In Situ STEM Observation of Pt Nanoparticles on Carbon Surface in the Air Condition

Electrocataysts for PEFCs are most commonly composed of Pt nanoparticles on carbon supports. Even though such electrocatalysts are highly active, their durability is still a problem. Especially, carbon corrosion at high potential followed by Pt agglomeration is a major issue to be overcome. Against the carbon corrosion, graphitization of carbon surface is one of possible solutions. In our studies, so far, the surface of carbon black, such as Vulcan and Ketjen black, was graphitized though the simple heat treatment, and then further Pt nanoparticles were deposited on such heat-treated carbon. As a result, durability of such electrocatalysts was successfully increased. On the other hand, we have found that degradation mechanism cannot be explained only by agglomeration and growth of Pt particles, according to the TEM observation before and after the electrochemically accelerated durability tests. Therefore, in this study, an accelerated durability test is newly introduced by using in-situSTEM observation in the air condition to understand the degradation mechanism.

  For our study, Hitachi 300 kV Cold-FE TEM (HF-3000) was used. Continuous SEM/STEM observation for 30 minutes was performed at 200 ^oC in the air condition. A standard catalyst, Pt on Vulcan carbon was first investigated, and still images of SEM made out of the movie are shown in Figure 1. As seen, Pt particles, whose size was about 2 nm, were initially well dispersed on carbon particles. They did not move around very much and were rather embedded into the carbon surface owing to carbon corrosion. Interestingly, in this case, TEM images themselves cannot give us information on degradation mechanism since the particle size is not much changed. Now, we have found that decreased PEFC performance is rather derived from the fact that embedded Pt particles lose their active sites.

  On the other hand, Pt particles on graphitized carbon were more agglomerated, but not much embedded into the carbon surface. Since the durability of PEFC performance is higher with graphitized carbon, we believe embedded Pt is a major reason for degradation. Details of comparison with electrochemically accelerated test are also discussed.