Polarisation losses

As a result of the transferred species, loss mechanisms occur. In terms of the first law of thermodynamics these losses are well known as polarisation losses. Polarisation losses are sensitively influenced by numerous mechanisms, which are strongly non-linear with respect to a change of the operational parameters like the current density, electrical potentials, temperature, pressure, gas compositions and material properties. These parameters are assumed to be constant in case of a differential cell area. Thus, the loss mechanisms are summarised in a constant area specific resistance ASR [ 2cm2]. A change of the local overpotential (EN(Uf) — Vceii) at constant ASR complies with a proportional change in the local current density. 

As Manassen et al. (1981) pointed out, the configurations represented in Fig. 10.5 have another disadvantage, namely the disparity between the small surface area desired to minimise the dark current in the storage electrode or half-cell, since this opposes and reduces the photocurrent, and the large surface area necessary to minimise storage polarisation losses and maximise storage capacity. 

The wiggler at the Japanese Photon Factory (a 2.5 GeV machine, table 4.1) is also a three-pole device with a centre field of 6T and Ac=0.5 A. This wiggler oscillates the electrons vertically and so therefore the radiation emitted has a vertical plane of polarisation. This is a useful attribute for diffraction measurements since in this case high angle reflections can be measured in the horizontal plane without significant polarisation losses (chapter 5), which simplifies the mechanical mounting of diffractometers. 

When the current is decreased, the combined effect of the changes in the partial pressures and the polarisation losses results in the increase of the cell voltage. It is observed that the cell voltage initially overshoots before settling to a steady-state value. The FU and the OU, which are proportional to the current, also decrease. The reverse phenomena are observed with the increase in the external load current. The sudden decrease in the load current results in the decrease in the rate of hydrogen and oxygen consumption and the rate of water vapour formation. In other words, the... 

The decrease in the current also results in the decrease in the polarisation losses and the decrease in the reaction entropy flow rate (5r) (due to reduced mass flow rates) and thereby results in the fall of the system temperature. The reverse phenomena are observed (at time 2000 s in Fig. 10.8) when the load current density is increased. 

The supported type allows the use of very thin layers but is subject to additional polarisation losses arising from restricted mass transfer through the porous substrate. On the other hand the self supported design requires thicker component layers and thus material costs are higher and electrolyte resistance larger. 

Some general comments were made in section 2(d) relating to polarisation losses at electrodes in prototype multi-cell devices. A more detailed analysis of the kinetics associated with platinum electrodes is now presented, as nearly all the published data on experimental high temperature electrolysers refers to systems incorporating platinum electrodes. This type of electrode is also often employed in commercial high temperature oxygen monitors although of course platinum electrodes would not be used in commercial fuel cells and electrolysers. 

At low operation temperatures, polarisation losses and the importance of catalysis of the electrode reactions Increase. At the cathode, mixed potentials can arise when traces of combustible substances determine the electrode potential in competition with oxygen, an effect, mentioned near the end of Section 2.3, whose cause was recognised by Hartung in 1981 [143], today the basis of the development of hydrocarbon sensors. For the anodes growing interest is directed to materials which accelerate the electrochemical oxidation of CO and hydrocarbons and is stable against fuel impurities. 

To minimise reactions between the cathode and the electrolyte, in Japan, most research efforts have focused on the A-site-deficient lanthanum manganite. In the USA and Europe, however, efforts [50] have been made to seek alternative cathodes, but with only limited success. Perhaps the most significant finding has been the use of composite cathodes in contact with the YSZ electrolyte. These composite cathodes minimise cathode/electrolyte interaction by mixing LSM and YSZ powders and laying down a thin layer of this mixture on the electrolyte [10]. Another step forward has been the use of an activation process to reduce the polarisation loss at the electrode [51]. 

In the case of high-temperature fuel cell anodes, conflicting evidence has been presented on the number and significance of the dissipative mechanisms detected as contributing to polarisation losses by impedance spectroscopy. At least three features may be distinguishable, dependent on the particular anode structure and the experimental conditions [17,19], This variability of the impedance spectra is... 

Electrochemical performance Full open circuit voltage Lott polarisation loss Insignificant gas leakage or cross-leakage (no or minimal sealing) No electrical short Uniform gas distribution between cells and across cell Easy gas access to reaction sites... 

Transport of gaseous species usually occurs by binary diffusion, where the effective binary diffusivity is a function of the fundamental binary difiiisivity -HaO. and microstructural parameters of the anode [3, 4]. In electrode microstructures with very small pore sizes, the possible effects of Knudsen diffusion, adsorption/desorption and surface diffusion may also be present. The physical resistance to the transport of gaseous species through the anode at a given current density is reflected as an electrical voltage loss . This polarisation loss is known as concentration polarisation, and is a function of several parameters, given as... 

This latter reaction scheme does not depend upon the adsorption of fuel gas, while the former one does. The implication is that anodic activation polarisation would be independent of what the fuel is in the latter scheme, while it would be a function of the type of fuel in the former case. Recent work has shown that the total polarisation loss with CO as a fuel is much greater than that with H2 as the fuel, and the difference cannot be attributed to differences in concentration polarisation [42], It is possible that the differences may be due to differences in the adsorption characteristics of H2 and CO. Thus, the preliminary conclusion is that adsorption of fuel gas must be an important step. 

An electrode model is especially advantageous if it can be used to relate the kinetic and mass transfer resistance to electrode geometry and microstructure for instance, to thickness, porosity, pore or particle size, contact areas of phases, and/or grain size of electrode and electrolyte materials. A well-tested and validated electrode model, therefore, may serve to assist in the design of optimised electrode structures or electrode/electrolyte interfaces to minimise polarisation loss. 

Figure 2.4 indicates the polarisation losses for PAFC with anode and cathode loading of 0.5 mg Pt/cm, pressure 1 atm, 100% phosphoric acid and operating temperature of 180°C. The anode losses are very small on pure Ha, whereas the cathode polarisation is greater with air and comparatively small for pure oxygen. As discussed, it is known that an increase in cell pressure enhances the performance of the cell (Eq. 2.72). 

At higher pressure and current density, the diffusion polarisation at the cathode decreases and reversible cell potential increases. At the cathode, increased oxygen and water pressures decrease activation polarisation. As shown in Fig. 2.5, an increase of 44 mV is observed when pressure and temperature are increased by 2.9 atm and 15°C, respectively. An increase in temperature exhibits a beneficial effect on cell performance because the polarisation losses diminish with increased temperature. The relationship between voltage gain and temperature change is given by ... 

Significant polarisation losses occur in MCFC and its operating conditions are almost same as those of PAFC. For an MCFC, the anode and cathode compartments are at the same pressure, and the change in reversible voltage can be given by... 

Minimum Polarisation Losses Usually polarisation losses in SOFC cause a major drop of open circuit voltage. Hence, the stack should be designed in a way to minimise any polarisation losses, gas leakage, short-circuiting, and cross-leakage of gases. 

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. 

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