IL-FE-SEM and ICP-MS Study for Degradation of Pt/C Catalyst
Authors: Kinumoto Taro , Ikeyama Misako , Matsuoka Miki , Tsumura Tomoki , Toyoda Masahiro 
Published: February 2020

The carbon black-supported Pt nanoparticle catalyst (Pt/C) is a representative cathode catalyst of polymer electrolyte fuel cell (PEFC). Unfortunately, Pt/C deteriorates with loss of the electrochemical surface area (ECSA) under potential cycling operations and start-stop cycles, and is one of the major reasons for the deterioration of cell performance. Several degradation mechanisms have been considered to be responsible for the loss of ECSA[1] and then, we have been investigating the degradation mechanisms of individual Pt particles in Pt/C by “identical location field emission scanning electron microscopy (IL-FE-SEM)”, which is our original technique to visualize as well as monitor the change of Pt nanoparticles at nanometer scale, in order to quantitative analysis the degradation process as well as design durable catalyst[2]. In this presentation, the potential pulse acceleration degradation test (ADT) of Pt/C was carried out in HClO4 solution, and simultaneously the degradation of individual Pt particles was monitored by using IL-FE-SEM. Furthermore, Pt dissolution amount in the solution was also measured by using inductively coupled plasma-mass spectrometry (ICP-MS) to discuss the material balance owing to the deterioration of Pt/C.
Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo, Pt weight = 46.7 wt%) was dispersed on a glassy carbon disk, and then Nafion thin film was formed on the catalyst (calculated thickness = 80 nm), which was employed as the test electrode. FE-SEM observation was carried out and the observation point was defined for IL-FE-SEM. After that, the test electrode was transferred to a glass half-electrochemical cell filled with 0.1 mol dm-3 HClO4 aqueous solution. Potential pulse ADT (0.6 V - 1.0 V, pulse time = 3 s, recommended by Fuel Cell Commercialization Conference of Japan; FCCJ) was carried out under N2 atmosphere at 60oC. IL-FE-SEM observation and cyclic voltammetry measurements were carried out after an arbitrary test period. We randomly selected 750 pieces of Pt nanoparticles to investigate the degradation mechanism. ICP-MS analysis was also carried out by sampling a tiny amount of the electrolyte solution during the ADT.
Figure 1 shows typical IL-FE-SEM images of Pt/C catalyst taken during ADT. Individual (specific) Pt particles in Pt/C were clearly monitored. The typical morphological changes were observed for Pt particles such as shrinkage and growth of the particles, disappearance, migration, coalescence, precipitation on carbon support. The typical degradation processes were observed for Pt particles such as shrinkage and growth of the particles, disappearance, migration, coalescence, precipitation on carbon support.
The ECSA decreased to 65% after 10,000th pulse. The total number of Pt particles was degreased from 750 to 697, showing the number of Pt particles is surely decreased during the ADT. Among the degradation processes, disappearance and shrinkage were recognized as main process, suggesting that dissolution of Pt should be severely taken place under the ADT. It was found from ICP-MS results that 2% of Pt was dissolved out from Pt/C catalyst after 10,000th pulse. In this presentation, the material balance of Pt during the ADT will be also discussed based on IL-FE-SEM and ICP-MS.

Y. Shao-Horn, et al., Top. Catal., 46, 285 (2007).

T. Kinumoto, et al., Electrochemistry, 83, 12 (2015).

Figure 1