C7h8

-4mV @ lOppm

In adding lOppm NO, voltage loss is negligible even in the long-term test. Whereas, in the case of N02, obvious cell voltage drop was observed. Fig. 1 shows the influence of N02 in the air on cell performance. Various concentrations of NO2 were added repeatedly on the same cell.

Cell voltage drop depends on N02 concentration and added duration time. It became more than 40mV with lOOppm for about 50 hours, about 45 mV with lOppm for about 1,000 hours, and about 20mV with lppm for about 700 hours.

On terminating NO2 addition, cell voltage recovered gradually to the initial voltage.

Overall cell voltage decline rate became about 3mV/I,000h, which is normal value at this temperature. This result indicates that N02 poisoning is not permanent but temporary.

In our other experiments, it is found that NO2 adsorbs on the Pt-Alloy catalyst, it reduces active surface area for oxygen reduction, and consequently causes cell voltage drop. In voltage recovering process, this adsorbed NO2 is probably reduced into N2, and then is released from cathode.

Cell voltage drop at O.lppm N02 was not observed on another cell. Considering the environmental standard, which requires less than 50~70ppb no2, the effect of NO2 in the air is negligible in practice. However, this influence is so significant that air inlet of PAFC should be kept enough distance from air exhaust outlets of gas engine and gas turbine. In addition, air has to be ventilated sufficiently at indoor parking lot.

When introduced SO2 concentration is less than lppm, sizable cell voltage drop was not observed. On the other hand, in adding 40ppm S02, cell voltage drops by about 20mV within 30 minutes. However, in terminating so2 addition, cell voltage recovers to the previous value also within 30 minutes. This result indicates that SO2 adsorbs physically on the surface of cathode catalyst, but it can be released quite easily from its surface.

(c) Toluene

Painting of floors, walls and piping near PAFC unit is a common work at field test sites. Therefore, we investigated the influence of toluene (C7H8), a common solvent of paint. It was revealed that toluene affects adversely on cell performance, and this process also occurs quickly like the case of S02. However, in this case, toluene seems to be combusted on cathode catalyst. In our other experiments, it is verified that catalyst combustion occurs at as low as 180%?.

Therefore, solvents like toluene have to.be removed by the active carbon filter, or it would be better to shut down PAFC unit for safety during paint works.

(2) INFLUENCE OF OPERA TING CONDITIONS

50(1

I) 2000 4000 6000

OPERATION TIME (hrs) Fig.l INFLUENCE OF N02 ADDITION

50(1

I) 2000 4000 6000

OPERATION TIME (hrs) Fig.l INFLUENCE OF N02 ADDITION

Osaka Gas Co., Ltd. has been operating more than 42 units of PAFC cogeneration system. In addition to field operation, we are also investigating the influence of operating parameters, such as temperature, current density, fuel utilization and load change by using these sub-scale single cells. Although real influence should be verified on short-stacks or real cell stacks, sub-scale single cell tests are effective to survey operating parameters respectively.

(a) TEMPERATURE

We examined the influence of operating temperature at 170, 185, 215°C. Fig.2 shows the relationship between operating temperature and cell voltage decline rate. As operating temperature increased, cell voltage decline rate increased exponentially.

(b) CURRENT DENSITY

We also examined the influence of current density at 100, 200, 300 mA/cm2. Although at lower current density higher cell voltage decline rate was expected because of higher cell voltage, lower cell voltage decline was observed at low current density in reverse. Considering our small number of tested cells, it cannot be concluded that higher current density has an adverse effect on cell performance. However, it can be said that lower current density as low as 100 mA/cm2 does not accelerate cell degradation.

(c) OTHER OPERATING CONDITIONS

The cell life test under higher fuel utilization at 94% did not show increase of cell voltage decline rate. In real cell stacks, fuel utilization influences more severely due to larger distribution of current and hydrogen concentration, but this result suggests the possibility of higher electrical efficiency PAFC system.

Load change from 150 mA/cm2 to 300 mA/cm2 are applied to the sub-scale cell more than 1,000 times, but sizable difference from normal condition was not observed either.

(3) LOW CURRENT DENSITY AND WW TEMPERATURE PAFC

(a) SHORT-STACK TESTS

The cell life of PAFC is said to be mainly dependent on following three factors,

1) Sintering of Electrocatalyst

2) Flooding of Electrode

3) Loss of Electrolyte (Phosphoric Acid).

If operating temperature is lowered, catalyst sintering and corrosion of carbon material and PTFE will be eased quite obviously. Since loss of electrolyte depends on the vapor pressure of phosphoric acid, lowering temperature is expected to extend the life of electrolyte exponentially.

Although lowering temperature is considered to accelerate corrosion of Table 3 SHORT-STACK SPECIFICATIONS carbon material due to lower concentration of phosphoric acid, our experiments on sub-scale cells indicate that above mentioned factors are more dominant in determining cell degradation.

In general, current density itself is supposed not to influence on cell

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