Anode-supported cells polarisations

Note that there is a potential loss across the electrolyte due to the electrolyte resistance. The potential steps at the Interfaces are now smaller compared to the OCV condition due to the losses originating from the polarisation resistance of the electrode processes. Figure 10.2c Illustrates the case of an anode-supported cell. The potential is given for both the position of the reference electrode where no current flows across the electrolyte (i.e. the electrical potential is constant across the electrolyte), and for a position far away from the electrode edges as in Figure 10.2b. 

Table 10,1 Contributions to ASR for a Rise-type anode-supported cell (Nl-YSZ/YSZ/tSM-YSZ) at 8S0°C tested in a plug flow-type configuration at S and 8S% fuel utilisation (FU). Rehji is calculated using a specific conductivity of YSZ of 0.045 S/cm, Sconnect is an estimation, Rp.eichem is the sum of typical anode and cathode polarisation resistances measured in separate electrode experiments, Rp.aiff is calculated using a diffusion coefficient of 10 cm /s, 30% porosity, a tortuosity factor of 3 and a thickness of 0.1cm, and fip,conver is Calculated using Eq. (10) with i = 0,5 A/cm ...

Figure 10.12 shows a number of ASR values obtained at different temperatures for an anode-supported cell together with values modelled from the available knowledge of the cell components [39]. The measured ASRs have been corrected for fuel utilisation. The electrolyte resistance and the electrode polarisations only approximately follow Arrhenius expressions in reality. The assumed values of activation energies and pre-exponentials are given in the figure caption. For the diffusion resistance, which is the only non-temperature activated term of the considered losses, a conservative estimate is used. 

If the reaction kinetics of the electrode is assumed to be very rapid, mass transfer and ohmic resistance are the dominant resistances. Assuming a reaction zone that coincides with the electrode-electrolyte interface, the diffusion fluxes in stationary operation can be expressed simply in terms of bulk gas partial pressures and gas-phase diffusivities. This is illustrated schematically in Figure 11.8, which compares anode- and cathode-supported cell designs for the simple case of a H2/O2 fuel cell. The decrease in concentration polarisation at the cathode, rjcc- is obvious in the case of an anode-supported cell, while the model shows that concentration polarisation at the anode, tiac is relatively insensitive to anode thickness. The advantage of the mass transfer-based approach is that analytical expressions are obtained for the polarisation behaviour. These are rather simple if activation overpotential is excluded but may still become elaborate in the case of an internally reforming anode where a number of reactions (discussed in Section 11.3) may occur simultaneously within the pores of the anode. 

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,... 

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. 

Most of the discussion in this chapter is centered on cells made with traditional materials such as YSZ electrolyte, Ni + YSZ anode, and LSM + YSZ cathode although its extension to other materials is essentially straightforward. The relative contributions of various polarisations vary widely among the different cell designs anode-supported, cathode-supported, and electrolyte-supported. Ohmic contribution is the smallest in electrode-supported cells due to the thin... 

Electrode-supported cells pass low ohmic polarisation losses as compared to the electrolyte-supported cells. In addition to this, anodic polarisation is quite less than the cathodic polarisation, hence, anode-supported SOFC is the preferred configuration. 

Fig. 16 Predictions of dependence of thermochemical stresses on the temperature gap between stress free temperature (T f) and working temperature, for anode supported LSCM/ 8YSZ/SCMC cells, at 800 °C, in air (thin lines) and for prospective operation of H20,H2/LSCM/8YSZ/ SCMC/air cells with H20 H2 = 1 1, and polarisations = 0.1 V for LSCM and ij, = -0.1 V for SCMC...

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