Effects of ozone treatment of carbon support on Pt-Ru/C catalysts performance for direct methanol fuel cell

This research is aimed to increase the activity and utilization of Pt-Ru alloy catalysts and thus to lower the catalyst loading in anodes for methanol electrooxidation. The Pt-Ru/C catalysts were prepared by chemical reduction. The support of Vulcan XC-72 carbon black was pretreated by ozone at different temperatures for different times. The specific surface area of the samples was evaluated by the standard BET method. The surface concentrations of oxygen were determined by XPS. The results showed that the surface concentrations of oxygen on the carbon were first decreased and then increased with pretreating times, and the specific surface area of the carbon was decreased with pretreating times at the same temperature. The specific surface area was increased with increasing temperature, and the surface concentration of oxygen was first decreased and then increased with increasing temperature for the same pretreating time. Pt-Ru/C catalysts supported by untreated and O₃ treated carbon black were characterized and tested for methanol electrooxidation. X-ray diffraction (XRD) was used to characterize the influence of carbon treated with ozone on Pt-Ru/C catalysts. It was found that the catalysts were composed only of f.c.c. Pt-Ru alloy particles without metallic Ru or Ru oxide. Cyclic voltammetry (CV) and Tafel curves were used for methanol electrooxidation on Pt-Ru/C catalysts in a solution of 0.5 mol/L CH₃OH and 0.5 mol/L H₂SO₄, showing that the catalytic activity of Pt-Ru/C catalysts supported by ozone treated carbon was higher than that by the untreated one. The ozone treatment time and temperature, which affect the performance of Pt-Ru/C catalysts, were discussed. Electrochemical measurements showed that the catalysts supported by the carbon after ozone treatment for 6 min at 140 °C had the best performance.     

Catalysts for Direct Methanol Fuel Cells

The activity of in house prepared carbon-supported Pt-Ru catalysts for methanol oxidation and carbon-supported RuSe for the oxygen reduction reaction in direct methanol fuel cells (DMFCs) was investigated. The composition of Pt-Ru/C was varied both in terms of weight loading (ratio of total metal content to carbon) as well as the ratio of Pt to Ru. The measurements were carried out in a half cell arrangement in sulphuric acid at various temperatures. The weight loading and ratio of Pt to Ru were varied in order to find out the optimum weight loading of precious metal and the temperature dependence of Pt to Ru ratio on methanol oxidation reaction. It has been found that there exists an optimum in the weight loading at 60 wt.% for carbon-supported Pt-Ru catalyst towards its maximum mass activity. While 1:1 Pt to Ru ratio exhibits a higher activity than 3:2 Pt:Ru above 60 °C, 3:2 ratio exhibits a higher activity at lower temperature. It has been observed that RuSe is inactive towards methanol and it is realised that RuSe is a potential candidate as methanol tolerant oxygen reduction catalyst. The activity of carbon supported RuSe for oxygen reduction reaction (ORR) was tested in sulphuric acid in the presence of methanol. Even though the mass specific activity of the RuSe catalyst is somewhat lower than that of Pt/C, the surface activity of carbon-supported RuSe is superior than that of carbon supported Pt which indicate the unfavourable size distribution of RuSe/C catalyst.     

CTAB assisted microwave synthesis of ordered mesoporous carbon supported Pt nanoparticles for hydrogen electro-oxidation

Mesoporous carbon with ordered hexagonal structure derived from the co-assembly of triblock copolymer F127 and resol was employed as the carbon support of Pt catalysts for hydrogen electro-oxidation. Structural characterizations revealed that the mesoporous carbon exhibited large surface area and uniform mesopores. The Pt nanoparticles supported on the novel mesoporous carbon were fabricated by a facile CTAB assisted microwave synthesis process, wherein CTAB was expected to improve the wettability of carbon support as well as the dispersion of Pt nanoparticles. X-ray diffraction and transmission electron microscopy were applied to characterize the Pt catalysts. It was found that the Pt nanoparticles were uniform in size and highly dispersed on the mesoporous carbon supports. The cyclic voltammograms in sulfuric acid demonstrated that the electrochemical active surface area of Pt catalysts prepared with CTAB was two times than that without CTAB.     

Ethylene diamine-grafted carbon nanotubes: A promising catalyst support for methanol electro-oxidation

In this paper, carbon nanotubes (CNTs) were modified by ethylene diamine (ED) and then used as the support of the Pt-Ru catalyst. The cyclic voltammetry (CV) and Fourier transform infrared spectroscopy were employed to study the interaction between ED and CNTs. The morphology and elemental composition of the as-prepared Pt-Ru/ED/CNT electrode were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The electrocatalytic properties of the Pt-Ru/ED/CNT electrode for methanol electro-oxidation were investigated by CV, polarization method and electrochemical impedance spectroscopy. The long-term stability of the Pt-Ru/ED/CNT electrode was also evaluated. Compared with the Pt-Ru/CNT electrode, the Pt-Ru/ED/CNT electrode exhibits excellent electrocatalytic properties and long-term stability. These results show that the ED-grafted CNTs are the promising catalyst support for methanol electro-oxidation.     

