Towards a Solid Oxide Fuel Cell/Electrolysis Cell Fabricated Entirely By Microwave Methods
Authors: Molero-Sánchez Beatriz , Addo Paul , Birss Viola , Morán Emilio 
Published: February 2020

There is great interest in the development of SOFCs that can be operated in reverse in the electrolysis mode, thus producing fuels and oxygen, while also serving as an electricity storage medium for renewable energy. In the solid oxide electrolysis cell (SOEC), steam can be converted to H2 and O2 while steam together with CO2 can generate syngas and O2. High temperature operation (700-950 oC) of water electrolysis cells significantly increases the performance of SOECs, as compared to PEM-based electrolysis systems.  However, an obstacle for operating at high temperatures is the lower stability of materials, although the significant progress made in the development of SOFC materials is a significant asset to SOECs as well [1]. Even so, the operating conditions in SOECS are quite different than in SOFCs, and thus new problems are emerging, including the delamination of the SOEC anode from the electrolyte, oxidation of Ni in the SOEC cathode as a result of the high levels of steam present, and sulfur poisoning of the Ni cathode [2]. Therefore, research is this field is moving towards the use of mixed ionic and electronic conducting oxides, which have been shown to be more stable as oxygen electrodes than conventional LSM materials [3].

Previous research in our group  has been focused on  the development of robust sulfur and coke tolerant electrode-supported SOFCs, based primarily on very promising metal oxide materials currently being developed in our group, which have shown very good catalytic activity for both H2/CO oxidation and O2 reduction. These are based on a La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr) mixed ionic-electronic conducting (MIEC) perovskite material [4, 5]. Because of the excellent performance of LSFC, efforts have been made to further improve its properties. Thus, the A-site of the perovskite was doped with Ca instead of Sr, producing La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr), shown to be a very promising oxygen and fuel electrode for reversible SOFCs [6-8].   The main goal of the introduction of Ca was to decrease the thermal expansion coefficient of this derivative of LSFCr, in order to more closely match that of a Gd-doped ceria (GDC) electrolyte[8].The partial substitution of Sr by Ca may also enable the introduction of structural inhomogeneities, as Ca doping of LaFeO3 is known to promote oxygen-vacancy ordering [9]. We have also demonstrated that the LCFCr material can be produced by microwave (MW) methods, showing that the pure phase can be obtained at a much lower synthesis temperature of only 300 oC,  the synthesis time can be cut down by ca. 50%, and there is a significant increase in its surface area (10.4 m2 g-1 vs 0.89 m2 g-1 ) [10].

In this work, we are focussed on working towards a solid oxide fuel cell/electrolysis cell that is fabricated entirely with the use of MW techniques, starting with the synthesis of the electrode/electrolyte powders and including the sintering of the full cell. An effective method has been developed for the MW co-sintering of the anode-electrolyte-cathode combination in one simple step. This approach, in which sintering temperatures as high as 1000 oC can be achieved in just a few minutes, would have a significant impact on both lowering material and cell manufacturing costs and on further enhancing the performance of these cells.  Thus, the LCFCr perovskite powders were first formed using MW methods and were then screen-printed on both sides of a gadolinia-doped ceria (GDC) electrolyte, followed by MW-assisted sintering of the cell.  It is shown that these LCFCr/GDC/LCFCr cells, sintered using only MW energy, gave performances that were very similar to cells fabricated using normal ceramic processing methods.  However, the time required to achieve this was decreased by ca. ten times, thus translating to significant manufacturing cost savings.

Acknowledgements:  We are very grateful to the SOFC Canada NSERC Strategic Research Network, as well as Carbon Management Canada, for the support of this work.  .

References:

[1] A. Hauch et al., Solid State Ionics, 192  547-551.

[2] A. Hauch et al., Journal of Materials Chemistry, 18 (2008) 2331-2340.

[3] M.A. Laguna-Bercero et al., Journal of Power Sources, 203  4-16.

[4] M. Chen et al., Journal of Power Sources, 236 (2013) 68-79.

[5] P. Addoet al., 11th Europeand SOFC and SOE forum, Luzerne, Switzerland, 2014, pp. B0314.

[6] P.K. Addo et al., ECS Transactions, 66 (2015) 219-228.

[7] B. Molero-Sánchez et al., ECS Transactions, 66 (2015) 185-193.

[8] B. Molero-Sánchez et al.,International Journal of Hydrogen Energy, 40 (2015) 1902-1910.

[9] V.V. Kharton et al.,Chemistry of Materials, 20 (2008) 6457-6467.

[10] B. Molero-Sánchez et al., Ceramics International, 41 (2015) 8411-8416.