL) conditions, followed by a multi-objective optimization targeting net electrical efficiency, external water intake, and external air flow. A suitable trade-off solution was selected for heat exchanger network (HEN) synthesis at EoL, and a mixed-integer linear programming model minimized the number of heat exchangers, determining optimal hot/cold stream matches under practical design constraints. These include forbidden matches, pressure drops, avoidance of high-temperature flow splitting/merging, and the creation of a hot box to reduce heat losses, enabling realistic, compact and marine-compatible layout. The resulting HEN was validated under BoL and part-load operation, confirming robustness and flexibility across the SOFC system's lifetime. The optimized system achieves 60.3% (BoL) and 50.2% (EoL) net electrical efficiency, producing 1788.3 kg/day of saturated steam and 1467.1 kg/day of hot water under EoL. A preliminary piping and instrumentation diagram was developed, incorporating safety and control loops, and valves and sensors. Overall, the study demonstrates the flexibility and robustness of SOFC-based cogeneration systems for efficient and practical maritime applications, advancing the integration of lower-emission technologies.

Three short stacks from SOLIDpower/SolydEra have been tested within Task 3.4. The main results are presented and discussed in this section. There are similarities between the different tests but as the third test was the most complete, the explanations given in the analysis of the third test give also the most complete description, including of observations mentioned for the first 2 stacks.

Correlating the microstructure of an energy conversion device to its performance is often a complex exercise, notably in solid oxide fuel cell research. Solid oxide fuel cells combine multiple materials and interfaces that evolve in time due to high operating temperatures and reactive atmospheres. We demonstrate here that operando environmental transmission electron microscopy can identify structure-property links in such devices. By contacting a cathode-electrolyte-anode cell to a heating and biasing microelectromechanical system in a single-chamber configuration, a direct correlation is found between the environmental conditions (oxygen and hydrogen partial pressures, temperature), the cell open circuit voltage, and the microstructural evolution of the fuel cell, down to the atomic scale. The results shed important insights into the impact of the anode oxidation state and its morphology on the cell electrical properties.

Green hydrogen can be generated from water and electricity from renewable energy sources (solar, wind). However, the efficient and continuous dispatch of green hydrogen for industrial processes is challenged by the intermittent supply of renewable electricity. This issue can be mitigated by developing innovative electrolysers characterized by improved efficiency, reliability and flexibility, in combination with optimized schemes for the BoP.In this context, the European project PROMETEO (Hydrogen PROduction by MEans of solar heat and power in high TEmperature Solid Oxide Electrolysers) proposes an innovative system based on Solid Oxide Electrolysis (SOE) making a highly efficient use of heat and power generated from solar energy. Concentrating Solar technologies with Thermal Energy Storage (TES) are used to drive hydrogen production when solar resource is not directly available and/or when power is less expensive. A fully-integrated optimized prototype (15 kg/day hydrogen production, 25 kWe SOE) where the SOE is combined with the TES and ancillary components will be constructed and validated. The design of the prototype considers multiple criteria: end-users’ needs, sustainability, regulatory, safety scale-up and engineering concerns. Particular attention is paid to partial load operation, transients and hot stand-by periods. A suitable TES, consisting of one tank filled with a molten salt, has been considered to generate the feed steam for the SOE at controlled temperature and rate when the power is available.Industrial end-users lead to techo-economic and sustainability studies to apply the technology in on-grid and off-grid scenarios and for different applications: utility for grid balancing, power-to-gas, and hydrogen as feedstock for the fertilizer/chemical industry. The project PROMETEO started in January 2021 and receives funding from the Clean Hydrogen Joint Undertaking (CH JU) under grant agreement No. 101007194. The JU receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research.

A novel setup has been designed and mounted to investigate the local electrochemical and thermal behavior of a cell from a 4-cell short stack under reversible operation solid oxide fuel cell/solid oxide electrolyzer (SOFC/SOE). It consists in the partition of the cell's oxygen electrode into 20 segments, where each segment is controlled by a specific electronic load. This allows to control the segments independently in galvanostatic or potentiostatic mode. The configuration also enables local electrochemical impedance spectroscopy (EIS) measurements of each segment. 20 thermocouples were added to measure the local temperature of the segments in order to gain a deeper insight into the cell thermal distribution. This helps correlating the local thermal and electrochemical responses to the operating parameters. This specific setup allowed mapping the cell's local behavior based on a multidimensional operating conditions matrix, including current density, temperature, feed gas composition, sweep gas flow and polarization. Moreover, a durability test of more than 1,000 h helped investigating the local long-term degradation in a reversible (day SOE, night SOFC) configuration. These results are of significant importance towards optimal operation of a reversible SOC with enhanced durability.

