PtNi Nanoparticles Encapsulated in Few Carbon Layers as High-Performance Catalysts for Oxygen Reduction Reaction

PtNi alloys have been demonstrated to be one of the most promising catalysts for oxygen reduction reaction (ORR) in fuel cell applications due to their high catalytic activity and efficient utilization of Pt. However, improving the durability of such catalysts remains a significant challenge. Herein, we report on the formation of PtNi nanoparticles with an average diameter of around 10 nm embedded in few carbon layers (PtNi@C) and their ORR catalytic performance. The synthesis procedure entails the use of a solvothermal method to form 2-methylimidazole–Pt-Ni composites with a MOF-like structure (designated as MPN) followed by a thermal annealing treatment. The presence of Pt during the solvothermal process catalyzes the deposition of Ni, and the Ni in the MPN catalyzes the formation of well-graphitized carbon at a temperature as low as 400 °C. The PtNi@C catalyst exhibits an ORR mass activity (MA) of 0.84 A mgpt–1 and a specific activity (SA) of 1.54 mA cm–2 at 0.9 V (vs RHE), representing 6.5-fold and 8.4-fold improvement over a commercial Pt/C catalyst. More significantly, the electrochemically active surface area of the PtNi@C catalyst shows little change after 5000 cycles of potential scans between 0.6 and 1.1 V in O2-saturated 0.1 M HClO4. This study demonstrates the feasibility of stabilizing Pt-based nanocatalysts through graphitic carbon encapsulation

Electrooxidation of Carbon Monoxide and Methanol on Platinum-Overlayer-Coated Gold Nanoparticles: Effects of Film Thickness

The electrooxidation of carbon monoxide and methanol on Pt-coated Au nanoparticles attached to 3-aminopropyl trimethoxysilane-modified indium tin oxide electrodes was examined as a function of Pt film thickness and Au particle coverage. For the electrodes with medium and high Au particle coverages, the CO stripping peak position shifts to more negative values with increasing Pt film thickness, from ca. 0.8 V (vs Ag/AgCl) at 1 ML to 0.45 V at 10 ML. Accompanying this peak potential shift is the sharpening of the peak width from more than 150 to 65 mV. For the electrode with low Au particle coverage, similar peak width narrowing was also observed, but the peak potential shift is much smaller, from 0.85 V at 1 ML of Pt to 0.65 V at 10 ML. These observations are compared with the CO oxidation on bulk Pt electrodes and on Pt films deposited on bulk Au electrodes. The film-thickness-dependent CO oxidation is explained by d band theory in terms of strain and ligand effects, the particle size effect, and the particle aggregation induced by Pt film growth. Corresponding to the increasing CO oxidation activity, the current density of methanol oxidation grows with the Pt film thickness. The peak potential and current density reach the same values as those obtained on a polycrystalline bulk Pt electrode when more than 4 ML of Pt is deposited on the Au particle electrodes with a particle coverage higher than 0.25. These results suggest that it is feasible to reduce Pt loading in methanol fuel cells by using Pt thin films as the anode catalyst

Electrooxidation of Carbon Monoxide on Gold Nanoparticle Ensemble Electrodes: Effects of Particle Coverage

Gold nanoparticles were attached to amine-functionalized indium tin oxide substrate to form particle ensemble electrodes with controlled particle coverage. Electrooxidation of carbon monoxide (CO) on these particle ensemble electrodes was studied in CO-saturated alkaline solutions by means of cyclic voltammetry, with an emphasis on the effects of particle coverage. CO oxidation half-wave potential was found to decrease with increasing particle density. However, the current density was significantly larger at lower particle coverage electrodes. On the basis of a model for electron transfer on a partially covered electrode, the observations were explained in terms of the change in reactant mass transport pattern with varying particle coverages:  CO diffusion is predominantly mixed spherical and linear at low particle coverages and changes to mostly linear at high particle coverages. The possibility of contributions from particle agglomeration is also briefly discussed

