Operando X-Ray Absorption Spectroscopic Study on Electrochemical Oxygen Reduction Mechanism of Novel Platinum-Based Nanostructured Catalysts

Abstract

Polymer electrolyte fuel cells (PEFCs) have been developed for electric device applications such as fuel cell vehicles (FCV). PEFC cathode catalyst requires an excess Pt due to the degradation of oxygen reduction reaction (ORR) activity. Many research efforts aim to design Pt-based nanomaterials with controllable composition and structure to increase their activity while reducing the amount of Pt, like core-shell structure. The core-shell structure is obtained by Adzic’s method (Cu-UPD)1, where the underpotential deposition (UPD) of Cu mediator on Pt and the displacement of Cu mediator by Pt are conducted. This method requires the highly precise potential control for the UPD of Cu by means of potentiostats. In contrast, our previous research demonstrated the spontaneous two-step method (Galvanic cell) to synthesize spherical Pd@Pt core-shell catalyst without a special control of electrochemical equipment and reducing/stabilizing agents2. In this study, this Galvanic cell method was applied for Pd@Pt core-shell nanowire catalysts. Compared with the conventional Cu-UPD method, this method can synthesize the catalysts on a larger scale and form more uniform monolayer Pt shells, resulting in a higher ORR activity.Ultrathin Pd nanowires were prepared using surfactants as templates3. Briefly, palladium nitrate, ODA, and DTAB were dissolved in toluene under strong magnetic stirring. The mixture was placed in an inert atmosphere and sonicated for 20 min. NaBH4 was added to the above solution as a reducing agent. One hour later, the organic phase was recovered using distilled water and chloroform as extractants. After centrifugation, Pd nanowires can be obtained. After mixing the catalyst with carbon, the Pd/C powder is dispersed in the N2-saturated 0.01M CuSO4 and 0.5 M H2SO4 solution, with continual stirring. Then, a Cu wire is immersed in the above mixture2. Near 5 hours later, Cu wire is extracted, and successively N2-saturated 10 mM K2PtCl4 solution is added to the suspension to obtain Pd-nanowire@Pt/C. Electrochemical tests were performed in 0.1 M HClO4, and the catalyst ORR activity was evaluated at 0.9 V. XAS measurements of Pt L-edge were also performed under the same conditions.In contrast to the previously reported method (Cu-UPD)4, our proposed method does not require electrochemical equipment, or reducers/stabilizers, and uniform Pt shells in Pd-nanowire@Pt/C can be obtained. As shown in Figure 1, Core-shell catalysts have a lower white line intensity than Pt/C, meaning that they are more difficult to oxidize. The ligand effect causes surface Pt compression, and after calculating, the first-shell Pt-Pt bond length in the Pd-NW@Pt Gal. catalyst is shorter than that of the nanoparticle, which means a stable structure and less oxygen species generation. So, the catalysts synthesized using the Galvanic Cells method can provide higher ORR activity (specific activities: 0.8 mA/cm2 and mass activities: 1.3 A/mgPt) compared to the Cu-UPD method. Acknowledgement This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References; [1] Zhang, J.; Mo, Y.; Vukmirovic M. B.; Klie R; Sasaki K.; Adzic R R.; J. Phys. Chem. B 2004 108 (30), 10955-10964.[2] Wang, X.; Orikasa, Y.; Inaba, M.; Uchimoto, Y.; et al. ACS Catal., 2020, 10, 430– 434.[3] Teng, X.; Han, W.; Ku, W.; Hucker, M.; et al. Angew. Chem., Int. Ed. 2008, 47, 2055– 2058.[4] Koenigsmann, C., Adzic, R. R.; et al. J. Am. Chem. Soc., 2011 , 133, 9783– 9795. Figure 1