Effects of the cathode gas diffusion layer porosity

The porosity of the cathode gas diffusion layer has two different effects on the

fuel cell performance: as the porous region provides the space for the reactants to

diffuse towards the catalyst layer region, an increase in the porosity means that the

onset of mass transport limitations occurs at higher current densities, i.e. it leads to

higher limiting currents. The adverse effect of a high porosity is an expected increase

in the contact resistance. Contact resistance occurs at all interfaces inside the fuel cell,

the most important one being the interface between the bipolar plates and the gas

diffusion layers. In this study, the solid phase conductivity is assumed constant and

does not change with the variation of GDL porosity. Hence, the location of the

gas-liquid interface along the flow channel direction at various porosities of the

cathode gas diffusion layer and its effect on cell performance were elucidated. Figure

3.11 shows the effects of the cathode gas diffusion layer porosity on the location of

the gas-liquid interface where the liquid water begins to condense along flow channel

direction at a cell voltage of 0.7 V and a relative humidity of the cathode of 80%. The

results in Fig. 3.11 show that the location of the gas-liquid interface appears early and

is closed to the flow channel inlet region in the gas diffusion layer as the gas diffusion

layer porosities decreased. These phenomena can be explained as that given smaller

porosity of the gas diffusion layer, the liquid water is more difficult to transport out of

the gas diffusion layer, hence, the liquid water will spread throughout the gas

diffusion layer. Once the liquid water block the pores of the porous media, causing the

hinder the diffusion transport of fuel gas to the cathode catalyst layer, thus limiting the

electrochemical reaction and causeing the decrease of cell performance. On the

contrary, the higher porosity of the cathode gas diffusion layer facilitates the transport

of water from the cathode catalyst layer to the flow channel. Hence, less water blocks

the pores of the gas diffusion layer, increasing the vacant pores space and helping the

reactant diffusion to the cathode catalyst layer. Nevertheless, the variation of the

gas-liquid interface location becomes small as the porosity changes from 0.7 to 0.8.

The phenomenon can be conjectured as that when the cathode gas diffusion layer

porosity is too big, the capillarity effect is not sensible, hence, the gas-liquid interface

location doesn’t seem to change obviously.

Figures 3.12 (a) and 3.12 (b) show I-V and I-P curves at various porosities of

the cathode gas diffusion layer at a relative cathode humidity of 80%. In Fig. 3.12 (a)

the I–V curves demonstrate that a better performance can be obtained by using a gas

diffusion layer of higher porosity. The remarkable drop in cell potential is caused by a

mass transfer or concentration loss, which is a consequence of shortage of the reactant

gas at high current density in which more reactant gas is required for fast reaction. A

gas diffusion layer of higher porosity has an ability of stronger diffusion transport,

which is beneficial in that it supplements the reactant gas to the cathode catalyst layer

and thus shifts the occurrence of the performance drop to a higher value of the current

density. However, Fig. 3.12 (a) also shows that when the cathode gas diffusion layer

porosity reaches the value of 0.8, the cell performance almost does not change

comparing with the value of 0.7. This phenomenon also conform the result of Fig.

3.11. The variation of power density is also shown in Fig. 3.12 (b). It reveals that the

power density increases with the cathode gas diffusion layer porosity. In closing, if the

variation of solid phase conductivity is neglected, the cell performance is enhanced as

the cathode gas diffusion layer porosity increase. But when the cathode gas diffusion

layer porosity reaches the value of 0.7, the cell performance is not enhanced.

Figures 3.13 and 3.14 illustrate the same oxygen and water mass fractions at

various porosities of the cathode gas diffusion layer, respectively, in the direction of

the flow channel at an operating voltage of 0.7 V and a cathode relative humidity of

80%. According to the model equations, a higher porosity of gas diffusion layer

accelerates the speed of oxygen diffusion to the cathode catalyst layer, as displayed in

Fig. 3.13. Because of the aperture of the pores being relatively large, meaning that it

may not block by the liquid water and the diffusion transport of reactant gas is easy to

pass through the pores of gas diffusion layer. Hence, the oxygen does not spread

throughout the gas diffusion layer, and can reach the cathode catalyst layer. On the

contrary, the reactant gas spread throughout the gas diffusion layer for a small

porosity of gas diffusion layer. The diffusion transport of the reactant gas and the

liquid water by the capillary force to pass though the gas diffusion layer is difficult.

Fig. 3.13 also shows that when the porosity value is 0.7, no matter what the porosity is

increased, the oxygen mass fraction does not change obviously because of the fuel gas

or water almost pass though the gas diffusion layer directly. Figure 3.14 shows a

similar result with Fig 3.13. That is, the water moves though the gas diffusion layer

more easily at higher porosity of gas diffusion layer and the water does not

remarkably occupy the pores and thus preventing the oxygen diffusion transport.