Synthesis and evaluation of carbon nanotube-supported RuSe catalyst for direct methanol fuel cell cathode

Ruthenium selenide (RuSe) supported on a carbon nanotube (CNT) material, i.e., RuSe/CNT, with a controlled composition (Ru:Se = 1:0.2) was synthesized using a modified polyol method as a model catalyst for direct methanol fuel cell (DMFC) cathode. The prepared electrocatalyst was physically characterized by means of Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) Spectroscopy and X-ray Photoelectron Spectroscopy (XPS), and its activity for oxygen reduction reaction (ORR) was examined using Linear-Sweep Voltammetry (LSV). In addition, the methanol tolerance was characterized using Electrochemical Impedance Spectroscopy (EIS). It was found that the prepared RuSe/CNT catalyst has good catalyst morphology, uniform and small particle size, and controllable catalyst composition. After subjecting to a proper heat treatment at 400 °C, the electrocatalyst exhibits a good oxygen reduction activity with high methanol tolerance. From both LSV and XPS analyses, it was concluded that a high Se3d_5/2 content plays an important role for oxygen reduction on RuSe/CNT. The EIS characterization also identified the presence of reaction intermediates during the oxygen reduction process. Based on the test results, the mechanisms underlying the dual function of the RuSe/CNT catalyst are proposed. The prepared catalyst was further evaluated for its potential application to DMFC. At 70 °C, the single-cell DMFC integrated with RuSe/CNT exhibited a performance much better than that incorporated with Pt/C counterpart when operated with a high-concentration (i.e., 6 M) methanol fuel. However, substantial improvements are still needed for practical applications.     

Testing of carbon supported Pd-Pt electrocatalysts for methanol electrooxidation in direct methanol fuel cells

In the present work, a detailed characterization of the electrochemical behavior of carbon supported Pd-Pt electrocatalysts toward CO and methanol electrooxidation in direct methanol fuel cells is reported. Technical electrodes containing an ionomer in their catalyst layer were prepared for this purpose. CO and methanol electrooxidation reactions were used as test reactions to compare the electrocatalytic behavior of bimetallic supported nanoparticles in acidic liquid electrolyte and in solid polymer electrolyte (real fuel cell operating conditions). Experimental results in both environments are consistent and show that the electrochemical behavior of carbon supported Pd-Pt depends on their composition, giving the best performance in direct methanol single fuel cell with a Pd:Pt atomic ratio of 25:75 in the catalyst.     

Recent progress of anode catalysts and their support materials for methanol electrooxidation reaction

This review paper summarizes the recent progress of anode catalysts for methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). The electrocatalytic activities of the noble and noble-free catalysts in different electrolyte media are compared and discussed. Noble-free catalysts exhibit high activity in alkaline medium, whereas Pt-based catalysts are the most active MOR catalysts in acidic medium. The types of catalyst support materials for DMFC anodes are also discussed and further divided into carbonaceous and non-carbonaceous materials. The ion and electron transport through the support materials and their effects on the overall performance are elaborated. Lastly, this paper highlights the major challenges in achieving the optimum DMFC performance from the aspect of tailoring the properties of MOR electrocatalysts to pave its way for commercialisation.     

Methanol-tolerant carbon aerogel-supported Pt-Au catalysts for direct methanol fuel cell

Pt-Au nanoparticles supported on carbon aerogel, namely 2:1 has been synthesized by the microwave-assisted polyol process. The structure of Pt-Au nanoparticles is characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The electrochemical property of Pt-Au catalysts for methanol oxidation is evaluated by cyclic voltammetry (CV). The results show that Au-modified Pt catalysts exhibit a high methanol tolerance and improved electrochemical catalytic activity, suggesting that carbon aerogel supported Pt-Au catalysts are better catalysts for the electrochemical oxidation of methanol than conventional Pt catalysts.     

