content, the acid absorption rate is higher than that of high PTFE content and also the absorbed acid quantity is also increased with decreasing FIFE content So, in the initial stage of acceleration test, three phase reaction boundaiy is easily formed in the low FIFE content region and the good performance is shown in relatively low 35 wt% FIFE. In Fig. 2, the electrode of low FIFE content is rapidly degradation with increasing acceleration test time. In 25 and 35 wt% FIFE contents the electrode performance is rapidly lowered. This means that the electrode of low PTFE contents is easily flooded with increasing acceleration test time. In the case of PTFE content more than 45wt.%, the performance of the electrode increases slightly in 5 hour acceleration test than that of in 0 hour acceleration test, but the performance is decreased in 24 hour acceleration test

Fig. 3 shows the electrode performance at 0.7Vrhb sintered at various temperatures. As stated above, the electrode sintered at 350°C showed the highest performance and it had current density of 305 mA/cm2 at 5 hrs of acceleration test. Electrode performance may degrade rapidly by flooding due to carbon corrosion and loss of PTFE although it has high initial performance. To the contrary, the electrode having low initial performance may have long life if it has high wetproofing ability to the electrolyte. In other words, when the wetproofing ability is too high, flooding by electrolyte will be little but the elcctrode performance will decrease because of insufficient formation of 3 phase interface.

Figure 4 presents the measured impedance in Nyquist representation for the unwetted Pt/C elcctrode specimen subjected to 250, 300, 350,450, 500 and 550mVRHE in 1MH2S0.) solution. One does not observe the constant phase element (CPE) appearing due to a porous structure of the electrode which has a theoretical slope of 45° in the high frequency range of the Nyquist plot indicating that the electrolyte does not infiltrate into the narrow pores of the unwetted Pt/C electrode. Thus, oxygen reduction occurring on the surface of the unwetted Pt/C electrode mainly contributes to the resulting cathodic current of the ac-impedance response. For the unwetted Pt/C elcctrode subjected to potentials from 250 to 550 mVRHE (Fig. 4), one high frequency capacitive arc and one low frequency inductive arc are observed. The occurrence of the low frequency inductive arc indicates that oxygen reduction proceeds via formation of intermediate states on the unwetted Pt/C electrode specimen. The magnitude of the overall impedance increases with decreasing overpotential for oxygen reduction.

The Nyquist plots for oxygen reduction on the prc-wetted Pt/C electrode specimen subjected to 250, 300, 350, 450, 500 and 550 mVjyjE in 1 M H2SO., solution are shown in Fig. 5. In contrast to the unwetted Pt/C elcctrode specimen, the impedance spectra obtained from potential range from 250 to 550 mVRHE consist of the CPE with a slope of 45°, one high frequency capacitive arc, and on low frequency capacitive arc. The appearance of the CPE in the high frequency range indicates that the electrolyte infiltrates into the narrow pores of the pre-wetted Pt/C electrode specimen. Considering that the overall impedance value of the pre-wetted Pt/C electrode specimen is smaller by three orders in magnitude than that of the unwetted Pt/C electrode specimen, oxygen reduction occurring within the narrow pores mainly contributes to the resulting cathodic current of the ac-impcdance response. The inductive to capacitive transition in impedance spectra with pre-wetting treatment implies the change in mechanism and kinetics of oxygen reduction.

0 0

Post a comment