Aqueous Phase Synthesis of Cu/Pt Core-Shell Nanoparticles Supported on Carbon Black By Controlling Cu and Pt Complexes for a PEFC Cathode Catalyst
Authors: Terui Yosuke , Yokoyama Shun , Tsuchida Shuzo , Seki Ryohei , Taniguchi Yasushi , Ueyama Yasuhiro , Takahashi Hideyuki , Tohji Kazuyuki 
Published: February 2020

Since polymer electrolyte fuel cell (PEFC) could be miniaturized and operate in low temperature, PEFC is researched and developed vigorously for utilizing to our personal living. Here, it is well known that reaction ratio of cathode is extremely slow than that of anode, and that optimum catalyst material for PEFC cathode is Platinum (Pt) because of its high catalyst ability. However, it is also true that Pt is one of the precious metals, which increase the cost of PEFC. Thus, high cost and low stability are the major problems for the commercial application of PEFC. Therefore, the amount of Pt of the cathode materials must be reduced.

To reduce the amount of Pt in PEFC cathode, substitution of the core of Pt nanoparticles with other transition metal is effective, since inside of nanoparticles does not affected to the catalytic reactions. Therefore, in this study, synthesis method of well-dispersed Cu core - Pt shell nanoparticles supported on Ketchen Black (KB) substrate (Cu@Pt/C catalyst) were developed. Especially, we develop the restricted deposition method of nanoparticles onto the surface of KB (thus, nanoparticles were “not” deposited in the micropore of KB).

The Cu@Pt/C catalyst was synthesized by utilizing the liquid phase reduction method. Copper(II) chloride dihydrate (CuCl2・2H2O) and chloroplatinic acid (H2PtCl6・6H2O) were used as the precursors. Trisodium citrate (C6H5Na3O7・2H2O) and Cetyltrimethylammonium bromide (CTAB) were used as complexing agent and dispersing agent of KB, respectively. Sodium borohydride (NaBH4) was selected reducing agent.

The surface potential of KB were positive because of the covering with hydrophilic group of CTAB. On the other hand, metal complex estimated by the calculation has negative charge ([(Cu2+)2(OH-)2(cit3-)2]). By utilizing these electrostatic interaction, 41.3% of Copper complex in original solution was successfully absorbed on the surface of KB.

Cu nanoparticles with a mean particle diameter of 12.9 nm were synthesized through the reduction of these Cu species, and these were galvanic displaced by Pt according to the following reaction (2Cu + [PtCl6]2- → Pt + 2Cu2+ + 6Cl-). Synthesized materials were analyzed by X-ray Diffraction (XRD), Scanning Transmission Electron Microscopy (STEM), Cyclic Voltammetry (CV) and Rotating Disk Electrode (RDE) analysis.

XRD results show that peak position of Pt , such as (111), were shifted to higher angle, which means that a part of Pt formed mixed phase with Cu instead of pure Pt and/or Cu metals. These peaks were not disappeared after acid treatment using 13M HNO3. SEM-EDS results indicated that the ratio of Cu:Pt were 27.1 : 72.9. These results suggested that the nanoparticles synthesized were Pt-Cu alloy core with Pt shells or Cu core with Pt-Cu shells.

Electrochemical active surface area (EASA) was calculated from the results of CV analysis. Specific Activity (SA) was calculated from Koutechy´–Levich plots which were obtained from linear sweep voltammograms obtained by RDE analysis. The calculated EASA and SA values were 42.0 [m2/g] and 20.4[A/m2@0.8V], respectively. Both values are smaller than that of the commercial catalysts (59.3 [m2/g] and 25.5[A/m2@0.8V] ). It is due to the presence of coarse particles of Cu@Pt catalyst and the inert material on the surface of particles. It is necessary to synthesize nanoparticles with critical radius and dispose the inert material. Another results will be reported in our session.