Mass transfer resistance, reduction temperature

The results also suggest that through AC impedance measurements, the performance drops caused by individual processes such as electrode kinetic resistance, membrane resistance, and mass transfer resistance can be correlated to either reduction or improvement in cell performance. If individual impedances are known, the contribution to the change in performance can be identified, which is very important in the design and optimization of high-temperature MEA catalyst layer components, structure down-selection, and MEA architecture. 

The graph shows that the MS3220 membrane performed better than the others at the same operating conditions the highest value of distillate flux was around 30 kg m h . By increasing the distillate temperature from 23 to 42°C, the flux decreased down to 22kgm h , because of the lower driving force available for the transport. An increase of the distillate flux was obtained when the feed velocity was increased, due to a reduction of the heat and mass transfer resistances. 

In reality, the Biot number for mass transport in most cases is considerably higher than the Biot number for heat transport, i.e. the ratio r is frequently larger than 40-50 [19]. This means that the catalyst pellet can usually be treated as isothermal, at a temperature level which is controlled by the interphase heat transfer resistance. However, this leads to a reduction of the general problem to the case which has already been treated in the previous section. 

A schematic representation of this polarisation curve is given in Rg. 15.2. The first part of the curve corresponds to activation loss, mainly the kinetics of oxygen reduction (with jo = 10 -10 A cm ) involved the second part is linear and due to ohmic loss, mainly the electrolyte resistance the third part is due to mass transfer or diffusion loss when the value of becomes close to /umcat or E(j) tends to zero. The optimal operating point is located in the linear part of the curve. These current-density-potential curves are very important for any type of fuel cell, because they summarise the influence of all the important parameters on the performance of a cell. Even though the equation is more complex in the case of high-temperature fuel cells, the general features of the current density vs potential characteristics are similar. 

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