Buildings are major energy consumers, accounting for a significant portion of global energy consumption. Integrating hydrogen systems, electrolyzers, accumulation, and fuel cells is proposed as a clean and efficient energy alternative to mitigate this impact and move toward a more sustainable future. This paper presents a systematic procedure for incorporating these technologies into buildings, considering building engineers, and stakeholders. First, an in-depth analysis of buildings’ main energy consumption parameters is conducted, identifying areas of energy need with the most significant optimization potential. Next, a detailed review of the various opportunities for hydrogen applications in buildings is conducted, evaluating their advantages and limitations. Performing a scientific review to find and understand the requirements of building engineers and the stakeholders has given notions of integration that emphasize the needs. As a result of the review process and identifying the needs to integrate hydrogen into buildings, a flowchart is proposed to facilitate decision-making regarding integrating hydrogen systems into buildings. This flowchart is accompanied by a matrix of variables that considers the defined requirements, allowing for combining the most suitable solution for each case. The results of this research contribute to advancing the adoption of hydrogen technologies in buildings, thus promoting the transition to a more sustainable and resilient energy model.

Introduction Hydrogen production from renewable energy sources, or green hydrogen, is essential to contribute to the decarbonization strategy planned by the European Commission by 2050 ¹ . Some hydrogen technologies currently under investigation are alkaline (AWE), proton exchange membrane (PEM) or solid oxide (SOE) water electrolysers, with AWE and PEM being the most mature to date. Nevertheless, one of the main challenges in water electrolysis is the lack of water sources, as some of these hydrogen technologies, such as AWE and PEM, require high purity water (type I) ² . Water treatments commonly used to obtain highly pure water are reverse osmosis, ion exchange processes, electrodeionisation and distillation, among others. However, these processes involve high energy consumption, high costs and significant environmental impact, mainly caused by water losses. Besides, green hydrogen production using AWE or PEM is problematic in terms of the required water consumption to achieve a certain electrolysis capacity, as usually, less than half of the treated water is recovered after these treatments ³ . In contrast, solid oxide electrolysis (SOEC) operates at elevated temperatures (600 – 850 °C), and is supplied by water vapor (steam electrolysis) rather than liquid water. Therefore, water purity requirements may be less stringent compared to other electrolysers, enabling the potential use of alternative water resources with varying levels of purity, which may require less demanding water treatments ^4,5 . This positions SOEC as a promising technology compared to other electrolysis options, highlighting the need for comprehensive studies on the use of non-conventional water sources and their impact on the performance and integrity of various SOEC components. SOEC technology is based on the application of an electrical current to split water (steam) into hydrogen and oxygen. Anode and cathode, where the redox reactions take place, are separated by an electrolyte, which is an excellent oxygen ion conductor and an electronic insulator ⁶ . Splitting of steam into hydrogen and oxygen ions occurs at the fuel electrode, being the latter transported through the electrolyte to the oxygen electrode, where oxygen ions react to form molecular oxygen. Moreover, solid oxide cells can be operated in reverse mode as solid oxide fuel cells (SOFC) to produce electrical power. As SOFC, the reactions and ion transport occur in the reverse direction, i.e., oxygen is reduced forming oxygen ions at the oxygen electrode, and these ions are transported through the electrolyte, oxidising hydrogen at the fuel electrode. Other advantages of this technology are the excellent electrical efficiency, consequence of faster kinetics and favourable thermodynamics at high temperatures, and the possibility to operate with other fuel reactants such as carbon dioxide to produce syngas ^2,7 . Several scientific research articles have been published studying the viability of different water sources in SOEC, mainly seawater ^5,8 . However, the degradation rates reported are above the suggested value recommended by Idaho National Laboratory for the commercialisation of this technology (~ 15%/kh vs 1%/kh, respectively) ^8,9 . Therefore, other water resources, such as wastewater from treatment plants or swamp water, could be an interesting option in SOEC. As a result, the Regional Spanish project entitle ‘GACSOEC’ was initiated with the aim of testing the viability of non-conventional water sources to generate green hydrogen using SOEC and consequently, proving the potential of this technology in areas with scarce water resources. For this purpose, three different water sources will be first treated, by the adsorption of impurities using active carbon, and subsequently tested in an