5 Investigation Results

5.1 CO Reduction in the CO Selective Oxidizer

Using a model gas corresponding to that of reformed methanol, the CO selective oxidation performance of various catalysts was evaluated. As shown in Figure 2, the Ru catalyst was found to be capable of reducing CO concentration in a wider operating temperature range and in a wider range of gas flow rates than that of the well-known Pt catalyst widely used for the CO selective oxidizer. These results indicate that, by using the Ru catalyst for the CO selective oxidizer, it' is possible to supply reformed gas with a stably low CO concentration, continuously, against the load variations imperative for automobile operation."'

Side reactions that occur in the CO selective oxidizer were studied to help to elucidate the reason for the excellent CO selective oxidation characteristics shown by the Ru catalyst. It is known that CO is generated by the reverse shift reaction, as expressed in Equation (I), which occurs in the CO selective oxidizer.1"

H2 + C02 — C0+H20 (1) In order to check the influence of reverse shift reaction, a model gas (C02=25%, Hj=75%) not containing CO was humidified and fed, without air, into a CO selective oxidizer. The CO concentration in the out going gas was then measured. It was clearly found that the amount of CO generated with the Ru catalyst was smaller than that with the Pt catalyst, as shown in Figure 3.

Also known to occur in the CO selective oxidizer, where a noble metal catalyst is used, are the methanation reactions expressed in Equations (2) and (3).

C02 + 4H2 — CH, + 2H20 (3) To check the relationship between the CO and CH4 concentrations formed, another model gas (C0=0.1 %, C02=25%, Hj=balance) containing CO was humidified and fed, without air, into a CO selective oxidizer. As the results in Figure 4 indicate, as the catalyst temperature rose, the CH, concentration tended to increase while the CO concentration tended to decrease. Additional investigations indicated that these reactions occurred more readily when the Ru catalyst was utilized for CO selective oxidation.'41

Detailed analysis produced further insight into the reverse shift reaction and methanation reactions, as well as the benefits of Ru utilization. In the case of an oxidizer using the Pt catalyst, CO is oxidized into COr but then a portion reverts back to CO via the reverse shift reaction. Though this reversal also occurs where the Ru catalyst is used, it is to a lesser extent In addition, with Ru, a portion of the CO generated by reverse shift reaction is converted to CH, as methanation reaction occurs. Consequently, the Ru catalyst is capable of producing lower levels of CO at the oxidizer outlet, owing, not only to reduced reverse shift reaction, but also to this CO to CH4 conversion. Thus, in view of these results, as well as obvious cost advantage over Pt, a compact, low cost methanol reformer, suitable for automotive use, can be devised by the application of Ru to the CO selective oxidizer.

5.2 Prevention of CO Poisoning with Alloy Electrocatalyst

In order to enhance the CO tolcrance of the PEFC anode elcctrocatalysts, the CO tolerances of several Pt based alloy electrocatalysts were evaluated. Proper alloying of the experimental catalysts were confirmed by means of X-ray diffraction prior to evaluation. Utilizing a Pt cathode electrocatalyst and anode gas containing lOOppm CO the samples were evaluated. Figure 5 shows a comparison of the cell voltage measurements obtained at a current density of 0.4A/cml Only the Pt-Ru electrocatalyst exhibited a higher CO tolerance than the unalloyed Pt electrocatalyst.

Once the Pt-Ru electrocatalyst was identified as superior in CO performance, the alloy ratio was then optimized, as was the active layer thickness and supporting process of the Pt-Ru electrocatalysts. Using the knowledge obtained by these optimization efforts, a new type of Pt-Ru electrocatalyst was prepared, and its performance in the PEFC was evaluated. As shown in Figure 6, cell performance equivalent to that of a Pt electrocatalyst was obtained, and performance similar to that in pure hydrogen was obtained, even with gas containing CO concentrations of 1 OOppm.15"6'

5.3 Prevention of the Effects of C02

In addition to CO, reformed gas also contains C02. The probable detrimental effects on the PEFC, anticipated by the presence of C02, were investigated. As a result, two phenomena were found, namely, CO poisoning and CO, barrier effect. CO generated from the C02 contained in the reformed gas was found to poison the electrocatalyst."> Additionally, the C02 was found to stagnate around the anode. Due to the greater specific gravity of the C02, this stagnation impeded the diffusion of H2 over the electrode's active layer and lowered performance.

As indicated earlier, the poisoning caused by CO can be successfully averted by the application of a Pt-Ru alloyed anode electrocatalyst to the PEFC. Regarding the control of the C02 barrier effect, optimization of the gas flow field shape was found to be the most effective. In other words, by combining the knowledge of electrocatalysts and gas flow fields, it is possible to prevent the adverse effects of C02 contained in the reformed gas."1'

5.4 Prevention of the Effects of Residual Methanol

Because reformed gas was also found to contain some residual methanol, its effects on cell performance needed to be clarified. Cell performance was measured using a model reformed gas containing various concentrations of methanol. It was found that as methanol concentrations increased cell performance decreased. As expected, Pt electrocatalyst poisoning was one of the sources of the deteriorating performance. This poisoning was caused by the dissolution of methanol in the anode, resulting in the formation of poisonous aldehyde. The occurrence of another phenomenon was also confirmed, methanol crossover. Methanol contained in the anode gas permeated into the electrolyte membrane, reaching the cathode and reacting with oxygen contained in the air, thus, lowering the potential of the cathode.

Nevertheless, it was concluded that these adverse effects of residual methanol can be reduced to an acceptable level with the employment of both a Pt-Ru anode electrocatalyst, and optimization of PEFC operating conditions.m

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