炭载体前处理对Pt-Ru/C催化剂性能影响

为研究水蒸气处理后热处理对炭黑表面特性的影响,提高DMFC阳极催化剂的催化活性,利用先水蒸气处理后热处理的Vnlcan XC-72炭黑为载体制备Pt-Ru/C催化剂,与水蒸气处理的和未经处理的炭载体制备Pt-Ru/C催化剂的性能进行比较.采用XPS和BET测试了处理后的炭粉表面的含氧浓度和比表面,结果表明：水蒸气处理后,炭载体比表面积增大,含氧浓度降低;水蒸气处理后热处理,炭载体比表面积进一步减小,含氧浓度增加.用XRD对催化剂的结构进行了表征,结果表明：水蒸气处理后热处理的炭黑为载体制备Pt-Ru/C催化剂结晶状态良好,催化剂颗粒较小.在0.5mol/L CH3OH和0.5mol/L H2SO4混合溶液中,利用玻炭电极测试了循环伏安曲线和阶跃电位曲线,结果表明：用先水蒸气处理后热处理的炭粉为载体制备的催化剂比仅水蒸气处理和未经处理的炭粉为载体制备的催化剂的活性最高.     

Pt-WO3 supported on carbon nanotubes as possible anodes for direct methanol fuel cells

The carbon nanotube (CNT) synthesised by the template carbonisation of polypyrrole on alumina membrane has been used as the support for Pt-WO₃, Pt-Ru, and Pt. These materials have been used as the electrodes for methanol oxidation in acid medium in comparison with E-TEK 20 wt% Pt and Pt-Ru on Vulcan XC72R carbon. The higher electrochemical surface of the carbon nanotube (as evaluated by cyclic voltammetry) has been effectively used to disperse the catalytic particles. The morphology of the supported and unsupported CNT has been characterised by scanning electron micrograph and high-resolution transmission electron micrograph. The particle size of Pt, Pt-Ru, and Pt-WO₃ loaded CNT was found to be 1.2, 2, and 5 nm, respectively. The X-ray photoelectron spectra indicated that Pt and Ru are in the metallic state and W is in the +VI oxidation state. The electrochemical activity of the methanol oxidation electrode has been evaluated using cyclic voltammetry. The activity and stability (evaluated from chronoamperometric response) of the electrodes for methanol oxidation follows the order: GC/CNT-Pt-WO₃-Nafion>GC/E-TEK 20% Pt-Ru/Vulcan Carbon-Nafion>GC/CNT-Pt-Nafion>GC/E-TEK 20% Pt/Vulcan carbon-Nafion>Bulk Pt. The amount of nitrogen in the CNT plays an important role as observed by the increase in activity and stability of methanol oxidation with N₂ content, probably due to the hydrophilic nature of the CNT.     

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.     

Physical and electrochemical characterization of platinum and platinum-ruthenium treated carbon nanotubes directly grown on carbon cloth

Dense carbon nanotubes (CNTs) were grown via a thermal chemical vapor deposition process on titanium treated carbon cloths. The catalysts in the form of either platinum (Pt) or platinum-ruthenium (Pt-Ru) nano-sized particles were then deposited on the CNT surface by potentiostatic electrodeposition. After the deposition process, the surface morphology of prepared specimens was examined by scanning electron microscopy and transmission electron microscopy, and the electrochemical characteristics of the specimens were also investigated by cyclic voltammetry in nitrogen saturated sulfuric acid aqueous solutions and in mixed sulfuric acid and methanol aqueous solutions. Well dispersed catalysts, Pt alone or Pt-Ru, were observed on the surfaces of the CNTs directly grown on carbon cloths. It was found that the electrochemical behavior of the specimen with Pt-Ru deposits was distinctly different from that of those with only Pt deposits. It was also observed that a more negative electrodeposition potential led to a better performance of the catalysts in terms of methanol oxidation efficiency and the suppression of carbon monoxide poisoning.     