electrochemical SOEC test station with commercial solid oxide cells. The cells’ performance will be evaluated to determine the degradation rate and ASR associated with the different water sources investigated in this project. Subsequently, post-mortem analyses of the cells will be conducted to identify the specific elements present in the water that contribute most significantly to the degradation of cell performance. Methodology Three water sources of different purity (tap water, swamp water and wastewater from a water treatment plant) were treated by adsorption, applying four different treatments based on active carbon filters, to eliminate impurities and reduce the metal content. Additional water treatment processes, such as reverse osmosis or radiation, might be integrated along the adsorption process to improve the water quality, depending on the electrochemical results. Compositional analyses were carried out before and after water treatments to determine the efficacy of the purification and to define the possible elements that might affect the performance of the cells. Post-treated water was stored at -4°C to avoid growth of microorganisms before electrochemical assays. Commercial anode supported solid oxide cells (5x5 cm ² ,with a 4x4 cm ² active area) were chosen to perform the different electrochemical experiments. The electrolyte consisted of a dense layer of 8 mol% yttria-stabilised zirconia (8YSZ) and a barrier layer of gadolinium-doped ceria (GDC), the oxygen electrode of lanthanum strontium cobalt ferrite (LSCF), and the fuel electrode of NiO-YSZ. The cells were mounted in a SOFC/SOEC cell test rig (Open Flanges ^TM , Fiaxell) inside a furnace and were operated in a cross-flow configuration whereby the gases were distributed via gas flow channels to both electrodes. A peristaltic pump was used to pump the treated water from the reservoir to the fuel inlet channel, where a steamer was placed to achieve a constant steam flow to the cell. Air and hydrogen were controlled by mass flow controllers. Outside the housing, the gas outlets were heated and insulated to prevent condensation, and a reservoir was installed to collect any unreacted water. During the heating ramp, nitrogen and air were used to purge and burn out any possible organic material present in the cell housing. Additionally, the test station was equipped with a potentiostat/galvanostat and a booster to perform the electrochemical analyses. The experiments were carried out at 800 °C with a 70/30 H ₂ O/H ₂ ratio and a current density of 0.5 A/cm ² during at least 50 h. Post-mortem cells were analysed using SEM/EDS and XRD to determine degradation phenomena such as delamination, impurities deposition or elements’ migration/reaction. Thus, a correlation between the electrochemical tests, the degradation effects and the quality and composition of water could be established. Discussion Several operational conditions, such as temperature, H ₂ O/H ₂ ratio, gas composition, and current density were kept constant to compare the effect of water quality on cell performance. Initially, short degradation tests of 50 h were conducted to discard those water samples leading to degradation rates above 5%. Subsequently, selected water samples will be subjected to longer degradation tests (≥ 100 h) to define the water quality that achieves the best electrochemical results (lower degradation rate and ASR). The aim is to reach a degradation rate below 10%/1000h and 0.1 – 2 Ω·cm ² ASR. Initially, a short degradation test with deionised water from a laboratory water purification system (Wasserlab) was performed to be used as a reference point for the rest of experiments with the treated water. The degradation rate of the commercial cell after 50 h test is 1.72% with a R _Ω = 0.32 Ω·cm ² and R _t = 0.5 Ω·cm ² (Figure 1). It is worth mentioning that commercial cells already report typical degradation values between 3.2 – 4.6% per 1000 h, so this value falls within the expected range. SEM/EDS analyses showed no delamination nor impurities deposition during the test. After this initial testing, tap water, swamp water and wastewater from a water treatment plant are treated using four different pre-treatment + water treatment. These treatments will produce water with different purity levels that will be subjected to electrochemical testing. Those water treatments yielding degradation rates above 10%/kh and/or 0.1 – 2 Ω·cm ² ASR will be discarded. Importantly, if the composition analyses after treatment reveals high level of some impurities already identified as harmful for SOEC performance, such as chromium or chlorine, those water samples will be excluded. Conclusions This project will focus on the electrochemical performance of solid oxide cells using non-conventional water sources that are previously purified with different water treatments mainly based on adsorption with active carbon. Tap water, swamp water and wastewater from a treatment plant were treated with four different pre-treatment + water treatment to achieve a certain level of purity and reduce impurities that could affect the performance of the cell, such as sulphur or chromium. Electrochemical experiments were initially carried out using deionised water to determine the degradation level