Methanol electro-oxidation on carbon-supported and Pt-modified Au nanoparticles

Methanol electro-oxidation on Pt-modified Au nanoparticles was studied in aqueous 0.5 M H₂SO₄/1 M CH₃OH. The Pt-modified Au nanoparticles showed lower values than pure Pt nanoparticles in area-specific current densities. The activity also decreased with lower amounts of Pt on the Pt-modified Au nanoparticles. This result indicates that the intrinsic activities of the Pt-modified Au nanoparticles were lower than that of Pt/C. This phenomenon may be due to the strengthened Pt–CO bonding or the weakened ensemble effect in the methanol electro-oxidation. However, in the Pt-specific current densities, the Pt-modified Au nanoparticles showed higher values than pure Pt nanoparticles. This enhanced electrocatalytic activity could be attributed to more reaction-favorable surface structures of Pt-modified Au nanoparticles, which most likely lead a high level of Pt utilization for the electro-oxidation reaction.     

Effect of reduction conditions on electrocatalytic activity of a ternary PtNiCr/C catalyst for methanol electro-oxidation

The effect of reduction conditions on a Pt₂₈Ni₃₆Cr₃₆/C catalyst was investigated by using two different reduction methods: hydrogen reduction and NaBH₄ reduction. In hydrogen reduced catalysts, dissolution of metallic Ni and Cr was observed during cyclic voltammetry (CV) tests, and a larger amount of Ni and Cr was dissolved when reduced at higher temperatures. For methanol electro-oxidation, the highest specific current density of 1.70 A m⁻² at 600 s of the chronoamperometry tests was observed in the catalyst reduced at 300 °C, which was ∼24 times that of a Pt/C catalyst (0.0685 A m⁻²). In NaBH₄ reduced catalysts, formation of an amorphous phase and a more Pt-rich surface was observed in X-ray diffraction and CV results, respectively, with increasing amounts of NaBH₄. When reduced by 50 times of the stoichiometric amount of NaBH₄, the PtNiCr/C catalyst (PtNiCr-50t) showed a current density of 34.1 A g_{noble metal}⁻¹, which was 81% higher than the 18.8 A g_{noble metal}⁻¹ value of a PtRu/C catalyst at 600 s of the chronoamperometry tests. After 13 h of chronoamperometry testing, the activity of the PtNiCr-50t (15.0 A g_{noble metal}⁻¹) was 110% higher than the PtRu/C catalyst (7.15 A g_{noble metal}⁻¹). The PtNiCr/C catalyst shows promise as a Ru-free methanol oxidation catalyst.     

Novel methanol electro-oxidation catalyst assisting with functional phthalocyanine supports

Novel electro-catalyst based on phthalocyanine stabilized Pt colloids has been developed for methanol electro-oxidation. Water soluble Cu²⁺ phthalocyanine functioned with sulfonic groups were selected as catalyst supports because of the relatively high catalytic activity of Pt catalyst and nearly the same catalytic selectivity complex with Cu-phthalocyanine, compared to others that chelated with Fe, Co and Ni ions. The as-resulting Pt-CuTsPc catalysts have average particle size of 2 nm and narrow size distribution. With the assistance of CuTsPc supports, the methanol electro-oxidation activity and poison tolerance of Pt catalyst have a significant increase. I_f/I_b ratio (anodic peak current density, forward to backward) of the Pt-CuTsPc/C catalysts also has obvious increase to 2.5, from value of 0.8 for pure Pt/C catalyst. The reaction Tafel slope of Pt-CuTsPc/C catalysts is 56.6 mV dec⁻¹, much smaller than that of the Pt/C catalyst. The transient current density on Pt-CuTsPc/C at 0.60 V is enhanced to 650% of that on the Pt/C catalyst while the enhancement factor R for comparison of steady-state current obtained on Pt-CuTsPc/C and Pt/C catalyst varies between 111% and 534% in the potential region of 0.3-0.75 V.     

