Enhancement of Electrocatalytic Oxygen Reduction Reaction on Pd3pb Ordered Intermetallic Catalyst in Alkaline Aqueous Solutions
Authors: Matsumoto Futoshi , Jeevagan Arockiam John , Gunji Takao , Tanabe Toyokazu , Kaneko Shingo 
Published: February 2020

Enhancement of the oxygen reduction reaction (ORR) was examined with Pd3Pb ordered intermetallic nanoparticles (NPs) supported on titania (Pd3Pb/TiO2). The Pd3Pb/TiO2 catalyst was synthesized by conventional wet chemical method with Pd and Pb ion precursors, a reducing agent and TiO2 powder under ambient temperature. Powder X-ray diffraction (pXRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy measurements indicated the formation of the ordered intermetallic phase of Pd3Pb in the NP forms on the TiO2 surfaces. Electrochemical measurements showed that the Pd3Pb/TiO2 catalyst can remarkably enhance the ORR in alkaline environment due to the unique surface of Pd3Pb NP and the strong interaction between Pd3Pb and TiO2 when compared with TiO2-supported Pd and Pt NPs. The onset potential of Pd3Pb/TiO2 was shifted towards to higher potential by 110-150 mV when compared to Pd/TiO2 and Pt/TiO2.

The catalyst-coated GC electrodes were used to study the ORR activities and kinetics at the Pd/TiO2 and Pd3Pb/TiO2 electrodes. Figure 1A shows the ORR polarization curves of Pd/TiO2 (a) Pd3Pb/TiO2 (b) and Pt/TiO2 (c) catalysts obtained in O2-saturated 0.1M KOH(aq) solution at a rotation speed of 2000 rpm. The polarization curves show a well-defined diffusion limiting current region from -0.8 V to -0.4 V. The onset potential of the Pd/TiO2 (a) for the ORR is -0.25 V. On the other hand, the Pd3Pb/TiO2 (b) shows the onset potential at -0.14 V, which is more positive than that of Pd/TiO2, indicating that the Pd3Pb NPs feature a significant enhancement in the electrocatalytic ORR in an alkaline environment when compared with Pd/TiO2. Further, the electrocatalytic ORR activity of Pd3Pb/TiO2 was also compared with Pt/TiO2 (c) catalyst. The Pt/TiO2 catalyst showed an onset potential at -0.22 V (Fig. 1A (c)). In order to evaluate the kinetic parameter of Pd3Pb/TiO2 during the course of ORR, the polarization curves were recorded at different disk rotation rate in rotating disk electrode (RDE) experiments. Fig. 1B depicts the dependence of ORR curves on the disk rotation rate for the Pd3Pb/TiO2-coated GC. The number of electrons involved in ORR can be determined from the slope of the Koutecky-Levich (K-L) plot. The diffusion coefficient of O2
(DO2
= 1.9 x 10-5 cm2 s-1) , the kinetic viscosity of the solution (0.01 cm2 s-1) and the bulk concentration of O2 (CO2
=1.2 x 10-6 mol cm-3) were used to estimate the number of electrons.  From the straight lines shown in the K-L plots (Fig.1C), the number of electron transferred in ORR was calculated to be 3.65-3.76 at around -0.08-0.28 V, indicating that the ORR is relatively dominated by a four-electron reduction of O2 in the potential region from -0.08 to -0.28 V. A chronoamperometric study was conducted to investigate the durability of the catalyst for the ORR. The current-time curves on Pd3Pb/TiO2 (a) and Pd/TiO2 (b) catalysts in O2 saturated 0.1 M KOH(aq) solution are shown in Fig. 1D. It is clear that for ORR on Pd3Pb/TiO2 (a) at constant applied potential of -0.1 V, the current densities of the catalysts change quickly in the initial stage and after reaching to 5 mAcm-2, the current density decrease slowly. After 90 min, the current density decreased to -3.9 mAcm-2 (to 78%). The above results clearly reveal that the Pd3Pb/TiO2 is more stable than that of Pd/TiO2 in terms of catalytic activities of oxygen.

Fig. 1 (A) ORR polarization curves of (a) Pd/TiO2, (b) Pd3Pb/TiO2 and (c) Pt/TiO2 catalysts in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s-1 and electrode rotation rate of 2000 rpm. (B) the dependence of ORR current density of the Pd3Pb/TiO2-coated GC electrode in a 0.1 M KOH solution on the electrode rotation rate. (C) K-L plots obtained from the ORR curves for the Pd3Pb/TiO2 at different potentials. (D) Chronoamperometry curves of (a) Pd3Pb/TiO2 and (b) Pd3Pb/CB catalysts in O2-saturated 0.1 M KOH at electrode potential of -0.1 V.

Figure 1