Quantitative TEM Analysis for the Pt Morphology in the Catalyst Layers of Polymer Electrolyte Membrane Fuel Cells

We carried out quantitative transmission electron microscopy (TEM) analysis for the Pt morphology in the cathode catalyst layer of polymer electrolyte fuel cells (PEFCs) for investigating the transportation of Pt species during the cell operation. The specimens for the TEM observation were cut off from the catalyst layer with approximately 100nm thickness without embedding it in a resin. The size and number of the Pt particles contained in the same volume of the catalyst layer were accumulated to obtain their size distributions. The distributions of Pt surface areas and volumes were also estimated from the size distributions, assuming that the Pt particles are sphere. The total volumes of the Pt particles estimated by the analysis corresponded to 65-86% of those calculated from the Pt loadings at the MEA preparations. The change in the Pt morphology before and after a potential cycling test without power generation was investigated. For the cycled MEA. the Pt surface area per weight (the specific Pt surface area) calculated from the TEM observation was nearly identical to the electrochemically active surface area (ECSA) by cyclic voltammetry(CV). This novel method for the TEM analysis provides the distributions of the Pt concentration in the whole catalyst layer as well as the Pt surface distribution and the Pt volume distribution in a given area of the catalyst layer without chemical analysis or spectroscopy. Those data can be used to understand the dependence of the microstructures of the catalyst layer on the cell performances.