Electrooxidation of Carbon Monoxide and Methanol on Platinum-Overlayer-Coated Gold Nanoparticles: Effects of Film Thickness

The electrooxidation of carbon monoxide and methanol on Pt-coated Au nanoparticles attached to 3-aminopropyl trimethoxysilane-modified indium tin oxide electrodes was examined as a function of Pt film thickness and Au particle coverage. For the electrodes with medium and high Au particle coverages, the CO stripping peak position shifts to more negative values with increasing Pt film thickness, from ca. 0.8 V (vs Ag/AgCl) at 1 ML to 0.45 V at 10 ML. Accompanying this peak potential shift is the sharpening of the peak width from more than 150 to 65 mV. For the electrode with low Au particle coverage, similar peak width narrowing was also observed, but the peak potential shift is much smaller, from 0.85 V at 1 ML of Pt to 0.65 V at 10 ML. These observations are compared with the CO oxidation on bulk Pt electrodes and on Pt films deposited on bulk Au electrodes. The film-thickness-dependent CO oxidation is explained by d band theory in terms of strain and ligand effects, the particle size effect, and the particle aggregation induced by Pt film growth. Corresponding to the increasing CO oxidation activity, the current density of methanol oxidation grows with the Pt film thickness. The peak potential and current density reach the same values as those obtained on a polycrystalline bulk Pt electrode when more than 4 ML of Pt is deposited on the Au particle electrodes with a particle coverage higher than 0.25. These results suggest that it is feasible to reduce Pt loading in methanol fuel cells by using Pt thin films as the anode catalyst

Electrooxidation of CO on Uniform Arrays of Au Nanoparticles: Effects of Particle Size and Interparticle Spacing

Uniform arrays of Au nanoparticles with controlled size and interparticle distance were synthesized by using polystyrene-b-poly(2-vinylpyridine) as a template and an Ar plasma treatment. These uniform arrays of nanoparticles are ideal model systems for studying the effects of particle size and interparticle distance on their catalytic activity. Electrooxidation of carbon monoxide (CO) on these particle arrays in CO-saturated 0.1 M NaOH was examined. On particle arrays with a particle size of ca. 4 nm and an interparticle distance varying from 28 to 80 nm, rotating disk electrode (RDE) voltammetric results show that the half-wave potential for CO oxidation shifted to more positive potentials as the interparticle distance increased. This apparent kinetic difference can be explained by the CO diffusion pattern change with the interparticle distance. On particle arrays with a similar interparticle distance but varying size from 2.4 to 9.0 nm, the electrooxidation of CO shows a particle size-dependent activity, with the highest activity obtained on 4.2 nm Au particles, as revealed by the Tafel plot. The Tafel slope also depends on the particle size, with the smallest slope obtained on 4.2 nm particles. The particle size-dependent catalytic activity was tentatively explained in terms of the size-dependent adsorption properties. A brief comparison was made with the results from gas phase CO oxidation on Au nanoparticles

Seed-Mediated Growth of Uniform Gold Nanoparticle Arrays

We demonstrated through electron microscopic characterizations that the semi-hexagonal Au nanoparticle arrays supported on solid surfaces prepared by a polymer template approach could be enlarged uniformly by using a solution-based seed-mediated growth. The semi-hexagonal particle arrangement is largely retained after the growth as evident by the similar Fourier transform patterns obtained before and after the growth, indicating that the growth occurs over the seed particles on the surface. The shape and size of the augmented particles can be tuned by varying the growth conditions. The particle density can be conveniently varied by using polymer templates with different chain lengths. The growth mechanism is briefly discussed. Strong surface-enhanced Raman signals of molecules adsorbed on the 40−60 nm grown particles are observed

In Situ Surface-Enhanced Raman Spectroscopic Studies of CO Adsorption and Methanol Oxidation on Ru-Modified Pt Surfaces

