Electrochemical stability of carbon nanofibers in proton exchange membrane fuel cells

This fundamental study deals with the electrochemical stability of several non-conventional carbon based catalyst supports, intended for low temperature proton exchange membrane fuel cell (PEMFC) cathodes. Electrochemical surface oxidation of raw and functionalized carbon nanofibers, and carbon black for comparison, was studied following a potential step treatment at 25.0 °C in acid electrolyte, which mimics the operating conditions of low temperature PEMFCs. Surface oxidation was characterized using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Cyclic voltammograms clearly showed the presence of the hydroquinone/quinone couple. Furthermore, identification of carbonyl, ether, hydroxyl and carboxyl surface functional groups were made by deconvolution of the XPS spectra. The relative increase in surface oxides on carbon nanofibers during the electrochemical oxidation treatment is significantly smaller than that on carbon black. This suggests that carbon nanofibers are more resistant to the electrochemical corrosion than carbon black under the experimental conditions used in this work. This behaviour could be attributed to the differences found in the microstructure of both kinds of carbons. According to these results, carbon nanofibers possess a high potential as catalyst support to increase the durability of catalysts used in low temperature PEMFC applications.     

Development of carbon nanotube and carbon xerogel supported catalysts for the electro-oxidation of methanol in fuel cells

Multiwalled carbon nanotubes and high surface area mesoporous carbon xerogel were prepared and used as supports for monometallic Pt and bimetallic Pt-Ru catalysts. In order to assess the influence of the oxygen surface groups of the support, the mesoporous carbon xerogel was also oxidized with diluted oxygen before impregnation. Various reduction protocols were tested, the best results corresponding to reduction with sodium borohydride. High dispersion catalysts were obtained, which showed quite good performance in the electro-oxidation of methanol. In particular, a remarkable increase in the activity was observed when the Pt-Ru catalysts were supported on the oxidised xerogel. This effect was explained in terms of the metal oxidation state, as shown by XPS. It has been shown that the oxidised support helps to maintain the metals in the metallic state, as required for the electro-oxidation of methanol. This effect was negligible in the case of the Pt catalysts.     

Carbon aerogel supported Pt-Ru catalysts for using as the anode of direct methanol fuel cells

A carbon aerogel (CA) loaded with platinum nanoparticles can achieve good catalytic performance in proton exchange membrane fuel cells. Pt-Ru bimetallic nanoparticles were loaded onto a carbon aerogel through a simple process. The PtRu/CA achieved good cell performance when used as a direct methanol fuel cell anode catalyst. The advantages of carbon aerogel may be attributed to the mesopore structure that can facilitate the mass transportation in the electrode. The Ru content in the catalyst has a great influence on its performance. The PtRu/CA with 25 at.% Ru achieves the best cell performance at 30 °C.     

Carbon nanofibers: Synthesis, characterization, and electrochemical properties

Carbon nanofibers (CNFs) have been synthesized by co-catalyst deoxidization process by a reaction between C₂H₅OC₂H₅, Zn and Fe powder at 650 °C for 10 h. These nanofibers exhibit diameters of ∼80 nm and lengths ranging from several micrometers to tens of micrometers. X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy indicate that as-prepared CNFs possess low graphitic crystallinity. The resultant CNFs as electrode shows capacity of ∼220 mAh/g and high reversibility with little hysteresis in the insertion/deintercalation reactions of lithium-ion. In addition, the possible growth of CNFs is discussed.     

Carbon supported Pt–Co alloys as methanol-resistant oxygen-reduction electrocatalysts for direct methanol fuel cells

The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co (Pt/C and Pt–Co/C) alloy electrocatalysts was investigated in sulphuric acid (both in the absence and in the presence of methanol) and in direct methanol fuel cells (DMFCs). In pure sulphuric acid Pt–Co/C alloys showed improved specific activity towards the oxygen reduction compared to pure platinum. In the methanol containing electrolyte a higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. The onset potential for methanol oxidation at Pt–Co/C was shifted to more positive potentials. Accordingly, Pt–Co/C electrocatalyts showed an improved performance as cathode materials in DMFCs.     

