Morphology Changes of Pt Supported on an Ordered Mesoporous Carbon with Network-Structure: In Situ TEM Observation of a PEFC Electrocatalyst
Authors: Nishino Hanako , Sato Satoko , Yamazaki Hiroko , Kamino Takeo , Uchida Makoto , Iiyama Akihiro , Koizumi Naoto , Miyao Toshihiro 
Published: December 2024

Development of highly durable and active electrocatalysts for polymer electrolyte fuel cells (PEFCs) is an important research topic for future hydrogen energy applications. In PEFCs used for fuel cell vehicles (FCVs), it is known that the high cathode potential causes corrosion of the catalyst support carbon, especially during startup and shutdown operations. It is also known that PFSA-based ionomers directly contacting the Pt surface cause poisoning and deactivation of catalytic activity for the oxygen reduction reaction. There has been interest in immobilization of Pt particles inside nanopores at adequate depth regions (accessible region) at which direct contact with ionomers is suppressed, without any significant limitation of mass transport of reactants and products1).
We have recently developed a new Pt/C electrocatalyst using an ordered mesoporous carbon having a network-structure (Net-OMC) with a three-dimensional macrostructure and ordered nanopore (several-nm size) arrangement with ca. 5 nm diameter2)3). In this study, morphologic changes of a Pt/Net-OMC electrocatalyst were studied using in situ TEM/STEM at elevated temperatures and in various gas atmospheres.
The Net-OMC was prepared as previously reported2). In this study, the annealing temperature of the Net-OMC sample was 1000 °C. Loading of Pt on the Net-OMC support was carried out using a colloidal technique2)3). The loading amounts of Pt for Pt/Net-OMC were 20 to 30 wt.%. The high-resolution analytical TEM (H-9500, Hitachi High-Tech) was used to observe the characteristics of the catalyst samples at elevated temperatures in various gas atmospheres. The sample was mounted on a heating element of a direct-heating type sample heating holder equipped with a gas supply injector4). During TEM measurements, the total pressure in the vicinity of the catalyst sample was controlled to around 0.2 Pa, and, in order to minimize damage caused by electron beam irradiation, the latter was limited to a short period of time. After sample measurement and cooling down to room temperature, the sample was transferred to the high-resolution STEM with a cs-corrector lens system (HD-2700, Hitachi High-Tech), without detaching the sample from the sample holder in order to observe secondary electron (SE) images at identical locations of the samples.
In situ TEM observations were performed from room temperature to 200 °C under various gas atmospheres. At the initial stage, highly dispersed Pt particles were predominantly located at the near outermost surface of nanopores of the Net-OMC support particles. It was found that most Pt particles in the nanopores of Net-OMC migrated 2-5 nm from their original depth positions with increasing temperature. Figure 1 shows the results of SE images for the Pt/Net-OMC catalyst after in situ TEM observations. SE images observed after heating in oxygen (Fig. 1(b) and hydrogen (Fig. 1(c)) flow revealed that Pt particles in the nanopores migrated from the surface to greater depths. Although corrosion of the Net-OMC support progressed to some extent, it was found that agglomeration and growth of Pt nanoparticles hardly progressed. When this carbon support is used as an electrocatalyst, even if catalyst deterioration progresses, it is expected to suppress aggregation of Pt particles due to the feature of accessible nanopores of the Net-OMC support. In the previous study by our group, the Pt/Net-OMC catalyst exhibited remarkably higher durability than a commercial Pt/C during potential step cycling measurement2)3). These results suggest that the more stable characteristics of the catalyst can be easily created by pre-heat treatment in the appropriate atmosphere, and indicate that the Net-OMC is a particularly effective support to combat Pt migration and aggregation.
This work was partially based on results obtained from project JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) Japan.
References
1) V. Yarlagadda, M. Carpenter, T. Moylan, R Kukreja, R. Koestner, W. Gu, L. Thompson, A. Kongkanand, ACS Energy Lett., 3, 618 (2018).
2) T. Miyao, H. Nishino, H. Yamazaki, S. Sato, K. Tamoto, M. Uchida, A. Iiyama, K. Shibanuma, N. Koizumi, 242nd ECS meeting Abstract, #I01D-1578(2022).
3) S. Sato, H. Yamazaki, H. Nishino, K. Tamoto, M. Uchida, A. Iiyama, K. Shibanuma, M. Sodeno, N. Koizumi, Y. Hoshikawa, T. Miyao, 244th ECS meeting Abstract, #I01D-1999(2023).
4) T. Kamino, H. Saka, Micros. Microanal. Microstructure, Y4, 219 (1993).

Figure 1