Cermet anodes polarisation

A power density of 0.37 W/cm at 650°C for an SOFC using a 2 pm Ni-YSZ cermet anode on top of a 0.5 pm functional layer of (Y203)o, 5(Ce02)og5 (YDC) on a 8 pm YSZ electrolyte.It was also shown that the polarisation resistance of the YSZ cermet without the YDC-layer was about 6 times higher. The result was interpreted as a direct electrochemical oxidation of CH4 facilitated by the YDC. This interpretation is in strong contrast to the findings that doped ceria in itself is about inert to direct oxidation of and that Ni-YSZ anodes will be quickly... 

This chapter first considers the complex mix of attributes required of SOFC anodes, including matching of thermal expansion coefficients, chemical compatibility with the electrolyte and the interconnect, porous structure to allow gas permeation, and corrosion resistance to the fuel and impurities therein. Then the nickel cermet anode is described in detail, especially its fabrication processes. Steady-state anode reactions of hydrogen and carbon monoxide are analysed, followed by a description of transient effects. Finally, behaviour under current load and operation on different fuels are discussed. The details of the anode reactions and polarisations are described in Chapter 9. 

The basic concepts of composite or single-phase MIEC electrodes are equally applicable to anodes. Traditionally, however, the typical anode used to date has been a composite mixture of Ni and YSZ. The presence of YSZ not only suppresses the thermally induced coarsening of Ni, but it also introduces MIEC characteristics. Other anodes currently under investigation are based on cermets of copper, which are being explored for direct oxidation of hydrocarbon fuels [39]. These types of anodes are in an early stage of development and thus their polarisation behavior is not discussed here. In so far as single-phase anodes are concerned, some work has been reported in the literature, most notably on La-SrTi03 [40, 41]. Work on this as well as other perovskite-based anodes is in its infancy, and is not elaborated upon further. The discussion in this chapter is confined to Ni + YSZ cermet anodes. 

Fig. 3 Dependence of anode polarisation resistance on cermet thickness for the fine cermet anodes at 1000 °C in 97 % H2 + 3 % H2O at OCV. R1 is electrode impedance, R2 and R3 are gas diffusion and gas conversion outside the anode structure, respectively [19].

This model raises the issue of the effective thickness of the electrochemically active portion of the anode structure. Primdahl and Mogensen [20] found no correlation between polarisation effects and electrode thickness down to 20 pm, and in more recent work [26] a depth of 10 pm for the active zone is sustained. Mathematical modelling [29] is in accord with this experimental evidence (Figure 6.11). Beyond that thickness, the cermet can be regarded as a passive contact layer, and in anode-supported intermediate temperature fuel cells, as also having a structural and mechanical function. It is therefore available as a site for fuel reactions such as reforming. Some studies with this as objective have already been reported, such as the incorporation of ruthenium as catalyst [30],... 

In most SOFCs, the main contribution to rjohm is from the electrolyte, since its (e.g. yttria-stabilised zirconia, YSZ) ionic resistivity is much greater than electronic resistivities of the cathode (e.g. Sr-doped LaMnOs, LSM), and the anode (e.g. Ni + YSZ cermet). For example, the ionic resistivity of YSZ at 800°C is 50 J2cra. By contrast, electronic resistivity of LSM is 10 Qcm and that of the Ni + YSZ cermet is on the order of 10 S2cm. Thus, the electrolyte contribution to ohmic polarisation can be large, especially in thick electrolyte-supported cells. The recent move towards electrode-supported cells, in which electrolyte is a thin film of 5 to 30 microns, reduces the ohmic polarisation. Also, the use of higher conductivity electrolyte materials such as doped ceria and lanthanum gallate lowers the ohmic polarisation. 

The best known type of SOFC anode-supported cells consists of a Ni/Zirconia cermet support layer, a thin functional anode, a thin dense yttria doped zirconia electrolyte layer and a thin Strontium doped lanthanum manganite (LSM) cathode layer (material alternatives are given in Fig. 8). Such cells have proved to be stable and durable for thousands of operation hours when operated at relatively mild conditions [9]. The degradation rate increases with increasing cell polarisation and increasing current density. The polarisation-dependent degradation has been identified to mainly originate from the cathode/electrolyte interface. However,... 

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