Morphology-Controlled Pt Alloy Nanoparticles for Oxygen Reduction Reaction
Authors: Nakamoto Tatsuichiro , Seki Ryohei , Yokoyama Shun , Tsuchida Shuzo , Takahashi Hideyuki , Tohji Kazuyuki 
Published: February 2020

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-NH4Cl and with-NH4Cl, respectively.

Transmission electron microscope (TEM) observation revealed that, in the without-NH4Cl, highly dispersed small NPs were deposited on carbon support. On the other hand, in the with-NH4Cl, catalytic NPs were obviously larger than those in the without-NH4Cl, 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-NH4Cl), the plate-like Pt alloy NPs (with-NH4Cl) and commercial Pt. Compared to the commercial Pt, electrochemical active surface area (ECSA) of the without-NH4Cl was almost the same and that of the with-NH4Cl was relatively smaller. This hierarchy is corresponding to particle diameters obtained by the TEM observation. Mass activities of our samples were higher than the commercial Pt, especially, the without-NH4Cl showed a 2-fold increase. The with-NH4Cl exhibited 4 times higher specific activity. Therefore, plate-like Pt alloy NPs will achieve drastically higher mass activity if ECSA is improved. In order to explain the ORR catalytic activities of the synthetic samples, a model of structure of the catalytic NPs was constructed (see Figure). The with-NH4Cl has larger diameter of the catalytic NPs, which indicates more atoms are located at the core of the particles. Since Pt atoms at the core are catalytically inactive, with-NH4Cl showed smaller ECSA than the without-NH4Cl. However, at the same time, the without-NH4Cl has more Cu atoms at the surface, and exposed Cu atoms are easy to leach out during potential scans in acidic solution. In Pt alloy catalysts, transition metals including Cu exhibit an important synergistic effect. When Cu atoms combine with Pt atoms, electronic state of surface Pt changes and surface adsorption energy is tuned, which contributes to an enhancement of ORR catalytic activity. As a result, with-NH4Cl which has larger diameter inhibited leaching of Cu atoms at the core, and sustained the synergistic effect to enhance specific activity. Moreover, plate-like morphology of the with-NH4Cl can be another factor of the high specific activity by selective exposure of active facet. It is reported that Pt nano-plates selectively expose active facet and exhibit high specific activity.

In conclusion, we adjusted concentration of ammonium chloride in aqueous synthetic solution to control reduction rate of Pt and morphology of Pt alloy NPs supported on carbon. Highly dispersed small Pt alloy NPs were synthesized in water, whereas plate-like Pt alloy NPs which showed high specific activity were synthesized in aqueous ammonium chloride solution. However, when the concentration of ammonium chloride is excess, the chemical reduction of Pt is inhibited and synthetic yield decreases. Thus, the addition amount of ammonium chloride requires careful consideration.

Figure 1