Ru-modified Pt is so far the best catalyst for the anode reaction of direct methanol fuel cells. The role of Ru is believed to promote carbon monoxide oxidation through a bifunctional mechanism and an electronic effect. However, direct experimental evidence for the electronic effect is sparse. In addition, whether Ru oxides or metallic Ru is the active component in the catalyst is still under debate. To address these issues, carbon monoxide adsorption and methanol oxidation on Ru-modified Pt thin film surfaces in acidic media were studied using in situ overlayer surface-enhanced Raman spectroscopy (SERS). Cyclic voltammograms show that with the presence of Ru, CO and methanol oxidation is significantly enhanced as evident by the negative shift of the oxidation potential. The addition of Ru to Pt surfaces did not change the Pt−CO stretching frequency, suggesting that the Pt−CO bond is not weakened significantly by Ru. Through combined SERS and cyclic voltammetry studies, we found that Pt and Ru oxides are inhibitors for methanol oxidation. In particular, Ru oxides on Ru-modified Pt strongly inhibit methanol oxidation on Pt sites, even when the Pt sites are in the metallic state

Carbon-Encapsulated Co Nanoparticle and Hollow Carbon Sphere Composites as High-Performance Catalysts for Oxygen Reduction Reaction

Nitrogen and cobalt doped carbon materials are promising alternative catalysts for the oxygen reduction reaction (ORR) to replace more expensive Pt-based catalysts. However, the activity and stability of these materials remain largely subpar to those of Pt-based materials. In this work, a N, Co doped carbon composite with a hierarchical structure of few-carbon-layer encapsulated Co nanoparticles attached on hollow carbon spheres was successfully synthesized and demonstrated to have excellent ORR activity and long-term stability in 0.1 M KOH. The optimal catalyst showed a half-wave potential of 0.865 V, and it only shifted negatively by 17 mV after 50000 potential cycles between 0.4 and 1.2 V (vs RHE). These performance markers are better than those of commercial Pt/C catalysts. The high density of accessible active sites, the modification of the electronic structure of Co-Nx active sites and the carbon layer work function, together with the nonplanar active site structure, are potential contributors to such a high performance

Synthesis and Oxygen Reduction Activity of Shape-Controlled Pt₃Ni Nanopolyhedra

Platinum-based alloys have been extensively shown to be effective catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Most of these catalysts are nanoparticles without shape control. Recently, extended Pt3Ni(111) surfaces prepared in ultrahigh vacuum were demonstrated to possess enhanced ORR catalytic activity as compared to the state-of-the-art carbon supported Pt (Pt/C) nanoparticle catalysts. How and whether this promising surface can be transformed into practical nanoscale electrocatalysts used in PEMFCs remain a challenge. We report a new wet-chemical approach of preparing monodisperse Pt3Ni nanoctahedra and nanocubes terminated with {111} and {100} facets, respectively. We further show that the ORR activity on the Pt3Ni nanoctahedra is ∼5-fold higher than that of nanocubes with a similar size. Comparison of ORR activity between carbon-supported Pt3Ni nanoctahedra and commercial Pt/C reveals that the Pt3Ni nanoctahedra are highly active electrocatalysts. This synthetic strategy may be extended to the preparation of other shape-controlled fuel cell electrocatalysts

Attachment of Cobalt “Picket Fence” Porphyrin to the Surface of Gold Electrodes Coated with 1-(10-Mercaptodecyl)imidazole

Self-assembled monolayers of 1-(10-mercaptodecyl)imidazole on Au electrodes were used to bind cobalt “picket fence” porphyrin (cobalt 5,10,15,20-tetrakis(α,α,α,α-2-pivalamidophenyl)porphyrin) to the electrode surface. The binding involved coordination of the cobalt center of the porphyrin to the pendant imidazole groups in the monolayer coating. Attempts to coordinate the Co(II) oxidation state of the porphyrin to the coatings were not successful. However, with the Co(III) oxidation state, substantial binding was achieved which persisted even when the Co(III) was reduced to Co(II). Absorption spectra of the attached porphyrin were obtained for both oxidation states of the cobalt center. The remaining axial coordination site on the attached cobalt porphyrin is accessible to ligands, for example, imidazole, in aqueous solution