Pt45Ru45M10/C (M = Fe, Co, and Ni) catalysts for methanol electro-oxidation

Pt₄₅Ru₄₅M₁₀/C (M = Fe, Co, and Ni) catalysts were synthesized and physical and electrochemical properties were analyzed by XRD, TEM, CO stripping and methanol electro-oxidation activity measurement. Among these catalysts, the Pt₄₅Ru₄₅Fe₁₀/C catalyst exhibited the highest mass activity of 2.6 A/g catal. while those of the Pt₄₅Ru₄₅Co₁₀/C and Pt₄₅Ru₄₅Ni₁₀/C catalysts were 2.2 and 2.5 A/g catal., respectively. In the case of specific activity, the catalysts exhibited much higher activities of 110 (130%), 120 (140%) and 150 (170%) mA/m² for the Fe, Co and Ni incorporated catalysts, respectively, than 88 mA/m² of a commercial PtRu/C catalyst.     

Co-catalytic effect of Ni in the methanol electro-oxidation on Pt-Ru/C catalyst for direct methanol fuel cell

This research is aimed to improve the utilization and activity of anodic catalysts, thus to lower the contents of noble metals loading in anodes for methanol electro-oxidation. The direct methanol fuel cell anodic catalysts, Pt-Ru-Ni/C and Pt-Ru/C, were prepared by chemical reduction method. Their performances were tested by using a glassy carbon working electrode through cyclic voltammetric curves, chronoamperometric curves and half-cell measurement in a solution of 0.5 mol/L CH₃OH and 0.5 mol/L H₂SO₄. The composition of the Pt-Ru-Ni and Pt-Ru surface particles were determined by EDAX analysis. The particle size and lattice parameter of the catalysts were determined by means of X-ray diffraction (XRD). XRD analysis showed that both of the catalysts exhibited face-centered cubic structures and had smaller lattice parameters than Pt-alone catalyst. Their sizes are small, about 4.5 nm. No significant differences in the methanol electro-oxidation on both electrodes were found by using cyclic voltammetry, especially regarding the onset potential for methanol electro-oxidation. The electrochemically active-specific areas of the Pt-Ru-Ni/C and Pt-Ru/C catalysts are almost the same. But, the catalytic activity of the Pt-Ru-Ni/C catalyst is higher for methanol electro-oxidation than that of the Pt-Ru/C catalyst. Its tolerance performance to CO formed as one of the intermediates of methanol electro-oxidation is better than that of the Pt-Ru/C catalyst.     

Highly active PtRu catalysts supported on carbon nanotubes prepared by modified impregnation method for methanol electro-oxidation

A simple and rapid synthesis method (denoted as modified impregnation method, MI) for PtRu/CNTs (MI) and PtRu/C (MI) was presented. PtRu/CNTs (MI) and PtRu/C (MI) catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It was shown that Pt-Ru particles with small average size (2.7 nm) were uniformly dispersed on carbon supports (carbon nanotubes and carbon black) and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. Cyclic voltammetry and chronoamperometry showed that the Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation activities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK) catalyst. The single cells with Pt-Ru/CNTs (MI) catalyst exhibited a power density of 61 mW/cm², about 27% higher than those single cells with commercial Pt-Ru/C (E-TEK) catalyst.     

Electrocatalytic properties of Pd clusters on Au nanoparticles in formic acid electro-oxidation

Pd clusters were formed on highly dispersed Au nanoparticles (∼3.5 nm in diameter) using a seed-mediated growth process. The structural information and electrocatalytic activities of these Pd clusters on Au nanoparticles were confirmed by high-resolution-transmission-electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The resulting nanoparticles, which had a uniform size (<5 nm in diameter), were highly dispersed on carbon particles, and Pd clusters (<0.44 nm in size, <2 atomic layers) were formed selectively on Au nanoparticles. XPS results show that the Pd 3d_5/2 peak shifted to lower binding energies and that the amount of surface oxide decreased as the Pd content was decreased on the Au nanoparticles. In formic acid electro-oxidation, these Pd clusters exhibit enhanced electrocatalytic activity relative to that of carbon-supported Pd nanoparticles. These results may be due to the modified electronic and geometric structure of the Pd clusters on the Au nanoparticle substrate.     