Reversible solid oxide cells (rSOC) can convert excess electricity to valuable fuels in electrolysis cell mode (SOEC) and reverse the reaction in fuel cell mode (SOFC). In this work, a five - cell rSOC short stack, integrating fuel electrode (Ni-YSZ) supported solid oxide cells (Ni-YSZ parallel to YSZ vertical bar CGO parallel to LSC-CGO) with an active area of 100 cm(2), is tested for cyclic durability. The fuel electrode gases of H-2/N-2:50/50 and H-2/H2O:20/80 in SOFC and SOEC mode, respectively, are used during the 35 reversible operations. The voltage degradation of the rSOC is 1.64% kh(-1) and 0.65% kh(-1) in SOFC and SOEC mode, respectively, with fuel and steam utilisation of 52%. The post-cycle steady-state SOEC degradation of 0.74% kh(-1) suggests improved lifetime during rSOC conditions. The distribution of relaxation times (DRT) analysis suggests charge transfer through the fuel electrode is responsible for the observed degradation. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

High-temperature electrolysis for reducing H2O (and CO2) to H2 (and CO) converts concentrated solar energy into fuels and chemical feedstock. We invented an integrated reactor concept comprising a solar cavity receiver for reactant heating, a solid oxide electrolyzer (SOE) stack for water electrolysis, and concentrated photovoltaic (PV) cells for the SOE stack's electricity demand. A numerical model compared thermoneutral and endo/exothermal operation of the SOE stack. Without heat recovery, we predicted a maximum solar -to-hydrogen (STH) efficiency of 19.85% (assuming 20% PV efficiency and 20% heat losses in the solar cavity receiver) and preferentially endothermal operation. Heat recovery further improved the perfor-mance. We demonstrated a 2.5 kW (17% electrical and 83% thermal input) reactor, incorporating a commercial 16-cell Ni/YSZ/LSM SOE stack into a double-helical solar cavity receiver, with 3.33% STH efficiency (assuming 20% PV efficiency). The experimentally supported analysis indicates that endothermal operation increases the performance and predicts STH efficiencies encouraging intensi-fied research and technology development.

Although the effects of impurities on the Ni-YSZ electrode of a solid oxide cell in fuel cell operation have been studied, reports on their effects during electrolysis operation are limited. Here, short-term experiments at various current densities and a durability test (1400 h) are performed to investigate the effects of HCl on the Ni-YSZ electrode of a solid oxide electrolysis cell (SOEC) operated in co-electrolysis mode. Without current bias, exposure to 10 ppmv of HCl was tolerated whereas significant degradation was observed when the SOEC was exposed to 5 ppmv of HCl under polarization, mostly related to an increase of the resistance associated with the charge-transfer processes. Prolonged exposure to HCl revealed an initial steady degradation before an increase by a factor of >10 caused by polarization resistance increases, thereby emphasizing the importance of prolonged poisoning tests. Stopping the HCl supply and operating the SOEC in an HCl-free atmosphere allowed for voltage stabilization but no performance recovery was observed. An increased serial resistance indicates that the desorption of Cl can also be detrimental. A low-frequency pseudo-inductive hook appeared in the electrochemical impedance spectra, possibly related to the increase of the charge-transfer processes' resistance and indicating modification to the electrochemical pathways.

The progress in the diffusion of solid oxide fuel cell (SOFC) as commercial devices is not paired by literature production. Articles describing the behaviour of SOFC stacks are rare because of confidentiality reasons for commercial suppliers while research centres prefer to focus on single components or low technology readiness level research. This article aim to fill this gap presenting the analysis of three short stacks run in operative conditions for 10 000 h each. The stacks are characterized through voltage vs time curves, electron microscopy, and electro-chemical impedance spectroscopy. Focus is given on the interconnect; notably on the different types of coatings, varying for composition (MnCo2O4, MnCo1.8Fe0.2O4) and deposition technique (atmospheric plasma spray-APS, physical vapour deposition-PVD, wet powder spraying-WPS). Nitriding of the steel substrate as a solution to improve the chromium retention properties is tested as well. Results: indicate that coating deposition technique is the most important parameter, with single repeat unit (SRU) containing PVD coating showing the lowest voltage degradation rate. Commercial ferritic stainless steel K41 confirmed to be a reliable choice if coupled with a coating. Moreover, SRU containing WPS coating demonstrated to be more reliable than expected from standard area specific resistance 4-probe test.

