Abstract:

Direct methanol fuel cell (DMFC) is a potential clean energy facility because of abundant resources, easy storage, and high safety of methanol. However, the low activity, poor durability, and high price of the catalysts hamper the development of DMFC. High-index faceted nanocrystals usually show high catalytic activity for the electro-oxidation of small organic molecules due to high densities of low-coordinated surface sites. Surface modification is an alternative approach for improving catalyst performance via ligand effect or electronic effect. Herein, we prepared tetrahexahedral Pd nanocrystals (THH Pd NCs) enclosed by {730} high-index facets via electrochemical square-wave potential deposition, and modified the THH Pd NCs with Ru using cyclic voltammetry (CV). The coverages of Ru (θ_Ru) were controlled by limiting the CV cycles. The electrocatalytic performance of the Ru-modified THH Pd NCs for methanol oxidation was studied using CV in an alkaline methanol solution. We found that Ru modification can greatly reduce the onset and peak potentials of methanol electro-oxidation from -0.33 to -0.39 V and from -0.16 to -0.26 V, respectively. The current densities at -0.3 V during methanol electro-oxidation increased with increasing θ_Ru from 0 to 0.08, and decreased with increasing θ_Ru from 0.08 to 0.27. When θ_Ru was 0.08, the current density on the Ru-modified THH Pd NCs reached 1.5 mA∙cm^-2, which was 10 times higher than that achieved for the THH Pd NCs. To detect the products at molecular level during methanol electro-oxidation, in-situ electrochemical Fourier-transform infrared (FTIR) spectroscopy was applied. The spectra of both THH Pd NCs and Ru-modified THH Pd NCs (θ_Ru = 0.08) showed that both CO₂ and formate were produced. The band intensities of CO₂ and formate on Ru-modified THH Pd NCs (θ_Ru = 0.08) were 1.6- and 1.2-times larger than those of THH Pd NCs, respectively. However, the band intensity of CO_ad during methanol electro-oxidation was almost equal on the two catalysts, implying that the "CO poison path" is not suppressed by Ru modification. These results indicate that a small amount of Ru (θ_Ru = 0.08) modification is beneficial for the electrocatalytic oxidation of methanol by enhancing the "formate path" at low potential. Overall, this study illustrates the influence of Ru modification of THH Pd NCs on its catalytic performance in methanol electro-oxidation, which throws light on the synthesis and application of catalysts with high activities.

Key words: High-index facet, Tetrahexahedral Pd nanocrystal, Surface modification, Methanol