Highly efficient CO2 bubble removal on carbon nanotube supported nanocatalysts for direct methanol fuel cell

In this paper, we investigate the CO₂ microbubble removal on carbon nanotube (CNT)-supported Pt catalysts in direct methanol fuel cells (DMFCs). The experiments involve the incorporation of near-catalyst-layer bubble visualization and simultaneous electrochemical measurements in a DMFC anodic half cell system, in which CH₃OH electro-oxidation generate carbon dioxide (CO₂) microbubbles. We observe rapid removal of smaller CO₂ bubble sizes and less bubble accumulation on a Pt-coated CNT/CC (Pt/CNT/CC, CC means carbon cloth) electrode. The improved half cell performances of the high CO₂ microbubble removal efficiency on the CNT-modified electrode (Pt/CNT/CC) were 34% and 32% higher than on Pt/CC and Pt/CP electrodes, respectively.     

PtRuMo/C catalysts for direct methanol fuel cells: Effect of the pretreatment on the structural characteristics and methanol electrooxidation

The influence of thermal treatment under different environments of PtRuMo/C catalyst has been investigated for CO and methanol electrooxidation in a half cell and in a DMFC single cell. The PtRuMo/C catalysts were synthesized following two step procedure while the thermal treatments consisted of heating at 300 °C in H₂ or He atmosphere for 1 h. Structural characteristics of the electrocatalysts have been studied employing a wide range of instrumental methods, including physicochemical techniques like X-ray diffraction, TEM, TPR, XPS, and electrochemical techniques like single cell studies and Fourier Transform Infrared Spectroscopy adapted to the electrochemical system for in situ studies. These electrocatalysts exhibited good dispersion and small particle size, which increased upon increasing thermal treatment. Moreover, thermal treatment, mainly under H₂ is responsible for the decrease of the lattice parameter and the increase of the spill over effect to Mo sites. These effects were also accompanied by increasing the proportion of the more reduced Ru species in this catalyst. The electrochemical characterization revealed that although all ternary catalysts were more active towards CO and methanol oxidation than the binary catalyst, the catalyst treated with H₂ improves its performance by ca. 15% higher with respect to the ternary catalysts treated either in He treatment or with no treatment. The enhancement in activity is associated with a change in the reaction path, which promotes the direct oxidation of CHO species to CO₂ without the production of the CO poisoning species. The synergistic effect of the three metals seems to be improved and the Mo–Pt and Mo–Ru interaction strengthened.     

A polyoxometalate-deposited Pt/CNT electrocatalyst via chemical synthesis for methanol electrooxidation

Polyoxometalate anion PMo₁₂O₄₀³⁻ (POM) is chemically impregnated into a Pt-supported carbon nanotubes (Pt/CNTs) catalyst that is prepared via a colloidal method. The POM-impregnated Pt/CNTs catalyst system (Pt/CNTs-POM) shows at least 50% higher catalytic mass activity with improved stability for the electrooxidation of methanol than Pt/CNTs or POM-impregnated Pt/C (Pt/C-POM) catalyst systems. The enhancement in electrochemical performance of the Pt/CNTs-POM catalyst system can be attributed to the combined beneficial effects of improved electrical conductivity due to the CNTs support, highly dispersed Pt nanoparticles on the CNTs, and increased oxidation power of the polyoxometalate that can assist oxidative removal of reaction intermediates adsorbed on the Pt catalyst surface.     

Design and preparation of CNT@SnO2 core-shell composites with thin shell and its application for ethanol oxidation

A novel electrocatalyst structure of carbon nanotubes (CNT) coated with thin SnO₂ and Pt (Pt/(CNT@SnO₂)) is reported. The CNT@SnO₂ composites with a thin shell (about 2 nm) are prepared by a simple chemical-solution method. The Pt/(CNT@SnO₂) catalyst is prepared by first microwave heating H₂PtCl₆ in NaOH ethylene glycol solution and then depositing Pt nanoparticles onto CNT@SnO₂ composites. High-resolution transmission electron microscopy and X-ray diffraction show that crystalline SnO₂ of about 2 nm thickness is coated uniformly on the surface of the CNTs. Pt nanoparticles of about 3.2 nm in diameter are homogenously dispersed on the SnO₂ surface. Electrochemical studies were carried out using cyclic voltammetry and chronoamperometry. The results showed that Pt/(CNT@SnO₂) catalysts have much higher catalytic activity and CO-tolerance for ethanol electro-oxidation than that of Pt/CNT.     

