In this work, we investigate the potential of a novel carbon composite as an electrode for high-voltage electrochemical double-layer capacitors. The carbon composite was prepared following a sustainable synthetic approach that first involved the pyrolysis and then the activation of a precursor formed by winery wastes and graphene oxide. The composite prepared in this way shows a very high specific surface area (2467 m²·g⁻¹) and an optimum pore size distribution for their use in supercapacitor electrodes. Graphene-biowaste-derived carbon composites are tested as active electrode materials in two different non-aqueous electrolytes, the ammonium salt-based conventional organic electrolyte and one imidazolium-based ionic liquid (1 M Et₄NBF₄/ACN and EMINTFSI). It was found that the presence of graphene oxide led to significant morphological and textural changes, which result in high-energy and power densities of ~27 W·h·kg⁻¹ at 13,026 W·kg⁻¹. Moreover, the devices assembled retain above 70% of the initial capacitance after 6000 cycles in the case of the organic electrolyte.

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 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.

Introduction The imminent energy crisis caused by the heavy dependence on fossil fuels has led to the search for a net zero-emission scenario that requires the development of reliable and efficient solutions for energy conversion and storage for the intermittent renewable energies. In this context, hydrogen has been identified as a potential energy carrier not only for energy storage, but also for the decarbonization of key industries and transport [1], [2]. There are various hydrogen production methods in different stages of development, being the water splitting process one of the most environmentally friendly, as it does not require fossil fuels as starting material. Thermochemical water splitting, photoelectrochemical methods and water electrolysis are among these techniques [3]. Water electrolysis is considered the cleanest technology when combined with renewable energies, producing the so-called green hydrogen. There are several electrolysis technologies with varying degrees of development, being the most common alkaline electrolysis (AEL), followed by proton exchange membrane electrolysis (PEMEL) and solid oxide electrolysis (SOEC) [3]. In the last decade, solid oxide cells (SOC) have generated great interest due to its high conversion efficiency, the possibility of working in fuel mode or electrolysis mode and the fuel flexibility (H ₂ O+CO ₂ co-electrolysis mode) [1], [2]. The operating temperature in SOC technology rises >650 ºC, favouring thermodynamics and kinetics of reactions, but demanding the use and development of cell and system materials, as well as designs, that stand the elevated temperatures and optimize the heat management. This is a point of improvement together with the scaling up of cell and stack manufacturing processes, to obtain stacks of higher power output that allow to obtain powerful electrolysis plants [4]. A dense electrolyte is required for a proper anion conduction in SOC, being yttria-doped zirconia (YSZ) the most used compound. The widely employed material for the hydrogen electrode is Ni used together with YSZ, getting the Ni-YSZ cermet [1]. For the air electrode, perovskite-type oxides are often used in the literature, such us lanthanum strontium manganite (LSM), combined with YSZ. Different fabrication methodologies have been used in SOC manufacturing, being the most standardized techniques tape casting and screen printing. Tape casting is used for thicker layers (20-250 µm), while screen printing is applied for thinner ceramic layers (2-50 µm). Both are processing methods that can be scaled up by adapting the production requirements [5]. During this work, planar anode supported cells were fabricated, employing tape casting for the hydrogen electrode and electrolyte and screen printing for the air electrode. The process for scaling-up each step is described, both in terms of volume produced as well as deposition method, making use of pre-industrial scale equipment. Methodology Cell fabrication In this work, a hydrogen electrode supported cell was prepared, being the thickest layer and acting as mechanical support of the cell. The hydrogen electrode is divided into a dense functional layer, which acts as the active part of the electrode, and a thick and a porous support layer, which acts as the mechanical support layer and facilitate the flow of gases. First, for the preparation of the cell components, the slurry for the different layers was prepared by ball milling a water solvent, Duramax D3005 (Dow Chemicals) dispersant and a 5M KOH basifying agent with powders: YSZ (Tosoh) for the electrolyte and YSZ and NiO (Vogler) for the hydrogen electrode. Subsequently, the obtained electrolyte and functional hydrogen slurries were mixed with the Duramax B1000 binder (Dow Chemicals) in a planetary mixer. For the porous support layer, the binder and cornstarch poreformer were added to the slurry to generate porosity, and this combination was firstly mixed in a helix stirrer prior planetary mixer. Subsequently, the defoaming program was optimized to allow a correct deairing. The electrolyte and hydrogen electrode layers were casted with a tape caster adjusting the gap to achieve the target thicknesses, being 20-30 µm for the electrolyte, 70-80 µm for the dense hydrogen electrode, and 175-185 µm for the porous hydrogen electrode. The electrolyte was firstly casted on a susbtrate film and dried. Then, the functional layer slurry was casted on top of it and left drying, obtaining a bilayer. Separately, the porous layer slurry was casted on a substrate film and dried. The casting speed was 1,8 m/min in electrolyte and 4,8 m/min in the functional and support layers. The drying temperature was 40 ºC in all cases. Subsequently, the green layers were detached and cut at the desired size. A bilayer and three support layers were calendered at 40 ºC to join them together to obtain the green half-cell. Finally, the green half-cell was heated up to 1500ºC necessary for sintering and densification of the ceramic layers. The air electrode layer was also divided into a functional layer and a support contact layer. The ink was prepared initially by mixing an ink vehicle with the powders LSM (Fuell cell materials) and YSZ in the ball milling in different ratios. Afterwards, these inks were further processed with a three-roll-mill. These inks were printed by screen printing with a target thickness of 20 µm in each layer (functional and support layer respectively) on the electrolyte of the sintered half-cell. The final cell was finally sintered up to 1100ºC. Up-scaling process The upscaling of