Hollow core/mesoporous shell carbon capsule as an unique cathode catalyst support in direct methanol fuel cell

Hollow core mesoporous shell (HCMS) carbon has been explored for the first time as a cathode catalyst support in direct methanol fuel cells (DMFCs). The HCMS carbon consisting of discrete spherical particles possesses unique structural characteristics including large specific surface area and mesoporous volume and well-developed interconnected void structure, which are highly desired for a cathode catalyst support in low temperature fuel cells. Significant enhancement in the electrocatalytic activity toward oxygen reduction reaction has been achieved by the HCMS carbon-supported Pt nanoparticles compared with carbon black Vulcan XC-72-supported ones in the DMFC. In addition, much higher power was delivered by the Pt/HCMS catalysts (i.e., corresponding to an enhancement of ca. 91–128% in power density compared with that of Pt/Vulcan), suggesting that HCMS carbon is a unique cathode catalyst support in direct methanol fuel cell.     

Two-step pyrolysis process to synthesize highly dispersed Pt-Ru/carbon nanotube catalysts for methanol electrooxidation

Highly dispersed Pt-Ru particles with different atomic ratios supported on carbon nanotubes were synthesized using an easy two-step synthesis method including adsorption and pyrolysis. In this method, the functionalized carbon nanotubes act as adsorption sites for metallic ions and subsequently act as nucleation center for catalyst deposition in the pyrolysis process. The deposited Pt-Ru nanoparticles disperse on the carbon nanotubes surface uniformly, and the bulk composition of the Pt-Ru particles can be adjusted simply by changing atomic ratios of the metallic solution for adsorption. Finally, the electrocatalytic activity of the as-prepared catalysts supported on carbon nanotubes toward oxidation of methanol was studied. Results showed that their electrocatalytic activity, having long-term stability, strongly depends on the atomic ratio of Pt to Ru. The higher the concentration of Pt in the binary system is, the greater the electrocatalytic activity will be.     

Electrical signal effect on electrochemical activities of metal catalysts electrically deposited on carbon nanotubes

The electrochemical deposition of Pt-Ru nanoparticles on carbon nanotubes (CNTs) supports and their electro-catalytic activities, were investigated. Pt-Ru catalysts of 4-12 nm average size were grown successfully on supports by step-potential plating methods. Electro-plating at 0.06 s step intervals was sufficient to obtain small-size 4.8 nm particles, showing good electrochemical activity. The catalysts’ loading contents were enhanced by increasing the plating time. The sizes and morphological structures of the Pt-Ru/support catalysts were evaluated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical behaviors of the Pt-Ru/support catalysts for methanol oxidation were investigated according to their characteristic current-voltage curves in a methanol solution. In the result, the electrochemical activity increased with increased plating time, reaching the maximum at 24 min, and then decreased. The improved catalytic activity was correlated to the small particle size and the higher specific surface area of the catalysts.     

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.     

Platinum and platinum-ruthenium nanoparticles supported on ordered mesoporous carbon and their electrocatalytic performance for fuel cell reactions

Highly ordered meso-porous carbon, denoted CMK-3 was synthesized by using mesoporous silicates, SBA-15 as the starting templating materials. The ordered mesoporous carbon was loaded with platinum and platinum-ruthenium nanoparticles using alternative synthesis techniques. The metal loaded ordered mesoporous carbon powders were characterized by transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX), X-ray diffraction, and nitrogen adsorption isotherm experiments. Micrometer-scale and centimeter-scale electrodes containing the mesocarbon/nanometal electrocatalysts were tested for some typical fuel cell reactions. While the nanometal/mesocarbon catalysts have well-defined and uniform properties in the nanometer scale, they have mixed electrocatalytic performance. A synthesized Pt/mesocarbon electrocatalyst outperformed a commercial electrocatalyst for oxygen reduction on a gas-diffusion electrode. The Pt-Ru/mesocarbon electrocatalyst synthesized, however, was not as effective for methanol oxidation.     

Preparation of Pt/mesoporous carbon (MC) electrode catalyst and its reactivity toward oxygen reduction

Highly structurally controllable mesoporous carbon (MC) support was synthesized through the self-organization of surfactants and carbon precursors, followed by carbonization. Then, Pt catalysts were successfully deposited on MC, in order to create a model of an ideal triple phase boundary in such a nano-space. The resulting Pt/MC catalysts showed better oxygen reduction reactivity with the existence of Nafion^® than without that. Handling of ionomers is a key to develop an ideal triple phase boundary within the pores of MC. Depending on the solvent where Nafion^® was diluted, the reactivity toward oxygen reduction was different. Due to its hydrophobic pores of MC, Nafion^® diluted by more hydrophobic solvent was able to access to the pores easily. As a result, Pt inside the mesopores was efficiently used. Furthermore, by changing a Pt precursor, oxygen reduction current started to increase at more positive potential, indicating the enhanced activity.     