Synthesis and optimization of PtRu/TiO2-CNF anodic catalyst for direct methanol fuel cell

Direct methanol fuel cell (DMFC) is a promising power source technology, but it has been unable to be successfully commercialized due to its high cost and low kinetic oxidation. Both problems stem from one of its main components, the catalyst. Therefore, this study is focused on determining and optimizing the electrocatalyst parameters of a high-performance DMFC. The electrocatalyst, PtRu/TiO₂-CNF, is produced by the deposition method and is subjected to electrochemical measurement and cyclic voltammetry (CV) to measure half-cell performance in a DMFC. The optimization process involved two main phases, a screening process followed by response surface methodology (RSM). The resulting optimum parameters were then used for the single cell performance testing. The results show that the mathematical model suggested by RSM is adequate for the optimization of the parameter levels. The optimum parameters suggested by RSM are a PtRu composition of 30.25% and a catalyst loading of 0.59 mg/cm², resulting in almost perfect agreement between the measured current density (603.06 mA/mg_PtRu) and the predicted value (600.63 mA/mg_PtRu). The current density obtained in this study is the highest among other researchers in the same field.     

Synthesis and characterization of PtNx/C as methanol-tolerant oxygen reduction electrocatalysts for a direct methanol fuel cell

Platinum nitride supported on carbon (PtN_x/C) is synthesized by the novel strategy of chelating the Pt precursor followed by pyrolysis and is characterized as a possible cathode electrocatalyst for direct methanol fuel cells (DMFCs). The prepared PtN_x/C is shown to possess high methanol tolerance and catalytic activity for the oxygen reduction reaction (ORR). The results indicate that the temperature of the heat treatment and the molar ratio of Pt to N in the precursor solution play important roles in the catalytic performance. A sample of PtN_x/C prepared at 700 °C with a Pt:N ratio of 1:2 shows a significant decrease in the potential loss associated with the mixed potential and the poisoning effect by adsorbed methanol, and this results in a high power density of 180 mW cm⁻². The performance is 30% higher than that of Pt/C under 4 M of methanol concentration.     

Investigation of Pt/WC/C catalyst for methanol electro-oxidation and oxygen electro-reduction

A Pt/WC/C catalyst is developed to increase the methanol electro-oxidation (MOR) and oxygen electro-reduction (ORR) activities of the Pt/C catalyst. Cyclic voltammetry and CO stripping results show that spill-over of H⁺ occurs in Pt/WC/C, and this is confirmed by comparing the desorption area values for H⁺ and CO. A significant reduction in the potential of the CO electro-oxidation peak from 0.81 V for Pt/C to 0.68 V for Pt/WC/C is observed in CO stripping test results. This indicates that an increase in the activity for CO electro-oxidation is achieved by replacing the carbon support with WC. Preferential deposition of Pt on WC rather than on the carbon support is investigated by complementary analysis of CO stripping, transmission electron microscopy and concentration mapping by energy dispersive spectroscopy. The Pt/WC/C catalyst exhibits a specific activity of 170 mA m⁻² for MOR. This is 42% higher than that for the Pt/C catalyst, viz., 120 mA m⁻². The Pt/WC/C catalyst also exhibits a much higher current density for ORR, i.e., 0.87 mA cm⁻² compared with 0.36 mA cm⁻² for Pt/C at 0.7 V. In the presence of methanol, the Pt/WC/C catalyst still maintains a higher current density than the Pt/C catalyst.     

Na-intercalated carbon nanotubes-supported platinum nanoparticles as new highly effective catalysts for preferential CO oxidation in H2-rich stream

Na⁺-intercalated carbon nanotubes (Na-CNTs) were obtained by impregnation of CNTs with sodium acetate followed by annealing at high temperatures under argon. Stable Na-CNTs-supported Pt catalysts (Pt/Na-CNT catalysts) were then prepared for hydrogen purification via preferential CO oxidation in a H₂-rich stream (CO-PROX). Characteristic studies show that the content of Na⁺ species in CNTs is increased with increased annealing temperature and the Pt nanoparticles with an average size of 2–3 nm are uniformly dispersed on the surfaces of Na-CNTs. An optimized Pt/Na-CNT catalyst with 5 wt% Pt loading can completely remove CO from 40 °C to 200 °C. This catalyst also exhibits long-term stability for 1000 h at 100 °C in feed gas containing 1% CO, 1% O₂, 50% H₂, 15% CO₂, and 10% H₂O balanced with N₂. The electron transfer between the Pt nanoparticles and Na⁺ species plays an important role in enhancing the CO-PROX performance of the catalyst.     

