Compositional Improvement of Ln[sub 0.435]Ba[sub 0.145]Sr[sub 0.4]Fe[sub 0.8]Co[sub 0.2]O[sub 3−δ] IT-SOFC Cathodes Performance by Multiple Lanthanide Substitution

Abstract

A series of materials based on the perovskite Ln 0.435 Ba 0.145 Sr 0.4 Fe 0.8 Co 0.2 O 3−␦ system ͑Ln = La 1−x−y Pr x Sm y ͒ has been characterized as intermediate temperature solid oxide fuel cell ͑IT-SOFC͒ cathodes. Among the different La, Pr, and Sm combinations, those containing at a time Sm and La or alternatively Pr and La show the lowest polarization resistance values. Within the same substitution degree, praseodymium-based compositions have lower electrode resistance than samarium-based ones. Promising electrode compositions based on different ratios of Pr and La were tested in fully assembled fuel cells, comprising a Ni-yttriastabilized zirconia ͑YSZ͒ anode and a YSZ/gadolinium-doped ceria bilayered electrolyte, and they were compared to a cell with a Pr 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3−␦ cathode. A maximum power density of 0.45 W cm −2 was achieved at 650°C for the most promising La 0.2175 Pr 0.2175 Ba 0.145 Sr 0.4 Fe 0.8 Co 0.2 O 3−␦ cathode-based fully assembled fuel cell. A systematic electrochemical study based on electrochemical impedance spectroscopy is shown for the fully assembled cells as a function of pH 2 , pO 2 , applied current, and operating temperature.High efficient and low emission power source systems are becoming of high interest for future power generation. In that scenario, solid oxide fuel cells ͑SOFCs͒ can play an important role although several issues are still to be solved. New electrolyte compositions or arrangements have been developed to operate at intermediate temperatures ͑500-700°C͒ 1-3 in conventional SOFC configurations but state-of-the-art cathode compositions, i.e., lanthanum-strontium manganite based cathodes, suffer from low activity when the operation temperature is decreased below 750°C. 4-7 Other lanthanidebased compositions having iron or/and cobalt show acceptable performances at intermediate temperatures. [8][9][10][11][12][13] Barium-based cathodes have enough activity and ionic conductivity for 500°C SOFC applications 14 but thermochemical stability could be an added limitation. Related stoichiometries have been tested with a large number of different A-position elements. 15,16 The most effective elements identified were further combined in a single material, aiming to develop multiple-active site cathodes 17,18 by using mixture designs.Aforementioned investigations comprised up to three different elements in the A-position of the A 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3−␦ perovskite cathode system. The relatively low content of cobalt has been chosen to minimize stability issues related to redox and spin transition processes related to cobalt-rich composition, 19 which can lead to abrupt chemical expansion and phase decomposition, whereas very active cathode materials has been reported for materials with small contents of cobalt. 9 From these previous experiments, it has been concluded that relatively small amounts of barium effectively reduce the electrode polarization resistance of multiple-lanthanide-based cathodes. In this work, deeper electrode screening included u...

Electrochemical properties of composite fuel cell cathodes for La5.5WO12−δ proton conducting electrolytes

“…This has been observed for high-temperature proton conductor-based cells employing both thick electrolytes 17 and thin supported electrolytes 28,29 and this behavior is in contrast with the inverse behavior widely reported for conventional (oxide-ion based) SOFC electrolytes. 30,31 This fact is not clearly understood although this may be related to two possible effects: (1) the activation of surface species through an imposed current/DC bias, which typically affects conventional SOFC cathodes while the nature of the partial electrode reactions is different for both types of SOFCs, and (2) the drop in the local concentration of hydrogen in the anode through the dilution with the formed water in conventional SOFC cells which causes a slight decrease in the cell voltage (Nernst potential) whereas this effect does not take place in the PC-SOFC anode.…”

Study of Pr and Pr and Co doped La2NiO4+ as cathodes for La5.5WO11.25− based protonic conducting fuel cells

“…However, when the Co dopant is also introduced in the B position together with the Pr, Ea is slightly reduced (1.44 eV). For this reason the best electrochemical properties are achieved for the LPNC cathode, which behaves somewhat better at low temperatures than the LPN electrode.The better performance of the doped compounds could be linked to the higher total conductivity and the reported catalytic effect of Co and Pr on oxygen reduction electrocatalysts[40,41].The impedance spectra recorded at 750 ºC in wet air for the three cathodes sintered at 1150 ºC are represented inFigure 8aand b (Nyquist and Bode plots, respectively). One or two different contributions (one or two separated arcs) can be distinguished.…”

Redox stability and electrochemical study of nickel doped chromites as anodes for H2/CH4-fueled solid oxide fuel cells

“…The variation of the relaxation frequencies of each process as a function of the electrode composition allows understanding 1 the changes of the different contributions to the total polarization resistance. The comparatively most performing electrode La 0.85 Sr 0.15 Cr 0.9 Ni 0.1 O 3-δ is mostly controlled by low frequencies (LF)processes, presumably ascribed to surface processes[45,46,47]. The worst electrode LaCr 0.9 Ni 0.1 O 3-δ shows(Figure 9) a much less important contribution of LF processes and is principally limited by medium frequencies (MF) processes, which might be related to coupled ion transport thorough the electrode bulk and reaction processes[48,49,50,51], as expected from the limited ionic conductivity of this compound.…”