Synthesis and characterization of PDDA-stabilized Pt nanoparticles for direct methanol fuel cells

Pt nanoparticles are synthesized by the alcoholic reduction of H₂PtCl₆ in the presence of a polycation, poly(diallyldimethylammonium chloride) (PDDA). The size of the PDDA-Pt nanoparticle colloids is in the range of 2-4 nm, depending on the PDDA to Pt ratio in the solution. The PDDA-Pt nonoparticles can be self-assembled to the sulfonic acid group, SO₃⁻, at the Nafion membrane surface by the electrostatic interaction, forming a self-assembled monolayer (SAM). The study shows that such SAM reduced the methanol crossover and enhanced the power output of direct methanol fuel cells (DMFC) by as much as 34% as compared to the cell based on an un-modified Nafion membrane. In addition, PDDA-Pt nanoparticles synthesized with low PDDA/Pt ratios show considerable catalytic activity for the methanol oxidation reaction (MOR) in comparison to a commercial Pt/C catalyst. However, the electrocatalytic activity of PDDA-Pt nanoparticles decreased significantly with the increase in the PDDA/Pt molar ratio, indicating that the excess PDDA inhibits the MOR.     

Novel spherical TiO2 supported PdNi alloy catalyst for methanol electroxidation

A novel PdNi/TiO₂ electrocatalyst for methanol oxidation is fabricated using spherical TiO₂ nanoparticles as support. The structural and electrochemical properties of the PdNi/TiO₂ catalyst are characterized by XRD, TEM and electrochemical analysis. The cyclic voltammograms of PdNi/TiO₂ catalyst show that there is a large methanol oxidation peak in about 0.882 V that is much bigger than that of the commercial PtRu/C catalyst in 0.7 V. The composite TiO₂ material has high catalytic activity without UV light illumination. The electrocatalytic activity and anti-poisoning capability of the PdNi/TiO₂ catalyst are promising, which may become a potential candidate for direct methanol fuel cell.     

直接甲醇燃料电池钯基催化剂研究进展

直接甲醇燃料电池(DMFC)阳极催化剂是直接甲醇燃料电池的关键材料之一。由于钯的价格便宜、储量丰富、在碱性条件下活性较高,成为取代铂作为DMFC的潜在的阳极催化剂。着重介绍了近年来钯基阳极催化剂在碱性条件下对甲醇的电氧化的研究进展,展望了其发展前景。     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Performance of ternary PtRuRh/C electrocatalyst with varying Pt:Ru:Rh ratio for methanol electro-oxidation

Highly dispersed ternary PtRuRh/C anode catalysts for direct methanol fuel cells were prepared with various contents and their electro-catalytic activities towards methanol oxidation at 25 °C and 60 °C were examined to investigate the influence of the catalyst composition. Electrocatalysts were prepared by a co-impregnation method using ethanolic solutions of metal precursors and carbon black followed by pyrolysis under reducing conditions. X-ray diffraction analysis revealed that the fcc peaks shifted to higher diffraction angles with increasing Rh content, indicating the alloying of Rh into the fcc structure. In terms of the mass specific current density, the activity towards methanol oxidation differed significantly depending on the catalysts composition and cell temperature. The catalyst prepared at a ratio of Pt:Ru:Rh = 1:1:2 exhibited the highest activity at 60 °C of 155 A (g-Pt)⁻¹ at 0.5 V vs. RHE.     

Effect of heat treatment on the performance of TiO2-Pt/CNT catalysts for methanol electro-oxidation

TiO₂-Pt/CNT catalysts before and after heat treatment were prepared. Their catalytic activities for methanol and CO electro-oxidation were studied in detail. The results showed that the proper amount of hydrous TiO₂ in TiO₂-Pt/CNTs (e.g. heated at 200 °C for 2 h) was favorable for enhancing the catalytic activity of Pt/CNTs, which provided evidence for bi-functional mechanism. The studies on the catalysts with different TiO₂/Pt molar ratio displayed that the optimum molar ratio varied with the increase of heat treatment temperature. It was found that the optimum molar ratio of TiO₂/Pt was at 1:2 for the catalysts without heat treatment and was at 1:1 for the catalysts by heat treatment at 500 °C. This fact was ascribed to the difference in compact degree between TiO₂ and Pt/CNTs before and after heat treatment. Considering the influence of heating temperature, it was found that TiO₂-Pt/CNT catalyst heated at 200 °C for 2 h had better catalytic activity for methanol oxidation.     

