Abstract: In accordance with established liquid feed direct methanol fuel cell one-dimensional model of methanol penetrating discussion of the impact on battery performance. The results showed that: the current density is low, methanol penetration of the impact of a larger battery performance; With the current density increases, methanol penetrating properties of the battery is getting smaller and smaller.
Key words: direct methanol fuel cell; mathematical model; methanol penetrating; have potential
Key words: 7rM911.4 literature sign code: A Article ID :1008-1267 (2008) 04-0023_03
Preliminary studies on direct methanol fuel cell set up along the perpendicular to the direction of proton exchange membrane of one-dimensional model of coupled equations of the components of the mass conservation equation, momentum equation and electric conduction equation. Anodic methanol oxidation kinetics of complex multi-step reaction mechanism snapped. Model also takes into account methanol penetration of the cathode reaction of oxygen and thus amended the parallel response theory. In the pre-published article, a detailed description of the material in the battery within the concentration distribution. This paper mainly introduces the cathode, anode potential distribution have been obtained at different current density of methanol penetrating impact on battery performance.
1. Have the potential distribution
Can be seen from Figure 1, in the work of the density of smaller batteries, the cathode potential had been higher than the anode potential, in the open when there is a high cathode-off potential. This is because the work when the battery current density smaller. For the anodic reaction of methanol flux is very small, a large number of methanol penetrate to the cathode catalyst layer oxidation reaction occurs. The formation of a high penetration of methanol-off potential.
When the battery operating current density (> 475 to cm-2) when. Anode potential had been greater than the cathode potential. The reason is that the battery work of larger current density, methanol penetration flux smaller (Figure 1), methanol had potential to penetrate smaller. The anodic oxidation kinetics of methanol have been caused by a greater potential.
When the battery operating current density of 797mA cm-2 when. Potential increased rapidly over the anode, when the battery is mainly affected by the anode concentration polarization control.
2 methanol penetrating impact on battery performance
Figure 2 reflects the concentration of methanol feed 2500moL/m3, oxygen pressure of 1.0 MPa (gauge pressure), batteries work when the temperature is 80 ℃. Methanol penetrating impact on battery performance.
Figure 2 ideal battery that there is no methanol penetration of direct methanol fuel cell (that is, direct methanol proton exchange membrane can be completely prevented through methanol).
From Figure 2 we can see that when the battery when the current density of less actual battery performance and the performance difference between the ideal of a larger battery. With the battery operating current density, the actual battery performance and the performance difference between the ideal battery is getting smaller and smaller. That is, methanol penetrating properties of the battery is getting smaller and smaller. This is because the work when the battery current density smaller. For the reaction of methanol is very small, penetrating to the battery cathode methanol more. In addition, the work of the battery when the current density smaller. Penetrate to the cathode of methanol oxidation reactions caused by the internal cathode current is the main factor had potential. Methanol penetration of the impact of a larger battery performance. With the current density increases, the amount for the reaction of methanol increased. And discharge current density is high. Electrode performance by ohmic polarization control, penetrating to the cathode catalyst layer of methanol oxidation to form the internal cathode current is not accounted for the vast majority of total current, thus penetrating methanol battery performance was less affected.
Figure 3-based model and the cathode catalyst layer response to the assumption that the establishment of parallel one-dimensional model of the DMFC battery performance that corresponds to the results of the comparison. From the figure we can see that when the battery current density smaller work, the two models have larger differences in battery performance. The reason is that at this time to the cathode penetrate a large amount of methanol. Cathode catalyst layer with a higher concentration of methanol, the cathode catalyst layer for oxygen reduction reaction activity of the effective surface area decreases, the existence of cathode methanol oxygen reduction reaction on the impact of the larger, so the two models have larger differences in battery performance .
With the battery work of the current density increases, the two models of battery performance has been gradually reduced the difference. This is because the work of the battery current density is high, to infiltrate into smaller cathode methanol flux. And when high-current battery-control step in the anode, so the two models of battery performance to be more nuanced.
Figure 4 pass accounted for the methanol permeability of methanol percentage of the total flux and current density. At low current density. Access to most of the anode diffusion layer to the cathode of methanol as a result of infiltration and wasted. With the current density increase. Methanol penetrating pass accounted for the total flux of methanol is getting smaller and smaller proportion. Methanol penetrating pass accounted for the total flux ratio of methanol as the reaction of methanol and methanol penetration flux and change. With the battery operating current density for the reaction of methanol to increase the total flux. The penetration flux reduced methanol. To the battery limits of the work of all the current density of methanol are used for reaction. Methanol almost no penetration to the battery cathode catalyst layer, when the reaction of methanol flux equivalent to the total flux of methanol.
Figure 5 shows the penetration of methanol flux and current density. Methanol penetration flux with current density increases. When the current density close to the limit when the current density. Methanol flux penetration with almost no changes in current density increase. This is because the current density of methanol penetration flux reflected in two aspects. On the one hand with the current density increases, the anode catalyst layer within the more water will drag effect as a result of electricity through the proton exchange membrane. On the other hand with the current density increases, methanol in the reaction of the anode catalyst layer consumption increased penetration of water had membrane carried by decreasing the concentration of methanol.
3. Conclusion
3.1 in the low current density. Access to most of the anode diffusion layer to the cathode of methanol as a result of infiltration and wasted. Methanol penetration of the impact of a larger battery performance.
3.2 With the current density increases battery life. For the reaction of methanol to increase the total flux, and the methanol penetration flux reduced to battery current density limit the work of all of methanol are used for reaction, almost no methanol penetrate to the battery cathode catalyst layer. Methanol penetration of battery performance was less affected.
