Improved Electrocatalytic Oxidation of Urea Using Ru-Ni Binary Nanoparticles Prepared By an Ionic Liquid/Metal Sputtering Technique

Abstract

Introduction Direct urea fuel cells are attracting as next generation power sources due to the high energy density, safety, and ease of transportation of urea in comparison with other candidate sources for fuel cells, such as hydrogen, alcohols and ammonia. Botte and co-workers reported that nickel electrodes exhibited electrocatalytic activity for urea oxidation in basic aqueous solution.[1] However further improvement in stability and activity for catalysts is necessary to achieve practical use. Recently we have reported the facile preparation of metal and alloy nanoparticles in ionic liquids without additional stabilizing agents using metal sputter deposition technique.[2]The extremely low vapor pressure of ionic liquids enables the sputter deposition of metal species onto the ionic liquids under reduced pressure. Noble metal nanoparticles of several nanometers in size, such as Au, Pd, and Pt, were successfully prepared by sputtering the corresponding metal targets, while simultaneous sputter deposition of different metals produced alloy nanoparticles such as AuPd and AuPt. Alloy nanoparticles exhibited unique physicochemical properties distinct from those of monometallic counterparts, and their catalytic activities could be modified by the control of chemical composition of nanoparticles. Here we report the preparation of Ru-Ni binary nanoparticles by sequential sputter deposition of Ru and Ni onto ionic liquids followed by heat treatment. Furthermore the electrocatalytic activity of thus-obtained nanoparticles is investigated for urea oxidation in alkaline media. Experimental Ru-Ni binary nanoparticles were synthesized by the sequential sputter deposition of Ru and Ni metals onto ionic liquids of 1-ethyl-3-methylimidazolium tetrafuloroborate using a sputter coater (SC-701HMCII, Sanyu Electron). A portion of acetonitrile solution dispersed with carbon black (CB) particles was mixed with thus-obtained ionic liquids, followed by heat treatment at 300 ˚C for 30 min. Thus-obtained Ru-Ni nanoparticles-immobilized CB particle catalyst (Ru-Ni/CB) were isolated by centrifugation. The Ni/Ru ratio in the catalyst was controlled by changing the amount of Ru sputter-deposited in the ionic liquid. Nanoparticles of Ru and Ni, loaded on CB in a similar manner, were also prepared for comparison. The electrocatalytic activity for urea oxidation was investigated with use of grassy carbon electrodes modified with Ru-Ni/CB catalyst particles in 0.1 mol dm-3 NaOH aqueous solution containing 0.1 mol dm-3 urea. Results and discussions TEM observation of Ru-Ni/CB revealed that small Ru-Ni binary nanoparticles with average diameter of 3.9 ± 2.1 nm were densely immobilized on the surface of CB particles without formation of large aggregation as shown in the inset of Fig. 1. The chemical condition of particles loaded on CB was investigated by XPS. Ni/CB showed a single peak of Ni 2p at 856.1 eV attributable to Ni2+, indicating that Ni was completely oxidized during preparation procedure of catalyst. In contrast, the Ni 2p signals of Ru-Ni/CB appeared at 854.2 and 860.1 eV, which were assigned to Ni0 and Ni3+, respectively. This suggests that Ni oxidation was partially prevented in Ru-Ni/CB probably due to the electronic interaction between Ni and Ru atoms. Figure 1 shows cyclic voltammograms of Ru-Ni/CB, Ni/CB, and Ru/CB for urea oxidation in 0.1 mol dm-3NaOH aqueous solution. As reported previously, Ni/CB exhibited the electrocatalytic activity for urea oxidation, in which anodic current for urea oxidation was increased at the potential more positive than 0.5 V vs Ag/AgCl, though no current was observed for Ru/CB. On the other hand, urea oxidation also occurred with use of Ru-Ni/CB at the potential more positive than 0.5 V vs Ag/AgCl, the intensity being about 2 times larger than that of Ni/CB at a constant potential. These results indicated that the electrocatalytic activity for urea oxidation was higher for Ru-Ni binary particles than pure Ni particles, in spite that Ru particles had no activity. The chemical composition of Ru-Ni/CB catalyst was found to influence the catalytic activity: Volcano-type dependence was observed between Ru content in Ru-Ni/CB and the current density at 0.6 V vs. Ag/AgCl. Consequently, we concluded that electrocatalytic activity for urea oxidation of Ni particles was improved by preparing Ru-Ni binary nanoparticles in which Ru and Ni were probably mixed in atomic or nanometer scales. REFERENCES [1] Botte, et al., Electrochimica Acta, 2012, 81, 292. [2] M. Hirano, et al., Phys. Chem. Chem. Phys., 2013, 15, 7286. Figure 1