Carbon supported PtBi catalysts for direct formic acid fuel cells

Carbon supported PtBi bimetallic catalysts (PtBi/C) prepared by depositing Bi on a commercial Pt/C catalyst and by codeposition of Pt and Bi have been compared for formic acid oxidation in a multi-anode direct formic acid fuel cell. Both types of catalyst gave much higher cell performances than the Pt/C, with only low amounts of Bi (Pt to Bi mole ratios of 11:1 and 14:1, respectively) required for optimum performance. The high Pt to Bi ratio for the best codeposited catalyst indicates that the Bi was concentrated at the surface, and this is consistent with X-ray diffraction and X-ray photoelectron spectroscopy results. However, cyclic voltammetry revealed a strong electronic effect that is inconsistent with surface decoration. The effects of the Bi have been attributed to selective blocking of sites at which CO is formed.     

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.     

A highly efficient synthesis approach of supported Pt-Ru catalyst for direct methanol fuel cell

A highly efficient synthesis approach of urea-assisted homogeneous deposition (HD) coupled with H₂ reduction has been employed to synthesize carbon-supported Pt-Ru catalyst with high metal loading of 60 wt%. The urea-assisted HD method permits in situ gradual and homogeneous generation of hydroxide ions, resulting in smaller Pt-Ru nanaoparticles deposited on the carbon support along with better particle size distribution compared with the catalyst prepared by the conventional impregnation-NaBH₄ reduction. The Pt-Ru catalyst prepared by the HD-H₂ method has demonstrated greatly enhanced catalytic activity towards methanol oxidation and much improved fuel cell performance compared with the one prepared by impregnation-NaBH₄ reduction. HD-H₂ method is simple with mild synthesis condition, but provides very efficient approach for the preparation of Pt-based catalysts for fuel cells.     

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.     

Noble metals supported on carbon black composites as catalysts for the wet-air oxidation of phenol

Pt, Pd and Ru catalysts supported on carbon black composites (CBC) were characterized in the wet air oxidation of phenol solution using a fixed-bed reactor working in a trickle-flow regime under relatively mild conditions: temperature, 393-433 K; pressure, 50-80 bar; liquid hourly space velocity (LHSV), 0.5-6 h⁻¹. The activity of the catalysts decreases in the following order: Pt/CBC>Pd/CBC≈Ru/CBC?CBC. The physicochemical properties of the CBC are affected by its reaction with oxygen during the oxidation process. Combustion of the CBC material in the aqueous phase proceeds at a lower temperature than that in the gas phase; its surface properties change according the same rules as during low temperature oxidation by gaseous air.     

Carbon supported Pt–Cr alloys as oxygen-reduction catalysts for direct methanol fuel cells

Electrocatalytic activities of various carbon-supported platinum–chromium alloy electrocatalysts towards oxygen reduction in 1 mol l⁻¹ H₂SO₄ and in 1 mol l⁻¹ H₂SO₄/1–3 mol l⁻¹ CH₃OH, were investigated by means of rotating disc electrode experiments and in solid polymer electrolyte direct methanol fuel cells. The activity of these electrocatalysts for methanol oxidation was evaluated using cyclic voltammetry. It was found that Pt₉Cr/C prepared by reduction with NaBH₄ exhibits the lowest activity for methanol oxidation and the highest activity for oxygen reduction in the presence of methanol, in comparison to commercial Pt/C, Pt₃Cr/C and PtCr/C electrocatalysts.     

