Interfacial resistance

Condensation of pure vapors under laminar conditions in the presence of noncondensable gases, interfacial resistance, superheating, variable properties, and diffusion has been analyzed by Minkowycz and Sparrow [Int. ]. Heat Ma.s.s Tran.sfer, 9, 1125 (1966)]. 

Amauroux, N, Petit, J. and Leger, L., Role of interfacial resistance to shear stress on adhesive peel strength. Langmuir, 17(21), 6510-6517 (2001). 

Each of these processes is characterised by a transference of material across an interface. Because no material accumulates there, the rate of transfer on each side of the interface must be the same, and therefore the concentration gradients automatically adjust themselves so that they are proportional to the resistance to transfer in the particular phase. In addition, if there is no resistance to transfer at the interface, the concentrations on each side will be related to each other by the phase equilibrium relationship. Whilst the existence or otherwise of a resistance to transfer at the phase boundary is the subject of conflicting views"8 , it appears likely that any resistance is not high, except in the case of crystallisation, and in the following discussion equilibrium between the phases will be assumed to exist at the interface. Interfacial resistance may occur, however, if a surfactant is present as it may accumulate at the interface (Section 10.5.5). 

Goodridue, F. and Bricknell, D.J. Trans. Inst. Chem. Eng. 40 (1962) 54. Interfacial resistance in the carbon dioxide-water system. 

One of the fundamental requirements for a high performance anode is to have excellent catalytic activity toward the electrochemical oxidation of the fuel (e.g., hydrogen). This is reflected as low anode polarization or interfacial resistance. This area has seen intensive research for quite some time and is covered very well by the reviews of McEvoy [2], Zhu and Deevi [3], and Jiang and Chan [4], In this section, the focus will still be on revealing the influences of processing and testing parameters on the obtained anode electrochemical performance. 

Like electrical conductivity, the anode composition (i.e., Ni to YSZ volume ratio) also influences the anode activity or polarization. The lowest anode interfacial resistance is usually obtained when the Ni to YSZ volume ratio is 40 60. For example, Kawada et al. [42] found that anode interfacial resistance reached a minimum when the Ni content was 40 vol%, as shown in Figure 2.12. This was verified by several other independent studies [25, 31, 43, 44], For example, Koide [25] found that the... 

FIGURE 2.13 (a) Maximum power and (b) cell total ohmic resistance (labeled as IR resistance ) and interfacial resistance (labeled as polarization ) at constant current density of 0.3 A/cm2 versus the volume percent of Ni in the Ni-YSZ cermet for electrolyte-supported cells with an active area of 2 cm2 operated at 1000°C. (From Koide, H. et al., Solid State Ionics, 132 253-260, 2000. Copyright by Elsevier, reproduced with permission.)... 

For electrolyte-supported cells, many studies indicate that anode resistance decreases significantly as anodic current passes through the anode. For example, van Herle et al. [55] found that anode resistance decreased dramatically from 2.4 to 0.5 and to 0.1 fl when the cell current increased from 0 to 95 and then to 567 mA (Figure 2.20). Similarly, Primdahl and Mogensen [39] studied the effect of anode overpotential on the anode interfacial conductance and found that the anode interfacial resistance decreased significantly as the anode overpotential increased, which was also verified by Jiang and Badwal [43],... 

However, now there is still the uncertainty as to the relative increase in anode/ electrolyte interfacial resistance under different current densities. Primdahl and Mogensen [39] found that the relative increase in anode interfacial resistance due to sulfur poisoning is independent of temperature and cell current density (up to 100 mA/cm2) when the anode was subject to 35 ppm H2S at 1000°C. Whether this is also the case when the cell temperature is lower (i.e., at 750°C), the H2S concentration is lower (i.e., 1 ppm), and the current density is higher (i.e., up to 1 A/cm2) is not clear at the current stage. 

The possible existence of an interface resistance in mass transfer has been examined by Raimondi and Toor(12) who absorbed carbon dioxide into a laminar jet of water with a flat velocity profile, using contact times down to 1 ms. They found that the rate of absorption was not more than 4 per cent less than that predicted on the assumption of instantaneous saturation of the surface layers of liquid. Thus, the effects of interfacial resistance could not have been significant. When the jet was formed at the outlet of a long capillary tube so that a parabolic velocity profile was established, absorption rates were lower than predicted because of the reduced surface velocity. The presence of surface-active agents appeared to cause an interfacial resistance, although this effect is probably attributable to a modification of the hydrodynamic pattern. 

The validity of using equations 12.17 and 12.18 in order to obtain an overall transfer coefficient has been examined in detail by King117 . He has pointed out that the equilibrium constant /7 must be constant, there must be no significant interfacial resistance, and there must be no interdependence of the values of the two film-coefficients. 

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

Since the electrolyte may contain associated species, we choose to define the general term current fraction as Is /Io, assuming that interfacial resistances, which may change during the course of an experiment, have been allowed for. Because the steady state current is not a linear function of the applied potential difference above some undefined potential, the above parameter is generally potential-dependent. However, because electrolytes display a linear, steady state, current-applied potential difference response up to at least 20 mV we may define a limiting current fraction, f+, as... 

The limiting current fraction is the maximum fraction of the initial current which may be maintained at steady state in the absence of interfacial resistances. In specific circumstances this parameter may be equal to the transport or transference number of particular species, but without a priori knowledge of the species present in an electrolyte it is preferable that values are referred to, rather than t+ or T+ values. For polyether electrolytes containing LiClO values of 0.2-0.3 are often observed. 

Even after all the above issues, that is, mechanical strength, ion conductivity, and interfacial resistance, have been resolved, SPEs still have to face the crucial issue of surface chemistry on each electrode if the application is intended for lithium ion technology, and there is no reason to be optimistic about their prospects. 

Results are similar for films deposited on YSZ however, there appears to be a difference between films deposited on ceria vs YSZ in terms of interfacial electrochemical resistance. As shown previously in Figure 6c, LSC films on YSZ often exhibit a second high-frequency impedance associated with oxygen-ion exchange across the electrode/electrolyte interface.That this difference is associated with the solid—solid interface has been confirmed by Mims and co-workers using isotope-exchange methods. As discussed in greater detail in sections 6.1—6.3, this interfacial resistance appears to result from a reaction between the electrode and electrolyte, sometimes detected as a secondary phase at the interface. 

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