AM for EnergyCeramic Additive Manufacturing

Research on ceramic 3D printed SOFCs now extends to include SOECs

3D printing the next generation of enhanced solid oxide fuel and electrolysis cells

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Electroceramic-based energy devices like solid oxide fuel and electrolysis cells are promising candidates to benefit from using 3D printing to develop innovative concepts that overcome shape limitations of currently existing manufacturing techniques. Building on a similar project conducted in 2017, a new study published by the Journal of Material Chemistry, reports on the fabrication a new family of highly performing SOECs – electrolyte-supported solid oxide cells – using ceramics stereolithography. Conventional planar and high-aspect-ratio corrugated electrolytes were 3D printed with yttria-stabilized zirconia to fabricate solid oxide cells. Corrugated devices presented an increase of 57% in their performance in fuel cell and co-electrolysis modes, which is directly proportional to the area enlargement compared to the planar counterparts. This enhancement by design combined to the proved durability of the printed devices (less than 35 mV/1000 h) represents a radically new approach in the field and anticipates a strong impact in future generations of solid oxide cells and, more generally, in any solid-state energy conversion or storage devices.

Solid oxide fuel cells (SOFCs) are zero-emission power generators able to convert hydrogen into electricity with efficiency (LHV) above 60% over the whole range of kilowatt scales. This efficiency can reach values as high as 90% (LHV) in combined heat and power units (CHP), with SOFCs being one of the most efficient energy generation devices currently existing. Alternatively, the same devices operated in reverse mode are energy storage units able to produce storable hydrogen from electricity and water. SOECs (solid oxide electrolysis cells) are highly efficient energy conversion devices, with higher production yields and lower specific electric energy than competing electrolysis technologies.

Solid oxide cells (SOCs) are ceramic-based multilayer electrochemical cells consisting of a gas-tight oxide-ionic conductor electrolyte with electrodes in both sides. The state of the art materials for SOCs are yttria-stabilized zirconia (YSZ) for the electrolyte, combined with YSZ-based composites as electrodes, namely, lanthanum strontium manganite (LSM-YSZ) for the oxygen electrode and Ni–YSZ for the fuel electrode.

Only few strategies have been explored to take advantage of a straightforward increase of the performance of the cells by modification of its geometry, likely due to the strict limitations in manufacturing complex ceramic shapes. For instance, an increase of the active area of the cells by corrugation of the electrolyte will directly reduce the internal resistance of the cell, i.e. its area-specific resistance, proportionally increasing their performance per projected area.

This study presents the fabrication of 250 μm-thick 8YSZ (8 mol% yttria-stabilized zirconia) electrolytes by SLA 3D printing with a corrugated architecture, which intrinsically increases around 60% the active area compared to an also printed planar counterpart. A comprehensive characterization of the electrochemical performance of both types of cells is presented in this work in a range of temperatures between 800–900 °C in fuel cell and CO2and steam co-electrolysis modes. The analysis of the impedance spectroscopy of the cells allowed the clear identification of the origin of the enhancement. The corrugated architecture is discussed here as a first example of the wide range of printable geometries that can be fabricated by the ceramic 3D printing approach proposed in this work, proving its unfair advantage in improving the performance of the so obtained cell.

Planar and corrugated YSZ ceramic pieces were fabricated by using CERAMAKER a ceramic 3D printer from 3DCERAM. Computer Assisted Design (CAD) software was employed to sketch planar and corrugated membranes of the same 2.00 cm in diameter (of which 1.6 cm is the diameter for the electrode deposition, determining the future active area of the cell) and 250 μm in thickness but with different effective surface areas of 2.00 and 3.15 cm2, respectively. Such membranes were monolithically integrated with external annular rings to enhance the mechanical stability and ensure good sealing of the membranes during the testing.

To obtain the dimensions here described after the sintering, a rescaling process is applied to take into account the shrinkage during the sintering process (initial design values are not reported for clarity reasons). STL files were automatically created by using DMC software to slice the design and control the 3D printer. The 3DMIX-8YSZ solvent-free UV-photocurable slurry from 3DCERAM, which is composed by 8YSZ ceramic powder, acrylate UV curable monomer, photoinitiator and dispersant, was employed. The substitution of solvents by photo-polymerizable binders allows to achieve high ceramic loading, good homogeneity and a low viscosity of the suspension, which is further improved by adding diluents.33 8YSZ slurry with high ceramic loading (ca. 50 vol%) was deposited over a 30 × 30 cm2 printing platform by a double doctor blade system able to homogeneously spread the paste.

The blades were adjusted to deposit a thin layer of 25 μm in thickness. After deposition of the photocurable slurry, a UV semiconductor laser (power around 500 mW) focused at the building platform reproduces, slice by slice, the pattern designed by CAD using mirror rastering with a speed of 5000 mm s−1. Under UV exposure, the photocurable slurry, containing a monomer and a photoinitiator active in the UV region,34 locally solidifies following a free-radical photopolymerization process.

Images of the self-standing 3D printed 8YSZ membranes. Top view (a and b) and cross-section (c and d) of the planar and corrugated membranes, respectively. Detail of the cross-section by SEM for the planar (e) and corrugated (f) electrolytes showing (in the inset) the steps defined with the layer-by-layer 3D printing process.

Symmetrical and full electrochemical cells were fabricated using previously optimized standard procedures. Commercial NiO–YSZ and LSM–YSZ pastes (Fuel cell materials, USA) were painted on 3D printed YSZ pieces as fuel and oxygen electrodes, respectively. Attachment temperatures of 1400 °C for 3 h and 1200 °C for 1 h  were employed for the fuel and oxygen electrodes, respectively.

Planar and corrugated 8YSZ freestanding membranes were fabricated by means of SLA 3D printing after sintering at high temperatures to obtain crack-free and homogeneous parts. Overall, the 3D printed YSZ parts are considered suitable for working as electrolytes in SOFC/SOEC applications. The performance of the planar and corrugated LSM–YSZ/YSZ/Ni–YSZ solid oxide fuel cells was evaluated by measuring polarization curves under hydrogen (fuel electrode) and synthetic air (oxygen electrode) atmospheres in the temperature range between 800 °C and 900 °C.

Electrolyte-supported solid oxide cells with both conventional (planar) and enhanced-area (corrugated) architectures were successfully fabricated with ceramic 3D printing technologies. 3D printed solid oxide cells with planar geometry presented a good performance (comparable to conventional cells) in both fuel cell and co-electrolysis mode. More interestingly, corrugated cells showed an improvement directly proportional to the increase of their active area achieved by 3D structuration. In this work, a direct increase of 60% on conventional SOFC technology (LSM–YSZ/YSZ/Ni–YSZ) was reached obtaining an excellent maximum power density of 410 mW cm−2 at 900 °C.

Similarly, a high current density of 600 mA cm−2 at 1.3 V was injected in a corrugated solid oxide electrolysis cell operating in co-electrolysis mode. Moreover, a remarkably low degradation of the enhanced cells was proved in durability tests of 600 h of duration even at high-current density conditions (j = 360 mW cm−2 at 850 °C). These exceptional results can be considered the first step for the fabrication of a radically new generation of solid oxide cells with enhanced performance related to their change in nature from planar to three-dimensional. This enhancement goes beyond the high-aspect-ratio of their corrugated electrolyte and includes 3D printed structural elements with embedded functionality and improved stackability. The 3D printing methodology of this work represents a versatile approach that increases the design freedom for high performing and durable complex devices and a step forward in the revolution of the additive manufacturing in the energy sector.

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