Thermal degradation of the support in carbon-supported platinum electrocatalysts for PEM fuel cells

The cathode catalyst layer in proton exchange membrane (PEM) fuel cells can contain nanometer-sized platinum particles dispersed on a high surface area carbon. In order to assess support stability, samples of carbon-supported catalysts were held at elevated temperatures under dry air conditions. The samples were weighed at regular intervals. These tests showed that the platinum particles were able to catalyze the combustion of the carbon support at moderate temperatures (125-195 °C). As the temperature increased, the rate of carbon combustion increased. The amount of carbon that was lost after extended oven exposure at a constant temperature was shown to depend on both the temperature and platinum loading. A simple first-order kinetic model was able to describe the results. With further work on a range of different carbon supports, this work is expected to help develop more stable catalyst supports for PEM fuel cells.     

Alternative platinum electrocatalyst supporter with micro/nanostructured polyaniline for direct methanol fuel cell applications

In this study, a series of micro/nanostructured polyanilines were synthesized and their morphology-dependent electrochemical properties for acting as a catalyst supporter for direct methanol fuel cell (DMFC) applications were investigated. These micro/nanostructures include submicron spheres, hollow microspheres, nanotubes, and nanofibers. Among the four micro/nanostructures, polyaniline nanofibers (PANF) manifest their superiority in high electrochemical active surface. Accordingly, PANF is adopted as the catalyst supporter thereafter. To couple with the use of the alternative catalyst supporter, this study also investigates the effect of reductant type on morphology and electrocatalytic properties of the PANF-supported Pt particles through a chemical reduction reaction. TEM images indicate that formic acid as a reductant results in well-dispersed Pt particles on the PANF surface. On the other hand, aggregations of Pt are observable when NaBH₄ is selected as a reductant. Moreover, the methanol oxidation current density measured with the Pt/PANF electrode being prepared by using formic acid is double that by using NaBH₄. Compared with Pt/XC-72, the Pt/PANF electrode possesses higher electrocatalytic activity and exhibits double power density. Moreover, Pt/PANF is superior to Pt/XC-72 in the aspect of operation stability based on a continuous discharge for 5 h.     

Synthesis of multi-branched porous carbon nanofibers and their application in electrochemical double-layer capacitors

Multi-branched carbon nanofibers with a porous structure have been synthesized on a Cu catalyst doped with Li, Na, or K. The products were characterized by field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. Using this new type of nanofiber as polarized electrodes, an electrochemical double-layer capacitor with a specific capacitance of ca. 297 F/g was obtained using 6 M KOH as the electrolyte.     

Highly dispersed Ag nanoparticles on functional MWNT surfaces for methanol oxidation in alkaline solution

Silver (Ag) nanoparticles were electrocrystallized on 4-aminobenzene monolayer-grafted multi-walled carbon nanotubes (MWNTs) by a potential-step method. The structure and nature of the resulting Ag/MWNT composite were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), the results show that the electrochemically synthesized Ag nanoparticles were homogeneously dispersed and well-separated from one another on the modified MWNT surfaces. The electrocatalytic properties of the Ag/MWNT electrode for oxidation of methanol in alkaline solution have been investigated by cyclic voltammetry (CV), and excellent electrocatalytic activity was observed. This may be attributed to the small particle size of the silver particles. The results imply that the Ag/MWNT composites have a good application potential in fuel cells.     

Carbon supported and unsupported Pt–Ru anodes for liquid feed direct methanol fuel cells

A comparative study of the use of supported and unsupported catalysts for direct methanol fuel cells has been performed. The effect of catalyst loading, fuel concentration and temperature dependence on the anode, cathode and full fuel cell performance was determined in a fuel cell equipped with a reversible hydrogen reference electrode. Although the measured specific activities of supported catalysts were as much as 3-fold greater than the unsupported catalysts, membrane electrode assemblies prepared with supported catalysts showed no improvement with loadings above 0.5 mg/cm². Fuel cells utilizing 0.46 mg/cm² supported catalyst electrodes performed as well as unsupported catalyst electrodes with 2 mg/cm². The temperature dependence and methanol concentration dependence studies both suggest increased methanol permeation through the thinner supported catalyst layers relative to the unsupported catalyst layers.