This study gives evidence that the time-dependent performance changes in anode supported cells for intermediate-temperature solid oxide fuel cells is essentially influenced by the mixed ionic-electronic conducting ͑MIEC͒ cathode. The impedance spectra recorded during 700 h of operation at 750°C were interpreted using an appropriate equivalent circuit model by ͑i͒ a distribution of relaxation time analysis followed by ͑ii͒ a complex nonlinear least squares fit. Four electrode polarization processes were separated by selective experimental parameters. The cathodic part, initially the smallest, is only discovered among the anodic contributions by a change in fuel gas composition from H 2 -H 2 O to CO-CO 2 and increases by 310% ͑15 m⍀ cm 2 at 11 h, 62 m⍀ cm 2 at 700 h͒. A Sr ͑and Co͒ depletion of the MIEC cathode composition La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3−␦ possibly caused this degradation. The anodic polarization has a proportion of 92% at the start and decreases to 73% ͑168 m⍀ cm 2 at 11 h, 173 m⍀ cm 2 at 700 h͒. The anode charge-transfer reaction initially causes 60% of the total polarization losses and 50% after 700 h. This is assigned to a change in the triple phase boundary and/or a degradation in ionic conductivity in the anode functional layer. The gas diffusion polarization remains constant at 58 m⍀ cm 2 . La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3−␦ ͑LSCF͒ as a mixed ionic and electronic conductor ͑MIEC͒ was reported as one of the best performing cathode materials for intermediate-temperature solid oxide fuel cells ͑IT-SOFC͒. Preliminary parameter studies concluded that the degradation rate for anode supported cells ͑ASCs͒ with composite La 0.65 Sr 0.3 MnO 3−␦ ͑LSM͒-yttria-stabilized zirconia ͑YSZ͒ cathode is smaller ͑⌬V/V = 0.5%/1000 h at 800°C͒ 1 compared to ASCs with LSCF cathodes ͑⌬V/V = 0.9-1.5%/1000 h at 800°C͒. [2][3][4] Our long-term study focuses on the LSCF cathode degradation course in a state-of-the-art ASC. Considering that, the contributions of ohmic and polarization losses of electrolyte, anode, and cathode have to be identified by high resolution impedance studies based on a combination of distribution of relaxation times ͑DRT͒ analysis followed by a complex nonlinear least squares ͑CNLS͒ fitting approach. This approach, in combination with a recently developed equivalent circuit model, supports a detailed examination of polarization losses in ASCs. 5 Thus, a time-resolved identification of each single polarization mechanism clarifies the share of the LSCF cathode.The importance of electrochemical impedance spectroscopy ͑EIS͒ for the ongoing development of SOFCs was recently summarized by Jensen et al. 6 This contribution presents different methods such as ͑i͒ the differential impedance analysis ͑DIA͒, 7 ͑ii͒ the deconvolution of impedance spectra into DRT first introduced by Schichlein 8 and applied in Ref. 5 and 9, and ͑iii͒ the analysis of difference in impedance spectra ͑ADIS͒ introduced by Hagen et al., 10 Barfod et al., 11 and Jensen et al. 12 The ADIS method analyzes the difference between p...
The time-related change of anode supported single cell performance for intermediate temperature SOFC was evaluated continuously up to 700 hours of operation. For the first time, cathodic and anodic polarization losses were separated by high resolution impedance studies based on DRT analysis (distribution of relaxation times) followed by a CNLS fitting approach. The cathodic polarization resistance (15mΩcm2 at t = 11 h), by far the smallest part, could only be visualized in the relevant frequency range by well selected and adapted experimental parameters. Within 700h the cathode resistance increases by 400% (63mΩcm2 at t = 700 h) undergoing a nonlinear degradation rate which can not explained by microstructural changes but probably by compositional changes.
i) Oxygen Surface Exchange k δ and (ii) Bulk Diffusion Coefficients D δ of a porous La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathode are determined by electrochemical impedance measurements on anode supported solid oxide fuel cells. The cathode polarization resistance of anode supported cells was separated by the method of distribution of relaxation times (DRT) followed by a complex nonlinear least square (CNLS) fitting approach using a reasonable equivalent circuit consisting of four physical related elements. The values for k δ and D δ were determined for cathodes fresh from the start of the cell operation up to 700h and compared to literature data on bulk samples. At an operation temperature of T = 750°C the cathode polarization resistance changed substantially with time and ended with an overall increase of 310%. The analysis of the cathode polarization resistance which was described by a Gerischer element revealed a rather constant value for k δ but a distinct decrease for D δ .
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