Perovskites Doped with Small Amounts of Noble Metals for IT-SOFCs

Abstract

Solid oxide fuel cells (SOFCs) have a great potential among the emerging energy conversion technologies because they can efficiently convert various types of fuel (hydrogen, methane, CO, biogas) into electrical energy in the 750-850 °C temperature range. The choice of electrodes remains a challenge because most metal oxides suffer from low chemical stability and low electrocatalytic activity toward oxygen reduction reaction (ORR) in the intermediate temperatures (IT) range (500-700 °C) [1]. In this view, perovskite oxides (ABO3) have shown excellent properties in terms of thermal stability, compatibility with other cell components, low production cost and, most importantly, great flexibility [1]. La0.6Sr0.4FeO3-δ, (LSF) perovskite oxide have been widely investigated for the high electrical conductivity and the electrochemical properties [2]. However, the main drawback is the poor activity towards ORR in the IT range, as most iron-based perovskite oxides [3]. The B-site doping with noble metal catalysts can sensitively improve the properties and make the compounds suitable for electrode application.The addition of small amounts of Ru or Pt into LSF structure can be an effective approach to improve the electrocatalytic properties [4,5]. Specifically, we synthesized B-site doped LSF powders with very low amounts (<5%mol) of Ru and Pt by using solution combustion synthesis (SCS) method. The synthesized powders were calcined at quite low temperature (750 °C) to get as high as possible surface area (>15 m2/g). All compounds showed a single perovskite phase. As reported in Fig. (a), the addition of Ru and Pt in the structure causes a progressive shift of the XRD peaks to lower 2θ angles indicating the perovskite lattice expansion due to the substitution of Fe4+/ Fe3+ sites with larger Ru and Pt ions.To assess the electrocatalytic activity, the compounds were investigated in oxidizing conditions using electrochemical impedance spectroscopy (EIS) analysis. Fig. (b) shows the Arrhenius plot of area specific resistance (ASR) values of LSFRu and LSFPt perovskite oxides. The ASR values were measured on LSFM/La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM)/LSFM (M=Ru, Pt) symmetric cells in the temperature range 550-800 °C. The insertion of a very low amount (1%mol) of Pt or Ru significantly reduces the polarization resistance, confirming the beneficial effect of noble metal ions on ORR activity. All these features suggest that very low Pt and Ru doping positively affects the structural and electrocatalytic properties valuably for cathodic applications.[1] Hwang J, Rao R.R. (2017). Science 358:751-756[2] Hansen KK, Mogensen M (2008). ECS Trans 13:153–160[3] Chavan SV, Singh RN (2013). J Mater Sci 48:6597–6604[4] Fan W, Sun Z. (2016) RSC Adv 6:34564[5]Shaoli G., Hongjing Wu, (2015),Catalysts, 5, 366-391 Figure 1