Preparation and characterization of successively deposited Pt/Ru bimetallic electrocatalysts for the methanol oxidation

Two types of Pt/Ru electrocatalysts, which have different structural characteristics, were prepared with different synthetic routes. That is, Pt/Ru electrocatalysts were synthesized by the coreduction and successive deposition methods, respectively. The structural and catalytic properties of Pt/Ru electrocatalysts were characterized by XRD, TEM, voltammetry and chronoamperometry. From the XRD analysis, coreduced and successively deposited Pt/Ru electrocatalysts had an alloyed structure. TEM analyses showed that all the electrocatalysts had a highly dispersed state on the Vulcan XC-72R substrate. From the voltammetry, the coreduced electrocatalysts displayed higher catalytic activity than the successively deposited electrocatalysts for the electrooxidation of methanol. These results explain why coreduced catalysts are better able to dehydrogenate methanol and have a greater CO tolerance than the successively deposited ones. But chronoamperometry showed that successively deposited Pt/Ru electrocatalysts had stability similar to that of the coreduced ones. Although the successively deposited electrocatalysts showed lower catalytic activity than the coreduced ones, their enhanced catalytic activity was obtained by the successive deposition method in the comparison of methanol oxidation current density with pure platinum electrocatalyst.     

Effect of heat treatment on PtRu/C catalyst for methanol electro-oxidation

The effect of heat treatment on a commercial PtRu/C catalyst was investigated with a focus on the relationship between electrochemical and surface properties. The heat treated PtRu/C catalysts were prepared by reducing the commercial PtRu/C catalyst at 300, 500, and 600 °C under hydrogen flow. The maximum mass activity for the methanol electro-oxidation reaction (MOR) was observed in the catalyst heat treated at 500 °C, while specific activity for the MOR increased with increasing heat treatment temperature. Cyclic voltammetry (CV) results revealed that the heat treatment caused Pt rich surface formation. The increase in surface Pt was confirmed by X-ray photoelectron spectroscopy; the surface (Pt:Ru) ratio of the fresh catalyst (81:19) changed to (87:13) in the 600 °C heat treated catalyst. Quantitative analysis of the Ru oxidation state showed that the ratio of metallic Ru increased with an increase in heat treatment temperature. On the other hand, RuO_xH_y completely reduced at 500 °C and the ratio of RuO₂ slightly decreased with increasing heat treatment temperature.     

Particle size effects of PtRu nanoparticles embedded in TiO2 on methanol electrooxidation

Size-controlled PtRu nanoparticles embedded in TiO₂ were prepared by simultaneous multi-gun sputtering from pure targets of Pt, Ru, and TiO₂. The mean diameter of the PtRu nanoparticles, as confirmed by their high-resolution transmission electron microscopic images, can be varied from ∼1.8 to ∼3.7 nm by changing the RF power ratio of PtRu and TiO₂. The transmission electron diffraction and grazing incidence wide angle X-ray scattering patterns of the PtRu nanoparticles embedded in TiO₂ confirmed that the PtRu exists as a mixed alloy structure consisting of both fcc and hcp phases, whereas the TiO₂ matrix is present as an amorphous phase. The size-controlled PtRu/TiO₂ electrodes were found to exhibit unique electronic properties depending on their size, which affected the potential of zero total charge and methanol oxidation reaction. The mass activity of PtRu/TiO₂ for methanol oxidation was determined by the interplay of the surface electronic factors at the metal-solution-interface and the value of the oxophilicity of the nanoparticles was increased by decreasing the particle size.     

Synthesis of sulfated ZrO2/MWCNT composites as new supports of Pt catalysts for direct methanol fuel cell application

Sulfated zirconia supported on multi-walled carbon nanotubes as new supports of Pt catalyst (Pt–S-ZrO₂/MWCNT) was synthesized with aims to enhance electron and proton conductivity and also catalytic activity of Pt electrocatalyst in terms of larger concentrations of ionizable OH groups on surfaces. Fourier transform infrared spectroscopy analysis shows that the sample surfaces were modified with sulfate. Transmission electron microscopy results show that the Pt and sulfated ZrO₂ particles dispersed relatively uniformly on the surface of the multi-walled carbon nanotube. X-ray diffraction shows that S-ZrO₂ and Pt coexist in the Pt–S-ZrO₂/MWCNT composites and S-ZrO₂ has no effect on the crystalline lattice of Pt. Pt–S-ZrO₂/MWCNT catalyst was evaluated in terms of the electrochemical activity for methanol electro-oxidation using cyclic voltammetry, steady-state polarization experiments and electrochemical impedance spectroscopy technique at room temperature. Pt–S-ZrO₂/MWCNT catalyst show higher catalytic activity for methanol electro-oxidation compared with Pt catalyst on non-sulfated ZrO₂/MWCNT support and commercial Pt/C (E-TEK).     