cell manufacturing is of great interest in terms of simplicity, repeatability, versatility and especially cost-effectiveness. However, the transition to larger manufacturing scales requires optimization of both the process with respect to operating conditions and working dynamics. In this work, cell fabrication follows the standard protocol previously explained with some changes in the process, which can be divided into volume scale-up and process scale-up. Volume scaling refers to the increase in slurry volume production. When manufacturing at low volume scale, a ball milling machine that can hold one milling jar was used, whereas a high volume pre-industrial ball mill admitted up to 4 milling jars simultaneously. The filling proportions in each jug were maintained but, in this case, double quantity can be produced. Initially, the slurry was casted with a lab scale tape caster which was the same used for low volume production. The working parameters used for casting were the same as for low volume production. Once the produced volume had been increased, the up-scaling process was performed using a pre-industrial roll-to-roll tape caster (HaikuTech, Lab-Scale Tape caster, Model CAM-L252). This equipment continuously cast the slurry on top of the substrate, having a 3,5 m long casting capacity and 2 m long drying tunnel. The slurry produced within the previous step was used to cast in this equipment. For this purpose, the working conditions to obtain the desired thickness were optimized. Subsequently, the tapes obtained followed the same calendering and sintering protocol as in lower scale production but optimizing the process to reduce time and harness the tape dimensions. Characterization and test Volume scaling In a first step, viscosity measurements of the slurries were carried out to study their stability over time and as a tool for quality control of the process. These measurements were useful not only to characterize the stability over time but also to identify the different viscosity values between low and high-volume production. Viscosity values were taken immediately after the preparation of the slurry and after 1 and 2 hours at rest. For low and high production volume, the viscosity values were 33.0 ± 0.7 Pa.s and 5.7 ± 0.1 Pa. s for the electrolyte, 148.0 ± 14,1 mPa.s and 94.2 ± 3.5 mPa.s for the functional hydrogen layer and 188.0 ± 4.5 Pa.s and 89.8 ± 4.0 Pa.s, respectively. These results showed that, regardless of the production volume, the slurry viscosities remained stable over time (up to 2 hours). This conclusion was interesting because it indicates that the slurries can be used without requiring additional mixing steps over time and without affecting the properties of the tapes. This finding could be appreciated on tapes produced after keeping at rest the slurry during these period of times (0, 1 and 2 h), and no differences in appearance or thickness were observed. The stability of the slurries will allow large volumes to be casted over longer periods of time during process upscaling. Moreover, the results showed a decrease in viscosity values when produced at higher volume. Scaling in process During process upscaling, different casting speeds were tested for each type of slurry, and the thickness homogeneity was studied. For this purpose, 1 m long tape was considered, measuring the thickness in the centre of 5x5 cm ² squares. In this way, in addition to studying the relationship between speed and final thickness, the suitable casting speed was selected (Figure 1). During this study, the drying temperature and gap remained constant, and the casting speed was varied. Once the appropriate speed was selected, the initial gap is adjusted to obtain the desired final thickness. During the tests, the speed was selected according to the final appearance of the tape in terms of imperfections and thickness homogeneity. In the case of electrolyte, the selected casting speed was 0.5 m/min, for the hydrogen functional layer was 1.5 m/min and for hydrogen support layer was 1.0 m/min. This graph displays how the thickness decreases linearly with increasing casting speed which may be attributed to shorter slurry deposition times at higher speeds. The thickness variability (± 1.2 µm for electrolyte, ± 3.6 µm for the functional layer and ± 8.5 µm for support layer) was considered suitable to be homogenised during the calendering step. The pre-industrial scale equipment allows to triple the length of tapes produced compared to small-scale production (12-14 m vs 3 m long for the electrolyte). In addition, the microstructure of the different casted layers was observed using SEM. As shown in Figure 2(C and D) , both the electrolyte and electrode functional layer were dense and without delamination and imperfections at both fabrication scales. Likewise, in the two production scales the porous support layer (E and F) has similar pore distribution with large number of pores between 2-12 µm thanks to the addition of pore former. Nevertheless, some differences can be seen in the layers manufactured at different production scales. The NiO-YSZ grain distribution is different, presenting lower grain size when manufactured at pre-industrial scale in both functional and support layers. This effect, especially in the functional hydrogen layer, could have an impact on the electrochemical results and could be related to the lower viscosity values in the slurries. Further electrochemical testing will reveal the influence of these differences on cell performance. Conclusions This work is focused on optimizing the solid oxide cell manufacturing process using advanced materials and its scaling-up. In this study, the scaling-up manufacturing was divided in volume upscaling and upscaling tape casting process, providing working conditions optimization to obtain large quantities of useful material for solid oxide cells fabrication. The stability in the viscosity of the slurries and the thickness homogeneity in the tapes allowed to know that they are stable and suitable for their use over time. The observed cell microstructure confirms these statements, obtaining solid oxide cells suitable for electrochemical characterization. These future electrochemical studies of both small and pre-industrial scale fabricated cells will reveal conclusions to be drawn about the impact of each mode of fabrication and to decide on future improvements in the cells and, therefore, in the manufacturing process. Bibliography [1] Int. J. Hydrogen Energy , vol. 33, no. 9, pp. 2337–2354, 2008 [2] Science (80-. ). , vol. 370, no. 6513, 2020 [3] J. Power Sources , vol. 203, pp. 4–16, 2012 [4] Int. J. Hydrogen Energy , vol. 44, no. 31, pp. 16976–16982, 2019 [5] Int. J. Hydrogen Energy , vol. 37, no. 4, pp. 3874–3882, 2012 Figure 1