Infiltration, Overpotential and Ageing Effects on Cathodes for Solid Oxide Fuel Cells: La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δversus Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ

Abstract

The two half-cell systems were compared to evaluate the influence of infiltration, overpotential and ageing on their performance and stability. Structure, microstructure and chemical composition were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with Energy Dispersive X-ray spectroscopy (EDX), whereas redox behavior was evaluated through temperature programmed reduction (TPR) and thermal gravimetric analysis (TGA) in combination with electrochemical impedance spectroscopy (EIS), which was also used to test the electrochemical performance. A decrease of polarization resistance for both infiltrated electrodes, compared to the reference ones, was highlighted at open circuit voltage (OCV), although the BSCF-based cathode was more benefitted more from infiltration than the LSCF-based one. Moreover, the application of cathodic overpotentials (−0.1 to −0.3 V) resulted in opposite effects on the LSCF-and BSCF-based cathodes, highlighting a different response of the oxygen vacancies to the applied voltage for the two perovskite compositions. The positive effect of the LSM layer was further confirmed by the observed improvement in long-term stability of both infiltrated perovskite-type systems. 1 They exhibit mixed ionic and electronic conductivity, with excellent charge and mass transfer properties 2 and high oxygen exchange surface coefficients. 3 Their excellent activity for oxygen reduction has been proved widely. [4][5][6][7][8][9][10] Although the structure and electrochemical processes of LSCF and BSCF are quite different, both of them have long-term stability and redox behavior issues to be clarified, in order to be used as cathode materials in commercial devices. Indeed, when used as cathodes in intermediate-temperatures solid oxide fuel cells (IT-SOFCs), they suffer from chemical and structural instability, which causes degradation during operation time. Moreover, the correlation between their redox behavior under electrochemical operating conditions is still unclear. Many studies have been devoted to investigating LSCF degradation and several causes have been reported, among them: i) mutual cations diffusion between interfaces with consequent atoms depletion and phase separation, [11][12][13][14][15][16] ii) LSCF grain coarsening, 17 iii) reactivity with YSZ-based electrolytes, even with ceria-based barrier layer, 18 iv) impurity contamination, namely Cr from metal interconnects and B from sealing when the cell is inside a stack.19-21 Some recent work performed on an LSCF/CGO (Ce 0.9 Gd 0.1 O 2-δ ) composite cathode on a YSZ electrolyte with a CGO barrier layer 11 affirms that Sr depletion, phase separation and cations inter-diffusion occur mainly during the fabrication process, with negligible contributions during long-term * Electrochemical Society Member.e Present Address: R&D Manufacturing Support Dept., Ansaldo Energia, Via Nicola Lorenzi 8, I-16152 Genoa. z E-mail: carpanese@unige.it operation (973 K). On the other hand, some previous works conversely found that LS...

Sucrose-Assisted Solution Combustion Synthesis of Doped Strontium Ferrate Perovskite-Type Electrocatalysts: Primary Role of the Secondary Fuel

“…The use of the two fuels allowed achieving high reducing power and high interaction of the fuel mixture with metal cations, favoring the stabilization of Co 2+ reduced state in Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ structure. The obtained oxygen vacancy-rich configuration positively affected oxygen mobility and cathodic performance [22,24].…”

“…It has been found that the variation of its molecular weight (MW) can modulate the microstructure and porosity development of the produced material [16,[18][19][20].The mixed-fuels approach in SCS was demonstrated to positively affect the electrocatalytic performance of perovskite-type materials in the oxygen reduction up to an order of magnitude [5,[21][22][23]. PEG (MW = 20,000) was already employed in the compresence of sucrose in two previous papers dealing with Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ powders as cathodes for solid oxide fuel cells prepared by SCS [22,24]. Optimal features and electrochemical properties were reached by a mixture of sucrose and PEG in a 1:2.5 molar ratio [22].…”

“…The use of the two fuels allowed achieving high reducing power and high interaction of the fuel mixture with metal cations, favoring the stabilization of Co 2+ reduced state in Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ structure. The obtained oxygen vacancy-rich configuration positively affected oxygen mobility and cathodic performance [22,24].Perovskite-type compounds with ABO 3 structure are a well-known class of functional materials with a wide range of applicability in catalysis, electronics, optics, piezoelectrics, and other technological fields [25][26][27]. The perovskite structure formation depends on the relative A-site and B-site cation radius according to the Goldsmith tolerance factor, which is the most widely recognized criterion [28].…”

Distribution of Relaxation Times and Equivalent Circuits Analysis of Ba0.5Sr0.5Co0.8Fe0.2O3−δ

“…After calcination at 950 • C, the abovementioned mixture was completely converted into the cubic (Pm-3m) perovskite phase (ICDD, 109462). More details are reported elsewhere [28,30].…”

Utilisation of methylcellulose as a shaping agent in the fabrication of Ba0.95Ca0.05Ce0.9Y0.1O3 proton-conducting ceramic membranes via the gelcasting method

“…The capacitances obtained for the MF and LF semicircles belong to the range 0.42-0.63 mF cm -2 and 85-302 mF cm -2 , respectively. These values indicate that MF and LF semicircles represent oxygen reduction reaction processes [40,41]. The polarisation resistance R p was calculated as the sum of R 2 and R 3 .…”