In situ voltammetric characterization of PEM fuel cell catalyst layers

In our work we have studied in-house made MEA:s (geometric area 5 cm2) based on commercial (E-TEK, Inc.) carbon supported Pt and Pt/Ru catalysts. A Nafion 115 membrane was coated with ink containing Pt/C catalyst and Nafion solution and dried layers were sintered by hot pressing. After post-processing the membranes were installed in a single cell test fixture. In addition, the ink used for coating was characterized in thin film form in sulphuric acid electrolyte and the results were compared to the fuel cell data. Current-voltage characteristics were measured after a running-in period of about 16 hours. After cooling the test fixture down to room temperature the cyclic voltammograms were taken in situ by using a two-electrode potentiostatic measurement circuit. The electrode under study is purged by an inert gas (N2) while the other electrode is purged by hydrogen gas so as to form a reversible hydrogen electrode. In this manner the surface processes taking place on both the anode and cathode side of the MEA could be measured and evaluated. From the hydrogen desorption charge on Pt the available Pt surface area has been determined and the ratio between measured area and calculated total Pt area was in the range of 30%. The ratios between electrochemical surface areas on the anode and cathode side compare well with the corresponding ratios obtained from calculations of total added Pt surface area. The relation between the voltammetrically obtained surface areas and the polarization data (current-voltage curves) is discussed. An enlargement of the active surface area could be noted after potential scans into the Pt oxide formation region This indicates the presence of a deactivation process, probably due to adsorbed organic material removed oxidatively during the extended polarization. When MEAs based on Pt/Ru alloy catalysts were measured the hydrogen adsorption charge decreased as a result of the lower hydrogen adsorption capability of Ru. In the double layer region the formation of Ru-oxides was also well manifested.