In this work, the effect of sulphur poisoning of the Ni-YSZ electrode of an SOEC operated in co-electrolysis mode was investigated. Short-term tests with exposure up to 5 ppmv of SO 2 were performed at OCV and under polarization ( 0.25 A c m − 2 ). The two-stages degradation pattern observed consisted of an initial fast voltage increase followed by a slower voltage increase similar to that of an SOFC exposed to H 2 S. Electrochemical impedance spectroscopy and the analysis of the distribution of relaxation times showed that both the catalytic and electrochemical reactions were affected by SO 2 . After extended periods in SO 2 -free reactant, only a partial recovery of the performances was observed even when exposure amounted to only 0.5 ppmv of SO 2 independently on the current density. A durability test at a constant polarization of 0.5 A c m − 2 showed a voltage ‘runaway’ behavior during successive exposures to 1 ppmv and 2 ppmv of SO 2 . This behavior originated from a drastic increase of the serial resistance, which almost completely recovered when the SO 2 supply was cut. This behavior was not observed during exposure to 0.5 ppmv of SO 2 , suggesting that, in these test conditions, the voltage ‘runaway’ behavior could be avoided at a sub-ppmv level. Successive exposure-recovery cycles were found to weaken the SOEC tolerance to SO 2 and a low frequency pseudo-inductive arc was observed in the impedance response during and after the second exposure to SO 2 .

While the global fuel utilization of solid oxide fuel cells (SOFCs) is limited by the stack aging rate, the fuel excess is typically used in a burner, and thus limiting the system electrical efficiency. Further, natural-gas-fueled SOFCs require treated water for the steam reforming process, which increases operational cost. Here, we introduce a novel micro anode off-gas recirculation fan that is driven by a partial-admission (21%) and low-reaction (15%) steam turbine with a tip diameter of 15 mm. The 30 W turbine is propelled by pressurized steam, which is generated from the excess stack heat. The shaft runs on dynamic steam-lubricated bearings and rotates up to 175 krpm. For a global fuel utilization of 75% and a constant fuel mass flow rate, the electrical gross DC efficiency based on the lower heating value was improved from 52 % to 57 % with the anode off-gas recirculation, while the local fuel utilization decreased from 75% to 61%, which is expected to significantly increase stack lifetime. For a global fuel utilization of 85%, gross efficiencies of 66% in part load (4.5 kWe) and 61% in full load (6.3 kWe) were achieved with the anode off-gas recirculation. The results suggest that the steam-driven anode off-gas recirculation can achieve a neutral water consumption.

While high performance of SOFC systems has already been achieved, there is still a need to increase the stack lifetime, while decreasing the system's total cost of ownership, both being linked. The increase in lifetime includes the minimization of performance degradation, but also avoidance of potentially detrimental events during operation, originating from the stack, outside the stack, or a combination of both. The present paper highlights how the detection of faulty operation as early as possible is important to implement adequate mitigation strategies, wherever applicable, according to the fault and its grade of severity. Three types of faults have been considered, fuel starvation, leakage and carbon deposition. Both hardware components and algorithms to monitor, detect, isolate, and finally correct faulty system operation have been developed and will be embedded in a real SOFC system.

Electrochemical Impedance Spectroscopy (EIS) is extensively used to characterize Solid Oxide Cells (SOCs) to extract information on the elementary loss mechanisms. However, these individual mechanisms usually overlap in the frequency domain, requiring dedicated data processing for unambiguous identification. A powerful method for discriminating process contributions is the analysis by the Distribution of Relaxation Times (DRT). The de-convoluted spectrum of SOC generally presents, six peaks from mHz to hundreds of kHz. DRT peak-to-process attribution is often obtained by experimental sensitivity analysis. In the study, six parameters have been systematically varied: temperature, current density, partial pressure of O2 at the oxygen electrode, partial pressure of steam at the fuel electrode, total flow rates and fuel composition. The effect of wires inductance and different cell geometries has also been analyzed. The study provides detailed information about the contribution of the elementary processes to the total losses over a wide range of operation regimes. It further refines peak-to-process attributions by using a dynamic numerical model that includes gas and solid phase transport coupled with charge transfer and chemical reactions. Moreover, the non-univocal literature attribution of processes in the middle frequency range is clarified: strongly overlapping peaks cannot be separated even by DRT.

