Carbon-supported Ag (Ag/C) catalysts with four different metal loadings were prepared by a citrate-protecting method. Oxygen reduction reaction (ORR) activities on these carbon-supported Ag-nanocatalysts (Ag/C) in alkaline solutions were studied. Four major findings are reported in this paper: (1) Test results indicate that the Ag/C catalysts promote predominately a four-electron pathway for ORR on electrodes over the swept potentials from 0.2 to −0.8 V vs Hg/HgO in O2-saturated 0.1 M NaOH solutions. (2) A novel marker for predicting ORR activities on the Ag/C catalysts based on the cyclic voltammetry characteristic curves has been identified: the ORR activities have a strong correlation with the intensity of the anodic peak at the potential of 0.230 V vs Hg/HgO in Ar-saturated 0.1 M NaOH solutions. (3) As the metal loading on carbon particles increases from 10 to 60 wt %, the peak intensities increase linearly, and the ORR onset potentials shift positively with maximum shift of 62 mV for 60% Ag on carbon support. (4) A hitherto unnoticed poisoning effect has been discovered: silicate has a significant poisoning effect on the ORR activities of the Ag/C catalysts.

The objective of this study is to investigate the effect of palladium particle size on catalytic activity for oxygen reduction reactions (ORRs) in alkaline solutions. Carbon-supported palladium catalysts with average Pd particle sizes from 3to16.7nm were obtained by treating a 20% Pd∕C catalyst in a normalH2∕normalN2 gas at elevated temperatures from 300to600°C , respectively. The specific activity of the Pd∕C catalysts for the ORRs, described by a kinetic ORR current (inormalk) normalized to the Pd active surface area, was found to increase continuously by a factor of about 3 with increased particle size from 3to16.7nm without reaching a maximum. The mass activity increased by a factor of 1.3 when Pd particle size increased from 3to5nm and then decreased with increased Pd particle size. The hydrogen peroxide yield on the Pd∕C catalysts was negligible, especially in the mixed kinetic-diffusion controlled region, indicating that the four-electron transfer path is dominant for the ORR on the Pd∕C in alkaline solutions.

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Carbon-supported Pt and Pd ( 20wt% metal) were employed as oxygen reduction catalysts in 0.1M NaOH solutions. Results from X-ray diffraction analysis show that the metals are in the form of Pt face-centered cubic (fcc) and Pd (fcc) crystal only. The metal particles were observed to distribute uniformly on carbon supports, and the average particle size of the Pd and the Pt was 3.1 and 2.3nm , respectively, from the transmission electron microscopy image analysis. The Pd∕C catalyst showed high activity toward the oxygen reduction reaction (ORR) both at room and elevated temperatures (up to 60°C ) compared with the Pt∕C . The activation energies for the ORR on the Pd∕C are slightly lower than that on the Pt∕C (39 vs 48kJmol−1 at the overpotential of 0.3V ). The four-electron path is dominant on both the Pt∕C and the Pd∕C . The normalH2normalO2 yields on both catalysts are analyzed in detail. As a cathode fuel cell catalyst, the Pd∕C catalyst is less sensitive to alcohol contamination as compared to Pt∕C . The challenges for using Pd∕C as an ORR catalyst in alkaline solutions are also discussed in this paper.

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