LSM-based cathode

The initial polarization behavior for the oxygen reduction on LSM-based cathodes is characterized by a well-known activation phenomenon, as shown in... 

Various strategies have been developed to improve the electrocatalytic activities of the LSM-based cathodes. Murray and Barnett [68, 69] showed that the addition of YSZ and gadolinia-doped ceria (GDC) phase to LSM significantly reduced the electrode polarization resistance. Figure 3.7 shows the electrode polarization resistance of the LSM/GDC tested in the air [69], The electrode polarization resistance... 

Perovskite related cathode materials have been shown to possess levels of ionic and electronic conductivity comparable to existing perovskite cathode materials. The development of these new cathodes is in the very early stages and significant research is required before these can compete with the established Lai.xSrxCoOs.g (LSC) and Lai.xSrxMnOg.g (LSM) based cathodes. However, one of the main advantages offered by these materials is the prospect of fast surface exchange and ionic diffusion at temperatures considerably lower than the established candidate materials thus enabling development of low temperature SOFC devices. 

In combination with a LSM-based cathode, a Ni-YSZ anode and YSZ electrolyte appeared blackened after sintering whereas cells with a LSCF cathode did not exhibit such a color change [73]. The black coloration was attributed to the diffusion of manganese from the cathode to the anode through the electrolyte. Chemical interaction between closely spaced coplanar anodes and cathodes could affect the cell performance and material compatibility studies could facilitate the selection of suitable ceU component materials. 

To improve the electrocatalytic activities of the LSM-based cathodes, ion conducting secondary phases such as GDC and YSZ are introduced to enhance the ionic conductivity. The electrode s polarisation resistance Re is reduced considerably when YSZ is added to LSM because YSZ reduces grain growth of LSM and enhances the triple phase boundary regions. Figure 3.15 shows the AFM images of the YSZ surface obtained after removal of LSM using HCl treatment. 

In addition to being able to catalyze the dissociation of O2. the material used for the cathode must be electronically conductive in the presence of air at high temperature, a property found primarily in noble metals and electronically conductive oxides. Ionic conductivity is also desirable for extending the reaction zone well into the electrode since the ions must ultimately be transferred to the electrolyte. Since precious metals are prohibitively expensive when used in quantities sufficient for providing electronic conductivity, essentially all SOFC prototypes use perovskite-based cathodes, with the most common material being a Sr-doped LaMnOs (LSM). In most cases, the cathode is a composite of the electronically conductive ceramic and an ionically conductive oxide, often the same material used in the electrolyte. 

In order not to be lost in engineering complexity, and to remain within the scope of the text we will consider only a few prototype materials and discuss them in the context of problems of principal interest. At present the standard SOFC works at 850-1000°C and is based on Y203-doped Zr02 (YSZ) as electrolyte, SrO-doped LaMn03 (LSM) as cathode, and a two phase mixture of Ni and YSZ as anode. Before we deal with the electrodes, let us consider first the zirconia electrolyte and then have a look at alternative electrolyte materials. 

Regarding the development of LSM based composite cathodes, several works have reported the preparation of LSM-SDC cathodes from the powders obtained by different synthesis methods (Ye et al., 2007 Chen et al., 2007 Xu et al., 2009). However, no reports on the... 

The current state-of-the-art SOFC anode-supported cells based on doped zircona ceramic electrolytes, ceramic LSM cathodes, and Ni/YSZ cermet anodes are operated in the temperature range 700-800°C with a cell area specific resistance (ASR) of about 0.5 O/cm at 750°C. Using the more active ceramic lanthanum strontium cobalt ferrite (LSFC)-based cathodes, the ASR is decreased to about 0.25 Q/cm at this temperature, which is a more favorable value regarding overall stack power density and cost-effectiveness. 

For intermediate temperature SOFCs, a composite cathode consisting of strontium-doped lanthanum manganite (LSM) and YSZ has shown good performance [10]. For use with ceria-based electrolytes [11], a (La,Sr)(Co,Fe)03 (LSCF)-based cathode has been developed [12],... 

Fig. 12 Le/i Effect of the flow configuration and methane conversion fraction (PR) on the stress. Case of an anode-supported cell with LSM-YSZ cathode and compressive gaskets, a Temperature profile and b First principal stress in the anode. The MIC is displayed in transparency, c First principal stress in the cathode (insert alxtve the symmetry line), d Contact pressure on the cathode GDL and compressive gasket and e vertical displacement along the z-axis, with an amplification factor of 2,000. Right column effect of creep in a cell based on a LSCF cathode and a temperature distribution, on b the evolution of the first principal stress in the anode support in operation and c during thermal cycling to RT and d evolution of the first principal stress in the GDC compatibility layer after thermal cycling. The profiles above and below the symmetry axis refer to different operation time [88, 89]. Reproduced here with kind permission from Elsevier 2012...

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