Many research groups use the half-cell test to measure the oxygen reduction reaction (ORR) activity of Pt catalysts, which is mainly used as electrocatalyst in the polymer electrolyte fuel cell (PEFC) electrode. The past decade has seen significant development from a variety of highly active platinum catalysts studies for the oxygen reduction reaction. However, these performances are almost exclusively obtained from the half-cell test and are difficult to get from the single-cell test. Because strong adsorption of sulfonated side chain in PFSA ionomer declines the oxygen reduction reaction (ORR) Pt. The ionomer, as binding material in electrode, diminishing the ORR activity by specific adsorption of sulfonate groups onto the Pt active sites existing on exterior surfaces of carbon support. In this work, effect of ionomer loading and Pt distribution on interior/exterior surface of carbon support is systematically investigated and the gap between the half-cell test and the single-test is closed. The Pt particles located onto the microporous HSC may cause the dramatically lowed catalytic poisoning as ionomer covering the Pt/HSC is inaccessible to the Pt catalysts on the interior surfaces. For an accurate verification of the location of Pt, both Pt/vulcan carbon (VC) and Pt/HSC was measured 180 and 0 degrees in scanning electron mode (SE) and scanning transmission electron microscope mode (STEM) at the same location. Each about 20 % and 60 % Pt particles on the exterior surface of the carbon supports was located in Pt HSC and Pt/VC. The ionomer-to-carbon weight ratio was increased from 0.1 to 0.4 for a practical demonstration of the membrane electrode assembly (MEA) environments and confirming to the Pt poisoning by ionomer on the exterior surface of carbon supports at half-cell test. For the enhanced particle distribution of the catalyst ink, the pH values of catalyst ink were adjusted to pH 11 by adding 1M KOH solution. Electrochemical measurement was conducted to both single-cell test and half-cell test for proving that ionomer poisoning decreases the mass activity of Pt catalysts. The MEAs for single-cell test were prepared by the decal method with the half-cell test ink, to compare the results. As the ICR increases, the ionomer builds up on exterior surfaces of carbon supports and so poisoned by the amount of Pt located on the exterior micropores. Pt/HSC that has the Pt particles located the interior surfaces of carbon supports was comparatively resistant to the ionomer poisoning and contributed to the ORR activity of the Pt catalysts. As a result, the ionomer reduces the ORR activity of Pt/C by specific adsorption of sulfonate groups onto the active sites existing on exterior surfaces of carbon supports rather than those on interior surfaces. The Pt particles located on the interior surfaces of carbon supports are in general inaccessible to the ionomer particles due to the narrower void in the carbon supports. The results of this study suggest an approach to reduce the Pt active sites poisoning of the ionomers, which is expected that the mass transport and the ORR activity characteristics of the PEFC electrodes will be improved by introducing it into full cells.

Reference

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