A proper amount of carbon nanotubes for improving the performance of Pt-Ru/C catalysts for methanol electro-oxidation

This study aims to improve the performance of the anode catalyst in a direct methanol fuel cell by using carbon black (XC) and mesoporous carbon (MC) as supporting materials for preparing Pt-Ru/XC and Pt-Ru/MC catalysts. This study investigates the effect of adding different amounts of bare carbon nanotubes (CNTs) or carbon nanotubes impregnated with Pt and Ru (abbreviated as Pt-Ru/CNT, containing 10 wt.% Pt and Ru) to the prepared catalysts. Experimental results reveal that 10 wt.% Pt-Ru/C with carbon black and mesoporous carbon prepared by the multiple impregnation method had smaller Pt-Ru grain sizes and a better dispersion or carbon supports due to low precursor concentrations in each impregnation. These, in turn, achieved better electro-catalytic performance for methanol oxidation. Adding CNTs or Pt-Ru/CNT to Pt-Ru/XC and Pt-Ru/MC obviously improves their electro-catalytic characteristics. The appropriate amounts of bare CNT and Pt-Ru/CNT added to Pt-Ru/XC and Pt-Ru/MC catalysts are 5% and 20%, respectively. The resulting catalysts (both containing 10 wt.% Pt and Ru) produce activities similar to those of the E-TEK Pt-Ru/C catalyst containing 20 wt.% Pt and Ru.     

Methanol resistant ruthenium electrocatalysts for oxygen reduction synthesized by pyrolysis of Ru3(CO)12 in different atmospheres

Novel ruthenium electrocatalysts for the oxygen reduction reaction (ORR) were prepared by pyrolysis of Ru₃(CO)₁₂ in three atmospheres: neutral (N₂), partially oxidative (air) and partially reductive (70:30 N₂/H₂), at temperatures in the 80–700 °C range. The materials were characterized by FT-IR spectroscopy, X-ray diffraction and scanning electron microscopy. A thermogravimetric analysis of the Ru₃(CO)₁₂ precursor in the three atmospheres was also performed. The electrocatalytic properties of the materials were evaluated by rotating disk electrode measurements in 0.5 mol L⁻¹ H₂SO₄. The kinetic parameters, such as the Tafel slope, exchange current density and charge transfer coefficient, are reported. The catalysts prepared in N₂ and N₂/H₂, in general, show a higher performance than those synthesized in air. In the two nitrogen containing atmospheres, a pyrolysis temperature of 360 °C seems to lead to better electrocatalytic properties for the ORR. The new electrocatalysts are also tolerant to methanol concentrations as high as 2.0 mol L⁻¹.     

Influence of metal oxides on Pt catalysts for methanol electrooxidation using electrochemical impedance spectroscopy

Carbon nanotubes used as supports for platinum catalysts deposited with metal oxides (CeO₂, TiO₂, and SnO₂) were prepared for their application as anode catalysts in a direct methanol fuel cell. Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy measurements were carried out in a solution of 0.5 M CH₃OH and 0.5 M H₂SO₄. Catalysts with the addition of CeO₂, TiO₂, and SnO₂ presented higher catalytic activity than pure platinum catalysts, and the catalysts with CeO₂ were the best among them. Electrochemical impedance spectra indicated that methanol electrooxidation on these catalysts had different impedance behaviors at different potential regions. The mechanism of methanol electrooxidation changed with increases of the potential. The promotion effect of the metal oxides lies in the oxidation of intermediate CO_ads on Pt at low potential regions.     

Performance of sputter-deposited platinum cathodes with Nafion and carbon loading for direct methanol fuel cells

One of the difficulties for a direct methanol fuel cell (DMFC) is low catalyst utilization efficiency because a certain amount of Pt loading is inactive as the catalyst. Sputter-deposited Pt electrodes are expected to improve mass activities for oxygen reduction reaction (ORR) compared with those prepared by a conventional method. Meanwhile, mass activities of sputter-deposited Pt cathodes for the ORR decreased with an increase in amount of Pt loading. In this study, the loading of protonic and electronic conductors to improve mass activities of sputter-deposited Pt electrode were investigated as cathodes for DMFCs.     