and ASR of the commercial cells using high purity water. This will set a baseline for the rest of experiments with lower water quality. Degradation rates below 10%/1000 h and 0.1 – 2 Ω·cm ² ASR are set as KPIs to prove the viability of these water sources for SOEC. References European Commission, A hydrogen strategy for a climate-neutral Europe , p. 1–23, (2020). P. Saravanan, M. R. Khan, C. S. Yee, and D. V. N. Vo, in New Dimensions in Production and Utilization of Hydrogen ,, p. 161–190, Elsevier (2020). T. Ohmi, Ultra-Clean Technology Handbook : Volume 1: Ultra-Pure Water , p. 944, CRC Press, (2020). M. Maddaloni et al., Water , 15 , 2569 (2023). Z. Liu et al., Appl. Energy , 300 , 117439 (2021). M. A. Laguna-Bercero, J. Power Sources , 203 , 4–16 (2012). M. Mueller, M. Klinsmann, U. Sauter, J. C. Njodzefon, and A. Weber, Chemie-Ingenieur-Technik , 96 , 143–166 (2024). C. K. Lim, Q. Liu, J. Zhou, Q. Sun, and S. H. Chan, J. Power Sources , 342 , 79–87 (2017). X. Zhang, J. E. O’Brien, and R. C. O’Brien, in ASME 2012 10th International Conference on Fuel Cell Science, Engineering and Technology Collocated with the ASME 2012 6th International Conference on Energy Sustainability, FUELCELL 2012 , p. 1–8 (2012). Figure 1

Introduction Global warming and energy crisis have motivated the drive for green energy transition and deployment of sustainable energy sources, supported by advances in energy storage and the emerging Power-to-X technologies (P2X). [1] Primary P2X technologies processes lie in the production of green hydrogen, the most prospective energy carrier. Solid oxide electrolysis cells (SOEC) is a promising hydrogen production technology, featuring high electrical efficiency, fuel flexibility (e.g. CO ₂ conversion) and reversible operation (same device working both as electrolyzer or fuel cell). CENER's Hydrogen Area is actively advancing in SOEC technology, with focus on the development of high performance kW-scale SOEC stack prototypes. The objective is to enhance scalability, reliability and efficiency while reducing both capital and operational costs associated with electrolyzers. For this purpose, some efforts are concentrated on developing scalable, reproducible and cost-effective fabrication methods for producing pre-industrial scale batches of proprietary 10 x 10 cm ² planar SOECs. These will serve as the foundation for assembling primary stacks and supporting the progression of multiple projects with increasing power generation targets. This contribution will outline the development of the fabrication process for the various components of a cell, namely the hydrogen electrode, electrolyte and air electrode. The process includes ink/slurries formulation, optimization of deposition method and sintering, electrochemical validation and finally, upscaling the cell size up to 10 x 10 cm ² . Material and methods SOEC cell fabrication was performed using different deposition techniques for the different cell components. Specifically, tape casting was employed for the hydrogen electrode and electrolyte (half-cell), while screen printing was applied for the oxygen electrode. Both methods rely on the preparation of ceramic slurries and inks respectively, prepared from selected powders and appropriate solvent and additives. Regarding the half-cells, the aim was to develop eco-friendly, water-based slurries. State of the art materials, i.e. Yttria Stabilized Zirconia (YSZ) and the ceramic-metallic compound NiO-YSZ, were selected for the electrolyte and hydrogen electrode, respectively. Slurry optimization was performed varying the composition of the different additives and mixing procedure (milling and mixing techniques, milling speed, times etc.). Slurry viscosity was measured to ensure reproducibility and slurry stability over time, indirectly analyzing particle dispersion or aggregation. Tape casting slurries were prepared to be deposited with a Dr. Blade, using various gap settings to produce thin green layers or tapes once the solvent dried. Multilayer casting allowed the electrolyte and the active layer of the hydrogen electrode to be deposited sequentially, obtaining a bilayer. After casting, the bilayers were calendered with the hydrogen electrode’s support layer tapes, cut to the desired dimensions and sintered at temperatures up to 1500 ºC to produce flat and robust half-cells to be used as substrate to deposit the air electrode. Screen-printing technique was employed to fabricate the air electrode, composed by Gadolinium doped ceria (GDC) and Lanthanum Strontium Cobalt Ferrite (LSCF), and the barrier layer based on GDC. Ink formulation involved a preliminary step, where LSCF and/or GDC powders were partially mixed with solvents and additives using a ball mill, then the ink was further processed through a triple-roll mill. These inks were applied to the substrate (sintered half-cell) by a semi-automatic screen printer, optimizing printing conditions. After printing the barrier layer/air electrode, the full cells were dried to remove the solvent and then sintered at temperatures up to 1400 ºC. Additionally, various characterization techniques were used to evaluate the microstructure (e.g., electrolyte densification, grain and pore distribution and cell delamination via scanning electron