Electrode Polarisations 2 Ohmic Polarisation

A first parameter to be studied is the applied potential difference between anode and cathode. This potential is not necessarily equal to the actual potential difference between the electrodes because ohmic drop contributions decrease the tension applied between the electrodes. Examples are anode polarisation, tension failure, IR-drop or ohmic-drop effects of the electrolyte solution and the specific electrical resistance of the fibres and yarns. This means that relatively high potential differences should be applied (a few volts) in order to obtain an optimal potential difference over the anode and cathode. Figure 11.6 shows the evolution of the measured electrical current between anode and cathode as a function of time for several applied potential differences in three electrolyte solutions. It can be seen that for applied potential differences of less than 6V, an increase in the electrical current is detected for potentials great than 6-8 V, first an increase, followed by a decrease, is observed. The increase in current at low applied potentials (<6V) is caused by the electrodeposition of Ni(II) at the fibre surface, resulting in an increase of its conductive properties therefore more electrical current can pass the cable per time unit. After approximately 15 min, it reaches a constant value at that moment, the surface is fully covered (confirmed with X-ray photo/electron spectroscopy (XPS) analysis) with Ni. Further deposition continues but no longer affects the conductive properties of the deposited layer. 

When stripping current is varied, stripping curves are shifted because ohmic drop and electrode polarisation vary with current density. Thus if the cut-off voltage is fixed, errors will be introduced depending on the steepness of the transition. Electrode polarisation can indeed become important as exchange current between 0.6 and 7.8 mA/cm with a 200 mV tension variation per decade of current has been measured (35) for the Ag-Agi Rbl interface. 

To,.eliminate ohmic drop and electrode polarisation the use of pulse read-out has met with some success (36). 

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. 

Electrode polarisations cause large voltage losses in SOFCs and need to be reduced to low levels for increased efficiency. The three polarisations described in this chapter are ohmic polarisation, concentration polarisation, and activation polarisation. The ohmic contribution stems from resistance to electron and ion flows in the materials, and is generally dominated by the electrolyte resistance, with the consequence that SOFCs employing thick (>100 micron) YSZ as electrolj te have high ohmic losses at temperatures below about 900 C. Now that thinner electrolytes are being used in electrode-supported cells, this resistance has dropped and it is possible to use YSZ down to about 700°C. 

ASR may be divided into ohmic resistance, R, and electrode polarisation resistance, Rp. The ohmic resistance originates from the electrolyte, the electrodes materials and the current collection arrangement. This is very much dependent on geometric factors such as thickness of the cell components and the detailed geometry of the contact between current collection and electrodes, and between electrodes and electrolyte as current constrictions may be important [41]. The electrode polarisation resistance is further divided into contributions from the various rate-limiting steps. Thus, ASR can be broken down in five terms ... 

To predict the local polarisation in a full-scale cell or stack at any point, its dependence on composition, pressure, and temperature of the gas flowing in the gas channel contacting the electrode must be known. In a large cell, these bulk gas properties vary from one point to the next. Electrode polarisation or overpotential - the difference between the local potential of the electrode under load and the potential at open circuit (equilibrium potential) - is also a local quantity because it depends not only on the bulk gas composition but also on the current density. In a large cell the current is usually distributed nonuniformly, as discussed in Sections 11.2-11.5. Similar to Eq. 7, one can express the local cell voltage under load, i.e., when current is passed, as the thermodynamic cell potential minus three loss terms the ohmic loss, the cathode polarisation, and the anode polarisation ... 

The major loss of fuel cell voltage is due to the ohmic and electrode polarisations. Eflfoits are made to reduce the polarisations, so that V approaches E and hence maximum efficiency is obtained. This can be done by improving the electrode stmcffire, using highly conducting electrol5die and better electrocatalysts. Hence, the actual and ideal potentials for a fuel cell are different due to various losses. 

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. 

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

Fig. 15.5 Schematic cross section of a typical set-up for performing electrochemical experiments under ultrathin electrolyte layers. The working electrode is made of a cylinder that is, together with the counter electrode ring positioned around it, embedded in an inert and insulating polymer, e.g. in epoxy. The Kelvin probe tip is positioned over the centre of exposed surface of the working electrode. This way Ohmic drops during polarisation can be minimised...

The difference E — E(i) is a measure of the change in gas phase compositions just outside of the electrodes. This difference must be accounted for in the overall description of cell performance. The voltage loss term (i) is known as the polarisation or overpotential, and is a function of current density it consists of a number of terms, with their origins related to various phenomena occurring in the cell, under a finite current. The different polarisations are termed (a) ohmic... 

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... 

As realised from the above issues in the comparison of test results on the electrodes and on the cells, it is a non-trivial task to break down the total loss measured on a single cell into its components using the results from the electrode studies. Impedance spectroscopy on practical cells is, however, a technique by which a partial break down can be made. Though the impedance spectra obtained in general are difficult to interpret due to the many processes involved, the spectra can at least provide a break down of the total loss into an ohmic resistance (Rj = Rgiyt + Rconnect) and a polarisation resistance reflecting losses due to chemical, electrochemical, and transport processes, as described in more detail in Chapter 9. 

In the 2-D cell simulation, as well as in simplified (quasi-2-D) stack-level simulations, it is usually assumed that each side of the electrode/interconnect is at equal potential over the whole 2-D plane of the cell. As mentioned above, this is justified because the ohmic voltage drop in the plane of the electrodes and interconnect layer is usually much smaller than the ohmic voltage drop across the electrolyte and the combined polarisation of the two electrodes. Nevertheless, in such a quasi-2-D stack model, individual fuel cells in the stack may have different cell voltages due to different temperature, fuel distribution, and other factors. However, the total current flow through each cell (integrated over the plane of the PEN elements and the gas flows) must be the same. The total stack output voltage is the sum of each individual cell voltage. 

Electrode-level models describe the performance of SOFC electrodes in detail. They take into account the distribution of species concentrations, electric potential, current, and even temperature in the electrode. Their purpose is to (i) interpret the performance (polarisation curve) of electrodes in terms of rate-limiting resistances such as kinetic (activation), mass transfer, and ohmic resistance and (ii) predict the local polarisation in full-scale cell and stack models. 

The objective of an electrode model is to analyse the point-to-point distribution of the reaction in an SOFC electrode, leading to current, potential, and species concentration distributions. The result of the analysis is a prediction of the polarisation of the electrode due to (i) kinetic resistance, (ii) mass transfer resistance, and (iii) ohmic resistance. 

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. 

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