Pt-Based Intermetallic Nanoparticles as Electrocatalysts for ORR

One of the barriers for the commercilization of fuel cells is the catalysts, in which both anode and cathode consist of Pt-based precious metal. Since Pt is currently costly and scarce, it poses a question of how to lower the Pt loading and increase the efficiency. Most of previous studies focused on Pt alloyed with some 3d-transition metals, such as Fe, Co, Ni, etc. However, the activity and stability are not good enough for fuel cell applications. Recently, ordered intermetallic nanoparticles have attracted some attention, since the ordered intermetallic phase provides definite composition and structure. They can provide predictable controls over structural, geometric, and electronic effects, which are not afforded by alloys. However, previous reports on ordered intermetallic are mainly used as anode catalysts, such as formic acid oxidation. And the synthesis procedure is very complex. More importantly, the particles are unsupported and it is not easy to clean the particle surface. In our studies, we present that carbon supported ordered intermetallic nanoparticles can be easily formed using a simple impregnation-reduction method followed by high temperature pretreatment. Ordered Pt 3 Co intermetallic cores with a 2-3 atomic-layer thick platinum-rich shell was found according to electron energy loss spectroscopical (EELS) mapping. These nanoparticles showed over 200% increase in mass activity and over 300% increase in the specific activity compared with the disordered Pt 3 Co alloy nanoparticles for the ORR. The stability tests showed a minimal loss of activity after 5,000 potential cycles and the ordered core-shell structure was maintained virtually intact according to EELS mapping. Two dealloying methods (electrochemical and chemical) were implemented on Cu3Pt/C nanoparticles to control the atomic-level morphology and activating the performance for oxygen reduction reaction (ORR). It is found that electrochemical dealloying method resulted in formation of a thin Pt skin of c.a. 1 nm with an ordered Cu 3 Pt core structure, while the chemical leaching gave rise to a spongy structure, with no ordered structure being preserved. Both dealloying methods yielded enhanced specific and mass activity toward ORR and higher stability relative to Pt/C. The chemical dealloyed nanoparticles show better mass activity than the particles directly electrochemical dealloyed after 50 potential cycles, while a slight lower specific activity. The mass activity even enhanced after 5000 potential cycles in both cases. These findings are important to build next-generation fuel cell catalytsts.