Visualizing Hydrogen Oxidation Reaction Deactivation on a Polycrystalline Pt Electrode Surface Suppressed by Melamine: A Scanning Electrochemical Microprobe Study

Pt supported on carbon black (Pt/C) is generally used as an anode and cathode catalyst in polymer electrolyte fuel cells. Owing to the highly complex surface microstructure of Pt/C, electrochemical studies employing single-crystal Pt surfaces are powerful but insufficient alone for a comprehensive understanding of the catalytic properties and surface modification effects by organic compounds. This study investigated the dependence of the hydrogen oxidation reaction (HOR) activity of a polycrystalline Pt electrode surface on grain crystallographic orientation and the influence of surface modification by melamine. As a prerequisite, the 2D HOR activity map obtained by a scanning electrochemical microprobe was successfully correlated with the 2D grain crystallographic orientation map determined by electron backscatter diffraction by using indentation marks made by a nanoindenter on the electrode surface as a guide. Low-activity grain regions had orientations close to (100), whereas moderate- and high-activity grain regions exhibited orientations close to (110) and (111), respectively. Then, HOR deactivation was monitored in situ by conducting square-wave potential cycling in a 0.1 M NaClO₄ + 0.01 M HClO₄ mixed solution with or without melamine. Changes in the HOR activity map revealed that melamine effectively suppressed HOR deactivation, even under oxidation and reduction conditions of the Pt electrode surface, by stabilizing the microstructures of the (111)- and (110)-oriented surface grains. These findings demonstrate that grain-resolved 2D activity maps provide essential information about the relationship between grain properties and catalytic activity for the design of efficient catalyst surfaces.