Investigation of PtCoCr/C catalysts for methanol electro-oxidation identified by a thin film combinatorial method

The ternary Pt-Co-Cr system was investigated for suitability as a methanol electro-oxidation reaction (MOR) catalyst by combinatorial synthesis and high-throughput screening method. A PtCoCr thin film library was prepared by a multi-target sputtering technique while parallel characterization was performed using a multichannel multielectrode analyzer. The highest MOR activity was observed in the Pt₃₀Co₃₀Cr₄₀ composition after a conditioning process. The high MOR activity of the thin film Pt₃₀Co₃₀Cr₄₀ composition was verified in a powder version of the alloy. In 20 h chronoamperometry tests, the MOR activity of the Pt₃₀Co₃₀Cr₄₀ powder catalyst alloy was , which was 160% higher than the value of the PtRu/C catalyst, suggesting that PtCoCr alloys are promising candidates as Ru-free MOR catalysts. The effect of the conditioning process was also investigated, revealing that dissolution and oxidation of surface Co and Cr occurs during conditioning. After conditioning, the activity of 900 °C reduced Pt₃₀Co₃₀Cr₄₀ catalyst dramatically increased by 26.8 times from at 600 s of the chronoamperometry tests.     

Performance of Pt/C catalysts prepared by microwave-assisted polyol process for methanol electrooxidation

Pt nanoparticles catalysts supported on the Vulcan XC-72 carbon black with different mean sizes have been synthesized by microwave-assisted polyol process and characterized by energy dispersive analysis of X-ray (EDAX), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results of physical examinations show that Pt nanoparticles have a narrow size distribution and are highly dispersed on the surface of carbon support, and Pt loading in Pt/C catalyst is the similar with the theoretical value. The results of cyclic voltammetry and chronoamperometry demonstrate that the Pt/C catalyst prepared by microwave-assisted polyol process at the pH value of about 12 exhibits the highest catalytic activity for methanol electrooxidation. The activity of Pt/C catalyst is also related to the microwave heating time, and the optimal heating time is 40 s in this work.     

The influence of carbon based supports and the role of synthesis procedures on the formation of platinum and platinum-ruthenium clusters and nanoparticles for the development of highly active fuel cell catalysts

A simple but effective solvent free method for the synthesis of platinum group metal nanoparticles on carbon nanotubes is presented. The initial work directly compares a typical wet chemical method and an organo-metallic chemical vapour deposition (OMCVD) technique for the production of 10 wt% Platinum on activated carbon and carbon nanotubes. The results obtained clearly showed that the wet chemical method produced materials with poorer physical-chemical characteristics and electrocatalytic activity. Also, carbon nanotubes were shown to be a more effective support regardless of the method of synthesis. Subsequent experimental work focused on the use of carbon nanotubes as a support, and the metal-organic chemical vapour deposition method as the synthesis technique. The method was successfully used to produce multiple samples with loadings of 20, 40 and 60 wt% Pt/CNT and a 40 wt% PtRu/CNT. HRTEM studies revealed stabilized clusters of platinum within CNT defects on samples synthesized using the OMCVD technique. The particle size distribution was relatively narrow, and the electrocatalytic activity was comparable or better than the benchmark Johnson Mathey 40 wt% Pt/C or 40 wt% PtRu/C.     

The influence of carbon nanofiber support properties on the oxygen reduction behavior in proton conducting electrolyte-based direct methanol fuel cells

Platinum nanoparticles supported on fishbone carbon nanofibers (CNFs) were synthesized and studied for the oxygen reduction reaction (ORR). The crystalline and textural properties of the CNFs were modified by synthesizing them at different temperatures, allowing the comparison of supports with either improved graphitization degree or improved porosity. A carbon black (Vulcan XC-72R) was used for comparison. Half-cell studies determined that the ORR activity is enhanced when using a CNF with improved graphitization, in contrast with CNFs with better textural properties such as surface area or pore volume. The catalysts were tested at the cathode of a direct methanol fuel cell corroborating the suitability of using highly graphitic CNFs, and a similar behavior was found in comparison with the state of the art carbon black used in this field.