Electrocatalytic Activity of Spots of Electrodeposited Noble-Metal Catalysts on Carbon Nanotubes Modiﬁed Glassy Carbon Xingxing Chen, Kathrin Eckhard, Min Zhou, Michael Bron, and Wolfgang Schuhmann* Analytische Chemie, Elektroanalytik & Sensorik, Ruhr-Universita ¨ t Bochum, Universita ¨ tsstrasse 150, D-44780 Bochum, Germany A strategy for the screening of the electrocatalytic activity of electrocatalysts for possible application in fuel cells and other devices is presented. In this approach, metal nanoclusters (Pt, Au, Ru, and Rh and their codeposits) were prepared using a capillary-based droplet-cell by pulsed electrodeposition in a diffusion-restricted viscous solution. A glassy carbon surface was modiﬁed with carbon nanotubes (CNTs) by electrophoretic accumulation and was used as substrate for metal nanoparticle deposi- tion. The formed catalyst spots on the CNT-modiﬁed glassy carbon surface were investigated toward their catalytic activity for oxygen reduction as a test reaction employing the redox competition mode of scanning elec- trochemical microscopy (RC-SECM). Qualitative informa- tion on the electrocatalytic activity of the catalysts was obtained by varying the potential applied to the substrate; semiquantitative evaluation was based on the determina- tion of the electrochemically deposited catalyst loading by means of the charge transferred during the metal nano- particle deposition. Qualitatively, Au showed the highest electrocatalytic activity toward the oxygen reduction reac- tion (ORR) in phosphate buffer among all investigated single metal catalysts which was attributed to the much higher loading of Au achieved during electrodeposition. Coelectrodeposited Au-Pt catalysts showed a more posi- tive onset potential (-150 mV in RC-SECM experiments) of the ORR in phosphate buffer at pH 6.7. After normal- izing the SECM image by the charge during the metal nanocluster deposition which represents the mass loading of the catalyst, Ru showed a higher electrocatalytic activity toward the ORR than Au. One topic of great interest in PEM (proton exchange mem- brane) fuel cell research over the last decades is the development of novel catalysts with low cost and high electrocatalytic activity. 1-4 Carbon supported Pt and its alloys have been established as state of the art catalysts for anode and cathode reactions in PEM fuel cells. 5-9 Different routes such as impregnation/reduction, colloidal methods, spontaneous reduction, and electroless deposition have been developed to prepare highly dispersed catalysts with small particle size in order to efﬁciently use the precious metal. 10-15 Besides, electrochemical deposition, especially the pulsed elec- trodeposition technique, 6,16-18 is proving to be an increasingly attractive route for the preparation of catalyst nanoparticles due to its simple operation, high purity of deposits, and easy control of the particle size. 19 In the case of electrocatalysis, electrodepo- sition is especially attractive due to the fact that deposition takes place only at electrically connected sites. 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