Methanol oxidation at carbon supported Pt and Pt-Ru electrodes: an on line MS study using technical electrodes

Methanol oxidation at technical carbon based electrodes in 0.05 M H₂SO₄ has been investigated by cyclic voltammetry using online MS under the conditions of an acid methanol fuel cell (DMFC). 5% Pt on Norit BRX and 30% Pt/Ru (40/60) on Norit BRX were used as catalysts. It is shown that methanol oxidation at technical electrodes can be characterized by a combination of cyclic voltammetry and mass spectroscopy. The onset potentials and potential dependences of the methanol oxidation rate can be determined directly by monitoring the formation of CO₂. Onset potentials of 0.5V and 0.25 V/RHE have been measured for Pt and Pt-Ru catalysts, respectively. The onset of methanol oxidation can be shifted to even more cathodic potentials (0.2V) if the Pt-Ru electrode reduces oxygen simultaneously. Carbon monoxide gas was also purged into the methanol containing electroyte during measurement in order to investigate the catalyst performance under more adverse conditions. C¹³-labelled methanol was used to distinguish between CO₂. formed from methanol (m/e = 45) and CO-oxidation (m/e = 44). Without CO the use of C¹³-labelled methanol enabled a distinction between methanol oxidation and carbon corrosion. The methanol oxidation at the platinum catalyst is severely inhibited by the presence of CO, shifting its onset to 0.65 V/RHE. In contrast the performance of the Pt-Ru electrode is not seriously affected under these conditions. It is concluded that Pt-Ru is an excellent catalyst for a methanol anode in an acid methanol fuel cell (DMFC).     

Preparation of platinum-ruthenium alloys supported on carbon by a sonochemical method

Pt and Pt-Ru alloys with several Pt/Ru ratios supported on carbon (Vulcan) were prepared using high-intensity ultrasound by reduction of H₂PtCl₆ and RuCl₃ precursors in an aqueous solution. This method of catalyst preparation was performed in absence of any surfactant or organic addictive. The particles formed were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX) and transmission electron microscopy (TEM). From the XRD studies, a decrease of metal particle size and of the lattice parameters was observed with the increase of the Ru content. The electroactivities were tested for the methanol oxidation reaction in acid electrolyte, and it was found that Pt-Ru catalysts were more activity than pure Pt.     

Preparation of Low Loading Pt/C Catalyst by Carbon Xerogel Method for Ethanol Electrooxidation

Low platium loading Pt/C catalyst was prepared by direct Pt-embedded carbon xerogel method. The Pt content of the as-prepared Pt/C is about 4.32 wt% and has a typical polycrystalline phase. Textural and structural characteristics of the catalysts were characterized by XRD, EDS and BET. Pt-embedded in carbon xerogel increases the specific surface area and pore volume of the X-Pt/C during carbon gelation and the carbonization process. Electrochemical characteristics of the catalysts for ethanol electrooxidation were measured. The results indicated that the as-prepared 4.32 wt% Pt/C has higher mass current density in ethanol electrooxidation as compared to the 20 wt% Pt/C. This may be due to the high roughness of the Pt surface that is formed during the carbon gelation and carbonization process.     

Highly efficient metal-free phosphorus-doped platelet ordered mesoporous carbon for electrocatalytic oxygen reduction

Platinum-free electrocatalysts especially, various heteroatom-doped carbon nanostructures have attracted particular attraction as plausible solution for commercializing fuel cell technology. In this direction, novel phosphorus-doped platelet ordered mesoporous carbon (P-pOMC) is developed for the first time as metal-free electrocatalyst for alkaline oxygen reduction reaction. The P-pOMC is synthesized by nanocasting method using platelet ordered mesoporous silica as template. Various characterizations reveal that the P-pOMC materials have covalently bound P atoms with carbon framework for facilitation of oxygen reduction reaction (ORR) and also have very high surface area with uniform distribution of short mesoporous channels for unhindered mass transfer. Combination of P doping and excellent surface properties empowers the newly-developed P-pOMC catalyst to show high ORR activity nearly equal to that of state of the art Pt catalyst along with superior long-term stability and excellent methanol tolerance.     

Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Enhancing the catalytic performance of Pt/C catalysts using steam-etched carbon blacks as a catalyst support

XC72 carbon blacks were treated using a steam-etching technique. Pt catalysts using steam-etched carbon blacks (SECBs) as supports were developed for oxygen reduction reactions in polymer electrolyte fuel cells (PEFCs). The use of the steam-etched carbon blacks as a catalyst support can improve the dispersion uniformity of Pt nanoparticles on the surface of the support and can decrease the Pt particle size, as indicated by transmission electron microscopy (TEM) data. The overall catalytic performance of the catalysts using SECBs was better than that of the catalysts using non-SECBs. The mass activity of the Pt catalysts using SECBs etched for 1 h had the best catalytic performance for both the catalysts using SECBs and those using non-SECBs. In addition, the electrochemically active surface area of the Pt/SECBs significantly increased after etching. TEM results revealed that the center of the carbon black was more easily etched than the surface.