LSM electrodes

The surface segregation of Sr is of particular interest as SrO affects the surface reactivity and the activation behavior of the LSM electrode. Jiang and Love [36] studied the activation behavior of Lao72Sr018Mn03 cathode after treatment of the LSM coating with diluted hydrochloric acid (HC1) solution. The etched solution... 

FIGURE 3.2 Cyclic voltammograms of an LSM electrode as a function of reverse potential at 900°C and oxygen partial pressure of 100 Pa. The scan rate is 200 mVs-1. (From Chen, X, J. et al., Electrochem. Solid-State Lett., 7 A144-A147, 2004. With permission.)... 

The impedance polarization performance of LSM electrode is closely related to the mechanism and kinetics of the oxygen reduction reactions. 02 reduction at SOFC cathodes is the most heavily studied subject, and this subject is sufficiently broad and complex to warrant its own review. Interested readers should consult the recent excellent articles by Adler [1] and Fleig [55], Here, only the polarization performance and its influencing factors are discussed. 

Figure 3.5 [36], For the 02 reduction reaction on freshly prepared LSM electrodes, the initial polarization losses are very high and decrease significantly with the cathodic polarization/current passage (see Figure 3.5b). Consistent with the polarization potential, the impedance responses at open circuit decrease rapidly with the application of the cathodic current passage. For example, the initial electrode polarization resistance, RE, is 6.2 Qcm2 and after cathodic current treatment for 15 min RK is reduced to 0.7 Qcm2 see Figure 3.5 (a). The reduction in the electrode polarization resistance is substantial. The analysis of the impedance responses as a function of the cathodic current passage indicates that the effect of the cathodic polarization is primarily on the reduction in the low-frequency impedance [10]. Such activation effect of cathodic polarization/current on the electrochemical activity of the cathodes was also reported on LSM/YSZ composite electrodes [56-58], Nevertheless, the magnitude of the activation effect on the composite electrodes is relatively small.

FIGURE 3.8 Impedance curves of pure LSM, 0.8 mgcm-2 GDC impregnated LSM and 5.8 mgcm-2 GDC impregnated LSM electrodes measured at 700°C in air. The impedance curves were measured after cathodic current treatment at 200 mAcm-2 for 120 min at the same temperature. 

Figure 3.16 shows typical SEM micrographs of the YSZ electrolyte surface in contact with an LSM cathode in the presence of a Fe-Cr ally at 900°C after cathodic polarization at 200 mAcnr2 for different periods [185], The LSM electrode coating... 

Figure 38. Schematic and geometry of patterned thin-film LSM electrode on YSZ studied by Horita and coworkers. After establishing steady-state cathodic polarization, the atmosphere surrounding the electrode is rapidly switched from 02- to 02-rich for 10 min at fixed total Por The sample is then quenched to room temperature and postmortem analyzed using secondary-ion mass spectrometry (SIMS) imaging. (Reprinted with permission from ref 230. Copyright 2000 Elsevier.)...

Figure 41. Linear-sweep voltammagrams of a porous LSM electrode on YSZ in air at 950 °C as a function of sweep rate. (Reprinted with permission from ref 233. Copyright 1998 The Electrochemical Society, Inc.)...

Figure 42. Impedance characteristics of porous LSM electrodes on YSZ, measured at zero bias and 945 °C in air, as a function of polarization history and processing conditions. U-11, U-12, and U-13 correspond to firing temperatures of 1100, 1200, and 1300 °C, respectively. Bold line initial impedance. Thin line impedance measured 2 min following cathodic polarization at 100 mA/cm for 30— 90 min. Dashed line impedance measured 30 min following cathodic polarization. (Reprinted with permission from ref 209. Copyright 1997 The Electrochemical Society, Inc.)...

A typical impedance spectrum obtained on LSM microelectrodes is shown in Fig. 42a. The arc represents the impedance due to the electrochemical reaction at the LSM microelectrode. A small ohmic drop caused by the YSZ electrolyte (and partly by the sheet resistance due to the finite electronic conductivity of the LSM electrode) is more than three orders of magnitude smaller than the electrode resistance and not visible in the figure. The impedance spectra for nominally identical microelectrodes turned out to be reproducible with a standard deviation <15%. The data of Fig. 42b display the relation between the electrode resistance Rei and the microelectrode diameter dme several series of experiments with different electrode thicknesses consistently revealed that the resistance Rei is approximately proportional to dmc 2. and hence to the inverse electrode area. 

From this observation it can be concluded that the rate-determining process directly involves the electrode area, i.e. occurs i) at the surface of the LSM ii) in the bulk of the thin LSM electrodes or iii) at the LSM/YSZ interface. From thickness-dependent measurements further information with respect to the rate-determining step could be expected, since a predominant bulk path with transport of oxide ions through LSM being rate-limiting should yield Rei oc tme (tme = microelectrode thickness). Hence, a sample with 60 pm microelectrodes of two different thicknesses... 

Cheng et al. (2008) reported LaCoOs nanopowders for intermediate temperature solid oxide fuel cells prepared by an aqueous gel-casting technique at 600 °C. The performance of La-Sr-Mn-O (LSM) electrode impregnated with as-synthesized LaCoOs nanopowders showed a significant improvement. 

When YSZ and LSM are assembled in a cell, the LSM electrode is porous, while the YSZ is dense. The major part of the oxygen ions that enter the electrolyte vacancies comes from reactions of dissociation and ionization of oxygen molecules at the Triple Phase Boundary (TPB), shown schematically in Figure 15-3. As its name indicates, three phases are present... 

The efficiency of the oxygen generator or SOFC depends on its microstructure. The LSM electrode material has to be porous and to have a small particle size to optimize the active surface area at the TPB. At the same time, the YSZ electrolyte has to be dense and air tight. Both have to reach their final microstmcture in one final heat treatment at 1200°C in order to produce the working device. Sol-gel processing is the key to assembling the consecutive layers. 

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