Kinetic Study of La0.75Sr0.25Cr0.5Mn0.5O3-Δ Nanoelectrodes for Symmetrical-SOFC

One of the main technological challenges of the solid oxide fuel cells (SOFC) is to reduce its operating temperature. Nanostructured materials open this possibility since the microstructure plays a fundamental role on the electrocatalytic activity of the electrodes. In this context, the understanding of the limiting mechanism steps for both O₂ reduction and H₂ oxidation reactions is a main objective in order to improve the design of the nanostructured electrodes.  

La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM) perovskite has demonstrated a promising combination of properties for its employ as electrode (anode and cathode, simultaneously) in symmetrical-SOFC, among which it can mention: phase and dimensional stability, high electrochemical activity in both reducing and oxidizing atmospheres, and chemical compatibility with various solid electrolytes. In recent works, we present the synthesis by combustion of pure-phase LSCM nanocrystallites [1], and the bottom-up building process of nanostructured LSCM electrodes employing La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ(LSGM) as electrolyte [2]. In particular, the influence of the electrode crystallite size on the electrochemistry behavior as both anode and cathode was studied by electrochemical impedance spectroscopy (EIS).

In this work, we present an electrode kinetics study of the nanostructured LSCM material by employing EIS measurements in different oxidizing and reducing atmospheres. Figures (a) and (b) present the EIS spectra of LSCM/LSGM/LSCM cell varying the oxygen and hydrogen partial pressures, i.e. pO₂ and pH₂ respectively. As it can be observed, when both pO₂ and pH₂ increase the electrode polarization resistance decrease, which indicates that the LSCM nanocrystallites are electrocatalytically active both for the oxygen reduction as well as hydrogen oxidation. Figures (c) and (d) show the evolution of the polarization resistance with the pO₂ and pH₂ obtained by fitting of the EIS spectra by using Electrical Equivalent Circuits. In both cases, two EIS arcs at high and low frequency (HF and LF, respectively) can be resolved showing complex mechanisms. In the H₂ oxidation, the HF contribution with n ≈ 0 is independent of H₂ concentration, probably due to a charge transfer process, while the LF contribution with n ≈ 0.5 could involve atomic species, e.g. H₂-dissociative adsorption or H-surface diffusion [3]. On the other hand, in the O₂ reduction, the HF contribution with n ≈ 0.25 is typically assigned to O-surface transport process whereas the LF arc with n ≈ 1 is generally attributed to O₂ mass transfer in the gas phase [4].

 References

[1] C.M. Chanquía, J.E. Vega-Castillo, A.L. Soldati, H. Troiani, A. Caneiro. J. Nanopart Res (2012) 14:1104.

[2] C.M. Chanquía, A. Montenegro-Hernández, H.E. Troiani, A. Caneiro. Journal of Power Sources 245 (2014) 377-388.

[3] P. Blennow, K.K. Hansen, L. R. Wallenberg, M. Mogensen. Solid State Ionics 180 (2009) 63–70.

[4] C.M. Chanquía, L. V. Mogni, H. Troiani, A. Caneiro. Journal of Power Sources 270 (2014) 457-467.

Figure Caption

 Figure1. (a,b) Normalized EIS spectra and (c,d) Log-Log plots of polarization resistance contributions of LSCM nanoelectrode collected varying pO₂ and pH₂, respectively.