Morphology-Controlled Pt Alloy Nanoparticles for Oxygen Reduction Reaction

Proton-exchange membrane (PEM) fuel cell, which can provide clean electricity with high efficiency, is essential for improving the sustainability of our society. Since oxygen reduction reaction (ORR) limits the performance of PEM fuel cells, precious platinum catalyst is crucial in the cathode electrode. Recently, Pt and its alloy nanoparticles (NPs) with controlled morphology have been attracted to reduce the amount of Pt loading. For example, nano-frames, nao-rods, nano-plates, etc. have been reported. As reduction rate and/or stability of a chemical reaction from ionic to metallic state play a key role to synthesize morphology-controlled NPs, in 233rd ECS meeting, we have shown a control technique of oxidation-reduction potential (ORP). However, the controllable range of ORP was revealed to be narrow, which suggests that controllability of the reduction rate needs to be extended. In order to delay the reduction rate for morphology control of Pt alloy NPs, solid precursor instead of ionic counterpart is hopeful. Herein, ammonium hexachloroplatinate which is generated in recycling process is chosen as Pt precursor. The ammonium hexachloroplatinate hardly dissolves in aqueous ammonium chloride solution, whereas it is known to dissolve well in water. In the present work, we demonstrate a facile synthesis method of morphology-controlled Pt alloy NPs by using solid Pt precursor. The ammonium hexachloroplatinate was chemically reduced with Cu ions on carbon support. Synthetic solution was adjusted to be water or ammonium chloride solution, and the samples were labelled as without-NH 4 Cl and with-NH 4 Cl, respectively. Transmission electron microscope (TEM) observation revealed that, in the without-NH 4 Cl, highly dispersed small NPs were deposited on carbon support. On the other hand, in the with-NH 4 Cl, catalytic NPs were obviously larger than those in the without-NH 4 Cl, and the morphology was not spherical but plate-like. X-ray diffraction (XRD) profiles indicated that crystal phases of both samples were mainly composed of Pt-Cu alloy crystal. ORR catalytic activities were measured for the small Pt alloy NPs (without-NH 4 Cl), the plate-like Pt alloy NPs (with-NH 4 Cl) and commercial Pt. Compared to the commercial Pt, electrochemical active surface area (ECSA) of the without-NH 4 Cl was almost the same and that of the with-NH 4 Cl was relatively smaller. This hierarchy is corresponding to particle diameters obtained by the TEM observation. Mass activitie