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Breaking the temperature limits of Solid Oxide Fuel Cells: Towards a new family of ultra-thin portable power sources

Periodic Reporting for period 4 - ULTRA-SOFC (Breaking the temperature limits of Solid Oxide Fuel Cells: Towards a newfamily of ultra-thin portable power sources)

Reporting period: 2020-10-01 to 2021-03-31

Solid Oxide Fuel Cells (SOFCs) are one of the most efficient and fuel flexible power generators. However, a great limitation on their applicability arises from temperature restrictions. Operation approaching room temperature (RT) is forbidden by the limited performance of known electrolytes and cathodes while typical high temperatures (HT) avoid their implementation in portable applications where quick start ups with low energy consumption are required.
The ULTRASOFC project aims at breaking these historical limits by taking advantage of the tremendous opportunities arising from novel fields in the domain of the nanoscale (nanoionics and nanoengineering) and recent advances in the marriage between micro and nanotechnologies. From the required interdisciplinary approach, the ULTRASOFC project addresses materials challenges to (i) reduce the operation to RT and (ii) technological gaps to develop ultra-low-thermal mass structures able to reach high temperature with extremely low consumption and immediate start up.
ULTRASOFC released a unique set of materials fully integrated in silicon substrates that allow operation at very low temperaturesominated perovskites (GB-enhanced or Vertically Aligned Nanocomposites-VANs) reduced in orders of magnitude their resistance working as cathodes. Beyond their application in μSOFCs. In particular, novel electrolytes based on BICUVOX have been proved for the first time as thin films enabling the operation at RT. On the other hand, nanoengineered interface-d, this new set of materials open the door for a new generation of iontronic-based devices.
Apart from breaking the state-of-the-art of low T solid state electrochemical cells, the ULTRASOFC project has covered the gap of knowledge existing for high temperature MEMS based on mixed ionic and electronic materials. In this regard, ULTRASOFC developed robust fabrication routes for self-sustained micromachined μSOFCs up to 1cm2 and their stacking with other components (silicon microreactors for hydrogen generation and silicon-based interconnects). Beyond the use in μSOFC, this approach has opened up new horizons and opportunities for research in adjacent fields like electrochemical transducers or chemical sensors (already proved within the ULTRASOFC project).
The activity of the project has been focused on:
i) developing low temperature electrolytes able to be integrated in microtechnologies. A new concept electrolyte has shown the potential to be used at temperatures approaching room temperature. BICUVOX has been selected as superior oxygen conducting electrolyte material, with forecasted operating temperature below 100 ºC. The high ionic conductivity, added to the high stability vs. pO2 proved in this LOW temperature range, anticipates the potential use of BICUVOX down to RT in a new family of non-heated solid state electrochemical systems. This opens up amazing possibilities for the fabrication of solid state devices fully integrated in CMOS technologies. (Journal of Materials Chemistry A 7.45 (2019): 25772-25778 (Hot Paper and cover image); Journal of the European Ceramic Society 39.2-3 (2019): 101-114.)
ii) understanding the effect of grain boundaries on the ionic conductivity and the catalytic properties of mixed-ionic electronic conductors with applicability in Solid Oxide Fuel Cells. In particular, it has been proven that strain-induced defects in grain boundaries of manganites deeply impact their functional properties by boosting their oxygen mass transport while abating their electronic and magnetic order. We were able to alter the grain boundary composition by tuning the overall cationic content in the films, which represents a new and powerful tool for engineering electronic and mass transport properties of metal oxide thin films useful for solid-state devices. (Advanced Materials 31.4 (2019): 1805360; Metal Oxide-Based Thin Film Structures. Elsevier, 2018. 409-439; Chemistry of Materials 30.16 (2018): 5621-5629; Solid State Ionics 299 (2017): APL Materials 7.1 (2019): 013205; Nature communications 12.1 (2021): 1-11).
iii) unveiling point defects concentration in transition metal oxide thin films. Especifically, the defect chemistry of a relevant material such as La1-xSrxFeO3-δ (LSF) with different Sr content has been studied by using a novel in-situ spectroscopic ellipsometry approach applied to thin films at intermediate-to-low temperatures. Through this technique, the concentration of holes in LSF has been correlated to measured optical properties and its evolution with temperature and oxygen partial pressure was determined. In this way, a systematic description of defect chemistry in LSF thin films in the temperature range from 350 to 500ºC has been obtained, which represents a step forward in the understanding of LSF for emerging low temperature energy and information technologies applications (Advanced Materials Interfaces 8.6 (2021): 2001881).
iv) integration of μSOFC into silicon technology to create a compact power generator for portable applications. The strategy followed consisted of a vertical stacking of all the components (fuel cells stack, fuel pre-processor unit, and a catalytic post-combustion unit), a heat management unit consisting of a heat exchanger and a thermal insulation. Each individual element has been designed and fabricated after the optimization of the manufacturing processes. Special effort has been performed in the improvement of the SOFC membranes, which is the most critical element of the device. Moreover, systematic work on sealing, encapsulation by 3D printed ceramic parts, design of microfluidic pathway and stacking of cells has been carried out (Journal of Physics: Conference Series. Vol. 1407. No. 1. IOP Publishing, 2019; Smart Sensors, Actuators, and MEMS VIII. Vol. 10246. International Society for Optics and Photonics, 2017; ACS applied materials & interfaces 13.3 (2021): 4117-4125).
The major achievements of the project represent clear advances beyond the state of the art. In particular, ULTRASOFC released BICUVOX thin films for the first time reducing the operating temperature of currently existing electrolytes below 100ºC, which was a target of the project. Moreover, ULTRASOFC proposes a new strategy for generating high diffusion pathways based on grain boundary and interface engineering. After unveiling the underpinnings of this new approach in the field of Nanoionics, it represents a breakthrough universally applicable to perovskite cathodes limited by oxygen diffusivity reducing their polarization resistances in orders of magnitude. Novel room-temperature thin-film based solid state devices based on this new set of materials are expected in the near future. Moreover, a new operando characterization technique for thin-film MIEC devices has been developed within the project. This technique already proved its unique capabilities to study defect chemistry and redox processes at never explored low temperatures.
Finally, ULTRASOFC releases a robust cell and stack design and fabrication process for μSOFCs that demonstrate the capability to integrate these devices into mainstream silicon technology and highlights their potential to be applied into portable applications.The fabrication of cells up to 1cm2, fully operative micro-reactors for fuel conversion and first short stacks can be considered a progress beyond the SoA and should end up with robust complete stacks in the near future.