Figure 2。 Schematic representation of charge and gases transport processes in the stack。

For the stack, the channels in interconnector are used to carry the fuel and air flows and the ribs collect current, which separate and define the channels, as shown in Fig。 2。 The width of the ribs or the channels is commonly about several millimeters [13, 24]。 The charge and gases transport paths of the stack in the parallel electrode surface direction is no less than that in the vertical electrode surface direction。 Therefore, at least, the ohmic and concentration losses of electrodes in the parallel electrode surface direction are the same important as those in the vertical electrode surface direction。 Thick electrode layer is benefit for reducing the ohmic and concentration losses in the parallel electrode surface direction due to increasing the cross section of the charge and gases transport paths。 On the other hand, thick electrode layer hinders the charge and gases transport in the vertical electrode surface direction because of elongating the charge and gases transport path。 Thus it is difficult to judge whether the performance of the ASC stack is better than that of the CSC stack under the same operating condition。 In practice, it is very necessary to clarify the advantage or disadvantage of ASC and CSC, which assists in understanding the effect of cell design on cell performance in the stack level and playing the full potential of the stack by optimizing the cell design。

In literature, there is only one study comparing the performance of the ASC stack with that of the CSC stack under the same operating condition [25]。 In that study, the computational domain consists of the fuel and air channels and the electrodes–electrolyte assembly but the ribs are completely ignored。 However, many studies have already shown the strong effect of the ribs on the charge and gases transport [13, 26, 27]。 For ASC stack, an oxygen depletion zone of 0。46 mm was found with a cathode rib width of only 0。8 mm due to thin cathode thickness limiting the oxygen diffusion to the area under rib [27]。 For the CSC stack, the minimum hydrogen concentration under anode rib is only about one third of that under anode channel [28]。 Therefore, the model developed in reported [25] can't accurately predict the performance of the ASC or CSC stack。

A 3D model was developed to predict the performance of the ASC or CSC stack。 The computational domain comprises the ribs, fuel channels, air channels and the electrodes–electrolyte assembly。 Detailed comparisons between ASC stack and CSC stack are made to illustrate the role of the cell design on the stack performance。

2。MODEL

A repeating cell unit of a SOFC stack is shown schematically in Fig。 3a。 Due to symmetry, we select half of the repeating unit of stack as our computational domain as shown in Fig。 3b。 The computational domain is comprised of (i) cathode-side interconnect plate and the air channels, (ii) electrodes–electrolyte assembly, (iii) anode-side interconnect plate and the fuel channels。

Figure 3。 Schematic of a SOFC stack。

2。1。Governing Equations

2。1。1。Charge Transport

Electronic and ionic current density are governed by charge continuity equation, which can be described as follows associated with the point form of Ohm’s law。

iel  (elel ) 0

iio  (ioio ) 0

are the electronic and ionic current density, respectively,  el

electronic  (ionic)  potentials,

el     represents  the  electronic  conductivity  of  electrodes  ,   while

represents the ionic conductivity of electrolyte。

The electronic conductivity of composite electrode el    can be estimated as [29]: