Understanding the Degradation of Pt Electrocatalysts in Proton Exchange Membrane Fuel Cells By 3D Identical Location STEM

Abstract

Proton exchange membrane fuel cells (PEMFCs) are promising energy conversation devices due to their high power/energy density, high efficiency and the absence of carbon dioxide emission. Yet, one of the largest obstacles for the commercial application of PEMFC is the degradation of platinum group metal (PGM)-based electrocatalysts during long term operation. Four degradation mechanisms have been widely reported, namely, 1) Ostwald ripening; 2) particle migration and coalescence; 3) carbon corrosion and 4) Pt dissolution and reprecipitation in the membrane. However, the role played by each degradation mechanism is not fully understood. In this work, scanning transmission electron microscopy (STEM) tomography coupled with identical location TEM (IL-TEM) are utilized to delineate the roles of each degradation mechanism for Pt catalysts on Vulcan XC72 carbon support. Compared with conventional TEM, 3D STEM tomography is capable of identifying the following: 1) interparticle distances; 2) 3D structure of carbon black, 3) specific positions of Pt catalysts on carbon support; and 4) shapes and morphologies of Pt catalysts[1]. In addition, IL-TEM allows a comparison of Pt catalysts and carbon support of the same region of interest (R.O.I), at the beginning of life and after potential cycling, which provides direct insight into the effects of potential cycling on the degradation of Pt catalysts. Figure 1(a-b) shows the 3D illustration of Pt catalysts on the Vulcan XC72 carbon support before and after potential cycling, respectively. We observe that volume shrinkage is insignificant in the bulk of the carbon black particles and that there are no Pt catalysts falling off after potential cycling (0.6V-1.0V vs RHE, 6000 cycles). In addition, we note the appearance of newly-formed Pt nanoparticles or ionic clusters (<1nm in diameter). A careful inspection regarding the formation mechanisms of these particles shows that particle migration and coalescence is not involved. Instead, these newly-formed particles are likely due to Pt dissolution and re-precipitation. The 3D IL-TEM experiments also show the coalescence of Pt NPs. However, the coalescence is unlikely due to long migration distances (>0.5nm). Instead, the growth of Pt nanoparticles result from Ostwald ripening and subsequent sintering. In summary, 3D STEM tomography coupled with IL-TEM (~1nm spatial resolution) provides unique insight into the roles of each degradation mechanisms for Pt electrocatalysts on Vulcan XC72 during potential cycling. Reference: [1]Horn, S. Y.; Sheng, W. C.; Chen, S.; Ferreira, P. J.; Holby, E. F.;Morgan, D. Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells. Top. Catal. 2007, 46, 285-305. Figure 1