Infiltrated La0.8Sr0.2Ga0.8Mg0.2O3-δ Based Cells Fed with Biogas

La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-d (LSGM) based porous/dense structures were optimized for infiltration of different metal catalysts. Porous anodic substrates with an open porosity larger than 65% were fabricated by using LSGM commercial powders (purchased by Praxair) with two different types of porogens: micrometric carbon and polymethilmetacrilate (PMMA) in a total weight percentage of 55%. The anodes were pre-sintered at 1250°C for 2 hours to get enough mechanical strength to be used as substrates for spin-coating deposition of micrometric layers of electrolyte. Micrometric dense layers of LSGM of thickness ranging from 10 to 25 micron were obtained depositing various spun layers and then co-sintered at 1450°C for 6 hours [1]. La_0.8Sr_0.2Fe_0.8Co_0.2O_3-d was used as cathode and deposited by using a screen-printing oil and firing at 900°C. A 17 M solution of metal (Ni, Co, Cu and their 1:1 mixtures) salts was used for infiltration,  dried and heated at 350°C for 30 minutes. Multiple infiltrations were necessary to get a metal catalyst amount of 25wt% that is the required value to get performing electrocatalytic performance. In Fig. 1 an SEM image (backscatted detector) of the cross section of Ni infiltrated cell is reported. The reduction behavior of impregnated LSGM powders  with the same amount of metal catalyst infiltrated in the scaffolds was examined by H₂-TPR techniques. Ex-situ catalyst activity measurement was used for the initial catalyst screening. The catalytic activity for CH₄ and CO₂ conversion followed the order Ni~Ni-Co>Co>Ni-Cu >>Co-Cu. Co is known to decrease the carbon deposition by different effects such as decreasing the particle size of metallic alloy and thanks to its low affinity toward carbon, the main drawback is the low catalytic activity in comparison with Ni [2]. However, the addition of Co to Ni slightly decreased the catalytic activity, thus Ni-Co alloys may couple the high activity of Ni with the high carbon and sintering resistance of Co yielding higher performing catalysts.  The catalysts selectivity of different catalysts for the dry reforming reaction, expressed as H₂/CO ratio, increased with temperature reaching a values of ~ 0.9, close to the thermodynamic value, both for Ni and Ni-Co catalysts. Stability tests were also performed on Ni and Ni-Co impregnated powders, both methane and carbon dioxide conversions were stable in Ni-Co without sign of conversion decrease, showing thus promising performance for the internal reforming of bio-gas in LSGM-based SOFCs. The electrochemical measurements were performed on infiltrated cells with the same amount of Ni and Ni-Co in the temperature range between 650 and 750°C. Measurements were performed both in H₂ and CH₄ and results compared. The maximum power density at 750°C in 100 cm³min^-1 of H₂ was 813 mw/cm² and 446 mw/cm² feeding 100 cm³min^-1 gas mixture with CH₄ /CO₂ ratio of 1.5 for Ni infiltrated cell (electrolyte thickness 13-15 µm). Similar performance were obtained for slightly thicker electrolyte (17-19 µm) Ni-Co based cell (max power density 654 mw/cm² in H₂ and 425 mw/cm² 100 cm³min^-1 gas mixture with CH₄ /CO₂ ratio of 1.5) confirming the results of the catalytic investigation on the corresponding infiltrated LSGM powders: the catalytic activity of Ni-Co infiltrated anodes was comparable to that of Ni infiltrated anodes. From the electrochemical impedance measurements at OCV and at 0.5 V of Ni and Ni-Co infiltrated cells, both ohmic and polarization resistances increase with time, thus a further investigation on the cell stability is in progress. Post-mortem analysis of Ni-Co infiltrated cell after 4 days of cell measurements do not show any clearly visible coke formation the anode substrate.Thus, the electrochemical tests in biogas of Ni and Ni-Co infiltrated cells showed promising performance confirming the results of the catalytic investigation.

References

[1] Z. Salehi, F. Basoli, A. Sanson, E. Mercadelli, S. Licoccia, E. Di Bartolomeo, Ceramics International, 40, 16455-16463 (2014).

[2] S. McIntosh, R. J. Gorte, Chem. Rev.104, 4845-4865 (2004).

Acknowledgments

The authors gratefully thank to the Italian Ministry for Education, University and Research (PRIN-2010-2011-Prot.2010KHLKFC).