Solid-oxide electrolyzer based power-to-methane is promising for large-scale energy storage as well as biogas upgrading by efficiently converting intermittent renewable power and CO2 (e.g., in biogas) into synthetic methane. Either air or pure oxygen can be employed in solid oxide electrolyzers for anode sweeping and thermal management. This work investigates the optimal conceptual design of a variety of power-to-methane layouts to (i) identify the effect of sweep-gas type on system performance and (ii) compare different concepts for biogas upgrading. Bi-objective optimization is performed to understand the trade-off between system efficiency and methane production with the effects of the key design variables. The results indicate that oxygen sweep only marginally affected the system performance (6% reduction in methane production at the same system efficiency). The methanation inside the electrolyzer helped achieve a higher system efficiency (over 90%) by maintaining the electrolyzer temperature as high as possible. Unlike most biogas-upgrading systems, which behaved similarly to the standalone power-to-methane system, with an efficiency range of 70-86%, the directbiogas-electrolysis performed within a wide efficiency range (52-88%) and a reduced methane yield (50% less than the other systems operating at 70% efficiency). The detrimental methane reforming inside the electrolyzer was limited by increasing the reactant conversion and the electrolysis pressure. The various solid-oxide electrolyzer based power-to-methane concepts showed promising results for biogas upgrading applications. The practical choice of biogas-upgrading concepts will depend on the requirement of operational flexibility to handle variable renewable power considering different gas storage and carbon capture technologies.

Power-to-fuel systems via solid-oxide electrolysis are promising for storing excess renewable electricity by efficient electrolysis of steam (or co-electrolysis of steam and CO2) into hydrogen (or syngas), which can be further converted into synthetic fuels with plant-wise thermal integration. Electrolysis stack performance and durability determine the system design, performance, and long-term operating strategy; thus, solid-oxide electrolyzer based power-to-fuels were investigated from the stack to system levels. At the stack level, the data from a 6000-h stack testing under laboratory isothermal conditions were used to calibrate a quasi-2D model, which enables to predict practical, isothermal stack performance with reasonable accuracy. Feasible stack operating windows meeting various design specifications (e.g., specific syngas composition) were further generated to support the selection of operating points. At the system level, with the chosen similar stack operating points, various power-to-fuel systems, including power-to-hydrogen, power-to-methane, power-to-methanol (dimethyl ether) and power-to-gasoline, were compared techno-economically considering system-level heat integration. Several operating strategies of the stack were compared to address the increase in stack temperature due to degradation. The modeling results show that the system efficiency for producing H-2, methane, methanol/dimethyl ether and gasoline decreases sequentially from 94% (power-to-H-2) to 64% (power-to-gasoline), based on a higher heating value. Co-electrolysis, which allows better heat integration, can improve the efficiency of the systems with less exothermic fuel-synthesis processes (e.g., methanol/dimethyl ether) but offers limited advantages for power-tomethane and power-to-gasoline systems. In a likely future scenario, where the growing amount of electricity from renewable sources results in increasing periods of a negative electricity price, solid oxide electrolyser based power-to-fuel systems are highly suitable for levelling the price fluctuations in an economic way.

This paper presents a model-based investigation to handle the fundamental issues for the design of co-electrolysis based power-to-methane at the levels of both the stack and system: the role of CO2 in co-electrolysis, the benefits of employing pressurized stack operation and the conditions of promoting internal methanation. Results show that the electrochemical reaction of co-electrolysis is dominated by H2O splitting while CO2 is converted via reverse water-gas shift reaction. Increasing CO2 feed fraction mainly enlarges the concentration and cathode-activation overpotentials. Internal methanation in the stack can be effectively promoted by pressurized operation under high reactant utilization with low current density and large stack cooling. For the operation of a single stack, methane fraction of dry gas at the cathode outlet can reach as high as 30 vol.% (at 30 bar and high flowrate of sweep gas), which is, unfortunately, not preferred for enhancing system efficiency due to the penalty from the pressurization of sweep gas. The number drops down to 15 vol.% (at 15 bar) to achieve the highest system efficiency (at 0.27 A/cm2). The internal methanation can serve as an effective internal heat source to maintain stack temperature (thus enhancing electrochemistry), particularly at a small current density. This enables the co-electrolysis based power-to-methane to achieve higher efficiency than the steam-electrolysis based (90% vs 86% on higher heating value, or 83% vs 79% on lower heating value without heat and converter losses).