(91a) Improved Activity of Carbon-Supported Platinum Electrocatalysts: Synthesis and Characterization
The particle size effect is known to play an important role in catalysis, particularly in the case of electrocatalysts comprising nanoparticles (NPs). The activity and mechanism for the oxygen reduction reaction (ORR) have been reported to be dependent on catalyst size. However, investigation of ORR catalyst particle size effects as a means to maximize activity while minimizing precious metal content is challenging because the commonly used technique of impregnation and reduction of Pt salts for the synthesis of C-Pt catalysts yields NP structures with broad particle size distribution. Recent attempts at systematic studies to investigate size-controlled NP structures have had limited success, especially with particle sizes less than about 1.5 nm, leaving size effects occurring in the sub-nanometer regime unexplored. This size region is interesting because significant changes from bulk properties can occur. In this work, we describe the colloidal synthesis and characterization of monodisperse Pt NPs from <1 nm to ~3 nm spanning the cluster-to-crystal transition and subsequent deposition onto carbon support (for batches ranging from approximately 20-atom clusters to 650-atom, 2.7-nm particles with narrow distribution). We are able to illustrate a particle size effect across this transition region that affects oxygen reduction reaction (ORR) activity and electrochemically active surface area. We report approximately 2x increased ORR activity for synthesized 1.5 nm diameter particles versus commercial catalyst. Images of these particles show a structure that is transitional between crystalline and amorphous. TEM, FFT, Tafel analysis, and EXAFS are used to characterize the particles, reaction mechanism, and to propose a reason for observed particle size effects. This investigation provides a novel method for synthesizing and supporting catalysts for use in proton exchange membrane fuel cells that yields catalysts with improved activity versus current commercially available catalysts.