(45d) Graphitic Mesoporous Carbon as a Support for Pt, Pt-M, and Pd Electrocatalysts to Achieve Stable Oxygen Reduction Catalysts

Proton exchange membrane fuel cell catalysts currently are limited by (1) the cost of platinum; (2) the current per mass of metal for the oxygen reduction reaction at the cathode; and (3) the catalyst stability during load cycling in transportation applications. Alloy catalysts offer a promising route to achieving higher activities by introducing strain into the surface atoms. The use of a highly graphitic carbon support, which has a much higher resistance to oxidation than amorphous carbon, is utilized to mitigate loss of active metal surface area. For transportation applications, catalyst support stability at 1.2V is required for an accumulated life time of the vehicle, with a maximum allowable performance loss of < 30 mV. Typical amorphous carbon supports, for example Vulcan carbon, easily oxidize at potentials of 1.2V, which leads to significant activity losses.

An emerging concept in catalyst design is to pre-synthesize metal nanocrystals coated with stabilizing ligands to control their morphology, and then to infuse the particles onto high surface area supports. This technique allows for the synthesis of catalyst particles on graphitized carbons, which do not bind efficiently with the precursors used for particle synthesis in the traditional wetness impregnation technique. We have synthesized a variety of pure metal and metal alloy catalysts for ORR by an arrested growth precipitation method, including Pt, PtPd, PtCu, and Pd nanoparticles with controlled sized (ranging from 3-6 nm) and composition. They were then infused onto mesoporous graphitic carbon (ordered and disordered) treated at various graphitization temperatures. The activities of the catalysts were determined by rotating disk measurements at 0.9 V, where the catalysts are in the kinetically controlled regime.

Monometallic and bimetallic catalysts for ORR were found to be stable for 1000 cycles from 0.5 to 1.2V. The corrosion resistant graphitic supports underwent little oxidation, and thus the morphology of the supported metals was found to be stable according to XRD, SEM-EDS, and XPS. The strong metal interaction with the π electron support sites further contributed to the stability. The combination of high activities and high stabilities for these metals with highly controlled shapes and composition on graphitic carbon is of high practical importance for fuel cell applications.