microscopy, SEM); crystal structure (using X-ray diffraction, XRD), and electrochemical performance (electrochemical impedance spectroscopy – EIS - and intensity-voltage curves) of the cells. Results In order to obtain proprietary robust SOEC cells, this study was initially centered on developing the fabrication process by tape casting (hydrogen electrode and electrolyte) and screen printing (air electrode). At first, the process was validated on button cells (20 mm diameter), subjected to electrochemical assays to optimize the fabrication protocol. Subsequently, the protocol will be upscaled and validated on square cells of 5 x 5 cm ² and 10 x10 cm ² size. Tape casting technique was optimized to obtain homogeneous and flexible tapes. For it, a comprehensive study was dedicated to slurry formulation, considering different additives (dispersing agents, binder, dearing and defoamer elements) and assessing the influence of their concentration on cell microstructure and electrochemical performance. Indirect measurements, such as slurry viscosity or stability, were used to evaluate slurry homogeneity and particle dispersion. Figure 1a) shows how slight changes in defoamer concentration led to a decrease in viscosity (black column). In addition, it was observed a decrease on viscosity values over time (grey column, after 1 h). This may be associated with slurry instability originated by particles decantation, which could be attributed to inadequate binder loading. Moreover, ball milling parameters were demonstrated to influence grain size and particle distribution. As an example, Figure 1b) shows the difference between two milling speeds on NiO grains on hydrogen electrode. As can be seen, the higher the milling speed the better NiO particles distribution. Moreover, casting parameters, such as gap between the Dr. Blade and the casting bed, casting rate or drying temperature, as well as calendaring conditions, were found important to produce dense, homogeneous and well adhered tapes. In the same way, the sintering process, involving different temperatures and plateaus, was found critical for obtaining appropriate microstructure and, especially, planar half-cells. Similar contraction ratios between the different layers are essential to achieve the desired planarity. In the same way, ink formulation was thoroughly studied for the fabrication of the air electrode. Different solids contents as well as GDC/LSCF proportions were considered to optimize the ink. The uniformity of particle size distribution as well as the consistent thickness and porosity of the printed air-electrode layer were achieved by setting a specific gap between the rollers during ink preparation, in addition to controlling the printing process. Among the parameters that can be controlled during the deposition process (such as snap-off, printing speed, and squeegee pressure), the snap-off (distance between substrate and screen) played a key role in achieving a uniform thickness across the full cell. As an example, Figure 2 illustrates the influence of snap-off on layer thickness. When a larger snap-off distance was used to print the GDC barrier layer, more defects were observed due to fluctuations in layer thickness, with areas where the GDC ink was clearly missing (Figure 2a). In contrast, a smaller snap-off distance allowed for better thickness control, resulting in a more homogeneous layer and higher quality in the screen-printed layers (Figure 2b). On the other hand, this study showed that high-solid content inks resulted in thicker films, as shown in Figure 2 (c, d) for GDC/LSCF screen-printed on electrolyte. The thickness and homogeneity improved when increasing the solid content from 40 wt % to 50 wt % (Figures 2c and 2d, respectively). Furthermore, sintering steps play a crucial role in layers porosity, being necessary to study the process to achieve the desired porosity for each layer. The resulting cells have been subjected to electrochemical tests in order to assess their performance and modify their composition and fabrication accordingly. Once the process is validated, it will be upscaled to square cells of 5 x 5 cm ² and eventually, to 10 x 10 cm ² . Conclusions In conclusion, this study is focused on optimizing the fabrication process of solid oxide cells, addressing each step in the process, including slurry and ink formulation, deposition technique, sintering conditions, and electrochemical validation. Various control parameters, such as viscosity, porosity or grain size, were checked to guide the optimization. Through comprehensive optimization of component concentrations, milling and deposition parameters, as well as sintering steps, robust planar cells with the desired microstructure — specifically in terms of thickness, grain distribution, and homogeneity — were successfully fabricated. The cells are currently undergoing electrochemical testing and based on these results, further refinements in the fabrication process will be implemented. The next technical challenge will involve upscaling to produce larger cells, with dimensions up to 10 x 10 cm². References Chem. Rev. 2024, 124, 10509-10576 Figure 1 Figure 2