Preparation and characterization of a PtRu/C nanocatalyst for direct methanol fuel cells

PtRu/C nanocatalysts were prepared by changing the molar ratio of citric acid to platinum and ruthenium metal salts (CA:PtRu) from 1:1, 2:1, 3:1 to 4:1 using sodium borohydride as a reducing agent. Transmission electron microscopy analysis indicated that well-dispersed smaller PtRu particles (2.6 nm) were obtained when the molar ratio was maintained at 1:1. X-ray diffraction analysis confirmed the formation of PtRu alloy; the atomic percentage of the alloy analyzed by the energy dispersive X-ray spectrum indicated an enrichment of Pt in the nanocatalyst. X-ray photoelectron spectroscopy measurements revealed that 83.34% of Pt and 79.54% of Ru were present in their metallic states. Both the linear sweep voltammetry and chronoamperometric results demonstrated that the 1:1 molar ratio catalyst exhibited a higher methanol oxidation current and a lower poisoning rate among all the other molar ratios catalysts. The CO stripping voltammetry studies showed that the E-TEK catalyst had a relatively higher CO oxidation current than did the 1:1 molar ratio catalyst. Testing of the PtRu/C catalysts at the anode of a direct methanol fuel cell (DMFC) indicated that the in-house PtRu/C nanocatalyst gave a slightly higher performance than did the E-TEK catalyst.     

High activity PtRu/C catalysts synthesized by a modified impregnation method for methanol electro-oxidation

A modified impregnation method was used to prepare highly dispersive carbon-supported PtRu catalyst (PtRu/C). Two modifications to the conventional impregnation method were performed: one was to precipitate the precursors ((NH₄)₂PtCl₆ and Ru(OH)₃) on the carbon support before metal reduction; the other was to add a buffer into the synthetic solution to stabilize the pH. The prepared catalyst showed a much higher activity for methanol electro-oxidation than a catalyst prepared by the conventional impregnation method, even higher than that of current commercially available, state-of-the-art catalysts. The morphology of the prepared catalyst was characterized using TEM and XRD measurements to determine particle sizes, alloying degree, and lattice parameters. Electrochemical methods were also used to ascertain the electrochemical active surface area and the specific activity of the catalyst. Based on XPS measurements, the high activity of this catalyst was found to originate from both metallic Ru (Ru⁰) and hydrous ruthenium oxides (RuO_xH_y) species on the catalyst surface. However, RuO_xH_y was found to be more active than metallic Ru. In addition, the anhydrous ruthenium oxide (RuO₂) species on the catalyst surface was found to be less active.     

Novel Pt-Ru nanoparticles formed by vapour deposition as efficient electrocatalyst for methanol oxidation: Part II. Electrocatalytic activity

The methods developed and described in paper—part I are employed to prepare nanometer size Pt-Ru particles on a Vulcan^® XC72R substrate with controlled metal loading. Transmission Electron Microscopy (TEM) confirmed uniform particles size (average diameter 2 nm) and homogeneous dispersion of the particles over the substrate. Energy Dispersive X-ray absorption (EDX) analysis confirmed the compositional homogeneity. The catalytic activity of these supported nanoparticles with regard to methanol electrooxidation is investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO-stripping voltammetry techniques at temperatures between 25 and 60 °C. Such investigation concerns supported catalysts prepared with ca. 10 and 18 wt.% overall metal loading (Pt + Ru) onto the Vulcan^® XC72R substrate. Comparative testing of our catalysts and a commercial Pt-Ru/Vulcan reveals markedly superior activity for our catalysts. In fact, we observe for the latter a five-fold increase of the oxidation current as compared to a commercial Pt-Ru/Vulcan with equal metal loading. One of the reasons for the greater activity is found to be the very high dispersion of the metals over the substrate, i.e. the large surface area of the active phase. Other reasons are plausibly ascribable to the varied Pt/Ru composition and/or reduced presence of contaminants at the catalyst surface.