Nitrogen 27。32 6。226 -0。9502

3。RESULTS AND DISCUSSION

In order to investigate the performance discrepancy between the ASC stack and the CSC stack, two models were established with the same settings as described above except the thickness of electrodes。 The ASC stack model has an anode thickness and a cathode thickness of 500 and 50μm, respectively。 The corresponding parameters for the CSC stack model are 50 and 500μm,  respectively。 It is some unexpected to find that the average current density of the ASC stack is only 5580 A m-2, a reduction of 20。7% from the CSC stack of 7033 A m−2。 The substantial discrepancy between the performances of the ASC stack and the CSC stack may be understood by the difference distributions of gases concentration and electronic potential in Fig。 4-5。

3。1The Distributions of Gases Concentration

Fig。 4 illustrates the oxygen concentration distribution in the air channel and cathode of the ASC stack and the CSC stack。 For the ASC stack, the oxygen concentration in the vertical electrode surface direction is almost uniform, which benefits from thin cathode。 However, on the other hand, the thin cathode simultaneously also limits the oxygen diffusion to the area under rib and leads to a wide oxygen depletion zone under the rib, as shown in Fig。 4a。 As a result, electrochemical reaction can't take place in oxygen depletion zone due to the lack of the oxygen。 For the CSC stack, cathode is the thickest layer。 Thick cathode provides a wide alleyway allowing oxygen to penetrate under the ribs。

Although the oxygen concentration of the CSC stack under rib is small, an oxygen depletion zone is avoided, as can be seen in Fig。 4b。 Thus, the effective reaction zone of the CSC stack is larger than that of the ASC stack, which is an important factor contributing to the reduced performance of the ASC stack。

Figure 4。 Distributions of O2 for (a) the ASC stack and (b) the CSC stack

Figure 5。 Distribution of H2 for (a) the ASC stack and (b) the CSC stack

Fig。 5 provides the hydrogen concentration in the fuel channel and anode of the ASC stack and the CSC stack。 As expected, the highest value of hydrogen concentration is located at the channel inlet, while a progressive hydrogen concentration decrease is observed moving to the cell outlet。 Comparing Fig。 5a and 5b, it is obvious that the hydrogen concentration distribution along fuel flow direction under channel for the ASC stack is similar to that for the CSC stack at the corresponding position。 For the ASC stack, the hydrogen concentration under rib is almost as big as that under channel, as shown in Fig。 5a。 While Fig。 5b shows a quite different situation。 There is a large hydrogen concentration gradient in the x direction for the CSC stack。 Although hydrogen concentration of the ASC stack under rib is bigger than that of the CSC stack at the corresponding position, the most hydrogen under rib of the ASC stack doesn't take part in the electrochemical reaction because of a wide oxygen depletion zone under rib, as shown in Fig。 4a。

3。2The Distributions of Electronic Potential

(a)(b)

Figure 6。 Distribution of electronic potential in cathode for (a) the ASC stack and (b) the CSC stack

(a) (b)

Figure 7。 Distribution of electronic potential in anode for (a) the ASC stack and (b) the CSC stack

The distribution of electronic potential in cathode for the ASC stack and the CSC stack is given in Fig。 6。 The ohmic loss in cathode for the CSC stack is 19 mV, which is less than one fifth of that for the ASC stack (105 mV)。 Notable cathode ohmic loss of the ASC stack is created by two main aspects:

(i) a narrow cross-section of current path in the x direction (ii) relatively small cathode electrical conductivity。 A very small electronic potential variation is observed in anode for the ASC stack, as shown in Fig。 7a。 The high anode electrical conductivity is believed to be responsible。 For the CSC stack, the ohmic loss in anode is about 13 mV, as given in Fig。 6b。 Large anode ohmic loss of the CSC stack is attributed to thin anode hindering current flow in the x direction。 The total electrodes ohmic loss for the CSC stack is 32 mV, which is only about one third of that for the ASC stack 106 mV。 This is another factor contributing to the reduced performance of the ASC。