The goal of 10 MT of green hydrogen production and 40 GW electrolyzer capacity targeted by European Union by 2030 implies the necessity of a fast development of high performing electrolyzers. Among the technologically mature electrolyzer choices, solid oxide cells (SOCs) can reach unrivaled conversion efficiencies, and reduced electricity consumption and operational costs. These benefits arise from their high operating temperatures of around 650-800 ºC, resulting in improved kinetics and favored thermodynamics. Furthermore, this allows operating the cell reversibly in both electrolysis (SOE) and fuel cell modes (SOFC). To reach these ambitious goals and to consolidate the role of green hydrogen in a net-zero emission energy system, it is crucial to coordinate efforts between industry, R&D and public/private stakeholders. In this context, the HyGrIn Lab from Grid Integration, Electrical Storage and Hydrogen Department at CENER focuses on the technological development of SOC technology. HyGrIn Lab belongs to the Spanish Solid Oxide Electrolyzer Consortium , where members with deep experience in SOC, viz. , Complutense University of Madrid, San Pablo CEU University, and the Aragonese Institute of Nanoscience and Materials (INMA, CSIC) coordinate R&D activities to cover the full value chain of SOC development, including new materials research, cell and stack fabrication and optimization, and system modeling and engineering. These agents cooperate on the search of technological advances in the improvement of scalability, reliability, efficiency, and reducing both capital and operational costs of electrolyzers, ultimately reducing the levelized cost of green hydrogen (LCOH). In this contribution, we will present the main advances on system modeling and engineering activities at CENER. Modeling tasks focus on design and optimization SOE (either water or co-electrolysis) and SOFC systems through process simulations. Furthermore, techno economic analysis (TEA) analize the viability of such systems in a wide range of operational scenarios, including various renewable energy sources, plant size and locations. Both commercial software (AspenTech®) and in-house libraries are employed to accomplish these tasks. In this contribution we share our most recent research focused on the TEA of MW-scale SOE plant and its integration possibilities in Spain. In this work, the LCOH was estimated in various scenarios and the major contributors to reduce this value were identified. Figure 1a shows the process flow diagram proposed for such system after optimizing the heat exchanger network, while Figure 1b shows the breakdown of the LCOH obtained for this system [1]. Our engineering efforts are centered around the design, construction, and commissioning of solid oxide system prototypes. Once the process is designed and optimized by simulation, conceptual, basic and detailed engineering steps are carried out. This process involves designing the equipment and selecting commercially available options, based on design specifications; making piping and instrumentation diagrams; proposing the system layout (see to Figure 1c); the electrical design; and working on process automation by PLC/HMI/SCADA development. Moreover, we also handle the construction of the system components (see progress in Figure 1d), as well as overseeing the final start-up and testing in various scenarios. In this contribution, we will share our recent work on the detailed engineering and start-up of a complete 10 kW SOFC system. Figure 1 a) Process flow diagram with the optimized heat exchanger network, and b) breakdown of the LCOH for the system under study in our recent publication [1]. OPEX= operational expenses, CAPEX= capital expenses. c) layout of the designed 10 kW SOFC system, as well as, d) current final prototype during the construction stage. Figure 1

The scale-up of solid oxide cells (SOCs) manufacturing is critical for its industrial implementation. However, it presents significant challenges, including obtaining defect-free, dimensionally uniform layers and reproducible microstructure, controlling slurry properties, and minimizing resource consumption and waste generation. This work details the successful transition of a half-cell fabrication route (Ni-YSZ/YSZ) from laboratory scale to a pre-industrial roll-to-roll system, representing one of the first demonstrations of water-based multilayer casting at this scale. The scale-up was performed using water-based slurries as a feasible alternative to conventional organic-based systems, by carefully adjusting both slurry preparation and deposition parameters (i.e. gap, casting speed and drying temperature). While implementing pre-industrial fabrication, reduced slurry viscosity, sample reproducibility and appropriate microstructure were observed, resulting in maintained electrochemical performance (0.38 ± 0.02 and 0.42 ± 0.04 W cm ⁻² , using LSM-YSZ oxygen electrode). In addition, the optimized multilayer deposition resulted in fivefold increase in production capacity, from ∼ 30 to > 150 half-cells per week, as well as cost-efficient material utilization (from 3.5 to 0.7 € material cost per half-cell). These findings demonstrate the feasibility of high-throughput water-based SOCs manufacturing by a more sustainable process and offer a pathway toward the cost-effective production of next-generation energy technologies.